# Need clarification on quantum indeterminism

Need clarification on "quantum indeterminism"

Hi,

I should start off by saying that I am not physicist, nor am I a physics student, however I have lately been in need of some clarification on basic quantum physics.

My question is this, does quantum mechanics contradict the idea of cause-and-effect?

I have read about Heisenberg's uncertainty principle, but I am confused as to whether it is stating that there is only one "real" possible location for a quanta, or that our trying to observe said quanta is what causes the "uncertainty"?

Perhaps another way of saying it would be: if we repeated the same experiment under the same conditions on the quantum level, could we possible end up with different results? (Hence "indeterminancy.")

Sorry if this sounds incoherent, as I stated earlier, I am no physicist.

Any answers (particularly ones with references) are appreciated in advance.

## Answers and Replies

bhobba
Mentor

If QM contradicts the idea of cause and effect depends on what you mean by cause and effect. If you mean some cause uniquely determines an outcome then yes it does - it only allows you to predict the probabilities of the outcome of an observation. If you mean does everything have an actual cause but you can not necessarily predict the outcome of that cause then no it doesn't - everything in QM has an actual cause - the so called wave-function collapse that is one of the central mysteries of QM has a cause - an observation - we simply can not predict the outcome.

There is also an interpretation of QM called Many Worlds that totally skirts the issue and cause and effect definitely is true regardless. But it is a bit too weird for many people including me.

The uncertainty principle is simply a mathematical theorem about the statistical relation between certain types of quantities. Since the outcome of observations in QM can not be predicted it puts some limits on how certain quantities such as position and momentum can vary in a statistical sense. If you know the position of a particle with great precision then a measurement of momentum will give results that vary a lot and conversely. However in our normal classical world this variance is so small its virtually undetectable.

Thanks
Bill

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DrChinese
Science Advisor
Gold Member

My question is this, does quantum mechanics contradict the idea of cause-and-effect?
Welcome to PhysicsForums, quantum7!

As the words are generally used, most physicists would say that quantum theory is indeterministic and there is no unique observer independent effect from a specific cause (system in a prepared state).

There is still debate as to whether such a formulation is possible. For example, there are a group of interpretations called "Bohmian" which have 2 important features: a) causes propagate faster than c, in fact they are instantaneous; b) apparent indeterminism is due to ignorance of initial conditions. Note that b) is not a property of most interpretations of quantum mechanics and essentially requires a).

Alright.

I think I understand, but please correct me if I'm wrong.

It's not that quanta are just acting randomly(without cause), otherwise things like Heisenburg's uncertainty principle would make little sense because anything that is truly random can't be divided up into, say, a 25:25:50 ratio. It would have to be 33:33:33, as that would be complete randomness.

It is more so that it is just seemingly random, given that we can not observe it without obscuring it. If we had some miraculous way of observing it without obscuring it, say like a God's eye perspective, than quantum mechanics would be as deterministic as Newtonian physics.

As I said earlier, please correct me if I misunderstood anything.

However, it seems that my current understanding(which is likely in error) contradicts what I've been reading in Stephen Hawking's "A Briefer History of Time". In the book Dr.Hawking claims that if we calculated all the variables involved in throwing a dart at a bulls eye, and then threw the dart accordingly there is still a chance we could miss based on quantum mechanics. The odds are infinitesimal that if we continued throwing the dart for billions of years that something as large as the dart would ever miss the bulls eye, however it does still remain in the realm of possibility.

It is more so that it is just seemingly random, given that we can not observe it without obscuring it. If we had some miraculous way of observing it without obscuring it, say like a God's eye perspective, than quantum mechanics would be as deterministic as Newtonian physics.
It appears that that is actually wrong.

It's the basis of what is referred to as 'hidden variable' theory. The idea that there is some sort of completely deterministic reality underneath the quantum 'fuzziness' and it's just our clumsy measurement that produces the probability effects.

In fact, it turns out that QM seems to be the real deal, a particle really has no specific location until it is measured. And once we've located it, it's momentum becomes indeterminate, not merely to us but to the particle itself.

This is what all the fuss is about with entanglement. Bell proved conclusively that it was logically impossible for an electron (photon, whatever) to have a 'real' intrinsic spin in the classical sense and that only the weird mathematics of QM predicted the outcome of actual experiment.

Check Leonard Susskind's lectures on YouTube (Stanford University)

anything that is truly random can't be divided up into, say, a 25:25:50 ratio. It would have to be 33:33:33, as that would be complete randomness.
No. The 33:33:33 is no more truely random than the 25:25:50.

For the purpose of this discussion, you can use a random() function on your computer as a
sufficiently nice approximation for something truely random, because it gives an equal
distribution of reals betwenn 0 and 1. But then apply some functions on it, like
sin(random()) or random()^2 and you will obtain other, quite nontrivial distributions.

Ok, the random on your computer is not truely random, but the point would
be the same if it would be - applying a function on a true random result does
not make it less random.

It appears that that is actually wrong.

It's the basis of what is referred to as 'hidden variable' theory. The idea that there is some sort of completely deterministic reality underneath the quantum 'fuzziness' and it's just our clumsy measurement that produces the probability effects.

In fact, it turns out that QM seems to be the real deal,...
Sorry; no. There exists hidden variable interpretations of quantum theory. In particular, there is the de Broglie-Bohm theory. And if I read suggestions that Bell's theorem shows that there are no hidden variables I look for smileys similar to Bell himself was one of the most important proponents of the dBB interpretation.

Welcome to PhysicsForums, quantum7!

As the words are generally used, most physicists would say that quantum theory is indeterministic
All quantum interpretations are based on the shroedinger equation which is deterministic- thus ruling out any arbitrary occurences. How the probabilites actually work I can't say, but they follow a strict deterministic world view, for how the world will proceed.

The measurements however are indeterministic in the location of the particle which is a difference in function .

The Heisenberg uncertainty principle is only related to the actual location of the particle not the question of how it will evolve.

How many times do you have to say this before people get it on this forum?

How many times do you have to say this before people get it on this forum?
I'm sure statements like this don't help to clarify anything. At best they're interpreted as (unfounded) arrogance and lead to you being ignored. So let's keep this and other discussions as rational as possible without dismissing fundamental questions, even if you think the answer is obvious.

I'm sure statements like this don't help to clarify anything. At best they're interpreted as (unfounded) arrogance and lead to you being ignored. So let's keep this and other discussions as rational as possible without dismissing fundamental questions, even if you think the answer is obvious.
Did you notice that it does comes at the end of an actual explanation

It's hard to explain further because one would have to define what we mean by probability. This terminology is often synonymous(misstakenly) with uncertainty as to the future outcome.

But the shroedinger model does not work like that-- the probabilites are exact which is why they are indeed deterministic. Another word for exact would be that the probabilites are precise.

That does not take away the fact that they still are probabilites-- It does sound paradoxical in connection with determinism but it's not. A anology would remove any confusion but I can't think of a good one at the moment.

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