Einstein’s theory of gravity has been tested in lots of ways, from the slow precession of Mercury’s orbit, to the detection of gravitational waves. So far the theory has passed every test, but that doesn’t necessarily mean it’s completely true. Like any theory, general relativity is based upon certain assumptions about the way the universe works. The biggest assumption in relativity is the principle of general equivalence. 

The equivalence principle was proposed by both Galileo and Newton, and basically states that any two objects will fall at the same rate under gravity. Barring things like air resistance, a bowling ball and a feather should fall at the same rate. Experiments that have tested the principle of equivalence show it’s a good approximation at the very least.

In Newtonian gravity, this just means that the gravitational force on an object is proportional to its mass, so even if the equivalence principle is only an approximation we could still use Newtonian gravity. But Einstein’s theory of relativity, gravity isn’t a force, but simply an effect of the warp and weft of spacetime. In order for this to be true, the equivalence principle can’t be approximately true, it has to be exactly true. If objects “fall” due to the bending of space itself, then everything must fall at the same rate, because they are all in the same spacetime.

But there’s an interesting twist to this principle. One of the things relativity predicts is that mass and energy are related. This is where Einstein’s most famous equation, E = mc², comes into play. Normally the “relativistic mass” of an object is effectively the same as its regular mass, but objects like neutron stars have such strong gravitational and electromagnetic fields that their relativistic mass is a bit larger than the mass of their matter alone. If the gravitational force on an object is proportional to its mass-energy, then a neutron star should fall slightly faster than lighter objects. If Einstein is right, then a neutron star should fall at exactly the same rate as anything else.

A few years ago, astronomers discovered a system of three stars orbiting closely together. Two of them are white dwarf stars, while the third is a neutron star. The neutron star is also a pulsar, which means it emits regular pulses of radio energy. The timing of these pulses are determined by the rotation of the neutron star, which is basically constant. Any variation in the timing of the pulses is therefore due to the motion of the neutron star in its orbit. In other words, we can use the radio pulses to measure the motion of the neutron star very precisely.

Each of the stars in this system is basically “falling” in the gravitational field of the others. Recently a team of astronomers observed this system to see if the neutron star falls at a different rate different from Einstein’s prediction. Their result agreed with Einstein. To within 0.16 thousandths of a percent (the observational limit of their data) the neutron star falls at the same rate as a white dwarf.

Once again, Einstein’s gravitational theory is right.

Paper: A. Archibald et al. Testing general relativity with a millisecond pulsar in a triple system. 231st meeting of the American Astronomical Society, Oxon Hill, Md. (2018)

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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 general relativity is a strongly verified model of gravity, it does have some theoretical problems. One of the biggest is the fact that it can’t be quantized in the way that other models can. As a result, the current state of physics has quantum theories for the fundamental forces of electromagnetism, and the strong and weak interactions, while gravity remains stuck in a largely classical regime. As a result there have been a great deal of effort to develop a quantum theory of gravity. Perhaps the most famous effort is string theory, but an alternative is an extension of general relativity known as conformal gravity. 

An example of a conformal transformation. Credit: MathGifs.

At a basic level, general relativity is based upon a set of mathematical symmetries. Starting with the equivalence principle, which states that all objects fall at the same rate regardless of their mass, relativity proposes that the laws of physics should be the same regardless of your frame of reference. If you move from one position to another (translation), change your orientation (rotation) or change your speed (Lorentz boost), the laws of physics should be the same. In mathematics we would say our equations are invariant under these kinds of transformations. A basic consequence of these symmetries is that the vacuum speed of light is the same in all reference frames. Conformal gravity adds another symmetry known as the conformal transformation. An example of a conformal transformation would be to warp a two-dimensional sheet while preserving the angles of your coordinate grid. The upshot of such a transformation is that it preserves all the local properties of regular relativity while allowing a change in the global structure. By adding this symmetry, the hope of conformal gravity is that you could map the spacetime of relativity into a form that could be quantized, thus producing a quantum theory of gravity.

