Last year astronomers made the first detection of gravitational waves from the merging of two black holes. It gave us an entirely new way to view the cosmos. Now we aren’t limited by the emission and absorption of light by matter. We can explore the universe through ripples in the fabric of spacetime itself. Through recent observations we can study the most mysterious aspect of spacetime, known as dark energy. 

Dark energy is what causes the universe to expand. It makes up about 70% of the universe, but we don’t really know what it is. One reason for this is that we don’t know exactly how much it expands the universe. Cosmic expansion is typically defined in terms of the Hubble parameter H₀. Because the universe expands, more distant galaxies appear to be moving away from us faster than closer galaxies. The velocity of a distant galaxy is related to its distance by v =H₀d. We can measure the speed of a galaxy through the redshift of its light. The greater the galaxy’s speed, the more its light is shifted toward longer (red) wavelengths.

Various methods used in the cosmic distance ladder. Credit: Wikipedia

Knowing the distance of a galaxy and its observed redshift, we can determine the Hubble parameter. When we do this for lots of galaxies, we get a value of about H₀ = 67.6 (km/s)/Mpc. But there is a catch. We can’t measure the distances to the furthest galaxies directly. We use what’s known as the cosmic distance ladder, where we use one type of measurement to determine the distance of nearby stars, use that result with other observations to measure distances to close galaxies, and use that result to measure more distant galaxies. Each step in the ladder has its own advantages and disadvantages, and if one rung in the ladder is off, it throws off all the other ones.

Fortunately, we have other ways to measure the Hubble parameter. One of these is through the cosmic microwave background. This remnant echo of the big bang has small fluctuations in temperature. The size of these fluctuations tells us the rate of cosmic expansion (among other things). Observations by the Planck satellite gave a Hubble parameter value of about H₀ = 67.7 (km/s)/Mpc.

But other methods of measuring the Hubble parameter give slightly different results. For example, one method looked at how light is gravitationally lensed by distant galaxies. Gravitational lensing can create multiple images of distant supernovae, and since each image takes a different path around the galaxy, they arrive at different times. The timing of these images can be used to determine the Hubble parameter, and the result is about H₀ = 71.9 (km/s)/Mpc. A different method using distant supernovae gives a result as high as H₀ = 73 (km/s)/Mpc. So what is the real value of the Hubble parameter?

This is where gravitational waves come in. All of the measurements of the Hubble parameter so far rely upon observations of light. Gravitational waves provide us an entirely new method to measure cosmic distances. As two black holes or neutron stars begin to merge, they spiral ever closer to each other, creating gravitational waves we can detect. The frequency of these waves depends upon their masses, and their masses determine how much energy they produce when they merge. By comparing the energy they produce with the strength of the gravitational waves we observe, we know their distance. This is similar to the way standard candles are used in optical astronomy, where we know the actual brightness of a star or galaxy, and compare it to the observed brightness to determine distance. In fact, this new method has been termed a standard siren.

But distance isn’t enough to determine the Hubble parameter. We also need to determine its speed away from us. We aren’t able to measure the redshift of gravitational waves, so we can’t used them to measure speed. But when two neutron stars merge they produce both gravitational waves and light. For one such merger, we not only observed the light produced, but also its redshift. From that we can find the Hubble parameter. Since the distance is found directly from gravitational waves, it doesn’t rely upon the cosmic distance ladder or an assumed model of cosmic expansion. From this event it found H₀ = 70 (km/s)/Mpc.

While that result points to a larger Hubble parameter, the uncertainty of the result is really large. Based on the data, it could be as large as 82 or as small as 62. But this is only one measurement. As more mergers are observed, we will get more precise results. So gravitational waves will help us pin down the Hubble parameter. It’s only a mater of time, and space.

