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Beginner's Guide to Quantum Entanglement
No Fortune writes "Einstein called it 'Spooky action at a distance.' This article describes, in scientific layman's terms, how spooky action is created." From the article: "Normally the photons exit the crystal such that one is aligned in a horizontally (H) polarized light cone, the other aligned vertically (V). By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics demands they differ."
...what the hell is the matter with you anyway?
Throwing that kind of physics at us on a Saturday evening when you *know* most of us are half drunk?
Bastard.
Mod me down with all of your hatred and your journey towards the dark side will be complete!
Lord help us... but anyway, you've got be half-drunk to even start getting quantum physics. Everyone knows that... well, they do when you ask, they didn't before you asked.
in scientific layman's terms
Ah, oxymoron terms... the best kind.
I've been waiting for Teach Yourself Spooky Action in 24 hours.
+1 dugg
Is that what the geek kids are calling it these days?
"Excuse me, but you stimulate the neurons in my hypothalamus. Would you like to come over to my place and study quantum entanglement?"
"Figure 5.2 is an enhanced photograph of a photon ..." - That is more than just misleading.
"But all your emitter and collector are belong to me!"
Suppose you take a coin and spin it on a frictionless surface in a vacuum, so that it's perfectly balanced and doesn't wobble. In theory, it will keep on spinning at the same rate forever.
Now suppose you take a second coin, identical in all respects to the first, and start both coins spinning at the same time--but with one of them 90 degrees out of phase compared to the other, so when one is "horizontal" when viewed from above, the other is "vertical".
Finally, suppose you have a way to move the coins without affecting their rotation. Move one of the coins as far away as you like from the other.
Reach out a finger and stop one of the coins. Suppose that at the instant you stopped it, the coin was horizontal. You now know that, at that particular instant, the second coin was vertical--not because the coins somehow "communicated" with each other, but simply because they both followed the same laws of physics up until you interfered.
Granted, I'm oversimplifying tremendously, but is this a semi-reasonable explanation of why quantum entanglement has nothing to do with instantaneous communication, or do I just need to get to sleep?
That's a too simple description of polarization. It doesn't work that way. Take a polarizing filter and shine a light through it. Add another polarizing filter but rotate it 90 degrees from the other. The light is cut off from passing all the way through both. So far, so good. Now here's the tricky part. Take a third polarizing filter and place it in between the two previous ones. Rotate it around. WOW! At some intervals you can now see through all three! With two if you rotate the second you get total blockage when the filter is at 90 and 270 degrees from the first. You get more blockage points around the 360 degrees with the in-between third one (Extra ponts: how many?)! Strange. Add another. You get even more blockage points. (How many now?) Very strange indeed. Does the experiment account for this, the real behavior of polarizing filters and not the simplistic one in the article?
I just had a look through digg and it lacks the one thing that makes me come back to slashdot - insightful comments. The comments were 50% juvenile drunken-louts-at-school level and the other 50% were people who didn't understand the question.
While we are on the subject of Quantum Mechanics. Check out Caltech's website on Quantum Computers.
I would also like to put you towards HP's Research on it.
The future is quantum mechanics, no matter the subject.
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>>The comments were 50% juvenile drunken-louts-at-school level and the other 50% were people who didn't understand the question.
Exactly!!! And what is your impression of Digg comments?
Brian Greene has a beautiful explanation of how your idea was proven incorrect in The Fabric of the Cosmos. Now comes my horrible attempt to further simplify it so it fits in a Slashdot post.
Your idea would be a fine alternative explanation, if there was only one property being measured. Essentially, there are an infinite number of ways to "stop the coin", different angles if you will. I.e. there are many different properties which can be measured, each of which have the same two possible values. If you measure the same property on both coins, you'll get opposite values, no matter how far apart the coins are.
But suppose you randomly choose which properties to measure on the two coins? You could randomly choose the same property to measure for both, thus guaranteeing the results will be opposite. If you measure two different properties, there's a 50% chance your results will be opposite, because there are only two possible values. Therefore, if these properties are indeed determined before you measure them, you should see opposite values more than 50% of the time.
But that doesn't happen! Experiments have confirmed that the same value is obtained exactly 50% of the time! Thus proving that the values of these properties can't have a pre-determined value.
Why does one photon have to "communicate" to the other? Take two photons, one is polarized 90 degrees from the other. You don't know anything else. At some point you observe one, and now know the polarization of the other. Why is their communication taking place?
To make an analogy,say I flip a coin and don't look at it. Then I cut the coin in half between the two sides (without looking at which side is which). I take one side across town to my friend, and keep one. I have no idea which side I have until I look at it, but once I do I also know which side my friend has across town. Where's the mystery here, because I've never been able to understand why there's any spooky action at a distance?
AccountKiller
A and B have agreed that certain measurements of quantum entangled particles will be made a various time intervals as determined by P.
Point C lies between A and B, closer to A than B. C sends quantum entangled particles of definite polarization to A and B. At the agreed-to intervals, A does polarization measuments of particles coming from C; B measures for same polarization at the same interval (accounting for the extra time for the entangle photon to reach B).
