A quantum mechanical thought experiment:
I have the traditional setup where electrons are shot at two slits of
the proper size and spacing, and the location of impact is recorded on
the oposite wall.  The distribution patern for the electrons will
match the interference patern of their wave function.  But if I put an
electron detector at each of the slits, there will be no more
interference patern, right? The idea is, observing the electrons at
the slits causes their wavefunctions to collapse before they reach the
wall.  But what constitutes an observation? is it simply an energy
exchange between the "detector" and the object? In other words, it has
nothing to do with information about the object being gathered (other
than that it is necessary to have an energy exchange to gather
information)? So if we ensure that the electron detector automatically
destroys all information about the electron after it detects it (thus
making the info inaccessible to me, the observer) there will still be
no interference pattern, right? ("no" would certainly be more
intriguing as an answer.)

Another scenario: The electron detectors are built so that they are
activated by a quantum mechanical event (schrodinger's cat style). 
This is done so that the probability that the electron detectors are
off is exponentially decreasing with time.  What will the electron
distribution on the oposite wall look like?  I'm guessing it will be a
linear combination of the with-interference and without-interferece
distribution, with coefficients corresponding to the probability the
electron detector off or on respectively. yes?
thank you!

---

Clarification of Question by ignignokt1-ga on 24 Oct 2005 23:12 PDT

I just posted this as a comment, but I might as well also offer it as
a clarification as well:

Thanks a lot for the second link, it's very insightful.  The third
experiment describe doesn't seem hard to set up, so I'm wondering why
it hasn't been performed, or maybe an analogous one has been
already..?  (would be interesting, but not crucial for an answer)

Now, what if we have the first setup in the second link, with Idler 1
blocked up.   There is then no interference pattern because now there
is a "threat of gathering information. So it really is the "threat" of
measurement that matters, and not measurement itself, and the presence
or lack of the idler detector doesn't even matter once idler 1 is
blocked off? This is what the link seems to imply.  I'm going to
submit my own interpretation and say that the idler 1 photon hitting
or not hitting the wall is the relevent measurement in this situation.
 We force one of the cases to interact with the outside world. The
wave function then collapses exactly when the photon would hit the
wall blocking it.  is there anything wrong with this interpretation. 
It seems to eliminate vague notions of a "threat of measurement".  (I
mean there's always a threat, it's so hard to draw the line) But wait!
what if we just arrange the experiment so the two idler beams to do
not join in the same trajectory, allowing for potential information on
the photon's action at the beam splitter to be available.  If the
so-called "threat of obtaining information"-interpretation is correct,
we should expect no signal interference pattern. If my interpretation
is correct there should still be interference until we place an idler
detector at idler 1 or idler 2. If my interpretation IS correct in
this scenario, the time of idler detection would have to be before the
signal detection in order to  eliminat the signal interference,
because other wise we have a measurement affecting an outcome
retroactively!!! right?
 
If you can answer this based on your own QM knowledge, then you rock
my world and I don't even need supporting sources.


I realize I've already presented well over 16 dollars worth in
seperate question, but really I'm just trying to frame the "big
question".  What I'm looking for is a concrete specification for what
constitutes a quantum mechanical measurement.  ie is it simply the
presence of gatherable information in the system, an energy exchange,
interaction with a concious observer or WHAT? Wikipedia asks the
question and provides no answer:
http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics#What_physical_interaction_constitutes_a_measurement.3F


****CRITERIA FOR ANSWER****:
answer the original questions I asked 
OR answer the questions in the second paragraph of this write-up
OR answer the "big question" concretely enough that I could apply it
to these experiments (don't know if modern physics has the answer yet,
so...)
OR give examples of real experiments that relate to the ones I've
mentioned, ones relevant to the "big question" in general.  What's
modern physics's status on this question and why's it taking so long?

much thanks!

---

Request for Question Clarification by hedgie-ga on 14 Nov 2005 10:50 PST

Hello Igni..

Since publication of the EPR paper the answer is NO for majority,
and for (almost) all physicists, it is so since publication of the Bell's theorem .

 Evolution of the concept is sketched here:

http://en.wikipedia.org/w/index.php?title=EPR_paradox&oldid=2346607

Current version of that page is less conceptual (i.e. is more mathematical) 

http://en.wikipedia.org/w/index.php?title=EPR_paradox

but both have good references.
Versions in between show certain diversity of opinions.

That answer is assuming the question was: 

"But what constitutes an observation? is it simply an energy
exchange between the "detector" and the object?"

Good formulation of a question is a prerequisite of a good answer:
You will get better response when you include just one question per GA querry.

So, if you decide to post another (simpler) querry, please note (in the 
given references) that we do not have two (or more) theories which differ
in prediction of what the experiment will show. We have one theory,
same predictions, all amply verified.

