waves

Factual Questions
1

How can radio WAVES travel through space? Surely some kind of medium is necessary. Am I stupid or just ill-informed (or both?)

2

Radio waves, like light waves, are a form of electromagnetic radiation and are therefore self-perpetuating (meaning that no medium is necessary. The waves are themseves the medium).

Barry

3

Ill-informed. Definitely not stupid. Stupid people don’t ask about things they don’t understand.

Here’s some info on the subject:
http://spaceboy.nasda.go.jp/note/kagaku/e/kag108_michelson_e.html

http://www1.union.edu/~malekis/Freshman%20Seminar/relativity_theory.htm

These probably won’t completely satisfy your curiosity, but they will give you a couple of names and terms for further Googling - or even what to look up in a physics book at the library.

4

Actually, it was investigation into the “medium” of space throught which EM waves travel that eventually led to Einstein’s thoery of Relativity. Experimenters trying to measure the so-called “ether wind” were perplexed when they found that the speed of light was always the same no matter what reference frame it was measured in. Einstein was genius enough to say, Hey, what if the experimental result is actually true-- what would that mean? And the rest is history.

5

Here’s how I think about it:

It’s fairly easy to demonstrate that a changing magnetic field creates an electric field. You just hook up a wire to a current meter and wave a magnet at it. The change in magnetic field at the wire creates an electric field inside the wire, which moves electrons along the wire and you can see the resulting current (flow of electrons) on the meter.

Similarly, a moving electrical charge (i.e. a current flow) creates a magnetic field. That means that a changing electrical field creates a magnetic field.

You can guess the consequence - an oscillating electric field creates an oscillating magnetic field nearby, which will then create an oscillating electric field just a bit further. It propagates through nothing! Light consists of alternating magnetic and electric fields.

6

Why “just a bit further”? I’ve been able to intuitively understand why an EM wave propagates via the fact that the E field points to a moving charge’s present position and not its retarded position, but I’ve never been able to see the “just a bit further” explanation.