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What is the speed of light? Here’s the history, discovery of the cosmic speed limit

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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Why is the speed of light the way it is?

It's just plain weird.

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio , and author of " How to Die in Space ." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights . 

We all know and love the speed of light — 299,792,458 meters per second — but why does it have the value that it does? Why isn't it some other number? And why do we care so much about some random speed of electromagnetic waves? Why did it become such a cornerstone of physics? 

Well, it's because the speed of light is just plain weird.

Related: Constant speed of light: Einstein's special relativity survives a high-energy test

Putting light to the test

The first person to realize that light does indeed have a speed at all was an astronomer by the name of Ole Romer. In the late 1600s, he was obsessed with some strange motions of the moon Io around Jupiter. Every once in a while, the great planet would block our view of its little moon, causing an eclipse, but the timing between eclipses seemed to change over the course of the year. Either something funky was happening with the orbit of Io — which seemed suspicious — or something else was afoot.

After a couple years of observations, Romer made the connection. When we see Io get eclipsed, we're in a certain position in our own orbit around the sun. But by the next time we see another eclipse, a few days later, we're in a slightly different position, maybe closer or farther away from Jupiter than the last time. If we are farther away than the last time we saw an eclipse, then that means we have to wait a little bit of extra time to see the next one because it takes that much longer for the light to reach us, and the reverse is true if we happen to be a little bit closer to Jupiter.

The only way to explain the variations in the timing of eclipses of Io is if light has a finite speed.

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Making it mean something

Continued measurements over the course of the next few centuries solidified the measurement of the speed of light, but it wasn't until the mid-1800s when things really started to come together. That's when the physicist James Clerk Maxwell accidentally invented light.

Maxwell had been playing around with the then-poorly-understood phenomena of electricity and magnetism when he discovered a single unified picture that could explain all the disparate observations. Laying the groundwork for what we now understand to be the electromagnetic force , in those equations he discovered that changing electric fields can create magnetic fields, and vice versa. This allows waves of electricity to create waves of magnetism, which go on to make waves of electricity and back and forth and back and forth, leapfrogging over each other, capable of traveling through space.

And when he went to calculate the speed of these so-called electromagnetic waves, Maxwell got the same number that scientists had been measuring as the speed of light for centuries. Ergo, light is made of electromagnetic waves and it travels at that speed, because that is exactly how quickly waves of electricity and magnetism travel through space.

And this was all well and good until Einstein came along a few decades later and realized that the speed of light had nothing to do with light at all. With his special theory of relativity , Einstein realized the true connection between time and space, a unified fabric known as space-time. But as we all know, space is very different than time. A meter or a foot is very different than a second or a year. They appear to be two completely different things.

So how could they possibly be on the same footing?

There needed to be some sort of glue, some connection that allowed us to translate between movement in space and movement in time. In other words, we need to know how much one meter of space, for example, is worth in time. What's the exchange rate? Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa.

Einstein's theories didn't say what that number was, but then he applied special relativity to the old equations of Maxwell and found that this conversion rate is exactly the speed of light.

Of course, this conversion rate, this fundamental constant that unifies space and time, doesn't know what an electromagnetic wave is, and it doesn't even really care. It's just some number, but it turns out that Maxwell had already calculated this number and discovered it without even knowing it. That's because all massless particles are able to travel at this speed, and since light is massless, it can travel at that speed. And so, the speed of light became an important cornerstone of modern physics.

But still, why that number, with that value, and not some other random number? Why did nature pick that one and no other? What's going on?

Related: The genius of Albert Einstein: his life, theories and impact on science

Making it meaningless

Well, the number doesn't really matter. It has units after all: meters per second. And in physics any number that has units attached to it can have any old value it wants, because it means you have to define what the units are. For example, in order to express the speed of light in meters per second, first you need to decide what the heck a meter is and what the heck a second is. And so the definition of the speed of light is tied up with the definitions of length and time.

In physics, we're more concerned with constants that have no units or dimensions — in other words, constants that appear in our physical theories that are just plain numbers. These appear much more fundamental, because they don't depend on any other definition. Another way of saying it is that, if we were to meet some alien civilization , we would have no way of understanding their measurement of the speed of light, but when it comes to dimensionless constants, we can all agree. They're just numbers.

One such number is known as the fine structure constant, which is a combination of the speed of light, Planck's constant , and something known as the permittivity of free space. Its value is approximately 0.007. 0.007 what? Just 0.007. Like I said, it's just a number.

So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is.

So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know.

Learn more by listening to the episode "Why is the speed of light the way it is?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Robert H, Michael E., @DesRon94, Evan W., Harry A., @twdixon, Hein P., Colin E., and Lothian53 for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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• voidpotentialenergy This is just my opinion but i think L speed is it's speed because the particle part of it is the fastest it can interact with the quanta distance in quantum fluctuation. Light is particle and wave so the wave happens in the void between quanta. Gravity probably travels in that void and why gravity seems instant. Reply
• rod The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*. Reply

Admin said: We all know and love the speed of light, but why does it have the value that it does? Why isn't it some other number? And why did it become such a cornerstone of physics? Why is the speed of light the way it is? : Read more

rod said: The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*.

• rod FYI. When someone says *the universe has chosen*, I am reminded of these five lessons from a 1982 Fed. court trial. The essential characteristics of science are: It is guided by natural law; It has to be explanatory by reference to natural law; It is testable against the empirical world; Its conclusions are tentative, i.e., are not necessarily the final word; and It is falsifiable. Five important points about science. Reply
• Gary If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ? Reply
• Gary Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ? Reply

Gary said: Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ?

Gary said: If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ?

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Learn About the True Speed of Light and How It's Used

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Light moves through the universe at the fastest speed astronomers can measure. In fact, the speed of light is a cosmic speed limit, and nothing is known to move faster. How fast does light move? This limit can be measured and it also helps define our understanding of the universe's size and age.

What Is Light: Wave or Particle?

Light travels fast, at a velocity of 299, 792, 458 meters per second. How can it do this? To understand that, it's helpful to know what light actually is and that's largely a 20th-century discovery.

The nature of light was a great mystery for centuries. Scientists had trouble grasping the concept of its wave and particle nature. If it was a wave what did it propagate through? Why did it appear to travel at the same speed in all directions? And, what can the speed of light tell us about the cosmos? It wasn't until Albert Einstein described this theory of special relativity in 1905 it all came into focus. Einstein argued that space and time were relative and that the speed of light was the constant that connected the two.

What Is the Speed of Light?

It is often stated that the speed of light is constant and that nothing can travel faster than the speed of light. This isn't entirely accurate. The value of 299,792,458 meters per second (186,282 miles per second) is the speed of light in a vacuum. However, light actually slows down as it passes through different media. For instance, when it moves through glass, it slows down to about two-thirds of its speed in a vacuum. Even in air, which is nearly a vacuum, light slows down slightly. As it moves through space, it encounters clouds of gas and dust, as well as gravitational fields, and those can change the speed a tiny bit. The clouds of gas and dust also absorb some of the light as it passes through.

This phenomenon has to do with the nature of light, which is an electromagnetic wave. As it propagates through a material its electric and magnetic fields "disturb" the charged particles that it comes in contact with. These disturbances then cause the particles to radiate light at the same frequency, but with a phase shift. The sum of all these waves produced by the "disturbances" will lead to an electromagnetic wave with the same frequency as the original light, but with a shorter wavelength and, hence a slower speed.

Interesting, as fast as light moves, its path can be bent as it passes by regions in space with intense gravitational fields. This is fairly easily seen in galaxy clusters, which contain a lot of matter (including dark matter), which warps the path of light from more distant objects, such as quasars.

Lightspeed and Gravitational Waves

Current theories of physics predict that gravitational waves also travel at the speed of light, but this is still being confirmed as scientists study the phenomenon of gravitational waves from colliding black holes and neutron stars. Otherwise, there are no other objects that travel that fast. Theoretically, they can get close to the speed of light, but not faster.

One exception to this may be space-time itself. It appears that distant galaxies are moving away from us faster than the speed of light. This is a "problem" that scientists are still trying to understand. However, one interesting consequence of this is that a travel system based on the idea of a warp drive . In such a technology, a spacecraft is at rest relative to space and it's actually space that moves, like a surfer riding a wave on the ocean. Theoretically, this might allow for superluminal travel. Of course, there are other practical and technological limitations that stand in the way, but it's an interesting science-fiction idea that is getting some scientific interest. 

Travel Times for Light

One of the questions that astronomers get from members of the public is: "how long would it take light to go from object X to Object Y?" Light gives them a very accurate way to measure the size of the universe by defining distances. Here are a few of the common ones distance measurements:

• The Earth to the Moon : 1.255 seconds
• The Sun to Earth : 8.3 minutes
• Our Sun to the next closest star : 4.24 years
• Across our Milky Way  galaxy : 100,000 years
• To the closest  spiral galaxy (Andromeda) : 2.5 million years
• Limit of the observable universe to Earth : 13.8 billion years

Interestingly, there are objects that are beyond our ability to see simply because the universe IS expanding, and some are "over the horizon" beyond which we cannot see. They will never come into our view, no matter how fast their light travels. This is one of the fascinating effects of living in an expanding universe. 

Edited by Carolyn Collins Petersen

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What Is the Speed of Light?

The speed of light is the rate at which light travels. The speed of light in a vacuum is a constant value that is denoted by the letter c and is defined as exactly 299,792,458 meters per second. Visible light , other electromagnetic radiation, gravity waves, and other massless particles travel at c. Matter , which has mass, can approach the speed of light, but never reach it.

Value for the Speed of Light in Different Units

Here are values for the speed of light in various units:

• 299,792,458 meters per second ( exact number )
• 299,792 kilometers per second (rounded)
• 3×10 8 m/s (rounded)
• 186,000 miles per second (rounded)
• 671,000,000 miles per hour (rounded)
• 1,080,000,000 kilometers per hour (rounded)

Is the Speed of Light Really Constant?

The speed of light in a vacuum is a constant. However, scientists are exploring whether the speed of light has changed over time.

Also, the rate at which light travels changes as it passes through a medium. The index of refraction describes this change. For example, the index of refraction of water is 1.333, which means light travels 1.333 times slower in water than in a vacuum. The index of refraction of a diamond is 2.417. A diamond slows the speed of light by more than half its speed in a vacuum.

