Is there inherent, observable randomness in the universe?

[Trantor]

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First, consider the result that says before the measurement the particle's spin is indeterminate. I believe that this is a function of the fact that when the particle's spin is measured, it is with respect to a random set of axes. If the particle had a definite spin, it would spin about a definite axis. This would produce a quantifiable disparity in the number of times you counted it as "spin-up" vs. "spin-down" as opposed to a particle which is forced to decide by your measurement what axis it is spinning with respect to.

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If the particle had a definite spin, ie as measured by some apparatus, then its spin is definite and if the axis of measurement were not aligned with the axis the measurement would have no spin component on that axis detected, if it was aligned it would measure all its spin along that axis. this is straightforward QM.


BUT, I'm not sure this is what you have in mind. If a spin is unmeasured and can be either up or down 50% then it can be up or down 50% relative to ANY orientation of axis. The uncertainty is not because we don't know the spin through absence of measurement but it is actually in some direction. It is in a supperposition of up and down relative to all orientations of axes.

[atrifix]

When quantum physics was first getting off the ground in the early 20th century, people were aware of statistics and indeterminism but thought it was just inaccuracy in our measuring devices--so no one proposed indeterminism as an actual theory, although they did use it in their calculations. The Heisenberg uncertainty principle states that these uncertainties are not caused by impairments in measuring devices, but are a fundamental part of the world. Many physicists had trouble coming to grips with this (Einstein is the most notable one), but repeated experiments in the later half of the century, involving single photons as well as single electrons, showed this to be more or less true.

Now, it may just be that our knowledge is limited. Nothing can ever be PROVEN in physics. It's possible that quantum mechanics will in fact be overturned for a deterministic theory. However, QM is the most successful physical theory to date, so it is probably correct given our current understanding.

Also, MWI is perfectly consistent with everything found so far in QM. If you consider many-worlds (possibly infinite) to be deterministic, then there is no contradiction.

[maurile]

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So you are saying that a minority of physicists believe there is an underlying deterministic process behind the uncertainty principle?

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There are a few deterministic interpretations of QM (e.g., Everett's many-worlds interpretation), but most physicists think these are more far-fetched than the indeterministic interpretations.

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Is it safe to say, then, that inherent randomness of the universe has not been *proven* (at least to the extent that something can be proven in physics), even though it is widely (but not unanimously) believed among leading physicists?

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Yes.

By the way, neither determinism nor indeterminism can ever be completely established. Where the universe looks random, it's always possible that there's an underlying pattern we just haven't discovered yet. Where the universe looks deterministic, it's always possible that the apparent pattern is accounted for merely by a series of (huge) coincidences.

[Darryl_P]

I wish I had a nickel for every time I've heard an atheist say to a theist "why can't you accept that it's just random?"

These guys think that's the end of the line. Something is random, here's the distribution, so we've explained it, end of story. If that's not ultimacy then I don't know what is.

Anyway, I thought it was agnostics who don't make any bold claims.

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Anyway, I thought it was agnostics who don't make any bold claims.

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thought wrong

http://en.wikipedia.org/wiki/Atheist

[atrifix]

In the philosophical world, "weak atheism" is usually referred to as agnositicism, and "strong atheism" is usually just referred to as atheism. These are typically beliefs about an omnipotent, omniscient, omnibenevolent god, not just any gods in general.

[chezlaw]

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Where I think you go wrong is in saying that there's something deterministic going on in the measurement process. While you could be right, there's no evidence to support this, and QM works just fine without it.

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I just wanted to see if it worked, I'm not making the mistake of saying something deterministic is going on, just that it could be and hence Bells work cannot prove that there are random events. [My main interest is that it gives me a way of thinking about QM that is reasonably intuitive]

QM looks so like late evaluation to me, which is independent of the determinism issue, that I don't see how QM could convince someone either way. Moving away from the computer analogy, it seems intuitive that as there is no use in evaluating states that are never used, any efficient universe should work this way.

The OP may notice he has now got the result he was after in the first place, although that's definitely not what motivates my interest.

Thanks for your help.

chez

[kevyk]

Here's the thought experiment that Bohm proposed:

Two entangled electrons (spin adds to zero) set off in opposite directions. At distance d to the right, electron 1 passes through a Stern-Gerlach magnet which is oriented to measure the spin as "up" or "down." It measures the spin of electron 1 to be "up." At distance 2*d to the left, electron 2 passes through another Stern-Gerlach magnet, which is oriented to measure a "right" or "left" spin. It measures the spin to be "right."

A "realist" looking at the first measurement would conclude that electron 2's "actual" spin was "down." The same experimenter looking at the second measurement would conclude that electron 1's "actual" spin was "left."

The point is that if each electron has an acutal, but unknowable spin, this result should not be possible. Yet, it is what actually happens when the experiment is conducted, and is the result predicted by the classical quantum theory. An electron's spin choice with repect to a given axis seems to be genuinely indeterminate before a measurement is made.

Of course, I will make the disclaimer that there are non-local hidden-variable theories which have not been ruled out--but the smart money seems to be on genuine randomness.

[Darryl_P]

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Of course, I will make the disclaimer that there are non-local hidden-variable theories which have not been ruled out--but the smart money seems to be on genuine randomness.

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Are these non-local hidden-variable theories the ONLY ones that exclude true randomness or would the MWI accomplish it also? Or is the MWI a hidden-variable theory?

Philosophically speaking I don't see how the idea of the observer influencing the event being observed (which I have no trouble accepting) is incompatible with determinism. Or is there a reason why I should?

[chezlaw]

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Of course, I will make the disclaimer that there are non-local hidden-variable theories which have not been ruled out--but the smart money seems to be on genuine randomness.

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Would the computer simulation idea be a hidden-value theory?

I thought not, in which case hidden-value or genuine randomness are not the only options.

chez