Paper: The LIGO Scientific Collaboration, et al. A gravitational-wave standard siren measurement of the Hubble constant. Nature 551, 85–88. doi:10.1038/nature24471 (2017)

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During the Archean Eon of planet Earth, when life was figuring out how to harness energy from the Sun, two black holes in a distant galaxy merged with a ripple of gravitational waves. Over the next 2.9 billion years these ripples traversed a vast and empty space, while on Earth a plucky little species of bipeds learned to use lasers and mirrors to measure gravitational vibrations smaller than the nucleus of an atom. When the gravitational ripples reach Earth, they become humanity’s third detection of merging black holes. 

With this third detection of gravitational waves, named GW170104, gravitational astronomy is coming into its own. Like the previous mergers, the initial black holes were stellar mass black holes (19.4 and 31.2 solar masses, respectively) and they merged to become a 48.7 solar mass black hole, releasing about 2 solar masses worth of energy as gravitational waves. This is similar to the other two mergers we’ve detected, and confirms that stellar mass black holes can be produced with a mass larger than 20 Suns. Observations of x-rays near black holes had previously demonstrated they could have masses between about 5 and 15 solar masses. The size of these mergers also support the existence of medium sized black holes, between the stellar mass size and the supermassive size seen in the centers of galaxies.

The gravitational waves of three confirmed black hole mergers, and one tentative merger. Credit: Credit: LIGO/B. Farr (U. Chicago)

This latest detection is also the most distant black hole merger we’ve observed, happening about 3 billion light years away, which is more than twice as distant as the previous mergers. This greater distance allows us to test Einstein’s theory in new ways, particularly a quantum aspect known as gravitons. In quantum theory, the forces between particles are caused by field quanta. For electromagnetism, these are photons. For the strong nuclear force, they are gluons. For gravity, the field quanta are known as gravitons. Gravitons are the only field quanta we haven’t observed. Gravity is the weakest of the four fundamental forces, so to directly observe a graviton you’d need something like a Jupiter-mass detector orbiting a neutron star. We aren’t likely to do that any time soon. But we do understand the theory of gravitons pretty well. One of the key predictions of relativity is that gravitons should be massless, like photons. As a result, gravitational waves should always propagate at the speed of light. With this new merger we can test this idea through a property known as dispersion.

Dispersion occurs when waves originating from the same source travel at different speeds. You can see this in a prism, where sunlight is spread out into a rainbow of colors. This is caused by the fact that the speed of light through glass varies with wavelength or color. A similar effect occurs in astronomy, when radio waves travel through the ionized plasma of interstellar space. We can actually use this effect to measure the distribution of gas and dust in our galaxy, for example. Light undergoes dispersion when traveling through plasma because the charged particles of the plasma interact strongly with light. When light travels through unionized gas, there isn’t any dispersion.

Since gravitational waves don’t interact strongly with other masses, there shouldn’t be any dispersion as they travel through the vacuum of space. However there is another way that dispersion might occur. If gravitons have mass, then gravitons with different energies would travel at different speeds. Over the course of 3 billion light years this dispersion would be big enough to observe. The latest merger showed no evidence of dispersion, which means gravitons (assuming they exist) appear to be massless. General relativity passes yet another test.

The next step for gravitational astronomy is to bring more detectors online. With the limited data we have, we can’t pin down the exact location of these mergers in the sky, so we can’t connect a merger event to things seen in the optical spectrum. More detectors will also let us gather more information about the rotation of black holes before their mergers, which will let us further test general relativity. We are still at the beginning stages of an entirely new field of astronomy, but already the power of this new field is starting to show.

Paper: B. P. Abbott et al. GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2. Phys. Rev. Lett. 118, 221101 (2017) DOI: 10.1103/PhysRevLett.118.221101

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Gravitational waves are produced when black holes, stars or planets orbit each other (among other things). As these masses move around each other, they create gravitational disturbances that radiate outward. While we’ve only recently detected gravitational waves directly, we’ve known the exist for decades because of a secondary effect known as inspiraling. As gravitational waves radiate away from two orbiting masses, they carry a bit of energy with them. As a result the two masses lose a bit of energy and move closer to each other. Over time they will spiral ever closer until they collide. 

