Stony Stevenson writes to tell us that University of Michigan physicists have been able to establish an "entanglement" between two atoms trapped more than a meter apart in different enclosures using light. This shows how two different atoms can have a sort of communication, something Einstein referred to as 'spooky action-at-a-distance'. "By manipulating the photons emitted from each of the two atoms and guiding them to interact along a fibre-optic thread, the researchers were able to detect the resulting photon clicks and entangle the atoms. Professor Monroe explained that the fibre-optic thread was necessary to establish entanglement of the atoms. But the fibre could be severed and the two atoms would remain entangled, even if one were 'carefully taken to Jupiter'."
My arm-chair understand of Entanglement suggests that it should violate causality. Consider the following thought experiment.
We have two pairs of quantum mechanically entangled electrons. We sent a single electron from each pair five light minutes in to space. A long with a small machine that measures that's designed to react when it an electron comes "de-entangled". When it senses this, it immediately the spin of the electron in the other pair.
Here on earth we have a Tsar Bombe linked to one of the electrons from one of the pairs. Five meters away, the other electron is linked to a button. When a person presses the button, it measures one of the electron, thus breaking its entanglement. That instantly breaks the entanglement of the other electron live light minutes away. The machine then breaks the entanglement of the other pair thus instantly triggering the Tsar Bombe destroying the hut and everything in 100 Sq miles.
The problem is that, as I understand it, this would happen ten minutes before I press the button. Whoops! You see, when I de-entangle the first electron the disentanglement on the other side happens five minutes in my past. When the machine disentangles the second electron, the other electron is five minutes in its past. Totalling to ten minutes. Can you see what I'm getting at? I'm assuming this argument isn't new - What mistake have I made here?
My armchair reaction was, "Do they even have equipment precise to the nanosecond that you would need to determine that information had traveled one meter, faster than light speed?"
> a small machine that measures that's designed to react when it an electron comes "de-entangled"
That's your mistake. There's no possible way to detect that an electron has suddenly become "de-entangled".
The only thing the machine can measure is the electron's spin in either of two axis. Now, say you measure it in the left-right axis and its spin comes up as left. What do you know now? You do know that if the corresponding entangled particle has been measured in the left-right axis, it would have come up as right. But this does not tell you whether it has actually been measured. There is no way to tell whether the other party has measured their particle. No information has been transferred. You can't violate causality, even with quantum entanglement.
that's the 'weirdest' part: an interaction which an instantaneous non-local effect *but* that cannot be used to communicate faster than C??
And you'd think with that inherent self-contradiction, physicists would acknowledge that there's something fundamentally fscked with their understanding of the universe.
Yeah, they'll tell you that faster-than-C communication breaks causality and "allows things to happen before they're caused".
So you tell them that no, in an objective reference frame, event A happens befo
Quantum mechanics is hard for people to understand because the effects we observe at the quantum level are fundamentally different from our experience with the macroscopic world. Consider a photon's polarization. If you polarize that photon up-down, then with 100% probability, the photon is polarized up-down. If you attempt to measure the photon's polarization left-right, you will discover that with a 0% probability, it has that polarization. So far so good right? If, however, you measure the polarization of the photon at 45 degrees, you now have a 50% probability that is polarized in that direction and 50% probability that is polarized at -45 degrees.
Now, extend this to entangled photons. You entangle two photons that are polarized up-down. You separate the photons by some distance. If you measure the polarization up-down, with 100% probability, you will discover that the polarization is up-down. No information transfered, nothing learned. Why? You already knew that the probability was 100% of being up down. Now, let's say that you measure the polarization at 45 degrees. With 50% probability, the polarization will be at 45 degrees instead of -45 degrees. Again, no information transfered. All you know now is that both particles have the same polarization. If someone else was holding on to the other entangled photon, they cannot know that the photon has "resolved" itself to a particular polarization value after the first photon has been measured. If someone told them the polarization of the first photon, then they could predict the value of the photon that they currently have, but that first requires someone to tell them (at the speed of light) what the polarization of their photon is. Again, no information transfered.
So what is entanglement useful for then? It could be used as a powerful method of sharing a secret. Suppose I give you a cloud of entangled photons. If I don't know anything about the photons, then their polarizations will be completely random. I could then say that each time I resolve a photon's polarization, I will send you a message that I have read the value of the photon. So, I read the polarization of one photon causing its field distribution to collapse to the value I have measured. I then send you a message saying I have read the first value. At this point, you read the value of the corresponding entangled photon. You know that we have the same values, and so we have our first bit of the secret key. If we repeat this process for each entangled photon, we would end up with a random secret key that we both share that has never been sent across the transmission medium.
