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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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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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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One of the predictions of general relativity is that the motion of large masses, such as a binary system of black holes or neutron stars, should produce gravitational waves.  When most people think of waves they typically think of water waves. Drop a pebble in a calm pond and you can watch the waves spread out over the surface of the water.  Gravity waves are similar, but are ripples in the fabric of space and time itself.  There is a project looking for these gravitational waves, known as the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

We actually have a great deal of evidence for gravity waves.  For one, they are a consequence of general relativity, which has passed every observational test we’ve made of it so far.  We also have indirect evidence of gravitational waves through an effect known as inspiralling.  When two stars orbit each other, they produce gravitational waves. The gravity waves in turn take away some of the energy from the binary system. This means that the two stars gradually move closer together, or inspiral. As the two stars inspiral, their orbital period gets shorter (because their orbits are getting smaller).  We have observed this effect with a pulsar orbiting a companion star, and the result is exactly that predicted by gravitational waves.

A schematic of the LIGO experiment.
Credit: LIGO/MIT

Still, we’d like to detect gravitational waves directly, which is where LIGO comes in.  Because gravitational waves create ripples in space, the distance between objects shift slightly as a gravitational passes by.  This means by making precise measurements of the relative positions of objects, we can in principle detect these waves.  The difficulty is that even gravity waves produced by binary black holes will only cause small distortions of an experiment here on Earth.  This is why LIGO uses laser interferometry along a beam 4 kilometers long.  Even then, the expected distortion would be about a billionth of a nanometer.

So far, the LIGO project hasn’t had a confirmed detection of gravity waves, but it is hoped that upgrades to the project currently under way will allow the project to be successful.

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