# Is this a fair comparison? (Electromagnetic waves)

1. Jun 17, 2013

### tolove

I remember having great confusion when I first read about electromagnetic waves. I remember being introduced to a historical text about ether when I was a child, and thinking it was an unsolved puzzle. I'd never really been able to shake the question "What does an electromagnetic wave propagate through?"

I was thinking today, though, is it fair to compare a simple particle mass to an electromagnetic wave? No one is confused about Newton's first law when we talk about baseballs and spaceships drifting through space. However, can we simply apply the concept of the first law of motion to electromagnetic waves, and view an electromagnetic wave as a baseball with strange properties?

Or is there a more complicated explanation as to why electromagnetic waves can propagate through space?

Last edited: Jun 17, 2013
2. Jun 17, 2013

### Staff: Mentor

Compare them in which way? What do you mean with "fair"?
An electromagnetic wave is not a baseball with strange properties, but it has a momentum.
There is an easier explanation. The Maxwell equations.

3. Jun 17, 2013

### tolove

By "fair," I mean a conceptual view that is accurate "enough."

If electromagnetic waves have momentum, then they have a mass, and a velocity that cannot change (c)?

Along with the first law, matter will continue on through space unless interrupted, correct?

Is that the solution to the strange concept of why light can travel through a vacuum? It's just a mass like any other, only very small and with strange (quantum) properties.

4. Jun 17, 2013

### WannabeNewton

No. An electromagnetic field has both linear and angular momentum but this doesn't imply it is some kind of matter.

5. Jun 17, 2013

### Staff: Mentor

No they have not.
In vacuum, that is right.
Sure, if no force acts on an object it will travel in a straight line.
No it is not "a mass". It is not matter either.

6. Jun 17, 2013

### Jano L.

Guys above are right. Newton's first law for massive objects as intended and understood before electromagnetic theory has nothing to do with electromagnetic waves.

But I think there is a connection between the two cases, from the modern standpoint. The connection is the law of conservation of momentum. This is valid for combined system matter + EM fields and in special cases also separately both for small bodies (the first Newton law is its special case) and for EM radiation far from source (also special case, when no charged bodies are in sight).