
Do physicists really believe in true randomness? - Xcelerate
http://www.askamathematician.com/2009/12/q-do-physicists-really-believe-in-true-randomness/
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jandrewrogers
The difficulty with mathematical randomness (versus unpredictability) in
physics is that you would not expect the Laws of Thermodynamics to exist in a
universe governed by the former. Some literature on algorithmic information
theory touches on this.

Consequently, if local determinism is not plausible (Bell) and randomness is
not plausible (Kolmogorov et al), that leaves non-local determinism as the
most sensible assumption. It does lend some credence to the idea that we
perceive a low dimensionality projection of a higher dimensionality space.

~~~
Houshalter
What about many-worlds? It appears randomly from the inside but actually
isn't.

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Russell91
This article completely misses the point. You can never disprove hidden
variable theories in a stochastic system, because hidden variable theories
make the exact same predictions as 'true random' theories - the difference is
only philosophical. What you can disprove are local hidden variable theories,
which were somewhat popular (and favored by Einstein, for example) prior to
Bell's thought experiment being empirically supported. See what Einstein
wanted was for all of quantum mechanic's weird results to be explainable by
local interactions that took place when two particles interact - so that when
they become entangled and subsequently drift apart, you don't have to have any
"spooky action at a distance" to explain the results of experiments you do on
them separately. However, what Bell's experiment shows is that the dice don't
get rolled until one of the particles is measured - and that the way in which
a measurement is performed on one particle affects its correlation with the
other. So in fact you do need instantaneous nonlocal interaction to explain
the real world. The result, unfortunately, has nothing to do with 'hidden
variables' vs. 'true randomness'. What it does say though, is that you can't
just explain away the weirdness of quantum mechanics as the result of some
yet-to-be-found local hidden variables.

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encoderer
"So in fact you do need instantaneous nonlocal interaction to explain the real
world." Do you just mean quantum entanglement, or some other "spooky action"?
(love that quote btw -- like many)

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Russell91
Haha, well the instances of Bell's experiment that I've seen so far all refer
to entanglement. It's pretty easy to think about some unknown quantity having
a probability distribution on its states. If you have an electron, and you
don't know the spin yet, you say that it can be either up or down. What's
special about entanglement between 2 particles is that it says the
distributions on the two are not independent. In the classic example, one can
only be spin up if the other is spin down - so they have a statistical
dependence on each other. Bell's experiment tells us that the strength of this
statistical dependence depends on how you measure the particles, which isn't
determined until measurement time, when the particles are far away. But that
being said, you could imaging all other sorts of spooky action at a distance -
like 2 particles that have never been local to one another having some sort of
statistical dependence. But that would be really hard to show until you found
what type of nonlocal statistical dependence to look for, so usually when
we're trying to figure out a good model we end up thinking about entanglement.

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pvnick
I love the idea of true randomness because it seems, to my uninformed mind, to
allow us the possibility of free will. Without true randomness, while we may
enjoy the illusion of free will, our decisions are ultimately left to
deterministic hidden variables. That makes me sad. But given true randomness,
we may in fact have some mechanism whereby we are truly autonomous. How that
might work is way beyond me and probably leads very quickly to pseudoscience
and quackery, but it's certainly fun to think about.

~~~
avalaunch
I'm not sure random will is equivalent to free will. Is a choice still a
choice if it's made randomly? Couldn't you say that the photon in the
experiment has free will then? What's the difference?

~~~
colechristensen
True randomness eliminates determinism. With determinism free will is ruled
out. That doesn't mean everything that shows true randomness has free will.

~~~
Xcelerate
> With determinism free will is ruled out.

This is a popular statement but I've never thought that to be the case.
Imagine you're creating a movie. You can pick the actors, the characters, the
plot, etc. When you play the movie, the playback is completely deterministic.
But that doesn't mean free will didn't design the film.

I could see something similar applying to reality. Your actions may be
predetermined, but that doesn't specify exactly _how_ they were determined.

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Totient
I think this article is side-stepping an important philosophical point as to
what randomness is.

