# From where does the energy for gravitational waves come from?

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As far as I understand, in the events detected by LIGO, about 4% of the total mass of merging binary black holes was converted to gravitational waves.

Where does this energy come from, i.e. what exactly gets converted into gravitational waves?

Is it simply the kinetic energy of the merging objects (velocities of these objects before merger are huge, up to 60% of c if I recall correctly), so does it mean emitting gravitational waves makes them orbit slower, but retain their original masses? Or do the compact objects really lose "real" mass, meaning they become lighter and in case of BHs their radius changes accordingly?

As an example, let's assume two BHs, both with 50 solar masses, orbiting each other far enough (say 1 light year) so that GWs nor kinetic energy has no significance to these initial mass measurements. During the merge, they should radiate about 5 solar masses in GWs. Would the resulting black hole have mass of 95 or 100 solar masses?

Radiating gravitational waves makes an inspralling binary orbit closer and faster. (Rob Jefferies)

The source of the energy for both increased kinetic energy, and the gravitational radiation is the same: gravitational potential energy. (PM 2Ring)

Two black holes at a distance of 1 light year have a huge amount of potential energy, about 10^48 Joules of potential energy. As they spiral, a significant amount of that energy is radiated as gravitational waves

This is real mass lost. The mass of resulting black hole is smaller than the sum of the two merging black holes, though at no point does any black hole itself become smaller.

As Rob correctly pointed out, the emission of gravitational waves reduces the orbital energy and result in an inspiral. This reduction in total energy also reduces the mass of the final BH, since $E=mc^2$. The bulk of the gravitational wave energy is emitted (and energy=mass lost) in the final chirp, when the separation approaches the Schwarzschild radius.

To quantify this, let's just make a simple energy budget calculation, starting from two equal-mass BHs of mass $M_ullet$ orbiting each other at distance $d$ on a circular orbit. Then the orbital energy is $$E_{mathrm{orbit}} = -frac{GM^2_ullet}{2d} = -M_ullet c^2 frac{R_s}{4d}$$ where $R_s=2GM/c^2$ the Schwarzschild radius of each BH and we have assumed that $dgg R_s$ such that the orbit is Keplerian. The total initial energy is then given by the rest mass energies plus the orbital energy as $$E_{mathrm{total}} = M_ullet c^2 left[2-frac{R_s}{4d} ight].$$ After coalescence, a remnant of mass $M_{r}$ emerges. The energy deficit is the difference between the initial and final energies egin{equation} delta E = M_ullet c^2 left[2-frac{R_s}{4d} ight] - frac{M_rc^2}{sqrt{1-v^2/c^2}}, end{equation} where $v$ is the speed of the remnant w.r.t. to the centre of mass of the progenitors. This energy has been lost by gravitational wave radiation. If this corresponds to a certain amount $mu$ of rest mass, then from $delta E = mu c^2$ we find $$M_r = sqrt{1-v^2/c^2}left[2M_ullet -mu - M_ulletfrac{R_s}{4d} ight].$$ Now for $v=0$ and $R_sll d$, the mass deficit $delta mequiv 2M_ullet-M_r$ is identical to $mu$: the radiated energy corresponds to the mass deficit; the final hole has 95$M_odot$ if $M_ullet=50M_odot$ and $mu=5M_odot$. In particular, the gravitational wave energy cannot be taken merely from the orbital energy as suggested by another answer.

The mass deficit is even larger than the radiated energy if the remnant has undergone a considerable velocity kick, such that $v eq0$ (caused by asymmetric gravitational wave radiation).

## From where does the energy for gravitational waves come from? - Astronomy

In the language of Albert Einstein's general theory of relativity, gravitational radiation or gravitational waves (GWs) are ripples in the geometry of space and time.'' A less abstruse way to describe gravitational radiation is by drawing an analogy to the electromagnetic spectrum (light, infrared, radio, microwave, x-ray, etc.). Just as these represent forms of the free radiation or waves associated with electricity and magnetism, so GWs represent the radiation associated with the force of gravity. Einstein actually predicted their existence in 1916, the same year his paper on general relativity theory was published. He even calculated the radiation emitted from a binary star system (the strongest source known at the time) and concluded that the radiation was so weak that it had a negligible practical effect.'' For the next half century gravitational radiation remained a theoretical curiosity that was of no practical astrophysical significance. In the last two decades astrophysicists have discovered several new potential sources and have come to believe that it not only may be possible to detect gravitational waves directly but also that their emission may even be the dominant process in the evolution of some astrophysical objects.

### GENERATION AND DETECTION

Electromagnetism and gravity are the only two fundamental, longrange forces in nature. Just as accelerated electric charges generate electromagnetic radiation, so do accelerated gravitational charges,'' that is, masses, generate gravitational radiation. Simply by analogy with electromagnetism, it is not surprising that gravitational waves are predicted by general relativity and every other viable theory of gravity. Furthermore, if gravity is to obey the laws of Einstein's special theory of relativity, then gravitational radiation must travel at the speed of light.

Because all electric charges have mass, one might expect gravitational radiation to be as abundant as electromagnetic radiation however, this is not the case. Consider the system indicated in Fig. 1 that depicts two particles of the same mass M and opposite electric charge ±Q oscillating at opposite ends of a spring of length L. The ratio of the power PG emitted in gravitational radiation to the power PEM emitted in electromagnetic radiation by this system is

PG / PEM = (GM 2 / Q 2 )(L / ) 2

where is the wavelength of the radiation. If the particles are electrons (which are responsible for most of the electromagnetic radiation we observe), the first factor alone is 10 -43 , which illustrates the incredible weakness of gravity. The second term in the preceding equation is proportional to the square of the ratio of the speed of the masses to the speed of light and is always less than 1. From this example it is clear that large, rapidly moving masses are the best sources of GWs.

Electromagnetic radiation is detected by a wide variety of instruments, all of which operate on the same principle: electromagnetic waves exert force on electric charges. Likewise, gravitational radiation can be detected by the force it exerts on masses. If a GW is incident on the spring system of Fig. 1 the masses will be driven into oscillation. The amplitude of the oscillations, however, is very small, again because of the weakness of gravity. In fact, the ratio of the energy absorbed from a gravitational wave to the energy absorbed from an electromagnetic wave of equal strength is given by exactly the same ratio as in the equation.

As is characteristic of electromagnetic waves, GWs have two possible polarizations and exert force on matter only in directions perpendicular to the direction of propagation of the wave. The energy in the wave decreases inversely with the square of the distance from the source. For a polarized GW the lines of force in the plane perpendicular to the direction of propagation are illustrated in Fig. 2. The resultant accelerations of four test masses, A, B, C, and D, are indicated by arrows. One-half cycle later the directions of the lines of force and accelerations are reversed. A representation of the other possible polarization is obtained by rotating the figure by 45 ° . The primitive gravitational wave detector of Fig. 1 is obtained by simply connecting masses A and B (or C and D) by a spring.

