'Ingenious' Experiment Closes Loopholes In Quantum Theory 214
Annanag writes: A Bell experiment in the Netherlands has plugged loopholes in the theory of quantum mechanics using a technique called entanglement swapping to combine the benefits of using both light and matter. It's Nobel-Prize winning stuff. Quoting: "Experiments that use entangled photons are prone to the ‘detection loophole’: not all photons produced in the experiment are detected, and sometimes as many as 80% are lost. Experimenters therefore have to assume that the properties of the photons they capture are representative of the entire set. ...
[In the new work], researchers started with two unentangled electrons sitting in diamond crystals held in different labs on the Delft campus, 1.3 kilometers apart. Each electron was individually entangled with a photon, and both of those photons were then zipped to a third location. There, the two photons were entangled with each other — and this caused both their partner electrons to become entangled, too.
This did not work every time. In total, the team managed to generate 245 entangled pairs of electrons over the course of nine days. The team's measurements exceeded Bell’s bound, once again supporting the standard quantum view. Moreover, the experiment closed both loopholes at once: because the electrons were easy to monitor, the detection loophole was not an issue, and they were separated far enough apart to close the communication loophole, too."
[In the new work], researchers started with two unentangled electrons sitting in diamond crystals held in different labs on the Delft campus, 1.3 kilometers apart. Each electron was individually entangled with a photon, and both of those photons were then zipped to a third location. There, the two photons were entangled with each other — and this caused both their partner electrons to become entangled, too.
This did not work every time. In total, the team managed to generate 245 entangled pairs of electrons over the course of nine days. The team's measurements exceeded Bell’s bound, once again supporting the standard quantum view. Moreover, the experiment closed both loopholes at once: because the electrons were easy to monitor, the detection loophole was not an issue, and they were separated far enough apart to close the communication loophole, too."
Loopholes in the experiments not the theory (Score:4, Informative)
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I'm not an quantum physicist, but the loopholes appear to be in the experiments intended to demonstrate the "spookiness" of quantum theory, not the theory itself:
I'm gonna argue with you on this new info not closing loopholes in the theory. Until there is proof (demonstrable and repeatable) to back a theory there are loopholes or gaps in the theory. Once the gaps and loopholes have been closed through experimentation the theory comes closer to being fact and not theory. The loopholes existed in the quantum entanglement experiments because of less than ideal methodology, testing conditions, apparatus, etc. in trying to apply the theory to reality.
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Does flipping one electron now flip the other? (Score:1)
Does the fact that the two separated electrons are now entangled mean that flipping one of the electrons will now flip the other? Supposedly, quantum entanglement can't be used for communications but I've never understood why. Even if flipping one electron *might* flip the other, it means you could communicate because error-correcting protocols work pretty well over noisy communications channels.
Re:Does flipping one electron now flip the other? (Score:4, Insightful)
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Because you don't get to 'flip' anything without breaking entanglement. You can just measure one electron and be sure that the same measurement will give you the same result in entangled one. It is like having two random number generators with the same seed - they always give the same (random) answer, but it does not allow you to transmit anything.
That's the "local hidden variables" theory, in which both particles are set with some quantum state at entanglement and don't interact later but which we know is false. If we angle the detectors, collapsing the quantum state at one end will cause correlation at the other end that can't be explained by hidden variables. The funny thing is though is that in order to measure the correlation you need both sets of measurements, which you have to transfer from one to the other at classical speeds so you don't get
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There's no control over which way the electron flips, so no way to send a message that way. And there's no way to measure whether or not an electron has or has not flipped, so no way to send a message that way, either.
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If site B flipped their part of the second pair into a known state...
You can't flip it into a state of your own choosing without first breaking the entanglement. I think.
And being a Dutch experiment... (Score:3)
...the electrons were moved between labs on a bicycle.
Ah, the Dutch! Whether it is a dike or a quantum theory, they can plug the holes ;-)
Loopholes closed in Quantum mechanics? (Score:2)
This is huge (Score:4, Informative)
Guys, this is huge. People have been doing versions of this experiment for decades, every time making it more refined, in order to be able to reach the striking conclusion with the fewest possible assumptions: that the world is not deterministic. The quantum randomness is not our ignorance, is a fundamental property of nature.
What they did was to violate a Bell inequality [wikipedia.org], without using the most questionable extra assumptions (called loopholes) people normally use to extract a conclusion from this experiment: that the separated laboratories are not somehow communicating to conspire to produce the desired outcome, or that the photons they detect are indeed a good representative of all the photons that were emitted in the experiment (normally people can detect only a small fraction of the photons).
I am a quantum physicist, and I know the science behind this experiment very well. If anybody wants to ask me anything, I'd be glad to oblige.
CS guy question: (Score:2)
Does this open the door up to FTL communication?
