I Would this experiment prove or disprove quantum randomness?

[john taylor]

I was told of a thought experiment by a friend of mine who says that his thought experiment will either prove or disprove randomness in quantum mechanics, and this was in fact verified by several other physicists, who said that if this experiment was ever successfully carried out it would either prove or disprove quantum randomness. His thought experiment imagines(in laymans terms), two teams of scientists who want to measure the position of 10 000 neutrons which will be fired, so that they travel 1 at a time. The scientists will measure the position of these neutrons by bouncing gamma rays off of the neutron to find its location, and they will measure it at exactly the same time. You would naturally expect both parties to measure the exact same position each time for each particle. However because the probability of the wave function is relative to each observer, the wave function collapse would have to collapse randomly relative to each of the two observers. Therefore if they observe the same location for each neutron 100% of the time it would prove determinism because if the wave function is randon collapsing 100%(all of the time) of the time to produce the same result for its location for both observers it is by definition not a random process(because you cannot consistently get the same result from two random process all of the time, that would suggest if this took place that it was not random but that it was fixed and that there was a defined process taking place underneath), however the only way for randmness to then be proven would be for the scientists to observe to conflicting results for the same particle at exactly the same time, for it's position, because it was shown earlier that it would not be random for two random relative processes(the collapse of the wave function), to yield the same result for the position of a large sample of particles, so the only way to disprove this would be to observe an instance in which tow conflicting results for the position of a particle is measured at exactly the same time. would this experiment be a proof if it could be done? Or could this experiment be a stepping stone to an experiment which would actually work?

 

[PeterDonis]

they will measure it at exactly the same time.

How? How will the experiment enforce this constraint?

 

[DrChinese]

Therefore if they observe the same location for each neutron 100% of the time it would prove determinism because if the wave function is randon [sic] collapsing 100%(all of the time) of the time to produce the same result for its location for both observers it is by definition not a random process(because you cannot consistently get the same result from two random process all of the time, that would suggest if this took place that it was not random but that it was fixed and that there was a defined process taking place underneath)

Your premise ("you cannot consistently get the same result from two random process all of the time") is false. Experiments on entangled particle pairs by different observers yield identical outcomes for identical observations (such as spin at some angle setting), and they don't need to be observed at exactly the same time. The outcomes themselves are random but correlated. In your view, that proves "determinism" of some kind. But it doesn't, that's just your assumption! The observers are not measuring independent attributes in either your example or mine. There is a predicted dependency between what the two observers see. The outcomes appears random, and unconnected to any currently known "cause".

It is a subject of debate and semantics as to whether there is true randomness or not (in quantum nature). There are interpretations of quantum mechanics that go in varying ways on this matter. The only thing that will answer it (experimentally) is to locate a precursor attribute that would allow us to predict the apparently random outcome in advance. Short of that, it is a matter of faith.

 

[cosmik debris]

Can you prepare 10000 neutrons in the same state? I thought the uncertainty principle would have something to say about that. Also I would've thought that probabilities of events were invariant.

Cheers

 

[PeterDonis]

Can you prepare 10000 neutrons in the same state? I thought the uncertainty principle would have something to say about that.

No, it doesn't. The uncertainty principle just says that there can't be a state that is simultaneously an eigenstate of multiple non-commuting observables. As long as you restrict yourself to eigenstates of just one observable (for example, spin along a particular direction), the uncertainty principle puts no limits on how many systems you can prepare in that state.

In the real world, no preparation apparatus is perfect, so yes, a real apparatus would not be able to prepare 10,000 neutrons in exactly the same state. But that has nothing to do with the uncertainty principle.

 

[john taylor]

It's a though experiment and so should be judged conceptually.

 

[john taylor]

So do you think it would work in concept or in principle or do you think this thought experiment has any value at all(for instance could be modified into an experiment which works)?

 

[PeterDonis]

So do you think it would work in concept or in principle

You haven't answered the question I posed in post #2. I think if you try to answer it, you will find that it can't be answered: there is no way to enforce the constraint of having two measurements at "exactly the same time". So your experiment can't be realized, even in principle.

@DrChinese also makes good points in post #3.

 

[john taylor]

I understand Mr Peter Donis, but let's say in principle you could enforce those constraints would the experiment work in principle?

 

[PeterDonis]

lets say in principle you could enforce those constraints

You can't, even in principle. That's my point.

 

[john taylor]

There are many thought experiments in which certain constraints are virtually impossible to enforce.

 

[PeterDonis]

There are many thought experiments in which certain constraints are virtually impossible to enforce.

If you can't enforce them even in principle, the thought experiment is meaningless.

 

[john taylor]

Well do you reckon this experiment could be modified into an experiment which works?

