Quantum entanglement and parallel universes

6 Comments
Posted October 23rd, 2008 in Quantum 2. Tags: , , , .

In 2007 I noticed a confusing post on Slashdot quoting from an article claiming that the no cloning theorem prevented entangled particles from being used for faster-than-light (FTL) communication. I had never heard of any FTL implications of the no cloning theorem, so I responded to this post to see if a less-dumb scientist could explain this connection to me (incidentally, I’m still waiting- someone please educate me!).

While the resulting conversation didn’t shed any light on the purported FTL implications of the no cloning theorem, DrVomact asked a question that eventually led to an enjoyable discussion (edited for clarity) about quantum entanglement and parallel universes.



Written by Dumb Scientist on June 04 2007, @03:34PM

I am a physicist, and I haven’t heard of a connection between the no-cloning theorem and using entanglement for FTL communication. Can anyone explain how they’re connected?

Before learning about entanglement, it’s necessary to understand a concept known as a quantum “superposition.” A quantum superposition is a bizarre state in which particles occupy two or more mutually incompatible states until they’re observed. For example, a particle can be in a superposition where it exists in both New York and Los Angeles at the same instant. But as soon as someone looks at the particle it ‘collapses’ so that it exists in only one city, not both. Quantum mechanics very explicitly states that you can’t predict which city the particle will appear in, even in principle. The exact city in which the particle exists prior to being observed is thus literally unknowable. (This paragraph wasn’t in the original post. Dumb mistake- I should have remembered to define ‘superposition’.)

Now consider two particles which are both in superpositions of being either horizontally or vertically polarized. The “spookiness” of entanglement lies in the fact that the two particles can be created in a state where (for example) their polarizations have to be opposite (perpendicular). Thus even though each particle is in a (literally unknowable) superposition of horizontal and vertical, if you measure the first particle and find that it’s horizontally polarized, that automatically means that a measurement on the second particle will show that it’s vertically polarized. This correlation occurs even if the second measurement is made so soon after the first that light doesn’t have time to travel from the first particle to ‘tell’ the second particle which way the first particle collapsed. There also doesn’t seem to be any distance restriction- the two particles could be on opposite sides of the galaxy and still be correlated.

At first glance, this seemed remarkable. At second glance, it just seemed like conservation of momentum. For example, suppose the two particles are formed by the decay of another particle that initially had zero angular momentum. Then the sum of the two resulting particles’ angular momenta needs to be zero so the final and initial momenta are the same (this is the same reason that guns have recoil). As a result, the polarizations must be equal and opposite- for a fairly mundane reason. The “spookiness” Einstein talked about lies in the fact that quantum mechanics says that both particles are literally horizontally and vertically polarized, up until the point where the first one is “collapsed” onto horizontal. Then the second particle instantly collapses onto vertical, which occurs even if both particles are separated by a great distance. Einstein took this spookiness to mean that quantum mechanics must be incomplete– that each particle really did have a well defined state before the collapse, but current quantum theory simply can’t describe it.

Amazingly, in 1966 a physicist named John Bell found a way to experimentally test the issue using “Bell inequalities”. Quantum mechanics correctly predicted the outcome of these experiments- carried out mainly in the early 1980s- up to high sigma values. In other words, the experiments convincingly proved Einstein wrong.

So we have to adjust to the reality of absurdities like ‘spooky’ FTL correlations. The problem with using these correlations for FTL communication is that each measurement just gives you a random horizontal or vertical outcome. In fact, the only “interesting” feature of making these measurements is that when you finally compare your results with the person who has the other entangled particles, you find that your results correlate perfectly. This isn’t useful for communication, though. Entanglement could only be used for FTL communication if quantum mechanics had small nonlinear terms which would allow one party to “bias” the collapse preferentially onto horizontal or vertical. This would allow for the transmission of binary data or Morse code. Unfortunately, decades of testing have shown that any nonlinearities in the Schrodinger or Dirac equations underlying quantum mechanics are very, very small. (Note that even though entanglement isn’t useful in an FTL sense, it can be used to facilitate teleportation.)

