Randomness

[Andrew Karpinski]

OK. So, random is commonly used in place of 'so complicated we cannot predict it', ie. in the roll of a die. Pretty much everything you may consider random (ie, seeing an old friend 'randomly' on the bus) is not actually random. There are any number of factors that caused that friend to get on the bus, you just don't know what they are.

Now, people have been telling me that randomness truly does exist, on a quantum level. Can anyone explain this to me? Also, does randomness exist in our everyday lives, and if so, is this simply because of the randomness on a quantum level? Are things on the quantum level random?

Thank you.

[soko]

Randomness is one of those things that may or may not exist and like god, is unknowable.

[atrifix]

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OK. So, random is commonly used in place of 'so complicated we cannot predict it', ie. in the roll of a die.

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Quantum randomness is essentially the same, except that even if you knew all of the causal factors, you still could not predict it.

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Also, does randomness exist in our everyday lives, and if so, is this simply because of the randomness on a quantum level?

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The randomness is so incredibly small that it wasn't observed until the early 20th century. Obviously you can predict where a billiard ball is going to go before you hit it. But it does affect every particle in the universe, so it can be pretty important.

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Are things on the quantum level random?

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Who knows? We're not gods.

[bunny]

I think that is the currently accepted theory, that it's all probabilistic and that there is an inherent randomness. I think Metric posted a link a while back to a paper which suggested (?) a way of framing quantum mechanics that was not probabilistic at a fundamental level - it was too technical for me to get through, I'm afraid, but it might give you what you are looking for...

[Piers]

Randomness is a property of observer centric models in general rather than the actual physics of the situation.

Quantum mechanics is an observer centric, inductive model.

(The wave or partial model of light is a deductive model because they start from the assumption that light is a wave or particle and then derive the defining equations.

In the quantum model we say don’t understand what’s really happening, so don’t try just collect together all the observations we can and combine them in a model that emphasises predictability as much as possible. )

Randomness is not a property of the real [img]/images/graemlins/confused.gif[/img] world; it is just a property of some types of model we might use to represents aspects of the world.

[ChrisV]

It depends on which interpretation of quantum mechanics is used. The equations of quantum physics function independently of what narrative is used about what is "really" going on underneath.

The "standard" Copenhagen Interpretation contains randomness. So does the many-worlds interpretation. The transactional interpretation, as far as I'm aware, is deterministic and therefore does not contain randomness.

The source of the randomness in the Copenhagen Interpretation is Heisenberg's Uncertainty Principle which states that a particle has no definite position and momentum, but rather a probability distribution of positions and momenta. Because these properties of a particle cannot be measured accurately, it is impossible to say with certainty when, for example, a radioactive atomic nucleus will decay. That is a random event with a certain probability of occurring.

This bothered Einstein, who famously said "I cannot believe that God would choose to play dice with the universe". Niels Bohr, one of the architects of the Copenhagen Interpretation, retorted "Einstein, don't tell God what to do".

For more on the different interpretations of quantum mechanics, see here.

[Metric]

To clarify, standard quantum theory introduces randomness in two ways -- you can get randomness by ignoring part of the "full" description of a system, but also the result of an observation of a system whose state is fully known can also be random (this is the part that is puzzling).

The paper bunny refers to says that really only the first type of randomness exists, and that the logic of the measurement process actually includes this notion (ignoring part of the full description of a system) in a very subtle but absolutely general way. I favor this interpretation strongly -- no exotic and other-worldly ideas are required to justify the formalism.

(the paper is "quantum mechanics of measurement" by Cerf and Adami for those interested in the details)

[Sadat X]

This is a very important question, IMO. Without randomness, wouldn't all of existence be predetermined?

Here's a wikipedia link about the philosophical implications of quantum theory as it relates to determinism.

[jason1990]

The answer to the question, "Does randomness exist?", depends on your definition of randomness. You began your post with one definition:

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OK. So, random is commonly used in place of 'so complicated we cannot predict it', ie. in the roll of a die.

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Obviously, this kind of randomness exists. But you are clearly asking about a different kind of randomness. To distinguish between them in this post, let me use "random" to mean the above and "actually random" to mean this second form you are asking about. So what is your definition of actually random? In your post, you tell us what it is not:

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Pretty much everything you may consider random (ie, seeing an old friend 'randomly' on the bus) is not actually random. There are any number of factors that caused that friend to get on the bus, you just don't know what they are.

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From this, I infer that an event is actually random if it is uncaused, in the sense that it is not the necessary consequence of any other events.

The outcomes of quantum experiments are certainly unpredictable, but the question is whether they are random or actually random. The idea that they are random (i.e. not actually random) is the theory of hidden variables. Bell's Inequality (which has been experimentally verified) implies that the theory of hidden variables is inconsistent with a local universe. (Deja vu! I just posted this in another thread.) In other words, if these events are not actually random, then the universe in non-local, in that distant events are capable of instantaneously affecting one another. Because of this, any deterministic interpretation of quantum mechanics must necessarily incorporate the concept of a non-local universe.

One example of an interpretation which holds that quantum events are not actually random is Bohmian mechanics, which is described in David Bohm's book, "The Undivided Universe". As you might infer from the title, the theory postulates (as it must) a non-local universe, in which all parts interact and affect one another instantaneously. Bohmian mechanics is a consistent interpretation of quantum theory, but it does not offer any predictions which differ from the standard interpretation. So there is no way to experimentally refute or verify it. It is merely an interpretation.

So we do not know if quantum phenomena are actually random, but we do know that if they are not, then our universe is non-local. Of course, we therefore do not know if there are any macroscopic phenomena which are actually random. Even if we accept that quantum phenomena are actually random, this would not necessarily imply anything about any macroscopic events.

However, if you believe in free will, then the answer is easy. Every choice you make, by the above definition, generates an actually random event.

[Andrew Karpinski]

Thanks guys, this is really a wealth of knowledge you are supplying here. The randomness I am searching for is fundamentally, I think, along the lines of 'has no cause'. That is, if you take two universes, which are identical in every way, is there something that may occur differently in one universe or the other, becuase it is 'random'.
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. Because these properties of a particle cannot be measured accurately, it is impossible to say with certainty when, for example, a radioactive atomic nucleus will decay. That is a random event with a certain probability of occurring.
'

Is this true randomness, or 'if a tree falls in the middle of the woods, and no one is around to see it, does it make a sound?' randomness?

BTW, can anyone recommend a 'quantum mechanics for idiots' type book for me? This discussion is fascinating but a fair bit of it is going over my head.