# How to tell if an object is moving

by Yoghurt114
Tags: object, space, speed
 P: 3 Simple question, to which I have no answer. Say, you encounter an object in space. How can you tell if it's moving, if you don't know your own location or speed, and you don't have anything you can use to navigate by or determine your location. Asked out of pure interest. Thanks.
 Sci Advisor P: 2,193 You cannot. First it is critical to understand that there is no absolute frame of reference, that is, you cannot simply say an object is "moving", but you must specify what is it moving relative to. If you'd like to know if an object is moving relative to you, well then that's quite simple. One way is to look at its parallax, i.e. is its position changing relative to some very distant objects (stars or quasars or something like that). Another way would perhaps be to look for any kind of doppler shift coming from any light emitted from the object.
 P: 3 The doppler shift is the answer I was looking for. Thanks. I was under the assumption the doppler shift only applies to ambulances and police cars ;> Never realized it also applies to electromagnetic waves, but it of course makes perfect sense.
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## How to tell if an object is moving

Doppler shift on incoming light if the object is something like a star or planet or nebula and very far away. If it is something like a space ship at close range, then normal radar or a laser range finder would work. And at a close enough distance you could see it moving relative to you.
 P: 3 By the way. If the object does not seem to have any doppler shift of EM waves in any direction. Wouldn't it be safe to assume it is not moving? That is. It is not moving relative to .... space?
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P: 10,386
 Quote by Yoghurt114 By the way. If the object does not seem to have any doppler shift of EM waves in any direction. Wouldn't it be safe to assume it is not moving? That is. It is not moving relative to .... space?
The object would only have no doppler shift from YOUR frame. I could be in another frame travelling away from it and it would have doppler shift for me. There is NOT a frame for space itself. The closest thing you could say would be to take a frame of the cosmic microwave background.

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