# Why gravitons?
**URL:** https://boards.straightdope.com/t/why-gravitons/620578
**Category:** Factual Questions
**Created:** [May 1, 2012, 11:31pm UTC](https://boards.straightdope.com/t/why-gravitons/620578 "2012-05-01T23:31:40Z")
**Posts on this page:** 8
**Page:** 2
### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 7, 2012, 6:01am UTC](https://boards.straightdope.com/t/why-gravitons/620578/21 "2012-05-07T06:01:16Z")
> [@](#):
>
> So if we amplified them somehow, we could hear them?
Well, and converted them to sound waves. But yes, you can find sound files of, say, simulated neutron star mergers online. They start off low, then chirp high, and abruptly cut off.
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [May 7, 2012, 12:37pm UTC](https://boards.straightdope.com/t/why-gravitons/620578/22 "2012-05-07T12:37:17Z")
> [@iamnotbatman](#):
>
> The exchange of virtual gravitons. That is worth emphasizing because the perturbative treatment of “forces” in quantum field theory makes use of a convenient mathematical formalism involving mathematical terms we find convenient to associate with gravitons. This should not be surprising, since the gravitational “force” is a result of space-time curvature, and gravitons are vibrations in that curvature. Therefore gravitons provide a natural set of basis states for describing the interaction between particles and space-time curvature. Of course, what is “really happening” is up for debate. Ultimately particles affect space-time curvature and that curvature affects their trajectories; there is an interaction, and that interaction can be most conveniently described mathematically by the exchange of virtual gravitons.
>
> “But a particle just travels along the shortest path through space-time”, you object. Gravity is not actually unique in this respect. In QED and QCD, particles follow paths of extemal action. In all of these cases, when you try to actually calculate the path or the interactions, the “forces” that define those paths/interactions are described perturbatively using virtual particles.
So gravitons (and photons, etc.) follow the shortest path **and** the longest path possible? Could this be the answer to the GR question of inertia- every object’s virtual gravitons are interacting with the rest of the universe?
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### Author:  [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)
#### Post date: [May 7, 2012, 2:13pm UTC](https://boards.straightdope.com/t/why-gravitons/620578/23 "2012-05-07T14:13:40Z")
It’s really more proper to say “stationary” action (my bad) rather than minimal/extremal/etc, although it’s really not a point worth worrying about. Basically the action must either be a minimum or a saddle point, but the action is never actually maximized. When you calculate the trajectory of a particle using the principle of stationary action, you do it by finding a path for which the action is stationary. Whether that path happens to maximize or minimize (or is just a saddle point of) the action is not really relevant. It is not trivial to make a general statement about when a path of stationary action will be a minimum/maximum/saddle-point, but it can be proven that it is never a maximum. It is also difficult to find simple examples (I think – maybe someone here knows one) where the action is not minimized. I can think of one. Put a particle on a sphere. Pick two points on the sphere. There are two paths of stationary action. One is the path of shortest length: an arc along a great circle connecting the two points. The second path is also along a great circle connecting the two points, but going around the “long way”. Both paths minimize the action, and indeed, we know the particle could just as well take the shorter or the longer path: either way it would be travelling in a straight line. But you’ll notice that the longer path is not “the path of maximum length,” because we could have added wiggles to the path to make it as long as we wanted, but those paths would not have stationary action.
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### Author:  [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)
#### Post date: [May 8, 2012, 11:50am UTC](https://boards.straightdope.com/t/why-gravitons/620578/24 "2012-05-08T11:50:31Z")
> [@iamnotbatman](#):
>
> It’s really more proper to say “stationary” action (my bad) rather than minimal/extremal/etc, although it’s really not a point worth worrying about. Basically the action must either be a minimum or a saddle point, but the action is never actually maximized.
I gave a simple example of a (local) maximum: reflection from a sufficiently convex spoon (the curvature has to be greater than that of a paraboloid). What is so hard about that?
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### Author:  [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)
#### Post date: [May 8, 2012, 12:09pm UTC](https://boards.straightdope.com/t/why-gravitons/620578/25 "2012-05-08T12:09:54Z")
I don’t get it. How can it be a local maximum if you can always add wiggles that make it larger?
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### Author:  [@wolfking](https://boards.straightdope.com/u/wolfking)
#### Post date: [May 8, 2012, 11:15pm UTC](https://boards.straightdope.com/t/why-gravitons/620578/26 "2012-05-08T23:15:15Z")
> [@iamnotbatman](#):
>
> It is also difficult to find simple examples (I think – maybe someone here knows one) where the action is not minimized.
Well here is an example that occurs all the time in daily life. Whenever you look at an object in a mirror and you also have direct line of sight to that object, you are presented with two different paths, one(direct line of sight) is minimal and the other(through the mirror) is not. However they are both _locally_ minimal.
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [May 8, 2012, 11:56pm UTC](https://boards.straightdope.com/t/why-gravitons/620578/27 "2012-05-08T23:56:01Z")
An aside about gravitons: since we don’t have a good quantum theory of gravity, what we surmise about gravitons is more or less by default of what we guess has to be true. Since quantum theory holds that all energy has to be quanticized, gravity is presumed to be too. And simply to not contradict the known behavior of gravity, gravitons would have to have zero rest mass, be electrically neutral and be bosons with a quantum spin of 2. But all this is simply by way of all the alternatives being ruled out; we don’t have a positive theory yet that predicts them.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 9, 2012, 12:46am UTC](https://boards.straightdope.com/t/why-gravitons/620578/28 "2012-05-09T00:46:47Z")
Well, maybe. The reason why the String Model first started getting any attention is that versions of it which accounted for all of the Standard Model particles also all turned out to predict the existence of a massless spin-2 particle, despite that not being one of the things they were designed to predict. Of course, there’s very little real progress in the String Model.
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