Can someone explain to me how photons are directed from one place to another without interference with (i.e., "observation" by) other atoms? This seems impossible, but then it pops up the question: why is this interference (which seems to qualify as "observation") not a problem?
Glass (especially the stuff they use in optical fibres) is pretty good at not interacting with photons, which is why it looks like glass.
Why doesn't glass interact with photons? Because it has a large band gap. There are no excitations to be had at optical wavelengths, so there's no way for the photon's energy to be transferred to anything else.
Some fraction of the photons used in this experiment will nonetheless have been absorbed along the way, but that's not a problem, we only count ones where we have a pair.
A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original. The few that remain absorbed will slightly warm up the glass which then regains thermal equilibrium by throwing off a photon at a much longer wavelength in a random direction.
> the new one is absolutely indistinguishable from the original
This is where I get stuck, because no one explains this. How do we know this is true? Is this because they match in all the parameters that we're aware of (say, momentum, spin, whatever), or is this because it has been proven (through a Bell-like test) that there cannot be any other parameters that we are potentially unaware of, and hence if they match in the known ones, then they are one and the same?
The reason we know the photons are indistinguishable is because quantum mechanics deals with configurations of particles at locations, and these configurations predict different outcomes depending on whether "photon A here and photon B there" is a different configuration than "photon B here and photon A there". If A and B are distinguishable, the configurations aren't the same, and you'll notice real quick when your experimental results don't line up with expectations based on them being the same. Mathematically it's as simple as the fact that x^2 + y^2 != (x+y)^2 for all x,y.
(Would appreciate if downvoters explained whether they downvoted because my understanding and thus explanation is bunk (hey, I'm not a physicist) or because they're generally against LessWrongthink-peddling...)
There could be other parameters but since we're not aware of them to us the photons are indistinguishable.
It's in a nutshell why your mirror image still looks remarkably like you. It's all 'brand new' photons moving in a new direction but they carry all the properties of the originals, the only thing that is different is that there are fewer of them because some of the absorbed ones never made it to re-emission because their interaction with the mirror substrate caused them to stay there.
No, it doesn't mean that there is no magical hidden property that actually distinguishes them, but that as far as we can tell they're indistinguishable. Given that nothing about the photons is different they must be the same right?
For more confusing theory, look up the single-electron universe.
Yup I've heard about the single-electron universe.
Okay, so doesn't what you said imply there could very well be a hidden variable that we're unaware of, that affects the two entangled photons as they are separated? What's ruling this out?
What Bell tests, of which this one is the best implementation so far, rule out is local hidden variable. Hidden variable that can propagate faster than light is not ruled out by the test. (For the most people, it is ruled out by being faster than light.)
A new photon is emitted after absorption by the old one, the new one is absolutely indistinguishable from the original.
Isn't this a lossless process of the sort forbidden by thermodynamics? What keeps us from creating a system where "identical" photons perpetually bounce between two electrons?
> Isn't this a lossless process of the sort forbidden by thermodynamics?
It is lossless on the single-particle level, but not in large thermodynamic ensembles of many, many particles. There, only most photons interact losslessly, while some get absorbed nonetheless, heat up the glass and give you thermodynamics.
How exactly unitary quantum mechanical time evolution transforms into ergodic, classical and thermodynamic time evolution in large systems is (relatively) open question.
The thing is that you can have essentially static periodic systems, where in entropy do not go down but simply don't increase. Overall in the universe entropy will however increase on average.
Replying to myself, more awake. That had an error in it, it should have been 'after absorption of the old one'. Remember to pay more attention when posting tired.
You're kind of there, but I think it's helpful to drop the notion of wave-particle duality you have since it's confusing you. Watch Feynman's videos about it (or just read his book called QED) if you want clarification, but I think it's best to for the most part think of the particles described in particle physics as particles with special rules and behaviors that wouldn't make sense if they were just tiny spheres, avoid thinking in terms of "waves", and learn about those special rules as you can. Anyway one way you can think of the direction is that it's on the most probable path based on the weighted sum of all possible (even absurd) paths through the local area, see https://en.wikipedia.org/wiki/Path_integral_formulation. At the classical level, absurd paths cancel each other out, and in general the path a photon takes from A to B will be along the curve representing the least amount of time needed. (Hence the "bend" of light as it enters water since photons propagate through water at a different speed.) Whether one actually observes the photon going from A to B depends on that configuration's probability.
What he said. Remember that ‘classical’ waves (water, sound, strings…) are all just large-scale assemblies of tiny particles interacting with each other locally.
Isn't this just because not all of the laser is perfectly parallel? The household laser is created by exciting atoms to emit photons in a cylinder with mirror caps. One of the caps is 'perfect', the other is 99% reflective. The photons go in random directions, but will bounce around within the cylinder until they get absorbed (and re-emitted in a random direction). Photons that are (near) parallel to the cylinder have a much higher chance of going through the imperfect mirror. Thus what shoots out the end is a (near) parallel stream of photons.
I have a question that's slightly off-topic from what OP asked. Is it possible to know which particles are entangled? If I have a pile of particles, can I say that a select group of them were still entangled while others are not?
Another question along the same lines: If I give my friend one of my entangled particles, can I tell if he has measured the spin by examining my half of the pair of entangled particles?
No to both questions. Both would allow you to violate the speed of light if it was possible.
