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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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How Does Light Travel Through Space? Facts & FAQ

Last Updated on Mar 15 2024

Light is such a fundamental part of our lives. From the moment we’re born, we are showered with all kinds of electromagnetic radiation, both colorful, and invisible. Light travels through the vacuum of space at 186,828 miles per second as transverse waves , outside of any material or medium, because photons—the particles that make up light—also behave as waves. This is referred to as the wave-particle duality of light.  

• What Is Light?

The wave-particle duality of light simply means that light behaves as both waves and particles . Although this has been long accepted as fact, scientists only managed to observe both these properties of light ¹ simultaneously for the first time in 2015.

As a wave, light is electromagnetic radiation—vibrations, or oscillations, of the electric and magnetic fields. As particles, light is made up of little massless packets of energy called photons ¹ .

• What Are Light Waves?

Waves are the transference of energy from one point to another. If we dropped a pebble into a small pond, the energy that the impact creates would transfer as a ripple, or a wave, that travels through the surface of the water, from one water particle to another, until eventually reaching the edge of the pond.

This is also how sound waves work—except that, with sound, it’s the pressure or vibrations of particles in the air that eventually reach our ears.

Unlike water and sound, light itself is electromagnetic radiation—or light waves—so it doesn’t need a medium to travel through.

• What Are Transverse Waves?

Light propagates through transverse waves. Transverse waves refer to a way in which energy is transferred.

Transverse waves oscillate at a 90-degree angle (or right angle) to the direction the energy is traveling in. An easy way to picture this is to imagine an S shape flipped onto its side. The waves would be going up and down, while the energy would be moving either left or right.

With light waves ¹ , there are 2 oscillations to consider. If the light wave is traveling on the X axis, then the oscillations of the electric field would be at a right angle, either along the Y or Z axes, and the oscillations of the magnetic field would be on the other.

• Can Anything Travel Faster Than Light?

The simple answer to this question is no, as far as we know at this time, nothing can go faster than the speed of light ¹ . Albert Einstein’s special theory of relativity states that “no known object can travel faster than the speed of light in a vacuum.”

Space and time don’t yet exist beyond the speed of light—if we were to travel that fast, the closer we get to the speed of light, the more our spatial dimension would shrink, until eventually collapsing.

Beyond this, the laws of physics state that as an object approaches the speed of light , its mass would become infinite, and so would the energy it would need to propel it. Since it’s probably impossible to create an infinite amount of energy, it would be difficult for anything to travel faster than light.

Tachyon, a hypothetical particle, is said to travel faster than the speed of light. However, because its speed would not be consistent with the known laws of physics, physicists believe that tachyon particles do not exist.

• Final Thoughts

Light travels through space as transverse, electromagnetic waves. Its wave-particle duality means that it behaves as both particles and waves. As far as we know, nothing in the world travels as fast as light.

• https://phys.org/news/2015-03-particle.html
• https://www.nature.com/articles/ncomms7407
• https://www.wtamu.edu/~cbaird/sq/2017/07/20/is-the-reason-that-nothing-can-go-faster-than-light-because-we-have-not-tried-hard-enough/
• https://www.physics.brocku.ca/PPLATO/h-flap/phys6_1f_1.png
• https://science.nasa.gov/ems/02_anatomy

Featured Image Credit: NASA, Unsplash

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About the Author Cheryl Regan

Cheryl is a freelance content and copywriter from the United Kingdom. Her interests include hiking and amateur astronomy but focuses her writing on gardening and photography. If she isn't writing she can be found curled up with a coffee and her pet cat.

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How Light Travels: The Reason Why Telescopes Can See the Invisible Parts of Our Universe

Due to how light travels, we can only see the most eye-popping details of space—like nebulas, supernovas, and black holes—with specialized telescopes.

• Our eyes can see only a tiny fraction of these wavelengths , but our instruments enable us to learn far more.
• Here, we outline how various telescopes detect different wavelengths of light from space.

Light travels only one way: in a straight line. But the path it takes from Point A to Point B is always a waveform, with higher-energy light traveling in shorter wavelengths. Photons , which are tiny parcels of energy, have been traveling across the universe since they first exploded from the Big Bang . They always travel through the vacuum of space at 186,400 miles per second—the speed of light—which is faster than anything else.

Too bad we can glimpse only about 0.0035 percent of the light in the universe with our naked eyes. Humans can perceive just a tiny sliver of the electromagnetic spectrum: wavelengths from about 380–750 nanometers. This is what we call the visible part of the electromagnetic spectrum. The universe may be lovely to look at in this band, but our vision skips right over vast ranges of wavelengths that are either shorter or longer than this limited range. On either side of the visible band lies evidence of interstellar gas clouds, the hottest stars in the universe, gas clouds between galaxies , the gas that rushes into black holes, and much more.

Fortunately, telescopes allow us to see what would otherwise remain hidden. To perceive gas clouds between stars and galaxies, we use detectors that can capture infrared wavelengths. Super-hot stars require instruments that see short, ultraviolet wavelengths. To see the gas clouds between galaxies, we need X-ray detectors.

We’ve been using telescopes designed to reveal the invisible parts of the cosmos for more than 60 years. Because Earth’s atmosphere absorbs most wavelengths of light, many of our telescopes must observe the cosmos from orbit or outer space.

Here’s a snapshot of how we use specialized detectors to explore how light travels across the universe.

Infrared Waves

We can’t see infrared waves, but we can feel them as heat . A sensitive detector like the James Webb Space Telescope can discern this thermal energy from far across the universe. But we use infrared in more down-to-Earth ways as well. For example, remote-control devices work by sending infrared signals at about 940 nanometers to your television or stereo. These heat waves also emanate from incubators to help hatch a chick or keep a pet reptile warm. As a warm being, you radiate infrared waves too; a person using night vision goggles can see you, because the goggles turn infrared energy into false-color optical energy that your eyes can perceive. Infrared telescopes let us see outer space in a similar way.

Astronomers began the first sky surveys with infrared telescopes in the 1960s and 1970s. Webb , launched in 2021, takes advantage of the infrared spectrum to probe the deepest regions of the universe. Orbiting the sun at a truly cold expanse—about one million miles from Earth—Webb has three infrared detectors with the ability to peer farther back in time than any other telescope has so far.

Its primary imaging device, the Near Infrared Camera (NIRCam), observes the universe through detectors tuned to incoming wavelengths ranging from 0.6 to 5 microns, ideal for seeing light from the universe’s earliest stars and galaxies. Webb’s Mid-Infrared Instrument (MIRI) covers the wavelength range from 5 to 28 microns, its sensitive detectors collecting the redshifted light of distant galaxies. Conveniently for us, infrared passes more cleanly through deep space gas and dust clouds, revealing the objects behind them; for this and many other reasons, the infrared spectrum has gained a crucial foothold in our cosmic investigations. Earth-orbiting satellites like NASA’s Wide Field Infrared Survey Telescope ( WFIRST ) observe deep space via longer infrared wavelengths, too.

Yet, when stars first form, they mostly issue ultraviolet light . So why don’t we use ultraviolet detectors to find distant galaxies? It’s because the universe has been stretching since its beginning, and the light that travels through it has been stretching, too; every planet, star, and galaxy continually moves away from everything else. By the time light from GLASS-z13—formed 300 million years after the Big Bang—reaches our telescopes, it has been traveling for more than 13 billion years , a vast distance all the way from a younger universe. The light may have started as ultraviolet waves, but over vast scales of time and space, it ended up as infrared. So, this fledgling galaxy appears as a red dot to NIRCam. We are gazing back in time at a galaxy that is rushing away from us.

Radio Waves

If we could see the night sky only through radio waves, we would notice swaths of supernovae , pulsars, quasars, and gassy star-forming regions instead of the usual pinprick fairy lights of stars and planets.

Tools like the Arecibo Observatory in Puerto Rico can do the job our eyes can’t: detect some of the longest electromagnetic waves in the universe. Radio waves are typically the length of a football field, but they can be even longer than our planet’s diameter. Though the 1,000-foot-wide dish at Arecibo collapsed in 2020 due to structural problems, other large telescopes carry on the work of looking at radio waves from space. Large radio telescopes are special because they actually employ many smaller dishes, integrating their data to produce a really sharp image.

Unlike optical astronomy, ground-based radio telescopes don’t need to contend with clouds and rain. They can make out the composition, structure, and motion of planets and stars no matter the weather. However, the dishes of radio telescopes need to be much larger than optical ones to generate a comparable image, since radio waves are so long. The Parkes Observatory’s dish is 64 meters wide, but its imaging is comparable to a small backyard optical telescope, according to NASA .

Eight different radio telescopes all over the world coordinated their observations for the Event Horizon Telescope in 2019 to put together the eye-opening image of a black hole in the heart of the M87 galaxy (above).

Ultraviolet Waves

You may be most familiar with ultraviolet, or UV rays, in warnings to use sunscreen . The sun is our greatest local emitter of these higher-frequency, shorter wavelengths just beyond the human visible spectrum, ranging from 100 to 400 nanometers. The Hubble Space Telescope has been our main instrument for observing UV light from space, including young stars forming in Spiral Galaxy NGC 3627, the auroras of Jupiter, and a giant cloud of hydrogen evaporating from an exoplanet that is reacting to its star’s extreme radiation.

Our sun and other stars emit a full range of UV light, telling astronomers how relatively hot or cool they are according to the subdivisions of ultraviolet radiation: near ultraviolet, middle ultraviolet, far ultraviolet, and extreme ultraviolet. Applying a false-color visible light composite lets us see with our own eyes the differences in a star’s gas temperatures.

Hubble’s Wide Field Camera 3 (WFC3) breaks down ultraviolet light into specific present colors with filters. “Science visuals developers assign primary colors and reconstruct the data into a picture our eyes can clearly identify,” according to the Hubble website . Using image-processing software, astronomers and even amateur enthusiasts can turn the UV data into images that are not only beautiful, but also informative.

