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Jim B.
01-20-2005, 08:20 AM
I know it always makes a good plot for science fiction: quantum realities, the theory that says all the different possibilities that could exist, do in other worlds. Now I know sci-fi notwithstanding, travel to these other worlds is probably impossible (though someone please correct on this if I am wrong). But what about communicating with them somehow? I know there this experiment where light is passed thru two slits, and it forms a multipattern with supposedly allows us to see some of these other planes.

If we could communicate with other quantum realities, think of the possibilities. It would certainly prove this theory of quantum mechanics, if nothing else. And we could communicate with dearly departed loves ones that remain alive in these other realities. Or we could talk to other realities that are more technologically advanced than us, and share in their technological advances. In short, if my assumptions about it are accurate, the possibilities in using it would be endless and amazing.

Anyways, my views of its applications aside, this is a technical question. I want to know if communication between these separate realities is possible and how. And of course, implied in all of this is whether this theory of quantum mechanics is true to begin with.

Thank you in advance to all who reply :)

jester21
01-20-2005, 08:22 AM
Thank you in advance to all who reply :)


Don't thank me yet.... luckily.. I have no idea what you are talking about :smack:

Jim B.
01-20-2005, 08:25 AM
Don't thank me yet.... luckily.. I have no idea what you are talking about :smack:
What part don't you understand, this theory of quantum mechanics, or the wording of my question?

jester21
01-20-2005, 08:27 AM
What part don't you understand, this theory of quantum mechanics, or the wording of my question?


lol... ANY theory of quantum mechanics. I failed college Physics I for god sakes ;o)

Mathochist
01-20-2005, 11:25 AM
I know it always makes a good plot for science fiction: quantum realities, the theory that says all the different possibilities that could exist, do in other worlds. Now I know sci-fi notwithstanding, travel to these other worlds is probably impossible (though someone please correct on this if I am wrong). But what about communicating with them somehow? I know there this experiment where light is passed thru two slits, and it forms a multipattern with supposedly allows us to see some of these other planes.

You're vastly misinterpreting the double-slit experiement. It's not that there's communication between two different choices that are made, but that the photon passing through the double-slit acts as a wave rather than a particle. The same effect is seen in water waves with no quantum theory at all, let alone a many-worlds interpretation

If we could communicate with other quantum realities, think of the possibilities. It would certainly prove this theory of quantum mechanics, if nothing else. And we could communicate with dearly departed loves ones that remain alive in these other realities. Or we could talk to other realities that are more technologically advanced than us, and share in their technological advances. In short, if my assumptions about it are accurate, the possibilities in using it would be endless and amazing.

Your assumptions aren't, at least partially since you only seem to have a hazy grasp of what quantum mechanics is in the first place. Communication would prove the truth of an interpretation of quantum mechanics. QM itself is as well-proven as any scientific theory.

Incidentally, there would be far too many other worlds to pick one with specific properties such as dead relatives not having died. In fact, what's to say that any such "possible world" exists? It's entirely likely that no matter how the quantum dice fell at any point in the past, grammy is still dead and you just have to suck it up and get over it like everyone else does.

Anyways, my views of its applications aside, this is a technical question. I want to know if communication between these separate realities is possible and how. And of course, implied in all of this is whether this theory of quantum mechanics is true to begin with.

I haven't looked into many-worlds interpretations in a while, but IIRC there really isn't any way of communicating. This is partly why I stopped paying attention, since by the proponents' very admissions there's no way whatsoever to test this interpretation. Whether other possible worlds with different quantum jumps in their past "exist" or not, we're in this world -- completely out of contact with the other ways things could have gone.

Thank you in advance to all who reply :)

Always glad to disillusion the overly-credulous.

JasonFin
01-20-2005, 11:47 AM
I know it always makes a good plot for science fiction: quantum realities, the theory that says all the different possibilities that could exist, do in other worlds. Now I know sci-fi notwithstanding, travel to these other worlds is probably impossible (though someone please correct on this if I am wrong). But what about communicating with them somehow? I know there this experiment where light is passed thru two slits, and it forms a multipattern with supposedly allows us to see some of these other planes.

If we could communicate with other quantum realities, think of the possibilities. It would certainly prove this theory of quantum mechanics, if nothing else. And we could communicate with dearly departed loves ones that remain alive in these other realities. Or we could talk to other realities that are more technologically advanced than us, and share in their technological advances. In short, if my assumptions about it are accurate, the possibilities in using it would be endless and amazing.

