What’s the real reason you can’t go faster than the speed of light?
bigthink.com
bigthink.com
- Steven Wright
However, the ones that would stay on the Earth would never see me arrive at my destination, as they would have to wait those millions of years. Is it correct?
But then another thing I don't understand in this model is the twin paradox. If I returned back to Earth what would be the Earth time and my local time? Would that agree? Or would I arrive millions years in the future?
Well, to be a stickler for the laws of physics, you can't move through space at the speed of light. In principal, you can get moving pretty quick though, but what if there's a space pebble between you and those stars? If you're going at half the speed of light, the collision force would be in the atomic bomb range. So that's a potential problem.
And yes, if you returned home, you'd find that tens of thousands of years had passed. (Not millions. It's not that far.)
This explanation is designed to ask you to see if from the point of view of spacetime, rather than asking what it looks like from any particular point of view. You can use the conversion factors to translate.
What those factors tell you is that if they could "see" you, they'd see you moving very, very slowly on your spaceship, because it's always moving at very near the speed of light. They'd see you aging very slowly, and arriving at the center of the galaxy (a thousand generations later) having aged barely at all. Then they'd see you coming back -- again aging very slowly -- and arriving back another thousand generations later. When you stop your ship at earth, they'd watch a clock on your ship suddenly start ticking at a normal rate.
Time in space-time is not measured by your clock. It's measured by all clocks that are at rest with respect to each other.
Everybody sees you arriving at that point (0,0,0,<very large number>), including yourself. That's the number on the clock at your destination when you arrived. You took a different route through space-time to arrive there, which is why your wristwatch reads 1.
(We're assuming that the earth clock and the center-of-the-galaxy clock are not moving relative to each other, which isn't quite true but close enough to avoid introducing those complications.)
Since you are the one accelerating and decelerating, you're the one whose wristwatch cannot be trusted. That's the half-assed solution to the twin paradox. A better solution to the twin paradox involves watching the "boosts" from one frame to another and doing the math to calculate what happens. But you do know that the non-moving clock at the origin is always going to the longest route in time, and anything moving relative to it will be moving more in space (and thus less in time).
Special relativity does not, in itself, forbid you going backwards in time. But it does tell you what would be involved in going backwards in time that way. They involve things like imaginary mass and imaginary energy -- things that pop out of factor sqrt(1-v^2/c^2) when v>c.
As far as we can tell, imaginary mass and imaginary energy are nonphysical. Relativity doesn't say it has to be that way, but nobody has ever seen them and they don't seem to exist.
There are hints of it in the Standard Model, which define mass and energy in terms of even powers of real numbers. Again, that doesn't prove that other things can't exist, only that we've never seen them, and would expect the universe to look very different if they did exist.
It might have something to do with the configuration of the universe at very early times, the low-entropy state that is the reason that the universe is interesting (and, in particular, capable of having us in it). But that is an open question.
Meanwhile, special relativity is sufficient to rule out time travel unless you want to make some assumptions that mass and energy are very different from anything we've ever seen them do.
There is no 'past' to return to if you could reverse the arrow of time. I don't think the universe has frames like a video that you can rewind to. Interactions are probabilistic with time going forwards and they would still be probabilistic were time inverted.
Source: I listen to podcasts
We're probably riding a "wave" of spacetime and if we are then the rest of the universe is as well.
If that is the case, when a moment passes it ceases to exist in the same way that a ripple in a puddle of water does. If you wanted to go backwards in time, you would have to take the whole universe with you.
Nope, don't push it that hard. Four-velocity is a nice mathematical abstraction, but has nothing to do with "speed" and "travels" as we know them.
So, information about an earthquake or asteroid strike, or even a plane crash if you don't prevent it, is fine. You can choose not to get on the plane, based on the information. But if you did stop the plane taking off, what physical event would there be to communicate?
That is not to say information does routinely go back, or does just because somebody wants for it to. The universe appears not generally to care what people want. But if any does go back, and organic systems could pick it up, it would be adaptively advantageous to be receptive to it.
Again, just because it could happen, in principle, doesn't mean it does. It would depend on some biological structure being just incidentally sensitive to the signal, and then refined by selection. It would be a Rube Goldberg sort of process, but study of parasite lifecycles will cure you of skepticism as to nature's tolerance for dodgy complexity and marginal usefulness.