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Is Time Travel (going backwards) Possible?
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<blockquote data-quote="freyar" data-source="post: 6042195" data-attributes="member: 40227"><p>Wowsers, get busy for a couple of days and watch the thread explode! I'm a bit tired and in the middle of some work, but I want to address a couple of points quickly.</p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p>That's a lot of words about the Doppler effect and cosmological redshift! Going through that, I'm finding some good explanations and a little bit of confusion, so I'll just try my best at explaining rather than pick through all the previous posts (I will try to reference them at appropriate points if I can keep them all in mind).</p><p></p><p>OK, as Umbran says, the usual Doppler effect is not something that "happens" to the photon but is due to the relative motion of the photon's source and receiver. At low relative speeds, you can think that the receiver moves past the peaks of the light wave (photon) more quickly/slowly if the receiver is moving toward/away from the source. At these low speeds, the relative change in the light's wavelength = v/c, the relative speed over the speed of light (assuming head-on motion). This picture isn't quite right at relativistic speeds (a significant fraction of the speed of light), but it is a reasonable picture if you also throw in some words about "relativistic time dilation" (the equation changes a lot, though). A proper explanation would traditionally come after a couple of years of university physics, but I could try to give you something better in a few days (when I'm less busy) if someone wanted to start a different thread. I'm happy to do physics Q&A when I'm not swamped at work. Anyway, Nellisir also, I think, gave something similar to this explanation but may have been mixing in some of what I'm about to say next.</p><p></p><p>All that said, the normal Doppler effect is <em>not quite</em> what happens in the cosmological expansion of the universe, at least not in Einstein's general relativity, although you can think about it that way when you talk about light moving from galaxy to galaxy. However, it's more useful (and generalizes better mathematically) to think that the galaxies are really not moving with respect to some grid you've laid down in space (at least not due to the expansion of the universe). Instead, it's the space inside the grid lines that's getting bigger. In this way of thinking, the photon/light wave itself really does get stretched out as space grows between successive peaks of the wave. In my work, I find this to be the clearest way of thinking about it, especially in the very early universe, which is just a big plasma without galaxies (or much other structure), so it's very hard to think about objects moving apart unless you go down to the single particle level. On the other hand, it's very easy to think about the overall plasma being at rest and just diluting as the universe expands --- and the photons redshift as it does so.</p></blockquote><p></p>
[QUOTE="freyar, post: 6042195, member: 40227"] Wowsers, get busy for a couple of days and watch the thread explode! I'm a bit tired and in the middle of some work, but I want to address a couple of points quickly. That's a lot of words about the Doppler effect and cosmological redshift! Going through that, I'm finding some good explanations and a little bit of confusion, so I'll just try my best at explaining rather than pick through all the previous posts (I will try to reference them at appropriate points if I can keep them all in mind). OK, as Umbran says, the usual Doppler effect is not something that "happens" to the photon but is due to the relative motion of the photon's source and receiver. At low relative speeds, you can think that the receiver moves past the peaks of the light wave (photon) more quickly/slowly if the receiver is moving toward/away from the source. At these low speeds, the relative change in the light's wavelength = v/c, the relative speed over the speed of light (assuming head-on motion). This picture isn't quite right at relativistic speeds (a significant fraction of the speed of light), but it is a reasonable picture if you also throw in some words about "relativistic time dilation" (the equation changes a lot, though). A proper explanation would traditionally come after a couple of years of university physics, but I could try to give you something better in a few days (when I'm less busy) if someone wanted to start a different thread. I'm happy to do physics Q&A when I'm not swamped at work. Anyway, Nellisir also, I think, gave something similar to this explanation but may have been mixing in some of what I'm about to say next. All that said, the normal Doppler effect is [i]not quite[/i] what happens in the cosmological expansion of the universe, at least not in Einstein's general relativity, although you can think about it that way when you talk about light moving from galaxy to galaxy. However, it's more useful (and generalizes better mathematically) to think that the galaxies are really not moving with respect to some grid you've laid down in space (at least not due to the expansion of the universe). Instead, it's the space inside the grid lines that's getting bigger. In this way of thinking, the photon/light wave itself really does get stretched out as space grows between successive peaks of the wave. In my work, I find this to be the clearest way of thinking about it, especially in the very early universe, which is just a big plasma without galaxies (or much other structure), so it's very hard to think about objects moving apart unless you go down to the single particle level. On the other hand, it's very easy to think about the overall plasma being at rest and just diluting as the universe expands --- and the photons redshift as it does so. [/QUOTE]
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