Anyways, my views of its applications aside, this is a technical question. I want to know if communication between these separate realities is possible and how. And of course, implied in all of this is whether this theory of quantum mechanics is true to begin with.

Thank you in advance to all who reply :)Sorry, but no. Any sort of classical communication between worlds is impossible in principle.

One way of looking at it is that the theory doesn't describe something like multiple physical copies of the earth. There is only one earth and one universe, but it is in an extremely large number of different states, all physically real.

There's a FAQ on the many-worlds interpretation here (http://www.hedweb.com/manworld.htm). It's already slightly outdated and the accuracy of a few of the answers is questionable, but it should give a good idea of the reasoning behind the theory.

In essence, the primary appeal is that the MWI is in some sense the automatic interpretation: that it looks like it should be analytically derivable if you fail to make any special provision for observation or measurement in the theory at all. Great progress has been made towards demonstrating this, some even in the few years since the FAQ was written, though it has not by any account been completely proven.

aahala
01-20-2005, 11:59 AM
Many of us communicate with quantum realities all the time. We read and post on these boards.:D

ninetypercent
01-20-2005, 12:26 PM
If we survive long enough, won't we eventually encounter other humans in the Universe?

Whack-a-Mole
01-20-2005, 12:39 PM
You're vastly misinterpreting the double-slit experiement. It's not that there's communication between two different choices that are made, but that the photon passing through the double-slit acts as a wave rather than a particle. The same effect is seen in water waves with no quantum theory at all, let alone a many-worlds interpretation.

This may be a nitpick but I do not think this is strictly correct. Certainly waves show this interference pattern but the experiment can be done such that equating the particles to waves does not hold.

Do the experiment by shooting a single electron at your double-slits. Wait five minutes and shoot a second electron and so on. Let the experiment run a long time and wonder of wonders you still see the interference pattern (this experiment has in fact been done with these results).

So, how does a single electron interfere with itself? In order to see what we see happen the electron needs to pass through BOTH slits. This is one reason why QM is so disconcerting and non-intuitive as our brains tell us one particle can ONLY go through ONE slit...not both.

I think this experiment is one pillar of the many worlds interpretation. The idea being that the particle travels all paths and as such there is a reality where the particle goes through the left slit and one where it goes through the right. Why they interfere with each other if these two realities must be distinct and non-interactive is way beyond me (hell...most of this is way beyond me anyway but it is fun to think about).

FriendRob
01-20-2005, 01:13 PM
This may be a nitpick but I do not think this is strictly correct. Certainly waves show this interference pattern but the experiment can be done such that equating the particles to waves does not hold.

Do the experiment by shooting a single electron at your double-slits. Wait five minutes and shoot a second electron and so on. Let the experiment run a long time and wonder of wonders you still see the interference pattern (this experiment has in fact been done with these results).

So, how does a single electron interfere with itself? In order to see what we see happen the electron needs to pass through BOTH slits. This is one reason why QM is so disconcerting and non-intuitive as our brains tell us one particle can ONLY go through ONE slit...not both.
But even with one slit we can still say the photon behaves as a wave. Indeed, it HAS to behave as a wave for the interference effects to occur.



I think this experiment is one pillar of the many worlds interpretation. The idea being that the particle travels all paths and as such there is a reality where the particle goes through the left slit and one where it goes through the right. Why they interfere with each other if these two realities must be distinct and non-interactive is way beyond me (hell...most of this is way beyond me anyway but it is fun to think about).

IMO this is where the MWI breaks down. If we think of the universe as branching into two different universes (photon passes through slit 1 in universe 1 and photon passes through slit 2 in universe 2), then how to explain the interference pattern? In neither universe does it pass through both slits, so in neither universe can there be any interference.

Now in the FAQ I see that they say worlds only split when a measurement is made. This version of the MWI seems to solve the problem I just mentioned, but at the cost of eliminating the original reason for the MWI: to answer how ONE photon can go through BOTH slits.

That said, I remember a paper by David Deutsch titled "Photographing Alternate Universes", or something like that. He described (not very convincingly, IMO) how to take a "photograph" of one of the alternate histories.
:dubious:

JasonFin
01-20-2005, 02:36 PM
So, how does a single electron interfere with itself? In order to see what we see happen the electron needs to pass through BOTH slits. This is one reason why QM is so disconcerting and non-intuitive as our brains tell us one particle can ONLY go through ONE slit...not both.

