# Is the quantum world truly random?

1. Apr 18, 2015

### Chuckstabler

The title sums up my question: Is the quantum world truly random? If so, doesn't this seem to contradict a primary assumption of science, determinism? How is it that something can truly be random? Are there any theories that deal with this? By this I mean to ask if there are any theories that treat what appears to be random quantum behavior as non random, determined by causes that we cannot directly observe?

2. Apr 18, 2015

### Orodruin

Staff Emeritus
Determinism is not the primary assumption. The primary assumption is testability and that a theory can be falsified.

These would be theories with so called "hidden variables". It has been shown, and tested, that this cannot be the case. I suggest a Google search for "Bell's inequality".

3. Apr 19, 2015

### phyzguy

Whether or not the universe is deterministic is not, "a primary assumption of science", but is a question to be answered by experiment. Experiments in the quantum realm appear to show that the universe is not deterministic and there is a component that is in fact truly random. A Uranium nucleus can sit happily for billions of years, and then one day suddenly decay, and it appears that there is no inner clock in the nucleus which caused it to decay, it just happened. Attempts to explain the randomness as determined by causes we cannot observe go by the name of "hidden variable theories". No hidden variable theory exists which explains the results of quantum mechanics, and the Bell inequalities put severe constraints on the type of hidden variable theories which are possible.

From a philosophical standpoint (I know we're not supposed to discuss philosophy here, but I can't resist), I'm continually mystified by the difficulty that people have in accepting the randomness inherent in quantum mechanics and in wanting the universe to be deterministic. Do you really believe that in the instant of the Big Bang, 13 billion years ago, that the entire history of the universe was written in the initial conditions, and that all we are doing is watching things play out, with no possibility of influencing the future?

4. Apr 19, 2015

### bhobba

It certainly does appear to show that, and it would be the most reasonable assumption. But the interesting thing is there are interpretations where its not random as well as random, as well as, strangely, interpretations that are deterministic but due to lack of knowledge introduces randomness (that is many worlds). It would seem, at least at our present state of knowledge, an unanswerable question.

Thanks
Bill

5. Apr 19, 2015

### bhobba

Is the quantum world truly random?
It looks that way but we don't really know.

If so, doesn't this seem to contradict a primary assumption of science, determinism?
That's not an assumption of science. A lot of stuff (some would say most of it rubbish) has been written about that, but the truth is its very simple as Feynman in his inimitable style clearly explains:

How is it that something can truly be random?
How can something not be? By this I mean, a priori, neither random nor not random is true or false.

Are there any theories that deal with this?
By this I mean to ask if there are any theories that treat what appears to be random quantum behaviour as non random, determined by causes that we cannot directly observe?
Yes eg Bohnian Mechanics.

Thanks
Bill

Last edited: Apr 20, 2015
6. Apr 19, 2015

### quawa99

If the quantum events are truly random we are still watching things play out still without any possibility of 'us' influencing the future.The only way the future would then be influenced is by the occurance of purely random events on which we humans or anything else in the universe cannot have any effect whatsoever.So whether people want universe to be deterministic or not we will always end up just watching things play out.

7. Apr 19, 2015

### bhobba

Have a look at the law of large numbers.

Thanks
Bill

8. Apr 19, 2015

### craigi

It's easy to see that you can generate a set of numbers deterministically which are indistinguishable from a set of numbers generated indeterministically. We define both sets of numbers as random.

We can choose whatever back story we like for that set of numbers. We have no way to prove it either way.

Last edited: Apr 19, 2015
9. Apr 19, 2015

### Chuckstabler

Okay guys, thanks for your replies! I was not trying to sound like I'm questioning the validity of Quantum mechanics, perhaps I should have worded my question better. Perhaps I should've worded my question better. It seems odd to me that, on the macroscopic scale, we can identify "causes" of every event. The "cause" of my decision to ask this question can be found in the way that electrical signals are summed across a network of connections among neurons with given weights defined by the number of ion channels on the synapse, the type of ion channel (determining whether the electrical signal will be an inhibiting one or an exciting one), and the total distance that the signal must passively act through in order to reach the axon hillock, with these factors being determined by previous experience (vast oversimplification, but it will suffice). You can follow this chain of events back to the big bang, at least for the macroscopic world. In the microscopic world however, this doesn't seem to hold up. In a way I guess it does though, because we can identify the probability that a certain outcome will happen based on prior conditions. Anyways, just wanted to clear that up. Thanks :)

10. Apr 19, 2015

### Staff: Mentor

"Severe constraints" is a nice way of phrasing it. One of these constraints is that any successful hidden variable theory must be no less offensive to our classical intuition than quantum mechanics. This is sufficient to cause many people to lose interest in the quest for such a theory.

(A fair case can be made that Bohmian mechanics is such a theory, although it has not been developed to cover anywhere near the range of phenomena that is covered by more orthodox formulations of QM without hidden variables ).

11. Apr 19, 2015

### Ilja

Wrong. "Hidden variable" theories exist and are well-known. The best known one is de Broglie-Bohm theory, also known as Bohmian mechanics.

The point of Bell's theorem is a quite different one: It shows that such things like hidden variable theories also need causal influences greater than the speed of light. This would be in contradiction with fundamental interpretations of the relativity principle, which claim that there should be really no preferred frame. Weaker interpretations of relativity, which claim only that all frames should be indistinguishable by observation, have no problem with postulating, together with hidden variables, also a hidden preferred frame. In particular, de Broglie-Bohm theory needs such a preferred frame for relativistic physics. So, the point of Bell's theorem is simply that this is not a failure of de Broglie-Bohm theory, but a necessary property of realistic interpretations.

12. Apr 19, 2015

### Ilja

This is wrong.

Essentially, for every theory which can be written in the form $H=\sum p_i^2 + V(q)$ the de Broglie-Bohm variant comes completely straightforward. This includes, in particular, all field theories of type $H = (\partial_t \varphi)^2 + F(\partial_i \varphi, \varphi)$. Variants for fermions and gauge fields have been proposed too. By the way, the first version of Bohm's theory for relativistic gauge fields was given in the original paper itself, for photons.

Additionally, it would be sufficient to have a de Broglie-Bohm theory for only some part of the degrees of freedom of the theory. This would be a less beautiful theory, but it would do the job of defining a realistic hidden variable theory.

13. Apr 19, 2015

### Staff: Mentor

I'm sorry, I may not have been clear enough in my wording - not altogether surprising in a parenthesized afterthought. I was responding to phyzguy's statement that there are no viable hidden variables theories by pointing that Bohmian mechanics is a fair counterexample.

I could perhaps have used the word "elaborated" instead of "extended".

14. Apr 20, 2015

### kochanskij

The Many Worlds interpretation of quantum mechanics is completely deterministic and does not involve any faster than light signals. MWI simply says that when a particle in a superposition interacts with another particle, it puts it into a superposition, which then interacts with still another one. This chain continues forever. Measuring devices and people join the superposition. The wave equation is always followed and a collapse never occurs.
There is nothing random in the MWI. Randomness is an illusion of our consciousness. A person is put into a superposition of states. Each version observes only one of the possible states of the particle. But no version knows which state he will see until he sees it. So it appears like one state became real at random and the other states disappeared. But in reality they are all still in existence.

15. Apr 20, 2015

### bhobba

That is not what it says at all. A mixed state results from decoherence and each 'part' becomes by interpretation, a world.. A mixed state is NOT a superposition. Outside it remains in superposition, but inside, due to decoherence, that is another matter.

Thanks
Bill

Last edited: Apr 20, 2015