radio waves travel through empty space

Electromagnetic Radiation

Claimed by Zoila de Leon (Fall 2023)

• 1.1 What is a Electromagnetic(EM) Radiation?
• 1.2 General Properties
• 1.3 Problem Solving Method and Equations
• 1.4 Fields Made by Charges and Fields Made by Monopoles
• 2 The EM Spectrum
• 3 Waves and Fields
• 4 A Mathematical Model
• 5 Connectedness: X-Rays
• 6.1 Practice Problems (new section by Zoila)
• 7 References

What is a Electromagnetic(EM) Radiation?

Electromagnetic radiation is a form of energy that is all around us and takes many forms, such as radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Before 1873, electricity and magnetism were thought to be two different forces. However, in 1873, Scottish Physicist James Maxwell developed his famous theory of electromagnetism. There are four main electro magnetic interactions according to Maxwell:

• The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them
• Magnetic poles come in pairs that attract and repel each other much as electric charges do
• An electric current in a wire produces a magnetic field whose direction depends on the direction of the current
• A moving electric field produces a magnetic field, and vice versa

General Properties

The four Maxwell's Equations provide a complete description of possible spatial patterns of electric and magnetic field in space.

• The Ampere-Maxwell Law
• Gauss's Law
• Faraday's Law

Other than Maxwell's Four equations, there are general properties of all electromagnetic radiation:

• Electromagnetic radiation can travel through empty space. Most other types of waves must travel through some sort of substance. For example, sound waves need either a gas, solid, or liquid to pass through in order to be heard
• The speed of light is always a constant (3 x 10^8 m/s)
• Wavelengths are measured between the distances of either crests or troughs. It is usually characterized by the Greek symbol λ (lambda).

Electromagnetic waves are the self-propagating, mutual oscillation of electric and magnetic fields. The propagation of electromagnetic energy is often referred to as radiation. We can also say that the 'pulse' of these moving fields result in radiation (7).

The equation for propagation is E=cB with c being the speed of light. This equation is derived from combining the two equations E=vB and B=u0e0vE, proving that v is equal to 3e8 meters/second.

Problem Solving Method and Equations

To go about solving/analyzing mathematically an electromagnetic field using Maxwell's equations,this is how we proceed (7)

• Establish the space and time in which the electric and magnetic fields are present
• Check that Maxwell's equations can be applied in the situation above
• Check when the charge accelerates, it produces these fields and therefore radiation
• Show how these fields would interact with matter

The equation of the Radiative Electric Field is: E= 1/(4πe0)*-qa/(c^2r) where a is the acceleration of the particle, c is the speed of light and r is the distance from the original location of the charge to right before the kink. This kink happens on the electric field because of the slight delay when the charge is moved.

Fields Made by Charges and Fields Made by Monopoles

We can differentiate fields made by charges and the ones made by magnetic monopoles. (7) For fields made by charges, when the charge is

• at rest, E=1/r^2 and B=0
• constant speed, E=1/r^2 and B=1/r^2
• accelerating, E=1/r and B=1/r

For fields made by magnetic monopoles, the first point would have E and B switched.

The EM Spectrum

EM spectrum is a span of enormous range of wavelengths and frequencies. The EM spectrum is generally divided into 7 different regions, in order of decreasing wavelength and increasing energy and frequency. It ranges from Gamma rays to Long Radio Waves. Following are the lists of waves:

• Visible Light
• Infrared Rays
• Long radio waves

Although all these waves do different things, there is one thing in common : They all travel in waves.

Infrared radiation can be released as heat or thermal energy. It can also be bounced back, which is called near infrared because of its similarities with visible light energy. Infrared Radiation is most commonly used in remote sensing as infrared sensors collect thermal energy, providing us with weather conditions.

Visible Light is the only part of the electromagnetic spectrum that humans can see with a naked eye. This part of the spectrum includes a range of different colors that all represent a particular wavelength. Rainbows are formed in this way; light passes through matter in which it is absorbed or reflected based on its wavelength. As a result, some colors are reflected more than other, leading to the creation of a rainbow.

Waves and Fields

As we learned in class, electric field is produced when an electron is accelerating. Likewise, EM radiation is created when an atomic particle, like an electron, is accelerated by an electric field. The movement like this produces oscillating electric and magnetic fields, which travel at right angles to each other in a bundle of light energy called a photon. Photons travel in a harmonic wave at the fastest speed possible in the universe.

Electromagnetic waves are formed when an electric field couples with a magnetic field. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

A wavelength (in m) is the distance between two consecutive peaks of a wave. Frequency is the number of waves that form in a given length of time. A wavelength and frequency are interrelated. A short wavelength indicates that the frequency will be higher because one cycle can pass in a shorter amount of time. Likewise, a longer wavelength has a lower frequency because each cycle takes longer to complete.

Waves can be classified according to their nature:

• Mechanical waves
• Electromagnetic waves

Mechanical Waves

Mechanical waves require a medium (matter) to travel through. Examples are sound waves, water waves, ripples in strings or springs.

Water Waves

Sound Waves

Electromagnetic Waves

Electromagnetic waves do not require a medium (matter) to travel through - they can travel through space. Examples are radio waves, visible light, x-rays.

Radio Waves

Visible Lights

A Mathematical Model

The position of the particle is defined by a sine wave:

y = ymaxsin(wt)

Where w is the angular frequency

Amplitude is the distance from the maximum vertical displacement of the wave to the middle of the wave. The Amplitude of the sinusoidal Wave is the height of the peak in the wave measured from the zero line. This measures the magnitude of oscillation of a particular wave. The Amplitude is important because it tells you the intensity or brightness of a wave in comparison with other waves.

The period of the wave is the time between crests in seconds(s).

T = 2pi/w-----(units of seconds)

Frequency is the number of cycles per second, and is expressed as sec-1 or Hertz(Hz). Frequency is directly proportional to energy and can be express as "

E = hv where E is energy, h is Planck's constant ( 6.62607*10^-34J) and v is frequency

f = 1/T f = w/2pi----(Units Hertz)

Wavelength is the distance between crests in meters. Wavelength is equal to the speed of light times frequency. Longer wavelength waves such as radio waves carry low energy; this is why we can listen to the radio without any harmful consequences. Shorter wavelength waves such as x-rays carry higher energy that can be hazardous to our health.

Wavelength and Frequency

The speed of light is the multiplication of the wavelength and frequency.

This diagram shows all properties of waves:

ENERGY FLUX

Is defined by the following equation:

Connectedness: X-Rays

Electromagnetic Radiation while commonly thought of as only including visible light, radio waves, UV waves, and gamma rays; also include X-rays. In 1895, X-rays were initially discovered by William Roentgen, who accidentally fell upon the most important discovery about his life (Figure 1). Roentgen was already working on cathode rays, and because of a fluorescent glow that occurred during his experiments, covered his experimental apparatus with heavy black paper. However, when he did this, he discovered a glow coming from a screen several feet away. Through many more experiments, he discovered that a new type of energy, not cathode rays, were the cause of the glow. He named them “x-rays” and received the 1901 Nobel Prize in Physics. Roentgen never patented his monumental discovery and as a result, numerous researchers set out to find a multitude of uses and capitalize on his work.

Primarily, people could now view objects that were hidden from plain view (i.e. scanners in airports). While X-rays are now used in 100’s of professions (security, chemistry, art galleries), its most important function is to view bones to determine abnormalities in humans. In fact, one of Roentgen’s first x-rays was of his wife’s hand (Figure 2). X-rays fall under the scope of electromagnetic radiation because, like all E.R. waves, it is comprised of photons. X-rays have wavelengths between 0.01 to 10 nanometers and fall between UV and Gamma Waves on the E.R. spectrum (Figure 3). There are two main methods in which an x-ray may be formed. Both require a vacuum-filled tube called an x-ray tube (Figure 4). With an anode on one end and a cathode on the other, an electric current is applied and a high energy electron is projected from the cathode, through the vacuum, and at the anode. In the characteristic x-ray generation approach, the electron from the cathode collides with an inner shell electron on an atom on the anode (Figure 5). Both of these electrons are ejected from the atom and an outer shell electron takes the place of the inner shell one. Because the outer electron must have a lower energy to fill the inner shell hole, it releases a photon with the equivalent energy of the difference between the two energy levels in the atom. This photon is the x-ray that is used to view objects such as bones.

In the Bremsstrahlung x-ray generation method, the electron from the cathode is slowed as it passes the nucleus of an atom at the anode (Figure 6). As it slows and its path is changed, the loses energy (kinetic energy). This energy is also released as a photon which is subsequently called an x-ray. Depending on the voltage and current of the tube and the material of the anode, different types (as in wavelengths and energy) of x-rays can be produced and each one. However, all X-rays will continue to pass through objects until it reaches a material dense that stops it. However, density of the material required depends on the energy of the x-ray. For example, during a medical x-ray, x-rays of a certain energy will pass through soft tissue (skin, organs, etc) but not through bones. The x-rays that pass through the soft tissue will strike the screen and the absence of the x-rays absorbed by the bones will cause a negative space on the screen. The areas where x-rays do not strike will form the image of the bone. While the principles remain the same, x-ray machines today use incredible sophisticated technology to specify the type of x-ray they want and have greatly increased in accuracy since Roentgen’s initial discovery.

• Information and photographs are pulled from references 1 through 5 cited below*

Already, during the Ancient Greek and Roman times, light was studied as the presence of deflection and refraction were noticed. Electromagnetic radiation of wavelengths in the early 19th century. The discovery of infrared radiation is ascribed to astronomer William Herschel, who published his results in 1800 before the Royal Society of London. Herschel used a glass Triangular prism (optics)|prism to refract light from the Sun and detected invisible rays that caused heating beyond the red part of the spectrum, through an increase in the temperature recorded with a thermometer. These "calorific rays" were later termed infrared.

In 1801, Rohann Ritter, discovered the presence of ultraviolet light using salts. It was known that light could darken some silver halides and while doing so, he realized that the region beyond the violet bar (therefore ultraviolet) was more effective in changing the color of the halides. However,in 1864, while summarizing the theories of his time accumulating into his famous set of Maxwell equations, James Clerk Maxwell managed to deduce the speed of light being around 3e8 meters per second. This was instrumental in creating the rest of the spectrum.

In 1887-1888 Physicist Heinrich Hertz not only tried to measure the velocity and frequency of electromagnetic radiation waves at other parts of the known spectrum of the time, but he was also able to prove that Maxwell's findings were correct. He did this on the microwave radiation as well.

The discovery of X-rays occurred in 1895 by Wilhelm Rontgen when his barium platinocyanide detector screen began to glow under the presence of a discharge that passed through a cathode ray tube although the latter was completely covered. Once he determined its possible use, he tried to look at his wife's hand using this new discovery. However x-ray spectroscopy was not institutionalized until later by Karl Manne Siegbahn.