Once you measure a particle it is no longer entangled, so if you could tell if a particle was entangled or not then you can violate the speed of light by instantly knowing an action taken on the other particle.
So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled? This outside observer would simply observe that the particle has spin "up" or "down" but would never know that it was entangled earlier by someone else. This observer would never know it was in a superposition unless they had knowledge that someone had previously done this. Only the person who entangled the particle can clue the observer in on the fact that they were entangled.
I can give you a particle, tell you it's in a superposition but you'd have no way to verify this. If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not. You only know about its current state. There is nothing in your observation that shows "hey the particle just changed from superposition to spin up"
Thanks for the replies! I watch "word science festival" and similar videos and I am fascinated by the quantum world but I have wondered these questions for a while and didn't know anyone who could answer them.
> So, then is it correct to say that an outside observer can never tell if a particle he/she is looking at is actually entangled?
Correct. Entangled means these two particles have correlated measurements. If you don't have that second particle then there is nothing to correlate with.
> This observer would never know it was in a superposition
Superposition and entangled are two different things.
A particle can be in a superposition and not be entangled. (But not the reverse.)
> I can give you a particle, tell you it's in a superposition but you'd have no way to verify this.
> If you observe the particle, perhaps you'd see it's in spin "up" but that would be meaningless because that observation wouldn't tell you anything about whether it was entangled or not.
Crucially, from an experimental point of view, we can tell whether a pair of particles was entangled by measuring both of them and comparing the results. To ensure that nothing else is messing with these results, eg. that there's no signal travelling between the particles, we can move them very far apart and arrange that each measurement takes place at almost the same time; that way, if any such signal existed, it wouldn't have time (even travelling at the speed of light) to allow the first measurement to influence the second measurement.
Importantly, each person doing the individual measurements has no idea what the other person's result is. They only know what they themselves have measured. In order to compare the results, the experimenters must either meet to compare notes, or send a message (eg. via radio) telling the other. Hence, even if the entangled particles interact instantaneously, there's no way to use the result of a measurement without waiting for that subsequent radio message from your collaborator, and that message will only travel at the speed of light. The quantum interaction has not sent any information faster than light.
Thanks for the reply. I've been doing well with understanding the implications of the test and the after effects but I struggled with understanding the state of the two particles themselves. Namely, the trouble was with understanding that no one but the people who entangled the particles are actually aware that they are entangled. There is no way for a stranger to look at two particles (given they were entangled by someone else) that those particles are entangled. They'll look like two normal particles. The answers here have helped me understand this.
I think all particles are basically entangled. When you get a photon from the sun it's probability of it being in a given state depends to an extent on the state of all the other photons coming out of the sun. But it's too complicated to figure and so appears random. It's only in simple experiments where exactly two particles are entangled and don't interact with others that you can figure out and notice the effects.
theorems and bad assumptions. first: speed of light is derived from 2 energy states and affects ONLY matter with rest mass. Once you understand all the energies involved it becomes clear where the GR effect comes from.
Einstein never said that speed of light is necessary for virtual particles.
Photons are NOT particles but waves with boundary condition and therefor appear particle like.
Photon entanglement is based on a string like (has 3 dimensional space of course) energy state in in our vaccum, but I can't explain here the BSM-SG photon model. Read it for yourself.
Interaction in general with the photons doesn't effect their polarisation and so the polarisation state remains unobserved. It's only when you check their polarisation that it is 'observed'. (It's not so much observation as a reduction in the possible states. When the photon is heading down the fiber it could be in any state and has a probability amplitude for being in various states. After it goes through a polariser and into a detector it's definitely in one particular state so you don't get the multitude of probability amplitudes).
Could someone please explain, how can they know beforehand the other electron in the other diamond is the first one's "sister"? What if those two are NOT entangled and will have identical spins? What defines entangled electrons? Is it a must that once they were e.g. in the same atom? Thank you. (I only completed 2 basic University physics courses which isn't enough :D )
They use a process called "entanglement swapping" that allows you to create entanglement between two particles that have never interacted. It's described partway through this article:
I think that if you had non entangled electrons you wouldn't get the 'spooky' result. You'd just get random electrons arriving without anything statistically significant of interest.
Why is it that none of these experiments seem to consider the "many worlds" understanding of "spooky action at a distance" (which is non-spooky, and action only at immediate contact)?
Roughly: both the samples are in both world A (up/down) and world B(down/up), and when you interact with them, you get split and are pulled into both world A and world B, and each copy of you when you interact pulls the thing you interact with also into A and B, and so when you compare results, the A-you compares the A-result and the B-you compares the B-result.
The splitting is not immediate or faster than light. But it moves at exactly the speed of information, so you can't race ahead of it and watch it happen. To interact with it is to become part of it.
Right, exactly. I experience great ennui when people discuss this stuff via popularizations.
Let's start a thread where we talk about how tcp/ip works ... but with a twist. The only admissible references in the discussion are color pictures aimed at non-programmers. Anybody tries to talk about what actually works, we label them the "shut up and program" crowd and decry their lack of imagination.
But it does subtract something from the discussion. It subtracts spookiness, it subtracts action at a distance, and it subtracts anything going faster than light.
