Viewing a single comment thread. View all comments

thatsjustfuntastic OP t1_jadnhev wrote

Reply to comment by PEVEI in eli5 if light is a wave what medium does it travel through? by thatsjustfuntastic

Wouldn't that make the electromagnetic field the medium?

2

PEVEI t1_jado2it wrote

This is one of those issues with using language to express rigorous concepts, because "medium" has a lot of meanings in English, but it really just means one thing in physics.

For ELI5 purposes a key difference is that in a medium the propagation of a wave is impacted by the movement of the medium. Imagine a tank of water with a whirlpool in it, rotating clockwise. If you generate a wave in that flow it's velocity through the medium is going to be impacted by the velocity of the flow of that medium.

Fields don't work that way.

8

RhynoD t1_jae6u5h wrote

> Fields don't work that way.

AFAIK they don't not work that way. A lot of particle physics and particle interactions and fields can be accurately modeled using carefully designed water tanks.

1

PEVEI t1_jae70d8 wrote

"The map is not the territory."

"All models are wrong, some are useful."

3

RhynoD t1_jae7yth wrote

Fair. But on the other hand, if all we have is models, is it not fair to say that fields do kind of work that way?

1

PEVEI t1_jae8vjv wrote

No, because a tank of water isn't the same as a field, it's just a useful way to model some aspects of how fields behave within certain limits. Outside of the limits its designed to accurately model, allegories are more misleading than illuminating.

Take Newton's version of gravitation for example, it's a great model! For everything you're going to encounter on Earth and can measure with the instruments of the day, it was very accurate. The only way you could tell that there were holes in the model was through observation of things like the precession of Mercury's orbit. The reality however is that Newton's model is deeply wrong, gravity isn't a fundamental force which acts instantaneously across space.

That model was then replaced by Einstein's model of gravitation, in the form of his field equations, which describes gravity as an apparent force emerging from the curvature of spacetime. This apparent force no longer acts instantaneously, and these corrections explained Mercury's orbit and a lot more.

If you use Newton's model you can build rockets to visit planets in the Solar system. If you use Newton's model to build a GPS system though, it would be useless, without Relativistic corrections the system fails in days at the most. Einstein's model seems pretty much perfect, you need to go to some pretty exotic places to see where the model breaks down, such as the interior of a black hole.

But it's important to remember that models are what we use to illustrate reality, or make predictions about reality, but models are not reality.

The map is not the territory.

4

Any-Growth8158 t1_jadpkrz wrote

No. The changing electric and magnetic fields are the wave. A electric and magnetic field can change without a medium, although until semi-recently many scientists thought that a medium was required for a wave to propagate. They hypothesized an all pervasive static medium called the aether. The Earth and all heavenly bodies traveled through it.

In the late 19th century the michael-morley experiment showed that there wasn't a static aether. It determined that light travels at the same speed in perpendicular directions. If light was propagating through a stationary aether then the light would have taken different times to travel one way vs its perpendicular since the Earth, the galaxy, and everything else is moving.

Relativity further poke a hole in the bottom of the boat for an aether since it doesn't accept a static absolute reference frame for anything. And a medium would not support everyone everywhere agreeing that the speed of light is the same.

5

PerturbedHamster t1_jae8iqa wrote

Not in the sense we usually use the word medium. For sound, the air is there wether or not there's a sound wave. The ocean is there wether or not there's a wave. But for an EM wave, there's no background EM field in empty space (at least classically) unless there's a wave going through.

2