Quantum mechanics is random in nature?

[Delta Kilo]

OK, but it is hardwired into the mathematical formalism of QM.

It is hardwired into Born rule only. This is the only place where apparent randomness is generated, everything else follows from that. At the moment it is simply postulated, and the way it is usually done does not allow internal state of the measuring apparatus to enter into the picture, thus creating an impression that randomness is conjured out of nothing.

Clearly that fact does not preclude the possibility that some more fundamental theory with some other mathematical formalism but without the baked-in randomness could also exist.

I think the formalism is fine as it. All it takes is to demote Born rule from postulate into a theorem and show that input from the environment/apparatus is necessary for the measurement to take place. That's it. This will banish randomness from the rules and move it to initial conditions instead, just like with the explanation of 2nd law of thermodynamics.

So far, so good... But until we have a candidate theory to consider, "so far" isn't very far at all.

Well, attempts have been made to derive Born rule. I understand there is no consensus, but there has been progress in studying decoherence, mesoscopic states etc.
Like this Nobel Prize winning work of 20 years ago:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.77.4887

A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high Q microwave cavity. The mesoscopic superposition was the equivalent of an “atom+measuringapparatus” system in which the “meter” was pointing simultaneously towards two different directions—a “Schrödinger cat.” The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement.

(emphasis mine) Here "superposition" = deterministic (not random) input, "statistical mixture" = random output, "decoherence phenomenon" is responsible for creating randomness during measurement. Clearly there are rules governing this evolution and Born rule has to be the consequence of these rules.

 

[vanhees71]

Well, attempts have been made to derive Born rule. I understand there is no consensus, but there has been progress in studying decoherence, mesoscopic states etc.
Like this Nobel Prize winning work of 20 years ago:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.77.4887

(emphasis mine) Here "superposition" = deterministic (not random) input, "statistical mixture" = random output, "decoherence phenomenon" is responsible for creating randomness during measurement. Clearly there are rules governing this evolution and Born rule has to be the consequence of these rules.

Concerning the status of ideas how one could derive the Born rule from the other postulates of QT, see (it's another Nobel laureate, by the way :-)):

S. Weinberg, Lectures on Quantum Theory, Cambridge University Press (2012)

BTW one of the best QT textbooks of the recent years. As always with all textbooks by Weinberg following the "no-nonsense approach" to physics, which is most important when it comes to the discussion of interpretations, although I don't share Weinberg's opinion that the issue on interpretation is undecided today. As I emphasized in this thread (and already many times before in this forum) I think that there is nothing unsolved and that the physically relevant interpretation is given by how it is used in scientific work to analyze and describes the outcomes of experiments or, more general, any kind of observations in nature. Philosophical speculations are irrelevant for physics!

 

[DrChinese]

Anyway, all I'm saying is every time when there is random output in QM there just happens to be a thermal bath conveniently located nearby and therefore randomness in QM is emergent phenomena which does not need to be hardwired into the theory at the fundamental level.

And all I am saying is that we can rule out the thermal bath as the source of quantum randomness, unless the spacelike separated thermal baths possesses global (non-local) attributes. But yes, there is a thermal bath nearby.

 

[Delta Kilo]

And all I am saying is that we can rule out the thermal bath as the source of quantum randomness, unless the spacelike separated thermal baths possesses global (non-local) attributes. But yes, there is a thermal bath nearby.

I looked at your argument again and didn't find it convincing.

We have a system consisting of 2 separated but entangled photons such that their polarization is unknown but identical (Type I PDC for example). Observing the photons' individual polarizations by the 2 *different* observers - at the same angle - always yields the same results!

The results are the same for both observers but different from one run to the next.. This is an example of randomness induced (I presume) by the unknown internal state of the measuring apparatus.

Therefore, none - and I mean none - of the outcome can be attributed to the state of the observer unless there is something mysterious being communicated from observer to observer.

