RE: Quantum theory of measurement

From: Hal Finney <hal.domain.name.hidden>
Date: Wed, 12 Oct 2005 11:59:39 -0700 (PDT)

Ben Goertzel writes:
> Hal,
> > It won't make any difference, because the CC is not used in the way you
> > imagine. It doesn't have to produce a record and it doesn't have to erase
> > any records.
>
> OK, mea culpa, maybe I misunderstood the apparatus and it was not the CC
> that records
> things, but still the records
> could be kept somewhere, and one can ask what would happen if the records
> were
> kept somewhere else (e.g. in a macroscopic medium). No?

I don't think this makes sense, at least I can't understand it.


> > The point is, there is no change to the s photon when we put the polarizer
> > over by p. Its results do not visibly change from non-interference
> > to interference, as the web page might imply. (If that did happen,
> > we'd have the basis for a faster than light communicator.) No, all
> > that is happening is that we are choosing to throw out half the data,
> > and the half we keep does show interference.
>
> Yes but we are choosing which half to throw out in a very peculiar way --
> i.e. we are throwing it out by "un-happening" it after it happened,
> by destroying some records that were only gathered after the events
> recorded in the data already happened...

You have to try to stop thinking of this in mystical terms. IMO people
present a rather prosaic phenomenon in a misleading and confusing way,
and this is giving you an incorrect idea. Nothing is un-happening.
No records are destroyed after they were gathered.

Forget that anybody told you this was a "quantum eraser" and think
about what really happens. When all the polarizers are in place, half
of the p photons get eaten and half get through. This gives us a way
to split up the s measurements into two halves. It turns out that each
half independently shows interference, but that the two interference
patterns are the opposite of each other. When you combine the two halves
back together, the peaks of one half fill in the valleys of the other,
and the data set as a whole shows no interference.

Look at it concretely as it might happen in the lab. We record a bunch
of s measurements and also record whether we get a coincidence with a p
photon getting through, in the CC. Maybe we write a little check mark
next to the s measurements where there was a p photon coincidence.

We go through afterwards to analyze the data. If we just plot all
the s measurements we see a smooth curve, no interference. Now we
go through and cross off the ones where there was no p coincidence.
We cross off s measurement number 1, then numbers 3 and 4, then 5, 7,
10 through 12, and so on. When we plot the remaining measurements,
now we see an interference pattern.

In other words, the coincidence with the p photon identifies a subset
of the s measurements which shows interference. The total collection
of s measurements still shows no interference.

There is no real "erasing" going on. Whoever coined the term "quantum
eraser" was a master of public relations, but unfortunately he confused
millions of lay people into getting the wrong idea about the physics.

Hal Finney
Received on Wed Oct 12 2005 - 15:00:27 PDT