Sometimes I feel that people writing about physics are having great fun with deliberately confusing people by making stuff sound magical and illogical.
This particular effect has nothing to do with whether or not anyone is looking at an object. It's the methods of making it visible that cause the effect.
I think that a big part of the problem is trying to make the new more layman friendly removing all the math. Quantum mechanics is unintuitive but the math is very clear, so "anyone" can get the same correct result for an experiment. When you remove the math, you keep only the unintuitive part.
Another part of the problem is linkbait, making slightly wrong interpretations make more interesting articles. Sometimes there are a few articles from different news sites about some subject, and it's very common that the more misleading gets more attention here.
For an interesting explanation of what is happening here, I like the stack of slightly rotated polarizers. (See this comment https://news.ycombinator.com/item?id=10437289 ) The math is equivalent, but the result is not linkbaity. You can even try it at home! If you think enough about it, it's completely weird, but the calculation are pretty straightforward.
Generally a decent layman's explanation of the experiment, but they had to conflate quantum mechanics with magic by saying "the atom knew it was being watched..."
It's very hard to handle these misconceptions once they're already so widespread.
We call those "heisenbugs" - one of the most terrifying and maddening things you can encounter prior to a launch. I'm almost positive that I remember reading a sort from a game dev who ran into one of these right before a release, where printing some variable caused it to have the correct value; in this case, it was left in the game.
Well, I know that I ran into a situation (and may once have posted about it) in a C program where I was doing printf to debug a program, and it ran fine. Whenever I removed them, the program would crash. Finally discovered that the printf debugging was forcing garbage collection which took care of a bad pointer somewhere else in the program. Honestly can't remember if I ever found the pointer or just left the printfs in the code...
Thank goodness we have Valgrind these days. Though I don't know how well it would handle a C app with a garbage collector.
Calling printf can (will) change what's beyond the stack of the calling function, so if the pointer in question pointed outside the active stack, printf may have put a safer value there by coincidence.
This happens all the time. I very recently had an issue where a deadlock was fixed by a print statement, because two lists were being lazily evaluated at the same time, and the print statement forced one of the lists to be evaluated earlier and finished by the time the deadlock was occurring.
The main problem with that series of videos, if I remember correctly, is that it only ever gives high-level descriptions of experiment outcomes. Those are interesting, but they aren't really good explanations. Then to top that off, they get some of the important details wrong.
For example, in the quantum eraser video referred to by the video you linked, it's said that there's an interference pattern for B when you don't measure the entangled associated photon A (and that there is one when you do measure A). That's not the case. If it was the case, we'd have a handy-dandy FTL communication mechanism.
What actually happens is that there's never a visible interference pattern for B in this experiment. Instead, you use the measurements of A to filter or split the measurements of B into two groups (e.g. Bs when A up, Bs when A down). Within each group of measurements, you'll find an interference pattern. The two interference patterns will complement each other so the sum is an apparent lack-of-interference; you need the As to do the separation.
I personally think the the quantum eraser experiment has a terrible misleading name. It's more of a "use entangled measurements to find the interference" experiment.
I do remember a data structures textbook in university that actually managed to confuse me about stuff I already understood really well. It wasn't offering a different perspective, it was just that badly written.
Atoms don't know they're being watched and using language like watched misleads people into thinking they do. Same issue as the double slit experiment. Watching is not passive, the correct term should be measured and the methods of measuring things affects them because you have to bounce particles off of things to measure them and thus measuring them (watching) changes their behavior.
They actually are coming up with crazy ways to indirectly obtain measurement without interference or at least less, but this is about the limits of my knowledge on the subject.
If I understand correctly, what's going on is... Suppose you have a system rotating between state |0> and state |1>, like f(t) = cos(t) |0> + sin(t) |1>. If you measure at time t, you get |0> with probability cos(t)^2. After measuring, the system starts rotating again, but reset to |0> (assuming you measured that). If you measure n times per rotation period, the chance of measuring the state |0> every time (i.e. of effectively keeping the system in the |0> state) is (cos(τ/n)^2)^n per period. That converges to 100% as n gets large.
So, by measuring more and more frequently, you can effectively stop a system like that from transitioning between states.
It's an interesting example of measurement unavoidably affecting quantum systems, where the problem clearly isn't due to kicking or perturbing the state.
The ball will slowly start rolling down one or the other side of the slope (because gravity).
You have a robot hand that keeps checking if the ball is still on top by grabbing it and then releasing it right in the middle. There is some leeway when the hand grabs the ball but none when it releases it.
__|__ _|_
| o | => |o|
So effectively, if you do this fast enough, you keep "resetting" the ball on top of the slope.
Not really. A minor problem is that it fails to capture the fact that this process will also hold the qubit stable in the |1> state. More importantly, it fails to capture the limiting behavior of the success rate; it should move smoothly towards 100% as the measurement frequency increases but the ball-and-slope would instead jump discontinuously from not working to working as the hand went from being too-slow to fast-enough.