Bummer. On the other hand, I think the existence of FTL communication would automatically lead to the feasibility of backwards-in-time communication, so at least we don’t have to worry so much about being killed by assassins hired over the time-phone by future versions of our own grandchildren…



Written by DrVomact on June 04 2007, @07:36PM

Interesting, but I’m afraid that your explanation is far too clear…and it’s typically a bad sign when I understand something that’s supposed to be as complicated as quantum mechanics…in other words, I’m probably not understanding it. Let me put it this way: I need some help to see the “spookiness”. You say:

…Thus even though each particle is in a (literally unknowable) superposition of horizontal and vertical, when you measure the first particle and find that it’s horizontally polarized, a measurement on the second particle will show that it’s vertically polarized….

Not being a physicist, I have no clue what you mean by “superposition of horizontal and vertical” polarization. But that’s OK…you’re referring to some sort of property or properties that particles can have, and that can be measured. If I understand correctly, you are saying that pairs of these particles are created in such a way that these qualities (whatever they are) must be in a contradictory relationship. So if you measure the (whatever) of one particle, you know that the other particle has the opposite (whatever) quality. But why is that mysterious, or spooky?

Let’s say I have a magic particle maker. When I press the trigger, it emits two particles in opposite directions. The particle maker is constructed in such a way that each time I shoot off a particle pair, one of the particles must be blue, and the other white. However, the colors are assigned randomly so that I can never predict whether the particle shot off to the left will be blue or white–I just know that the particle shot off to the right is of the opposite color. Now let’s say that I’ve set up a game of catch-the-particle with Joe and Bill. Joe is standing a mile to my left, and Bill is a mile to my right. Due to the nature of my particle maker, I know that if Joe receives a blue particle, Bill has just got beaned by a white one, and vice versa. That doesn’t seem spooky to me…it doesn’t even seem particularly interesting.

I have a feeling that what I’m missing is tied up with the word “unknowable” that you used in your explanation. It’s not at all clear to me what you meant by that…if some object has a quality that can be measured, then that quality is not “unknowable” just because I haven’t measured it yet. I’m sure it’s not news to you that “unknown” is not the same as “unknowable”. So what did I miss? I think this is the crux of the matter…if you could make this clear to me, I’d be mighty grateful.



Written by Dumb Scientist on June 04 2007, @11:02PM

First of all, photons (particles of light) behave as though they’re waves. There’s a lot of subtlety hidden in this statement, but it’s not relevant now: just picture a single particle of light traveling into the screen while “waving” up and down pretty much like a water wave. This is a vertically polarized photon, and it can be easily distinguished from a horizontally polarized photon which travels into the screen while “waving” left to right instead of up and down. When I say “easily distinguished”, I mean that you can place a detector in the photon’s path that produces a “click” if the photon is horizontally polarized, and a “beep” if the photon is vertically polarized.

Now, according to the math of quantum mechanics, it’s possible for a photon to be in a “superposition state” of horizontally and vertically polarized. This is sometimes described by saying that the photon is traveling into the screen while “waving” at, say, a 45° angle- somewhere in between horizontal and vertical. This isn’t wrong, strictly speaking, but describing the superposition state with a physical geometric angle is very misleading- it obscures the very strangeness we seek to understand. Instead, I’ll say that the math literally implies that the photon is in two states (horizontal and vertical) at once- but when the photon hits the detector it randomly “collapses” onto one of those states. The important point is that quantum mechanics says the photon was in two states at once before the collapse, and that it’s impossible- even in principle- to say which state the photon was in before it hits the detector. Theories which attempt to describe the state of the photon before the collapse (which would eliminate the spookiness and make the situation analogous to your blue-white particle maker) are known as “hidden variable theories.” I don’t think any physicists consider these theories plausible, despite their intuitive appeal.

This “superposition then random collapse” is a bizarre property of quantum theory. But it becomes even more bizarre when you create two entangled photons- two photons that are each in their own superpositions, but created such that if one photon is horizontal, the other is vertical. Now, a measurement performed on one particle doesn’t just collapse its polarization down to vertical or horizontal, it also collapses the other particle into the opposite polarization.

Again, calling this phenomenon “spooky” depends on you believing me when I say that the photons are in a literally unknowable state before they are measured. Before the 1980s, it was plausible to believe that the whole concept of a “superposition” was mere fiction: the misguided result of equations that produced good answers for some problems, but implied a nonsensical property like entanglement in other cases. This is certainly the opinion Einstein held, so you’re in good company.

The problem is that in the 1980s, Aspect et. al. performed a series of experiments that have (nearly) conclusively proven that quantum mechanics is correct: particles do not have precisely determined states before they are measured. The details of these experiments are horrendously complicated (I don’t understand the nitty gritty of them myself, to be honest).

