An interesting discussion about Physics and the Multiverse

A discussion at the First Things Blog that started off like this:

"In fact, as philosopher Alexander Pruss explains, the multiverse theory leads to an intriguing conclusion: given a multiverse, it is just as reasonable to assert the resurrection of Jesus as it is to assert the existence of China.

Suppose one thinks both (a) that the multiverse should be invoked in order to explain the origins of life, because the probabilities in one universe are too low (or, presumably, to explain fine-tuning of constants) and (b) the resurrection of Christ is too weird to believe. Well, in an infinite (naturalistic, I suppose) multiverse, someone very much like Christ does in fact get resurrected—it is very unlikely that the particles should move in such a way as to reverse death, but in an infinite multiverse even such unlikely things will happen. Isn’t that an interesting thought? (It reminds one of David Lewis’s observation that on his view the Greek gods exist, though he thought—I don’t know with what justification—that they didn’t exist in our world.)

And, I add, such a thing will happen in infinitely many universes, given an infinite naturalistic multiverse: In infinitely many universes, a monotheistic religious leader named “Jesus” is crucified and rises again on the third day, with all the details being as Christians claim. In our universe, it is claimed by otherwise credible witnesses that this happened—and these witnesses are not contradicted by other alleged eye-witnesses. Why not take their claim at face value, and say that we just are in one of the infinitely many universes where it happens? "

But ended up as a discussion about the theory of the Multiverse with the physicist Stephen M Barr:

http://www.firstthings.com/blogs/firstthoughts/2010/04/05/the-multiverse-and-the-resurrection-of-christ/

Stephen M. Barr April 5th, 2010 10:33 am

A few comments. (1) It is less misleading to say that the multiverse is a single universe with many, diverse PARTS that are not in contact with each other, than to say it contains many “universes”. (2) There is no reason why the multiverse has to be “infinite”. The multiverse could be exponentially larger than the part of the universe we can see, but still finite. That would allow it to explain some of the things it has been invoked to explain. If it is exceedingly large, but still finite, then most of what Alexander Pruss says does not apply. (3) Even if the multiverse were infinite, Pruss’s argument doesn’t really work. Yes, it is true that in an infinite universe even things that have exceedingly low probability of happening on a particular planet will happen on some planet somewhere — indeed, on an infinite number of planets. But that does not make it any less unlikely to happen on this planet. Suppose, for example, that the chance of my drawing a straight flush ten times in a row is 10 to the minus 60. If there is an infinite number of inhabited planets, there may be an infinite number where poker has been invented, and an infinite number where someone has drawn ten straight flushes in a row. That does not make it any more likely that I or anyone else on THIS planet will draw ten straight flushes in a row. The events on different planets are statistically independent of each other.

Barry Arrington April 5th, 2010 10:59 am

Joe, a few weeks ago I put up a post on this subject at uncommondescent.com: Multiverse Mavens Hoisted on Own Petard.

http://www.uncommondescent.com/wp-admin/post.php?action=edit&post=12213

The materialists had some surprising responses in the discussion thread.

Joe Carter April 5th, 2010 11:22 am

Stephen Barr (1) It is less misleading to say that the multiverse is a single universe with many, diverse PARTS that are not in contact with each other, than to say it contains many “universes”.

Let me see if I can clarify what you’re saying. We have two options:

(1) The multiverse is single set that contains diverse parts that are not in contact with each other.(2) The multiverse is a single set that contains diverse universes that are not in contact with each other.

If #1 is less misleading, then it would follow that it is misleading to speak of our own universe as a distinct entity. We can’t really know if our “part” is separate from the multiverse itself or if it is merely a subpart of another system, etc.

How then can we talk about such things as, say, the consistency of natural laws? If we are but a part of a multiverse then it is possible that the other parts of the system may bleed over and interfere/interact with our own.

(2) There is no reason why the multiverse has to be “infinite”. The multiverse could be exponentially larger than the part of the universe we can see, but still finite. That would allow it to explain some of the things it has been invoked to explain. If it is exceedingly large, but still finite, then most of what Alexander Pruss says does not apply.

But wouldn’t it have to be nearly infinite to explain away the anthropic evidence?

It seems if the multiverse is large enough to be used to explain the probability of an extreme improbable event such as the fine-tuning of our universe (or our part of the multiverse) then it seems like it should be large enough to explain away sub-improbable events like a resurrection.

