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Comment by jacknews | Hacker News Reader

I find the popular descriptions of entanglement to be unenlightening. In this case we go from "a pair of socks" to an "unsystematic phenomenon that occurs simultaneously in two locations".

What? Where did the socks go?

My preferred analogy to show that the universe is weird is a pair of coins. You throw one to your friend 10m away, and toss the other in the air for yourself. Your's lands 'heads', while the coin to your friend is still tumbling through the air. At that point you can be sure that your friend's coin will land 'tails', even though it hasn't landed yet, since they are correlated (to land opposite). Somehow the coins 'communicate' through space and time, or act as though they're parts of a single entity. There's no mechanistic explanation for how this happens, it's just the way the universe appears to work. That's the mystery, and quantum mechanics at least allows you to calculate it, if not really understand it.

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This is the best explanation of entanglement that I've read, and I think I finally "get it".

It also makes me think that the problem is not in understanding "why is Coin1 correlated with Coin2", but in finding the latent variable that Coin1 and Coin2 are both correlated with. Is that something being researched currently?

> Is that something being researched currently?

Sort of, but actually more no.

The way the equations in quantum physics work, this just happens, and there isn't any accounting for distance or space. That's just how it is.

Now, that seems to contradict our intuitive understanding of physics. Things can't be just linked like that without some force or information traveling between them.

So there are a few different interpretations of that quantum weirdness:

1. That's the way things really are, and our natural perception that things don't work that way may have helped us evolve to this point, but is actually wrong on the quantum scale

2. There is a hidden variable (the communication part, like a pilot wave) and we just haven't found it yet.

It has to do with the combinations of states in the observable you are measuring (in the photon polarization example above, it would be spin or angular momentum). You have to have some way of setting up the system in the entangled state that you want.

Your belief is equivalent to a “hidden variable” theory, that the coins have some property stored in them that ensures the outcome is predetermined.

Unfortunately, while that theory is very intuitive, it’s also been conclusively proven wrong through statistical methods.

Instead of a 50:50 chance of landing between two states, it’s actually closer to throwing three pairs of entangled coins at your friend and into the air, but both of you only having the dexterity to catch one. Any remaining coins scatter on the ground and in doing so lose their entanglement. For the sake of identification, let’s say all three pairs of entangled coins are different colors. You choose to both catch coins of the same color, and you see that they show opposite sides 100% of the time. Great! The analogy still holds.

Here’s the problem. Now you start just catching a random coin from each set of three and collecting the results. You do this many times. Let’s assume there’s a hidden variable. In each set of coin throws, there are eight possible predetermined outcomes. Two of these outcomes are that one of you catches a coin from a set predetermined to be TTT while the other catches a coin predetermined to be one of HHH. The other six outcomes are equivalent to one of you catching from a set of TTH and the other catching one of HHT. There are no other possible sets of hidden outcomes.

In the first two instances, you are 100% guaranteed to get a matching Heads/Tails pair by catching coins at random. In the remaining six, the two of you will record a matching pair 5/9 of the time (2/3 of the time you have 2/3 chance of a match, 1/3 of the time you have 1/3 chance of a match).

This means that well over 50% of the time you will catch matching pairs of coins. This is an inescapable consequence of any hidden variable theory. However, in practice, it turns out you have matching pairs exactly 50% of the time. There’s no getting around this. You perform the experiment hundreds of thousands of times, and your long term average is 50%. Given the above probabilities, there is no possible way for there to be a hidden variable predetermining the outcome. As far as we can tell, the only thing that explains the 50% rate of matching HT pairs is that at the moment a coin is caught and observed, they both spontaneously choose opposite sides to display.

> There's no mechanistic explanation for how this happens, it's just the way the universe appears to work.

Do you mean that a mechanistic explanation isn't known yet or that one doesn't exist?

I suppose, define "mechanistic", but one has not been discovered yet AFAIK, I could be wrong.

There are also many constraints (eg Bells Theorem) on what such an explanation could look like.

One thing to add to your explanation because there is ALWAYS someone who ask: this "spooky action at distance" cannot be used to communicate faster than light.

To add to this:

- Even if you throw the coin to your friend at the speed of light, you can still know what your friend's coin will land on before it happens. (Picture your friend a million light-years away, and you tossing your coin a few feet up. You look at yours to see 'heads', and look into the distance knowingly.)

- Even though you know this information, you can't tell your friend ahead of time what their coin will land on, because that information can't travel faster than the speed of light (which the coin is also traveling at).

- This holds even if you throw a coin at a super-slow speed to your friend and then transmit to them what their coin will land on: they'll know before it happens technically, but the information itself still travels at the speed of light.

But what if you simply change the State of a particle - as a method of communication?

Is that possible? To change the state of a quantom entangled particle? So that the other user can perceive the state change instantaneously? I guess not but it wasn't clear to me.

I believe the caveat here is that once either particle is observed, both are "constantized" into their final states (and manual changes to either no longer affects the other). For example, until you look at your coin, it could either be heads or tails (and your friend's coin could also be tails or heads). Once you look at it and see that it's heads, it'll forever be heads -- and your friend's coin will forever be tails, regardless of whether they look at their coin when it arrives.

The opposite is true: if you don't look at your coin and your friend does, then tells you his coin is tails, you can know your coin is heads. You can't change it to be tails and tell your friend he must've seen wrong -- his is still tails and yours is still heads.

Obviously that's a super-simplified version (because I don't understand it all myself), but the gist is: once either entangled particle is observed, the bond between the two is broken and any manual modification of your coin (like flipping it over manually) no longer has any effect on your friend's coin, or vice versa.

Seems obvious to me there is a mechanistic explanation our perception is incapable of realizing. We can see the results so there is hope.

> Seems obvious to me there is a mechanistic explanation our perception is incapable of realizing. We can see the results so there is hope.

'Hope'? Confirmation bias primes people to see what they want to see.

You just described a zero sum equation. If you have 10 and you take away 7 the other value must be 3.

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