Questioning Light Speed: An Advanced Physics Mystery

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SUMMARY

The discussion centers on the misconception regarding the relative speeds of light sources in the context of special relativity. Specifically, it clarifies that while two light sources moving in opposite directions may appear to have a relative speed of 2c, this is not applicable when considering their velocities in a reference frame. According to Einstein's theory, the velocities of objects do not add linearly as they do in classical mechanics, and the concept of a reference frame for a photon is non-existent, as photons cannot be at rest.

PREREQUISITES
  • Understanding of Einstein's theory of special relativity
  • Familiarity with the concept of reference frames in physics
  • Basic knowledge of classical mechanics and velocity addition
  • Awareness of the properties of light and photons
NEXT STEPS
  • Study the principles of Einstein's special relativity in detail
  • Learn about the relativistic velocity addition formula
  • Explore the concept of reference frames and their significance in physics
  • Investigate the nature of photons and their behavior in different reference frames
USEFUL FOR

Students of physics, educators teaching advanced concepts, and anyone interested in understanding the implications of special relativity on the behavior of light and motion.

wilders
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Hi everyone,

First I want to say that I hardly know anything about advanced physics, and I'm just looking to ask something that I've been thinking about for a while. So I'd also appreciate if you kept the explanation somewhat understandable :P

Anyway, here's my question:

If light from one source goes in the opposite direction of light of another source, wouldn't that mean that they would go 2 times the speed of light (2c), RELATIVE to each other?
I'm pretty sure this is not the case, but what I do not understand is why it isn't the case.

Hopefully you understand what I mean x_x
Thanks in advance :)
 
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When we say "velocity of object A relative to object B," we mean "the velocity of A as 'seen' by B" (or more technically "the velocity of A in a reference frame in which B is at rest"). This can never be greater than c.

You're asking about "the difference in the velocities of A and B as 'seen' by a third object C which is not moving along with either A or B." This is a different thing, and it's OK for this to be greater than c. In fact it can be as great as 2c, as in your example.

The reason they're different is that velocities don't "add" in relativity the same way as they do in classical mechanics:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html

There's another issue in that the "velocity of an object relative to a photon", using the definition in the first paragraph above, doesn't make any sense, because there is no reference frame in which a photon is at rest.

https://www.physicsforums.com/showthread.php?t=511170
 

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