11 ms·

A classical analogy for entanglement: suppose I have two balls in a bag. They are identical in every way, except one is red and the other is blue. I randomly gr

• kolinko 5y ago
  The analogy you mentioned is exactly the wrong one - it suggest that it’s just a matter of a hidden variable. A proper (but less elegant) would be: you have two balls with the same color or a pattern. You take one out. If you check the color first, you will find the other’s color the same, but the pattern sometimes different. If you check the pattern first, you will find the pattern the same, but the color sometimes different.
  • tylerhou 5y ago
    > it suggest that it’s just a matter of a hidden variable. I disagree. Suppose that I create a machine that chooses which ball to place in each box. This machine makes the choice based on some measurement of a quantum particle (electron spin). Then the colors of the ball are entangled with the state of the quantum particle, which cannot be described by some local hidden variable.
    • lisper 5y ago
      Only if you can completely isolate the balls so their states don't decohere. That is not practically possible to achieve, particularly since in your scenario you reach into the bag and touch the balls. As soon as you interact with the balls in any way, you become entangled with them and the behavior of the system becomes classical.
      • serverholic 5y ago
        You didn't add anything to his example. This is just purely to be pedantic.
        • lisper 5y ago
          No. The only way you can actually observe entanglement is in an isolated entangled system (this is the reason quantum computers are hard to build). It is true that at a philosophical level there is no difference, but from the point of view of physics, which is to say, what is observable, isolation is crucial. Non-isolated systems behave classically, notwithstanding that they are actually quantum systems.
          • tylerhou 5y ago
            Would you claim that when Einstein developed his theories of relativity, they were invalid (from the point of view of physics) because their consequences were not yet observable? For example, Einstein used thought experiments to develop special relativity in 1905, but since kinematic time dilation was only experimentally confirmed in 1971, his work was not a contribution to physics until then?
            • lisper 5y ago
              The difference is that the limits on observing relativistic effects in 1905 were technological, whereas in QM you cannot observe the effects of entanglement in a non-isolated system even in principle. This is a fundamental constraint imposed by the theory itself. You can't get around it even with arbitrarily advanced technology.
  • naasking 5y ago
    > The analogy you mentioned is exactly the wrong one - it suggest that it’s just a matter of a hidden variable. It is equivalent to a hidden variable, just a non-local one.
    • kolinko 5y ago
      his analogy suggests there is a local hidden variable.
      • naasking 5y ago
        That too could be a valid analogy if "randomly grab one in each hand" isn't actually random, but only appears random. This would be analogous to superdeterminism.
• tsimionescu 5y ago
  The problem with your classical analogy for entanglement is that it doesn't match the data. Or rather, it only matches the data for quantum properties that are similarly blue or red. The non-classical properties of entanglement start appearing once you start measuring combinations of the redness and blueness of those balls. Let's say that instead of looking at the balls, you pass them through some machine that will let a red ball pass through with some probability P that you control; if the ball is blue, the machine will let it pass with probability 1-P. Let's say further that you have three such machines. You set the first machine to P=1. You pass each ball falling from this machine through a second machine, which has P = 0. You will never see a ball pass through to the end - if it were red, it would pass the first machine, but not the second; if it were blue, it would not pass the first machine at all. But, let's say you now put a third machine between the other two, and you set P = 0.5. With classical balls, nothing changes - a blue ball doesn't make it past the first machine, while a red ball goes through the first, may or may not pass the second, and never makes it through the third regardless. However, a quantum ball actually has a chance to pass through the 3 machines if you set it up this way. In fact, that chance is pretty large - more than half of the balls will start passing once you add the middle filter machine. Still, this is easy to explain if we assume that the middle machine actually paints the ball instead of just detecting its color. This is where the entanglement experiment comes in: if you pass the pair of balls through the three machines, with ball 1 passing through machines P=1 and P=0.5, and ball 2 passing through P=1, you will find that sometimes both balls make it through, even though both balls can't be red at the same time, and they can't communicate about passing through the P=0.5 machine (you can repeat the experiment with the balls being taken arbitrarily far away before passing through the filters).
  • tylerhou 5y ago
    Bell's inequality (as you allude to) describes how transformations on quantum wave functions cannot behave classically. But classical wave functions can certainly be entangled as entanglement is a property of a wave function, not transformations on wave functions.
    • tsimionescu 5y ago
• PeterisP 5y ago
  Reviewing Bell's theorem - described in this article - has resulted in experimental evidence that all classic analogies in the style of "some state was embedded in each particle at the moment of entanglement and the measurement just revealed something about what was in that single particle locally at that time" can not be true. Bell's theorem describes the highest possible upper bound of correlations for spin measurements along different axis if it was as you say. But it turns out that in practice they are more correlated than what would be possible according to Bell's theorem, ergo, that analogy (which, in general, is plausible and reasonable) is not compatible with the physical reality we live in.
  • GoblinSlayer 5y ago
    The classical analogy don't model superposition, that's what violates Bell's inequalities, but it illustrates the correlation aspect of entanglement well.