(3) Even if the multiverse were infinite, Pruss’s argument doesn’t really work. Yes, it is true that in an infinite universe even things that have exceedingly low probability of happening on a particular planet will happen on some planet somewhere — indeed, on an infinite number of planets. But that does not make it any less unlikely to happen on this planet.

I originally forgot to include the link to the post which address, somewhat, that objection:
When in infinitely many universes, some set of testimonies not contradicted by any witnesses is true, and in infinitely many universes, the equivalent set of testimonies not contradicted by any witnesses is false, we should suspend judgment. But then I should suspend judgment over the existence of China if a multiverse obtains. For I only know of China on testimony, and in infinitely many universes the testimony is true, and in infinitely many it’s false.

Suppose, for example, that the chance of my drawing a straight flush ten times in a row is 10 to the minus 60. If there is an infinite number of inhabited planets, there may be an infinite number where poker has been invented, and an infinite number where someone has drawn ten straight flushes in a row. That does not make it any more likely that I or anyone else on THIS planet will draw ten straight flushes in a row. The events on different planets are statistically independent of each other.

True, but Pruss’ point is that if we were to hear from eyewitnesses that a person has drawn a straight flush ten time in a row and there were contradictory eyewitnesses, then we’d have no reason to think that, however improbable, we weren’t one of the infinite number of universes that it occurred on.

The point isn’t necessarily that the multiverse makes the resurrection more likely as it does, bur rather, as Pruss says, “highlights the serious problems that multiversists have with probabilistic reasoning.”

We can’t simply use the multiverse theory to explain away one improbable event (fine-tuning) but then deny that it can be used to explain other improbable events. If there are an infinite (or near infinite) number of universes, then we should expect that events of very low probability have occurred an infinite (or near infinite) number of times.

Stephen M. Barr April 5th, 2010 1:31 pm

Joe, I am not sure that you have clarified my statement (1). The multiverse idea is actually very simple. The simplest version of the multiverse idea is that the universe is very large compared to the part we can see. We can only see out to a distance of about 10 billion light years, because light from places farther away than that has not had time to reach us since the time of the Big Bang. So we have a “horizon”. The idea is that the universe is vastly (maybe even infinitely) larger than the part within our horizon. If that is the case, then there are a vast (maybe even infinite) number of places in the universe that are too far from each other to see each other or know what conditions are like in each other’s regions. It is analogous to saying that the earth is a big place containing many little tribes too far from each other to know anything about each other. In and of itself, this is a perfectly reasonable idea. I should add that there are very strong pieces of evidence (as well as convincing theoretical considerations) in favor of an idea called “cosmic inflation”. If inflation is right, then the universe probably IS exponentially larger than the part we can see.

Now add to this the idea that conditions in parts of the universe sufficiently remote from each other to be out of touch with each other might be so different that the laws of physics APPEAR superficially to be different. That is the multiverse idea. People who entertain this (very reasonable) possibility generally assume that the deepest, most fundamental laws of physics are the same everywhere. But the same fundamental laws may be realized in different ways in different places. (In essence, the same equations can have many different solutions.)

You ask, “But wouldn’t it have to be nearly infinite to explain away the anthropic evidence?”

What do you mean by “nearly infinite”? Something is either infinite or it’s finite. If you’re “nearly dead”, you ain’t dead, you’re alive. If the universe is “nearly infinite”, it isn’t infinite, it’s finite. To explain some of the anthropic coincidences the universe only needs to be VERY large, it doesn’t need to be infinite.

The passage you quote from Pruss, which you say “addresses, somewhat, that objection”, i.e my third point, does not, as far I can tell, do so at all.

You quote Pruss as saying that his argument “highlights the serious problems that multiversists have with probabilistic reasoning.” There are problems with probabilistic reasoning if the universe is infinitely big. But, as I noted, the multiverse does not have to be infinitely big, it only needs to be very large, in which case there is no problem with probabilistic reasoning.

You say, “We can’t simply use the multiverse theory to explain away one improbable event (fine-tuning) but then deny that it can be used to explain other improbable events.” Yes we can, if the arguments are essentially different. The “fine-tunings” that the multiverse can explain are circumstances required to have life appear. In most parts of the multiverse (by far), the highly improbable tunings do not occur and there is no life. But any sentient being in the universe must find himself in one of those very few and far between parts of the universe where the tunings have occurred. That is a perfectly good way to explain why we see certain “unlikely” conditions being satisfied — IF those unlikely conditions are conditions for life to exist at all and therefore for beings to exist who can note of the conditions around them. That doesn’t mean it is a way to explain any and all unlikely occurrences. It in no way explains the Resurrection, or the miracles of Jesus, for example. But why do we want a naturalistic explanation of those things anyway?

