NIST Physicists Show Ion Pairs Perform Enhanced 'Spooky Action'
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In the classical version of an information experiment, if i randomize placement of objects A and B into 2 boxes, and send one of those boxes to someone else - upon opening my box I instantly know whether the other box contains object A or B. The Quantum Mechanical version of the above experiment is very similar, except there could be several different degreees of freedom to measure on upon opening my box (what angle of spin measure on, etc..)
So to me, that doesn't suggest that something "traveled" to the other box, just like in the Classical version.
Rather, somehow I only "knew" the system at the beginning without knowing "any of it's parts" (due to entanglement & superposition).
Then, just observing a degree of freedom in one the parts finally reveals what the corresponding degree of freedom in the other part was.
This (intuitively) makes more sense to me than saying "information traveled" and "action at a distance"
[1] Black hole Firewalls with Sean Carroll and Jennifer Ouellette https://www.youtube.com/watch?v=_8bhtEgB8Mo&t=3634s
But exactly this is not the case, exactly this is the difference between classical physics and quantum physics. Classically you place one coin in each of the two boxes, heads in one, tails in the other. Nothing strange happens here, the outcome is determined when setting up the boxes. When you look in one of the box, you will see either heads or tails as determined by the setup. The other box will have the coin showing the opposing side, also determined by the setup.
The quantum case is very different. You again place one coin in each of the two boxes, again one heads and one tails, but this time the coins are constantly flipping. It is no longer predetermined whether you will get heads or tails and only when you look in one of the boxes does the coin in there stops flipping and you see either heads or tails. But more importantly the other coin stops flipping at the very same time so that you will see the opposing side when you look into the second box.
The correlation is in both cases established when the boxes are setup, the coins will always show opposing sides. The quantum cases of course also requires both coins flipping synchronously. But in the classical case the exact outcome is also established during the setup while in the quantum case the exact outcome is only established when one looks in the first box.
I'm not sure this corresponds to my understanding of the wave function before observation.
Just because the probabilities for each state is less than 1, to me doesn't mean that it's "constantly flipping". Only that upon measurement there's a probability that one of those states gets defined/constrained.
This is what i think susskind meant by understanding the whole (probabilities of the wave function of the whole system), without understanding the parts "the particles" i.e. it's kind of not right to think of just the parts here. It's only the system+parts that must be thought of together (which of course is why one of the reasons no-one truly understands it).
This was only meant as a classical model how the outcome is not predetermined, I did not want to imply that the wave function oscillates back and forth between two states or something like that. I could also have said that you place two coins in each box and one randomly explodes when you look in the box. And of course the corresponding coin in the second box also explodes simultaneously.
I didn't use any notion of "predetermination" when describing the quantum mechanical version. I said that "there could be several different degrees of freedom to measure on" I meant this to exactly correspond to "the exact outcome is only established when one looks in the first box".
The only notion of "predetermination" I ascribe to is that of the whole system, which is completely described by the quantum states before observation, but as i said this tells us nothing of the parts it's composed of.
(I am choosing in my description to put less emphasis on the particles/parts (which normally get all the attention), and more on the quantum system)
This (intuitively) makes more sense to me than saying "information traveled" and "action at a distance"
You can not get away from non-locality in quantum mechanics. Because the outcome is only determined when a measurement is performed, this »information« has to travel to the entangled partner when the measurement is made. And this must happen instantaneously. In the classical example you can avoid this because the outcome is already determined during the setup.
I said that "there could be several different degrees of freedom to measure on" I meant this to exactly correspond to "the exact outcome is only established when one looks in the first box".
This are three different things. You can of course decide to measure the spin along any axis you like, that is you can choose the observable. When you do this, then you are not measuring different degrees of freedom, you are measuring the same degree of freedom, the spin, in different bases. But, and that is what I meant, even if you always measure the spin along the same axis, say along the positive x axis, then the spin of an electron in a superposition of spin either +1 or -1 along this axis is only determined at the time of the measurement. Before that measurement the electron had no definite spin along this axis.
I didn't/am not claiming that the effects are local.
> only determined at the time of the measurement... Before that measurement the electron had no definite spin along this axis.
That is what I'm saying as well. I didn't claim otherwise.
So to me, that doesn't suggest that something "traveled" to the other box, just like in the Classical version.
I read this as you saying nothing travels from the measurement to the entangled partner. But this is what happens, »Someone just looked inside me and saw heads, please show tails when someone looks inside you!« travels from the measurement to the entangled partner. And if that happens instantaneously, then this is a non-local effect.
That is why I thought you wanted to say nothing travels, neither slowly nor instantaneously and therefore non-locally.
At any rate, "intuitive" strikes me as a bad test for QM.
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).
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.
> "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?
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.
