# EM waves- self propagating in a vacuum?

kasap
First of all, I understand that an EM wave is created by a charged particle that is moving.

What I don't understand is how it propagates itself when travelling through a vacuum as there is no medium for it to interact with. Can this be explained by anything other than photon theory? I really can't wrap my head around the idea of self propagation.

Sorry in advance if this question is a load of rubbish, thanks.

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## Answers and Replies

Homework Helper
Gold Member
First of all, I understand that an EM wave is created by a charged particle that is moving.

What I don't understand is how it propagates itself when travelling through a vacuum as there is no medium for it to interact with. Can this be explained by anything other than photon theory? I really can't wrap my head around the idea of self propagation.

Well, you must.

Close to the oscillating charge the field is dependent on the oscillating charge, but as the field expands it continually renews itself by alternately generating E and H fields. The cause of the field, the oscillating charge, is essentially "forgotten"!

kasap
Thanks to both of you for your replies,
Well, you must.

Close to the oscillating charge the field is dependent on the oscillating charge, but as the field expands it continually renews itself by alternately generating E and H fields. The cause of the field, the oscillating charge, is essentially "forgotten"!

Thank you this makes more sense now, I looked up Maxwell's equations and saw that a changing magnetic field gives rise to an electric field and vice versa. But what causes them to change, is it not that the source (the oscillating charge) is constantly changing position? Sorry in advance if this is a stupid question.

mikeph
What causes them to change? That is just how they are. Is it any more mysterious than the fact that charges and current create the fields in the first place?

Physics isn't really in the habit of answering "why" questions.

Mentor
2021 Award
But what causes them to change, is it not that the source (the oscillating charge) is constantly changing position?
The charge need not continue to change position. If it briefly changes position and then remains at rest then there will be a brief wave which will propagate at the speed of light forever (in principle).

Mentor
But what causes them to change, is it not that the source (the oscillating charge) is constantly changing position? Sorry in advance if this is a stupid question.

The initial disturbance is created by the moving charge, but once created it will continue to propagate. Intuitively, it's not all that different from what happens when you toss a rock into a pool of still water - the ripples keep spreading long after the rock has quietly sunk to the bottom.

Gold Member
What causes them to change? That is just how they are. Is it any more mysterious than the fact that charges and current create the fields in the first place?

Physics isn't really in the habit of answering "why" questions.

DaleSpam andf Nugatory have done well in answering 'why'. It's a really cool topic although I would look at Faradays basic ideas on Bf and how they are created by moving charges (from your 'static' frame of reference) as opposed to Maxwell's equations for more simplicity in understanding how these EM waves propagate through space.