Q: If gravity is the reaction matter has on space, in that it warps space, why do physicist’s look for a gravity particle? Wouldn’t gravity be just a bi-product of what matter does to space?

Physicist: Isn’t that weird?

The name “quantum mechanics” comes from the fact that, at its most base, quantum mechanics requires all particles and energies to come in discrete (one might say “quantized”) packets.  At some point a bunch of physicists starting asking awkward questions like; the matter is quantized, the energy is exchanged in quantized packets, so why do we assume the force is smooth and continuous?

Compounding this awkward line of questioning was the fact that photons were already known to carry electromagnetic force.  Literally, photons are little oscillating bits of electric and magnetic fields, which is exactly what the electric and magnetic forces are.  So the next obvious question was “do the other forces have ‘force carriers‘?”

You’re damn right they do.  Photons for electromagnetism, W and Z bosons for the nuclear weak force, and gluons for the nuclear strong force.  There’s every force but gravity!  Each of the carriers were predicted by the (then new) study of “quantum field theory”, and have since been observed.  The theory itself is gorgeous and works beautifully.  In fact, it barely makes sense to think of anything in the universe (including space) as not being quantized.

So, some physicists are looking for evidence of the existence of gravitons (the gravity particle), because it would really tie things together nicely.  There are a couple drawbacks however…  In order for something to be detected it has to do something.  Gravity is a really, really weak force, and a graviton is the smallest amount of that force that can exist.  Most physicists have already given up any hope of detecting the graviton directly, and instead are looking at extremely indirect methods.  The drawback there is that the graviton (if it exists, and if our theories hold up) is a very strange particle, and is described using amazingly nasty math (even more nasty than the normally nasty math of quantum field theory).  So it’s difficult to even figure out what those indirect methods should be.

To actually answer the question: some physicists are looking for the graviton because it “fits”.

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11 Responses to Q: If gravity is the reaction matter has on space, in that it warps space, why do physicist’s look for a gravity particle? Wouldn’t gravity be just a bi-product of what matter does to space?

  1. Cris says:

    Just discovered this site and I absolutely adore it! Just a short follow-up on this one: do we NEED theoretically to find a graviton? That is: (I know I’d be nice to have all “forces” following the same quantum field logic, but) is our present “geometrical” understanding of gravity not enough to understand observed phenomena? Could gravity be not a force but JUST a result of the geometry of spacetime?

  2. The Physicist The Physicist says:

    There’s a problem in physics that stems from the fact that that approach (gravity = geometry) seem to work perfectly for everything we do in general relativity. But at the same time, it causes problems when quantum mechanics enters the picture. Basically, one or the other or both need to change.

  3. The Cool Dude says:

    If the Graviton existed, wouldn’t it have to radiate from all matter? If it was a particle of determinate size, which could be quantized, wouldn’t the physical SIZE of an object affect how gravity works on it?

    I’m sorry for posting on everything, you’re probably sick of me by now

  4. James Anderson says:

    We know that where there is Mass there is gravity. The assumption is that gravity is created by Mass but has anyone considered that maybe gravity only exists where there is no mass. Almost like a positive air pressure shooting to a lower pressure area gravitons are propelled towards mass, striking all matter in the way actually pushing it. We generally assume that gravity ‘pulls’ us towards Mass. Maybe gravitons will never be detected on Earth because in such close proximity to such a large mass they are very sparse.. Maybe we are looking in the wrong place. Deep space maybe? Does anyone have any information on the experiments that have taken place thus far in search of the graviton? I am very interested in this subject and if anyone could point me in the right direction that would be greatly appreciated.


  5. Xerenarcy says:

    gravitons… heh, i remember the last time i thought they existed. fell out of relevance with most alternate theories i’ve read into. basically gravitons seem obsolete…

    once we have established a natural symmetry between acceleration and gravitation, to justify the need for a new particle for gravity is near impossible without making predictions about how acceleration should differ from gravitation. and if it doesn’t make a prediction about expected differences, it should predict gravitons’ behavior identically under acceleration. to my knowledge this is not the case; gravitons have been confined to gravitation.

    this disconnect between relativity and QM with ‘a particle for every force’ is further exemplified by gravitational (and accelerational) time dilation:
    would gravitons be affected by gravitational time dilation or would gravitons cause gravitational time dilation?
    (assuming acceleration != gravity) would gravitons in an accelerated frame of reference introduce a new type of force or time dilation?

    as i mentioned on another article yesterday, to me it is becoming more and more likely that answering what role time plays in acceleration under relativity will reveal more about gravity than anything else – there are demonstrable holes in QM and relativity where the two are supposed to overlap, and most if not all involve time varying or behaving strangely.