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It is easy to prove that it is not possible to send information by doing measurements on a part of an entangled state, so no, I'm afraid this is completely useless for FTL communication.
Re:This is huge, how about superdeterminism? (Score:2)
OK, I'll bite. You said in your post that the world is not deterministic. Does the new experiment disprove superdetermism?
Just to show where I'm getting this from I did glance just now at the wikipedia article on Bell's theorem and, I quote:
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This is true, superdeterminism is a way out of the conclusion of the experiment.
That is why I said they only did the experiment without using "the most questionable extra assumptions". The assumption that the world is not superdeterministic is very reasonable, IMHO. Without it, one cannot even do science. For example, in a superdeterministic world, the wavefunction of a photon will depend on which measurements you are making on it, so there isn't such a thing as "the" wavefunction of the photon, and it is n
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Your scheme would work, if one could deterministically entangle the electrons at a distance, but one can not.
The issue is that this technique (called entanglement swapping, look it up), gets the photons, and tries to entangle them; sometimes you succeed, sometimes you fail. For them, this is not a problem, because they can just discard the cases where they failed and do the experiment with the other electrons, but for you it is, as you need to be able to entangle them or not at will.
In general, it is a simp
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How do you communicate a bit using this distinction? Let's say I want to send '0', how do I do it?
It is a problem. Let's say the probability of getting them entangled is 0.5 (I'm too lazy to look it up now). Then you do the thing one time, you don't entangled the electrons, awesome, and they read '0'. Then you want to send them another '0', you do the thing, now they do get entangled, and they read '1'. Shit, now what? You cannot control what they are going to read! They do know the results of your measurem
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Yes, this is exactly the point of quantum key distribution [wikipedia.org], using Ekert's protocol.
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No, the state that describes the system is the electrons together with the photons, you are doing a measurement on the photons, you are doing a measurement in a part of the state.
Look, this is a very fundamental property of quantum mechanics, it just doesn't make any sense for it to fail. The whole thing would go down in flames.
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Actually, it is being swapped; the initial state is (left electron entangled with left photon) and (right electron entangled with right photon), and the final state is (left electron entangled with right electron), with the photons destroyed. So the entangled was swapped from electron-photon to electron-electron.
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What do you mean by the experiment?
One needs counterfactual definiteness to derive the Bell inequalities, so yes, one needs to assume that (among other things) to conclude the world is not deterministic. But the experiment itself is just collecting measurement statistics, so it does not need to assume anything like that.
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Yes. The outcome of the Wigner's friend [wikipedia.org] experiment (Described in more detail by David Deutsch in section 8 of this paper [googlecode.com]). In that case, the Copenhagen interpretation predicts that we would see no interference, whereas the Many-Worlds interpretation would say that we would see interference.
These experiments you mention have no chance of testing that, because they are looking for completely different stuff. What the interpretations of quantum mechanics differ on is how very large and complex quantum systems
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Yes, one can have determinism without locality. This is done, for example, in Bohmian mechanics [wikipedia.org]. But I would warn you against giving up locality in this way. What we need is an extremely conspiratorial kind of action-at-a-distance to be able to predict (actually postdict) the observed results. The choice of measurement they are going to make in one of the diamonds must determine, faster than light, the result of the measurement being done on the other diamond. Needless to say, very few people (mostly philos
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What I can say for sure is that at this point ER=EPR is pure speculation, so I wouldn't be so eager to draw conclusions for it. If you want to know more about its implications, you should ask some quantum gravity guy (i.e., not me).
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ER=EPR is designed to avoid superluminal representations of the Poincare group (which is the symmetry group of Special Relativity, and which has "c" as its sole free parameter, corresponding to that of a massless particle; photons are expected to be massless).
Avoidance of non-locality even gets a explicit mention in section 3.1 of the Malcadena & Susskind paper http://arxiv.org/abs/1306.0533 [arxiv.org]
So, no, ER=EPR does not satisfy non-locality.
(It's mostly designed to try to preserve AdS/CFT in the face of the A
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Technically speaking, Bell's theorem needs determinism and locality to apply, so to conclude that the world is not deterministic, one does need to assume that the world is local. Since I know of no serious scientist that wouldn't assume the world is local*, I can safely say the conclusion of the experiment is that the world is deterministic.
When you write
Even stochastic theories violate Bell's inequality when they insist on maintaining locality.
I think you meant that they respect Bell's inequality no? Otherwise your argument wouldn't make sense. I think what you're talking about is a class of theo
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Thanks for sending me the paper, now I know what you're talking about.
First of all, let us clear up something: what Bell showed is that any deterministic and local theory (you can call it local hidden variable theory if you want, it's just a name) will respect Bell inequalities (in the experiment in question, they will achieve a value of the correlation S that will respect the Bell inequality S <= 2). Since we can violate the inequality in Nature (they achieve S = 2.42), the assumptions must be wrong.
