Physics Help Forum Is speed of light fixed?

 Light and Optics Light and Optics Physics Help Forum

 Aug 27th 2010, 09:25 PM #1 Junior Member   Join Date: Aug 2009 Posts: 3 Is speed of light fixed? Hello, I'm wondering if the speed of light is fixed? If it's fixed, what are the evidence and if NOT, then what are the implications. I'm trying to convince a friend of mine(creationist) that light is not decaying. I've told him about E=mc^2, but the objection was what if the threshold for nuclear reaction is still well above the c limit. Also, what are the implications for light decaying? (supposing it is decaying). thanks
 Aug 30th 2010, 06:14 AM #2 Junior Member   Join Date: Jul 2010 Posts: 10 hi light changes velocity when it enters mediums of different density, the denser the medium the slower the velocity. The implications are that you won't be able to observe 'c' in mediums other than vacuum. light decaying, i have never heard of this before, and what's up with his argument that you quote him as a 'creationist'?
 Aug 31st 2010, 12:08 AM #3 Senior Member   Join Date: Feb 2009 Location: Singapore Posts: 206 the speed of light is constant in a vacuum and constant in any given medium for that medium, at least so far as mankind has been able to observe. __________________ To each his own.
 Sep 2nd 2010, 08:23 AM #4 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,354 Is your creationist friend trying to explain why it's possible to see objects that are over 5000 LY away, while maintaining that the universe is only 5000 years old? The problem with that conjecture is that c is intimately bound to other constants, which in turn affect how nuclear reactions occur (Planck's constant, for example). Any significant change in c would affect how nuclear reactions within stars ocur, which consequently would make stars that are 1 million LY away look and act fundamentally different from stars that are only a few tens or hundrdes of LY away. But we don't see that. And if you consider that the your friend is arguing that the universe is less than 1 millionth the age that scientists think (5 thousand years vs 12 billion), take your E= mc^2 equation and imagine what hapens if c is a million times faster. Finally, if the speed of light has for some reason slowed by a factor of 1 million in the last 5000 years, then certainly we would continue to see it changing over the past 100 years - it should have slowed by another factor of 30% in just the last 100 years according to your frend's hypothesis. But we don't see that at all - the speed as measured is remarkably constant.

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