What is the evidence for GR and against the graviton theory?

[belliott4488]

There is such a thing as a true theory. I think, therefore, I am; therefore the universe is real, and has real laws. It is true that we can never prove one of our made up theories to be exactly in accordance with a true law of the universe, but the goal in science is to keep approaching the truth in the hopes that someday, we actually do know the true laws. Perhaps we are overly ambitious as a species by our pattern of always believing the latest theory up until the point where it is disproven. But once a better theory arises, we can reject an old theory.

Sorry, I think you're conflating a matter of faith with science. A professor I once heard teaching intro-level college Physics (I was a TA, not a student) used to emphatically attempt to drive home the point that no (finite) number of experiments can ever prove a theory true, but it takes only one* experiment to prove it false.

(*Technically, it takes two, since a second one is needed to confirm the results of the first, but that's not really a second experiment; it's a repetition of the first one.)

If you think there are "true" theories floating out in Heaven and that it is our objective to try to approach these ideal theories with our imperfect ones, well - there were certainly a lot of Greek philosophers who would have agreed with you (as would have Newton, it should be pointed out), but I believe that way of thinking is of little relevance to modern science.

All modern scientific theories are conditional, in the sense that they are accepted only as long as they continue to make predictions that agree with observation, but we must always remain open to the possibility that a new experiment will prove even our best theory false. That's why we no longer call the axioms of our theories "Laws", as Newton did in the belief that he was discovering the true laws of God.

GR so far has produced no predictions that are not verified by observation - in that sense it is as "true" as any theory we've ever had. The desire to create a Quantum Field Theory of gravity (and the graviton) comes about because we expect that eventually we will find an experiment whose outcome is not correctly predicted by GR (at the Planck scale, most likely), but no such theory exists yet, in the sense of its being able to make testable predictions. For that reason, it makes no sense to speak of "choosing a graviton theory over GR" - we all hope to have a QFT for gravity some day, but it will necessarily include the predictions of GR in the classical limit.

 

[Vanadium 50]

I've spoken with several physicists on the matter, none of which have been able to point to any evidence that strongly supports GR over the standard model.

What makes you think the standard model has anything to say about gravity at all?

You are drawing-up a conflict that doesn't exist.

Exactly.

 

[Vanadium 50]

My understanding (I'm not a particle physicist) is that the graviton model consists of taking the predictions of GR and determining what properties a graviton would have to match them.

That's essentially correct. What one does is ask what sort of quantum theory would have the the weak-field limit of GR as it's long-distance and weak-field limit.

Because the quantum theory is invalid in the strong-field case, there's no way to match this to the strong-field case of GR.

 

[Vanadium 50]

1) GR was proposed to explain the bending of light by gravity. Although it was a WACKY idea, people chose to accept it because it explained the observable events when nothing else did. Also, people may have been more open to the concept in light of the current political situation.

Virtually none of this is correct. Let me address these points one by one.

GR was not proposed to explain the bending of light by gravity. GR was published in 1916, and the famed Eddington eclipse expedition was in 1919. The deflection of light by gravity in fact is predicted classically: Cavendish discussed the possibility of Newtonian black holes in 1783, as did Laplace in 1796.

2) It was discovered that all of the other fundamental forces are mediated by gauge bosons in the standard model, and all of their associated bosons have been observed. This theory makes intuitive sense.

This is an oversimplifcation. "Mediated by gauge bosons" means that one can take the classical field, and when treated quantum mechanically there emerge discrete excitations of the field. For example, if you start with the discrete electromagnetic fields, you get a photon out. But the existence of a photon doesn't mean that we toss out classical electromagnetism any more than the existence of a graviton means we toss out it's classical field theory: GR.

3) The idea that gravity may also be mediated by a boson is logical, and was proposed, but since we already have a working model for gravity and we can't find a graviton...GR stays. Of course, we have no equipment capable of detecting the graviton, so we have no way to disprove GR.

I challenge you to calculate the perihelion advance of Mercury using gravitons.

The reason people use GR and not a quantum theory of gravity is that they can get answers out of GR, and these answers match observations. We don't have an internally consistent quantum theory of gravity - what you call "graviton theory".

4) The evidence for GR is not actually evidence for GR. It's really just evidence that gravity bends light. But the graviton can also explain that.

Well, that and:

• Perihelion advance of Mercury
• Gravitational time dilation
• Shapiro delay
• Equivalence of free-fall
• Nordtvedt effect nonexistence

So as far as I can tell, both theories explain the observations, and the only reason to believe in GR is it was proposed first.

Are you able to do either of these calculations? I suspect not, because nobody can do calculations in "graviton theory", as the theory is internally inconsistent. What people do is they say "this corresponds to GR in the weak-field long-distance limit" and then do the GR calculation.

If you can't do the calculations yourself, how can you tell that one set is better than the other?