9 ms·

Entanglement Made Simple

91 points by beefman 10y ago

skc 10y ago
Every time I read anything on how wierd quantum mechanics is I wonder how theoretical physicists can stay sane. It's all so bizarre that I don't see how anyone can look that deeply into it and not come away thinking our entire existence is one big pointless hoax.
- deciplex 10y ago
  If you really want to understand quantum mechanics in an intuitive sense (i.e. enough to understand why it works, but not really enough to generate your own robust predictions at a very granular level) I recommend http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ There is a (subtly, and not in a terribly important way) flawed conclusion regarding personal identity toward the end, and parts of it are a bit long-winded (I guess esp if you don't already enjoy reading Yudkowsky as I do), but it is solid. tl;dr: QM's reputation is more a function of it being poorly taught (cf this article), not really the theory itself - ultimately it makes the world less confusing, as any sound scientific theory should.
- stared 10y ago
  It is not bizarre; it is being presented as such, though. The actual mathematics of quantum mechanics is simple (vectors, matrices). What is confusing is: - trying to describe quantum mechanics without mathematics (since we have no direct intuition/experience, many analogies do not hold), - inventing some profound philosophy to describe facts. For example, my entry on that: https://johncarlosbaez.wordpress.com/2015/03/13/quantum-superposition/ https://johncarlosbaez.wordpress.com/2015/03/13/quantum-supe... (not entanglement, but - an electron being in two places at once). [Full disclaimer: I am an ex theoretical physicist. I don't claim I am sane, though.]
  - tim333 10y ago
    It's not bizarre if you skip the tricky bits like how does it work, why don't the equations work for gravity, are there many worlds and so on.
  - shanacarp 10y ago
    >[Full disclaimer: I am an ex theoretical physicist. I don't claim I am sane, though.] You sound sane :)
- vixen99 10y ago
  Isn't that thought only thinkable if you believe the human brain is the best possible apparatus available for comprehending the universe? Something bigger and better may have a different view.
  - lomnakkus 10y ago
    I'm pretty sure we can say with near certainty that any intelligence that has a) evolved, and b) exists on a macro-scale, will also have problems comprehending quantum theory.
- givan 10y ago
  It looks bizarre because we are looking at it through our current paradigm, we need to change our view of the universe and probably ourselves to understand it and it will stop looking bizarre. Quantum mechanics understanding will probably be a scientific revolution, something comparable to understanding the earth is round or the sun does not circle earth. There are a few scientist that are trying to break the old model of thinking. http://www.theepochtimes.com/n3/1714898-breaking-through-conventional-scientific-paradigm/ http://www.theepochtimes.com/n3/1714898-breaking-through-con...
- Chathamization 10y ago
  I'm not sure it's more bizarre than, say, matter being made up of atoms. As others have said, it tends to be taught in an overly confusing way - perhaps intentionally so. This article is a decent example of that.
- tim333 10y ago
  It is very odd but the psychological take on that is very much down to the individual. If everything was Newtonian determinism it would be rather dull. Behaviour that is so mysterious we can't really understand it is quite interesting.
deciplex 10y ago
I'm not sure trying to instill an "intuitive" sense of quantum mechanics by appealing to classical principles in this way is such a great idea. The example of entanglement given here implies that each system was a circle or a square "all along" i.e. from the moment of being entangled. That's not actually how it works, and thinking about it that way will lead to misunderstandings later on, should you try to study QM more deeply. You will have to unlearn the lesson taught here. It also leaves you unequipped to reason about quantum computing, by the way. Also: > 1. A property that is not measured need not exist. > 2. Measurement is an active process that alters the system being measured. These are both misleading enough that they can probably just be called 'wrong' as well. We shouldn't assign mysterious properties to "measurement" as though it's some magical thing that just makes quantum mechanics happen. Just call it what it is: entangling the state of your brain with the results of an experiment. That conveniently also provides for a good jumping off point into Many Worlds / Relative-State Formulation of QM.
- millstone 10y ago
  Describing measurement as "entangling your brain" is a particular viewpoint that is not widely accepted. Most physicists would agree that the LHC's particle collision data constitute measurements, even though most of those results have never been looked at by a human. Measurement is at its most mysterious in the MW interpretation, because measurements are probabilistic and probabilities are hard to get out of a purely deterministic theory like MW.
  - deciplex 10y ago
    It's not clear what point you're trying to argue. Yes, the particle collision data constitutes a measurement in the sense that it is entangled with whatever medium the data is being stored on (and, in practice, a while lot else), even if a human hasn't looked at it yet. Measurement as we usually mean it when talking about this stuff in the context of QM means a human looking at a thing, but sure that need not always be the case. I mean if you want to split hairs we can go back and forth all day, but that doesn't strike me as an interesting or enlightening avenue of discussion.
