# Electrons settling into a lower energy state: How long does the light stay on for?

1. ### robfrias

2
So I was pondering an electron jumping to a lower energy state and releasing some light. And then I became stuck trying to figure out how long the light stays on for. (ie, how many waves of light will the "event" cause.) If it's as simple as it looks, it would make sense that one jump would correspond to one complete wave (ie. one complete EM field oscillation).

Is this true, or can one electron jump create a series of waves or wave sets ?

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3. ### f95toli

2,364
Re: Electrons settling into a lower energy state: How long does the light stay on for

A single jump will result in a single photon being released.
You can't really uses classical EM in when dealing with single photons, so your idea of "waves" does not really work.

4. ### robfrias

2
Re: Electrons settling into a lower energy state: How long does the light stay on for

Not so sure why not. Every single article I've read on visible light describes it quite specifically as an EM wave, with a period/frequency, and a corresponding amplitude. The fact that it has associated with it a particle interpretation should not invalidate its wave function.

In general , people talk of rays of light, which I have a pretty hard time reconciling with the notion of a space/time varying EM field. I imagine light just like radio waves emanating from a radio tower. That is, in terms of ever expanding spheres or bubbles of a perfectly invisible medium that only collapse or pop once some other electron receives the energy (ie photon) of that bubble. Is my conception wrong?.

Basically my eye is nothing more than a compact array of cellular antennas limited to receive a very narrow range of super duper high EM frequencies.

5. ### f95toli

2,364
Re: Electrons settling into a lower energy state: How long does the light stay on for

And most of the time the "wave description" is good enough. But in the case of single photons this classical description does not really work anymore, you need to use the full quantum mechanical machinery: quantum electrodynamics (QED). Keep in mind that photons are not really "particles" in the usual meaning of the word, they are quite different from say a basket ball or even a proton, so it is not really a case of "replacing" waves with particles. It is actually quite difficult to pin down what a photon really "is", the most physically accurate description I can think of is a "localized excitation of the vacuum".

Also, do not confuse "wave function" (as used in QM) and "waves" (in the EM sense), they are different things (and photons do not even have wave functions, at least not in the usual QM meaning of the word).
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6. ### ytuab

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Re: Electrons settling into a lower energy state: How long does the light stay on for

I am also confused about the photons. What is their nature? Real waves or patricles?

Classically, the photons can be expressed by the electromagnetic waves.
Also in QM(QED), the photons satisfy the Maxwell's equation which is the same as the classical one.

In QED, the electromagnetic waves can be expressed by the waves which have two polarization vectors perpendicular to the movement direction(k).
The "particle" photons are "embedded" in these waves as creation or annihilation operators.
But the electronic (or the magnetic) fields in QED satisfy the same Maxwell equations as the classical ones.
(So also from the QED viewpoint, when the angle between the axis of polarization of light and the polarizing filter is
$$\theta$$, the amplitude of the transmitted light is $$\cos \theta$$, isn't it?)

Even in the QM(not the classical mechanics), it is complicated.
QED is one of the "relativistic" QM. So it is a "local" theory which doesn't violate the causality (micro causality).

But if the causality isn't violated at all, the nonlocal phenomena such as the collapse of the wavefunctions and the entanglement wouldn't occur. So (though I forget where I saw this), in QM, it is said that the "macro-causality" is violated, but the "micro-causality" isn't violated.

[But I think the "macro-phenomenon" is an assembly of the "micro-phenomenon". So is it inconsistent? ]

7. ### LostConjugate

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Re: Electrons settling into a lower energy state: How long does the light stay on for

Well if the electron makes one jump you do not end up with a "complete oscillation" as OP was asking. Draw it out. You get half of an oscillation. So a photon is like a half of an oscillation, with no nodes.

It still causes an electro-magnetic field as the electric field is changing as it goes to the top of the bump and then back down. When this "bump" comes into contact with a free electron, that electron would jump as it follows the changing electric force.

I would say that an electron that moves relative to other objects emits light to those objects.