Understanding the Relationship Between Determinism and Quantum Mechanics

[revo74]

Is it wrong to say that QM is random? Aren't we too ignorant of what's going on at the level of QM to claim with certainty what's going on?

How could the Universe be deterministic at the level of GR, but we different at the level of QM. They are both connected, only describing different scales.

 

[maverick_starstrider]

Is it wrong to say that QM is random? Aren't we too ignorant of what's going on at the level of QM to claim with certainty what's going on?

How could the Universe be deterministic at the level of GR, but we different at the level of QM. They are both connected, only describing different scales.

Quantum Mechanics is not random, but the outcomes of a specific measurement are probabilistic and that is a key difference. Its mathematical description is entirely deterministic up until a measurement is made at which point it tells us the relevant probabilities for various outcomes. Furthermore, we no undoubtedly that should there lie a deterministic core (what is called a hidden variable theory) it would have to violate locality (and thus relativity). It's pretty far from my field of expertise but I believe it is possible to recover much of GR within the framework of QFT. The fact that GR should have equations that are deterministic and QM should not is not particularly interesting regardless. The reason being that larger quantum systems tend to have their probabilistic nature drowned out (decoherence) which is why large quantum systems can be treated within the framework of Newton's Classical Mechanics, which is deterministic, to great accuracy. I don't believe particle physicists and cosmologists would be surprised at all if it turned out that the equations of GR were just an effective field theory of an ultimately quantum one.

 

[DrChinese]

Is it wrong to say that QM is random? Aren't we too ignorant of what's going on at the level of QM to claim with certainty what's going on?

The answers to these questions are what is called "interpretation dependent". There is absolutely NO evidence that quantum phenomena are anything other than fully random. There is substantial evidence that it is not possible to complete our understanding of what is going on at the quantum level by additional knowledge. However, these statements are explainable by some deterministic interpretations. However, they will not be causally local.

 

[maverick_starstrider]

fully random

Again, I'm not a fan of the term "fully random" since it implies, in everyday usage, that essentially any outcome is possible or even that all outcomes are equally likely, that it lacks order or description; chaotic (this word also in the layman sense). Which is of course not the way quantum is at all. Quantum mechanics can tell you that if you perform a certain experiment then 73.31873...% of the time you will get outcome A and 26.68127..% of the time you will get outcome B. In this sense it is very deterministic and accurate (if you repeat the exact same experiment many many times you will find the probabilities predicted by QM to be extremely accurate). However, on any given experiment it not only can't say which outcome will occur but it says that it can't be known at the most fundamental level (reality itself doesn't know). In this sense it is "random". However, in our made up example it does give you extremely accurate probabilities for outcome A and B and in addition tells you that outcomes C, D and E will never occur.

 

[unusualname]

@maverick_starstrider let me make it simple, QM is not purely deterministic, does that help?

 

[maverick_starstrider]

@maverick_starstrider let me make it simple, QM is not purely deterministic, does that help?

Well I'm just saying if you're talking to a non-physicist "probabilistic" and "random" will likely be interpreted differently. When layman talk about random they tend to mean something either has a uniform random distribution or an unknown distribution. Neither of which is an aspect of QM.

 

[unusualname]

Well I'm just saying if you're talking to a non-physicist "probabilistic" and "random" will likely be interpreted differently. When layman talk about random they tend to mean something either has a uniform random distribution or an unknown distribution. Neither of which is an aspect of QM.

Ok, point taken, and you've actually been very reasonable in your posts above (so apologies for the bluntness of my post).

I believe there is one deterministic law in the universe, it is Schrödinger Evolution, the rest is non-deterministic (check my home page for exactly how I think it works)

 

[DrChinese]

if you repeat the exact same experiment many many times you will find the probabilities predicted by QM to be extremely accurate). However, on any given experiment it not only can't say which outcome will occur but it says that it can't be known at the most fundamental level (reality itself doesn't know). In this sense it is "random".

My point is that it appears random to every test humanly possible. That would indicate NO evidence that it is NOT random. The rest is semantics. If a pattern is found in the future, so be it. But it existeth not at this time!

 

[DrFurious]

There is absolutely NO evidence that quantum phenomena are anything other than fully random.!

You've hit on one of the great unsolved problems in QM. Wiki "quantum chaos" or the "correspondence principle" for more. The answer isn't so clear cut.

 

[LukeD]

Depending on interpretation, it'd be yes or no.

Completely Random, if your interpretation says that the "wave" characteristic exists or that all other possible worlds exist.

--Not-- at all Random, if your interpretation says that classical physics arises out of an unknowable -deeper- classical-type physics in which the usual classical variables only exist as equilibrium variables. In that case, the interpretation is not random dynamics, but ultimately unknowable dynamics.