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Physics

Quantum Physics

July 9, 2012

A roll of the dice: Quantum mechanics researchers show that nature is unpredictable

Jul 09, 2012

Many of the predictions we make in everyday life are vague, and we often get them wrong because we have incomplete information, such as when we predict the weather.

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But in quantum mechanics, even if all the information is available, the outcomes of certain experiments generally can't be predicted perfectly beforehand.

This inability to accurately predict the results of experiments in quantum physics has been the subject of a long debate, going back to Einstein and co-workers, about whether quantum mechanics is the best way to predict outcomes.

Researchers from the University of Calgary's Institute for Quantum Information Science along with researchers from the Perimeter Institute in Waterloo and the Eidgenössische Technische Hochschule (ETH) in Zürich/Switzerland have published a paper in Physics Review Letters that suggests quantum theory is close to optimal in terms of its predictive power. The research in this paper looks at measurements on members of maximally entangled pairs of photons that are sent into Stern-Gerlach-type apparatus, in which each photon can take one out of two possible paths.

"In our experiment, we show that any theory in which there is significantly less randomness is destined to fail: quantum theory essentially provides the ultimate bound on how predictable the universe is," says Dr. Wolfgang Tittel, associate professor and GDC/AITFIndustrial Research Chair in Quantum Cryptography and Communicationat the University of Calgary.

Dr. Renato Renner, Professor at the ETH in Zürich adds: "In other words, not only does God 'play dice,' but his dice are fair."

Randomness in quantum theory is one of its key features and is widely known, even outside the scientific community, says Tittel. "Its appeal is its fundamental nature and broad range of implications: knowing the precise configuration of the universe at the big bang would not be sufficient to predict its entire evolution, for example, in contrast to classical theory."

Explore further:

Use of laser light yields versatile manipulation of a quantum bit

More information: "An experimental bound on the maximum predictive power of physical theories" is by Terence E. Stuart,Joshua A. Slater, Roger Colbeck, Renato Renner and Wolfgang Tittel is available: prl.aps.org/abstract/PRL/v109/i2/e020402

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antialias_physorg

4.7 / 5 (6)
Jul 09, 2012

Well, that should put the final lid on the coffin for causality/determinism...its all just correlation and randomness.

At least this shows that we have free will (not in the sense that we can will anything at all - but that 'will' cannot be predetermined/precomputed for an infinitely long time)

Report

JRDarby

4 / 5 (5)
Jul 09, 2012

aa,

If everything is determined, there is no free will. If everything is random, there is no free will. In any case, free will is an entirely separate issue from predictability of the progression of events from an initial cause.

Besides, the article didn't say that everything was random--only that there was a bound to predictability. That could mean that there is free will, or there is no free will, or there is free will sometimes and not others. In the first instance, there seems to be a bound to deterministic predictions. In the second instance, there may be a bound to our abilities to comprehend our actions--to predict the consequences of our options for actions and act accordingly--and what is freedom without this predictive power but randomly firing in any direction you "want" to go? In the third instance, maybe it just means that our actions are generally free (or determined), but that there are exceptions to the rule.

Who knows? Neither of us, for sure.

Report

Pyle

4 / 5 (2)
Jul 09, 2012

In any case, free will is an entirely separate issue from predictability of the progression of events from an initial cause.

Only if you believe in a spiritual dimension. If you believe that the "soul" is just wetworks between the ears then free will is not any different from "the predictability of the progression of events from an initial cause".

Everything that a person chooses is a result of things that have happened; whether they be memories stored, delusions created by misfiring neurons, or whatever. In a world without quantum randomness it may have been possible to have enough information to predict all future events, including decisions. This study says in our universe it just ain't so. At the quantum level, even with "perfect" information the result is random.

But true, random or determined, I guess free will has left the building.

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daywalk3r

3.8 / 5 (16)
Jul 09, 2012

Well, that should put the final lid on the coffin for causality/determinism...its all just correlation and randomness.

Yes, nature is indeed unpredictable to an absolute degree, but that does not necessarily mean it has do anything with true "randomness".

Just because we can/will not be able to get any useful information from bellow a certain threshold (sub-Planck/sub-Schwarzschild) does not mean there it nothing happening down there.

Yes, there is no way how to probe/measure anything past those barriers, so the next easiest thing is to just call it "random", but that does not make you any wiser, nor is it a final solution towards ultimate understanding.

I would even go as far as stating that "random" is just a concept developed by our minds and has no direct relation to reality. It essentially means "unpredictable", but it is tied to a speciffic observer's refference frame.

True randomness would mean it is unpredictable from ANY possible refference frame. GL prooving that.

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antialias_physorg

5 / 5 (2)
Jul 09, 2012

Yes, nature is indeed unpredictable to an absolute degree, but that does not necessarily mean it has do anything with true "randomness".

