Doppler Effect: Light Blueshifted When Source Moving Towards Observer

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Discussion Overview

The discussion revolves around the Doppler effect, specifically addressing the blueshift of light when a source is moving towards an observer. Participants explore the implications of relative motion in the context of special relativity, questioning the concept of "rest" and how it applies to the observer and the light source.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant asserts that if both they and the light source are moving towards each other, the light will appear blueshifted, and questions the scenario of being at rest while the source moves towards them.
  • Another participant emphasizes the importance of defining "rest" in relativity, suggesting that there is no absolute rest and that precision in formulations is crucial.
  • A participant claims to have worked out the equations related to the blueshift but is challenged on the correctness of their calculations.
  • There is a reiteration that a source moving towards an observer will indeed result in a blueshift, with one participant acknowledging a realization of their earlier error.
  • Multiple participants question the notion of being "at rest," indicating that the relationship between the observer and the light source is not straightforward and cannot be assumed to be at rest relative to one another.
  • A later post suggests that the situation remains the same regardless of whether the observer or the source is moving, reinforcing the relativity of motion.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the concept of "rest" in relation to the Doppler effect. While some participants agree on the outcome of blueshift when the source approaches, there is no consensus on the definitions and implications of being at rest.

Contextual Notes

Participants express uncertainty regarding the definitions of "rest" and the implications of relative motion, indicating that assumptions about rest frames are not universally accepted. The mathematical correctness of the claims made by participants remains unresolved.

kent davidge
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If I and the light source are both moving toward one another, I see light blueshifted. What if I'm at rest and the source moving towards me?
 
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At rest relative to what? You have asked enough questions in the relativity forums to know that there is no absolute rest and that being precise in formulations is important. You should also post your own work, not just say that you "seem to get" something.
 
kent davidge said:
I've worked out the equations

Not correctly. If you did it correctly, a source moving towards you would be blueshifted.
 
Orodruin said:
At rest relative to what?
Self evident. I do not believe I would have to make that explicit to a experienced physicist like you.
Vanadium 50 said:
a source moving towards you would be blueshifted.
Indeed, as soon as I posted this question, I realized where I was going wrong, and got the correct result.
 
Orodruin said:
At rest relative to what?
kent davidge said:
Self evident. I do not believe I would have to make that explicit to a experienced physicist like you.
Not only is the answer not self-evident, but I am unable to imagine any possible answer. As you posed the problem, there are only two things to consider: you and the light source, and these certainly are not at rest relative to one another.
 
The original poster says they've found their error so this thread is closed.
The key point here is that it is the exact same physical situation whether we think in terms of the light source approaching the observer or the observer approaching the light source; neither is more "at rest" than the other.
As always, if you have more to say on the topic, PM me or any other mentor.
 

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