### SOURCES

Considering the inherent weakness of gravity, laboratory sources of gravitational radiation are nonexistent. For example, a 1-ton steel bar spun so rapidly that it is on the verge of being ripped apart by centrifugal force radiates less than 10 -30 W. (This problem was considered by Einstein in 1918.) By contrast, existing detectors are only sensitive enough to detect such a source (at a distance of one wavelength) if it emits more than 10 6 W. Current hopes for directly detecting GWs are pinned on astrophysical sources in which massive bodies undergo tremendous accelerations. Short term binary star systems emit strongly (10 25 -10 29 W) at frequencies from 10 -4 -10 -3 mHz, but there are currently no detectors capable of detecting such sources even nearby. Even though not directly detectable, the energy loss from the short term binary pulsar PSR 1913+16 due to gravitational radiation has been measured by precise timing observations of the pulsar orbit. These measurements agree with the predictions of general relativity to within 1%, a result which is an important confirmation of the existence of GWs. A much stronger source is gravitational collapse to a black hole during which a large fraction of the mass of an entire star may be accelerated to velocities approaching the speed of light. It is expected that as much as 10 49 W of GWs will be emitted from such a source in the form of a pulse of duration 0.001 s. It has also been conjectured that massive black holes (10 5 -10 9 M ) are located in the nuclei of many galaxies and quasars. During their formation these objects could emit as much as 10 54 J of energy in the form of GWs at frequencies between 10 -1 and 10 -5 Hz. Several more exotic sources of low frequency gravitational radiation have been suggested, for example, quantum gravity fluctuations in the early universe, a quantum chromodynamic phase transition from free quarks into nucleons and oscillating cosmic strings. The period of GWs from these sources range from days to years and are out of the range of laboratory detectors.

### DETECTORS

The first attempts to detect gravitational waves were made in the 1960s by Joseph Weber who acoustically suspended a massive (1400-kg) aluminum cylinder and monitored the level of excitation of its lowest vibrational mode. Resonant at 1660 Hz, this detector was designed to be sensitive to the burst of gravitational radiation predicted to accompany the formation of a solar mass black hole. His 1969 claim of detection of GWs was not confirmed and it is now the consensus of opinion within the scientific community that GWs have not yet been directly observed. Several cryogenically cooled cylinders are now in operation with a sensitivity adequate to detect the GWs emitted from gravitational collapse occurring anywhere in the Milky Way galaxy. None has been detected. Laser interferometers that detect the relative displacement of two mirrors (separated by hundreds of meters) of a Michelson interferometer are potentially more sensitive detectors that respond to a wider range of frequencies. It is hoped that several of these instruments will be in operation in the 1990s with a sensitivity sufficient to detect gravitational collapse in galaxies outside our own. At lower frequencies (10 -3 Hz), radar ranging of spacecraft and the monitoring of solid Earth vibrations have been employed but these methods are not yet sensitive enough to detect such events as the formation of massive black holes in distant galaxies. The effect of GWs on the arrival times of pulsars has been used to place upper limits on a background of GWs with periods of a few years. These limits already have provided important constraints on some models of cosmic strings.

### GRAVITATIONAL WAVE ASTRONOMY

The observations of the binary pulsar PSR 1913+16 have established the existence of gravitational radiation, but as an astrophysical source of GWs this system is the least interesting object imaginable, that is, two point particles in orbit around one another. The main motivation of researchers in the field is not simply to observe GWs directly and thereby confirm their existence, but rather to be able to use them to probe deeply into the regions of strong gravitational fields and dense matter that may block other forms of radiation. Only when GWs are detected from one or more of the spectacular astrophysical phenomena mentioned above will gravitational wave astronomy be considered a legitimate branch of astrophysics.

## Gravitational Waves: Theoretical Interpretation

General relativity states that gravity is expressed as a space-time curvature, and it predicts the existence of gravitational waves.

Gravitational waves are propagating gravitational fields produced by the motion of massive objects. They are often called ripples of space-time curvature. Gravitational fields produced by massive particles control the motion of matter or light in space-time in a manner similar to how electric fields produced by charged particles control how other charged particles move. It is important to detect gravitational waves because they would bring new information about distant galaxies which electromagnetic waves cannot. It could also directly prove general relativity.

## How matter emits gravitational waves

According to General Relativity, when any objects with mass accelerate, they emit gravitational waves. This is analogous to the electromagnetic waves emitted by accelerating charged particles.

For example, consider a gravitational wave propagating in the z direction in Cartesian coordinates. The wave only deforms space-time in the direction perpendicular to its propagation direction. Deformation of space-time means shrinking and stretching of the physical length between two points. Gravitational waves affect space-time in such a way that certain deformation patterns occur periodically.

If at time t, space is stretched in the x-direction at the maximum amplitude, then at time t+T/2 (where T is the period), one-half period later, space will have shrunk by the maximum amplitude and then will again stretch to its maximum at time t+T, one full period later. The theory also states that in the y-direction, space is deformed in the opposite phase of the x-direction. Thus, if space is stretched out in the x-direction, space is shrunk in the y-direction.

## Stretching and Shrinking

The relative length change of two points resulting from gravitational wave is expressed as

## Sources of detectable gravitational wave

### Binary system

The magnitude of the gravitational waves depends on the distance from the source and the second derivative of the mass distribution (mass times acceleration in case of one particle). Thus, one must consider very massive objects moving violently as the candidates for the sources of detectable gravitational waves. The most promising candidates are compact binaries consisting of either two neutron stars, two black holes or one neutron star and one black hole. They are small and heavy, which allows them to orbit at a closer distance and at a high orbital frequency, which means that the second derivative of the mass distribution of the system is large. Therefore, the system emits strong gravitational waves. Compact binaries have another interesting aspect associated with gravitational radiation (explained in a later section).

### Rotating neutron stars

If non axis-symmetric along the rotation axis, a rotating neutron star will emit gravitational waves. If symmetric, the second derivative of the mass distribution holds constant at zero in the system, which leads to no gravitational wave emission.

### Supernovae

Supernovae are a good gravitational source. They are compact and have large accelerations. Similar to rotating neutron stars, if a supernova's explosion has axial symmetry, gravitational waves will not be emitted for the constant mass distribution. Initial density and temperature fluctuations and other factors may direct asymmetric collapse. If a supernova, which is exceptionally bright, is observed in gravitational wave, we will be able to test a prediction of general relativity which states that the speed of gravitational wave is the same speed as light.

### Stochastic background

This comes from the density fluctuation of the early stage of the universe. Measuring the background would tell us about the nature of the Plank-size universe and provide clues for testing the various cosmological models. Although stochastic background is interesting, it is so weak that modern technology is far from achieving this task.

## Relation between the motion of the source and the period and the amplitude of the consequent gravitational wave

The magnitude of gravitational waves is proportional to the second derivative of the mass distribution of the emitting system and inversely proportional to the separation of the source and observing point. The next question that arises is how the period of a gravitational wave is related to that of the motion of the source. If the binaries are in a circular orbit, the resulting gravitational waves have a frequency that is twice that of the binary system--that is, the period of the gravitational wave is one half of the orbital period.

The magnitude changes with the orientation. However an angle-averaged estimate of the signal strength is expressed in the following formula.

To see why this is proportional to the second derivative of the mass distribution of the system, recall that the derivative is proportional to and inversely proportional to .

