Travel at 0.5c & Observe Redshifted Light Traveling at c

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Discussion Overview

The discussion revolves around the observation of redshifted light while traveling at 0.5c away from the sun. Participants explore the implications of this scenario on the speed of light and the nature of redshift, considering both classical and relativistic perspectives.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant suggests that traveling at 0.5c away from the sun would lead to observing redshifted light, concluding that light does not have a speed limit, but rather that the speed limit is a spacetime limitation to the inertial frame of reference.
  • Another participant asserts that light travels at the same speed in all inertial frames, noting that while the energy of the light may change (redshifted or blueshifted), the speed remains constant at c.
  • A different participant challenges the initial claim, stating that under Einstein's relativity, the measurements of the speed of light are invariant, and that the initial reasoning may be based on an incorrect application of Galilean relativity.
  • One participant attempts to clarify the syntax of the original claim, suggesting that if the observer is moving at 0.5c relative to Earth, the light would still travel at c relative to both the Earth and the spaceship.
  • Another participant expresses confusion regarding the original statement and requests clarification on the intended meaning.

Areas of Agreement / Disagreement

There is no consensus among participants. Some agree on the invariance of the speed of light, while others question the implications of redshift and the interpretation of relativistic effects.

Contextual Notes

Participants reference concepts such as time dilation, length contraction, and the relativity of simultaneity, indicating that these factors are relevant to the discussion but remain unresolved in terms of their application to the original claim.

Frederick T
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It's come to my attention that if we were to travel in a spaceship at 0.5c directly away from our sun, I would observe that the light from the sun as being red shifted. What I would also observe was that this red shifted electromagnetic wave would in fact travel away from me in the spaceship traveling at 0.5c at the speed of light with its new redshifted properties. I would be lead to conclude that light does not, in fact, have a speed limit, but the speed limit exists only as a spacetime limitation to the inertial frame of reference.
 
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You can conclude that. Light travels at the same speed in all inertial frames. What you observe as red shifted light ie lower energy might be observed in another inertial frame as blue shifted light and yet it’s the same light, the same photons. In all cases, the speed is the speed of light but the energy is different.
 
Frederick T said:
What I would also observe was that this red shifted electromagnetic wave would in fact travel away from me in the spaceship traveling at 0.5c at the speed of light with its new redshifted properties.
No you would not. This would be true under Galilean relativity, but that is not an accurate description of reality. You need to use Einstein's relativity, and time dilation, length contraction, and the relativity of simultaneity conspire to make your measurements of the speed of light invariant. This has been extensively tested - see the FAQ linked in the sticky thread in this forum.

This is covered in all introductions to SR.

Edit: having read Nugatory's and Dale's responses below, it's possible I'm misreading the OP. The above response may or may not be relevant to the question actually being asked.
 
Last edited:
Frederick T said:
wave would in fact travel away from me in the spaceship traveling at 0.5c at the speed of light with its new redshifted properties.
The syntax is a bit tangled here, but if what you mean is:
1) relative to the earth, the spaceship is moving at .5c and the flash of light is moving at c.
2) relative to the spaceship, the Earth is moving backwards at .5c and the flash of light is moving at c.
then you have it right.
 
Frederick T said:
What I would also observe was that this red shifted electromagnetic wave would in fact travel away from me in the spaceship traveling at 0.5c at the speed of light with its new redshifted properties
I don’t understand what you mean to say here. Can you please clarify?
 

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