- 10y ago
biswaroop 10y ago
I always understand entanglement as 'cannot be factored'. It's really just that mathematical and simple. Say there are particles a and b that can be in states 1 or 2 entangled: a1 * b2 - a2 * b1, vs not entangled: (a1+a2)*(b1+b2) So, when you apply a measurement operator on the state of particle a, you know something about particle b in the entangled case. Schrodinger said: "the best possible knowledge of the whole doesn't necessarily include the best possible knowledge of the parts." An entangled state should be treated as a whole, and not a sum of parts, kind of like how a book is treated as a continuous story, not a sum of letters.
- justifier 10y ago
  " Another way of expressing the peculiar situation is: the best possible knowledge of a whole does not necessarily include the best possible knowledge of all its parts, even though they may be entirely separate and therefore virtually capable of being ‘best possibly known,’ i.e., of possessing, each of them, a representative of its own. The lack of knowledge is by no means due to the interaction being insufficiently known — at least not in the way that it could possibly be known more completely — it is due to the interaction itself. Attention has recently been called to the obvious but very disconcerting fact that even though we restrict the disentangling measurements to one system, the representative obtained for the other system is by no means independent of the particular choice of observations which we select for that purpose and which by the way are entirely arbitrary. It is rather discomforting that the theory should allow a system to be steered or piloted into one or the other type of state at the experimenter's mercy in spite of his having no access to it. " Schrödinger, 1935 (o) where i disagree with schrödinger is thinking there are two systems stead one that simply contains the entanglement.. if that's what he's going to call it.. " When two systems, of which we know the states by their respective representatives, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled. " Schrödinger, 1935 (o) (o) http://plato.stanford.edu/entries/qt-entangle/ http://plato.stanford.edu/entries/qt-entangle/
acqq 10y ago
A horrible article, it can only contribute to confusion than to present what is known. No, there are no squares and circles, and no cakes. It isn't simpler an there's no insight to be gained by thinking about the cakes and squares and circles. Much better learning about what is. For example, to get some intuition for the things you can't directly observe try considering what's going on in the "simple" act of water drop forming and detaching. https://youtu.be/c4MUTij8f6I?t=120 https://youtu.be/c4MUTij8f6I?t=120 It's waves all the way down.
fiatjaf 10y ago
> Entanglement arises in situations where we have partial knowledge of the state of two systems. What happened to the notion that entangled states were about particles being in many places at the same time?
- lmm 10y ago
  Superposition is a necessary precondition for entanglement but not what entanglement itself is about.
cousin_it 10y ago
> The interesting effects, which EPR considered paradoxical, arise when we make measurements of both members of the pair. When we measure both members for color, or both members for shape, we find that the results always agree. Thus if we find that one is red, and later measure the color of the other, we will discover that it too is red, and so forth. On the other hand, if we measure the shape of one, and then the color of the other, there is no correlation. Thus if the first is square, the second is equally likely to be red or to be blue. That's not paradoxical, that's just two objects with the same random shape and the same random color. Quantum entanglement is stranger than that, it can't be explained by hidden correlated information about both objects. Here's an example of quantum entanglement. Imagine that Alice and Bob live far from each other and can't communicate. There are three possible yes/no questions you can ask them, but each person can only answer one question. These facts are known: 1) If you ask Alice and Bob the same question, they always give opposite answers. 2) If you ask Alice question 1 and ask Bob question 2, the probability that both will say yes is 5%. 3) If you ask Alice question 2 and ask Bob question 3, the probability that both will say yes is 5%. 4) If you ask Alice question 1 and ask Bob question 3, the probability that both will say yes is 20%. Under classical assumptions, there's no way for Alice and Bob to prepare a strategy beforehand that would lead to such probabilities. The reason is that case 4 is completely covered by cases 2 and 3, but has higher probability than the two of them combined. Here's a proof: (4) => A1Y and B3Y => A1Y and B3Y and (A2N or A2Y) // tautology => A1Y and B3Y and (B2Y or A2Y) // by (1) => (A1Y and B2Y and B3Y) or (A1Y and A2Y and B3Y) // logical operations => (A1Y and B2Y) or (A2Y and B3Y) // logical operations => (2) or (3) Now, as strange as it sounds, in a quantum world Alice and Bob can defeat that proof and prepare a strategy beforehand. When you ask Alice a question, she will consult her prepared photon in a box, and measure it in a way that depends on which question you asked. The measurement will mess up the photon's state, so you don't get to ask another question. Elsewhere, Bob will do the same with his photon. The two photons were prepared together, but are completely separated at the time of the game. Before you ask, it's provable that such tricks cannot be used to send information faster than light. Quantum "spooky action" is somehow more powerful than hidden correlations, but less powerful than outright communication. It's pretty subtle and most popular accounts don't get it right.
