# Determinism and Quantum Mechanics

1. Aug 8, 2013

The way I saw it, we lived in a deterministic universe. And all the claimed randomness in the quantum world was just hidden factors about the nature of reality that we didn't know. Then I heard about Bell's inequality and how it takes out the possibility of hidden factors.

So i saw this video on Bell's inequality

From what I gather, the hidden factors the person in this video talks about aren't the hidden factors I thought about when asking myself this question. The hidden factors I thought of were our lack of knowledge about the true nature of these quantum objects.

Also, I think it makes sense to say that if the seemingly random quantum world makes up the every day deterministic world (like me dropping a ball and it falling) that we see, then the quantum world should have hidden properties that we don't know exist which make it seem as if it is random. And if time were to be reversed (And every single factor is reversed along with time) and these experiments re-done, then they would have the exact same outcome.

So is it impossible for these hidden factors to exist? Do our experiments prove that randomness exists or do they show only that randomness could exist and we don't know because of these hidden factors?

Last edited by a moderator: Sep 25, 2014
2. Aug 8, 2013

### DrChinese

Based upon Bell's Theorem and experimental outcomes, it is now known that either we live in a deterministic non-local universe OR an indeterministic local universe. We do not have any current way to determine which of these is the better description.

So you still have wiggle room.

3. Aug 9, 2013

Thank you for the reply. I appreciate it :) I've been trying so hard to get my head around this. I think I'll stick with the deterministic non-local view for now.

Last edited: Aug 9, 2013
4. Aug 9, 2013

### vanhees71

Hm, I'd say we have a very good description of the everything not related to gravity, and that's the Standard Model of elementary particle physics. This is a local microcausal relativistic quantum field theory. We know that it is incomplete, because it doesn't incorporate gravity. Further cosmology, based on general relativity, describing space, time, and gravitation in a classical way, hints strongly towards the existence of dark matter, consisting of particles not described by the Standard Model. Also the value of the cosmological constant ("dark energy") is a mystery and demands an extreme fine tuning of the standard-model paramters due to the quadratic divergence of the Higgs mass. On the other hand, the more the experimentalists at the LHC measure with great precision, the better the Standard Model becomes more confirmed. Only very recently there is some hint from the decay of B mesons to $\mathrm{K}^* \mu^+ \mu^-$ which may indicate physics beyond the standard model. On the other hand the very rare decay $\mathrm{B}_s \rightarrow \mu^+ \mu^-$ is in accordance with the standard-model prediction with high accuracy.

That said, we must take the Standard Model as the best theory of matter and its interactions we have today, and that's a local microcausal quantum field theory. Together with the violation of Bell's equality, which has been empirically tested to amazing accuracy while confirming at the same time the predictions of quantum theory this seems to hint at an indeterministic universe, governed by local interactions.

As far as I know, there is no completely satisfying non-local model and as long as such a thing has been established and is at least as successful as the Standard Model, I tend to believe that Nature behaves indeterministically and is best described by local relativistic QFT.

5. Aug 9, 2013

### the_pulp

Sorry DrChinese or whoever wants to answer, but what is the status of Time Symmetric Quantum Mechanics (another interpretation of QM). With that interpretation, we could add the following option:

3) Determinism, Locality but no Causality

Am I right or am I confusing everything?

Thanks!

6. Aug 9, 2013

### phyzguy

Saado: I have a philosophical question for you. A completely deterministic universe implies that we have no free will, and that the universe is just unfolding in a way completely determined by some long-ago set of initial conditions. Yet it certainly "feels like" I have free will. I admit that it is possible that my feeling of having free will is just an illusion, and things are just unfolding in a deterministic way that I have no control over. However, it seems to me that the simpler hypothesis is that the universe is not deterministic, and that I do in fact have free will. Given this, I welcome the conclusions of quantum mechanics which say that the universe is not deterministic. However, you (and I have met many others who agree with you) seem to want the universe to be deterministic, and you go to great lengths to find "loopholes" which allow the universe to be deterministic, in direct opposition to the evidence of your senses that you do in fact have free will. Can you explain to me why you feel this way?

7. Aug 9, 2013

### jfizzix

We could also live in a non-deterministic non-local universe. It's just that the opposite has been ruled out (or will be once we have a loophole-free violation of a Bell/CHSH inequality).

8. Aug 9, 2013

### DrChinese

Ah, I would agree with this configuration as being viable.

I happen to classify those as part of the non-realistic group, which means that I consider them also to be non-deterministic. But now we are treading into semantics, because what is indeterministic to me may be deterministic to you. I say it is indeterministic because outcomes are not dependent on any set of variables, regardless of "when" they are. A random element is introduced which is not, in principle, based on any configuration.

9. Aug 9, 2013

### DrChinese

Also viable.

10. Aug 9, 2013

### The_Duck

It depends on what exactly you want "free will" to mean, but under a reasonable interpretation determinism and free will are compatible.

Introspection is nice, but it's not a tool for drawing conclusions about fundamental physics.

11. Aug 9, 2013

### ZapperZ

Staff Emeritus
For this topic to qualify to be discussed on the physics forums, it must have a substantial physics content. It cannot solely be based on philosophy or a matter of taste. Things being discussed must have either physics-based theory, or experimental support, or can be measured. It isn't sufficient to insist that there are "hidden variables". One must show how such a thing can be detected and measured, and how it would deviate from what quantum mechanics predicts.

For example, one should look at the issue of quantum contextuality, and how such a thing is measured. If one believes in such hidden variables, or that there are already some determined values before we make such a measurement, then one must tackle a bunch of experiments that seem to contradict such a thing. The latest ones that I've highlighted:

https://www.physicsforums.com/showpost.php?p=4129344&postcount=155
https://www.physicsforums.com/showpost.php?p=3372804&postcount=139

Zz.

12. Aug 9, 2013

I feel this way for the same reason that Einstein felt this way. I just don't like the idea of randomness. Maybe because it's fundamentally counter intuitive. Maybe because it sounds better that we are able to predict every outcome without relying on probabilities. It seems that I'm trying to find loopholes but I'm just curious as to why randomness appears to exist at a quantum level and not in our every day lives. And I want to know if it's "certain" or there's still room for debate that randomness exists. If it is "certain" like how some people have claimed that randomness exists; then I welcome its truth with open arms. Even though I don't like the idea.

I'm not too bothered about not having, or having free will. Free will at its core can not exist with determinism and can not exist with randomness. Because in both situations, you either have no control over the inevitable outcome or you have no control of the random nature of the quantum world.

I think my question was answered by DrChinese who said it's not yet "certain" either way. Meaning the possibility of hidden variables still exist. I just needed to know what the experiments say.

I'm sorry if this topic diverged from the actual science and went to philosophy.

I'm 17 and so have a lot to learn

Last edited: Aug 9, 2013
13. Aug 10, 2013

### bhobba

Does it? Or does it imply theoretically its the case but due to chaotic effects we can never predict anything well enough. For me a universe that for all practical purposes is not deterministic is just the same as one that isn't.

Thanks
Bill

14. Aug 10, 2013

### bhobba

The Kolmogorov probability axioms are very elegant and look pretty straight forward to me:
https://en.wikipedia.org/wiki/Probability_axioms

My personal view for what its worth is its a pretty meaningless distinction because randomness may be the result of an underlying determinism like classical statistical physics and truly random systems can display quite deterministic behavior via the law of large numbers.

Thanks
Bill