# Gravity and Gravitons

Glenn
If a spacestation were rotating to simulate 1g earth conditions, would gravitons be involved? Is this a gravity well like the rubber sheet analogy. How does the equivalence principle play into all this.

Please keep the answer in laymans terms if at all possible. I want some hopes of understanding it ;-)

-Thanks,
Glenn

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pervect
Staff Emeritus
You only really need gravitions if you're trying to quantize gravity. You don't need quantum gravity to describe a rotating spacestation, so it's best not to even think about gravitions, especially since we don't even have a theory of quantum gravity yet.

You could start talking about the geometry of space-time for a rotating space-station using classical GR. The geometry is non-euclidean, this shows up in many ways, one of them being that light beams don't travel in straight lines in the rotating reference frame. This would be significantly more produtive than talking about gravitions, and rather interesting if one has the right background. However, it's far from the simplest approach. Only a very few people are going to be interested or able to follow the classical GR approach.

The simplest answer is to use standard newtonian physics, and to point out that the observer on the space-station needs to accelerate constantly to maintain his posiition, and that this centripetal acceleration gives the space-station dweller the illusion of gravity, just like he was in an linearly accelerating rocketship.

A slightly more sophisticated answer is to use Lagrangian mechanics, so that one can talk about generalized forces. The use of generalized forces allows one to talk about the apparent "centrifugal" and "coriolis" forces that the space station dweller experiences. This requires more advanced physics, but I think it actually gets closer to what the space-station dweller experiences. The only problem with this is that someone will probably pipe up and say that "centrifugal force isn't a force", and explaining the nature of a generalized forces rigorously requires a lot of math, though it's fairly easy to grasp an intuitive concept of it.

Glenn said:
If a spacestation were rotating to simulate 1g earth conditions, would gravitons be involved?
That would be my guess.
Is this a gravity well like the rubber sheet analogy. How does the equivalence principle play into all this.
The rubber sheet analogy can be misleading. It can give you an idea of what the geometry of the spacetime is but it wouldn't, for example, be useful in describing a uniform gravitational field since that is a frame dependant notion. The equivalence principle comes in by being able to describe the objects as moving on this "rubber sheet".

pervect said:
You could start talking about the geometry of space-time for a rotating space-station using classical GR. The geometry is non-euclidean,
The geometry of spactime does not change when you change from an inertial frame of referance in flat spacetime to a rotating frame of reference.

Pete

Glenn said:
If a spacestation were rotating to simulate 1g earth conditions, would gravitons be involved? Is this a gravity well like the rubber sheet analogy. How does the equivalence principle play into all this.

Please keep the answer in laymans terms if at all possible. I want some hopes of understanding it ;-)

-Thanks,
Glenn
you cant mix the two up. Gravity in quantum mechanics and in general relativity are two different things. IF one if found out to be true, the other one will be ultimetelly false. You cant have both. One theory (quantum theory) talks about a vitrual particle interaction between matter particles. This virtual particle is called a graviton. On the other hand, general relativity talks about bending of space time, and this is the cause of gravity. This is not an analogy, by the looks of theorys today and experimental results, this is a true effect that takes place. We will actually be given more insight in about a month, when Gravity Probe B experiments begin being examined.
About your question with the equivalence principle, this is nothing hard to understand. All Einstein said was that there cannot be a gistinguishable factor between acceleration and gravity. Gravity in a sence can be simulated with acceleration and you cant tell the difference between the two.