So we have no choice but to abandon the picture that each particle is like a pair of particles which are produced by a machine in pairs of “blue and white.” Each particle is literally blue and white, and there is no way for anyone- no matter how advanced their technology- to be able to tell which is which before they’re measured. It’s not a matter of our inability to describe the situation fully, it’s because the particles are literally in both states at once. Seem a little more spooky now?



Written by DrVomact on June 05 2007, @01:33AM

Thank you for that very lucid explanation, Dumb Scientist.

I should lay my cards on the table. I’m not a physicist, I am something far worse: a philosopher. No I’m not kidding, I have a degree that proves that I, for at least a very short time (my dissertation defence took 3 hours), managed to convince 3 other philosophers that I was right about something. (You have to have spent some time around philosophers to understand why that’s considered a noteworthy achievement.)

Anyway, words like “knowability” and claims about “reality” are the sorts of words that get philosophers excited (well, as excited as they get), so I’ve spent a large portion of my life in sort of a stand-off with the quantum mechanics. A lot of them are plainly fools who write popular books to make money…it’s pretty easy to pick them out. However, there are also some obviously very intelligent and serious people who talk about this stuff, who devote their lives to studying it, and yet utter what appear to be completely crazy propositions, propositions that should clearly be dismissed out of hand as nonsense.

Though I’d like to, I can’t do that–I can’t simply dismiss what appears to be the dominant opinion among the leading scientific luminaries of the day. On the other hand, I can’t bring myself to agree with it either, because I don’t really understand it. Call me stubborn, but I cannot, in good conscience, agree with anything I don’t understand.

I must admit to a major handicap that colors my understanding (or lack of it) of this subject: acute dysmathia. Math and I have never gotten along…I can handle logic (deduction, argument, that sort of thing) just fine; I can use words like razors (especially when I’m more awake than I am now). But numbers and funny squiggles have never meant anything to me. It’s much the same with music; they tell me that there are different notes, that they are called “A” and “G” or whatever, but I can’t repeatably tell one from the other, nor can I recognize melodies–much less discuss the finer points of syncopation.

I have often resented the reliance placed on mathematics by today’s physics; it’s as though a mere tool had become an end in itself. As you mentioned, one opinion I have taken comfort in is that “well, this quantum wave collapse looks good in their mathematical equations, it makes for an internally consistent mathematical description of what they’re trying to explain, but it doesn’t have anything to do with the world I live in. If it did, they could show it to me, and explain it in real words and not mathematical squiggles. But now, you’re telling me, there’s evidence to support the assertion that an object can be in two mutually inconsistent states at once. That is disturbing.

The immediate reply that pops into my head is that if you are empirically demonstrating this “45 degree” spin state, then isn’t that itself a measurement that should precipitate the quantum collapse? –But I’m sure someone’s thought of that.

For the sake of argument, let’s assume that you perform the experiment for me, and I not only understand it, I’m convinced that it shows what you say it shows. You have demonstrated that a “particle” can have two contradictory characteristics (or states, or whatever) at once. In other words, you’ve demonstrated what appears to be a paradox.

Isn’t a such a paradox cause to re-examine our fundamental assumptions, to ask ourselves whether it’s not time to take a totally new view of the universe? I’m not qualified to do that, though I have some vague intuitions about what might have gone wrong. For a couple of hundred years, physicists have vacillated between describing light–and then matter–alternatively in terms of either waves or particles. Mathematical models–and observed results–seemed to support both hypotheses in turn. But aren’t both particles and waves merely metaphors? We derive the notion of a wave from watching ripples in a pond. Then we find that we can apply this mental model of a wave to both sound and light. But not even sound is exactly like a ripple on the surface of a pond–it’s three dimensional, and consists of pulses of compressed and relatively rarefied air. Light “waves” truly have a merely metaphorical connection to water ripples. (Especially since we gave up on the aether.) And particles…well one is tempted to imagine them as little tiny solid balls whirling around in space…but of course any physicist will tell you that’s a ridiculously naive way to think about, say, electrons.

Where am I going with all this? Well,I think I’m going to bed…I’m currently being possessed by the demon of insomnia, and no doubt I’ll rue tomorrow what I wrote today. But I’d like to throw out this one suggestion: if we don’t stick to trying to understand the subject of your “45 degree” experiment as being something like a little marble or a ripple, does this change our understanding of the experiment? Could it be that the experiment says, “throw away your notions about particles and waves”, instead of “the universe is really crazy”? (Yes, I realize that both could be true.)

