Is "measurement" or "observation" a simplified term for a complex process? It can't be as simple as a human "looking" at something since there's nothing special about humans (that I know of). I really wish I had the time and capability to understood how quantum entanglement really works. It seems a lot like magic if you take the layman explanations at face value.
To "observe" can mean any sort of interaction between the particle and a larger, connected physical system. To observe something, you have to have a physical interaction with it (it is hit by a photon of light which then hits your eyes, to use the most basic example) and so its quantum state decoheres into a particular state.
Most scientists are fine with the idea that it doesn't matter if the larger physical system has any sort of "consciousness" or "observation"; isolated particles have quantum behaviors, interacting particles decohere towards more normal-seeming physics. However, there are theories that shouldn't be immediately discounted that do still ascribe a role to the particular observer, indicating everything from reality being relative to consciousness being a fundamental aspect of the universe. I wouldn't take these too seriously, but I wouldn't dismiss them out of hand either.
Of course, that's not addressing the worst thing about quantum mechanics: once you've finally rid yourself of the pseudoscience and poor metaphors that make it sound like magic, it seems for just a second like a normal, intuitive process that's just been obscured through poor explanation--then you learn a little more and realize it's far more bizarre than you'd imagined.
>seems for just a second like a normal, intuitive process that's just been obscured through poor explanation--then you learn a little more and realize it's far more bizarre than you'd imagined.
This is a great explanation of the process of learning about quantum mechanics.
I'm a lay person when it comes to QM: I get my information from Brian Greene's excellent books. I kind of think it is going too far to say that it's pseudoscience and poor metaphors that make it seem like magic. Readers of the popular scientific accounts learn that the various slit experiments with partial observation etc show that these subatomic particles behave like spreading waves of probability until they are observed at which point it's like a sample is taken from the probability distribution. That seems very magical to me and has done to many scientists over the last century, right?
This is a Hard Problem, to define what process is a "measurement" and what is not. But if we think about it very simply: a photon hitting a detector (or analog film strip) can be a measurement.
Since this process converts quantum information to classical information (a digital or analog signal), we know that it must lose information. Only by making many measurements can we infer statistical properties of the entire quantum state. This is exactly what happens with the two-slit experiment.
I personally subscribe to the "shut up and calculate" interpretation of QM (or perhaps the projection-valued operator approach when I'm feeling mathematical). The above intuition has served me well so far.
> Since this process converts quantum information to classical information (a digital or analog signal), we know that it must lose information. Only by making many measurements can we infer statistical properties of the entire quantum state. This is exactly what happens with the two-slit experiment.
Wow, I've never heard it explained that way. That's brilliant - did you come up with it? Interference patterns are the result of a huge number of individual photons interfering with each other at an enormous rate, and the result are laid out spatially...really lovely.
I can't say I can claim any credit for the idea, no, this is what is you learn once you chase the QM rabbit hole deep enough. But it's maybe not a common explanation in Intro to QM class or in popular science writing?
Actually (technical nitpick) it's even more lovely than what you describe: people have done the two slit experiment with verifiably single-photon sources... and you still get interference patterns. Really, we know that only a single photon exists between source and detector for the entire lifetime of the photon, and yet you get interference. The only explanation is that it interferes with itself, which is what QM says happens.
The original "Wow! QM!" two slit experiments were done with electrons, which were thought of as competely particle-like, whereas light was thought of as completely wave-light. QM of course says both are both.
I had thought the pattern represented interference between the probability distributions of where the photon might be found (which themselves follow "wavelength" and produce wave-like behavior). With two slits you get two distinct probability distributions that overlap spatially at the screen, which when added together cause denser/less-dense areas of photon detection and looks like an interference pattern.
That would have explained why streams of single photons "self-interfere" (they wouldn't--it's the underlying probability of their path that interferes with itself, so the pattern would eventually appear at any emission rate) but that seems so simplistic as to be unlikely to be the truth of the matter, especially with delayed choice and quantum erasure effects.
Do you happen to know a good layman's source for a solid explanation of what we know about double slit right now? I'd really like to understand it, if only at a high level.
Are you still comfortable with this explanation if the photons are released with a 1-second gap between them and the tally is visibly shown building up into the interference effect on a computer screen? "Interfering with each other at an enormous rate" might mislead you a bit; neither "each other" nor "enormous rate" is particularly important in the double-slit interference pattern.
Actually, you can't know this until you get a lot of measurements to see the pattern. E.g. if you just put one photon through you get one dot, and you'd be hard pressed to infer anything from that. So in the end you still need a lot of measurements to detect interference.
Also I bet you could mess with the interference pattern by pulsing the light source. This would be the lesser known form of the Uncertainty principle, where energy and time are unknowable.
>Imagine at home you put one glove in your coat without looking (and noticing it's only one of the two). After exiting the train you notice it's cold and you pull out that single glove. At this very instant you know it's either the left or the right glove, and you therefore know which one is left at home. However, no information was transmitted by your "measurement". Of course in quantum mechanics this is more complicated because of the not entirely measurable wave function, but this is the basic idea.