Wouldn’t that kind of destroy their religious significance?

Joe Carter April 5th, 2010 2:20 pm

The simplest version of the multiverse idea is that the universe is very large compared to the part we can see.

I think most people would find your explanation reasonable, but still consider applying the term “multiverse” to be misleading.

The word universe, which is derived from the Latin to “uni” (one) and versus (to turn), mean “turned into one.” The term has historically referred to the sum total of all physical objects in existence. Even if the universe contains places that are too far from us to even know what they are, they are still (I would think) part of our universe.

That is why I think the term mulitverse is misleading if we are referring to parts of a whole, rather than distinct universes. Multiverse would literally mean “to turn into many” which is, if I understand what you are saying, these areas are located within the same cosmos as us.

I think it also becomes confusing when some people are using the term to refer to mulitple distinct universes to use it to refer merely to parts of a larger whole.

You ask, “But wouldn’t it have to be nearly infinite to explain away the anthropic evidence?”

What do you mean by “nearly infinite”? Something is either infinite or it’s finite. If you’re “nearly dead”, you ain’t dead, you’re alive. If the universe is “nearly infinite”, it isn’t infinite, it’s finite. To explain some of the anthropic coincidences the universe only needs to be VERY large, it doesn’t need to be infinite.

You’re right, of course. I should have chosen my words more carefully on that point.
I actually had two ideas in mind, one serious and one tongue-in-cheek.

The more snarky answer is that “nearly infinite” is “a number large enough to get the result needed.” If it turned out a larger number was needed, they would merely tack on one more, ad infinitum, to get the desired result.

But the point I meant by my question was that to determine the probability of an event there must be some boundary condition. For example, let’s say that the universe has existed for roughly 4.32*10^17 seconds. That is an actual finite number. But what if the probability requires, for instance, a time period that is larger than that, a number that exceeds the bounds of the finite timeline? That number may still be finite but it has taken on many of the qualities of an infinite number.

The passage you quote from Pruss, which you say “addresses, somewhat, that objection”, i.e my third point, does not, as far I can tell, do so at all.

In your original point you said, “Yes, it is true that in an infinite universe even things that have exceedingly low probability of happening on a particular planet will happen on some planet somewhere — indeed, on an infinite number of planets. But that does not make it any less unlikely to happen on this planet.”

But wouldn’t it make the probability 50%? Let’s imagine we have two sets of probabilities:

1) An infinite number of planets in which the resurrection did occur.2) An infinite number of planets in which the resurrection did not occur.

Both of these sets are the same size, so there is an equal likelihood that they have obtained.
How would a person determine that ot was less likely to happen on this planet? Since there are an infinite number of universes that will be exactly the same in all respects except for the fact that the resurrection did/did not occur, how can they determine the probability?

Also, as Pruss notes, the fact that some people that some eyewitnesses claim to have seen the even in question would seem to tip (at least on the Bayensian scheme) toward the affirmative.
There are problems with probabilistic reasoning if the universe is infinitely big. But, as I noted, the multiverse does not have to be infinitely big, it only needs to be very large, in which case there is no problem with probabilistic reasoning.

I agree with that in theory, but how does it work in practice? If we’re talking about a large, but not infititely big, number then what are the contraints on it? It seems that allowing the number to expand to fit whatever is needed is simply treating it as “Infinity minus 1.” How do scientists prevent the category confusion that may result from treating a finite numbers as if it were “nearly infinite?” ; )

Yes we can, if the arguments are essentially different. The “fine-tunings” that the multiverse can explain are circumstances required to have life appear. In most parts of the multiverse (by far), the highly improbable tunings do not occur and there is no life. But any sentient being in the universe must find himself in one of those very few and far between parts of the universe where the tunings have occurred. That is a perfectly good way to explain why we see certain “unlikely” conditions being satisfied — IF those unlikely conditions are conditions for life to exist at all and therefore for beings to exist who can note of the conditions around them.

But shouldn’t we account for anthropic bias? Just because we are here to observer our existence does not change the fact that the conditions were “unlikely.” Instead, we should consider what would be the likelihood of the event occurring in a random sampling of the universe. Once we account for observer bias, we realize that the event is even more unlikely than we might ordinarily imagine.