Well, we know from Bell's theorem that it cannot be attributed to the state of individual photon either. In other words there must be something mysterious being communicated from one photon to another. In which case one observer, being presented with superposition, can randomly choose one outcome and it will be mysteriously communicated from one photon to another.The other observer will then be presented with a resulting pure state and won't have any choice but to agree with the first one.

If the observers contributed to the uncertainty - to the randomness - then that would show up in experiments such as above. It doesn't.

I don't see why it should.
Uncertainty and randomness are two different notions. Uncertainty tells whether the system object+measuring apparatus is initially in pure pointer state or in superposition. Randomness then chooses which particular pointer state from those present in superposition it is going to evolve to. The observer contributes to the latter but not to the former.

 

[DrChinese]

Well, we know from Bell's theorem that it cannot be attributed to the state of individual photon either. In other words there must be something mysterious being communicated from one photon to another. In which case one observer, being presented with superposition, can randomly choose one outcome and it will be mysteriously communicated from one photon to another.The other observer will then be presented with a resulting pure state and won't have any choice but to agree with the first one.

We don't know that something is communicated from photon to photon or not, especially considering that the photons don't need to have ever co-existed or have ever shared a common light cone). You are accurate that the total measurement context could be:

SpatiallySeparatedAlice+EntangledPhotonA+SpatiallySeparatedBob+EntangledPhotonB+[who knows what else]

In this case, the observers must be in some kind of nonlocal contact. So you are actually stating the Bohmian interpretation. They call "randomness induced by the unknown state of the observers/universe" a pilot wave.

 

[vanhees71]

Well, there's always a common cause of photons being entangled. I guess what you are after is entanglement swapping, but also for this you first need entangled photons, which are produced in some local process in this entangled state (nowadays usually via parametric downconversion).

 

[David Lewis]

All it takes is to demote Born rule from postulate into a theorem and show that input from the environment/apparatus is necessary for the measurement to take place. That's it. This will banish randomness from the rules and move it to initial conditions instead.

Does a Geiger counter placed near a radioactive atom register both decayed and not decayed at the same time, or does the presence of the Geiger counter cause the state to be either one or the other?

 

[vanhees71]

A Geiger counter counts decays and not "non-decays". The Geiger counter doesn't cause the decay but registers it. Maybe I don't understand the question right, but isn't this obvious?

 

[Grinkle]

A Geiger counter counts decays and not "non-decays". The Geiger counter doesn't cause the decay but registers it. Maybe I don't understand the question right, but isn't this obvious?

To my reading, this was David Lewis' point. If it wasn't his point, then I prefer my reading of his post.

 

[Delta Kilo]

Does a Geiger counter placed near a radioactive atom register both decayed and not decayed at the same time, or does the presence of the Geiger counter cause the state to be either one or the other?

Well, the first part of your question is a matter of interpretation, the answer would be yes in MWI and no in BM.
The second part I guess is also yes, except of course a brick will serve just as well.
If an atom decays somewhere in the interstellar space far away from everything else, then, until the pieces actually hit something else (and it may take a while), it exists in superposition of decayed and non-decayed. When an atom or a decay product collides with another particle it gets entangled with it. It still remains in superposition but now it involves another atom as well. Eventually the number of other particles involved becomes sufficiently large for the process to be irreversible but it will take much longer. (well, that is how I understand it)

 

[entropy1]

So, I take it the formalism leaves room for a deterministic as well as a strictly random interpretation. My take on that is that you can't get strict randomness from determinism, but you can get determinism overlaying strict randomness (in the formalism at least). So randomness is the more fundamental factor in my view. My preference goes to strict randomness. However, the formalism leaves it open to interpretation.

 

[Zafa Pi]

"Individual events resulting from identical preparations are not reproducible. ...
Whether this nonreducibility is due to nondeterminism in nature, or merely to limitations (practical or fundamental) in the preparation procedure, is a question we cannot, and need not, answer here. The statistical approach is applicable in any case." Ballentine, page 44. (My emphasis)

Randomness (undefined), intrinsic?, lack of knowledge?, unknown algorithm?, which is the fairest of them all? I won't tell, I can't tell, and neither will the great John Bell.
Keep chasing fairies, ladies and gentlemen, it's good exercise.