How to Measure the Speed of Light

One way of measuring the speed of light uses great distances, such as distant points on the Earth or known distances between the Earth and astronomical objects. For example, you can measure the speed of light by measuring the time it takes for light to travel from a light source to a distant mirror and back again. The other way of measuring the speed of light is solving for c in equations. Now that the speed of light is defined, it is fixed rather than measured. Measuring the speed of light today indirectly measures the length of the meter, rather than c .

In 1676, Danish astronomer Ole Rømer discovered light travels at a speed by studying the movement of Jupiter’s moon Io. Prior to this, it seemed light propagated instantaneously. For example, you see a lightning strike immediately, but don’t hear thunder until after the event . So, Rømer’s finding showed light takes time to travel, but scientists did not know the speed of light or whether it was constant. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave that travelled at a speed c . Albert Einstein suggested c was a constant and that it did not change according to the frame of reference of the observer or any motion of a light source. In other words, Einstein suggested the speed of light is invariant . Since then, numerous experiments have verified the invariance of c .

Is It Possible to Go Faster Than Light?

The upper speed limit for massless particles is c . Objects that have mass cannot travel at the speed of light or exceed it. Among other reasons, traveling at c gives an object a length of zero and infinite mass. Accelerating a mass to the speed of light requires infinite energy. Furthermore, energy, signals, and individual photos cannot travel faster than c . At first glance, quantum entanglement appears to transmit information faster than c . When two particles are entangled, changing the state of one particle instantaneously determines the state of the other particle, regardless of the distance between them. But, information cannot be transmitted instantaneously (faster than c ) because it isn’t possible to control the initial quantum state of the particle when it is observed.

However, faster-than-light speeds appear in physics. For example, the phase velocity of x-rays through glass often exceeds c. However, the information isn’t conveyed by the waves faster than the speed of light. Distant galaxies appear to move away from Earth faster than the speed of light (outside a distance called the Hubble sphere), but the motion isn’t due to the galaxies traveling through space. Instead, space itself it expanding. So again, no actual movement faster than c occurs.

While it isn’t possible to go faster than the speed of light, it doesn’t necessarily mean warp drive or other faster-than-light travel is impossible. The key to going faster than the speed of light is to change space-time. Ways this might happen include tunneling using wormholes or stretching space-time into a “warp bubble” around a spacecraft. But, so far these theories don’t have practical applications.

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• Helmcke, J.; Riehle, F. (2001). “Physics behind the definition of the meter”. In Quinn, T.J.; Leschiutta, S.; Tavella, P. (eds.). Recent advances in metrology and fundamental constants . IOS Press. p. 453. ISBN 978-1-58603-167-1.
• Newcomb, S. (1886). “The Velocity of Light”. Nature . 34 (863): 29–32. doi: 10.1038/034029c0
• Uzan, J.-P. (2003). “The fundamental constants and their variation: observational status and theoretical motivations”. Reviews of Modern Physics . 75 (2): 403. doi: 10.1103/RevModPhys.75.403

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Speed of Light Calculator

Table of contents

With this speed of light calculator, we aim to help you calculate the distance light can travel in a fixed time . As the speed of light is the fastest speed in the universe, it would be fascinating to know just how far it can travel in a short amount of time.

We have written this article to help you understand what the speed of light is , how fast the speed of light is , and how to calculate the speed of light . We will also demonstrate some examples to help you understand the computation of the speed of light.

What is the speed of light? How fast is the speed of light?

The speed of light is scientifically proven to be the universe's maximum speed. This means no matter how hard you try, you can never exceed this speed in this universe. Hence, there are also some theories on getting into another universe by breaking this limit. You can understand this more using our speed calculator and distance calculator .

So, how fast is the speed of light? The speed of light is 299,792,458 m/s in a vacuum. The speed of light in mph is 670,616,629 mph . With this speed, one can go around the globe more than 400,000 times in a minute!

One thing to note is that the speed of light slows down when it goes through different mediums. Light travels faster in air than in water, for instance. This phenomenon causes the refraction of light.

Now, let's look at how to calculate the speed of light.

How to calculate the speed of light?

As the speed of light is constant, calculating the speed of light usually falls on calculating the distance that light can travel in a certain time period. Hence, let's have a look at the following example:

• Source: Light
• Speed of light: 299,792,458 m/s
• Time traveled: 100 seconds

You can perform the calculation in three steps:

Determine the speed of light.

As mentioned, the speed of light is the fastest speed in the universe, and it is always a constant in a vacuum. Hence, the speed of light is 299,792,458 m/s .

Determine the time that the light has traveled.

The next step is to know how much time the light has traveled. Unlike looking at the speed of a sports car or a train, the speed of light is extremely fast, so the time interval that we look at is usually measured in seconds instead of minutes and hours. You can use our time lapse calculator to help you with this calculation.

For this example, the time that the light has traveled is 100 seconds .

Calculate the distance that the light has traveled.

The final step is to calculate the total distance that the light has traveled within the time . You can calculate this answer using the speed of light formula:

distance = speed of light × time

Thus, the distance that the light can travel in 100 seconds is 299,792,458 m/s × 100 seconds = 29,979,245,800 m

What is the speed of light in mph when it is in a vacuum?

The speed of light in a vacuum is 670,616,629 mph . This is equivalent to 299,792,458 m/s or 1,079,252,849 km/h. This is the fastest speed in the universe.

Is the speed of light always constant?

Yes , the speed of light is always constant for a given medium. The speed of light changes when going through different mediums. For example, light travels slower in water than in air.

How can I calculate the speed of light?

You can calculate the speed of light in three steps:

Determine the distance the light has traveled.

Apply the speed of light formula :

speed of light = distance / time

How far can the speed of light travel in 1 minute?

Light can travel 17,987,547,480 m in 1 minute . This means that light can travel around the earth more than 448 times in a minute.

Speed of light

The speed of light in the medium. In a vacuum, the speed of light is 299,792,458 m/s.

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How to travel at (nearly) the speed of light.

One hundred years ago, on May 29, 1919, measurements of a solar eclipse offered proof for Einstein’s theory of general relativity. Even before that, Einstein had developed the theory of special relativity, which revolutionized the way we understand light. To this day, it provides guidance on understanding how particles move through space — a key area of research to keep spacecraft and astronauts safe from radiation.

The theory of special relativity showed that particles of light, photons, travel through a vacuum at a constant pace of 670,616,629 miles per hour — a speed that’s immensely difficult to achieve and impossible to surpass in that environment. Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light.

Scientists suspect magnetic reconnection is one way that particles are accelerated to nearly light speed. This illustration depicts the magnetic fields around Earth, which snap and realign, causing charged particles to be flung away at high speeds. Find out all three ways that this acceleration happens .

Image Credit: NASA

Can anything travel faster than the speed of light?

Does it matter if it's in a vacuum?

In 1676, by studying the motion of Jupiter's moon Io, Danish astronomer Ole Rømer calculated that light travels at a finite speed. Two years later, building on data gathered by Rømer, Dutch mathematician and scientist Christiaan Huygens became the first person to attempt to determine the actual speed of light, according to the American Museum of Natural History in New York City. Huygens came up with a figure of 131,000 miles per second (211,000 kilometers per second), a number that isn't accurate by today's standards — we now know that the speed of light in the "vacuum" of empty space is about 186,282 miles per second (299,792 km per second) — but his assessment showcased that light travels at an incredible speed.

According to Albert Einstein 's theory of special relativity , light travels so fast that, in a vacuum, nothing in the universe is capable of moving faster. 

"We cannot move through the vacuum of space faster than the speed of light," confirmed Jason Cassibry, an associate professor of aerospace engineering at the Propulsion Research Center, University of Alabama in Huntsville.

Question answered, right? Maybe not. When light is not in a vacuum, does the rule still apply?

Related: How many atoms are in the observable universe?

"Technically, the statement 'nothing can travel faster than the speed of light' isn't quite correct by itself," at least in a non-vacuum setting, Claudia de Rham, a theoretical physicist at Imperial College London, told Live Science in an email. But there are certain caveats to consider, she said. Light exhibits both particle-like and wave-like characteristics, and can therefore be regarded as both a particle (a photon ) and a wave. This is known as wave-particle duality.

If we look at light as a wave, then there are "multiple reasons" why certain waves can travel faster than white (or colorless) light in a medium, de Rham said. One such reason, she said, is that "as light travels through a medium — for instance, glass or water droplets — the different frequencies or colors of light travel at different speeds." The most obvious visual example of this occurs in rainbows, which typically have the long, faster red wavelengths at the top and the short, slower violet wavelengths at the bottom, according to a post by the University of Wisconsin-Madison . 

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When light travels through a vacuum, however, the same is not true. "All light is a type of electromagnetic wave, and they all have the same speed in a vacuum (3 x 10^8 meters per second). This means both radio waves and gamma rays have the same speed," Rhett Allain, a physics professor at Southeastern Louisiana University, told Live Science in an email.

So, according to de Rham, the only thing capable of traveling faster than the speed of light is, somewhat paradoxically, light itself, though only when not in the vacuum of space. Of note, regardless of the medium, light will never exceed its maximum speed of 186,282 miles per second.

Universal look

According to Cassibry, however, there is something else to consider when discussing things moving faster than the speed of light.

"There are parts of the universe that are expanding away from us faster than the speed of light, because space-time is expanding," he said. For example, the Hubble Space Telescope recently spotted 12.9 billion year-old light from a distant star known as Earendel. But, because the universe is expanding at every point, Earendel is moving away from Earth and has been since its formation, so the galaxy is now 28 billion light years away from Earth.

In this case, space-time is expanding, but the material in space-time is still traveling within the bounds of light speed.

Related: Why is space a vacuum?

So, it's clear that nothing travels faster than light that we know of, but is there any situation where it might be possible? Einstein's theory of special relativity, and his subsequent theory of general relativity, is "built under the principle that the notions of space and time are relative," de Rham said. But what does this mean? "If someone [were] able to travel faster than light and carry information with them, their notion of time would be twisted as compared to ours," de Rham said. "There could be situations where the future could affect our past, and then the whole structure of reality would stop making sense."

This would indicate that it would probably not be desirable to make a human travel faster than the speed of light. But could it ever be possible? Will there ever be a time when we are capable of creating craft that could propel materials — and ultimately humans — through space at a pace that outstrips light speed? "Theorists have proposed various types of warp bubbles that could enable faster-than-light travel," Cassibry said.

But is de Rham convinced?

