In the New Scientist magazine dated 27th March the front cover story is about Quantum Entanglement. Fantastic stuff, much of it beyond me. (and most people I guess) But the article says this:

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They were talking about measurments of a single quantum system.
Article by Michael Brooks

Now hang on a minute. This is very weird, which is what the quantum world is all about it seems,
but it is explained that this was a thought experiment. This makes it all the more strange. How do you come up with a 'time travel' result from a thought experiment? Doesn't common sense kick in to say this cannot be right? Or doesn't common sense matter in the quantum world!!

Can anyone enlighten us in laymans terms what is happening here?

Also, given enough resources, money and time could this thought experiment actually be demonstrated?




Why is the title of this post 'Sausage'? I Don't know. I'm sure some future event will reveal why it is so.

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Sounds wrong to me, first off how are they capturing and holding still one photon and then coming back to measure that same photon out of the zillions of little photons zooming around, how did they find the exact same photon. Sounds like theve been using the magnetic containment bottles to make bongs with again. :roll:

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doesn't the Uncertainty Principle state that you cannot observe something without changing it?

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As far as I know, this thought experiment goes back to the "paradox" proposed by Einstein, Podolsky and Rosen (a.k.a. "EPR paradox").

(I confess that I cannot guarantee that my following explanation is entirely correct and understandable... ops

This "paradox" regards the interpretation of Quantum Mechanics (QM) and it is a paradox if we assume as valid three assumptions:

1) QM is complete
In QM, information on the state of a system is contained (mathematically) in the "wavefunction". This assumption means that ALL the information about the system is contained in the wavefunction.

2) Effects are local
Two systems separated in space cannot affect each other instantaneously.

3) "Reality" of physical properties
Physical objects have properties that are "real", i.e. present even if they are not observed or measured in a particular measurement (for example, the mass of a particle).

One word about measurement in QM:
In the theory, the effect of a measurement is to select one term in the wavefunction with a certain probability. If the measurement is repeated on identical systems, the results of the measurements follow a statistical distribution.

Now, let's consider a source of photons. We can setup the source two emit at the same time two photons in opposite directions with opposite polarizations. The wavefunction for the system of the two photons looks like this:

A(left,+)B(right,-) + A(left,-)B(right,+)

A(left,+)B(right,-) means that when photon A, going to the left, has positive polarization, then photon B, going to the right, has negative polarization.

A(left,-)B(right,+) is the opposite case: if A has negative polarization, then B has positive polarization.

The sum of these two terms means that both cases have the same probability of being produced by our source of photons. This is an "entangled" state.

Now, let's setup the measurement. As devices to measure the polarization of a photon we use polarizers: depending on its polarization the photon will pass through the polarizer with a certain probability.
In our thought experiment we will use two polarizers: one on the left of the source, and one on the right.
We set them up in such a way that photon A has 50% probability to pass, whatever its polarization. The polarizer on the right is setup in the same way

QM predicts for this experiment that whenever A passes through the polarizer on the left, photon B will not pass. In this case, the measurement selected the term A(left,+)B(right,-) (+ = pass, - = does not pass).
And when A does not pass, B will (term A(left,-)B(right,+) ).

Now, performing this experiment yields results that agree with this prediction (and experiments have been actually done, as far as I know; see teletransport of photons).

The problem is how to interpret these results, given the original assumptions.

Is the polarization of photon A already determined before the measurement (assumption 3)? This contradicts assumption 1, which implies that all the information that we can ever know about the polarization of A is that its positive with 50% probability (before the measurement).

Is the polarization of A undetermined before the measurement, and "pinned" by the measurement (a possible interpretation following assumption 1)? This would contradict assumption 3 (polarization as intrinsic and "real" propertiy of a photon) and possibly assumption 2 (the information about the polarization of A is somehow transmitted to B without delay).

What the EPR "gedankenexperiment" shows, is that the three assumptions are not compatible. So the problem is: which can we give up.

It is very hard to give up the completeness of QM because of its success in explaining and predicting experimental results.

It is very hard to give up the "locality" of effect due to the success of Special and General Relativity.

And it is very hard to give up the "reality" of physical properties ("Something is not there until it is observed" does not sound very good...).

Nowadays the trend among physicists is to weaken the second assumption. The reasoning goes like this:
Even if the two photons were lightyears apart, they are still one system, therefore we cannot consider them as two separate systems (due to the entaglement). And this does not contradict Relativity, because anyway these "non-local" effects do not really imply instantaneous transfer of information.

But the question has not been solved in a satisfactory way, yet.

