Quantum Communication

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Quantum Communication

Postby Sir_Elderberry » Sat Dec 08, 2007 1:33 am UTC

Ok, this has been mulling around my mind ever since I understood the following (I am stating the below to clarify my premises; any of which may be wrong):

-It is possible to entangle particles
-When entangled, particles are in a superposition where they have no definite spin. If either is measured, both collapse into identical states.
-This happens instantaneously regardless of differences in space.

However, I was also told:
-This does not violate relativity because information cannot be transferred.

Now, here's the thought experiment:

I entangle some photons and give you half of them, so that each of mine corresponds to one of yours. They are sorted into a 2x* lattice, in which * is pretty unimportant for this discussion. When I want to send you a message, I collapse the photons I wish to, and your photons also collapse in the corresponding squares. We predertimne that those in the first row are dots, and those in the bottom row are dashes. So I tap out top, top, top, bottom, bottom, bottom, top, top, top, or, ... --- ..., or SOS. You are, at this point, in Alpha Centauri and instantly realize I am in danger.

Are my premises wrong or is there a more fundamental flaw with my device that goes deeper than engineering difficulties?
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Re: Quantum Communication

Postby Aluminus » Sat Dec 08, 2007 1:42 am UTC

I thought you don't get to choose which state the entangled particles collapse to, that it was just random.
If you could communicate instantly across 4 lightyears of space, you would violate the whole "information can't nothing can travel faster than light thing"
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Re: Quantum Communication

Postby Sir_Elderberry » Sat Dec 08, 2007 1:43 am UTC

But isn't the fact that they collapse itself information?
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Re: Quantum Communication

Postby Aluminus » Sat Dec 08, 2007 1:51 am UTC

Time to call in the big guns:
a totally reliable source wrote:Although two entangled systems appear to interact across large spatial separations, no useful information can be transmitted in this way, so causality cannot be violated through entanglement. This is the statement of no communication theorem.

The no-communication theorem says, that you cannot statistically work out what's going on at the other end of the entanglement. Not even if you made the measurement in an infinitesimal amount of time. There's just no way. No actual information is transferred across space (or time for that matter) when the states collapse.
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Re: Quantum Communication

Postby Sir_Elderberry » Sat Dec 08, 2007 2:04 am UTC

Ok, I'm confused about why, though. The systems collapse. That is, after all, the point of the problem. If the systems collapse, then at the very least, the fact one collapsed tells you that the other one may have collapsed due to my involvement, or maybe it happened by chance. If I give you a million photons and collapse all of mine, it's incredibly unlikely that they all did that of their own volition, and far more likely that I am instead trying to signal something.
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Re: Quantum Communication

Postby btilly » Sat Dec 08, 2007 3:04 am UTC

Sir_Elderberry wrote:Ok, this has been mulling around my mind ever since I understood the following (I am stating the below to clarify my premises; any of which may be wrong):

-It is possible to entangle particles
-When entangled, particles are in a superposition where they have no definite spin. If either is measured, both collapse into identical states.
-This happens instantaneously regardless of differences in space.

However, I was also told:
-This does not violate relativity because information cannot be transferred.

This is all true.
Sir_Elderberry wrote:Now, here's the thought experiment:

I entangle some photons and give you half of them, so that each of mine corresponds to one of yours. They are sorted into a 2x* lattice, in which * is pretty unimportant for this discussion. When I want to send you a message, I collapse the photons I wish to, and your photons also collapse in the corresponding squares. We predertimne that those in the first row are dots, and those in the bottom row are dashes. So I tap out top, top, top, bottom, bottom, bottom, top, top, top, or, ... --- ..., or SOS. You are, at this point, in Alpha Centauri and instantly realize I am in danger.

Are my premises wrong or is there a more fundamental flaw with my device that goes deeper than engineering difficulties?

There is a flaw, and it is instantly obvious if you subscribe to the Everett hypothesis. (If you do not subscribe to that interpretation, it still follows from the fact that all interpretations of QM make identical predictions about what we see.)

In the Everett hypothesis there is no such thing as collapse. Instead what happens is that when you observe something, you become entangled with the quantum mechanical system. Which effectively splits one observer into parallel observers who (for well-understood thermodynamic reasons) cannot ever interact.

So if you have 2 entangled photons, you look at one, and I look into the other, then we get entangled. Suppose, for the moment, that the photons were polarized one of two ways. Well we have up with 2 copies of you and 2 copies of me. Thanks to quantum entanglement, when we meet up with you, the right copies of us will meet up to form 2 consistent descriptions of what happened. However your observation of your photon in no way, shape, or form affected the photon that I could see. The "spooky action at a distance" from entanglement is just a prediction about what will happen later when we meet up again in sub light interactions.