The direct detection of gravitational waves showed how two black holes entered such a death spiral, merging to become a single black hole. We’ve also observed the inspiraling of a pulsar orbiting a companion star. The two haven’t collided yet, but will eventually collide in about 300 million years. So what about smaller masses? As the Earth orbits the Sun, does it produce gravitational waves? Does the Moon? Yes, but the effect is extremely tiny, so we can’t observe it directly.

Take the Earth, for example. The amount of gravitational energy lost by two masses in a circular orbit is pretty easy to calculate, and for the Earth and Sun it comes out to be about 200 watts. That tiny loss of energy means that the Earth moves closer to the Sun by about the width of a proton each day due to gravitational waves. Of course gravitational waves aren’t the only thing that affects Earth’s orbit. Because the Sun is radiating away about 5 million tons of mass every second, the gravitational attraction of the Sun is decreasing, the Earth actually moves away from the Sun by about 1.6 centimeters per year. Even that tiny effect is far more significant that the inspiraling effect of gravitational waves.

Technically all orbiting masses generate gravitational waves, so everything is inspiraling over time. But unless the masses are really large, such as stars and black holes, the effect is negligible. As far as planets are concerned, we can effectively ignore it.

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Now that gravitational waves have been observed, the race is on to design better and more sensitive gravitational telescopes. The LIGO telescope measures gravitational waves by precisely measuring the distance between reflectors. As gravitational waves pass through LIGO the distance changes very slightly. One way to improve over LIGO is to create a more sensitive telescope in space following similar designs, such as the proposed eLISA mission. But there are other ideas that are also worth considering, such as designs using atomic clocks. 

An alternative design for gravitational wave detection. Credit: Kolkowitz, et al.

While atomic clocks can measure time very precisely, they can also measure the frequency of laser light very precisely. If two satellites containing atomic clocks were put into a common orbit, laser signals from each satellite could be measured by the atomic clock in the other. If a gravitational wave passed by, it would cause a small oscillation between the satellites, which could be seen an a periodic Doppler shift of the laser signals.

One advantage of such an experiment is that it could be tuned to gravitational waves of a particular frequency, rather than having a range of frequencies such as LIGO. Such a narrow band sensitivity would make it a poor detector of black hole mergers, but it could detect gravitational waves from periodic sources such as binary neutron stars. In a recent paper outlining the idea, the authors propose such atomic clocks could be included in an eventual eLISA mission.

Right now this is just an idea, but in the new world of gravitational wave astronomy, a lot of ideas could soon become reality.

Paper: S. Kolkowitz, et al. Gravitational wave detection with optical lattice atomic clocks. arXiv:1606.01859v1. (2016)

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On 26 December 2015 the LIGO observatory saw another merger of two black holes. This time the black holes were smaller, with masses of about 8 and 14 Suns. As a result, we captured the inspiraling of the black holes for a longer time. Gravitational wave astronomy is now fully under way. 

The cleaned up observation of the black hole merger. Credit: LIGO

Because of the size and distance of this merger (about 1.4 billion light years) this particular merger is fainter than the first. It was seen a periodic fluctuation buried within the LIGO noise, so the data has to be matched to computer simulations to really determine its properties. It’s statistical validity dances around the usual five sigma range, so there is no doubt the signal is real.

More than the first merger, this is a textbook example of a merger. We captured nearly thirty orbits of the two black holes as they danced ever closer to each other. We can see not only the steady gravitational waves of their orbits, we can also see how their orbital periods get shorter, orbiting ever faster as they approach the merger. This is textbook behavior. It is exactly the type of event we expected to see. The merging of stellar-mass binaries.

Overall this new observation is further confirmation not only of general relativity, but of central aspects of astrophysics. Black holes are real, they occasionally merge just as we predicted, and we can now start using gravitational waves as an astronomical tool.

Paper: B. P. Abbott et al. GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence. Phys. Rev. Lett. 116, 241103 (2016)  DOI: 10.1103/PhysRevLett.116.241103

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The results of gravitational waves from LIGO are still warm off the press, and the race is on for the next generation of gravitational wave detectors. 

