What is the electromagnetic field?

[bendavis78]

What is "the" electromagnetic field?

I'm not a professional scientist, just a software engineer with a fascination for how our world works. I thought I'd plug into some of the smart minds here to see if I could make some sense of some things I'm not quite understanding.

I've been reading Hawking's "Grand Design", and have become fascinated with the famous double-slit experiment. I understand this experiment is sort of a kick-off point for quantum mechanics. So, I wanted to understand the nature of light, specifically the wave model. From what I understand, a wave travels through a medium (eg, sound waves through air), so when I looked for what medium light waves travel through, I get a lot of references to "The electromagnetic field".

This is where I get confused. My current understanding of electromagnetic fields is that they are generated by particles of matter (eg, the magnetic field of the earth). So what to people mean when they say THE magnetic field? Is there a single, grand magnetic field that spans throughout the universe? I imagine there must be, otherwise we wouldn't see the light from distant stars, right? But then how does this field exist within a vacuum where there is no matter?

 

[G01]

A field is, by definition, defined at all points in space. So, if you want, think about the electromagnetic field existing everywhere. However, if there are no charged particles around, then the field value is just 0.

 

[The_Duck]

As G01 says we can indeed think of a single electromagnetic field extending through all of space. Imagine you have a little charged particle that you carry around with you. At any point in space, you can set it down and measure the electric force on it. This tells you the magnitude and direction of the electric field at that point. Then you can move the charged particle around and there will be some magnetic forces which you can measure to determine the magnetic field. So you can measure "the electric field" and "the magnetic field" anywhere in space.

The reason to talk about the electromagnetic field separately from the charged particles that produce it is that the field really does take on a life of its own. A charged particle at rest produces an electric field, but if you wiggle the charged particle around you can shake parts of the field loose from the particle and they go propagating off independently into space as electromagnetic waves.