On 9/16/2011 23:37, Warp wrote:
> Darren New<dne### [at] sanrrcom>  wrote:
>> It doesn't matter if it hits a measuring device. It only matters if you look
>> at the measurement.
>
>    And how does the particle know that the measurement was looked at?

Clearly individual particles don't "know" anything in the traditional sense. 
I suspect if we knew *why* it worked that way, we'd be much closer to a GUT.

>    But that has nothing to do with whether someone "looked" at the measurement
> or not. It has to do with whether the two possible paths of the emitted
> particle were kept separate or whether they were merged before the particle
> hit the measurement device.

You misunderstand. The actual particle hitting the measurement device and 
being checked for interference fringes is *not* the particle being measured. 
*That* particle takes exactly the same path in both cases.

> If I understand correctly, the interference
> pattern would disappear if the emitted particles are kept separate even if
> nobody "looks" at the result. It has nothing to do with an observer, only
> with how the original particles and the emitted particles interact.

I'm not sure what the "emitted" particle is here.

The experiment says basically this:

1) Emit one particle.
2) Run it thru two slits.
3) Split it into an entangled pair, A and B.
4) Let A hit the detector that looks for interference.
5) Some time later, bounce B off a half-silvered mirror,
    such that if it goes through, you can tell what slit it came from,
    and if it doesn't, you can't tell which slit it came from.

The behavior of whether A creates an interference fringe is
determined by whether you can tell which slit B came through. You never need 
to touch A, look at A, or figure out which slit A came through. Two "A" 
photons will cause interference if they weren't in the machine at the same 
time, as long as you don't look at two *other* photons *after* you've 
already determined where the two "A" particles hit the screen. Indeed, you 
could put the detectors for "B" so far from the experiment that thousands of 
"A" particles could have gone through the machine and been detected one at a 
time before you even look at the first "B" particle.

>    While at macroscopic levels it's hard to understand how particles affect
> each other from a significant distance (and even time) this way, it kind of
> makes sense, if you imagine that the particles are somehow "bound" together
> even though they are in their own separate paths: This way what happens to
> one particle affects what happens to the other. If the path of the
> "measurement" particle is merged, it affects the original particle (even
> through time, via some quantum oddity), and if it's not merged, it also
> likewise affects it.

Yes. Except that you can take the second measurement after the first 
particle had already given you an answer. And it works even over time-like 
distances (i.e., if the second detector is so far away that you'd need FTL 
signals for it to affect the first detector), even if you discount the fact 
that it's actually affecting stuff already measured.

>    However, what does not make sense even in this context is that the particle
> somehow "knows" whether someone "looked" or not.

Nope, that doesn't really make sense. :-) Even the multi-worlds 
interpretation of it doesn't make sense, if you actually think about it with 
common sense.

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> >    But that has nothing to do with whether someone "looked" at the measurement
> > or not. It has to do with whether the two possible paths of the emitted
> > particle were kept separate or whether they were merged before the particle
> > hit the measurement device.

> You misunderstand. The actual particle hitting the measurement device and 
> being checked for interference fringes is *not* the particle being measured. 
> *That* particle takes exactly the same path in both cases.

  No I don't. When I say "emitted particle" I'm talking about the secondary
particle emitted at the slit towards the measurement device that (possibly)
tells which slit the original particle went through.

> > If I understand correctly, the interference
> > pattern would disappear if the emitted particles are kept separate even if
> > nobody "looks" at the result. It has nothing to do with an observer, only
> > with how the original particles and the emitted particles interact.

> I'm not sure what the "emitted" particle is here.

  It's particle B in your list. If the two possible paths that B could take
are merged, the interference pattern appears, but if they are not merged,
the interference pattern disappears. This regardless of whether someone
"looks" at the result of B or not.

  Clearly what happens to B's path affects A (even if this effect happens
through space and time). It's not dependent on whether someone "looks" at
it.

