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Couple of questions on quantum physics from non-physicist

  1. Mar 23, 2012 #1
    Hi, thats my frist post here, I just made my registration. I'm 21 yaers old and I'm studying for a lowyer so my knowage on physics is quite poor. Anyway I watched a documentary on quantum physics today and my mind got messed up !
    Here are my questions:

    Quantum entanglement- Is it true that all particles in the universe are connected ?

    a) So lets say if I heat my body by running - then all the athoms in the universe will respond somehow [changing their energy levels?!] ?
    b) How strong is the entanglement? I mean would a "jiggle" break the entanglement of two electrons?

    Ok sorry if my questions are stupid, and sorry for my bad english [I'm bulgarian]. :blushing:
     
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  3. Mar 23, 2012 #2

    HallsofIvy

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    No, "quantum entanglement" does not mean any of those things.

    It can be shown that if two electrons, for example, are in the same energy level in an atom, they must have opposite "spins". If you "release" them from the atom (very carefully!) they will still have opposite spins- they are "entangled". If, for example, you test one of those electrons on earth and find it has "spin up", you know immediately that its "companion" electron, which perhaps is past Pluto by this time, has "spin down".

    The "paradoxical" part of this is that some interpretations of quantum theory assert that an electron does not have a specific spin until it is tested. Observing the electron forces it into one of "spin up" and "spin down" and so, at the same time, causes that "companion" electron, however far it is, is also force into a specific spin.
     
  4. Mar 23, 2012 #3
    10x... I wanted to ask, because I googled it and I found all kind of garbage [like quantum mysticism ] and I felt really confused.
    I found a video of Michio Kaku [who I think is great scientist ] talking about the possability of using entanglement for comunication such as "telepathy". Now I would guess that for this to become reallity - then the brains of two people must be connected with quantum entanglement [which sounds impossible, but...]
    http://video.answers.com/michio-kaku-on-easier-ways-of-telepathy-517069322 [Broken]
    Now he says that it would be possible to use entanglement to buld "telepahty", but there are easyer ways. Anyway so guessing one can entangle every particle of the two brains - then does it mean that chages in one of the brains will cause change in the other brain [the "telepathy" effect]
     
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  5. Mar 23, 2012 #4

    epenguin

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    So can another non physicist who never got this thing ask?:

    So my understanding is that the other guy could be making an observation on these electrons before he gets a message from me about what I saw. Say I observe electrons till I get four ups in a row and then stop observing, that means "A", etc. So he will see four downs and thus get a message from me faster than light. Oh, but he has no way to know I've stopped. Er, how did anyone think I could transmit a message faster than light? :confused: And why are we not allowed to think the electrons that separate did have a spin, each opposite from the same atom even if we don't know which this or that one has till we find out in an measurement?
     
    Last edited: Mar 24, 2012
  6. Mar 23, 2012 #5
    A lot of people say there is universal interconnectiveness, and there are also a lot of people that say there is no such thing.

    Anyway, I read this in "Quantum Theory: Concepts and Methods" by Asher Peres: http://books.google.co.nz/books?id=...=Quantum Theory: Concepts and Methods&f=false

    "... two particles of the same type are always entangled, even if they were prepared independently, far away from each other, in different laboratories."
     
  7. Mar 24, 2012 #6
    Is this the Copenhagen interpretation?

    I googled this universal interconnectiveness and I must say that most of the articles and videos I found were looking really unreliable [and the people explaining them were a bit strange]. Are there any of the leading scientist who deny it ? [I would like to read what they say about it]
     
  8. Mar 24, 2012 #7
    I wouldn't try googling the subject, as I've found it most unhelpful purely due to the amount of junk the search engine spits up.

    Many people who wrote chapters for this book - http://books.google.co.nz/books?id=...C8Q6AEwAA#v=onepage&q=Entangled World&f=false - believe in universal entanglement.

    Brian Cox does. And as far as I gather from his email response, Alexander Lvovsky does too.
     
