Starship said:
Few physicists know the complex math of general relativity.
I doubt it's true that "few" physicists know it--all cosmologists would have to, as should any physicist studying theories of quantum gravity. It may be that most physicists are required to study the basics as part of a general physics education in graduate school, I'm not really sure.
Starship said:
I'm not sure whether it's required in What is meant with time-travel? Does this mean that we can run the movie backwards? Can we go back to the stone age?
What does "run the movie backward" mean? The idea of slicing the block of ice into cross-sections and using them as frames in a movie was just a sort of schematic way of thinking about the difference between our ordinary notion of moving time and spacetime in relativity, the block of ice with the strings embedded in it is really the fundamental thing, just as spacetime as a whole with various worldlines embedded in it is really the fundamental thing in general relativity. You can sometimes do a "foliation" on spacetime which means cross-sectioning 4D spacetime into a series of 3D "spacelike hypersurfaces", but this isn't possible for all possible spacetimes, and in spacetimes where you can there's no unique way to do it (so two events which are part of the same hypersurface in one foliation would be part of different hypersurfaces in another).
But for any worldline, you can at least talk about the "proper time" experienced along that worldline, ie the time that would be read on a clock that followed that worldline. So backwards time travel in this context would mean that the clock's worline loops around in such a way that when it reads 4:00, it passes next to the point on the worldline where it read 3:00.
Starship said:
Anyway, here is a very good argument against time travel.
http://mb-soft.com/public/time.html
That's a pretty ill-informed argument actually. For example, they say:
Speculate now, that time travel into the past was possible, and that people a thousand years in our future learned how to do it and control it. For the moment, let's take a big example, that they decided to transport a medium sized hill from their time to ours, a thousand years earlier.
No matter WHAT process might be involved, it is certainly clearly necessary to know exactly where each atom was at some specific instant in time. This would be an astoundingly difficult accomplishment, but we will assume that those people a thousand years in our future would have figured out some way to do it.
Since we would need to somehow re-construct the structure of the hill, we would certainly need to know the position of each atom REALLY accurately. If two atoms were supposed to be right next to each other (as in a compound, like hydrogen and oxygen forming water molecules), The location of the atoms would obviously have to be known to about 10-12 cm (or one one-trillionth of a centimeter). This figure comes from nuclear physics, where this is a common distance of interaction between individual atoms.
But time travel as envisioned by physicists does not involve an object disappearing from one time and being reconstructed in another, like the transporter in star trek--instead, it involves a physical object taking a continuous trip through a twisty region of spacetime which results in it ending up in the past, there are no discontinuities where it disappears or reappears. Since the entire rest of the section of that paper is based on this disappear/reappear premise, it doesn't make sense. The paper also assumes that history could be "changed" by the presence of a time traveler, but in the "static spacetime" view, whatever the time traveler does in the past should have been part of history all along, so it wouldn't have changed anything. See
this thread for a discussion of how paradoxes can be avoided by postulating that only self-consistent histories would be allowed by the laws of physics.