Richard Feynman's "QED"

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On Sat, 17 Sep 2005 05:34:27 -0700, William Sommerwerck wrote:

> Bob Cain's admonishments finally got me to read Feyman's "QED". I strongly
> recommend it to everyone in this group. Especially the first chapter, which
> briefly discusses the philosophy of science. <ahem>
>
> As for the contents... There are typos on pages 98 and 106 (which might have
> been caught in later printings). Also, Feynman correctly explains the
> difference between "explain" and "describe", then proceeds to (usually) use
> the former when he means the latter.
>
> These are _very_ minor points. My principal gripe is with Bob Cain's claim
> that QED is a theory that shows how particles can have wave-like properties
> in a "reasonable" way, without having to twist things around. I disagree
> with that.
>
> QED is an attempt to come up with a single coherent theory that describes
> both macro and micro effects. As such, it seems to work extremely well. The
> problem with Bob's claim is that pesky little timer. The timer introduces
> phase into the theory -- and phase is a property associated with waves, not
> particles. As I see it, the particle/wave duality has been incorporated into
> the theory, but in a way that includes both behaviors in a manageable
> fashion. That is, you don't need one set of calculations for "waves", the
> other for "particles" -- the single theory describes both forms of behavior,
> particularly how particle-like "things" can produce wave-like effects. *
> That's good.
>
> Feynman -- who explicitly states that light (EM radiation) is "really" a
> particle -- doesn't directly address the point that a theory (in his view)
> treating EM energy as being "particulate" has a timer attached to it. The
> timer seems to be necessary if you're going to account for wave-like
> effects, but Feynman seems to be trying to sweep this necessity under the
> rug. **
>
> He also contradicts what I've read in so many books on physics (which
> doesn't make it true, of course), that light is neither a wave nor a
> particle -- it just looks like one or the other, depending on the
> experiment. "QED" also neglects to cover this point -- Feynman doesn't
> explain (as far as I could tell) how (or whether) QED theory predicts this.
>
> Oh, one other thing... If I don't say this, it's inevitable someone is going
> to deliberately misread what I wrote... I do not claim or imply that QED is
> incorrect in any way, or does not produce useful results. I am simply
> commenting on how Bob Cain and Richard Feynman have described it. I'd like
> to see a deeper discussion of the points I've covered.
>
> * I don't remember reading anything about the opposite. Is it there? Did I
> miss it?
>
> ** Feynman says that these lectures on QED give a correct, but not
> necessarily complete, description. Okay. No problem. I do wish he'd
> explained the rationale of the timer, though. It makes sense to me that it
> should be needed; I just wonder why he left out any discussion.

And which newsgroup did you choose to write your critique of "Abbey Road"?

d

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On Sat, 17 Sep 2005 06:33:03 -0700, William Sommerwerck wrote:

>> And [for] which newsgroup did you choose to write
>> your critique of "Abbey Road"?
>
> If you'd look at the arguments about "reconstruction" filters, you'd
> understand where this seemingly OT post came from.

In that case, it might have had more relevance had you posted it to that
thread.

And reconstruction filters need no inverted commas.

d

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> "William Sommerwerck" <gizzledgeezer@comcast.net> wrote:

>> If you'd look at the arguments about "reconstruction" filters, you'd
>> understand where this seemingly OT post came from.

> Who cares? Your posts are tiresome and pedantic.

Yes. Because no one bothers to respond -- pro or con -- intelligently.

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On Sat, 17 Sep 2005 08:27:20 -0700, William Sommerwerck wrote:

>>>> And [for] which newsgroup did you choose to write
>>>> your critique of "Abbey Road"?
>
>>> If you'd look at the arguments about "reconstruction" filters, you'd
>>> understand where this seemingly OT post came from.
>
>> In that case, it might have had more relevance had you posted it to that
>> thread.
>
> I originally intended to, but decided it was more-appropriate to break it
> out.
>
>
>> And reconstruction filters need no inverted commas.
>
> They're not inverted commas, they're quotation marks, commonly used to
> indicate the writer doesn't like the term, sees ambiguity in it, etc, etc,
> etc.

There is no ambiguity.

d

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On Sat, 17 Sep 2005 09:15:47 -0700, William Sommerwerck wrote:

>>>> And reconstruction filters need no inverted commas.
>>>
>>> They're not inverted commas, they're quotation marks, commonly used to
>>> indicate the writer doesn't like the term, sees ambiguity in it, etc,
> etc,
>>> etc.
>>
>> There is no ambiguity.
>
> There is to me!