X-Ray Light

Since 1999, the orbiting Chandra X-Ray Observatory is the most sensitive radio telescope ever built. During one observation that lasted a few hours, its X-ray vision saw only four photons from a galaxy 240 million light-years away, but it was enough to ascertain a novel type of exploding star . The observatory, located 86,500 miles above Earth, can produce detailed, full-color images of hot X-ray-emitting objects, like supernovas, clusters of galaxies and gases, and jets of energy surrounding black holes that are millions of degrees Celsius. It can also measure the intensity of an individual X-ray wavelength, which ranges from just 0.01 to 10 nanometers. Its four sensitive mirrors pick up energetic photons and then electronic detectors at the end of a 30-foot optical apparatus focus the beams of X-rays.

Closer to home, the Aurora Borealis at the poles emits X-rays too. And down on Earth, this high-frequency, low-wavelength light passes easily through the soft tissue of our bodies, but not our bones, yielding stellar X-ray images of our skeletons and teeth.

Visible Light

Visible color gives astronomers essential clues to a whole world of information about a star, including temperature, distance, mass, and chemical composition. The Hubble Telescope, perched 340 miles above our planet, has been a major source of visible light images of the cosmos since 1990.

Hotter objects, like young stars, radiate energy at shorter wavelengths of light; that’s why younger stars at temperatures up to 12,000 degrees Celsius, like the star Rigel, look blue to us. Astronomers can also tell the mass of a star from its color. Because mass corresponds to temperature, observers know that hot blue stars are at least three times the mass of the sun. For instance, the extremely hot, luminous blue variable star Eta Carina’s bulk is 150 times the mass of our sun, and it radiates 1,000,000 times our sun’s energy.

Our comparatively older, dimmer sun is about 5,500 degrees Celsius, so it appears yellow. At the other end of the scale, the old star Betelgeuse has been blowing off its outer layer for the past few years, and it looks red because it’s only about 3,000 degrees Celsius.

A View of Earth

Scientists use different wavelengths of light to study phenomena closer to home, too.

Detectors in orbit can distinguish between geophysical and environmental features on Earth’s changing surface, such as volcanic action. For example, infrared light used alongside visible light detection reveals areas covered in snow, volcanic ash, and vegetation. The Moderate Resolution Imaging Spectroradiometer ( MODIS ) infrared instrument onboard the Aqua and Terra satellites monitors forest fire smoke and locates the source of a fire so humans don’t have to fly through smoke to evaluate the situation.

Next year, a satellite will be launched to gauge forest biomass using a special radar wavelength of about 70 centimeters that can penetrate the leafy canopy.

💡 Why is the sky blue? During the day, oxygen and nitrogen in Earth’s atmosphere scatters electromagnetic energy at the wavelengths of blue light (450–485 nanometers). At sunset, the sun’s light makes a longer journey through the atmosphere before greeting your eyes. Along the way, more of the sun’s light is scattered out of the blue spectrum and deeper into yellow and red.

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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FREQUENTLY ASKED QUESTIONS

What is a light-year.

Light-year is the distance light travels in one year. Light zips through interstellar space at 186,000 miles (300,000 kilometers) per second and 5.88 trillion miles (9.46 trillion kilometers) per year.

We use light-time to measure the vast distances of space.

It’s the distance that light travels in a specific period of time. Also: LIGHT IS FAST, nothing travels faster than light.

How far can light travel in one minute? 11,160,000 miles. It takes 43.2 minutes for sunlight to reach Jupiter, about 484 million miles away. Light is fast, but the distances are vast . In an hour, light can travel 671 million miles.

Earth is about eight light minutes from the Sun. A trip at light-speed to the very edge of our solar system – the farthest reaches of the Oort Cloud, a collection of dormant comets way, way out there – would take about 1.87 years. Keep going to Proxima Centauri, our nearest neighboring star, and plan on arriving in 4.25 years at light speed.

When we talk about the enormity of the cosmos, it’s easy to toss out big numbers – but far more difficult to wrap our minds around just how large, how far, and how numerous celestial bodies really are.

To get a better sense, for instance, of the true distances to exoplanets – planets around other stars – we might start with the theater in which we find them, the Milky Way galaxy

Our galaxy is a gravitationally bound collection of stars, swirling in a spiral through space. Based on the deepest images obtained so far, it’s one of about 2 trillion galaxies in the observable universe. Groups of them are bound into clusters of galaxies, and these into superclusters; the superclusters are arranged in immense sheets stretching across the universe, interspersed with dark voids and lending the whole a kind of spiderweb structure. Our galaxy probably contains 100 to 400 billion stars, and is about 100,000 light-years across. That sounds huge, and it is, at least until we start comparing it to other galaxies. Our neighboring Andromeda galaxy, for example, is some 220,000 light-years wide. Another galaxy, IC 1101, spans as much as 4 million light-years.

Based on observations by NASA’s Kepler Space Telescope, we can confidently predict that every star you see in the sky probably hosts at least one planet. Realistically, we’re most likely talking about multi-planet systems rather than just single planets. In our galaxy of hundreds of billions of stars, this pushes the number of planets potentially into the trillions. Confirmed exoplanet detections (made by Kepler and other telescopes, both in space and on the ground) now come to more than 4,000 – and that’s from looking at only tiny slices of our galaxy. Many of these are small, rocky worlds that might be at the right temperature for liquid water to pool on their surfaces.

The nearest-known exoplanet is a small, probably rocky planet orbiting Proxima Centauri – the next star over from Earth. A little more than four light-years away, or 24 trillion miles. If an airline offered a flight there by jet, it would take 5 million years. Not much is known about this world; its close orbit and the periodic flaring of its star lower its chances of being habitable.

The TRAPPIST-1 system is seven planets, all roughly in Earth’s size range, orbiting a red dwarf star about 40 light-years away. They are very likely rocky, with four in the “habitable zone” – the orbital distance allowing potential liquid water on the surface. And computer modeling shows some have a good chance of being watery – or icy – worlds. In the next few years, we might learn whether they have atmospheres or oceans, or even signs of habitability.

One of the most distant exoplanets known to us in the Milky Way is Kepler-443b. Traveling at light speed, it would take 3,000 years to get there. Or 28 billion years, going 60 mph.

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How Does Light Travel?

Sound & Light (Physics): How are They Different?

The question of how light travels through space is one of the perennial mysteries of physics. In modern explanations, it is a wave phenomenon that doesn't need a medium through which to propagate. According to quantum theory, it also behaves as a collection of particles under certain circumstances. For most macroscopic purposes, though, its behavior can be described by treating it as a wave and applying the principles of wave mechanics to describe its motion.

Electromagnetic Vibrations

In the mid 1800s, Scottish physicist James Clerk Maxwell established that light is a form of electromagnetic energy that travels in waves. The question of how it manages to do so in the absence of a medium is explained by the nature of electromagnetic vibrations. When a charged particle vibrates, it produces an electrical vibration that automatically induces a magnetic one -- physicists often visualize these vibrations occurring in perpendicular planes. The paired oscillations propagate outward from the source; no medium, except for the electromagnetic field that permeates the universe, is required to conduct them.

A Ray of Light

When an electromagnetic source generates light, the light travels outward as a series of concentric spheres spaced in accordance with the vibration of the source. Light always takes the shortest path between a source and destination. A line drawn from the source to the destination, perpendicular to the wave-fronts, is called a ray. Far from the source, spherical wave fronts degenerate into a series of parallel lines moving in the direction of the ray. Their spacing defines the wavelength of the light, and the number of such lines that pass a given point in a given unit of time defines the frequency.

The Speed of Light

The frequency with which a light source vibrates determines the frequency -- and wavelength -- of the resultant radiation. This directly affects the energy of the wave packet -- or burst of waves moving as a unit -- according to a relationship established by physicist Max Planck in the early 1900s. If the light is visible, the frequency of vibration determines color. The speed of light is unaffected by vibrational frequency, however. In a vacuum, it is always 299,792 kilometers per second (186, 282 miles per second), a value denoted by the letter "c." According to Einstein's Theory of Relativity, nothing in the universe travels faster than this.

Refraction and Rainbows

Light travels slower in a medium than it does in a vacuum, and the speed is proportional to the density of the medium. This speed variation causes light to bend at the interface of two media -- a phenomenon called refraction. The angle at which it bends depends on the densities of the two media and the wavelength of the incident light. When light incident on a transparent medium is composed of wave fronts of different wavelengths, each wave front bends at a different angle, and the result is a rainbow.

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Science Struck

How Does Light Travel Through Space and Other Media?

Light is one of the most enigmatic of entities in the universe. You are not the first person to carefully reflect upon this question about light travel. From Galileo, Newton, to Einstein, every one of the great minds has thought about this question and thanks to them, we have an answer today, in terms of the classical theory of electromagnetism.

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The Cosmic Speed Limit

According to Einstein’s special theory of relativity, the speed of light is constant in vacuum and no object can exceed it. In short, nothing can travel faster than the speed of light. It is the cosmic speed limit for information exchange.

Scientists have measured the speed of light to be 299,792,458 meters per second. It is the highest speed that can be achieved by any entity. So far, no object has been found to travel at a velocity that exceeds the speed of light.

One of the central pillars of modern physics, the special theory of relativity, will turn out to be wrong, if light speed is exceeded by any other entity. In short, light is the fastest thing in our universe and no information can travel faster than it. How does light achieve this phenomenal speed? How does it travel at all? Let us find out.

The Nature of Light: Wave/Photons

One can understand the phenomenon of light travel, once its nature is understood. The first glimpses of the nature of light were provided by the ingenuity of James Clark Maxwell. It came as a brilliant revelation to him as he was constructing a theory that describes the electric and magnetic forces between stationary and moving charged objects.

He discovered that electricity and magnetism were two sides of the same coin. His theory combined electricity and magnetism into the unified force of electromagnetism and light was found to be an electromagnetic wave. This theory pictures light as a transverse electromagnetic wave, traveling through space. This is the classical perspective.

