# Is a macroscopic pair where I observe one of them quantum entanglement?

Of what little I know/understand about quantum entanglement can somebody confirm if the following experiment is a good analogy to quantum entanglement of pair of particles? PS: please don't laugh as this could be very very lame!

I take an orange and an apple (of similar shape & weight). I put each of them in a non transparent bag separately and seal it off. I put both the bags in a box. I close my eyes and shake it thoroughly such that I no longer know which bag contains what fruit. Then I randomly pick one bag and take a flight to other part of the country.

Now given the above setup, I will not know what fruit is in my bag. In other words the fruit in my bag could be both apple/orange at the same time until I open the bag and see what is inside. As soon as I open it it is determined (similar to wave function collapsing) that I have an orange (for example) and there by making the fruit in the bag that was left in the box an apple (or vice versa).

Does this in anyway come close to what they are doing with quantum entanglement?

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No. What you are describing is what is known as a hidden variables theory. Quantum entanglement behaves quite differently. – user2963 Mar 3 '12 at 2:08

Your explanation looks like more the start of the development of the Bell's Theorem for discrete variables. The best explanation of entanglement that I read its the John Preskill's one; its on page 10 of the following link:

http://www.theory.caltech.edu/~preskill/ph229/notes/chap4_01.pdf

It's very simple and clear to understand; this you can explain to your friend too. And, if you want a video explanation, the Scientific American one its very good too:

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ohh great. The video really nailed it for me. Thanks for the link. – Shreedhar Kotekar Mar 11 '12 at 2:41

Someone please correct me if I am wrong, as my understanding in the field is also very limited.

With my current understanding, what the poster was talking about is actually the concept of uncertainty principle. Without observation, an entity will be in an undertermined state regarding its properties. Once the object is observed, then the wave function would collapse and a single state is determined. This is the foundation of quantum mechanics, and it's the state of quantum entanglement pair before observation.

An analogy of quantum entanglement with an orange and apple would be. Assuming that a state of the orange and apple is whether or not they are peeled. Initially you would put an apple and an orange each in a bag and seal it off, without knowing they are peeled or not (some super entity will be peeling and unpeeling both at the same time inside each bag). Then you take a random bag and bring it to the other side of the world with you (large distance not required). You open the bag and you would find the orange or apple peeled or unpeeled, and the other one would be in the opposite state. E.g. if you brought apple and found that it's unpeeled, then the orange on the other side of the world will be peeled. (This is in the event that an entangled pair will have opposite state when observed. However, this is not always the case. In general, observing an entangled entity would determine its and its entangled pair's associated states.)

Quantum entanglement is essentially a phenomenon of a entangled pair of two entities sharing a set of a undetermined properties until one is observed. At which, the other one's state will also be determined and is no longer uncertain.

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I don't understand what the "peeling" analogy is for. – Siyuan Ren Mar 3 '12 at 3:07
I guess what I was trying to describe with peeling is a description of a state of an entity. That state will be uncertain in both of the entities in the entangled pair until observation. Of course, I should add that the states are not always going to be the opposite of each other (will edit in post). Once again though, my knowledge is limited, and if you or someone have better understanding, please enlighten! Thanks! – gtr32x Mar 3 '12 at 3:15
Thanks for the answer but I am not sure I fully follow... which is fine given this is way over my head :). However does it not mean that the state of the entangled pair are only uncertain for an outside observer? For the particles itself will they not know what state they are in right? – Shreedhar Kotekar Mar 3 '12 at 4:44
No, they will not. That is what makes the quantum situation different than classical: there is no definite state (hidden variable) of the particles before the measurement. – user2963 Mar 3 '12 at 4:55
@Shreedhar I would strongly recommend against trying to find a classical analogy for entanglement: it is impossible, because entanglement goes beyond what is classically possible - or rather the classical worldview is what remains when entanglement is excluded. Instead, just try to understand a simple case of entanglement, at least from the mathematical point of view. Look at the answers here for a very good explanation: physics.stackexchange.com/questions/3489/… – user2963 Mar 3 '12 at 5:01

It's a very good analogy to "spooky action at a distance" experiment.

It's not a good analogy to quantum entanglement. We don't need an analogy to quantum entanglement. "Quantum entanglement" means "correlation" or "information", or something like that.

If you try to do a spooky action at a distance experiment with apples and oranges, it's very difficult, because apples and oranges are so much quantum entangled with the environment.

If you manage to shuffle the fruits so that the environment does not know which is which, then according to quantum mechanics, there must be some spooky action at a distance happening, when you look into a fruit bag.

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You're in good company in thinking that this is what quantum entanglement is about - this is what Einstein thought, and he wrote a famous paper along with Boris Podolsky and Nathan Rosen (usually called the "EPR paper", after their initials), which made exactly this argument. However, Bell's theorem (which was discovered later) is generally accepted as showing that this analogy doesn't work - statistically speaking, it seems that looking in one bag does have to physically affect what's in the other one, rather than just changing your knowledge about it.

There are various ways that people have attempted to get around Bell's theorem, so the view you put across is still held by some people in the theoretical physics community (and may ultimately turn out to be correct after all), but Bell's theorem does make it a bit tricky, and the majority of physicists do not currently believe that quantum reality works in the way you describe.

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