If you understand all of that, then you'll understand that "collapse" is just quantum entanglement with a complex system. But being "collapsed" in no way shape or form affects the photon.
Sir_Elderberry wrote:Ok, I'm confused about why, though. The systems collapse. That is, after all, the point of the problem. If the systems collapse, then at the very least, the fact one collapsed tells you that the other one may have collapsed due to my involvement, or maybe it happened by chance. If I give you a million photons and collapse all of mine, it's incredibly unlikely that they all did that of their own volition, and far more likely that I am instead trying to signal something.

Hopefully this explanation will have helped you. The long and short of it is that I can't tell whether you collapsed any particular photon. Since I can't tell that, I can't tell whether you collapsed one or a million photons. And therefore you can't signal anything to me.
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Re: Quantum Communication

Postby Sir_Elderberry » Sat Dec 08, 2007 3:12 am UTC

btilly wrote:Stuff


Thank you sir. I can't say that I have a very deep understanding, but I can see enough of what you're getting at to understand where I went wrong.

(This has bothered me for years. Thank you, xkcd.)
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Re: Quantum Communication

Postby Herman » Sat Dec 08, 2007 4:34 am UTC

That's a pretty interesting explanation, and I'm sure it's more rigorously correct, but here's a slightly simpler explanation I heard:

Two particles are entangled s.t. they each have the same state, but both are as yet unmeasured. I measure one, which puts the other into the same state. But I can't control which state I will measure. Now, in order to get a message, my partner at the other end has to measure (and thus collapse) his particle too. That's the key. He'll measure the same state I got, but then, he also might have measured that state if I didn't do anything. As far as he knows, there's no way to tell if I collapsed the particle or if he just did now. So no information is sent.

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Re: Quantum Communication

Postby Strilanc » Sat Dec 08, 2007 5:40 am UTC

A simpler explanation would be that you can't measure if a photon is collapsed. Measuring it would collapse it.
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Re: Quantum Communication

Postby gmalivuk » Sat Dec 08, 2007 5:56 pm UTC

Yeah, all that's measured is the spin. Measuring your photon doesn't tell you whether you've just collapsed it by measuring it, or whether you're measuring it in the same state as its "partner" was put into by "already" being measured. The only way to check is to ask the other observer, which has to take place at light-speed.

This isn't to say we might not discover some exotic way to exploit quantum mechanics to get ftl communication, but "spooky action at a distance" isn't the way to do it.
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Re: Quantum Communication

Postby Sir_Elderberry » Sat Dec 08, 2007 7:43 pm UTC

Ok, thanks. My mistake seemed to be that I thought that you could measure the photon as not having a state, and thus know that I had not sent anything, or vice versa.
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Re: Quantum Communication

Postby Meowsma » Sat Dec 08, 2007 8:33 pm UTC

Image

This book focuses a large part on quantum entanglement and instantaneous information transfer (that's the issue in the Schroedinger's Kittens thought experiment). Great book for any quantum mechanics enthusiast or dabbler.

But yeah, GREAT detail in that book.

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Re: Quantum Communication

Postby thoughtfully » Sat Dec 08, 2007 11:49 pm UTC

Schrodinger's Kittens totally rocks. Probably my favorite physics book. But there is a book that is devoted to entanglement that is very good also. It is titled, simply enough, entanglement.
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Re: Quantum Communication

Postby Indon » Sun Dec 09, 2007 5:33 am UTC

gmalivuk wrote:This isn't to say we might not discover some exotic way to exploit quantum mechanics to get ftl communication, but "spooky action at a distance" isn't the way to do it.


I dunno. If said 'spooky action' could be used to interact with something - anything, really, on a non-quantum scale, which I suppose is the hard part - else, then just determining the state could be used to transmit information without needing to care what specific state results; just that the particle's state has been determined.
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Re: Quantum Communication

Postby Sir_Elderberry » Sun Dec 09, 2007 5:39 am UTC

Indon wrote:
gmalivuk wrote:This isn't to say we might not discover some exotic way to exploit quantum mechanics to get ftl communication, but "spooky action at a distance" isn't the way to do it.


I dunno. If said 'spooky action' could be used to interact with something - anything, really, on a non-quantum scale, which I suppose is the hard part - else, then just determining the state could be used to transmit information without needing to care what specific state results; just that the particle's state has been determined.


