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Q & A: speeds of light and electrons

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Most recent answer: 08/06/2013
Q:
How can light be the fastest thing that there is when it is a byproduct of an occuring electron shift. Wouldn't it stand to reason that the electrons would have to be moving faster than the emitted photon?
- Terry (age 49)
Salinas, Ca, USA
A:

We agree that a change of an electron's state is a common source of light. I'm mystified by the idea that then "it [would] stand to reason that the electrons would have to be moving faster than the emitted photon". Why? Motions in your throat and mouth are a common source of sound. Does your tongue then have to travel faster than sound?

Mike W.


(published on 08/06/2013)

Follow-Up #1: movement of light sources

Q:
I had asked how light could be the fastest thing there is when it is the result of an electron shift and as a byproduct, which is the key to my reasoning,wouldn't the source of the photon have to be faster than something it emitted. It was pointed out to me that sound comes from movements in our mouth and throat and the person posed the question that does our tongue have to move faster than sound to produce sound? That is a move in the right direction of reason, except, that it is not our tongue that produces sound it is the vibrations of our vocal cords. I do not believe that these vibrate faster than sound but I do not know for certain. Still, this does not seem to be the same thing to me. Light is an energy source and a "byproduct" of that electron shift. How is the photon emitted, as in what causes this to happen, at all, and my first question remains, that wouldn't the source of movement have to be faster than the emitted photon? I appreciate anyone's thoughts on this.
- Terry (age 49)
california
A:

Your question remains a mystery to me. Almost any wave you wish to think of (water waves, waves down a violin string, ...) can be triggered by motions that are slower than the wave speed. Yes, that includes sound waves from either the vocal chords or from the tongue.

Ok, so let's look at electromagnetic waves like light. Instead of visible light, let's think of low frequency EM waves traveling at exactly the same speed as light, following exactly the same wave equation, but with an easier source to picture. This EM radiation can come from an antenna- just a wire in which electrons flow back and forth. The intensity of the wave grows as those electrical currents get bigger (faster electron motions), but the wave speed doesn't change. The speed of the wave remains exactly the same no matter how slowly the electrons flow back and forth.

You don't need to even think about photons to understand this. It's all in Maxwell's equations for classical electromagnetism.

(How fast might the electrons flow in a typical case? Say you've got some standard ac current, e.g. 1 amp, flowing in a copper wire with about a square millimeter cross section.The average velocity of the electrons is only around 10-2 cm/s, quite  slow. )

You use the phrase "stands to reason" but you haven't explained what sort of reasoning goes against the actual content of all our physical theories.

Mike W.

 


(published on 08/06/2013)

Follow-up on this answer.