People think Quantum Physics is spooky, but I don't get it -- I really don't. Can anyone please explain to me (or point me at a link) that will tell me how this is any different than having two billiard balls, one is red and one is blue. Without looking at them, you put them both into boxes and ship them off to opposite sides of the globe. Now, one box is opened, and the ball is blue. So you know when the other box is opened, the ball they got will be red.
That's not spooky, bizarre, or even strange. It's not counterintuitive. So how is it different than quantum entanglement? I do not know, but I would like to.
how this is any different than having two billiard balls, one is red and one is blue. Without looking at them, you put them both into boxes and ship them off to opposite sides of the globe. Now, one box is opened, and the ball is blue. So you know when the other box is opened, the ball they got will be red.
If I may tweak your analogy: imagine two billiard balls, shipped off to opposite sides of the globe. you can measure either their color (red-blue) or their pattern (solid-stripe). If you measure the color of one, and it comes up blue; if the other ball's color if measured, it will come up red (and vice-versa). If you measure the pattern of one, and it comes up solid; then if the other one's pattern is measured, it will come up stripy (and vice-versa). But measuring one aspect destroys any correlation in the other: if you measure the color of one of them, and it comes up red; and the other guys measure the *pattern* of the other, and it comes up solid, and then you measure the pattern of the first, it will not necessarily be striped: it might be solid or striped, with 50-50 probability. The measuring of the color destroyed the pattern information in the first ball.
It's more like you have a bag of blue and red billiard balls, you pull out two randomly without looking at either ball's color, place each in a box and ship them halfway across the world. The two boxes are opened up and observed, and each time one box contains a red ball the other box will always contain a blue ball.
What's even weirder is that in the quantum mechanical world, it's not that your picking two particles that are either in one state or the other with equal probability and it turns out that you always pick up opposite states. Rather it's that you have two particles that are both in both possible states at the same time. When you measure the particle it collapses into one of the two known states, but up until then it is in a superposition of both. And when you do that to one of the two entangled particles, the other particle will also collapse into one of the two states at the exact same time and you will know exactly which one the other particle will be in based on what state your own particle is in.
How entanglement works though is that you have two billiard balls that are not red or blue but both simultaneously. That is unless you measure it.
So you take your boxes too each side of the world and look in one that sets that ball to say red, the other turns blue instantly, and when you say instantly you really mean it, it is faster than light, faster than what should be the infinite speed, it is instant.
That is weird.
However, your example is accurate in describing why quantum entanglement doesn't break causality. You see you can't predict what colour the ball is going to be so you can't go to one end with eight boxes and say 'right ill make this byte the number 172.' then set your balls to 10101100 leading to the other boxes instanteously being set as well.
All you can do is measure the 8 boxes find out which are red and blue at either end confirm that they are entangled, thats it. No information transfer no causality breaking.
This is also why the initial posts idea falls down. You might know which particle is entangled with which but you can't measure its status without breaking the entanglement. So you could say tell the person 'measure it in 10 minutes and see if its broken down.' and yes you confirm that the entanglement breaks down instantaneously but you rather defeat the point by already giving the information. Either that or the person can guess when it breaks down but measuring it causes it to break down and bam you defeat the point again.
Entanglement has some kind of instant effect but it can not be used to send information and thus causality is preserved.
The "faster than the speed of light" thing surprises me. Not because of how c functions in relationship to matter and energy, but because the physicists, whose discipline has now had a full 100 years to digest these complexities, and personally, eight or more years of post-secondary education hammering home the need to state things carefully, fail to state that the fact of the violation of the speed of light for an effect can not itself be established at faster than the speed of light.
Two physicists in a similar reference frame measure two entangled particles in different light cones (any interaction would therefore need to travel faster than ligth). The entanglement effect says that if one measures red, the other measures blue. How do they confirm this? The information about their measurements must travel *at the speed of light* until information from the distinct measurements meets up. At *this point in time* they know if the entaglement effect conformed with theory or did not conform with theory. They can't posssibly determine this conclusion faster than the speed of light between the positions where the measurements were taken.
It interests me that the effect can travel faster than light, but the conclusion about the effect can not, yet I've never seen a physicist discuss this. The discussion always goes entanglement, faster than light, spooky, bada bing. It's possible that the entanglement effect doesn't resolve itself until information about the two experimental measurements (which converges in obedience with the speed of light) actually meets up. Perhaps the disentanglement takes place only *after* the results of the two experiments meets up. That would involve the experiment (and experimenters) having become entangled in the experiment. Weird? In the realm of the very tiny, that's never stopped mother nature before.