A better example might be... a series of polarizing filters? If you have a series of polarizers but skip all the way from vertical to horizontal with nothing in between, no light gets through. If you put a diagonal polarizer between the horizontal and vertical, some light gets through. If you have a big long series of very gradual steps from vertical polarizer to horizontal polarizer, almost all the light gets through. More frequent measurements causing the zeno effect is like to adding more gradations of polarizer direction causing the all-light-gets-through effect.
Er, I am fairly sure that polarizer example doesn't work, and still no light gets through, as polarizers don't rotate the light, but simply cut off components that are orthogonal to the polarization direction. So stacking slightly rotated filters through to 90 degress would still leave you with no light.
EDIT: On second thought, it might just be you wording making it seem weird. The analogy works if you don't look at the whole stack, but the after adding each polarizer. Still, the same effect is had as simply gradually rotating the 2nd polarizer.
Following a vertical polarizer with a horizontal polarizer will block all light. But putting a diagonal polarizer in between will result in some of the light getting through. You can find videos and explanations of this effect on youtube [2] [3].
Hah, nice! Looking at it now, of course it makes sense. I guess I just short-circuited by thinking of the polarizer as blocking light only, and not that the reduction in intensity is the result of the projection of the previous wave direction onto the new wave plane!
EDIT: not that this makes the quantum effects any clearer ;)
*while illuminated by a bright laser. By dimming the laser the quantum behaviour returned, and you watching the atoms or not had no effect on the phenomenon.
Despite the misleading title, I imagine affecting the quantum behaviour of atoms with lasers has all kinds of nice use cases!
Affecting the quantum behavior of atoms with lasers is pretty much the entire field of ultracold quantum gases :) A handful of Nobel prizes have been awarded for it.
One of the key points of the article is that measurement is necessary to freeze the state. Where do you see watching the atoms or not had no effect on the phenomenon?
Layman question: what constitutes observation in the world of quantum physics ? The article suggests that stronger light means less quantum effects, but isn't observation a binary thing, so something is either watched or not.
Observation means: Interaction with a 'classical system'. Now because of entanglement this also works if we look at chains of interactions: Interaction with another single atom for example is no observation, but if this other atom interacts with a 'classical system' at some point earlier or later we have again an 'observation'.
Now 'classical systems' are systems made of so many elements that we can only make statistical statements about it because we can't ever determine the state of each of it's constituents.
I suspect for some time now that this is also the source for the "quantum randomness". In principle, everything is deterministic, but to observe something we always have interactions with a "big classical system" (in the end it's always our brain). And because we don't know the exact state of those systems but only statistical averages, quantum mechanics looks random for us, even if it's perfectly deterministic in itself. But that's my personal view of the matter.
The first couple of paragraphs are a great summary. Prof. Binney has some interesting observations on the non-physicality of quantum measurement (the formalism).
One of the postulates of QM is that after a measurement the system is in a well-defined state (the one we measured). But this is aphysical.
It's an artifact of the deliberate choice to formally model measurement in a way that simultaneously recognizes that all measurements disturb the measured system, while also wanting to abstract away the particular hardware used.
So, I'm starting to see wavefunction collapse and the whole Copenhagen interpretation as artifacts of the the formalism, not any kind of physical truth.
Any effect that is conditional on the state and thermodynamically irreversible counts as a measurement.
One way to do a partial measurement is to make the conditional effects less orthogonal to each other, e.g. cause a target qubit to conditionally rotate by 22 degrees instead of a full 180 degrees.
Another possible concept for a partial measurement is not distinguishing between all of the cases. For example, if a system can be in the x=-1, x=0, or x=1 cases then it's possible to do a measurement that distinguishes x=-1 from x in {0,1} without collapsing all the way down to x=0 or x=1.
If you make the opening shot in billiards, the balls are all over the table in the end. In principle you could now make another shot which restores the original, ordered position, but in practice it's impossible. So the opening shot is called 'irreversible'.
Same thing goes for many things, for example letting some hydrogen gas out of a gas bottle: The gas will never go back into the bottle by itself (even if there is a very very small probability that this may happen, in practice it never will). Because the behavior of this kind of systems is described in thermodynamics those systems are called "thermodynamically irreversible".
In the end it simply means that its much more easy to break things than to assemble them again (which also leads to the concept of the always increasing entropy).
Obviously it is not about observation by a human; I don't think anyone on here thinks we are special magical creatures when it comes to quantum physics.
However, not all interactions with another particle count as an observation, and I think the poster asked for a more precise description of the boundary, and why it would not be a boundary but actually appears to be a continuum.
> I don't think anyone on here thinks we are special magical creatures when it comes to quantum physics.
The general public does, most explanations of the double slit experiment you can find lead people to believe that mind affects reality because the word observation confuses people into thinking consciousnesses is involved.
Would there be a difference in result when only the computer looked at the individual positions, logged the measurements, determined them and noted the level of quantum behaviour to test this hypothesis?
I read this thread on the physics stackexchange
http://physics.stackexchange.com/questions/110488/consciousn...