 

[Jilang]

Well, the first part of your question is a matter of interpretation, the answer would be yes in MWI and no in BM.
The second part I guess is also yes, except of course a brick will serve just as well.
If an atom decays somewhere in the interstellar space far away from everything else, then, until the pieces actually hit something else (and it may take a while), it exists in superposition of decayed and non-decayed. When an atom or a decay product collides with another particle it gets entangled with it. It still remains in superposition but now it involves another atom as well. Eventually the number of other particles involved becomes sufficiently large for the process to be irreversible but it will take much longer. (well, that is how I understand it)

It's an interesting idea, but how likely is it that pieces flying apart would spontaneously recombine?

 

[davenn]

It's an interesting idea, but how likely is it that pieces flying apart would spontaneously recombine?

depends on the particles
eg ...
for alpha particle decay, very easily ... The alpha particle is just a helium nucleus and with readily recombine with free electrons
to form a stable Helium atom

 

[Zafa Pi]

depends on the particles
eg ...
for alpha particle decay, very easily ... The alpha particle is just a helium nucleus and with readily recombine with free electrons
to form a stable Helium atom

I thought that alpha and beta decay refer to atom that undergoes radioactive decay by emitting an alpha particle or an electron. The alpha particles and electrons themselves are not decaying.

 

[atyy]

A Geiger counter counts decays and not "non-decays". The Geiger counter doesn't cause the decay but registers it. Maybe I don't understand the question right, but isn't this obvious?

How can you show that the Geiger counter does not cause the decay?

Above you claimed that philosophical speculations are irrelevant for physics. Why is it relevant for physics that the Geiger counter caused or did not cause the decay? If you cannot show that the Geiger counter does not cause the decay, then it is you that is promoting the irrelevant philosophical speculations.

 

[davenn]

I thought that alpha and beta decay refer to atom that undergoes radioactive decay by emitting an alpha particle or an electron. The alpha particles and electrons themselves are not decaying.

yes that's correct, and I didn't state anything to the contrary,
but that wasn't the comment/question I was answering

 

[davenn]

How can you show that the Geiger counter does not cause the decay?

I counter that with ... can you show us that it does ??

I agree with vanhees71 ... the counter is just reacting/detecting particle emission

 

[DrChinese]

How can you show that the Geiger counter does not cause the decay?

There are plenty of ways to demonstrate this. For example, the number of clicks increases in the presence of radioactive material, but is absent when there is no such material nearby.

Your comments have veered well off the thread subject.

 

[atyy]

I counter that with ... can you show us that it does ??

I agree with vanhees71 ... the counter is just reacting/detecting particle emission

That is not a counter, since I never claimed (in the minimal interpretation) that it does. In the minimal interpretation, whether it does or does not is irrelevant.

 

[atyy]

There are plenty of ways to demonstrate this. For example, the number of clicks increases in the presence of radioactive material, but is absent when there is no such material nearby.

Your comments have veered well off the thread subject.

And do decays occur when there is no Geiger counter to register them?

 

[vanhees71]

How can you show that the Geiger counter does not cause the decay?

Above you claimed that philosophical speculations are irrelevant for physics. Why is it relevant for physics that the Geiger counter caused or did not cause the decay? If you cannot show that the Geiger counter does not cause the decay, then it is you that is promoting the irrelevant philosophical speculations.

If the observation would cause a radioactive substance to decay, it wouldn't decay, if nobody is "looking". This is obviously not true according to observations, because fortunately radioactive substances decay, no matter whether you look at them, and it's pretty hard to prevent it from decaying (there are examples like bound beta decay, where the Pauli principle has dramatic effects, but that's not the point here).

