5 ms·

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 t

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."
  - EliRivers 10y ago
    It was a very wrong analogy. I could say that a barn full of pigs is an analogy for a car engine. Just saying its an analogy doesn't fix it. An analogy is meant to help people understand something; the analogy presented helped them misunderstand. You've done a lot better in your correction of it, but in doing so you've had to basically render the analogy worthless. There is no analogy for what's happening. It's simpler and clearer to just state the facts.
  - ddlatham 10y ago
    How can you tell the difference between Bob's card being in an indeterminate state vs. being in a determinate state that is not yet known? Or what is the difference?
    - adekok 10y ago
      That explanation requires a deeper understanding of QM which is difficult to explain via analogy.
      - EliRivers 10y ago
        Oh God yes. Two topics analogies should never be used for when attempting to impart understanding; QM and pointers.
  - Luc 10y ago
    > As Richard Feynman said, "QM is not only stranger than you can imagine, it's stranger than you can possibly imagine." Don't think Feynman said that. It's kind of the antithesis of 'the pleasure of finding things out'. It seems to be a quote from an essay by JBS Haldane. 'The Universe is not only queerer than we suppose, but queerer than we can suppose.'
  - Chathamization 10y ago
    > 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. That's not necessarily true, there are perfectly valid deterministic interpretations of quantum mechanics.
Chathamization 10y ago
It's more akin to taking a pair of pennies and giving one to two people in different houses, both of whom have their eyes closed. If the two people immediately open their eyes and look at the penny, they find the two pennies match 80% of the time. OK, no problem, someone sets them so that they match 80% of the time before giving them out. But if they flip the pennies over before opening their eyes, the pennies match only 50% of the time. So clearly something is effecting the state of the pennies after they've been separated. My understanding of Bell's inequality is that experiments had two electrons traveling in two different directions, through two different measuring devices measured ended up being measure as spin-up or spin-down when measured at different angles. There were certain levels of correlation when the measurements were done at different angles, but those didn't fit when compared to other correlations (IE, you couldn't have 90 and 45 degrees match 85% of the time, 45 and 0 match 85% of the time, and 90 and 0 match only 20% of the time). Worth noting that this doesn't necessarily mean, as some claim, that "the cards" aren't 100% red or blue at any given moment in time. There are certain valid interpretations that are deterministic (De Broglie-Bohm, for example). I believe Bell was in favor of deterministic interpretations.

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."
- EliRivers 10y ago
  It was a very wrong analogy. I could say that a barn full of pigs is an analogy for a car engine. Just saying its an analogy doesn't fix it. An analogy is meant to help people understand something; the analogy presented helped them misunderstand. You've done a lot better in your correction of it, but in doing so you've had to basically render the analogy worthless. There is no analogy for what's happening. It's simpler and clearer to just state the facts.
- ddlatham 10y ago
  How can you tell the difference between Bob's card being in an indeterminate state vs. being in a determinate state that is not yet known? Or what is the difference?
  - adekok 10y ago
    That explanation requires a deeper understanding of QM which is difficult to explain via analogy.
    - EliRivers 10y ago
      Oh God yes. Two topics analogies should never be used for when attempting to impart understanding; QM and pointers.
- Luc 10y ago
  > As Richard Feynman said, "QM is not only stranger than you can imagine, it's stranger than you can possibly imagine." Don't think Feynman said that. It's kind of the antithesis of 'the pleasure of finding things out'. It seems to be a quote from an essay by JBS Haldane. 'The Universe is not only queerer than we suppose, but queerer than we can suppose.'
- Chathamization 10y ago
  > 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. That's not necessarily true, there are perfectly valid deterministic interpretations of quantum mechanics.

Chathamization 10y ago

It's more akin to taking a pair of pennies and giving one to two people in different houses, both of whom have their eyes closed. If the two people immediately open their eyes and look at the penny, they find the two pennies match 80% of the time. OK, no problem, someone sets them so that they match 80% of the time before giving them out. But if they flip the pennies over before opening their eyes, the pennies match only 50% of the time. So clearly something is effecting the state of the pennies after they've been separated. My understanding of Bell's inequality is that experiments had two electrons traveling in two different directions, through two different measuring devices measured ended up being measure as spin-up or spin-down when measured at different angles. There were certain levels of correlation when the measurements were done at different angles, but those didn't fit when compared to other correlations (IE, you couldn't have 90 and 45 degrees match 85% of the time, 45 and 0 match 85% of the time, and 90 and 0 match only 20% of the time). Worth noting that this doesn't necessarily mean, as some claim, that "the cards" aren't 100% red or blue at any given moment in time. There are certain valid interpretations that are deterministic (De Broglie-Bohm, for example). I believe Bell was in favor of deterministic interpretations.