Arthur T Knackerbracket has processed the following story:
Detecting a graviton — the hypothetical particle thought to carry the force of gravity — is the ultimate physics experiment. Conventional wisdom, however, says it can’t be done. According to one infamous estimate, an Earth-size apparatus orbiting the sun might pick up one graviton every billion years. To snag one in a decade, another calculation has suggested, you’d have to park a Jupiter-size machine next to a neutron star. In short: not going to happen.
A new proposal overturns the conventional wisdom. Blending a modern understanding of ripples in space-time known as gravitational waves with developments in quantum technology, a group of physicists has devised a new way of detecting a graviton — or at least a quantum event closely associated with a graviton. The experiment would still be a herculean undertaking, but it could fit into the space of a modest laboratory and the span of a career.
[...] Currently, Albert Einstein’s general theory of relativity attributes gravity to smooth curves in the space-time fabric. But a conclusive graviton detection would prove that gravity comes in the form of quantum particles, just like electromagnetism and the other fundamental forces. Most physicists believe that gravity does have a quantum side, and they’ve spent the better part of a century striving to determine its quantum rules. Nabbing a graviton would confirm that they’re on the right track.
But even if the experiment is relatively straightforward, the interpretation of what, exactly, a detection would prove is not. The simplest explanation of a positive result would be the existence of gravitons. But physicists have already found ways to interpret such a result without reference to gravitons at all.
[...] It’s hard to experimentally probe gravity because the force is extremely weak. You need huge masses — think planets — to significantly warp space-time and generate obvious gravitational attraction. By way of comparison, a credit card-size magnet will stick to your fridge. Electromagnetism is not a subtle force.
One way to study these forces is to disturb an object, then observe the ripples that travel outward as a consequence. Shake a charged particle, and it will create waves of light. Disturb a massive object, and it will emit gravitational waves. We pick up light waves with our eyeballs, but gravitational waves are another matter. 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.
[...] 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 (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.
Among a handful of engineering hurdles involved in opening that window, the highest would be putting a heavy object into its ground state and sensing it jumping to its next-lowest-energy state. One of the groups pushing the state of the art on this front is at ETH Zurich, where Fadel and his collaborators cool tiny sapphire crystals until they display quantum properties. In 2023, the team succeeded in putting a crystal into two states simultaneously — another hallmark of a quantum system. Its mass was 16 millionths of a gram — heavy for a quantum object, but still half a billion times lighter than Pikovski’s bar. Nevertheless, Fadel considers the proposal to be achievable. “It wouldn’t be too crazy,” he said.
[...] Now graviton chasers find themselves in a peculiar position. On the main facts, everyone is in agreement. One, detecting a quantum event sparked by a gravitational wave is — surprisingly — possible. And two, doing so would not explicitly prove that the gravitational wave is quantized. “Could you make a classical gravitational wave that would produce the same signal? The answer is yes,” said Carney, who along with two co-authors analyzed this type of experiment in Physical Review D in February.
[...] “This is an exciting paper,” said Alex Sushkov, an experimental physicist at Boston University. “These are hard experiments, and we need bright, smart people to move in this direction.”
It might motivate subsequent experiments that would take physicists deeper into the quantum gravity era, just as scattering experiments once took them deeper into the era of the photon. Physicists now know that quantum mechanics is much more than quantization. Quantum systems can take on combinations of states known as superpositions, for instance, and their parts can become “entangled” in such a way that measuring one reveals information about the other. Experiments establishing that gravity exhibits these phenomena would provide stronger evidence for quantum gravity, and researchers are already exploring what it would take to carry them out.
None of these tests of gravity’s quantum side are completely ironclad, but each would contribute some hard data regarding the finest features of the universe’s weakest force. Now a frigid quantum bar of beryllium appears to be a prime candidate for an experiment that will mark the first step down that long and winding road.
(Score: 2) by VLM on Monday November 04, @12:31PM (1 child)
Maybe as a modest proposal they could build a very large double slit experiment and graph the output of a large array of scales. That would be mildly impressive.
(Score: 4, Insightful) by JoeMerchant on Monday November 04, @01:25PM
Did it occur to the physicists working on this method that they might be "doing it the hard way"?
You can wash all the windows on the Petronas Towers by building bamboo scaffolding for the window washers to climb... however, there are also easier ways.
Which would seem to be this chilled beryllium approach, more akin to hanging a washing platform on cables and lowering it down.
There was a time we barely knew anything about how to work with electricity in practical terms.... I feel like we're at a similar "go fly a kite in a thunderstorm" stage with much of the quantum realm.
🌻🌻🌻 [google.com]
(Score: 3, Interesting) by Username on Monday November 04, @03:59PM (1 child)
There is a whole lot of unknown that lies slightly out of our perception that we know exist but really have no clue about it.
There might be living beings on our planet that we don't even know exist since we cannot see or hear them, that might very well know and see how gravity functions. Might be thousands of them flying at lightspeed, zipping through physical objects like a bird through air.
We have such a limited view of the universe.