In 1900, Paul Villard discovered Gamma rays although he initially thought that they were particles similar to alpha and beta particles which were emitted during radiation. These 'particles' were later proven to be part of the electromagnetic spectrum.

Practice Problems (new section by Zoila)

1. Elert, Glenn. "X-rays." X-rays – The Physics Hypertextbook. N.p., n.d. Web. 08 Apr. 2017. http://physics.info/x-ray/

2."X-rays." X-rays. N.p., n.d. Web. 08 Apr. 2017. http://www.physics.isu.edu/radinf/xray.htm

3. "Basics of X-ray PhysicsX-ray production." Welcome to Radiology Masterclass. N.p., n.d. Web. 08 Apr. 2017. http://www.radiologymasterclass.co.uk/tutorials/physics/x-ray_physics_production#top_2nd_img

4. "X-Rays." Image: Electromagnetic Spectrum. N.p., n.d. Web. 08 Apr. 2017. https://www.boundless.com/physics/textbooks/boundless-physics-textbook/electromagnetic-waves-23/the-electromagnetic-spectrum-165/x-rays-597-11175/images/electromagnetic-spectrum/

5. "This Month in Physics History." American Physical Society. N.p., n.d. Web. 08 Apr. 2017. https://www.aps.org/publications/apsnews/200111/history.cfm

6. Editors, Spectroscopy. “The Electromagnetic Spectrum: A History.” Spectroscopy Home, 27 Oct. 2017, www.spectroscopyonline.com/electromagnetic-spectrum-history?id=&sk=&date=&&pageID=4.

7. Chabay, Ruth W., and Bruce A. Sherwood. Matter & Interaction II: Electric & Magnetic Interactions, Version 1.2. John Wiley & Sons, 2003.

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23.2: Electromagnetic Waves and their Properties

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learning objectives

• Explain the meaning and importance of Maxwell’s equations

Maxwell’s Equations

Maxwell’s equations are a set of four partial differential equations that, along with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits.

Named after esteemed physicist James Clerk Maxwell, the equations describe the creation and propagation of electric and magnetic fields. Fundamentally, they describe how electric charges and currents create electric and magnetic fields, and how they affect each other.

Maxwell’s equations can be divided into two major subsets. The first two, Gauss’s law and Gauss’s law for magnetism, describe how fields emanate from charges and magnets respectively. The other two, Faraday’s law and Ampere’s law with Maxwell’s correction, describe how induced electric and magnetic fields circulate around their respective sources.

Each of Maxwell’s equations can be looked at from the “microscopic” perspective, which deals with total charge and total current, and the “macroscopic” set, which defines two new auxiliary fields that allow one to perform calculations without knowing microscopic data like atomic-level charges.

Gauss’s Law

Gauss’s law relates an electric field to the charge(s) that create(s) it. The field (E) points towards negative charges and away from positive charges, and from the microscopic perspective, is related to charge density (ρ) and vaccuum permittivity (ε 0 , or permittivity of free space) as:

\[\nabla \cdot \mathbf { E } = \dfrac { \rho } { \epsilon _ { 0 } }\]

Gauss’s Law basically says that a net amount of charge contained within a region of space will generate an electric field that emanates through the surface that surrounds that region.

Example of Gauss’s Law : A positive charge contained within a region of space creates an electric field that emanates from the surface of that region.

Gauss’s Law for Magnetism

Gauss’s law for magnetism states that there are no “magnetic charges (or monopoles)” analogous to electric charges, and that magnetic fields are instead generated by magnetic dipoles . Such dipoles can be represented as loops of current, but in many ways are similar in appearance to positive and negative “magnetic charges” that are inseparable and thus have no formal net “magnetic charge.”

Magnetic field lines form loops such that all field lines that go into an object leave it at some point. Thus, the total magnetic flux through a surface surrounding a magnetic dipole is always zero.

Field lines caused by a magnetic dipole : The field lines created by this magnetic dipole either form loops or extend infinitely.

The differential form of Gauss’s law for magnetic for magnetism is

\[\nabla \cdot \mathbf { B } = \mathbf { 0 }\]

Faraday’s Law

Faraday’s law describes how a time-varying magnetic field (or flux) induces an electric field. The principle behind this phenomenon is used in many electric generators. Both macroscopic and microscopic differential equations are the same, relating electric field (E) to the time-partial derivative of magnetic field (B):

\[\nabla \times \mathbf { E } = - \frac { \partial \mathbf { B } } { \partial \mathbf { t } }\]

Ampere’s Circuital Law (with Maxwell’s correction)

Ampere’s law originally stated that magnetic field could be created by electrical current. Maxwell added a second source of magnetic fields in his correction: a changing electric field (or flux), which would induce a magnetic field even in the absence of an electrical current. He named the changing electric field “displacement current.”

Maxwell’s correction shows that self-sustaining electromagnetic waves (light) can travel through empty space even in the absence of moving charges or currents, with the electric field component and magnetic field component each continually changing and each perpetuating the other.

Electromagnetic Waves : Electric (red) and magnetic (blue) waves propagate in phase sinusoidally, and perpendicularly to one another.

The microscopic approach to the Maxwell-corrected Ampere’s law relates magnetic field (B) to current density (J, or current per unit cross sectional area) and the time-partial derivative of electric field (E):

\[\nabla \times \mathbf { B } = \mu _ { 0 } \mathbf { J } + \mu _ { 0 } \epsilon _ { 0 } \frac { \partial \mathbf { E } } { \partial t }\]

The Production of Electromagnetic Waves

Electromagnetic waves are the combination of electric and magnetic field waves produced by moving charges.

• Explain the self-perpetuating behavior of an electromagnetic wave

Electromagnetic waves

Electromagnetic radiation, is a form of energy emitted by moving charged particles. As it travels through space it behaves like a wave, and has an oscillating electric field component and an oscillating magnetic field. These waves oscillate perpendicularly to and in phase with one another.

Electromagnetic Wave : Electromagnetic waves are a self-propagating transverse wave of oscillating electric and magnetic fields. The direction of the electric field is indicated in blue, the magnetic field in red, and the wave propagates in the positive x-direction. Notice that the electric and magnetic field waves are in phase.

The creation of all electromagnetic waves begins with a charged particle. This charged particle creates an electric field (which can exert a force on other nearby charged particles). When it accelerates as part of an oscillatory motion, the charged particle creates ripples, or oscillations, in its electric field, and also produces a magnetic field (as predicted by Maxwell’s equations).

Once in motion, the electric and magnetic fields created by a charged particle are self-perpetuating—time-dependent changes in one field (electric or magnetic) produce the other. This means that an electric field that oscillates as a function of time will produce a magnetic field, and a magnetic field that changes as a function of time will produce an electric field. Both electric and magnetic fields in an electromagnetic wave will fluctuate in time, one causing the other to change.

Electromagnetic waves are ubiquitous in nature (i.e., light) and used in modern technology—AM and FM radio, cordless and cellular phones, garage door openers, wireless networks, radar, microwave ovens, etc. These and many more such devices use electromagnetic waves to transmit data and signals.

All the above sources of electromagnetic waves use the simple principle of moving charge, which can be easily modeled. Placing a coin in contact with both terminals of a 9-volt battery produces electromagnetic waves that can be detected by bringing the antenna of a radio (tuned to a static-producing station) within a few inches of the point of contact.

Energy and Momentum

Electromagnetic waves have energy and momentum that are both associated with their wavelength and frequency.

• Relate energy of an electromagnetic wave with the frequency and wavelength

Electromagnetic radiation can essentially be described as photon streams. These photons are strictly defined as massless, but have both energy and surprisingly, given their lack of mass, momentum, which can be calculated from their wave properties.

Waves were poorly understood until the 1900s, when Max Planck and Albert Einstein developed modern corrections to classical theory.

Planck theorized that “black bodies” (thermal radiators) and other forms of electromagnetic radiation existed not as spectra, but in discrete, “quantized” form. In other words, there were only certain energies an electromagnetic wave could have. In his work he developed what is now known as “Planck’s constant,” which is approximately equal to 6.626×10 -34 J·s.

The energy (E) of a photon can be related to its frequency (f) by Planck’s constant (h):

\[\mathrm { E } = \mathrm { hf } = \frac { \mathrm { hc } } { \lambda }\]

The ratio of speed of light (c) to wavelength (λ) can be substituted in place of f to give the same equation to energy in different terms. Note that energy cannot take any value: it can only exist in increments of frequency times Planck’s constant (or Planck’s constant times c divided by wavelength). Energy of a wave is therefore “quantized. ”

Wavelength : Wavelength of the sinusoidal function is represented by λ.

Momentum is classically defined as the product of mass and velocity and thus would intuitively seem irrelevant to a discussion of electromagnetic radiation, which is both massless and composed of waves.

However, Einstein proved that light can act as particles in some circumstances, and that a wave-particle duality exists. And, given that he related energy and mass (E=mc 2 ), it becomes more conceivable that a wave (which has an energy value) not only has an equation to mass but a momentum as well.

And indeed, Einstein proved that the momentum (p) of a photon is the ratio of its energy to the speed of light.

\[\mathrm { p } = \dfrac { \mathrm { E } } { \mathrm { c } } = \dfrac { \mathrm { hf } } { \mathrm { c } } = \dfrac { \mathrm { h } } { \lambda }\]

Substituting E with hc/λ cancels the c terms, making momentum also equal to the simple ratio of Planck’s constant to wavelength.

The Speed of Light

The speed of light in a vacuum is one of the most fundamental constant in physics, playing a pivotal role in modern physics.

• Relate speed of light with the index of refraction of the medium

The speed of light is generally a point of comparison to express that something is fast. shows a scale representation of the time it takes a beam of light to reach the moon from Earth. But what exactly is the speed of light?

Light Going from Earth to the Moon : A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.

It is just that: the speed of a photon or light particle. The speed of light in a vacuum (commonly written as c) is 299,792,458 meters per second. This is a universal physical constant used in many areas of physics. For example, you might be familiar with the equation:

\[\mathrm { E } = \mathrm { mc } ^ { 2 }\]

where E = Energy and m = mass. This is known as the mass-energy equivalence, and it uses the speed of light to interrelate space and time. This not only explains the energy a body of mass contains, but also explains the hindrance mass has on speed.

There are many uses for the speed of light in a vacuum, such as in special relativity, which says that c is the natural speed limit and nothing can move faster than it. However, we know from our understanding of physics (and previous atoms) that the speed at which something travels also depends on the medium through which it is traveling. The speed at which light propagates through transparent materials (air, glass, etc.,) is dependent on the refractive index of that material, n:

\[\mathrm { v } = \dfrac { \mathrm { c } } { \mathrm { n } }\]

where v = actual velocity of light moving through the medium, c = speed of light in a vacuum, and n = refractive index of medium. The refractive index of air is about 1.0003, and from this equation we can find that the speed of visible light in air is about 90 km/s slower than c.

