Quantum mechanics and randomness

I’ve never been comfortable with the idea of randomness and as such I believe in a deterministic Universe.

My query is whether or not this belief really has to be in contradiction of the current general Scientific understanding that randomness underlies all physical hapenings.

It seems to me a little arrogant for scientists to claim that just because we can’t see a reason for something to happen, there isn’t a reason and a suficient cause for it to happen.

Would it be possible to take Quantum mechanics as correct, but just say that rather than events being random, we just don’t understand why they happen.

We do a similar thing with coin tossing: for all intents and purposes they result is unpredictable, but most people accept that, in theory at least, it is possible to predict the result if you know enough about the conditions.

I hope that there is a definitive answer to this, but there is always a possibility that it will turn into a debate, so mods, feels free to move, if necessary.

The idea that the quantum randomness isn’t really random is usually described as the “hidden variables” interpretation of QM. I think, though, that someone had managed to prove that there cannot be hidden variables, and that there is true randomness. I wish I could remember who, perhaps a cursory web search on “hidden variables” could find it. I’m at work or I’d do it myself.

It is my understanding that the current dominant theory is that there is indeed true randomness at the quantum level.

This is roughly what I thought, but it is really whether this ‘proof’ is completely solid, that I am interested in.

I guess I “cannot believe that God plays dice with the Universe”.

OK, nobody’s here anyway, they’re all home watching the World Cup, so I can surf with impunity…

I found this article:

An excerpt:

The point of the article, though, is that maybe there are indeed hidden variables, and that “Bell overlooked a large class of possible hidden variables whose behaviour is consistent with the existing experimental findings.”

There were three main ideas of where the particle was before the measurement of where it actually was -

  1. Realist - it’s at a point, but quantum mechanics is incomplete, there are hidden variables that fully describe the system.

  2. Orthodox - the ‘Copenhagen Interpretation’ - it’s not anywhere, but measuring forced the particle to “decide” on a position.

  3. Agnostic - submitted by Pauli - one cannot answer this, ie. there is no way of telling.

I think scientists are generally agreed on number 2 now.

Revtim - a quick google search says it was John Bell in 1964 who worked with hidden variables, but from what I can gather, his arguements didn’t prove thier existence in quantum mechanics, but the arguement continues.

Bell’s theorem (also known as Bell’s inequality) has profound implications for philosophy. I have seen it misapplied to validate all manner of silliness, including but not limited to holistic universe and (my favorite) telepathy. Essentially, Bell shows that if you assume 1)Quantum Mechanics, 2) local causality, and 3) objective reality, then you reach a contradiction. This means that (at least) one of your assumptions is invalid. 1) QM and GR are the two most accurate theories science has ever produced. We are not willing to deny QM. We really don’t want to lose 2) local causality or 3) objective reality, but one must go. One of them is an illusion. It is objective reality that is sacrificed. Something does not have a (defined) value until it is measured.

Yup! The current consensus is that QM is truly random. Many great minds can’t cope with this though, and Einstein was firmly against the idea of ‘God playing dice’.

For a good but lengthy discussion, try the QM FAQ

It then goes on to show that all known theories will introduce strange effects, such as non-locality.

I recomend a search for “Bells inequality” and “EPR Paradox”.
On preview I see that Revtim has come back, with a highly interesting article. I’ll have to read it tonight!

I agree with you that the implications of QM can be disturbing and a deterministic universe has somewhat of a more comfortable feeling to us mere mortals.

Unfortunately (for our comfort) it seems that true randomness is at the base of it all. The whole notion of a quantum computer runs on this idea and they have actually gotten a quantum computer to work (albeit an extremely simple one) so it’s not just theory. If our particle really did exist at a given point at all times and was merely hidden from us behind Heisenberg’s Uncertainty Principle then a quantum computer wouldn’t work. The whole idea behind the computer is that a particle takes all paths available to it and only settles down, so to speak, upon our measuring it. Because of this a quantum computer should be able to calculate vast amounts of numbers simultaneously (the particle is everywhere [within certain limitations] at once thus tracing all paths of the calculation in one go) rather than one at a time as current computers do.