This leads to the above equation. Therefore, if the frequency of the orbital period increases, the resulting gravitational wave will also increase its frequency and amplitude. And this is achieved by self-energy loss of gravitational radiation.

## Gravitational radiation and self-energy loss of binary system

According to general relativity, gravitational radiation takes energy from its source. The energy loss results in the smaller separation of the binaries and therefore shorter orbital frequency. This comes from the fact that the rate of change in orbital period of a binary system is negatively proportional to the rate of change in gravitational potential energy of the binary system:

As the system radiates gravitational energy, it decreases its orbital period. Therefore, the resulting gravitational wave has a waveform of increasing frequency and amplitude. Furthermore, the radiation energy is proportional to the square of the amplitude of the associated gravitational wave. This means that the radiation power increases faster and faster as the system loses its energy due to gravitational radiation and thus resulting in the larger rate of change in orbital period. In fact, the final stage of the binary system consisting of two neutron stars has such a large radiation energy that the neutron stars will lose all their potential energy and finally collide. The waveform resulting from the collision will be very rough. Looking at the waveform, we can also obtain information about the mechanism of astronomical events.

Collision of two neutron stars

Binary Neutron Star Merger, calculated by Maximilan Ruffert

Black Hole - Neutron Star Binary Mergers

## Why is it important to measure the gravitational wave?

Gravitational waves are most unique in that they propagate without interacting with matter. This allows us to obtain new information about the universe that electromagnetic waves fail to provide. The amplitude and frequency of gravitational waves describe the frequency and mass of the emitting source. The shape of the final phase of a binary system might give some new insight in astronomy. Stochastic background would reveal the mass distribution of the early plank-scale universe and the evolution of the early universe.

## Gravitational Waves

Gravitational waves originate in much the same way as electromagnetic waves do. If you accelerate an electric charge, like an electron, it will produce an electromagnetic wave. The energy for this wave comes from the kinetic energy of the electron.

In the same way, if you accelerate a mass, it will emit a gravitational wave, and the energy for that wave will come from the masses kinetic energy.
In fact, since the above electron also has a mass, it will emit gravitational waves as well as electromagnetic ones. Gravitational waves however are very very weak and the electromagnetic waves account for the vast majority of the lost kinetic energy.

Gravitational waves originate in much the same way as electromagnetic waves do. If you accelerate an electric charge, like an electron, it will produce an electromagnetic wave. The energy for this wave comes from the kinetic energy of the electron.

In the same way, if you accelerate a mass, it will emit a gravitational wave, and the energy for that wave will come from the masses kinetic energy.
In fact, since the above electron also has a mass, it will emit gravitational waves as well as electromagnetic ones. Gravitational waves however are very very weak and the electromagnetic waves account for the vast majority of the lost kinetic energy.

## Ep. 71: Gravitational Waves

When he put together his theories of relativity, Einstein made a series of predictions. Some were confirmed just a few years later, but scientists are still working to confirm others. And one of the most fascinating is the concept of gravitational waves. As massive objects move in space, they send out ripples across the Universe that actually distort the shape of matter. Experiments are in place and in the works to detect these gravitational waves as they sweep past the Earth.

### Transcript: Gravitational Waves

Fraser Cain: Pamela you made it back to Illinois?

Dr. Pamela Gay: I am back in the middle of the country. It was great being able to record face to face with you last week.

Fraser: That was really fun and the AAS meeting was a riot too I have to say. Although I donâ€™t think Iâ€™ve ever written so much in my life as I did over those 4 days. I think I put out about more than thirty articles in 4 days.

Pamela: You put out more than Phil and I combined.

Fraser: I think so. But it was definitely a valuable experience and the next one is going to be in St. Louis?

Fraser: Yeah, itâ€™s across the river from you, right? Whatâ€™s the date on that?

Pamela: Itâ€™s going to start Memorial Day Weekend and run the first week of June.

Fraser: OK. So weâ€™ll probably be gearing up to do the same thing.

Pamela: And weâ€™ll have another meet-up except this time in the middle of the country for everyone else in the middle of the country.

Fraser: Right the meet-up was super fun. That was great to see all of the people who had driven down there. Some people came quite a ways. It was just great to see everybody and talk. That was a lot of fun.

Pamela: We canâ€™t thank George enough for helping put everything together for us.

Fraser: It was great. And in the end we did have about sixty people were there?

Pamela: Yeah, it was sixty.

Fraser: It was quite a crowd. The one last little piece of administration is that weâ€™re going to be probably posting this show a little later on Mondays. We have a few more things that we have to do these days to kind of bring the whole show together and it takes a little more time. So before we tried to post Sunday night and have it ready for Monday.

Now it will be later on and maybe even into the evening on Monday. So no promises but I think it will still be Mondays so I think the day still is the same.

When he first put together his theories of relativity, Einstein made a series of predictions. Some were confirmed just a few years later, but scientists are still working to confirm others.

One of the most fascinating is the concept of gravitational waves. â€œAs objects move in space like black holes, they send out ripples across the universe that can actually distort the shape of matterâ€. Experiments are in place and in the works to detect these gravitational waves as they sweep past the earth. So Pamela, what did Einstein predict?

Pamela: Just like rolling a rock around on a stretchy sheet. Not that I would know why you would do this, but for educational purposes us teachers do this a lot. If you roll a rock around on a stretchy sheet, you can actually end up getting waves on the stretchy sheet.

What he figured was, if you take an object, a planet, a star, a black hole and you rolled it around in the fabric of space and time you can also build up waves in space and time. These waves as they propagate through space can actually cause objects to contract and expand out as the waves pass through them.

Fraser: So the waves that he was predicting rippling on this sheet of rubber could just include a vacuum. But if there happened to be things on that sheet of rubber like planets or stars or anything, those would also be distorted. So itâ€™s not like the underlying space underneath is different from the stars and planets and so on it all just gets distorted.

Pamela: And this is actually kind of new to our way of thinking. When we talk about the expansion in the universe we understand that things that are gravitationally bound together, they are going to stay their same size.

But this space that theyâ€™re embedded within is expanding because of the Hubble Constant and because of dark energy and because of a lot of other things. With gravitational waves, everything is expanding and contracting. If a gravitational wave passes through me, I will expand and contract.

The way it works out is that for every one meter long object youâ€™re going to get an expansion or contraction thatâ€™s actually way smaller than a proton like a hundred thousandth the width of a proton. Thatâ€™s one of the little pieces of an atom.

So Iâ€™m not really worried about getting noticeably expanded or contracted by gravitational waves. But when you start looking at planet sized objects, then you start to be able to actually get measurable expansions and contractions from these gravity waves.

Fraser: But the expansion and contraction would be hard to perceive because everything around you is being expanded and contracted at the same time, right?

Pamela: But the wave is actually moving at the speed of light. So itâ€™s not like the entire planet experiences the wave all at once. If you have a high speed enough way of measuring this, then I can see me expand and contract and then you way over in Vancouver a thousand plus miles away youâ€™re going to expand and contract later in time if thatâ€™s the direction the wave is passing in. Or youâ€™ll expand and contract first and then Iâ€™ll do it later.

