It might be possible to detect gravitons after all

(quantamagazine.org)

175 points | by elsewhen12 hours ago

11 comments

  • DemocracyFTW21 hour ago
    I choked on this part:

    > The discussion recalls a messy, largely forgotten episode from the dawn of the quantum era. In 1905, Einstein interpreted experimental data to mean that light is “quantized,” coming in discrete particles now called photons. Others, including Niels Bohr and Max Planck, thought that the classical, wave nature of light might still be saved. [...] Most physicists presume that everything in the world is quantized, including gravity. But proving that assumption will entail a new war, one that has only just begun.

    1) No, that is not a "messy, largely forgotten episode", rather, it is frequently re-told and almost a required Inshallah of every piece on quantum physics.

    2) Please spare me that "war" simile, it only shows you're an American who can not write too well. War on drugs. War on poverty. War on whatever. The Browser Wars. Dude get a grip. Don't always "killer feature", "shot him dead", "waged war on the germs in her refrigerator". We have to fight a "war" to find out whether spacetime is quantum? Rilly??

  • ta937548298 hours ago
    So I thought gravity was basically the curvature of spacetime. But if there's a "gravity" particle, those two things seem mutually exclusive?

    Can someone who understands this please explain it to me, thanks!

    • whatshisface8 hours ago
      Our ability to solve integrals is much more limited when the dx represents a slight change in a function, rather than a small change in a real number. As a result, a lot of things that are easy to say in English such as "quantized curvature in spacetime," or "strongly coupled gauge theory," turn into a big mess when they're written down more precisely. One of the consequences of this limitation is that we have a model for quantized vibrations in spacetime that only works when they do not interact with each other. General relativity says that no, gravitational fields do interact with each other - so the picture we have at present is incomplete. The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.

      String theory and loop quantum gravity fit into this picture by trying to replace the integral over something we can't handle with an integral that matches it at large scales, but turns into something more tractable at small scales. Maybe the fact that we still can't make sense of the integral is Nature's way of telling us that she does not do the integral either...

      • actionfromafar12 minutes ago
        I don't know if it was just sheer luck that your comment fit my particular flavor of ignorance perfectly, but it struck me as great writing! I think I learned a little thing today. When I read the article I thought, too bad I can't ever understand anything of this, but now my personal model of the universe is just a little bit richer.
      • Aardwolf5 hours ago
        > The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.

        Can you please elaborate, the first part of the sentence says graviton is for non-self-interacting gravity, the second part of the sentence says graviton is for self-interacting (if 'its' in 'its self-interaction' refers to the graviton).

        I don't intend to nitpick the sentence, just trying to understand the theory and I don't even know if particle means self interaction or the opposite and can't parse it here either...

        If the answer is graviton is for non-self-interacting: what is the model for the other case (where gravity does self interact) and what would cause that self interaction if not the graviton?

        • FeepingCreature4 hours ago
          > Can you please elaborate, the first part of the sentence says graviton is for non-self-interacting gravity, the second part of the sentence says graviton is for self-interacting (if 'its' in 'its self-interaction' refers to the graviton).

          The point is, we know gravitation does self-interact. But our best model, the graviton, doesn't model self-interaction. So the model is probably accurate in regimes where you'd expect little self-interaction anyways.

      • Iolaum3 hours ago
        Even with gravity being "self interactive" can't we have stable particles ("gravitons"?) that behave like solitons do?

        https://en.wikipedia.org/wiki/Soliton

        • gradschoolfail1 hour ago
          If you’re young (or if you’re old but your mind remains flexible) i urge you to think hard about this problem.. i do and i will! my mind is already quite inflexible so i’m not going say anything definite about this question out of fear of saying something dumb/misleading. Your q hides monsters ready to snipe any serious mind of the planet!

          That said, an easier question to ponder (and many have tried) might be:

            Do photons self-interact? With each other? In free space?
          