-- 
                                                          - Warp

Post a reply to this message

On 9/16/2011 3:07 PM, Darren New wrote:
> On 9/16/2011 14:11, Warp wrote:
>> Now, how does the football (or atom, or particle) "know" that what those
>> emitted particles hit is not "measurement"?
>
> Well, that is indeed the fundamental problem exposed by Schrodinger's cat.
>
>> measurement device that can tell the difference?
>
> It doesn't matter if it hits a measuring device. It only matters if you
> look at the measurement.
>
No, no, no.

Observer in the case of quantum mechanics is not a "person" looking at 
the thing, its "any object/particle that interacts, to take a 
measurement." Whether or not something living looks at the result is 
*not* what produces the effect. Geeze, if that interpretation where 
true, then quantum effects would never have happened *at all* for 
billions of years prior to the formation of life. *Any* interaction 
"measures" the state of the particle *period*. Schrodinger's cat is just 
a goofy anthropomorphizing of the process, which a lot of people, 
including apparently people on here, get wrong, because people see 
"observer" and assume that has to be "person", or something that 
otherwise thinks. That is just plain wrong.

Post a reply to this message

Patrick Elliott <sel### [at] npgcablecom> wrote:
> Schrodinger's cat is just 
> a goofy anthropomorphizing of the process, which a lot of people, 
> including apparently people on here, get wrong, because people see 
> "observer" and assume that has to be "person", or something that 
> otherwise thinks. That is just plain wrong.

  Clearly the cat is a good measurement device for whether the poison was
released or not. That's not the point. The point is whether the cat is
dead or alive from outside of the box.

-- 
                                                          - Warp

Post a reply to this message

On 9/17/2011 10:13 AM, Warp wrote:
> Patrick Elliott<sel### [at] npgcablecom>  wrote:
>> Schrodinger's cat is just
>> a goofy anthropomorphizing of the process, which a lot of people,
>> including apparently people on here, get wrong, because people see
>> "observer" and assume that has to be "person", or something that
>> otherwise thinks. That is just plain wrong.
>
>    Clearly the cat is a good measurement device for whether the poison was
> released or not. That's not the point. The point is whether the cat is
> dead or alive from outside of the box.
>
Sigh. Outside doesn't matter. The cat is a hypothetical. Just because 
"you" don't know the state, until you open the box, doesn't mean it 
hasn't collapsed, if you are dealing with a literal cat. But that isn't 
what is intended here. The cat is the state itself, in the thought 
experiment, so whether it is one or the other *requires* an outside 
influence. A real cat would already invalidate the experiment, as would 
anything else you might use, like a sheet of radiation sensitive 
material, where you want to "observe" if the material decayed yet, or 
not. It happens, in such cases, whether you open the box now, or 4 
million years from now. That you don't know what happened isn't relevant.

But, if you are talking about the cat as a "state", and the observation 
as checking to see if the state changed, the situation is, in principle, 
valid. But, you have to make everything hypothetical to have it mean 
anything. Did the laser fire (the poison release)? Did it emit a 
particle in the right direction (no leaks in the box)? Did that particle 
split as intended (the cat breathed it, maybe it was a really short 
lived poison)? When it did, what happened to the pair you are testing 
(was the cat effected)? The cat is just one of the cogs in the system. 
Assuming event #1 happened, as planned, something happens in the rest of 
the system, whether you did anything yourself or not. The *real* trick 
is the fact that you can create conditions where you "know" what the 
outcome will be, instead of just looking into the box to find out. 
Normally, you can't, because all the stuff going on happens "in the box".

Post a reply to this message

On 9/17/2011 9:44, Warp wrote:
> Darren New<dne### [at] sanrrcom>  wrote:
>>>     But that has nothing to do with whether someone "looked" at the m
easurement
>>> or not. It has to do with whether the two possible paths of the emitt
ed
>>> particle were kept separate or whether they were merged before the pa
rticle
>>> hit the measurement device.
>
>> You misunderstand. The actual particle hitting the measurement device 
and
>> being checked for interference fringes is *not* the particle being mea
sured.
>> *That* particle takes exactly the same path in both cases.
>
>    No I don't. When I say "emitted particle" I'm talking about the seco
ndary
> particle emitted at the slit towards the measurement device that (possi
bly)
> tells which slit the original particle went through.

