# Graviton source

If gravitational force is caused by a particle (tensor boson) which is massless and so travels at c, why doesn't matter ever exhaust, or even seem to reduce, its supply of these particles?

Drakkith
Staff Emeritus
Force carrier particles aren't literal particles that go streaming out from an object to exert a force. It's very difficult to explain, but the simplest answer I can give you is that these carrier particles are closer to a convenient way of looking at the math than actual particles.

jtbell
Mentor
If gravitational force is caused by a particle (tensor boson) which is massless and so travels at c, why doesn't matter ever exhaust, or even seem to reduce, its supply of these particles?

If the electrostatic force is caused by a particle (photon) which is massless and so travels at c, why doesn't a charged object ever exhaust, or even seem to reduce its supply of these particles?

I expect the answer to the gravitational case is the same (or very similar) to the electrostatic case, which is that the "force-carrying particles" are "virtual" ones, i.e. artifacts of a certain procedure (the "perturbation expansion") for calculating the effects of those forces. In the electrostatic case, there are other ways of doing the calculation (at least in principle) besides using virtual photons, so the virtual photons aren't "necessary." I expect the situation is similar with the gravitational case.

I think it's also clear that you can't treat gravitons as regular particles by the sheer fact that even though they move with c *away* from the heavy object, their effect is to *pull* at whatever they meet.

Drakkith
Staff Emeritus
I think it's also clear that you can't treat gravitons as regular particles by the sheer fact that even though they move with c *away* from the heavy object, their effect is to *pull* at whatever they meet.

They don't even *move*, as they're virtual particle in this context, not real particles.

True, but any change will propagate with c away from the source. That could be conceived to be a particle-like property, i.e. an indication that there is a constant "stream" (not that it is, just saying).

Honestly, I myself am still uncomfortably vague about the nature of particles. I understand they are excitations of a field, but whenever I think I'm getting a good mental handle on them, there's an aspect to it that confuses me again. Like for example virtual particles.

Staff Emeritus
That's fair, but maybe your shouldn't attempt to explain them to others until you are comfortable and no longer confused.

Drakkith
Staff Emeritus
True, but any change will propagate with c away from the source. That could be conceived to be a particle-like property, i.e. an indication that there is a constant "stream" (not that it is, just saying).

If so, then you could ask, "What rate are they being emitted?" But that concept makes no sense when it comes to virtual, force carrying particles. The electromagnetic and gravitational interactions are continual processes that display no particle-like behavior outside of E&M waves.

Thank you all very much for your clarifications and musings. (Actually, Vanadium, hearing from the similarly "confused" is helpful, or at least interesting.)

I like your explanation, Drakkith. Almost seems to say the math is defining more than describing reality. It jives with what I've read since posting my admittedly unenlightened question: that virtual particles aren't particles at all, but disturbances in fields. As Matt Strassler puts it: "A particle is a nice, regular ripple in a field, one that can travel smoothly and effortlessly through space, like a clear tone of a bell moving through the air. A 'virtual particle', generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields." Note that in his simile he likens them to compression waves.

Intrigued by your rhetorical parody of my question, rumborak. Coulumb force/interaction is the attraction or repulsion (not emission) of particles. I guess your analogy is lost on me a little.