Which is what I was saying, that it doesn't matter how the chips fall, only that they do.
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Re: Quantum Communication

Postby thoughtfully » Sun Dec 09, 2007 6:33 am UTC

This might ckear things up a bit.
Mighty Jalapeno (another thread) wrote:Well, the first two paragraphs hereexplain what it is and how it won't work, but it's still a cool idea.

For those too lazy to click:

    Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated. This leads to correlations between observable physical properties of the systems. For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin-up, the other one will always be observed to be spin-down and vice versa, this despite the fact that it is impossible to predict, according to quantum mechanics, which set of measurements will be observed. As a result, measurements performed on one system seem to be instantaneously influencing other systems entangled with it. But quantum entanglement does not enable the transmission of classical information faster than the speed of light (see discussion in next section below).

    Observations on entangled states naively appear to conflict with the property of relativity that information cannot be transferred faster than the speed of light. Although two entangled systems appear to interact across large spatial separations, no useful information can be transmitted in this way, so causality cannot be violated through entanglement. This is the statement of no communication theorem. Although no information can be transmitted through entanglement alone, it is possible to transmit information using a set of entangled states used in conjunction with a classical information channel. This process is known as quantum teleportation. Despite its name, quantum teleportation cannot be used to transmit information faster than light, because a classical information channel is required.
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Re: Quantum Communication

Postby roundedge » Sun Dec 09, 2007 1:11 pm UTC

One analogous scenario is that there are two gift wrapped boxes on the table, and both you and your friend know that either both boxes contain an XBOX 360, or both contain a Wii. You each take a box, and travel a long distance away from each other. Then on christmas, you open your boxes, and you both know what the other guy got for christmas. Spooky action at a distance is only counterintuitive if you subscribe to the interpretation that the contents of the boxes are only determined once you open them, otherwise it's actually a pretty unremarkable phenomenon.

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Re: Quantum Communication

Postby Owehn » Sun Dec 09, 2007 9:01 pm UTC

One analogous scenario is that there are two gift wrapped boxes on the table, and both you and your friend know that either both boxes contain an XBOX 360, or both contain a Wii. You each take a box, and travel a long distance away from each other. Then on christmas, you open your boxes, and you both know what the other guy got for christmas. Spooky action at a distance is only counterintuitive if you subscribe to the interpretation that the contents of the boxes are only determined once you open them, otherwise it's actually a pretty unremarkable phenomenon.


Except that the true significance of Bell-type experiments is in the measurements along different axes, as if one of you had measure how much of a Wii the gift was, and the other measured how much of an XBox, with any complex linear combination of states possible. (Actually, it would be more accurate to say that one of you measured the gift's Wiiness, and the other its (Wii+Xbox)ness.) Whereas the Christmas gift can be explained by assigning "local hidden variables" to the boxes (namely their contents), appropriate Bell experiments exhibit higher correlations than are possible within any hidden variables theory.
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Re: Quantum Communication

Postby LoopQuantumGravity » Mon Dec 10, 2007 10:07 am UTC

Owehn wrote:
One analogous scenario is that there are two gift wrapped boxes on the table, and both you and your friend know that either both boxes contain an XBOX 360, or both contain a Wii. You each take a box, and travel a long distance away from each other. Then on christmas, you open your boxes, and you both know what the other guy got for christmas. Spooky action at a distance is only counterintuitive if you subscribe to the interpretation that the contents of the boxes are only determined once you open them, otherwise it's actually a pretty unremarkable phenomenon.


Except that the true significance of Bell-type experiments is in the measurements along different axes, as if one of you had measure how much of a Wii the gift was, and the other measured how much of an XBox, with any complex linear combination of states possible. (Actually, it would be more accurate to say that one of you measured the gift's Wiiness, and the other its (Wii+Xbox)ness.) Whereas the Christmas gift can be explained by assigning "local hidden variables" to the boxes (namely their contents), appropriate Bell experiments exhibit higher correlations than are possible within any hidden variables theory.


So you're telling me that quantum mechanics ruined Christmas?
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Re: Quantum Communication

Postby Owehn » Mon Dec 10, 2007 3:15 pm UTC

Yes, I'm sorry: All those years of Christmases, expectation values through the roof, waiting to see what Super Paws's present states would be when you collapsed the wave-packaging, and now you realize the sham that the uncertainty really is just classical ignorance of the hidden gifts.
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Re: Quantum Communication

Postby Hefty One » Sat Dec 15, 2007 5:18 am UTC

So if you were around 4 light years away and in trouble....how could they help anyway?

