Re: Double Slit variant

From: Caroline Thompson (ch.thompson1_at_virgin.net)
Date: 11/14/04 

Date: Sun, 14 Nov 2004 13:08:01 +0000 (UTC)

"Blake Winter" <blake.winter@houghton.edu> wrote in message
news:87423d2a.0411121006.3602e941@posting.google.com...

> Let's say we have a source of back-to-back equal energy,
> opposite momentum photons ... These photons are clearly
> entangled. Now let's put a double slit diffraction
> experiment on two sides of the source, so that we can do
> double slit diffraction with each photon from the same event.
> We see the usual patterns build up, and nothing particularly
> odd happens.
>
> [double slit experiment] *source* [double slit experiment]
>
> However, let's say that on the experiment on the left we
> decide to determine which slit the particle passes through.
> It strikes me that because of the correlation between
> the photons, if we know which direction the photon going
> left went, we know that the photon going right had to go
> the opposite direction, and therefore we know which
> slit it passed through. Now, we know that on the experiment
> on the left the interference pattern disappears, but would
> not the interference pattern disappear on the right even
> though we have changed nothing there? In other words,
> we change the probability distribution on the experiment
> in the left by placing a detector in each slit. From
> conservation laws, we can then figure out which slit
> the photon on the right went through. Then the probability
> distribution on the right should no longer correspond to
> the usual double slit pattern.
>
> This seems reasonable to me (as in, I don't see why my
> conclusion is wrong).

Didn't Popper suggest a rather similar experiment?

> However, it does seem to violate locality in a noticeable
> way, because by measuring the probability distribution
> on the right, we can tell whether a measurement of which
> slit is made on the left without comparing our data from
> the right with the data from the left. This would open the
> possibility of sending real information faster than light,
> which seems like a bad thing. So my conclusion would be
> offhand that this couldn't happen, but I can't actually
> figure out quantum mechanically why this wouldn't work.
> Are there any ideas?

I don't know what QM has to say on the matter, but it wouldn't work for the
good reason that light is really just a wave. The existence of photons is
an illusion created by the detectors. Moreover, there is no conclusive
evidence that quantum entanglement *ever* happens, since all existing
experiments have had "loopholes".

The struggle to find conclusive evidence, in the form of loophole-free Bell
tests, has been going on now for over 30 years. For a realist view on this
see Emilio Santos' latest article:
Santos, E, "Bell´s theorem and Increasing empirical support to local
realism?", http://arXiv.org/abs/quant-ph/0410193

I don't agree with all that he says but his conclusion is logical:
" The validity of local realism may be either refuted by a single
loophole-free experiment or increasingly confirmed by the passage of time
without such an experiment."

For the latest ideas for loophole-free tests see:
R. García-Patrón Sánchez, J. Fiurácek , N. J. Cerf , J. Wenger , R.
Tualle-Brouri , and Ph. Grangier, "Proposal for a Loophole-Free Bell Test
Using Homodyne Detection", Phys. Rev. Lett. 93, 130409 (2004)
http://arxiv.org/abs/quant-ph/0403191

for which more detail is available in:
R. Garcia-Patron, J. Fiurasek, N.J. Cerf, "Loophole-free test of quantum
non-locality using high-efficiency homodyne detectors",
http://arxiv.org/abs/quant-ph/0407181

Caroline
Caroline H Thompson

ch.thompson1@virgin.net
http://freespace.virgin.net/ch.thompson1/

Relevant Pages

  • Re: Photons shapeshifting to wave prior to measurement
    ... >>some of the photons pass near enough the edges of the slit to ... >>'in constant motion'would cause even photons following ... >>Diffraction patterns can be seen with a single slit. ... >>think I remember that the edge of shadows display interference ...
    (sci.physics)
  • Re: Photons shapeshifting to wave prior to measurement
    ... >> certain prefered directions, causing 'diffraction patterns', even under ... >> As far as I know, people have looked at the quality of the photons as ... >> the slit as simply 'solid objects' rather than looking at what any STM ... A single slit will produce an interference pattern. ...
    (sci.physics)
  • Re: Wave as wave, particle as particle
    ... When photon numbers are counted each emission or transition event is merely *assumed* to be caused by one photon, the number of events then counted and assumed to equal the number of photons. ... In experiments that intend to measure interference effects, such as in the double slit experiment, you are measuring interference of light and thus the properties of light called phase and/or polarity, I see no integer numbers associated with those qualities, only angles. ... Though QM provides a counterpart to classical wave addition, it provides no mechanism, it is thus nothing more than a renaming the classical parameters along with the claim that they are nonclassical. ...
    (sci.physics)
  • Re: Is Faster-than-light Communication Possible?
    ... > entangled photons, and for each entangled pair, one photon goes to ... > slit apparatus, and both are on opposite sides of the source. ... > see interference patterns. ...
    (sci.physics)