As mentioned earlier, the speed of light (usually of light in a vacuum) is used in many areas of physics. Below is an example of an application of the constant c.

The Lorentz Factor

Fast-moving objects exhibit some properties that are counterintuitive from the perspective of classical mechanics. For example, length contracts and time dilates (runs slower) for objects in motion. The effects are typically minute, but are noticeable at sufficiently high speeds. The Lorentz factor (γ) is the factor by which length shortens and time dilates as a function of velocity (v):

\[\gamma = \left( 1 - \mathrm{ v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 } \gamma = \left( 1 - \mathrm { v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 } \gamma = \left( 1 - \mathrm { v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 }\]

At low velocities, the quotient of v 2 /c 2 is sufficiently close to 0 such that γ is approximately 1. However, as velocity approaches c, γ increases rapidly towards infinity.

The Doppler Effect

The Doppler Effect is the change in a wave’s perceived frequency that results from the source’s motion, the observer, and the medium.

• Give examples of daily observations of the Doppler effect

The Doppler effect is a periodic event’s change in frequency for an observer in motion relative to the event’s source. Typically, this periodic event is a wave.

Most people have experienced the Doppler effect in action. Consider an emergency vehicle in motion, sounding its siren. As it approaches an observer, the pitch of the sound (its frequency) sounds higher than it actually is. When the vehicle reaches the observer, the pitch is perceived as it actually is. When the vehicle continues away from the observer, the pitch is perceived as lower than it actually is. From the perspective of an observer inside the vehicle, the pitch of the siren is constant.

The Doppler Effect and Sirens : Waves emitted by a siren in a moving vehicle

The difference in the perceived pitch depending on observer location can be explained by the fact that the siren’s position changes as it emits waves. A wave of sound is emitted by a moving vehicle every millisecond. The vehicle ‘chases’ each wave in one direction. By the time the next wave is emitted, it is closer (relative to an onlooker ahead of the vehicle) to the previous wave than the wave’s frequency would suggest. Relative to an onlooker behind the vehicle, the second wave is further from the first wave than one would expect, which suggests a lower frequency.

The Doppler effect can be caused by any kind of motion. In the example above, the siren moved relative to a stationary observer. If the observer moves relative to the stationary siren, the observer will notice the Doppler effect on the pitch of the siren. Finally, if the medium through which the waves propagate moves, the Doppler effect will be noticed even for a stationary observer. An example of this phenomenon is wind.

Quantitatively, the Doppler effect can be characterized by relating the frequency perceived (f) to the velocity of waves in the medium (c), the velocity of the receiver relative to the medium (v r ), the velocity of the source relative to the medium (v s ), and the actual emitted frequency (f 0 ):

\[\mathrm { f } = \left( \dfrac { \mathrm { c } + \mathrm { v } _ { \mathrm { r } } } { \mathrm { c } + \mathrm { v } _ { \mathrm { s } } } \right) \mathrm { f } _ { 0 }\]

The Doppler Effect : Wavelength change due to the motion of source

Momentum Transfer and Radiation Pressure Atom

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation.

• Explain formation of radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation. EM radiation (or photon, which is a quantum of light) carries momentum; this momentum is transferred to an object when the radiation is absorbed or reflected. Perhaps one of the most well know examples of the radiation pressure would be comet tails. Haley’s comet is shown in.

Halley’s Comet : As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus. The streams of dust and gas thus released form an atmosphere around the comet (called the coma), and the force exerted on the coma by the Sun’s radiation pressure and solar wind cause the formation of an enormous tail that points away from the Sun.

Although radiation pressure can be understood using classical electrodynamics, here we will examine the quantum mechanical argument. From the perspective of quantum theory, light is made of photons: particles with zero mass but which carry energy and – importantly in this argument – momentum. According to special relativity, because photons are devoid of mass, their energy (E) and momentum (p) are related by E=pc.

Now consider a beam of light perpendicularly incident on a surface, and let us assume the beam of light is totally absorbed. The momentum the photons carry is a conserved quantity (i.e., it cannot be destroyed) so it must be transferred to the surface; thus the absorption of the light beam causes the surface to gain momentum. Newton’s Second Law tells us that force equals rate of change of momentum; thus during each second, the surface experiences a force (or pressure, as pressure is force per unit area) due to the momentum the photons transfer to it.

This gives us: pressure = momentum transferred per second per unit area = energy deposited per second per unit area / c = I/c, (where I is the intensity of the beam of light).

Laser Cooling

There are many variations of laser cooling, but they all use radiation pressure to remove energy from atomic gases (and therefore cool the sample). In laser cooling (sometimes called Doppler cooling), the frequency of light is tuned slightly below an electronic transition in the atom. Because light is detuned to the “red” (i.e., at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion (typical setups applies three opposing pairs of laser beams as in ).

The Magneto Optical Trap : Experimental setup of Magneto Optical Trap (MOT), which uses radiation pressure to cool atomic species. Atoms are slowed down by absorbing (and emitting) photons.

In each scattering event, the atom loses a momentum equal to the momentum of the photon. If the atom (which is now in the excited state) then emits a photon spontaneously, it will be kicked by the same amount of momentum, only in a random direction. Since the initial momentum loss was opposite to the direction of motion (while the subsequent momentum gain was in a random direction), the overall result of the absorption and emission process is to reduce the speed of the atom. If the absorption and emission are repeated many times, the average speed (and therefore the kinetic energy ) of the atom will be reduced. Since the temperature of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms. Simple laser cooling setups can produce a cold sample of atomic gases at around 1mK (=10 -3 K) starting from a room temperature gas.

• Maxwell’s four equations describe how electric charges and currents create electric and magnetic fields, and how they affect each other.
• Gauss’s law relates an electric field to the charge(s) that create(s) it.
• Gauss’s law for magnetism states that there are no “magnetic charges” analogous to electric charges, and that magnetic fields are instead generated by magnetic dipoles.
• Faraday’s law describes how a time-varying magnetic field (or flux ) induces an electric field. The principle behind this phenomenon is used in many electric generators.
• Ampere ‘s law originally stated that a magnetic field is created by an electrical current. Maxwell added that a changing electric flux can also generate a magnetic field.
• Electromagnetic waves consist of both electric and magnetic field waves. These waves oscillate in perpendicular planes with respect to each other, and are in phase.
• The creation of all electromagnetic waves begins with an oscillating charged particle, which creates oscillating electric and magnetic fields.
• Once in motion, the electric and magnetic fields that a charged particle creates are self-perpetuating: time-dependent changes in one field (electric or magnetic) produce the other.
• Max Planck proved that energy of a photon (a stream of which is an electromagnetic wave ) is quantized and can exist in multiples of “Planck’s constant” (denoted as h, approximately equal to 6.626×10 -34 J·s).
• \(\mathrm { E } = \mathrm { hf } = \frac { \mathrm { hc } } { \lambda } \)describes the energy (E) of a photon as a function of frequency (f), or wavelength (λ).
• \(\mathrm { p } = \frac { \mathrm { E } } { \mathrm { c } } = \frac { \mathrm { hf } } { \mathrm { c } } = \frac { \mathrm { h } } { \lambda }\) describes the momentum (p) of a photon as a function of its energy, frequency, or wavelength.
• The maximum possible value for the speed of light is that of light in a vacuum, and this speed is used for a constant in many area of physics.
• c is the symbol used to represent the speed of light in a vacuum, and its value is 299,792,458 meters per second.
• When light travels through medium, its speed is hindered by the index of refraction of that medium. Its actual speed can be found with: \(v=\frac{c}{n}\).
• The Doppler effect is very commonly observed in action.
• The Doppler effect can be observed in the apparent change in pitch of a siren on an emergency vehicle, according to a stationary observer.
• The observer will notice the Doppler effect on the pitch of the stationary siren when moving relative to its pitch, or if the medium moves when the observer is stationary.
• Photons carry momentum (p = E/c). When photons are absorbed or reflected on a surface, the surface receives momentum kicks. This momentum transfer leads to radiation pressure.
• Electromagnetic radiation applies radiation pressure equal to the Intensity (of light beam) divided by c (speed of light).
• Laser cooling uses radiation pressure to remove energy from atomic gases. The technique can produce cold samples of gases at 1mK or so.
• differential equation : An equation involving the derivatives of a function.
• flux : A quantitative description of the transfer of a given vector quantity through a surface. In this context, we refer to the electric flux and magnetic flux.
• electromagnetic wave : A wave of oscillating electric and magnetic fields.
• phase : Waves are said to be “in phase” when they begin at the same part (e.g., crest) of their respective cycles.
• photon : The quantum of light and other electromagnetic energy, regarded as a discrete particle having zero rest mass, no electric charge, and an indefinitely long lifetime.
• wavelength : The length of a single cycle of a wave, as measured by the distance between one peak or trough of a wave and the next; it is often designated in physics as λ, and corresponds to the velocity of the wave divided by its frequency.
• frequency : The quotient of the number of times n a periodic phenomenon occurs over the time t in which it occurs: f = n / t.
• special relativity : A theory that (neglecting the effects of gravity) reconciles the principle of relativity with the observation that the speed of light is constant in all frames of reference.
• refractive index : The ratio of the speed of light in air or vacuum to that in another medium.
• doppler effect : Apparent change in frequency of a wave when the observer and the source of the wave move relative to each other.
• classical electrodynamics : A branch of theoretical physics that studies consequences of the electromagnetic forces between electric charges and currents.

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13.1: Electromagnetic Waves

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Did you ever wonder how a microwave works? It directs invisible waves of radiation toward the food placed inside of it. The radiation transfers energy to the food, causing it to get warmer. The radiation is in the form of microwaves, which are a type of electromagnetic waves.

What Are Electromagnetic Waves?

Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. Like other waves, electromagnetic waves transfer energy from one place to another. The transfer of energy by electromagnetic waves is called electromagnetic radiation . Electromagnetic waves can transfer energy through matter or across empty space.

Q: How do microwaves transfer energy inside a microwave oven?

A: They transfer energy through the air inside the oven to the food.

May the Force Be With You

A familiar example may help you understand the vibrating electric and magnetic fields that make up electromagnetic waves. Consider a bar magnet, like the one in the Figure below. The magnet exerts magnetic force over an area all around it. This area is called a magnetic field. The field lines in the diagram represent the direction and location of the magnetic force. Because of the field surrounding a magnet, it can exert force on objects without touching them. They just have to be within its magnetic field.

Q: How could you demonstrate that a magnet can exert force on objects without touching them?

A: You could put small objects containing iron, such as paper clips, near a magnet and show that they move toward the magnet.

