Velocity dependent equations and frames of reference

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Discussion Overview

The discussion revolves around the implications of velocity-dependent equations, particularly de Broglie's wavelength, in different frames of reference. Participants explore how these concepts apply to observers in various states of motion, including acceleration, and the resulting observable characteristics of particles like electrons.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that wavelength is not frame invariant, similar to kinetic energy, suggesting that different observers may measure different wavelengths based on their relative motion.
  • There is a contention regarding whether an accelerating observer would see an electron emit electromagnetic radiation (EMR), with some asserting that it would not, while others question this assertion.
  • A participant raises the idea that any velocity or energy-dependent property is frame dependent, leading to different observable characteristics in different frames.
  • Some participants emphasize that the differences in observed properties are not unique to special relativity but are also present in classical mechanics.
  • Concerns are expressed about the understanding of relativity, particularly from a beginner's perspective, highlighting the complexity of these concepts.
  • There is a discussion about the nature of physical quantities having different values in different frames, with some participants noting that invariants exist but are not the only relevant quantities.
  • One participant suggests that studying light clocks could provide a clearer understanding of relativistic effects, including length contraction and time dilation.

Areas of Agreement / Disagreement

Participants exhibit disagreement on whether an observer would see an electron emit EMR when accelerating. While some assert it would not, others challenge this view, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

Participants express varying levels of familiarity with the concepts discussed, with some indicating that they are new to physics and find relativity particularly challenging. This suggests a potential gap in foundational knowledge that may affect the discussion.

victorhugo
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With a velocity dependent equation such as de Broglie's λ=h/mv
There's just so many questions, where do I start...
In your frame of reference, an electron might be standing still but from an outside frame it could be moving at 0.1c
does that mean you'd see different wavelengths? What about when your frame accelerates, does the electron emit EMR?
 
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victorhugo said:
does that mean you'd see different wavelengths?
Yes, wavelength is not frame invariant just as kinetic energy is not.

victorhugo said:
What about when your frame accelerates, does the electron emit EMR?
No.
 
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Yes, wavelength is not frame invariant just as kinetic energy is not.

Prepare for questions from a level 1 newbie physicist... And of course, thank you so much for always answering my silly questions :)

1. [This question is covered in another thread split off from this one]

2. That makes sense, but does that mean that any velocity/energy dependent property of anything can only be described compared to a certain frame of reference? That is, having different observable characteristics at different frames.

3. So an observer seeing the electron accelerate, just not relative to its own 'spaceship' frame, would not see it emit EMR? To me it seems that it would, maybe I wasn't clear on my question enough. If we were to accelerate the electron alone or inside a jar, why would it make a difference whether it emits EMR or not?
 
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victorhugo said:
That makes sense, but does that mean that any velocity/energy dependent property of anything can only be described compared to a certain frame of reference? That is, having different observable characteristics at different frames.
It does not mean anything else than that it is frame dependent.

victorhugo said:
So an observer seeing the electron accelerate, just not relative to its own 'spaceship' frame, would not see it emit EMR? To me it seems that it would, maybe I wasn't clear on my question enough. If we were to accelerate the electron alone or inside a jar, why would it make a difference whether it emits EMR or not?
You did not specify that the electron was also accelerating.
 
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It does not mean anything else than that it is frame dependent.

Could you please elaborate on that then?
 
victorhugo said:
Could you please elaborate on that then?
I do not see how this can be misinterpreted. It is not something particular to SR and it is present already in classical mechanics.
 
Orodruin said:
I do not see how this can be misinterpreted. It is not something particular to SR and it is present already in classical mechanics.
Eg The wavelength (and anything else that is velocity dependent) of a particle is frame dependent? That is, different observers disagree on the wavelength because they observe different velocities?
 
victorhugo said:
Eg The wavelength (and anything else that is velocity dependent) of a particle is frame dependent? That is, different observers disagree on the wavelength because they observe different velocities?
Obviously, I do not see why you find this strange. Energy is different in different frames already in classical mechanics. Do you have a problem with that?
 
Orodruin said:
Obviously, I do not see why you find this strange. Energy is different in different frames already in classical mechanics. Do you have a problem with that?
Well, I'm still in school and started learning physics not even 2 years ago. We don't learn about any of this at school, only simple things like projectile motion, torque, basic particle physics etc.
No problems, I'm just finding all relativity strange, as expected.
 
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victorhugo said:
Well, I'm still in school and started learning physics not even 2 years ago. We don't learn about any of this at school, only simple things like projectile motion, torque, basic particle physics etc.
No problems, I'm just finding all relativity strange, as expected.

Well, let us start with velocity itself - it is a measurable quantity. Do you find it strange that it can have different values in different inertial systems?
 
  • #11
There is nothing a priori wrong with physical quantities that have different values in different frames. What we require is that the values of the properties obey the same laws in all frames. We like quantities that are the same in all frames (so much that we give them a name, invariants) because we can compute them in any frame and obtain their value in any frame, but they are by no means the only tool in the box.
 
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  • #12
In that case the light clock is better to study since it allows you to derive the full Lorentz transforms. Those include both length contraction, time dilation and the relativity of simultaneity. After that I recommend looking up Minkowski diagrams, which are a very useful tool for seeing what is going on as viewed in different frames.
 
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