How can light exert pressure?

[K^2]

Particles are not changing their properties because a scientist is watching them. Nothing is exiting our eyes and interacting with them. It's the act of measuring that destroys some of the data, and it's machines that do it.

It's way more complicated than that. You should read up on quantum eraser one of these days. Suffice it to say, you can make a measurement and instead of collapsing the wave function, the wave function of the measurement device becomes entangled to the thing you are measuring. In fact, in pure Quantum Mechanics there is no such thing as collapse at all. You just get things more and more entangled. Collapse itself has more to do with stat mech and what you define to be your observer. It's an interesting topic.

A quantum mechanical wave function can contain superpositions of many fundamental states if you don't measure the system but the moment you perform the measurement you cause it to collapse into one of these discrete states or another.

Not exactly. There isn't such a thing as "fundamental state". There are basis states for the measurements. For example, if you are measuring spin of a particle, you can choose an axis along which you are measuring. If you measure along z axis, the state will collapse into a "spin-up" or "spin-down" state. But you can also measure along x or y, or any other axis. (Obviously, the very choice of z is arbitrary.) And then the system will still collapse into one of two states, but they will be a different pair of states. And each of these can be written as a superposition of the original "spin-up" and "spin-down".

[lajoswinkler]

What I simply wanted to point out was that people staring at stuff doesn't change anything. It's one of the most persistent and stupid misunderstandings of the quantum mechanics with lots of bad consequences.

[K^2]

What I simply wanted to point out was that people staring at stuff doesn't change anything. It's one of the most persistent and stupid misunderstandings of the quantum mechanics with lots of bad consequences.

It does, though. If you make an observation, you've made a measurement. And making a measurement changes the wave function. You can't get around that.

[WestAir]

K^2,

Lajoswinkler is trying to argue the "if a tree falls and there's no one around to hear it" argument. He's saying that a scientists eyes don't emit EMR, rather it observes visible light that comes to it, therefore a watching eye can't physically interact with the wave function experiment. Which is true to a point, but because the wave function is still being interacted with by something, it's obviously still being "observed" - whether the scientist is there to hear the tree fall or not.

Lajoswinkler will correct me if I misunderstood his point.

[Fractal_UK]

What I simply wanted to point out was that people staring at stuff doesn't change anything. It's one of the most persistent and stupid misunderstandings of the quantum mechanics with lots of bad consequences.

It's important to realise that it isn't simply "destructive" imaging of a state as you implied before, i.e. firing a photon at an object and the energy of that photon causes that object to change in some way. It is something fundamental about the measurement process that means that making an observation changes that system from existing as a superposition of states with some probability amplitude to one that exists in a defined state. Also, it isn't that the system will just continue on doing what it was before, after you make an observation you may have radically altered the time evolution of that quantum system from what it would have been if you had not ever made that measurement.

[K^2]

He's saying that a scientists eyes don't emit EMR, rather it observes visible light that comes to it, therefore a watching eye can't physically interact with the wave function experiment.

Ah, well, that's a common mistake. QM doesn't obey locality. Which might seem like a very crazy thing, but it's really alright. All observable interactions are still local.

Of course, the reason is that it's essentially a mathematical trick. MWI gets you all the same results without loss of locality.

Edit: I should probably clarify this a bit. If you change the state of, say, an electron, you change the wave function of every single electron in the entire universe at the same time. Instantly. There is no speed-of-light delay. So one shouldn't be surprised that just an act of watching system from afar can change the wave function of that system.

Edited September 30, 2013 by K^2

[Tommygun]

Edit: I should probably clarify this a bit. If you change the state of, say, an electron, you change the wave function of every single electron in the entire universe at the same time. Instantly. There is no speed-of-light delay. So one shouldn't be surprised that just an act of watching system from afar can change the wave function of that system.

I never knew that, so could you use that as a way of communication?

[K^2]

I never knew that, so could you use that as a way of communication?

Nope. In fact, there is a theorem that says that you cannot. Though, you can do neat things with it. For example, quantum teleportation. It allows you to send quantum information via classical channel.

