Quantum uncertainty, and Schrodinger's cat

[ryan]

Oh, so you do you not believe that the cat is in a superposition of being alive and dead?

What cat?

Schrodinger's cat

 

[Juma]

What cat?

Schrodinger's cat

Of course not. It doesnt exist.

 

[ryan]

Schrodinger's cat

Of course not. It doesnt exist.

If the Schrodinger thought experiment actually happened, do you believe that the cat would be in a superposition of being both dead and alive?

 

[skepticalbip]

Of course not. It doesnt exist.

If the Schrodinger thought experiment actually happened, do you believe that the cat would be in a superposition of being both dead and alive?

Schrodinger's thought experiment is a thought experiment. It can not actually be conducted because there is much more in reality that would be involved. The experimental conditions were supposed to have the cat's state solely dependent on the decay state of the radioactive particle. It was intended to show the absurdity of the idea that quantum uncertainty meant that quantum events were in a superposition of all allowed states until observed (the observation collapsing the wave function). If you accept the Copenhagen interpretation then you accept that the cat in the thought experiment would be both "dead and alive" until the box was opened.

So the question is, does quantum uncertainty mean that quantum events are in superposition of all allowed states until observed or does it mean, as Schrodinger maintained, that quantum events are always in some definite state but we can only know the probability of it being in any given state.

ETA:
Einstein's thought experiment of riding on a photon can not be conducted either because of concerns of reality. However, anyone criticizing this experiment because it doesn't account for all of the problems reality throws up preventing it, just doesn't understand thought experiments.

 

[Bomb#20]

A cop pulled Werner Heisenberg over on the freeway. "Excuse me sir," says the cop, "Did you know you that I just recorded your speed at exactly 98 miles per hour?"

"Oh great," says Heisenberg. "Now I'm lost".

Very good!

And in the same spirit...

 

[ryan]

If the Schrodinger thought experiment actually happened, do you believe that the cat would be in a superposition of being both dead and alive?

Schrodinger's thought experiment is a thought experiment. It can not actually be conducted because there is much more in reality that would be involved. The experimental conditions were supposed to have the cat's state solely dependent on the decay state of the radioactive particle. It was intended to show the absurdity of the idea that quantum uncertainty meant that quantum events were in a superposition of all allowed states until observed (the observation collapsing the wave function). If you accept the Copenhagen interpretation then you accept that the cat in the thought experiment would be both "dead and alive" until the box was opened.

So the question is, does quantum uncertainty mean that quantum events are in superposition of all allowed states until observed.

From what I have been reading, that is not what is being refuted by the opposing interpretations, except for the hidden variable theory. The two interpretations, objective collapse theory (wave function collapse) and the Copenhagen interpretation both agree that there is an initial superposition. But the objective collapse theory says that the collapse happens spontaneously and almost instantly.

Apparently, those who have claimed to have built quantum computers have been able to keep particles in a superposition of states (the quantum bit).

 

[skepticalbip]

Schrodinger's thought experiment is a thought experiment. It can not actually be conducted because there is much more in reality that would be involved. The experimental conditions were supposed to have the cat's state solely dependent on the decay state of the radioactive particle. It was intended to show the absurdity of the idea that quantum uncertainty meant that quantum events were in a superposition of all allowed states until observed (the observation collapsing the wave function). If you accept the Copenhagen interpretation then you accept that the cat in the thought experiment would be both "dead and alive" until the box was opened.

So the question is, does quantum uncertainty mean that quantum events are in superposition of all allowed states until observed.

From what I have been reading, that is not what is being refuted by the opposing interpretations, except for the hidden variable theory. The two interpretations, objective collapse theory (wave function collapse) and the Copenhagen interpretation both agree that there is an initial superposition. But the objective collapse theory says that the collapse happens spontaneously and almost instantly.

This would be a different idea of quantum uncertainty than Schrodinger's thought experiment addressed, so a different problem. However, a new thought experiment where a cat's state was solely dependent on the state of a quantum event would still have the cat both "alive and dead" during the time (however short) that the quantum event was in superposition even though in this one an observation wouldn't be required to collapse the wave function.

Apparently, those who have claimed to have built quantum computers have been able to keep particles in a superposition of states (the quantum bit).

ETA:
To add a modified version of my earlier post:

So the question is, does quantum uncertainty mean that quantum events are in superposition of all allowed states or does it mean, as Schrodinger maintained, that quantum events are always in some definite state but we can only know the probability of it being in any given state.