So,
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What? No, come on, this doesn't make any sense! The theorems of 1964 and 1976 are the same! In 1976 Bell chose to conflate both assumptions into one that he calls "local causality", but local causality is exactly the same as the conjunction of determinism and locality. Wiseman does not agree with Maudlin at all, he is just charitably explaining where Maudlin made a trivial mistake.
Come on, In my previous reply I thought you had an interesting point about the locality of quantum mechanics, but now this is ju
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The state change only becomes effective when the results from the two
labs are brought together and are jointly analyzed, which can happen
centuries later. Bohmians like Maudlin tend to confuse such changes in
distributions with a change in the world, because the notions of states
and wave functions are reified, and considered as some real thing out
there
Note here that locality is maintained by not having any appropriate change in the world until the two labs bring their results together! This is what I take Wiseman to be referring to when he talks about giving up on correlation. In the Nature article I linked earlier:
No, it's not. Werner is talking about the nonlocality of the wavefunction collapse, whereas Wiseman is talking about abandoning Reichenbach's common cause principle.
But one can go further, by recalling that local causality rests on two principles: Einstein’s principle of relativistic causality, and the principle of common cause. Thus Bell’s 1976 theorem can be restated as: either causal influences are not limited to the speed of light, or events can be correlated for no reason.
...
Those who hold Einstein’s principle to be inviolable (the localists) must conclude that some events are correlated for no reason. A challenge for them is: if correlations do not necessarily imply a cause, when should scientists look for causes, and why?
and from the arxiv.org paper,
In conclusion, for a proper appreciation of the foundational importance of Bell’s
theorem to physics, information science, and the philosophy of causation, one should be
familiar with both the 1964 Bell’s theorem and the 1976 Bell’s theorem, even though
they are logically equivalent. The former proves that quantum phenomena are either
nonlocal (in a “causation by agents” sense) or undetermined, while the latter proves
that quantum phenomena violate local causality (in a “common cause for correlations”
sense).
Let me clarify what they are talking about: Bell's theorem follows from local causality. Local causality itself can be derived either from the conjunction of determinism and locality, or from the conjunction of Reichenbach's common cause principle and locality. So, if you want to keep locality, you have to give up de
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I'm not seeing much of a disagreement with me in your latest reply. For the most part, you appear to be restating in your own words things I've also said. I think we now agree on what the choice is: locality, or common cause. If you want to maintain locality then you have to deny a common cause in these entanglement experiments. That is, even though the 'entangled' particles demonstrate properties that are highly correlated, the correlation nevertheless lacks a common cause. Or, has a common cause that occurs AFTER the experiment is performed (or thereabouts). Do you agree with this characterisation?
Good that we have cleared things up. I can agree with your characterisation if you are more specific about "common cause": what we have to give up is Reichenbach's common cause principle, which is not the only sort of common cause imaginable. In fact, we know the correlations exist because of the entangled state, so the state is some kind of common cause, just not Reichenbach's.
And to conclude, I'd like to bet that you are not a physicist (probably a philosopher?), if you think it is in any way tenable to abandon locality.
I'm in favour of letting people see the results of their bets. I have a background in physics, but my main area is indeed philosophy, so well done :)
I would be interested to hear why you think abandoning locality would be a big problem.
Ahá! For once the stereotypes worked =)
Well, for starters, it is hard to reconcile nonlocality and relativity; it requires the
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Hmm, polarisation-preserving fibers are commonplace in quantum optics experiments, never heard of any problem with them. One thing you should keep in mind is that there are no "classical light waves", only photons, so one would need a very strange kind of interaction to be able to preserve the polarization of "macroscopic states" while fucking up the polarisation of individual photons.
But this point is moot anyway, because the rules of the game allow the experimenters to do anything before the choice of set
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This isn't "one of Bell's theories". But you are correct, this is not ruled out. But please keep in my that superdeterminism cannot be ruled out in principle, so I don't think it is an interesting assumption.
Experimental issues (Score:3, Interesting)
This experiment has a big problem, as an applied optics (polarization specific) expert, they use polarization entanglement, but then run the light through fiber optics.
The problem is that fiber optics (even polarization preserving designs) have a terrible issue with preservation of polarization states.
I haven't read the paper in detail yet, but I don't know how they can mitigate this issue...
The entanglement works with the right electrons (Score:3)
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Science doesn't explain why at all. How could it?
No number of cannonballs dropped off of towers will tell you why they fall.
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"No number of cannonballs dropped off of towers will tell you why they fall."
Umm, gravity.
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Why does gravity exist?
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Things fall because of gravity. Thats an explanation. Why does gravity exist is another question entirely. There's probably endless levels of reality to go down through.
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Gravity is not just an explanation-- it's the LAW.