So in trying to look for gravity waves, we actually look for this â€œit happens there now it happens hereâ€ change in when we make the measurements.

Fraser: Did Einstein know why this would happen? Was this just something that popped out of his calculations?

Pamela: Well it did come out of his calculations, but it actually makes sense when you start thinking of gravity as being a geometric effect. There are two different ways to look at gravity.

You can see it either as a standard force where youâ€™re flinging bosons all over the place and the bosons wandering from object to object are what are conveying the force. This is a standard model way of looking at things that all forces are mediated by bosons.

Fraser: So you have particles zipping back and forth, which are creating the force.

Pamela: Right. Or at least carrying the force.

Fraser: Carrying the force.

Pamela: Thereâ€™s another way to look at gravity. The other way to look at gravity is to see it as a geometric property of how the universe is put together.

When the sun sits in the center of our solar system, it is actually warping space and time around it to create this three-dimensional bowl. If we were able to see the grid of space and time from outside, that grid would get denser around the sun and things would fall into the denser part of the grid as they are attracted to the sun, and the planets are just rolling around inside this bowl.

In this different way of looking at the universe where really space and time is a fabric that can be deformed, that can be stretched, that can be squinched together.

In that view, once you start getting a massive object, something that deforms the fabric around it, that moving object can create waves just like a moving object on a normal stretchy surface can create waves. Here it just sort of comes out of looking at space geometrically and seeing masses as creating deformities in the geometry of space.

Fraser: How far could these waves propagate?

Pamela: Thatâ€™s the really cool thing. There is nothing out there to stop them. If I try to shine a flashlight from me to you, that flashlight beam is going to get stopped by the earth. Itâ€™s going to get stopped by scattering in the air. Itâ€™s going to get stopped by a lot of things. In fact light from the most distant galaxies is getting affected left and right. Its getting gravitationally bent by intervening objects. Itsâ€™ color is being affected by the dust and matter that it goes through. Itsâ€™ color is even sometimes changed by the effects of gravitational objects that it passes near.

All of these different effects alter the light that is between us and the most distant objects in the universe. Now a gravitational wave just doesnâ€™t care. Itâ€™s just going to blow through the universe, expanding and contracting everything that gets in its path. But itself, it is not going to be changed at all.

Fraser: So it doesnâ€™t care about dust. It doesnâ€™t care if itâ€™s going through a vacuum or itâ€™s going through planets. Whether itâ€™s going through an area that has high gravity like could it just pass through a black hole?

Fraser: And not even notice and not get sucked into a black hole?

Pamela: Unlike light and matter, gravitational waves are the only thing that can just blindly pass through anything.

Pamela: So it gives us a tool to find out about events on the edge of the universe that we might not otherwise have any way of knowing about.

Fraser: Okay, so it sounds like a great tool so why havenâ€™t we been able to use it so far?

Pamela: Itâ€™s that whole â€œa two meter long object is only going to get deformed about a hundred thousandth the width of a proton.â€

Pamela: Yeah, thatâ€™s real small. Itâ€™s really, really hard to detect and the problem is there are so many things that are going to interfere with our detections. Weâ€™re trying, we really are trying, but these are stubborn and elusive creatures. We have however detected them indirectly.

Fraser: Okay, well letâ€™s talk about that then. What are the ways that weâ€™ve detected it out in space? I think I know where this is going.

Pamela: Well, waves carry energy. They carry momentum with them. This is why when you get hit with a large ocean wave it actually knocks you on your butt sometimes. Itâ€™s because the energy in that wave is being transferred to you.

Now that energy had to come from somewhere and the energy in gravitational waves has to come from the systems that they are in. When we look at really high math systems they contain a couple of different objects. You canâ€™t have a gravitational wave if you just have this lone black hole hanging out spinning by itself. It has to be interacting with something that creates an asymmetry.

Fraser: Okay. And would that be because like I know that we talked about finding extra solar planets around stars, we can see the star because it is being yanked back and forth by the planet. We can see its motion this is sort of the way the wave lengths of its light is changing as the star is being pulled away and towards us.

And in the case of two compact objects, the two of them are going to be orbiting one another right? They will be moving in space or in some common point of gravity.

Pamela: Itâ€™s the motion that is so important here. If you can imagine you have a perfectly still cup of coffee and you very carefully drop cream in and you want to stir it in. The most effective way to stir it in is to take your flat spoon and move it away so that you muck up the fluid and get it moving the most.

Now if you take your pencil and put your pencil in end first and roll it between your hands like youâ€™re trying to start a fire so that the pencil is rotating about its center axis but isnâ€™t moving left, right, forward, backward, any of those things. Itâ€™s just rotating about that axis, that pencil is going to do nothing to mix up your coffee unless it really has a rough surface.

Fraser: And so we want the situation where you have two very massive objects moving very quickly in space.

Pamela: Around and around, moving the fabric of space and time around.

Fraser: All right. So we can have two neutron stars or two black holes or two white dwarves or some combinationâ€¦

Pamela: Theyâ€™re really just low energy ones. As the earth goes around the sun, weâ€™re creating gravitational waves with an energy of about three hundred watts. But thatâ€™s kind of boring and small and the sun is giving off like ten to the twenty-sixth watt thatâ€™s a one followed by twenty-six zeroes worth of watts compared to something my garage flood light gives off. So you canâ€™t really detect that.

Fraser: So what are the objects that astronomers have seen so far?

Pamela: Back in the seventies a grad student and his advisor came across an object that contained two neutron stars, one of which was a pulsar. Pulsars are fast rotating neutron stars that we can measure the rotation cycles because for reasons that we think are associated with their magnetic field they have a hot spot that flashes past us and beams light at us as many as several hundred times a second in some cases.

We can use these flashes, these pulses that make these neutrons starsâ€™ pulsars to measure the very careful, very small details of the dynamics of the system because those pulses are going to get Doppler shifted. They are going to get sped up and slowed down depending on whether the pulsar is moving toward us or away from us in its orbit. We can measure changes in the orbit by watching this over long periods of time.

What the graduate student and his advisor were able to observe was the period was actually decaying in this system. The two neutron stars were getting closer and closer to one another over time and the only way thatâ€™s going to happen is if the system is somehow radiating energy. The rate at which the stars were getting closer and closer to one another matched with what you would expect if they were radiating energy in the form of gravitational waves.

Fraser: So regular energy gets converted into gravitational wave energy that gets radiated out and causes sort of a loss of energy in the system.

Pamela: Exactly. We have these stars that are orbiting, shaking up. They are rippling the fabric of space and time.

Just as it takes energy for you or I or ripple our sheets as we are trying to spread them out over our bed, it takes energy for these high mass objects to ripple the fabric of space and time and so theyâ€™re losing energy and that energy is propagating through space causing objects to contract and expand and hopefully someday be observed here on earth directly.

Right now, all weâ€™ve observed is these systems losing energy and we have assumed that energy is getting lost to gravitational waves.

Fraser: Right, but can we see that distortion of the gravity waves that would be changing the size or the shape of the pulsars?

Pamela: We hope. We havenâ€™t done it yet but weâ€™re trying.

