[extropy-chat] HISTORY: Solved & Unsolved Riddles

[Eliezer S. Yudkowsky]

John K Clark wrote:

> P.A.M. Dirac Wrote in  Principles of Quantum Mechanics:
> 
> "Each photon then interferes only with itself. Interference
> between two different photons can never occur."

I don't know whether Dirac said this, but this can't possibly be right. 
There is no such thing as a *particular* photon, any more than there is a 
*particular* dollar in an electronic bank account.  If photon 1 goes to A 
and photon 2 goes to B, it is exactly the same point in configuration 
space as if photon 2 goes to A and photon 1 goes to B, and we add the 
complex amplitudes.  The universe simply does not distinguish between 
photon 1 and photon 2; the universe's bookkeeping just tracks the final 
state "photon at A, photon at B".  All of quantum mechanics is set up to 
operate on configuration spaces, not particles, and cannot be factorized 
into particles.  "Particles" are just a way of doing bookkeeping on the 
connectivity of the configuration space.  We know from the laws of physics 
that, regardless of what our brains might like to think, there is no such 
thing as the "same" photon.  The probability of physics someday 
discovering a way to distinguish between the two photons is even less than 
the probability of physics someday discovering a way to make the photons 
go faster than c; it would even more strongly violate the fundamental way 
that reality seems to be structured.

If Dirac said this, he can't possibly have meant what the sentence seems 
to say.  Perhaps he was talking about some specific experimental setup.

-- 
Eliezer S. Yudkowsky                          http://singinst.org/
Research Fellow, Singularity Institute for Artificial Intelligence