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There is no necessity that reality consists of endless levels. There can be a "rock bottom" of existence that just "is", and cannot be explained.
If it cannot _ever_ be explained, not by any level of understanding and technology, then you're really just using a non-traditional description of theology.
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Theology isn't an experimental science. Physics is. We can be sure that something works in physics even if we don't understand it. How are we supposed to be sure that anything in theology works in the real world?
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Gravity is inertia through curved spacetime. It happens that mass has a local effect on spacetime that distorts geodesics.
And now, all together...
What's inertia, and why does it happen like that? What's spacetime? What does it really mean for spacetime to be curved? Why does mass curve spacetime? What is mass?
You can get at least partial answers to some of these if you ask somebody who knows more than I do, of course.
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Sorry, that's Newtonian. Einstein's gravity exists because mass warps space-time. Now as to why it does that.....
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Actually I worked for awhile on a variation of that, but it depends on the presence of mass slowing time, so that particles experience a slight preference to end up in slower time than in faster time. You've got to rewrite the equations to describe space as flat, and all the bending to be handled by variations in the speed of time (WHAT???, but yes). I was assured that this was a reasonable and valid thing to do. I never did learn enough quantum theory to try to convert this into a general theory of grav
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Sorry, I never worked out the math. I did talk to a physics grad student about the concept (of refactoring the equations to put all of the distortion into time) and he said it was valid. I was never an advanced math student, and tensors baffle me, so I'm not the person to work out the math. I just throw out the idea from time to time to see if someone else will develop it.
But I do think it would work.
Re:Is quantum mechanics a theory? (Score:4, Insightful)
Gravity exists because spacetime, curved by massive bodies, effectively changes what it means to have inertial reference frames from the more intuitive Newtonian notion. Take away the massive bodies and spacetime flattens, straight lines are Euclidean, and gravitational attraction goes away. Gravity, then, exists due to the interaction between mass and spacetime.
Of course, you could ask why that interaction exists, and keep asking the question as more explanations are found. I don't know that that'd ever end, but I guess you could eventually hit some inherent axiom or self-referential property of nature. If you're asking for some ultimate underlying conscious intention, though, you may find yourself disappointed, or at least you should accept the possibility that such a question may simply not apply.
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A theory is a piece of supposition about the universe (normally dressed in mathematical formalism), backed up by observation. It helps us predict future events.
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A mathematical description of what happens is exactly what one wants. If you cannot show anything, what happens and is not covered by the mathematical description, and you can deduct things what will happen, that wasn't known before, then you have a theory.
If you want an answer to the question why, you are not looking for science.
Re:Is quantum mechanics a theory? (Score:5, Interesting)
Even most physicists don't understand Feynman's point that QM is called "mechanics" for a reason: it's a set of mechanical rules for getting the right answer. It tells you nothing about how the universe operates behind the scenes so as to produce the same answer as QM. Feynman's little easy-to-understand book on QED should be read by everyone who thinks that QM is more than a tool for performing calculations. (And read Tegmark's book for an example of what happens when an intelligent person reads meaning into QM.)
Regarding the actual article: at first sight, this looks like a great experimental verification of something that no one (as far as I know) doubted; but it's always good to confirm another prediction of QM that appears bizarre to us.
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What eludes us is the interpretation. Or in other words, we didn't find a way to fit this theory in our limited human minds. But the theory works, and it explains things, with maths.
The interpretation is important, but only because it makes working with the theory more intuitive and therefore allow us to progress faster. However, the true essence of any theory in modern physics lie in its mathematical description and how well it matches reality.
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It's true that a model is not necessarily an explanation. Epicycles had predictive value (if tuned well), but almost all agree it's not how planetary motion "works". Newton gravity is considered a better explanation because it's the simplest working model: Occum's Razor.
If a better model comes along, we may make that the top model, ie, the "best explanation so far". Maybe there is no gravity, only something that happens to fit our gravity model. We do more experiments to find that out by testing the model m
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"Newton gravity is considered a better explanation because it's the simplest working model"
Yes, and no. The simplest has a certain charm to it, but that doesn't make it more correct. What makes a theory more correct, aside from predictive value, is how well the elements and relationships of the theory correspond to elements and relationships of the physical system.
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There *are* interpretations of quantum mechanics which *do* explain the "meaning" of the equations. (I'm guessing that's what you mean by "why".) Unfortunately there are several different interpretations that are consistent with the math. My favorite is the Everett-Graham-Wheeler multiworld model, but it's not the only alternative, and so far there seems to be no way to choose between them. But there are only a few interpretations, so most possible ideas of how things could work, and what it all means,
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"No, if you want "why" or "truth" look into the philosophy department."
Thats a bit defeatist. Science has always been about explaining the "why". Why is the sky blue, why do the planets going around the sun etc.