And it adds this whole world-collapsing unnecessary machinery that has no practical applications or physical evidence. I myself would prefer Bohmian mechanics if we had to choose a single interpretation. Moreover, it's possible to define a non-stochastic version of the theory (no gods playing dice), which would explain QM somewhat like Classical Mechanics explains Thermodynamics; and it's falsifiable by looking for non-equilibrium conditions (admittedly those conditions will be very hard to achieve).
My understanding was that MWI allows superpositions of macroscopic objects (the universal wavefunction), whereas the Copenhagen interpretation doesn't (macroscopic objects cause wavefunction collapse), which is in principle testable [1] (although not with current technology).
a) Nobody said "unforeseeable". Macroscopic superpositions are entirely foreseeable, we just don't have the technology to keep them in a coherent state.
b) The link only says that some people have argued that position. I think all they're saying is that "wavefunction collapse caused by some arbitrary division between microscopic and macroscopic objects" is a stronger claim than MWI
Many worlds interpretation: adding approximately zero explanatory value, at the low low cost of infinite parallel universes.
I think the best way to understand MWI is that a lot of physicists/philosophers are more comfortable with the idea of inhabiting an infinity of forking but "real" (essentially classical) universes than they are with inhabiting a single "unreal" universe in which QM is taken seriously.
That's a pretty uncharitable interpretation of it. If you look at the actual physics, MWI is incredibly simple: the wavefunction just evolves as the wavefunction evolves. That's it. There's some amplitude over here, and there's some amplitude over there, and this possibility and that possibility are mutually exclusive etc (with some "weird" quantum correlations, of course.)
The Copenhagen interpretation's "wavefunction collapse" is just like a Bayesian update -- the wavefunction you're left with is like a conditional probability. Maybe it makes you happy to throw away the "other universe" bits of the wavefunction and normalise probabilities on your own branch, but you obviously don't have to, and any discussion of when the collapse happens (or worse, whether it's a physical process that propagates across the universe faster than light) is obviously very silly business.
Any other intepretation requires new, undiscovered physics. You have to make additional assumptions in order for the equations of quantum mechanics not to give rise to many world.
Because standard academic discourse in physics journals is not aimed at providing intuitive explanations, it is aimed to provide "shut up and calculate" results and try not to spend much time on questions of what is real. It wouldn't surprise me if the "shut up and calculate" crowd was the dominant object-level belief among physicists, even if most of them could agree that by just assuming quantum mechanics applies universally, that is to say the universal wavefunction is real, then that implies MWI and any collapse postulate on top of that is an unneeded detail stripped by Occam's Razor. To the ideologically motivated this might be frustrating because for the MWI proponent (I'm sort of one) framing everything with MWI seems to simplify a lot of layman explanations. But physicists in the field don't need those explanations framed that way, they need the math and data and engineering setup which I'm not sure is any easier to learn and comprehend and apply if your interpretation belief is one way or another. I'm mainly pro-MWI-for-laymen (practicing physicists can believe what they like, disbelieving can be helpful too if it motivates them to find experimental falsification!) just because MWI trivially dispels pretty much all the pop culture misconceptions about quantum mechanics.
An extension of this is "shut up and test it". MWI has an experiment. I'm ambivalent about it being feasible since on my (layman) reading it sounds like you need to turn back time by proxy to execute it.
I'm not sure your explanation for the lack of spookiness in many worlds works. The essence of most Bell's Theorems experiments is you start with entangled photons (or similar), they travel a long way apart and then go through polarisers at either end and into detectors. The odd thing is that the number getting through both slits depends on the angle between the polarisers. But how does the photon at polariser 1 know what the orientation of polariser 2 is at the time it's pair photo gets there? You can spin the polariser and the chances of it getting through depends on the orientation when the photon gets there, not when it set off. You'd think it can't but experimentally it does. I don't see how that weirdness goes away with many worlds.
(Bell's theorem is basically maths to figure out if the photons have to 'know' the angle between the polarisers rather than just say start off with the same polarisation.)
For as far as I can tell, the solution is that whenever there is a choice to be made, all choices are made and you get separate branches/parallell universe bubbles that expand at the speed of light, interacting (or not) with other universe bubbles. The bubble in which entangled electron A had spin up interacts with the other in which electron B had spin down and vice versa.
So essentially you have an ever growing set of merging and splitting history branches, always growing for every point in space, continously.
I've got no idea how to make that work for your example of polarizers, or for the example of delayed quantum eraser experiments, etc... The surface of the bubbles apparantly have to carry all of that information and there must be some mechanism for them to decide which ones interact with which others.
They aren't exactly bubbles expanding at the speed of light (necessarily). They are more like: when you get a particle interaction whose outcome is determined by a fact which has more than one answer, you get more than one outcome. And that applies recursively as those particles whizz off and interact with anything else.
It's possible though unlikely that they could be expanding no faster than the speed of a walking scientist - if nothing else in the universe is yet causally determined by the outcome.
Many worlds makes it clear how it can't: because there is no FTL transmission. There is only stuff that has been pulled into the superposition (seeing a definite answer) and stuff that hasn't yet (for which both answers are still superposed). When the latter comes in contact with the former, it enters the superposition, which from the outside perspective is a split. From the inside perspective, it looks like "the superposition collapsed". From the outside, one side gets "open" and the other gets "closed". From the inside, this looks like getting a random answer.
You can see intuitively why this can't convey information. You always get both answers (in different "worlds").