Well, that makes you the last one left. Read up a bit - you'll get there.

d

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William Sommerwerck wrote:
> Bob Cain's admonishments finally got me to read Feyman's "QED". I strongly
> recommend it to everyone in this group. Especially the first chapter, which
> briefly discusses the philosophy of science. <ahem>

Fun stuff. One of the coolest reads in my library.
Completely approachable by a layman.

> QED is an attempt to come up with a single coherent theory that describes
> both macro and micro effects. As such, it seems to work extremely well. The
> problem with Bob's claim is that pesky little timer. The timer introduces
> phase into the theory -- and phase is a property associated with waves, not
> particles.

Every particle has a frequency of oscillation as proposed by
DeBroglie and verified by experiment. Anything that
oscilates has a phase.

> Feynman -- who explicitly states that light (EM radiation) is "really" a
> particle -- doesn't directly address the point that a theory (in his view)
> treating EM energy as being "particulate" has a timer attached to it. The
> timer seems to be necessary if you're going to account for wave-like
> effects, but Feynman seems to be trying to sweep this necessity under the
> rug. **

LOL! He was very good at that. Renormalization, which he
invented, is just such a sweeping (of infinities no less.)
He suspects that it isn't valid math but it works to amazing
precision anyway.

He also admits that the path integral or "sum over
histories" which is the basis of his formulation of QED and
his path to the Nobel prize is on shaky mathematical
grounds. It still works amazingly well, and Feynman
Diagrams, schematics for path integrals, are on every
particle physicist's blackboard regardless.

> He also contradicts what I've read in so many books on physics (which
> doesn't make it true, of course), that light is neither a wave nor a
> particle -- it just looks like one or the other, depending on the
> experiment. "QED" also neglects to cover this point -- Feynman doesn't
> explain (as far as I could tell) how (or whether) QED theory predicts this.

That would require going to a mathematical depth
inappropriate for the audience to which that book speaks.

> Oh, one other thing... If I don't say this, it's inevitable someone is going
> to deliberately misread what I wrote... I do not claim or imply that QED is
> incorrect in any way, or does not produce useful results. I am simply
> commenting on how Bob Cain and Richard Feynman have described it. I'd like
> to see a deeper discussion of the points I've covered.

Never before, and probably never again, have my name and
Richard Feynman's been joined by an "and." You just made my
day! :-)

> ** Feynman says that these lectures on QED give a correct, but not
> necessarily complete, description. Okay. No problem. I do wish he'd
> explained the rationale of the timer, though. It makes sense to me that it
> should be needed; I just wonder why he left out any discussion.

Didn't he mention DeBroglie's equation?


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

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jnorman34@comcast.net wrote:
> i have read QED several times over the past many years, and i agree it
> is one of the best semi-layperson examinations of the topic, as
> presented by one of the foremost authorities on the subject. feynman
> does not attempt to offer the idea that physics actually understands
> "how" reality can be this way - he simply tries to describe the
> observed results in regard to prevailing QED theory. i think you are
> making a basic error however in indicating that the timer introduces
> phase into the mechanics of the interaction between photons and
> electrons - the timer concept is merely a tool that allows you to come
> up with quick answers to predicted results in a given situation.
> feynman also does not try to describe the idea that photons are both
> wave and particle - he states explicitly many times that in every
> experiment, we ALWAYS detect photons as particles. wave nature of
> photons is exhibited only through the actions of individual photon
> particles. and it is prudent to remember that QED was written for
> laypersons, and feynman states many times that he is simplifying the
> true picture so the reader can grasp both the general description of
> photon/electron interaction and the profound mystery inherent in the
> actual nature of subatomic reality. as you say, keep reading, but do
> not mistake that you are going to "get there" in terms of
> understanding, because NOBODY understands it. the nature of reality is
> surely the most intriguing anf significant topic that humans can study.
>
Very nicely said.

-Bill

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> i have read QED several times over the past many years, and i agree it
> is one of the best semi-layperson examinations of the topic, as
> presented by one of the foremost authorities on the subject. feynman
> does not attempt to offer the idea that physics actually understands
> "how" reality can be this way - he simply tries to describe the
> observed results in regard to prevailing QED theory.

Correct.


