Can light act like an object in motion and stand still in an ideal environment?

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Discussion Overview

The discussion centers around the hypothetical scenario of a light source moving directly away from an observer at the speed of light and whether the light emitted would behave like an object acted upon by opposing forces, potentially standing still. The conversation explores implications related to relativity, visibility of the light source, and the nature of light in an ideal environment.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that a light source cannot move at the speed of light, even in an ideal experiment.
  • Others propose that if one assumes the scenario is possible, it leads to the creation of new, non-standard laws of physics, which may not be suitable for discussion in the forum.
  • A participant suggests that the original question lacks a valid premise, but offers a modified version where the light source moves at nearly the speed of light, leading to a discussion about the behavior of light in that context.
  • It is noted that light always travels at speed c in a vacuum, regardless of the motion of the source, and that red-shifting could affect visibility.
  • One participant raises the possibility of massless particles decaying into other massless particles, questioning the implications of conservation laws and quantum field theory on this scenario.
  • There is a mention of red-shift potentially preventing the detection of emitted photons if they are shifted to zero frequency.

Areas of Agreement / Disagreement

Participants generally disagree on the feasibility of the original scenario and its implications. There is no consensus on whether the modified scenario provides a valid basis for further exploration.

Contextual Notes

The discussion is limited by assumptions regarding the nature of light and the constraints of relativity. The implications of quantum field theory and conservation laws are also noted but remain unresolved.

Securityman
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In an ideal environment, if a light source is moving directly away from you at the speed of light will the light act like an object that is acted upon by two opposing forces and stand still? Would we be able to see the light source? Thanks!

Secman
 
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Securityman said:
moving directly away from you at the speed of light

That's not possible, even in an ideal experiment.
 
jtbell said:
That's not possible, even in an ideal experiment.

Which part? The speed of light part? If so can we assume it's possible for this system?
 
Securityman said:
Which part? The speed of light part?

Yes.

If so can we assume it's possible for this system?

If we do this, we can't use relativity to make predictions about the consequences.
 
Securityman said:
In an ideal environment, if a light source is moving directly away from you at the speed of light will the light act like an object that is acted upon by two opposing forces and stand still? Would we be able to see the light source? Thanks!

Secman

Securityman said:
Which part? The speed of light part? If so can we assume it's possible for this system?
If you assume that it's possible for your system, then you get to make up your own laws of physics and you can make up anything you want. No one is going to be able to second-guess what you might dream up. But you're not allowed to speculate on such matters on this forum. This forum is to learn about relativity and how it explains the real world, not to pretend that the world is different so you can make up your own fantasy.
 
Securityman said:
In an ideal environment, if a light source is moving directly away from you at the speed of light will the light act like an object that is acted upon by two opposing forces and stand still? Would we be able to see the light source?

As phrased, the question has no answer, as there is no such thing as a light source "moving directly away from you at the speed of light". However, there is a very closely related question which does make sense and does have an answer:
In an ideal environment, if a light source is moving directly away from you at almost (say 99.99999999999999999999%) the speed of light will the light act like an object that is acted upon by two opposing forces and stand still? Would we be able to see the light source?

And the answer is:
1) No, the light will not almost stand still. In a vacuum, light travels at the speed c regardless of the velocity of the source or the target.

2) Because of the first answer, yes, the light will get to you so you'll be able to detect it. The light will be red-shifted, and if it's red-shifted enough it won't register on your eyes, so you may not be able to literally "see" the light source, but if you have the right instruments (for example an infrared night vision device for moderate red-shifts) you will be able to detect it.
 
Last edited:
I'd like to point out that the OP said only "light source" and never mentioned anything about a flashlight or some-such thing that has mass.

It requires a bit more, then, than simply the axioms of special relativity to rule out the possibility of a massless particle (moving away from you) decaying into more massless particles (one of which is moving towards you). Perhaps momentum or energy conservation prevents this, but I cannot confirm this without taking out a pen and paper...especially if we include the possibilities of many (n-particle) decays.

At a more fundamental level, perhaps quantum field theory prevents such decays?

EDIT: I had not considered the red-shift effect. Perhaps that itself would prevent this decay since any potential photon emitted would be red-shifted to 0-frequency.
 

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