Atom or Nucleus?

[Farsight]

OK OK. But to show you what I mean, look at this David Deutsche lecture about quantum interference.

 

http://www.quiprocone.org/Protected/Lecture_2.htm

 

In the first paragraph he assumes that photons are classical impenetrable spheres, and then builds his whole case for parallel universes and quantum computers upon that supposition. He avoids mentioning that his beam splitter can be modelled on wave interference.

 

http://www.quiprocone.org/Protected/Worked_Examples2.pdf

 

"Suppose that photons are classical impenetrable spheres. Two photon beams each of crosssectional area 1mm² and each containing 3 × 10¹⁵ photons per second pass through each other at right angles for one minute, and no photon is detected by an array of very sensitive photon detectors that surrounds the intersection region everywhere except within the beams. Calculate an approximate upper bound on the diameter of a photon.

[Qfwfq]

What I can say, without even following the links, is:

 

Suppose that photons are classical impenetrable spheres.

 

This is a model, phenomenologists are constantly working out things according to different ones so that the fit with experimental data can be compared.

 

Currently, nobody takes this model quite seriously for the photon.

 

Calculate an approximate upper bound on the diameter of a photon.

 

Particle physicists usually talk about he cross section of an event. It is defined in terms of probability but can be related to a fictuous geometrical quantity, exactly as that exercise requires to compute. In some cases it isn't a totally fictuous thing; in all cases it is a fundamental manner of working out the results of accelerator experiments according to quantum field theory and comparing with actual data.