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Do physicists really believe in true randomness?
- j2kun 13y agoIsn't this just a question of whether they believe in quantum mechanics?
- pavpanchekha 13y agoBelieving in quantum mechanics is much like believing in gravity or relativity—it is definitely a part of the universe we live in, and no mainstream physicists disagree on its correctness.
- tedsanders 13y agoHowever, there is a degree of belief in the interpretation of quantum mechanics that you choose. It's odd that the author of this article espouses the Many Worlds interpretation without pointing out that it's not a consensus viewpoint. Also oddly, the Many Worlds interpretation is known for being a deterministic interpretation yet the author promotes it as a non-deterministic interpretation.
- pavpanchekha 13y agoBelieving in quantum mechanics is much like believing in gravity or relativity—it is definitely a part of the universe we live in, and no mainstream physicists disagree on its correctness.
- pavpanchekha 13y agoBelieving in quantum mechanics is much like believing in gravity or relativity—it is definitely a part of the universe we live in, and no mainstream physicists disagree on its correctness.
- panic 13y agoIt's not about "belief" -- you can perform the experiment described in the article and see for yourself.
- cwzwarich 13y agoThere are deterministic interpretations of quantum mechanics, under which the usual randomness in quantum mechanics is not truly random.
- ngoldbaum 13y agoBut 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.
- Grieverheart 13y agoBell's theorem is based on the assumption of free will, see eg http://en.wikipedia.org/wiki/Superdeterminism http://en.wikipedia.org/wiki/Superdeterminism
- dllthomas 13y agoEeeeh, not quite "free will", but "choices that are not structured relative to the thing being measured", which is a much weaker assumption (though still an assumption).
- cwzwarich 13y agoThe Copenhagen interpretation is arguably the most popular interpretation of quantum mechanics and is nonlocal.
- ngoldbaum 13y agoTrue, but there are no hidden variables in the copenhagen interpretation.
- davorak 13y agoNo, the many worlds interpretations of quantum mechanics, those that I am familiar with, do not contain any randomness.
- baddox 13y agoI'm struggling to understand the distinction between randomness and unpredictability. The many worlds interpretation still implies unpredictability, doesn't it?
- viraptor 13y agoThanks 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.
- livingparadox 13y agoCan 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.
- pokpokpok 13y agoI really don't think you read the entire article, but: "As another aside, Bell’s Inequality only proves that the reality assumption and locality (nothing can travel faster then light) can’t both be true. However, locality (and relativity) work perfectly, and there are almost no physicists who are willing to give it up. Except for Bohm, who’s an ass."
- analog31 13y agoForgive me, but I'm reaching back into my recollection of this subject from when I was a physics student. The presence or absence of true randomness has to be turned into a testable hypothesis, or it's not a scientific question. I think the point of the experiment as described is to find such a test, where randomness, or its absence, produce different outcomes. Whether it achieves that goal is for people smarter than me to figure out.
- yk 13y agoThere 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.
- platz 13y agoI 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 13y agoI 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.)
- aye_priori 13y agoPerhaps 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 13y agoIn 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
- ealloc 13y agoBohmian 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.
- deleted 13y ago[deleted]
- nanidin 13y ago> 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 13y agoI had the opposite reaction.
- pvnick 13y agoI 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 13y agoI'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?
- moyix 13y agoThere's a pretty good essay from Scott Aaronson [1] that tries to disentangle the question of randomness from unpredictability, by saying that unpredictable events (which he says are the only things we know of physically that might permit something like free will) are events to which we can't assign a probability distribution. It separates out the notions of "random" events like coin flips, which we know will go heads with p=.5 and tails with p=.5, from truly unpredictable events, where we can't say even in principle what the probabilities of the outcomes might be. These latter events are where he finds some wiggle room for free will. [1] http://www.scottaaronson.com/papers/giqtm3.pdf http://www.scottaaronson.com/papers/giqtm3.pdf
- pvnick 13y agoThat's a good point, I'm not quite sure how to answer that. I know almost nothing of quantum mechanics. My logic is simply that if you had a large enough calculator and nothing was truly random that you could predict us having this conversation from the moment of the big bang. That's incompatible with true freedom of will (which someone is sure to ask me to define, I swear metaphysics suffers from death by definitions). But given true randomness: 1) Person faced with ethical decision 2) Random events somewhere in the brain 3) _____________ 4) Unpredictable autonomous decision! Or something like that. Edit: moyix's is a much better response than mine
- Russell91 13y agoThis 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.
- encoderer 13y ago"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)
- Russell91 13y agoHaha, 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.
- ced 13y agoJohn 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
- jandrewrogers 13y agoThe 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.
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- Houshalter 13y agoWhat about many-worlds? It appears randomly from the inside but actually isn't.
- Totient 13y agoI 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.
- bradleyjg 13y agoI was under the impression that violation of Bell's Inequality generated a trilemma -- locality, realism, and counterfactual definiteness.
- ASneakyFox 13y agoI 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.
- Xcelerate 13y agoHere'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.