Relative velocity of electron moving

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Discussion Overview

The discussion revolves around the concept of relative velocity, specifically focusing on the relative velocity of an electron moving at 0.85c in the same direction as a photon. The scope includes theoretical considerations of special relativity and the implications of measuring velocities in different frames of reference.

Discussion Character

  • Debate/contested

Main Points Raised

  • Some participants assert that from the electron's frame, the photon moves away at c, as light always moves at c relative to an inertial frame.
  • Others argue that the question of relative velocity "with respect to the photon" cannot be answered because it implies transforming to the photon's rest frame, which is not possible.
  • It is noted that discussing motion "relative to a photon" is problematic since a photon does not have a rest frame.
  • A participant expresses confusion regarding a proposed answer of -ve c, questioning its validity and suggesting a misunderstanding of the responses provided.
  • Another participant reinforces the idea that the flaw in the original question lies in the assumption that one can measure velocity relative to a photon.

Areas of Agreement / Disagreement

Participants generally agree that it is impossible to define a rest frame for a photon and that the concept of relative velocity in this context is flawed. However, there remains disagreement regarding the interpretation of the question and the validity of the proposed answer of -ve c.

Contextual Notes

The discussion highlights limitations in understanding the implications of special relativity, particularly regarding the definition of rest frames and the nature of light's velocity.

rajan rana
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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?
 
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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.
 
rajan rana said:
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?

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:
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.
 
thanks for replying sir. but the answer given is -ve c is it right
 
rajan rana said:
thanks for replying sir. but the answer given is -ve c is it right

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.
 

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