Gravitational waves are detected by carefully measuring the distances between reflecting mirrors. As a gravitational wave passes through a detector, the distances shift slightly due to the warping of space by the gravitational wave. Of course on Earth there are all sorts of things that can cause the mirrors to shift, from minor tremors to the rumbling of a nearby truck. Most of the “noise” is actually larger than the signal you want to detect. It is like trying to perform a careful experiment where every now and then someone walks by and starts pounding on your lab table.

There are ways to reduce the level of noise, and ways to distinguish a real gravitational signal from other things, but all those background events limit the sensitivity of LIGO. The obvious solution is to simply put the whole thing in space. Floating in space, there aren’t any ground vibrations to bother you. Problem solved!

Except it isn’t quite so simple. On Earth the LIGO detectors are assembled within a rigid structure, which is easy to do when you’re on the ground. We can’t do the same thing in space. For example, LIGO’s mirrors are spaced about 4 kilometers apart. To make a rigid structure in space would require the construction of the largest space-based object by far. Even if we did construct such a monstrosity, gravitational torsion and the heating and cooling of the support structure would create far more noise than even LIGO has.

A better idea is to simply let each mirror float freely in space. That way the system will simply orbit the Earth or Sun in a predictable way, and any short-term deviation will be due to a gravitational wave disturbance. At least that’s how it should work, but we couldn’t be sure until it’s tried. Enter LISA Pathfinder.

LISA Pathfinder’s initial noise tolerance exceeds expectations by a factor of 100. Credit: Spacecraft: ESA/ATG medialab; data: ESA/LISA Pathfinder Collaboration

Pathfinder was designed as a proof of concept. It is a single spacecraft containing two blocks about a meter apart. Reflecting lasers of the block allows us to measure how far apart they are. The blocks are designed to float freely within the spacecraft, and were enclosed in gas-filled containers to dampen any vibrations once in orbit. The key test of Pathfinder was to determine just how much of an issue background noise would be. What was found is that the noise is much smaller than expected. In fact, at key frequency ranges, the noise limit was primarily due to the thermal noise (brownian motion) from the gas surrounding the blocks. Over time this noise would die down as gas is purposely leaked away from the containers.

This is a great result, and it’s a big step toward a new era of space-based gravitational astronomy.

Paper: M. Armano et al. Sub-Femto-gg Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results. Phys. Rev. Lett. 116, 231101 (2016). doi: 10.1103/PhysRevLett.116.231101

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While the evidence for black holes is pretty conclusive, the extreme nature and odd physics of black holes has encouraged skepticism about their existence in certain circles. While highly dense objects exist, they argue, that doesn’t mean such objects are black holes. On some level they have a point, because there are proposed objects that are black-hole like, but not true black holes, such a gravastar. 

A gravastar is an extremely dense object where the behavior of quantum gravity somehow kicks in to prevent the formation of a singularity and event horizon, which are the most contentious aspects of black holes. In some models it is assumed there is a minimum scale (Planck length) where gravity stops functioning in the usual way, while in others dark energy kicks in at small scales to prevent the formation of a true black hole. In either case a gravastar would look quite similar to a black hole.

LIGO’s detection of a black hole merger. Credit: LIGO

When gravitational waves were detected this year, it was seen as definitive proof of black holes. The gravitational “chirp” and ringdown detected by LIGO was an exact match of a black hole merger, and even allowed us to determine the masses of the initial and final black holes. It confirmed the existence of gravitational waves, which was the last great prediction of general relativity. Since general relativity predicts black holes quite clearly, the result is pretty definitive. But it is true that black holes should depend upon quantum gravity, which we don’t yet fully understand. If quantum gravity resulted in gravastars, would the LIGO detection look any different? It turns out the answer is yes, but not in a way we can currently detect.