However, light can also be alternately perceived as a particle, known as a photon, if you look at it, from the perspective of quantum mechanics, which quantizes all matter and energy. In fact, according to quantum field theory, everything behaves like a particle and a wave. Not just photons, but electrons, which were earlier thought to be particles, are now known to behave as waves. Both, particle and wave viewpoints are equivalent, and for this discussion, we will focus on the wave perspective, which provides a simpler explanation of light travel.

How Light Travels Through Space

So light is a kind of wave. However every wave, waves something. That is, every wave like a sea wave, travels through a medium, in which it causes disturbances and creates undulations.

If a sea wave is a ripple or a disturbance that travels through water, then what is the medium in which light travels as a wave? The answer is none. This is where light is different from any other kind of a wave. It can travel through vacuum and it does not require a medium to propagate.

How does light manage to do this? The answer lies in the fact that it is an electromagnetic wave, carrying energy. Maxwell discovered how light travels through vacuum. Here is an illustration of an electromagnetic wave, that illuminates its nature and clearly presents its component parts.

Here’s an explanation of what makes light travel possible, in a nutshell. Any charged object has an electric field associated with it. When that electric field changes, a changing magnetic field is created. Consequently, the changing magnetic field again creates an electric field. This continues ad infinitum, creating an electromagnetic wave, traveling in space. This is how light travels, as a disturbance or ripple in the electromagnetic field.

n short, an accelerated charge radiates electromagnetic waves. Mathematically, this relation can be understood through the solution of Maxwell’s equations for free space.

Visible light, radio waves, and even X-rays are all electromagnetic waves of different wavelengths and frequencies. The wavelength of visible light is such that our eyes are tuned to it, just like a radio receiver is tuned to radio waves. Ergo, our eyes can perceive that part of the electromagnetic spectrum.

Light travels in straight lines in vacuum. However, in a material medium, light shows two properties of reflection and refraction. When a light wave cannot penetrate an object, it gets reflected back and when a light wave travels inside a medium, it bends or gets refracted.

When light travels through a medium, it interacts with its electric field. Depending on the nature of that field, one can predict the degree of bending or refraction that light undergoes, when passing through it.

Here’s how you perceive light, through your eyes. Light entering inside the white of your eyes (cornea), through the lens, bends or refracts, to get focused on your retina. The retina converts the incident light into electric signals, from which your brain creates an image, that you perceive.

Light Travel Through Different Media

Though the speed of light is constant in vacuum, when traveling through denser media, it slows down considerably. The degree of bending or slowing down of light velocity in different media, is measured by the refractive index of a particular medium. Light undergoes bending or slowing down, when entering a denser medium, from a rarer one and speeds up when entering a rarer medium, from a denser one. Let us see what happens in both cases.

As illustrated in the diagram presented below, when light enters a denser medium, it slows down and bends towards the normal. Here, the angle of refraction (r) is lesser than the angle of incidence (r < i).

As demonstrated by the diagram presented below, when entering a rarer medium, from a denser medium, light speeds up, and bends away from the normal. Here, the angle of refraction is greater than the angle of incidence (r > i).

If the refractive index of one medium is known, the other one can be deduced from Snell’s law, if the angles of incidence (i) and refraction (r) are known. It’s stated as: sin i/sin r = n2/n1 where, ‘i’ is the angle of incidence, ‘r’ is the angle of refraction, and n1 is the refractive index of the medium of incidence, while n2 is the refractive index of the medium of refraction.

To fully understand how the dynamic nature of the electromagnetic field leads to the creation of waves, you will have to dig deep into Maxwell’s equations, presented in the graphic below. A good starting point is The Feynman Lectures on Physics, Vol.II. If God is perceived to be the laws of nature, one could say the following.

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Does Light Travel Forever?

Most recent answer: 01/23/2013

Hi Raja, Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears. In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through). There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us! David Schmid

(published on 01/23/2013)

Follow-Up #1: How far does light go?

Light just keeps going and going until it bumps into something.  Then it can either be reflected or absorbed.  Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.   

Light has some interesting properties.   It comes in lumps called photons.  These photons carry energy and momentum in specific amounts related to the color of the light.  There is much to learned about light.   I suggest you log in to our website and type  LIGHT into the search box.   Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" ,  the answer is yes.

(published on 12/03/2015)

Follow-Up #2: less than one photon?

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

(published on 02/04/2016)

Follow-Up #3: stars too far away to see?

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

(published on 07/22/2016)

Follow-Up #4: light going out to space

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

(published on 09/01/2016)

Follow-Up #5: end of the universe?

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

(published on 03/26/2017)

Follow-Up #6: seeing black holes

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

(published on 01/29/2018)

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How Do Light Rays Travel In Space

Table of Contents:

Sun makes slow light . a href=”www.

The photons of energy have finally, after 100,000 years, come to the end of their journey inside the Sun. They have now reached a zone which is transparent to light. The photons escape into space, and travel at the classic speed of light — around 300,000 kilometres per second. The very surface of the Sun is called the ‘photosphere’, because that’s where the photons have escaped from.

Video advice: Light traveling to The Earth from the Sun in Real Time

Ever wonder what it would look like from a third world perspective of light particles from our Sun traveling to the Earth in real time? Say no more! The video shows exactly that!

After thousands upon thousands of years, they make their way into what’s called the ‘radiative zone’. The radiative zone is huge — it stretches up from the outer core (that’s at the 24 per cent mark) to about 70 per cent of the way to the surface. It’s called the radiative zone, because here, energy travels by radiation. The gamma rays are still being absorbed and re-radiated, but each time they’re being re-radiated at longer wavelengths. They gradually get converted from gamma rays down to visible light. The temperature in the radiative zone is around 7 million°C near the bottom, and about 2 million°C near the top.

J O Bird BSc, CEng, MIEE, CMath, FIMA, FCollP, MIEIE, P J Chivers BSc, PhD, in Newnes Engineering and Physical Science Pocket Book, 1993.

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Light raysJ O Bird BSc, CEng, MIEE, CMath, FIMA, FCollP, MIEIE, P J Chivers BSc, PhD, in Newnes Engineering and Physical Science Pocket Book, 1993Publisher SummaryThis chapter focuses on light rays and its behavior. Light is an electromagnetic wave and the straight line paths followed by narrow beams of light, along which light energy travels, are called rays. Light always travels in straight lines although its direction can be changed by reflection or refraction. The behavior of light rays may be investigated by using a ray-box. This consists merely of a lamp in a box containing a narrow slit that emits rays of light. There are two laws of reflection, namely, the angle of incidence is equal to the angle of reflection and the incident ray, the normal at the point of incidence, and the reflected ray lie in the same plan. When a ray of light passes from one medium to another, the light undergoes a change in direction. This displacement of light rays is called refraction. In general, when entering a denser medium from a less dense medium, light is refracted toward the normal and when it passes from a dense to a less dense medium, it is refracted away from the normal.

Light from the sun travels through space to Earth’s atmosphere. What will light waves do when they move from empty space into matter made up of different gases? – They slow down, which may cause them to refract. The speed of light in vacuum is the highest possible speed (according to Einstein). It’s ~~300000km//sec When light goes into a denser medium (and any medium is denser than vacuum) it slows down. For instance the speed in air is: 1/1.000293=99.97% of this speed, which seems like almost the same, but it isn’t quite so. In water it goes down to ~~75% and in glass (depending on the sort of glass) to 2//3 or even 1//2 of the speed in vacuum. Diamond even reduces the speed to about 40%. If light hits a boundary (vacuum to denser) at an angle it will bend, or be refracted. Since not all colours have the same retardation, red light tends to be bent less than violet, with the other colours of the spectrum somewhere in between. This – in our daily life – creates rainbows.

Video advice: How Light Travels Through Time

If you shine a light at a spaceship traveling at half the speed of light, will the way they see the light be different? If you were to travel faster than the speed of light, would you age slower?

If light hits a boundary (vacuum to denser) at an angle it will bend, or be refracted. Since not all colours have the same retardation, red light tends to be bent less than violet, with the other colours of the spectrum somewhere in between. This – in our daily life – creates rainbows.

Quick Answer: How Does Light Rays Travel

Light rays Light always travels in straight lines although its direction can be changed by reflection or refraction. When a ray of light passes from one medium to another, the.

Electromagnetic surf is waves that may traverse matter or through empty space. However, when light travels in matter, it interacts using the atoms and molecules within the material and slows lower. Consequently, light travels fastest in empty space, and travels slowest in solids.

How does the light ray travel short answer? Do light rays move? Where does light always travel? Does light travel instantaneously? Is light a ray or particle? How does light go through glass? How does the light have to enter the glass? Why do waves move light? How does light travel through empty space? How does light travel without a medium? How fast is the speed of dark? How does light have no mass? Does time stop in a black hole? Is light invisible? How is light made? What exactly is light? Do mirrors absorb light? Can light pass through mirror? Does light go through water?

Video advice: Science Video for Kids: How Does Light Travel?

Light travels in waves. Light rays cannot bend to go around something. The light rays are either reflected, absorbed, or refracted. Watch out this video to understand these terms in detail! #Education #Science #Kids

Can light rays travel through space?

Like all forms of electromagnetic waves, light can travel through empty space, as well as through matter . Light can be absorbed. Absorbed light energy is converted into some other form, such as thermal or heat energy.

How does light can travel in space where no matter exists?

So the answer to your question is simply, light travels through the vacuum the same way matter does .

How does light travel through the vacuum of space?

Light travels as a wave. But unlike sound waves or water waves, it does not need any matter or material to carry its energy along. ... Nothing travels faster than light energy. It speeds through the vacuum of space at 186,400 miles (300,000 km) per second .

Why is there no light in space?

Because space is a near-perfect vacuum — meaning it has exceedingly few particles — there's virtually nothing in the space between stars and planets to scatter light to our eyes. And with no light reaching the eyes, they see black. —What color is the sunset on other planets?