That doesn’t mean it is a way to explain any and all unlikely occurrences. It in no way explains the Resurrection, or the miracles of Jesus, for example. But why do we want a naturalistic explanation of those things anyway? Wouldn’t that kind of destroy their religious significance?

To answer your last question, we first need to answer whether the multiverse theory is a “naturalistic explanation.” Personally, I don’t believe it is. If the laws of nature are different enough in other parts of the universe, then they may be indistinguishable from what we consider to be “supernatural phenomenon.” If they were to “break into” our part of the universe in the same way that God “breaks into” the physical realm, then it may difficult, if not impossible, to say whether the cause was God or the multiverse.

Barry Arrington April 5th, 2010 2:21 pm

Dr. Barr, you say that the other parts of universe are “over the horizon.” If I understand your metaphor, one of the implications of your assertion is that we cannot, even in principle, test empirically the hypothesis that there are other universes (or parts of our universe if you prefer) that are “over the horizon.” How then could this hypothesis be falsified? And if it cannot, in principle, be falsified, is the statement “there are other universes over the horizon” not more properly classified as a metaphysical statement?

R Hampton April 5th, 2010 3:58 pm

String Theory suggests that it may be possible to empirically detect other universes — gravity can leak from one to affect another, producing “gravitational waves, dark-matter-type effects, or even the big bang itself.”

Could CERN’s LHC Detect the Existence of a Parallel Universe?

More exciting than the discovery of Higgs Boson, who’s function is giving mass to the particles of matter, could be the possible creation of tiny, particle-sized black holes. Real data from these experiments will rewrite the theorists’ Guide to the Quantum Universe. According to current physics these nano black holes could not be created at the energy levels the LHC is capable of producing. They could only be created if a parallel universe actually exists, providing the extra gravitation needed to generate the nano black holes.

R Hampton April 5th, 2010 4:06 pm

Reported evidence of gravitational interaction: New Proof Unknown “Structures” Tug at Our Universe

(Edit: The article is about Dark Flow, Wikipedia page on it here).

In 2008 scientists reported the discovery of hundreds of galaxy clusters streaming in the same direction at more than 2.2 million miles (3.6 million kilometers) an hour. This mysterious motion can’t be explained by current models for distribution of mass in the universe. So the researchers made the controversial suggestion that the clusters are being tugged on by the gravity of matter outside the known universe…

Will Wilson April 5th, 2010 5:16 pm

Some confusion seems to be arising over the definition of “multiverse”. This may be due to the fact that physicists frequently mean different thing when using the word. Page 14 of this paper may be helpful in explaining:

http://arxiv.org/abs/0704.0646

As I understand it, Stephen is talking about a Level I multiverse and Joe is talking about a Level II multiverse, in Tegmark’s terminology.

Note that a Level I multiverse doesn’t really address the anthropic problems.

Will Wilson April 5th, 2010 5:21 pm

Well, that’s unfair to Stephen actually. There are conceivable ways in which a Level I multiverse *could* address fine-tuning arguments, but people trying to refute fine-tuning arguments rarely use a Level I multiverse to do it, because doing so involves abandoning the principle of isometry of space, and consequently, via Noether’s Theorem, conservation of momentum.

It’s been a while since I studied symmetries in physical laws, so take my last claim there with a grain of salt.

Stephen M. Barr April 5th, 2010 6:33 pm

Dear Joe, As to terminology, I think we are making the same point. I would say that a “universe” is a totality of things that directly or indirectly interact with each other physically. The so-called “multiverse” is really just the universe. It is made up of many parts, which some people designate “universes”, but which really be thought of as parts of one universe. I am objecting to calling the parts “universes”, because I think the whole is the universe. You (for the same reason) are objecting to calling the whole a multiverse, because the whole should be called the universe. I am not bothered by calling the universe, in such a scenario, the multiverse, as long as one doesn’t think the name means “many universes”. It should be thought of as short for “multi-part universe”.

