4 ms·
Why does this not describe the "entanglement" phenomenon: 1) Alice, Bob, and Charlie are in a room. 2) Alice has only an apple and a pear, seen by Charlie. 3
by nonbel 10y ago
Why does this not describe the "entanglement" phenomenon:
1) Alice, Bob, and Charlie are in a room.
2) Alice has only an apple and a pear, seen by Charlie.
3) Charlie leaves the room.
4) Alice gives one of the two fruits to Bob.
5) Bob leaves the room to meet up with Charlie and starts eating the pear.
6) Charlie instantly knows that Alice has the apple, no matter where she has ended up in the universe (which is not spooky at all).
- danbruc 10y agoBecause Bob leaves the room with either an apple or a pear and this choice is made inside the room. In quantum physics Bob would leave the room with half an apple and half a pear, Charlie would ask Bob to eat the pear, and in this moment Bob's half of the apple would teleport to Alice and Alice's half of the pear to Bob. Charlie could also have asked Bob to eat the apple. Very roughly speaking, this picture is actually flawed. Charlie can not choose the fruit to eat in quantum mechanics. In this picture you would also transmit Charlie's choice to Alice faster than the speed of light which is not possible in quantum mechanics. The important part of this picture is that Bob does not leave the room with a predetermined fruit, the choice happens later outside of the room but Alice still always ends up with the other fruit, Alice and Bob never end up with the same fruit.
- mirimir 10y agoI get how "teleport" isn't QM. But the issue for both is locality violation.
- nonbel 10y agoThanks, > "Charlie would ask Bob to eat the pear" This seems to be the key part. So when they run experiments on this, a certain result can be forced? > "The important part is that Bob does not leave the room with predetermined fruit." Right, is this aspect measured somehow or only inferred from the theory?
- danbruc 10y agoI tried to set this straight in my comment. Charlie does not get to choose the fruit, the fruit is chosen at random. I just allowed Charlie to make the choice to emphasize that the choice happens outside of the room. Had I just said the choice happens at random, then it would not be obvious why this random choice could not also be made in the room with Alice. But then we were back at Bob leaving the room with either an apple or a pear. Have I look at my other comment with coins in boxes [1], that picture captures the situation better and in a way that is not easily translated to fruits. [1] https://news.ycombinator.com/item?id=13981054 https://news.ycombinator.com/item?id=13981054
- beojan 10y ago> I just allowed Charlie to make the choice to emphasize that the choice happens outside of the room. The problem is that allowing Charlie to make the choice allows him to use entanglement to transmit information faster than the speed of light.
- nonbel 10y agoI think, as others have pointed out, these analogies just aren't perfect enough. Do you know where I can find a data source for one of these tests? I checked a few recent papers from wikipedia but they only reported (very high level) summary statistics.
- stouset 10y ago> This seems to be the key part. So when they run experiments on this, a certain result can be forced? No, a result can't be forced. The analogy being used isn't very close. I'm going to summarize an explanation used in (IIRC) The Elegant Universe that may be helpful. Mulder and Scully each get shipped a box that's entangled with the other's box. These boxes have three windows. You're allowed to open only one window, at which point you'll see a red or green light. These boxes further have the property that they're entangled: if Mulder and Scully open the same window, they'll see the same color. Mulder and Scully open the same window on each box, and sure enough, they see the same color. Mulder claims that it's spooky action at a distance, Scully claims that the boxes are simply preprogrammed with a preset color pattern: one of "red, red, green", "green, red, green", "red, red, red", and so on. Mulder thinks for a bit and devises an experiment. They now acquire a large number of these boxes, each paired with a box in the other's possession (and in such a way they can identify which of their two boxes correspond to one-another). They proceed to open doors randomly, recording the results. Now they compare notes. In the case where they both opened the same door on an entangled pair of boxes, they always get the same result. So far so good. But what happens when they look at the results as a whole? Well, Scully's theory was that these boxes are preprogrammed and the data we've collected falsifies this theory. The only possible patterns, ignoring specific color and order, are ones where two windows reveal the same color and one window reveals another color (any ordering of GGR or RRG) or where all three windows open to the same color (GGG or RRR). In the latter case, their results will obviously match 100% of the time. In the former case, 2/3 of the time their results will match 2/3 of the time (for 4/9ths) and 1/3 of the time their results will match 1/3 of the time (for an additional 1/9th), adding up to 5/9ths. So if Scully's theory is true, when opening doors at random, they should have recorded the same result at least 5/9ths of the time (more if some boxes are programmed with all windows revealing the same color). Unfortunately for Scully, their records show that their results matched exactly 50% of the time. No matter how complicated your "programmed" hidden variable theory is (pseudorandom generators with the same seed, etc.), it is fundamentally disproven if the observed match rate is less than 5/9ths. The simplest explanation that fits this experimental result is that both boxes choose to display a color truly randomly when a given window is opened, but they both arrive at the same random color when the same window is opened! Again, this statistical argument works against all hidden variable theories.