I think this experiment is one pillar of the many worlds interpretation. The idea being that the particle travels all paths and as such there is a reality where the particle goes through the left slit and one where it goes through the right. Why they interfere with each other if these two realities must be distinct and non-interactive is way beyond me (hell...most of this is way beyond me anyway but it is fun to think about).This is the argument for the many-worlds-interpretation that David Deutsch uses in his popular writings. It's not an argument that is commonly seen in the serious physics literature. It's very elegant in how it reflects the general feel of more technical arguments for this interpretation, but taken literally as a physical theory about reality it is less convincing. The problem is that a major advantage of the many-worlds interpretation is its ability to explain why entities that fundamentally obey a wave equation nonetheless appear to us to have nearly-precise positions, velocities, and energies. Making the fundamental assumption that they actually do have fundamental positions wastes this explanatory work.

The physicist Julian Barbour (http://www.amazon.com/exec/obidos/tg/detail/-/0195145925/qid=1106246801/sr=8-13/ref=sr_8_xs_ap_i13_xgl14/103-8136702-6652664?v=glance&s=books&n=507846) does advocate taking this view literally; he proposes that particles are fundamental and wave-like behavior is strictly a result of interference. Though mathematically consistent, this leads to the awkward conclusion that particles interact in a way that leads to wave-like behavior which due to certain other processes often appears to us like particle-like behavior. It's much more common to view the wave behavior as truly fundamental; on this interpretation, wave-particle "duality" is an illusion that gained acceptance due to our poor understanding of quantum mechanics.

One problem is that particles don't behave like people expect them to behave溶ot even in individual "worlds." Newton's laws of mechanics simply don't hold, at all, except as a large-scale limit on average behavior. For technical reasons related to the Heisenberg uncertainty principle, particles with true physical positions simply cannot have true physical velocities or energies, even in single worlds, so non-classical behavior is unavoidable. In individual worlds, particles can take any path whatsoever from one point to another, unconstrained by conservation of energy or momentum.

The reason we see more predictable behavior is that in quantum mechanics a world (a classical state of a system) is considered to have a complex phase, and worlds with equal but opposite phases can cancel each other out. The worlds that remain uncanceled are largely those where something akin to classical mechanics appears to hold, at least at large scales and on average. This can be mathematically derived from the basic mathematics of the theory (see Feynman's path integral, though I couldn't find any good nontechnical cites). One can say that the virtual particles sometimes invoked in quantum-mechanical explanations exist because violations of energy conservation that last a very short time can still have measurable consequences.

But even with one slit we can still say the photon behaves as a wave. Indeed, it HAS to behave as a wave for the interference effects to occur.



IMO this is where the MWI breaks down. If we think of the universe as branching into two different universes (photon passes through slit 1 in universe 1 and photon passes through slit 2 in universe 2), then how to explain the interference pattern? In neither universe does it pass through both slits, so in neither universe can there be any interference.

Now in the FAQ I see that they say worlds only split when a measurement is made. This version of the MWI seems to solve the problem I just mentioned, but at the cost of eliminating the original reason for the MWI: to answer how ONE photon can go through BOTH slits.In modern versions of the many-worlds interpretation, world-splitting is not viewed as an occult process that takes place everywhere simultaneously at the most convenient moment. It is an ordinary physical process that happens in a continuous fashion, at specific places and times, and which propagates via classical information channels.

Quoting from the FAQ I linked to earlier, on "Why do worlds split?"Worlds, or branches of the universal wavefunction, split when different components of a quantum superposition "decohere" from each other [7a], [7b], [10]. Decoherence refers to the loss of coherency or absence of interference effects between the elements of the superposition. For two branches or worlds to interfere with each other all the atoms, subatomic particles, photons and other degrees of freedom in each world have to be in the same state, which usually means they all must be in the same place or significantly overlap in both worlds, simultaneously.

For small microscopic systems it is quite possible for all their atomic components to overlap at some future point. In the double slit experiment, for instance, it only requires that the divergent paths of the diffracted particle overlap again at some space-time point for an interference pattern to form, because only the single particle has been split.

Such future coincidence of positions in all the components is virtually impossible in more complex, macroscopic systems because all the constituent particles have to overlap with their counterparts simultaneously. Any system complex enough to be described by thermodynamics and exhibit irreversible behaviour is a system complex enough to exclude, for all practical purposes, any possibility of future interference between its decoherent branches. An irreversible process is one in, or linked to, a system with a large number of internal, unconstrained degrees of freedom. Once the irreversible process has started then alterations of the values of the many degrees of freedom leaves an imprint which can't be removed. If we try to intervene to restore the original status quo the intervention causes more disruption elsewhere.