That doesn't really explain the mechanics of the splitting process.
It's "And to explain this, we postulate that a whole new universe appears fully formed out of nowhere, with an identical history but a distinguishable future, because it does."
I suppose it's possible. But it doesn't seem very parsimonious - because you effectively have to double the energy in the multiverse for every distinguishable quantum event.
Does conservation of energy not apply in the multiverse? Are universes - as many as you need - free at point of sale?
Because whether the spin is up or down when you observe it is completely random. Both observers at either end would believe they looked at it first without any other information, and the stream of bits would be completely random AND 100% correlated with other.
Ok, so you will always have perfect correlation and since you spun the polarizer after the photons started moving that implies spooky action at a distance.
Because interpretations are just that, interpretations. They are intended to let us understand it intuitively, but the math behind them is always the same.
Interpretations affect how you apply that math to model reality, and hence the predictions you make. Math does not make any predictions about the outcome of an experiment on its own, it's just a formal system.
It doesn't make any different predictions from the Copenhagen interpretation. That makes the question of which of the two (or of the various other interpretations) is "right" untestable, and therefore unscientific.
I'm not sure any of the interpretations of quantum mechanics are actually distinguishable in any way we can measure, so the best thing to do would probably be to pick the one which makes things easiest to work with.
I think Occam's razor is the cause. Many worlds suffers from the appearance of violating Occam's razor. Of course Occam's razor is just a rule of thumb, but it seems to inhibit physicists being comfortable with the many worlds interpretation.
Those words provoke little controversy when spoken about matter beyond the visible universe. Why? Because the simplest mathematical description we have of the objects we can see also includes those which in principle we cannot. We grant them the same ontological status as the Earth we stand on -- few would claim them as mere fictions to ease our task of prediction.
Occam's Razor properly applied limits the amount of information in the description, not the amount of things the description describes. For example, pi's infinite decimal expansion is spared by this information theoretic razor. If you reject the multiverse, your full description of the universe must contain one bit for every binary "choice" the universe made, every qubit that collapsed. Quite a few bits that turns out to be. The multiverse description suffers no such flaw, as it contains only the initial conditions and the rules by which those conditions change over time.
It doesn't create worlds. There's always only exactly one fuzzy-quantum-thing. It's just that some parts of it no longer interact with other parts of it.
It's amazing how many people think that Quantum Mechanic's "two states" (e.g. in the Schrödinger's cat example [1]) just mean "it's actually in one state, and we just don't know which until we look".
You don't need quantum mechanics for that. That's an ordinary cat (dead or alive) put in a box by someone else without telling you, e.g. the very classical, and intuitive "I don't know yet" thing we deal with in everyday life -- which poses absolutely no problem and no questions about the nature of the universe, the kind quantum mechanics do.
The "two states" in QM is a different beast. It's not "a box we don't know yet what's inside".
[1] Well, actually all possible states, but let's stop at the simplified 2 state cat example.
> It's amazing how many people think that Quantum Mechanic's
> "two states" (e.g. in the Schrödinger's cat example [1])
> just mean "it's actually in one state, and we just don't
> know which until we look".
Why is that amazing? Seems like the absolutely most simple and logical conclusion. In the Schrödinger's Cat example, there the only reason to assume the cat is in a dual state is if you want to make a more complex interpretation of the situation.
The cat was first invoked as a kind-of-mocking thought experiment and a point of critique towards (the Copenhagen interpretation of) QM, IIRC.
(The answer to the thought is that yes, indeed, the cat would already be in a single state prior to observation, because the large macro system would have decohered and collapsed into a single state prior to an observer checking the status of the system.. unless I'm mixing something up.)
>Seems like the absolutely most simple and logical conclusion.
It's simple, logical and wrong.
The idea with the cat was to magnify (in the macro world) a quantum phenomenon from the micro-world were the state of the object is absolutely and measurably NOT "either/or". It's both (all) states at the same time.
>In the Schrödinger's Cat example, there the only reason to assume the cat is in a dual state is if you want to make a more complex interpretation of the situation.
No, the reason to assume the cat is in a dual state is that the particle determining its fate IS in a dual state (superposition), a state contrary to everyday intuition.
Now, you could say that the cat is dead or alive in some split one of "infinite universes" or a few other explanations, e.g. that we have collapsed to a single state earlier for some reason, but not the crude dead-or-alive-we-just-don't-know-yet thing in a single physical universe...
It doesn't matter if it is wrong, it is still the simplest explanation to Schrödinger's Cat. So there is nothing amazing about people arguing for that view.
Saying that it is actually the particle that is in a super position doesn't change anything, since we cannot observe the super position.
Therefore the particle can also be assumed to be in 1 of 2 states, which is yet to be determined. Nothing about the situation requires anything to be in a superposition.
So again, my point is that there is absolutely nothing amazing about people still holding on to the classical view.
The amazing thing is that then you have no explanation to the real, practical, measurable results of the two-slit experiment. You know, there's a reason for why many intelligent people with some understanding of physics and logic decided to drop the classical view.
Iirc, you can replicate the two-slit effect using drops of oil on water. This by introducing a wave in the water and watch it carry the oil drops as distinct units.
Except that such a (pilot) wave is exactly the kind of hidden variable that Bell's theorem disputes.