Goodnight



Written by Dumb Scientist on June 05 2007, @10:44AM

However, there are also some obviously very intelligent and serious people who talk about this stuff, who devote their lives to studying it, and yet utter what appear to be completely crazy propositions, propositions that should clearly be dismissed out of hand as nonsense.

I agree, as do most physicists. Quantum mechanics is difficult to learn, not only because the math is complicated, but also because it’s seemingly nonsensical. If it’s any comfort, every quantum physicist since the 1920s has gone through the same stage of incredulity that you find yourself in now. In fact, that very incredulity has caused physicists to test quantum mechanics to a greater degree of precision than any other theory I can think of off the top of my head. In every experiment, designed to disprove every bizarre property of quantum mechanics, quantum theory has prevailed…

Though I’d like to, I can’t do that–I can’t simply dismiss what appears to be the dominant opinion among the leading scientific luminaries of the day. On the other hand, I can’t bring myself to agree with it either, because I don’t really understand it. Call me stubborn, but I cannot, in good conscience, agree with anything I don’t understand.

It took me a long time to believe in black holes (even after most physicists thought they were conclusively proven to exist) so we agree on this principle.

I must admit to a major handicap that colors my understanding (or lack of it) of this subject: acute dysmathia. Math and I have never gotten along…I can handle logic (deduction, argument, that sort of thing) just fine; I can use words like razors (especially when I’m more awake than I am now).

I’ve always found math and logic to be extremely similar, if not synonymous. They both start with a small collection of axioms and use deductive (and occasionally inductive) logic to arrive at a conclusion. Have you ever tried to learn symbolic logic? If so it’s quite similar to math, and I suspect it would lessen the transition shock because it encodes information into symbols in similar ways.

As you mentioned, one opinion I have taken comfort in is that “well, this quantum wave collapse looks good in their mathematical equations, it makes for an internally consistent mathematical description of what they’re trying to explain, but it doesn’t have anything to do with the world I live in.

Interestingly, wave function collapse is not an internally consistent description of reality. The inconsistency arises because the “collapse” of a quantum state is a completely different process than the normal time evolution of a quantum state; physicists say that it is a “non-unitary” process because it annihilates some of the quantum state (a state which is originally horizontal and vertical collapses onto vertical only, thus annihilating the horizontal part of the state). This is opposed to ordinary “unitary” processes that occur at all other times which don’t affect “how much” of the state exists.

This paradox was resolved (in my opinion) by Everett and Wheeler when they reformulated quantum mechanics without the “collapse” axiom- instead of collapsing onto horizontal or vertical, the entire universe splits in two. In one universe, the observer sees the collapse result in a horizontal state, while the other universe sees a vertical state. This is sometimes referred to as the “many worlds interpretation” of quantum mechanics. It involves fewer axioms, has no internal mathematical inconsistencies such as those created by the notion of wave function collapse, and results in the same physical predictions. The only caveat is that it predicts a nearly infinite number of parallel universes, leading some physicists to say that the many worlds interpretation is cheap in terms of axioms, but expensive in terms of universes.

I mention this only because I find it interesting, but it really doesn’t change any part of the argument here. We still can’t predict what polarization will be observed, it’s just that the many worlds interpretation explains that unpredictability by saying that we can’t be sure which universe we will find ourselves in after the measurement splits the universe into two copies.

The immediate reply that pops into my head is that if you are empirically demonstrating this “45 degree” spin state, then isn’t that itself a measurement that should precipitate the quantum collapse? –But I’m sure someone’s thought of that.

A horizontally polarized photon will definitely produce a click, and a vertically polarized photon will definitely produce a beep. A photon in a 45° spin state will click or beep randomly but over many measurements 50% of the photons produce clicks and 50% produce beeps.

But that doesn’t make the 45° spin state different or special. The exact same random click/beep phenomenon would occur with horizontally polarized photons if you tilted the horizontal and vertical detectors by 45° so the “horizontal” detector clicks when a +45° photon hits it, and the “vertical” detector beeps when a -45° (perpendicular to the +45° state) photon hits it. In this case, horizontally polarized photons would randomly click or beep.