In this classical picture the decision which glove was in your pocket was made when you put it into your pocket. Still in your pocket, the glove already interacted with the world like a right or left glove e. g. the glove bulged the pocket in a certain way and moved fluff around in your pocket.
If the gloves would be quantum objects, the glove would be in a superposition state. At the same time the content in your pocket would be the right and left glove.
Hence it would interact with your pocket in both ways before measured leaving your fluff in your pocket also in an undetermined (superposition) state.
Things get more complicated here because we talk about huge classical objects with a lot of atoms. And here it is practically impossible to obtain such a superposition because the glove interacts a lot with the environment. E. g. by looking at the bulge of your pocket you might be able to determine the glove type without opening the pocket.
There are a lot of (justified) comments on that answer. A crucial point of quantum uncertainty is that it's different from classical intuition. Particles behave like waves (e.g. cause interference with themselves) until observed. That doesn't happen with the glove.
The essence of the answer is that correlation doesn't imply causation (true).
The essence of the comment is that the correlation in a quantum case may be stronger than in a classical case such as the case with gloves (also true).
The comment doesn't invalidate the answer unless you think it does imply causation (non-locality). It just says that the answer is not the whole truth: that would be the complete mathematical description of QT with corresponding procedures to connect the math with observations.
The glove analogy obviously has its limits. It doesn't make it wrong. Otherwise, the accepted quantum theory is also wrong because it doesn't describe all known phenomena (consider gravity).
> "Particles behave like wave until observed"
Consider photon: it is neither wave nor particle in a classical sense. The math is such that in some cases it is easier to describe it as wave (interference pattern), in others—as particle (photoelectric effect).
It is surprising that the familiar concepts from a human scale (such as wave, particle) are useful in such a wide range of physical phenomena. Though only because they are more familiar does not make them more real than e.g., the concepts produced by the equations of the quantum theory.
All models are wrong—some are useful. (In a sense that there is no territory under a map, only more complete or different maps)
Let me give you a couple pointers; I did my Master's thesis in quantum transport.
(A) Measurement and observation is NOT a simplified term for a well-understood complex process. It is an atomic term for a not-well-understood (therefore maybe simple or complex, we don't know) process which is really extremely simple if we take it at its surface meaning and don't poke inside it too far.
Let me take the simplest example, the Stern Gerlach experiment. This is really simple: put two long magnets next to each other with a "gap" between them, and preferably give them very different shapes; this creates an "inhomogeneous magnetic field" between them. It turns out a spinning electric charge, when it comes into such a field, should get deflected based on the direction that it's spinning.
Fire a beam of electrons at this apparatus, and you'll notice something interesting. Let me give some coordinates: if the gap between the two magnets is "horizontal" and the electrons go through it "forwards", they split into two beams, half going "up" and half going "down". We say that these have 'spin up" and "spin down" but that depends on a bunch of little arbitrary choices. If you put another apparatus "horizontal" in front of either beam, you'll notice that those ones going "up" all go "up" through the second set of magnets; likewise for the ones going "down".
So there's a lot to unpack here: first off, if they were normal spinny things, then this separation into two beams is really weird! Because what about an electron that's spinning "forward" or "left"? Why wouldn't nature recognize that mathematically it's spinning just as much clockwise as anticlockwise, vertically, and not deflect it up or down at all? So there should classically be lots of particles deflected between these two beams: it is very strange that this does not happen! In fact we can perform the experiment. We can use a second Stern-Gerlach magnet, this time oriented vertically, so that it deflects electrons into 2 beams going left or right, call these "spin left" and "spin right." Now we take the spin-left electrons and first put them through a vertical Stern-Gerlach magnet, make sure they keep going left, great. We just rotate the magnet and we find half of them go "up" and half of them go "down." Nature doesn't know the difference between "left" and some sort of 50/50 mixture of "up" and "down"; those are the same to Nature, at least where an electron's spin is concerned. And that finding is very robust: "up" is a 50/50 mix of "left" and "right" so if you use another Stern-Gerlach magnet on the electrons that went left and then up, you do NOT see them all go left again! They will all go up if it's horizontal, but they will not all go left if it's vertical. Instead half of them go left and half of them go right!
Now, we have a very folksy understanding of what we mean when we measure these things: we stick a very sensitive electron detector in some place, connect it to a counter, and we watch the counter erratically tick upwards. Of course it is so sensitive that it ticks upwards due to all sorts of other noise sources, even without a signal, but when we turn on the electron gun we start to see that in some places, pointed at the place where the beam hits the gap between the magnets, it starts rising much faster than the noise would provide, and in some places it doesn't rise any faster at all; it's all attributable to noise. That's how we know there are these two beams coming out; we move this detector around and see some peaks in the detection rate.
