# Maxwell's EM theory and principle of relativity

• B
What was the problem between Maxwell's EM theory and the principle of relativity? Why went the theory against the principle?

I understand that the EM theory says that Light was a wave and ether is it's medium.

On the other hand the principle of relativity says that there is no state of absolute rest.

## Answers and Replies

Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Gold Member
What was the problem between Maxwell's EM theory and the principle of relativity? Why went the theory against the principle?

I understand that the EM theory says that Light was a wave and ether is it's medium.

On the other hand the principle of relativity says that there is no state of absolute rest.
There is no conflict between Maxwell’s theory and the principle relativity. In fact, it is the prime example of a relativistic field theory.

The conflict is between EM theory and Galilean relativity, in which all speeds change under transformation between inertial frames.

There is no conflict between Maxwell’s theory and the principle relativity. In fact, it is the prime example of a relativistic field theory.

The conflict is between EM theory and Galilean relativity, in which all speeds change under transformation between inertial frames.
If I understand it correctly then light's speed is not changing and that's against the Galilean relativity. Is it correct?

Dale
Mentor
2020 Award
If I understand it correctly then light's speed is not changing and that's against the Galilean relativity. Is it correct?
Yes

Yes
Thank You so much people. You are amazing!

Ibix
Science Advisor
2020 Award
It's important to distinguish between Galilean relativity, which underlies Newtonian physics, and Einsteinian relativity, which underlies modern physics. Both systems respect the principle of relativity. Also, both systems have a speed that's the same in all inertial reference frames - in Galilean relativity it's infinite speed and in Einstein's relativity it's a finite speed, the one at which light propagates.

The problem in the latter half of the 19th century was that electromagnetic theory did not appear to respect the principle of relativity. As Orodruin notes, it turned out not to respect Galilean relativity, but Einstein's version wasn't known at the time.

It didn't initially occur to anyone that the problem was with Newton and Galileo. An obvious solution was to propose that there was a preferred frame for electromagnetism, and a plausible way to introduce this is to propose some medium (the ether) and guess that Maxwell's equations are a special case that work in the frame where the ether is at rest. So we started searching for evidence of the ether, hoping to develop evidence that would lead to a more general form of Maxwell's equations.

However, we never found evidence of an ether. But explaining why we hadn't found it so far lead to (what we regarded as) a set of patches to Maxwell's equations. This is why the core maths of special relativity is named after Lorentz. Einstein's initial triumph was to re-derive the Lorentz transforms, starting from just the Principle of Relativity and the assumption that the speed of light is the same in all inertial reference frames, and to show that they could apply to everything (not just electromagnetism) and to explain how we had missed that fact. Nasty run-on sentence left in to highlight just how broad this success was.

So the initial problem was that Maxwell's equations didn't respect Galilean relativity. Trying to fix Maxwell didn't work out well, and we ended up fixing Galilean relativity to make Einsteinian relativity instead. Maxwell's equations still don't respect Galilean relativity, but they do respect Einsteinian relativity.

• Sorcerer and HAF
It's important to distinguish between Galilean relativity, which underlies Newtonian physics, and Einsteinian relativity, which underlies modern physics. Both systems respect the principle of relativity. Also, both systems have a speed that's the same in all inertial reference frames - in Galilean relativity it's infinite speed and in Einstein's relativity it's a finite speed, the one at which light propagates.

The problem in the latter half of the 19th century was that electromagnetic theory did not appear to respect the principle of relativity. As Orodruin notes, it turned out not to respect Galilean relativity, but Einstein's version wasn't known at the time.

It didn't initially occur to anyone that the problem was with Newton and Galileo. An obvious solution was to propose that there was a preferred frame for electromagnetism, and a plausible way to introduce this is to propose some medium (the ether) and guess that Maxwell's equations are a special case that work in the frame where the ether is at rest. So we started searching for evidence of the ether, hoping to develop evidence that would lead to a more general form of Maxwell's equations.

However, we never found evidence of an ether. But explaining why we hadn't found it so far lead to (what we regarded as) a set of patches to Maxwell's equations. This is why the core maths of special relativity is named after Lorentz. Einstein's initial triumph was to re-derive the Lorentz transforms, starting from just the Principle of Relativity and the assumption that the speed of light is the same in all inertial reference frames, and to show that they could apply to everything (not just electromagnetism) and to explain how we had missed that fact. Nasty run-on sentence left in to highlight just how broad this success was.

So the initial problem was that Maxwell's equations didn't respect Galilean relativity. Trying to fix Maxwell didn't work out well, and we ended up fixing Galilean relativity to make Einsteinian relativity instead. Maxwell's equations still don't respect Galilean relativity, but they do respect Einsteinian relativity.
Thank You for clarifying