I've just read Brian Greene's book "The Fabric of the Cosmos," and I have a couple of questions I hope I can get some help with. He talks about the equivalence of gravity and acceleration, specifically, "the force one feels from gravity and the force one feels from acceleration are the same. They are equivalent. Einstein called this the principle of equivalence." (pg. 67) And again, "Since gravity and acceleration are equivalent, if you feel gravity's influence, you must be accelerating." On page 341, he says "...just as the electromagnetic force is transmitted microscopically by photons, the gravitational force should be microscopically transmitted by another class of particles, gravitons (the most elementary, quantum bundles of gravity)." My question is: given this equivalence of gravity and acceleration, and given the theoretical role of gravitons in mediating the gravitational force, what role, if any, might gravitons play in acceleration? Are gravitons somehow generated when something is accelerated that produce the gravity-like effects? My second question concerns the possibility of past-directed time travel. Dr. Greene says, on pg. 459, "The fact is, no one has shown that the laws of physics absolutely rule out past-directed time travel." But my question is: does not the law of conservation of mass/energy rule out past-directed time travel? If a thing were to leave the universe of today and travel backward in time to the universe of a previous day, would not today's universe have less mass/energy, and the previous universe's day have more? Would this not violate the conservation law? I'm no scientist, just an interested layman trying to understand things that are way over my head. Can anyone set me straight on these things?
SciFi-like time travel implies that the past & future now exist in some sense, which does not seem plausible. SciFi-like time travel results in all sorts of paradoxes unless you are willing to accept some many worlds view of reality or some fierce restrictions on where you will be when you take a trip in time. The paradoxes go away if you traveling backwards or forwards 100 years requires that you end up at least 100 light years from earth. While the World Line concept views reality as a static 4D geometrical object, this view seems to be a usable model rather than a reality. For those not familiar with the World Line concept. Our normal perception of point-like particles is that they move in a 3D space. The GR World Line models describes a point-like particle as a static 4D curve. In this view, the past present & future all exist as complex stationary geometric objects. BTW: The claim that the laws of physics do not rule out time travel is a bit questionable. At the quantum level, there seems to be no observable arrow of time. While not observable, there might very well be an arrow of time at the quantum level. Else why are classical level events not time reversable? Id est: You cannot unscramble an egg. I think gravitons are a Quantum Theory view of gravity. They might not have been detected yet. Maybe they do not exist. Some theory (EG: Loop Quantum Gravity) which marries General Relativity & Quantum Theory is yet to be developed. It may or may not require gravitons. In spite of the apparent equivalence of gravitationa & acceleration effects, there is no need for acceleration to involve graviton-like particles. Remember that GR is only a very useful model of reality. It is not reality.
Unbeliever: Welcome to sciforums. Gravity and acceleration are not quite the same thing. Gravity is generated by concentrations of mass or energy, which in Einstein's theory cause spacetime to curve. Einstein's theory is not a quantum theory, and so it does not discuss gravitons. The graviton is a theoretical particle that must somehow tell objects about the local curvature of spacetime. Presumably, gravitons would be emitted by one object with mass and absorbed by another, which would then feel a "gravitational force". Merely accelerating in the absence of a curved spacetime caused by mass would not involve the emission of gravitons, I don't think, though I am not an expert on this. Perhaps there would be a compensating flow of energy/mass in the other direction if something time-travelled. The whole issue is quite complicated, though. When you talk about "the universe of today", you need to realise that the universe of today for a person travelling past you at walking pace is a different universe of today from your one, because the ideas that you both have about which events are simultaneous are different. The very concept of "today" depends on your state of motion. I think that conservation of energy is a difficult thing to define universally.
