calamity said:
I believe, at least from what Einstein showed, we can safely ascertain that time is not a steady state. It speeds up, it slows down, all of it depending on circumstance. Even when we ignore Einstein's equations and just go by our own experiences, we've all felt time slow down at certain times of duress. Likewise it speeds up when we are doing something we really enjoy. So, at the very least, we know that the experience of passing time is dependent on perception.
That seems to be correct. I still kinda think that Augustin still has the ultimate word on time. To paraphrase: so long as I don't think about it, I know exactly what time is. But when I focus in and try to figure it out, time becomes utterly mysterious. It's one of the weirdest things it's possible to think about, at least in my view.
One interesting upshot to Maudlin's view (which is actually more mainstream than the article suggests) is that past time must be finite. That is, there must be a definite starting moment. I'm a bit too hungover right now to try to reproduce the proof, but here's the gist:
One might think that one could, in imagination, trace the chronology of the universe backward, rather as if time is on a line, and we can (again, just in imagination) walk it backwards infinitely. The problem with this is that, if so, we would not be here now. Since the arrow of time points one direction, this way of thinking about time is flawed. The exercise would be more like trying to start at an infinite time in the future, and working back to this moment. Now one thing to keep in mind is that each moment is real, so it cannot elapse in any shorter interval than it actually did. Once you get all these facts before you, it's just obvious that we could never work backward from a point in time infinitely far into the future to this moment. No matter how far we go, there would still be an infinite amount of time to traverse.
Applying this reasoning to our present moment, we see that time had to have a start a finite amount of time in the past. We can walk backward through the chronology of the universe to a starting point, a point beyond which there simply is no time, no more past to discover.
Now, if that doesn't make your head utterly shrink, nothing will.
calamity said:
What I would like to know is if someone ran a 100 yard dash on earth in 10 seconds flat, would they run it in 3 seconds if doing it on a spaceship moving close to C?
The way I understand it, it would still seem to the person on the ship as if the race took ten seconds. But from the perspective of someone on earth, it would seem to take much longer.
To see why, imagine a time-keeping device that consists of a laser beam bouncing between two mirrors, and each time it strikes one mirror, an interval is "clicked" forward (for ease of understanding, imagine the mirrors are placed far enough apart that it takes half a second for the beam to bounce from one mirror to the other, so as the beam strikes mirror 1, an attached device clicks forward one second). As this device accelerates, the laser beam has to traverse a longer distance. It's like throwing a rock at a car that is standing still just in front of you, and then throwing another as it drives away. The second throw will go over a longer distance, and if the rock travels at the same speed in both throws, the one when the car is driving away takes longer). From the perspective of someone in the same acceleration frame as the two mirrors, time keeps flowing at the same rate. But from the perspective of someone in a slower frame, time flows much more slowly
for the accelerated frame. That's why, if we sent a spaceship at a speed approaching C to a nearby star, the trip might seem to take a few years for those on board, but to us on earth, the trip might take centuries.
