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Photons are to Electrons as Gravitons are to ... what?

What is it that 'emits' a graviton? And what 'absorbs' it?

I've been looking for a good layman's description of how gravitons interact with ... things ... and have been unable to find one. The closest I've got is this boat and brick example. Which is good, as far as it goes. But has obvious flaws (negative momentum particle? the layman was following along great, to that point).

I'm hoping for something more like:

"When an electron in a higher-than-normal orbit falls back to its normal orbit, it emits a photon, a packet of energy, which carries that energy through space at the speed of light until it encounters another electron which absorbs it, causing the electron to move to a higher-than-normal orbit"

But for Gravitons:

"When a ____[electron substitute for gravitons] is in a ____ [state or action replacing the orbit change of electrons], it emits a Graviton, a packet of ____ [would it still be energy, or something else?], which carries that Gravity(?) through space at the speed of light until it encounters another _____[?] which absorbs it..."

How does an individual graviton work, in this type of description?

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  • $\begingroup$ See here for how the momentum transfer works. For the other part of your question, theoretically all particles exchange gravitons, while all charged ones exchange photons. $\endgroup$
    – J.G.
    Jan 19, 2022 at 8:49

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As long as you appreciate these are all deeply flawed "lies to children" mental pictures, and need not "represent" a consistent "story"...

Nothing of these things happen, or even remotely so, any more than a snake biting its tail represents cyclic organic compounds (another celebrated dream metaphor).

The brick thrown from boat to boat metaphor asks you to suspend your disbelief when the brick traveling from boat to boat sucks momentum from the recipient (absorber) boat and pulls it towards the emitter boat. This does not happen in conventional mechanics that you have build intuition for, but in quantum mechanics, a deeply counterintuitive theory, where you mostly follow the math.

Such pictures, including terms like photons or gravitons, are cartoon mnemonics for well-understood and applied calculational recipes: they are shorthand for mathematical expressions in perturbation theory, a calculational technique that yields good answers. Highly perceptive people have often been led astray by them.

In electromagnetism, two magnets emit and absorb gazillions of low energy virtual photons at and from each other, resulting in a mutual attraction (if they are aligned properly).

In quantum gravity, a speculative/hypothetical type of theory, an apple and the earth emit and absorb gagillions of "soft" gravitons at and from each other, creating a mutual attraction, classically understood as gravitational field attraction. Repulsion is not possible here.

In electromagnetism, (quantum) atomic transitions are described by photons than by emissions and absorption of classical electromagnetic waves.

In gravity, microscopic quantum phenomena are not observable directly (or have been observed), so you observe gravitational classical fields from and to everything that has energy (including light, and photons) or mass; or classical gravitational waves from fast-moving massive black hole or neutron star binaries, etc.

Because (in some highly technical way outranging our scope here) gravity is "weak", in comparison, it is freakishly hard/impossible to observe the type of quantum microscopic transition you are envisioning. So your pre-fab paragraph needs heavy deconstructive revision,

  • When a mass or clump of energy is in any state, it emits an indefinite number of real or virtual gravitons, packets of energy and momentum which thus carry that Gravity through space at the speed of light until it encounters another clump, spaceship, nebula, or photon which absorbs it, picking up the energy and momentum transferred.

however, I'm not quite sure what you were going to do with that picture. Unlike real photons which are routinely observable in the lab by several gadgets and techniques, single gravitons are not even remotely observable; F Dyson is famous for having doggedly sought a proof for this.

When theorists yap breathlessly about gravitons, they involve them in elaborate quantum calculations which might contribute to indirect shifts of recondite quantities possibly providing "smoking guns" for their (the gravitons') existence.

But it is quite hard to develop intuition on something unobserved and arguably disconnected from observable phenomena.

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    $\begingroup$ While this does not address the basic problem that these images are lies to children, you can make electromagnetic attraction more intuitive. Instead of throwing bricks toward another boat, you throw a boomerang away from it. It circles around and hits the other boat from the far side. $\endgroup$
    – mmesser314
    Jan 19, 2022 at 5:33
  • $\begingroup$ No! This is a common fallacy. The boomerang imparts momentum to the air, a medium to change direction… A nasty rabbit hole of the type decried… think about it. $\endgroup$ Jan 19, 2022 at 9:39
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    $\begingroup$ If you call it a fallacy, you are taking the idea to seriously. Exchanging photons isn't the same as throwing bricks either. But if you want to use that thought to illustrate the idea, you might as well use boomerangs too. The concept you want to illustrate is that exchanging photons can exchange momentum in either direction. You want to relate that to familiar classical ideas. Boomerangs and bricks aren't perfect, but it is better than bricks alone. $\endgroup$
    – mmesser314
    Jan 19, 2022 at 15:15
  • $\begingroup$ Yes, indeed, it's better than bricks alone, but would lead the exacting literalist (who might take the idea too seriously!) to strike back. Instead, you might as well throw a ball to a wall away from the absorber bat; have the ball ricochet, travel to the absorber boat and past it, ricochet again off a wall behind it; and hit it to bring about boat convergence, hence an attractive force. Then the literalists will come back and wonder about attraction versus repulsion... you get the picture. There is no end to shadow-boxing with metaphors... $\endgroup$ Jan 19, 2022 at 15:50
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In the analogy between photons and gravitons, (real) photons make up electromagnetic waves such as light and radio waves; and (real) gravitons make up gravitational waves such as are detected at LIGO. Just as photons are emitted by accelerating charges, gravitons are emitted by accelerating masses, at least in principle. In practice we do not have a complete quantum theory of gravity, so the details of which masses emit gravitons and under which circumstances are still somewhat hazy. But clearly if gravitons exist, large masses like neutron stars and black holes can emit them -- we can directly detect gravitational waves from such objects.

