If they are real, what exactly do gravitons do?

LitleBang

Buckethead, I don't think velocity causes dilation. The acceleration getting to the velocity is what causes the time dilation.

benk99nenm312

This is very interesting. Nice job. I lack some of the math to understand parts of this but I get the just of it. So, is it a consencus that gravitons interact with others or is it still debated?

tom.stoer

Let me ask another question: is there consensus what gravitons are?

Max™

No, they're unobserved, there is just a graviton shaped hole in current quantum theories which suggests we should find one.

Vanadium 50

benk99nenm312, let me try again. Your view of what a graviton is has nothing to do with what physicists mean when they say the word "graviton".

I can have a static electric field, and charged objects feel a force in that field. I can also have excitations in that field, and they are called photons. That doesn't mean that charged objects must emit photons - a charged object doesn't have to glow.

I can have a static gravitational field, and massive objects feel a force in that field. I can also have excitations in that field, and they are called gravitons. That doesn't mean that massive objects must emit gravitons for exactly the same reason a charged object doesn't have to glow.

Your complaints about gravitons are based on a model that has nothing to do with what physicists mean when they say the word "graviton".

benk99nenm312

But if the charged object in the field wants to interact with another charged object, it mediates photons between them, right?

Vanadium 50

No. That would mean it glows.

You might have heard virtual photons are exchanged. Virtual photons are not real photons. You can't see them; you can't measure them; you can't count them; you can't detect them. You don't even need them to do the calculation. They are mathematical artifacts - on par with drawing an auxiliary line in geometry.

benk99nenm312

So you are saying that gravitons are mathematical artifacts?

A photon is real when it is not mediating the electromagnetic force. When is a graviton real?

tom.stoer

That is exactly my question: what are gravitons? physical particles? virtual obejcts derived from perturbation theory (which in inconsistent for GR)? a mathematical concept in Hilbert space? ...

My feeling is that the failure of naive perturbation theory for GR forces us to think about different concepts. If you look at some recent approaches for quantum gravity (loops, dynamical triangulations) you wan't find anything like a graviton in the fundamental concepts. That does not mean that these theories are wrong, it simply means that the concept of ordinary quantum field theory cannot be applied to quantum gravity.

Max™

That is well known, I thought. If it did work perfectly, we'd have a very good candidate for a unified field theory.

As for LQG, the actual links between nodes in spin foam models could be described as gravitons at least metaphorically.

Trying to fit things to actual physical particles isn't a helpful idea though, you can speak sensibly of wave-like or particle-like behavior, but assuming there is a neat little transverse wave diagram or point like particle down in there is misleading at best.

We know that whatever this stuff is, it behaves in certain ways which we can often find easily understandable metaphors for in our day to day experience.

We still aren't sure what this stuff actually IS exactly though.

Applying the methods which have successfully produced the Standard Model to Gravity is very difficult, but a few things happen when you do.

When you include the SSB/Higgs Mechanism, you get a Spin 0 Massive Boson shaped "hole" in the theory, and Spin 2 Massless Boson shaped "holes". The relationship between other symmetry groups and their bosons suggests the Spin 0 one should be the Higgs, which should then couple to other particles in certain ways to produce their observed masses.

The nature of Gravity in that it has no "poles" plus seems to propagate at light speed suggests the Spin 2 one should be a Graviton. This is at odds with the smooth continuum of spacetime described by GR, and one of the big problems seeking resolution which drives String/LQC/etc Theories.

tom.stoer

Max!

I fully agree with you - but reading the other posts I don't think that the colleagues have the same opinion.

I think that the graviton in LQG (if this concept makes sense at all) is the fundamental excitation of a spin network.

But the majority seems to have something in mind that is derived from ordinary quantum field theory, something like quantized plane waves. This is INCONSISTENT in GR. A plane wave in LQG is a very complex spin network state. I do not understand Rovelli's arxiv papers completely, but I understand enough to see that it is not the "ordinary graviton" as the majority may expect.

The problem with ST is that it works fairly well perturbatively (and therefore ST is able to tell us what the perturbative graviton is), but I do not see results for fully dynamical spacetimes with propagating gravitational fields (not necessarily plane waves).

Tom

Vanadium 50

No, I am saying that virtual gravitons are mathematical artifacts. Just like virtual photons.

Just like when the photon is.

benk99nenm312

I don't understand. If the graviton were real at any given time, what would it do? What would be its effects? What would we measure, detect, observe? Why haven't we observed it then?

Max™

Are you falling towards the center of the Earth currently?

There's your answer, I think.

benk99nenm312

So are you saying that, when real, gravitons have no effect what-so-ever on me or anything else.

This would indicate to me that they are not 'real'.. ever. That's why I'm confused.
(The only way something has absolutely no effect on anything else is if it doesn't exist.)

Max™

Again, it's a very misleading way to think of things.

Mathematically it is convenient to describe electromagnetic fields in terms of particles shooting photons at each other, and it turns out that light, real photons, actually has electromagnetic properties.

The difference being you don't see magnets "glowing" at each other literally.

So right now your butt is "glowing" at the earth with virtual gravitons in a convenient mathematical description.

So a gravity wave (literally a ripple in spacetime) would be akin to a photon in that it would be a defined waveform with gravitational properties instead of electromagnetic properties.

There's more truth and understanding to be found in that comparison if you really consider the nature of a magnetic field and light.

benk99nenm312

Yes, now I see. This helps define a graviton. Now the OP's question is somewhat answered. So is some of mine. Thanks.

So here's my question. If a satelite is positioned behind earth, as before, then how does a virtual graviton emitted from the sun interact with the satelite? Is the answer simply that it does not exist? That it is a mathematical artifact, and therefore is useless to apply in real nature (which is by the way, what we do, right?)? We try to answer what is happening in nature, so a mathematical description is great, but if a virtual graviton is not a viable conceptual explanation, GR seems much more appropriate. So again what I guess I really want answered is, how does a graviton reach a satelite behind earth?