The QM interpretions are an issue in philosophy of science (or metaphysics) and
have their roots in the  uncritical use of the concept of probability
in early formulations of the QM .
(This is an issue not just in QM but also in the Statistical Mechanics
(as lampooned e.g. by the 'improbability drive' in Hitchikers Guide to
Galaxy)).

My question is: Are you happy with references you got (above and
below) or do you want to post another GA querry?

You might find the following interesting from this website:

http://galileo.phys.virginia.edu/classes/751.mf1i.fall02/02_751_Intro.htm

To illustrate how weird this really is, consider a beam of photons
split into two by a half silvered mirror, the two half-beams than
follow widely separated paths until they are reunited by a suitable
sequence of mirrors to interfere with each other.  Sending one photon
at a time, we will eventually build up a diffraction pattern of some
sort.  So if we think of the initial photon as a ?wave packet? it will
split into two half ?wave packets? which will finally interfere with
each other.  Now suppose I put 100% efficient photon detectors on both
paths. If I send photons through the apparatus one at a time, I get a
series of clicks from the two detectors: path 1 clicks, path 1 clicks
again, path 2 clicks, etc.: a random series. I never get both clicking
with one photon. (We can dim the light enough so that the photons are
far apart, that is, they definitely come one at a time.) What does
this tell us about the nature of the wavefunction?

You might be inclined to think that the photon goes at random, but
half the time it goes along one path, half the time the other. That is
to say, the photon really is on one of the paths, we just don?t know
which until we detect it, and the wavefunction represents our
ignorance. We do know that once we detect the photon on one path,
there?s zero probability of finding it on the other path?so that part
of the wavefunction has gone!  But was it really there in the first
place for that particular photon? Yes: the other half wavepacket must
have been there, because if I hadn?t captured the photon with a
detector in the way, the two half wavefunctions would have gone on to
interfere with it to give the diffraction pattern.  So this line of
thinking is wrong: we cannot say that the photon ?really is? on one of
the two paths before we detect it.

As an addition to my above comment, I came across the actual experiment that
I had in mind which shows that the "threat of obtaining information about 
the path traveled forces the electron to travel a single path" even though
no intervention (detectors) occurs in the actual beams producing the 
interference pattern!!! 

See: http://www.tardyon.de/ko2.htm

Thanks a lot for the second link, it's very insightful.  The third
experiment describe doesn't seem hard to set up, so I'm wondering why
it hasn't been performed, or maybe an analogous one has been
already..?  (would be interesting, but not crucial for an answer)

Now, what if we have the first setup in the second link, with Idler 1
blocked up.   There is then no interference pattern because now there
is a "threat of gathering information. So it really is the "threat" of
measurement that matters, and not measurement itself, and the presence
or lack of the idler detector doesn't even matter once idler 1 is
blocked off? This is what the link seems to imply.  I'm going to
submit my own interpretation and say that the idler 1 photon hitting
or not hitting the wall is the relevent measurement in this situation.
 We force one of the cases to interact with the outside world. The
wave function then collapses exactly when the photon would hit the
wall blocking it.  is there anything wrong with this interpretation. 
It seems to eliminate vague notions of a "threat of measurement".  (I
mean there's always a threat, it's so hard to draw the line) But wait!
what if we just arrange the experiment so the two idler beams to do
not join in the same trajectory, allowing for potential information on
the photon's action at the beam splitter to be available.  If the
so-called "threat of obtaining information"-interpretation is correct,
we should expect no signal interference pattern. If my interpretation
is correct there should still be interference until we place an idler
detector at idler 1 or idler 2. If my interpretation IS correct in
this scenario, the time of idler detection would have to be before the
signal detection in order to  eliminat the signal interference,
because other wise we have a measurement affecting an outcome
retroactively!!! right?
 
If you can answer this based on your own QM knowledge, then you rock
my world and I don't even need supporting sources.


I realize I've already presented well over 16 dollars worth in
seperate question, but really I'm just trying to frame the "big
question".  What I'm looking for is a concrete specification for what
constitutes a quantum mechanical measurement.  ie is it simply the
presence of gatherable information in the system, an energy exchange,
interaction with a concious observer or WHAT? Wikipedia asks the
question and provides no answer:
http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics#What_physical_interaction_constitutes_a_measurement.3F


****CRITERIA FOR ANSWER****:
answer the first questions I asked 
OR answer the questions in the second paragraph of this write-up
OR answer the "big question" concretely enough that I could apply it
to these experiments (don't know if modern physics has the answer yet,
so...)
OR give examples of real experiments that relate to the ones I've
mentioned, ones relevant to the "big question" in general.  What's
modern physics's status on this question and why's it taking so long?

much thanks!