"We can imagine being able to communicate at the speed of light with systems outside our solar system ," de Rham said. "But sending actual physical humans at the speed of light is simply impossible, because we cannot accelerate ourselves to such speed.

"Even in a very idealistic situation where we imagine we could keep accelerating ourselves at a constant rate — ignoring how we could even reach a technology that could keep accelerating us continuously — we would never actually reach the speed of light," she added. "We could get close, but never quite reach it."

Related: How long is a galactic year?

This is a point confirmed by Cassibry. "Neglecting relativity, if you were to accelerate with a rate of 1G [Earth gravity], it would take you a year to reach the speed of light. However, you would never really reach that velocity because as you start to approach lightspeed, your mass energy increases, approaching infinite. "One of the few known possible 'cheat codes' for this limitation is to expand and contract spacetime, thereby pulling your destination closer to you. There seems to be no fundamental limit on the rate at which spacetime can expand or contract, meaning we might be able to get around this velocity limit someday."

— What would happen if the speed of light were much lower?

— What if the speed of sound were as fast as the speed of light?

— How does the rubber pencil illusion work?

Allain is similarly confident that going faster than light is far from likely, but, like Cassibry, noted that if humans want to explore distant planets, it may not actually be necessary to reach such speeds. "The only way we could understand going faster than light would be to use some type of wormhole in space," Allain said. "This wouldn't actually make us go faster than light, but instead give us a shortcut to some other location in space."

Cassibry, however, is unsure if wormholes will ever be a realistic option.

"Wormholes are theorized to be possible based on a special solution to Einstein's field equations," he said. "Basically, wormholes, if possible, would give you a shortcut from one destination to another. I have no idea if it's possible to construct one, or how we would even go about doing it." Originally published on Live Science.

Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester. 

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How does a photon know to travel at the speed of light?

25 August 2021

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How does a photon “know” to travel at the speed of light?

Elaine Patrick Cyffylliog, Denbighshire, UK

I don’t know, ask Erwin Schrödinger. He was a relative of mine on my mother’s side. He told her mum, but she couldn’t understand it either. We’ve been in the dark ever since.

Yang Guijen Balik Pulau, Penang, Malaysia

The laws of the universe require that all the energy and matter particles occupying its space must abide by its rules – so as to maintain a viable home and playground for all. One of these rules is that if you are a massless particle of electromagnetic origin, and you want to play in vacuum space, then you must move at the speed of light, 299,792,458 metres per second, consistently.

If you are a particle with mass, however, then there are other rules that you can follow.

@kbachmann, via Twitter

Wouldn’t any speed travelled by photons be, by definition, the speed of light?

Ian Glendinning Vienna, Austria

All massless particles always travel at a speed represented by the letter c , whereas massive particles can travel at any speed between zero and c . Since photons are massless, they travel at c , which is called the speed of light because the photon was the first known example of a massless particle.

So the short answer to the question is that a photon knows to travel at the speed of light because it is massless.

Ken Appleby Ledbury, Herefordshire, UK

What we call photons are actually interactions of electromagnetic fields. Between interactions, photons don’t exist. You can’t watch a photon in transit, only detect an excitation of the electromagnetic field when it happens.

Photons don’t exist as particles. There are no particles, just interactions of quantum fields. Maxwell’s equations embody and explain in elegant mathematics the empirical results of Faraday’s experiments into electrostatic and magnetic fields – fields being a novel concept of Faraday’s invention. The equations reveal the existence of electromagnetic waves, which are always observed to travel at the constant speed c , regardless of the motion of emitter, receiver or observer. It was this apparent paradox that Einstein’s special relativity paper resolved, by dispensing with the notions of simultaneity, absolute space and time.

So at root, the answer to your question is just simply that that is reality. That is what we observe. The reasons are illuminated by the equations of electrodynamics, but ultimately it is an empirical observation. At least, so far.

James Bailey Southampton, Hampshire, UK

This question is the wrong way round. A photon is a packet of electromagnetic radiation. A very small part of the spectrum of that radiation (wavelengths of around 400 to 750 nanometres) is detectable with our eyes and we call this light. It is like asking why light takes 1/299,792,458 of a second to travel 1 metre, when in fact we just find it more convenient to define it as that, rather than use the old definition of a metre as a ten-millionth of the distance from the equator to the North Pole.

The really interesting question for me is why does electromagnetic radiation travel at 300,000 kilometres per second, and that brings us back to the question of time that has been raised before. Does light travel through time? If so, what exactly is it that it is travelling through? Or does time itself do the moving and is constantly sweeping past us like the wind while everything else stands still?

Does time actually exist as anything or is it just a convenient invention to allow us to talk about how things are moving?

To answer this question – or ask a new one – email [email protected] .

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Why you can't travel at the speed of light

A lbert Einstein is famous for many things, not least his theories of relativity. The first, the special theory of relativity, was the one that began the physicist's reputation for tearing apart the classical worldview that had come before. Special relativity, a way of relating the motion of objects in the universe, led scientists to re-evaluate their assumptions about things as fundamental as time and space. And it led to important revelations about the relationship between energy and matter.

Special relativity was published by Einstein in 1905, in a paper titled "On the Electrodynamics of Moving Bodies". He came to it after picking on a conflict he noticed between the equations for electricity and magnetism, which the physicist James Clerk Maxwell had recently developed, and Isaac Newton's more established laws of motion.

Light, according to Maxwell, was a vibration in the electromagnetic field and it travelled at a constant speed in a vacuum. More than 100 years earlier, Newton had set down his laws of motion and, together with ideas from Galileo Galilei, these showed how the speed of an object would differ depend on who was measuring it and how they were moving relative to the object. A ball you are holding will seem still to you, even when you're in a moving car. But that ball will seem to be moving to anyone standing on the pavement.

But there was a problem in applying Newton's laws of motion to light. In Maxwell's equations, the speed of electromagnetic waves is a constant defined by the properties of the material through which the waves move. There is nothing in there that allows the speed of these waves to be different for different people depending on how they were moving relative to each other. Which is bizarre, if you think about it.

Imagine someone sitting in a stationary train, throwing a ball from where he's sitting to the opposite wall, a few metres further down the train from him. You, standing on the station platform, measure the speed of the ball at the same value as the person on the train.

Now the train starts to move (in the direction of the ball), and you again measure the speed of the ball. You would rightly calculate it as higher – the initial speed (ie, when the train was at rest) plus the forward speed of the train. On the train, meanwhile, the game-player will notice nothing different. Your two values for the speed of the ball will be different; both correct for your frames of reference.

Replace the ball with light and this calculation goes awry. If the person on the train were shining a light at the opposite wall and measured the speed of the particles of light (photons), you and the passenger would both find that the photons had the same speed at all times. In all cases, the speed of the photons would stay at just under 300,000 kilometres per second, as Maxwell's equations say they should.

Einstein took this idea – the invariance of the speed of light – as one of his two postulates for the special theory of relativity. The other postulate was that the laws of physics are the same wherever you are, whether on an plane or standing on a country road. But to keep the speed of light constant at all times and for all observers, in special relativity, space and time become stretchy and variable. Time is not absolute, for example. A moving clock ticks more slowly than a stationary one. Travel at the speed of light and, theoretically, the clock would stop altogether.

How much the time dilates can be calculated by the two equations above. On the right, Δt is the time interval between two events as measured by the person they affect. (In our example above, this would be the person in the train.) On the left, Δt' is the time interval between the same two events but measured by an outside observer in a separate frame of reference (the person on the platform). These two times are related by the Lorentz factor (γ), which in this example is a term that takes into account the velocity (v) of the train relative to the station platform, which is "at rest". In this expression, c is a constant equal to the speed of light in a vacuum.

The length of moving objects also shrink in the direction in which they move. Get to the speed of light (not really possible, but imagine if you could for a moment) and the object's length would shrink to zero.

The contracted length of a moving object relative to a stationary one can be calculated by dividing the proper length by the Lorentz factor – if it were possible for an object to reach the speed of light its length would shrink to zero.

It is important to note that if you were the person moving faster and faster, you would not notice anything: time would tick normally for you and you would not be squashed in length. But anyone watching you from the celestial station platform would be able to measure the differences, as calculated from the Lorentz factor. However, for everyday objects and everyday speeds, the Lorentz factor will be close to 1 – it is only at speeds close to that of light that the relativistic effects need serious attention.

Another feature that emerges from special relativity is that, as something speeds up, its mass increases compared with its mass at rest, with the mass of the moving object determined by multiplying its rest mass by the Lorentz factor. This increase in relativistic mass makes every extra unit of energy you put into speeding up the object less effective at making it actually move faster.

As the speed of the object increases and starts to reach appreciable fractions of the speed of light (c), the portion of energy going into making the object more massive gets bigger and bigger.

This explains why nothing can travel faster than light – at or near light speed, any extra energy you put into an object does not make it move faster but just increases its mass. Mass and energy are the same thing – this is a profoundly important result. But that is another story.

• Albert Einstein
• A short history of equations
• Isaac Newton

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Why Does Gravity Travel at the Speed of Light?

As with so much in physics, it has to do with einstein’s theory of general relativity..

The dead cores of two stars collided 130 million years ago in a galaxy somewhat far away.

The collision was so extreme that it caused a wrinkle in space-time — a gravitational wave. That gravitational wave and the light from the stellar explosion traveled together across the cosmos. They arrived at Earth simultaneously at 6:41 a.m. Eastern on August 17.

The event prompted worldwide headlines as the dawn of “multimessenger astronomy.” Astronomers had waited a generation for this moment. But it was also the first-ever direct confirmation that gravity travels at the speed of light.

The Speed of Gravity

We all know light obeys a speed limit — roughly 186,000 miles per second. Nothing travels faster. But why should gravity travel at the same speed?

That question requires a quick dive into Albert Einstein’s general relativity, or theory of gravity — the same theory that predicted gravitational waves a century ago.

Einstein overthrew Isaac Newton’s idea of “absolute time.” Newton thought time marched onward everywhere at an identical pace — regardless of how we mortals perceived it. It was unflinching. By that line of thinking, one second on Earth is one second near a black hole (which he didn’t know existed).

Newton also thought gravity acted instantaneously. Distance didn’t matter.

It’s All Relative

But then Einstein showed that time is relative. It changes with speed and in the presence of gravity. One of the ramifications of that is that you can’t have simultaneous  actions at a distance . So information of any kind has a finite speed, whether it’s a photon — the light-carrying particle — or a graviton, which carries the force of gravity.