According to the models, gravastars are so dense they have collapsed almost to the point of being a black hole. The merger of two gravastars would still have a chirp and ringdown of gravitational waves. The ringdown of a gravastar merger would differ slightly from that of black holes, but only at the tail of the ringdown. Of course that part of the ringdown seen by LIGO is buried in the background noise of the data. Thus, the gravastar supporters would argue, the LIGO event detected either a gravastar or black hole merger, but can’t distinguish one from the other.

Does that mean the existence of black holes is in limbo? Personally I don’t think so. While gravastar models argue against black holes, there’s no compelling argument for gravastars. While they do resolve certain theoretical conundrums black holes have, gravastar models have problems of their own. Not the least of which is the fact that they depend upon heuristic arguments of quantum gravity that may or may not be valid. So on the whole I don’t find the gravastar model particularly compelling. There’s also the risk of playing the denialism game regarding black holes, where no amount of evidence will ever be seen as sufficient. To be clear, I don’t think gravastar supporters are playing the denialism game. It is good to be skeptical of new work, and the gravastar model is one way to test the limits of our observations.

That’s all part of the challenge of doing science.

Paper: Vitor Cardoso, et al. Is the Gravitational-Wave Ringdown a Probe of the Event Horizon? Phys. Rev. Lett. 116, 171101 (2016) arXiv:1602.07309 [gr-qc]

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The discovery of gravitational waves from two merging black holes has raised a number of questions about what would happen if two black holes merged near our solar system. While the real answer is complex, we can do a back of the envelope calculation. 

The observed merger released three solar masses of energy as gravitational waves in a fraction of a second. That’s a huge amount of energy, but it was released 1.3 billion light years away from us. Like light, the energy of a gravitational wave decreases by the square of the distance, so very little energy actually reached us. The basic design of advanced LIGO is a set of mirrors spaced 4 kilometers apart. When the gravitational waves passed through LIGO, the separation of the mirrors shifted by less than a hundredth of a proton’s width, or one part in 10²¹.

The amount of shift caused by a gravitational wave is due to its amplitude, not its energy. While the energy of gravitational waves follow the inverse square relation, the amplitude of gravitational waves follows the inverse distance relation. In other words, if we were half as far away from the merger we’d have seen four times the energy, but only twice the shift. As long as we aren’t too close to the merger where things become complicated and nonlinear, this relation will give us a good idea of just how strongly the gravitational waves will affect us.

For example, suppose the two black holes were a billion times closer. At a distance of 1.3 light years the gravitational waves of the merger would be a billion times greater, raising the shift to one part in 10¹². The arm of advanced LIGO would have shifted by four nanometers, or about half the width of a hydrogen atom. Huge by optical standards, but not really noticeable. The entire Earth would shift in diameter by about a hundredth of a millimeter. Such a shift might trigger some seismic activity that was on the edge of happening anyway, but it wouldn’t be the end of the world. If we put the black holes even closer, their mass alone would start to disrupt the Oort cloud, regardless of any gravitational waves. So we can safely say that merging black holes will never have a serious effect on us.

But just for fun let’s look at how close we could get. Before the merger the two black holes have a diameter of about 212 and 170 kilometers respectively. After the merger the final black hole has a diameter of about 365 kilometers. If we were really close to the black holes, the tidal forces alone would kill us, so let’s assume we’re at least 10,000 kilometers from ground zero. At that distance the shift caused by the gravitational waves would be about one part in a thousand. If you were floating in space you would likely feel that, since a person would experience a shift of a millimeter or two. Would it hurt, or possibly harm you? That’s hard to say. It would really depend on how resilient humans are to gravitational wave distortion, and we don’t have any experimental data on that. If I were to guess I’d say as long as your space suit held up you’d be fine.

If you were really 10,000 kilometers from two orbiting solar mass black holes their gravity would pose a much greater threat than any gravitational waves. While gravitational waves can carry a great deal of energy, they only interact weakly with matter. In many ways it’s amazing that we can detect gravitational waves at all.

