Freethought & Rationalism Archive

excreationist · 03-02-2002, 01:11 AM

It might take me a long time to get my hands on one of them.... (I try to avoid paying for books since they are expensive)

Lykil · 03-02-2002, 10:32 AM

I understand that it is difficult to explain here, but still, let me just summarize my problems: Let’s say you have two entangled particles, that you move to a great distance of each other. According to the EPR exp. (if I haven’t misunderstood it completely) it should be possible to measure only one of them, and instantly deduce what the other particle would be, since they always come in pairs, and the one particle is the opposite of the one measured. I believe that the EPR showed very clearly (assuming the ‘fuzzy state’ is a fact) that there was instantanious communication between the particles. Since this communication is directly related to our measurement of the particles, why don’t we get the crucial data, that allows for the deduction?

Quote:

ohwilleke: More particularly, it is theoretically (not just practically) impossible to know what state a quanta will be in what it is observed under certain circumantances. Yet, by observing one twinned particle in one place, you can get this theoretically impossible to know knowledge about the other.

This seems to support faster-than-light (FTL) communication. The explanation I’ve been given though is that the particles first have to travel the distance that they are separated, at sub-light speed, and therefore the communication isn’t really faster-than-light, it’s just put on hold until after the particles have arrived/been measured. Vaguely similar to a closed letter being opened at a certain point. No one would say that letters communicate faster-than-light (especially not where I live), even though the information is hidden from the outside world before the letter is ‘measured.’

As for a sensible interpretation of the FTL communication, does anyone have any comments on the idea of a different dimension (a fifth or something) that ties the particles together so that the communication isn’t really FTL, but only the distance is zero through this strange dimension otherwise invisible to us? I don’t know how much of this is science and how much is just this guy trying to fool me into thinking he knows a lot

Quote:

excreationist: Basically I'm just looking for evidence that this intermediate fuzzy state must exist.

Exactly. If we compare the particles in the tunnel to – say – billiard balls, all that seems to happen is the device used to measure the particles deflecting them, and thus changing the results, rather than instantiating (?) them.

As for books, I can’t afford to buy them right now, so if anyone has an interesting URL or knows a link to a great website, please: POST IT HERE!

Regards
Mandark

Ipecac · 03-03-2002, 08:53 AM

Quote:

Originally posted by Kosmon:
However, it is impossible for data (information) to be transferred faster than the speed of light... so instantaneous communication across space would only be possible to an extent. This is also why it would be impossible to figure out the spin of one particle here and instantaneously be able to deduce the spin of its partner across the universe.

This is simply not true. It has been shown that <a href="http://www.compu-web.com/nimtz.htm" target="_blank">using quantum tunneling you can transmit data faster than light. </a>

Fixed link, it works now.

[ March 03, 2002: Message edited by: Ipecac ]

Malaclypse the Younger · 03-03-2002, 08:58 AM

Quote:

Originally posted by Ipecac:


This is simply not true. It has been shown that <a href="http://leader.linkexchange.com/33/X1265205/showiframe?" target="_blank">using quantum tunneling you can transmit data faster than light. </a>

The link is defective, so I'm unable to evaluate the claim.

Much depends on the definition of "information". It is certainly true that quantum events are nonlocally correlated, which requires that the quanta have "information" about each others' states.

However, AFAIK, it is known that according to our present understanding of QM one cannot use quantum entanglement to nonlocally transmit "information" in the sense of a message of the traditional sort, such as a string of characters.

[ March 03, 2002: Message edited by: Malaclypse the Younger ]

Ipecac · 03-03-2002, 09:06 AM

Quote:

Originally posted by Malaclypse the Younger:
The conclusion of QM is that quanta do "communicate" nonlocally. However, that communication only establishes correlation.

To detect quantum nonlocality, I must first determine the other observer's results and correlate them with my own. I must use a local (speed-of-light) method of communication to determine the other observer's results.

Without the other observer's results, I see only what appears to be random results. Essentially, nonlocal quantum correlation turns one set of random results into another set of random results. However, by the definition of random, those two sets of results are indistinguishable without the corresponding information.