I get the sense that you think the idea of positing a size of the universe that is fantastically large, but still finite, somewhat artificial and arbitrary. It seems simpler to posit a universe that is infinite and be done with it. (e.g. You say, “If we’re talking about a large, but not infititely big, number then what are the contraints on it? It seems that allowing the number to expand to fit whatever is needed is simply treating it as “Infinity minus 1.”) Well, it is not quite so arbitrary as all that. There are, as I noted, strong empirical evidence that universe underwent a period of exponential “inflation” just after the Big Bang. During that inflation, the size of the universe (assuming it to be finite) would have grown exponentially with time, i.e. its time would have been proportional to e^(Ht), where H is the “Hubble parameter” during the inflationary period. So in a time 60/H (60 “Hubble times”), the size of the universe would have increased by a factor of e to the 60th power (60 “e-foldings”), which is about 10 to the 26th power. We know that there was at least that much inflation. But suppose that inflation lasted twice as long as that — 120 Hubble times — then the universe would have increased by e to the 120th power, which is equal to (e to the 60th power) TIMES (e to the 60th power), i.e. roughly 10 to the 52. If inflation had lasted three times as long, then the universe would have increased by e to the 180th power. In other words, inflation is a mechanism which generates universes that are fantastically large. Making inflation last just a little longer multiplies the size of the universe by a huge factor.
You say that the laws being different in different parts of the universe is “supernatural”. As I tried to emphasize, the fundamental laws are not different in different places in a mutiverse scenario, they just look superficially like they are. An analogy: In a cold place people may only encounter H2O in the form of ice. In a warm place maybe only in the form of water.

Superficially, they think they are dealing with different substances obeying different physical laws: water obeys equations describing fluid flow, ice obeys laws describing crystalline things. But deep down they are obeying the same laws governing H2O molecules. In several types of well-motivated theories that people study in particle physics today, the same fundamental laws can give rise to very different “effective laws” at a superficial level. So this is not a particularly radical or daring idea. There is absolutely nothing supernatural about it.

For example, consider a simple “supersymmetric SU(5) grand unified theory”. (There is some empirical evidence in favor of supersymmetric grand unified theories.) In that theory, there are several “degenerate ground states”, i.e. solutions to the equations having the lowest possible energy, in which things look very different. In the different ground states there are effectively different forces, particles, etc. But all the ground states are solutions of the same fundamental laws. There is no reason why regions of the universe so far from each other that they are not in “causal contact” should be in (or near) the same “ground state” — in fact, they shouldn’t all be. So, in this theory, parts of the universe very far from each other will have different particles and forces, etc., but deep down they are all obeying the same fundamental equations.

Advice to all people reading this: if you don’t understand very well what I am saying, read up on the subject until you do, and only then make philosophical/theological critiques of the multiverse idea. Philosophizing and theologizing about technical matters insufficiently grasped is very dangerous!!!

Many popular attacks on the multiverse scenario are based on popular misconceptions — and so I am happy to clear some of those up here.

Stephen M. Barr April 5th, 2010 7:17 pm

Dear Barry Arrington,


Many scientists would agree with you that the multiverse idea is metaphysics rather than physics. I would make two points. First, well-tested theories may tell us that there are phenomena that we cannot directly have access to and study. For example, General Relativity implies that when matter is dense enough a black hole forms. It also tells us that we cannot look at what is going on inside a black hole — information cannot get out. Does that make the existence of the region of spacetime inside the black hole a matter of metaphysics? The theory of strong interactions tells us that inside protons there are quarks, and also tells us that we cannot produce a quark in isolation. Are quarks matters of metaphysics?

Suppose that I am trapped on an island so that I can never get off the island as long as I live. I see the ocean extending as far as the eye can see. It would be quite reasonable for me to conclude that the ocean extends even farther than the horizon. How far, I cannot say. But it is more reasonable for me to suppose that there is more ocean than to suppose that the ocean just happens to end and the land begin exactly at the limit of my sight. Is the existence of “more ocean” a matter of metaphysics? Whatever you call it, it may be the most plausible among all the untestable hypotheses about what lies beyond my range of sight. Our picture of reality involves many extrapolations beyond what is testable, based on what is most plausible given what we do know.

In other words, it is often quite reasonable to extrapolate beyond what we can see, if that gives us a simpler and more sensible picture of reality.

Metaphysics? Physics? I don’t care what it is called. The question is whether, given everything we know, the multiverse is a plausible hypothesis.

This brings me to my second point. My second point is that the burden of proof is on anyone who is using anthropic coincidences to argue for God. If the multiverse scenario is a plausible hypothesis (whether testable or not) that would explain a certain fact without positing the existence of God, then it shows that the fact in question is not in itself a compelling reason to believe in God. This is common sense. If the prosecutor adduces some fact as pointing to the defendant’s guilt, the existence of a plausible alternative explanation undercuts the prosecutor’s case, whether or not the defendant can prove the alternative explanation to be correct.

