Was QED intended as a joke?
z@z
| Extracts from http://www.corepower.com/~relfaq/light_mass.html :
| : A massless particle can have energy E and momentum p because mass is
| : related to these by the equation m^2 = E^2/c^4 - p^2/c^2 which is
| : zero for a photon because E = pc for massless radiation.
|
| What about photons in media with refraction coefficients n > 1. I
| suppose that the momentum of a photon is then p = f*h/(c*n) and that
| a photon transfers a part of its momentum to a lens when it gets into
| it. If this is true, the general validity of m^2 = E^2/c^4 - p^2/c^2
| is refuted, isn't it?
The explanation of light refraction by Huygens' principle is
a first-rate example of an effective (i.e. simple and elegant)
physical explanation.
This classical explanation of refraction requires that waves
propagate at different velocities depending on the refraction
coefficients of the media and that the waves can freely move
over distances larger than their wavelength.
In QED, the velocity of photons does not depend on constitutive
constants of the medium as in Maxwell's theory but is always c.
The progagation delays corresponding to refraction coefficients
are explained by regular absorption and reemission of photons.
The refraction coefficient of water at 20 degree Celsius is
n = 1.333 for visible light of a wave length of 590 nanometer.
So according to common sense physics, the wave length of photons
shrinks from around 590 nm to around 590 nm / 1.333 = 440 nm
when the photons penetrate water. But the diameter of water
molecules is only around 0.3 nm. So how can Huygen's principle
work, if photons are continuously absorbed and reemitted by
molecules?
The situation is especially strange as the path-integral method
of QED is essentially the same as Huygens principle. So in this
respect, QED is not even self-consistent!
A further problem is that water molecules are assumed to absorb
(and reemit at exactly the same frequency) a continuous spectrum
and not only discrete lines corresponding to discrete energy
levels.
Yes, I know, the work of Bohr, Heisenberg, Dirac, Feynman, and
others has demonstrated that only a fool can be naive enough to
believe that ordinary logic, conservation laws and similar common
sense principles are relevant to the strange world of quanta.
Sometimes I wonder whether Feynman, a gifted entertainer with
common sense, really believed in what he taught mankind. Maybe
virtual particles were at least partially intended as a good joke.
In claiming that particles can propagate backwards in time (from
the future to the past!) Feynman either ignores our philosophical
heritage or he makes fun of it (and of us).
But in proposing his path-integral method as an alternative to
Heisenberg's and Schroedinger's formulations of QM, Feynmen has
shown that QM is (apart from the Planck-Einstein-Bohr quantum
concept and from integrated experimental facts) essentially not
much more than an unwarranted generalization of the old principle
of Huygens.
Wolfgang Gottfried G.
Liechtenstein - 1999/11/11
Stephen
> Sometimes I wonder whether Feynman, a gifted entertainer with
> common sense, really believed in what he taught mankind. Maybe
> virtual particles were at least partially intended as a good joke.
No, they were intended to represent terms in a series. Feynman
diagrams, and the imagery of virtual particles used in these diagrams, are
a very useful tool for stating and solving problems.
--
"The end of our foundation is knowledge of causes,
and secret motions of things; and the enlarging of the bounds
of human empire, to the effecting of all things possible."
- Francis Bacon, "New Atlantis".
Gregory L. Hansen
Stephen <stephe...@deathtospam.hotmail.com> wrote:
>In article <80f4ee$pfq$1...@pollux.ip-plus.net>, "z@z" <z...@z.lol.li> wrote:
>
>
>> Sometimes I wonder whether Feynman, a gifted entertainer with
>> common sense, really believed in what he taught mankind. Maybe
>> virtual particles were at least partially intended as a good joke.
>
>
> No, they were intended to represent terms in a series. Feynman
>diagrams, and the imagery of virtual particles used in these diagrams, are
>a very useful tool for stating and solving problems.
I know where virtual particles came from, but I'm not so sure what the
thinking is on them in modern physics. People talk about virtual photons
carrying the electromagnetic force and virtual mesons carrying the weak
force, for instance. And there are experiments (I'm involved in one) that
even measure the coupling constants for those particles. The way it's
talked about sure makes it seem like there really are ghostly particles
flying around (not to mention ghost particles...). But I know they're
mathematical terms in a series, which makes me wonder if they've been
unjustly reified.
--
"That's not an avocado, that's a grenade!" -- The Skipper
Gerry Quinn
The stuff fields are made of is neither particles nor waves, but has
aspects of both. I would say the particle aspects are as real as any
others. We know something leaks out over all of space anyway, so
something is flying around.
- Gerry Quinn
Tom Roberts
> The explanation of light refraction by Huygens' principle is
> a first-rate example of an effective (i.e. simple and elegant)
> physical explanation.
Yes. And QED provides a one-level-deeper explanation of why Huygens'
principle is valid.
> In QED, the velocity of photons does not depend on constitutive
> constants of the medium as in Maxwell's theory but is always c.
> The progagation delays corresponding to refraction coefficients
> are explained by regular absorption and reemission of photons.
Sort of. It's significantly better to think of this as interference of
all possible interactions between the photons and the charged particles
of the medium (see below).
> The refraction coefficient of water at 20 degree Celsius is
> n = 1.333 for visible light of a wave length of 590 nanometer.
> So according to common sense physics, the wave length of photons
> shrinks from around 590 nm to around 590 nm / 1.333 = 440 nm
> when the photons penetrate water. But the diameter of water
> molecules is only around 0.3 nm. So how can Huygen's principle
> work, if photons are continuously absorbed and reemitted by
> molecules?
It is your "sound bite" approach to physics which is at fault. When
one considers the properties of photons in water as an interference
effect among all the electrons and protons in the water, the importance
of the diameter of the individual molecules vanishes. This is a _bulk_
effect, and not an individual-molecule effect.
If you think about it, this is a bulk effect classically as
well. Think more about it, and you will see that the accuracy
of the classical description pretty much requires this to be a
bulk (i.e. multiparticle) effect in QED.
> The situation is especially strange as the path-integral method
> of QED is essentially the same as Huygens principle. So in this
> respect, QED is not even self-consistent!
Not true. Learn what QED really is. As I have said before, you need to
actually _LEARN_ what these physical theories say before making such
conclusions.
> A further problem is that water molecules are assumed to absorb
> (and reemit at exactly the same frequency) a continuous spectrum
> and not only discrete lines corresponding to discrete energy
> levels.
Actually, to compute the behavior of photons in water, no atomic
transitions are used at all. Your model of "absorbtion and subsequent
emission" is blatantly violated in QED, because the emission can occur
_BEFORE_ the absorbtion (one must sum over all Feynman diagrams, and
each diagram includes an integral over all possible 4-momenta; when
expressed in space-time this implies that the interactions are not
well-ordered in time, and all possible orderings occur with equal
likelihood).
> In claiming that particles can propagate backwards in time (from
> the future to the past!) Feynman either ignores our philosophical
> heritage or he makes fun of it (and of us).
He did not ignore it, he (and the other founders of QED) discovered
that this "heritage" (as you call it) is not applicable here. Note
that that "heritage" was acquired in everyday experience, which does
not include quantum effects at all (directly). Only a fool would
expect/insist that our common-sense heritage would apply in regimes
far removed from the domain in which that heritage was acquired;
this applies to the quantum world, and to relativiity, and to extreme
low or high temperatures, and to extreme high or low pressures, and
to....
Your criticisms would be more effective if you actually _UNDERSTOOD_
what you are attempting to criticise. But then, your criticisms would
vanish.
Tom Roberts tjro...@lucent.com