Not necessarily. Let's say you have two pair of entangled particles, pairs AA and BB. I have one half of the pair AB and you have the other, AB. Let's say we take our two particles and separate them by a light week. If I manipulate one particle(A) so that its state changes in a predermined way when you observe this state change(on your A) you do the same thing on the other half of the other pair(your B). I should see the same state on my other particles when I observe it(on my B). It theoretically should take no longer than for me to observe the change and initiate the same change on the other particle. You could theoretically reduce this experiement to a couple of light seconds in distance to test the theory. This shouldn't be impossible to test now that we have created entangled molecules.

Malaclypse the Younger · 03-03-2002, 09:28 AM

Quote:

Originally posted by Ipecac:
[QB]Not necessarily. Let's say you have two pair of entangled particles, pairs AA and BB...

First of all, we need to be rigorous about what we mean by "manipulating" an entangled particle.

To "manipulate" an entangled particle means to perform a "measurement" on that particle which will collapse a mixed state into an eigenstate (a particular state).

According to QM, once you perform a measurement on one of two entangled particles and determine its eigenstate, the other particle will be in the correlated eigenstate.

For instance let us assume that we are measuring the vertical polarization of two correlated photons.

Before the measurement, each photon is in a mixture of possible polarization states. It's state vector is sqrt (2) * up + sqrt (2) * down; it's in a mixture of the up and down state.

When we measure one photon, we reduce this mixture of states to a definite state, either up or down; it's state vector becomes either 1 * up + 0 * down or 0 * up + 1 * down (the "eigen" in "eigenstate" refers to the "1").

Since the photons are correlated, when I determine the eigenstate of one photon, I know that the eigenstate of the correlated photon is the opposite. So I know that if I measure an up state, my partner will measure a down state and vice versa.

However, since I don't know what state I will get when measuring a photon, I can't use the information I receive to transmit a message. It is important to note that I can't make the photon be in either an up state or a down state; I can merely transform the mixed state to some random eigenstate.

So what both my partner and I see when measuring the states of photons are merely a random collection of up and down states. It is only when we transmit our various measurements to each other locally (speed-of-light) that we can detect the correlation. Until we do so, we merely have a random collection of up/down states with no way to assign meaning to the results.

[ March 03, 2002: Message edited by: Malaclypse the Younger ]

Ipecac · 03-03-2002, 10:31 AM

Quote:

Originally posted by Malaclypse the Younger:
However, since I don't know what state I will get when measuring a photon, I can't use the information I receive to transmit a message. It is important to note that I can't make the photon be in either an up state or a down state; I can merely transform the mixed state to some random eigenstate.

We can't make a photon be in a particular state today. This does not mean we will never be able to do it. Also, now that we have been successful in creating entangled molecules it might be easier to manipulate these objects. Certainly we can manipulate the states of a molecule.

Malaclypse the Younger · 03-03-2002, 11:22 AM

Quote:

Originally posted by Ipecac:


We can't make a photon be in a particular state today. This does not mean we will never be able to do it. Also, now that we have been successful in creating entangled molecules it might be easier to manipulate these objects. Certainly we can manipulate the states of a molecule.

True indeed.

IIRC, Nick Herbert proved that our current formulation of QM precludes superlumunal information transfer (although I haven't seen the proof). If we had evidence that we could directly manipulate the quantum state of a photon, we would have to substantively adjust our current formulation of QM--which is not impossible.

However, based on today's understanding of QM, there is no rational justification for the belief that superluminal information transfer is possible.

Lykil · 03-04-2002, 09:20 AM

Quote:

Originally posted by Malaclypse the Younger:


First of all, we need to be rigorous about what we mean by "manipulating" an entangled particle.

To "manipulate" an entangled particle means to perform a "measurement" on that particle which will collapse a mixed state into an eigenstate (a particular state).

According to QM, once you perform a measurement on one of two entangled particles and determine its eigenstate, the other particle will be in the correlated eigenstate.

For instance let us assume that we are measuring the vertical polarization of two correlated photons.

Before the measurement, each photon is in a mixture of possible polarization states. It's state vector is sqrt (2) * up + sqrt (2) * down; it's in a mixture of the up and down state.

When we measure one photon, we reduce this mixture of states to a definite state, either up or down; it's state vector becomes either 1 * up + 0 * down or 0 * up + 1 * down (the "eigen" in "eigenstate" refers to the "1").

Since the photons are correlated, when I determine the eigenstate of one photon, I know that the eigenstate of the correlated photon is the opposite. So I know that if I measure an up state, my partner will measure a down state and vice versa.

