Which gravity theory is right?

[Joza]

In Einsteins's relativity, gravity is described as a force arising from the warping of space-time by the presence of matter.

But, in string theory, it is described as a force arising from the exchange of bosons, the graviton, right?

Surely, both cannot be right? Are these compatible?

 

[DaveC426913]

Sure they can. These are simply two models describing the same thing.

But in fact, we have not done enough experiments to determine which of the two is more adept at describing gravity.

 

[belliott4488]

Einstein's theory (General Relativity, or GR) is a classical theory, whereas one of the goals of String Theory is to create a theory of gravity that is consistent with both GR and Quantum Field Theory (which describes all other known fundamental interactions). In that sense GR has to String Theory the same relationship that Classical E&M theory (Maxwell Eqs.) has to Quantum Electrodynamics.

(Actually, a Quantum Theory of Gravity was not one of String Theory's original goals, but rather it just came about as a nice bonus, which is what gives encourages many people to be optimistic about its correctness. Witten has even countered the claims that String Theory makes no testable predictions by saying that, in fact, it predicts the existence of gravity, as described in the classical limit by GR.)

 

[jtbell]

And as far as I know, no one has yet even proposed an experiment that can distinguish between the two, mainly because string theory has not progressed far enough to make generally-accepted experimental predictions that are different from those of general relativity.

 

[AlephZero]

Surely, both cannot be right? Are these compatible?

I would bet they are both wrong, but so far, nobody has found out why. That's the way science works, in the long term.

Of course they can be compatible. They can both be "near enough right" to be useful, or Einstein's theory of gravity might be a useful approximation to a string theory of gravity, just like Newtonian mechanics is a useful approximation to relativistic mechanics even though we know Newtonian mechanics is "wrong".

 

[rewebster]

I would bet they are both wrong, but so far, nobody has found out why. That's the way science works, in the long term.

Of course they can be compatible. They can both be "near enough right" to be useful, or Einstein's theory of gravity might be a useful approximation to a string theory of gravity, just like Newtonian mechanics is a useful approximation to relativistic mechanics even though we know Newtonian mechanics is "wrong".

Isn't that the one scientists use to send stuff to Mars, etc.?



I tend to look at all three as being 'not right' but still 'not totally wrong'.

 

[D H]

None of them (Newtonian, GR, string) are "right". "Rightness" (i.e., proof) is the domain of mathematics, not physics. String "theory" doesn't even qualify as a theory, yet. Mass is axiomatic and gravity has no mechanism in GR, and that's not "right" in the minds of many physicists. Too much detail is also not right, in a sense. Nobody in their right mind would use either string theory or GR to describe the geopotential (http://cddis.nasa.gov/926/egm96/egm96.html), for example.

 

[AlephZero]

Isn't that the one scientists use to send stuff to Mars, etc.?

Quite possibly. But Newtonian mechanics doesn't explain the orbit of Mercury, and it doesn't explain the behaviour of the clocks on GPS satellites. Relativity does explain both of them, to a practical degree of accuracy.

Approximate theories are fine, but if you use them it's a good idea to know what the approximations are.

Being a ME not a research physicist, I don't have a professional opinion on how well GR agrees with experiment - but there are clearly some bits of the jigsaw puzzle missing at the quantum mechanics level.

 

[D H]

What scientific theories are not approximations?

 

[pervect]

In Einsteins's relativity, gravity is described as a force arising from the warping of space-time by the presence of matter.

But, in string theory, it is described as a force arising from the exchange of bosons, the graviton, right?

Surely, both cannot be right? Are these compatible?

It's not so clear that they both cannot be "right". See for example http://xxx.lanl.gov/abs/astro-ph/0006423

A pedagogical description of a simple ungeometrical approach to General Relativity is given, which follows the pattern of well understood field theories, such as electrodynamics. This leads quickly to most of the important weak field predictions, as well as to the radiation damping of binary pulsars. Moreover, certain consistency arguments imply that the theory has to be generally invariant, and therefore one is bound to end up with Einstein's field equations. Although this field theoretic approach, which has been advocated repeatedly by a number of authors, starts with a spin-2 theory on Minkowski spacetime, it turns out in the end that the flat metric is actually unobservable, and that the physical metric is curved and dynamical.

So one can recover most of GR with a theory based on spin-2 bosons. There are a few issues with this approach, though.

The equivalence is only local. GR predicts the possibility of more complex topolgies (wormholes) than a quantum theory does (wormholes, closed universes, etc). So if we see a physical example of such a complex topology, that would support GR, and would suggest very strongly that the geometrical formulation is right. One would have to put any non-trivial topologies into the "spin-2 boson" theory by hand.

There may be other issues as well. The spin-2 theory as outlined is a bit difficult to deal with because it has entities in it that don't transform as tensors.