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Re: Quantum Communication

Postby Sir_Elderberry » Sat Dec 15, 2007 6:22 am UTC

Hefty One wrote:So if you were around 4 light years away and in trouble....how could they help anyway?


Well...ah...maybe all I need is economic aid, and I can wait 4 years. Bail money and stuff.
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Re: Quantum Communication

Postby Owehn » Sat Dec 15, 2007 6:27 am UTC

Hefty One wrote:So if you were around 4 light years away and in trouble....how could they help anyway?


I wouldn't get help from you until four years later. (Well, really eight, because I first have to tell you I'm in trouble, and then you have to send help.) Even though the popular way of thinking about entanglement is "Anything you do to one particle instantly effects the same change in the other", that really isn't true, and to the extent that it is it doesn't allow you to communicate at faster-than-light speeds.
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Re: Quantum Communication

Postby ocean_soul » Wed Dec 26, 2007 4:52 pm UTC

Aluminus wrote:I thought you don't get to choose which state the entangled particles collapse to, that it was just random.
If you could communicate instantly across 4 lightyears of space, you would violate the whole "information can't nothing can travel faster than light thing"


"...Nothing classical information, matter,...(a lot of other stuff)... can travel faster than light...".
In fact everything you can call "stuff" can't travel faster than light. But the collapse still occurse instantly.
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Re: Quantum Communication

Postby sleepyrohan » Mon Jan 06, 2014 11:16 pm UTC

btilly wrote:Hopefully this explanation will have helped you. The long and short of it is that I can't tell whether you collapsed any particular photon. Since I can't tell that, I can't tell whether you collapsed one or a million photons. And therefore you can't signal anything to me.

I have a doubt regarding this based on the quantum eraser experiment (See "Quantum Eraser | Quantum Mechanics ep 4" by "Looking Glass Universe" on youtube).
From what I understand, in the double slit experiment, photons behave like a wave and interfere with themselves causing a waveform like pattern on the screen. However, when the corresponding entangled photons are "observed", the photons in the experiment stop behaving like a wave, and there is no waveform pattern.

Can't this phenomenon be utilized for FTL communication? Won't it allow the receiver to detect when the sender starts "observing" the photons, and hence the sender can encode a message if there is a predefined protocol for communication?

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Re: Quantum Communication

Postby gmalivuk » Wed Jan 08, 2014 8:38 pm UTC

Double-slit interference is a positional thing, and entanglement is a spin thing (basically), so I'm not sure why measuring spin would have any effect on the position of a different photon.
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Re: Quantum Communication

Postby Tass » Wed Jan 08, 2014 9:33 pm UTC

gmalivuk wrote:Double-slit interference is a positional thing, and entanglement is a spin thing (basically), so I'm not sure why measuring spin would have any effect on the position of a different photon.


Position can be entangled too. If you send a spin-entangled par of electrons through a non-uniform magnetic field, then their position will depend on their spin, which is indeterminate until you measure either the spin or the position of one of them. Still doesn't help with FTL communication, though.

Edit: Removed answer to specific question about quantum erasure being used to transmit ftl. It was wrong. It is to late night for me to do quantum mechanics now.

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Re: Quantum Communication

Postby gmalivuk » Wed Jan 08, 2014 10:12 pm UTC

Ah, interesting.

However, it is still true that nothing you can do can determine (faster than light) whether or not your partner has observed his or her particle yet.
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Re: Quantum Communication

Postby some_dude » Thu Jan 09, 2014 9:29 pm UTC

While no useful information can be transferred by using entanglement does the Universe really distinguish between useful and useless information? How does it do that? It seems like FTL transmission of information, whether it's useful information or not, would violate SR, "something" still seems to get transmitted FTL. Also it seems like causality would be in trouble. Say a mad scientist places a bomb in Paris and another bomb in Washington. Which one of these that detonates is based on the measured spin of an electron that is entangled with another electron in a lab orbiting Alpha Centauri ~4 light-years away. Someone else measure that particle shortly before the mad scientist measures his/hers, the result of the first measurement is up which causes the mad scientist to measure down corresponding to Washington. Could that first measurement, even though the result was random, not be said to have caused the bomb in Washington 4 light-years away to go off?
(I'm not a crackpot, I'm asking why I'm wrong not actually arguing that entanglement violates SR).

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Re: Quantum Communication

Postby gmalivuk » Thu Jan 09, 2014 10:36 pm UTC

some_dude wrote:While no useful information can be transferred by using entanglement does the Universe really distinguish between useful and useless information? How does it do that?
It doesn't need to. No information whatsoever originates at one place and arrives at another faster than light.

