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.

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

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.

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.

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. 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.

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).

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.

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:

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 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—not 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.

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?”

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, 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.

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.

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:
[ul]
[li]It applies completely different physical rules to systems depending on whether or not they are observed.[/li][li]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.[/li][li]It requires faster-than-light influences—though only of a subtle and bizarre sort that cannot possibly be used to transfer information, even in principle.[/li][li]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.[/li][li]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).[/li][li]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.[/li][li]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.[/li][/ul]
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:
[ul]
[li]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).[/li][li]It applies a single set of rules to all physical phenomena, whether or not they are observed—the ordinary wave equation simply applies to everything, all the time, without exception.[/li][li]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.[/li][li]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).[/li][li]It does not require faster-than-light influences to explain predictions or experimental results.[/li][li]It can be extended to relativistic quantum mechanics without difficulty (some interpretations can’t).[/li][li]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.[/li][li]It requires only extremely simple and limited metaphysical assumptions, beyond the mathematical framework of quantum theory that all interpretations share.[/li][li]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.[/li][li]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).[/li][/ul]
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—the 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.

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.

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?

“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.

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”.

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?

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.)

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 Schrödinger’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.

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.