Scientists Propose Groundbreaking Method to Detect Single Gravitons | Hacker News Reader

Scientists Propose Groundbreaking Method to Detect Single Gravitons

scitechdaily.com

18 pointsamichail13 comments

I haven't read this paper yet, but it does look interesting.

For context, there is a famous lecture by Freeman Dyson [1] in which he studies the question of whether it is possible in principle to detect an individual graviton. He considered a couple of different classes of detectors and his conclusions were mostly pessimistic. The essential problem with many detectors (e.g., something like LIGO) is that any detector that is sensitive enough to detect a single graviton would be so massive that it would collapse into a black hole.

I'll be curious to see how this new work fits into Dyson's framework.

[1]: http://publications.ias.edu/sites/default/files/poincare2012...

You may also enjoy https://arxiv.org/abs/gr-qc/0601043

This is cool... One of the team’s proposed innovations is to use available data from LIGO as a data/background filter...

“The LIGO observatories are very good at detecting gravitational waves, but they cannot catch single gravitons,” notes Beitel, a Stevens doctoral student. “But we can use their data to cross-correlate with our proposed detector to isolate single gravitons.”

Wasn't there a new way to measure gravitational waves; Ctrl-F hnlog for LIGO and LISA:

"Mass-Independent Scheme to Test the Quantumness of a Massive Object" (2024) https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13... .. https://news.ycombinator.com/item?id=39048910 :

> This yields a striking result: a mass-independent violation of MR is possible for harmonic oscillator systems. In fact, our adaptation enables probing quantum violations for literally any mass, momentum, and frequency. Moreover, coarse-grained position measurements at an accuracy much worse than the standard quantum limit, as well as knowing the relevant parameters only to this precision, without requiring them to be tuned, suffice for our proposal. These should drastically simplify the experimental effort in testing the nonclassicality of massive objects ranging from atomic ions to macroscopic mirrors in LIGO.

From https://news.ycombinator.com/item?id=30847777 .. From "Massive Black Holes Shown to Act Like Quantum Particles" https://www.quantamagazine.org/massive-black-holes-shown-to-... :

> Physicists are using quantum math to understand what happens when black holes collide. In a surprise, they’ve shown that a single particle can describe a collision’s entire gravitational wave.

> "Scale invariance in quantum field theory" https://en.wikipedia.org/wiki/Scale_invariance#Scale_invaria...

"New ways to catch gravitational waves" https://news.ycombinator.com/item?id=40825994 :

- "Kerr-Enhanced Optical Spring" (2024) https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13... .. "Kerr-enhanced optical spring for next-generation gravitational wave detectors" (2024) https://news.ycombinator.com/item?id=39957123

- "Physicists Have Figured Out a Way to Measure Gravity on a Quantum Scale" with a superconducting magnetic trap made out of Tantalum (2024) https://news.ycombinator.com/item?id=39495482

- "Measuring gravity with milligram levitated masses" (2024) https://www.science.org/doi/10.1126/sciadv.adk2949

"Physicists Have Figured Out a Way to Measure Gravity on a Quantum Scale" https://news.ycombinator.com/item?id=39495482#39495570 .. https://news.ycombinator.com/item?id=30847777 :

> Does a fishing lure bobber on the water produce gravitational waves as part of the n-body gravitational wave fluid field, and how separable are the source wave components with e.g. Quantum Fourier Transform/or and other methods?

"Distorted crystals use 'pseudogravity' to bend light like black holes do" https://news.ycombinator.com/item?id=38008449 :

"Deflection of electromagnetic waves by pseudogravity in distorted photonic crystals" (2023) https://journals.aps.org/pra/abstract/10.1103/PhysRevA.108.0... :

> We demonstrate electromagnetic waves following a gravitational field using a photonic crystal. We introduce spatially distorted photonic crystals (DPCs) capable of deflecting light waves owing to their pseudogravity caused by lattice distortion

In experiments that detect gravity waves, I recall the problem of Heisenberg Uncertainty Principle limiting our ability to see an individual graviton. I don't recall much detail and probably didn't understand fully when I read it. Something about our detectors and what an individual graviton effect would cause, falls within that limit; hence undetectable.