What I find much more amazing is that the thought experiment of Schrödinger's cat is so often used to explain the implications of quantum mechanics, because it was meant as a way to show that all that superposition stuff clearly doesn't happen in normal life, so something has to be wrong / missing in quantum mechanics.
The key fact that most people miss out on when explaining Schrodinger's cat is that the box is not simply "a box", say, a cardboard box or a plastic box or something. It's a magic box that completely isolates the cat from the entire rest of the universe. No neutrinos, no photons of any frequency, no quantum interaction of any kinds between the inside and the outside of the box, probably not even any gravity (though how important that is is currently unknown). The "cat in the box" is effectively in a separate universe until we open the box.
That point has to be hammered on, rather than neglected, because in reality it is completely true that no such box exists, and almost certainly no such box could exist, and it's very important when explaining this to people that it's a thought experiment, a metaphor, not something that actually happens in the macroscopic world.
Thank you. I've been trying to get this point across to people for decades. I think that the use of the word observe has been a real hindrance.
Please everyone, stop using the word 'observe' to describe what happens when the box is opened. It has been used by physicists to mean the same as 'measure' which really means something like 'touch'. The problem is that for all but a tiny fraction of the world's population 'observe' simply means 'see' and is something that can be done without 'touching' and without being seen oneself.
Um, the original formulation needed no magic box. The original formulation is also equivalent to setting up a gun that fires at a cat you're watching the whole time iff the next photon you send at a beam splitter is detected by one detector over the other.
Consider the cat in the box. Is it alredy dead OR alive (classical interpretation) or is it both at the same time (quantum mechanics) before you open the box and look?
The classical interpretation needs so-called hidden variables which determine the state of the cat.
In the quantum mechanics interpretation the state is determined when the interaction (measurement, looking at it) takes place. Before that there are several parallel realities of what could be.
In the case of the cat we cannot actually determine which interpretation is correct. We need a different kind of experiment:
As far as I understood (no physicist here) they use two entangled objects. E.g. two photons which are generated by a special process so that one polarized one way and the other one polarized the other way but we don't know which is which before the measurement.
Then you need to measure both photons separately and determine (statistically) which has which polarization.
These measurements need to be spaced out far enough such that faster than light speed communication would be required for the photon measured first to tell the second photon what was happening.
So far it still doesn't help to exclude the possibility of hidden variables since the measurement setup is static.
Another trick is needed.
They use a random generator to determine at both ends what to measure (which polarization direction) shortly before doing the actual measurement. So, when the photon is generated it doesn't know what would be acutally measured.
When you tabularize the all the possible combinations of how the photons could be measured and calculate the possible probability for the measurement results, there is a difference between what we would expect from classical physics when the status is determined at photon generation and stored inside the photons in a hidden variable and the result we would get from quantum mechanics without a hidden variable but with (faster than light) spooky entanglements.
The experiments show that there is no hidden variable. This is a BIG thing because this means that classical physics are not enough to explain the universe.
This is how I think about it (don't take my word for it, I don't really understand QM)
QM is like lazy evaluation. The result is not computed until someone actually forces it. This is different from the result being computed but you just don't know what it is.
Another way is to think of entangled states is like two virtual addresses which alias to the same physical address. The addresses could be very far apart (similar to entangled particles being very far apart), and their contents are coupled but "uncomputed", until one of the virtual addresses is accessed, faulting in the physical page. At this point, both addresses instantly point to the same contents.
IANAP, but I think using "lazy evaluation" as an analogy is missing one of the most interesting aspects of quantum physics: it's not just that the particles (or what have you) are in an indeterminate state. The fact that they're in an indeterminate state changes the way they behave. The reason it's possible to build up interference patterns using single electrons is because the various probabilities for its path between the electron gun and target are interfering with each other. I'm pretty sure a typical "lazy evaluation" system will not exhibit that type of behaviour :).
It's not. When you're in a superposition of two states, it only appears to be "indeterminate" under measurements which are not orthogonal to the state. To measurements which are orthogonal, the state is 100% determinate.
What's more, measuring with a non-orthogonal measurement makes the state orthogonal to that measurement, and not orthogonal to the other measurement!
This is totally different to how lazy evaluation works.
The actual description of "how it's different" in QM is going to depend on the interpretation you choose, but classical vs. QM is basically the difference between "I don't know yet" vs. "it isn't yet"
To elaborate a bit, Copenhagen Interpretation of QM states that the cat is both alive and dead. It will only settle down on a form, dead or alive, once we observe it. You could read farther on the subject [1].
Sure! One of the major advantages of quantum computing is that you can act on the superposition, i.e. construct some sort of operation which goes "Map the dead cat state to x, and the live cat state to y." This becomes useful when you can generate a superposition of all possibilities, and then perform operations which make it more probable to see the states you desire. In the classical version, the final result of mapping the dead cat would be only x or y
The basic cat scenario isn't "interesting" yet, you're right, it's just "we don't know." Importantly you would expect this result even if the cat was observed. You'd still observe a dead cat at the end, or chances at a live cat at different points during the experiment. (The original thought experiment killed the cat randomly over a time span using radioactive decay to release poison randomly during the time).
The science part here, is that a hypothesis has been proposed about the inner workings, and the "so what?" objection is a valid question. If the "so what?" is nothing, it's just an idea.