I hope people realize that I am playing the devil’s advocate. I believe that one make arguments in favor of God’s existence from the anthropic coincidences and fine tunings. I do that in my book, and in an article some years ago in First Things entitled “Anthropic Coincidences”. But I also think that the multiverse idea is a reasonable one, in and of itself. It is not some desperate atheist ploy. It may appear that way to people who have insufficient background in physics to judge what cosmological and particle physics scenarios are reasonable and which aren’t.

Stephen M. Barr April 5th, 2010 7:23 pm

Dear Will Wilson,

Tegmark is an extreme example. Most physicists who would invoke a multiverse scenario to explain anthropic coincidences (e.g. Steven Weinberg, Leonard Susskind, Andrei Linde, Lawrence Hall) would be thinking of the same kind of multiverse I am talking about here.
You are right that momentum conservation is tied (via Noether’s Theorem) to space-translation symmetry — i.e. to the idea that the fundamental laws of physics are the same in every place. But the principle of momentum conservation does not rule out the multiverse idea.


Stephen M. Barr April 5th, 2010 7:29 pm

Dear R. Hampton,

Most particle physicists think it highly unlikely that the LHC will produce tiny black holes or tell us about parallel universes. Also, it seems at the moment very unclear to most particle physicists that we shall ever be able to test superstring theory. Lots of very sexy possibilities of what might be seen experimentally that are regarded as VERY long shots by particle physicists
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Crude showed me this from an article: "In fact, uncertainty looms over Hawking’s entire legacy. Unlike Einstein’s theories, which have been confirmed many times by experiment, Hawking’s ideas about singularities and black hole evaporation will probably never be observed. There is a small chance—Hawking himself puts the probability at less than 1 percent—that the Large Hadron Collider, the enormous new particle accelerator near Geneva, might detect miniature black holes. If Hawking is right (and for the sake of those who fear the LHC might spawn a planet-devouring mini black hole, he’d better be), those black holes would evaporate almost as soon as they appeared. Such a discovery would validate one of Hawking’s signature insights and could easily provide the tangible evidence needed to snag a Nobel Prize. "

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R. Hampton April 5th, 2010 8:53 pm

I understand. However, should another universe interact with ours – by gravity or some other force – I suspect we will find a definitive means of detection (not necessarily in our lifetime). The Human species has a tremendous ability to find ingenious solutions to (seemingly) intractable scientific problems.

Joe Carter April 5th, 2010 9:19 pm

Stephen Barr It may appear that way to people who have insufficient background in physics to judge what cosmological and particle physics scenarios are reasonable and which aren’t.

I certainly agree that that those of us with an insufficient background should be careful about making claims about science. But in fairness to those who may have impressions about the theory, we are basing our (mis)understanding on the claims of physicists and popular science journals.

(For example: http://discovermagazine.com/2008/dec/10-sciences-alternative-to-an-intelligent-creator/article_view?b_start:int=0&-C=)

I’ve been trying to follow this issue for years and yet today was the first time I’ve heard anything about the additional universes being merely parts of our own universe. I had always also heard that there were likely an infinite number of them. Your version sounds both reasonable and unobjectionable—and completely different from the popular conception of the theory. I should have guessed that the real version wasn’t as speculative as the version that trickles down to the popular level.

However, while I agree that laymen should tread carefully in discussions about technical matters, I think skepticism may be warranted when scientists make claims that appear—on the surface at least—to be illogical. For example, a recent article in Scientific American begins:

A team of scientists has succeeded in putting an object large enough to be visible to the naked eye into a mixed quantum state of moving and not moving.

I’ll admit that I don’t understand quantum mechanics. But I do have a reasonably good grasp of the law of non-contradiction. I suspect that what is being referred to is a part of an object that is moving while another part is not moving. But the sentence, as written, proposing a contradictory claim. The problem is likely semantic rather than empirical but it is still likely to lead people to be skeptical and misunderstand the claims and advances of cutting-edge physics.

Now if they really do mean that the entire object is moving (A) and not moving (not-A) at the same time, then I would assume that they are misinterpreting the data. My confidence in the law of non-contradiction is absolute; the idea that it could be empirically refuted is absurd. So even without understanding the technical details, a laymen like me would have justification for being skeptical of the claim as stated.

I think something similar occurs when people hear about the popular version of the multiverse theory. For instance, I’m fairly convinced that an actual infinite cannot exist. Therefore any claims that begin with, “An infinite number of universes. . . “ automatically raises a red flag in my mind.

R Hampton April 5th, 2010 9:31 pm

Ah, but in our universe quantum states are not contradictory – that is, impossible – states. Granted that most find this truth difficult to comprehend, but such is God’s Creation. As for God’s motives, Science is simply not equipped to answer.