- 10y ago
Ruud-v-A 10y ago
Here’s an analogy: I put a red card in one envelope, and a blue card in a different envelope. I send one envelope to Alice (but I don’t say which one) and one to Bob. Alice and Bob could be on opposite sides of the planet. When Alice opens her envelope, she knows which colour she has, but she also instantly knows which colour Bob has -- without communication with Bob. Bob might not even have opened his envelope yet. There is no faster than light communication or magic going on.
- EliRivers 10y ago
  This analogy is fundamentally misleading. In this analogy, the blue card was always blue in the envelope, and the red card was always red. In this analogy, this information was set right from the start. It's really misleading.
  - adekok 10y ago
    It's an analogy. The weirdness of QM comes from the following, slightly more complicated, situation. You have two cards, a red one and a blue one. You also have two envelopes. Without looking at anything, you put one card in each envelope. You label them, and send one each to Alice and Bob. Now, we know for sure that when Alice opens her envelope and looks at the card, we immediately know which card Bob has. There is no information being exchanged, so Alice knows what card Bob has immediately when she opens her envelope. The weirdness of QM comes from looking at the intermediate states. i.e. the states before Alice opens her envelope. There are (waving wands) ways to tell what the intermediate states could be, by varying certain (QM) magical properties of the experiment. The weirdness of QM is this. Every single idea you have about the intermediate state is wrong. What do I mean by that? You could say there are "hidden variables". i.e. one envelope "really has" a red card in it, and the other envelope "really has" a blue card in it. This interpretation is wrong. Every experiment shows that the envelopes are in a "superposition" of states. That is, the envelopes don't contain either a red card or a blue card. They carry... something... that is maybe red and maybe blue. That seems strange, but OK. What happens when Alice opens her envelope? When Alice opens her envelope, the card which is "maybe blue, maybe red" suddenly becomes either 100% blue, or 100% red. The weird part is the next step. When Alice opens her envelope, Bob's card immediately becomes the opposite color. This happens no matter how far away Bob is from Alice. So is information traveling faster than the speed of light? No. We knew Alice's card was red or blue, so Bob's card must have been blue or red. Lo and behold, Bob's card turns out to be one of those colors. And yes, Bob's card really is in an indeterminate state prior to Alice opening her envelope. And once Alice opens her envelope, Bob's card really is 100% either red or blue. The confusion comes from what is really happening behind the scenes. i.e. What is the real explanation? The current QM research shows that the answer is "Uh... right... we're not actually sure." Somehow the two cards are connected (QM people say "entangled"), because when you look at one, the other immediately "decides" (?) what color it is. But there's no information being passed between the cards. There's nothing exchanged between the cards. Yet somehow Bob's card "knows" that Alice has looked at her card. This is profoundly strange and confusing. No one has any idea what's going on. Physicists have many theories, but it's difficult to either prove or disprove most of them. That takes time, money, expertise, etc. So we're left with "QM is strange". As Richard Feynman said, "QM is not only stranger than you can imagine, it's stranger than you can possibly imagine."
eternalban 10y ago
I found this video [1] to be a remarkably engaging and informative (but Bohr biased) overview of this topic. Alain Aspect's comment regarding "if you just want answers you simply compute" in context of "understanding" entanglement is spot on. [1]: https://youtu.be/-LklvINk_L4 https://youtu.be/-LklvINk_L4
- eternalban 10y ago
  This is a much better upload of the same film: https://youtu.be/BFvJOZ51tmc https://youtu.be/BFvJOZ51tmc
tim333 10y ago
The author "makes it simple" by using a crappy get out for explaining the seeming faster than light influence in quantum entanglement: >This “spooky action at a distance,” as Einstein called it, might seem to require transmission of information — in this case, information about what measurement was performed — at a rate faster than the speed of light. >But does it? Until I know the result you obtained, I don’t know what to expect. I gain useful information when I learn the result you’ve measured, not at the moment you measure it. And any message revealing the result you measured must be transmitted in some concrete physical way, slower (presumably) than the speed of light. The thing is when the results are obtained you can write them down or similar. Unless you are presuming the written results and other records change as you mail them to each other you are left with the faster than light type paradox. I'm with Einstein on that one in that something odd is happening. My guess is that quantum mechanics is the fundamental nature of the universe and the appearance of space and time are a resulting epiphenomenon. As a result of that the particles appear far apart but are in some ways are still connected.
- pwendelboe 10y ago
  As I understand it though, because the result is random you can't actually transmit meaningful information faster than light. You can know what the other measurement was but because it was random it's rather useless information. But I don't know maybe I have it wrong...