The measurement of a photon’s polarization depends on the relative angle between the polarization and the detector’s orientation. The photon is always detected if the relative angle is 0. If the relative angle is exactly 90°, it will never be detected. For intermediate angles, the statistical odds of a detection are higher when the relative angle is small, and the odds are 50/50 when the relative angle is 45°.

For the sake of argument, let’s assume that you perform the experiment for me, and I not only understand it, I’m convinced that it shows what you say it shows. You have demonstrated that a “particle” can have two contradictory characteristics (or states, or whatever) at once. In other words, you’ve demonstrated what appears to be a paradox. Isn’t a such a paradox cause to re-examine our fundamental assumptions, to ask ourselves whether it’s not time to take a totally new view of the universe?

No more so than time dilation and relativity of simultaneity are proof that we should re-examine the assumptions that went into special relativity. Quantum superpositions are weird, sure, but what would you expect? Our intuition is based on instincts inherited through countless generations of creatures who interacted with the world on a length scale from 0.1 mm to 100 km (the smallest visible dust grain size to the distance to the horizon as seen from the top of a mountain), and at speeds of up to ~200 m/s (approximate tornado windspeed). We shouldn’t necessarily expect our intuition to apply to a radically smaller domain (quantum mechanics becomes important on very small scales like a millionth of a meter) or very fast speeds (where time slows down and simultaneous events are different for different observers).

That’s not to say that this strangeness isn’t reason to go over the theory with a fine-tooth comb. Unfortunately, most of this work was done before I was born. From what I can tell, most 20th century scientists simply refused to believe in this nonsense, so they kept designing experiments to disprove it. Quantum mechanics kept stubbornly being right. After over 80 years of testing the basic theory, I think we have to accept that quantum mechanics reflects the way the universe works, at least until someone can come up with a better experiment.

Where am I going with all this? Well,I think I’m going to bed…I’m currently being possessed by the demon of insomnia, and no doubt I’ll rue tomorrow what I wrote today. But I’d like to throw out this one suggestion: if we don’t stick to trying to understand the subject of your “45 degree” experiment as being something like a little marble or a ripple, does this change our understanding of the experiment? Could it be that the experiment says, “throw away your notions about particles and waves”, instead of “the universe is really crazy”? (Yes, I realize that both could be true.)

We actually have thrown away the notions of particles and waves. Think of it this way: according to quantum mechanics, objects can occupy a continuous spectrum between “particle” and “wave.” Only extremely fast, massive “quantum states” have properties that look like particles. Very slow or very light quantum states have properties that look like waves. In between, they exhibit properties of both. But these extremes (particle and wave) do not define the situation very well; they are simply extreme limits.



Written by DrVomact on June 05 2007, @09:13PM

On my way home from work tonight, I remembered what really bothers me about quantum mechanics. (It’s a long commute.) I’m not really vitally worried about wave/particle stuff, or what the exact characteristics or behaviors of the quantum objects are. What bugs me is “collapse”. The way I understand what I’ve read about QM, I’m required to believe that quantum objects are in some sort of indeterminate state until they are observed. Observation works a change on the observed object (and if “entanglement” is true, it may change another object at a distance, but let’s not worry about that now).

I’ve never understood this. Sometimes, it seems as though the quantum mechanics mean that the change is strictly mechanical–that is, it’s a side effect of the instrument used to observe the object’s behavior–going through a detector somehow changes the particle. But that seems trivial to me–of course something really tiny is going to be affected in some way if you do something to measure it. At other times, it seems to me that the assertion of “collapse” is something much more mysterious, that it has to do with someone learning about the particle’s characteristics. I’ve read some QM articles in which the author seems to be asserting that the act of someone becoming aware of a particle’s behavior somehow changes that behavior. In other words, if the instrument is turned on, and nobody happens to be watching it, then no “collapse” takes place. It’s just when the physicist is paying attention that “collapse” happens.

That has got to be wrong, because nobody but a philosopher could possibly say anything so completely silly. (Like David Hume for example, who thought that the world might possibly go away when he closes his eyes, or Berkeley, who averred that the universe only exists because God pays attention to it.) So if it’s not a mere mechanical side-effect of instrumentation or a mystical by-product of the physicist’s careful observation, what causes quantum “collapse”? (In case you’re wondering, I am not pulling your leg–I am really this confused.)