 

[DrClaude]

You would naturally expect both parties to measure the exact same position each time for each particle. However because the probability of the wave function is relative to each observer, the wave function collapse would have to collapse randomly relative to each of the two observers.

I don't get this. Why wouldn't the two scientists measure the same position?

And what do you mean by "the wave function is relative to each observer"?

 

[PeterDonis]

do you reckon this experiment could be modified into an experiment which works?

I don't see what "works" would mean. I think the way you are trying to analyze the experiment is confused. @DrChinese and @DrClaude have both pointed out examples of this.

I also think it's going to be very difficult to have a useful discussion based on second hand information. You say:

I was told of a thought experiment by a friend of mine who says that his thought experiment will either prove or disprove randomness in quantum mechanics, and this was in fact verified by several other physicists, who said that if this experiment was ever successfully carried out it would either prove or disprove quantum randomness.

If physicists actually believe such an experiment would prove something, there should be a peer-reviewed paper somewhere that says so and explains why. That's what we should be basing our discussion on.

 

[Paul Colby]

You would naturally expect both parties to measure the exact same position each time for each particle.

Isn't this is false from the get go? I could have 1000 hydrogen atoms in their ground state and use electrons instead of neutrons. They'd get 1000 different results when referenced to the same point relative to each atom.

 

[DaveC426913]

You would naturally expect both parties to measure the exact same position each time for each particle.

HUP will prevent this. The more accurately you try to measure the position, the less you can know about its momentum.
So, you can't know you're measuring it at the same time. It's spread out.

 

[Nugatory]

I understand Mr Peter Donis, but let's say in principle you could enforce those constraints would the experiment work in principle?

You are missing the point of the question. The thing that prevents us from preparing 10000 particles in exactly the same position eigenstate (there's some mathematical sloppiness in this choice of words but that can't be helped in a math-free thread, and it doesn't affect the argument) is quantum randomness in the form of the HUP. Thus, to set up this experiment - even as a thought experiment - we have to start from the premise that there is no quantum randomness. It is not at all surprising that when we start with that premise we get results that show that there is no quantum randomness... but these results tell us nothing about whether the premise is in fact valid.

 

[PeterDonis]

The thing that prevents us from preparing 10000 particles in exactly the same position eigenstate (there's some mathematical sloppiness in this choice of words but that can't be helped in a math-free thread, and it doesn't affect the argument) is quantum randomness in the form of the HUP.

This way of putting it might be misleading. The preparation procedure in the experiment as described in the OP is that 10,000 neutrons are "fired". This usually means something close to a momentum eigenstate (how close depends on how well collimated the source is). It is nothing like preparing a particle in a position eigenstate. Also, as I pointed out in post #5, the HUP does not place any limitations on preparing multiple particles in the same state; the HUP only says there aren't simultaneous eigenstates of non-commuting observables, and places limits on how "close" a given state can be to eigenstates of multiple commuting observables. Here we are just talking about measuring a single observable, position, so the HUP is irrelevant.

The way I would describe the in principle problem with the proposed experiment is that it is impossible, even in principle, to make two position measurements "at exactly the same time".

 

[zonde]

His thought experiment imagines(in laymans terms), two teams of scientists who want to measure the position of 10 000 neutrons which will be fired, so that they travel 1 at a time. The scientists will measure the position of these neutrons by bouncing gamma rays off of the neutron to find its location, and they will measure it at exactly the same time. You would naturally expect both parties to measure the exact same position each time for each particle. However because the probability of the wave function is relative to each observer, the wave function collapse would have to collapse randomly relative to each of the two observers. Therefore if they observe the same location for each neutron 100% of the time it would prove determinism because if the wave function is randon collapsing 100%(all of the time) of the time to produce the same result for its location for both observers it is by definition not a random process(because you cannot consistently get the same result from two random process all of the time, that would suggest if this took place that it was not random but that it was fixed and that there was a defined process taking place underneath), however the only way for randmness to then be proven would be for the scientists to observe to conflicting results for the same particle at exactly the same time, for it's position, because it was shown earlier that it would not be random for two random relative processes(the collapse of the wave function), to yield the same result for the position of a large sample of particles, so the only way to disprove this would be to observe an instance in which tow conflicting results for the position of a particle is measured at exactly the same time. would this experiment be a proof if it could be done? Or could this experiment be a stepping stone to an experiment which would actually work?

As I understand in each iteration of this experiment there is neutron from which several gamma photons are bounced and then some of them are detected by one observer and some other are detected by other observer. And there is requirement that both observations happen at exactly the same time (probably in lab's rest frame, right? or possibly in neutron's rest frame).
But different observers are detecting different gamma photons that bounced from neutron at different times. How do you imagine you could satisfy your requirement about the same time? And more importantly why do you expect inconsistent observations? Because QM certainly does not predict inconsistent observations.