          After thinking thru this question yourself, you might be more prepared to consider gravitons (=“spin-2 massless bosons”) ditto for me!
      • gradschoolfail7 hours ago
        This needs to be emphasized more, by the TFA too — most (theoretical) physicists think that detecting gravitons is an engineering exercise that has no implications* for quantum gravity (as understood by the public)

        >The model of non-self-interacting gravity is a particle we call a "graviton,"

        This needs to be emphasized even more, because it has

        >when the dx represents a slight change in a function

        *see the discussion around sharikous’ comment below

        https://news.ycombinator.com/item?id=42003116

        • lazide47 minutes ago
          Because they don’t want to run the risk of being wrong, eh?
    • LeoPanthera7 hours ago
      In theory, if gravitons exist, they should reproduce the same effects as the curvature of spacetime at larger scales. So, while they seem contradictory, they're actually complementary. Gravitons would be the "quantized" particles that, in large numbers, create the effect we observe as curved spacetime.

      The problem is that nobody has successfully combined these two views into a single unified theory, known as "quantum gravity". General Relativity and quantum mechanics don't naturally fit together, and that's why we don't yet fully understand gravity in a way that reconciles both the spacetime curvature and graviton perspectives.

    • pdonis7 hours ago
      > I thought gravity was basically the curvature of spacetime.

      Classically, it is. But most physicists believe that there is a quantum theory of gravity that underlies the classical theory, and that that quantum theory will include, at some level of description, a spin-2 gauge boson that mediates the quantum gravitational interaction, called the "graviton". Our classical theory of gravity, General Relativity, would then be the classical limit of that quantum theory, just as classical Maxwell electrodynamics is the classical limit of quantum electrodynamics.

    • renegade-otter3 hours ago
      I just watched this a few days ago on the Space Matters channel about gravitational waves: https://www.youtube.com/watch?v=9bg2NINW8a0

      Not just some dumbed down Discovery show - it pushes the limits of what a layperson can understand.

    • blenderob59 minutes ago
      > So I thought gravity was basically the curvature of spacetime. But if there's a "gravity" particle, those two things seem mutually exclusive?

      It does not have to be either or. It can be both. Both models can be useful to understand the nature of gravity and make predictions about natural phenomenon.

    • gary_08 hours ago
      Electromagnetism is both a continuous wave and a discrete particle, so it makes sense to me that a continuous spacetime curvature could also be a discrete particle at the same time. (Keeping in mind we're not talking about tangible shapes but mathematical models that describe aspects of reality that are hard for humans to intuitively conceptualize.)

      Of course, our idea of how to reconcile quantum gravity with general relativity is much less developed than our understanding of electromagnetism and the nuclear forces.

      • somat2 hours ago
        My understanding is the particle model of electromagnetism, the photon, really only shows up where the em field interacts with matter(electrons really), the em field itself is not quantized, or at least not quantized at the level of the photon. Not that this really matters(intentional), we can only interact with the em field as matter so that is what matters.
      • Willingham8 hours ago
        When you mention nuclear forces, are you referencing weak force and strong force? Do we understand these forces at the same level that we understand electromagnetism?
        • btilly7 hours ago
          Yes. The Standard Model has completely explained all experiments involving them for around 50 years now.

          In fact the outstanding success of the Standard Model has posed its own problems - the lack of deviations from it makes it hard for experiments to point in a useful direction for better theories to be developed along.

          • jcranmer6 hours ago
            That's not quite accurate. There are a few things that the Standard Model doesn't exactly account for--neutrino oscillation being the most famous. The trouble is that these issues aren't really big enough to suggest new physics, and the experiments aren't good enough to really suggest how much patching actually needs to be done.
          • elashri6 hours ago
            > the lack of deviations from it makes it hard for experiments to point in a useful direction for better theories to be developed along.

            We have anomalies (deviations from standard model) in many measurements done by several experiments. This is a good summary [1] from them up until now (sorry for the pay-walled)

            [1] https://www.nature.com/articles/s42254-024-00703-6

        • 7 hours ago
          undefined
      • dist-epoch1 hour ago
        The most elegant description of electromagnetism is also in terms of curvature, but the curvature of a certain mathematical structure called "connection on fiber bundles" and the math field is called differential geometry.
    • ajkjk7 hours ago
      Not an expert, but: the curvature of spacetime is modeled as a tensor field (the metric tensor). That field can have (classical) waves in it, which is what LIGO detects (I believe). Then you can certain hypothetically quantize that field, in which case it definitely has to be a spin-2 particle and it seems likely that there will be a way to do it since all the rest were.

      The "geometry" comes from the fact that the way we measure distances (or, well, experience time) uses the metric tensor field to do it. But it is still ultimately just a value attached to every point like any other field.