“In relativity, there is a ‘speed of information’ — the maximum speed that you can send information from one point to another,” says University of Wisconsin-Milwaukee physicist Jolien Creighton, an expert on general relativity and member of the LIGO team that first spotted gravitational waves.

Creighton explains that in electromagnetism, when you shake an electron, it creates a change in the electric field that spreads out at the speed of light. Gravity works the same way. Shake a mass and the change in the gravitational field — the gravitational wave — propagates at that same speed.

“So the fact that the speed of gravitational waves is equal to the speed of electromagnetic waves is simply because they both travel at the speed of information,” Creighton says.

There’s an easy way to picture this, too. Imagine the sun vanished right now. Earth wouldn’t just drift into space instantly. After eight minutes, Earth would go dark and simultaneously push off in a straight line.

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Have we made an object that could travel 1% the speed of light?

University Distinguished Professor of Astronomy, University of Arizona

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Have we made an object that could travel at at least 1% the speed of light? – Anadi, age 14, Jammu and Kashmir, India

Light is fast . In fact, it is the fastest thing that exists, and a law of the universe is that nothing can move faster than light. Light travels at 186,000 miles per second (300,000 kilometers per second) and can go from the Earth to the Moon in just over a second. Light can streak from Los Angeles to New York in less than the blink of an eye.

While 1% of anything doesn’t sound like much, with light, that’s still really fast – close to 7 million miles per hour! At 1% the speed of light, it would take a little over a second to get from Los Angeles to New York. This is more than 10,000 times faster than a commercial jet.

The fastest things ever made

Bullets can go 2,600 mph (4,200 kmh), more than three times the speed of sound. The fastest aircraft is NASA’s X3 jet plane , with a top speed of 7,000 mph (11,200 kph). That sounds impressive, but it’s still only 0.001% the speed of light.

The fastest human-made objects are spacecraft. They use rockets to break free of the Earth’s gravity, which takes a speed of 25,000 mph (40,000 kmh). The spacecraft that is traveling the fastest is NASA’s Parker Solar Probe . After it launched from Earth in 2018, it skimmed the Sun’s scorching atmosphere and used the Sun’s gravity to reach 330,000 mph (535,000 kmh). That’s blindingly fast – yet only 0.05% of the speed of light.

Why even 1% of light speed is hard

What’s holding humanity back from reaching 1% of the speed of light? In a word, energy. Any object that’s moving has energy due to its motion. Physicists call this kinetic energy. To go faster, you need to increase kinetic energy. The problem is that it takes a lot of kinetic energy to increase speed. To make something go twice as fast takes four times the energy. Making something go three times as fast requires nine times the energy, and so on.

For example, to get a teenager who weighs 110 pounds (50 kilograms) to 1% of the speed of light would cost 200 trillion Joules (a measurement of energy). That’s roughly the same amount of energy that 2 million people in the U.S. use in a day.

How fast can we go?

It’s possible to get something to 1% the speed of light, but it would just take an enormous amount of energy. Could humans make something go even faster?

Yes! But engineers need to figure out new ways to make things move in space. All rockets, even the sleek new rockets used by SpaceX and Blue Origins, burn rocket fuel that isn’t very different from gasoline in a car. The problem is that burning fuel is very inefficient.

Other methods for pushing a spacecraft involve using electric or magnetic forces . Nuclear fusion , the process that powers the Sun, is also much more efficient than chemical fuel.

Scientists are researching many other ways to go fast – even warp drives , the faster-than-light travel popularized by Star Trek.

One promising way to get something moving very fast is to use a solar sail. These are large, thin sheets of plastic attached to a spacecraft and designed so that sunlight can push on them, like wind in a normal sail. A few spacecraft have used solar sails to show that they work, and scientists think that a solar sail could propel spacecraft to 10% of the speed of light .

One day, when humanity is not limited to a tiny fraction of the speed of light, we might travel to the stars .

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• Space Travel + Astronomy

Here's What Actually Happens When You Travel at the Speed of Light, According to NASA

NASA created a fun video to answer all of our burning questions about near-light-speed travel.

Ever wish you could travel at the speed of light to your favorite destinations ? Once you see the reality of that speed, you may rethink everything.

"There are some important things you should probably know about approaching the speed of light," NASA's video, Guide to Near-light-speed Travel , explains. "First, a lot of weird things can happen, like time and space getting all bent out of shape."

According to the video, if you're traveling at nearly the speed of light, the clock inside your rocket would show it takes less time to travel to your destination than it would on Earth. But, since the clocks at home would be moving at a standard rate you'd return home to everyone else being quite a bit older.

"Also, because you're going so fast, what would otherwise be just a few hydrogen atoms that you'd run into quickly becomes a lot of dangerous particles. So you should probably have shields that keep them from frying your ship and also you."

Finally, the video tackles the fact that even if you were moving at the speed of light, the "universe is also a very big place, so you might be in for some surprises." For example, your rocket's clock will say it takes about nine months to get from Earth to the edge of the solar system. An Earth clock would say it took about a year and a half. Fortunately, NASA astronauts have a slew of tips for avoiding jet lag along the way.

"If you want to get to farther out vacation spots," the video explains, "you'll probably need more than a few extra snacks. A trip to the Andromeda Galaxy, our nearest large neighbor galaxy, can take over one million years. And a trip to the farthest known galaxy where it currently sits might take over 15 billion years, which is more vacation time than I think I'll ever have."

The video doesn't explain how your rocket will travel at the speed of light. Our technology just isn't there yet, but maybe the aliens will share that tech with us soon. Until then, you can track the first crew launch of Artemis II , a rocket that will fly around the moon in 2024 before making its first lunar landing in 2025.

A Vacuum Full of Infinite Energy

By Harnessing the Unlimited Vacuum Energy In Space, We Could Finally Reach Light Speed

Invisible vacuum energy is all around us. We could use it to power propulsion, enhance nanostructures, and build levitating devices.

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This landmark experiment, first devised by Casimir just after World War II—and only realized 25 years ago—paved the way for scientists to witness the manifestations of quantum theory in a real, practical way. Quantum fields and their vibrations power our modern-day understanding of physics, from subatomic interactions to the evolution of the entire universe. And what we learned, thanks to Casimir’s work, is that infinite energy permeates the vacuum of space. There are many ideas in the science fiction universe that propose using vacuum energy to power a starship or other advanced kind of propulsion, like a warp drive. While these ideas are still dreams, the fact remains that a simple experiment, devised in 1948, set fire to our imaginations and our understanding of the universe.

Casimir , a Dutch physicist, had spent his graduate years with his advisor, Niels Bohr, one of the godfathers of quantum physics , and had picked up a liking for this new, extraordinary theory of the cosmos. But as quantum theory evolved, it started to make extremely strange statements about the universe . The quantum world is weird , and its ultimate weirdness is normally invisible to us, operating at scales well below our normal human perception or experimentation. Casimir started to wonder how we might be able to test those ideas.

He went on to discover a clever way to measure the effects of ever-present infinite quantum fields merely using bits of metal held extremely close together. His work showed that quantum behavior can manifest in surprising ways that we can measure. It also showed that the strangeness of quantum behavior is real and can’t be ignored, and what quantum mechanics says about the workings of the universe —no matter how bizarre—must be believed.

Quantum Fields Are Otherworldly, But Very Real

One of the lessons of the quantum world is that particles , like electrons, photons, neutrinos, and whatnot, aren’t what they seem to be. Instead, each of the particles that we see in nature is actually just a piece of a much larger, grander entity. These grander entities are known as quantum fields, and the fields soak every bit of space and time—all throughout the universe—the same way that oil and vinegar soaks a piece of bread.

There is a quantum field for every kind of particle: one field for the electrons , one for the photons, and so on. These fields are invisible to us, but they make up the fundamental building blocks of existence. They are constantly vibrating and buzzing. When the fields vibrate with enough energy, particles appear. When the fields die down, the particles disappear. Another way to look at this is to say that what we call a “particle” is really a localized vibration of a quantum field. When two particles interact, it’s really just two pieces of quantum fields interacting with each other.

.css-2l0eat{font-family:UnitedSans,UnitedSans-roboto,UnitedSans-local,Helvetica,Arial,Sans-serif;font-size:1.625rem;line-height:1.2;margin:0rem;padding:0.9rem 1rem 1rem;}@media(max-width: 48rem){.css-2l0eat{font-size:1.75rem;line-height:1;}}@media(min-width: 48rem){.css-2l0eat{font-size:1.875rem;line-height:1;}}@media(min-width: 64rem){.css-2l0eat{font-size:2.25rem;line-height:1;}}.css-2l0eat b,.css-2l0eat strong{font-family:inherit;font-weight:bold;}.css-2l0eat em,.css-2l0eat i{font-style:italic;font-family:inherit;} There’s no such thing as a true vacuum; wherever you go, there are always vibrating quantum fields.

These quantum fields are always vibrating, even when those vibrations aren’t strong enough to produce a particle. If you take a box and empty out all of the stuff—all the electrons, all the photons, all the neutrinos, all the everything—the box is still filled with these quantum fields. Since those fields vibrate even in isolation, that means the box is filled with invisible vacuum energy, also known as zero-point energy—the energy of these fundamental vibrations.

In fact, you can calculate how many vibrations are in each of these quantum fields ... and the answer is infinity! There are small ones, medium ones, big ones, and gigantic ones, all flopping on top of each other continuously, as if spacetime itself was boiling at the subatomic level. This means that the vacuum of the universe really is made of something. There’s no such thing as a true vacuum; wherever you go, there are always vibrating quantum fields.

A Simple Experiment Involves Multiple Infinities

This is where Casimir’s experiment comes in: If you take two metal plates and stick them really, really close together, the quantum fields between those plates must behave in a certain way: the wavelengths of their vibrations must fit perfectly between the plates, just like the vibrations on a guitar string have to fit their wavelengths to the length of the string. In the quantum case, there are still an infinite number of vibrations between the plates, but—and this is crucial—there are not as many infinite vibrations between the plates as there are outside the plates.

How does this make sense? In mathematics, not all infinities are the same, and we’ve developed clever tools to be able to compare them. For example, consider one kind of infinity where you add successive numbers to each other. You start with 1, then add 2, then add 3, then add 4, and so on. If you keep that addition going forever, you’ll reach infinity. Now consider another kind of addition, this one involving powers of 10. You start with 101, then add to it 102, then 103, then 104, and keep going.

