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Author Topic: What is the maximum speed of information?  (Read 20611 times)

Offline Geezer

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What is the maximum speed of information?
« Reply #50 on: 15/01/2011 05:05:22 »
Eweooooo!

It's not quite as cut and dried as I thought. One thing that I didn't happen to see any mention of is that the setup attempts to measure both particles at the same time, so you don't really know who won the duel.

I found myself wondering what would happen if one path was deliberately slightly longer so that the a side was always measuring first for part of the experiment, then vice versa for the other part, would it alter the outcome at all. At least it might simplify the detector logic.
 

Offline QuantumClue

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What is the maximum speed of information?
« Reply #51 on: 15/01/2011 05:26:02 »
Geezer

I cannot help but read this last one as invoking perhaps some kind of determinism, is this what you are hinting at?

I wouldn't really know ;D but I don't think that's what I mean to imply. I'm wondering if there could be some sneaky false assumption in the logic, although I can't imagine how that would get past so many clever scientists. I asked the question more because I don't fully understand it myself than as a direct challenge to the experiment.

However, as you bring it up, if there was some sort of determinism at play, would the experiment point to that as a possible explanation?




I don't know personally.

What did you mean by ''ignoring us?'' Can you be a tad more technical mate?
 

Offline Geezer

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What is the maximum speed of information?
« Reply #52 on: 15/01/2011 05:48:33 »
What did you mean by ''ignoring us?'' Can you be a tad more technical mate?

Having read a bit more on the subject, I think that would be the same as saying there is no superluminal link between the entangled particles and hidden variables account for the experimental results.

It's interesting that Bell himself might have been slightly concerned that it could all be a consequence of superdeterminism, although I personally don't subscribe to that philosophy.
 

Offline QuantumClue

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« Reply #53 on: 15/01/2011 07:09:00 »
Ah yes, of course, I forgot about Superdeterminism. I like it.

It's not for people though who believe that quantum mechanics is built on a randomized world... personally, I have never fully detatched from some super, underlying principle which governs everything.
 

Offline QuantumClue

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What is the maximum speed of information?
« Reply #54 on: 16/01/2011 11:52:16 »
It struck me this morning a solution to the idea of all our discussions so far.

Accoridng to this equation:

vp=c/√1-ω0

the group velocity has a solution where a system of information can travel at superluminal speeds. Usually the speed of information will travel at the group velocity which travels at lightspeed, so if someone wanted to invoke the idea that at times information can travel at superluminal speeds, then it must violate the usual group velcity.
 

Offline JP

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« Reply #55 on: 16/01/2011 14:50:52 »
The group velocity is not the same as the information velocity.  It certainly can exceed the speed of light, but the information-containing part of a pulse of light always moves at the speed of light or slower.
 

Offline QuantumClue

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« Reply #56 on: 16/01/2011 15:05:04 »
Of course it applys to information. Bits representing up and down spins, may have a transfer of information, and according to that equation, one for exceeding lightspeed.
 

Offline JP

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« Reply #57 on: 16/01/2011 15:32:31 »
No.  No it doesn't.  I've extensively studied group velocities in light pulses, and it isn't information velocity.  Period.

If you want to start up a discussion of why it isn't, I could go into it, but I think it might derail this thread.
 

Offline QuantumClue

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« Reply #58 on: 16/01/2011 15:45:35 »
What does information mean to you? I see information as being part of the system. It could be tangible, it could be ethereal, but in effect it is all about what makes that system up... what is it to you?
 

Offline JP

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« Reply #59 on: 17/01/2011 00:43:27 »
All the information from a pulse arrives when you detect that a pulse has been turned on.  Put another way, it's the it's when you know the shape of the incoming pulse from the amount of light you've received. 

For a more rigorous definition, all the information is contained in the points of non-analyticity of a pulse, and arrives when those points arrive.
 

Offline QuantumClue

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« Reply #60 on: 17/01/2011 14:26:49 »
I would have been wrong anyway:

http://en.wikipedia.org/wiki/Phase_velocity

States that its not a violation of SR nor is it to be assumed information can travel this fast.
 

Offline JP

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« Reply #61 on: 17/01/2011 15:41:15 »
True, but phase velocity is different from group velocity.  (Neither carries information faster than light, however.)

