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Author Topic: Why is the photon regarded as a massless particle?  (Read 22924 times)

Offline JP

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Re: Why is the photon regarded as a massless particle?
« Reply #50 on: 25/09/2012 12:54:45 »
hubble_bubble, a photon does always travel at c in a vacuum, by definition.  In general relativity, this means with respect to the patch of space immediately around the photon.  It's a bit like running on a treadmill.  If you run at a constant speed with respect to the conveyor belt under your feet and someone slowly turns the speed up, you'll move at different speeds with respect to someone who's not on the treadmill.  You'll eventually start going backwards even though you're gong a constant speed with respect to the belt beneath your feet. 

This happens because light always moves at c over regions of flat space-time.  Gravity makes space-time curved, but over any tiny portion of a curve, it looks flat (much like the earth's surface looks flat to us even though it's not).  So over a very small neighborhood, light moves at c.  Over a large neighborhood, you have to account for curvature, such as when comparing what two observers at different points in curved space-time see.
 

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Re: Why is the photon regarded as a massless particle?
« Reply #50 on: 25/09/2012 12:54:45 »

 

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