"Quantum mechanics describes a fucking ton of things"
It doesn't really explain anything. Its a convenient probabilistic bucket to put stuff in that we don't really understand but like to give it a name anyway.
"What" is for engineers. Physicists should always be aiming to answer the "why". Anyone can
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It's been about explaining the "why" questions that really translate to "what's happening that results in this phenomena".
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The difference between quantum mechanics and just any convenient probabilistic bucket is that it has excellent predictive value. Using the known principles of quantum mechanics, we can calculate what we'll observe in all sorts of experiments, and the experimental agreement is extremely good.
As it happens, there's a lot of numbers that are vital in making these predictions, and there's a lot of them with no obvious relation to each other, which look completely arbitrary. In science, when we learn someth
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You're trolling, right? There's a big difference between the two things...
Even if the entanglement experiment didn't work every time, it did enough to demonstrate that it's extremely unlikely that the results can be explained through chance. There's nothing wrong with this.
Hopefully if someone else repeated the entanglement experiment, they'd obtain similar results. The criticism of psychology study was the inability to reproduce the study and obtain similar results.
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yep, and they replicated their result 245 times. The typical psych experiment doesn't have that luxury; it takes too long and too much effort.
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I'm not sure I consider those replications, as they are all using the same equipment, and if there were a systematic problem, then there might well be a systematic effect.
OTOH, this is, essentially, a replication of experiments done previously, with a couple of added features, and it's results are consistent with those prior experiments. *THAT* I do consider a replication.
Given the nature of publishing, the study that confirms this one will also need to have some changes. It shouldn't be that way, but jou
Re: Wait, physics doesn't work either? (Score:2)
Systemic
Re: Wait, physics doesn't work either? (Score:2)
You are coming upon an old conundrum. Does every particle "know" about every other particle or not? How does a rotating reference frame know it is rotating? The side that you are taking will be disproven if gravity waves are found. That is a vast oversimplification but I think the questio
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The difference is, that we are speaking here about a failure rate of singular events, which is a technical problem, and requires a larger population of events for sufficient confidence.
That is contrary to the results the previous article, that regardless of the population size, we cannot reproduce the result, which is a systemic problem.
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That is contrary to the results the previous article, that regardless of the population size, we cannot reproduce the result, which is a systemic problem.
No. That is not how psychology works. It is not ethical to conduct psychology experiments on arbitrarily large groups of people. You cannot say that the experiment was expanded to a point where you can make a statement about it "regardless of the population size".
It is shameful that you were modded up for that comment, it shows how little people on slashdot understand about psychology.
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Certainly not. Since psychology doesn't work. Period.
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No. That is not how psychology works.
Certainly not. Since psychology doesn't work. Period.
It is a safe bet that if you are willing to make such a sweeping and silly generalization that you haven't studied psychology yourself much - if at all. You encounter successful applications of psychology in your daily life regularly without realizing it, and the influence of psychology on other sciences is also significant.
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indeed, hypothesis tested by experiment. But lack of reproducibility calls into question the effectiveness of the testing.
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There are sciences where we can't conduct experiments, astronomy being the obvious one. Hypothesis tested by observation includes these.
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That's probably a better phrase, I agree. While the definition of "experiment", as I understand it, does include "set up a telescope and see what's there" (being "an act for the purpose of discovering") the common perception includes setting up equipment to ensure that what you're watching turns out to be interesting :)
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You're mixing up generally irreproducible results and a situation where a proportion of reported results are not replicated.
Modern psychology is very much a science, using the scientific method. However, due to the difficulty of studying it, the requirements for publishing a result are low enough that many of them turn out to be incorrect (not reproducible). That these results are eventually found to be incorrect is a validation of the scientific nature of psychology.
ANY subject involving a complex, diffi
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It is shameful that you were modded up for that comment, it shows how little people on slashdot understand about psychology.
The more they understand about it, the worse it looks.
Quite the opposite most of the time. A big problem here is that a lot of people (particularly people on slashdot, though this happens in many other circles as well) think they know a lot about psychology because they have read a lot of angry rants against it, even though they have never had formal exposure to the fundamentals or history of psychology.
If you have some interest in the outcome, for example if you yourself are a psychologist, it would be considered a minimal level of integrity to disclose that ... since we're speaking of ethics now.
I am not a psychologist. I have friends and colleagues who are, and I took psychology as an undergrad. My work is in a more contemporary hard science.
It
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To borrow, and mangle, a quote from B.W.:
"Psychology is not rocket science. Hell, it isn't even sociology."
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obvious troll is obvious.
Entanglement (Score:2, Informative)
A lot of quantum confusion can be dispelled by realizing that particles don't really exist. There are a bunch of phenomenae that look like particles, but also look like wavelike perturbations of a field. Since we don't really have any good mental analogies of what's "really" happenening, we have to fall back on mathematical descriptions. So the general concept is that you can glom two waves/particles together so that you cannot describe them individually any more. From my limited understanding, I don't thin
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There are a bunch of phenomenae that look like particles, but also look like wavelike perturbations of a field.