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  • $\begingroup$ So if an object with mass were entirely stationary (yes, I know that 'stationary relative to what?' is a complication to this question, but for the sake of argument...) it would have no gravity? $\endgroup$
    – Harthag
    Jan 19, 2022 at 15:28
  • $\begingroup$ @Harthag : No, it would emit no gravitational waves; just as a stationary electron emits no electromagnetic waves. I was talking about real gravitons, which, like any real particles, can be detected and have a definite amount of energy. Virtual particles, by contrast, are really just book-keeping devices in quantum field theory. Trying to read too much into how virtual particles behave is a mistake -- they are not real, and the colorful pictures involving virtual particles are just deeply flawed stories, as another answer says. $\endgroup$
    – Eric Smith
    Jan 19, 2022 at 22:13
  • $\begingroup$ Your original question did use real photons (emitted from orbital transitions, and carrying real amounts of energy) so that's why I answered describing real gravitons. $\endgroup$
    – Eric Smith
    Jan 19, 2022 at 23:24
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I have come up with a rather speculative theory that explains what you are looking for in a slightly different way.

Under Newton, gravity was an attraction between bodies (say planets), so there was a search for a particle emitted by one planet that attracted another planet. But this idea changed under General Relativity where we now say that gravity is caused by the warping of spacetime. You could say that gravity IS the warping of spacetime. Because a single planet can cause the warping of spacetime, gravity does not require two planets, just one. So the question becomes, what is it inside a planet that would cause the warping of spacetime?

We know that this warping is caused by "mass". But what is mass? A planet made of tin has less gravity than a planet made of lead, even if they both have the same size and same number of atoms inside. So we must look deeper into what is inside those atoms. We have protons, neutrons and electrons. And the protons and neutrons are made up of quarks and gluons. A planet made of lead has more quarks, gluons and electrons than a planet made of tin. So we are left with the choice of quarks, gluons and electrons as potential sources for this warping.

Scientists have told us that in the first fraction of a second after the big bang, there was only quarks, gluons and gravity; that's it. There were no electrons, so I think they can be eliminated from contention.

We know that a simple bar magnet works because the spin of the electrons inside that magnet generates a magnetic field outside of the magnet. I contend that gravity works in a similar way, but it is the spin of gluons inside the planet that generates this field surrounding the planet.

Now comes the interesting part. There is a theory known as Loop Quantum Gravity. In this theory, space itself is particulate. Scientists have told us that energy causes a warping or distortion of these tiny particles of space. My theory holds that energy, as in the energy of the excited gluon field surrounding a planet, actually causes a dilation (or swelling) of this particulate space. We could call this the warping of space.

As a further step, I believe that it is this particulate space that defines the speed of time. That is, light must travel between these particles. Therefore, if these particles are dilated then light, and thus time, travels more slowly. So the dilation of these particles is in fact a warping of space and time, or spacetime.

So in the end, the particles that you are thinking are called gravitons are actually gluons, which we know exist. It is the spin of these gluons that leads to gravity.

You can read my paper here. All of the references are inside. I believe that this answers your questions about the behavior of gravitons (i.e. gluons) to create gravity. Have fun.

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  • $\begingroup$ So does anything increase or decrease the spin of gluons? or cause them to align or prevent them from aligning, as with a magnetic field? $\endgroup$
    – Harthag
    Jan 19, 2022 at 0:01
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    $\begingroup$ -1. This answer is full of misconceptions and I believe it should be deleted. Gluons have nothing to do with gravity, they mediate the strong nuclear force, which ends up being irrelevant at large distances. Gravitons are a completely different thing with a completely different behavior (for example, gluons have spin 1, while gravitons must have spin 2). Please notice that we deal with mainstream physics here, not with personal theories that wildly challenge well-established knowledge. $\endgroup$ Jan 19, 2022 at 0:25
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    $\begingroup$ I concur, this answer contains a large number of misconceptions. For example, the source of gravity in General Relativity is the stress-energy tensor, not "mass". For another example, I don't know where the poster got the idea that there were no electrons created in the big bang, but that's not part of any model I'm familiar with (lepton number is conserved in the standard model). $\endgroup$
    – Eric Smith
    Jan 19, 2022 at 1:59
  • $\begingroup$ @Harthag Gluons spin at c, the speed of light. In normal conditions, they spin randomly. However it has been shown that it is possible to polarize proton spin in frozen hydrogen, and it has been shown that polarized proton spin results in polarized gluon spin. (See my paper for more on this.) $\endgroup$ Jan 19, 2022 at 2:37
  • $\begingroup$ @EricSmith Physicist John Wheeler said "Spacetime tells matter how to move, matter tells spacetime how to curve" I don't see any mention of a stress energy tensor in that statement. My theory merely provides a possible explanation for the second half of that statement; how exactly does matter tell spacetime how to curve? $\endgroup$ Jan 19, 2022 at 2:42

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