On a related point, I've never seen a physicist comment on whether it is possible to take two particles of unknown histories and prove they are not entangled. I suspect this can only be done by taking measurements which shuffle the quantum deck. Entangled particles are always introduced as an exceptional state of matter, produced painstakingly only in laboratory equipment for the purpose of conducting this experiment.
Is it not possible that most of the particles in the universe are entangled with most of the other particles of the universe? If there is no physical demonstration that two particles *are not* entangled, on what basis could you answer "no"? As a simpler case, is it possible to construct three particles A, AB, and B where AB is entangled with both A and B?
It just bugs me that the typical account of this effect rarely gets past the word spooky before exposition ceases, as if the very phrase "faster than light" causes some kind of cerebral blood flow trauma in any person who has devoted eight years of higher education in grappling with the consequences of E=mc^2.
It interests me that the effect can travel faster than light, but the conclusion about the effect can not, yet I've never seen a physicist discuss this. The discussion always goes entanglement, faster than light, spooky, bada bing.
Well, look harder. This effect is at the heart of a lot of interpretations of quantum mechanics.
In my preferred interpretation, the Many Minds Interpretation, there's nothing going at the speed of light. The fact that you'll find that the other one has measured the opposite of wha
If we put entangled photon pairs down different fiber lines, and include a birefringent component to split the beam into polarized components... Each photon ought to essentially split itself. We wouldn't know which path a given photon took until we measured it, but we would know what the properties were supposed to be based on the waveform collapse. In this case, the observation would be the exact same as it the photon actually had a discrete property which caused it to choose one path as it hit the crysta
People think Quantum Physics is spooky, but I don't get it -- I really don't. Can anyone please explain to me (or point me at a link) that will tell me how this is any different than having two billiard balls, one is red and one is blue. Without looking at them, you put them both into boxes and ship them off to opposite sides of the globe. Now, one box is opened, and the ball is blue. So you know when the other box is opened, the ball they got will be red.
It's ``spooky'' to some since the ball decides -randomly- at the point of observation which color to display. The color is not known or set (or defined), in any way, before that observation. (so the `other' ball has no way of knowing what that -random- choice was, but somehow still manages to choose the proper color). [ie: in your example, the balls already have their color before they're separated; in quantum mechanics, they randomly choose the color upon observation].
First thing that pops to mind is ``how do they -know- that it's random?''; maybe the balls had their colors pre-set all along (like in your example). Well, there are various logical puzzles you can play where if things are -random- you'd get one result, and if things are pre-set, you'd get another result---and it does appear like the choice is -random- and not pre-set.
Google for ``Free Will Theorem''; it's a fun read:-)
There's a lot of stuff about "no hidden variables" (ie: it's not that ``there's something [a deeper knowledge of things] we don't understand yet'' that's hidden from us... it's that the choice truly is random (there are no `hidden variables'); and somehow the other particle knows about that random choice at faster than speed of light). You cannot use this to send information though (since the choice is random---you only know what the other particle's choice is... but you can't force it to choose something in particular).
To resolve the confusion (and how I like to view things), it helps to picture the two particles as really being different sides of the -same- particle, that, from our perspective, just exists [we can observe] at two different locations. Picture the world from the particle's perspective---if you're moving at the speed of light, time stands still for you, therefore, from your perspective, you can traverse the universe at infinite speed---from your perspective, you can instantly react to events anywhere in the universe (from the outsider's perspective, they just see you as moving at the speed of light...). I guess it's one of those things that are hard to explain, but easy to visualize.
[...] how this is any different than having two billiard balls, one is red and one is blue.
Exactly! That's the question everybody should ask when they hear about "spooky action", but for some reason, I have rarely seen it asked.
The answer is: there's a difference that can be seen in the thought experiment proposed by Einstein and some other people, which is explained in this Wikipedia article: EPR paradox [wikipedia.org].
However, when I first read this article, I didn't understand any of it, because it assumes lots o
The problem is that, as I understand it, this would happen ten minutes before I press the button. Whoops! You see, when I de-entangle the first electron the disentanglement on the other side happens five minutes in my past. When the machine disentangles the second electron, the other electron is five minutes in its past. Totalling to ten minutes. Can you see what I'm getting at? I'm assuming this argument isn't new - What mistake have I made here?
I'm not sure, but I think you just invented time travel!
Ok, your comment is badly mangled, but I think I get the gist of it and I'll try to explain.