If we could just settle on calling them wavicles, people would be less likely to get hung up because of their preconceived notions of waves and particles. They're both and neither.
These things are subjected to so many tests, maybe we should call them "testicles."
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Yes, but the preconceived notions of waves and particles are useful in understanding these things. You just have to keep in mind that everything that small is both, but not at the same time.
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If you're measuring one attribute of wave behavior, you've got waves. If you're measuring one attribute of particle behavior, you've got particles. If you try to measure one of each, you'll get waves or particles, depending on what actually winds up being measured.
Re:Wait, physics doesn't work either? (Score:5, Interesting)
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See, what you're missing is simply that it's turtles all the way down.
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Re:Wait, physics doesn't work either? (Score:5, Interesting)
"in terms of what other system could we try to explain the observed phenomena that we call entanglement?" Specifically (while I realize it cannot be used to transmit information), how is it faster than light? Is the concept of locality a defensible one?
Interestingly enough like most effects of quantum mechanics, entanglement does not have an easy macroscopic analog to compare. One way to think about it is that it is a type of emergent behavior because of the rules that QM appear to follow.
More specifically, entanglement is kind of emergent behavior that is a logical consequence of conservation rules and quantum superposition states. If you believe in the QM rules regarding conservation (e.g, conservation of say spin), and QM rules involved with superposition wave function collapse (e.g., so called "observation"), the emergent consequence of these rules is a behavior we call entanglement.
The macroscopic analog is sort of as follows. Suppose you have 1 balls and 2 boxes. By some method hidden from you, the ball into one of the two boxes and it is sealed. If you believe in conservation of balls, The two boxes are now entangled. You can move them arbitrarily far apart and then open one box, if it has a ball, you instantly know the other doesn't have a ball.
Where this breaks down is how you put the ball in the box. In the QM version of this, the method of which you put the ball into the box doesn't really put the ball into the box, it simply puts a type of probability of a ball into a box. Interestingly enough, the box can act sort of like a 1/2 ball in the box until you open it and then it "collapses" and is either a ball or not ball. The strange part is how can it act as if there is a 1/2 ball in the box before you open it? If you think of the decision being made when you seal the box, there is some sort of locality, but if you think of the decision being made when you open the 1/2 filled box, there is non-locality and you need to use a concept like entanglement as an emergent behavior.
That is 1/2 ball in the box (part particle, part wave) is QM and nobody really understands that part, so there's really not an analogous macroscopic system on which to understand it, because the systems we are familiar with don't follow those rules.
On the concept of locality, it's really unknown. We generally think of distance and time (warped by general relativity) as the way we measure locality (e.g., light cones, etc), but there isn't a clear idea if there isn't a macro-dimension or holographic way that alters our understanding what is local or non-local. Using current theories, we already speculate that there are singular violations of locality (e..g, EPR's or worm-holes, etc), and we don't understand the fabric of space-time (e.g quantum gravity) well enough to say if our current theories about this are descriptive enough to yield our current intuitions about space-time locality or if it will be as weird as QM.
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Wow. +1 "Enlightening".
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Locality doesn't work. These experiments can be done with electrons, and we can measure the spin. If one electron has spin up, the other will have spin down if measured along the same axis. If the measurement axes are different, you can calculate how likely one electron is to have spin down if the other measures as spin up based on assumptions. It turns out that the observed probabilities don't match the ones you'd get if they had what you'd expect if the relation was based on locality. Look up Bell's [wikipedia.org]
Re: Wait, physics doesn't work either? (Score:2)
The Bell's theorem is based on the assumption that if there are local variables that are hidden, the result measured spins of the particles would be linearly correlated to the angles these particles are created from.
I do not see how this assumption is valid though. Particles are not billiard balls. Particles may have properties that force their spin into specific correlations. It is stupid to accept that local variables would mean linear correlation.
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Entanglement may actually have a macroscopic analog, just not one we have experienced yet. Spend some time Googling the "ER=EPR Conjecture".
The TL;DR is "entanglement and wormholes are different manifestations of the same underlying thing"
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On the other, I can now rephrase my question thusly: "in terms of what other system could we try to explain the observed phenomena that we call entanglement?"
Math. Specifically, complex linear algebra.
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I think the answer is "we don't really know." One of the criticisms of the standard model is that, although it has fantastic predictive ability, it doesn't have much explanatory ability. It can't even tell you something as seemingly simple as why an electron has the rest mass that it does.