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LIGO announced that they have detected gravitational waves from a black hole merger. If verified it will be the ultimate confirmation of Einstein’s theory of general relativity. But what are gravitational waves, and why is their detection such a big deal? 

Water is perhaps the most common example of a wave. Drop a stone into a calm pond and you can see ripples expand over the surface of the water. This occurs because water is a fluid. When the stone is dropped into the water, it pushes the water around it out of the way. The water closest to the stone is pushed into the surrounding water, causing it to bunch up a bit. As the bunched up water tries to go back to its original state, it pushes into water further out. Thus a ripple moves through the water.

This is the basic process of any wave. A disturbance in a material affects the region around it causing the disturbance to move through the material. Thus we have ripples in water, sound waves in air, and even seismic waves from earthquakes. Since it takes time for the disturbance to move through the material, waves move through the material at a finite speed.

For a long time it was thought that waves could only move through a physical material, like sound through air. When it was shown in the 1600s that light travels at a finite speed, it sparked much debate over whether light was made of “particles” as Newton suggested, or whether it was a wave traveling through some material. In the early 1800s experiments showing light’s wave behavior seemed to answer the question in favor of waves. We now know light’s behavior is more subtle than that, but in the 1800s it seemed clear that light was definitely a wave. So there must be a material through which light propagates.

Light passing through two small slits creates an interference pattern, proving the wave behavior of light. Credit: Wikipedia user Jordgette.

The most popular candidate was known as the luminiferous aether. The difficulty with the aether was that it would have to be completely invisible and insubstantial to physical objects, so there was no way to detect it directly. The only evidence that the aether existed came from the fact that light travels in waves. Odd as the aether was, it clearly had to exist since waves always move through some material.

Then in the late 1800s it was found that the speed of light was always the same regardless of ones motion through the aether. This is a deeply un-wavelike behavior. If light really moved through the aether at a particular speed, the Earth’s motion through the aether should make the speed of light appear faster or slower at different times of the year. An unchanging speed of light meant our assumption about the aether must be wrong.

If the Earth moved through the aether, we could measure its effect. Credit: Wikipedia

In the early 1900s it was shown that light waves could be explained through special relativity. Instead of moving through a material, the energy fields of electricity and magnetism could be disturbed like a fluid. Light waves are thus waves of electromagnetic energy moving through space. Because physical objects are made of charged particles that have electromagnetic fields, this meant they could never travel through space faster than the speed of light. Relativity showed us that waves didn’t need a physical material to travel through. Waves could move through fields of energy.

But if physical objects couldn’t move faster than light, what about gravity? In Newton’s model of gravity, masses exert forces on each other instantly. As a result, energy could pass from one mass to another with infinite speed. It seemed odd that light energy could only travel at the speed of light while gravitational energy could travel instantly. This puzzle led Einstein to develop a general theory of relativity.

The basic idea of special relativity is that no frame of reference can be favored over any other. This allows the speed of light to be the same in all frames of reference, but it does so by making the behavior of space and time relative to the observer rather than an absolute background. Even the concept of “now” is relative. In general relativity the central idea is the principle of equivalence. Since all bodies fall at the same rate regardless of their mass, a body floating freely in space must be equivalent to a body falling freely. As a result, gravity is not a force between masses, but rather an effect of spacetime curvature. Under Eisntein’s model space and time become flexible and relative, and take on a fluid-like behavior. Masses moving through space should create disturbances in spacetime, just as running your hand through water creates ripples. If general relativity is correct, then there must be gravitational waves.

A visualization of gravitational waves. Credit: NASA

Over the years various tests of general relativity have confirmed the theory works, and so it has widely assumed that gravitational waves exist. But observing gravitational waves directly has been notoriously difficult. Even the strongest of gravitational waves would be extraordinarily weak, and since they are a warping of spacetime itself, effects such as the finite speed of gravity are impossible to measure. The best evidence we’ve had so far has been indirect evidence. In the 1970s observations of a pulsar orbiting another star found that it slowly spirals closer to its companion. According to relativity this is because gravitational waves radiate away from the binary system, causing it to lose energy and spiral closer together.

