
How Dyson Saw Feynman - dnetesn
http://nautil.us/issue/59/connections/another-side-of-feynman
======
tzs
> On the Sunday Feynman was up at his usual hour (nine a.m.), and we went down
> to the physics building, where he gave me another two-hour lecture of
> miscellaneous discoveries of his. One of these was a deduction of Maxwell’s
> equations of the electromagnetic field from the basic principles of quantum
> theory, a thing which baffles everybody including Feynman, because it ought
> not to be possible.

Physics stackexchange discussion of this [1].

That includes a link to a 2001 paper, "Feynman's derivation of Maxwell
equations and extra dimensions" [2].

Here's a paper Dyson wrote about it in 1989 [3].

That was discussed here on HN [4].

[1] [https://physics.stackexchange.com/questions/391744/does-
feyn...](https://physics.stackexchange.com/questions/391744/does-feynmans-
derivation-of-maxwells-equations-have-a-physical-interpretation)

[2] [https://arxiv.org/abs/hep-ph/0106235](https://arxiv.org/abs/hep-
ph/0106235)

[3] [http://fermatslibrary.com/s/feynmans-proof-of-the-maxwell-
eq...](http://fermatslibrary.com/s/feynmans-proof-of-the-maxwell-equations)

[4]
[https://news.ycombinator.com/item?id=11067435](https://news.ycombinator.com/item?id=11067435)

~~~
westoncb
What I'd really like to hear about is Feynman's thought process for arriving
at this...

~~~
stan_rogers
Murray Gell-Mann described it completely: first, he wrote down the question,
then he thought very hard, then he wrote down the answer. Feynman was acutely
aware of the very different ways in which people internally model things -
something that was, no doubt, heavily reinforced by the initial reception of
Feynman diagrams at the Shelter Island Conference - so he didn't place any
special importance on his own internal "sketching" models when describing
things to others. (If anything, he probably viewed them about the same way he
viewed Maxwell's mechanical model of electromagnetism - a useful thinking tool
for someone who happened to think best in a particular mode, but ultimately
nonsensical.) If he couldn't think of at least a couple of alternative ways of
coming to the same answer, he usually left it as a sort of exercise for the
student.

~~~
DoctorOetker
"first, he wrote down the question, then he thought very hard, then he wrote
down the answer."

this is like:

Step 1: Start

Step 2: ???

Step 3: PROFIT!

" he didn't place any special importance on his own internal "sketching"
models when describing things to others."

So the answer was more like: Feynman's priority was to show the most concise
derivation, not to show the path he stumbled around to arrive at a result.

Indeed I believe most people once they have a rough path to a result,
immediately try to simplify it to the crux of the argument before
presentation, all the unnecessary assumptions are trimmed off, just like a
mathematician tries to find a proof that relies on the least axioms.

Sometimes I see people explain how they came up with their result, but I
suspect even these detours are often substantially cleaned up before
presentation. Others simply no longer remember the original thought process,
due to time passing, or because while working out the longwinded original path
the simpler shortcut simply hits them. That instant high of a realization also
tends to pollute your memory of stumbling towards it.

Another problem is that big problems were often in the back of a persons mind
for a long time, and before a solution materializes, small insights or even
vague associations -that may be seperated by years- ultimately combine, so a
sudden insight is often just the last piece, with some of the supporting
lemmas never disclosed before (as of seemingly no value when the smaller
supporting insights were conceived)

Perhaps there is a text somewhere, where Feynman explains how he originally
arrived at it, but if so I haven't seen it. I too would like to see it if it
is known or ever turns up...

------
tempestn
> In the evening I mentioned that there were just two problems for which the
> finiteness of the theory remained to be established; both problems are well-
> known and feared by physicists, since many long and difficult papers running
> to fifty pages and more have been written about them, trying unsuccessfully
> to make the older theories give sensible answers to them. When I mentioned
> this fact, Feynman said, “We’ll see about this,” and proceeded to sit down
> and in two hours, before our eyes, obtain finite and sensible answers to
> both problems. It was the most amazing piece of lightning calculation I have
> ever witnessed, and the results prove, apart from some unforeseen
> complication, the consistency of the whole theory. The two problems were the
> scattering of light by an electric field, and the scattering of light by
> light.

~~~
DoctorOetker
I'm familiar with the scattering of light by light, but I am not familiar with
the scattering of light by a electric field, where can I learn more?

~~~
SiempreViernes
[https://en.wikipedia.org/wiki/Compton_scattering](https://en.wikipedia.org/wiki/Compton_scattering)
and the famous low energy limit
[https://en.wikipedia.org/wiki/Thomson_scattering](https://en.wikipedia.org/wiki/Thomson_scattering)

~~~
DoctorOetker
I'm familiar with Compton scattering (I did the experiment), but that is
scattering from a charge(d particle). I would have expected Dyson to write
"light scattering from an electric charge" or perhaps "from an electron" if he
meant Compton scattering.

