Darren New <dne### [at] sanrrcom> wrote:
> clipka wrote:
> Of course this is true with flourescence, because you're emitting a
> different frequency than you absorbed. It's not true in general. See, for
> example, a laser, where all re-emitted light is specifically in the same
> direction and phase as the absorbed photon that triggers the re-emission.

Nay. If a laser would work this way, it wouldn't be a lAser ("Light
*Amplification* by stimulated emission of radiation"). The laser effect is a
totally different story.

In a laser, the lasing medium are "pumped" in some way by an external source;
typically, this is a high-energy (i.e. short-wavelength) radiation source
outside the lasing medium, like an UV lamp. *That* is what the electrons in the
lasing medium actually absorb.

The emission then happens in a totally different, lower-energy spectral band,
and can happen spontaneously; or it can be stimulated by light of that very
frequency and will then have the same direction and phase as the stimulating
photon; but the stimulating photon is *not* absorbed.


> Maxwell's equations are a statistical summarization of the actual behavior.

Nay. They are an *exact* description of the *waveforms* which *determine*
statistical behavior (the "probability wave" of a particle between actual
interactions).


> Sort of. What would keep interference from working between an absorbed
> photon and a re-emitted photon?

The fact that the photon has at last stopped being a wave (having just a certain
*probability* to be somewhere) and started being a particle (actually
interacting *somewhere* particular).

It is also described of the "probability wave" of the photon having "collapsed".

That's the moment where Maxwell can go home, and have a break until another
photon is emitted.


> > But as soon as a photon is absorbed by an electron,
> > the photon's probability wave collapses,
>
> This is incorrect, as far as I understand it. OK, well, for the kind of
> "absorb" you're talking about, where the photon turns into a higher energy
> band of electron "orbit", that might be right. But not for simple "change of
> direction" kind of absorption.

What on earth should a "change of direction kind of absorption" be?

Strictly speaking, there is no such thing as a change of direction in a photon
anyway - there is just interference of its probability wave with itself,
because of disturbances in spacetime ("gravitation lenses") or the
electromagnetic field (due to the presence of charged particles). Which, at a
non-relativistic, non-quantum newtonian scale, can be described by the concept
of "light rays", but in quantum world there is no such thing.


> > so the re-emitted light's probability wave has no way of interferencing with it,
>
> Also incorrect. You can get interference between two photons that aren't
> even in the same light cone any more.

I'm not sure about this one - but if this is actually the case, then the effect
is obviously limited to the time when *both* photons "travel", i.e. exhibit
probability wave nature. As soon as one of the photons is absorbed, the other
can't interfere with it anymore.


> I think we're talking about different types of absorption and re-emission.
> I'm talking individual photons interacting with individual electrons. I.e.,
> I'm talking about the scale where it's nonsensical to argue whether it's the
> "same" photon or a "different" photon.

Individual photons can't interact with individual electrons unless they collapse
their probability wave and choose a particular electron to interact with.

At that moment, the interaction of the photon with the electron constitutes a
quite precise "measurement" of the photon's location (which turns out to be
identical to the electron's, with only the wavelength and the electron diameter
posing a bit of uncertainty), and therefore according to Heisenberg's
uncertainty relation the photon's impulse (and therefore direction) gets
somewhat wishy-washy.


> Light "bounces" off the electrons of an atom. Whether you want to call it
> "absorb and re-emit" or whether you want to call it "bounce" simply depends
> on whether you want to think of it as the same photon or a different photon,
> which is a question that makes no sense.

Fine. So we have "bounce" back in the dictionary, which was my initial intention
in this argument.

But the question *does* make sense from a quantum physics point of view: There's
a difference between a photon's *probability wave* "bouncing off" a whole bunch
of electrons all at once (by interferencing with itself due to the disturbance
in the EM field caused by the charged particles), or a photon actually
*interacting* with an individual electron in an "absorb and re-emit" kind of
fashion.

The former doesn't do anything to any individual electron. The latter does.

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