Relative velocity of electron moving

1. Jul 19, 2013

rajan rana

an electron is moving with a velocity of 0.85c in the same direction as that of photon.the relative velocity of the electron with respect to photon is......... and why?

2. Jul 19, 2013

Staff: Mentor

From the electrons frame, the photon moves away at c. This is because light ALWAYS moves at c relative to an inertial frame.

From an observer at rest, the light moves away from the electron at 0.15 c.

3. Jul 19, 2013

ZapperZ

Staff Emeritus
This question "with respect to the photon" cannot be answered, because it requires that one transforms to the photon's REST FRAME", and we can't do that!

Zz.

Last edited: Jul 23, 2013
4. Jul 19, 2013

HallsofIvy

Just to put my oar in (and basically repeat what Drakkith and ZapperZ said) when we talk about "A"s velocity "relative to B", we mean A's velocity as measured in B's rest frame. As ZapperZ said, it is impossible to talk about any motion "relative to a photon" because a photon does NOT HAVE a "rest frame". On the other hand, the speed of a photon relative to anything that does have a rest frame is c, as Drakkith said.

5. Jul 22, 2013

rajan rana

thanks for replying sir. but the answer given is -ve c is it right

6. Jul 23, 2013

ZapperZ

Staff Emeritus
You appear to not have understood the responses that you were given.

Whoever gave you that "answer" have not understood the flaw in your question.

Zz.