Again, if you keep this series going on forever, you’ll also reach infinity. But in a sense you’ll “get” to infinity faster. So by carefully subtracting these two sequences, you can get a measure of their difference even though they both go to infinity.

Using this clever bit of mathematics, we can subtract the two kinds of infinities—the ones between the metal plates and the ones outside—and arrive at a finite number. This means that there really are more quantum vibrations outside the two plates than there are inside the plates. This phenomenon leads to the conclusion that the quantum fields outside the plates push the two plates together, something called the Casimir effect in Hendrik’s honor.

The effect is incredibly small, roughly 10 -12 Newtons, and it requires the metal plates to be within a micrometer of each other. (One Newton is the force which accelerates an object of 1 kilogram by 1 meter per second squared.) So, even though Casimir could predict the existence of this quantum effect, it wasn’t until 1997 that we were finally able to measure it, thanks to the efforts of Yale physicist Steve Lamoreaux.

🦎 Quantum Physics In Action Perhaps most strangely, the creature with the deepest connection to the fundamental quantum nature of the universe is the gecko. Geckos have the ability to walk on walls, and even upside-down on ceilings. To accomplish this feat, a gecko’s limbs are covered in countless, microscopic hair-like fibers. These fibers get close enough to the molecules of the surface it wants to climb on for the Casimir effect to take action. It creates an attractive force between the hair and the surface. Each individual hair provides only an extremely tiny amount of force, but all the hairs combined are enough to support the gecko.

In this experimental setup, which can fit on a kitchen countertop, the plates don’t magically pull themselves together. Instead it’s the infinite vibrating quantum fields of spacetime pushing them together from the outside.

We don’t normally see or sense or experience the Casimir effect. But when we want to design micro- and nano-scale machines , we have to account for these additional forces. For example, researchers have designed micro-scale sensors that can monitor the flow of chemicals on a molecule-by-molecule basis, but the Casimir effect can disrupt the operations of this sensor if we didn’t know about it.

Scientists Are Exploring the Potential of Vacuum Energy

For several years, researchers have been investigating the possibility that we really can extract vacuum energy and use it for energy. A 2002 patent was awarded for a device that captures the electric charge from the Casimir experimental setup’s two metal plates, charging a storage battery. The device can be used as a generator. “To continuously generate power a plurality of metal plates are fixed around a core and rotated like a gyrocompass,” according to the patent.

💫 Scientists Believe Light Speed Travel Is Possible. Here’s How.

The U.S. Defense Department’s Defense Advanced Research Projects Agency (DARPA) gave researchers $10 million in 2009 to pursue a better understanding of the Casimir force. Though progress in actually using vacuum energy continues to be incremental, this line of energy research could give rise to innovations in nanotechnology, such as building a device capable of levitation, researchers said at the time.

At the University of Colorado in Boulder, Garret Moddel ’s research group has developed devices that produce power “that appears to result from zero-point energy quantum fluctuations,” according to the group’s website . Their device essentially recreates Casimir’s experiment, generating an electrical current between the two metal layers that researchers could measure, despite applying no electrical voltage.

As for Casimir himself, who was immersed in a quantum revolution unfolding at Leiden University, he had a tendency to downplay the importance of his own work. In his autobiography, Haphazard Reality , Casimir said, “The story of my own life is of no particular interest.” And his monumental 1948 paper designing his experiment ends with the simple statement, “Although the effect is small, an experimental confirmation seems not infeasable and might be of a certain interest.”

In fact, his initial insight did not make a big splash on the scientific community, nor were there glowing popular press accounts of his experiment. Part of the reason was Casimir’s own modesty, and another is that he soon left academic research to pursue a career in industry. But despite these humble beginnings, his work cannot be understated.

Today, we continue to refine Casimir’s original experimental setup, searching for any cracks in our theories, and we use it as a foundation to explore ever more deeply the fundamental nature of the cosmos.

Paul M. Sutter is a science educator and a theoretical cosmologist at the Institute for Advanced Computational Science at Stony Brook University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page .

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‘A Lot of Chaos’: Bridge Collapse Creates Upheaval at Largest U.S. Port for Car Trade

A bridge collapse closed Baltimore’s port, an important trade hub that ranks first in the nation by the volume of automobiles and light trucks it handles.

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Shipping in the Port of Baltimore

Monthly cargo handled by the Port of Baltimore

By Peter Eavis and Jenny Gross

• March 26, 2024

The Baltimore bridge disaster on Tuesday upended operations at one of the nation’s busiest ports, with disruptions likely to be felt for weeks by companies shipping goods in and out of the country — and possibly by consumers as well.

The upheaval will be especially notable for auto makers and coal producers for whom Baltimore has become one of the most vital shipping destinations in the United States.

As officials began to investigate why a nearly 1,000-foot cargo ship ran into the Francis Scott Key Bridge in the middle of the night, companies that transport goods to suppliers and stores scrambled to get trucks to the other East Coast ports receiving goods diverted from Baltimore. Ships sat idle elsewhere, unsure where and when to dock.

“It’s going to cause a lot of chaos,” said Paul Brashier, vice president for drayage and intermodal at ITS Logistics.

The closure of the Port of Baltimore is the latest hit to global supply chains, which have been strained by monthslong crises at the Panama Canal, which has had to slash traffic because of low water levels; and the Suez Canal, which shipping companies are avoiding because of attacks by the Houthis on vessels in the Red Sea.

The auto industry now faces new supply headaches.

Last year, 570,000 vehicles were imported through Baltimore, according to Sina Golara, an assistant professor of supply chain management at Georgia State University. “That’s a huge amount,” he said, equivalent to nearly a quarter of the current inventory of new cars in the United States.

The Baltimore port handled a record amount of foreign cargo last year, and it was the 17th biggest port in the nation overall in 2021, ranked by total tons, according to Bureau of Transportation Statistics.

Baltimore Ranks in the Top 20 U.S. Ports

Total trade in 2021 in millions of tons

Baltimore ranks first in the United States for the volume of automobiles and light trucks it handles, and for vessels that carry wheeled cargo, including farm and construction machinery, according to a statement by Gov. Wes Moore of Maryland last month.

The incident is another stark reminder of the vulnerability of the supply chains that transport consumer products and commodities around the world.

The extent of the disruption depends on how long it takes to reopen shipping channels into the port of Baltimore. Experts estimate it could take several weeks.

Baltimore is not a leading port for container ships, and other ports can likely absorb traffic that was headed to Baltimore, industry officials said.

Stephen Edwards, the chief executive of the Port of Virginia, said it was expecting a vessel on Tuesday that was previously bound for Baltimore, and that others would soon follow. “Between New York and Virginia, we have sufficient capacity to handle all this cargo,” Mr. Edwards said, referring to container ships.

“Shipping companies are very agile,” said Jean-Paul Rodrigue, a professor in the department of maritime business administration at Texas A&M University-Galveston. “In two to three days, it will be rerouted.”

But other types of cargo could remain snarled.

Alexis Ellender, a global analyst at Kpler, a commodities analytics firm, said he expected the port closure to cause significant disruption of U.S. exports of coal. Last year, about 23 million metric tons of coal exports were shipped from the port of Baltimore, about a quarter of all seaborne U.S. coal shipments. About 12 vessel had been expected to leave the port of Baltimore in the next week or so carrying coal, according to Kpler.

He noted that it would not make a huge dent on the global market, but he added that “the impact is significant for the U.S. in terms of loss of export capacity.”

“You may see coal cargoes coming from the mines being rerouted to other ports instead,” he said, with a port in Norfolk, Va., the most likely.

If auto imports are reduced by Baltimore’s closure, inventories could run low, particularly for models that are in high demand.

“We are initiating discussions with our various transportation providers on contingency plans to ensure an uninterrupted flow of vehicles to our customers and will continue to carefully monitor this situation,” Stellantis, which owns Chrysler, Dodge, Jeep and Ram, said in a statement.

Other ports have the capacity to import cars, but there may not be enough car transporters at those ports to handle the new traffic.

“You have to make sure the capacity exists all the way in the supply chain — all the way to the dealership,” said Mr. Golara, the Georgia State professor.

A looming battle is insurance payouts, once legal liability is determined. The size of the payout from the insurer is likely to be significant and will depend on factors including the value of the bridge, the scale of loss of life compensation owed to families of people who died, the damage to the vessel and disruption to the port.

The ship’s insurer, Britannia P&I Club, part of a global group of insurers, said in a statement that it was “working closely with the ship manager and relevant authorities to establish the facts and to help ensure that this situation is dealt with quickly and professionally.”

The port has also increasingly catered to large container ships like the Dali, the 948-foot-long cargo vessel carrying goods for the shipping giant Maersk that hit a pillar of the bridge around 1:30 a.m. on Tuesday. The Dali had spent two days in Baltimore’s port before setting off toward the 1.6-mile Francis Scott Key Bridge.

State-owned terminals, managed by the Maryland Port Administration, and privately owned terminals in Baltimore transported a record 52.3 million tons of foreign cargo in 2023, worth $80 billion.

Materials transported in large volumes through the city’s port include coal, coffee and sugar. It was the ninth-busiest port in the nation last year for receiving foreign cargo, in terms of volume and value.

The bridge’s collapse will also disrupt cruises traveling in and out of Baltimore. Norwegian Cruise Line last year began a new fall and winter schedule calling at the Port of Baltimore.

An earlier version of this article misstated the Port of Baltimore’s rank among U.S. ports. It was the nation’s 17th biggest port by total tons in 2021, not the 20th largest.

How we handle corrections

Peter Eavis reports on business, financial markets, the economy and companies across different sectors. More about Peter Eavis

Jenny Gross is a reporter for The Times in London covering breaking news and other topics. More about Jenny Gross

Vande Bharat Express: Trail Runs At 160 Kmph To Begin, Mumbai-Ahmedabad Travel Time To Be Reduced

T he Western Railway (WR) has been given the green light to test faster Vande Bharat Express trains , potentially slashing travel times between Mumbai and Ahmedabad.

These trains, known for their speed and efficiency, will undergo trial runs at a maximum speed of 160 kilometers per hour (kmph), up from the current 130 kmph. If successful, passengers can expect to save around 45 minutes on their journeys, as reported by TOI.

The Commission of Railway Safety, part of the Ministry of Civil Aviation of the Indian government, has authorized confirmatory oscillograph car runs (COCR) for the 16-coach Vande Bharat train sets in both directions along the Mumbai Central-Vadodara-Ahmedabad route of the WR.