The "so what?" part is where the science happens. In trying to promote the idea (hypothesis) as a working idea with evidence (theory) We look for side effects of the idea that are expected to be different than our existing ideas (theories). We expect to see a cat when we open the box, and it's alive or dead. That's not interesting. But let's actually ask a science-like question of the consequences. These aren't related to the quantum mechanics questions really, but it's the sort of "consequences" questions that do get asked.
Maybe we can do the experiment over a longer time, like a week. We put the cat in the box with food and water. In the original experiment the poison is released at a random time during the week. So under our existing understanding (theory) we expect, if we run the experiment a hundred times we would see that random amounts of food and water are left pretty much evenly from 0% to 100% depending on when the cat randomly actually died.
Here is a different idea (hypothesis) about the side effects: If the cat really is alive and dead as hypothesized, it will use only the part of the food the 'alive' part would use, which is reasonably predictable. It changes by percentage over the week from 100% (alive) to 0% (fully dead). This means we should always see the same, or similar amounts of food and water left at the end. This is different than the usual prediction!
Now we have a new idea, a hypothesis that relies on consequences of the first hypothesis. So we run 100 experiments, and unlike our current understanding of cat mortality, we find that in this case the remaining food and water values a grouped up at about 50%! We can even do a control where the cat is observed the entire week, and we find that under these "observed" conditions our expected spread of values does happen. We've proven that our previous idea isn't enough to explain everything, and we have tested a prediction of the new idea, so our previous theory isn't "wrong" but it isn't entirely accurate, and the new hypothesis has gained evidence to become a new more complete theory.
Now in the real world at the cat scale, we definitely would NOT expect this to be the case, and real science is much more complicated. It's easy to make errors in predictions, or have "confounding" factors that throw off our investigations. But the initial investigations into spooky action were every bit as bizarre and unexpected. We're learning how things work at the atomic scale, and it turns out that while they mostly behaved as we though they did, there are some odd edge cases that really turn things on their head. Much like we don't poison cats in our day to day life, just in thought experiments (I hope), we don't interact with spooky action in science a whole lot either. But as we learn more, it informs our understanding, and frequently leads some unexpected concequences.
What's missing with the cat example is a measurement axis beside dead/alive.
Pretend instead that we measured the cat with a different measurement that was only somewhat correlated with the cat being alive, such that this test would return "true" with a 75% chance. A subsequent second measurement using the original alive/dead criteria would now return "alive" with a 70% probability! That is, we have altered the in-between state of the cat (in the cat's favor!) by our first measurement. It is this that is unique to quantum mechanics.
A pseudo-classical analogue of this phenomenon can be observed with polarizing filters. Analogous to the cat's initial radiation dose is a 45°-polarized light beam. Analogous to a measurement of the cat being alive/dead is a 0°-polarized filter – 50% of light makes it through the filter. However if a 15°-polarized filter is placed between these, one will instead observe that 70% of light makes it through the 0° filter! (You can also test this easily at home with three polarizing filters; two held at 90° to each other, and a third placed in between at 45° – the third will allow light to pass through the others.) This is solely due to the nature of electromagnetic waves, which very closely parallels that of quantum fields.
Replace "light beam" with "single photon" in the above paragraph and you get a fully-quantum analogue of my modified Schrödinger's cat experiment ;)
This is Leonard Susskinds attempt at writing a book that tells you the bare minimum you need to know to really understand what's happening. (experience with calculus and linear algebra extremely helpful but not necessarily required)
Question. How is this any different to writing A on one piece of paper, then B on another piece of paper, and selecting one randomly, then giving the other piece to someone else. Then the other person travels a light year away from us, and we both look at our pieces of paper simultaneously, at pre-agreed time. If mine says "A" then I know the other one says "B", and vice-versa. I instantly know the state of something that is a light year away, even though no information was actually transmitted. Isn't this how quantum entanglement works in principle?
The whole point of the Bell experiment is that it proves that which one has the "A" and which one the "B" wasn't pre-decided when the pieces of paper were separated.
>Isn't this how quantum entanglement works in principle?
Not it's not which is what makes the whole thing interesting. The whole point of Bell's Theorem and the experiments is to demonstrate that that hypothesis doesn't work.
This is a good question!
It's different because if you pass quantum entangled information to the other person, you will be able to cooperate more effectively with them on certain games using that information. It's explained very well in the second paragraph of this article: http://www.scottaaronson.com/blog/?p=2464 .
In a simplified way, this is what Bell's theorem tells you: That quantum informations behaves fundamentally different than "precalculated" random information.
I'm missing the part where it is explained how they cooperate better in the 3rd and 4th paragraph. Especially since sharing independent bits of equal probability 0 and 1 isn't comparable to sharing entangled qubits (at least it isn't explained how they are equal).
In a sense 'maybe not'. This would violate the bell test anyways as you predetermine the outcome by writing it down. Someone already knows what is on the paper. This is thus also subluminal communication as the outcome was already determined at the start (assuming you traveled at subluminal velocities). The idea of the entanglement states that the spin of the electrons is determined at the same time which guarantees their connection, not the outcome of the spin.
Instead of invoking the idea of action-at-a-distance and then reflexively prepending the "spooky" tag to it, why doesn't anyone ever ask what a purely local physical description would entail, and then prepend something like "spooky" to that?