The problem is that we can't currently control what state the two disentangle into, we can merely guarantee that they share a state in common. Special relativity doesn't explicitly deny something happening faster than the speed of light, just data being transmitted faster than that limit. Because we can't determine anything from the two entangled electrons other than they share a common state, we can't actually get any data out of the system, thus there is no discrepancy. There's also the fact that determining if they are entangled is itself a measurement and thus the act of checking for entanglement breaks the entanglement. We can only verify they are entangled by checking after the fact that they both have the same state when we measure them, otherwise there is no way to know if they are entangled or not.
1. Place an entangled photon generator exactly half way between earth and mars. 2. Do not aim the photon outputs (beams) at earth and mars, but aim the beam at a 90 degree angle to earth and mars. 3. Immediately measure the polarization of one of the photons so that then, both photon polarizations are known.
Ok. Since now you measured the photon polarization, the photons cease to be entangled. Therefore you just have generated a photon of random spin (well, actually one randomly selected of two spins, where th
My arm-chair understand of Entanglement suggests that it should violate causality.
Quantum entanglement [wikipedia.org] can't violate causality. The reason for this is that entanglement can't transmit information alone, it needs to be performed in conjunction with a classical, non-entangled information channel. This is explained in the No-Communication Theorem [wikipedia.org]. It boils down to the fact that you can't tell the difference between random fluctuations in the particles and the signal you are trying to transmit, in order to separate the two you need to transmit some additional information by classical means. Take a look at this discussion on quantum teleportation [wikipedia.org].
The end result is that information transmitted through entanglement travels at the fastest speed allowed by conventional means. Until we create a warp drive that limit is the speed of light.
", in order to separate the two you need to transmit some additional information by classical means" No, you do not need to transport it seperatly, per se. You only need to have the receive understand how to interpret the spins. This can even be done even if the spin direction is completely random.
I've always wondered if we would one day be able to use entangled photons to peer beyond the event of a black hole. Keep one particle in an observable state and send one through the black hole. Something is bound to happen and it might give us some insight into what exists beyond the event horizon. This experiment sounds like a step toward that possibility.
The problem with that idea is that, as I understand it, you'd have to wait the age of the universe before you got a result. As you approach the event horizon of a black hole, you experience ever-increasing relativistic time dilation; time passes normally for you, but the rest of the universe appears to be speeding up. To an outside observer, you're playing out a modern example of Xeno's paradox; the closer you get to the event horizon, the less distance you are covering. So when you drop your entangled photo
I've said this a few times now, but I'll repeat it: You Can't Transmit Information Across A Quantum Entanglement. (Usual caveats: to the best if our knowledge at the present time).
There is no response in the second atom. If two particles are entangled, no measurement or manipulation of one can change the measurement outcome statistics of the other. You just know that if you measure them a certain way, the results will be correlated. It can seem like a subtle difference...
Greg Egan has a good version: paraphrased, you have a coin on Earth, and a coin on Mars. They're entangled. You flip them. You get random results.
Now you turn on a widget on Earth. You continue to flip them. You continue to get random results, at both ends. But now they're the same random results.
The key fact is: you don't know that this is happening, until you can get a communication from Earth to Mars or vice versa describing what the results are. Once you do, you can compare the results, and say: hey, during this time period both coins were producing identical results! Maybe the widget was turned on! Or it could be just chance, of course. The coins are random, after all.
So while it's interesting, it's not useful as a communications medium.
(It is, however, great for a means of generating encryption keys. Earth wants to send a message to Mars? Earth turns on the widget, waits a bit, turns it off again. It then sends a message saying, the sequence from X to Y is the encryption key, here's a message encrypted with it. During that period, the coin on Mars has produced the same random sequence of bits as the one on Earth --- so you get the same key at both ends, without having to transmit it! But you still haven't transferred any actual information until you transmit the encrypted message, via conventional means.)
An ansible [wikipedia.org] is a device described in science fiction for superluminal communication. It's usually portrayed as a pair (or more) of devices closely connected, as if separated from a common origin.
I'm looking forward to a day when ansible devices are as common as symmetric key crypto, which will likely be the only way to secure their communications, other than the "conservation of info" already built in to quantum entanglement.
That's interesting, but mostly irrelevent. You can't transmit information across an entanglement. Faster-than-light communication is, to the best of our knowledge at the present time, still as impossible as it ever was.
I believe the ansible was a device that used entanglement to provide faster than light communication without breaking the laws of physics. It was later proven (60's?) that under existing quantum theory entanglement cannot transmit information, so the ansible fell out of favor with some authors, particularly those trying stay true to science.
That's kinda what it sounds like to me anyway... but I'm not all that knowledgeable in the area of quantum physics... I barely understand common physics. But at least I read the article... and it sounds like they have created the atomic equivalent of two cans and a string without the string.
