Slashdot Mirror
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!
Did you mean: fleabiteus ?
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
I call it saturday night.....
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?
If you do, they will find your wave function collapsed. In seven days.
I doubt that we will ever figure out - and I suspect that even if we did figure out we couldn't do much about it
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.
And the top article on digg links to newsforge, a stablemate of slashdot. I didn't know of any other site which puts in links to newsforge. It's a pretty obscure site outside the slashdot community.
I only had a quick look, and correct me if I am wrong, but digg seems to want people to register to view discussions. From the POV of making money from a site this is a better way to go. So would OSDN care if people moved over to digg? Dunno, have to think about this.
http://michaelsmith.id.au
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.
\
>>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?
Dupes and the terrible editing give slashdot its charm. Come on. Also great trolling here on slashdot.... superb. Slashdot has an entire culture around it now. The crap on the front page doesn't even matter anymore. Cmdrtaco knows it. Thats why he doesn't give a shit about dupes and everything else. You could post a story that is nothing but random letters and still generate 100+ comments.
The best education consists in immunizing people against systematic attempts at education. - Paul Feyerabend
Yep and even now when no one is on K5, it still loads slower than here.
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
Okay, I've got it now. Or at least, I've got enough of it to realize that I need to go take some more physics classes to have a decent chance of getting it . . .
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.
I'm obviously don't have my PhD so bear with me.
The part that I *do* get is:
You cannot measure a system without altering it. That is, if you stick a multimeter in a computer you may crash it. The instrument of measurement is too course to see the state of a system without altering it. Shed light on electrons and they'll 'fly away'. In quantum physics, we're dealing with such elementary particles that absolutely every means of actually measuring the system will interfere with it.
It is statistically correct to say 'the particle is 50% here and 50% there', if chances are 50-50 for it being in one place or another.
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.
Two rockets fly in opposite directions at the spead of light for a year. One of them is known to carry a closed envelope saying "white", the other one carries an envelope saying "black". The envelopes are in a time-locked safe. We don't know which rocket carries which envelope. Statistically we might as well say both rockets carry an envelope saying 'grey'. After a year of travel, the captain of the first rocket opens his envelope and reads a single word. Instantly he knows what the contents of the envelope of the other rocket are. Yikes! Spooky action at a distance?
Someone hit me with a clue-bat, *please*?
Visit http://ringbreak.dnd.utwente.nl/~mrjb/growingbettersoftware to download your free copy of the book
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.
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.
until you measure one. Firstly, for quantum entanglement to be there, both photons must have come from a single event, like an electron-positron collision. They come out of that event with no particular polarization, but rather a quantum superposition of polarizations. This is evidenced by the fact that they have a 50-50 chance of passing through any polarization filter, regardless of its orientation.
However, once one of them *has* passed through a polarization filter, the other one must have a polarization of 90 degrees of from the other. So if two people distant from one another have two filters set 90 degrees from each other, and if one photon of an entangled pair passes through one filter, the other must pass through the other filter.
I too am not a physicist, but still enjoy reading about topics like this. I want to make a comment however, by way of analogy, on the nature of Heisenberg Uncertainty and DeBroglie Wavelength.
When I was in highschool, our biology teacher asked us to look throgh a microscope at a drop of water under a slide. In the view I could see a fuzzy/blurry-looking speck of dust, which was apparently jiggling. The teacher explained that the jiggling of the speck was due to it being battered by water molecules in a phenomenon known as "Brownian Motion".
I've never heard Heisenberg Uncertainty or DeBroglie Wavelength described in that manner before, but I want to ask if these phenomena could likewise be considered a form of Brownian Motion.
Ie. the intrinsic DeBroglie wavelength of a small particle could be due to it being buffeted by some minute forces occurring in the space surrounding it (aka. the Quantum Vacuum), and likewise the associated Heisenberg uncertainty would be the fuzziness/blurriness from that jiggling.
In school, I never understood why an electron's orbital was called a probability cloud, because it was just so counterintuitive. But if you use that jiggling analogy, one could visualize a tetherball attached to a post, where the ball is the electron, the post is the nucleus, and the tether is the charge attraction between them. Once again, the buffeting from the surrounding Quantum Vacuum would cause the tetherball to bop around under the constraints of the tether. If you used time-lapse photography, that tetherball might show up as a Probability Cloud, rather than in a single position.
I think good science should always strive to make the explanations as intuitive as possible, rather than hiding behind cryptic phrases such as "spooky action" and "counter-intuitive quantum behavior". When science can't explain things intuitively, then in my opinion science is failing to do its job, and more efforts need to be made to come up with better analogies. Good analogies make the difference between enlightenment and ignorance.
Comments, anyone?
For a few years I've wondered if you could use entanglement to communicate back in time.
Anyone familiar with "realistic" time travel predictions knows if you have a worm hole and spin move one end around really fast then each end will be in a different 'time frame'. Entering one end will bring you out in a different time.
By the same thinking I wonder could you take a pair of entangled particles and move one around really fast......then spinning one should cause the other to respond, but in a different time.
Anyone know why this wouldn't work?