Tunneling is the one possibility as far as I know, and it doesn't seem likely to be able to do so.
 

Offline QuantumClue

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« Reply #62 on: 17/01/2011 15:47:21 »
Yes, I understand that.
 

Offline SkyWriting

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What is the maximum speed of information?
« Reply #63 on: 18/01/2011 09:23:45 »
By the way, Quantumclue also mentions Fred Allan Wolf.  I would recommend taking anything from Dr. Wolf with a large dose of skepticism.  He's a big proponent of quantum pseudoscience and publishes a lot of books that are very poorly regarded by mainstream science.

My experience with "Wolf" leads me to the conclusion that he's not well adjusted. Logically anyway.
 

Offline SkyWriting

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What is the maximum speed of information?
« Reply #64 on: 18/01/2011 09:32:18 »
True, but phase velocity is different from group velocity.  (Neither carries information faster than light, however.)
Tunneling is the one possibility as far as I know, and it doesn't seem likely to be able to do so.

The most likely solution is that entanglement verifies that observation changes reality. 
Then the information need not "exceed" the speed of light because the "destination" or outcome changes based on the observation of one of the two entangled particles.
 

Offline lightarrow

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« Reply #65 on: 18/01/2011 14:22:07 »
For a more rigorous definition, all the information is contained in the points of non-analyticity of a pulse, and arrives when those points arrive.
This is interesting, do you have any link or some more about it?
 

Offline JP

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« Reply #66 on: 18/01/2011 14:45:53 »
Hmm... I can point you to some work on it, but I don't have any papers handy.

Kurt Oughstun's thesis work involved looking at the leading edge of pulses and seeing how fast they moved through media--the result is that at least a tiny piece of the very front of the pulse moves at the speed of light through any medium, and that piece carries information.  Dan Gauthier at Duke University also did some work on this.  I think some of it is in his paper in Nature: http://www.nature.com/nature/journal/v425/n6959/full/nature02016.html
I recall a talk of his where he showed that if you encoded bits of information by turning a signal on and off, the best you could do at detecting those on/off bits was the speed of light, even if the group velocity was superluminal.

As for analytic functions, the wiki has a nice article (http://en.wikipedia.org/wiki/Analytic_function), as do most advanced books on complex analysis.  The take home point from analytic functions is that you only need an infinitesimally small piece of the signal (in theory) to reconstruct the entire thing, so the leading edge of the pulse should carry all the pulse's information, and the above research seems to show that's limited to the speed of light.
 

Online yor_on

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What is the maximum speed of information?
« Reply #67 on: 21/01/2011 15:27:24 »
The problem for me is the assumption that we have a indeterminate state before the measurement. If there is one it all makes sense, but what if there isn't. What if the state was set at the beam splitter for example? How would one disprove such a statement? It all depends on how you look at a 'interaction'. The definition we seem to use is that a 'interaction' only becomes one in our measuring, excluding most of what's happening outside our measuring.

If that is wrong, then an interaction is whatever change a relation for whatever we later will be measuring, including those 'interactions' untouched by us.
==

Or you will have to assume two 'states'. One 'changeable' like a beam splitter and one 'defining' it finally, aka our 'measurement'.
 

Offline Airthumbs

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« Reply #68 on: 31/01/2011 20:29:21 »
Ok help me here please, so two little entangled particles get sent away to some measuring devices.  One is told to spin to the right, and as a result the other spins to the left? yes or no.....



 

Offline Geezer

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« Reply #69 on: 31/01/2011 21:00:44 »
Ok help me here please, so two little entangled particles get sent away to some measuring devices.  One is told to spin to the right, and as a result the other spins to the left? yes or no.....


Well, kinda! ;D

I don't think it gets "told", so you can't know in advance its direction. But whatever way it was, its chum does the opposite (apparently at the same time!) That's why Einstein called it "spooky".

(BTW - I'm sure I have some of the details wrong here, so please don't "but Geezer said" me.)
 

Online yor_on

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What is the maximum speed of information?
« Reply #70 on: 31/01/2011 23:00:25 »

You take two prisms, similar to those inside a pair of binoculars, glue them together, and then send a short laser pulse through the prism cube you've made, so that half of the light is reflected. Or you can use a half transparent mirror, separating the light pulse into two, one reflected back, its other part passing through. This down-convert the 'photon' into half its energy at the same time as it creates two particles from one, exactly the same but, as I understands it, now of oposite spin. There are also polarizing beam splitters that do the same using polarization instead. As I understands it a photons polarization is just another description of its spin, although I'm not a hundred percent sure on that one, it seems to differ in the amount of 'states' a photons spin can have (3) as compared to the polarizations (2)?