In QM jargon we say that the components (or vectors in the underlying Hilbert state space) have become permanently orthogonal due to the complexity of the systems increasing the dimensionality of the vector space, where each unconstrained degree of freedom contributes a dimension to the state vector space. In a high dimension space almost all vectors are orthogonal, without any significant degree of overlap. Thus vectors for complex systems, with a large number of degrees of freedom, naturally decompose into mutually orthogonal components which, because they can never significantly interfere again, are unaware of each other. The complex system, or world, has split into different, mutually unobservable worlds.
That said, I remember a paper by David Deutsch titled "Photographing Alternate Universes", or something like that. He described (not very convincingly, IMO) how to take a "photograph" of one of the alternate histories.
:dubious:Deutsch is a very smart guy, but I suspect your skepticism is justified. No doubt he has some defensible way as describing what he is proposing as technically a photograph "of" an alternate history, but advocates of other interpretations would describe it as a reflection of some altogether different phenomenon. Deutsch has a tendency, which I have noticed in many of his papers (some available here (http://www.qubit.org/people/david/index.php?path=Home), to ignore alternative interpretations of what is going on. This doesn't mean anything he says is straightforwardly wrong, but it does drive the point home that the standard description of quantum mechanics is largely a historical accident. If scientists in the 1930s had known what we know today about decoherence phenomena, a more literal interpretation in many-worlds terms could well have become the norm, and quantum mechanics might have seemed much less philosophically problematic.

Omphaloskeptic
01-20-2005, 03:09 PM
The idea of communicating with other "worlds" (quantum branches) is sometimes called the "Everett-Wheeler telephone" (sadly, as already explained, it's forbidden by standard quantum theory). This might help if you want to Google for some more explanations.

CJJ*
01-20-2005, 03:57 PM
Quite frankly (and I'm a layman here), I have never understood the compelling philosophical need for the many-worlds interpretation of quantum mechanics. What are the serious defects such an interpretation is trying to patch?

My understanding of the Copenhagen interpretation (CI) is a little different from how popular literature explains it, but I think the main point is that the wave function represents a coding of all the known information; it is not in and of itself a real thing. This is no different than, say, interpreting Newton's laws as a definition of the concept of force: Force itself is not real in the sense that it cannot be measured other than thru use of Newton's laws. Similarly, the wave equation exists only as something defined by the laws of QM.

In classical mechanics, the exact initial state of a system can be unambiguously set. The CI, however, says that this can never be done: You cannot, for example, guarantee the position and velocity of an electron prior to its action in the aforementioned double-slit experiment; if you try to, Heisenberg's uncertainty relation (which is derivable from the postulates of QM, and is therefore not an added axiom) kicks in. As such, I think the CI would say that a single electron in the double-slit experiment passes thru one and only one of the slits, though you cannot predict exactly which one. The interference pattern emerges only after multiple trials in which the initial conditions of the electrons used are never identical (whether or not the electrons are shot all at once or sequentially with a 5 min. or so gap between is therefore immaterial).

I would guess such an interpretation could be verified by shooting a single electron thru the slits and checking, but my guess is this is way beyond the threshold for experimental error to detect. Moreover, one might argue the CI itself forbids any experiment from being able to detect even this, as it would essentially be a confirmation of the particle nature of the electron. For me, this is too strict an interpretation of the CI, and it hardly seems a justification for granting reality to mathematical constructions that could be interpreted as generating multiple-worlds.

I guess my interpretation comes down to saying that singly electrons act as particles, but given the inability to guarantee non-conjugate quantities associated with this particle (such as position and velocity)--an inability built into the foundation of QM--electrons demonstrate some wave properties in experiments where they are used en masse.

I would be grateful to the QM experts here to explain whether or not my understanding of CI is correct and point out defects which are more reasonably solved by the MWI. Thx:-)

Mathochist
01-20-2005, 05:24 PM
Quite frankly (and I'm a layman here), I have never understood the compelling philosophical need for the many-worlds interpretation of quantum mechanics. What are the serious defects such an interpretation is trying to patch?

It's a philosophical problem, not a physical one, strictly speaking. The question is what QM tells us about the fundamental nature of reality.