This is where there are these two neat gravitational twin observatories that act as a single observatory, truth be told, called LIGO. Laser Interferometer Gravitational-Wave Observatory. Itâ€™s actually a pair of different facilities one located out on the west coast and one located out on the east coast in Washington state and Louisiana state respectively here in the United States.

Each facility consists of a pair of twin arms off at right angles to one another. Inside of these arms there are tubes that have no air in them. They are complete vacuums and theyâ€™re shooting laser beams back and forth down these tubes. At the ends of the tubes the lasers can interfere with one another. When laser beams are allowed to interfere in specific ways you get nice little diffraction patterns.

You can very precisely measure the distance between where the laser is emitted, where it reflects, and where it eventually ends up getting detected by looking at these interference patterns. You hope to use the fact that you have two beams so one on one axis hopefully is going to get contracted and the other beam on the other axis is hopefully going to get expanded at the same moment and weâ€™ll be able to measure this.

In a certain period of time corresponding to the amount of time that it takes to get from Hanford, Washington to Livingston, Louisiana or visa versa, weâ€™ll see the exact same thing happen on the other coast of the United States.

Fraser: Oh, I get it. The beams are at a right angle and if the length of one of these arms gets shortened or lengthened just a slight little bit, itâ€™s going to throw off the precision of the two lasers, how theyâ€™re interacting with each other in a measurable way.

So you might get the one gravity wave passing over the one facility and then a fraction of a second later itâ€™s going to hit the second facility and in theory they should see the exact same length change in the one facility as they see in the other facility.

Pamela: Now the problem is, you also have things like UPS trucks. You have things like slight ground tremors, or all sorts of things all over the planet that are constantly causing bumps and jitters and skitters and all sorts of little motions in the system. All of these motions can wipe out the actual gravitational waves.

Fraser: I guess thatâ€™s why they have to have the two facilities. If you just have one, then any little fluctuation, any bump, Iâ€™m sure me jumping up and down over the facility would probably mess up their measurement enough to make it look like a gravitational wave. But by having the two, then they can try and see one change and then the second. So have they turned up anything yet?

Fraser: Is that just because it doesnâ€™t exist, or does that mean they havenâ€™t turned up anything because itâ€™s not sensitive enough?

Pamela: Itâ€™s a complicated issue. Theyâ€™ve been working on LIGO for a long time. It has a new set of instruments. Theyâ€™ve been working very hard to tune everything to get everything lined up and find things. But, itâ€™s hard work and theyâ€™re still working on tuning the system.

Theyâ€™re still trying to figure out how to calibrate for everything thatâ€™s happening here on the planet Earth thatâ€™s mucking with their system. And thereâ€™re going to get there, hopefully. Itâ€™s been a long ride. When I was at Michigan State University as an undergrad one of my classmates was actually a summer intern down in Louisiana working on LIGO. Theyâ€™ve been working to do this for a lot of years.

Fraser: How many events and what kinds of events were they hoping to see by now?

Pamela: It depends on where you look. Right now theyâ€™re estimating that once everything is fully operational, they should be seeing something a couple times a year. It also depends on what the universe decides to throw at us.

Fraser: Right and what kind of thing would they be seeing?

Pamela: Well, for instance when two black holes merge. Thatâ€™s going to send out an amazing, whopping amount of gravitational waves. In some cases super novas that are asymmetric can send out gravitational waves. If you have an asymmetrical disk outside of a super massive black hole, you may also be able to see that.

All sorts of different things produce specific sets of gravitational waves. The shape of the waves the distribution of the waves is very specific to the type of object you are observing. Hopefully, some of these will become apparent.

There are also theorists out there that are constantly tuning their models. Thatâ€™s the other thing that is fun to watch. The theorists come up with an idea â€œIf this happens, we should seeâ€, and then they lay out their plan and exactly what types of things will produce what size, or what type of black hole. Weâ€™re still working on honing in on that as well. Or what size, or what type of gravitational wave weâ€™re still on homing in on that.

Fraser: But even when they donâ€™t see something thatâ€™s still very interesting because that just means that this prediction might be incorrect or that prediction might be incorrect and that just lets them continue to focus their prediction.

At least theyâ€™ve got some kind of instrument that is checking that they can compare their theories against which the string theorists should be so lucky, right?

Pamela: Yeah, we are in the position where there have been a few events where Iâ€™ve seen press releases that have said, â€œLIGO did not detect something. That means the gravitational waves from this specific event that were also detected in light, could not have been bigger thanâ€ and then they give error bars.

So now we know at least the gravitational waves are smaller than a certain amount. And thatâ€™s still new information that we didnâ€™t have prior to LIGO being constructed.

Fraser: But once they did make very specific predictions on how strong these should be.

Pamela: The problem is figuring out the physics of how exactly do things merge. What are the time scales? What are the fluid dynamics of your combining two different objects that are spinning, orbiting each other, or there might be a disk of material around them. It starts to get complicated when youâ€™re not dealing with non-spinning clean black holes.

Fraser: All right. So weâ€™ve learned about LIGO and weâ€™re still waiting on the result from them, are there any other missions or any other experiments in the works?

Pamela: Well there are some other ground-based observatories. But the greatest hope that we have rests in a mission called LISA.

LISA is going to be the space-based version of that and LISA in fact stands for Laser Interferometer Space Antenna.

Fraser: So the Hubble Space Telescope is the optical detector of gravitational waves?

Pamela: Basically, except thereâ€™s not going to be a lot of optical detectors involved. All they need to be able to see is their own laser.

Fraser: So itâ€™s kind of the same thing as LIGO. Youâ€™re going to have space craft separated by a long distance pointing lasers at each other hoping to see that contraction and expansion.

Pamela: Itâ€™s a really neat system. If youâ€™ve ever played with Tinker Toys, LISA really looks like a giant Tinker Toy construction where you have these three little disks (and theyâ€™re not all that little), on orbit separated by huge amounts and each disk is connected to the other two disks in this equilateral triangle with laser beams.

What theyâ€™re looking for is any gravitational cause that will change the separation of these three spacecraft from one another. Now unfortunately, even though itâ€™s now in space, itâ€™s not as simple as you might think. There will still be stuff interfering with out ability to detect gravitational waves. This is not going to be an easy task no matter what.

Once you get into space you have to start worrying about solar particles, drift in the spacecraft over time where their orbits arenâ€™t quite as precise as we thought. The Earthâ€™s gravitational field isnâ€™t perfectly symmetric as you fly over different parts of the planet the gravitational pull is going differ from place to place. All of these different things have to be completely accounted for in trying to make sense of the data that we get out of LISA.

Fraser: Now what will be the sensitivity of LISA?

Pamela: LISA is actually going to be so sensitive that it may be able to, although itâ€™s not designed for this, actually be able to detect a continuum of gravitational waves that started back when the universe had just formed.

Whatâ€™s neat about gravitational waves is, if they were given off during the â€œBig Bangâ€ during the whole period of inflation and during the three hundred thousand years leading up to the formation of the cosmic microwave background we can detect them.

We canâ€™t detect anything before the cosmic microwave background in light because the universe was opaque. We couldnâ€™t see through it. But, gravitational waves donâ€™t care about the universe being opaque. They just go.