I believe that they are the same though, in that a photon only have two degrees of freedom in reality, disallowing one of the spins, whereas in theory particles of spin 1 should have three. the explanation to that is that even though 'point particles' have three states of freedom (dimensions), as the photon is massless it only have two states left. All Bosons, meaning particles without 'rest-mass' comes in whole integers, photons for example are of spin 1. 


"Theory predicts the existence of two bosons whose s differs from 1. The force carrier for gravity is the hypothetical graviton; theory suggests that it has s=2. The Higgs mechanism predicts that elementary particles acquire nonzero rest mass by exchanging hypothetical Higgs bosons with an all-pervasive Higgs field. Theory predicts that the Higgs boson has s=0. If so, it would be the only elementary particle for which this is the case." And "QFT (Quantum field theory) defines the s_{z} = 0-state to corresponds to a non-physical degree of freedom. Such photons will exhibit a negative probability-distribution. The reason for these troubles are the fact that a photon has zero-restmass."

Now, all particles of matter, also called fermions, have a half-integer spin. All known elementary fermions have spin ½, including protons, neutrons, electrons, and quarks.

But it's confusing all the same.
I'm sure JP could answer that one though :)

(Phieww, had to rearrange it as my comments suddenly made very little sense, after me filling the material in, not that unusual even when such non-withstanding I have to admit:)

==Quote


The head-on collision of a quark (the red ball) from one proton (the orange ball) with a gluon (the green ball) from another proton with opposite spin; spin is represented by the blue arrows circling the protons and the quark. The blue question marks circling the gluon represent the question: Are gluons polarized? The particles ejected from the collision are a shower of quarks and one photon of light (the purple ball).

===End of quote.

Okay, with the risk to bore you physics savvy to death :) Let's dive into what spin is, as I understands it :) First of all you need to consider a vector, or a 'angular momentum'. All angular movements can be considered as being of two 'forces', working together on whatever there is, creating that 'invisible angle' of momentum/force. Like this. 2 forces shown  | _    That then will produce a combined angular, 'invisible force', working inbetween those two. Can you see how I mean? Think of them as two leashes leading down to your dog. The dog (Sebastian) will feel it as one leash diagonally stretched between the two actual leashes if the same restraint is applied to both leashes.

(Da*n those two-legged bas*s he growls as he tries to get up to speed:)

Spin is a similar idea.

" Angular momentum is a vector quantity (something that has both a magnitude and a direction, just like a velocity) that can take on only certain values in quantum mechanics. Another thing we know about angular momentum is that, in quantum mechanics, it cannot take on just any old values, but only certain specific ones.  If a particle has three units of total angular momentum, then its projection can be any of (-3, -2, -1, 0, 1, 2, 3) and that is it: projections must differ by an integer number of units. 

Very weird, but quite a handy fact: if you know that a particle's angular momentum can take on only two different projection values, then you know its total angular momentum must be 1/2, and the projection values are (-1/2, 1/2).  If you know there are three projection values, then you know the total angular momentum is 1, with projections (-1, 0, 1). Spin acts like this, so everything you've just learned about angular momentum is also true of spin."

And

"A particle with integral spin (0,1,2,...) is not in any way limited by other
particles of its own kind.  At one location, you can have a huge number with
exactly the same energy, moving in exactly the same way.  Photons of light,
neutrinos, and pions are such particles.  These tend to be the communication
particles, the particles that spend most of their time passing between
things.  These are called Bose-Einstein particles, or bosons for short.