Ah. With that clarification in mind... The interference pattern disappear
s 
if you don't actually look at the "emitted" particle at all. If you let t
he 
emitted particle hit the detector, then you don't use that detector's 
results, you get no interference pattern - you get just a level output. 
That's what this paragraph means:

"""
Note that the total pattern of all signal photons at D0, whose entangled 

idlers went to multiple different detectors, will never show interference
 
regardless of what happens to the idler photons.[3] One can get an idea o
f 
how this works by looking carefully at both the graph of the subset of 
signal photons whose idlers went to detector D1 (fig. 3 in the paper[1]) 
and 
the graph of the subset of signal photons whose idlers went to detector D
2 
(fig. 4), and observing that the peaks of the first interference pattern 

line up with the troughs of the second and vice versa (noted in the paper
 as 
'a π phase shift between the two interference fringes'), so that the
 sum of 
the two will not show interference.
"""

In this particular experimental setup, if you don't look at which detecto
r 
it hit, you can't tell because you can't separate out the inteference fro
m 
it's half-phase-shifted brother.

>>> If I understand correctly, the interference
>>> pattern would disappear if the emitted particles are kept separate ev
en if
>>> nobody "looks" at the result. It has nothing to do with an observer, 
only
>>> with how the original particles and the emitted particles interact.
>
>> I'm not sure what the "emitted" particle is here.
>
>    It's particle B in your list. If the two possible paths that B could
 take
> are merged, the interference pattern appears, but if they are not merge
d,
> the interference pattern disappears. This regardless of whether someone

> "looks" at the result of B or not.

In this experiment at least, you *have* to look at B in order to determin
e 
which category A falls into. If you don't look at B at all, you don't get
 
interference fringes, or more precisely, you get two sets of interference
 
fringes that are half a fringe offset.

>    Clearly what happens to B's path affects A (even if this effect happ
ens
> through space and time). It's not dependent on whether someone "looks" 
at
> it.

In this case, not *quite*, I think. (Again, you're skirting the edges of 
my 
understanding... :-)  There have been other experiments proposed (by 
Wheeler, for example) that don't involve a second entangled photon at all
, 
but are hard to set up because you need devices that are light-seconds lo
ng 
to make them work. :-)

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

Post a reply to this message

On 9/17/2011 9:58, Patrick Elliott wrote:
> Observer in the case of quantum mechanics is not a "person" looking at the
> thing, its "any object/particle that interacts, to take a measurement."

Nope.  Contradicted by experimental evidence. Go read the other branch of 
the thread, and read the quantum erasure articles.

> Whether or not something living looks at the result is *not* what produces
> the effect. Geeze, if that interpretation where true, then quantum effects
> would never have happened *at all* for billions of years prior to the
> formation of life.

No, quantum *collapse* wouldn't have happened. :-)

> *Any* interaction "measures" the state of the particle
> *period*.

Again, this is contradicted by experimental evidence.

Unless you have something else to point to, that shows where an experiment 
where the scientists took a measurement but then didn't look at it still 
caused the collapse?

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

Post a reply to this message

On 9/18/2011 1:02, Patrick Elliott wrote:
> The *real* trick is the fact that you can create conditions
> where you "know" what the outcome will be, instead of just looking into the
> box to find out.

That's exactly what the quantum eraser does, and if you don't look at the 
result, you don't get interference. That's precisely the point I'm making. 
You get interference at time T by taking a measurement at time T+D, where D 
is a timelike separation from T.

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> In this case, not *quite*, I think. (Again, you're skirting the edges of my 
> understanding... :-)  There have been other experiments proposed (by 
> Wheeler, for example) that don't involve a second entangled photon at all, 
> but are hard to set up because you need devices that are light-seconds long 
> to make them work. :-)

  Btw, I don't understand why the double slit experiment cannot be used to
send information faster than c.

  Suppose you set up the delayed-choice experiment on orbit around Alpha
Centauri such that you simply shoot a beam of light towards the slits,
behind which there's a detector surface which can be used to see if there's
an interference pattern or not. Then you put the photon splitters at the
slits and send the secondary light particles created this way towards Earth.