Because conformal gravity is an extension of general relativity, it keeps Einstein’s theory as an approximation. Conformal gravity agrees with all the usual tests of relativity such as Mercury’s orbital precession, the gravitational deflection of light, and gravitational redshift. However its global structure is different, which gives it testable predictions. For one, conformal gravity doesn’t allow for a cosmological constant. This would mean dark energy could not be a property of spacetime, but instead would be caused by an energy field. It’s also thought that the model could also account for dark matter. From the perspective of conformal gravity, “dark matter” is a property of spacetime’s global structure rather than some kind of invisible stuff.

Given that conformal gravity provides a path to quantum gravity and accounts for dark matter and dark energy without the introduction of an invisible something, what’s not to love? To begin with, there are actually several varieties of conformal gravity. Depending on how you introduce conformal symmetry you can get different forms of the model. Each has some advantages and disadvantages. There’s no one clear model that addresses all the issues with general relativity. There also isn’t much observational evidence to support it. Currently, observations of cosmic expansion agree with the cosmological constant model of general relativity (but doesn’t rule out an energy field model), and modified gravity models don’t agree well with the behavior of dark matter. So conformal gravity isn’t ruled out by observations, but there’s no compelling evidence to support it over other models.

Depending on who you talk to, opinions of conformal gravity range from the greatest gravitational theory ever to something hovering near fringe theory. To be honest, I’m not familiar enough with conformal gravity to make a judgement. From what I know of the model I can see some interesting ideas, but I’m not convinced it can solve all the problems some claim it can.

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A new black hole model will not “break” relativity. Relativity works just fine, thank you very much, and no computer simulation of a five dimensional black hole will change that fact. So why are science headlines making such a claim? It all starts with a new paper in the Physical Review Letters that focuses on the cosmic censorship hypothesis.

A troublesome aspect of black holes in general relativity is that all of a black hole’s mass collapses down to a region of infinite density known as a singularity. Other models like classical electromagnetism also have singularities, but they tend to be more of a mathematical remnant we can deal with. In relativity, the gravity within a black hole becomes so strong that matter is inevitably compressed into a physical singularity. At that point relativity “breaks down” in that it simply can’t tell us what’s going on. All physical models “break down” when you have a physical singularity. This is nothing new, and doesn’t mean that relativity is wrong, simply that there is a limit to its usefulness as a model.

The general consensus about black hole singularities is that a more general quantum theory of gravity will address the issue, likely by eliminating singularities from the equation. But until then one work around is to note that a black hole’s singularity is enclosed by the event horizon. Since nothing can escape the event horizon of a black hole, the singularity is safely hidden from the universe, so we don’t really have to worry about it. This is known as the cosmic censorship conjecture. There are difficulties with this idea when it comes to quantum theory and the information paradox, but that’s another story. If a singularity weren’t hidden by the event horizon, then it would be a naked singularity, and the cosmic censorship conjecture would be violated. Computer simulations have shown that naked singularities can form according to general relativity, but the known examples require unusual ideal conditions that aren’t likely to happen in the real universe. So it’s generally thought that real black holes can’t have naked singularities.

This new work looks at black holes in a 5-dimensional version of relativity (rather than the usual 4-dimensional relativity of our universe). In 5D relativity black holes that aren’t spherical can form, and the paper looks specifically at torus-shaped black holes with a ring singularity in the middle. From computer simulations the team found that thin torus black holes could develop an instability in such a way that a naked singularity could form. Interesting work, but not earth-shattering news.

So to sum up, singularities in general relativity are a problem  (which we knew). Naked singularities can form under special situations (which we knew). Naked singularities can form in hypothetical 5D versions of relativity as well (not really surprising). Naturally the headlines read “New Model Breaks Relativity!”