But without a direct observation of gravitational waves, there is the chance that general relativity could be wrong. We could, for example, come up with a model that gives us all the effects of Einstein’s theory without gravitational waves. It wouldn’t be as elegant as general relativity, but it would work. Gravitational waves are an absolute necessity for general relativity, and if they don’t exist the model is wrong. So even though we expect gravitational waves to exist, without proof there would always be a small bit of doubt about relativity.

That’s why we’ve been looking for gravitational waves for so long, and that’s why the result is so important.

This post originally appeared on Forbes.

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Two black holes circle each other in a gravitational dance. Spiraling closer over thousands of years, they eventually get so close that they can no longer keep dancing. In a fraction of a second these two black holes merged into a single, larger black hole. It’s an event that happens fairly regularly throughout the universe. But this time, a group of humans 1.3 billion light years away measured the ripples in space and time produced during the merger. 

It’s hard to overstate the significance of our first direct detection of gravitational waves. On the one hand the discovery announced in Physical Review Letters confirmed what we’ve suspected for decades: gravitational waves exist. By itself that’s not a big deal, since they are a natural result of general relativity, and we’ve had indirect evidence of gravitational waves since the 1970s. The direct detection of gravitational waves is yet another confirmation of what we’ve already known. On the other hand, this opens up an entirely new window to the universe.

The event as seen in the two LIGO detectors (above) compared to the numerical model fits (below). Credit: B. P. Abbott et al.

The paper released today has been peer reviewed, which is comforting given the BICEP2 incident. It’s also a remarkably strong result given the extreme sensitivity necessary to detect gravitational waves. The advanced LIGO experiment consists of two detectors located in Louisiana and Washington. To qualify as a real detection, there must be a nearly simultaneous event in both detectors with the same basic form. In the above image, the event in question matches up quite well. It also matches the expected signal as calculated from numerical simulations of merging black holes. This is a strong, clear signal confirming gravitational waves.

The data is good enough that we actually know quite a bit about the merging black holes. The larger black hole had a mass of about 36 Suns, while the smaller one had a mass of about 29 Suns. When the two black holes merged they formed a single black hole of about 62 Suns. You might notice those numbers don’t add up. That’s because in the process of merging, about 3 solar masses worth of energy was radiated away as gravitational waves. That’s a huge amount of energy to release in a fraction of a second, which is why we can detect it so clearly from more than a billion light years away. We also know some broader characteristics, such as how fast the final black hole rotates, roughly where in the sky the merger occurred and the cosmological redshift of the event (which is how we know its distance).

While the detection of gravitational waves is the biggest news, this is also further confirmation that black holes are real. If they weren’t black holes their merger would create a burst of light or neutrinos like a stupendous supernova, which wasn’t seen. This is also the first clear observation of a black hole merger.

In the history of human civilization humans looked up at the sky and saw light. When Galileo first raised his telescope to the sky he saw light. Over the centuries we’ve widened the range of what light we can observe. We’ve launched telescopes into space to see types of light not visible from the ground. With the exception of some neutrinos and cosmic particles, the field of astronomy is rooted in our ability to observe and analyze light.

But now we can listen to the very fabric of space and time.

Paper: B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration).Observation of Gravitational Waves from a Binary Black Hole Merger. Phys. Rev. Lett. 116, 061102 (2016)

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Gravitational waves is the last major prediction of general relativity to be directly observed. We have indirect evidence of gravitational waves through phenomena such as binary pulsars, but so far attempts at direct observations have yielded nothing. Currently the main effort to detect these waves focuses on the Laser Interferometer Gravitational-Wave Observatory (LIGO), which uses laser interferometry to measure tiny shifts in the position of masses. The LIGO uses laser interferometry along a path 4 kilometers long, but even then, the expected distortion would be about a billionth of a nanometer. This is about the same level as the background noise of LIGO itself, so finding a gravitational signal in the noise is difficult at best.

The eLISA project would use interferometry across three spacecraft.

