# Light question

1. Mar 6, 2014

### wilders

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, wouldnt that mean that they would go 2 times the speed of light (2c), RELATIVE to eachother?
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

2. Mar 6, 2014

### Staff: Mentor

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.