> i think you are
> making a basic error however in indicating that the timer introduces
> phase into the mechanics of the interaction between photons and
> electrons - the timer concept is merely a tool that allows you to come
> up with quick answers to predicted results in a given situation.

That's like saying that because a kitchen timer helps us produce tasty food,
it isn't really a timer.

Without that little timer, QED wouldn't work. And as Feynman explicity
points out, the timer runs faster for higher-energy (ie, shorter-wavelength)
photons. If that isn't phase, I don't know what is.

The phasing shows up again and again in his descriptions of the thought
experiments. If the timer didn't produce the phasing effects -- which appear
as variations in the angles of the various vectors -- you can't get
"phasier" than that! -- the theory would lose its predictability.


> feynman also does not try to describe the idea that photons are both
> wave and particle - he states explicitly many times that in every
> experiment, we ALWAYS detect photons as particles.

Only if we're looking to detect photons as photons. There are all sorts of
experiments that show light is a wave.


Again, my gripe is not with QED, but the fact (and is a fact) that Feynman
says light is "really" a particle, then fails to explain the rationale of
the timer.

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william - inre:
" If that isn't phase, I don't know what is. "

not necessarily - remember that you can perform the double-slit
experiment one photon at a time, and still acheivee the same result of
wave interference pattern - this is tantamount to demonstrating that
each individual photon interferes with itself, and is not what we
normally think of as wave interference.


and

" There are all sorts of
experiments that show light is a wave."

every experiment we can do indicates that photons are particles. every
method of detecting photons indicates that they are particles. you
cannot say/know anything about a photon's nature or location or
anything between the time it is emitted and the time it is absorbed.
all wave-seeming results are the build up of individual photons.

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William Sommerwerck wrote:

> PS: One of the reasons Feyman is so easy to respect is that he is so quick
> to point out what he _doesn't_ know or _doesn't_ understand. Most people
> aren't like that.

Hear, hear! OTOH, as the man who probably understood more
physics, both in breadth and in depth, than any other man in
this glorious last century of physics, such admissions were
of little threat to his ego. :-)


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

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jnorman34@comcast.net wrote:
> william - inre:
> " If that isn't phase, I don't know what is. "
>
> not necessarily - remember that you can perform the double-slit
> experiment one photon at a time, and still acheivee the same result of
> wave interference pattern - this is tantamount to demonstrating that
> each individual photon interferes with itself, and is not what we
> normally think of as wave interference.

Right. The Feynmanesque way of viewing the single photon
pattern is that a photon is a rotating phasor simultaneously
taking all possible paths to get to a given point on the
screen where the complex phasor values that each path gives
at that point are all summed (and the magnitude taken) to
get the probability of it hitting that point. The phase
shift along each path, before summing, is a function of the
length of the path (and whatever else it might do along the
way.) When you calculate all this out for every point on
the screen, the pattern emerges from the probabilities.

If this sounds suspicously like a view that turns a particle
into a wave you are hearing correctly.

> every experiment we can do indicates that photons are particles.

Not quite. Every experiment that looks for the wave
properties finds them and every experiment that looks for
particle properties also finds them. Doing an experiment
that simultaneously shows both properties is the problem.
This is expressed in Bohr's complimentarity principle which
is more a conjecture than a proof and says (under the
Copenhagen interpretation of QM) that such an experiment is
not possible.

Afshar did an experiment earlier this year that seems to
simultaneously show both properties and it has been the
subject of lots of debate but I don't think it has yet been
shown to be specious. For more, see:

http://en.wikipedia.org/wiki/Afshar_experiment

And his blog at:

http://irims.org/blog/index.php/questions

> all wave-seeming results are the build up of individual photons.

Even a single photon gives a result, in the form of a
probability distribution, that is wave-seeming. Of course,
to actually see that distribution you have to look, as you
say, at the results of lots of individual photons.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

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> > Please note your first sentence -- "every experiment we can do
> > indicates that photons are particles". Yes, of course -- by
> > defintion, photons are particles. That doesn't mean that light is
> > made of particles.
>
> There are many measurments that show very well that light is a particle
> phenomina. The first clear one was published 100 years ago. There are
> as many that show thet light is a wave phenomina.

Perhaps my comment was a little too subtle and could have stood some
clarificatin.

By defintion, photons are particles. If we perform an experiment _designed_
to show that light is particles, we will get that result. Similarly, an
experiment designed to demonstrate the wave nature of light reveals _that_.
Neither "proves" that light is one or the other.