How does the light from the Sun travel to Earth?

The Sun's energy gets to the Earth through radiation , which you can prove just by standing outside and letting the sun's rays warm your face on a sunny day. ... The energy lost is emitted as light or electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels.

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Three ways to travel at (nearly) the speed of light.

Katy Mersmann

1) electromagnetic fields, 2) magnetic explosions, 3) wave-particle interactions.

One hundred years ago today, on May 29, 1919, measurements of a solar eclipse offered verification 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.

One of NASA’s jobs is to better understand how these particles are accelerated. Studying these superfast, or relativistic, particles can ultimately help protect missions exploring the solar system, traveling to the Moon, and they can teach us more about our galactic neighborhood: A well-aimed near-light-speed particle can trip onboard electronics and too many at once could have negative radiation effects on space-faring astronauts as they travel to the Moon — or beyond.

Here are three ways that acceleration happens.

Most of the processes that accelerate particles to relativistic speeds work with electromagnetic fields — the same force that keeps magnets on your fridge. The two components, electric and magnetic fields, like two sides of the same coin, work together to whisk particles at relativistic speeds throughout the universe.

In essence, electromagnetic fields accelerate charged particles because the particles feel a force in an electromagnetic field that pushes them along, similar to how gravity pulls at objects with mass. In the right conditions, electromagnetic fields can accelerate particles at near-light-speed.

On Earth, electric fields are often specifically harnessed on smaller scales to speed up particles in laboratories. Particle accelerators, like the Large Hadron Collider and Fermilab, use pulsed electromagnetic fields to accelerate charged particles up to 99.99999896% the speed of light. At these speeds, the particles can be smashed together to produce collisions with immense amounts of energy. This allows scientists to look for elementary particles and understand what the universe was like in the very first fractions of a second after the Big Bang. 

Download related video from NASA Goddard’s Scientific Visualization Studio

Magnetic fields are everywhere in space, encircling Earth and spanning the solar system. They even guide charged particles moving through space, which spiral around the fields.

When these magnetic fields run into each other, they can become tangled. When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection. The rapid change in a region’s magnetic field creates electric fields, which causes all the attendant charged particles to be flung away at high speeds. Scientists suspect magnetic reconnection is one way that particles — for example, the solar wind, which is the constant stream of charged particles from the Sun — is accelerated to relativistic speeds.

Those speedy particles also create a variety of side-effects near planets.  Magnetic reconnection occurs close to us at points where the Sun’s magnetic field pushes against Earth’s magnetosphere — its protective magnetic environment. When magnetic reconnection occurs on the side of Earth facing away from the Sun, the particles can be hurled into Earth’s upper atmosphere where they spark the auroras. Magnetic reconnection is also thought to be responsible around other planets like Jupiter and Saturn, though in slightly different ways.

NASA’s Magnetospheric Multiscale spacecraft were designed and built to focus on understanding all aspects of magnetic reconnection. Using four identical spacecraft, the mission flies around Earth to catch magnetic reconnection in action. The results of the analyzed data can help scientists understand particle acceleration at relativistic speeds around Earth and across the universe.

Particles can be accelerated by interactions with electromagnetic waves, called wave-particle interactions. When electromagnetic waves collide, their fields can become compressed. Charged particles bouncing back and forth between the waves can gain energy similar to a ball bouncing between two merging walls.

These types of interactions are constantly occurring in near-Earth space and are responsible for accelerating particles to speeds that can damage electronics on spacecraft and satellites in space. NASA missions, like the Van Allen Probes , help scientists understand wave-particle interactions.

Wave-particle interactions are also thought to be responsible for accelerating some cosmic rays that originate outside our solar system. After a supernova explosion, a hot, dense shell of compressed gas called a blast wave is ejected away from the stellar core. Filled with magnetic fields and charged particles, wave-particle interactions in these bubbles can launch high-energy cosmic rays at 99.6% the speed of light. Wave-particle interactions may also be partially responsible for accelerating the solar wind and cosmic rays from the Sun.

Download this and related videos in HD formats from NASA Goddard’s Scientific Visualization Studio

By Mara Johnson-Groh NASA’s Goddard Space Flight Center , Greenbelt, Md.

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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Could A Telescope Ever See The Beginning Of Time?

The James Webb Space Telescope, or JWST for short, is one of the most advanced telescopes ever built . Planning for JWST began over 25 years ago, and construction efforts spanned over a decade. It was launched into space on Dec. 25, 2021, and within a month arrived at its final destination: 930,000 miles away from Earth. Its location in space allows it a relatively unobstructed view of the universe .

The telescope design was a global effort , led by NASA, and intended to push the boundaries of astronomical observation with revolutionary engineering. Its mirror is massive – about 21 feet (6.5 meters) in diameter. That’s nearly three times the size of the Hubble Space Telescope, which launched in 1990 and is still working today.

It’s a telescope’s mirror that allows it to collect light. JWST’s is so big that it can “see” the faintest and farthest galaxies and stars in the universe. Its state-of-the-art instruments can reveal information about the composition, temperature and motion of these distant cosmic objects.

As an astrophysicist , I’m continually looking back in time to see what stars, galaxies and supermassive black holes looked like when their light began its journey toward Earth, and I’m using that information to better understand their growth and evolution. For me, and for thousands of space scientists, the James Webb Space Telescope is a window to that unknown universe.

Just how far back can JWST peer into the cosmos and into the past? About 13.5 billion years.

Time Travel

A telescope does not show stars, galaxies and exoplanets as they are right now. Instead, astronomers are catching a glimpse of how they were in the past . It takes time for light to travel across space and reach our telescopes. In essence, that means a look into space is also a trip back in time.

This is even true for objects that are quite close to us. The light you see from the Sun left it about 8 minutes, 20 seconds earlier. That’s how long it takes for the Sun’s light to travel to Earth .

You can easily do the math on this. All light – whether sunlight, a flashlight or a light bulb in your house – travels at 186,000 miles (almost 300,000 kilometers) per second . That’s just over 11 million miles (about 18 million kilometers) per minute. The Sun is about 93 million miles (150 million kilometers) from Earth. That comes out to about 8 minutes, 20 seconds.

But the farther away something is, the longer its light takes to reach us. That’s why the light we see from Proxima Centauri , the closest star to us aside from our Sun, is 4 years old; that is, it’s about 25 trillion miles (approximately 40 trillion kilometers) away from Earth, so that light takes just over four years to reach us. Or, as scientists like to say, four light years .

Most recently, JWST observed Earendel, one of the farthest stars ever detected . The light that JWST sees from Earendel is about 12.9 billion years old.

The James Webb Space Telescope is looking much farther back in time than previously possible with other telescopes, such as the Hubble Space Telescope . For example, although Hubble can see objects 60,000 times fainter than the human eye is able, the JWST can see objects almost nine times fainter than even Hubble can .

The Big Bang

But is it possible to see back to the beginning of time?

The Big Bang is a term used to define the beginning of our universe as we know it. Scientists believe it occurred about 13.8 billion years ago . It is the most widely accepted theory among physicists to explain the history of our universe.

The name is a bit misleading, however, because it suggests that some sort of explosion, like fireworks, created the universe. The Big Bang more closely represents the appearance of rapidly expanding space everywhere in the universe. The environment immediately after the Big Bang was similar to a cosmic fog that covered the universe, making it hard for light to travel beyond it. Eventually, galaxies, stars and planets started to grow.

That’s why this era in the universe is called the “cosmic dark ages.” As the universe continued to expand, the cosmic fog began to rise , and light was eventually able to travel freely through space. In fact, a few satellites have observed the light left by the Big Bang, about 380,000 years after it occurred. These telescopes were built to detect the splotchy leftover glow from the Big Bang , whose light can be tracked in the microwave band.

However, even 380,000 years after the Big Bang, there were no stars and galaxies. The universe was still a very dark place. The cosmic dark ages wouldn’t end until a few hundred million years later, when the first stars and galaxies began to form.

The James Webb Space Telescope was not designed to observe as far back as the Big Bang, but instead to see the period when the first objects in the universe began to form and emit light. Before this time period, there is little light for the James Webb Space Telescope to observe, given the conditions of the early universe and the lack of galaxies and stars.

Peering back to the time period close to the Big Bang is not simply a matter of having a larger mirror – astronomers have already done it using other satellites that observe microwave emission from very soon after the Big Bang . So, the James Webb Space Telescope observing the universe a few hundred million years after the Big Bang isn’t a limitation of the telescope. Rather, that’s actually the telescope’s mission. It’s a reflection of where in the universe we expect the first light from stars and galaxies to emerge.

By studying ancient galaxies, scientists hope to understand the unique conditions of the early universe and gain insight into the processes that helped them flourish. That includes the evolution of supermassive black holes, the life cycle of stars, and what exoplanets – worlds beyond our solar system – are made of.

Adi Foord is an Assistant Professor of Astronomy and Astrophysics at the University of Maryland, Baltimore County. This article is republished from The Conversation under a Creative Commons license . Read the original article .

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How does astronomy use the electromagnetic spectrum?

There is more to light than meets the eye, and it teaches us a lot about the universe.

• Radio waves
• Submillimeter waves
• Ultraviolet

Additional resources

Bibliography.

Parts of the electromagnetic spectrum invisible to human eyes reveal a vast amount of information about the universe, but it took a long time for astronomers to learn how to view it.

For thousands of years, humans were looking up at the star-studded night sky using just their eyes sensitive to the optical wavelength of the electromagnetic spectrum . The first telescopes, invented in the early 17th century, enhanced the ability of human eyes by magnifying distant objects. 

But as physicists started discovering in the 19th century that there are other, invisible, types of light in the natural world around us, astronomers realized that there must be such light emanating also from the universe . 