To receive a signal you have to measure something. That can be ones and zeros streaming from a wire or light scattering off a distant smoke signal. To make a measurement you have to collapse the wave function. Once the wave function is to more, you have no chance of sending anything else. So maybe we could send a single bit with a single entangled state. Perhaps the trick would be to get a whole lot of them. The fact that the universe is self consistent lends credibility to causality.
We should probably not use words like "communication" to describe entanglement, because it only confuses people. Connection and correlation do not equal classical communication.
Before complaining, please know what you are talking about... A quick search on wikipedia would tell you: Einstein received his Nobel Prize for works on Quantum Theory!
http://en.wikipedia.org/wiki/Albert_Einstein [wikipedia.org]: Einstein received the 1921 Nobel Prize in Physics "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."
http://en.wikipedia.org/wiki/Photoelectric_effect [wikipedia.org]:The photoelectric effect is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or visible light. (...) The photoelectric effect helped further wave-particle duality, whereby physical systems (such as photons, in this case) display both wave-like and particle-like properties, a concept that was used in quantum mechanics. Albert Einstein mathematically explained the photoelectric effect and extended the work on quanta that Max Planck developed.
No. You can't transfer information across an entanglement. Faster than light communication is as impossible as it ever was; and causality has not yet been knowingly violated.
The tugging of the rigid wire isn't an instantaneous transfer of motion. Each atom must tug on the one next to it, etc. At no time does this transfer of motion exceed the speed of light.
BTW, I've heard this question posed more often as a pair of scissors with the blades as long as the Solar System. Close the short end and the tips should be moving faster than light. Except they don't, because as you get further out to the tips it requires more and more energy to move them faster. They'll get close, but never exceed c.
As *both* a geek and a sports fan, it's because The #5 (out of 110 Division 1-A teams) ranked University of Michigan football team lost to Appalachian State last Saturday, 34-32. UM is the first ranked team (e.g., Top 20) in the 100+ year history of college football to lose to a Division I-AA team.
For a more geek-friendly comparison, UM's loss was as shocking as if the MPAA and RIAA announced that all the movies and music they "owned" were going to be released into the public domain next Monday.
Because researchers at Appalachian State subsequently proved that the atoms would remain entangled even if carefully taken two points beyond Jupiter, perhaps by blocking a field goal attempt shortly after the asteroid belt.
Don't usually reply to AC's, but no, the speed limit arises not because of something we noticed in "particle accelerator experiments" it is because of the geometry of space time, which is different than the euclidean geometry that we expereince at low speeds and energies. If you could send something out faster than the speed of light, then you can truly send things into the past and there by violate causality. If you want to know why this is, study Minkowskian geometry, and particularly its Lorentian coordin
Entanglement and causality? (Score:3, Interesting)
My arm-chair understand of Entanglement suggests that it should violate causality. Consider the following thought experiment.
We have two pairs of quantum mechanically entangled electrons. We sent a single electron from each pair five light minutes in to space. A long with a small machine that measures that's designed to react when it an electron comes "de-entangled". When it senses this, it immediately the spin of the electron in the other pair.
Here on earth we have a Tsar Bombe linked to one of the electrons from one of the pairs. Five meters away, the other electron is linked to a button. When a person presses the button, it measures one of the electron, thus breaking its entanglement. That instantly breaks the entanglement of the other electron live light minutes away. The machine then breaks the entanglement of the other pair thus instantly triggering the Tsar Bombe destroying the hut and everything in 100 Sq miles.
The problem is that, as I understand it, this would happen ten minutes before I press the button. Whoops! You see, when I de-entangle the first electron the disentanglement on the other side happens five minutes in my past. When the machine disentangles the second electron, the other electron is five minutes in its past. Totalling to ten minutes. Can you see what I'm getting at? I'm assuming this argument isn't new - What mistake have I made here?
Simon.
Re: (Score:3, Insightful)
Re:Entanglement and causality? (Score:5, Funny)
(Sorry, couldn't resist...)
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Re:Entanglement and causality? (Score:5, Informative)
The only thing the machine can measure is the electron's spin in either of two axis. Now, say you measure it in the left-right axis and its spin comes up as left. What do you know now? You do know that if the corresponding entangled particle has been measured in the left-right axis, it would have come up as right. But this does not tell you whether it has actually been measured. There is no way to tell whether the other party has measured their particle. No information has been transferred. You can't violate causality, even with quantum entanglement.
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Re:Entanglement and causality? (Score:5, Interesting)
And IMHO, that's the 'weirdest' part: an interaction which an instantaneous non-local effect *but* that cannot be used to communicate faster than C??