Okay, as long as someone other than me brought God into this discussion, I’ll give you my understanding of what’s going on:

The phrase “God playing dice” suggests something so random that even God cannot predict the outcome. I agree that, to a person who believes in an all-knowing God, that’s ridiculous.

But that is NOT what quantum theory suggests. Quantum theory says that there are no “hidden variables” inside this physical, deterministic universe. It seems to me that quantum theory DOES allow for hidden variables which are outside of this universe. Namely… God!

In other words I do not believe that God plays dice to see the outcome of each quantum event. Rather, He sets up the dice, and personally decides, chooses, and CAUSES the outcome of each quantum event.

Keeve, I don’t believe in a God, I was just using a quote from old Al. You talk about a “physical, deterministic universe” as though it is undisputed, but surely if the universe is deterministic, then there is no room for randomness.

I can live with things being counter-intuitive, and I could probably live with the idea of there being randomness in the universe, if I knew why it is necessary to say that things are random rather than just unexplained (unexplainable).

It seems that there may be some indications that universe is deterministic in the past for the given observer. Freeman Dyson had a lecture here earlier on quantum mechanics positing, basically, that the Copenhaugen Interpretation was too rigid in formulation as quantum effects were predictors of future events and not formulations of the past. I’ll include a brief quotation I found on Paul Davies’ website in Dyson’s own words about a few proposed thought experiments:

The whole metanexus site, actually, is a pretty good introduction to current thinking about the philosophical implications of quantum mechanics.

The quote “God does not play dice [with the universe]” was said by Albert Einstein - he believed in hidden variables and he meant nothing is random in the universe.

Well, here’s a couple of thoughts.

One is that the Bell inequalities only rule out (if memory serves) local hidden variables, not global ones (although I may have this backward, as I can never remember). What this means is basically what Dr Matrix said. Also, the article that Revtim linked to is interesting and offers another possibility.

The second is that there is an alternative formulation of QM due to David Bohm which is deterministic and agrees with experiment (as far as I remember; I don’t know much about it), but which has other undesirable properties; it postulates a “carrier wave” which we can’t see, and while it’s deterministic there are some issues with making it relativistic and also perhaps with instantaneous action at a distance (I could be wrong about this; I’m not an expert, as I said, because NO ONE teaches this stuff). I don’t honestly see either the carrier wave or the action at a distance problem as insuperable objections, especially given that non-relativistic Schrodinger mechanics has action at a distance anyway. I’d love to learn more of this theory, but because it’s so disfavored, that’d be rather difficult and frankly I’ve got more important things to work on.

Sure…with a picture of the computer and everything.

g8rguy, Stanford’s department of philosophy (of all places) has a great discussion regarding Bohmian mechanics and hidden variable theories. Relevant links:
Bohmean Mechanics
Kochen-Specker Theorem

In fact, spending hours there isn’t difficult at all.

I should note that when I say they got an extremely simple quantum computer to work I don’t mean the computer itself which is undoubtedly complex to build and make function. What I mean by ‘simple’ in this case is its computational ability. In the case of the linked article above the quantum computer correctly factored the number 15 to the result 3 and 5 and this calculation (that you or I can do in our heads without trying) represents the most complex quantum calculation to date. That may not seem too big a deal but it shows that the theroy of quantum computing can be a reality.

For interested parties, the links are rather technical, but the discussion doesn’t hinge on technical developments per se, so you can understand the conclusion if you blindly accept their math.

Boy, he must be busy! No wonder he doesn’t perform miracles anymore.

Thanks for the terrific links, erl! I should have known that you’d have something to say on this…

When I have time, I’ll look through them more carefully, but it appears on the quick glance I’ve given them that it may indeed be the case that Bohm’s formulation is ignored rather unjustly.

Global hidden variables would violate local causality. Local causality is to dear to sacrifice. So, no global variables – hidden or otherwise.