Is quantum mechanics imply nature is deterministic or probabilistic?

[Pipsqueakalchemist]

TL;DR

I’m confused about what the true nature of quantum mechanics is so id like people to explain it to me

So initially I thought quantum mechanics was deterministic in the equations but was probabilistic in measurement. I’m aware of bell’s inequality which rules out hidden variables unless you assume super determinism. But recently I’ve come across something called decoherence and some people have told me that it solves the measurement problem. My understanding is that information of the quantum object is lost during measurement to the environment so that means the Schrödinger equation doesn’t give us all the information to make the predictions that’s why it seems random when we measure. So does bell theorem account for decoherence? Or does decoherence doesn’t change the fact the universe is fundamentally probabilistic.

 

[atyy]

Summary:: I’m confused about what the true nature of quantum mechanics is so id like people to explain it to me

There are several interpretations about the true nature of quantum mechanics. The most common interpretation avoids asking questions about its "true nature" and accepts it as a practical recipe for an observer to predict the probabilities of measurement outcomes.

So initially I thought quantum mechanics was deterministic in the equations but was probabilistic in measurement.

That's true of the formalism.

I’m aware of bell’s inequality which rules out hidden variables unless you assume super determinism.

Bell's inequality does not rule out nonlocal hidden variables. It rules out local hidden variables. And yes, when ruling out local hidden variables, several assumptions are made, including no superdeterminism.

But recently I’ve come across something called decoherence and some people have told me that it solves the measurement problem.

It is a common misconception that decoherence solves the measurement problem. It does not. However, it is probably part of the solution to the measurement problem. The measurement problem is that quantum mechanics requires a subjective division between observer and the system being measured. Decoherence means that in many cases, we get consistent results even if we place that subjective division between observer and system at different places.

My understanding is that information of the quantum object is lost during measurement to the environment so that means the Schrödinger equation doesn’t give us all the information to make the predictions that’s why it seems random when we measure. So does bell theorem account for decoherence? Or does decoherence doesn’t change the fact the universe is fundamentally probabilistic.

In decoherence, we have the observer and the total quantum system as usual. The total quantum system evolves deterministically. The difference is that here we make the total quantum system contain the environment and the quantum system (ie. total quantum system = environment + quantum system). So information is lost when we focus on the quantum system, but information is not lost when we consider the total quantum system. The measurement problem is not solved, since we still need an observer to make a measurement on the total quantum system.

 

[Pipsqueakalchemist]

There are several interpretations about the true nature of quantum mechanics. The most common interpretation avoids asking questions about its "true nature" and accepts it as a practical recipe for an observer to predict the probabilities of measurement outcomes.

That's true of the formalism.

Bell's inequality does not rule out nonlocal hidden variables. It rules out local hidden variables. And yes, when ruling out local hidden variables, several assumptions are made, including no superdeterminism.

It is a common misconception that decoherence solves the measurement problem. It does not. However, it is probably part of the solution to the measurement problem. The measurement problem is that quantum mechanics requires a subjective division between observer and the system being measured. Decoherence means that in many cases, we get consistent results even if we place that subjective division between observer and system at different places.

That's not correct. In decoherence, we have the observer and the total quantum system as usual. The quantum system evolves deterministically. The difference is that here we make the total quantum system contain the environment and the quantum system (ie. total quantum system = environment + quantum system). So information is lost when we focus on the quantum system, but information is not lost when we consider the total quantum system. The measurement problem is not solved, since we still need an observer to make a measurement on the total quantum system.

But what about decoherence? I thought that the Schrödinger equation gives us all the information needed to make the prediction and that we can’t gain anymore information and that it’s random in outcome. But decoherence imply that there are information that are lost or at least that’s my understanding of it. So does decoherence imply we don’t have enough information and solves the measurement problem?

 

[Demystifier]

But what about decoherence? I thought that the Schrödinger equation gives us all the information needed to make the prediction and that we can’t gain anymore information and that it’s random in outcome. But decoherence imply that there are information that are lost or at least that’s my understanding of it. So does decoherence imply we don’t have enough information and solves the measurement problem?

By decoherence information is not really lost, but only hidden in the environment. As @atyy said, it does not solve the measurement problem.

 

[Pipsqueakalchemist]

So it’s possible that there are non local hidden variables that affect the measurement hence why it seems random? Could it also be possible that nature is truly probabilistic?

 

[Pipsqueakalchemist]

So it’s possible that there are non local hidden variables that affect the measurement hence why it seems random? Could it also be possible that nature is truly probabilistic?

Also is this non local hidden variables the bohemian mechanics interpretation of quantum mechanics?

 

[DennisN]

So initially I thought quantum mechanics was deterministic in the equations but was probabilistic in measurement.

That's correct.

Though I'd like to add that not all measurements are probabilistic, e.g. the electric charges of particles are fixed, the magnitude (amount) of quantum spin is fixed, while the spin direction is not. Also, when you measure the spin direction of a particle, e.g. along the x-axis, and then measure it again along the x-axis without doing anything else in between, you will get the same result as in the first measurement.

There have been many discussions on PF regarding determinism and probabilities in quantum mechanics. It's a bit like the question if a die roll (a 6-sided die) is deterministic and/or probabilistic.

It's both, in a sense.
The outcome of a die roll (assuming perfect random behavior) can not be predicted, so it's probabilistic.
But the statistics of the outcomes of many die rolls is known, i.e. over time each outcome will occur with the probability 1/6, so the general outcome of many rolls of the die is sort of deterministic.

That's how I see it.

 

[Demystifier]

So it’s possible that there are non local hidden variables that affect the measurement hence why it seems random? Could it also be possible that nature is truly probabilistic?

Yes and yes. It looks random because in practice we don't know the exact initial conditions of the hidden variables.

 

[Demystifier]

Also is this non local hidden variables the bohemian mechanics interpretation of quantum mechanics?

Yes. (Bohmian, not bohemian.)