Re: EP and Unruh - effect (Layman)
- From: ebunn@xxxxxxxxxxxxxxxxxxxxxx
- Date: Wed, 27 Jul 2005 21:32:52 +0000 (UTC)
In article <9OygLjMk3e5CFw8z@xxxxxxxxxxxxxxxxxxxx>,
Oz <Oz@xxxxxxxxxxxxxxxxxxxx> wrote:
>Eugene Stefanovich <eugenev@xxxxxxxxxxxx> writes
>
>>I don't understand your point. You agree that accelerated electron
>>radiates.
>
>Interesting point, but is it right?
>
>If I accelerate an electron in a straight line (using a linear
>accelerator for example), doe it radiate?
>
>I think not.
Yes, it does. This is experimentally confirmed all the time.
The tricky questions arise when we think about gravitational
accelerations as opposed to those in a linear accelerator. Since we
can't easily produce gravitational accelerations anywhere near as
large as those in a linac, we don't have experimental data, and we
have to rely on theoretical predictions.
Theory predicts pretty robustly that a free-falling charge does
radiate. Specifically, if you drop an electron in the Earth's
gravitational field, an observer standing still on the Earth's surface
will detect radiation from the electron.
On the other hand, suppose you have an electron sitting on a table
on the Earth's surface. An observer at rest with respect to
the Earth clearly will not detect radiation; all there'll be is
a static electric field.
So far so good. None of this is terribly surprising. Now let the
observer be in free fall. As she falls past the electron on the table,
does she see radiation? Presumably the answer must be yes, since in this
frame the electron is accelerating upwards, and accelerating charges
radiate. Otherwise, we'd have a clear violation of the equivalence
principle.
At this point, some people get very worried. How can the answer to
the question "Is it radiating?" depend on what frame you're in?
Surely it's either radiating or it's not, right? Well, that's not so
obvious. In classical physics, you check whether something is
radiating or not by measuring properties of the electromagnetic field.
The electromagnetic field is different in different frames, so maybe
it can be radiating in one frame but not another. In quantum physics,
you check whether something is radiating by counting photons. But as
Unruh pointed out, how many photons you count, even in vacuum, depends
on whether you're in an inertial or an accelerated frame.
So although the situation seems a bit strange, it's not obvious that
there's a contradiction. Stephen Parrott tries
(http://arxiv.org/abs/gr-qc/9303025) to sharpen the problem down to a
definite contradiction by a clever observation: if the electron is
radiating, it must be losing energy, so something must be supplying it
with energy. In particular, suppose that the electron isn't just
sitting on a table, but is being suspended at a fixed point above the
Earth's surface by a little rocket engine. If the electron is
radiating, the rocket should have to burn extra fuel to hold it up.
He argues that an electron at rest in the Earth's frame will *not*
have to consume extra fuel to stay up, but that an electron
accelerating at the same rate through empty space *will* have to. He
concludes that the equivalence principle does not apply to charged
particles.
Personally, I find his argument unconvincing, because it relies on
assumptions about the back-reaction on the charged particle due to its
own electromagnetic field. It's a strange fact about theoretical
physics that there is no consistent, well-behaved theory giving this
back-reaction on point charges, so he has to make some assumptions
that I think are unjustified. Still, it's a very thought-provoking
argument.
-Ted
--
[E-mail me at name@xxxxxxxxxx, as opposed to name@xxxxxxxxxxxxxxxxxxx]
.
- References:
- EP and Unruh - effect (Layman)
- From: we_pretty
- Re: EP and Unruh - effect (Layman)
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- EP and Unruh - effect (Layman)
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