Written by Dumb Scientist on June 05 2007, @10:01PM

I agree that quantum collapse is bizarre, and leads to paradoxes such as Schrodinger’s Cat and Wigner’s Friend. My personal answer is that there is no such thing as collapse. I use the term because it’s part of the standard “interpretation” of quantum mechanics, but I actually believe that the many worlds interpretation (that I detailed in my last post) is more likely to be true. I say this because:

  1. Quantum collapse is introduced in the standard interpretation as an arbitrary axiom. Therefore, if any different formulation of quantum mechanics can eliminate this axiom while producing identical predictions about real experiments, we should choose the new formulation based on Occam’s razor.
  2. Because quantum collapse is introduced as an axiom, it is not justified in any specific physical manner. In the many worlds interpretation, collapse is simply seen as “the decoherence caused by interacting with a system containing a very large number of particles.” This phenomenon, known as einselection, is rigorously described on a fundamental level. It is a purely mechanical process, not a mystical one. It has nothing to do with knowledge. In fact, it emerges as an obvious consequence of letting an individual particle interact with an object composed of a very large number of particles.
  3. Quantum collapses are a fundamentally non-unitary process, as opposed to every other process in quantum mechanics. It’s inconsistent with the rest of the theory in that respect.
  4. Abandoning quantum collapse by choosing the Many Worlds Interpretation of Everett and Wheeler (and more recently Deutsch) results in exactly the same experimental predictions as the “standard interpretation.”

Now, the many worlds interpretation is weird. It implies that a nearly infinite number of parallel universes exist, each representing the state of the universe if a particle had gone this way or that way, collapsed onto horizontal or vertical, etc. Because we humans are made up of particles, that means that there is a different universe for every possible event in history. There is a universe where the Nazis won WW2, where dinosaurs never became extinct, where Paris Hilton is a college professor, etc. But this weirdness is, in my mind, more than offset by the elegant way that it simplifies quantum theory. Not all physicists agree with me, but the many worlds interpretation is not a fringe view by any stretch of the imagination, and it seems to be growing more accepted with each year…



Written by DrVomact on June 07 2007, @09:02PM

As an enthusiastic fan of fantasy/science fiction from an early age, the “many worlds” (MW) hypothesis appeals to me; how nice, if it were true! However, as a fugitive philosopher, I must remain unsmilingly skeptical.

1. Quantum collapse is introduced in the standard interpretation as an arbitrary axiom. Therefore, if any different formulation of quantum mechanics can eliminate this axoim while producing identical predictions about real experiments, we should choose the new formulation based on Occam’s razor.

With all respect sir, I must frown on your uttering the name of Occam in the same decade of your life as the phrase “many worlds”. I truly cannot think of a more complex hypothesis! Perhaps the MW theory is no more strange than “collapse”, but it’s quite a bit more extravagant. I mean…all those universes!

2. Because quantum collapse is introduced as an axiom, it is not justified in any specific physical manner. In the many worlds interpretation, collapse is simply seen as “the decoherence caused by interacting with a system containing a very large number of particles.” This phenomenon, known as einselection, is rigorously described on a fundamental level. It is a purely mechanical process, not a mystical one. It has nothing to do with knowledge. In fact, it emerges as an obvious consequence of letting an individual particle interact with an object composed of a very large number of particles.

I am pleased to learn that there are no epistemic requirements for MW. That is, I think you’re saying that MW does not rely on the activity of observers–that the generation of new universes occurs whether or not someone is observing quantum events. Do I understand this correctly? If so, that is an improvement. I can’t, however, concur with your claim that MW is “obvious”.

4. Abandoning quantum collapse by choosing the Many Worlds Interpretation of Everett and Wheeler (and more recently Deutsch) results in exactly the same experimental predictions as the “standard interpretation.”

This sounds as though neither hypothesis is empirically verifiable; both explain observed phenomena equally well, correct? I suppose that is no reason to reject either theory; I don’t know what gravity “really” is, but Newton’s–and later, Einstein’s–theories of gravity both had “explanatory power”.