An electric field is similar to a magnetic field. It is an area of electrical force surrounding a positively or negatively charged particle. You can see electric fields in the following Figure below. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them.

How an Electromagnetic Wave Begins

An electromagnetic wave begins when an electrically charged particle vibrates. The Figure below shows how this happens. A vibrating charged particle causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field. The two types of vibrating fields combine to create an electromagnetic wave.

How an Electromagnetic Wave Travels

As you can see in the Figure above, the electric and magnetic fields that make up an electromagnetic wave are perpendicular (at right angles) to each other. Both fields are also perpendicular to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. However, unlike a mechanical transverse wave, which can only travel through matter, an electromagnetic transverse wave can travel through empty space. When waves travel through matter, they lose some energy to the matter as they pass through it. But when waves travel through space, no energy is lost. Therefore, electromagnetic waves don’t get weaker as they travel. However, the energy is “diluted” as it travels farther from its source because it spreads out over an ever-larger area.

Electromagnetic Wave Interactions

When electromagnetic waves strike matter, they may interact with it in the same ways that mechanical waves interact with matter. Electromagnetic waves may:

• reflect, or bounce back from a surface;
• refract, or bend when entering a new medium;
• diffract, or spread out around obstacles.

Electromagnetic waves may also be absorbed by matter and converted to other forms of energy. Microwaves are a familiar example. When microwaves strike food in a microwave oven, they are absorbed and converted to thermal energy, which heats the food.

Sources of Electromagnetic Waves

The most important source of electromagnetic waves on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes.

Launch the Light Wave simulation below to help you visualize light as a transverse wave moving through the electromagnetic field. Be sure to adjust the wavelength band slider to observe waves of different sizes, such as Radio Waves and X-Rays. There is also an illustration of objects of comparable sizes next to the electromagnetic spectrum to help you imagine the sizes of these invisible light waves.

Interactive Element

• Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. They transfer energy through matter or across space. The transfer of energy by electromagnetic waves is called electromagnetic radiation.
• The electric and magnetic fields of an electromagnetic wave are areas of electric or magnetic force. The fields can exert force over objects at a distance.
• An electromagnetic wave begins when an electrically charged particle vibrates. This causes a vibrating electric field, which in turn creates a vibrating magnetic field. The two vibrating fields together form an electromagnetic wave.
• An electromagnetic wave is a transverse wave that can travel across space as well as through matter. When it travels through space, it doesn’t lose energy to a medium as a mechanical wave does.
• When electromagnetic waves strike matter, they may be reflected, refracted, or diffracted. Or they may be absorbed by matter and converted to other forms of energy.
• The most important source of electromagnetic waves on Earth is the sun. Many other sources of electromagnetic waves depend on technology.
• What is an electromagnetic wave?
• Define electromagnetic radiation.
• Describe the electric and magnetic fields of an electromagnetic wave.
• How does an electromagnetic wave begin? How does it travel?
• Compare and contrast electromagnetic and mechanical transverse waves.
• List three sources of electromagnetic waves on Earth.

Explore More

Watch the electromagnetic wave animation and then answer the questions below.

• Identify the vibrating electric and magnetic fields of the wave.
• Describe the direction in which the wave is traveling.

Additional Resources

Study Guide: Wave Optics Study Guide

Real World Application: Printing with Light, The Incredible Hulk

PLIX: Play, Learn, Interact, eXplore: Energy Levels: Bohr's Atomic Model

Electromagnetic spectrum

The electromagnetic spectrum is a range of wavelengths of electromagnetic radiation. From long to short wavelength, the EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.

Energy is propagated through space in the form of electromagnetic (EM) waves, which are composed of oscillating electric and magnetic fields. EM waves do not require a substance (like air or water) to travel through, meaning that — unlike sound — they can travel through empty space. In a vacuum, all EM waves travel at the same speed: the speed of light (which is itself an EM wave). Like all waves, an EM wave is characterised by its wavelength, and the range of wavelengths we observe, from very long to very short, is what we refer to as the EM spectrum. We divide up the EM spectrum roughly according to how the waves behave when they interact with matter and each division has a name. So we have: radio waves, which have the longest wavelengths; microwaves; infrared; visible light; ultraviolet; x-rays; and finally gamma rays, which have the shortest wavelengths. Celestial objects such as stars, planets and galaxies all emit EM waves at various wavelengths and so different telescopes are designed to be sensitive to different parts of the EM spectrum. EM radiation in and around the visible part of the spectrum is often referred to broadly as ‘light’, with shorter wavelengths referred to as ‘bluer’ and longer wavelengths referred to as ‘redder’.

By combining observations at different wavelengths, we can develop a more complete picture of the structure, composition and behaviour of an object than visible wavelengths alone can show. This makes Webb especially important, alongside existing telescopes such as the NASA/ESA Hubble Space Telescope . Webb's sensitive instruments provide excellent coverage of near- and mid-infrared wavelengths, enabling high-quality research into celestial objects that emit this kind of light, and providing critical extra details about targets also being studied at longer and shorter wavelengths.

The eighteen hexagonal mirrors that together make up Webb's primary mirror are coated in a thin layer of gold. This gives the telescope a distinctive look, but more importantly it's perfect for reflecting the infrared wavelengths to which Webb's instruments are sensitive. Webb has four instruments which all work in the infrared range: MIRI , which detects mid-infrared light, and NIRSpec ,  NIRCam and NIRISS , which are all sensitive to near-infrared light.

Even before cooperating with other telescopes, MIRI and NIRCam are frequently used together to obtain a more complete picture of a target over the whole of Webb's wavelength range. This image of the Pillars of Creation shows the different details which are highlighted in the different wavelength ranges — stars, particularly newly-formed stars, show up in near-infrared, while dust is placed on display by MIRI.

Electromagnetic spectrum

The electromagnetic spectrum is a range of wavelengths of electromagnetic radiation. From long to short wavelength, the EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.

Energy is propagated through space in the form of electromagnetic (EM) waves, which are composed of oscillating electric and magnetic fields. EM waves do not require a substance (like air or water) to travel through, meaning that — unlike sound — they can travel through empty space. In a vacuum, all EM waves travel at the same speed: the speed of light (which is itself an EM wave). Like all waves, an EM wave is characterised by its wavelength, and the range of wavelengths we observe, from very long to very short, is what we refer to as the EM spectrum. We divide up the EM spectrum roughly according to how the waves behave when they interact with matter and each division has a name. So we have: radio waves, which have the longest wavelengths; microwaves; infrared; visible light; ultraviolet; x-rays; and finally gamma rays, which have the shortest wavelengths. Celestial objects such as stars , planets and galaxies all emit EM waves at various wavelengths and so different telescopes are designed to be sensitive to different parts of the EM spectrum. EM radiation in and around the visible part of the spectrum is often referred to broadly as ‘light’, with shorter wavelengths referred to as ‘bluer’ and longer wavelengths referred to as ‘redder’.

By combining observations at different wavelengths, we can develop a more complete picture of the structure, composition and behaviour of an object than the visible wavelengths alone can show.

For more than three decades, Hubble has studied the Universe using its 2.4-metre primary mirror and its five science instruments. They observe primarily in the ultraviolet and visible parts of the spectrum, but also have some near- infrared capabilities. Hubble observes in different wavelength bands, one band at a time, each providing different information on the object under study. Each of these wavelengths is reproduced in a different colour and these are combined to form a composite image that well resembles the true emission from that celestial object.

By exploring this image , you can see how astronomers have used a set of single-colour images to construct the colour picture of a ring of star clusters surrounding the core of the galaxy NGC 1512. Each image represents a specific colour or wavelength region of the spectrum, from ultraviolet to near infrared, and shows the wide wavelength range covered by Hubble. Astronomers chose to study NGC 1512 in these colours to emphasise important details in the ring of young star clusters surrounding the core.

Astronomers use multi-wavelength imagery to study details that might not otherwise be present in visible images. For example, a new multiwavelength observation of Jupiter released in 2020 by Hubble in ultraviolet/visible/near-infrared light of Jupiter gave researchers an entirely new view of the giant planet. These observations provided insights into the altitude and distribution of the planet’s haze and particles and showed Jupiter’s ever-changing cloud patterns. The planet’s aurorae are only visible in the ultraviolet; however, the structure of the red spot is well studied at visible wavelengths.

To celebrate the telescope’s 25th anniversary in 2015, Hubble unveiled two new beautiful portraits of the popular Pillars of Creation, revealing how different details can be studied in visible and near-infrared observations. While the visible light captures the multi-coloured glow of gas clouds, the infrared image penetrates much of the obscuring dust and gas to uncover countless newborn stars.

We invite you to watch this Hubblecast that explores how Hubble’s observations differ across different wavelengths of the electromagnetic spectrum, and how these observations will be complemented by those of the James Webb Space Telescope.

If you are using content from the ESA/Hubble Word Bank, it should be credited as "ESA/Hubble". Our full copyright requirements are provided here.

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Can humans hear in space?

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The Medium of Space

Before we begin to talk about space , we should probably define it. For the purposes of this discussion, we'll consider space to be the region of the universe outside of the Earth's atmosphere.

You've probably heard that space is a vacuum . A true vacuum refers to the complete absence of matter. But how can space be a vacuum? Space contains stars , planets, asteroids , moons and comets , just to name a few cosmic bodies. Isn't that a lot of matter? How can space contain all these massive bodies and still be a vacuum?

It's because space is big. Between these large objects are millions of miles of emptiness. This empty space -- sometimes called interstellar space -- is practically devoid of all matter, so it's effectively a vacuum.

Sound waves can travel only through matter. Since there's almost no matter in interstellar space, sound can't travel through it. The distance between particles is so great that they would never collide with each other. Even if you could get a front seat for the explosion of the Death Star , you wouldn't hear anything at all.

Technically, you could argue that there are ways a human could hear in space. Let's address a few scenarios:

• Radio waves can travel through space. So, if you're wearing a space suit that contains a radio unit and one of your buddies sends you a radio message that there's pizza in the space station , you'd be able to hear it. That's because radio waves aren't mechanical -- they're electromagnetic . Electromagnetic waves can transmit energy through a vacuum. Once your radio receives the signal, it can convert the signal into sound, which will travel through the air in your space suit without a problem.
• Let's say that you're drifting through space while wearing a space suit and you accidentally bump your helmet into the Hubble Space Telescope . The collision would make a sound that you could hear, even though you're in space. That's because the sound waves would have a physical medium to move through: Your helmet and the air inside your space suit. You'd still be surrounded by a vacuum, so an independent observer wouldn't be able to hear anything, no matter how many times you rammed your head against a satellite .
• Imagine that you're an astronaut on a space shuttle mission. You've decided to step out into space but forgot to put on your space suit. You press your face against the space shuttle. You wouldn't have any air in your ears, so you couldn't hear in the traditional sense. However, you might be able to make out a few sounds through bone conduction before the perils of space caused you to expire. In bone conduction, sound waves travel through the bones of the jaw and skull to the inner ear, bypassing the eardrum. There's no need for air, so you could hear your fellow astronauts partying inside the shuttle for about 15 seconds. After that, you'd likely be unconscious and well on your way to asphyxiation.