    • cryptonector6 hours ago
      That gravity is curvature of spacetime is one view of two equivalent views, but it is the standard view. The other view is that you have flat spacetime with different distortions (of things other than spacetime) than the distortions you get in curved spacetime. The Schwarzschild metric essentially lets you do exactly that projection of curved spacetime to flat, and vice-versa. When you watch an animation like https://www.youtube.com/watch?v=hF7zltx7Ecc or https://www.youtube.com/watch?v=E1mD4C7dBKc you're watching a flat spacetime representation of GR's effects, and the reason for using flat spacetime in these representations is <drum-roll/> that that is what us humans understand.

      So if you take the gravity curves spacetime view, then gravity is not a force and all that. But if you take the alternative view then gravity is a force. Now, I'll leave what the distortions are that gravity produces in flat spacetime for another time, or for the reader. But I'll say this: this view is both controversial (perhaps replies will show this) and not (see above -and many other- animations).

      • NitpickLawyer6 hours ago
        I last took a physics course when Pluto was a planet, so excuse my possibly outdated question, but isn't the detection of gravitational waves proof of gravity being a force?

        I follow a few educators/communicators in this field and I have a feeling they're using this "gravity isn't really a force" to bridge the gap between their deep understanding and us mortals that don't poses the language / understanding to get the entire meaning behind it. Is that feeling correct or am I missing something?

        • Wololooo5 hours ago
          So the main issue here is how people were presenting it, in Quantum field theory, as stated by other people, each force is associated with a field and has at least one force carrier, the exact number is linked to the specifics of the mathematical framework underlying it

          To that extent you can build 3 fundamental forces, electro magnetic, weak (that are called together electroweak) and the strong force. You have an extra force carrier through the Higgs that allows you to give mass to everyone.

          Now you need to consider gravity because you know that gravity exist and since everything under the sub is quantised, well so should gravity.

          The main issue with gravity is that it is interpreted so far as a curvature of space time, it's mainly fine for big items, but the implications for quantum field theory is that you should modify the small integral element that you use (space shouldn't have the same size) except that you look locally at space that is mainly flat... And changing the integral does not lead to well behaved behaviours.

          You can start to introduce new fields but doing so also causes an issue...

          Funnily enough even in the standard model something is missing, everything mostly fits, but that's the trick, mostly, neutrinos have mass and this in itself is a problem because the Higgs mechanism doesn't provide mass to them ...

          Long story short, people take shortcut when explaining the messy gritty part of it, which is "fine" but not really, and from a simple standpoint one would like to have a simple field from which gravity is born, which might be but so far, to my simpleton understanding, this hasn't been too successful, unless some form of string theory is realised. But the pre requisite for this is a form of supersymmetric theory existing which is currently disfavored, but could exist in the unproved energy scales from here to the plank energy scale.

          Sorry this ended being a tad long and I'm not sure this is clarifying things.

        • ithkuil2 hours ago
          Fwiw gravitational waves were predicted by Einstein himself (Einstein, Albert, Ueber Gravitationswellen, 1918) as a consequence of general relativity.

          The core idea is that when you move a mass, its contribution to the spacetime geometry changes, but the effects of the change of the geometry doesn't apply instantaneously to all the universe but instead the change propagates at the speed of light.

          So that explains why any sudden movement of a mass creates a "crest" that moves through space at the speed of light.

          Furthermore, the sources of fast movement of extremely heavy mass just happen to involve an object that wiggles back and forth in a periodic way because those events involve heavy objects orbiting other heavy objects.

          That's the reason we can measure a wave with multiple crests and we can talk about a wave length of the gravitational waves: the wave length of the gravitational waves matches the period of the orbit of the heavy mass.

        • stouset6 hours ago
          My naïve understanding is that you can model gravity as a force in a flat, static spacetime. Equivalently you can model gravity as a forceless distortion of curved spacetime. Both models can be translated faithfully into one another, so you can solve problems related to gravity in either domain.
          • trevordixon2 hours ago
            My naive understanding is that forceless spacetime distortion predicts somewhat different things than the old model. That's how general relativity finally explained the procession of Mercury's orbit for example.
        • whatshisface6 hours ago
          It's a bit of an internet meme, gravity can take momentum away from one object and transfer it to another, and that's what Newton said a force was. The meme is that the way it happens makes "changing momentum" (3-momentum, the one Newton was talking about) and "going straight" (geodesically, in curved space-time) hard to separate in English.
    • yarg7 hours ago
      To my understanding (not the best) there's a huge disconnect between the physics of the very small (quantum mechanics and the standard model) and that of the very large (general relativity).