However, I note that in your article, you call the “collapse” theory “bizarre” and MW “weird”. It seems that one sort of strangeness just appeals to you more than the other. (Do you like science fiction, too?) I’ve learned that when scientists (or philosophers) say such strange things, it’s usually because someone is holding a gun to their heads (at least metaphorically). They’re trying to explain something that is itself disturbing; if the explanation that occurs to them is also unpalatable, they may nevertheless be driven to embrace it, because they believe that any explanation is better than none at all

So here is where I am with respect to understanding Quantum Mechanics: something has been observed that is so extremely strange, that even two such strange theories as “collapse” and MW look like good life preservers in a stormy sea. My problem now is that I can’t properly appreciate the emergency–I can’t understand what’s been observed that makes apparently sensible people behave like this. It’s as though I were sitting in an excursion boat, enjoying the scenery, when suddenly a bunch of geeky-looking fellows jump overboard, clutching life jackets. I have the queasy feeling that maybe I should go looking for one of those jackets (preferably one labeled “MW”), but I don’t see any reason to jump overboard just yet.



Written by Dumb Scientist on June 12 2007, @04:16PM

As an enthusiastic fan of fantasy/science fiction from an early age, the “many worlds” (MW) hypothesis appeals to me; how nice, if it were true!

I, too, seem to have been born with the gene for “sci-fi fan” turned on. Unfortunately, as I continue to learn about physics, I find myself less able to suspend disbelief than in years past. As a result, I usually prefer shows that either skip the technobabble completely (such as Firefly and Battlestar Galactica) or manage to get their facts straight occasionally. Stargate Atlantis is the only currently airing show I can think of that even comes close, and it’s only reasonably correct 30% of the time or so…

With all respect sir, I must frown on your uttering the name of Occam in the same decade of your life as the phrase “many worlds”. I truly cannot think of a more complex hypothesis! Perhaps the MW theory is no more strange than “collapse”, but it’s quite a bit more extravagant. I mean…all those universes!

MW is extravagent, but only in the sense that it implies the universe is larger and stranger than we thought it was before. That’s been happening for millenia- every new paradigm in science (heliocentricity, relativity, the realization that our sun is just another star, etc) has shown us that the universe is stranger than we thought it was before.

So most physicists have stopped trying to apply common sense to physics theories. If Einstein had used common sense to say “Of course everyone experiences time at the same rate!”, then we wouldn’t have special relativity today. The only criteria I use are:

  1. Does the theory match experiment?
  2. Is the theory mathematically consistent?
  3. Does the theory have fewer axioms than its nearest competitors?

In this sense, the MW interpretation wins. It is mathematically consistent, involves one fewer assumption than the conventional “collapse” interpretation of quantum mechanics, and it describes experimental results just as well. It’s possible that other interpretations are more accurate (such as Cramer’s “Transactional Interpretation”) but for now I’m leaning towards MW.

I am pleased to learn that there are no epistemic requirements for MW. That is, I think you’re saying that MW does not rely on the activity of observers–that the generation of new universes occurs whether or not someone is observing quantum events. Do I understand this correctly? If so, that is an improvement. I can’t, however, concur with your claim that MW is “obvious”.

Yes, the “collapse” effect in MW is a purely physical phenomenon. It’s difficult to translate the math into english, which is why I’ve said strange things like “generates new universes.” This is a clumsy (and probably overly dramatic) way of describing the process, but it’s the best I can do.

A more accurate way of describing the process would be to say that coupling an isolated quantum system to a much larger system (like a detector) dramatically reduces the off-diagonal terms of the density matrix describing the original quantum system. Since these off-diagonal terms describe interference between the various eigenstates of the quantum system (horizontal and vertical polarization, for example), this process effectively prevents the two eigenstates from interfering with each other. Because the two eigenstates no longer interact, some physicists interpret the resulting density matrix as saying that the two outcomes are now in two “parallel universes” which no longer interact.

But, as you can tell, it’s a lot easier to say “it splits the universe in two.”

This sounds as though neither hypothesis is empirically verifiable; both explain observed phenomena equally well, correct? I suppose that is no reason to reject either theory…

Yes. They’re simply interpreting the math of quantum mechanics in different ways, and the math always gives the same answer for real experiments.

However, I note that in your article, you call the “collapse” theory “bizarre” and MW “weird”. It seems that one sort of strangeness just appeals to you more than the other.

That’s true. The strangeness inherent to MW is of a variety that only contradicts “common sense.” The strangeness inherent to the collapse interpretation is of a variety that involves mathematical inconsistencies and additional (apparently unnecessary) axioms introduced to explain the “collapse” that the MW interpretation explains without any additional axioms.

