Quantum ‘spookiness’ passes toughest test yet
nature.com
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https://news.ycombinator.com/item?id=10130203
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https://news.ycombinator.com/item?id=10135260
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https://news.ycombinator.com/item?id=10139598
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Is that an accurate (given that it's an analogy) description of how entanglement works, or does this work disprove that idea and is what was mentioned as Einstein's idea?
Now, considering two photons, you don't know which basis Alice or Bob will choose in advance. If you just say "the photon is polarized horizontally", then their outcomes will be random if they measure in the "X" basis. If you include so-called "hidden variables" for all possible measurements, then you're basically at Einstein's idea.
(I've simplified some things above, so please don't quote me on it ;)
The phenomenon of entanglement is that measurements of certain things are either perfectly correlated or anti-correlated. Eg. Measurement of spin along some axis on electron A will always be -1 when measurement of spin along the same axis of electron B is +1.
The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin, and give one to you randomly, then you go a million light years away and look at your coin and find you have the 1973 coin, you will know I have the 1972 coin. Although this is only a simplified analogy of entanglement the important point is that the state was prepared before we separated, and no information has been communicated. When you try to understand this in terms of classical physics, it's weird. But it's not classical physics. I wrote more about it here:
https://news.ycombinator.com/item?id=10133674
Also check out the leonard susskind messenger lectures for a quick overview and his stanford continuing ed lectures on youtube for the long version :)
If that was the case (the coin analogy) it would be perfectly understandable by classical physics.
In fact it wouldn't even need physics to understand it, just common sense.
If that was the case there wouldn't be any EPR paradox etc.
The coin analogy is very misleading in this way -- and doesn't convey the problems physicists have had with explaining this phenomenon.
The actual mechanism by which this occurs is only "spooky" if you try and interpret it classically. But if you just accept that "hey, it's quantum mechanics" then it's very easy to understand. Same as I can't visualise a 5-sphere but I have mathematical tools to work with one.
I can't "picture" the mechanism that allows measurements of quantum states to be anti-correlated, but I can use maths to express that behaviour and make predictions about how the universe behaves.
I think that if the explanation doesn't also convey the spookiness, it's not right.
>The actual mechanism by which this occurs is only "spooky" if you try and interpret it classically. But if you just accept that "hey, it's quantum mechanics" then it's very easy to understand.
That's just pushing the spookiness under the carpet.
The mathematical meaning of the statement "two things are entangled" is that you cannot factor the states into the product of two separate quantities. So there is only one equation for the two particles. So once you "collapse" this for one particle (whether or not collapse is a physical action) it is by necessity collapsed for the other particle, because there is no other state that could not be collapsed.
The similarity to 2 coins is: if i have a 1972 coin and a 1973 coin
The two coins metaphor illustrates the anti-correlation of two particles and why that doesn't allow faster-than-light communication. By observing one, you can deduce the other.
Of course it isn't a metaphor for the whole process, such that you can derive implications from the metaphor that will hold in the quantum case. But the point of the 'spooky action at a distance' is that there is no metaphor based in classical physics that can do so.
So physicists, and lecturers (in my limited experience) use metaphors to explain certain parts of the behavior.
It's like when a physicist talks about a quantum being a wave or a particle. Both are metaphors from classical physics. It is neither, and both metaphors only illuminate very specific properties.
The approach of saying 'that metaphor has been proved wrong', just leaves us with no way of explaining stuff. It is worth keep reinforcing that all metaphors are wrong, but your response seemed overly-negative to me.
>The measurement of one half of an entangled pair doesn't modify the probability distribution of the other half of an entangled pair.
That's just saying it's local. Not relevant.
The only for it to make sense classically. It doesn't need to make sense, it just is. It's how QM works, you can't understand it by some classical mechanism.
Basically this:
https://en.wikipedia.org/wiki/Hidden_variable_theory#Declara...
Take a coin and split it in two. One side has heads, the other has tails. The key thing is you don't look at the two halves.
Now, take one half at random, and send it to a person very far away. Too far away to communicate easily at light speed.
Then, have the other guy look at what he has. He will (of course) determine that it has either heads or tails. Which means (of course) that the one you have is the opposite, either tails or heads.
Nothing weird yet, right?
The quantum weirdness comes in when you start doing variations of the tests to see what state the coin halves are in before you do the final measurement.
It turns out that any model you can think of (hidden variables, etc.) has physical consequences. When you look for those physical consequences, you don't see them.
What does that mean?
In this case, your coin half and the other coin half are in an "indeterminate" state. They both have some properties of "heads" and they both have some properties of "tails". But their state is not determined. It really is an unknown state.
And why move one half far away?
Because you can do tricks where you both measure the coin halves at the same time, or near the same time. If (somehow) one measurement sent information from one coin to the other... that information has to transfer at light speed.
But it doesn't. When one half is measured, the other half instantly becomes the opposite state. So far as anyone can tell, this process is instantaneous. And any "communication" between the two halves has infinite speed.
The difficulty here is setting up systems so that they're isolated, and nothing interferes with the test. Then, doing all of the little measurements testing the edge conditions of your theory.
In short, everything you think you know is wrong. The coin halves really aren't anything until you look at one of them. Then, both of them instantly become determined as heads/tails.
"The researchers started with two unentangled electrons sitting in diamond crystals held in different labs on the Delft campus, 1.3 kilometres apart. Each electron was individually entangled with a photon, and both of those photons were then zipped to a third location."
> To get around this, the researchers started with the diamonds over a kilometer apart in two different labs at the Delft University of Technology. They entangled each electron spin with a photon, then sent the photons over fiber optic cable to a third lab somewhere in between. There, the photons were entangled, which in turn caused the electrons in the diamonds to be entangled as well.
> The whole process was horribly inefficient, with a success rate of 6.4 × 10−9, mostly because photons kept getting lost in the fiber optic cables. But, over the course of nine days, the setup managed to successfully entangle the nitrogen vacancies 245 times.
Some time ago I thought how strange these all would look to someone who had no knowledge in physics at all. But then I got it: two particles in different galaxies being entangled is not much different from calling someone on the other side of the globe using a cellphone if you don't have prior knowledge/beliefes about either.
It's easy to say "come up with better explanations". It's not very helpful, though.
The purpose of criticism is not to be directly helpful by proposing better solutions, but to help eliminate the wrong ones.
And what is more important - it's very hard to judge if the explanation is rational. Every science is built on some axioms which define what is rational and what is not. That's why I guess the progress happens step by step (each next step expanding what one would consider rational).