Related: Astronomy: The oldest scientific discipline

Today, astronomers know that the majority of radiation, or light, present in the universe is invisible to human eyes. By looking at the universe in all possible wavelengths, scientists are piecing together a complex picture of the unfathomably vast cosmic environment that we are a part of. It took, however, decades, for instruments to be developed that could detect this invisible radiation from celestial sources. 

Here we explain what different parts of the electromagnetic spectrum teach us about the universe.

What do radio waves teach us about the universe?

Radio astronomy studies cosmic radiation with the longest wavelengths (from less than 0.4 inches to several miles, or 1 centimeter to several kilometers) and was the first kind of astronomy developed that relies on wavelengths other than optical light. 

The discovery that radio waves from bodies in the universe lash our planet was made completely by accident. In 1933, a young American radio engineer Karl Jansky, an employee of the famous telephone company Bell Laboratories, was tasked to search for sources of unexplained hiss that sometimes interfered with transmissions of radio messages across the Atlantic Ocean. Jansky found that while some of this noise was coming from sources on Earth , such as nearby thunderstorms, there was a type of signal, constantly picked up by his experimental antennas, that appeared to be coming from what we know today is the center of our Milky Way galaxy, the region where the black hole Sagittarius A* resides. Systematic exploration of the radio universe began soon thereafter.

Astronomers have discovered since that radio waves are emitted by spinning electrons and emanate from all sorts of environments that have the ability to make those electrons spin, Affelia Wibisono, an astronomer at the Royal Observatory Greenwich in the U.K., told Space.com. 

"Typically, when you detect radio waves, you're looking at electrons moving through a magnetic field," Wibisono said. "But ionized gas can emit radio waves as well."

By tracing the structure of radio wave-emitting clouds, astronomers were able to map out the entire structure of our galaxy, the Milky Way, as well as other nearby galaxies . They could determine areas with high concentrations of hot young stars , but also study objects obscured by dust, such as black holes that hide in galactic centers. Highly magnetized bodies, such as fast-spinning stellar remnants called pulsars are prime targets for radio astronomy as they send out powerful flashes of radio waves as they spin like superfast cosmic lighthouses. 

Famous radio telescopes 

As radio waves are the type of electromagnetic radiation with the longest wavelengths, radio telescopes have to be rather large. Vast arrays of radio-antennas, such as the Karl G. Jansky Very Large Array in New Mexico that consists of 28 dishes each 82-foot-wide (25 meters), are the technological standard today. By combining multiple antennas, astronomers create telescopes that have immense apertures that equal the distance between the array's most distant parts, thus enabling the scientist to detect the faintest signals with the best possible resolution.

The Square Kilometer Array (SKA), currently constructed across two locations in Australia and South Africa, will be the world's largest radio telescope by a significant margin once it comes online around 2028. With its thousands of dishes and dipole antennas spanning thousands of square miles of remote land, SKA will survey large areas of the sky at once and detect the faintest signals coming from the farthest reaches of the universe. 

The Event Horizon Telescope , famous for taking photographs of black holes , is also a radio telescope, or rather a worldwide network of radio telescopes with an aperture equalling the size of our planet. 

Unlike some other types of wavelengths, radio waves mostly penetrate Earth's atmosphere with ease, allowing astronomers to base their equipment on the planet's surface. 

However, due to the ubiquity of radio communication technologies in the modern world, radio telescopes are at risk of getting confused by human-made signals. SKA, for example, will therefore be surrounded by a radio-quiet zone where no cell phones and no radio equipment will be allowed. 

Constantly searching for better ways to study the universe, astronomers are now seriously considering building a radio telescope on the far side of the moon . Removed from Earth-based sources of human-made radio noise, as well as from Earth's ionosphere (the upper part of the atmosphere which contains ionized gas that absorbs and distorts some cosmic radio signals), such an observatory would provide scientists with the deepest and most undisturbed views into the earliest epoch of the universe.  

What do microwaves teach us about the universe?

The next electromagnetic spectrum band after radio waves are microwaves. As microwaves cover wavelengths between 3.3 feet and 0.04 inches (1 meter and 1 millimeter), the first discoveries of cosmic microwaves were actually made by radio telescopes. 

Microwaves have a special, although rather limited place in astronomy. According to the European Space Agency (ESA), the whole sky glows uniformly in microwaves with what has been identified as the cosmic microwave background . 

This uniformness is unseen in other wavelengths, which reveal the sky in dots and regions of varying brightness. In fact, cosmic microwave radiation is so odd that the researchers who first discovered it in the 1960s (completely by accident during experiments with echo balloons) originally thought it was produced by a telescope defect. 

Subsequent research, however, confirmed that the microwave hum was coming from space and that it was nothing less than the residue of radiation released by the Big Bang , the enormous explosion which created the universe some 13.8 billion years ago. 

This radiation was originally released in the form of highly energetic, short-wavelength X-rays, but since it took so long to reach us, the so-called redshift effect caused by the expansion of the universe has stretched this wavelength all the way into microwaves.  

Microwaves reveal the universe as it looked in its earliest stages. The most sensitive surveys were able to go as far as distinguishing the denser regions of gas and dust that subsequently produced the first galaxies. 

 Famous microwave telescopes

Microwaves get mostly absorbed by Earth's atmosphere, which means they are best studied by space-based telescopes.

In 1989, after the initial crude Earth-based detections of cosmic microwaves, NASA sent the first dedicated microwave-observing satellite — the Cosmic Background Explorer (COBE) — into space. COBE measured differences in the temperature of the microwave background in various regions. COBE's successor, the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2003, further improved the level of detail of this cosmic microwave map. These observations helped to determine the universe's age with greater precision, according to the European Space Agency (ESA) , and define the amounts of different types of matter that the universe contained in its earliest years. 

ESA's Planck mission , launched in 2009, then completed the task of creating the most accurate map of the cosmic microwave background, which, ESA said, is to some extent "definitive," as some of the measurements cannot be further improved.  

What do submillimeter waves teach us about the universe?

The submillimeter wavelength sits between the millimeter and infrared ranges. As the name suggests, submillimeter waves have lengths shorter than 1 mm, or 0.04 inches, and up to a few hundred micrometers. Observations in this range partially overlap with the longest wavelengths of the infrared spectrum.

The use of submillimeter wavelengths in astronomy is relatively recent, according to Astronomy Cast . Detectors used to detect submillimeter radiation are quite similar to those used in radio astronomy, but thousands of times smaller. Technology therefore had to progress enough to make these detectors possible.

Speaking to the Astronomy Cast, an astronomy podcast, American astronomer Pamela Gay said that the use of submillimeter waves in astronomy is limited to certain types of objects and phenomena. 

Submillimeter waves penetrate through clouds of molecular gas and dust into star-forming regions, which are obscured from the view of of optical telescopes.

In submillimeter waves, astronomers can observe universe's "natural lasers," regions where highly charged electrons emit laser light as they discharge some of their energy, said Gay. These natural lasers, sometimes called masers , are usually observed in a special type of pulsating variable stars called the Mira stars .

Submillimeter waves are also good at pointing astronomers to some interesting types of organic molecules and do a good job analyzing cold objects such as comets in the solar system , said Gay.

Famous submilimeter telescopes

Because submillimeter waves get absorbed by water in Earth's atmosphere, observatories that study sources of submillimeter radiation in the universe need to be built in high and dry places to prevent water vapor from obscuring their views. In essence, you will find submillimeter telescopes in the same places on Earth where you find the best optical telescopes.

The Atacama Large Millimeter/submillimeter Array (ALMA) , operated by the European Southern Observatory is located on the Chajnantor plateau in northern Chile at an altitude  of 16,400 feet (5,000 m). 

The Submillimeter Array on Hawai'i's Maunakea, which is operated by the Harvard Smithsonian Center for Astrophysics, sits somewhat lower, at 13,450 feet (4,100 m) above sea level.

What does infrared light teach us about the universe?

Unlike submillimeter waves, infrared light spans a vast range of the electromagnetic spectrum from 0.04 inches (just below 1 millimeter) on the side bordering with microwaves to 0.75 micrometers on the side bordering with the visible light. 

The NASA-led James Webb Space Telescope (JWST), launched on Christmas Day, 2021, thrust infrared astronomy into the spotlight with its ability to see the farthest reaches of the universe. 

Infrared light, which is essentially heat, was the first non-visible wavelength discovered, completely by accident, by British astronomer William Herschel in 1800 during his experiments with the visible light spectrum. It took, however, a long time for infrared detectors to become sensitive enough to provide the breathtaking views of the cosmos that JWST is now known for. 

The first crude observations of celestial objects in the infrared spectrum focused on the moon and the sun . Astronomers in the second half of the 19th century were able to measure the temperature of the sun 's atmosphere as well as the various temperature zones on the moon's surface. By the turn of the century, technology progressed to the level that it was possible to detect heat from the solar system's giant planets Jupiter and Saturn , according to A brief history of infrared astronomy .

Infrared astronomy, however, didn't fully take off until the second half of the 20th century when more sophisticated detectors were developed, allowing astronomers to analyze heat sources across the Milky Way.

Related: James Webb Space Telescope images: 12 astonishing views of our universe (gallery

As the JWST has plentifully demonstrated since the release of its first images in July, 2022, infrared light is good at many things.

Thanks to its ability to penetrate through dust and gas, infrared light reveals what's going on inside of thick dust and gas clouds where stars form. Stars emerging in the middle of these clouds are not yet hot enough to emit visible light, but are warm enough to be detected by infrared sensors.

With such advanced technology as the JWST, astronomers can observe matter that is only several degrees warmer than absolute zero, the temperature of minus 459.67 degrees Fahrenheit (minus 273.15 degrees Fahrenheit), where the motion of atoms stops.

When viewing the Milky Way in infrared light, a hidden galaxy of failed stars, called brown dwarfs, emerges. Brown dwarfs are bodies that are too big to be called planets but are not quite massive enough to ignite nuclear fusion in their cores. Bodies in the farther reaches of the solar system that receive too little solar illumination also spring into view. Even the interstellar medium , the cool gas and dust dispersed between stars and galaxies, can be mapped in the infrared spectrum.