However, since I don't know what state I will get when measuring a photon, I can't use the information I receive to transmit a message. It is important to note that I can't make the photon be in either an up state or a down state; I can merely transform the mixed state to some random eigenstate.

So what both my partner and I see when measuring the states of photons are merely a random collection of up and down states. It is only when we transmit our various measurements to each other locally (speed-of-light) that we can detect the correlation. Until we do so, we merely have a random collection of up/down states with no way to assign meaning to the results.

[ March 03, 2002: Message edited by: Malaclypse the Younger ]

Thanks for clearing that up. I think a lot of the misunderstanding about communicating faster than light stems from not understanding this bit clearly enough, and also from not knowing exactly what you mean by 'communication.' At least that was my biggest problem.

p.

Jesse · 03-04-2002, 02:16 PM

Something I posted on QM and the EPR experiment on another thread a while ago:

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Although people usually focus on indeterminacy when discussing the mysteries of quantum mechanics, the real mystery is that QM poses a lot of problems for a realist view of reality, i.e. one where the properties of the world exist independently of our measurement. This is most obvious in the EPR experiments where properties of entangled particles are measured at different locations. What we find is that there are regular correlations between measurements made on particle A and measurements made on particle B which are inexplicable if we picture the particles as classical objects with definite properties that cannot communicate faster than light--this is what Einstein called "spooky action at a distance."

It is sometimes imagined that the uncertainty principle, which prevents us from knowing simultaneously the value of two noncommuting variables (like position and momentum), is just a limitation on measurement; maybe the particle has a definite position and momentum at any given time, but each time we try to measure the position it changes the particle's momentum in a random way, and each time we measure the particle's position it offsets the momentum. However, the EPR experiment shows it is much worse than that. The correlations between entangled particles are such that they cannot be explained by any picture of the world in which the particles have definite values for each noncommuting variable at every time, unless the particles can somehow communicate instantaneously so as soon as you measure one the other "knows" which property you measured and adjusts its own properties. This is the result known as "Bell's Theorem," which says that no local theory of hidden variables can explain the results of the EPR experiment.

In Huw Price's Time's Arrow and Archimedes' Point he offers a little story to help us see what's so strange about the EPR results:

Quote:

By modern standards the criminal code of Ypiaria [pronounced, of course, "E-P-aria"] allowed its police force excessive powers of arrest and interrogation. Random detention and questioning were accepted weapons in the fight against serious crime. This is not to say the police had an entirely free hand, however. On the contrary, there were strict constraints on the questions the police could address to anyone detained in this way. One question only could be asked, to be chosen at random from a list of three: (1) Are you a murderer? (2) Are you a thief? (3) Have you committed adultery? Detainees who answered "yes" to the chosen question were punished accordingly, while those who answered "no" were immediately released. (Lying seems to have been frowned on, but no doubt was not unknown.)

To ensure that these guidelines were strictly adhered to, records were required to be kept of every such interrogation. Some of these records have survived, and therein lies our present concern. The records came to be analyzed by the psychologist Alexander Graham Doppelganger, known for his work on long distance communication. Doppelganger realized that among the many millions of cases in the surviving records there were likely to be some in which the Ypiarian police had interrogated both members of a pir of twins. He was interested in whether in such cases any correlation could be observed between the answers given by each twin.

As we now know, Doppelganger�s interest was richly rewarded. He uncovered the two striking and seemingly incompatible correlations now known collectively as Doppelganger�s Twin Paradox. He found that

(8.1) When each member of a pair of twins was asked the same question, both always gave the same answer;

and that

(8.2) When each member of a pair of twins was asked a different question, they gave the same answer on close to 25 percent of such occasions.

It may not be immediately apparent that these results are in any way incompatible. But Doppelganger reasoned as follows: 8.1 means that whatever it is that disposes Ypiarians to answer Y or N to each of the three possible questions 1, 2, and 3, it is a disposition that twins always have in common. For example, if YYN signifies the property of being disposed to answer Y to questions 1 and 2 and N to question 3, then correlation 8.1 implies that if one twin is YYN then so is his or her sibling. Similarly for the seven other possible such states: in all, for the eight possible permutations of two possible answers to three possible questions. (The possibilities are the two homogeneous states YYY and NNN, and the six inhomogeneous states YYN, YNY, NYY, YNN, NYN, and NNY.)

