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Question about entanglement.

  1. Feb 22, 2012 #1
    That quote is from this article..

    I thought that's exactly what entanglement meant. Can someone please explain entanglement better then? I am miss understanding Schrodinger's cat experiential.
  2. jcsd
  3. Feb 22, 2012 #2


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    Entanglement generally means if you measure one of the particles you will automatically know what the state of the other particle is. It does not imply "manipulating one particle we can at the same time change the other".

    It is usually applied to the polarization angle of a pair of photons, created at the same time with some type of relationship to each other. Often the pair will be the at same polarization angle due to the process the created them. Entanglement means that the measurement of the polarization of one of the photons will tell you the polarization of the other photon, even if it is a long ways away.

    Hope this is of some help.
  4. Feb 22, 2012 #3
    You make it sound as if the two photons were just given the same polarization intially, and your measurement of one of them just reveals what that polarization is. There are problems with this view due to Bell's theorem; see e.g. this explanation.
  5. Feb 22, 2012 #4


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    I would say that what you see is as if the results of a measurement on Alice is instantly transmitted to Bob, or vice versa. You cannot factually state which direction something acts though. Further, it is not as if any change to Alice is sent to Bob - just the results of an observation. So moving Alice does not move Bob (or vice versa).
  6. Feb 22, 2012 #5
    Then what is so insane about entanglement. Im so bummed. Its way less exciting than I thought. I feel like I just found out santa isn't real.
  7. Feb 22, 2012 #6
    "Specifically they showed that according to the theory I could put a particle in a measuring device at one location and, simply by doing that, instantly influence another particle arbitrarily far away" These types of explanations are misleading.
  8. Feb 22, 2012 #7
    It is actually as exciting as you thought, it's just a bit more subtle than you would have expected. Let's start with this basic property of entanglement: if you have a pair of entangled photons, and you perform a polarization experiment on one of them while your friend performs the same experiment on the other one hundreds of miles away, you two will get the exact same result. This fact is not in and of itself surprising, because there are multiple possible explanations for this fact:

    1. It could be that when the two photons were created together, they were given the same polarization, which is why they behave the exact same way when you send them through polarizers oriented at the same angles. This possibility is not "mystical" in anyway, and this kind of thing happens in classical mechanics all the time. For instance, you could flip a coin, and then cut the coin in two and put the two pieces in seperate envelopes and send them hundreds of miles away from each other. Then if people open the envelopes, they will always get the same result: either both halves are heads or both halves are tails! This is hardly a surprising physical phenomenon

    2. It could be the photons are not just behaving according to some common attribute they were both given when they were created. This raises some interesting questions: how did photon 2 "know" that photon 1 performed a certain way in the experiment, so that it should do the same thing? Especially if the two particles are seperated by such a great distance that there is no possibility of communication.

    Obviously, you would naturally assume that explanation 1 is more likely to be correct. But J.S. Bell proved that explanation 1 seems to logically lead to certain experimental predictions. And then people tested those predictions, and they found explanation 1 failed. You can read more about the famous Bell's theorem in numerous sources; this is my favorite. It is the fact that explanation 1 does not seem to work that makes entanglement so amazing.
  9. Feb 23, 2012 #8
    The article is an interesting one of a Bose-Einstein condensate which is forced quantum-mechanically to respond by partitioning two particles away from itself in exact opposite directions (to preserve the momentum of the condensate itself). The idea behind the experiment presents a vehicle to study quantum-mechanically entangled particles.
    The partial quote you gave is misleading and displays the bias ( interpretation ) of the scientist quoted. There are many interpretations of entanglement, they all 'work' if you look at entanglement in that particular way. The math, the formulae are always the same. You can have a room of scientists, all with their own interpretation of entanglement or more generally QM and it won't affect the experiment. The results will be the same, and satisfy all perspectives.
    Entangled particles-measure one of the two for a particular property then, if you measure the other particle for the same property, you will get the same result. If you don't measure the other particle for the same property you can say nothing about any properties of either particle WRT the other.
    This is not an interpretation, it is just bare bones what happens when you conduct an experiment with entangled particles.
  10. Feb 23, 2012 #9
    This is the wrong way to look at it. Entanglement doesn't say that the individual states of the two particles is connected. It says that there is a single state for the whole system which specifies both particles. If you measure a single particle, you learn something about the state of the system, and this can help you predict the results of a measurement on the other particle.

    Suppose you have two spin 1/2 particles, and the system is in an eigenstate of 0 total spin. The state of particle 1 and the state of particle 2 cannot be treated as if they were separate, independent things. Rather, there is only a state of the system, which might be something like
    1/sqrt(2) (|UD> + |DU>) or 1/sqrt(2) (|UD> - |DU>)
    This state cannot be factored into two independent states.
  11. Feb 23, 2012 #10
    That is exciting because I am reading "three roads to quantum gravity" by lee smolin and that is what he says. He said that the universe is not made up of objects, its made up of processes. And being an observer inside a system effects the measurement.

    Can someone explain the implications of this more clearly? And how it pertains to entanglement.
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