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."
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?
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
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
Now, the big issue that I haven't figured out is to what extent the noncommunication theorems are valid (the ones that argue that spooky action at a distance does not lead to spooky communication at a distance).
Personally I doubt the noncommunication theorems, but this may be due to my understanding. They seem entirely untestable for the reason that if you take things from a Copenhagen viewpoint, the fact is that the obvserver can't test this without breaking the experiment.
LedgerSMB: Open source Accounting/ERP
That tutorial isn't very good. The science is accurate, but it's presented in a tone that makes it seem like things "obey" physical laws, rather than physical laws accurately describing what things do. That makes it hard for a student to fill in the gaps with our imagination, because laws seem arbitrary rather than reflecting observed consistent behavior.
--
make install -not war
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
Well, fortune should have it that I have stumbled upon this article half (well, in fact, at least three-quarters) drunk, and as I read the article, I was thinking "gee, I'm half (well, in fact, at least three-quarters) drunk, and I _can_ understand this shit", so either, yes, Anonymous Coward, your postulate is correct, or this begginers guide is quite well written (perhaps better titled "Drunk's Guide to Quantum Entanglement").
What I do not understand, however (possibly due to my drunken state) is why it should be necessary that information flows between the two photons that are entangled. Could it not be that the parameters at the time of the split cause them to behave in particular fashions that will always complement each other? E.g. if I send a baseball in one direction spinning topwise, while at the same isntant sending a baseball in another direction spinning bottomwise, their spins will be opposite and continue to do so, without any interaction between the baseballs.
Again, sorry if the answer is blindingly obvious and I am merely oblivious.
"I think it would be a good idea" Gandhi, on Western Civilisation
> if I send a baseball in one direction spinning topwise, while at the same isntant sending a
> baseball in another direction spinning bottomwise, their spins will be opposite and continue
> to do so, without any interaction between the baseballs.
Yes, but baseballs are not subatomic particles. Among other things, looking at which direction they're spinning hardly changes their spin at all. The traditional line of thinking is that the laws of physics are different at the macroscopic level versus the subatomic level, but I suspect the real issue is that they *apply* differently because of the certain scale-related difference. Baseballs have a much larger mass, for instance, so gravity is a real issue for them; whereas, gravity hardly has any impact on an individual electron at all. On the other hand, a baseball has an almost exactly balanced charge, so electromagnetic force has very little impact on it; whereas, for an electron, that's a fairly big deal. These forces, though, are the two we understand best; nuclear force and weak force presumably also apply rather differently to an electron versus a baseball, but I'm not sure we understand all the details.
The whole deal with information "flowing" is anchored in the Heisenberg principle. The models I have read all suppose that if we don't know the direction of a particle's spin, it's spin direction is not merely unknown to us but actually indeterminate, i.e., could go either way. I don't fully understand all the arguments for this, but it is related in some way to the wave/particle duality of light, wherein before you measure the positions of the individual photons two beams of light interact as if they were waves, creating interference patterns, but upon being measured the photons do turn out to have very specific positions.
Cut that out, or I will ship you to Norilsk in a box.
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?