Comment Re:Not the Big Bang (Score 1) 127
How does spacetime know how fast something is going through it? If there is nothing else other than spacetime and a single photon, what regulates the photon's speed? What is the speed relative to?
The easiest way to answer that is by saying you're thinking about it incorrectly. Our every day experiences leads us to believe that distances are absolute. That time periods are absolute. And if we're talking about the relative speeds in human experience, that's a very, very good approximation.
Turns out reality is weirder. It's not that spacetime "knows" how fast something is traveling through it. It's that space and time don't behave like our senses lead us to believe. So, from our perspective, if energy to use to accelerate a ship wasn't a problem, and we're here on Earth observing it head someplace 10 light-years away, the very earliest that ship will get to its destination will be just over 10 years, as no matter how much energy is used to accelerate it, it's going to just asymptotically approach c, but never reach it.
However, the thing is, it's not really a speed-limit, of how you would think of it. From the perspective of the people on the ship, if energy isn't a problem, you can get to your destination as quickly as you want. Under 10 years? Sure. You can get there in under a second (we'll assume we've figured out how to ensure everyone won't die from accelerating from 0 to close to c, then back to 0 again all within a second). You can get there in a millisecond. If you put more energy into acceleration, you can always get there faster. However, you still never go faster than light. It just so happens that, as you accelerate, you disagree with the people on Earth about how far your destination is. It used to be 10 light-years away, but now, after that huge acceleration, it's only 100 meters away, and you can cover that small distance really quickly at near the speed of light. To you, an infinitesimally small amount of time has passed. To the people on Earth, over 10 years.
TL;DR; It's not that that spacetime is preventing you from going faster, it's that spacetime is a 4-D Minkowsky space, and we're approximating it as a 3D Euclidean space + time in everyday life. That's a great approximation, until you start going really fast. Then it's just not good anymore.