Question: does A's collapsing of the state of the entangled photon mean that B will see less entangle photons with that polarization? In other words, will it mean that successful polarization pass-thru's at A's filter has the consequence that B will notice a distinct drop of photons with that polarization passing through its filter? If that were the case, then communication is occuring between A and B in the time it takes for light to travel the shorter distance from C to B.
Change quantum particles to entangle coin tosses. A has the head filter activated, meaning all heads that pass thru A result in no heads at B. With the filter on a A, does B see less heads than he would at other, 'normal', intervals?
If you post it, they will read.
Goodness, no, this is not an accurate analogy. Here's a formally equivalent analogy that should raise your suspicions:
Take two bullets and fire them in opposite directions from identical guns. If you measure the distance of bullet #1 at time t, you will find, amazingly, that bullet #2 has traveled exactly the same distance, but in the opposite direction. Hopefully that doesn't strike you as an amazing result.
What both of our thought experiments say is that if you establish a correlation in a composite system (the two bullets, or your two coins), and you expand the system without doing anything to mess the correlation up, then, amazingly (not!) the correlation will be preserved no matter how large the system gets.
What QE involves is something different: it says you can create a correlation after you have expanded the system, and in less time than it would take any kind of signal or force to cross the distance involved (in fact, instantaneously as far as anyone knows). The correlation can't be used for communication because you can only verify the results of the correlation by communicating the results of measurements on the two parts of the system, which, of course, you can only do at the speed of light.
The whole business arises from the fact that we don't yet understand what happens when the "wavefunction collapses." We know that measuring a quantum system instantly transfers it from the quantum state it was in into a new one (the one consistent with our measurement). So far as we know, this happens instantly over the entire volume that the wavefunction occupies. The problem with this is that it seems dangerously close to violating relativity, because it seems something is being transmitted instantaneously over finite, possibly large, distances.
Relativity is not yet in trouble because we have no good theory of quantum measurement, no knowledge of how a wavefunction collapses, so we can't apply the restrictions of relativity to the internal workings of the collapse. Relativity may never be in trouble, because the collapse may be an epiphenomenon, an event that seems to involve transmission of information but which really doesn't.
Here's an example of an epiphenomenon: point the world's biggest laser at the Moon and look through a telescope at the dot. Aim the laser at one side of the Moon, and then swing it over to the other side quickly. If it takes you 0.25 seconds to move the laser's aim, how long will it take the dot to "travel" across the face of the Moon? 0.25 seconds, clearly, for a "speed" of 22,000 km/s. If you can change the aim of the laser in less than about 0.15 seconds your dot will "travel" across the surface of the Moon faster than the speed of light.
But that's because nothing is really moving. The "motion" of the dot is just a fiction in your mind you create to help describe what you're seeing, because what you are seeing looks superficially similar to what you see when a real object moves. But there's no more real motion here than there is horizontal motion when a group in a stadium does "the wave". In the same way, the "transmission" of information in a QE experiment may turn out to be an epiphenomenon of a higher order, something that "looks" like transmission but really isn't.
The hidden variables theory of quantum mechanics was disproven by a physicists named John Bell. In his method, he began by assuming that these "hidden variables" existed, then, using geometric arguments and the postulates of quantum mechanics, derived a set of inequalities which showed no physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.
It's not intuitive at all, but Bell's argument is sound. Entanglement and action-at-a-distance is real, and not due to the system's state being pre-determined by hidden variables.
/*No comment*/ #No comment
The part that I don't get (so kindly link me to an explanation) is, just because there is no way of measuring where a given particle is, that doesn't mean it's in two places at the same time. It just means we don't know.
What you're hankering after is a "hidden variable model". There is a variable that we can't observe, but it has a definite value. Unfortunately, no simple hidden variable model can explain observations. There are lots of ways of demonstrating this, but all have some complexity.
One of my favourties is due to John Conway and some other people, and it goes like this.
Physics tells us that if we pick any set of three directions at right anfles (eg up, backwards and left) and measure the squared spin of a simple particle (like an electron) in each of them, we get two 0s and a 1 in suitable units. The order of the three measurements doesn't matter.
Now, Conway et al found a set of points on a sphere (ie a set of directions) out of which you can choose lots of triples that are all at right angles. What you can't do is label these points 0 and 1 in such a way that every such triple has two 0s and a 1. So there can't be a hidden variable for the squared spin in each direction, because which one you get depends on which other ones you measure, even though these measurements don't interfere with each other. Using entangled particles and a bit of jiggery pokery you could even do the three measurements at the same time and far apart so there would be no time for information interchange.
A similar, although more subtle effect occurs in EPR. You give each "rocket captain" a choice of directions to measure the polarisation in, and you find a degree of correlation that you could not expect purely from a hidden variable model.
Check Wikipedia for "Bell Test Experiments".
The short answer is yes, of course it has been tested. No one would accept such a theory, or quantum mechanics in general for that matter, without experimental results that agree with its findings.