- #1
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Help with a little "Gedanken" experiment please?
Hello.
I was reading the chapter on special relativity from Brian Greene's The Elegant Universe, just to aid my understanding of the basics. SR isn't coming easily to me. During the discussion of the speed of light, and the build up to the postulate that the speed of light is constant in all inertial frames of reference, Green gives an example of two people playing catch. He does so to illustrate the pitfalls of the Galilean addition of velocities when applied to situations involving light. Basically the scenario is a guy's playing catch with his friend when a thunderstorm arrives and they duck for cover. When it passes, they resume the game but the friend has gone wild for some reason and tosses the guy a hand grenade instead of the ball. They are initially separated by distance d.
The guy turns to run away, and Green notes that our common sense would suggest that the velocity of approach of the grenade is the velocity with which it is thrown minus the velocity at which the guy flees (call this v). Then Green asks us to reconsider the situation replacing the grenade with a "laser gun". He points out that, absurd as it may seem, the shooter sees the photons chasing the guy at c, and the guy sees them approaching at c, NOT at c-v (for this would violate the principle of relativity: Maxwell's electrodynamics is supposed to be frame invariant). Nevertheless, it seemed paradoxical to me, even though I have studied SR in school before, when I swallowed it without question.
It occurred to me that if I could nail down exactly why I thought it was crazy, my understanding of how SR requires us to alter our notions of space and time would deepen. Upon a second reading, something *clicked*. I thought of it this way: in the frame of the fleeing guy, we regard him as stationary, jogging on the spot, with the world whizzing by him at v. His crazy friend is receding behind him at v, but this relative motion between observer and source doesn't affect the velocity of the oncoming photons. So assuming the guy turned to flee just as his friend fired, then the very first photon to emerge would have only distance d to cover (remember the fleeing dude is stationary in this frame). In the other frame (the rest frame of the shooter), the photon is still chasing him at c, but has a longer distance to cover! So given that the laser beam travels at the same velocity in both frames, but has to go farther in the latter, it would seem that the shooter would predict that more time would elapse before the guy got hit than the guy himself would predict. In other words, the fleeing guy predicts he will get hit BEFORE the shooter's predicted time! It's nuts. From the point of view of both observers, the guy has to either have been hit already, or not yet! THEN I realized what the resolution was. My statement above:
"In other words, the fleeing guy predicts he will get hit BEFORE the shooter's predicted time!"
is valid only if you consider time to be absolute and immutable, ticking away the seconds dispassionately, to use the colourful language of Brian Green. If you don't make that assumption about time, then you can instead conlude that the shooter sees the guy get hit "when" the guy feels himself get hit, but the elapsed time up to that event is different as measured by the two friends, ie they don't agree on how long it took. The shooter thinks it took longer. The fleeing guy's watch appears to have been running slow (to the shooter), for he has registered less elapsed time. It may seem mundane to you guys, but this was a profound realization for me, because I had gone from the postulate that c is constant in all inertial frames STRAIGHT to the conclusion that the elapsed times for an event to occur would be different as perceived by observers in relative motion at constant velocity. There was no hoopla in arriving at the conclusion. Much to my satisfaction, it arose as a necessary consequence of the postulate. The relative nature of space and time no longer seemed so hard to believe.
That having been said, I wanted to make sure there were no flaws in my reasoning. The only way to do that, I thought, was to set up the situation, and try to derive the exact factor by which time had slowed down for the fleeing guy as perceived by the shooter. I hoped that the answer I would get would be the familar relativistic gamma, but I haven't had any success. Sorry for the huge preamble, here's my work:
Fleeing guy's frame S' (moves at speed v wrt shooter's frame: S)
The photon that is emitted just as the guy turns to run has only to cover distance d (because the guy is stationary in this frame). So the time to impact will be:
[tex] t' = d/c [/tex]
S-frame:
The shooter sees the photon close the gap between himself and the target, but since the target is in motion, that distance is larger. If it takes time t to impact, it has to cover a distance:
[tex] ct = d + vt [/tex]
(note, My entire problem may be that the RHS of this eqn is wrong, but I don't know how to correct it. My reasoning leads me to arrive at that expression.)
[tex] ct - vt = d = ct' [/tex]
[tex] t(c - v) = ct' [/tex]
[tex] t = \frac{ct'}{c - v} [/tex]
[tex] t = \frac{t'}{1 - v/c} [/tex]
Umm...that's not the right answer.
