# Relative motion and light

## Main Question or Discussion Point

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

## Answers and Replies

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jtbell
Mentor
moving directly away from you at the speed of light
That's not possible, even in an ideal experiment.

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?

jtbell
Mentor
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.

ghwellsjr
Gold Member
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
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.

Nugatory
Mentor
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 infra-red night vision device for moderate red-shifts) you will be able to detect it.

Last edited:
Matterwave