  9. Mar 24, 2012 #8
    Copenhagen intepretation does assert that. It is the macroscopic apparatus that is considered the system that collapses the wave function, in that interpretation. But many other interpretations also say certain properties of a quantum system are simply potentials to become definite upon measurement (i.e. tested).
     
  10. Mar 24, 2012 #9
    I've never understood this. You said the spins are conserved when they are 'released' from the atom, so one is definitely up and one is definitely down. You can't know which electron has which spin until you check and see (seems pretty reasonable). Once you check the state of one then you will know the other since it was released from the atom in the opposite state. How does this mean the two are still related or connected some how? Isn't it just conserving what ever spin it had when it was released?

    Also, how can someone just say that each electron has both states and 'snaps' into an up or down depending on what you observe the first one to be? That seems like a completely unreasonable thing to say that can't be proven wrong? Kind of like saying, when no one looks at my bed it disappears... I can't prove this doesn't happen since I have to be looking at it to see.

    Or am I completely misunderstanding it?
     
  11. Mar 24, 2012 #10
    Well I'm no way near understanding quantum phusics- but from my research I learned the same info... so we might both be wrong, but we might both be right... we wont know untill someone observe our posts [a bit like in quantum world, isnt it :D ]

    Ok so there are some scientist who believe in the universal quantum entanglement, waht about scientist who does not belive in it ? What about people like Penrose, Kaku, Hawking ?

    I today did read about scientists who managed to entengle two diamonds. so here is the question: If I damage one of the diamonds [say hit it with hammer] would the other diamond respond by taking the same damage like the diamond I hit [say If i brake one diamond in to two parts- would the other diamond split in to two parts as well as respond]?
     
  12. Mar 25, 2012 #11

    Jano L.

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    I agree with you, it is a statement that seems not to be falsifiable. It has bothered many famous physicists, Einstein, Bohm, Bell, Jaynes, ...

    Another way to look at this is just to assume that the two particles are in states that have some mutual correlation, because they were once interacting. However, some modern experiments seem to violate Bell's inequality, which suggests that the states and the processes of measurement may not be of the simplest kind, so the mystery continues.
     
  13. Mar 27, 2012 #12
    Hello,

    In Quantum Mechanic the main concept is that the classical way to label objects assigning to them a certain number of definite properties is, in general, meaningless.

    Classically if for example you are describing a billiard bowl, you think it has a position, a velocity, an angular momentum, an energy etc...

    Quantum mechanically object are entirely described by one ray [itex]|\psi\rangle \in H[/itex] , where [itex]H[/itex] is an infinite-dimensional Hilbert space. This ray does not, in general, corresponds to any definite classical quantity: it has not one position, nor one momentum etc...
    It could happen that this ray has some definite classical properties, but there are quantities that can never be definite at the same time, like position and momentum.

    Now let's go back to the two entangled electron example. When they are in the atom, they have a defined total spin, [itex]S_T=0[/itex]. If you separate them carefully they will continue to have [itex]S_T=0[/itex] even when they are far from each other: they are entangled.
    The point is that total spin cannot be definite at the same time of the two spin [itex]S_1 , S_2 [/itex] of the two electrons.

    None of the two electrons have a definite spin: it's not about classical ignorance, they physically do not have a definite spin!

    When you perform a measurement on one of the two electrons you are changing the ray that was describing the system in another ray with definite spins [itex]S_1 , S_2 [/itex] but with no definite total spin any more. The two spins resulting will always be opposite, but since they were not definite before the measurement, it could be that the first is up and the second is down or the opposite.

    I hope this helps a bit :smile:

    Ilm
     
  14. Mar 27, 2012 #13
    If that interpretation would be wrong it would imply the existence of hidden variables according to which there is a theory/mechanism/underlying physical process that objectively sets one electron in one definite spin state and the other in the opposite definite spin state immediately after the emission from the atom. This is however neither in accordance with quantum formalism nor with Bell's inequality *experimental* violation which ruled out hidden variables. The only thing we can do nowadays is to accept that these interpretations of QM are the only that have stood the test of time through experimental evidence. In an entangled state there are no things like "two electrons having opposite spins", there is only one and the same object having spin=0. Until you measure it....
     