However, you might find the phase/state/configuration space formulation of quantum mechanics more intuitively satisfying. You can imagine a quantum system as being a particular state space with rules restricting how you
Re:Wait, physics doesn't work either? (Score:5, Interesting)
"Entanglement" is a philosophically difficult arena. According to quantum theory, there is just one wavefunction for the entire Universe. However, we as observers are part of that wavefunction observing another part of that wavefunction with a really, really, big chunk of the whole wavefunction effectively unobservable but still coupled to the observer (part of the wavefunction), the measuring apparatus (part of the wavefunction), and the "experiment" (yep, part of the wavefunction. Everything is "entangled", but quantum mechanics also predicts that large systems approximated with a random phase condition will behave like a classical system, and the usual rule is that we treat a measurement apparatus as a classical system that breaks the entanglement of a measured systems and forces it "unpredictably" into a separable state. But even this is words, not equations although random phase approximations are indeed equations and are used frequently in field theory.
The only coherent explanation of this that I am aware of is the process of:
a) Starting with a density matrix (or other representation) for "the Universe".
b) Use the Nakajima-Zwanzig approach of splitting the (fully entangled) density matrix up into two parts -- a "system" that you will continue to treat as a quantum system, and a "bath" -- everything else -- which would also include the measuring apparatus if you were trying to describe an experiment. One then accepts the fact that one cannot know or prepare the state of the bath (which is really, really big being the rest of the Universe and everything) and so one makes a statistical approximation of the bath (taking the trace) which essentially eliminates the pesky entanglement but also breaks useful things like unitarity and in a sense, conservation laws. One them creates projection-valued operators and transforms the equations for the system into stochastic or semiclassical equations of motion.
c) IIRC your final result is quantum mechanics for the system expressed as a non-Markovian integrodifferential equation that is almost impossible to solve. However, if one makes a Markov approximation (forces it to be time-local, delta-correlates time) you end up with a decent explanation for things like spontaneous decay as an "exponential" process rather than a punctuated unitary process. You go one way, you can make it into a Langevin equation, go another you can make it more like Fokker-Planck.
The lovely thing about this approach is that it renders moot all sorts of nonsense, such as EPR paradoxes and "wavefunction collapse". It is perfectly clear that in the Universal wavefunction no such paradox or collapse can occur. They are simply expressions of our ignorance of phase and state whenever we try to isolate some part of the whole and pretend that it is a standalone "system" that can ever be decoupled from everything else. Schrodinger's cat paradoxes disappear as there is no paradox in the Universal wavefunction, only when we try to project the state of the cat against our ignorance of phase and interaction with the outside Universe. The cat, if you like, cannot be entangled separately from its preexisting entanglement with the rest of the Universe, and we only get into trouble when we have to force it by partitioning the system in order to get a chunk small enough to work with.
Hope this helps. I doubt it will. Very few people seem to be in touch with Nakajima-Zwanzig and the Generalized Master Equation these days, and don't treat problems like this as OPEN quantum systems as opposed to closed systems with a classical measurement apparatus, which is a place you only get to on the far side of the N-Z GME ritual.
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Re:Wait, physics doesn't work either? (Score:5, Interesting)
The real question is - exactly wtf is entanglement anyway? I can find lots to read about what it looks like and how it behaves... but what's the underlying mechanism? Is there even the most speculative explanation of it?
Here's the best answer I can give you - I think it's true, and not so over-simplified as to be wrong.
The universe has some underlying state. We don't have direct access to that state - not only is it not directly observable, it's not directly related in any intuitive way to the state we can observe. There's this arbirtary-seeming transform between underlying state and what we observe (it only seems odd or arbitrary because all our intuitions are based on human-scale observables, and are not at all directly informed by this underlying state). This underlying state seems to be well-defined and deterministic, forwards and backwards in time. The observable universe is not.
Entanglement is a feature of how observations relate to underlying state - a feature of the transform. In very simple experiments we can measure specific properties of, say, an electron. We can't measure all of them, for a given electron, because the transform just doesn't work that way, but we can measure some. However, that's deceptive, because you can't really track that property of that electron over time, in non-trivial cases. If e.g. two electrons interact, become entangled, your observations are now a function of both electrons' underlying state, and that's a different transform from 2 non-entangled electrons.
There are two key concepts here. The first is that the whole notion of "particle" is a handy but false oversimplification. It can lead you to all sorts of false intuitions about how particles behave. Fundamentally, individual e.g. electrons don't have unique identities. The underlying state is a single electron field, which other fields can interact with, in a way that can sometimes be simplified as "particle interactions", for a simpler mental model, but you can't go too deep with that model. An example: "two electrons collide in an accelerator, and two electrons leave, which is which?" That question is "not even wrong", it's just nonsense. Thinking of electrons as billiard balls colliding is simply not a helpful model, as it just misses the point of the interaction.