With a focus on safety, the Commissioner of Railway Safety (Western Circle) has mandated that these trials take place during daylight and fair weather conditions. Additionally, Railway Protection Force personnel will be stationed at level crossing gates to prevent pedestrian access and identify any areas lacking proper barricading. Railway officials will be present on platforms during trial runs to ensure public safety.

Before the trials commence, locomotive pilots will undergo training and medical checks. During the trial runs at 160 kmph, no other trains will be allowed to cross paths at enroute stations. Special precautions will also be taken to prevent trespassing, particularly in the Mumbai Central-Virar suburban section.

The WR has been instructed to inform the Commission of Railway Safety of the trial dates. Once implemented, these faster Vande Bharat trains promise to revolutionize travel between Mumbai and Ahmedabad, offering commuters quicker and more efficient journeys.

March Madness 2024 at Sky Harbor: What to know if you're flying during the Final Four

With the NCAA Men's Final Four 2024 basketball tournament coming to Phoenix this week, Phoenix Sky Harbor International Airport is preparing for a travel rush.

While the Phoenix airport is well equipped to handle big crowds for major sporting events − Super Bowl 2023 was here last year − travelers should keep in mind that crowds at the airport mean it will take longer to navigate it.

Higher than normal passenger traffic is expected as people arrive for the Final Four tournament that will be played April 6 and 8 in at State Farm Stadium in Glendale, and prepare to leave on April 9.

Here's what to know if you'll be flying through the Phoenix airport during March Madness 2024.

How long are airport TSA wait times? Here's how to find out in real time

Where is the Final Four in Phoenix?

The 2024 NCAA Final Four tournament will be played April 6 and 8 at State Farm Stadium in Glendale.

The tournament was last held in metro Phoenix in 2017 when the venue was known as the University of Phoenix Stadium.

Other Final Four events include the March Madness Music Festival at Hance Park in downtown Phoenix and the Final Four Fan Fest at the Phoenix Convention Center downtown.

What is the busiest day at Phoenix airport for Final Four?

The highest Final Four passenger traffic days are expected on Thursday, April 4; Friday, April 5; and Tuesday, April 9, airport spokesman Eric Everts said. They did not give an estimate of how many people are expected to fly in and out during the Final Four.

Final Four 2024 car rentals

Make your Final Four car rental reservation before you arrive. Everts said the airport expects an increase in car rentals during the Final Four and walk-up customers may have limited options.

Does Phoenix light rail go to the airport?

Yes. People can ride the Valley Metro light rail between Sky Harbor Airport and downtown Phoenix.

To ride the light rail from the airport, take the free PHX Sky Train from Terminal 3 or Terminal 4 to the 44th Street Station, then purchase a pass from the Valley Metro fare vending machine and take the light rail westbound to downtown Phoenix. Cost is $2 per ride or $4 per day.

Use the Third Street/Washington station for events at the convention center, Footprint Center and Chase Field. Use the Roosevelt/Central Avenue or McDowell/Central Avenue stops for Hance Park.

Where do you pick up passengers at PHX?

Everts suggests picking up and dropping off passengers at the 24th Street or 44th Street Sky Train station.

The PHX Sky Train connects Terminals 3 and 4 to these stations, and Everts said pickups and dropoffs near the stations are a "convenient alternative" to using the curbsides at the terminals, which tend to get congested during peak travel times. It's free to ride the Sky Train.

How early should I get to Phoenix airport?

Because Sky Harbor is expecting more passengers than usual on Sunday, April 7, and Tuesday, April 9, Everts advised travelers to follow the "5-4-3-2-1 rule" to make the process of catching their flights as smooth as possible. That means allowing:

• Five hours before departure to check out of your hotel.
• Four hours before departure to return a rental car.
• Three hours before departure to check in at the ticket counter.
• Two hours before departure to get in line at the security checkpoint.
• One hour before departure to arrive at your gate.

Phoenix airport travel tips

Everts offers these tips for navigating Sky Harbor Airport:

• Check your flight status before heading to the airport. Check with your airline or go to skyharbor.com , which also has information about what dining, shopping and services are near your gate .
• Stash your Final Four program in your carry-on bag. Glossy magazines are made with materials that can trigger security scanners and slow down the process of checking luggage.
• Get through security more quickly with the free  PHX Reserve  program, where you can reserve a place in the TSA line up to six days in advance. You can also  check security wait times online .
• Anyone with questions about the airport can ask one of Sky Harbor's Navigator volunteers, identifiable by their purple uniforms. NCAA volunteers will also in the terminals to answer questions.
• Travelers with a hidden disability like dementia or autism can reserve a Compassion Cacti lanyard , which signals to airport staff that its wearer needs more time, assistance or patience in the traveling process. Sky Harbor also has a sensory room for people who need a calm space, plus other services for travelers who need a little extra care .

Sky Harbor Airport parking: Complete guide to terminal, economy and off-site lots

Michael Salerno is an award-winning journalist who’s covered travel and tourism since 2014. His work as The Arizona Republic’s consumer travel reporter aims to help readers navigate the stresses of traveling and get the best value for their money on their vacations. He can be reached at  [email protected] . Follow him on X, formerly Twitter:  @salerno_phx .

Support local journalism.   Subscribe to  azcentral.com  today.

Total solar eclipse 2024 live updates: Watch video and see photos of the sky spectacle

EAGLE PASS, Texas − In a moment of cosmic alignment that was precisely predicted yet undeniably mysterious , the moon crept across the face of the sun as seen from Earth, casting its shadow onto the United States on Monday.

And then it was over: A total eclipse of the sun has come and gone.  

The partial shadow crossed into the U.S. in Texas just after noon local time, and a bit more than an hour later, the minutes of “totality” began. The total eclipse of the sun then traced a path from southern Texas to northern Maine, where it exited into Canada and the ocean beyond.

The rare event was an astronomical experience like no other. Monday's eclipse was unusually accessible to millions of people. It was widely anticipated not only for its remarkable period of darkness, but for its rare timing: No total solar eclipse would be visible from the contiguous United States again until 2044. While millions of Americans may have seen today’s eclipse, a small group in Texas was among the very first.

Edge of eclipse shadow began in south Texas

Alejandra Martinez, a seventh-grade science teacher from the south Texas border city of Eagle Pass, peered up at the gray, gauzy sky, anxiously awaiting the solar eclipse’s entrance into the USA.

Sitting with a telescope under a camping canopy in a corner of the county airport, Martinez, 42, and two other science enthusiasts had been recruited by NASA to participate in an effort called Citizen CATE 2024, where more than 30 teams perched in the eclipse’s path will record the shadowy trek across the country. 

In this spot, less than five miles from the U.S.-Mexico border, she and her team would be among the first people in the U.S. to witness the phenomenon. Eagle Pass sits near the very beginning of the eclipse’s track from southwest to northeast across the country. It also sits very near dead-center of it, where the period of “totality” will be the longest. That total shadow was still more than an hour away when the edge of the shadow began to darken the sky.

Martinez, dressed in a “This Totality Rocks” T-shirt and eclipse-themed skirt, watched as the penumbra crept across the airfield, about 10 minutes past noon local time. She put on her eclipse glasses and peered up at the sky. The gauzy clouds parted. “There it is! There it is!” she yelled. “First contact. It’s begun!”

Clouds came and went as the shadow grew for the next hour. As totality approached Eagle Pass, the sky shaded a dull gray then a heavy brown and the temperature dropped a few degrees. Only a sliver of the sun remained.

A few seconds before 1:27 p.m. CT, the crowd counted down, “3..2..1…” The sun was gone, completely covered by the moon. Darkness spread over the small airport grounds. “Oh my god,” Martinez said, a hand covering her heart. “This is so cool!”

How to tell if you have eye damage from looking at a partial eclipse?

For those who took a chance and looked at the sun without protective eyewear during the eclipse, concerns may arise if their vision starts to seem impaired. Have they suffered eclipse blindness?

Looking directly at the sun even briefly without the proper safety glasses can result in eye damage, either temporary or permanent. The body's natural squinting reaction in normal circumstances prevents people from staring directly at the sun, but that reflex may not be activated during partial eclipses, even though a partly shielded sun can still burn the retina.

The American Academy of Ophthalmology says pain won't be an indication of damaged eyes because the retina lacks pain nerves. Worrisome signs are instead blurry vision, headaches, light sensitivity, blind spots in the central vision, distorted vision and changes in color perception. Anyone experiencing those symptoms should see an ophthalmologist, the AAO says.

What was the timeline of the 2024 solar eclipse?

The total eclipse began in Mexico at about 11:07 a.m. PDT, Monday before crossing into Texas at 1:27 p.m. CDT. It ended in Maine at 3:35 p.m. EDT. Even those who weren't in the path of totality could see a percentage of the eclipse.

To find out exactly when the eclipse occurred in your area, you can search by  USA TODAY's database by ZIP code  for a viewing guide.

Students in Washington get hands-on learning

Dozens of students spilled out of Ida B. Wells Middle School in Washington, D.C., gripping cardboard solar eclipse glasses and chatting with excitement. “Why is the sun shaped as a moon?” one student asked after looking toward the sun. “Why is it not getting dark?’’ asked another.

Teacher Troy Mangum, who the students called “Mr. Mango,’’ slowly explained the science behind the experience as he cautioned others to put on their glasses. Students at the school in northwest D.C. had had lessons about the eclipse leading up the day’s teaching outside the classroom.

“Anytime we can get hands on or dynamic learning … it’s invaluable,’’ said Mangum, a social emotional learning coach. “We talked about it so much. It’s nice to see them get excited about qeeky Mr. Mango type of stuff.”

Skylar Cruel, 12, had come outside with her science class. “I just really wanted to see it,’’ she said. “You get to see it turn from light to dark.”

Steps away at Whittier Elementary School, which focuses on science, technology, engineering and math (STEM), younger students put on their glasses and tilted their heads up toward the sun behind their school.

Michael Haile joined his daughter, Markan Worku, 9, a student at Whittier, and brought along his 5-year-old son, Keab. He called it a memorable experience for his children. Markan, who had been learning about solar eclipses in her third-grade class, simply said, “This is cool!’’ 