  15. Mar 27, 2012 #14

    Jano L.

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    How did you arrive at such a conclusion?

    Generally, measurements of projections of spin only verify that scalar products of vectors in Hilbert space predict correctly frequencies of outcomes. For that, we have to do a lot of measurements on different atoms.

    Such measurements do not falsify definite properties of individual atoms.
     
  16. Mar 27, 2012 #15
    That conclusion is very easy to obtain.

    Let us only consider the spin of the system.
    The state, before the measurement of the spin of one electron, has zero total spin.
    We know that

    [itex]
    |S_1=1/2\, ; \, S_2 = 1/2\, ; \, S=0\, , S_z =0 \rangle = \frac{1}{ \sqrt{2} } {\Large ( }|S_1=1/2\, ; \, S_2 = 1/2\, ; \,S_{z\, 1}= 1/2 \, ; \, S_{z\, 2} = -1/2 \rangle - |S_1=1/2\, ; \, S_2 = 1/2\, ; \,S_{z\, 1}= -1/2 \, ; \, S_{z\, 2} = 1/2 \rangle{\Large ) } \, .
    [/itex]

    Then none of the two electron has definite spin.

    You may wonder how do we know the initial condition is [itex]S=0[/itex] and not a state with definite spin for the two electrons.
    Well, we could simply measure total spin and obtain zero with frequency 1 :smile:

    Ilm
     
  17. Mar 27, 2012 #16

    Jano L.

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    You said

    I think it is a challenging task for believer in quantum completeness to show that _one_atom_ does not have spin (magnetic moment).

    EDIT
    The only thing your example shows is that your mathematical vector is not an eigenvector of spin operator sigma_z of the particle, which only means that quantum theory predicts the ensemble of measurements of atoms described by this vector will not be univalent.

    The important thing is, that predictions of quantum theory are about results of measurements, while your claim is about physically having a magnetic moment prior the measurement.
     
  18. Mar 27, 2012 #17
    I have not said that one atom does not have definite spin (that is nonetheless generally true). I was talking about electrons, that in our example were in a state with zero total spin and so with no definite single particle spin along any direction.

    The most important thing is however another. In quantum mechanic if two observables do not commute, then a system can never have definite value of both at the same time.
    If a particle is in some point, then it has not definite momentum.

    That’s completely different from classical uncertainty. The latter is accounted for describing a system not with a pure state but with a density matrix.

    Physical systems have properties before measurement (they are in some definite state), simply they haven't necessarily the same property your are going to measure.

    Take the two electron system we were talking about. Measuring the total spin will always result in [itex]S=0[/itex] (so my claim was about the result of a measurement ^^). Then this is a definite property of the system.
    If we now measure the spin of one single electron, we are measuring a property that was not definite before the measurement (but it will be definite after the measurement).

    Before the measurement none of the two electrons had the spin aligned along any direction because if this was not true we couldn't have measured [itex]S=0[/itex] with probability 1.

    Ilm
     
    Last edited: Mar 27, 2012
  19. Mar 27, 2012 #18

    Jano L.

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    Sorry, I was a bit too quick in reading your post.

    You are right that when we measure spin projection of one electron, when it interacts in an atom, we can only measure total spin projection of the system of electrons. Then we do no find anything about the spin of individual electrons.

    The fact that you can describe or predict the result of such measurement by quantum state which is superposition is not sufficient to disprove that "physically electrons have spin". It is better to say "in quantum theory, there are states when two electrons do not have definite spin".

    That is because quantum state vector describes magnetic moment, but it is not clear that it is a complete description.
     
  20. Mar 28, 2012 #19
    Well, I only stated what quantum mechanics says on the system.

    It could be that QM is wrong, and almost certainly is not a complete theory.

    Nonetheless I think that, as long as QM point of view is the most predictive we have, we must think as if QM was real. :smile:

    Ilm
     
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