# FeaturedI Quantum mechanics is random in nature?

1. Aug 1, 2016

### entropy1

I heard from many sources that quantummechanics is purely random in nature. Has this been demonstrated?

If so, what is the proof?

2. Aug 1, 2016

### micromass

1) What sources?
2) What do you mean with purely random? Or what do the sources mean with it?

3. Aug 1, 2016

### entropy1

1) Scientists (mostly on the internet).
2) Phenomena like the wavefunction collapse are considered 'purely random'. I must add however, that the measurement can be set up in such a way, that there are (strong) statistical correlations established, for instance in the case of commuting observables. However, we can't predict what will be measured in case of collapse. Now, I wonder whether the fact that the measured value cannot be predicted is reason enough to call it "random", or if there are other prerequisites to do so. That we can't predict something only says something about its predictability, not about its properties, right? If we can establish that collapse is random, we have to base that on the propery of being statistical in its nature, right?

4. Aug 1, 2016

5. Aug 1, 2016

### entropy1

I understand that. Unluckily, I can't recall my sources. Is it possible to consider my question as-is?

(I will try to retrace my sources)

6. Aug 1, 2016

### micromass

It'll have to do, I guess. But I thought it was important since it all depends on your definition of "at random". So while I don't doubt that you try your best to stay faithful to the scientists their account, it might still happen that what you said in your OP is not the same as what the scientists really said or meant!

I'll give you one scientist which might have said what you implied in your OP:

At the end he makes some philosophical statements about randomness and determinism in quantum mechanics, I do need to add that I don't think it is completely proven the way he presents it. I consider it very likely that things really do behave the way he said it though.
More specifically, he states that a hidden variable theory is wrong. I think Feynman was incorrect there as this cannot be proven. I consider it very likely to be the case that nature doesn't have hidden variables though.

7. Aug 1, 2016

### vanhees71

Feynman would be very upset if he's know that you say his remarks are "philsophical" ;-). SCNR.

8. Aug 1, 2016

### micromass

Yes, I know. But he makes fun of a "pompous philosopher" at the end. I was refering to that segment.

9. Aug 1, 2016

### entropy1

I think by "purely random", I mean: "Uncaused". Feyman mentioned in the video that having in principle knowledge of which slit a particle will pass in a double-slit experiment (so, even without actually measuring it), would destroy the interference pattern. So, the theory dictates that it is not possible (it seems so). What I would like to know is if the math is also dictating that for collapse! That is, do we know there is absolutely no physical cause determining the outcome of a collapse?

And even if there is no pysical cause for collapse, there still is a correlation between outcomes of collapse. Could this correlation be described in terms of hidden variables? It seems odd that there even is a correlation if there doesn't exist a mechanism to produce it.

10. Aug 1, 2016

### DrChinese

What would it take to convince you something is "really random" - past it appearing random and there being no currently known causes?

11. Aug 1, 2016

### entropy1

Good question! That is what I don't know! I thought other people were proposing it! If so, I'd like to know how they come to that!

Perhaps I could add: How can we demonstrate that (for instance) collapse is not 'induced' by 'other causes'?

Last edited: Aug 1, 2016
12. Aug 1, 2016

### DrChinese

I would say past attempting to discover such a cause (and failing to do so) - none. Obviously there are interpretations/theories (Bohmian Mechanics) which posit a cause. However, those cannot be demonstrated - that is part and parcel of the theory.

So we are back to the original question. The most common viewpoint "QM appearing to be random" means we live in a world of random quantum events without a cause.

But this common view would be updated were there to be evidence to the contrary in the future. Presumably that would mean that Bohmian non-locality was specifically demonstrated.

13. Aug 1, 2016

### Staff: Mentor

The randomness follows directly from the axioms of quantum mechanics, and in that sense it is dictated by the math. However, it is possible that there is more to it; it might be that the axioms could be derived from some deeper underlying theory that we don't yet know. That hypothetical deeper theory need not involve randomness. (An analogy: I get excellent agreement with experiment using the axiom "When tossed, my coin will randomly come up heads or tails with 50% probability each way" but the behavior of the coin is governed by deterministic Newtonian mechanics).

However, this discussion is altogether sterile unless and until we have a specific candidate theory in mind.

It does seem odd, or at least at odds with our classical intuition. That's the motivation for looking for a deeper underlying theory in the first place. However, we have to find one before we can sensibly talk about it.

14. Aug 1, 2016

### entropy1

So non-randomness is not yet ruled out, I understand? Then there would be no evidence for randomness yet, as I take it.

15. Aug 1, 2016

### DrChinese

There is no end of evidence for randomness in the quantum world, and no evidence for any hypothetical underlying cause to explain such events. So I disagree with your 2.

Or to put it on another level: there is equal evidence for an underlying cause for apparent quantum randomness as for the existence of unicorns and mermaids. As far as I know, nothing could rule out the future discovery of non-local hidden variables (your 1).

16. Aug 1, 2016

### entropy1

But can you assert that just because something (ie. collapse) is behaving randomly, it is in its nature random? (I hope I am not getting too philosophical here)

17. Aug 1, 2016

### Staff: Mentor

This is a highly philosophical question on what randomness is, rather than the mathematical concept. It reads a little bit like you were looking for evidence to support an ideological point of view, rather than evidence for insights.

Even the seemingly resolution of randomness in Nugatory's example isn't one. It simply transforms the randomness to the point where initial conditions on coin tosses are made. Whatever the future might show, it looks hard to get rid of randomness as mathematical concept. And if you visit a casino, you better won't rely on an underlying deterministic process.

18. Aug 1, 2016

### entropy1

I was expecting a remark of this kind (with all due respect). I conclude one is free to take either side, given good arguments. There are many good arguments to defend randomness, and none to defend non-randomness (HV).

It occurred to me that randomness maybe can't be proven except for its (random) behaviour. So (many) indications for random behaviour would make a strong case.

However, wouldn't it be a circumstantial one?

Last edited: Aug 1, 2016
19. Aug 1, 2016

### Staff: Mentor

I don't think anything in physics is a "real truth". Newtons gravity pretty well describes the fall of the famous apple. Considering GR, it is wrong. But nobody bothers GR when talking about the apple. At last, it isn't even clear that the apple will always have to land on earth. However, a theory that predicts it won't in $1$ of say $10^{50}$ cases will have it difficult to get established. And it won't even matter, since you cannot test it. It is all about satisfactory models that a) describe what has been found, b) describe what will be found and most important c) can be tested. We observe plenty of phenomena that are perfectly described by the mathematical model of randomness. Whether you call it true or not simply isn't relevant. Nobody cares.

20. Aug 1, 2016

### entropy1

I will confess right here my reason for wondering about the answer to my question: I, personally, have a hunch that the apparent randomness is in fact apparent, and can be described by non-random factors. However, I know very little of the matter, so I wanted to have my hunch ruled out to get rid of it. I can't help having the hunch. I deliberately am trying to be very careful with my words here, but that is the reason. I'm sure pretty much of the work has been done already by brilliant minds, of which I am not one, for all that matters.

Last edited: Aug 1, 2016