JasonFin
01-20-2005, 08:17 PM
Quite frankly (and I'm a layman here), I have never understood the compelling philosophical need for the many-worlds interpretation of quantum mechanics. What are the serious defects such an interpretation is trying to patch?

My understanding of the Copenhagen interpretation (CI) is a little different from how popular literature explains it, but I think the main point is that the wave function represents a coding of all the known information; it is not in and of itself a real thing. This is no different than, say, interpreting Newton's laws as a definition of the concept of force: Force itself is not real in the sense that it cannot be measured other than thru use of Newton's laws. Similarly, the wave equation exists only as something defined by the laws of QM.

In classical mechanics, the exact initial state of a system can be unambiguously set. The CI, however, says that this can never be done: You cannot, for example, guarantee the position and velocity of an electron prior to its action in the aforementioned double-slit experiment; if you try to, Heisenberg's uncertainty relation (which is derivable from the postulates of QM, and is therefore not an added axiom) kicks in. As such, I think the CI would say that a single electron in the double-slit experiment passes thru one and only one of the slits, though you cannot predict exactly which one. The interference pattern emerges only after multiple trials in which the initial conditions of the electrons used are never identical (whether or not the electrons are shot all at once or sequentially with a 5 min. or so gap between is therefore immaterial).

I would guess such an interpretation could be verified by shooting a single electron thru the slits and checking, but my guess is this is way beyond the threshold for experimental error to detect. Moreover, one might argue the CI itself forbids any experiment from being able to detect even this, as it would essentially be a confirmation of the particle nature of the electron. For me, this is too strict an interpretation of the CI, and it hardly seems a justification for granting reality to mathematical constructions that could be interpreted as generating multiple-worlds.

I guess my interpretation comes down to saying that singly electrons act as particles, but given the inability to guarantee non-conjugate quantities associated with this particle (such as position and velocity)--an inability built into the foundation of QM--electrons demonstrate some wave properties in experiments where they are used en masse.

I would be grateful to the QM experts here to explain whether or not my understanding of CI is correct and point out defects which are more reasonably solved by the MWI. Thx:-) "Copenhagen" interpretation is ambiguous. Though often used as a synonym for the most common mainstream interpretation of quantum mechanics taught in textbooks and referenced in the popular literature, it technically refers primarily to the views of Niels Bohr, which included certain radical views about the nature of reality which are not generally accepted. Bohr felt it was impossible in principle to speak meaningfully about microscopic objects except in relation to the macroscopic instruments used to measure them.

The more mainstream interpretation, as distinguished from the Copenhagen interpretation, is ambivalent as to whether the wave function is a real thing; different people will give you different answers. Different people also disagree about whether the world can accurately be described in terms of a wave-particle duality. The defining attribute of the type of interpretation of quantum mechanics taught in textbooks is that physical systems behave in a fundamentally different way when they're being measured than when they aren't. When left alone, a system evolves according to a wave equation, which usually makes the positions of particles become spread out in space. When measured, a system discontinuously and instantaneously jumps into a state such that whatever characteristics are being measured take on precise values.

There's one extremely significant difference between mainstream interpretations of quantum mechanics, including the Copenhagen interpretation, and what you describe. It is generally accepted that when we know the wave function of a system, we know all that can possibly be known about the system. It doesn't represent our ignorance. It is not merely unknowable which slit an electron traveled through; there is in fact no truth to the statement that it took one path rather than another while it was unobserved.

This may seem radical, but there are extremely good reasons for believing the world is like this. The problem is that there are predictions that provably cannot be reproduced and experimental results that cannot be explained if it is assumed that any physically relevant facts exist that are not included the wave function, such as classical positions, momenta, etc. These are known as hidden variables. Any theory at all that includes hidden variables cannot explain the correlations between distant results in some experiments without resort to faster-than-light (or backwards-in-time) signals that behave in a very mathematically awkward way.

As to why the many-worlds interpretation is arguably superior, some of the defects of the mainstream interpretation are:

It applies completely different physical rules to systems depending on whether or not they are observed.
It must rely on outside knowledge to make predictions, since the theory itself does not include any rules at all for determining what an observer is or whether a system should be regarded as observed.
It requires faster-than-light influences葉hough only of a subtle and bizarre sort that cannot possibly be used to transfer information, even in principle.
It is non-deterministic: even given perfect knowledge, the state a system will be in after it is measured can be predicted only in terms of probabilities.
It does not reproduce the predictions of classical physics in the appropriate macroscopic limit (though many people still say that it does; there are unique aspects of the quantum-mechanical behavior of chaotic systems that are not widely known).
It incorporates a process accompanying measurement that is irreversible in time, despite the fact that the governing equations of both classical and quantum mechanics are time-symmetric.
It cannot be used to explain unambiguously what aspects of our description of a system are actually physically real, let alone what the real state of a system actually is.