A purely local description would consist of a universe filled with nothing but [zero-dimensional] mathematical points that somehow affect each other in some bizarre way. But the physics involved would be entirely unknown, because mathematical points have precisely zero extent, with the side effect being that two mathematical points either a) occupy the same location, making them physically indistinguishable, or b) occupy different locations, making them absolutely disjoint (ie, "infinitely apart").
Let's think about 2 points approaching each other in a classical Newtonian universe. Now they are a meter apart and now they are a millimeter apart and now they are a nanometer apart. In each of these cases, they are perfectly disjoint, and have no possible way of communicating to each other. But now they both occupy the same location, and all possible classical physical descriptions have broken down!
The only rational solution is that the universe can only have a fundamentally nonlocal description. That is, the fundamental units of reality are spatially extended fields that do not suffer from the same requirement of pure mutual exclusivity that naive Bohr-esque classical models suffer from.
Abandoning this requirement is what gave de Broglie the impetus to develop his own theoretical model that would soon thereafter inspire Schrodinger to develop a mathematical infrastructure, which, when combined with Heisenberg's matrix mechanics via Dirac's new algebraic formalism, would become the modern, canonical quantum theory that we have today.
Edit: downvotes land me in negative territory for this :( ... Any explanation as why this reply is so bad? I am honestly curious!
Edit #2: I guess I should have just said "Hey, Schrodinger's cat!" or "Wow, how about that Many Worlds Interpretation!", then I would have inspired the peanut gallery to jump into some pseudo-philosophical discussion that has been repeated time and again. But alas, I have resorted to a thoughtful consideration of the deeper ideas of QM, and have found myself relegated to the footnotes of the "Hackernews-iverse" :P!!!
A purely local description is generally understood to be field-mediated interaction, where field can be at the same place as the particle it acts on, without being the same thing.
There is a semantic quibble at play here that actually makes all the difference in the world when it comes to discussions about QM that inevitably end up in pseudo-intellectual balderdash. A strictly local physical description can only involve classical Newtonian pictures, such that all of the nonlocal effects are superadded onto these pictures and were elegantly explained away by Newton as such: "Hypothesis non fingo" -- in other words, "Hehe, your guess is as good as mine".
In modern times, when we observe nonlocal effects between objects (such as with gravity), we first measure the effect, and only later do we invent something called a "field" whose definition consists of nothing other than the observed measurements themselves.
This is still nothing other than the same "Hypothesis non fingo" as before, except that now, the question of whether fields can possibly be explained in terms of classical physical pictures does not exist anywhere in the entire physics profession. That is, those kinds of hand-waving, natural language questions just don't have any place in the discourse of those who can rightfully be called credentialed, working physicists.
To such physicists, a particle is simply a set of quantum numbers that exists in n-dimensional Hilbert space (or "mathspace"). Again, whatever field is said to exist at the particle's location is nothing more than the measurement that is recorded by some apparatus that exists in [classical] spacetime. So, it is of course necessarily true that the particle and the [part of the] field [that affected it] must exist at the same location.
But to then say that this is therefore a meaningful account of something called "local physics" is, IMO, fairly absurd, because the entire point of the inventions called fields is to say: "Hey, I have no idea what accounts for instantaneous, space-bridging forces like gravity, so I am just going to leave it to the philosophers to offer up speculations so that the masses may be entertained."
Of course, this can be countered by the argument that gravity is fundamentally different from the electromagnetic force, because electromagnetism necessarily depends on the supposed propagation of things called photons. But this argument depends crucially on the notion that our mathematical shorthand for the quantization of the EM field (ie, photons) is at the same time a meaningful physical description that exists in the framework of a classical spacetime picture that is based fundamentally on the quality of locality.
However, QED (quantum electrodynamics; the quantum description of electromagnetism) in no way exists in any such classical framework. It is all rather the very same "mathspace" previously mentioned. The "field" is nothing but whatever bit of data that some classical measuring device recorded there.
p.s. Thanks for the upvotes :)... sometimes it pays to complain, I guess!!!
I don't understand why there have to be hidden variables in order for their not to be anything spooky about this. This is just how entanglement works. There's no way to understand some kind of classical "mechanism" for this. It's no more spooky than, say, magnetism. There is a state where some measurements of some properties are correlated. Big deal!
Magnetic effects are local, since photons are sent between the objects; ie. distant objects don't interact with each other, they interact with photons locally, which then "carry" the force to the other object, where they again act locally. In fact, magnetism is a realtivistic effect; it's how the electrostatic force behaves when things are moving.
"Spooky action at a distance" is something that's non-local, ie. there's nothing being transmitted between the objects; they "just know". Even if there were something being transmitted, it would have to be going faster than the speed of light.
A great example is gravity; in Newton's model, gravity is spooky action at a distance, because objects (eg. planets) are in a pure vacuum, yet everything is aware of the location of everything else. General relativity replaces this with local interactions: objects are interacting with the spacetime around them, curving it; movement follows these curves; and changes propagate outwards at the speed of light, as (gravitational) waves.
Magnetic effects are local, since photons are sent between the objects; ie. distant objects don't interact with each other, they interact with photons locally, which then "carry" the force to the other object, where they again act locally.