Stephen M. Barr April 5th, 2010 10:42 pm

Dear Joe,

The statement you quote (about an object being in a state that is a mixture of a state where it is moving and a state where it is not moving) is not based on a misinterpretation of the data, nor is it based on a misunderstanding of quantum mechanics, nor is it saying anything that violates the principle of non-contradiction, nor is it saying that a part of the object is moving and part is not moving. The paradox does not arise from the philosophical incompetence of theoretical physicists, or the inability of journalists to state things clearly. Nature itself acts very mysteriously at the quantum level. In this case, you must blame nature, not the physicists and not physics journalists.

The statement that annoys you is merely stating the paradox at the heart of quantum mechanics. That paradox is nothing new. It has been with us since the nineteen twenties and has been the subject of endless debate among physicists and philosophers.

I would say the same thing about the quantum paradoxes: either one must take the trouble to master the subject well enough to understand the issues, or one must leave it alone altogether.

There is no more mysterious a question than what than what quantum mechanics is saying about reality.

JB in CA April 6th, 2010 1:45 am

Stephen Barr: I really don’t follow what you’re saying. How is “a mixed quantum state of moving and not moving” not “saying anything that violates the principle of non-contradiction” (as you state)?

Will Wilson April 6th, 2010 2:32 am

Stephen,

I’m a bit confused. It seems like the version of the multiverse that you’re talking about is that which concerns portions of “this” universe beyond our light horizon, but if you want to combat the anthropic argument with that, it seems like you need to give up isometry of space.

I must be misunderstanding a portion of your argument. Can you explain?

Will Wilson April 6th, 2010 2:37 am

Of course, there are interpretations of quantum mechanics that involve neither wave function collapse nor multiple universes. One of the more remarkable prejudices of the physics world is that such theories are frequently not even considered (though my understanding is that that’s begun to change in the last decade or so).

Steve Barr April 6th, 2010 9:04 am

Dear JB in CA, What fails in quantum mechanics is not logic but our concepts. The question is whether velocity (or equivalently momentum) is something that can always be predicated of an object. An object that is in such an ambiguous state does not have a definite velocity. But one cannot even begin to discuss these issues without some understanding of the formalism of quantum mechanics. The mathematics of quantum mechanics is clear; what it is saying if translated into ordinary speech is far from clear. There is much dispute about that. So one must learn some of the mathematical formalism before one can begin to ask what it means. A discussion of it in a comment box on a blog is futile.

Dear Will, The only interpretations that involve neither many worlds nor wavefunction collapse are modifications of quantum mechanics, such as Bohmian mechanics, which have serious problems being extended to the full range of physical systems.

As to “isometry of space”, it is not really whether space has translation symmetry (which you are calling isometry of space) that determines whether momentum conservation holds, but whether the form of the Lagrangian is invariant under spatial translations. Consider spacetime near a massive body, such as the earth. Space is NOT translation invariant near the earth, because spacetime is curved in the vicinity of the earth (which is why things are gravitationally attracted toward the earth, according to General Relativity). The curvature of spacetime is greater ten thousand miles from the earth than it is twenty thousand miles away, for example. So space near the earth is NOT translation invariant. That does NOT imply, however, that momentum conservation is violated. If one takes into account all the momentum that is there, including the momentum carried by the gravitational field, then momentum is conserved. Your understanding of Noether’s theorem and its implications seems to be a bit off.

Will Wilson April 6th, 2010 1:11 pm

Stephen,

What are the problems with de Broglie-Bohm wave mechanics? I’m pretty sure there are extensions of it it that are consistent with special relativity, for instance. Are your disagreements with it experimental, theoretical, or conceptual?

Thank you for your clarification regarding translational symmetry of physical laws, but you still haven’t explained exactly what your proposed multiverse looks like. Is it, as I asked before, portions of “this” universe beyond our light horizon? If so, do you agree that physical constants need to take on other values in these inaccessible areas of the universe in order for the fine-tuning argument to be addressed?

How do you propose that physical constants vary with location?

Stephen M. Barr April 6th, 2010 4:39 pm

Dear Will,

The version of the multiverse that I find plausible is, as you say, the portions of this universe beyond our horizon. The way “constants” can vary, is that their values depend on fields whose value can be different in different places.

The problem with Bohmian mechanics is that it is hard to extend it to relativistic theories. in the words of Steven Weinberg, “It does not seem possible to extend Bohm’s version of quantum mechanics to theories in which particles can be created and destroyed, which includes all known relativistic quantum theories.”