Ref: No-Communication Theorem.

[drakesdoom]

Now I do clearly admit the specifics of current quantum theory are beyond my current understanding, that and women everything else I know completely. However it seems to me that one you are arguing over semantics and two you are arguing over who is right on the very edge of science where even those that specialize in the field are still running experiments to get a handle on it. Maybe this should just be let sat for a few years?

[K^2]

This isn't the edge of science. QM is well established and has been so for a long time. There is some debate on interpretations, but it's purely philosophical. It has no impact on science.

[GregoryNeal]

This thread has taken a huge turn from what the OP originally posted.

[lajoswinkler]

It does, though. If you make an observation, you've made a measurement. And making a measurement changes the wave function. You can't get around that.

No, it doesn't. If I have two machines doing the same experiment on particles and I look at one, I won't change anything. I think you actually understand the problem, but you don't understand what I'm trying to say.

K^2,

Lajoswinkler is trying to argue the "if a tree falls and there's no one around to hear it" argument. He's saying that a scientists eyes don't emit EMR, rather it observes visible light that comes to it, therefore a watching eye can't physically interact with the wave function experiment. Which is true to a point, but because the wave function is still being interacted with by something, it's obviously still being "observed" - whether the scientist is there to hear the tree fall or not.

Lajoswinkler will correct me if I misunderstood his point.

Yes, that was my point. Two machines can make a measurement and print out the results without anyone ever being in the room. The results will be no different from other machines which were measuring stuff while they were watched by people. And if the printed results come in a sealed envelope, they won't change upon opening even if they were not ever acknowledged by people. It is absurd, but quacks are actually using this misinterpretation to fool a lot of naive folks.

It's important to realise that it isn't simply "destructive" imaging of a state as you implied before, i.e. firing a photon at an object and the energy of that photon causes that object to change in some way. It is something fundamental about the measurement process that means that making an observation changes that system from existing as a superposition of states with some probability amplitude to one that exists in a defined state. Also, it isn't that the system will just continue on doing what it was before, after you make an observation you may have radically altered the time evolution of that quantum system from what it would have been if you had not ever made that measurement.

But the observation has nothing to do with people. It's the machines that do it. You're fixing on the method and missing the main point.

[K^2]

No, it doesn't. If I have two machines doing the same experiment on particles and I look at one, I won't change anything.

It does change. Look up Quantum Eraser. The question is only in whether the decoherence has had time to take place.

A measurement device taking a measurement does not result in wave function collapse. If it did, we wouldn't have Schrodinger's Cat. We do have experimental verification of measurement systems becoming entangled with the measured object without collapse. Again, Quantum Eraser makes use of that principle in a very clever way.

[Fractal_UK]

A measurement device taking a measurement does not result in wave function collapse. If it did, we wouldn't have Schrodinger's Cat.

We don't have Schrodinger's cat, Schrodinger's cat is what happens when you try and pretend that a classical system is in fact a quantum system. The result is amusing but devoid of direct physical meaning.

All he is saying is that if you perform a double slit experiment and detect which slit the particle passes through, it doesn't matter if a scientist looks at the computer screen recording that information or not. You have still generated wave function collapse by entangling your quantum system to your large classical detector.

Edited September 30, 2013 by Fractal_UK

[Kryten]

In the quantum eraser experiment the effect is still seen after photons are 'observed' only by detectors. Lajos is talking about the kind of people that point to such experiments as proof that, for example, the souls exist (to act as the 'outside observer' they think is necessary); or, more commonly that whatever cancer-curer or mind-control device they're trying to sell works.

[K^2]

We don't have Schrodinger's cat, Schrodinger's cat is what happens when you try and pretend that a classical system is in fact a quantum system. The result is amusing but devoid of direct physical meaning.

All he is saying is that if you perform a double slit experiment and detect which slit the particle passes through, it doesn't matter if a scientist looks at the computer screen recording that information or not. You have still generated wave function collapse by entangling your quantum system to your large classical detector.