 

[atyy]

If the observation would cause a radioactive substance to decay, it wouldn't decay, if nobody is "looking". This is obviously not true according to observations, because fortunately radioactive substances decay, no matter whether you look at them, and it's pretty hard to prevent it from decaying (there are examples like bound beta decay, where the Pauli principle has dramatic effects, but that's not the point here).

But this seems to lead to the conclusion that the moon is there when nobody is looking!

 

[vanhees71]

Well, several conservation laws tell you that the moon is there when nobody is looking. This is one of those pseudoproblems you can think about if you are a philosopher ;-)).

 

[DrChinese]

And do decays occur when there is no Geiger counter to register them?

This question has nothing whatsoever to do with quantum mechanics. You may as well ask if rocks stick to the Earth when there is no scale to weigh them.

 

[atyy]

Well, several conservation laws tell you that the moon is there when nobody is looking. This is one of those pseudoproblems you can think about if you are a philosopher ;-)).

So particles have trajectories?

 

[atyy]

This question has nothing whatsoever to do with quantum mechanics. You may as well ask if rocks stick to the Earth when there is no scale to weigh them.

Do they?

The question is intinmately related to quantum mechanics. The usual answer is that in the minimal interpretation, quantum mechanics is silent on such issues. If we are also silent on such issues, then quantum mechanics is random - that is one version of Bell's theorem.

If the moon is there when we are not looking, then Bell's theorem says nothing about whether quantum mechanics is fundamentally random.

 

[vanhees71]

So particles have trajectories?

How do you come to this conclusion?

 

[DrChinese]

Do they?

The question is intinmately related to quantum mechanics. The usual answer is that in the minimal interpretation, quantum mechanics is silent on such issues. If we are also silent on such issues, then quantum mechanics is random - that is one version of Bell's theorem.

If the moon is there when we are not looking, then Bell's theorem says nothing about whether quantum mechanics is fundamentally random.

That you ask "do they" shows that this discussion has veered away from quantum mechanics.

To the question of whether a Geiger counter is the cause of atomic decay, the answer is: of what relevance is the answer to observed randomness of quantum behavior?

Bell says nothing about whether quantum mechanics is fundamentally random regardless of whether particles have observable attributes when not observed.

 

[vanhees71]

It's utter nonsense! No matter if you put a counter somewhere in Chernobyl radioactive elements decay according to their half-life. You can measure today and then take away the counter and measure again in 10 years at the same place. Then you'll see that the stuff decayed, no matter whether your counter was there or not.

 

[atyy]

That you ask "do they" shows that this discussion has veered away from quantum mechanics.

To the question of whether a Geiger counter is the cause of atomic decay, the answer is: of what relevance is the answer to observed randomness of quantum behavior?

The answer is relevant to the randomness of quantum behaviour.

If we are agnostic about whether the moon is there when we are not looking, we are taking an operational view of quantum mechanics. Under the operational view, assuming signal locality, Bell's theorem guarantees operational randomness.

But if the moon is there when we are not looking, then we are assuming reality. Assuming reality, Bell's theorem guarantees nonlocality.

Thus the answer to the question in the OP depends intimately on how we answer the question as to whether the moon is there when we are not looking.

 

[vanhees71]

Also in quantum theory there are conservation laws telling you that the moon doesn't vanish, only because nobody is looking at her. This is now REALLY and empty discussion. Bell used this example to ridicule the collapse idea. He's also asking whether you need a "conscious observer", and what that might mean. Has there be the first collapse, when human beings made an observation or is an amoeba, making an observation enough?

 

[Grinkle]

Thus the answer to the question in the OP depends intimately on how we answer the question as to whether the moon is there when we are not looking.

In what sense is it practical to not look? To look means to interact with some other particle. It doesn't mean to look in the animal sense and to comprehend.

 

[vanhees71]

When I look at the moon my eyes just detect light that's reflected by the moon no matter whether I look at her or not.

 

[atyy]

How do you come to this conclusion?

If the moon is always there, then a particle is always there too. That means a particle has a trajectory.