And so itâ€™s possible once LISA is in orbit we may see this whole range of twittering in different frequencies of gravitational waves that are a signature of how our universe was formed and thatâ€™s just kind of cool.

Fraser: It sounds like gravity waves offer astronomers a whole new way of seeing. Itâ€™s not just the whole visible spectrum, itâ€™s a brand new way to be able to see the universe and in many cases, I guess they could look at something in infrared or visible and then also look at it with gravity waves.

Pamela: Whatâ€™s so cool about this is if youâ€™ve ever looked at a pond on a really still day, you can tell when the motorboat goes by because you see the waves. You can tell when thereâ€™s a bunch of rocks between you and the motorboat because of the patterns in the waves. You can actually start to guess whatâ€™s going on out in the middle of the lake without actually having to look up and looking at the middle of the lake by looking at the waves that are hitting the shore.

Well, we can look out across the universe and see whatâ€™s going on in other parts of the cosmos without actually being able to see whatâ€™s going on in other parts of the cosmos with normal light. Itâ€™s a whole new tool.

Fraser: I talked to a researcher one time and he said it was like youâ€™re listening. That it is such a completely different way. If youâ€™re using your telescope, youâ€™re seeing and if youâ€™re using gravity waves, youâ€™re listening.

Pamela: Itâ€™s a new sense. Itâ€™s something thatâ€™s going to be really fun to play with as we begin to develop the technology to do this efficiently.

Fraser: Well, letâ€™s hope that we make a detection in the next few years and this whole new science opens up.

Pamela: The real thing to hope for is added budget for NASA. LISAâ€™s not doing so well in the budget. We hope to be able to launch it somewhere around 2015 but itâ€™s still in the formulation phase.

The money isnâ€™t there to start building a spacecraft yet so hope that NASA all around gets more funding so projects like LISA will have the funding they need to allow astronomers to explore the universe in a whole new way.

Fraser: All right, so NASA if youâ€™re listening, make sure you fund LISA. Letâ€™s see if that helps. All right, well thanks Pamela. Weâ€™ll talk to you next week.

Pamela: Sounds good Fraser, talk to you later.

This transcript is not an exact match to the audio file. It has been edited for clarity.
Transcription and editing by Cindy Leonard

## What are gravitational waves?

Researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever detection of gravitational waves on September 14, 2015. Here, a technician works on some of the optics for a LIGO detector. Image via LIGO.

Gravitational waves are ripples in the structure of spacetime. Much as a ship traveling across the surface of a calm sea leaves a wake behind it, so moving objects in the universe create gravitational waves. The “ships” in the case of gravitational waves are extremely violent and cataclysmic events far off in the cosmos: black hole mergers, neutron star collisions, supernovae. All of these generate waves in the structure of spacetime, stretching and squeezing it as the ripples travel across the universe.

Because gravitational waves are extremely weak as observed from our earthly vantage point, the technology to detect them has become available only in recent years. Like all waves, gravitational waves diminish in size with distance, shrinking to faint echoes of those distant “shipwrecks” – those distant violent events in the cosmos – by the time they reach us. From our location, many light-years from a black hole merger or a neutron star collision, the waves compress and stretch space, and everything in it, by a thousandth of the diameter of a proton as they pass through the Earth. That’s a billionth of a billionth of a meter. We require very advanced technology indeed to see that change. It’s like seeing the distance between the sun and its closest neighbor among the stars – Alpha Centauri, 4.3 light years distant – change by the thickness of a human hair.

It was Albert Einstein who, in his General Theory of Relativity of 1915, first postulated the existence of gravitational waves. His suggestion that gravity travels in waves seemed logical: every type of light on the electromagnetic spectrum, from ultraviolet to visible to radio, travels in waves. Sound travels in waves. Why should gravity not be propagated in the same way? Einstein calculated that extremely violent events in the cosmos would cause space to ring like a bell. This was distinct from the idea of the static, unchanging gravitational fields that are generated by any object that has mass, like a star or a planet.

However, for decades after 1915, Einstein himself was unconvinced of the existence of gravitational waves. In 1936, he and colleague Nathan Rosen published a paper entitled Do Gravitational Waves Exist? which, initially, was rejected by one journal because of a mathematical error.

It was the error that had caused the authors to conclude that gravitational waves don’t exist. When Einstein had corrected the error, the paper’s conclusion became exactly the opposite! Although the evidence now pointed to their existence, Einstein remained unconvinced, and believed that even if gravitational waves did exist, they would be so very weak that humans could never develop the technology to detect them.

Albert Einstein in 1912. His general theory of relavity is fundamental to modern cosmology. It was Albert Einstein who, in his General Theory of Relativity of 1915, first postulated the existence of gravitational waves.

It should be noted that Einstein was not the only theorist who worked on gravitational waves. Important contributions were made by other famous scientists, among them Robert Oppenheimer, Roger Penrose, Karl Schwarzschild, Arthur Eddington, Kip Thorne and Richard Feynman. But it was Feynman who, in January 1957, finally convinced the doubters that not only do gravitational waves do exist, but they can carry energy as well, explaining this by using something he called his Sticky Bead argument.

Feynman’s work directly paved the way for today’s gravitational wave detectors.

Yet it would be another 50 years before the first gravitational waves were detected. Developing the concepts and the technology to do so took decades of hard work by many scientists. Finally, LIGO, the Laser Interferometry Gravitational-wave Observatory situated at two sites in the United States, started observing in 2002. It took several upgrades to LIGO, between 2002 and 2015, to give it the sensitivity to make its historic first detection.

The first detection, of two black holes merging some 1.3 billion light years distant, came in September 2015 and was announced to the world in February 2016 after months of work verifying that the signal, which had lasted a mere tenth of a second in perfect agreement with Einstein’s predictions, was real. Incredibly, LIGO had not yet begun its official observing run when the detection came: after its latest in a series of upgrades to improve its range and sensitivity, LIGO had been turned on for engineering tests. The black hole merger was detected almost immediately the detector was operational.

Another key prediction of Einstein was that gravitational waves would travel at the speed of light. By measuring the difference in time between when the gravitational wave signal arrived at the two LIGO observatories – in Hanford, Washington, and Livingston, Louisiana, separated by nearly 2,000 miles (3,000 km) – scientists were able to determine that Einstein’s prediction was completely correct. Gravitational waves do indeed propagate at the speed of light.

LIGO was joined in 2018 by the European Virgo detector in Italy, which has greatly improved the ability of scientists to pinpoint the location on the sky where the gravitational waves originated. Since then, LIGO/Virgo have detected some 50 black hole mergers, but also eight neutron star collisions and six neutron star-black hole collisions. Some of these may end up being to due to so-called “terrestrial interference”: vibrations from passing traffic and even distant ocean waves can cause false positives.

On January 14, 2020, LIGO also detected an event of completely unknown origin, which does not fit any models or predictions, perhaps, excitingly, pointing to the existence of a hitherto-unknown cosmic phenomenon.

Very soon the Japanese KAGRA observatory will join Virgo and LIGO in the detection of gravitational waves. In the 2030s, the European Space Agency will launch LISA, a space-based gravitational wave detector, which should enable the detection of low-frequency gravitational waves emanating from supermassive black holes and from supernova explosions. China has started work on building three gravitational wave observatories, its avowed intent to become the world leader in Earth- and space-based gravitational wave detection.