So despite the wisdom of Hollywood filmmakers, it's impossible to hear noises in space. We suggest the next time you watch a science fiction film, you plug up your ears whenever anything happens within the vacuum of space. It'll make the film seem more realistic and probably work as a great conversational topic with your friends once the movie's over.

To learn more about space, sound and other topics, tune in to the links below.

Related HowStuffWorks Articles

• How does going to the bathroom in space work?
• How Hearing Works
• How long can a human survive in outer space?
• How NASA Works
• How Satellites Work
• How Space Suits Work
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• How Space Tourism Works

More Great Links

• Mad Sci Network
• The Physics Classroom Tutorial
• Bone conduction. Encyclopedia Britannica. Retrieved February 26, 2008, from Encyclopedia Britannica Online. http://www.britannica.com/eb/article-9080583
• Mad Sci Network. http://www.madsci.org/
• spaceflight. Encyclopedia Britannica. Retrieved February 26, 2008, from Encyclopedia Britannica Online. http://www.britannica.com/eb/article-9384200
• Sound. Encyclopedia Britannica. Retrieved February 25, 2008, from Encyclopedia Britannica Online. http://www.britannica.com/eb/article-9109557
• The Physics Classroom Tutorial. Glenbrook South Physics. http://www.glenbrook.k12.il.us/gbssci/phys/Class/BBoard.html
• Weir, Laila. "High-Tech Hearing Bypasses Ears." Wired. September 16, 2004. http://www.wired.com/science/discoveries/news/2004/09/64963

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Radio Waves

WHAT ARE RADIO WAVES?

Radio waves have the longest wavelengths in the electromagnetic spectrum. They range from the length of a football to larger than our planet. Heinrich Hertz proved the existence of radio waves in the late 1880s. He used a spark gap attached to an induction coil and a separate spark gap on a receiving antenna. When waves created by the sparks of the coil transmitter were picked up by the receiving antenna, sparks would jump its gap as well. Hertz showed in his experiments that these signals possessed all the properties of electromagnetic waves.

You can tune a radio to a specific wavelength—or frequency—and listen to your favorite music. The radio "receives" these electromagnetic radio waves and converts them to mechanical vibrations in the speaker to create the sound waves you can hear.

RADIO EMISSIONS IN THE SOLAR SYSTEM

Astronomical objects that have a changing magnetic field can produce radio waves. The radio astronomy instrument called WAVES on the WIND spacecraft recorded a day of bursts of radio waves from the Sun's corona and planets in our solar system.

Data pictured below show emissions from a variety of sources including radio bursts from the Sun, the Earth, and even from Jupiter's ionosphere whose wavelengths measure about fifteen meters in length. The far right of this graph shows radio bursts from the Sun caused by electrons that have been ejected into space during solar flares moving at 20% of the speed of light.

RADIO TELESCOPES

Radio telescopes look toward the heavens to view planets, comets, giant clouds of gas and dust, stars, and galaxies. By studying the radio waves originating from these sources, astronomers can learn about their composition, structure, and motion. Radio astronomy has the advantage that sunlight, clouds, and rain do not affect observations.

Since radio waves are longer than optical waves, radio telescopes are made differently than the telescopes used for visible light. Radio telescopes must be physically larger than an optical telescopes in order to make images of comparable resolution. But they can be made lighter with millions of small holes cut through the dish since the long radio waves are too big to "see" them. The Parkes radio telescope, which has a dish 64 meters wide, cannot yield an image any clearer than a small backyard optical telescope!

A VERY LARGE TELESCOPE

In order to make a clearer, or higher resolution, radio image, radio astronomers often combine several smaller telescopes, or receiving dishes, into an array. Together, these dishes can act as one large telescope whose resolution is set by the maximum size of the area. The National Radio Astronomy Observatory's Very Large Array (VLA) radio telescope in New Mexico is one of the world's premier astronomical radio observatories. The VLA consists of 27 antennas arranged in a huge "Y" pattern up to 36 km across (roughly one-and-one-half times the size of Washington, DC).

The techniques used in radio astronomy at long wavelengths can sometimes be applied at the shorter end of the radio spectrum—the microwave portion. The VLA image below captured 21-centimeter energy emissions around a black hole in the lower right and magnetic field lines pulling gas around in the upper left.

THE RADIO SKY

If we were to look at the sky with a radio telescope tuned to 408 MHz, the sky would appear radically different from what we see in visible light. Instead of seeing point-like stars, we would see distant pulsars, star-forming regions, and supernova remnants would dominate the night sky.

Radio telescopes can also detect quasars. The term quasar is short for quasi-stellar radio source. The name comes from the fact that the first quasars identified emit mostly radio energy and look much like stars. Quasars are very energetic, with some emitting 1,000 times as much energy as the entire Milky Way. However, most quasars are blocked from view in visible light by dust in their surrounding galaxies.

Astronomers identified the quasars with the help of radio data from the VLA radio telescope because many galaxies with quasars appear bright when viewed with radio telescopes. In the false-color image below, infrared data from the Spitzer space telescope is colored both blue and green, and radio data from the VLA telescope is shown in red. The quasar-bearing galaxy stands out in yellow because it emits both infrared and radio light.

Next: Microwaves

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Radio Waves. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/05_radiowaves

Science Mission Directorate. "Radio Waves" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/05_radiowaves

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Ask an Explainer

How does sound travel through space.

Sound can't be carried in the empty vacuum of space because sound waves need a medium to vibrate through such as air or water. Until recently, we thought that since there is no air in space, that no sound could travel and that is still true but only up to a point. Space isn’t actually completely empty, there are large areas of gas and dust that do have the potential to carry sound waves . However, because the particles are so spread out, the sounds waves they produce are at such a low frequency, humans are incapable of hearing them. 

Why isn’t there any sound in space? An astronomer explains why in space no one can hear you scream

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

How far can sound travel through space, since it’s so empty? Is there an echo in space? – Jasmine, age 14, Everson, Washington

In space, no one can hear you scream.

You may have heard this saying. It’s the tagline from the famous 1979 science fiction movie “ Alien .” It’s a scary thought, but is it true? The simple answer is yes, no one can hear you scream in space because there is no sound or echo in space.

I’m a professor of astronomy , which means I study space and how it works. Space is silent – for the most part.

How sound works

To understand why there’s no sound in space, first consider how sound works. Sound is a wave of energy that moves through a solid, a liquid or a gas.

Sound is a compression wave . The energy created when your vocal cords vibrate slightly compresses the air in your throat, and the compressed energy travels outward.

A good analogy for sound is a Slinky toy . If you stretch out a Slinky and push hard on one end, a compression wave travels down the Slinky.

When you talk, your vocal cords vibrate. They jostle air molecules in your throat above your vocal cords, which in turn jostle or bump into their neighbors, causing a sound to come out of your mouth.

Sound moves through air the same way it moves through your throat. Air molecules near your mouth bump into their neighbors, which in turn bump into their neighbors, and the sound moves through the air. The sound wave travels quickly , about 760 miles per hour (1,223 kilometers per hour), which is faster than a commercial jet.

Space is a vacuum

So what about in space?

Space is a vacuum, which means it contains almost no matter. The word vacuum comes from the Latin word for empty .

Sound is carried by atoms and molecules. In space, with no atoms or molecules to carry a sound wave, there’s no sound. There’s nothing to get in sound’s way out in space, but there’s nothing to carry it, so it doesn’t travel at all. No sound also means no echo. An echo happens when a sound wave hits a hard, flat surface and bounces back in the direction it came from.

By the way, if you were caught in space outside your spacecraft with no spacesuit, the fact that no one could hear your cry for help is the least of your problems. Any air you still had in your lungs would expand because it was at higher pressure than the vacuum outside. Your lungs would rupture. In a mere 10 to 15 seconds , you’d be unconscious due to a lack of oxygen.

Sound in the solar system

Scientists have wondered how human voices would sound on our nearest neighboring planets, Venus and Mars. This experiment is hypothetical because Mars is usually below freezing , and its atmosphere is thin, unbreathable carbon dioxide . Venus is even worse – its air is hot enough to melt lead, with a thick carbon dioxide atmosphere.

On Mars, your voice would sound tinny and hollow, like the sound of a piccolo . On Venus , the pitch of your voice would be much deeper, like the sound of a booming bass guitar. The reason is the thickness of the atmosphere. On Mars the thin air creates a high-pitched sound, and on Venus the thick air creates a low-pitched sound. The team that worked this out simulated other solar system sounds , like a waterfall on Saturn’s moon Titan.

Deep space sounds

While space is a good enough vacuum that normal sound can’t travel through it, it’s actually not a perfect vacuum, and it does have some particles floating through it.

Beyond the Earth and its atmosphere, there are five particles in a typical cubic centimeter – the volume of a sugar cube – that are mostly hydrogen atoms. By contrast, the air you are breathing is 10 billion billion (10 19) times more dense. The density goes down with distance from the Sun, and in the space between stars there are 0.1 particles per cubic centimeter. In vast voids between galaxies , it is a million times lower still – fantastically empty.

The voids of space are kept very hot by radiation from stars. The very spread-out matter found there is in a physical state called a plasma .

A plasma is a gas in which electrons are separated from protons. In a plasma, the physics of sound waves get complicated . Waves travel much faster in this low-density medium, and their wavelength is much longer.

In 2022, NASA released a spectacular example of sound in space . It used X-ray data to make an audible recording that represents the way a massive black hole stirs up plasma in the Perseus galaxy cluster, 250 million light years from Earth. The black hole itself emits no sound, but the diffuse plasma around it carries very long wavelength sound waves.

The natural sound is far too low a frequency for the human ear to hear, 57 octaves below middle C, which is the middle note on a piano and in the middle of the range of sound people can hear. But after raising the frequency to the audible range, the result is chilling – it’s the sound of a black hole growling in deep space.

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13.1 Types of Waves

Section learning objectives.

By the end of this section, you will be able to do the following:

• Define mechanical waves and medium, and relate the two
• Distinguish a pulse wave from a periodic wave
• Distinguish a longitudinal wave from a transverse wave and give examples of such waves

Teacher Support

The learning objectives in this section will help your students master the following standards:

• (A) examine and describe oscillatory motion and wave propagation in various types of media.

Section Key Terms

Mechanical waves.