91 points by beefman 10y ago

skc 10y ago

Every time I read anything on how wierd quantum mechanics is I wonder how theoretical physicists can stay sane. It's all so bizarre that I don't see how anyone can look that deeply into it and not come away thinking our entire existence is one big pointless hoax.

deciplex 10y ago
If you really want to understand quantum mechanics in an intuitive sense (i.e. enough to understand why it works, but not really enough to generate your own robust predictions at a very granular level) I recommend http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ There is a (subtly, and not in a terribly important way) flawed conclusion regarding personal identity toward the end, and parts of it are a bit long-winded (I guess esp if you don't already enjoy reading Yudkowsky as I do), but it is solid. tl;dr: QM's reputation is more a function of it being poorly taught (cf this article), not really the theory itself - ultimately it makes the world less confusing, as any sound scientific theory should.
stared 10y ago
It is not bizarre; it is being presented as such, though. The actual mathematics of quantum mechanics is simple (vectors, matrices). What is confusing is: - trying to describe quantum mechanics without mathematics (since we have no direct intuition/experience, many analogies do not hold), - inventing some profound philosophy to describe facts. For example, my entry on that: https://johncarlosbaez.wordpress.com/2015/03/13/quantum-superposition/ https://johncarlosbaez.wordpress.com/2015/03/13/quantum-supe... (not entanglement, but - an electron being in two places at once). [Full disclaimer: I am an ex theoretical physicist. I don't claim I am sane, though.]
- tim333 10y ago
  It's not bizarre if you skip the tricky bits like how does it work, why don't the equations work for gravity, are there many worlds and so on.
- shanacarp 10y ago
  >[Full disclaimer: I am an ex theoretical physicist. I don't claim I am sane, though.] You sound sane :)
vixen99 10y ago
Isn't that thought only thinkable if you believe the human brain is the best possible apparatus available for comprehending the universe? Something bigger and better may have a different view.
- lomnakkus 10y ago
  I'm pretty sure we can say with near certainty that any intelligence that has a) evolved, and b) exists on a macro-scale, will also have problems comprehending quantum theory.
givan 10y ago
It looks bizarre because we are looking at it through our current paradigm, we need to change our view of the universe and probably ourselves to understand it and it will stop looking bizarre. Quantum mechanics understanding will probably be a scientific revolution, something comparable to understanding the earth is round or the sun does not circle earth. There are a few scientist that are trying to break the old model of thinking. http://www.theepochtimes.com/n3/1714898-breaking-through-conventional-scientific-paradigm/ http://www.theepochtimes.com/n3/1714898-breaking-through-con...
Chathamization 10y ago
I'm not sure it's more bizarre than, say, matter being made up of atoms. As others have said, it tends to be taught in an overly confusing way - perhaps intentionally so. This article is a decent example of that.
tim333 10y ago
It is very odd but the psychological take on that is very much down to the individual. If everything was Newtonian determinism it would be rather dull. Behaviour that is so mysterious we can't really understand it is quite interesting.