      The disconnect seems to be unresolvable (I don't understand this part at all) and so efforts are being made to quantise gravity and incorporate it into the standard model.

    • dcl7 hours ago
      Yes. I've seen lots of twitter/X posts lately about how Gravity is not actually a force. But how can that be true if there is a force carrying "gravity" particle? Or is the word 'force' being used loosely here?
      • cyberax6 hours ago
        > Yes. I've seen lots of twitter/X posts lately about how Gravity is not actually a force.

        That is true. Classically, gravity is a fictitious force, merely a result of inertia from moving in a curved space-time.

        > But how can that be true if there is a force carrying "gravity" particle? Or is the word 'force' being used loosely here?

        Because we _suspect_ that the classical view is not correct. And there's a quantum description that may or may not involve curved space-time.

        It's not impossible that the spacetime curvature is a mathematical artifact of a deeper theory. Merely a kinematic explanation, just like epicycles.

        It's also possible that the space-time _is_ really curved, and gravitons simply cause the curvature by somehow coupling with it. And then other matter experiences this, in the manner described above.

      • InDubioProRubio2 hours ago
        In a sim, it would fall into the "configurable parameter" category and dynamically altered parameter whos laws depended on locations are function lookups. And to execute performant, it would be a constant factor field only updated onAlteration with fun(x)

        So you have a thing, that gets interpolated updated with various functions, that overlap, and those functions only get updated at lightspeed, cause caching.

        Cachesize limit should show as farway gravity sources getting bundled into lower density information functions.

      • 7 hours ago
        undefined
      • XorNot7 hours ago
        You can make a lot of pseudo particles in semiconductors which definitely exist, but also aren't "real" - i.e. semiconductor electron holes are capably modelled as positive particles which can move freely with momentum/position within a semiconductor.
    • 1024core7 hours ago
      Sort of like how light is both a wave and a particle...?
    • exe342 hours ago
      it's important to realise that particles are an artefact of living in a monkey sized body. at the basic level, the equations are useful if they match observations, not if they make sense intuitively.

      https://arxiv.org/abs/1204.4616

    • swayvil6 hours ago
      Maybe all particles are twists in spacetime.
    • ruthmarx5 hours ago
      > So I thought gravity was basically the curvature of spacetime.

      That's just part of the picture.

      I always thought that Veritasium video did more harm than good.

  • DebtDeflation30 minutes ago
    The fact that QED and QCD are renormalizable while gravity is not is probably trying to tell us something deeper than we think.

    Relevant paper:

    https://arxiv.org/pdf/0709.3555

    You can read the first two paragraphs of the Introduction and then skip to the last sentence of the Conclusion if you want to bypass all the math.

  • DemocracyFTW21 hour ago
    Another goodie:

    > 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.

    By way of comparison, even an Olympic pool-size balloon of hot air will float.

    • tsimionescu52 minutes ago
      It's not clear what problem you have with that comparison. It's a classical example of just how weak gravity is compared to the electromagnetic interaction. A whole planet's worth of mass is weaker than the EM field generated by a tiny magnet. And the strong and weak nuclear interactions are even stronger still.
    • tecleandor9 minutes ago
      But the whole atmosphere is pushing it up, isn't it? (Well, not the "whole" atmosphere, but...)
  • DemocracyFTW245 minutes ago
    Oh and next up is "Timeline: The Photon Wars".

    Now I want to shoot someone.

    • omnicognate32 seconds ago
      The only thing making me feel like that is your obnoxious spamming of this post. Four top-level comments now and not a single interesting thing said. Stop it, please.
  • whatshisface9 hours ago
    I'm with the debaters on this one, the energy levels of a bound quantum system are predetermined to change in quantized intervals irrespective of if they are coupled to a classical or quantum field. What theory of gravity is this experiment intended to falsify?

    It would be great to have an independent gravitational wave detector though.

    • sharikous8 hours ago
      the statistics would be different. Check out Rabi oscillations (classical EM) vs Jaynes-Cummings model (quantized EM) and phenomena like quantum antibunching (only possible for quantized EM)
      • whatshisface7 hours ago
        How would mergers produce antibunched gravity?
  • choilive9 hours ago
    > physicists are debating what it would really prove.