  6. Anders says:

    I know the force carrier of Electromagnetism is effected by Gravity as light can get bent by gravitational fields and because light can’t escape a black hole. I was wondering would Gravitons be effected by Gravity as well or would Gravitons be unaffected by Gravity? Could Gravitons be used to study the inside of a black hole?

  7. The Physicist The Physicist says:

    They are likewise “bent”. Gravity is a seriously complicated thing.

  8. Angel says:

    That is a very complicated and problematic issue. Higgs bosons, as we know them, are believed to be responsible for the mass of a particle, yet at the same time, Higgs bosons appear to have mass themselves. We know that gravity affects electromagnetism, which at the same time can affect strong or weak nuclear forces. Assuming gravitons did exist, it would not be odd to have them be affected by the force they themselves carry.

    It is complicated because we have the first issue of assuming that gravity is a force of attraction. The problem is that according to Einstein, gravity is nowhere caused by mass attraction, but rather by dilations and distortions of space. Gravity is rather a force of movement and acceleration. However, we do not know what is it that mediates this force of acceleration whenever we encounter spatial distortion. Just like electromagnetism, we know what it does and how it does it, but we didn’t know what mediated or carried that force. Now we know bosons do, but gravity is still the only force that doesn’t have a quantum explanation that coheres and corresponds with relativity. Thus we need a quantum gravity theory, which hasn’t been found yet. Physicists belief the existence of such gravitons can solve the problem and can give a basis to a good quantum gravity theory. Physicists are not exactly required to find gravitons in order to develop quantum gravity, but is still looks like an essential step to. Otherwise, it becomes much more complicated than it already is.

    @Xerenarcy: acceleration is distinguishable by Newtonian mechanics. Gravity is only a kind of acceleration.

  9. Mirthdog says:

    I am assuming I’m not the only idiot here. Please excuse my unintentional lack if verbiage.

    1. Does gravity actually need a “quantum theory”, or can it not be the slate that QT’s are written on?

    2. Would equations containing gravity be altered if gravity were input as a reaction as opposed to a force? I do not know what it is a reaction of, or if whatever gravity was “reacting to” would merely be a replacement for gravity, mathematically speaking.

    3. Yes, I did drop out of high school.

  10. Angel says:


    Gravity is a force produced by the tensors that curve space time. If we were to input gravity as a reaction, it would instead be in the form of acceleration or momentum.

    Gravity needs a quantum theory for the reason that we have discovered that all the fundamental forces of nature are carried by fundamental particles, with the exception of gravity. There is a hypothetical particle called graviton, which is the particle that we would conclude to carry out the reactions of gravitational force. The problem ends up being that we don’t have evidence that it exists, and if it did exist, we wouldn’t be able to understand it the way we understand other particles. The efforts to try to understand gravitational force at the quantum level, which is necessary in physics to understand the incompatibility of quantum mechanics and relativistic mechanics and discover the law that generalizes both, and also necessary to understand the exact initial conditions of the universe previous to the Big Bang, are what lead us to the necessity of finding a quantum gravity theory.

  11. Paul Jeynes says:

    I think it is a good question. I asked Steven Weinberg the same question when he was talking in London ( I was a research student) and he immediately responded “Well, what would you put on the right side of Einsteins field equations?” explaining further that there you have the energy momentum tensor where matter or particles goes into the theory. And that part is quantum , so are we just going to put only the expectation (average) values for the matter part? That would be a possible approximation but surely unsatisfactory as a full theory. It is hard to just have quantum probabilities for only matter and not spacetime because they occur here together in one very fundamental formula.
    I have come to recognise spacetime as a deformation in spacetime fabric similar to electromagnetism as waves in EM field. All these wavefields need some kind of quantisation. Trying to keep the spacetime out of this would general framework when it is so similar would thus be perverse and also not lead to a consistent theory anyway.

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