# How can an electromagnetic wave propagate on itself even if there is no source that has created it? [duplicate]

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How can an electromagnetic wave propagate on itself even if there is no source that has created it?

## marked as duplicate by John Rennie, sammy gerbil, StephenG, stafusa, Chris♦Feb 3 '18 at 10:21

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• There is always a source, it is only that sometimes it is neglected. – DanielC Feb 3 '18 at 8:28
• Speaking quantum mechanically, only photons are produced. What you see is actually a beam of those. There is always a source for a photon (such as an excited atom transitioning to the ground state). – Sreekar Voleti Feb 3 '18 at 8:32
• -1 Not clear what you are asking. Please provide more information. – sammy gerbil Feb 3 '18 at 8:34

## 1 Answer

It is not the electromagnetic wave that propagates. It is space itself that propagates the wave. Case in point if you move a magnet by hand and you have a compass needle some distance away, the compass needle will take a time before it reacts. That time happens to be the speed of light. There is no light involved and no electromagnetic radiation. The only energy input involved is in mechanically moving the magnet.

• What is the difference between your first two lines ? I can't distinguish . – Vijay Kumar Feb 3 '18 at 8:43
• An electromagnetic wave is a disturbance in space, not some energy beam radiating from a source. – PeterS Feb 3 '18 at 9:00
• @PeterS, you're just muddling your own thoughts - there is no distinction between a disturbance in space and an energy beam that radiates. – Steve Feb 3 '18 at 9:44
• I thought I just demonstrated that with the example of moving a magnet in space. There is no beam radiated from the magnet. Therefore any disturbance detected at a distance must be from space itself deforming and propagating the disturbance. – PeterS Feb 3 '18 at 10:22