..and it is said most professional physicists don't believe in this 'consciousness (human or otherwise) causes collapse' Neumann-Wigner interpretation (and that of John Wheeler and Henri Stapps) and most amateurs do, but has it been tested in this way also by ruling out far out psi effects like retropsychokinesis by deleting or encrypting the data in such a way it can only be verified by a 'team' of independent artificial scientists (networked statistical software)?
Many measurements == little change? It sounds vaguely familiar of overloaded computing systems. Maybe we'll run timing attacks on whatever the quantum world is.
I really wish more science writers would explain what "observation" actually means. This article is better than most, but generally one leaves quantum-effect discussions with the impression that atoms can literally tell when a human brain is perceiving them, presumably through magic science brain waves.
I guess it depends on the presumed audience. As a layman, observation is a passive event. After reading through some of the comments, I can see that physicists have a different meaning for that word.
Natural languages have evolved around the human scale of things and humans actions on those things. When you try to describe nature using these concepts you will always get weird and unintuitive explanations.
I'm curious as to how the scientists confirmed that tunneling was taking place when the atoms were not being illuminated. Does anyone know how this is typically done?
I hope I understood your question correctly. It's very very very difficult to take a photograph to catch the atom while tunneling.
The atoms in this experiment form a lattice, and the lattice acts a trap, so the atoms can be only in some fixed spots. To go from one spot to another spot, it has to have some minimal energy, so it's equivalent to climbing a hill to go from one valley to another valley. The atom usually get that energy by random movements, but in this experiment they are very cold, to avoid this possibility, so they are trapped in one of those spots.
By quantum mechanics, they can travel from one spot to another spot without enough energy. This is equivalent to going from one valley to another valley without energy to climb the hill, so the tunneling nickname. The probability is something like exp(- distance * energy_difference * constants) so it's harder to go to a spot far away and it harder to pass a tall hill (I must be missing a the time variable there, look at Wikipedia for the exact expression.) And it's exponentially harder, not jut harder.
To answer your question. It's very very very difficult to take a photographs of the atoms while tunneling. If you turn off the laser and you take a photograph from time to time, you see that some atoms jump randomly from one spot to another between photos while "no one" is watching :). If you turn on the laser, the "laser" is watching, so the atoms don't move between photographs. But don't expect to get a photograph of the atoms while tunneling.
EDIT: after a coffee and a shower, I changed "impossible" to "very very very difficult"
I re-read the article, and combined with your answer, it makes much more sense now. What was originally confusing me, was how the scientists were confirming the presence of tunneling when the laser was off.
I was mistakenly assuming that they were using some special method to infer that tunneling was taking place.
But of course they don't need to, like you said.
The tunneling can be inferred from the change in positions of atoms in different photographs. They don't actually need to see the actual tunneling in-progress (even if it were somehow possible/not very difficult to do so). Makes sense.
Thanks for that answer, it's very helpful for someone like me whose last brush with quantum physics was the physics/chemistry courses in year 1 of engineering.
what i really don't quite get (amongst a large number of other things as well), is how you could have observed an atom without disrupting it's quantum state...
The Zeno effect is basically due to disruption of the quantum state. You measure repeatedly in some basis and this basically pins the state down (in this basis representation) even though it would not stay in this state if you did not interact with it.
No, I'm definitely thinking of the fact that observation inhibits change in a quantum system. I'm fairly certain I was taught this was the case and was experimentally verified. 'Long ago' here means ~20 years ago.
It says at the start of the article that it has already been experimentally confirmed for spin, this experiment confirms with respect to motion. I don't know when the spin one was done, maybe someone can provide more info.
What I don't know is if the different quantum properties are independent of each other. If they are then presumably one would have to verify this Zeno effect on each property of a quantum particle. So maybe we can now say that the Zeno effect has been experimentally verified for two quantum properties now. Looked at this way you were both correct and incorrect.
If spin and momentum are 'connected' in that if you experimentally verify some meta-property about one it verifies it for the other (such as, in this case, observing a property of a quantum particle freezes that particle) then this has been proved True and you were correct.
You can do this e.g. with photon polarization. Measuring here is very simple since it is achieved by sending your light through a linear polarizer.
If you send light through a chiral medium, the orientation of the polarization would normally rotate. However if you measure the polarization often enough (by putting enough polar filters that are all oriented in the same direction in between), this rotation will not occur.
This experiment is rather simple since you only need a standard optical bench and not any low temperature setup.
Maybe that folk wisdom originates from demons and the Nephilim, back from before the Deluge - that is, from our highly advanced ancestors who could manipulate energy and matter much better than we do before their planetary-scale hydroengineering project turned into a disaster?
You really should read the rest of the article. The imaging laser does nothing to impede normal movement, only quantum tunneling, which only happens frequently/reliably at near-absolute-zero temperatures anyway.
This particular effect has nothing to do with whether or not anyone is looking at an object. It's the methods of making it visible that cause the effect.