Re: [Phys-L] Feynman videos : QED for a general audience

[John Denker <jsd@av8n.com>]

In the second video, the fourth question is extraordinarily
interesting:

>   Are the clicks from a photomultiplier
>   caused by the photomultiplier and in
>   no way related to light?

Here's an amusing exercise:

  1) Ask yourself, how would *you* answer that question?

  2) Also, how do you think *I* would answer that question?
     See next message.

  3) Then listen to Feynman's answer:
       https://www.youtube.com/watch?v=kMSgE62S6oo#t=5584


========================


I would have given a very different answer.  There is an important,
truly fundamental concept here:  By way of analogy, think about
measuring the state of an electron in an atom:
  -- If you measure momentum, you see momentum.
  -- If you measure position, you see position.
  -- If you measure energy, you see energy.
  -- et cetera.

The same applies to the electromagnetic fields.  Contrary to what
Feynman implies, a photomultiplier is *not* the only way to measure 
the field.  Contrary to what he assumes, it is not the most sensitive
measurement you can do.  One super-important alternative is a voltmeter.

Loosely speaking, photon number is like energy and scales like the
*square* of the voltage.  So for weak fields, a voltmeter is incomparably
more sensitive than a photon counter.

So to answer the question, I would say that obviously the photons are
/related/ to the light;  an ideal photomultiplier won't click without
light.  However, the idea that the field is quantized comes almost
entirely from the photomultiplier.
  -- If you measure photon number, you see photon number.
  -- If you measure voltage, you see voltage ... *NOT* photon number.
  -- et cetera.

I say Feynman got this wrong, conceptually wrong, fundamentally wrong.
He's in good company;  Einstein got it wrong, too.  Spectacularly wrong.
Very unfortunately, a lot of people believed Einstein.  He got a Nobel 
prize for getting the wrong answer.  It set back the development of
physics by decades.  More than 80 years later, Glauber got the prize 
for cleaning up the mess.

Feynman said that all experiments show the EM field to be quantized.
This is just not true.  All /photomultiplier/ experiments show the
field to be quantized, but this depends at least as much on the 
photomultiplier as on anything else.

Feynman said Planck assumed that the energy was quantized.  I disagree.
As far as I can tell, Planck never assumed any such thing.  Indeed he
warned against such assumptions.  He observed that the black-body equation
/could/ be derived from such an assumption, but he well knew that there 
were lots of other assumptions that would lead to the same result.  In 
particular, AFAICT you are always better off looking at the /phase space/ 
rather than looking at the energy per se.  Look at the dimensions on 
Planck's constant already!  It has phase-space dimensions, not energy 
dimensions.  Planck understood this.

For the next level of detail, including diagrams:
  https://www.av8n.com/physics/coherent-states.htm