− Deborah Berry

In Michigan, a bonding experience for a mother and her three kids

In Michigan, eclipse enthusiasts packed a watch party along the Detroit Riverwalk near Cullen Plaza. They sat on the grass, on benches and berms. For Janelle Ikonen of Waterford, it was a chance to bond with her three children − Hannah, 9; Elim, 12; and Isaac, 14 − whom she took out of school for the day.

“I wanted to experience a unique event with them,” said Ikonen, 47. “It’s one of my big deals to me, to experience things with my kids. … I hope they’ll remember spending time with me.”

In Grand Rapids, where 94% coverage of the sun was expected, viewers gathered on the city's “Blue Bridge,” which offered a view of the eclipse over the Grand River. With no cloud cover over the city, visitors had an unobstructed scene. The Grand Rapids Public Museum set up a series of solar telescopes.

Jill and Wes Johnson, 50 and 14, were among those awaiting max coverage. They’d considered traveling from Grand Rapids to a place with total coverage, but decided 94% coverage was worthwhile enough to stay home.

“I think people are excited to see something unique,” Jill Johnson said.

− Georgea Kovanis and Arpan Lobo, Detroit Free Press

US women's soccer team takes in eclipse ahead of title game

COLUMBUS, Ohio − The U.S. women’s national soccer team took a break from SheBelieves Cup preparation to watch the eclipse. 

Several team members, including World Cup veterans Crystal Dunn, Alyssa Naeher, Naomi Girma and Trinity Rodman, coaches and staff members donned eclipse glasses and went outside their hotel to see the rare event.

As the sky darkened and the moon moved in front of the sun, someone yelled, “Come on, keep going!” Columbus is just outside the path of totality, so all but a sliver of the sun wound up being covered. 

The USWNT is in Columbus for the final of the SheBelieves Cup on Tuesday night against Canada.

− Nancy Armour

Witnessing the first total eclipse from a cruise ship

Passengers aboard Holland America's Koningsdam cruise ship eagerly gathered on the top decks at 10 a.m. local time to watch the spectacle of a solar eclipse.

Positioned off the coast of Mazatlan, Mexico − the first city in North America to witness the total eclipse − the ship provided an ideal vantage point for viewing the celestial event. Accompanied by an astronomer, the captain meticulously adjusted the ship's position to ensure the best possible view.

Dan Fueller, an experienced eclipse observer who had previously seen similar phenomena from ships in 1991 and 1998, emphasized the advantages of viewing the eclipse from a cruise ship. He noted the flexibility of the ship's mobility allowed for optimal positioning along the eclipse's path, ensuring unobstructed views.

"You're on a cruise ship and (the best view) is a little bit farther to the west or a little bit farther to the east − along the path, there's a clear sky − the ship can move. If you're on land and there's not a road there, you better have a tank."

− Joshuan Rivera

Partial eclipse 'like a little Pac-Man taking a bite out of the sun'

In Parthenon, Arkansas, the eclipse began near the Buffalo National River. Five friends traveled from Florida to be on a rock outcropping to witness the celestial event.

“Yes! I see it!” Angie Printiss exclaimed.

“Oh ya! You can see it,” said Kate Estes. “It’s like a little Pac-Man taking a bite out of the sun.”

At 20 minutes before totality, the sky got noticeably darker. The shadow of crescent sun shapes played off the limestone rocks.

“It looks like water − like fish scales,” Estes said. “That is so weird, when the trees move you can really see it.”

− Jennifer Portman

New Englanders travel for better view of the eclipse

Crowds ballooned into the thousands midday Monday in the center of St. Johnsbury, Vermont, as the nearly cloudless sky and warm sun boosted temperatures to an unseasonably 60 degrees.

Jim and Kim Cooper drove over from New Hampton, New Hampshire, their first time doing more than just passing through the town locals call St. Jays. Their son had proposed to his girlfriend in Tennessee during the 2017 eclipse and then had an eclipse-themed wedding, but the Coopers had never seen a total eclipse for themselves, so they decided to come to the closest place where they could be assured of a good view.

Jeffrey Breau had seen that 2017 eclipse for himself and was so impressed that he convinced family members to join him this time. Breau’s girlfriend, Nell Hawley, came up with him from Cambridge, Massachusetts, while his brother Alex and his girlfriend Diana Ventura drove up from New York City. The Breau brothers' father, Walter, joined them in taking in their first eclipse.

“I would have been OK watching it in the yard,” Walter said, though he would only have seen a partial eclipse from there. “That’s what you think now,” Jeffrey said quickly. “Talk to me in two hours.”

− Karen Weintraub

Kids bring homemade eclipse viewers to National Mall

Lucas and Lucy Porto Banco didn’t need to buy fancy eclipse glasses – the 7- and 10-year-old siblings made their own eclipse viewers from two cardboard shoe boxes left over from a couple of their mom’s recent shoe purchases.With the help of a YouTube tutorial video, they made two holes in each box with a whittling knife and taped tin foil over. Then they used dry spaghetti to poke holes in the tin foil for the sun’s light to pass through and project onto the dark inside of the box.The kids put together the viewer in little more than five minutes, Lucy said. “Maybe 15, with covering all the gaps in the shoe box,” she said.Even with his own eclipse-viewing creation, Lucas said he may still take advantage of his eclipse glasses. “I’m going to do both,” he said.

− Cybele Mayes-Osterman

What is an 'umbra?' − and other eclipse words

Astronomers and eclipse enthusiasts used a bunch of specialized words Monday as the nation experienced its first solar eclipse since 2017 and the last one for until 2044. Here are a few of those terms and what they mean, according to the American Astronomical Society .

Baily's Beads – This refers to the beaded appearance of the moon's edge as the eclipse nears totality. Shafts of sunlight shine through deep valleys on the moon's surface, which causes the effect known to observers as Bailey's Beads.

Umbra – The darkest part of the moon’s shadow, within which the entirety of the sun's bright face is blocked. An observer standing in the umbra sees a total solar eclipse.

Penumbra: The lighter part of the moon's shadow, making up its outer portion, where light can reach.

Corona – The sun’s upper atmosphere, visible as a pearly glow around the eclipsed sun during totality.

Totality – The maximum phase of a total solar eclipse, during which the moon’s disk completely covers the sun's bright face. It can last from a fraction of a second to a maximum of 7 minutes, 31 seconds, depending on viewing location and other factors.

− Dale Denwalt, The Oklahoman

A last-minute search for glasses for 'a perfect spot'

Patsy Faoro walked toward a half dozen people gathered at the gate of Turner Farm Park in Great Falls, Virginia. The people were awaiting the opening of an eclipse viewing event complete with telescopes and educational presentations.

Faoro, who lives near the park in this upscale Northern Virginia neighborhood, just wanted glasses to safely view the eclipse.

She visited CVS, Safeway and a 7/11 convenience store, all of which had advertised the availability of glasses to view the eclipse. None of the retailers had any glasses remaining when Faoro checked this morning.

“We have this great big open space that’s a perfect spot,” said Faoro, who viewed her first eclipse in the 1970s. “We just need some glasses.”

− Ken Alltucker

Indianapolis Motor Speedway event draws tens of thousands

Tory Patrick, 41, is hoping to give her four children a once-in-a-lifetime learning opportunity. Patrick and her family traveled 160 miles to Indianapolis Motor Speedway from their home in Granger, Indiana. They were expected to have 97% totality at home, but it’s not the same as total coverage, she said.

“They will be getting the full experience that they can’t read about in textbooks,” Patrick said.

Patrick and her sister Cally Gannon, 39, from Cincinnati, met in Indianapolis to view the eclipse from the speedway. They camped outside IMS over the weekend ahead of the event and on Sunday afternoon saw a double rainbow over the speedway’s iconic pagoda. An estimated 50,000 tickets were sold for the day’s festivities, presented in partnership with NASA and Purdue University.

Elsewhere in the stands, Ryan Harper, 30, knows today is special.

“We’re right in totality,” he said. “This sometimes doesn’t happen in people’s lifetimes.

— Sarah Bowman and Jade Thomas

Family travels from Baltimore to Rochester, N.Y., to view the eclipse

When Maze Pelham of Baltimore was a fifth-grader, he watched news of the 2017 solar eclipse and vowed to witness the next notable solar eclipse. That eclipse, and that day, has come. And this is why he and his parents, Janice and Sonney Pelham, left Maryland around 1 a.m. on Monday and drove nearly seven hours to Rochester, New York.

“I said to myself I would go to the next one,” Maze Pelham said while seated with his parents on lawn chairs at downtown Rochester’s Parcel 5 park, where he and hundreds of others are awaiting a view of the full eclipse. “It’s a promise I made to myself.”

– Democrat and Chronicle, part of the USA TODAY Network

In Brooklyn: Eclipses glasses, check. Lawn chair, check.

Ralph Emerson held a pair of paper sunglasses in his hands while riding the subway to Brooklyn's sprawling Prospect Park on Monday. He exited the train with a folded lawn chair strapped across his shoulder, and said he felt energized and full of anticipation.

"This is special," Emerson, 61, said. "I haven't woken up with this much excitement since the Super Bowl."

Walking to the park's botanic garden, Emerson said appreciating natural phenomena makes it easier to manage the stresses that come with living in a big city. Also like a big sports game, the eclipse will draw lots of people together to experience something much larger than themselves, in community, he said.

"A lot of times it's tragedies that bring us together, but sometimes things like this do," he said.

− Claire Thornton

Texas Eclipse Festival cut short by weather

The Texas Eclipse Festival was canceled Monday in Burnet County, 50 miles northwest of Austin, because of weather concerns. The festival, which included bands and other events, began Sunday and was supposed to wrap up Tuesday.

"We regret to inform you of the severe weather forecast, including risks of high winds, tornadic activity, large hail, and thunderstorms for later today, including during the eclipse, Tuesday, and Wednesday," festival organizers said on their website. "Your safety is our top priority."

Festivalgoers were urged to leave early for safety and to beat traffic. Guests, however, were allowed to stay "provided they pack and are prepared to depart after totality."

Traffic jams, collisions mar eclipse watchers' travel

Traffic crashes and road delays were reported across the nation as Americans flocked to prime locations to gaze up at the rare solar eclipse . Local roads and highways in states including Texas, Ohio, Illinois, Indiana, Vermont and New Hampshire were overwhelmed with eclipse chasers, many of whom were from out of state. The most intense bottlenecks appeared to be in the path of totality , where spectators will be able to view the full eclipse.