Strange, very strange.
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Re: (Score:3, Interesting)
And you'd think with that inherent self-contradiction, physicists would acknowledge that there's something fundamentally fscked with their understanding of the universe.
Yeah, they'll tell you that faster-than-C communication breaks causality and "allows things to happen before they're caused".
So you tell them that no, in an objective reference frame, event A happens befo
Re:Entanglement and causality? (Score:5, Insightful)
"Half of what we know about physics is wrong. The trouble is, we don't know which half." -Gary Skouson (AFAIK)
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Re:Entanglement and causality? (Score:4, Informative)
Now, extend this to entangled photons. You entangle two photons that are polarized up-down. You separate the photons by some distance. If you measure the polarization up-down, with 100% probability, you will discover that the polarization is up-down. No information transfered, nothing learned. Why? You already knew that the probability was 100% of being up down. Now, let's say that you measure the polarization at 45 degrees. With 50% probability, the polarization will be at 45 degrees instead of -45 degrees. Again, no information transfered. All you know now is that both particles have the same polarization. If someone else was holding on to the other entangled photon, they cannot know that the photon has "resolved" itself to a particular polarization value after the first photon has been measured. If someone told them the polarization of the first photon, then they could predict the value of the photon that they currently have, but that first requires someone to tell them (at the speed of light) what the polarization of their photon is. Again, no information transfered.
So what is entanglement useful for then? It could be used as a powerful method of sharing a secret. Suppose I give you a cloud of entangled photons. If I don't know anything about the photons, then their polarizations will be completely random. I could then say that each time I resolve a photon's polarization, I will send you a message that I have read the value of the photon. So, I read the polarization of one photon causing its field distribution to collapse to the value I have measured. I then send you a message saying I have read the first value. At this point, you read the value of the corresponding entangled photon. You know that we have the same values, and so we have our first bit of the secret key. If we repeat this process for each entangled photon, we would end up with a random secret key that we both share that has never been sent across the transmission medium.
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Re:Entanglement and causality? (Score:5, Interesting)
That's not spooky, bizarre, or even strange. It's not counterintuitive. So how is it different than quantum entanglement? I do not know, but I would like to.
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Re:Entanglement and causality? (Score:5, Informative)
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Re:Entanglement and causality? (Score:5, Informative)
What's even weirder is that in the quantum mechanical world, it's not that your picking two particles that are either in one state or the other with equal probability and it turns out that you always pick up opposite states. Rather it's that you have two particles that are both in both possible states at the same time. When you measure the particle it collapses into one of the two known states, but up until then it is in a superposition of both. And when you do that to one of the two entangled particles, the other particle will also collapse into one of the two states at the exact same time and you will know exactly which one the other particle will be in based on what state your own particle is in.
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Re:Entanglement and causality? (Score:5, Insightful)
So you take your boxes too each side of the world and look in one that sets that ball to say red, the other turns blue instantly, and when you say instantly you really mean it, it is faster than light, faster than what should be the infinite speed, it is instant.
That is weird.
However, your example is accurate in describing why quantum entanglement doesn't break causality. You see you can't predict what colour the ball is going to be so you can't go to one end with eight boxes and say 'right ill make this byte the number 172.' then set your balls to 10101100 leading to the other boxes instanteously being set as well.
All you can do is measure the 8 boxes find out which are red and blue at either end confirm that they are entangled, thats it. No information transfer no causality breaking.
This is also why the initial posts idea falls down. You might know which particle is entangled with which but you can't measure its status without breaking the entanglement. So you could say tell the person 'measure it in 10 minutes and see if its broken down.' and yes you confirm that the entanglement breaks down instantaneously but you rather defeat the point by already giving the information. Either that or the person can guess when it breaks down but measuring it causes it to break down and bam you defeat the point again.
Entanglement has some kind of instant effect but it can not be used to send information and thus causality is preserved.
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blood flow trauma (Score:4, Interesting)
The "faster than the speed of light" thing surprises me. Not because of how c functions in relationship to matter and energy, but because the physicists, whose discipline has now had a full 100 years to digest these complexities, and personally, eight or more years of post-secondary education hammering home the need to state things carefully, fail to state that the fact of the violation of the speed of light for an effect can not itself be established at faster than the speed of light.
Two physicists in a similar reference frame measure two entangled particles in different light cones (any interaction would therefore need to travel faster than ligth). The entanglement effect says that if one measures red, the other measures blue. How do they confirm this? The information about their measurements must travel *at the speed of light* until information from the distinct measurements meets up. At *this point in time* they know if the entaglement effect conformed with theory or did not conform with theory. They can't posssibly determine this conclusion faster than the speed of light between the positions where the measurements were taken.