 

[bilby]

“About your cat, Mr. Schrödinger—I have good news and bad news.”

 

[ryan]

“About your cat, Mr. Schrödinger—I have good news and bad news.”

 

[Underseer]

No, I'm not.

Heisenberg's Uncertainty principle exists because any attempt to measure the position and velocity/direction of a particle necessarily involves bouncing another particle off of it or having it pass through a field. In either case, your measuring device is causing a significant change in the thing you're trying to measure, so simply by measuring it, you are changing it enough that your measurements are no longer valid.

That's a widespread misconception perpetuated by many bad popularizations of QM.

"... the uncertainty principle is inherent in the properties of all wave-like systems, and that it arises in quantum mechanics simply due to the matter wave nature of all quantum objects. ...
Mathematically, in wave mechanics, the uncertainty relation between position and momentum arises because the expressions of the wavefunction in the two corresponding orthonormal bases in Hilbert space are Fourier transforms of one another (i.e., position and momentum are conjugate variables). A nonzero function and its Fourier transform cannot both be sharply localized."

(Source)

ETA: It's straightforward to measure a particle's position or momentum or spin or whatever without bouncing anything off it or doing anything at all to it that can change it. You just have to generate two particles in an entangled state, such that the sum of their properties of interest is known based on a conservation law. Then you bounce something off the other particle from the one you want to measure without affecting.

I've been through an introductory Quantum class and have used those wave functions.

The Heisenberg uncertainty principle exists because of what I just said. The act of taking a measurement changes the thing you are measuring to some degree. This is why we use probability functions to describe subatomic particles in the first place. Looking at the dial on a measuring device is not what collapses the wave function and can't be unless you believe that sentient minds have supernatural powers.

 

[skepticalbip]

I've been through an introductory Quantum class and have used those wave functions.

The Heisenberg uncertainty principle exists because of what I just said. The act of taking a measurement changes the thing you are measuring to some degree.

Heisenberg’s uncertainty principle is a fundamental limit of possible precision due to the matter-wave nature of all quantum objects, "fundamental limit" meaning the best possible even without a measurement technique affecting the event. It applies in purely mathematical analysis even without measurement.

This is why we use probability functions to describe subatomic particles in the first place. Looking at the dial on a measuring device is not what collapses the wave function and can't be unless you believe that sentient minds have supernatural powers.

Seems like you disagree with the Copenhagen interpretation and side with Schrodinger’s interpretation.

But we use the probability function because quantum events are probabilistic. e.g. the tunneling effect; we can trap an electron in an energy well. Even though it doesn’t have enough energy to escape ("go over the wall") we can calculate the probability of it “tunneling” through the energy barrier and appearing beyond it using the wave function that extends beyond the barrier. Measurement confirms that the wave function correctly predicted what we should measure. This understanding is quite useful and has been applied in such things as tunnel diodes.

 

[Bomb#20]

I've been through an introductory Quantum class and have used those wave functions.

The Heisenberg uncertainty principle exists because of what I just said.

Indeed he knows not how to know who knows not also how to 'unknow'. - R.F. Burton

The act of taking a measurement changes the thing you are measuring to some degree.

Counterexample already provided.

This is why we use probability functions to describe subatomic particles in the first place.

There could perfectly well be a Heisenberg Uncertainty Principle in a fully deterministic universe, witness the de Broglie/Bohm interpretation of QM. Likewise, there could perfectly well have been a stochastic universe with no specific lower bound on the product of uncertainty in momentum and position, say, a Newtonian universe on a background of random noise. An act of taking a measurement would change the thing you are measuring to some degree, but it wouldn't provide any special linkage between position and momentum, or between energy and time, or between any other pair of complementary observables.

Looking at the dial on a measuring device is not what collapses the wave function and can't be unless you believe that sentient minds have supernatural powers.

That's a separate issue -- I make no claims about which interpretation is correct, if any, or about when the wave function collapses, if it does. (And I may be the only person in the world who prefers GRW on aesthetic grounds. ) But while sentient minds collapsing the wave function by looking at a dial strikes me as singularly improbable, whether that's how the universe works isn't up to us, and labeling such a circumstance with a content-free derogatory term like "supernatural" doesn't make it any more improbable.

 

[Cheerful Charlie]

Schroedinger meant his cat as a reductio ab absurdum of the idea that a thing such as a cat can have a wave function, the sum of all wave functions of the cat's constituent particles. Over some years now physicists and engineers have struggled to create quantum computers with little real success. We live in a sea of heat energy that collapses wave functions making keeping a few cubits working long enough to do more than the simplest calculations impossible to achieve. For the same reasons its hard to keep a cubit working, a cat's wave function would not last a tiny fraction of a second. This is all a waste of time once one realizes that.