Turning now to 8.2, Doppelganger saw that there were six ways to pose a different question to each pair of twins: the possibilities we may represent by 1:2, 2:1, 1:3, 3:1, 2:3, and 3:2. (1:3 signifies that the first twin is asked question 1 and the second twin question 3, for example.) How many of these possibilities would produce the same answer from both twins? Clearly it depends on the twins� shared dispositions. If both twins are YYN, for example, then 1:2 and 2:1 will produce the same response (in this case, Y) and the other four possibilities will produce different responses. So if YYN twins were questioned at random, we should expect the same response from each in about 33 percent of all cases. And for homogeneous states, of course, all six posible question pairs produce the same result: YYY twins will always answer Y and NNN twins will always answer N.

Hence, Doppelganger realized, we should expect a certain minimum correlation in these different question cases. We cannot tell how many pairs of Ypiarian twins were in each of the eight possible states, but we can say that whatever their distribution, confessions should correlate with confessions and denials with denials in at least 33 percent of the different question interrogations. For the figure should be 33 percent if all the twins are in inhomogeneous states, and higher if some are in homogeneous states. And yet, as 8.2 describes, the records show a much lower figure.

Doppelganger initially suspected that this difference might be a mere statistical fluctuation. As newly examined cases continued to confirm the same pattern, however, he realized that the chances of such a variation were infinitesimal. His next thought was therefore that the Ypiarian twins must generally have known what question the other was being asked, and determined their answer partly on this basis. He saw that it would be easy to explain 8.2 if the nature of one�s twin�s question could influence one�s own answer. Indeed, it would be easy to make a total anticorrelation in the different question cases be compatible with 8.1�with total correlation in the same question cases.

Doppelganger investigated this possibility with some care. He found, however, that twins were always interrogated separately and in isolation. As required, their chosen questions were selected at random, and only after they had been separated from one another. There therefore seemed no way in which twins could conspire to produce the results described in 8.1 and 8.2. Moreover, there seemed a compelling physical reason to discount the view that the question asked of one twin might influence the answers given by another. This was that the separation of such interrogations was usually spacelike in the sense of special relativity; in other words, neither interrogation occurred in either the past or the future light cone of the other. (It is not that the Ypiarian police force was given to space travel, but that light traveled more slowly in those days. The speed of a modern carrier pigeon is the best current estimate.) Hence according to the principle of the relativity of simultaneity, there was no determinate sense in which one interrogation took place before the other.

This is the problem posed by the EPR experiment in a nutshell. As the great physicist Richard Feynman said, "Nobody understands quantum mechanics�do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?' because you will go 'down the drain' into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that." The weirdness of QM, and the difficulty with imagining "how can it be like than" in a way consistent with the view that reality exists before we observe it, extends to other famous experiments and thought-experiments, like the <a href="http://www.time-travel.com/twoholes.htm" target="_blank">Double-slit experiment</a> and the Schroedinger�s Cat experiment. (is the cat �really� alive or dead before it is measured?) But the EPR experiment shows most clearly what the basic problem here is for a realist.

A number of different "interpretations" of quantum-mechanical weirdness have emerged over the years, with none yielding any new physical predictions (and thus being experimentally indistinguishable) but each offering a different way to conceptualize what�s "really" going on in these sorts of experiments. <a href="http://www.tcm.phy.cam.ac.uk/~mdt26/qmint.html" target="_blank">Here</a> is a page which gives some good links on these various interpretations, and I�ll attempt my own summary here:

1. The Copenhagen Interpretation

Basically, the Copenhagen interpretation says that we shouldn�t worry about how what�s really going on in the first place�science can only deal with correlating and predicting the results of various measurements, but it can�t tell us anything about what goes on when we�re not looking. This is basically a logical positivist perspective, and it was preferred by Bohr.

2. "Objective Collapse" interpretation

Here the wave-particle duality is taken literally�the world exists as a wavelike potential when it�s not being observed, but somehow measurements periodically "collapse" the wavefunction into a definite state. Some versions of this suppose that it�s consciousness that does the collapsing, others suppose that an entangled system collapses once it reaches a certain limit in mass. Unlike the other interpretations, these might actually be expected to yield different predictions than orthodox QM�so far, there�s no evidence for anything like this though.

