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Old 22-09-2011, 03:21 PM
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sjastro
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Quote:
Originally Posted by pmrid View Post
OK. I'm struggling here.
Photon A and I both leave our respective destinations at the same time, heading towards one-another. At the point of departure we are, say, 5 light years apart. Our relative starting positions.

After 1 year, I have travelled a light year towards the source of photon A. In the same year, Photon A has travelled a light year in my direction.
At the end of year 1, we are therefore 3 light years apart, not 5. To my observing eye, hasn't the distance between us reduced at the rate of 2 light years/year?

Peter
An understanding of special relativity is required here.

At very high velocities approaching and including the speed of light (=c), velocities don't add vectorially.

For example if two objects approach each other at a velocity v, then their approach velocity relative to each other is 2v.

At high velocities their approach velocity is 2v/(1+(v/c)^2)

If each object (photon) approaches each other at a speed c, then the
approach velocity is 2c/(1+(c/c)^2)= c

Regards

Steven
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