They’re trying to explain something that is itself disturbing; if the explanation that occurs to them is also unpalatable, they may nevertheless be driven to embrace it, because they believe that any explanation is better than none at all

That’s certainly a possibility. My only real response is that I am willing to provisionally accept an explanation if it fits all available experimental data, is mathematically consistent, and involves the smallest number of arbitrary axioms possible. If an explanation with fewer axioms shows up, or if new experimental evidence contradicts the previous explanation, I’ll drop it like the proverbial hot potato.

Last modified February 6th, 2012
.
    
.

6 Responses to “Quantum entanglement and parallel universes”

  1. DrVomact posted on 2008-12-25 at 13:39

    Thanks for keeping this around…I’d forgotten I’d understood QM even this well (with your help). My final word on the matter: If quantum mechanics is true, then magic works. And I rather like that.

  2. j posted on 2009-05-21 at 16:31

    The “Reference Frame” in the newest Physics Today is blowing my mind.

    • (Ed. note: N. David Mermin wrote an article in the May 2009 issue of Physics Today titled “What’s Bad About This Habit?” The article isn’t available to the public, but Mermin’s main point is that scientists have a bad habit of treating their abstractions as though they’re real, rather than mere tools for making predictions. He calls this practice “reifying abstractions,” spends a lot of time discussing quantum abstractions like wave functions, and claims that the quantum measurement problem is caused by this bad habit, among other things.)

      I sort of agree with him. He’s right to say that quantum theory is useful in the sense that its abstractions are so ridiculous that they can’t be reified with a straight face, and that insight probably extends to more mundane abstractions like electric fields (to some degree).

      But reifying abstractions is necessary. That’s the whole point of science, in fact. One notices a strange phenomenon, and invents an abstraction which accounts for the phenomenon. The next step is to reify the abstraction– to say “let’s assume this abstraction is real, and thus applies to other phenomena as well– what other effects does it predict?”

      If the abstraction only applies to a single phenomenon, it’s probably not useful. So we have to take abstractions seriously otherwise new predictions are impossible. I’m also not sure his example of entanglement is a good one. The BBCJPW teleportation protocol was only developed because someone said “Hey, let’s take entanglement seriously– what can we do with it if it actually exists?”

      On the other hand, I’d probably have to say that “string theorists” scarcely qualify as physicists at all because they’ve gotten so lost in their abstractions that any connection to experimental data got lost years ago…

      • j posted on 2009-05-26 at 21:10

        Well, the ‘blowing my mind’ part was really the quantum measurement ‘problem,’ because I’ve thought a bit about that. But reifying abstractions isn’t necessary – we treat them as abstractions that help us calculate things. Your statement: “let’s assume this abstraction is real, and thus applies to other phenomena as well- what other effects does it predict?” doesn’t need the antecedent – the abstraction doesn’t have to ‘be real’ to apply well to other phenomena. But I mean, ultimately some reification is necessary for us to have any intuitive grasp of things.

      • Your statement: “let’s assume this abstraction is real, and thus applies to other phenomena as well- what other effects does it predict?” doesn’t need the antecedent – the abstraction doesn’t have to ‘be real’ to apply well to other phenomena.

        In Platonic realism, yes, the “reality” of an abstraction is different from how many phenomena are satisfactorily explained by that abstraction.

        But in a pragmatic scientific sense, I think they’re nearly indistinguishable. For instance, one abstraction I believe in is the planet Saturn. The abstraction of a planet with rings in the outer solar system is plausible because it explains images taken by many different telescopes and perturbations in the orbits of nearby planets. If the Saturn abstraction was only supported by a blurred image from a single telescope, I’d be less likely to believe it.

        This principle also extends to everyday life. If you hear disembodied voices, you ask other people if they hear them as well. If they do hear the voice, it’s probably a “real” voice coming from a hidden loudspeaker. If they don’t, it might be a sign of schizophrenia.

        In actual practice, the reality of an abstraction is gauged by how many diverse datasets support it. The only sense in which I can agree with Mermin’s point is that we shouldn’t assume that abstractions apply to datasets that haven’t been experimentally tested. But I don’t think that’s what he’s saying…

  3. Tim Palmer recently suggested that quantum uncertainty is the result of an as-yet-undiscovered fractal nature underlying quantum mechanics. I don’t understand his paper yet but I’m already fascinated.

Leave a reply

Comments at a DH4 level or higher are appreciated.

You may use HTML tags like <a href=""> <blockquote> <em> <strong> <code> in your comment.

.
.
.

Switch to our mobile site