Again, you are confusing effects of decoherence with collapse. Every classical system is a statistical limit of a quantum system. And macroscopic system can have very weird macroscopic effects because of this fact. Semiconductors, superconductors, lasers. If quantum physics stopped working at classical scales we'd have none of these.

In pure Quantum Mechanics, you must decide on what you are calling your observer before you interpret the results. And whether you choose the machine that does the experiment, the computer that stores it, or lab technician who looks at the results as your observer matters. You get completely different results and you get collapse happening at completely different times. It's only when you take into account the statistical mechanics and effects of decoherence that all of these become equivalent. You can say that collapse happened during initial measurement, during recording, or during observation. But only if all of these things happened after the decoherence has taken place.

Quantum Eraser specifically demonstrates what happens if you make these assumptions prior to decoherence of the system. You make mistakes. That's because it is wrong to assume that measurement causes the collapse. Collapse is caused by observer and choice of observer matters. In QE the observer is outside of the sub-system by design, and so you get no collapse in the sub-system. If the system has no observer, there is no collapse ever and no events happen in the system. This is easy to see if you diagonalize the Hamiltonian of the system. Amplitude of each eigen state remains constant. Id est, absolutely nothing happens in the system. It never changes.

Edit: If you ever get confused by QM in Copenhagen Interpretation, look at it from the MWI standpoint. It almost always helps.

Lajos is talking about the kind of people that point to such experiments as proof that, for example, the souls exist (to act as the 'outside observer' they think is necessary); or, more commonly that whatever cancer-curer or mind-control device they're trying to sell works.

Substituting one fallacy with another is never helpful. If you don't understand QM, don't try to explain it.

Edited October 1, 2013 by K^2

[Fractal_UK]

Again, you are confusing effects of decoherence with collapse. Every classical system is a statistical limit of a quantum system. And macroscopic system can have very weird macroscopic effects because of this fact. Semiconductors, superconductors, lasers. If quantum physics stopped working at classical scales we'd have none of these.

I'm not confusing anything. Decoherence is simply the mechanism by which observed wavefunction collapse occurs. Whether a system is macroscopic or not is largely irrelevant, you can have numerous macroscopic systems that behave quantum mechnically, even if macroscopic systems would traditionally be thought of as being classical. Scales are not quantum mechanical or classical, rather the properties of a given system determine whether its behaviour is dominated by quantum mechnical or classical effects. The latter may occur due to us approaching the classical limit of some huge system fundamentally described by quantum mechanics but that doesn't mean it's valid to try and assign some |Alive> and |Dead> states to a classical system like a cat and assume that's in some way meaningful.

[K^2]

Decoherence is not the mechanism of collapse. You can have collapse without decoherence and vice versa. Again, consider MWI where there is no such thing as collapse but there is such thing as decoherence, since decoherence is interpretation-independent.

And any classical state is described by some ensamble of quantum states. So long as you keep these ensambles separate, you can assign classical state labels to a quantum system. There are plenty of examples of that in quantum computing where amplification of states is required. Heck, if you get right down to it, Stern-Gerlach is the basic example. We assign labels of classical state - magnetization, to a quantum particles, do measurements, and everything's fine.

Again, where the problem happens is decoherence. If your classical states correspond to huge ensambles of quantum states, decoherence is effectively instant. Any kind of interaction between sub-system and the overall system results in there being no difference between assuming collapse happens during initial measurement or when the box was opened.

[Fractal_UK]

Decoherence is not the mechanism of collapse. You can have collapse without decoherence and vice versa. Again, consider MWI where there is no such thing as collapse but there is such thing as decoherence, since decoherence is interpretation-independent.

You seem to be intent on misreading what I have written in order to make some kind of a point and honestly, it's getting a bit silly. Re-read my post. I said decoherence is the mechanism by which observed wavefunction collapse occurs. It doesn't matter whether wavefunction collapse represents something that has real physical meaning or whether does not even occur, the fact remains that by making observations, we still observe an apparent wavefunction collapse and this effect is explained by decoherence.

[K^2]

No, it does not. Collapse occurs even in a system without decoherence. If I take a true isolated system, there is no decoherence. There is still wave function collapse, however, so long as you select an observer.

