# Which one is moving? [duplicate]

If there are two objects moving relative to each other at a constant velocity, is there any experiment the bodies themselves can do to determine which one is moving? EDIT: The comments say "NO". So please can anyone explain this:

If we could make bodies produce sound and while one produces other calculates the received frequency. So, does Doppler effect give same frequencies for both the cases or is there a way to perceive the difference?

• – sammy gerbil Aug 22 '18 at 14:18
• I deleted some comments that were answering the question and their responses. Please keep in mind that comments are meant for suggesting improvements to the question or requesting clarification on it, and should not be used for posting answers. – David Z Aug 23 '18 at 1:09
• How is this a duplicate? There is no Doppler Effect in the question you addressed above! – uvrichest Aug 23 '18 at 15:46

The principle of relativity is one of the most basic assumptions built in to standard physics. Fundamentally, it says that there's no single choice of a "correct" state of motion, which means that there cannot be any "experiment the bodies themselves can do to determine which one is moving" because there is no correct answer to that question — the only things that matter are relative velocities (because there is no such thing as "absolute" velocity). The issue of "which one is moving" depends on which frame you're in, and any frame is as correct as any other frame. So that's why the answer to the main question is "NO".

But there's another different question here, having to do with the Doppler effect in air, and it brings up an interesting subtlety. Only relative velocities matter, but now you have three things — the two bodies and the air — and the relative velocities between each pair of them will come into the physics. In principle, you could decide if one is at rest relative to the air. But there's nothing absolute about the air; it's just another object in physics. The wind could start to blow, and now suddenly the other object might be the one that's at rest relative to the air. Moreover, someone could drive by and decide that all three are moving with respect to her. But the principle of relativity says that she should be able to calculate the Doppler shift and — assuming she correctly accounts for the fact that the air is moving relative to her — get the right answer.

No. We have no way of determining an absolute, fixed frame of reference. Everything is relative - it's equally correct to say object A is fixed and object B has velocity v (from the reference frame of A), or that object B is fixed and object A has velocity -v (from the reference frame of B), or something entirely different from the frame of reference of object C.

• How about making one release waves and other one check the frequency? Doppler effect gives different frequencies in different cases, does it not? – uvrichest Aug 22 '18 at 13:53
• @uvrichest Sure, but both measurements are equally correct... from a certain point of view. – Nuclear Wang Aug 22 '18 at 13:55
• Could you explain? Which point of view? – uvrichest Aug 22 '18 at 13:55

Well, the physical laws are identical, so any experiment whatsoever gives the same results. Or you are saying that there are some 'preferred' frames, which has been ruled out centuries ago.

• What about Doppler Effect? Does it not give different results in different references? – uvrichest Aug 22 '18 at 14:11
• No. When I said any, it included the Doppler effect. @uvrichest – Trebor Aug 22 '18 at 14:12