Science: Detecting gravitons

Since the early 1900s, the possibility of gravity particles, gravitons, was discussed and debated. But since the force of gravity is so weak, it would take a huge detector about the size of Jupiter parked next to a neutron star to get the job done in a decade or two. In other words, there was no plausible way to detect one of the little buggers. And, even if a graviton exists and is detected, what it actually is would probably still be debated for another decade or two (or three).

But, a physicist and colleagues at the Stevens Institute of Technology in New Jersey think they have figured out a way to actually build a detector on Earth in a lab to look for gravitons. It's kind of a simple thing. Very simple actually. Super simple. Quanta magazine writes:

It took decades of effort and the construction of the colossal, miles-long detectors that make up the Laser Interferometer Gravitational-Wave Observatory (LIGO) to first sense a rumble in space-time in 2015 — one sent out by a collision between distant black holes.

Detecting a single graviton would be harder still, akin to noticing the effect of just one molecule in an ocean wave.

Igor Pikovski, a theoretical physicist now at the Stevens Institute of Technology in New Jersey, has been mulling over these developments since 2016. At the time, he and three collaborators noted that a vat of superfluid helium — which displays quantum properties despite having a large mass — could be set up to reverberate in response to certain gravitational waves.

It would take another conceptual leap to go from a gravitational wave detector to a detector for individual gravitons. In the recent paper, which appeared in Nature Communications in August, Pikovski and his co-authors outlined how the graviton detector would work.

First, take a 15-kilogram bar of beryllium (about 33.1 lb) or some similar material and cool it almost all the way to absolute zero, the minimum possible temperature. Sapped of all heat, the bar will sit in its minimum-energy “ground” state. All the atoms of the bar will act together as one quantum system, akin to one hulking atom.

Then, wait until a gravitational wave from deep space passes by. The odds that any particular graviton will interact with the beryllium bar are low, but the wave will contain so many gravitons that the overall odds of at least one interaction are high. The group calculated that approximately one in three gravitational waves of the right sort (neutron star collisions work best since their mergers last longer than black hole mergers) would make the bar ring with one quantum unit of energy. If your bar reverberates in concert with a gravitational wave confirmed by LIGO, you will have witnessed a quantized event caused by gravity.

See, physics stuff is easy peasy. All we needed was to build the LIGO to get the first part in place. Now all we need to do is get some liquid helium, dump in some beryllium and then sit back, eat popcorn and wait for an alarm thingy to alert the boffins that a gravity wave hit Earth at almost exactly the same time the cold beryllium started to vibrate. 

GW = gravity wave detected by LIGO

That's all there is to it! So simple I could do it.

And, for the deep divers in the crowd, clocks that physicists use are capable of measuring really small intervals of time, on the order of 247 zeptoseconds (247 x 10^-21 second). For some context, light travels only about 0.000075 millimeters (~2.95^-6 inch) in 247 zeptoseconds. So the point in time between LIGO detecting a wave can be very precisely correlated with when the cold beryllium started vibrating. 

By Germaine: Physicist-lite, very lite