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"Speed of Light"
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<blockquote data-quote="Umbran" data-source="post: 6278248" data-attributes="member: 177"><p>Well, there's the trick - pick two particles close enough together that there are no other particles in the space, then the relative speeds will be under that of light. Pick two distant particles... then you have a whole lot of other particles int eh field of view, and you can see the issue.</p><p></p><p></p><p></p><p>Ah, there's the phrase we can use. The particle velocities aren't meaningful in that frame, because the frame isn't constant! The frame is expanding!</p><p></p><p>One way to think of it is thus: when we pick a frame of reference, we don't actually pick just a central point. We pick that central point, and some distant points - usually implicitly we pick "relative to the distant stars" which are fixed for most human purposes. The reference points of the frame, Einstein tells us, are arbitrary, so for most cases they don't matter. But we are now talking about a case where it does matter.</p><p></p><p>When we are talking about cosmological expansion, we are no longer talking about small distances or times. We are talking about distances and times which span... all. There is no larger, fixed outer edge of the frame of reference to which we can refer, and we have to worry about the fact that the frame itself is no longer constant!</p><p></p><p>There are other ways to explain the difference.. I remember a really cute video I saw recently. Let me see if I can find it again...</p></blockquote><p></p>
[QUOTE="Umbran, post: 6278248, member: 177"] Well, there's the trick - pick two particles close enough together that there are no other particles in the space, then the relative speeds will be under that of light. Pick two distant particles... then you have a whole lot of other particles int eh field of view, and you can see the issue. Ah, there's the phrase we can use. The particle velocities aren't meaningful in that frame, because the frame isn't constant! The frame is expanding! One way to think of it is thus: when we pick a frame of reference, we don't actually pick just a central point. We pick that central point, and some distant points - usually implicitly we pick "relative to the distant stars" which are fixed for most human purposes. The reference points of the frame, Einstein tells us, are arbitrary, so for most cases they don't matter. But we are now talking about a case where it does matter. When we are talking about cosmological expansion, we are no longer talking about small distances or times. We are talking about distances and times which span... all. There is no larger, fixed outer edge of the frame of reference to which we can refer, and we have to worry about the fact that the frame itself is no longer constant! There are other ways to explain the difference.. I remember a really cute video I saw recently. Let me see if I can find it again... [/QUOTE]
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