Someone else measure that particle shortly before the mad scientist measures his/hers
There is no precise meaning of "shortly before" for places separated by such a distance. Depending on how I'm moving, that other measurement could seem to me to have happened anywhere from 4.37 years before the mad scientist's measurement, to 4.37 years after.
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Re: Quantum Communication

Postby some_dude » Fri Jan 10, 2014 4:14 pm UTC

There is no precise meaning of "shortly before" for places separated by such a distance. Depending on how I'm moving, that other measurement could seem to me to have happened anywhere from 4.37 years before the mad scientist's measurement, to 4.37 years after.


But the radial velocity of Alpha Centauri wrt. the Solar System is only ~20 km/s so shouldn't their time coordinates be almost similar? (assuming that the lab has a reasonable orbital velocity)

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Re: Quantum Communication

Postby gmalivuk » Fri Jan 10, 2014 6:59 pm UTC

I'm not moving along with either Earth or Alpha Centauri in this scenario (which as you say can be treated as though those two places are essentially comoving). If I'm going nearly lightspeed toward Alpha Centauri, then I'd calculate their measurement as having happened nearly 4.37 years before the one on Earth, and if I'm going nearly lightspeed toward the Sun I'd calculate the Centauri measurement as happening nearly 4.37 years afterward.
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Re: Quantum Communication

Postby Eebster the Great » Fri Jan 10, 2014 7:06 pm UTC

But while entanglement is not useful for superluminal communication, it could theoretically be useful for compressing classical communication channels or sending confidential information over insecure channels with superdense coding. Theoretically, anyway.

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Re: Quantum Communication

Postby gmalivuk » Fri Jan 10, 2014 9:01 pm UTC

Well sure, there are plenty of ways it could be useful, it's just that none of them are faster than light.
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Re: Quantum Communication

Postby Ciber » Sat Jan 11, 2014 2:51 am UTC

I feel like the complexity of the whole quantum entanglement thing is way overblown. All it's saying is that if you have two items that you know the state of, then you can infer the state of one of the items by looking at the other.
It's like if you get two pennies that you know are both facing the same way. You don't know if they are both face up or face down, but if you look at either of them then you know what the other one is. Cause if one is face up and you know that both are the same way, then you know that the other is also face up.

I also think that saying "The Speed Of Light" is bad practice. People should start saying "The Speed Of Change", because 90% of the time thats the actual speed that they want. How fast photons move does not define how fast information moves, it just happens to travel at the same speed because of the laws of physics. Seriously, if we just replaced what they teach in high school with this truth we would have way less misunderstandings.

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doogly
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Re: Quantum Communication

Postby doogly » Sat Jan 11, 2014 3:50 am UTC

Nope, apparently it is underblown! You're missing a critical distinction. It is not like any analogy wherein you have "two items," and a state of one and the other. Entanglement is when you have a single state, which is a tensor product. But there are not two states. This may sound like nitpicking (it's the tensor product of two things, don't be an ass doogly, you may say) but the difference between these situations is the crux of the matter.

If it were a classical correlation, like two sides of a penny, it would obey the Bell Inequalities. Violating these is what lets us know that quantum mechanics is doing something special.

"Speed of change" is also weird because lots of changes can happen at other speeds.
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Re: Quantum Communication

Postby PM 2Ring » Sat Jan 11, 2014 7:23 am UTC

doogly wrote:Nope, apparently it is underblown! You're missing a critical distinction. It is not like any analogy wherein you have "two items," and a state of one and the other. Entanglement is when you have a single state, which is a tensor product. But there are not two states. This may sound like nitpicking (it's the tensor product of two things, don't be an ass doogly, you may say) but the difference between these situations is the crux of the matter.

If it were a classical correlation, like two sides of a penny, it would obey the Bell Inequalities. Violating these is what lets us know that quantum mechanics is doing something special.


QFT


doogly wrote:"Speed of change" is also weird because lots of changes can happen at other speeds.


How do you feel about "space / time scale factor" ?

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Re: Quantum Communication

Postby Tass » Sat Jan 11, 2014 8:03 am UTC

doogly wrote:"Speed of change" is also weird because lots of changes can happen at other speeds.


How about "cosmic speed limit" or simply "c"

I'd generally use "light speed" only when talking to laymen, otherwise I'd use c.

Oh and I could also also quote the rest of Dooglys post for truth. Bells inequality shows us that Einstein was wrong and that it is not just hidden variables.


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