Can the Doppler Effect Explain Redshift in Light?

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Discussion Overview

The discussion revolves around the Doppler effect in light and its relation to redshift, particularly in the context of special relativity. Participants explore the implications of traveling at relativistic speeds towards a light source and how this affects the observed wavelengths of light.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion about how the Doppler effect applies to light, particularly regarding the shortening of wavelengths and the shift towards red or blue.
  • One participant states that while the speed of light remains constant, the observed wavelengths change, leading to a blue shift when moving towards the source.
  • There is a discussion about the counterintuitive nature of the constancy of the speed of light relative to the observer's motion, with some participants noting that this concept requires special relativity for understanding.
  • One participant suggests that the constancy of light speed is an assumption, while another counters that it is an observation supported by experimental evidence.
  • References to Einstein's postulates and the experimental basis for special relativity are made, indicating differing interpretations of his views.
  • A participant mentions finding a video that helped clarify their understanding of the topic.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the interpretation of the constancy of the speed of light and its implications. Some view it as an assumption, while others argue it is an established observation. The discussion remains unresolved with multiple competing views present.

Contextual Notes

Participants reference the need for Lorentz transformations and the limitations of Galilean velocity addition at relativistic speeds, indicating a complex interplay of concepts that may not be fully resolved in the discussion.

Combsbt
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I do not fully understand the Doppler effect in light.
A theoretical question:

If I was traveling near the speed of light, towards a source of light. Would all the wavelengths in the visible spectrum be shortened and therefor, everything shifting towards red?

I thought the speed of light was supposedly constant, no matter the relative velocity of the observer.
 
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Combsbt said:
If I was traveling near the speed of light, towards a source of light. Would all the wavelengths in the visible spectrum be shortened and therefor, everything shifting towards red?
The observed wavelengths will be shorter, which means a shift towards blue.

I thought the speed of light was supposedly constant, no matter the relative velocity of the observer.
The speed is constant. It's the observed wavelength that changes, not the speed.
 
Oh yeah, red is longer wavelength...

What I don't understand is how the speed of the wave propagation can remain constant relative to the observer. It seems that if you are moving towards the source, your relative velocity would be higher.
 
Combsbt said:
What I don't understand is how the speed of the wave propagation can remain constant relative to the observer. It seems that if you are moving towards the source, your relative velocity would be higher.
That the speed of light is independent of the speed of the source or observer is counterintuitive. It requires special relativity to make sense of it all.
 
Oh I see, so that property is an assumption.
 
Combsbt said:
Oh I see, so that property is an assumption.

No, it's an observation.
 
Vanadium 50 said:
No, it's an observation.

Einstein said:
"The insight fundamental for the special theory of relativity is this: The assumptions relativity and light speed invariance are compatible if relations of a new type ("Lorentz transformation") are postulated for the conversion of coordinates and times of events... The universal principle of the special theory of relativity is contained in the postulate: The laws of physics are invariant with respect to Lorentz transformations (for the transition from one inertial system to any other arbitrarily chosen inertial system). This is a restricting principle for natural laws..."

Einstein seemed to think differently.
 
Einstein assumed the constancy of the speed of light in deriving the special theory of relativity. Since then it has been tested (observed) experimentally in various ways. The link below refers to many of them.

Experimental Basis of Special Relativity
 
Combsbt said:
Einstein seemed to think differently.

Rather than playing word games and "quote the physicist", I think your time would be better spent understanding the physics, rather than the connotation of the word. There are already many sources (and many threads on here) on why the simple Galilean addition of velocities will not work at relativistic speeds.

Zz.
 
  • #10
Found a video on youtube with a visual explanation of special relativity. Seemed to clear some things up for me with my original question. Here it is if anyone is interested.

_http://www.youtube.com/watch?v=C2VMO7pcWhg&feature=related
 

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