The problem is that allowing Charlie to make the choice allows him to use entanglement to transmit information faster than the speed of light.
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.
Edit: I like danbruc's answer better.
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)
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.)
I love modern physics for lines like this.
Say you have a pair of entangled spins in a superposition of the two anti-parallel states first spin up and second spin down or first spin down and second spin up with equal probability for both states, i.e. each spin is up or down with a probability of 50 % but the two spins always have opposing orientations.
If you measure one of the spins, then there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured because the result of the measurement somehow had not yet enough time to reached the second spin, supposedly because this state change is limited to travel at the speed of light.
It is as if the spins had secretly picked on of the two possible outcomes when they were created but you just do not know which one until you perform a measurement. But this is not the case, this is what is called a local hidden variable theory and is ruled out by Bell test experiments. The spins have no definite orientation until you measure the first one but after the first measurement both spins instantaneously have a definit orientation no matter how far separated.
https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_m...
[...] there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured [...]
If an interpretation makes use of non-locality to explain this fact is a different matter, but experimentally it looks exactly like measurements instantaneously affecting entangled partners.
The point of the EPR experiment is that when we observe A at L we will observe A' at L'. And when we observe B at L we will observe B at B'. In other words "collapse is instantaneous".
In the many worlds interpretation, nothing is actually happening faster than light. At L we split into an A and a B observer. At L' we split into an A' and a B' observer. And then when those observers meet up, QM just describes how those observers match up to generate two consistent stories.
However this is a very, very important for theoretical physics. General Relativity is an inherently local theory. QM is inherently nonlocal. In decades of trying, nobody has figured out how to come up with a theory that bridges them.
But to add, there are at least ideas, it is not the case that we made no progress at all. I would like to highlight the ER=EPR conjecture [1] by Leonard Susskind and Juan Maldacena. It roughly states that entangled particles are connected by worm holes and that they are therefore not actually separated in space. Personally I prefer a less sensationalist sounding description, that entanglement defines what it means for points in space to be close together or far apart. This avoids constantly visualizing pairs of particle connected by glowing worm holes from science fiction movies.
My understanding is that there is a growing consensus on this
Furthermore you can still speak of particles just as you can speak of atoms, because it's useful to call the bumps something other than bumps all the time.
Think about it, think about all physics you have ever done, in school or wherever. You have probably never dealt with a situation where something was created or destroyed. You need variables to describe all the things you are dealing with, how do you add new variables or get rid of some old variables halfway in your calculation when something is created or destroyed?
This is what those fields in Fock space are for, they are mathematical tools to keep track of the number of particles you have in specific states. You do not start with a field and discover that it has quanta behaving like particles, you start with particles and just describe them with fields in order to be able to handle particle creation and annihilation.
Here is Nima Arkani-Hamed giving the Salam lectures 2012, a five day series on the state of fundamental physics. I marked [1] about 30 seconds where he is very clear about this. You can also start here [2] and watch about 10 minutes of this lecture where he quickly redevelops the ideas of quantum field theory.
Even if you can not follow all the details, you should easily be able to follow the explanation where the fields come from and why. The particles vs fields issue is explicitly discussed three or four times within those 10 minutes. You may of course want to watch the entire lecture series, it is really good and is pretty non-technical in the beginning. The last day I linked to is actually the exception if I remember correctly, the first four days are mostly without math.
Here are some quotes from a piece on the experiment I mentioned above. from https://www.sciencenews.org/blog/context/entanglement-spooky...
> Einstein insisted that the handedness of the glove must be determined in advance by some physical law. He was perplexed by the possibility that Alice’s choice of hand could have something to do with whether Bob’s glove would fit his fingers.
> Suppose you prepared entangled photons and sent them to Alice and Bob in such a way that if Alice measured hers to be vertically polarized, she instantly knows that Bob’s will be horizontally polarized. . . .
> . . . The photon does not have an orientation until Alice detects it. Same for Bob’s. But once Alice makes a measurement, the outcome of Bob’s measurement is certain.
> Here’s where a lot of confusion clouds entanglement commentary. Contrary to what you might have read in a magazine with “New” and “Yorker” in the title, Alice’s measurement does not “instantaneously” influence Bob’s photon. No signal is sent, no influence transmitted. For all Alice knows, Bob might have measured his photon first. In fact, if the measurements are made at nearly the same time, there might be no objective way to say who made the first measurement. (A space traveler flying along at nearly the speed of light might see Bob’s measurement first, while another traveler flying in a different direction would see Alice’s first.)
By all apparences, it seems to violate causality. It's one of those things that make you suddenly stop and bemusedly wonder if maybe we've finally found a kludge in the Universe's code.
edit BTW, if you'd like to know more about this experiment and why it has ridiculous implications, check this PBS clip: https://youtu.be/8ORLN_KwAgs