Back in 2015, the two detectors that make up the experiment called LIGO made waves. Well, technically they detected waves — they picked up the gravitational waves generated by two merging black holes. It was no small feat, but LIGO is no small experiment; in order to suss out the signal, the observatories had to use laser beams several kilometers long. Now, some researchers believe they can build instruments that can detect gravitational waves even LIGO can’t see — instruments that would be small enough to fit on a table-top.
To understand what makes this such an impressive claim, first you have to know what an interferometer is. Interferometers like those used by LIGO, which is short for Laser Interferometer Gravitational-Wave Observatory, take advantage of how beams of light interact with each other to take measurements. Light behaves like a wave with peaks and troughs, and when two waves of light interact, their waves will combine either constructively or destructively. For example, when the peaks of two different waves sync up, they’ll produce a taller peak. But when a peak and a trough come together, they’ll cancel each other out. These combinations produce an interference pattern that can be measured and analyzed.
LIGO’s two detectors each use a laser with a beam that’s split and sent down perpendicular paths each 4 kilometers long. At the end of the path is a mirror, which bounces the beam back towards the beam splitter. For some interferometers’ purposes that would be more than enough distance, but each of LIGO’s arms actually has a second mirror by the beam splitter, which bounces the laser back over and over again, making each beam of light travel 1,200 kilometers before they’re allowed to recombine and produce an interference pattern. Any tiny vibration that moves the mirrors affects the interference pattern, and LIGO can pick up vibrations 10,000 times smaller than a proton, making its interferometers the most sensitive in the world. But even LIGO’s incredible sensitivity still isn’t enough to detect relatively low-frequency gravitational waves.
To detect those with lasers, we would have to build interferometers in space with baselines that are hundreds of thousands of kilometers long. So, some scientists at the University College London decided to explore another method of making interferometers. One that involves quantum mechanics and diamonds.
The proposed device would use nanoscale diamonds with defects, called nitrogen-vacancy centers or N-V centers. In an N-V center, a nitrogen atom takes the place of a carbon atom and an empty spot in the diamond lattice is left open next to it. This defect can be treated like two unpaired electrons and its spin can be manipulated. In fact, these crystals are already used in one approach to quantum computers. The researchers propose a device that would trap the crystals and use microwaves to put their spins in superposition, meaning the same N-V center exists simultaneously in two states. Weird, I know — but that’s the quantum realm for you.
Anyway, when a magnetic field is applied to the crystals, the two spin states should separate and travel along different paths before meeting up again. Like LIGO’s split laser beams, tiny changes in space should create a pattern that can be measured and analyzed. The researchers believe a device like this that’s as small as 1 meter long could reveal low frequency gravitational waves, and even help us study the quantum character of gravity.
Of course, there’s one enormous catch: the technology to build an interferometer like this doesn’t exist yet. The scientists are confident that it can be realized in the near future, but until the necessary breakthroughs arrive, I’ll just pin my hopes on the next big laser interferometer — ESA and NASA’s space-based LISA, a laser interferometer made up of three different spacecraft which is set to launch sometime in the early 2030s. Either way, I can’t wait for what comes next.
LIGO’s detectors are so sensitive they can pick up vibrations in the Earth from sources thousands of miles away. That’s why it uses two detectors, one in Louisiana and one in Washington State, each acting as a noise filter for the other. Since its groundbreaking 2015 discovery, LIGO has been finding more gravitational waves. Earlier in 2020, it detected a neutron star collision.
Which approach do you prefer: lasers or quantum mechanics? Let us know in the comments.