Timeline for Does light in vacuum actually travel at the speed of light? [duplicate]
Current License: CC BY-SA 3.0
28 events
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| Mar 1, 2016 at 6:43 | comment | added | Qmechanic♦ | The question formulation (v4) seems to conflate massive objects (clocks, persons, observers, etc) and massless objects (photons), which have different physical properties. | |
| Mar 1, 2016 at 0:53 | review | Reopen votes | |||
| Mar 1, 2016 at 6:29 | |||||
| Mar 1, 2016 at 0:35 | history | edited | Yevgeny Simkin | CC BY-SA 3.0 |
a slight elucidation of my original question
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| Feb 29, 2016 at 15:04 | comment | added | WillO | @GeniaS: Yes, your imagined scenario is incorrect. If you and I are in motion relative to each other, I will see your clocks slowed down, not speeded up. | |
| Feb 29, 2016 at 7:50 | history | closed | Qmechanic♦ | Duplicate of Does a photon in vacuum have a rest frame? | |
| Feb 29, 2016 at 7:49 | history | edited | Qmechanic♦ | CC BY-SA 3.0 |
edited title
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| Feb 29, 2016 at 7:48 | history | protected | Qmechanic♦ | ||
| Feb 29, 2016 at 7:42 | answer | added | user12029 | timeline score: 1 | |
| Feb 29, 2016 at 7:42 | history | edited | Qmechanic♦ |
edited tags
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| Feb 29, 2016 at 7:37 | comment | added | Yevgeny Simkin | firstly... I appreciate everyone's patient participation in what is obviously a very silly question - I just wish I understood the root of the silliness. But I still see an inherent contradiction. Particle A is moving at c-1 in orbit around the earth. It witnesses life on earth progressing at some notably accelerated rate (because its relative time is slower). When it reaches c what does life on earth start to look like? In my imagined scenario life on earth accelerates from fast(er) to infinity. Is this incorrect? | |
| Feb 29, 2016 at 7:14 | answer | added | Fabrice NEYRET | timeline score: 0 | |
| Feb 29, 2016 at 5:14 | answer | added | Anders Gustafson | timeline score: 0 | |
| Feb 29, 2016 at 4:27 | comment | added | knzhou | @WillO In this limit, objects in the sequence of frames see objects that are at rest at $O$ increasingly length contracted, so that their length indeed goes to zero in the limit. That's all the OP wanted to know. | |
| Feb 29, 2016 at 4:26 | comment | added | knzhou | @WillO Oh, come on. I was going to type out "a series of frames with their relative velocity to a fixed frame $O$ approaching the speed of light" but didn't because it was clunky and obvious what I meant. We all understand basic SR here. | |
| Feb 29, 2016 at 4:21 | comment | added | Asher | @GeniaS. Let's say you measure time with a light clock: you pulse a laser at a mirror half a light-second away, and when it gets back (one second later) you move your second hand one second forward and send another pulse. If your measure of time changed with your velocity, you'd expect that you'd measure "different rates of time" at "different speeds," but that doesn't happen because $c$ is constant in all inertial frames. One consequence of this is that light always travels faster than you do, and we can verify that experimentally. | |
| Feb 29, 2016 at 4:17 | comment | added | WillO | @knzhou: In other words, the "limit" you're talking about doesn't exist in the first place, and if it did, it would be uninteresting for the reasons in Asher's comment. | |
| Feb 29, 2016 at 4:15 | comment | added | WillO | @knzhou: Actually, what you've said is entirely wrong. There is no such thing as "frames that get closer and closer to the speed of light", because there is no such thing as the (absolute) speed of a frame. (That's why it's called "relativity".) And even if it made sense to talk about this limit (which it doesn't), you still wouldn't learn anything from it, because the dimensionality of the frame does not vary continuously with "speed", so you learn nothing about what happens in the limit by looking at what happens very close to the limit. | |
| Feb 29, 2016 at 4:14 | comment | added | Asher | @knzhou something "approaching infinity" or "approaching zero" at a limit is very different from being actually infinite or actually zero. No matter how fast your spaceship travels, the onboard clock will always record time because you never reach $c$ | |
| Feb 29, 2016 at 4:11 | comment | added | knzhou | @WillO, you're being too strict here. Sure, you can't consider a photon frame, but you can consider a series of frames that get closer and closer to the speed of light. In this limit, everything Genia said is correct. | |
| Feb 29, 2016 at 3:55 | answer | added | Koray | timeline score: 1 | |
| Feb 29, 2016 at 3:55 | comment | added | WillO | As for why you perceive it, you perceive it for the same reason you perceive anything else --- it collides with you. | |
| Feb 29, 2016 at 3:51 | comment | added | WillO | In other words, I can (perfectly accurately) say that this particular photon was on the surface of the sun eight minutes ago and is now in my eyeball. A different observer might say this photon reached my eye one minute ago, or will reach it one minute from now, because that observer assigns different coordinates to different events than the ones I assign. A photon can't speak this language in the first place, because it assigns no coordinates at all and has no notion of "eight minutes ago" or "twelve minutes ago". But the events still take place, no matter who can or can't describe them. | |
| Feb 29, 2016 at 3:46 | comment | added | WillO | @GeniaS: The photon follows a perfectly well-defined path through spacetime. You and I are perfectly capable of assigning a location and a time to every point on that path. The photon is not. But the path still exists, quite independent of who can and who can't label its points. | |
| Feb 29, 2016 at 3:42 | comment | added | Yevgeny Simkin | @Asher, thanks... I apologize if I'm obtuse, but is it possible to explain this in some visualisation that doesn't require math for a layperson to understand? | |
| Feb 29, 2016 at 3:40 | comment | added | Yevgeny Simkin | @WillO, Ok, but if my thought experiment is correct then there's a problem of how it has time to "get" anywhere at all (relative to its surroundings which aren't moving at the speed of light). Consider what I'm describing. A photon "moves" at the speed of light (and is therefore stationary in time - it's not aging relative to us). So how is it that we perceive it at all? | |
| Feb 29, 2016 at 3:39 | comment | added | Asher | There is a fundamental difference between massless particles and massive particles that cannot be "crossed" in any way; massless particles always travel at $c$ and massive particles never travel at $c$, and since the effects of special relativity rely on a ratio of $v/c$ there is an asymptotically infinite gamma for massive particles and an undefined gamma for massless particles. In other words, massive particles and massless particles can never have the same reference frame (the same gamma with respect to another frame). | |
| Feb 29, 2016 at 3:34 | comment | added | WillO | Light travels at the speed of light. It does not have an associated frame, which means that if it were able to describe such things, it would not be able to assign numbers to your location in space or the time when you woke up this morning. That won't interfere with your ability to wake up. There are all kinds of things in this Universe that I am incapable of assigning numbers to, but they function just fine all the same. | |
| Feb 29, 2016 at 2:33 | history | asked | Yevgeny Simkin | CC BY-SA 3.0 |