(Facepalm)

Paper: Pau Figueras, et al. End Point of Black Ring Instabilities and the Weak Cosmic Censorship Conjecture. Phys. Rev. Lett. 116, 071102 (2016)  arXiv:1512.04532 [hep-th]

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On September 14, 2015 at 09:50:45 UTC advanced LIGO observed GW150914, a chirp of gravitational waves caused by the merging of two stellar-mass black holes. Just 0.4 seconds later, the Fermi gamma ray telescope observed a faint burst of gamma rays lasting about a second. While there is a small chance this is simply due to chance, it looks like the gamma ray burst was triggered by the merging black holes. 

A gamma ray burst (GRB) is a transient emission of gamma rays typically lasting less than two seconds. On average, about one gamma ray burst occurs every day. They appear randomly in all directions of the sky, and this means they aren’t produced in our galaxy. If they were, then GRBs would mostly be found along the plane of the Milky Way. They are thought to be caused by things like colliding neutron stars, or possibly the capture of a neutron star by a black hole.

This particular GRB (named GW150914-GBM) was observed by the Fermi Gamma-ray Burst Monitor, which can observe 70% of the sky at any given time. That’s great for observing these short-lived events, but it means that locating the source of a particular event is a bit imprecise. The most likely location of GW150914-GBM falls within the likely location of the gravity wave source GW150914. There are other aspects of the GRB that would tend to support a simultaneous event. While the burst was faint it had a hard x-ray spectrum, which would seem to rule out known sources within our own galaxy. There is still a possibility that some extragalactic event such as the collision of neutron stars just happened to occur in the same general direction 0.4 seconds after the gravitational wave event, but that doesn’t seem likely. Given its faintness there’s also a small chance that this could be a false-alarm.

If we assume the two events have the same cause that would mean the burst also occurred 1.3 billion light years away. From its apparent peak brightness we can calculate its peak luminosity. It turns out the peak luminosity of this event is an order of magnitude dimmer than any previous short GRB event. This would support the idea that it was not caused by a neutron star collision.

If this GRB was caused by merging black holes, it would be quite surprising. Stellar mass black hole binaries aren’t expected to have a disk of material around them that could emit gamma rays. We’ll need more data to be sure. Fortunately there will be plenty of opportunity to observe similar events over the next few years.

Paper: V. Connaughton, et al. Fermi GBM Observations of LIGO Gravitational Wave event GW150914 (preprint)

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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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A supernova is a dying star. In a last moment of brilliance a supernova can shine brighter than an entire galaxy. There is still much we don’t understand about supernovae, in part because of how difficult they are to observe. Since we can’t predict when a star will explode, we typically discover supernovae after they have brightened significantly. Ideally we’d like to know where and when a supernova will occur before it happens so we can observe one in its entirety. Thanks to the effects of general relativity, we can. 

In general relativity, spacetime is warped by the presence of mass. Because of this warping, light can be deflected from a straight path, an effect known as gravitational lensing. The effect was first observed by Arthur Eddington in 1919, but is most commonly observed when light from a distant galaxy is deflected by a closer galaxy on its way to reaching us. This lensing effect often produces multiple images of a distant galaxy as its light reaches us along different paths.

Light from a distant galaxy is lensed to produce multiple images of the galaxy. Credit: NASA & ESA.

The speed of light is finite, so it takes time for light from a distant galaxy to reach us, often billions of years. Since the images of a galaxy come to us along different paths, each having a slightly different distance, the time it takes light to reach us along each path is different. This means each image of the galaxy we see is from a slightly different time. Normally this is insignificant, since galaxies change very slowly. But a supernova occurs over a short period of time, so it can appear in one image of the galaxy first, only to reappear in another image years later.

In 2014, astronomers observed a supernova in a galaxy known as MACS J1149.5+2223. Because of gravitational lensing, at least three images of this galaxy can be seen. Using computer models they calculated the path distances of these multiple images, and determined that the supernova should appear in another galaxy image sometime between 2015 and 2020. They’ve been keeping an eye on the galaxy, and this month they were rewarded with a new appearance of the same supernova. The supernova has been nicknamed Refsdal after the Norwegian astronomer Sjur Refsdal, who first proposed the idea of time delayed supernovae in 1964.

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