One of the major problems with LIGO is that it is ground based. Any vibration of the ground, such as a truck driving on a road miles away, can cause noise in the signal. A better alternative would be to put the LIGO project in space. This is the idea behind the Evolved Laser Interferometer Space Antenna (eLISA), which would precisely measure the position of masses orbiting in space. While the project isn’t scheduled to launch until at least 2034, tomorrow the first test project will launch. Known as LISA Pathfinder, the spacecraft will put two masses in free fall about 38 centimeters apart.

You might think the spacecraft itself will be in free fall, so what’s the big deal. In this case the two masses need to be completely untouched by the spacecraft. While the masses are in free fall, the spacecraft will adjust its position to stay around them. A laser interferometer housed in the spacecraft will measure the relative positions of the two masses to within a hundredth of a nanometer.

Making such precision measurements in space is a big challenge, and LISA Pathfinder is an important step toward (hopefully) measuring gravitational waves.

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Gravitational waves are the last great prediction of general relativity to be directly observed. According to Einstein’s theory of gravity, a mass in motion creates ripples in space and time. Ripples caused by large masses like binary neutron stars or black holes should be large enough for us to detect across light years. But despite recent rumors, no gravitational waves have been detected so far.

We’ve had indirect evidence of gravitational waves for some time. In 1974 Russell Hulse and Joseph Taylor observed the motion of a pulsar orbiting another star. Pulsars are rapidly rotating neutron stars that emit radio pulses in our direction. The timing of these pulses is very precise, so they can be used to measure the pulsar’s position and motion extremely accurately. What they found was that the orbit of the pulsar degraded over time as energy radiated away from the system. The rate of this energy decay matched the predictions of general relativity, confirming the existence of gravitational waves.

The gradual decay of a binary pulsar orbit due to gravitational waves.

Around this same time, Joseph Weber attempted to detect gravitational waves directly. He noted that as gravitational waves pass through an object it is squeezed and stretched very slightly. Once distorted, the object would spring back to its original shape, which would create small sound waves within the object. Although these sound waves would be small, Weber calculated that they would be detectible with piezoelectric sensors mounted on a large aluminum cylinder. The problem is that gravitationally induced sound waves would be small enough to be lost in the noise of thermal vibrations naturally occurring in the aluminum cylinders. So Weber used two cylinders spaced about 1,000 km apart. A true gravitational wave would create a sound wave event in both detectors, unlike random thermal noise.

When Weber first performed the experiment, it seemed to work. He found about two dozen coincident events within a three month period. Within a year he claimed to have found a few hundred detection events. A slight difference in the timing of the events at each detector also seemed to imply the gravitational waves were coming from the center of our galaxy. This was big news, since it could imply a binary black hole in the center of our galaxy.

Soon other teams were building their own aluminum cylinders and undertaking their own gravitational wave experiments. But these results found only random noise. By the end of the 1970s it was generally thought that Weber had not actually detected gravitational waves. So why did Weber think he had?

The effect of a gravitational wave.

It seems to stem from the subtlety of statistics. Suppose you and a friend each roll a dice a hundred times and write down your results. When you compare your results, you’d probably find that for some rolls you had the same outcome, such as both rolling a 3 on your 15th roll. You might even find two or three in a row that matched. While they would look like “simultaneous events” they occur because of random chance. The same thing can occur with Weber’s detectors, where random fluctuations happen to match by chance. Distinguishing a real signal from randomly paired events is hard, and it turned out that Weber’s analysis wasn’t sophisticated enough to distinguish the two.

While Joseph Weber’s experiment failed to detect gravitational waves in the end, the project did stir up interest in gravitational wave detection. Modern efforts to detect gravitational waves such as LIGO have expanded on Weber’s idea of measuring motion induced by gravitational waves. Which just goes to show that even a failed experiment can be a success.

Paper: J. Weber. Gravitational-Wave-Detector Events. Phys. Rev. Lett. 20, 1307 (1968)

This post originally appeared on Forbes.

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