I still read Dyson's sentence as light scattering from an electrostatic field
(not clear if it would scatter from a homogenous field, or only gradients in
electric field).

Could others please chime in, and confirm SiempreViernes response, or
enlighten us both?

EDIT:

Could this refer to pair production?

The electric field of the highly positive nucleus affecting the virtual
electron positron pair before it collapses and re-emits the scattered photon?

~~~
SiempreViernes
I don't think pair production makes more sense, it's probably Compton
scattering because all QFT interactions using Feynman diagrams are expressed
as particle interactions.

~~~
DoctorOetker
why would such interactions not be expressible to express as a particle
interaction?

first draw a Feynman diagram of an incoming photon the splits into electron
and positron which recombine into an outgoing photon (an electron loop in the
middle of the diagram)

then on the bottom add an incoming proton which interact with a photon to
either the electron or the positron of the electron loop...

~~~
SiempreViernes
I'm not saying pair production can't be expressed as a particle interaction;
I'm saying all interactions with fields in QFT are expressed as particle
interactions, so photon electron scattering is a perfectly valid
interpretation of

"light scattering from an electric charge"

thinking of it more, I have a hard time seeing the conversion of a photon into
two "electrons" fairly being termed "scattering".

~~~
DoctorOetker
"

so photon electron scattering is a perfectly valid interpretation of

"light scattering from an electric charge"

"

But look up, you misquote Dyson: Dyson didn't write "light scattering from an
electric charge" but wrote "light scattering by an electric _field_ " .

You also misread me:

1) a photon can not be converted into two "electrons" and I did not write this
(but in the past positrons were called positive electrons).

2) the electron positron pair (from a single photon, _internal to the feynman
diagram_ ) is only intermediate but as charges are influenced by a strong
electric field (say a nucleus), before annihilating back into a single photon.

3) the incoming particles are a photon and a nucleus, and the outcoming
particles are again a photon and a nucleus, this is obviously scattering

This is different from Compton scattering (scattering of light from a
practically free electron) or its low energy limit Thomson scattering, but in
the case of scattering from atoms still high enough energy to eject an
electron.

This is different from Coherent Compton scattering, which is scattering of
light from an atom, or rather from an electron that remains bound to the atom,
such that instead of the compton wavelength of the electron, the compton
wavelength of the atom is used: "Compton found that some X-rays experienced no
wavelength shift despite being scattered through large angles; in each of
these cases the photon failed to eject an electron.[5] Thus the magnitude of
the shift is related not to the Compton wavelength of the electron, but to the
Compton wavelength of the entire atom, which can be upwards of 10000 times
smaller. This is known as "coherent" scattering off the entire atom since the
atom remains intact, gaining no internal excitation." (From Wikipedia page on
Compton scattering)

I still interpret Dyson's "light scattering from an electric field" as
probably referring to the Meitner-Hupfeld effect:

[https://en.wikipedia.org/wiki/Meitner%E2%80%93Hupfeld_effect](https://en.wikipedia.org/wiki/Meitner%E2%80%93Hupfeld_effect)

Here's the Feynman diagram I have in my head:

[https://www.overleaf.com/read/dszzcbcqdqtk](https://www.overleaf.com/read/dszzcbcqdqtk)

------
vecter
> On the third day of the journey a remarkable thing happened; going into a
> sort of semistupor as one does after forty-eight hours of bus riding, I
> began to think very hard about physics, and particularly about the rival
> radiation theories of Schwinger and Feynman. Gradually my thoughts grew more
> coherent, and before I knew where I was, I had solved the problem that had
> been in the back of my mind all this year, which was to prove the
> equivalence of the two theories. Moreover, since each of the two theories is
> superior in certain features, the proof of equivalence furnished a new form
> of the Schwinger theory which combines the advantages of both. This piece of
> work is neither difficult nor particularly clever, but it is undeniably
> important if nobody else has done it in the meantime. I became quite excited
> over it when I reached Chicago and sent off a letter to Bethe announcing the
> triumph. I have not had time yet to write it down properly, but I am
> intending as soon as possible to write a formal paper and get it published.
> This is a tremendous piece of luck for me, coming at the time it does. I
> shall now encounter Oppenheimer with something to say which will interest
> him, and so I shall hope to gain at once some share of his attention. It is
> strange the way ideas come when they are needed. I remember it was the same
> with the idea for my Trinity Fellowship thesis.

> My tremendous luck was to be the only person who had spent six months
> listening to Feynman expounding his new ideas at Cornell and then spent six
> weeks listening to Schwinger expounding his new ideas in Ann Arbor. They
> were both explaining the same experiments, which measure radiation
> interacting with atoms and electrons. But the two ways of explaining the
> experiments looked totally different, Feynman drawing little pictures and
> Schwinger writing down complicated equations. The flash of illumination on
> the Greyhound bus gave me the connection between the two explanations,
> allowing me to translate one into the other.