Does this change that theoretical limit?

I believe LIGO-style detectors would still have this issue. The procedure that is proposed here is more like a gravitational analog of the photo-electric effect. In the photo-electric effect you have an electron which is bound to an atom until a photon with sufficiently high energy knocks it out. In the proposed experiment here they would prepare a bar of a very pure material (e.g., aluminum) and cool it until it is in its ground state. The length of the bar would be chosen so that its resonant frequency would correspond to the frequency of a gravitational wave event. When a gravitational wave passes by, a graviton could excite an acoustic mode in the bar.

Incidentally, these kinds of resonant bars were one of the original ideas for gravitational wave detectors back in the 1960s and 70s. A physicist named Joseph Weber claimed to have detected a number of gravitational wave events back in the 1960s, but it was later determined that these were noise. [1]

[1]: https://en.wikipedia.org/wiki/Weber_bar

Edit 3: I jumped to some conclusions about what their actual apparatus would be, so some of this doesn't really address what the authors have written (see the link to the actual paper and preprint below). I'm mostly leaving this because it's already written, right in some parts, and the link at the start of "Edit 2" is worth a glance to anyone interested in an ELI12 on Weber-bar style detectors. I also am much less confident know that I know roughly how a telephone works!

It's more in analogy to MiniBooNE. In that, experimenters shoot a high-energy neutrino beam through a suitable medium, recording the electromagnetic radiation from a Z-boson interaction that gives a kick to a target particle. Sometimes these are electrons that give off flashes of Cherenkov radiation; sometimes the neutrino interaction is somewhat more complicated in the radiation ultimately produced. MiniBooNE's detector is colocated with a major neutrino beam generator.

Gravitationally, ideally one would produce a beam of very small merging black hole binaries to be shot through a detector medium. Binaries throw off gravitational waves with frequency proportional to the size of the orbit; a tiny orbit means high frequency, and a local merger is interesting in other ways. If gravitons are the quanta of such HF gravitational waves, the individual graviton energies should be sufficient to scatter particles in a suitable medium (e.g., a light particle like an electron might throw off Cherenkov radiation, which we could detect). We could also rely upon the https://en.wikipedia.org/wiki/Gravitational_memory_effect to leave fingerprints in the less excited bits of the detecting medium.

Since we don't have that ideal, we are stuck with astrophysical sources of gravitational waves, and so roughly the authors' idea <https://www.nature.com/articles/s41467-024-51420-8> (more conveniently at <https://arxiv.org/abs/2308.15440>) is comparable with what the MiniBooNE experimentalists did in arriving at their particular mineral oil: some media produce a "ladder" of events from a small particle deflection to a detectable signal, and all the ladder rungs rely on discrete energy spectra (in this neutrino detection example, the oils form a natural scintillator that produces light given small movements in the medium of charged particles characteristic of certain weak current interactions; in the gravitational case the idea is to use an ultra-low-temperature condensate). Careful tuning "should" pick out monochromatic gravitons from the astrophysical menu http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.ht... -- then it's a matter of waiting a lonnnnnnng time for a detection.

The Weber Bar was a classical stress-strain measuring device; I don't think Weber was thinking in terms of quantum anything as much as how a shock through a suitable substance could produce electrically- or acoustically-measurable side effects, somewhat like piezoelectric crystals. Ultimately his ideas about (classical) gravitational wave detection were so misguided as to be pseudoscientific. I am not very surprised that in pop sci reporting an analogy was drawn to them rather than to the difficulties in detecting Z-bosons but I'm surprised they get much mention (Weber 1960 is cited!) in the actual paper. I'm an optimist so think it's in part because the good science Weber did was tube engineering for very early masers and lasers, and it's not hard to imagine a (backwards) connection from this paper about stimulating high amplitude coherent states of gravitational waves to grasers <https://en.wikipedia.org/wiki/Gravity_laser>.

