HorseloverFat: Yes, negative inertia is part of the definition of negative mass. Thanks for pointing that out. The implication of this, is that if you apply a force to the left on such an object, it’ll accelerate to the right. Fun, eh?
Jeff_42: Yes, there are ways (or rather, there may be) to get from one point to another without actually “moving”, and hence avoiding the time dilation. You can still arrange to travel back in time with such methods-- That’s why I mentioned “FTL or the equivalent”. I explained this once already in this thread. Feel free to ask me further questions about that. As to your two photons from your ray gun, photons will always take a path which will minimize the amount of travel time: This is called Fermat’s Principle. If photons take more than one path from one point to another, such as through a lens, the two paths take the same amount of time. Hence, you can’t have a photon go around the Earth, unless there’s some strong gravitational fields or other effect causing that path to be as sort or shorter than the path straight through.
AHunter3:
Yes, unless those extra dimensions are timelike, instead of spacelike. The distance formula for three dimensions is d[sup]2[/sup] = x[sup]2[/sup] + y[sup]2[/sup] + z[sup]2[/sup]. In four dimensions, however, if the fourth is timelike, it’s d[sup]2[/sup] = x[sup]2[/sup] + y[sup]2[/sup] + z[sup]2[/sup] - c[sup]2[/sup]t[sup]2[/sup] : note the minus sign.
Ozone: A stoppage of motion (even if allowed by QM) wouldn’t stop time… It’s really fast speeds that screw things up, not really slow.
HorseloverFat: the stretch/scrunch idea is one way, but it’d take a LOT of energy. There’s another proposal floating around that would be a lot more efficient, using something best described as a warp bubble (whaddaya know, that’s what Star Trek called it, too…) Unfortunately, you still need negative mass. It’s believed that all methods require that stuff somewhere or another.
Jeff_42: The restriction on going back to when the machine was invented applies to some of the proposed methods, such as wormholes, but not necessarily to all of them. How do you know that future folks aren’t visiting us?
I hope this makes things clearer, and at worst, it helps my post count 