  On earth we receive these two secondary beams, and here we make a delayed
choice of whether to merge them or not before measurement. If we merge them,
then the interference pattern appears at Alpha Centauri, and if we don't
merge them (but measure which slit the photons went through) the interference
pattern disappears. This way we can send binary data from Earth to Alpha
centauri: To send a 0 we eg. merge the paths, and to send a 1 we don't.
The guys at Alpha Centauri can then read this info by seeing when the
interference pattern appears and when it doesn't.

  Since the particles are entangled, this happens without any delay.
(The secondary beams my take 4 light years to arrive here, but once they
do, we can send data back immediately, without another 4 ly delay. If the
streams are kept constantly on, after the initial 4 ly delay information
can be send continuously from Earth to Alpha Centauri with zero delay.)

  But apparently this is not possible. Why not?

-- 
                                                          - Warp

Post a reply to this message

On 9/18/2011 9:37, Warp wrote:
>    On earth we receive these two secondary beams, and here we make a delayed
> choice of whether to merge them or not before measurement. If we merge them,
> then the interference pattern appears at Alpha Centauri, and if we don't
> merge them (but measure which slit the photons went through) the interference
> pattern disappears.

Except you can't tell, in the case of this specific experiment, which 
electrons are part of the interference pattern and which electrons are part 
of the non-interference pattern until after you've detected the "B" idler 
photons. You're going to get all the photons, and what you have to do to get 
the interference pattern is to ignore the photons where I erased the 
information about my half of the experiment. Remember that you don't get an 
interference pattern from just one particle. You get an interference pattern 
statistically, when you let lots of particles build up. There's no way to 
look at one individual particle and say "is that part of an interference 
pattern?"

The way the particular experiment on the wiki page is described, 1/4th the 
particles randomly create an interference pattern, 1/4th randomly create an 
interference pattern offset 1/2 a fringe from the first one, and 1/2 of the 
particles create no interference pattern. But you don't know which group 
each particle is in until you look at the idler "B" emitted photon to see 
which way it went.  If you *influence* the emitted particle, you break the 
entanglement, and now you have results uncorrelated with the emitted 
particle, which (I think) means you'll get an interference fringe from the 
"A" particles because you erased what would let you distinguish them.



It's the same as a simpler question: Why can't I send any information at all 
over a quantum channel? Any experiment you concoct to measure a quantum 
property one way or the other to communicate is going to run up against the 
fact that the property you're measuring is influenced by the property you 
aren't measuring.

Let's say I decide to measure polarization, diagonal for 1, 
horizontal/vertical for 0. I send you half an entangled stream, then measure 
diagonal polarization or H/V polarization for each bit. The problem is that 
if you measure the H/V polarization when I measure the diagonal 
polarization, you just get random nonsense. You already have to know which 
polarization I'm going to measure in order to measure the same polarization. 
And if we both measure the same polarization, we'll get the same answers, 
but the answers will be a random stream of bits.




Or, for a super-simple analogy, let's say I have a glass table. You can see 
the underside of the glass table instantly, no matter where you are. So you 
go far away, and I stand next to the glass table. However, the only thing 
you can see through the glass of the table is a coin I have flipped.

So if I flip a coin and it lands heads up, I know instantly that you 
instantly saw the tails side of the coin. If it lands tails up, I know 
instantly that you know instantly that it landed heads up. That's FTL.

Now, what can we do with that data? Not much. It's a random stream of bits 
over which I have no control. If I set the coin down in a specific way, you 
can't see it. I know you saw the compliment of what I saw, but I have no way 
to influence that. If I try to influence it, the whole process collapses and 
you can no longer see the results from far away.




That's why, if you look at quantum cryptography, the quantum part of the 
process is used to generate a OTP, then to ensure the OTP hasn't been 
intercepted by someone else. But the actual communication, including the 
part where you ensure the OTP hasn't been intercepted, happens over normal 
non-quantum channels.

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

Post a reply to this message