Webb was built with the aim to detect the first light that lit up the universe a few hundred million years after the Big Bang. Although this light had been emitted in the optical wavelength range, the accelerating expansion of the universe had stretched this light into the infrared range thanks to the effect known as redshift. Optical telescopes, even if they were as sensitive as Webb, could therefore no longer see this light.

But the James Webb Space Telescope sees only a small fraction of the infrared spectrum, the so-called mid and near-infrared light, which spans wavelengths from 28.5 micrometers to 0.6 micrometers where the visible spectrum begins.

NASA's recently retired flying telescope SOFIA was a specialist in the longer wavelength type of infrared light, the so-called far infrared, which reaches all the way to 612 micrometers and is best for observing the cool interstellar medium.

Famous infrared telescopes

Both, the James Webb Space Telescope and SOFIA, the current and recently retired (respectively) infrared astronomy flagships, had their predecessor. 

NASA's Spitzer Space Telescope surveyed the universe in the mid-infrared and parts of the far infrared spectrum from 2003 to 2020. ESA's Herschel spacecraft complemented this work in the far-infrared spectrum between 2009 and 2013. 

An earlier airborne telescope, the Kuiper Airborne Observatory , studied the infrared sky from 1974 to 1995. 

What does optical light teach us about the universe?

Optical astronomy has made enormous leaps since those first early 17th-century telescopes. Enhancing the natural abilities of the human eye beyond imagination, 21st-century optical telescopes are still the backbone of astronomy research. 

From giant telescopes occupying remote mountain tops and highland plateaus to orbiting super-eyes such as the iconic Hubble Space Telescope , optical observatories reveal the universe with an ever-increasing level of detail. Some, on the other hand, focus on scanning vast swaths of the sky at once to spot unexpected phenomena, such as supernova explosions of dying stars or approaching asteroids . 

Optical telescopes show the universe as it would appear to human eyes. Colors in optical images correspond to the colors human eyes would see. Images from other types of  telescopes, such as those imaging the universe in infrared and ultraviolet light, have to be processed by astronomers on the ground, with colors artificially assigned to different wavelengths. 

To be visible in the optical wavelengths, objects need to either emit their own visible light or be illuminated by other objects. Planets, moons and asteroids in our solar system are only visible to optical telescopes (and to human eyes) because of the vicinity of our sun. 

Optical light can't pass through obstacles, such as thick clouds of dust, which hide some of the most interesting areas of the universe (such as centers of galaxies where supermassive black holes devour huge amounts of material or star-forming nebulas).

Optical light is also somewhat affected by Earth's atmosphere, even though not as much as the infrared and submillimeter wavelengths. While infrared and submillimeter radiation gets mostly absorbed, optical rays get a little dispersed by the molecules in the atmosphere, which means that observed objects don't appear as sharp as they would if the atmosphere wasn't present. This atmospheric blurring limits the accuracy of observations that Earth-based optical telescopes can achieve, even though modern adaptive optics systems installed on the world's best telescopes can to a certain extent make up for this shortcoming. 

Aside from complex, costly machines in space and on remote mountain tops, optical astronomy is the most accessible method of observing the sky for amateur skywatchers. Decent backyard telescopes can be purchased for a few hundred dollars and Space.com provides plenty of guides on how to pick the best one for you.

Related: Best telescopes for seeing planets in 2023

Best telescopes 2023: For stargazing galaxies, nebulas and more

Famous optical telescopes 

The Hubble Space Telescope is the undisputed king of optical astronomy and the source of many images that have gained iconic status. The telescope, launched in 1990, is still going strong and still may have a decade or so of life and fabulous astronomy ahead of it.

The Very Large Telescope (VLT) operated by the European Southern Observatory (ESO) in Chile is one of the most advanced Earth-based optical telescopes. VLT consists of four main telescopes, each with a 27-foot-wide (8.2 meter) mirror, and four 5.9-foot-wide (1.8 m) auxiliary telescopes. The four main telescopes can each detect light that is four billion times fainter than what human eyes can see. The telescopes can also work together as a so-called interferometer , which increases the resolution to a level that would be achievable with a single telescope with a 426-foot-wide (130 m) mirror. 

ESO is currently building the next-generation Extremely Large Telescope (ELT), also in Chile. With a single 130-foot-wide (39.3 m) mirror , ELT will be the world's largest optical telescope. Once completed, the observatory will be able to gather 100 million times more light than the human eye and provide images 16 times sharper than the Hubble Space Telescope, according to ESO. 

The twin Keck Telescopes on the Hawaiian island of Maunakea are fitted with 32.8-foot-wide (10 m wide) mirrors that forced the technical teams that designed and built them in the late 1980s to develop some ingenious technical solutions. Since it wasn't possible at that time to accurately operate a single solid mirror of such a size, engineers made the Keck mirrors from 36 hexagonal segments that work together as a unit with the help of an active optics system. This segmented mirror design is quite similar to the one used for the 21-foot-wide (6.5 m) mirror of the James Webb Space Telescope.

The Large Binocular Telescope in Arizona features the world's largest non-segmented mirror, measuring 28 feet (8.4 m) in diameter. 

The Gran Telescopio Canarias on the Spanish island of La Palma off the coast of western Africa, is the world's largest single-aperture optical telescope, featuring a 10.4 m wide mirror. 

What does ultraviolet light teach us about the universe?

The great Hubble is also the world's main observer of ultraviolet light that emanates from sources in the universe. Ultraviolet light has shorter wavelengths and carries higher energies than visible light and points astronomers to hot, energetic processes, such as those taking place in young stars and in young star-forming galaxies . Massive stars that orbit each other in binary systems also emit ultraviolet light and so do powerful auroras on giant gaseous planets like Jupiter.

Ultraviolet light gets absorbed by the ozone layer in Earth's atmosphere, which is good for organisms living on Earth (as these wavelengths are known to cause tissue-damage and cancer). For astronomy, however, the limited ability of ultraviolet light to penetrate the atmospheres means that telescopes designed to study it need to orbit in space.

Famous ultraviolet telescopes

Apart from the Hubble Space Telescope, solar observatories such as the European Solar Orbiter or NASA's Solar Dynamics Observatory carry ultraviolet imagers to observe highly energetic processes on the sun. NASA's Jupiter explorer Juno also carries an instrument for studying ultraviolet light. 

What do X-rays teach us about the universe?

Things get even more heated and energetic with X-rays. Discovered accidentally by German physicist Wilhelm Roentgen in 1895, these matter-penetrating rays are generated in vast amounts during some of the most extraordinary processes in the universe, such as when supermassive black holes or extremely massive neutron stars suck in matter from their surroundings, or during supernova explosions of dying stars. 

X-rays come from the hottest places in the universe including black hole and neutron stars' accretion disks where matter spirals at extreme speeds. High-temperature plasma that fills space between galaxies in galaxy clusters also emits X-rays, and so do stars including our sun. 

Astronomers recently discovered that comets can emit X-rays, Wibisono said, and that Jupiter, in addition to its ultraviolet aurora, also produces an aurora that shines in X-rays. 

"X-rays are a really powerful part of the spectrum because you get fluorescence in X-rays," said Wibisono. "Rocky surfaces of moons and planets give off X-rays for fluorescence. The atmospheres around terrestrial planets also fluoresce and X-rays, the gas giants scatter solar X-rays, so they act like a mirror to the solar X-rays."

Fluorescence is the ability of a surface to absorb and subsequently emit light that originally arrived from another source. 

Infamous for their potential to cause DNA mutations that may lead to cancer, X-rays get, just like ultraviolet rays, fortunately filtered out by Earth's atmosphere. X-ray astronomy could therefore only take off once humans were able to send objects to space. Astronomers knew prior to that that the sun is a powerful source of X-rays, but the first instruments capable of detecting other sources of cosmic X-rays were only launched aboard sounding rockets in the 1960s.

One of the problems with the detection of cosmic X-rays is their ability to penetrate matter. Just like they penetrate human tissue to reveal broken bones, X-rays also pass through mirrors that astronomers may want to use to concentrate them. 

Building sensitive X-ray detectors therefore requires some engineering ingenuity. Scientists have to design mirrors for X-ray telescopes in a way that the energetic rays hit the reflecting surface at a shallow angle "like a stone skipping across the surface of a pond," according to NASA . 

X-ray telescopes require multiple mirrors positioned at gradually increasing angles to deflect the X-rays onto a detector. Such contraptions, however, tend to be rather chunky and require large satellites to accommodate them. NASA's Chandra, for example, at 45-feet-long (13 m), is the largest satellite launched by the Space Shuttle , about a three feet (1 m) longer than Hubble. 

The matter-penetrating ability of X-rays, however, also has its advantages, as these rays easily escape from dust-shrouded regions, such as galactic centers where black holes munch on the infalling matter.

Famous X-ray telescopes

NASA's Chandra X-ray observatory is the current flagship X-ray telescope. In space since 1999, Chandra travels around Earth on an elliptical orbit that takes it as far as 83,000 miles (133,000 km) away from the planet's surface where no residual atmosphere obstructs the X-ray views. During its more than two decades in orbit, Chandra has imaged jets of matter shooting from supermassive black holes in galactic centers and even traced the separation of dark matter from normal matter in the collisions of galaxies in galactic clusters. 

ESA also has its X-ray space observer, the XMM-Newton space telescope , also launched in 1999. 

What do gamma-rays teach us about the universe?

— The 10 biggest telescopes on Earth — James Webb Space Telescope's 1st year in space has blown astronomers away — Best telescopes for kids 2023: Astronomy for all the family

Gamma-rays are the highest energy type of radiation present in the universe. Just like X-rays, they come from extremely hot and energetic processes in the universe, such as supernova explosions and accreting black holes. Even more capable of penetrating matter than X-rays, gamma-rays are also produced during nuclear explosions on Earth, and, in smaller quantities, in thunderstorms and during radioactive decay. Stars such as our sun also produce occasional gamma-ray flashes in the form of solar flares .