    • desmosxxx 5y ago
      Correlation aspect sure, but perhaps not the process lf entanglement
• Kranar 5y ago
  >They are identical in every way, except one is red and the other is blue. I randomly grab one in each hand and show my hands closed. Now the states of the ball are entangled: as soon as you see the color of one ball, that "determines" the color of the other. This gets used to explain entanglement but it really has absolutely nothing to do with it. This is nothing that the ancient Greeks wouldn't have known. Not to pick on you specifically, but do people really think it took a major revolution in physics in order to understand that if there are two balls, one is blue and one is red, then if you see one of the balls is red, you can conclude the other ball is blue? It's something that I think humans can solve at the age of 3. The failure in your explanation is right when you state that "one of the balls is red and the other is blue". The entire point of entanglement is that such a statement is not possible, that's a strictly classical interpretation. Rather, both balls are in a superposition of being both red and blue simultaneously, and it is not possible in principle to assign a color to either one of them until the moment a measurement is made.
  • oolonthegreat 5y ago
    so true. I have a similar beef about the popular explanation for uncertainty principle: "well you see the light hits the particle very hard so we know where it was but we don't know where its gone now". urgh.
    • tines 5y ago
      As someone who doesn't know any better than the explanation you have a beef with, I would love if you could explain in layman's terms why it's wrong and what a more accurate understanding is. I always thought that analogy was exactly how it worked, but it seems I have been misled unawares.
      • mannerheim 5y ago
        It presupposes that position and momentum have definite states that are just uncertain to us, while in most interpretations of quantum mechanics (e.g. Copenhagen and many worlds) the particle exists as a wavefunction, lacking a specific position or momentum, but instead existing as a probability density function in this space. The uncertainty principle here then relates to how much this probability density function 'peaks' in position space or momentum space. A higher peak in one space results in a wider spread in the other. This is because position and momentum are Fourier transforms of each other.
• renox 5y ago
  In this case this analogy is very bad because what your describing is an 'hidden local variables'..
• canjobear 5y ago
  This is exactly the analogy that Bell's Theorem refutes!
• Tomminn 5y ago
  If you wanted to make this quantum, one is red and the other is blue only if you're looking for a red ball when you stick your hand in the bag. If you're looking for a purple ball when you stick your hand in the bag, then one is purple and the other is green. That's the spooky part.
• phscguy 5y ago
  As others here have already noted, this analogy is wrong. But I think people here haven't given a convincing example of a system that behaves differently due to entanglement than it would if the behaviour were simply conditionally random (the ball example behaves identically if the balls are entangled or otherwise just classically random). The issue in finding a good example is that the effect of entanglement is rather subtle and hard to interpret intuitively. Here is an example that may make it more obvious: There exists a game that can be played cooperatively between two players that share two random bits. It is possible to win this game only 75% of the time if the bits are not entangled. If the bits are entangled there is a strategy for winning the game about 85% of the time. The details of the game and a good explanation can be found here: https://www.scottaaronson.com/blog/?p=2464 https://www.scottaaronson.com/blog/?p=2464 Basically, there is a game that involves sharing two bits, if they are entangled, it can be won 85% of the time. If they are not entangled but otherwise random (like the red and blue ball example), it can be won only 75% of the time.
• kgwgk 5y ago
  A more faithful analogy: I have two boxes with a small hole and ball inside. If I look through the holes, I will see that one ball is blue and the other is red. If I touch them through the holes, I will feel that one ball is hot and the other is cold. However, I cannot look and touch at the same time. And once I check the color or the temperature of one of the balls (so I know it for both) the link is broken. If I look first, I’l see that one is red and the other is blue. But if now I touch them, each one will by hot or cold with 50/50 probability. Finding the temperature of one doesn’t tell me anything bout the temperature of the other. And when I touch a ball I don’t know its color anymore: if I look at it again it could be red or blue with 50/50 probability.
• bawana 5y ago
  agree. The issue that is confusing in explanations of entanglement is the use of a statistical measurement to a model of an individual event. Entanglement is a state made after the observations of many events. Before there is a red ball and a blue ball, there is a precursor, purple ball. That purple ball splits and two balls wrapped in paper are created. They can travel great distances over long times to separate locations, A and B. When some humans start to unwrap them, they are amazed at the correlation-one blue is always matched by one red. So until the balls are unwrapped, how are the humans to describe them? They use the word superposition-unfortunately, many interpret that as a SIMULTANEOUS existence of both states in each ball. Whereas in reality there are many events - the humans at location A see both colors as do the humans at location B. That does not mean that individual particles assume both states. A similar confounding statistic was taught in grade school. The average family has 2.5 children. Yet half a live child was never born.
  • sooheon 5y ago
    > Whereas in reality there are many events - the humans at location A see both colors as do the humans at location B This is one of many interpretations, I don't think it's fair to label conjecture as "reality" yet.
• danbmil99 5y ago
  The balls in a bag experiment is exactly the kind that does use hidden variables that are local. No information has to be transmitted in either direction. Bell showed that the correlation is even greater than you can get using that sort of thinking. Reality is more like this: Bob and Alice each get a pair of bags, one black and one white. They open one of the bags in which they get either a red ball or a blue ball. If they both choose the white bags, their balls are different colors. However if either or both of them choose the black bag, the colors of the balls are the same. If you think about it, there's no way to put the balls in the bags to satisfy these conditions in all cases. This is a simplification of what's going on with Bell's theorem.
  • electrondood 5y ago
    This is an excellent analogy, thanks.
  • Epokhe 5y ago
    Thanks for the example. Could you point me to a resource where it explains why the reality is like that? If that’s an implication of a formula of quantum theory(which the article also mentioned briefly), I would like to learn about it and be able to derive this implication myself.
    • danbmil99 5y ago
      A more fleshed-out & professional explanation can be found here: https://www.scottaaronson.com/democritus/lec11.html https://www.scottaaronson.com/democritus/lec11.html scroll down to the section titled Relativistic Causality