deciplex 10y ago

I'm not sure trying to instill an "intuitive" sense of quantum mechanics by appealing to classical principles in this way is such a great idea. The example of entanglement given here implies that each system was a circle or a square "all along" i.e. from the moment of being entangled. That's not actually how it works, and thinking about it that way will lead to misunderstandings later on, should you try to study QM more deeply. You will have to unlearn the lesson taught here. It also leaves you unequipped to reason about quantum computing, by the way. Also: > 1. A property that is not measured need not exist. > 2. Measurement is an active process that alters the system being measured. These are both misleading enough that they can probably just be called 'wrong' as well. We shouldn't assign mysterious properties to "measurement" as though it's some magical thing that just makes quantum mechanics happen. Just call it what it is: entangling the state of your brain with the results of an experiment. That conveniently also provides for a good jumping off point into Many Worlds / Relative-State Formulation of QM.

millstone 10y ago
Describing measurement as "entangling your brain" is a particular viewpoint that is not widely accepted. Most physicists would agree that the LHC's particle collision data constitute measurements, even though most of those results have never been looked at by a human. Measurement is at its most mysterious in the MW interpretation, because measurements are probabilistic and probabilities are hard to get out of a purely deterministic theory like MW.
- deciplex 10y ago
  It's not clear what point you're trying to argue. Yes, the particle collision data constitutes a measurement in the sense that it is entangled with whatever medium the data is being stored on (and, in practice, a while lot else), even if a human hasn't looked at it yet. Measurement as we usually mean it when talking about this stuff in the context of QM means a human looking at a thing, but sure that need not always be the case. I mean if you want to split hairs we can go back and forth all day, but that doesn't strike me as an interesting or enlightening avenue of discussion.
10y ago

biswaroop 10y ago

I always understand entanglement as 'cannot be factored'. It's really just that mathematical and simple. Say there are particles a and b that can be in states 1 or 2 entangled: a1 * b2 - a2 * b1, vs not entangled: (a1+a2)*(b1+b2) So, when you apply a measurement operator on the state of particle a, you know something about particle b in the entangled case. Schrodinger said: "the best possible knowledge of the whole doesn't necessarily include the best possible knowledge of the parts." An entangled state should be treated as a whole, and not a sum of parts, kind of like how a book is treated as a continuous story, not a sum of letters.

justifier 10y ago
" Another way of expressing the peculiar situation is: the best possible knowledge of a whole does not necessarily include the best possible knowledge of all its parts, even though they may be entirely separate and therefore virtually capable of being ‘best possibly known,’ i.e., of possessing, each of them, a representative of its own. The lack of knowledge is by no means due to the interaction being insufficiently known — at least not in the way that it could possibly be known more completely — it is due to the interaction itself. Attention has recently been called to the obvious but very disconcerting fact that even though we restrict the disentangling measurements to one system, the representative obtained for the other system is by no means independent of the particular choice of observations which we select for that purpose and which by the way are entirely arbitrary. It is rather discomforting that the theory should allow a system to be steered or piloted into one or the other type of state at the experimenter's mercy in spite of his having no access to it. " Schrödinger, 1935 (o) where i disagree with schrödinger is thinking there are two systems stead one that simply contains the entanglement.. if that's what he's going to call it.. " When two systems, of which we know the states by their respective representatives, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled. " Schrödinger, 1935 (o) (o) http://plato.stanford.edu/entries/qt-entangle/ http://plato.stanford.edu/entries/qt-entangle/