Some of these are presented in the popular (and sometimes scientific) literature as fundamental features of quantum mechanics, though in fact they only characterize the way the theory is usually interpreted. The many-worlds interpretation, in its modern form, has many advantages:

It can state unambiguously which objects described by the theory are physically real (i.e. that the wave function is an accurate description of reality).
It applies a single set of rules to all physical phenomena, whether or not they are observed葉he ordinary wave equation simply applies to everything, all the time, without exception.
It can predict (or should be able to, in principle) the results of experiments without ever needing to make difficult judgment calls about which set of rules to apply.
It can be applied usefully in extremely alien environments in which the familiar concepts other interpretations make use of do not exist (some interpretation along these lines is thus practically required for studying quantum cosmology).
It does not require faster-than-light influences to explain predictions or experimental results.
It can be extended to relativistic quantum mechanics without difficulty (some interpretations can't).
It can go further than other interpretations in explaining why the wave function corresponds to the probabilities we observe in the way that it does.
It requires only extremely simple and limited metaphysical assumptions, beyond the mathematical framework of quantum theory that all interpretations share.
Without needing to make additional assumptions, it can go further than other interpretations towards explaining how the interactions of microscopic quantum-mechanical objects give rise to the classical reality we observe.
It is strictly deterministic: a given initial state leads to one and only one final state which can in principle be predicted perfectly given full knowledge of the initial state (this final state is sometimes one in which many worlds exist).

With respect to your example, I don't know whether the double-slit experiment has ever been performed with single electrons launched one-at-a-time at controlled times. Doing so would be much more difficult than it sounds; an electron isn't a macroscopic object you can grab onto. This is not regarded as an important problem葉he obvious conclusion is that if we were to graph the results of many such trials, we would see a normal interference pattern. It is relatively easy to produce an apparatus that emits electrons, with well-defined momenta, at such a low rate (though still at uncontrollable intervals) that most of the time at most one electron is traveling through the apparatus at once and the arrivals of electrons are detected when they occur as individual events. If actual control over the intervals (which should be possible in principle) were found to make a difference, it would be a serious violation of the laws of quantum mechanics, independent of interpretation.

Mathochist
01-20-2005, 08:59 PM
(CI) is non-deterministic: even given perfect knowledge, the state a system will be in after it is measured can be predicted only in terms of probabilities.

If you're suggesting that MWI is any better, you're making a huge dodge. MWI is non-deterministic in its completion, but the results which obtain in any given "world" are still just as non-deterministic, and the portion of "worlds" at an event with a given observation give the exact same probabilities.

(MWI) can state unambiguously which objects described by the theory are physically real (i.e. that the wave function is an accurate description of reality).
(MWI) applies a single set of rules to all physical phenomena, whether or not they are observed葉he ordinary wave equation simply applies to everything, all the time, without exception.
(MWI) can predict (or should be able to, in principle) the results of experiments without ever needing to make difficult judgment calls about which set of rules to apply.

There are variations on single-world interpretations which also avoid the so-called "measurement problem", like that proposed by Roger Penrose. Further, these ideas are in theory testable. Can you take a wild guess which I'll lean towards between a testable theory and MWI?

It does not require faster-than-light influences to explain predictions or experimental results.

"Influence" is a loaded term. You (mildly) disclaimed earlier, but here you use the implication that there's some signal travelling in observations of entangled states.

It requires only extremely simple and limited metaphysical assumptions, beyond the mathematical framework of quantum theory that all interpretations share.

Blatantly false. The whole of the theory is metaphysical, and it assumes the existance of uncountably many alternate (and unobservable and untestable) "worlds" spinning off every instance. That's far from "extremely simple and limited".

Whack-a-Mole
01-20-2005, 09:37 PM
With respect to your example, I don't know whether the double-slit experiment has ever been performed with single electrons launched one-at-a-time at controlled times. Doing so would be much more difficult than it sounds; an electron isn't a macroscopic object you can grab onto. This is not regarded as an important problem葉he obvious conclusion is that if we were to graph the results of many such trials, we would see a normal interference pattern. It is relatively easy to produce an apparatus that emits electrons, with well-defined momenta, at such a low rate (though still at uncontrollable intervals) that most of the time at most one electron is traveling through the apparatus at once and the arrivals of electrons are detected when they occur as individual events.