(Score: 2) by Reziac on Tuesday November 05, @02:16AM
Welcome to Nopalgarth.
https://en.wikipedia.org/wiki/The_Brains_of_Earth [wikipedia.org]
And there is no Alkibiades to come back and save us from ourselves.
(Score: 3, Interesting) by stormwyrm on Monday November 04, @07:34PM (3 children)
The last experiment I'd heard of that proposed to detect the quantization of gravity was this one:
https://www.nature.com/articles/s41467-024-51420-8 [nature.com]
It seems to require cooling a one ton object to millikelvin temperatures. With so much matter cooled to the ground state we then have some chance of detecting the very small predicted influence of a graviton from some gravitational wave event within a reasonable timeframe. I think the present world record for stuff cooled that close to absolute zero is of the order of milligrams, so this isn't quite feasible yet, and it also requires sensors that are much more sensitive than what we have today. It's not so unrealistic though and could possibly be done within the next five to ten years with further advances in technology. Sabine Hossenfelder has a video where she discusses this:
https://youtu.be/rq8R0MmFq8w?si=1G2P9-UnfMx5tIT1 [youtu.be]
Numquam ponenda est pluralitas sine necessitate.
(Score: 3, Insightful) by JoeMerchant on Monday November 04, @07:47PM (2 children)
I'm gathering that gravitons are supposed to be rather large, or to say it another way: a single graviton would seem to be quite a bit more influential than say, a photon?
Different types of influence, apples and oranges, or maybe raisins and avocados, but...
If gravity affects single atoms of oxygen, holding them down to the Earth, it would seem that that single atom of oxygen is demonstrating some gravitational effects from each and every other sub-atomic particle in the Earth... which would be an awful lot of gravitons...
Then there's always the electrostatic analog to gravity: a single electron exerts a force at a distance on other charged particles, near and far... are there electrotrons that carry that force?
🌻🌻🌻 [google.com]
(Score: 2) by stormwyrm on Monday November 04, @08:25PM (1 child)
Numquam ponenda est pluralitas sine necessitate.
(Score: 2) by JoeMerchant on Tuesday November 05, @03:08AM
Interesting to me is that both electrostatic and gravitational forces decrease with the inverse square of distance - but of course we lack "negative mass" so gravity just builds and builds while electrostatics tend to mix themselves in a mostly balanced soup self-cancelling the bulk of their effects at distance.
That inverse square relationship is the same with photons leaving a point source, the intensity of their "illumination" falls off as the photons spread in their travels through 3-space - it's a very nice mapping to the pure mathematical topological or whatever you want to call it result of a growing sphere - which has a surface area that increases as the square of the radius.
🌻🌻🌻 [google.com]
(Score: 2) by pkrasimirov on Monday November 04, @10:15PM (4 children)
> doing so would not explicitly prove that the gravitational wave is quantized
Then what? We already know the bar will be affected by the wave and will move, like everything around it.
(Score: 1) by khallow on Tuesday November 05, @01:21AM (3 children)
There's two aspects here. First, they haven't detected a graviton yet. Detecting them means one can then figure out where they're coming from and image that to some degree as well. Second, any insight into quantization of gravity no matter how meager will greatly help our efforts to unify disparate theories.
(Score: 2) by Reziac on Tuesday November 05, @02:20AM (2 children)
I once heard a physicist talking about the matter, and he stated that gravity cannot be particles, it must be a field, because its effect is instantaneous. Were it not so, everything would fly apart before the gravitons could arrive. Or so went his argument.
Side thought: how does a graviton make things sticky??
And there is no Alkibiades to come back and save us from ourselves.
(Score: 2, Funny) by Anonymous Coward on Tuesday November 05, @03:59AM
Are you sure he was a physicist, or if he was, maybe you experienced one of those Star Trek "rifts in the space-time continuums" and this discussion happened 130 years ago?
(Score: 3, Interesting) by stormwyrm on Tuesday November 05, @07:27AM
Has he never heard of Einstein's General Theory of Relativity? According to General Relativity the speed of gravity is also c, and this has been observationally verified since observations of binary pulsars like the Hulse-Taylor pulsar were made in the 1970s, and further strongly constrained by the observations of neutron star mergers that have been done since gravitational wave observatories like LIGO came online in recent years. Gravitational effects thus also occur at the speed of light, and yes, this has also been borne out by the way we see large-scale structure formation happening in the universe. Things indeed do fly apart when they are beyond a certain limit set by the propagation speed of gravity and thus we can't have gravitationally bound structures bigger than a certain size at this stage in the universe's evolution because of it.
No one's sure exactly how gravitons make things sticky though... That's what a quantum theory of gravity is supposed to answer and we don't really have a good one yet. In quantum field theories the exchange of virtual particles like photons leads to a transfer of momentum that looks like a force acting at a distance. General Relativity though tells us that the presence of matter/energy warps spacetime's geometry and what looks like a force acting at a distance is just objects trying to find the path of least resistance through a distorted geometry. How this happens at very small scales is not well understood and none of our theories as yet have any good answers that can be tested. If we can detect gravitons somehow, what we might be able to see of their behaviour could constrain what theories we have.
Numquam ponenda est pluralitas sine necessitate.