What do we mean when we say something is a wave? A wave is a disturbance that travels or propagates from the place where it was created. Waves transfer energy from one place to another, but they do not necessarily transfer any mass. Light, sound, and waves in the ocean are common examples of waves. Sound and water waves are mechanical waves ; meaning, they require a medium to travel through. The medium may be a solid, a liquid, or a gas, and the speed of the wave depends on the material properties of the medium through which it is traveling. However, light is not a mechanical wave; it can travel through a vacuum such as the empty parts of outer space.

A familiar wave that you can easily imagine is the water wave. For water waves, the disturbance is in the surface of the water, an example of which is the disturbance created by a rock thrown into a pond or by a swimmer splashing the water surface repeatedly. For sound waves, the disturbance is caused by a change in air pressure, an example of which is when the oscillating cone inside a speaker creates a disturbance. For earthquakes, there are several types of disturbances, which include the disturbance of Earth’s surface itself and the pressure disturbances under the surface. Even radio waves are most easily understood using an analogy with water waves. Because water waves are common and visible, visualizing water waves may help you in studying other types of waves, especially those that are not visible.

Water waves have characteristics common to all waves, such as amplitude , period , frequency , and energy , which we will discuss in the next section.

Misconception Alert

Many people think that water waves push water from one direction to another. In reality, however, the particles of water tend to stay in one location only, except for moving up and down due to the energy in the wave. The energy moves forward through the water, but the water particles stay in one place. If you feel yourself being pushed in an ocean, what you feel is the energy of the wave, not the rush of water. If you put a cork in water that has waves, you will see that the water mostly moves it up and down.

[BL] [OL] [AL] Ask students to give examples of mechanical and nonmechanical waves.

Pulse Waves and Periodic Waves

If you drop a pebble into the water, only a few waves may be generated before the disturbance dies down, whereas in a wave pool, the waves are continuous. A pulse wave is a sudden disturbance in which only one wave or a few waves are generated, such as in the example of the pebble. Thunder and explosions also create pulse waves. A periodic wave repeats the same oscillation for several cycles, such as in the case of the wave pool, and is associated with simple harmonic motion. Each particle in the medium experiences simple harmonic motion in periodic waves by moving back and forth periodically through the same positions.

[BL] Any kind of wave, whether mechanical or nonmechanical, or transverse or longitudinal, can be in the form of a pulse wave or a periodic wave.

Consider the simplified water wave in Figure 13.2 . This wave is an up-and-down disturbance of the water surface, characterized by a sine wave pattern. The uppermost position is called the crest and the lowest is the trough . It causes a seagull to move up and down in simple harmonic motion as the wave crests and troughs pass under the bird.

Longitudinal Waves and Transverse Waves

Mechanical waves are categorized by their type of motion and fall into any of two categories: transverse or longitudinal. Note that both transverse and longitudinal waves can be periodic. A transverse wave propagates so that the disturbance is perpendicular to the direction of propagation. An example of a transverse wave is shown in Figure 13.3 , where a woman moves a toy spring up and down, generating waves that propagate away from herself in the horizontal direction while disturbing the toy spring in the vertical direction.

In contrast, in a longitudinal wave , the disturbance is parallel to the direction of propagation. Figure 13.4 shows an example of a longitudinal wave, where the woman now creates a disturbance in the horizontal direction—which is the same direction as the wave propagation—by stretching and then compressing the toy spring.

Tips For Success

Longitudinal waves are sometimes called compression waves or compressional waves , and transverse waves are sometimes called shear waves .

Teacher Demonstration

Transverse and longitudinal waves may be demonstrated in the class using a spring or a toy spring, as shown in the figures.

Waves may be transverse, longitudinal, or a combination of the two . The waves on the strings of musical instruments are transverse (as shown in Figure 13.5 ), and so are electromagnetic waves, such as visible light. Sound waves in air and water are longitudinal. Their disturbances are periodic variations in pressure that are transmitted in fluids.

Sound in solids can be both longitudinal and transverse. Essentially, water waves are also a combination of transverse and longitudinal components, although the simplified water wave illustrated in Figure 13.2 does not show the longitudinal motion of the bird.

Earthquake waves under Earth’s surface have both longitudinal and transverse components as well. The longitudinal waves in an earthquake are called pressure or P-waves, and the transverse waves are called shear or S-waves. These components have important individual characteristics; for example, they propagate at different speeds. Earthquakes also have surface waves that are similar to surface waves on water.

Energy propagates differently in transverse and longitudinal waves. It is important to know the type of the wave in which energy is propagating to understand how it may affect the materials around it.

Watch Physics

Introduction to waves.

This video explains wave propagation in terms of momentum using an example of a wave moving along a rope. It also covers the differences between transverse and longitudinal waves, and between pulse and periodic waves.

• After a compression wave, some molecules move forward temporarily.
• After a compression wave, some molecules move backward temporarily.
• After a compression wave, some molecules move upward temporarily.
• After a compression wave, some molecules move downward temporarily.

Fun In Physics

The physics of surfing.

Many people enjoy surfing in the ocean. For some surfers, the bigger the wave, the better. In one area off the coast of central California, waves can reach heights of up to 50 feet in certain times of the year ( Figure 13.6 ).

How do waves reach such extreme heights? Other than unusual causes, such as when earthquakes produce tsunami waves, most huge waves are caused simply by interactions between the wind and the surface of the water. The wind pushes up against the surface of the water and transfers energy to the water in the process. The stronger the wind, the more energy transferred. As waves start to form, a larger surface area becomes in contact with the wind, and even more energy is transferred from the wind to the water, thus creating higher waves. Intense storms create the fastest winds, kicking up massive waves that travel out from the origin of the storm. Longer-lasting storms and those storms that affect a larger area of the ocean create the biggest waves since they transfer more energy. The cycle of the tides from the Moon’s gravitational pull also plays a small role in creating waves.

Actual ocean waves are more complicated than the idealized model of the simple transverse wave with a perfect sinusoidal shape. Ocean waves are examples of orbital progressive waves , where water particles at the surface follow a circular path from the crest to the trough of the passing wave, then cycle back again to their original position. This cycle repeats with each passing wave.

As waves reach shore, the water depth decreases and the energy of the wave is compressed into a smaller volume. This creates higher waves—an effect known as shoaling .

Since the water particles along the surface move from the crest to the trough, surfers hitch a ride on the cascading water, gliding along the surface. If ocean waves work exactly like the idealized transverse waves, surfing would be much less exciting as it would simply involve standing on a board that bobs up and down in place, just like the seagull in the previous figure.

Additional information and illustrations about the scientific principles behind surfing can be found in the “Using Science to Surf Better!” video.

• The surfer would move side-to-side/back-and-forth vertically with no horizontal motion.
• The surfer would forward and backward horizontally with no vertical motion.

Check Your Understanding

Use these questions to assess students’ achievement of the section’s Learning Objectives. If students are struggling with a specific objective, these questions will help identify such objective and direct them to the relevant content.

• A wave is a force that propagates from the place where it was created.
• A wave is a disturbance that propagates from the place where it was created.
• A wave is matter that provides volume to an object.
• A wave is matter that provides mass to an object.
• No, electromagnetic waves do not require any medium to propagate.
• No, mechanical waves do not require any medium to propagate.
• Yes, both mechanical and electromagnetic waves require a medium to propagate.
• Yes, all transverse waves require a medium to travel.
• A pulse wave is a sudden disturbance with only one wave generated.
• A pulse wave is a sudden disturbance with only one or a few waves generated.
• A pulse wave is a gradual disturbance with only one or a few waves generated.
• A pulse wave is a gradual disturbance with only one wave generated.

What are the categories of mechanical waves based on the type of motion?

• Both transverse and longitudinal waves
• Only longitudinal waves
• Only transverse waves
• Only surface waves

In which direction do the particles of the medium oscillate in a transverse wave?

• Perpendicular to the direction of propagation of the transverse wave
• Parallel to the direction of propagation of the transverse wave

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Science News Explores

Explainer: how heat moves.

Here are the three processes by which energy can be transferred from one place to another

Heat is being transferred from the hot end of this rod to the cold end via conduction, but the hot end of the rod is also radiating heat via that orange glow.

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By Sid Perkins

September 30, 2016 at 6:15 am

Throughout the universe, it’s natural for energy to flow from one place to another. And unless people interfere, thermal energy — or heat — naturally flows in one direction only: from hot toward cold.

Heat moves naturally by any of three means. The processes are known as conduction, convection and radiation. Sometimes more than one may occur at the same time.

First, a little background. All matter is made from atoms — either single ones or those bonded in groups known as molecules. These atoms and molecules are always in motion. If they have the same mass, hot atoms and molecules move, on average, faster than cold ones. Even if atoms are locked in a solid, they still vibrate back and forth around some average position.

In a liquid, atoms and molecules are free to flow from place to place. Within a gas, they are even more free to move and will completely spread out within the volume in which they are trapped.

Some of the most easily understood examples of heat flow occur in your kitchen.  

Put a pan on a stovetop and turn on the heat. The metal sitting over the burner will be the first part of the pan to get hot. Atoms in the pan’s bottom will start to vibrate faster as they warm. They also vibrate farther back and forth from their average position. As they bump into their neighbors, they share with that neighbor some of their energy. (Think of this as a very tiny version of a cue ball slamming into other balls during a game of billiards. The target balls, previously sitting still, gain some of the cue ball’s energy and move.)

As a result of collisions with their warmer neighbors, atoms start moving faster. In other words, they are now warming. These atoms, in turn, transfer some of their increased energy to neighbors even farther from the original source of heat. This conduction of heat through a solid metal is how the handle of a pan gets hot even though it may be nowhere near the source of heat.

Convection occurs when a material is free to move, such as a liquid or a gas. Again, consider a pan on the stove. Put water in the pan, then turn on the heat. As the pan gets hot, some of that heat transfers to the molecules of water sitting on the bottom of the pan via conduction. That speeds up the motion of those water molecules — they are warming.

As the water warms, it now begins to expand. That makes it less dense. It rises above denser water, carrying  away heat from the bottom of the pan. Cooler water flows down to take its place next to the hot bottom of the pan. As this water warms, it expands and rises, ferrying its newly-gained energy with it. In short order, a circular flow of rising warm water and falling cooler water sets up. This circular pattern of heat transfer is known as convection .

It’s also what largely warms food in an oven. Air that’s warmed by a heating element or gas flames at the top or bottom of the oven carries that heat to the central zone where the food sits.

Air that’s warmed at Earth’s surface expands and rises just like the water in the pan on the stove. Large birds such as frigate birds (and human flyers riding engineless gliders) often ride these thermals — rising blobs of air — to gain altitude without using any energy of their own. In the ocean, convection caused by heating and cooling helps to drive ocean currents. These currents move water around the globe.

The third type of energy transfer is in some ways the most unusual. It can move through materials — or in the absence of them. This is radiation.