(*collapses on the chair exhausted* )

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papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

I'm hardly an expert on this matter, basically just an overinformed layman, but I think it might be somehow relevant to some of the points above that from the photon's (extremely skewed!) reference frame, no time would pass between test A and test B (I'm not referring to papageno's A & B photons here). Perhaps this might help understand how the results of the second test can affect the first test?


One of the related things I find terribly weird also involves polarization. Apparently, if you arrange two planes polarized perpendicular to each other (I'm going to make some extremely simple diagrams here, imagine that the light passes through perpendicular to each figure, and on to the next figure (if it does)):
| -
light will not pass through, because it's vertically polarized by the first, and vertically polarized light cannot pass through the horizontally polarized plane.
But if you insert an extra piece of material diagonally polarized,
| \ -
the light manages to pass through! It's vertically polarized by the first, and then...?? I don't know. Haven't tried it myself, but this is what I've read. And I think I might find it even more perplexing & astounding than QE, even if it isn't as impressive (particularly to the layperson). :-k #-o

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Just a comment in response to the query as to the reality of measuring a photon. This is quite possible. In fact, 'photon traps' have been 'invented', making the execution of certain thought experiments relatively easy.

I'd definitely say that common sense doesn't apply well to the quantum world!

As far as I know, the frame of reference of a photon cannot be used (it is definitely not an inertial frame of reference).

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papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

Hello again
Thank you to papageno and JohnOwens for their answers. I sort of follow them!
They say QM is going to be the next 'big thing' in science and I'm sure there are many many years of amazing discoveries to come.

One thing to note is that the article doesn't say what the time difference between the two measurements was. Because it doesn't say, that would suggest the difference was extremely small, (say less than 1000th of a second) and JohnOwens suggest the difference could be 0 seconds. But, could it be longer? Over one second? Longer??

For the sake of completeness I'll quote the next paragraph from the article.

From New Scientist magazine 27th March 2004

Yes it does. However I believe they are saying that the change they caused during the second measurement had an effect on and caused the first mearsuement.

Perhaps it was something like this:

Scientist A: Okay lets take our first measurement.
Scientist B: ...and the result is...'a roll to the left'.
Scientist A: Hmmm. Not quite what we expected but fair enough. Make a note scientist B.

Sometime undisclosed time later.

Scientist A: Time to take another measurement.
Scientist B: ...and the result is...'a push to the left'
Scientist A: Hmmm.
Scientist B: Hmmm. That's funny.
Scientist A: Well, that explains the first measurment, but it's illogical captain.
Scientist B: You said it scientist A
Scientist C: What are you two chaps thinking about??


PS. I had sausage for breakfast this morning and thought wouldn't it be fun to called a thread on the BABB 'Sausage'. That would get it viewed a few times. Well, wouldn't you know it, I had already done it. Quantum time entanglement mechanics at work again.

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81.5% of all statistics are made up on the spot.

It's a matter of which basis (coordinate system) you use to make your measurement. Light that is polarized at 45 degrees can be thought of as a superposition of vertically polarization and horizontal polarization. It has a 50% chance of being in each, and a polarizer at either of those (V or H) orientations would pass a photon 50% of the time.

Once you've measured it (let's say it's vertical) you "collapse the wave function" - it's no longer in a superposition, it's now vertical. It would not pass through a H polarizer. But now a polarizer at 45 degrees is not in the correct basis. Vertical is the equivalent of a superposition of +45 degrees and -45 degrees. So you can apply the above argument all over again, with the new basis set.

Polarization is easier to understand than quantum entanglement. Please trust me on this one.

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I'm sure you meant to say QE here. QM has been a big thing for a number of years now.

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"I have a cunning plan that cannot fail."
S. Baldrick

Perhaps you are referring to the "sausage instability" encountered in nuclear fusion?

I had a sausage the other day, but it turned out that the pig, named Sweety, was alive and well living with a family of vegetarians in Dayton, Ohio and was having a grand time in the local wallow. Sigh, I ended up having to eat breakfast again ... corn flakes.

I would like to propose the Sausage Theory of Universal Faze Formation (STUFF).

Precepts:

1. The universe is shaped like a sausage.
2. The universe is actually shrinking as it warms up caused by a red shift in the quantum elements, sometimes called the Big Burner (BB).
3. Matter is constantly lost through seepage of ‘Oily Matter’ (OM).
4. After enough OM is lost, the universe will eventually go through a cool down period.
5. Outside the Y-Axis Universal Membrane (YUM) exists a much larger and stranger universe were STUFF is for breakfast.


I’m sure there is a moral here somewhere.