IANAP, but that is probably not a correct picture. Photons carry energy and therefore if you imagine electrically or magnetically charged objects sending out a constant stream of photons in order to mediate electric or magnetic forces you quickly get a problem with conservation of energy.
In fact, magnetism is a realtivistic effect; it's how the electrostatic force behaves when things are moving.
This also seems at least not accurate to me - relativity turns electric forces into magnetic forces and vice versa when you change reference frames, but they are not the same thing, they are two different aspects.
I'm not saying magnetism and entanglement are related, I'm saying that when we talk about forces between things, when we get down to it we have no more an understanding of how force "works" than we do how entanglement "works". We want everything to be like little machines with cogs and wheels and pulleys. But what is force? What is the thing that repels two negative charges? There's no "mechanism", that's just what force is and what it does. We're so comfortable accepting all this other shit about how the universe operates why should entanglement be any different?
Science is more than just accepting what we (experimentally) observe, since that would lead to a collection of facts rather than a theory. By looking for mechanisms, underlying causes, etc. we can find simple rules to explain seemingly unconnected phenomena. No scientist expects those rules to be intuitive; that's the realm of pop-sci "quantum woo" journalism. However, we do expect those rules to be consistent.
General relativity and quantum mechanics are notoriously incompatible, so all of our attempts to combine them so far have been inconsistent, either internally (ie. the maths doesn't work) or externally (ie. they're toy models which can't explain real experiments).
Bell showed that entanglement cannot be explained by local hidden variables, which was thought to be inconsistent with special relativity, since it seemingly allowed particles to communicate faster than light. More recently, work on quantum computation and quantum teleportation have clarified our ideas in terms of information: the speed of light is only a limit for information, and entanglement doesn't transfer information. Explanations like many-worlds are also based around information.
The theory is the maths. Maths makes predictions, observation confirms those predictions. End of story. Local hidden variables are not required any more than a medium is required for an electro magnetic wave to propagate.
And yes, it's correct that entanglement transfers no information! So there's nothing spooky about it; or more correctly it might seem spooky but there's no action at a distance.
The researches don't know how to explain the results in a way that is consistent with our intuitive understanding of the world, so I'd say most people consider that spooky.
One of the equally reasonable explanations is that the measurement results made by Alice and Bob are not actually determined until their compare their results. Or FTL "communication", or time-travel. Or that their sources of randomness are actually pre-determined.
I'm curious what would you consider spooky if not time-travel or whole universe pre-determinism?
Right they don't now how to explain it classically. In much the same way as electromagnetic waves for example are spooky if you try to explain them as propagating mechanically in the same way sound does. Light travelling in a vacuum?? Spooky! This in't time travel, there is no effect and no action at a distance. It's just how that state works. In the same way as I can't visualise a 5 dimensional sphere, I can't create a classical model that explains entanglement, but I can make predictions mathematically, observe the effects and then know that's how things are. Once you go beyond things that make sense intuitively, all you have is maths!
The problem with your argument is that you can rationalise anything that way - "that's just the way it is". Which is why I asked what you would consider spooky. If you don't consider anything within the realm of observable phenomena spooky, then you are essentially just defining spooky differently than other people.
It's not the "spooky" part that I have a problem with, it's the "action at a distance" part. Things can be spooky or not spooky subjectively, but there is no action at a distance.
there is a classical explanation for it, look into the BSM-SG model from Stoyan Sarg. Most brillian physicist ever lived. If you find a error in his theory, tell me, because for me it makes sense and everybody I asked just ignores him but could not tell me a error eighter. If its so wrong, should be easy to disprove:
- find a logical error
- find math error
- find something that is not explained in this framework
Yes, and he was right not to be easy with that, because there are no faster than light effects. Entanglement is a state in which some measurements of objects are correlated. End of story. There's no "wires" connecting them, it's just how shit works. It's no more spooky than how light travels in a vacuum. If you try to understand light as propagating the same as sound, it's spooky. But light works differently than sound. Measurement of very small things works differently than measurement of very large things. It's just not that big of a deal. Like it's important and interesting but everyone gets hung up on this spooky bullshit.
There is something that looks rather like a faster than light effect - in the two photons experiments the correlations depend on the angle between the polarisers even if they are far enough apart that a light speed signal could not relay the polariser angle in time. Whether you see that as spooky is a bit in the eye of the beholder. But it's certainly a surprising thing.
My guess would be its like that because quantum mechanics is the fundamental nature of the universe and space and time are an apparent effect of that. So in odd situations like the entanglement experiments quantum mechanics works as usual but something about space and time doesn't work like you might think it would. A bit like space and time being a leaky abstraction over quantum mechanics to make a programming analogy.
I'm happy that BSM-SG replaced QM for me :) They are looking at huge, huge objects and think they are fundamental particles and wonder why it behaves strangely... apart from fundamental errors they have.
That's helpful, thanks. What I was after is how the apparatus in this experiment maps onto the CHSH description given by Scott (if indeed, it does). It seems clear that the x / y values are used to determine the basis of measurement of the respective detectors, the a / b values are output based on whether each detector fires, and it's the correlations based on difference between the bases that allows you to beat the game.
The basis of measurement here is analogous to the angle of a polarising filter in a photon detector, but the detectors in this experiment are directly measuring the spin of the nitrogen vacancies (so what is the physical implementation of the different basis of measurement?).
http://www.scottaaronson.com/blog/?p=2464