Suppose we lived in a deterministic universe (e.g. one that actually ran on
Newtonian mechanics and classic electromagnetism).

If you're of a mildly Bayesian persuasion, you 'believe' in randomness - even
in a deterministic universe - because probability represents your knowledge.
Sure, all the rules for the universe's evolution over time are deterministic,
but you don't know the initial conditions, so you consider some probability
distribution over initial conditions. Thus, the results of future events are
'random.'

The counterargument might go something along the lines of "that's not _really_
random the way quantum mechanics is, because Bell's inequality demands
violating either hidden-variables or locality and violating locality is worse.
Without local variables, you can't meaningfully talk about having a
'underlying' deterministic universe."

Except you (sort of can). Bell's inequality implies that we can't have a
theory with local laws of physics for single universe. But we can have a
theory with local laws of physics for a multiverse which is the approach many-
worlds takes.

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ealloc
Bohmian Mechanics is an interpretation of Quantum mechanics which does not
have any "true" randomness.

It is effectively a hidden variable theory consistent with quantum mechanics.

So at least _some_ physicists do not believe in true randomness.

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viraptor
Thanks for posting something from this site. I've never heard of it before,
but can't stop myself from checking out yet another article. Most of them are
really well written.

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aye_priori
Perhaps someone else more knowledgeable can chime in -

Isn't true randomness unverifiable? In other words, it seems (to me) that
there is no body of evidence that would verify that a particular event was
"caused" by (i.e. was the result of) true randomness (as opposed to the result
of hidden events/variables)

~~~
andbberger
In the particular context of this post, it isn't. The local hidden variable
theory is conclusively wrong. Not sure if there are recent results about
nonlocal hidden variables, but last time I checked we couldn't say anything
about that.

There's a great (and pedagogical enough) discussion of this at the back of
Griffith's Introduction to Quantum Mechanics

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platz
I thought some scientists hadn't really closed the book on bell's inequality,
and though most agree with it there were some gotchas that leave room for
doubt as to correctness

[https://news.ycombinator.com/item?id=5507848](https://news.ycombinator.com/item?id=5507848)

~~~
pdonis
_I thought some scientists hadn 't really closed the book on bell's
inequality_

That isn't what the post you linked to, or the links it gives, says. Bell's
inequality _is_ violated. The book _is_ closed on that. (Technically, there
are a few holdouts who won't be convinced unless we do the experiments with
100% accurate detectors, but they've been done with detectors that are better
than 90% accurate and the inequality is violated.)

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livingparadox
Can someone explain this a bit better? As far as I can tell, the argument is
basically "these particle effects should be predictable. I can't predict them,
therefore they're impossible to predict". That sounds like nonsense to me...
And I don't understand the cos equation.

The argument seems to assume that the experiment is perfectly controlled, and
given that we're still figuring out quantum effects, this seems to be in
impossible task (Make sure no outside effects can change the experiment,
without knowing all the outside effects). So, to me, all the experiment proves
is we don't understand what's going on.

My belief: There is no such thing as true randomness or chaos. Only order we
don't understand.

~~~
yk
There are two different kinds of randomness [1], one is lack of knowledge, the
dice you could calculate or the precise times of keystrokes which could be
easily measured. The other is 'true' randomness, that is the claim that no
experiment or calculation could predict the value of the random variable.
Bells inequality can (sometimes) distinguish between the two. And it turns out
that quantum mechanics allows to measure Bells inequality. The result of these
experiments is, that there is no straight forward way to introduce 'hidden'
variables such that the randomness would just be a lack of knowledge. And it
does so in a way that allows for experimental errors. (Sorry for being quite
vague, it is quite late here and it would take some time to remind myself of
the details.)

So if you want to claim that there is no such thing as true randomness, then
you are essentially calling for another revolution of physics on a similar
scale as the quantum revolution at the start of the last century. It is
entirely possible that this happens, but it contradicts the most fundamental
theories of physics we have.

[1] Actually three, it is not obvious what optimal strategies have to do with
either of the definitions above.

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ced
John Bell himself wrote an entertaining paper railing on the orthodox
perspective: Against Measurement.

[http://www.tau.ac.il/~quantum/Vaidman/IQM/BellAM.pdf](http://www.tau.ac.il/~quantum/Vaidman/IQM/BellAM.pdf)

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ASneakyFox
I don't understand how the photon thing proves randomness. How did they
generate random angles?

I feel like the expirement is saying "we generate random numbers then prove
random exists". Ok and a horse is a horse.

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j2kun
Isn't this just a question of whether they believe in quantum mechanics?