Just like many other types of astronomy, gamma-ray astronomy came about by accident. In the 1960s, American military satellites were looking for signs of the USSR's testing of nuclear weapons, when they detected inexplicable flashes of extremely energetic gamma-rays. Lasting from fractions of seconds to several minutes, these gamma-ray bursts , as they became known, were coming regularly from all parts of the universe. 

It took until the 1990s for astronomers to figure out that these bursts come from extremely powerful explosions that mark the birth of new black holes when massive stars die. The shorter types of gamma-ray bursts are produced in collisions of superdense stellar remnants called the neutron stars.

Gamma-ray bursts point astronomers to the fact that a cataclysmic event has just occurred somewhere in the universe. By measuring the intensity of the burst, astronomers can learn something about the intensity and distance of the event. However, they need to search for the source of the flash afterward, using other types of telescopes. When they manage to locate the region in the sky where the burst has come from, they can then observe the area in other parts of the electromagnetic spectrum to gain more insight into the processes involved.

Famous gamma-ray telescopes

NASA's space telescopes Fermi and Swift together with ESA's Integral are the world's current gamma-ray burst spotting workhorses. However, only Swift, which covers about 9% of the sky, has the ability to locate sources of these giant explosions. 

Astronomers are therefore looking for new approaches to gamma-ray burst detection. In 2021, a team of scientists from Hungary and Slovakia launched a tiny cubesat called GRB Alpha , that has been successfully detecting gamma-ray bursts ever since. In October 2022, GRBAlpha made an accurate detection of the peak intensity of the brightest gamma-ray burst ever seen, while the event completely blinded detectors on NASA's Fermi. 

The researchers envision that a fleet of such cubesats would make it possible to find sources of gamma-ray bursts across the entire sky through the so-called triangulation, the same method used to pinpoint a location on Earth with the help of GPS . 

Follow Tereza Pultarova on Twitter @TerezaPultarova . Follow us on Twitter @Spacedotcom and on Facebook . 

Astronomy Cast, produced by the Planetary Science Institute in collaboration with Universe Today has a great series on the different kinds of astronomy based on the electromagnetic spectrum. You can listen to their radio astronomy , submillimeter astronomy , infrared astronomy , optical astronomy , ultraviolet astronomy , X-ray astronomy and gamma-ray astronomy .

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ESA: Seeing with infrared eyes: A brief history of infrared astronomy, July 2020, accessed April 2023 from: https://sci.esa.int/web/herschel/-/59550-a-brief-history-of-infrared-astronomy

Harvard University: History of X-Ray Astronomy, accessed April 2023 from: https://chandra.harvard.edu/xray_astro/history.html

ESA: History of X-ray astronomy in Europe: From Exosat to ATHENA, accessed April 2023 from: https://sci.esa.int/web/athena/-/60759-history-of-x-ray-astronomy-in-europe-from-exosat-to-athena

Pinkau, K. History of gamma-ray telescopes and astronomy, Experimental Astronomy, Volume 25, Issue 1-3, pp. 157-171, August 2009: https://ui.adsabs.harvard.edu/abs/2009ExA....25..157P/abstract

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Tereza is a London-based science and technology journalist, aspiring fiction writer and amateur gymnast. Originally from Prague, the Czech Republic, she spent the first seven years of her career working as a reporter, script-writer and presenter for various TV programmes of the Czech Public Service Television. She later took a career break to pursue further education and added a Master's in Science from the International Space University, France, to her Bachelor's in Journalism and Master's in Cultural Anthropology from Prague's Charles University. She worked as a reporter at the Engineering and Technology magazine, freelanced for a range of publications including Live Science, Space.com, Professional Engineering, Via Satellite and Space News and served as a maternity cover science editor at the European Space Agency.

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• rod How does BB explain the origin of the electromagnetic spectrum? For example, why should there be optical light vs. nothing to see at all? Consider some past discussions on issues like this in science. https://forums.space.com/threads/one-of-the-greatest-damn-mysteries-of-physics-we-studied-distant-suns-in-the-most-precise-astronomical-test-of-electromagnetism-yet.59494/ https://forums.space.com/threads/how-do-we-know-the-fundamental-constants-are-constant-we-dont.59359/ https://forums.space.com/threads/what-makes-newtons-laws-work-heres-the-simple-trick.57302/ Reply
• Pentcho Valev The underlying assumptions are that the universe is expanding and that the speed of light is constant. However the expanding-universe theory is too preposterous and will soon be abandoned. Then the only reason behind the cosmological (Hubble) redshift will turn out to be GRADUALLY DECREASING SPEED OF LIGHT. And cosmologists will have to start from the scratch. Reply
• murgatroyd Good overview article, thanks! Reply
• rod "This uniformness is unseen in other wavelengths, which reveal the sky in dots and regions of varying brightness. In fact, cosmic microwave radiation is so odd that the researchers who first discovered it in the 1960s (completely by accident during experiments with echo balloons) originally thought it was produced by a telescope defect. Subsequent research, however, confirmed that the microwave hum was coming from space and that it was nothing less than the residue of radiation released by the Big Bang, the enormous explosion which created the universe some 13.8 billion years ago." According to cosmology calculators, depending upon the input value for H0, the universe radius when the CMBR became light was about 40-41 million light years radius. In the spectrum of the CMBR, where is the H-alpha line and He showing the universe then was filled with H and He gas? The same applies to the H1 21-cm line for H gas filling the universe during the cosmic dark ages and showing zero-metal gas? Reply

rod said: According to cosmology calculators, depending upon the input value for H0, the universe radius when the CMBR became light was about 40-41 million light years radius. In the spectrum of the CMBR, where is the H-alpha line and He showing the universe then was filled with H and He gas? The same applies to the H1 21-cm line for H gas filling the universe during the cosmic dark ages and showing zero-metal gas?

Helio said: If I understand it, the photons were highly scattered during those 40 million years until the universe cooled, due to expansion, to about 3000K (2.73 x 1100). Suddenly, the electrons became bound to the protons, forming the first atoms. Thus, all those photons were released to go flying in every direction without all that scattering. So we’re not seeing emissions from h or he, hence no emission or absorption lines.

rod said: However, Helio, others do not see this as you present. Hydrogen absorption lines in the cosmic microwave background spectrum, https://www.researchgate.net/publication/226565853_Hydrogen_absorption_lines_in_the_cosmic_microwave_background_spectrum, August 2004. "We have calculated the intensities of the subordinate hydrogen lines formed during the recombination epoch at redshifts 800≲z≲1600. We show that an allowance for the angular momentum splitting of hydrogen atomic energy levels and the dipole transition selection rules can reveal absorption features in the cosmic microwave background recombination spectrum in the submillimeter wavelength range." Impact of inhomogeneous CMB heating of gas on the HI 21-cm signal during dark ages, https://arxiv.org/abs/1810.05908

Admin said: Frequencies of light invisible to the human eye reveal a vast amount of information about our universe. But it took decades for scientists to learn how How does astronomy use the electromagnetic spectrum? : Read more

rod said: However, Helio, others do not see this as you present. Hydrogen absorption lines in the cosmic microwave background spectrum, https://www.researchgate.net/publication/226565853_Hydrogen_absorption_lines_in_the_cosmic_microwave_background_spectrum,

Helio said: In their short conclusion, they seem to confirm the spectral distribution that would exist for the Recombination event. Recombination, of course, is a major prediction of the BBT, giving us the CMBR. That they can see some Hβ and Hγ lines is interesting, and not contrary to BBT.

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Venus Facts

Venus is the second planet from the Sun, and our closest planetary neighbor. It's the hottest planet in our solar system, and is sometimes called Earth's twin.

Quick Facts

Venus is the second planet from the Sun.

Venus is a bit smaller than Earth. It's 7,521 miles (12,104 kilometers) across, and Earth is 7,926 miles (12,756 kilometers).

On Venus, the Sun would rise in the west and set in the east, because Venus spins backward compared to Earth.

Except for Earth, Venus has by far the fewest impact craters of any rocky planet.

Venus is named for the ancient Roman goddess of love and beauty, who was known as Aphrodite to the ancient Greeks. Most features on Venus are named for women.

Introduction

Venus is the second planet from the Sun, and Earth's closest planetary neighbor. Venus is the third brightest object in the sky after the Sun and Moon. Venus spins slowly in the opposite direction from most planets.

Venus is similar in structure and size to Earth, and is sometimes called Earth's evil twin. Its thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system with surface temperatures hot enough to melt lead. Below the dense, persistent clouds, the surface has volcanoes and deformed mountains.

How Venus Got Its Name

The ancient Romans could easily see seven bright objects in the sky: the Sun, the Moon, and the five brightest planets: Mercury, Venus, Mars, Jupiter, and Saturn. They named the objects after their most important gods.

Venus is named for the ancient Roman goddess of love and beauty, who was known as Aphrodite to the ancient Greeks. Most features on Venus are named for women. It’s the only planet named after a female god.

Potential for Life

Thirty miles up (about 50 kilometers) from the surface of Venus temperatures range from 86 to 158 Fahrenheit (30 to 70 Celsius). This temperature range could accommodate Earthly life, such as “extremophile” microbes. And atmospheric pressure at that height is similar to what we find on Earth’s surface.

At the tops of Venus’ clouds, whipped around the planet by winds measured as high as 224 mph (360 kph), we find another transformation. Persistent, dark streaks appear. Scientists are so far unable to explain why these streaks remain stubbornly intact, even amid hurricane-force winds. They also have the odd habit of absorbing ultraviolet radiation.

The most likely explanations focus on fine particles, ice crystals, or even a chemical compound called iron chloride. Although it's much less likely, another possibility considered by scientists who study astrobiology is that these streaks could be made up of microbial life, Venus-style. Astrobiologists note that ring-shaped linkages of sulfur atoms, known to exist in Venus’ atmosphere, could provide microbes with a kind of coating that would protect them from sulfuric acid. These handy chemical cloaks would also absorb potentially damaging ultraviolet light and re-radiate it as visible light.