The reason for this is, I think, not hard to understand. Bohm (as I see it) destroys the “wave-particle duality” of standard quantum mechanics, in which the same entity is both a wave and a particle. Bohm, however, has two distinct entities, one of which is wavelike and the other particle-like, with the wave guiding the particle. It is essentially a realization of de Broglie’s “pilot wave” idea. Relativistic quantum field theory (RQFT), however, depends essentially on wave-particle duality.

Faraday explained force as being due to fields. Maxwell then showed that fields can have waves propagate in them. Quantum mechanics then showed that these waves are also particles. So (roughly speaking) forces fields waves particles. In quantum field theory, which is quantum mechanics applied to fields, one therefore finds that forces can be understood as being the result of the exchange of particles a la Feynman, closing the loop, as it were. This leads to a beautiful synthesis whereby force and matter (particles) are seen as manifestations of one underlying reality, namely quantum fields.

RQFT works fantastically well. Its picture of forces being mediated by particles cannot just be thrown out. But if one abolishes wave-particle duality as Bohm basically does, it is hard to see how one can arrive at the same results. That is why, as Weinberg says, Bohm doesn’t seem extendable to RQFT.

Peter G Kinnon April 7th, 2010 4:29 am

The issue of multiverses has here become side-tracked into a fruitless discussion on the interpretation of quantum mechanics.

Feynman himself remarked “nobody understands quantum mechanics”Of course the discipline has important practical implications, with regard to which he advised “just do the calculations”. Sound advice.

With regard to the positing of an infinity of multiverses, each having different physics, there is no doubt that this is largely a device proposed to avoid the invokation of deities to account for the “fine tuning” we observe. The existence of multiverses of this kind is not, of course, inconceivable, nor is the similarly extravagant Everett parallel universes model. There is, however, a better and possibly more useful explanation.

As detailed in chapter 11 of my recent book “Unusual Perspectives, this fine tuning is actually far stronger and more extensive than is generally recognised and extends beyond the “wow” numbers to the chemistry of the observed universe. This also being “just right” to allow (and in fact make inevitable) the exponential evolution of technology for which our species has been the vehicle.

This, together with closely related topics, is discussed in detail in my recent work “Unusual Perspectives” The electronic edition of can be freely downloaded from the eponymous website

Alexander R Pruss April 21st, 2010 12:55 pm

The metaphysically really interesting distinction is between the infinite and the finite. If the multiverse is very big but finite, well that’s not very metaphysically interesting (except in regard to fine-tuning arguments), though of course it may be very interesting in other ways. So, yes, my argument only relates to the infinite case.

As for the claim that what happens in different universes is independent, the argument linked to is just the most vivid of a series of arguments for the same conclusion that probabilities don’t make sense in an infinite multiverse. A more precise version of the argument is here, and there I do in fact explicitly discuss independence:

http://alexanderpruss.blogspot.com/2010/03/another-argument-against-actual.html

The issue is this. We have basically two ways of computing probabilities. One is from transition probabilities in indeterministic or quasi-indeterministic processes. The other is by counting. When we can count, the results of counting trump the results we get from transition probabilities. Suppose ten people independently toss a coin, and you’re one of them, and I am in another room. You ask me on the phone: “How likely is it that I have heads?” I will say: “1/2.” But now you give me one more piece of information: “Seven out of ten people in the room have heads.” This piece of information trumps the initial transition-probability-derived estimate. Now I should say: “7/10.” Interestingly, given the actual distribution of heads in the room, the transition probability is irrelevant. Even if all the coins were loaded so that the chance of heads was 1/3, my estimate should still be 7/10.

So the principle is: once you know the actual distribution of the outcome in a given population, the transition probabilities become irrelevant. But now suppose that the population of coin-tossers is infinite. Then whatever the transition probability, as long as it’s strictly between 0 and 1, and as long as all the coins are independent, we will (almost surely, i.e., with probability 1) get the same distribution: infinitely many heads and infinitely many tails. So in the infinite case, if my principle that distribution trumps transition probabilities, we get the same estimate for the probability of your having heads regardless of whether the coin is loaded. That, of course, is absurd, and hence it is impossible to have an infinite number of coin tosses. Or so the argument says.

Of course, maybe the trumping principle is false. I am more suspicious of it right now than I was when I first wrote the posts, but I haven’t seen a really good criticism of it yet.