Consider visible light, a form of radiation. It passes through some types of glass and plastic. X-rays, another form of radiation, readily pass through flesh but are largely blocked by bone. Radio waves pass through the walls of your home to reach the antenna on your stereo. Infrared radiation, or heat, passes through the air from fireplaces and light bulbs. But unlike conduction and convection, radiation doesn’t require a material to transfer its energy. Light, X-rays, infrared waves and radio waves all travel to Earth from the far reaches of the universe. Those forms of radiation will pass through plenty of empty space along the way.

X-rays, visible light, infrared radiation, radio waves are all different forms of electromagnetic radiation . Each type of radiation falls into a particular band of wavelengths. Those types differ in the amount of energy they have. In general, the longer the wavelength, the lower the frequency of a particular type of radiation and the less energy it will carry.

To complicate things, it’s important to note that more than one form of heat transfer may occur at the same time. A stove’s burner not only heats a pan but also the nearby air and makes it less dense. That carries warmth upward via convection. But the burner also radiates heat as infrared waves, making things nearby warm up. And if you’re using a cast-iron skillet to cook a tasty meal, be sure to grab the handle with a potholder: It’s gonna be hot, thanks to conduction!

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Can Humans Hear Sound in Space?

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Is it possible to hear sounds in space? The short answer is "No." Yet, misconceptions about sound in space continue to exist, mostly due to the sound effects used in sci-fi movies and TV shows. How many times have we "heard" the starship Enterprise or the Millennium Falcon whoosh through space? It's so ingrained our ideas about space that people are often surprised to find out that it doesn't work that way. The laws of physics explain that it can't happen, but often enough producers don't really think about them. They're going for "effect."

Plus, it's not just a problem in TV or movies. There are mistaken ideas out there that planets make sounds , for example. What's really happening is that specific processes in their atmospheres (or rings) are sending out emissions that can be picked up by sensitive instruments. In order to understand them, scientists take the emissions and "heterodyne" them (that is, process them) to create something we can "hear" so they can try to analyze what they are. But, the planets themselves aren't making sounds.

The Physics of Sound

It is helpful to understand the physics of sound. Sound travels through the air as waves. When we speak, for example, the vibration of our vocal cords compresses the air around them. The compressed air moves the air around it, which carries the sound waves. Eventually, these compressions reach the ears of a listener, whose brain interprets that activity as sound. If the compressions are high frequency and moving fast, the signal received by the ears is interpreted by the brain as a whistle or a shriek. If they're lower frequency and moving more slowly, the brain interprets it as a drum or a boom or a low voice.

Here's the important thing to remember: without anything to compress, sound waves can't be transmitted. And, guess what? There's no "medium" in the vacuum of space itself that transmits sound waves. There is a chance that sound waves can move through and compress clouds of gas and dust, but we wouldn't be able to hear that sound. It would be too low or too high for our ears to perceive. Of course, if someone were in space without any protection against the vacuum, hearing any sound waves would be the least of their problems. 

Light waves (that aren't radio waves) are different. They do not require the existence of a medium in order to propagate. So light can travel through the vacuum of space unimpeded. This is why we can see distant objects like planets , stars , and galaxies . But, we can't hear any sounds they might make. Our ears are what pick up sound waves, and for a variety of reasons, our unprotected ears aren't going to be in space.

Haven't Probes Picked Up Sounds From the Planets?

This is a bit of a tricky one. NASA, back in the early 90s, released a five-volume set of space sounds. Unfortunately, they were not too specific about how the sounds were made exactly. It turns out the recordings weren't actually of sound coming from those planets. What was picked up were interactions of charged particles in the magnetospheres of the planets; trapped radio waves and other electromagnetic disturbances. Astronomers then took these measurements and converted them into sounds. It is similar to the way that a radio captures the radio waves (which are long-wavelength light waves) from radio stations and converts those signals into sound.

Why Apollo Astronauts Report Sounds Near the Moon

This one is truly strange. According to NASA transcripts of the Apollo moon missions, several of the astronauts reported hearing "music" when orbiting the Moon . It turns out that what they heard was entirely predictable radio frequency interference between the lunar module and the command modules.

The most prominent example of this sound was when the Apollo 15 astronauts were on the far side of the Moon. However, once the orbiting craft was over the near side of the Moon, the warbling stopped. Anyone who has ever played with a radio or done HAM radio or other experiments with radio frequencies would recognize the sounds at once. They were nothing abnormal and they certainly didn't propagate through the vacuum of space. 

Why Movies Have Spacecrafts Making Sounds

Since we know that no one can physically hear sounds in the vacuum of space, the best explanation for sound effects in TV and movies is this: if producers didn't make the rockets roar and the spacecraft go "whoosh", the soundtrack would be boring. And, that's true. It doesn't mean there's sound in space. All it means is that sounds are added to give the scenes a little drama. That's perfectly fine as long as people understand it doesn't happen in reality. 

• Can a Planet Make a Sound in Space?
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• 10 Sounds We Hate Most
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Inside the quest to map the universe with mysterious bursts of radio energy

Astronomers still don’t know what causes fast radio bursts, but they’re starting to use them to illuminate the space between galaxies.

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When our universe was less than half as old as it is today, a burst of energy that could cook a sun’s worth of popcorn shot out from somewhere amid a compact group of galaxies. Some 8 billion years later, radio waves from that burst reached Earth and were captured by a sophisticated low-frequency radio telescope in the Australian outback. 

The signal, which arrived on June 10, 2022, and lasted for under half a millisecond, is one of a growing class of mysterious radio signals called fast radio bursts. In the last 10 years, astronomers have picked up nearly 5,000 of them. This one was particularly special: nearly double the age of anything previously observed, and three and a half times more energetic. 

But like the others that came before, it was otherwise a mystery. No one knows what causes fast radio bursts. They flash in a seemingly random and unpredictable pattern from all over the sky. Some appear from within our galaxy , others from previously unexamined depths of the universe. Some repeat in cyclical patterns for days at a time and then vanish; others have been consistently repeating every few days since we first identified them. Most never repeat at all. 

Despite the mystery, these radio waves are starting to prove extraordinarily useful. By the time our telescopes detect them, they have passed through clouds of hot, rippling plasma, through gas so diffuse that particles barely touch each other, and through our own Milky Way. And every time they hit the free electrons floating in all that stuff, the waves shift a little bit. The ones that reach our telescopes carry with them a smeary fingerprint of all the ordinary matter they’ve encountered between wherever they came from and where we are now. 

This makes fast radio bursts, or FRBs, invaluable tools for scientific discovery—especially for astronomers interested in the very diffuse gas and dust floating between galaxies, which we know very little about. 

“We don’t know what they are, and we don’t know what causes them. But it doesn’t matter. This is the tool we would have constructed and developed if we had the chance to be playing God and create the universe,” says Stuart Ryder, an astronomer at Macquarie University in Sydney and the lead author of the Science paper that reported the record-breaking burst. 

Many astronomers now feel confident that finding more such distant FRBs will enable them to create the most detailed three-dimensional cosmological map ever made—what Ryder likens to a CT scan of the universe. Even just five years ago making such a map might have seemed an intractable technical challenge: spotting an FFB and then recording enough data to determine where it came from is extraordinarily difficult because most of that work must happen in the few milliseconds before the burst passes.

But that challenge is about to be obliterated. By the end of this decade, a new generation of radio telescopes and related technologies coming online in Australia, Canada, Chile, California, and elsewhere should transform the effort to find FRBs—and help unpack what they can tell us. What was once a series of serendipitous discoveries will become something that’s almost routine. Not only will astronomers be able to build out that new map of the universe, but they’ll have the chance to vastly improve our understanding of how galaxies are born and how they change over time. 

Where’s the matter?

In 1998, astronomers counted up the weight of all of the identified matter in the universe and got a puzzling result. 

We know that about 5% of the total weight of the universe is made up of baryons like protons and neutrons— the particles that make up atoms, or all the “stuff” in the universe. (The other 95% includes dark energy and dark matter.) But the astronomers managed to locate only about 2.5%, not 5%, of the universe’s total. “They counted the stars, black holes, white dwarfs, exotic objects, the atomic gas, the molecular gas in galaxies, the hot plasma, etc. They added it all up and wound up at least a factor of two short of what it should have been,” says Xavier Prochaska, an astrophysicist at the University of California, Santa Cruz, and an expert in analyzing the light in the early universe. “It’s embarrassing. We’re not actively observing half of the matter in the universe.” 

All those missing baryons were a serious problem for simulations of how galaxies form, how our universe is structured, and what happens as it continues to expand. 

Astronomers began to speculate that the missing matter exists in extremely diffuse clouds of what’s known as the warm–hot intergalactic medium, or WHIM. Theoretically, the WHIM would contain all that unobserved material. After the 1998 paper was published, Prochaska committed himself to finding it. 

But nearly 10 years of his life and about $50 million in taxpayer money later, the hunt was going very poorly.

That search had focused largely on picking apart the light from distant galactic nuclei and studying x-ray emissions from tendrils of gas connecting galaxies. The breakthrough came in 2007, when Prochaska was sitting on a couch in a meeting room at the University of California, Santa Cruz, reviewing new research papers with his colleagues. There, amid the stacks of research, sat the paper reporting the discovery of the first FRB.

Duncan Lorimer and David Narkevic, astronomers at West Virginia University, had discovered a recording of an energetic radio wave unlike anything previously observed. The wave lasted for less than five milliseconds, and its spectral lines were very smeared and distorted, unusual characteristics for a radio pulse that was also brighter and more energetic than other known transient phenomena. The researchers concluded that the wave could not have come from within our galaxy, meaning that it had traveled some unknown distance through the universe. 

Here was a signal that had traversed long distances of space, been shaped and affected by electrons along the way, and had enough energy to be clearly detectable despite all the stuff it had passed through. There are no other signals we can currently detect that commonly occur throughout the universe and have this exact set of traits.

“I saw that and I said, ‘Holy cow—that’s how we can solve the missing-baryons problem,’” Prochaska says. Astronomers had used a similar technique with the light from pulsars— spinning neutron stars that beam radiation from their poles—to count electrons in the Milky Way. But pulsars are too dim to illuminate more of the universe. FRBs were thousands of times brighter, offering a way to use that technique to study space well beyond our galaxy.

There’s a catch, though: in order for an FRB to be an indicator of what lies in the seemingly empty space between galaxies, researchers have to know where it comes from. If you don’t know how far the FRB has traveled, you can’t make any definitive estimate of what space looks like between its origin point and Earth. 

Astronomers couldn’t even point to the direction that the first 2007 FRB came from, let alone calculate the distance it had traveled. It was detected by an enormous single-dish radio telescope at the Parkes Observatory (now called the Murriyang ) in New South Wales, which is great at picking up incoming radio waves but can pinpoint FRBs only to an area of the sky as large as Earth’s full moon. For the next decade, telescopes continued to identify FRBs without providing a precise origin, making them a fascinating mystery but not practically useful.