(In reality the gravitational waves we get here from verrrrrry extragalactic binary mergers won't be effectively monochromatic, so the detection idea seems to die in Section III: you won't be able to correlate a LIGO/VIRGO-style detection with this experiment with any reliability -- the "beep!" from the device (assuming it's not error) is as likely to be from the background. But maybe there are lots and lots of highly-similar EMRIs out there building up the black hole mass hierarchy, who knows.)

Edit: paid more attention to their Table I and how they got there, and well, their smaller devices (mostly leftmost columns) are basically "build better Weber bars". I wonder if there's any hope of someone getting hands on nine tonnes of niobium and getting it to ~ 1 mK. Anyway, good luck to them, publish the low-hanging-fruit results fast!

Edit 2: https://web.mit.edu/klmitch/classes/8.224/project/resonant.h... (Re the NIOBE resonant bar §1.10 of <https://dcc-llo.ligo.org/public/0125/P1600131/001/optomechan...> (disappointingly not 9 tonnes of niobium!) among others) - but decades of experimentalists trying and no confident result...

The problem is in the extreme weakness of the gravitational interaction compared to electromagnetism (and even compared to neutrino interactions), not the uncertainty principle. You can use small magnets to hang big things on a fridge door, even though the magnet and the object are being pulled floorwards by the gravitation of the entire planet. You need ENORMOUS vats of super-pure water surrounded by sensitive photodetectors <https://neutrinos.propg.ufabc.edu.br/wp-content/uploads/2019...> (Super-Kamioande, Japan) to occasionally spot light a particle strongly bounced by a neutrino from a significant nearby neutrino-generator. There are bazillions of neutrinos streaming through the chamber in that maintenance photograph. But there are A LOT more gravitons passing through, and that detector will almost never certainly spot an electron being sped up by them!

The idea in the paper is to replace water with something clever that is purer, much closer to absolute zero, and much more voluminous than Super-K. The clever bit is that it doesn't have to be more voluminous (by a lot) than our entire planet because interactions within the medium can (the authors argue) in principle magnify the signal of a deflection by a very precisely characterized (notably in frequency) graviton.

A somewhat unkind way of analogizing it is that they hope to see an avalanche and attribute it to (only) the landing of a very specifically shaped snowflake during a blizzard.

> Of course, there’s a catch with catching gravitons. The necessary sensing technology doesn’t quite yet exist.

That's, uh, kind of a big detail to not mention until the fourth paragraph from the bottom.

It's because there's no need to build it until you need to do this experiment though - this is the sort of bounds-pushing which top-end physics always does (i.e. before the LHC was built, sensors as good as ATLAS didn't exist, before LIGO the necessary interferometers didn't exist).

What they're proposing to build thought is however a logical refinement of established and demonstrated physical principles - it's entangling the vibrational state of a the big block of Aluminum to a system which you can more easily read out without disturbing it (examples in the literature seem to include circulating currents in super-conducting loops - so you can spectroscopically probe the state of the vibrating thing by looking at shifts in the emission/absorption of your superconductor, which is much easier).

Basically it is an experimental setup which is achievable with technology we would want to build anyway as a refinement of existing work.

It's easy to forget sometimes that the people taking it for granted that the technology doesn't exist yet are also the people whose work results in that technology being created. Of course it doesn't exist yet. Needing to invent better sensors, or better magnets, for the sake of running an experiment like this, is what causes those technologies to exist.

That can seem like a big deal at first, but it's pretty normal since these things are at the cutting edge of technology. Theories or highly simplified versions of experimental techniques are often proposed decades before the technology finally catches up to the level where those theories can be properly tested, or where those experimental techniques can be performed at a useful level.

For example, a lot of the technology needed to make the JWST did not exist when work on it started, it was developed specifically for JWST, or happened to be developed as they were working on it. Same goes for a lot of the capabilities of particle accelerators.

Reply on news.ycombinator.com