It should be t as measured by the shooter is GAMMA times the fleeing guy's proper time, right? Can somebody please help me fix this up?
Hello.
I was reading the chapter on special relativity from Brian Greene's The Elegant Universe, just to aid my understanding of the basics. SR isn't coming easily to me. During the discussion of the speed of light, and the build up to the postulate that the speed of light is constant in all inertial frames of reference, Green gives an example of two people playing catch. He does so to illustrate the pitfalls of the Galilean addition of velocities when applied to situations involving light. Basically the scenario is a guy's playing catch with his friend when a thunderstorm arrives and they duck for cover. When it passes, they resume the game but the friend has gone wild for some reason and tosses the guy a hand grenade instead of the ball. They are initially separated by distance d.
The guy turns to run away, and Green notes that our common sense would suggest that the velocity of approach of the grenade is the velocity with which it is thrown minus the velocity at which the guy flees (call this v). Then Green asks us to reconsider the situation replacing the grenade with a "laser gun". He points out that, absurd as it may seem, the shooter sees the photons chasing the guy at c, and the guy sees them approaching at c, NOT at c-v (for this would violate the principle of relativity: Maxwell's electrodynamics is supposed to be frame invariant). Nevertheless, it seemed paradoxical to me, even though I have studied SR in school before, when I swallowed it without question.
It occurred to me that if I could nail down exactly why I thought it was crazy, my understanding of how SR requires us to alter our notions of space and time would deepen. Upon a second reading, something *clicked*. I thought of it this way: in the frame of the fleeing guy, we regard him as stationary, jogging on the spot, with the world whizzing by him at v. His crazy friend is receding behind him at v, but this relative motion between observer and source doesn't affect the velocity of the oncoming photons. So assuming the guy turned to flee just as his friend fired, then the very first photon to emerge would have only distance d to cover (remember the fleeing dude is stationary in this frame). In the other frame (the rest frame of the shooter), the photon is still chasing him at c, but has a longer distance to cover! So given that the laser beam travels at the same velocity in both frames, but has to go farther in the latter, it would seem that the shooter would predict that more time would elapse before the guy got hit than the guy himself would predict. In other words, the fleeing guy predicts he will get hit BEFORE the shooter's predicted time! It's nuts. From the point of view of both observers, the guy has to either have been hit already, or not yet! THEN I realized what the resolution was. My statement above:
"In other words, the fleeing guy predicts he will get hit BEFORE the shooter's predicted time!"
is valid only if you consider time to be absolute and immutable, ticking away the seconds dispassionately, to use the colourful language of Brian Green. If you don't make that assumption about time, then you can instead conlude that the shooter sees the guy get hit "when" the guy feels himself get hit, but the elapsed time up to that event is different as measured by the two friends, ie they don't agree on how long it took. The shooter thinks it took longer. The fleeing guy's watch appears to have been running slow (to the shooter), for he has registered less elapsed time. It may seem mundane to you guys, but this was a profound realization for me, because I had gone from the postulate that c is constant in all inertial frames STRAIGHT to the conclusion that the elapsed times for an event to occur would be different as perceived by observers in relative motion at constant velocity. There was no hoopla in arriving at the conclusion. Much to my satisfaction, it arose as a necessary consequence of the postulate. The relative nature of space and time no longer seemed so hard to believe.
That having been said, I wanted to make sure there were no flaws in my reasoning. The only way to do that, I thought, was to set up the situation, and try to derive the exact factor by which time had slowed down for the fleeing guy as perceived by the shooter. I hoped that the answer I would get would be the familar relativistic gamma, but I haven't had any success. Sorry for the huge preamble, here's my work:
Fleeing guy's frame S' (moves at speed v wrt shooter's frame: S)
The photon that is emitted just as the guy turns to run has only to cover distance d (because the guy is stationary in this frame). So the time to impact will be:
[tex] t' = d/c [/tex]
S-frame:
The shooter sees the photon close the gap between himself and the target, but since the target is in motion, that distance is larger. If it takes time t to impact, it has to cover a distance:
[tex] ct = d + vt [/tex]
(note, My entire problem may be that the RHS of this eqn is wrong, but I don't know how to correct it. My reasoning leads me to arrive at that expression.)
[tex] ct - vt = d = ct' [/tex]
[tex] t(c - v) = ct' [/tex]
[tex] t = \frac{ct'}{c - v} [/tex]
[tex] t = \frac{t'}{1 - v/c} [/tex]