Some of the Russian Venera probes did, indeed, detect particles in Venus’ lower atmosphere about a micron in length – roughly the same size as a bacterium on Earth.

None of these findings provide compelling evidence for the existence of life in Venus’ clouds. But the questions they raise, along with Venus’ vanished ocean, its violently volcanic surface, and its hellish history, make a compelling case for missions to investigate our temperamental sister planet. There is much, it would seem, that she can teach us.

Size and Distance

Venus orbits the Sun from an average distance of 67 million miles (108 million kilometers), or 0.72 astronomical units. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. From this distance, it takes sunlight about six minutes to travel from the Sun to Venus.

Earth's nearness to Venus is a matter of perspective. The planet is nearly as big around as Earth. Its diameter at its equator is about 7,521 miles (12,104 kilometers), versus 7,926 miles (12,756 kilometers) for Earth. From Earth, Venus is the brightest object in the night sky after our own Moon. The ancients, therefore, gave it great importance in their cultures, even thinking it was two objects: a morning star and an evening star. That’s where the trick of perspective comes in.

Because Venus’ orbit is closer to the Sun than ours, the two of them – from our viewpoint – never stray far from each other. The ancient Egyptians and Greeks saw Venus in two guises: first in one orbital position (seen in the morning), then another (your “evening” Venus), just at different times of the year.

At its nearest to Earth , Venus is about 24 million (about 38 million kilometers) distant. But most of the time the two planets are farther apart; Mercury, the innermost planet, actually spends more time in Earth’s proximity than Venus.

One more trick of perspective: how Venus looks through binoculars or a telescope. Keep watch over many months, and you’ll notice that Venus has phases, just like our Moon – full, half, quarter, etc. The complete cycle, however, new to full, takes 584 days, while our Moon takes just a month. And it was this perspective, the phases of Venus first observed by Galileo through his telescope, that provided the key scientific proof for the Copernican heliocentric nature of the solar system.

Orbit and Rotation

Spending a day on Venus would be quite a disorienting experience – that is, if your spacecraft or spacesuit could protect you from temperatures in the range of 900 degrees Fahrenheit (475 Celsius). For one thing, your “day” would be 243 Earth days long – longer even than a Venus year (one trip around the Sun), which takes only 225 Earth days. For another, because of the planet's extremely slow rotation, sunrise to sunset would take 117 Earth days. And by the way, the Sun would rise in the west and set in the east, because Venus spins backward compared to Earth.

While you’re waiting, don’t expect any seasonal relief from the unrelenting temperatures. On Earth, with its spin axis tilted by about 23 degrees, we experience summer when our part of the planet (our hemisphere) receives the Sun’s rays more directly – a result of that tilt. In winter, the tilt means the rays are less direct. No such luck on Venus: Its very slight tilt is only three degrees, which is too little to produce noticeable seasons.

Venus is one of only two planets in our solar system that doesn't have a moon, but it does have a quasi-satellite that has officially been named Zoozve . This object was discovered on Nov. 11, 2002, by Brian Skiff at the Lowell Observatory Near-Earth-Object Search (LONEOS) in Flagstaff, Arizona, a project funded by NASA that ended in February 2008.  

Quasi-satellites, sometimes called quasi-moons, are asteroids that orbit the Sun while staying close to a planet. A quasi-satellite’s orbit usually is more oblong and less stable than the planet's orbit. In time, the shape of a quasi-satellite’s orbit may change and it may move away from the planet. 

According to the International Astronomical Union (IAU), the organization that names space objects, Zoozve is the first-identified quasi-satellite of a major planet. Earth also has quasi-satellites, including a small asteroid discovered in 2016 . 

Based on its brightness, scientists at NASA’s Jet Propulsion Laboratory (JPL) estimate Zoozve ranges in size from 660 feet (200 meters) to 1,640 feet (500 meters) across. 

Interestingly, Zoozve also orbits relatively close to Earth but does not pose a threat to our planet. For the next 175 years, the closest Zoozve will get to Earth is in the year 2149 when it will be about 2.2 million miles (3.5 million kilometers) away, or about 9 times the distance from Earth to the Moon.  

How Zoozve Got Its Name  

After the discovery in 2002, Skiff reported his finding to the Minor Planet Center , which is funded by a Near-Earth Object (NEO) Observations Program grant from NASA’s Planetary Defense Coordination Office . At that time, it was given the provisional name 2002 VE68. Skiff said he didn’t realize the asteroid’s importance and forgot about the object until a radio show host reached out to him in 2023 about naming it Zoozve. 

Soon after Skiff’s discovery, a team of astronomers, including Seppo Mikkola with the University of Turku in Finland and Paul Wiegert with the University of Western Ontario in London, determined that the object was the first of its kind to be discovered. They think that Zoozve may have been a companion to Venus for at least 7,000 years, and that Earth’s gravity helped push Zoozve into its present orbit. 

The name Zoozve comes from a child's poster of the solar system. The artist, Alex Foster, saw “2002 VE68” on a list of solar system objects, wrote down “2002 VE,” and then misread his own handwriting as “Zoozve.”  

Latif Nasser, co-host of the WNYC Studios show Radiolab, tracked down the source of the mistake with the help of Liz Landau , a NASA senior communications specialist. Nasser suggested that Skiff request that the IAU officially name the asteroid Zoozve. Skiff agreed, and the name Zoozve was approved in February 2024. 

Venus has no rings.

A critical question for scientists who search for life among the stars: How do habitable planets get their start? The close similarities of early Venus and Earth, and their very different fates, provide a kind of test case for scientists who study planet formation. Similar size, similar interior structure, both harboring oceans in their younger days. Yet one is now an inferno, while the other is the only known world to host abundant life. The factors that set these planets on almost opposite paths began, most likely, in the swirling disk of gas and dust from which they were born. Somehow, 4.6 billion years ago that disk around our Sun accreted, cooled, and settled into the planets we know today. Better knowledge of the formation history of Venus could help us better understand Earth – and rocky planets around other stars.

If we could slice Venus and Earth in half, pole to pole, and place them side by side, they would look remarkably similar. Each planet has an iron core enveloped by a hot-rock mantle; the thinnest of skins forms a rocky, exterior crust. On both planets, this thin skin changes form and sometimes erupts into volcanoes in response to the ebb and flow of heat and pressure deep beneath.

On Earth, the slow movement of continents over thousands and millions of years reshapes the surface, a process known as “plate tectonics.” Something similar might have happened on Venus early in its history. Today a key element of this process could be operating: subduction, or the sliding of one continental “plate” beneath another, which can also trigger volcanoes. Subduction is believed to be the first step in creating plate tectonics.

NASA’s Magellan spacecraft, which ended a five-year mission to Venus in 1994, mapped the broiling surface using radar. Magellan saw a land of extreme volcanism – a relatively young surface, one recently reshaped (in geologic terms), and chains of towering mountains.

The Soviet Union sent a series of probes to Venus between 1961 and 1984 as part of its Venera program (Venera is Russian for Venus). Ten probes made it to the surface, and a few functioned briefly after landing. The longest survivor lasted two hours; the shortest, 23 minutes. Photos snapped before the landers fried show a barren, dim, and rocky landscape, and a sky that is likely some shade of sulfur yellow.

Volcanoes and tectonic forces appear to have erased most traces of the early surface of Venus. Newer computer models indicate the resurfacing may have happened piecemeal over an extended period of time. The average age of surface features could be as young as 150 million years, with some older surfaces mixed in.

Venus has valleys and high mountains dotted with thousands of volcanoes. Its surface features – most named for both real and mythical women – include Ishtar Terra, a rocky, highland area around the size of Australia near the north pole, and an even larger, South-America-sized region called Aphrodite Terra that stretches across the equator. One mountain reaches 36,000 feet (11 kilometers), higher than Mt. Everest. Notably, except for Earth, Venus has by far the fewest impact craters of any rocky planet.

Other notable features of the Venus landscape include:

• A volcanic crater named Sacajawea for Lewis and Clark’s Native American guide.
• A deep canyon called Diana for the Roman goddess of the hunt.
• “Pancake” domes with flat tops and steep sides, as wide as 38 miles (62 kilometers), likely formed by the extrusion of highly viscous lava.
• “Tick” domes, odd volcanoes with radiating spurs that, from above, make them look like their blood-feeding namesake.
• Tesserae, terrain with intricate patterns of ridges and grooves that suggest the scorching temperatures make rock behave in some ways more like peanut butter beneath a thin and strong chocolate layer on Venus.

Venus’ atmosphere is one of extremes. With the hottest surface in the solar system, apart from the Sun itself, Venus is hotter even than the innermost planet, charbroiled Mercury. The atmosphere is mostly carbon dioxide – the same gas driving the greenhouse effect on Venus and Earth – with clouds composed of sulfuric acid. And at the surface, the hot, high-pressure carbon dioxide behaves in a corrosive fashion. But higher up in the atmosphere, temperatures and pressure begin to ease.

Magnetosphere

Even though Venus is similar in size to Earth and has a similar-sized iron core, the planet does not have its own internally generated magnetic field. Instead, Venus has what is known as an induced magnetic field. This weak magnetic field is created by the interaction of the Sun's magnetic field and the planet's outer atmosphere. Ultraviolet light from the Sun excites gases in Venus' outermost atmosphere; these electrically excited gases are called ions, and thus this region is called the ionosphere (Earth has an ionosphere as well). The solar wind – a million-mile-per-hour gale of electrically charged particles streaming continuously from the Sun – carries with it the Sun's magnetic field. When the Sun's magnetic field interacts with the electrically excited ionosphere of Venus, it creates or induces, a magnetic field there. This induced magnetic field envelops the planet and is shaped like an extended teardrop, or the tail of a comet, as the solar wind blows past Venus and outward into the solar system.

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