Dyson is obviously brilliant, and I'm sure he's being humble to a fault here,
but it's amazing how valuable being in the right place at the right time is.
This piece of work brought Dyson to fame:

> Oppenheimer rewarded Dyson with a lifetime appointment at the Institute for
> Advanced Study, "for proving me wrong", in Oppenheimer's words.[0]

[0]
[https://en.wikipedia.org/wiki/Freeman_Dyson](https://en.wikipedia.org/wiki/Freeman_Dyson)

~~~
SiempreViernes
Yeah, Dyson is like 85 % of the reason we have Feynman diagrams and not some
other way of calculating QFT processes.

See this for more:
[http://www.press.uchicago.edu/ucp/books/book/chicago/D/bo353...](http://www.press.uchicago.edu/ucp/books/book/chicago/D/bo3534300.html)

------
RcouF1uZ4gsC
I am always amazed by the sheer brainpower at the Manhattan Project. Has that
level of talent ever been assembled before or since?

~~~
racer-v
Try the 1927 Solvay Conference on Electrons and Photons:

[https://www.reddit.com/r/pics/comments/87a8ea/1927_group_pho...](https://www.reddit.com/r/pics/comments/87a8ea/1927_group_photograph_from_the_solvay_conference/)

Schrödinger, Pauli, Heisenberg, Dirac, de Broglie, Bohr, Planck, Curie,
Lorentz, Einstein... the number of Nobel Prizes here is left as an exercise.

~~~
westoncb
Those are a lot of the same people from the Manhattan project though, no?

Has something on that scale happened since, with an unrelated set of people?

~~~
racer-v
Actually this conference was largely from a previous generation of scientific
thinkers. According to their Wikipedia bios, Niels Bohr was the only one of
those 10 names involved in the Manhattan Project. Einstein wrote a letter
alerting President Roosevelt to the possibility of a German atomic bomb which
may have helped inspire the project. Heisenberg was also involved in the war
effort but "knew little of the Manhattan Project, so, if he were captured, he
would have little intelligence value to the Germans".

~~~
raattgift
Wow.

You quote -- without attribution -- wikipedia, and you get the antecedent
wrong.

I'll italicize the exact words you quote after their preceding sentence in the
Werner Heisenberg article on wikipedia:

"Godusmit was selected for this task because he had physics knowledge, he
spoke German, and he personally knew a number of the german scientists working
on the German nuclear energy project. He also _knew little of the Manhattan
Project, so, if he were captured, he would have little intelligence value to
the Germans_."

Note that the "he" in the second sentence is _Godusmit_ , not Heisenberg.

Heisenberg was, according to Meitner, straightforwardly a supporter of the
Nazi regime. Whether or not he was ideologically committed to the party,
Heisenberg _was_ most definitely involved in the war effort -- on the German
side! [ [https://www.nytimes.com/2002/01/07/us/letter-may-solve-
nazi-...](https://www.nytimes.com/2002/01/07/us/letter-may-solve-nazi-a-bomb-
mystery.html) \-- first sentence, "The leader of the Nazi atomic bomb program,
Werner Heisenberg, revealed its existence ..." a paragraph later: "Heisenberg
never expressed moral qualms about building a bomb for Hitler or hinted that
he might be willing to sabotage the project, the documents reveal" with a
follow-up here [https://www.nytimes.com/2002/02/07/world/new-twist-on-
physic...](https://www.nytimes.com/2002/02/07/world/new-twist-on-physicist-s-
role-in-nazi-bomb.html) ].

Further (and much more detailed) discussion from 2016 at APS Physics:
[https://www.aps.org/units/fps/newsletters/201607/heisenberg....](https://www.aps.org/units/fps/newsletters/201607/heisenberg.cfm)

~~~
racer-v
Thanks for this correction! I had hurriedly checked Wikipedia bios for each
participant and completely missed the main point of Heisenberg's wartime
activities.

[https://en.wikipedia.org/wiki/Werner_Heisenberg#1945:_Alsos_...](https://en.wikipedia.org/wiki/Werner_Heisenberg#1945:_Alsos_Mission_and_Operation_Epsilon)

It makes the story even more interesting to see Bohr and Heisenberg
participating on opposite sides.

------
skookumchuck
What a marvelous series of letter. I would sure love to have dinner with these
people!

------
VectorLock
I don't know what it is about this website but it constantly makes its Chrome
tab freeze up and so scrolling is as painful as walking on LEGOs.

~~~
tbabb
Same. I don't know if they're trying to mine bitcoin on my browser or what,
but it inspired me to block JS entirely for all of nautil.us.

------
richardfeynman
This is fantastic.