Sorry if I misstated the experiment when I said the electrons popped off once every five minutes. My aim was to indicate, as you mentioned, that you can do the double slit experiment with electrons such that you can be certain each electron is by itself and represents an individual event. The experiment you more accurately described has been done.

As an aside, reading further through the article below, I was surprised to see that the experiment has been done with things as big as Carbon-60 and Carbon 70. This leads me to wonder how big do you have to get before the QM world stops doing its bizzare tricks and the more "normal" macro world we know kicks in? Is it a hard line or fuzzy?

But in 1961 Claus Jnsson of Tbingen, who had been one of Mllenstedt's students, finally performed an actual double-slit experiment with electrons for the first time (Zeitschrift fr Physik 161 454). Indeed, he demonstrated interference with up to five slits. The next milestone - an experiment in which there was just one electron in the apparatus at any one time - was reached by Akira Tonomura and co-workers at Hitachi in 1989 when they observed the build up of the fringe pattern with a very weak electron source and an electron biprism (American Journal of Physics 57 117-120).

<snip>

Since then particle interference has been demonstrated with neutrons, atoms and molecules as large as carbon-60 and carbon-70.

SOURCE: Physics Web (http://physicsweb.org/articles/world/15/9/1)

Omphaloskeptic
01-20-2005, 09:51 PM
There are variations on single-world interpretations which also avoid the so-called "measurement problem", like that proposed by Roger Penrose. Further, these ideas are in theory testable. Can you take a wild guess which I'll lean towards between a testable theory and MWI?
Apples and oranges. CI and MWI are both interpretations of a single underlying physical theory, which I'll call "standard quantum mechanics" (SQM). Neither CI nor MWI is technically "testable" because they are just intuitively convenient ways of understanding the formal theory. SQM vs. PQM (Penrose's theory) is empirically testable. Deciding between CI/MWI (if you accept SQM) is just a matter of personal preference (though one may turn out to provide more or different insight into the theory than the other).

(NB: "Copenhagen" and "Many Worlds" both mean rather different things to different people, which makes it difficult to talk about. So maybe I'm just using different definitions of CI and MWI than you are. Since you consider MWI untestable, though, I assume you're talking about the "interpretation" part rather than the underlying theory.)

Mathochist
01-20-2005, 09:59 PM
As an aside, reading further through the article below, I was surprised to see that the experiment has been done with things as big as Carbon-60 and Carbon 70. This leads me to wonder how big do you have to get before the QM world stops doing its bizzare tricks and the more "normal" macro world we know kicks in? Is it a hard line or fuzzy?

Penrose's theory is, I think, a good starting point. I mainly know it in the form of a superposition of position states of "lumps" of matter. Basically, he notes that there is a difference in the energies of the gravitational fields of the two configurations. This energy uncertainty is related to an uncertainty in time, since energy and time are conjugate variables (like position and momentum). The conjecture is that this is the "characteristic time" over which the superposition decoheres into a statistical mixture: one state or the other. The greater the energy difference, the shorter the time. He extrapolates this to the case of Schrdinger's Cat, by noting that such a macroscopic object as a cat creates an enormous difference in energy between the living and the dead states, so the decoherence timeframe is tiny.

The test he proposes is basically to take a crystal and use an interferometer to bump it with a photon. The superposition is of two photon states, one of which strikes the crystal and bumps it exactly half the lattice length, the other of which doesn't strike the crystal. This means that the crystal (and every atom in it) is in a superposition of two position states. The energy difference for each atom is added up, which is why you use a crystal to multiply the energy difference for each atom by the number of atoms and cut the time scale down from a rather large amount (for an atom) to something on the order of the time it takes for the crystal to spring back to the rest state. I don't have the details of the rough calculations on hand, though I've seen them at half a dozen talks he's given over the last decade. The idea is that if he's right, the interferometer will behave one way and if he's wrong it won't. It's down to the technical question of how to engineer the experimental setup with a fine enough calibration.

Mathochist
01-20-2005, 10:04 PM
I assume you're talking about the "interpretation" part rather than the underlying theory.