In Oklahoma, a crash around 7:30 a.m. local time narrowed eastbound lanes on I-40, a major transcontinental highway that runs from California to North Carolina. The crash occurred not far from several state parks that were hosting eclipse events, and it's also within driving distance of the full eclipse path, which includes parts of southeastern Oklahoma and Arkansas.

Multiple crashes were reported on an interstate in Waco, Texas, a city in the path of totality, where several festivities and eclipse viewing events were scheduled. The crashes all occurred on I-35, which runs from southern Texas to Minnesota.

"Expect major traffic delays in this area," the Texas Department of Transportation said in a statement .

− Christopher Cann

Airport draws a crowd of jets for eclipse

EAGLE PASS, Texas − Maverick County International Airport in Eagle Pass, Texas, usually sees maybe three private planes a day. By 10 a.m. CT on Monday, more than 10 planes crowded the tarmac, including Cessnas, Pipers and at least one corporate jet − all there to catch an early glimpse of the total solar eclipse. Eagle Pass will be one of the first American cities to witness the phenomenon. Some watchers, like Brandon Beck, 43, flew in Sunday night from San Diego and spent the night in a sleeping bag on the tarmac next to his friend’s Mooney single-prop piston four-seater.“We’re so lucky to be on a planet where the sun is the perfect size and perfect distance to create that effect,” he said. “It’s obligatory. We have to see it.”

First the Cherry Blossoms, then the eclipse

WASHINGTON − The Cherry Blossoms brought Ali and Aurore Youssouf to the city all the way from Paris, but the pair were delighted to learn that their trip would overlap with the eclipse. The day is extra special for Aurore Youssouf, 43, who has never seen an eclipse before. On Monday morning, the couple arrived by chance at the National Mall early enough to snag a few pairs of free eclipse glasses distributed by the Smithsonian National Air and Space Museum for the occasion.“We didn’t even know. We just ended up here and found out that they were giving away some glasses,” said Ali Youssouf, 46.

Awaiting eclipse − and hail − in Texas

KERRVILLE, Texas − Forecasts called for considerable cloudiness, rain and possible hail, but retired NASA engineer Jeff Stone stayed positive as he and his wife prepared to host more than a dozen people for a watch party at their hilltop home in Texas Hill Country. Stone, an eclipse enthusiast who had met family in Missouri to watch the solar eclipse of 2017, said he recently reviewed video footage of that event and said current conditions weren’t that much different than they were back then.

“I’m outside and seeing patches of blue among scattered clouds, so we’re remaining hopeful,” he said.

About 150 miles to the northeast in Gatesville, lead pastor Eric Moffett of Coryell Community Church said a similar forecast did little to deter an expected gathering of about 800 visitors for the church’s family-oriented “Eclipse at the Crosses” event. Many, if not most, were from out of town or even out of country − Canada, Wales, France, Italy. Many locals were staying home to avoid the crowd, he said.

“It’s partly cloudy, but the sun is shining on me right now,” he said. “We are praying that it stays that way.”

− Marc Ramirez

What is the difference between a solar eclipse and a lunar eclipse?

A  total solar eclipse  occurs when the moon comes in between the Earth and the sun, blocking its light from reaching our planet, leading to a period of darkness lasting several minutes. A total lunar eclipse occurs when the moon and the sun are on exact opposite sides of Earth. The Earth blocks the sunlight that normally reaches the moon. Instead of that sunlight hitting the moon’s surface, Earth's shadow falls on it.

A lunar eclipse can last for a few hours, while a solar eclipse lasts only a few minutes. Solar eclipses also rarely occur, while lunar eclipses are comparatively more frequent. While at least two partial lunar eclipses happen every year, total lunar eclipses are still rare, NASA says. Another major difference: No special glasses or gizmos are needed to view the a lunar eclipse, and people can directly stare at the moon.

− Saman Shafiq

Solar eclipse, lunar eclipse: What is the difference?

Can you drive during the solar eclipse?

The  2024 solar eclipse  will shroud much of the United States in darkness on Monday, leaving many people to wonder: Is it safe to drive during the solar eclipse?

It is safe to drive during an eclipse as long as you don't look up at the sky. AAA is telling drivers to be focused on the road if they are operating a car during the total solar eclipse. But the automobile insurance company is advising Americans who want to safely view the total eclipse to "find a safe place to park (not on the side of a road or highway) away from other traffic and then wear your eclipse glasses," said Aixa Diaz, a spokeswoman from AAA.

"Pack your patience, whether traveling a great distance or locally," Diaz said. "People will be out and about to catch a glimpse of the eclipse."

A similar scenario  played out in 2017 during the nation's last total solar eclipse : Congestion in some areas lasted for up to 13 hours after totality,  according to Transportation Research News , a National Research Council publication. An analysis of traffic patterns from that year suggests the worst of the traffic – on interstates and rural back roads alike – will kick off after the eclipse ends and everyone tries to leave all at once.

− Kayla Jimenez

When will the next solar eclipse happen?

It will be 20 years before there's a chance to  witness a total solar eclipse  in the United States again. According to NASA, after Monday's total solar  eclipse , the next one viewable from the contiguous U.S. will be on Aug. 23, 2044.

Unfortunately, the 2044 total  solar eclipse  won't have the broad reach across the U.S. as the 2024 eclipse. The path of totality during the 2044 eclipse will only touch three states, according to the Planetary Society, a nonprofit involved in research, public outreach and political space advocacy. The eclipse will begin in Greenland, sweep through Canada and end around sunset in Montana, North Dakota and South Dakota.

But another eclipse scheduledfor Saturday, Aug. 12, 2045, will trace a path of totality over California, Nevada, Utah, Colorado, New Mexico, Oklahoma, Kansas, Texas, Arkansas, Missouri, Mississippi, Louisiana, Alabama, Florida and Georgia.

− Mary Walrath-Holdridge, Gabe Hauari and Eric Lagatta

Eclipse and pets: It's not the sky that's the problem

An  eclipse  itself isn't dangerous for domestic animals such as dogs and cats, but experts say it's probably best to not bring pets to  experience the April 2024 total solar eclipse . Experts' biggest concern is not what’s happening in the sky but on the ground as crowds of excited and  eager people gather , said Dr. Rena Carlson, president of the American Veterinary Medical Association. Dogs especially will take their cues from their owners rather than  the celestial event.

“Rather than the effects of the eclipse, I would be more worried about the excitement and all of the people,” Carlson said.

− Elizabeth Weise

A guide to the solar eclipse for kids

Here's an easy-to read, illustrated guide to help kids understand what the  April 8 total solar eclipse  means, where it will be visible and how you can watch it safely. It also shows them how an ordinary kitchen colander can easily be used to view a solar eclipse − the colander's holes can project crescent images of the sun onto the ground. There are even pages that can be downloaded and colored.

Trump's eclipse experience lit up internet in 2017

Seven years ago, when the last  total solar eclipse  crossed over North America, a photo of then-President Donald Trump  seemingly gazing at the sun with his naked eyes set the internet ablaze . During the afternoon of Aug. 21, 2017, Trump, who was president at the time, was joined by then first lady Melania Trump and their son Barron  to watch the rare phenomenon from the Truman Balcony  at the White House. Photos taken by members of the media captured all three donning eclipse glasses while looking at the eclipse.

But at one point during the viewing, the former president was captured in photos and videos without proper eye protection, seemingly gazing directly at the sun. Looking at an eclipse before or after the brief phase of totality without proper eyewear  can lead to eye damage .

− Natalie Neysa Alund

When is the next total solar eclipse? What to know about the next eclipse's path, timing

How to make a solar eclipse viewer

People who didn't manage to get their  hands on glasses  are not completely down and out. There are other safe ways to view the eclipse , say experts, and a lot of them only require a little bit of craftiness and items you can find lying around the house.

Steps to make the cereal box eclipse viewer (And here is a video aid) :

• Get an empty, clean cereal box.
• Cut a white piece of cardboard that will fit snuggly in the bottom of the box, or secure it permanently by gluing it in place.
• Cut the top of the cereal box, removing both ends and leaving the center intact.
• Put a piece of tape across the center of the top to securely hold it closed.
• Tape a piece of heavy-duty foil or double a single layer for additional strength, covering one of the openings at the top of the cereal box. The other opening will remain open for viewing.
• Using a small nail (approximately 3mm in diameter) push a hole in the foil.
• Cover the entire box with construction paper, leaving the single-viewing opening and the foil uncovered.

The finished box should be held with the pin-hole side facing the sun. It may take a little practice pointing the box. With your back facing the sun, look through the viewing opening. A small image of the sun, about ½ cm in diameter, can be seen projected on the white paper inside the box.

− Mary Walrath-Holdrid

What is the path of the solar eclipse called?

Those in the direct line of the eclipse will be treated to an incredible sight as the moon completely blots out the sun's disk, ushering in uncharacteristic darkness and revealing the sun's elusive outermost layer called the corona. That moment is called "totality" and this year the path of totality crosses through 13 U.S. states.

The last total solar eclipse to cut through North America was in August 2017.

How to look at eclipse without glasses

A total solar eclipse offers skygazers the rare opportunity to witness the eclipse with the naked eye. However,  solar eclipse glasses  are still needed until totality is reached. 

Certified solar eclipse  glasses are crucial for spectators to avoid the sun's retina-damaging rays. But when the moon moves completely in front of the sun and blocks its light, you'll know it's  safe to remove the glasses  for a short period of time.

How fast does an eclipse shadow travel?

According to a post from the  National Weather Service  in Indianapolis on X, an eclipse shadow travels at speeds from 1,100 to 5,000 mph. Near the equator, it travels at around 1,100 mph and increases in speed as it approaches the poles.

The eclipse party is over? Time to recycle those glasses!

Astronomers Without Borders , or AWB, a non-profit organization running its second nationwide  eclipse glasses recycling drive . Her business will join over 300 schools, museums, city governments, commercial businesses, community organizations and local libraries that will collect and ship an estimated  millions of glasses  to be repurposed for use by underserved communities around the world in  future solar eclipses .

AWB launched its first run of the program after the last solar eclipse in 2017 when volunteer centers across the country collected millions of the glass. After vetting them for reuse, more than  half a million glasses were distributed to Africa, Asia and South America.

People can visit  AWB's website  to find their nearest participating collection center and learn more about recycling the glasses, said Andrew Fazekas, the organization's communication manager.  

"We figure there's probably going to be tens of millions of glasses out there at the very least," he said. "And most of them are probably very gently used."