"Entanglement" happens just when the "particle" mental model fails: you can no longer pick two disjoint areas in the electron field and consider them as independent "electrons", but instead you have to reason about two areas which may be quite disconnected in space and time. E.g., you might know for sure that one electron is spin-up, and one spin-down, but have 0 information about which is which. None of that matters to the underlying state: there's just one electron field, and the only truly correct way to reason about it it to reason about the whole field all the time, and so this is only half of "WTF is entanglement".
The second concept gets too much into the math to explain well, but in a hand-wavy way it's this: "what is measurement?". There are older interpretations about measurement causing wavestate collapse and so on, but they're wrong because of that word "cause". Measurement is simply the observer becoming entangled with the observed. Measuring one entangled electron doesn't "cause" the other electron to do or become anything. The underlying state is unchanged, which is why there's no faster-than-light effect. In some cases, this is an overly pedantic distinction, but it matters when the difference between QM and intuition matters. In a two-slit experiment where you see an interference pattern at your detector, if you add a measuring device to one slit suddenly you don't see that interference pattern. Informally we might say the second observer "caused" this change, but formally that's wrong, it's just that a system with 2 slits and 2 detectors behaves differently from a system with 2 slits and one detector, and it doesn't matter which detector the electron passes first, because (see above) an "electron" as a discrete particle is fiction anyway, and both detectors are entangled with the electron field already, or they couldn't measure an electron anyhow.
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Your "underlying state" seems equivalent to a "hidden variables" theory. Note that the Bell inequality says that if the universe is local (no action at a distance, special relativity, etc.) then hidden variable theories cannot reproduce the observations of quantum mechanics.
It's possible the universe really does have a deterministic nature that is hidden from us, but if that's true then the laws of physics are not local. We tend to shy away from that option because of the success of special relativity, an
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our "underlying state" seems equivalent to a "hidden variables" theory.
No, it's just the sloppiness of English trying to represent math, or perhaps my lack of facility with one of those in trying to craft a metaphor.
To extend my above metaphor: there's no hidden "observable" state. The underlying state is not "this one spin-up, that one spin-down" (which is forbidden), because there are not electron identities anyhow, but instead "exactly one of them is spin-up". As you measure one of them, there are now three entangled things: the two electrons and your detector, and there'
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You can get a very limited idea of entanglement by thinking of two envelopes, one red card, and one black card. If you put one card in each envelope, you've got envelopes entangled in the sense that if you open one you know the color of the card in the other, wherever it may be.
Now, we figure that we can open the envelopes in different ways with different but related results (if we open them at the bottom, the cards change color, and if we open right along a side it's random which card is which color, a
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Everything in physics works in both time directions (you have to swap some signs +/- when you reverse time, but it all works). Causality as "a chain of related events over time" is a real thing, even if what you place in the chain may be somewhat arbitrary, but the direction, which is cause and which is effect, isn't so well defined. At the QM scale it's arbitrary. In human experience, a film played in one direction looks different than in the other because, ultimately, of the energy input from the Sun br
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I take a photon, I split it into two identical photons
Do you? That's very clever of you. People have won Nobel prizes for less.
I usually describe this as the "flock of starlings" effect. If all you can see is a flock and not the individual bird, the flock appears to jump and leap and disappear and reappear. But it is simply the effect of a detector that can only see flocks and not birds.
Yes, you do keep describing it that way. And you've been told several times why you're wrong, but you just won't listen.
However protons are not fundamental particles anymore, deep inelastic scattering showed they are made of smaller particles. So you never detected the proton AT ALL, you simply detected the net result of the effects of these sub-proton particles. That net result jumped around, not the proton. Likewise you could not have 'set' the position of the proton, because it does not exist! It was just an effect of multiple smaller particles on the detection mechanism.
Okay, let's ignore the fact that you're just plain wrong. Why do electrons, which are fundamental particles, behave the same way?
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European starlings, or African starlings?
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No. And no. This is just plain impossible.
What they did was to violate a Bell inequality [wikipedia.org], without cutting any corners (is this corner-cutting that is named as "loopholes" in the experiment). This proves that the world is not deterministic (if you believe in relativity).
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There's no way to communicate FTL this way, as you can only observe. For quantum secure communication, take one each of lots of entangled particles, and measure them in order as you need randomness to encrypt your message. Send it. Only the person with the entangled particles can determine what the randomness was. It's something like a high-tech one-time pad, except that it can't be copied without knowing how you're measuring the particles, and if it's copied it's destroyed. A message can be intercept
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No, you can't detect whether an electron is entangled or not. More correctly, you can't detect what it is entangled with since, IIUC it's always entangled with SOMETHING. This is actually one of the things that makes it difficult to do the experiment, as keeping the electron from shifting what its entangled with is difficult. (Actually, different characteristics of the electron can be entangled with different targets.)
Every interaction between particles yields an entanglement, you just can't usually figu