Questioning Light Speed: An Advanced Physics Mystery

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In the discussion on light speed, the question arises about whether light from two sources moving in opposite directions could be perceived as traveling at twice the speed of light (2c) relative to each other. It is clarified that while the velocities of objects can appear to exceed c when viewed from a third reference frame, the relative velocity of one object to another cannot exceed the speed of light. This distinction is crucial because, in relativity, velocities do not simply add as they do in classical mechanics. Additionally, the concept of measuring an object's velocity relative to a photon is invalid, as there is no frame of reference where a photon is at rest. Understanding these principles is essential for grasping advanced physics concepts related to light speed.
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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