 

[barbos]

ETA: It's straightforward to measure a particle's position or momentum or spin or whatever without bouncing anything off it or doing anything at all to it that can change it. You just have to generate two particles in an entangled state, such that the sum of their properties of interest is known based on a conservation law. Then you bounce something off the other particle from the one you want to measure without affecting.

That's cheating and lame interpretation of uncertainty principle.
What uncertainty principle really says is physical system can not be in a state where both position and momentum have zero uncertainty. In other words it's not possible to prepare your system to have certain position and momentum at the same time.

 

[tantric]

isn't there a school of thought whereby said functions are applied on a macro scale? what is that called?

 

[fromderinside]

isn't there a school of thought whereby said functions are applied on a macro scale? what is that called?

The point is still dodged. It can't be observed because it can't be measured. Think for a moment about the Voxel.

 

[Bomb#20]

ETA: It's straightforward to measure a particle's position or momentum or spin or whatever without bouncing anything off it or doing anything at all to it that can change it. You just have to generate two particles in an entangled state, such that the sum of their properties of interest is known based on a conservation law. Then you bounce something off the other particle from the one you want to measure without affecting.

That's cheating

[The referees confer...] "Disregard flag. First Down." - The Universe

and lame interpretation of uncertainty principle.

It's not an interpretation of the uncertainty principle in the first place -- it's a demonstration that a regrettably popular interpretation of the uncertainty principle is wrong. What it shows is that since the universe doesn't let you violate the uncertainty principle, but the universe does let you violate the alleged "No measuring without affecting" rule, the two cannot be one and the same. (Also, note that the measurement technique I described can't be used as a way to sidestep the uncertainty principle, because the initial position of the two particles and the sum of their momenta will themselves be sufficiently uncertain to obey the uncertainty relation.)

What uncertainty principle really says is physical system can not be in a state where both position and momentum have zero uncertainty. In other words it's not possible to prepare your system to have certain position and momentum at the same time.

Yes, that's my point.

 

[tantric]

I know how it works - I made an 4.0 in my Vision of Buddhism class, that's just my reality model. I was asking for the the school of physics that supports this - something bell?

 

[Speakpigeon]

People can consider weird the result of the double slit experiment but it is still the case that there is no logical problem with it whatsoever: Different experimental conditions produce different results. No big deal. The fact that some people do make logically problematic claims about these results doesn't imply anything about them.

Logic isn't the problem. The problem is understanding. Science seeks to understand that which we don't yet understand. Exactly how is it that the electron in a double slit experiment behaves as a particle when we try to watch it and like a wave when we don't? The Nobel committee is waiting for someone to answer that question. Obviously there are quite a few pretty sharp physicists trying to answer this question so any "obvious" answer has likely already been tested.

Yes, the problem is understanding.

All we have is interpretation, not evidence.

We can have acceptable evidence like in other scientific experiments by putting a camera inside the box, as already suggested. When we open the box, if the cat is dead we will see that the death of the cat has been recorded by the camera at some time before the opening of the box. If the cat is still alive when we open the box, all that the camera recording will show will be a cat alive until the box is opened (once the box is open the cat will bolt). A camera cannot help.
EB

Putting a camera in the box as an observer voids the thought experiment. The thought experiment is all about quantum weirdness, which changes when there is an observer (see the double slit experiment above). Your logical solution in this case is assuming that it doesn't

The thought experiment is about what goes on inside the box before the door is opened and the observer can have a look. As long as the door remains closed, the camera does not provide any observation of what is inside the box so the thought experiment remains essentially unaltered. The camera just becomes part of the system and there is a superposition of probability waves that specify the possible states of the camera at the end of the experiment.

Another question is that of possible interferences between possible states. An experiment with photons going through two slits is quite different. I don't see how in the Shrödinger experiment we could even observe interferences. Do you know that?
EB

 

[Speakpigeon]

Someone explain to me again how the detector, which observes whether the atom decayed, does not detect whether the atom decays and collapse the state of the whole system...

What detector? How would you know that a putative detector inserted inside the box detected something if not by opening the box, which would then collapse everything if it wasn't already done. So, you couldn't tell when it collapsed. I think this is the problem of insisting on a realist interpretation when by design any observer has to wait outside the box till the box opens, at which point he can't tell when exactly the waves collapsed.
EB