Then, in 2015, one particular radio wave flashed—and then flashed again. Over the course of two months of observation from the Arecibo telescope in Puerto Rico, the radio waves came again and again, flashing 10 times. This was the first repeating burst of FRBs ever observed (a mystery in its own right), and now researchers had a chance to determine where the radio waves had begun, using the opportunity to home in on its location.

In 2017, that’s what happened. The researchers obtained an accurate position for the fast radio burst using the NRAO Very Large Array telescope in central New Mexico. Armed with that position, the researchers then used the Gemini optical telescope in Hawaii to take a picture of the location, revealing the galaxy where the FRB had begun and how far it had traveled. “That’s when it became clear that at least some of these we’d get the distance for. That’s when I got really involved and started writing telescope proposals,” Prochaska says. 

That same year, astronomers from across the globe gathered in Aspen, Colorado, to discuss the potential for studying FRBs. Researchers debated what caused them. Neutron stars? Magnetars, neutron stars with such powerful magnetic fields that they emit x-rays and gamma rays? Merging galaxies? Aliens? Did repeating FRBs and one-offs have different origins, or could there be some other explanation for why some bursts repeat and most do not? Did it even matter, since all the bursts could be used as probes regardless of what caused them? At that Aspen meeting, Prochaska met with a team of radio astronomers based in Australia, including Keith Bannister, a telescope expert involved in the early work to build a precursor facility for the Square Kilometer Array, an international collaboration to build the largest radio telescope arrays in the world. 

The construction of that precursor telescope, called ASKAP, was still underway during that meeting. But Bannister, a telescope expert at the Australian government’s scientific research agency, CSIRO, believed that it could be requisitioned and adapted to simultaneously locate and observe FRBs. 

Bannister and the other radio experts affiliated with ASKAP understood how to manipulate radio telescopes for the unique demands of FRB hunting; Prochaska was an expert in everything “not radio.” They agreed to work together to identify and locate one-off FRBs (because there are many more of these than there are repeating ones) and then use the data to address the problem of the missing baryons. 

And over the course of the next five years, that’s exactly what they did—with astonishing success.

Building a pipeline

To pinpoint a burst in the sky, you need a telescope with two things that have traditionally been at odds in radio astronomy: a very large field of view and high resolution. The large field of view gives you the greatest possible chance to detect a fleeting, unpredictable burst. High resolution  lets you determine where that burst actually sits in your field of view. 

ASKAP was the perfect candidate for the job. Located in the westernmost part of the Australian outback, where cattle and sheep graze on public land and people are few and far between, the telescope consists of 36 dishes, each with a large field of view. These dishes are separated by large distances, allowing observations to be combined through a technique called interferometry so that a small patch of the sky can be viewed with high precision.  

The dishes weren’t formally in use yet, but Bannister had an idea. He took them and jerry-rigged a “fly’s eye” telescope, pointing the dishes at different parts of the sky to maximize its ability to spot something that might flash anywhere. 

“Suddenly, it felt like we were living in paradise,” Bannister says. “There had only ever been three or four FRB detections at this point, and people weren’t entirely sure if [FRBs] were real or not, and we were finding them every two weeks.” 

When ASKAP’s interferometer went online in September 2018, the real work began. Bannister designed a piece of software that he likens to live-action replay of the FRB event. “This thing comes by and smacks into your telescope and disappears, and you’ve got a millisecond to get its phone number,” he says. To do so, the software detects the presence of an FRB within a hundredth of a second and then reaches upstream to create a recording of the telescope’s data before the system overwrites it. Data from all the dishes can be processed and combined to reconstruct a view of the sky and find a precise point of origin. 

The team can then send the coordinates on to optical telescopes, which can take detailed pictures of the spot to confirm the presence of a galaxy—the likely origin point of the FRB. 

Ryder’s team used data on the galaxy’s spectrum, gathered from the European Southern Observatory, to measure how much its light stretched as it traversed space to reach our telescopes. This “redshift” becomes a proxy for distance, allowing astronomers to estimate just how much space the FRB’s light has passed through. 

In 2018, the live-action replay worked for the first time, making Bannister, Ryder, Prochaska, and the rest of their research team the first to localize an FRB that was not repeating. By the following year, the team had localized about five of them. By 2020, they had published a paper in Nature declaring that the FRBs had let them count up the universe’s missing baryons. 

The centerpiece of the paper’s argument was something called the dispersion measure—a number that reflects how much an FRB’s light has been smeared by all the free electrons along our line of sight. In general, the farther an FRB travels, the higher the dispersion measure should be. Armed with both the travel distance (the redshift) and the dispersion measure for a number of FRBs, the researchers found they could extrapolate the total density of particles in the universe. J-P Macquart, the paper’s lead author, believed that the relationship between dispersion measure and FRB distance was predictable and could be applied to map the universe.

As a leader in the field and a key player in the advancement of FRB research, Macquart would have been interviewed for this piece. But he died of a heart attack one week after the paper was published, at the age of 45 . FRB researchers began to call the relationship between dispersion and distance the “Macquart relation,” in honor of his memory and his push for the groundbreaking idea that FRBs could be used for cosmology. 

Proving that the Macquart relation would hold at greater distances became not just a scientific quest but also an emotional one. 

“I remember thinking that I know something about the universe that no one else knows.”

The researchers knew that the ASKAP telescope was capable of detecting bursts from very far away—they just needed to find one. Whenever the telescope detected an FRB, Ryder was tasked with helping to determine where it had originated. It took much longer than he would have liked. But one morning in July 2022, after many months of frustration, Ryder downloaded the newest data email from the European Southern Observatory and began to scroll through the spectrum data. Scrolling, scrolling, scrolling—and then there it was: light from 8 billion years ago, or a redshift of one, symbolized by two very close, bright lines on the computer screen, showing the optical emissions from oxygen. “I remember thinking that I know something about the universe that no one else knows,” he says. “I wanted to jump onto a Slack and tell everyone, but then I thought: No, just sit here and revel in this. It has taken a lot to get to this point.” 

With the October 2023 Science paper, the team had basically doubled the distance baseline for the Macquart relation, honoring Macquart’s memory in the best way they knew how. The distance jump was significant because Ryder and the others on his team wanted to confirm that their work would hold true even for FRBs whose light comes from so far away that it reflects a much younger universe. They also wanted to establish that it was possible to find FRBs at this redshift, because astronomers need to collect evidence about many more like this one in order to create the cosmological map that motivates so much FRB research.

“It’s encouraging that the Macquart relation does still seem to hold, and that we can still see fast radio bursts coming from those distances,” Ryder said. “We assume that there are many more out there.” 

Mapping the cosmic web

The missing stuff that lies between galaxies, which should contain the majority of the matter in the universe, is often called the cosmic web. The diffuse gases aren’t floating like random clouds; they’re strung together more like a spiderweb, a complex weaving of delicate filaments that stretches as the galaxies at their nodes grow and shift. This gas probably escaped from galaxies into the space beyond when the galaxies first formed, shoved outward by massive explosions.

“We don’t understand how gas is pushed in and out of galaxies. It’s fundamental for understanding how galaxies form and evolve,” says Kiyoshi Masui, the director of MIT’s Synoptic Radio Lab. “We only exist because stars exist, and yet this process of building up the building blocks of the universe is poorly understood … Our ability to model that is the gaping hole in our understanding of how the universe works.” 

Astronomers are also working to build large-scale maps of galaxies in order to precisely measure the expansion of the universe. But the cosmological modeling underway with FRBs should create a picture of invisible gasses between galaxies, one that currently does not exist. To build a three-dimensional map of this cosmic web, astronomers will need precise data on thousands of FRBs from regions near Earth and from very far away, like the FRB at redshift one. “Ultimately, fast radio bursts will give you a very detailed picture of how gas gets pushed around,” Masui says. “To get to the cosmological data, samples have to get bigger, but not a lot bigger.” 

That’s the task at hand for Masui, who leads a team searching for FRBs much closer to our galaxy than the ones found by the Australian-led collaboration. Masui’s team conducts FRB research with the CHIME telescope in British Columbia, a nontraditional radio telescope with a very wide field of view and focusing reflectors that look like half-pipes instead of dishes. CHIME (short for “Canadian Hydrogen Intensity Mapping Experiment)” has no moving parts and is less reliant on mirrors than a traditional telescope (focusing light in only one direction rather than two), instead using digital techniques to process its data. CHIME can use its digital technology to focus on many places at once, creating a 200-square-degree field of view compared with ASKAP’s 30-degree one. Masui likened it to a mirror that can be focused on thousands of different places simultaneously. 

Because of this enormous field of view, CHIME has been able to gather data on thousands of bursts that are closer to the Milky Way. While CHIME cannot yet precisely locate where they are coming from the way that ASKAP can (the telescope is much more compact, providing lower resolution), Masui is leading the effort to change that by building three smaller versions of the same telescope in British Columbia; Green Bank, West Virginia; and Northern California. The additional data provided by these telescopes, the first of which will probably be collected sometime this year, can be combined with data from the original CHIME telescope to produce location information that is about 1,000 times more precise. That should be detailed enough for cosmological mapping.

Telescope technology is improving so fast that the quest to gather enough FRB samples from different parts of the universe for a cosmological map could be finished within the next 10 years. In addition to CHIME, the BURSTT radio telescope in Taiwan should go online this year; the CHORD telescope in Canada, designed to surpass CHIME, should begin operations in 2025; and the Deep Synoptic Array in California could transform the field of radio astronomy when it’s finished, which is expected to happen sometime around the end of the decade. 

And at ASKAP, Bannister is building a new tool that will quintuple the sensitivity of the telescope, beginning this year. If you can imagine stuffing a million people simultaneously watching uncompressed YouTube videos into a box the size of a fridge, that’s probably the easiest way to visualize the data handling capabilities of this new processor, called a field-programmable gate array, which Bannister is almost finished programming. He expects the new device to allow the team to detect one new FRB each day.

With all the telescopes in competition, Bannister says, “in five or 10 years’ time, there will be 1,000 new FRBs detected before you can write a paper about the one you just found ... We’re in a race to make them boring.” 

Prochaska is so confident FRBs will finally give us the cosmological map he’s been working toward his entire life that he’s started studying for a degree in oceanography. Once astronomers have measured distances for 1,000 of the bursts, he plans to give up the work entirely. 

“In a decade, we could have a pretty decent cosmological map that’s very precise,” he says. “That’s what the 1,000 FRBs are for—and I should be fired if we don’t.”

Unlike most scientists, Prochaska can define the end goal. He knows that all those FRBs should allow astronomers to paint a map of the invisible gases in the universe, creating a picture of how galaxies evolve as gases move outward and then fall back in. FRBs will grant us an understanding of the shape of the universe that we don’t have today—even if the mystery of what makes them endures. 

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