~~~
cwzwarich
There are deterministic interpretations of quantum mechanics, under which the
usual randomness in quantum mechanics is not truly random.

~~~
ngoldbaum
But Bell's inequality says that these hidden variable theories are
incompatible with the results of experiments. Real experiments. Like these:
[http://en.wikipedia.org/wiki/Bell_test_experiments](http://en.wikipedia.org/wiki/Bell_test_experiments)

It may be the case that there really is a hidden variable, but it would need
to communicate it's value nonlocally, which I'd argue is stranger than the
universe just being somewhat random.

~~~
cwzwarich
The Copenhagen interpretation is arguably the most popular interpretation of
quantum mechanics and is nonlocal.

~~~
ngoldbaum
True, but there are no hidden variables in the copenhagen interpretation.

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bradleyjg
I was under the impression that violation of Bell's Inequality generated a
trilemma -- locality, realism, and counterfactual definiteness.

------
Xcelerate
Here's how I like to think of this (I have made some huge simplifications by
the way):

Basically, you have two entangled photons that travel in opposite directions.
Let's say one heads toward New York and the other heads toward California. The
particles arrive at their destination at the same time.

Each experimenter has a little filter (called a polarizer) that he can rotate,
and each experimenter rotates his filter to whatever angle he wants the
instant before the photon strikes it. Each experimenter then measures whether
the photon passed through his filter or got absorbed.

Because each experimenter rotates his filter at the last possible instant
before the photon strikes it, there's no way that the guy in New York could
know what angle the guy in California chose, because to know this would
require that information to travel faster than the speed of light.

Now let's ask the question: what is the probability that both photons had the
same measurement? (That is, what is the probability that either both photons
passed through the filter or both photons were absorbed). From looking at data
from multiple runs of this experiment, it turns out this probability is a
function of _both_ filter angles.

But since nothing can travel faster than light, how is it possible that the
probability is a function of two independently chosen angles? Well the
simplest way this can happen is something like P = f(θ1)g(θ2). You can see
here that the total probability is a separable function. In other words, the
total probability can be computed using functions of two separate angles. It
would be like computing the probability that two separate baseball players
both hit the ball; you don't need any faster than light information transfer
to figure this out.

However, in actual experiments, it turns out that P = f(θ1, θ2), and this
function is _not_ separable. What this means is the total probability is a
function of both angles _together_ \-- you can't compute independent
probabilities and combine them.

What does this mean? It means that somehow each filter "knew" the angle the
other filter was rotated to, instantly. So reality is "non-local". BUT there's
another option if you don't like that: the universe "knew" in advance what
angles the experimenters would choose, and let each photon know this before
they separated. This would give the correct experimental results, and you
wouldn't have to give up locality. This is called superdeterminism, and it
hasn't been ruled out yet, but let's be honest: is the universe really working
that hard to conspire against us? (At least one Nobel-prize holding physicist
thinks so).

EDIT: Also, why did this article instantly drop like 40 spots on HN? Is it
because I shouldn't post comments on articles I submit? (I didn't write the
original article by the way.) It's kind of annoying to spend twenty minutes
writing a comment that nobody's going to see.

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nanidin
> If, on the other hand, you try to predict something like the moment that a
> radioactive atom will radioact, then you’ll find yourself at the corner of
> Shit Creek and No

I love it!

~~~
kzrdude
I had the opposite reaction.

