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In String Theory it is predicted that as a result of the closed strings we have spin-2 gravitons.

1) How do we know there must be an excitation of spin-2 particles?

2) Why does a spin-2 particle HAVE to be a graviton?

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Related: physics.stackexchange.com/q/14484/2451 Related question by OP: physics.stackexchange.com/q/23926/2451 –  Qmechanic Apr 22 '12 at 14:35
    
^ The perfect question to go to after seeing a correct answer to this question –  Olly Price Apr 22 '12 at 15:44

3 Answers 3

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You ask:

In String Theory it is predicted that as a result of the closed strings we have spin-2 gravitons.

1) How do we know there must be an excitation of spin-2 particles?

String theories have been chosen and are being extensively studied because they can have a representation of spin two particles . One searches for such theories because when attempting to quantize gravitation the carrier particle, equivalent to the photon or gauge bosons, that appears in the quantum field theories studied, has spin two.

So only theories that can accommodate spin two particles can be candidates for a theory of everything.( strong, weak, electromagnetic and gravitational forces).

2) Why does a spin-2 particle HAVE to be a graviton?

It does not HAVE to, but if the theory under study does not have a spin two particle to be a candidate for describing the graviton, it cannot turn to be a Theory of Everything (TOE), which is the holy grail of theoretical physics.

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Thanks @anna v. Do some Physicists oppose String Theory because of the fact that the spin-2 particle could be something other than the graviton? –  Olly Price Apr 22 '12 at 15:46
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Theoreticians who oppose string theory have their own pet theories,but in the mathematics of all theories the objective is to accommodate the Standard Model and gravitation. Since all current theorists work with Quantum Field Theory as guide the graviton will be described by spin 2, because that is what comes out from the unsuccessful efforts to quantize independently the gravitational field. String theories are the only that can accommodate everything mathematically, at the moment. –  anna v Apr 22 '12 at 16:19
    
@OllyPrice: The string theory graviton was not identified as a graviton because it was spin-2. This is insufficient evidence by itself, you have to check that it has gravitational ward identities. –  Ron Maimon May 1 '12 at 21:08

have a look at http://en.wikipedia.org/wiki/Graviton. This explains that a spin 2 particle must be equivalent to general relativity. The details of why this is are far over my head I'm afraid.

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No worries, there seem to be so few people that really have an in depth explanation as to why we say that there is this 2-spin particle there, a shame. –  Olly Price Apr 22 '12 at 19:01

The reason is that in string theory, every operator on the world sheet creates a physical particle or a superposition of particles. When you make a small change in the background metric that the string is moving along, this changes the action by a certain operator, which means that a background geometry is equivalent to some particle in the string theory. The coordinate invariance means that if you change the metric in a coordinate way, nothing happens, and this gives the gravitational ward identity. This is worked out in chapter 2 of Green Schwarz Witten, and it is an insight most straightforwardly explained by Tamiyaki Yoneya in the late 1970s and early 1980s.

The reason it is spin 2 is the same as in GR--- it's because it's a small change in a metric, and a metric is spin 2 (two symmetric indices).

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