It interests me that the effect can travel faster than light, but the conclusion about the effect can not, yet I've never seen a physicist discuss this. The discussion always goes entanglement, faster than light, spooky, bada bing. It's possible that the entanglement effect doesn't resolve itself until information about the two experimental measurements (which converges in obedience with the speed of light) actually meets up. Perhaps the disentanglement takes place only *after* the results of the two experiments meets up. That would involve the experiment (and experimenters) having become entangled in the experiment. Weird? In the realm of the very tiny, that's never stopped mother nature before.
On a related point, I've never seen a physicist comment on whether it is possible to take two particles of unknown histories and prove they are not entangled. I suspect this can only be done by taking measurements which shuffle the quantum deck. Entangled particles are always introduced as an exceptional state of matter, produced painstakingly only in laboratory equipment for the purpose of conducting this experiment.
Is it not possible that most of the particles in the universe are entangled with most of the other particles of the universe? If there is no physical demonstration that two particles *are not* entangled, on what basis could you answer "no"? As a simpler case, is it possible to construct three particles A, AB, and B where AB is entangled with both A and B?
It just bugs me that the typical account of this effect rarely gets past the word spooky before exposition ceases, as if the very phrase "faster than light" causes some kind of cerebral blood flow trauma in any person who has devoted eight years of higher education in grappling with the consequences of E=mc^2.
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Well, look harder. This effect is at the heart of a lot of interpretations of quantum mechanics.
In my preferred interpretation, the Many Minds Interpretation, there's nothing going at the speed of light. The fact that you'll find that the other one has measured the opposite of wha
In that case (Score:3, Interesting)
In this case, the observation would be the exact same as it the photon actually had a discrete property which caused it to choose one path as it hit the crysta
Re:Entanglement and causality? (Score:4, Funny)
(Yeah, I know that doesn't really happen, but some bad explanations of entanglement could lead you to think that it could.)
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Re:Entanglement and causality? (Score:4, Insightful)
It's ``spooky'' to some since the ball decides -randomly- at the point of observation which color to display. The color is not known or set (or defined), in any way, before that observation. (so the `other' ball has no way of knowing what that -random- choice was, but somehow still manages to choose the proper color). [ie: in your example, the balls already have their color before they're separated; in quantum mechanics, they randomly choose the color upon observation].
First thing that pops to mind is ``how do they -know- that it's random?''; maybe the balls had their colors pre-set all along (like in your example). Well, there are various logical puzzles you can play where if things are -random- you'd get one result, and if things are pre-set, you'd get another result---and it does appear like the choice is -random- and not pre-set.
Google for ``Free Will Theorem''; it's a fun read
There's a lot of stuff about "no hidden variables" (ie: it's not that ``there's something [a deeper knowledge of things] we don't understand yet'' that's hidden from us... it's that the choice truly is random (there are no `hidden variables'); and somehow the other particle knows about that random choice at faster than speed of light). You cannot use this to send information though (since the choice is random---you only know what the other particle's choice is... but you can't force it to choose something in particular).
To resolve the confusion (and how I like to view things), it helps to picture the two particles as really being different sides of the -same- particle, that, from our perspective, just exists [we can observe] at two different locations. Picture the world from the particle's perspective---if you're moving at the speed of light, time stands still for you, therefore, from your perspective, you can traverse the universe at infinite speed---from your perspective, you can instantly react to events anywhere in the universe (from the outsider's perspective, they just see you as moving at the speed of light...). I guess it's one of those things that are hard to explain, but easy to visualize.
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Exactly! That's the question everybody should ask when they hear about "spooky action", but for some reason, I have rarely seen it asked.
The answer is: there's a difference that can be seen in the thought experiment proposed by Einstein and some other people, which is explained in this Wikipedia article: EPR paradox [wikipedia.org].
However, when I first read this article, I didn't understand any of it, because it assumes lots o
Re:Entanglement and causality? (Score:5, Funny)
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Re:Entanglement and causality? (Score:5, Informative)
Ok, your comment is badly mangled, but I think I get the gist of it and I'll try to explain.
The problem is that we can't currently control what state the two disentangle into, we can merely guarantee that they share a state in common. Special relativity doesn't explicitly deny something happening faster than the speed of light, just data being transmitted faster than that limit. Because we can't determine anything from the two entangled electrons other than they share a common state, we can't actually get any data out of the system, thus there is no discrepancy. There's also the fact that determining if they are entangled is itself a measurement and thus the act of checking for entanglement breaks the entanglement. We can only verify they are entangled by checking after the fact that they both have the same state when we measure them, otherwise there is no way to know if they are entangled or not.