All of the gravitational wave events detected so far agree perfectly with Einstein’s predictions and with computer simulations derived from his calculations. Einstein would surely have been amazed that he was wrong, that human intellect and ingenuity has indeed triumphed and created the technology he thought impossible. He would also probably have regretted doubting his own work in predicting the existence of gravitational waves. But he would also, surely, have been happy that the detection of gravitational waves is also a confirmation of his theory of Relativity. There are now few places left to run for those who doubt Einstein’s greatest triumph.

Gravitational-wave astronomy is a completely new science and one which promises to unlock many of the universe’s mysteries. It’s no exaggeration to say that a revolution in our view of the universe is underway. In the future, it might even be possible to detect gravitational waves from the Big Bang itself, to hear the sound of Creation ringing out across billions of years.

If you would like to keep up to date with the latest gravitational wave events, the University of Birmingham in the U.K. has created this page, which is a database of LIGO and Virgo detections during their current observing run. The database is also available as a free app for Android/Apple phones, downloadable from their respective stores.

Computer simulation of two merging black holes producing gravitational waves. Image via Werner Benger/ Wikimedia Commons.

Bottom line: First postulated by Albert Einstein in 1916 but not observed directly until September 2015, gravitational waves are ripples in spacetime.

## Where Did Those Gravitational Waves Come From? There's a Map

With today’s historic and incredibly exciting announcement of the first ever detection of gravitational waves came the news that these waves were generated by two merging black holes approximately 1.3 billion light-years away — an astrophysical event that is mind-blowing in its own right. So the next question that comes to mind is, unsurprisingly, in which direction did this black hole merger occur?

As it turns out, scientists of the Laser Interferometer Gravitational-Wave Observatory (LIGO) already have an answer, albeit a very general one.

As previously reported, LIGO is composed of two stations — one in Washington and the other nearly 2,000 miles away in Louisiana. The reason for having 2 stations is logical: should a gravitational wave pass through our volume of space (yes, these spacetime ripples pass through the Earth and us unimpeded), it must be detected by both stations to be confirmed as being &ldquoreal&rdquo and not a false positive caused by some kind of local disturbance near one of the stations. Secondly, having 2 stations allows LIGO scientists to triangulate the signal to derive a very general idea as to where these waves are coming from.

Thursday’s grand announcement pointed out that the Livingston station (Louisiana) &ldquoheard&rdquo the gravitational wave "chirp" 7 milliseconds before the Hanford station (Washington) on Sept. 14, 2015. As gravitational waves travel at the speed of light, this timing difference confirmed that the two detections were indeed the same event. Scientists were immediately able to deduce the direction the gravitational waves were traveling.

Now, the LIGO collaboration has released a map of the Southern Hemisphere skies, giving us a glimpse at the promising future of gravitational wave astronomy. In the map, contours have been added that represent the different probabilities for where the signal originated. The outermost purple line represents a 90 percent certainty that the signal’s source (the colliding black holes) is contained within that area. The innermost white contour line highlights a possible source region to a 10 percent certainty.

The band of stars through the middle of the image is the edge-on disk of the Milky Way and the Large Magellanic Cloud and Small Magellanic Cloud (two small nearby galaxies) can also be seen in the bottom portion of the image. It is worth noting that, although there is some uncertainty in the black hole’s distance, its location is far beyond our own galaxy and Local Group of galaxies.

As more gravitational wave detectors go online and their observations added to LIGO’s detections, better precision of the locations of gravitational wave sources will be pinpointed, making for highly detailed gravitational wave maps of the cosmos. So though it may not be precise, this is the first map of its kind where inside its contours two black holes merged 1.3 billion years ago.

## What causes gravitational waves?

• when a star explodes asymmetrically (called a supernova)
• when two big stars orbit each other
• when two black holes orbit each other and merge

An artist’s animation of gravitational waves created by the merger of two black holes. Credit: LIGO/T. Pyle

But these types of objects that create gravitational waves are far away. And sometimes, these events only cause small, weak gravitational waves. The waves are then very weak by the time they reach Earth. This makes gravitational waves hard to detect.

## Teaching the Activity

Each student will need a copy of the student worksheet. Students will need to view the four graphs listed above to complete the worksheet. The student worksheet provides space for the students to record their answers but not for the calculations that the questions will require. Teachers should encourage students to use additional paper to methodically record their manipulations of the equations. Such documentation will facilitate the discovery of computational mistakes. "Showing your work" is a life skill in professional science.

The student worksheet is self-contained and the teachers guide provides solutions and some additional insights for teachers. After watching Einstein's Messengers the students will be ready to undertake the activity. The teacher will need to decide how the students should do the work. If the students work individually or in small groups, the activity will probably require the majority of a 90-minute class period or most of two 50-minute periods. The amount of class time could be reduced by assigning a portion of the activity as homework.

Another key decision for the teacher is the amount of review and practice to provide on the algebra techniques required by the questions in the student worksheet. Several of the worksheet items involve manipulations of fractional exponents. Students might attack the worksheet with greater confidence and accuracy if they have reviewed exponent operations first.

## Contents

Ordinary gravitational waves' frequencies are very low and much harder to detect, while higher frequencies occur in more dramatic events and thus have become the first to be observed.

In addition to a merger of black holes, a binary neutron star merger has been directly detected: a gamma-ray burst (GRB) was detected by the orbiting Fermi gamma-ray burst monitor on 2017 August 17 12:41:06 UTC, triggering an automated notice worldwide. Six minutes later a single detector at Hanford LIGO, a gravitational-wave observatory, registered a gravitational-wave candidate occurring 2 seconds before the gamma-ray burst. This set of observations is consistent with a binary neutron star merger, [7] as evidenced by a multi-messenger transient event which was signalled by gravitational-wave, and electromagnetic (gamma-ray burst, optical, and infrared)-spectrum sightings.

### High frequency Edit

In 2015, the LIGO project was the first to directly observe gravitational waves using laser interferometers. [8] [9] The LIGO detectors observed gravitational waves from the merger of two stellar-mass black holes, matching predictions of general relativity. [10] [11] [12] These observations demonstrated the existence of binary stellar-mass black hole systems, and were the first direct detection of gravitational waves and the first observation of a binary black hole merger. [13] This finding has been characterized as revolutionary to science, because of the verification of our ability to use gravitational-wave astronomy to progress in our search and exploration of dark matter and the big bang.

There are several current scientific collaborations for observing gravitational waves. There is a worldwide network of ground-based detectors, these are kilometre-scale laser interferometers including: the Laser Interferometer Gravitational-Wave Observatory (LIGO), a joint project between MIT, Caltech and the scientists of the LIGO Scientific Collaboration with detectors in Livingston, Louisiana and Hanford, Washington Virgo, at the European Gravitational Observatory, Cascina, Italy GEO600 in Sarstedt, Germany, and the Kamioka Gravitational Wave Detector (KAGRA), operated by the University of Tokyo in the Kamioka Observatory, Japan. LIGO and Virgo are currently being upgraded to their advanced configurations. Advanced LIGO began observations in 2015, detecting gravitational waves even though not having reached its design sensitivity yet. The more advanced KAGRA started observation on February 25, 2020. GEO600 is currently operational, but its sensitivity makes it unlikely to make an observation its primary purpose is to trial technology.