Now, a lot of our explanations of how the electron can do all of these weird interactions with the magnets, depend on saying that the electron does not just take one path at one time! Instead maybe it is "spread out" in space or it "takes all the paths available to it" or something -- these funky interpretations make the rest mathematics used to describe the electron unbelievably simple, you just have these "unitary transforms" and this "linear evolution" and all of that complicated quantum mechanics stuff is super-simple mathematically. But when we measure, we just see this counter jittering upward with highly unpredictable increments but with some very predictable average rate. Most of those clicks we'd like to think are actual electrons which have made up their mind to take this path or that path and have successfully made it to the counter. How this happens, is something of a mystery. If the electron takes all paths, why does it end up here or there? If it's spread out so that it's half on this detector and half on that detector, why do we see the counter increment and not, say, fuzz between the two numbers, having half-incremented and half-not? Why isn't our world more fuzzy, if it's made out of these fuzzy probabilities and amplitudes at its core? And yet why, when we use scanning tunneling microscopes, do these electron clouds of these atoms look like little balls, as if those electrons really aren't spread out over all that space but occupy one single place all of the time?
The mystery comes because there's a lot of really easy ways to explain this funky Stern-Gerlach stuff, but most of them view the world in a way that's alien to our own. The measurement problem is "we know how measurement works pragmatically, and it never showed us this alien world before, so how is this alien world 'collapsing' into the familiar world that we all know and love?"
(B) Entanglement has to do with strange correlations between remote systems which you can only notice when they are brought back together. My favorite example is a game where 3 people compete as a team in several trials where we secretly put them at cross-purposes to each other, call it "Betrayal." We split the team of 3 people into 3 separate rooms and we prohibit communication between team members. Each room has a screen that we display a goal on, and two buttons labeled 1 and 0. Sometimes the displayed goals for an individual and the actual goals for the team will be at odds; the individual never gets any reward in these cases: it's only, "if the team gracefully recovers from our meddling 100 times in a row, we will give them all a big cash prize."
Okay, so how do these work? Once the people are settled in their rooms, 1/4 of the time we will broadcast a "control round" where we tell them all "make the sum of your three button-presses even," and start a countdown timer. They win if they all push exactly one of their buttons once before the time is up, and the sum of their pushes is even. Really simple. Then 3/4 of the time we will choose one of them at random to be a "traitor" to the other two: we tell the traitor, "make the sum of your three button-presses even," but we tell the others "make the sum of your three button-presses odd," and the team wins only if they each push exactly one of their two buttons once, and the sum is odd."
It's easy to prove that you cannot win this game more than 75% of the time classically; each of the 4 situations is represented by some equation among the 6 correlated random variables, but when you add all 4 equations together you find out that they reduce to 0 = 1, an obvious contradiction, so they can't all be simultaneously satisfied no matter how you correlate the random variables. It is also easy to prove that if they all start out with a class of entangled states called a "GHZ state", such as
|+++> + |---> = |000> + |011> + |101> + |110>
then they can either measure their state to get an even sum, or any two of them can perform the unitary transform mapping |+> to |+> and also mapping |-> to i |->, yielding the state
|+++> - |---> = |001> + |010> + |100> + |111>,
and then any measurement must yield an odd sum. The two who are told to make the sum odd can do this with absolutely no help from the one who does nothing to make the sum even. In theory, the only limit to your accuracy is how long you can keep these GHZ states away from outside noise and disturbance. And of course we can account for that by only requesting that you pass, say, only 90% of the trials successfully -- with enough trials we can still prove that a classical team with their 75% upper bound on success in individual trials will almost always fail whereas a quantum team whose tech is good enough to get to a 95% success rate will almost always pass enough trials.
But, you can't use this spooky collaboration to transfer information faster than light. And that's precisely because you can't discover that the two measurements are correlated until you compare them! We instantly correlate but we aren't instantly aware of our correlation; I can't figure that out until you send me a message saying, "hey, is your set of numbers X?" and I say "yes it is! woah! spooky!"
A measurement is an interaction of the quantum system under test/consideration with another system / component that is classical (non-quantum, e.g. big).
Why? Because otherwise we would have to specify the exact details of every measurement device. Model number wouldn't be enough. What is the full quantum state of photon detector with its enormous number of atoms? Unusable.
Entanglement isn't some spooky thing. In fact it's required for us to be able to measure anything. The measuring device's quantum state must become entangled with the quantum system it's measuring. We can't really consider each one separately, but we try anyway, and with great success if we do it right.
I like James Binney's take on this: QM is adult physics. It's an admission that when you measure something, you disturb it. When you measure something really small, you disturb it alot, and can't idealise that away as you can for classical macroscopic systems.
EDIT> I should add that for a measuring device, we want it to be correlated to the system under test. I.e. I could make a thermometer that always read 5C (uncorrelated to environment), but it would be a bad thermometer. Quantum entanglement is how this correlation is set up. But it means that now the future evolution of the quantum system depends on the (unknown to us) exact state of the measuring device.
Human observation is fine. The fact that observing a state changes it is one of the "weird" things about quantum physics. Again, as far as I know since I'm not an expert, this is related to the fact that observation forces the wave function to collapse to a specific value, which then eliminates the "waviness" of the particle.
This is demonstrated by the single-particle double-slit experiment [0] and the eraser variation [1].
Basically an observation is when an isolated particle becomes entangled with a larger system of particles (ie, a measurement device or person). At that point, it starts to behave more classically from the perspective of the system with which it is now entangled.
