susskind described entanglement in a bit different way than I'd heard - "that entanglement allows one to know everything there is to know about a system of particles (the whole), while knowing nothing about it's parts."
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"
The objects in boxes scenario you have described is useful to think about when coming to grips with QM and entanglement. However, one needs to be careful, because there is an additional subtlety: QM can have measurement scenarios that are not only entangled, but also non-local [1]. This means that it is provably impossible to endow the variables that you are measuring with prior, local assignments, e.g. like in the boxes scenario. So there is actually another phenomenon present in QM that makes it weird, and is strictly speaking distinct from entanglement.
locality is looking suspect for other reasons too, as it's one of the only ways to avoid 'black hole firewalls' [1] (the other way to avoid firewalls involves giving up conservation of information which would be bad for QM theory).
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"
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.
> the coins are constantly flipping ... the other coin stops flipping at the very same time
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).
I'm not sure this corresponds to my understanding of the wave function before observation.
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'm not sure your description of the difference between classical and quantum contradicts what I said. In fact I think they are compatible
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.
Well, then I just misunderstood you. I should probably better have quoted this sentence in my last comment.
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.
Very little about QM is intuitive at the ontological level; what matters is that it's logical at the mathematical and practical level. Lasers work, the math works, the ontology doesn't.
sure, but I'm not sure we should be saying that it doesn't have an ontology (even if it is a very weird one), just because the only model that we've been able to come up with thus far is mostly a calculational one (as QM "calculates" probabilities).
It doesn't have an ontology with a current prayer of being falsifies, which is the same thing if you're interested in the science and not the popsci. Exceptions exist, hence AdS/CFT correspondence and M-Theory, but they are exceptions. Even then, all that can be said is various tools have been created with value beyond their fields, not that they're in any way falsifiable yet.
At any rate, "intuitive" strikes me as a bad test for QM.
Yeah. But in retrospect, QM chaos seems to underlie a lot of biology, from evolution to neural development to immune response. But there's been no need to think much about QM except as a source of randomness. So far, anyway.
Fingers crossed At the very least, if QM emerges into your end of things, it should do so in clearly defined ways, and at scales/energies which have some hope of being probed.
What you're describing is a local hidden variable theory. In this case, the "hidden variable" is who has which fruit. It's "local" because there's no remote influence -- Bob eating his fruit doesn't change which one Alice has.
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.)
Ordinary entanglement is having a strange pair of coins. Toss them both and they will never come up both heads or both tails.
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)
That does describe entanglement. To say that A and B are entangled simply means the state of A is correlated with the state of B. Entanglement is only "spooky" or "weird" because (from Charlie's point of view) Bob is in a superposition of the states |Has Pear> and |Has Apple> between steps 4 and 5.
Because Bob leaves the room with either an apple or a pear and this choice is made inside the room. In quantum physics Bob would leave the room with half an apple and half a pear, Charlie would ask Bob to eat the pear, and in this moment Bob's half of the apple would teleport to Alice and Alice's half of the pear to Bob. Charlie could also have asked Bob to eat the apple.
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.
I tried to set this straight in my comment. Charlie does not get to choose the fruit, the fruit is chosen at random. I just allowed Charlie to make the choice to emphasize that the choice happens outside of the room. Had I just said the choice happens at random, then it would not be obvious why this random choice could not also be made in the room with Alice. But then we were back at Bob leaving the room with either an apple or a pear.
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.
I think, as others have pointed out, these analogies just aren't perfect enough. Do you know where I can find a data source for one of these tests? I checked a few recent papers from wikipedia but they only reported (very high level) summary statistics.
> This seems to be the key part. So when they run experiments on this, a certain result can be forced?
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.
more like, someone puts both fruit in black bags, mixes them up and gives one to alice and one to charlie. alice does not know which one she got until bob eats his fruit or until charlie sees it.
Oh cool! This experiment is very fun, not just from the results side. I was able to get a tour with the photonic side of this set-up (not the atomic side, as this paper elucidates). The photons are made entangled, then split and sent into some fiber-optic cables that run about in the tracts in the hallways. The rooms used to measure the entanglements are sufficiently far apart, but due to budgets, are at right angles from the source. We only went to one of the rooms, but the measurement devices are housed in marijuana grow pods you can buy, as they tend to keep the temperature stable, have access for the wires and cryogenic tubes, will keep out/in EM noise, and are pretty cheap. There was a line on the floor of that room, off in the corner, that had where the 'light speed' signal from the other measurement room stopped when the measurement (in the room we were in) was made, proving that the measurements could not interact. If you ever get a chance, get a tour at NIST, it is well worth whatever strings you have to pull. Getting to see The United States Second (where the US measures all of out time from) was a real highlight of my scientific career.
One think I've been wondering about spooky action is if entanglement reactions are bound to the speed of light. As in, if one particle of the entangled pair is manipulated, is there a "signal" that is "transmitted" at some speed? Or is it "instant" in the sense that it transcends lightspeed? Or does this question even make sense in this context? Quantum stuff gets weird fast since we have very little basis in which to intuit it.
The idea works like this. A pair of particles is entangled then separated to positions L and L'. At L we can observe A or B. At L' we can observe A' or B'. Until the moment of observation, it is indeterminate which. L and L' are too far apart and close in time for information to be communicated by light.
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.
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.
As far as I can tell, I am admittedly not a physicist, this is at least a contentious issue and probably, again as far as I can tell, a popular misconception. Those fields in quantum field theory are mathematical tools, one starts with particles and then defines fields to mathematically handle system where particles are created and annihilated.
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.
Just because "Particles are epiphenomena arising from fields." You are completely missing the point that the bumps in the field are what we call particles. There can be distances between the bumps/particles and that means that the bumps are "separated", because it matters whether the bumps are next to each other or far away... This is what we mean by separation. No one is saying the field itself doesn't fill all of space (which it does)
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.
no-one knows what is really "there" when unobserved. fields don't get us out of having to interpret the wave equation (the wave equation is a whole different kind of "waviness", it's not the same thing as the field); and the wave equation only lets us calculate probabilities. so we don't know. but it's true that some things don't behave like particles when we're not looking.
Although I'll certainly butcher the phrasing, my understanding is: measuring a property of one of the entangled particles lets you know the same property of the other particle (an experiment showed this happens faster than the speed of light by measuring the two particles when they were 1.3km apart). This is particularly mysterious because it seems that the property of the particle is not determined before this measurement.
> 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.)
It is instantly but you can not use it to transmit information.
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.
a side note, there isn't a universal "spin orientation detector" that reveals a spin orientation on the unit circle. You have to choose how to align the magnets so that the spin measured is either "up" or "down"; you could align the magnets any way you like, but they are fixed along an axis for the duration of the observation
One has to be careful what one means here. I meant it in the following specific sense and we have good experimental evidence for this.
[...] 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.
They are not bound by speed of light. Nothing is exchanged or transmitted, the collapse of wavefunction is genuinely instant in both places. The drawback is that you cannot choose or predict which outcome you will measure, so you can't really use it to transmit information instantly.
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
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"