Physics Question
I can't believe I didn't think of this question myself, dammit. But I just saw this elsewhere and didn't buy the response:
So: as we all know it's supposed to be true that if A leaves in a spaceship that travels near the speed of light and B stays on Earth, A will be younger than B upon his return. But wait: how can the laws of physics tell the difference between A and B? They are travelling at the same speed relative to each other, right? And there's no privileged third frame of reference, right? So WTF?
Rilkefan? Little help?
I can't believe I didn't think of this question myself, dammit. But I just saw this elsewhere and didn't buy the response:
So: as we all know it's supposed to be true that if A leaves in a spaceship that travels near the speed of light and B stays on Earth, A will be younger than B upon his return. But wait: how can the laws of physics tell the difference between A and B? They are travelling at the same speed relative to each other, right? And there's no privileged third frame of reference, right? So WTF?
Rilkefan? Little help?
posted by Winston Smith at
5:41 PM
11 Comments:
What's the response by which you are puzzled? Does it have something to do with the claim that the traveller returns, thus establishing 'home' as a special reference frame?
[ Assuming, just for fun, that space-time is a 4D box, the 'return' is actually to a point with the same spatial coordinates but a different time coordinate ]
The point of that response, IIRC, is that the traveller occupies two different intertial frames of reference (the one on the way out and the one on the way back), while the layabout only occupies one throughout.
Rather than rephrase all the stuff myself, I'll just leave you with the phrase "twin paradox" and let you have at the phrase with a search engine. After all, I ain't no fizzisist neither
It's no mystery. To return home, B has to turn around, thereby undergoing accelleration. He's no longer in a simple inertial frame of reference relative to A. You have to get General Relativity into the mix to handle this one correctly.
Without that turning around, each would continue to see the other as younger than himself due to time dilation.
Only one of them is accelerating. It's the acceleration, not the speed or distance, that matters. Since gravity is equivalent to acceleration, someone on a higher gravity planet would also experience time dilation.
Oh, right. That makes sense.
So time flies on Jupiter, but drags on Pluto? And if one could stand in a black hole would all the civilazations of men rise and fall before his very eyes?
Special Relativity
General Relativity
Haven't thought about this in a long time, but acceleration doesn't simply solve this - consider the case where twin 0 goes and comes right back, and twin 1 goes and comes back later, though it is true I think that duration in shifting frames marks the young twin For A and B, where A stays home, there are three frames to consider - F_A, F_B_going, F_B_coming. Something funny happens in B's perspective when observing A when switching from going to coming, but what A and B see in inertial frames makes sense at the end of the journey, whether calculating in SR or GR (I think).
An interesting twist that occurs to me is thinking about this in a closed universe - B goes straight and eventually comes back to A. This is in a way a simpler problem, as we skip F_B_coming and one acceleration. I'll have to mull on that.
A poem about GR and multiple-observer paradoxes.
If acceleration accounts for time dilation (e.g. the travelling twin's u-turn), what effect does a change in acceleration have? If the travelling twin's accleration is rising at a constant rate, would the time dilation he experiences thereby increase exponentially?
His acceleration could only rise up to a certain limit, because when you get close to lightspeed (around 90% as I recall), your mass starts to increases dramatically, so that you need exponentially increasing energy to maintain constant acceleration, let alone rising acceleration. The energy involved in accelerating to near lightspeed is enormous, I've seen references that say that accelerating a 1 ton mass to 90% of c requires ~30 gigatons (that's the energy of 30,000 1 megaton bombs, but with 100% of the energy converted into kinetic energy). Then, of course, you have to decelerate if you want to be able to stop and turn around.
Isn't the speed of light itself the reference point? So that a clock nearer the speed of light would seem to tick slower than an observer's clock, and a clock further from the speed of light would appear to tick faster than the observer's clock.
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