A particle with half-integral spin (1/2,3/2,5/2,...) is very limited by
other particles of its own kind.  If two such particles are at the same
location, something must be different about them.  They may be "spinning" in
different directions.  They may have different energies.  They may be moving
in different directions.  They cannot be identical in all ways.  Protons,
neutrons and electrons are the most common such particles.  These tend to be
the particles that build matter.  These are called Fermi-Dirac particles, or
fermions for short"

"In 1924 Wolfgang Pauli introduced what he called a "two-valued quantum degree of freedom" associated with the electron in the outermost shell. This allowed him to formulate the Pauli exclusion principle, stating that no two electrons can share the same quantum state at the same time. The physical interpretation of Pauli's "degree of freedom" was initially unknown. Ralph Kronig, one of Landé's assistants, suggested in early 1925 that it was produced by the self-rotation of the electron. When Pauli heard about the idea, he criticized it severely, noting that the electron's hypothetical surface would have to be moving faster than the speed of light in order for it to rotate quickly enough to produce the necessary angular momentum. This would violate the theory of relativity."
 
"Inspired by the photon picture of light waves, Bose was interested deriving Planck's radiation formula, which Planck obtained largely by guessing. Using the particle picture of Einstein, Bose was able to derive the radiation formula by systematically developing a statistics of massless particles without the constraint of particle number conservation. He was quite successful, but was not able to publish his work, because no journals in Europe would accept his paper... in 1926, Einstein completed the Bose-Einstein statistics by extending Bose's work to the case of massive particles with particle-number conservation.

The Bose-Einstein statistics was not completely without troubles, because not all the particles obey this statistics. It was Paul A. M. Dirac who found out that the Bose-Einstein system particles are totally symmetric under permutation of particles. This observation of course led to the Fermi-Dirac statistics. It is interesting to note how intensely Dirac was interested in permutations from his book entitled "Principles of Quantum Mechanics."

Let us go back to the photon statistics formula derived by Bose. There is a factor "2" sitting on the numerator of this formula. The usual explanation is that it is because photons are massless particles. Then why not 1 or 3 ? Bose argued that the photon can have two degenerate states. This eventually led to the concept of photon spin parallel or anti-parallel to the momentum.

The question of why the photon spin should be only along the direction of momentum has a stormy history. Eugene Wigner (1939) showed that the internal space-time symmetry of massless particles is isomorphic to the symmetry of two-dimensional Euclidean space consisting of one rotation and two translational degrees of freedom. It is not difficult to associate the rotational degree with the photon spin either parallel or anti-parallel to the momentum, but what physics is associated with the translational degrees of freedom. These translational degrees were later identified as gauge transformations. This does not solve the whole problem because there is one gauge degree of freedom while there are two translational degrees of freedom. How do they collapse into the one gauge degree of freedom? This problem was not completely solved until 1990."

"If/when electrons decide to share a single orbital, we get pairs (as opposed to, say, triplets) because the electrons have total spin s=½ so there are two possible spin projections sz={−½ or +½}.

If electrons had total spin s=1 then there would be three possible spin projections sz={−1, 0, or +1} ... and you would find orbitals with three electrons in them. You can’t occupy a given orbital more times than are allowed by the spin multiplicity because of the (Pauli) exclusion principle.")

"In 1922 the Dutch physicists Otto Stern and Walther Gerlach made a discovery remarkably similar to that of Erasmus Bartholin, but instead of light rays their discovery involved the trajectories of elementary particles of matter. They passed a beam of particles (atoms of silver) through an oriented magnetic field, and found that the beam split into two beams, with about half the particles in each beam, one deflected up (relative to the direction of the magnetic field) and the other down."

And the reason they did so came to be known as their 'spin'." One outcome of quantum field theory was a quantization of the electromagnetic field, the necessity of which had been pointed out by Einstein as early as 1905. On an elementary level, Maxwell’s equations are inadequate to describe the phenomena of radiation. The quantum of electromagnetic radiation is called the photon, which behaves in some ways like an elementary particle, although it is massless, and therefore always propagates at the speed of light. Hence the "spin axis" of a photon is always parallel to its direction of motion, pointing either forward or backward"

Some from here. but it's my own mix I'm afraid. Hope you could make some sense from it.
« Last Edit: 17/02/2011 23:44:29 by yor_on »
 

Online yor_on

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What is the maximum speed of information?
« Reply #71 on: 31/01/2011 23:37:48 »
And when it comes to phase or group velocity enabling 'FTL information' I have this very nice quote.

------------On 12-May-2006 by Guest_carbonlife------------------------


Whenever you see the words "faster than light", you should ask two questions:

Q1: Are they talking about the speed of light in a VACUUM?

A1: No, they're usually talking about the speed of light in some sort of material like glass, which is typically a third slower than light in a vacuum. The speed of light in a vacuum is the real speed limit.