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Ok. Since now you measured the photon polarization, the photons cease to be entangled. Therefore you just have generated a photon of random spin (well, actually one randomly selected of two spins, where th
Re:Entanglement and causality? (Score:5, Interesting)
The end result is that information transmitted through entanglement travels at the fastest speed allowed by conventional means. Until we create a warp drive that limit is the speed of light.
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not really (Score:3, Interesting)
No, you do not need to transport it seperatly, per se. You only need to have the receive understand how to interpret the spins. This can even be done even if the spin direction is completely random.
Entanglement and black holes... (Score:5, Interesting)
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So when you drop your entangled photo
Re:Entanglement and black holes... (Score:5, Informative)
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Re:Entanglement and black holes... (Score:4, Informative)
Greg Egan has a good version: paraphrased, you have a coin on Earth, and a coin on Mars. They're entangled. You flip them. You get random results.
Now you turn on a widget on Earth. You continue to flip them. You continue to get random results, at both ends. But now they're the same random results.
The key fact is: you don't know that this is happening, until you can get a communication from Earth to Mars or vice versa describing what the results are. Once you do, you can compare the results, and say: hey, during this time period both coins were producing identical results! Maybe the widget was turned on! Or it could be just chance, of course. The coins are random, after all.
So while it's interesting, it's not useful as a communications medium.
(It is, however, great for a means of generating encryption keys. Earth wants to send a message to Mars? Earth turns on the widget, waits a bit, turns it off again. It then sends a message saying, the sequence from X to Y is the encryption key, here's a message encrypted with it. During that period, the coin on Mars has produced the same random sequence of bits as the one on Earth --- so you get the same key at both ends, without having to transmit it! But you still haven't transferred any actual information until you transmit the encrypted message, via conventional means.)
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Finally (Score:4, Funny)
Re:Finally (Score:5, Funny)
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And then the moment she measures you up, it's over!
Ansible (Score:5, Informative)
I'm looking forward to a day when ansible devices are as common as symmetric key crypto, which will likely be the only way to secure their communications, other than the "conservation of info" already built in to quantum entanglement.
Re:Ansible (Score:5, Informative)
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Quantum Bluetooth? (Score:3, Funny)
spooky action from a distance (Score:4, Funny)
The big problem with entanglement. (Score:3, Interesting)
"a sort of communication" (Score:5, Insightful)
Re:FedEx, UPS, etc. are gonna make a fortune (Score:5, Funny)
Probably not.
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Re:FedEx, UPS, etc. are gonna make a fortune (Score:4, Informative)
Before complaining, please know what you are talking about... A quick search on wikipedia would tell you: Einstein received his Nobel Prize for works on Quantum Theory!
http://en.wikipedia.org/wiki/Albert_Einstein [wikipedia.org]: Einstein received the 1921 Nobel Prize in Physics "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."
http://en.wikipedia.org/wiki/Photoelectric_effect [wikipedia.org]: The photoelectric effect is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or visible light. (...) The photoelectric effect helped further wave-particle duality, whereby physical systems (such as photons, in this case) display both wave-like and particle-like properties, a concept that was used in quantum mechanics. Albert Einstein mathematically explained the photoelectric effect and extended the work on quanta that Max Planck developed.
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Re:FedEx, UPS, etc. are gonna make a fortune (Score:5, Funny)
I believe the article said " carefully taken to Jupiter" so that rules out UPS, FedEx, and especially the post office...
-=Geoskd
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I found a better company (Score:5, Funny)
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Re:I found a better company (Score:5, Funny)
I believe that's grounds for a permanent ban from Slashdot...
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Re:FedEx, UPS, etc. are gonna make a fortune (Score:5, Funny)
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Re:Someone explain this to me... (Score:5, Informative)
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Re:Someone explain this to me... (Score:4, Interesting)
BTW, I've heard this question posed more often as a pair of scissors with the blades as long as the Solar System. Close the short end and the tips should be moving faster than light. Except they don't, because as you get further out to the tips it requires more and more energy to move them faster. They'll get close, but never exceed c.
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For a more geek-friendly comparison, UM's loss was as shocking as if the MPAA and RIAA announced that all the movies and music they "owned" were going to be released into the public domain next Monday.
Cheers.
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If you could send something out faster than the speed of light, then you can truly send things into the past and there by violate causality. If you want to know why this is, study Minkowskian geometry, and particularly its Lorentian coordin