    Well, if we can detect the graviton before we have a working quantum theory of gravity, it would mean that gravity is in fact quantized and that we just need to figure it out. This would be a very big deal.

    • itishappy8 hours ago
      They can detect an interaction, but they can't prove that it's quantized without (I believe) sub-Poissonian statistics[0], which requires detecting enough events and with enough certainty that it would require planet-scale machinery.

      > 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(opens a new tab) in February.

      [0] https://en.wikipedia.org/wiki/Photon_statistics#Sub-Poissoni...

      • choilive4 hours ago
        It sounds like the article is saying we could detect many events without using a planetary scaled detector It mentions a single detector being a 15kg Be bar chilled to near absolute zero. Certainly very very difficult, but not in the realm of sci-fi.
    • toomuchtodo8 hours ago
      > This would be a very big deal.

      Gravity drive?

  • est9 hours ago
    That's a facinating read. I wonder what are the possible applications of "quantized gravity" ? GPS without satellites?
    • whatshisface8 hours ago
      If the Romulans were talking to the Klingons over a link secured by the quantum no-cloning theorem, the Enterprise could use quantized gravity to entangle the ship's computer with their adversaries' quantum radios in a way that no matter-based shielding could prevent.
    • MathMonkeyMan9 hours ago
      constraints on theories of everything
    • Vecr9 hours ago
      Cheap GPS receivers already have to do a bunch of tricks to get to the "okay" state they're currently at.

      Military devices either use GPS, star tracking, dead reckoning, or some combination. For submarines, detecting gravity variations could also be used, but it wouldn't rely on the quantization of gravity.

      In many places on land, you can use terrain landmarks.

      Since most things are already either covered, or have improvements in development, I don't really see investment for your idea.

      The improvements might even use "quantum" or "gravity", but I don't think the use of "quantum gravity" is very likely.

      • JumpCrisscross8 hours ago
        You’re mixing up general relativity with quantum gravity.
        • Vecr8 hours ago
          I'm not exactly sure what you're saying. I know you can have gravitons without "quantum gravity" (as incomplete theories). I'm responding to a commenter on applications.

          In a global position finding system, gravitational effects could be used, as could be quantum effects. Maybe even both in the same system.

          It seems really doubtful to me a practical system would depend on anything graviton related.

          • JumpCrisscross7 hours ago
            You buried your lede. The “tricks” you describe relate to GR. I missed that you’re essentially saying “no.”
  • ars9 hours ago
    I don't understand what a graviton is. The article implies that it's something that communicates changes in gravity? Is that correct?

    How does it communicate the magnitude of the change? By having lots of gravitons? Or does it have something akin to a frequency?

    • whatshisface9 hours ago
      A graviton is the smallest possible unit of a gravitational wave. The amplitude of the wave corresponds to the number of gravitons, like you said, and its frequency to their frequency (quantum particles have frequencies that are related to their momenta). We're aware that light, at least, works like that.
      • hhdhdbdb4 hours ago
        A gravitational wave requires an event like a black hole merger, or basicially somerhing to move and change the field, right?

        In this case, how does the fact that a big object is still influencing space/time around it communicate that fact when it is not moving. Is that still gravitrons?

      • russdill8 hours ago
        Gravitons impacting and imparting momentum seems like it would have a bunch of observational implications. Does a massive object cast a graviton shadow? Is the momentum positive or negative?
        • whatshisface8 hours ago
          Classical waves do all of those things too.
  • cyberax6 hours ago
    TIL that Freeman Dyson looks like a house elf!
  • DemocracyFTW247 minutes ago
    > At the time, many physicists rejected Einstein’s theory. Some wouldn’t come around for two decades. In their view, the experiment fell far short of conclusive proof. It was, rather, an opening argument in a decades-long war fought to determine the true nature of light.

    Dude. There were two world wars in that time. "ITS A WARRRRRRR!!!!!", and you're an American writer infected with TV screaming speech. "The MONSTER ... LURKING in the CENTER of our MILKY WAY", a.k.a. Saggitarius A*. WE FOUGHT A WAR TO FIND THE TRUE NATURE OF LIGHT. A WARRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR!!!

    This the reason I don't watch TV anymore.