Yes, which as I noted elsewhere renders the question one of philosophy rather than of physics. My point is that these stated advantages are ones that can never be tested since they're mere interpretation, while there are in-principle-testable refinements of the underlying theory (with its unitary/nonunitary "measurement problem") which answer them. Basically, as regards the "two mechanisms", MWI just pushes one into an unobservable realm and says that it's all unitary. As far as observations go, there are still two different mechanisms going on and still purely nondeterministic characters to the theory.

lektrikpuke
01-21-2005, 01:01 AM
To communicate you would need either of two possibilities (let's overlook the fact that there may be no one to communicate with):

1: A telephone. Okay, maybe not a telephone, but something that could interpret the signals (whatever) we send and make it intelligible at their end (again I'm assuming they are there and they're intelligent enough to build, let alone operate a telephone).

2: A bomb. Again, I'm being cute, but something that we can use to influence they're environment in such a way that they can interpret as intelligible. Although, on the surface this may appear to be similar to option one, in that interpretation is involved, a bomb is a little more like a one-sided conversation, wherein, it is hoped they can't communicate back.

Personally, I opt for the 2nd option. You get lots of free land as a side effect.

JasonFin
01-21-2005, 11:32 AM
Yes, which as I noted elsewhere renders the question one of philosophy rather than of physics. My point is that these stated advantages are ones that can never be tested since they're mere interpretation, while there are in-principle-testable refinements of the underlying theory (with its unitary/nonunitary "measurement problem") which answer them. Basically, as regards the "two mechanisms", MWI just pushes one into an unobservable realm and says that it's all unitary. As far as observations go, there are still two different mechanisms going on and still purely nondeterministic characters to the theory.Some supporters of the many-worlds interpretation (as on this FAQ (http://www.hedweb.com/manworld.htm)) would say that the existence of multiple worlds can be derived from the mathematical formalism of quantum mechanics, and that the only metaphysical assumption they add is to take that formalism seriously. In reality, the situation is more complicated: it's more accurate to say that if the interpretation is true, it must in principle be derivable from the mathematical formalism.

Assuming the equations we use to describe quantum mechanics are accurate, this means the interpretation is testable, at least in a weak way: it must be possible given sufficient computing power (or, more realistically, knowledge of sufficiently good approximations) to demonstrate via simulation that quantum systems brought into contact with a complex environment that is also modeled quantum-mechanically either do or do not cause that environment to split into different "worlds."

A significant amount of work has been done along those lines in the past 20 years. The phenomenon of "decoherence" is a recognized physical effect that was first discovered in the theoretical study of quantum computers. Roughly speaking, it refers to how the wave function of a system, when interacting with a much larger system, can split into distinct components that no longer interfere quantum-mechanically with one another in a measurable way. This causes quantum computation to fail, and it is also considered to be a physical description of the dynamics via which worlds can "split."

The much harder part is to show that these distinct branches of the wave function will under normal conditions resemble classical worlds, but recently there has been great progress along these lines. This paper (http://www.arxiv.org/abs/quant-ph/0105127) is an excellent (but pretty technical) summary of the present state of our understanding. It describes the effect of decoherence from the perspective of several different interpretations, including the MWI and others. From the abstract:Decoherence is caused by the interaction with the environment. Environment monitors certain observables of the system, destroying interference between the pointer states corresponding to their eigenvalues. This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Einselected pointer states are stable. They can retain correlations with the rest of the Universe in spite of the environment. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly non-local "Schrodinger cat" states. Classical structure of phase space emerges from the quantum Hilbert space in the appropriate macroscopic limit: Combination of einselection with dynamics leads to the idealizations of a point and of a classical trajectory. In measurements, einselection replaces quantum entanglement between the apparatus and the measured system with the classical correlation.

Mathochist
01-21-2005, 11:46 AM
if the interpretation is true, it must in principle be derivable from the mathematical formalism.

Assuming the equations we use to describe quantum mechanics are accurate, this means the interpretation is testable

I really must call bullshit here. The predictions of observations are entirely within the scope of the mathematical structure of the theory, and MWI makes no different predictions than the old-school CI. The only difference is pure metaphysics. Try reading, say, anything by Jeff Bub.

(decoherence) causes quantum computation to fail, and it is also considered to be a physical description of the dynamics via which worlds can "split."

Decoherence is perfectly explicable by an addition (which has been done) to basic axiomatic QM. In particular, it's explicable without reference to worlds "splitting off". To say that worlds split over decoherence makes the argument no more convincing than to say they split over the Von Neumann nonunitary evolution. Occam's Razor makes quick work of all those stubbly alternate worlds.

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