Q2: Are they talking about phase velocity or group velocity? If the article doesn't say, then either the reporter didn't ask the right questions, or the researcher is trying to mislead investors.

Group velocity (q.v.) is the speed of an actual information-pulse, when you factor out reflections and stray ripples.

Phase velocity (q.v.) is an illusory movement of wavelets which SEEM to move faster than light, but actually don't. You can see an example of this at the front of a moving row-boat traveling faster than its own wake. The wake falls behind the boat on either side -- that's the group velocity. If you 'send a signal' by throwing a pebble in the water, the ripples fall behind the boat -- again, group velocity -- the speed of an information-carrying wave in water.

But if you look at the bow wave, you see wavelets that SEEM to be traveling faster than the larger wave they're part of. They appear at the back of the bow wave, move quickly to the front, and disappear. These are phase-velocity waves, and their high velocity is illusory. They don't carry information, because they don't appear until an information-carrying wave has already moved past, and disappear when they reach the front of the msin wave.

Most materials have a constant index of refraction, which tells you how much light slows down in that medium ( Snell's Law of refraction ). In a 'tricky' medium, the index of refraction can be shifted up or down using laser pulses, or can even be made negative. The catch is that the laser pulse which performs the trick are themselves traveling slower than light in a vacuum, so none of the ripple effects behind them can carry information faster than the causative wave -- what you get is a 'bow wave' with some phase-velocity ripples running around inside it.


However there are always a few reporters who like to play with people's heads, by not explaining group velocity vs. phase velocity, and by playing with words so it sounds like somebody is doing FTL research. Again, general relativity only prohibits transfer of information faster than light in a vacuum. Quantum mechanics has a similar principle called the No-Communication Theorem -- which merely says that although quantum-entangled particles can 'agree' at a distance, they can't carry usable information. If you send one entangled particle to Alpha Centauri and measure the other one, the two particles will will 'agree' ( e.g. if one is spin up, then the other is spin-down. But since the measurement at Centauri comes out completely random, you still have to wait 4.2 years to compare results by radio, in order to 'decode' the information.

Every physics student has thought "there's gotta be a way...", and has eventually realized [often by trying it] that the 'usual tricks' don't work. "


-------------On 12-May-2006 by Guest_carbonlife------------------------------

Better add this too.

"The group velocity of a wave (e.g. a light beam) may also exceed c  in some circumstances. In such cases, which typically at the same time involve rapid attenuation of the intensity, the maximum of the envelope of a pulse may travel with a velocity above c. However, even this situation does not imply the propagation of signals with a velocity above c, even though one may be tempted to associate pulse maxima with signals.

The latter association has been shown to be misleading, basically because the information on the arrival of a pulse can be obtained before the pulse maximum arrives. For example, if some mechanism allows the full transmission of the leading part of a pulse while strongly attenuating the pulse maximum and everything behind (distortion), the pulse maximum is effectively shifted forward in time, while the information on the pulse does not come faster than c without this effect....

If the wave is travelling through an absorptive medium, this does not always hold. Since the 1980s, various experiments have verified that it is possible for the group velocity of laser light pulses sent through specially prepared materials to significantly exceed the speed of light in vacuum.

(Eh sorry. The guy must have meant the speed of light for that specific 'medium', not faster than in a vacuum. A lot of physicists seems to define the speed of light as always being 'c', but 'c' regulated by what medium it propagates in. Da*n, and that one is taken from the 'Group velocity wiki' too? Kind'a embarrassing that one. Well rest assured that there exist no materials allowing a speed faster than the one one expected for light in a vacuum, as far as I know :)

However, superluminal communication  is not possible in this case, since the signal velocity remains less than the speed of light. It is also possible to reduce the group velocity to zero, stopping the pulse, or have negative group velocity, making the pulse appear to propagate backwards.

However, in all these cases, photons continue to propagate at the expected speed of light in the medium... Anomalous dispersion happens in areas of rapid spectral variation with respect to the refractive index. Materials that exhibit large anomalous dispersion allow the group velocity of the light to exceed c and/or become negative."
« Last Edit: 01/02/2011 00:01:52 by yor_on »
 

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What is the maximum speed of information?
« Reply #71 on: 31/01/2011 23:37:48 »

 

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