[sumghai]

I'm surprised that nobody in this thread (up until this point and to my knowledge) has mentioned the Crookes radiometer:

[K^2]

Yeah, but its motion isn't due to light pressure.

[lajoswinkler]

It does change. Look up Quantum Eraser. The question is only in whether the decoherence has had time to take place.

A measurement device taking a measurement does not result in wave function collapse. If it did, we wouldn't have Schrodinger's Cat. We do have experimental verification of measurement systems becoming entangled with the measured object without collapse. Again, Quantum Eraser makes use of that principle in a very clever way.

It does not change. If it changed anything, any experiment involving particles colliding and waves interfering would be completely useless because sometimes there'd be a scientist in the room, and sometimes not. In fact, the world would not work properly because lots of different phenomena would be compromised just because a guy is staring at something.

Schrödinger's cat is a thought experiment. A teaching model, an impossible analogue od quantum world in macrocosmos. Schrödinger made it up to show what happens with particles/waves. He could've taken an elephant or a mushroom. It's just a model.

Wave function collapses happen in the quantum world, not with cats. Cat is alive or dead, and electrons can have two different properties at the same time before a measurement of those properties is made.

Sorry, but you really don't understand the issue at hand.

We don't have Schrodinger's cat, Schrodinger's cat is what happens when you try and pretend that a classical system is in fact a quantum system. The result is amusing but devoid of direct physical meaning.

All he is saying is that if you perform a double slit experiment and detect which slit the particle passes through, it doesn't matter if a scientist looks at the computer screen recording that information or not. You have still generated wave function collapse by entangling your quantum system to your large classical detector.

Exactly my point here. Thanks for the clarification.

In the quantum eraser experiment the effect is still seen after photons are 'observed' only by detectors. Lajos is talking about the kind of people that point to such experiments as proof that, for example, the souls exist (to act as the 'outside observer' they think is necessary); or, more commonly that whatever cancer-curer or mind-control device they're trying to sell works.

Bingo.

[K^2]

It does not change. If it changed anything, any experiment involving particles colliding and waves interfering would be completely useless because sometimes there'd be a scientist in the room, and sometimes not. In fact, the world would not work properly because lots of different phenomena would be compromised just because a guy is staring at something.

You are being silly. Does General Relativity not work because you have to chose a frame of reference, and because outcome depends on the choice? Why should QM not work because it depends on choice of observer?

Schrödinger's cat is a thought experiment. A teaching model, an impossible analogue od quantum world in macrocosmos. Schrödinger made it up to show what happens with particles/waves. He could've taken an elephant or a mushroom. It's just a model.

Wave function collapses happen in the quantum world, not with cats. Cat is alive or dead, and electrons can have two different properties at the same time before a measurement of those properties is made.

Sorry, but you really don't understand the issue at hand.

Do you understand how a measurement is made? Do you understand that there are experiments where the measurement of the spin is conducted by a microscopic system and where we can actually verify that no collapse happens, but rather the measurement device goes into a superposition of states? Now, we can't verify Schrodinger Cat on the scale of the cat because of the aforementioned decoherence, but we can verify it on microscopic systems and it has been verified.

I know it's a thought experiment. I also know that it was designed to show that QM is absurd. Problem is, the world really works that way.

There is no collapse at measurement, because measurement process can be completely described by a Hamiltonian. All it does it takes a superposition of states on a particle and transfers it to superposition of states in the measuring device. And I can do this for every step of the way. You tell me a process, and I can show you that there is no collapse. Measurement, recording of the result, and so on. Unless you actually chose an observer, you won't get a collapse. If you insist that there is no observer in the system, no collapse happens at all.

You do understand that collapse is part of interpretation and not part of the actual QM, right?

[lajoswinkler]

I don't really understand what are we actually discussing right now. All I'm saying is that one set of rules works in the tiny world, and other set works in the big world.

The particles don't give a damn whether they're stared at by me, a cat, a tree or a jar of pickles. That's all I'm saying, and I have the feeling this is turning into an armchair phylosophy. It's starting to go towards solipsism.