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"Speed of Light"
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<blockquote data-quote="freyar" data-source="post: 6256909" data-attributes="member: 40227"><p>I love when these threads come up on ENWorld. I wish I had more time available to answer them more often! In any event, I teach a university course each year on relativity and use it every day in my physics life, so I have way too much to say on the subject for a single post. I'll just try to mention a few things here.</p><p></p><p>1) What happens if c becomes infinite? Well, besides breaking electromagnetism and therefore us, it does change the notion of past & future in our universe. But it would actually change it to the notion most people have already: there would be a definite "present" that is the same everywhere, plus a definite past and future the same everywhere. In Einstein's relativity, that's not true. Your future is made up only of the events you can travel to at speed c or less. The reverse is true for the past.</p><p></p><p>2) As noted, the speed of light in materials may be different than c. This isn't due to the "ultimate speed limit" changing --- c is the same --- but due to the material affecting light. In terms of photons, yes, you can think of the photons bouncing off of electrons in atoms. That's not quite accurate, though, as the "photons" are large compared to atoms for visible light, microwaves, radio, etc. It's better to think of light as waves in this case, and the change in speed is due to the response of the wave to the material's response to the wave.</p><p></p><p>3) Virtual particles do not affect the speed at which light travels. If they did, it would violate relativity!* It is true that we can think of virtual particles as having measurable effects, but that's not one. Some of tomBitoni's links mention effects where you can think of virtual particles acting like matter and changing the speed at which light travels, but that's always in the presence of some other matter, like a nucleus or metal plates. That doesn't violate relativity because of the other stuff present.</p><p></p><p>*I should note that you can make theories of physics that violate relativity in small, subtle ways that would not have yet been detected by experiments. In those theories, you could ask about virtual particles changing the speed of light, but you could also ask about whether light always travels at speed c even before you think about virtual particles!</p></blockquote><p></p>
[QUOTE="freyar, post: 6256909, member: 40227"] I love when these threads come up on ENWorld. I wish I had more time available to answer them more often! In any event, I teach a university course each year on relativity and use it every day in my physics life, so I have way too much to say on the subject for a single post. I'll just try to mention a few things here. 1) What happens if c becomes infinite? Well, besides breaking electromagnetism and therefore us, it does change the notion of past & future in our universe. But it would actually change it to the notion most people have already: there would be a definite "present" that is the same everywhere, plus a definite past and future the same everywhere. In Einstein's relativity, that's not true. Your future is made up only of the events you can travel to at speed c or less. The reverse is true for the past. 2) As noted, the speed of light in materials may be different than c. This isn't due to the "ultimate speed limit" changing --- c is the same --- but due to the material affecting light. In terms of photons, yes, you can think of the photons bouncing off of electrons in atoms. That's not quite accurate, though, as the "photons" are large compared to atoms for visible light, microwaves, radio, etc. It's better to think of light as waves in this case, and the change in speed is due to the response of the wave to the material's response to the wave. 3) Virtual particles do not affect the speed at which light travels. If they did, it would violate relativity!* It is true that we can think of virtual particles as having measurable effects, but that's not one. Some of tomBitoni's links mention effects where you can think of virtual particles acting like matter and changing the speed at which light travels, but that's always in the presence of some other matter, like a nucleus or metal plates. That doesn't violate relativity because of the other stuff present. *I should note that you can make theories of physics that violate relativity in small, subtle ways that would not have yet been detected by experiments. In those theories, you could ask about virtual particles changing the speed of light, but you could also ask about whether light always travels at speed c even before you think about virtual particles! [/QUOTE]
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