- mirimir 10y agoIt would be spooky if, in the middle of step 5, Malory exchanged Alice's apple for a banana. And Charlie knew that. Edit: I like danbruc's answer better.
- beojan 10y agoThat does describe entanglement. To say that A and B are entangled simply means the state of A is correlated with the state of B. Entanglement is only "spooky" or "weird" because (from Charlie's point of view) Bob is in a superposition of the states |Has Pear> and |Has Apple> between steps 4 and 5.
- roywiggins 10y agoOrdinary entanglement is having a strange pair of coins. Toss them both and they will never come up both heads or both tails. Give one to a friend, walk to the ends of the earth, toss the coin, then call your friend: comparing notes, they've come up different again... spooky! Maybe the coins colluded when they were made, and there's no spooky action at a distance, just some hidden information inside the coins. This is what the Bell tests disprove. You can make a series of measurements that result in data that could not have been agreed upon by the coins ahead of time. (In quantum mechanics, the entangled particles are never perfectly correlated or anticorrelated like this, but they coincide or not at a rate much different than ordinary particles)
- gdavisson 10y agoWhat you're describing is a local hidden variable theory. In this case, the "hidden variable" is who has which fruit. It's "local" because there's no remote influence -- Bob eating his fruit doesn't change which one Alice has. Bell's theorem places limits on the correlations that a theory like this can explain. Here's a simple example of a Bell violation: 1) Alice and Bob get together and conspire. They can do anything they want here. 2) Alice and Bob are then taken to separate interrogation chambers, to be quizzed by separate interrogators. 3) Each interrogator randomly (and independently) chooses one of three yes-or-no questions to ask their subject. Both interrogators are working from the same list of 3 questions. Alice and Bob are asked the questions and the questions and their answers are recorded. 4) This whole process is repeated many times. When the results are tabulated, it's found that every time Alice and Bob were asked the same question, they gave the same answer. But when they were asked different questions, 75% of the time they gave different answers. How could Alice and Bob have achieved this? Classically, they couldn't. In order to always give the same answer to the same question, they'd have to (either directly or indirectly) agree beforehand on which answer to give to which question. Since there are three possible questions but only two possible answers to them, any strategy they pick must give the same answer to (at least) two of the three questions. Therefore, if their interrogators pick different questions there's at least a 1/3 chance they'll pick the questions with the same answer. But with quantum entanglement, the chance of getting the same answer from both is can be only 1/4. Something weirder is going on. (BTW, for completeness, here's how the quantum results I described can be achieved: Alice and Bob are replaced by electrons, prepared in the (entangled) singlet state. The "interrogations" are measurements of their spin (up or down) on the 0°, 120°, or 240° axes, with one of the detectors set up upside-down. This is not the way it's actually done, but it's easier to explain & see the paradox this way.)
- eveningcoffee 10y agoWhat would happen if Alice and Bob somehow knew what questions will be asked?
- return0 10y agomore like, someone puts both fruit in black bags, mixes them up and gives one to alice and one to charlie. alice does not know which one she got until bob eats his fruit or until charlie sees it.