3. The Bohm-de Broglie interpretation

Bell�s theorem shows that no local hidden variable theory can explain the results of the EPR experiment, but that leaves open the possibility of a nonlocal hidden variables theory where particles can communicate faster than light. This is the route taken by Bohm and de Broglie�s interpretation. In the Ypiarian story, this would be like the twins having a psychic link which allows one to know what question the other was asked, and adjust his own answer accordingly.

4. Transactional interpretation

The EPR experiment can also be explained if you assume the future can affect the past, so that the particle�s original properties are affected by the measurements that will be made on them later, once they are separated. In the Ypiarian story, this would mean that the twin�s choices to commit or not commit various crimes would be affected by which questions they would be asked much later when they�re interrogated. This isn�t as strange as it sounds, since all the laws of physics we currently know of are time-symmetric (they look the same forwards as they do backwards) and apparently the apparent "arrow of time" emerges solely from statistical mechanics, perhaps because the universe started off in a very low-entropy state. Huw Price�s book Time�s Arrow and Archimedes� Point, which I quoted from above, deals with this problem; not surprisingly, he favors a version of this interpretation.

5. The Many-Worlds interpretation.

This interpretation takes the mathematical formalism of QM literally and proposes that the wavefunction is all there is. This means that when I measure the state of a particle that�s in superposition, instead of "collapsing" it into a definite state, I just become entangled with it and enter into a superposition myself; basically, I "split" into two versions of myself, one of whom observes one state and another of whom observes another. In popular accounts this is sometimes explained in terms of the entire universe splitting into parallel histories all the time, but it�s a bit more subtle than that, since different "worlds" can interfere with each other and cannot be viewed as totally "parallel,� although thermodynamics may explain the appearance of splitting through a phenomenon called <a href="http://ii-f.ws/ubb/ultimatebb.php?ubb=get_topic&f=10&t=000355" target="_blank">decoherence</a>. For technical reasons this interpretation preserves locality (see question 12 of the <a href="http://www.hedweb.com/everett/everett.htm#local" target="_blank">Everett FAQ</a>), and it�s also 100% deterministic to boot (although it suggests an odd kind of subjective indeterminacy in which my first-person experience randomly chooses which split copy to become�hence a variation of this interpretation is the <a href="http://www.poco.phy.cam.ac.uk/~mjd1014/" target="_blank">many-minds interpretation</a> which deals with this issue a little more explicitly). This interpretation usually seen as theoretically the most elegant, and is often implicitly assumed in quantum cosmology, although physicists are often agnostic about whether other worlds/histories are actually "real." Many-worlds also helps make sense of quantum computation, which can be understood elegantly in terms of the computer performing different computations in different worlds and then combining the results through interference:
<a href="http://www.doc.ic.ac.uk/~ids/quantum_computing.html" target="_blank">www.doc.ic.ac.uk/~ids/quantum_computing.html</a>
<a href="http://www.herwig-huener.de/quantum.html" target="_blank">www.herwig-huener.de/quantum.html</a>
<a href="http://www.qubit.org/" target="_blank">www.qubit.org/</a>
<a href="http://www.qubit.org/people/david/Articles/Frontiers.html" target="_blank">www.qubit.org/people/david/Articles/Frontiers.html</a>
<a href="http://www.innerx.net/personal/tsmith/ManyWorlds.html#expmw" target="_blank">www.innerx.net/personal/tsmith/ManyWorlds.html#expmw</a>

So, those are the various interpretations�as I said, the main problem is that none of them really gives any new testable predictions, which is a bit unsatisfying. There�s some good reason to think that a theory of quantum gravity would transform our understanding of QM somewhat, so perhaps such a theory will depend on a modified version of one of these interpretations that is testable in some way. In any case, Bell�s inequality shows definitively than no classical, realist picture of the world can explain the EPR results, so whatever the truth turns out be, it�s guaranteed to violate our cherished assumptions in one way or another (faster-than light signalling, the future affecting the past, parallel universes�take your pick!)

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03-02-2002, 01:11 AM	#11
excreationist Veteran Member Join Date: Aug 2000 Location: Australia Posts: 4,886	It might take me a long time to get my hands on one of them.... (I try to avoid paying for books since they are expensive)

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