acqq 10y ago

A horrible article, it can only contribute to confusion than to present what is known. No, there are no squares and circles, and no cakes. It isn't simpler an there's no insight to be gained by thinking about the cakes and squares and circles. Much better learning about what is. For example, to get some intuition for the things you can't directly observe try considering what's going on in the "simple" act of water drop forming and detaching. https://youtu.be/c4MUTij8f6I?t=120 https://youtu.be/c4MUTij8f6I?t=120 It's waves all the way down.

fiatjaf 10y ago

> Entanglement arises in situations where we have partial knowledge of the state of two systems. What happened to the notion that entangled states were about particles being in many places at the same time?

lmm 10y ago
Superposition is a necessary precondition for entanglement but not what entanglement itself is about.

cousin_it 10y ago

> The interesting effects, which EPR considered paradoxical, arise when we make measurements of both members of the pair. When we measure both members for color, or both members for shape, we find that the results always agree. Thus if we find that one is red, and later measure the color of the other, we will discover that it too is red, and so forth. On the other hand, if we measure the shape of one, and then the color of the other, there is no correlation. Thus if the first is square, the second is equally likely to be red or to be blue. That's not paradoxical, that's just two objects with the same random shape and the same random color. Quantum entanglement is stranger than that, it can't be explained by hidden correlated information about both objects. Here's an example of quantum entanglement. Imagine that Alice and Bob live far from each other and can't communicate. There are three possible yes/no questions you can ask them, but each person can only answer one question. These facts are known: 1) If you ask Alice and Bob the same question, they always give opposite answers. 2) If you ask Alice question 1 and ask Bob question 2, the probability that both will say yes is 5%. 3) If you ask Alice question 2 and ask Bob question 3, the probability that both will say yes is 5%. 4) If you ask Alice question 1 and ask Bob question 3, the probability that both will say yes is 20%. Under classical assumptions, there's no way for Alice and Bob to prepare a strategy beforehand that would lead to such probabilities. The reason is that case 4 is completely covered by cases 2 and 3, but has higher probability than the two of them combined. Here's a proof: (4) => A1Y and B3Y => A1Y and B3Y and (A2N or A2Y) // tautology => A1Y and B3Y and (B2Y or A2Y) // by (1) => (A1Y and B2Y and B3Y) or (A1Y and A2Y and B3Y) // logical operations => (A1Y and B2Y) or (A2Y and B3Y) // logical operations => (2) or (3) Now, as strange as it sounds, in a quantum world Alice and Bob can defeat that proof and prepare a strategy beforehand. When you ask Alice a question, she will consult her prepared photon in a box, and measure it in a way that depends on which question you asked. The measurement will mess up the photon's state, so you don't get to ask another question. Elsewhere, Bob will do the same with his photon. The two photons were prepared together, but are completely separated at the time of the game. Before you ask, it's provable that such tricks cannot be used to send information faster than light. Quantum "spooky action" is somehow more powerful than hidden correlations, but less powerful than outright communication. It's pretty subtle and most popular accounts don't get it right.

10y ago

Ruud-v-A 10y ago

Here’s an analogy: I put a red card in one envelope, and a blue card in a different envelope. I send one envelope to Alice (but I don’t say which one) and one to Bob. Alice and Bob could be on opposite sides of the planet. When Alice opens her envelope, she knows which colour she has, but she also instantly knows which colour Bob has -- without communication with Bob. Bob might not even have opened his envelope yet. There is no faster than light communication or magic going on.

