Why is it "forbidden" to use EM waves as a way of detecting motion in two different inertial frames?

Question

Constant motion can not be detected by neither particles (because of inertia) nor mechanical waves ( because they need a medium ). However when you consider light for example and assume it does not need a medium to travel through you could detect constant motion. For example, when someone is traveling at the speed of light in the x direction, and fire some photons in the x direction the person would see light as standing still while someone who is not moving will see the light passing at c. So why would we consider constant velocity to always be relative and so on undetectable.

Clarify: Its obvious that the discovery was made, however for Einstein to actually get anywhere he needed to assume c to be constant. Which is exactly what I am wondering about, how did he come up with this. So that is why I am asking about constant relative motion and trying to come up with a thought experiment which proves the distinction between being stationary and constant velocity reference frames. However every single object with or without inertia which includes particles and mechanical waves are shown to give no proof of one or the other the one being in constant velocity. Which brings me to a light wave traveling through no medium and then would suddenly give us a way to detect constant motion, because now the so called ether wind is not causing the wave of light to go slower because the "wind" is moving in the opposite direction, no, it is now possible to say that light moves at a different speed for one of the two experiments on board the two and then proof that the one where c is slower, is the one actually moving and the one other one stationary. So from knowing this to be forbidden there is only one logical conclusion c must be constant for all observers, its the part where knowing this to be forbidden to c is constant for all observers is where I am stuck.

Ps: please don't just give me a mathematical proof, this doesn't really help me, Much appreciated!

Answer 1

It's been tried, and it doesn't work. The Michelson–Morley experiment and subsequent refinements don't detect motion.

You are wise to reject mathematical proof in physics. Mathematical objects are human constructs, not physical. We choose which objects to use as models for physics by matching them to observations like the Michelson–Morley experiment. With that discipline, mathematics can illuminate physics, but it never proves anything. The phenomena have the last word.

Here, the phenomena reject your reasoning, and the trouble is in your mathematical model of velocity addition. The model accepted in physics is what matches experimental results.

Here's the first paragraph from Einstein's On the Electrodynamics of Moving Bodies.

It is known that Maxwell's electrodynamics as usually understood at the present time when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy, producing a current at the places where parts of the conductor are situated. But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise assuming equality of relative motion in the two cases discussed to electric currents of the same path and intensity as those produced by the electric forces in the former case.

Einstein describes a simple experiment that demonstrates that, in electromagnetism, only relative motion matters. You can easily do this experiment. But this is all very mysterious in the absolute space of Newtonian mechanics.

The remainder of the paper is devoted to explaining how postulating the independence of the velocity of light from the velocity of the source can reconcile the relativity of motion with electrodynamics.

This simple experiment was not the only evidence Einstein knew of, but it was where he chose to start in elucidating relativity. Relativity was not dogma imposed via revelation: the phenomena forced physicists (often against their prejudices) to accept it.

Answer 2

In 1905, a number of experiments combined had already shown that the speed of light is always the same in all directions. It followed from Maxwell's equations (which are based on many electro-magnetic experiments), stellar aberration, and the Michelson-Morley experiment. The experiments showed that light, and EM waves in general, could not move with constant speed wrt. some aether medium, but always traveled with constant speed wrt. the observer instead. Nowadays there are many more experiments that confirm the same.

Einstein simply took that experimental result as a postulate, and worked through the logical consequences. The result was Special Relativity.

Because the speed of light is always constant, it cannot be used to detect movement wrt. some aether medium, which makes the concept of an aether superfluous. Together with the Principle of Relativity, that the physics laws are the same in all inertial frames, it follows that it is not possible to detect absolute linear velocity.

Answer 3

You can detect relative motion easily just by looking at something. What you can't do is to detect absolute inertial motion, because the very idea of absolute inertial motion is meaningless. By the way, your question assumes someone is moving relative to somebody else at c- that's impossible.

Answer 4

This part of your question is entirely incorrect

For example, when someone is travelling at the speed of light in the x direction, and fire some photons in the x direction the person would see light as standing still...

For two reasons:

1. Observers can never travel at the speed of light.
2. In ALL frames, light ALWAYS travels with speed $c$. The person could be travelling at 299,792,457.99999999 m/s and the photons he/she shoots out would still move at 299,792,458 m/s.