As I said: "it's all correlation AND randomness" - not "it's all randomness"

The point is that if you do not have randomness of any degree then you can have absolute information (any nonrandom sequence has a finite information value). But since that seems not to be part of the deal (as per the article) there has to be some truly random element out there.

Everything that a person chooses is a result of things that have happened

Only to a certain degree (i.e. there is correlation - but not causation).

In any case, free will is an entirely separate issue from predictability of the progression of events from an initial cause.

As I said: free not in the sense of "unbounded" but simply in the sense of "not ultimately predetermined".

Report

daywalk3r

3.4 / 5 (15)
Jul 09, 2012

The point is that if you do not have randomness of any degree then you can have absolute information (any nonrandom sequence has a finite information value)

The probability of someone being able to acquire "absolute information" is about exactly same as the probability of him knowing the last digit of Pi with 100% certainty.

Unless you want to argue that the next sequence is only random until it's calculated for the first time.. Because, you know.. How can something be truely random, if it can be predicted? ;o)

The bottom line still stands, that the evaluation of "randomness" is observer dependant, so until it can be prooven that an event is "random" for ANY possible observer, true randomness will remain a matter of faith.

there has to be some truly random element out there

Last time I checked, none of the elements of the periodic table were random or "just by chance" :-P (j/k)

there is correlation - but not causation

How does that work? :-O

Example please :-)

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vacuum-mechanics

1 / 5 (3)
Jul 09, 2012

Randomness in quantum theory is one of its key features and is widely known, even outside the scientific community, says Tittel. "Its appeal is its fundamental nature and broad range of implications: knowing the precise configuration of the universe at the big bang would not be sufficient to predict its entire evolution, for example, in contrast to classical theory."

By the way, talking about randomness in quantum mechanics, we know that it is the outcome of the character of particle wave duality nature. In contrast to classical theory in which both particle and wave nature were separately observed, and may be this paper could give some idea.

http://www.vacuum...id=17=en

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antialias_physorg

5 / 5 (1)
Jul 10, 2012

Unless you want to argue that the next sequence is only random until it's calculated for the first time.. Because, you know.. How can something be truely random

There's a very fine distinction between random and randomly distributed. The digits in Pi aren't random, but they are (as far as we know) randomly distributed.
To give an opposite example: A coinflip is random but the probability distribution of many outcomes is not.

The difference between the two examples is: Pi is part of an artificially created fully causal system (math), while a coinflip is part of reality (which is acausal)

Last time I checked, none of the elements of the periodic table were random or "just by chance"

Their decay seems to be. Vacuum fluctuations seem also to fit the bill.

Report

antialias_physorg

5 / 5 (1)
Jul 10, 2012

there is correlation - but not causation

How does that work?

Causation is the notion that everything is ONLY dependent on past (and fully determined by it). Acausality says that things may or may not be dependent on things in the past or the future. (While, for completeness' sake, an anticausal event - which is hypothetical - would be only dependent on events in the future.)

At a very fundamental level the definitions we have don't make much sense.
E.g. the Uncertainty Principle does not allow for infinitely tight bounds on momentum and position. This means that there is no way to make an infinitely precise measurement from which to deduce a causal chain because no such precise state exists. There is a very fundamental lack of information at that level - and it has nothing to do with our ability to perform good measurements.

There's just probability densities - from which you can only calculate correlations.

Report

Tausch

1 / 5 (1)
Jul 24, 2012

There is a very fundamental lack of information at that level - aa

At a very fundamental level the definitions we have don't make much sense. - aa

Still. Admirable and heroic endeavors of sense stemming from human existance and experience.

Great thread and commentary.

Perhaps we will find our definition of true randomness.
Humans are an excellent potential source for finding a working definition.

Report

Satene

1 / 5 (1)
Jul 24, 2012

The predictions of quantum mechanics aren't equivalent with (results of) experiments. For example, this is what the quantum mechanics predicts for double slit experiment with gamma ray photons - the probabilistic density of photons hitting some place at the target. This is what you actually get during this experiment. It means, no matter if you would do the same experiment with lightweight photons or heavyweight gamma ray photons, the quantum mechanics predicts the very same outcome of experiment. The physicists somehow managed to ignore this difference for one hundred years....

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Satene

1 / 5 (1)
Jul 24, 2012

The downvoting of the above comment just illustrates the mechanism, in which this ignorance propagates to another generations of physicists. The people dealing with physics don't want to hear that their theories don't work well, so they tend to ignore all unpleasant comments about it without action. Because the physicists cannot realize, that their pet theory may work well with high degree of precision for description of some phenomena, whereas it can be quite wrong/incomplete in another aspects, all similar warnings are ignored as an unsubstantiated ones. And because the community of physicists lacks the feedback from outside, they've absolutely no problem with it, until their money are going.

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hcnap

1 / 5 (1)
Aug 17, 2012

Randomness, Probability and Statistics are used when there is no exact formula available or we cannot recreate the exact circumstances of an event. Nature does behave in a uniform manner. Just that we don't really understand nature.

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