EliRivers 10y ago
This analogy is fundamentally misleading. In this analogy, the blue card was always blue in the envelope, and the red card was always red. In this analogy, this information was set right from the start. It's really misleading.
- adekok 10y ago
  It's an analogy. The weirdness of QM comes from the following, slightly more complicated, situation. You have two cards, a red one and a blue one. You also have two envelopes. Without looking at anything, you put one card in each envelope. You label them, and send one each to Alice and Bob. Now, we know for sure that when Alice opens her envelope and looks at the card, we immediately know which card Bob has. There is no information being exchanged, so Alice knows what card Bob has immediately when she opens her envelope. The weirdness of QM comes from looking at the intermediate states. i.e. the states before Alice opens her envelope. There are (waving wands) ways to tell what the intermediate states could be, by varying certain (QM) magical properties of the experiment. The weirdness of QM is this. Every single idea you have about the intermediate state is wrong. What do I mean by that? You could say there are "hidden variables". i.e. one envelope "really has" a red card in it, and the other envelope "really has" a blue card in it. This interpretation is wrong. Every experiment shows that the envelopes are in a "superposition" of states. That is, the envelopes don't contain either a red card or a blue card. They carry... something... that is maybe red and maybe blue. That seems strange, but OK. What happens when Alice opens her envelope? When Alice opens her envelope, the card which is "maybe blue, maybe red" suddenly becomes either 100% blue, or 100% red. The weird part is the next step. When Alice opens her envelope, Bob's card immediately becomes the opposite color. This happens no matter how far away Bob is from Alice. So is information traveling faster than the speed of light? No. We knew Alice's card was red or blue, so Bob's card must have been blue or red. Lo and behold, Bob's card turns out to be one of those colors. And yes, Bob's card really is in an indeterminate state prior to Alice opening her envelope. And once Alice opens her envelope, Bob's card really is 100% either red or blue. The confusion comes from what is really happening behind the scenes. i.e. What is the real explanation? The current QM research shows that the answer is "Uh... right... we're not actually sure." Somehow the two cards are connected (QM people say "entangled"), because when you look at one, the other immediately "decides" (?) what color it is. But there's no information being passed between the cards. There's nothing exchanged between the cards. Yet somehow Bob's card "knows" that Alice has looked at her card. This is profoundly strange and confusing. No one has any idea what's going on. Physicists have many theories, but it's difficult to either prove or disprove most of them. That takes time, money, expertise, etc. So we're left with "QM is strange". As Richard Feynman said, "QM is not only stranger than you can imagine, it's stranger than you can possibly imagine."

eternalban 10y ago

I found this video [1] to be a remarkably engaging and informative (but Bohr biased) overview of this topic. Alain Aspect's comment regarding "if you just want answers you simply compute" in context of "understanding" entanglement is spot on. [1]: https://youtu.be/-LklvINk_L4 https://youtu.be/-LklvINk_L4

eternalban 10y ago
This is a much better upload of the same film: https://youtu.be/BFvJOZ51tmc https://youtu.be/BFvJOZ51tmc

tim333 10y ago

The author "makes it simple" by using a crappy get out for explaining the seeming faster than light influence in quantum entanglement: >This “spooky action at a distance,” as Einstein called it, might seem to require transmission of information — in this case, information about what measurement was performed — at a rate faster than the speed of light. >But does it? Until I know the result you obtained, I don’t know what to expect. I gain useful information when I learn the result you’ve measured, not at the moment you measure it. And any message revealing the result you measured must be transmitted in some concrete physical way, slower (presumably) than the speed of light. The thing is when the results are obtained you can write them down or similar. Unless you are presuming the written results and other records change as you mail them to each other you are left with the faster than light type paradox. I'm with Einstein on that one in that something odd is happening. My guess is that quantum mechanics is the fundamental nature of the universe and the appearance of space and time are a resulting epiphenomenon. As a result of that the particles appear far apart but are in some ways are still connected.

pwendelboe 10y ago
As I understand it though, because the result is random you can't actually transmit meaningful information faster than light. You can know what the other measurement was but because it was random it's rather useless information. But I don't know maybe I have it wrong...