### Low frequency Edit

An alternative means of observation is using pulsar timing arrays (PTAs). There are three consortia, the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA), which co-operate as the International Pulsar Timing Array. These use existing radio telescopes, but since they are sensitive to frequencies in the nanohertz range, many years of observation are needed to detect a signal and detector sensitivity improves gradually. Current bounds are approaching those expected for astrophysical sources. [14]

### Intermediate frequencies Edit

Further in the future, there is the possibility of space-borne detectors. The European Space Agency has selected a gravitational-wave mission for its L3 mission, due to launch 2034, the current concept is the evolved Laser Interferometer Space Antenna (eLISA). [15] Also in development is the Japanese Deci-hertz Interferometer Gravitational wave Observatory (DECIGO).

Astronomy has traditionally relied on electromagnetic radiation. Originating with the visible band, as technology advanced, it became possible to observe other parts of the electromagnetic spectrum, from radio to gamma rays. Each new frequency band gave a new perspective on the Universe and heralded new discoveries. [16] During the 20th century, indirect and later direct measurements of high-energy, massive, particles provided an additional window into the cosmos. Late in the 20th century, the detection of solar neutrinos founded the field of neutrino astronomy, giving an insight into previously inaccessible phenomena, such as the inner workings of the Sun. [17] [18] The observation of gravitational waves provides a further means of making astrophysical observations.

Russell Hulse and Joseph Taylor were awarded the 1993 Nobel Prize in Physics for showing that the orbital decay of a pair of neutron stars, one of them a pulsar, fits general relativity's predictions of gravitational radiation. [19] Subsequently, many other binary pulsars (including one double pulsar system) have been observed, all fitting gravitational-wave predictions. [20] In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the first detection of gravitational waves. [21] [22] [23]

Gravitational waves provide complementary information to that provided by other means. By combining observations of a single event made using different means, it is possible to gain a more complete understanding of the source's properties. This is known as multi-messenger astronomy. Gravitational waves can also be used to observe systems that are invisible (or almost impossible to detect) to measure by any other means. For example, they provide a unique method of measuring the properties of black holes.

Gravitational waves can be emitted by many systems, but, to produce detectable signals, the source must consist of extremely massive objects moving at a significant fraction of the speed of light. The main source is a binary of two compact objects. Example systems include:

• Compact binaries made up of two closely orbiting stellar-mass objects, such as white dwarfs, neutron stars or black holes. Wider binaries, which have lower orbital frequencies, are a source for detectors like LISA. [24][25] Closer binaries produce a signal for ground-based detectors like LIGO. [26] Ground-based detectors could potentially detect binaries containing an intermediate mass black hole of several hundred solar masses. [27][28] binaries, consisting of two black holes with masses of 10 5 –10 9 solar masses. Supermassive black holes are found at the centre of galaxies. When galaxies merge, it is expected that their central supermassive black holes merge too. [29] These are potentially the loudest gravitational-wave signals. The most massive binaries are a source for PTAs. [30] Less massive binaries (about a million solar masses) are a source for space-borne detectors like LISA. [31] systems of a stellar-mass compact object orbiting a supermassive black hole. [32] These are sources for detectors like LISA. [31] Systems with highly eccentric orbits produce a burst of gravitational radiation as they pass through the point of closest approach [33] systems with near-circular orbits, which are expected towards the end of the inspiral, emit continuously within LISA's frequency band. [34] Extreme-mass-ratio inspirals can be observed over many orbits. This makes them excellent probes of the background spacetime geometry, allowing for precision tests of general relativity. [35]

In addition to binaries, there are other potential sources:

generate high-frequency bursts of gravitational waves that could be detected with LIGO or Virgo. [36]
• Rotating neutron stars are a source of continuous high-frequency waves if they possess axial asymmetry. [37][38]
• Early universe processes, such as inflation or a phase transition. [39] could also emit gravitational radiation if they do exist. [40] Discovery of these gravitational waves would confirm the existence of cosmic strings.

Gravitational waves interact only weakly with matter. This is what makes them difficult to detect. It also means that they can travel freely through the Universe, and are not absorbed or scattered like electromagnetic radiation. It is therefore possible to see to the center of dense systems, like the cores of supernovae or the Galactic Centre. It is also possible to see further back in time than with electromagnetic radiation, as the early universe was opaque to light prior to recombination, but transparent to gravitational waves. [41]

The ability of gravitational waves to move freely through matter also means that gravitational-wave detectors, unlike telescopes, are not pointed to observe a single field of view but observe the entire sky. Detectors are more sensitive in some directions than others, which is one reason why it is beneficial to have a network of detectors. [42] Directionalization is also poor, due to the small number of detectors.

### In cosmic inflation Edit

Cosmic inflation, a hypothesized period when the universe rapidly expanded during the first 10 −36 seconds after the Big Bang, would have given rise to gravitational waves that would have left a characteristic imprint in the polarization of the CMB radiation. [43] [44]

It is possible to calculate the properties of the primordial gravitational waves from measurements of the patterns in the microwave radiation, and use those calculations to learn about the early universe. [ how? ]

As a young area of research, gravitational-wave astronomy is still in development however, there is consensus within the astrophysics community that this field will evolve to become an established component of 21st century multi-messenger astronomy. [45]

Gravitational-wave observations complement observations in the electromagnetic spectrum. [46] [45] These waves also promise to yield information in ways not possible via detection and analysis of electromagnetic waves. Electromagnetic waves can be absorbed and re-radiated in ways that make extracting information about the source difficult. Gravitational waves, however, only interact weakly with matter, meaning that they are not scattered or absorbed. This should allow astronomers to view the center of a supernova, stellar nebulae, and even colliding galactic cores in new ways.

Ground-based detectors have yielded new information about the inspiral phase and mergers of binary systems of two stellar mass black holes, and merger of two neutron stars. They could also detect signals from core-collapse supernovae, and from periodic sources such as pulsars with small deformations. If there is truth to speculation about certain kinds of phase transitions or kink bursts from long cosmic strings in the very early universe (at cosmic times around 10 −25 seconds), these could also be detectable. [47] Space-based detectors like LISA should detect objects such as binaries consisting of two white dwarfs, and AM CVn stars (a white dwarf accreting matter from its binary partner, a low-mass helium star), and also observe the mergers of supermassive black holes and the inspiral of smaller objects (between one and a thousand solar masses) into such black holes. LISA should also be able to listen to the same kind of sources from the early universe as ground-based detectors, but at even lower frequencies and with greatly increased sensitivity. [48]

Detecting emitted gravitational waves is a difficult endeavor. It involves ultra-stable high-quality lasers and detectors calibrated with a sensitivity of at least 2·10 −22 Hz −1/2 as shown at the ground-based detector, GEO600. [49] It has also been proposed that even from large astronomical events, such as supernova explosions, these waves are likely to degrade to vibrations as small as an atomic diameter. [50]