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Artist, Heather Oliver             

Spacetime ‘Branes: The Multiverse

by David Darling

Today I have the pleasure of hosting my friend David Darling, an astronomer and well-known science writer, who will update us on the multiverse. Dr. Darling has written many books of popular science, including Life Everywhere: The Maverick Science of Astrobiology (in which he mentions my views on Rare Earth and the Anthropic Principle). He also maintains a much-visited website, The Worlds of David Darling that contains The Internet Encyclopedia of Science. His latest book, Megacatastrophes!: Nine Strange Ways the World Could End, his second collaboration with Dirk Schulze-Makuch, will appear next spring.

The multiverse, or theory of many universes, is very much in the news right now because some recent work strongly suggests that it might be true. The basic, mind-boggling idea is that “out there” is more than just the bubble of space-time we happen to live in – what we call the Universe. There are trillions and trillions (and trillions and trillions…) of other universes. Don’t even bother trying to imagine them all or your head might explode.

Surprisingly, the word “multiverse” has been around for a long time. It was coined way back in 1895 by the American philosopher William James, although he probably had something quite different in mind than what modern scientists are talking about.

And what are they talking about? Here’s the first problem we run into in tackling the multiverse concept. When scientists talk about the multiverse they can mean different things. To a cosmologist – someone interested in the origins and evolution of the universe as a whole – the multiverse is a consequence of the nature of the vacuum, which isn’t as empty as we usually suppose. The cosmologist’s multiverse stems from something called chaotic inflationary theory, which itself is a variety of the theory of cosmic inflation. In a nutshell, our universe is like a bubble of spacetime that spawned from a great foaming ocean of spacetime that’s always existed and always will exist. In it’s first few moments, our universe expanded at a fantastic rate before settling down to a more sedate rate of growth. But beyond our universe are other, similar bubbles – other universes – each expanding and each with their own physical constants and laws. One estimate puts the number of such universes at an outrageous 10 to the power 10 to the power 10 million (in other words 1 followed by 10 to the 10 million zeros – aargh!).

On the other hand, to a quantum physicist – someone who deals with the very smallest things in nature – the concept of the multiverse is a different beast. If you believe in something called Everett’s many-worlds interpretation of quantum mechanics, which a lot of quantum scientists do, every time an observation is made at the quantum (super-tiny) level, the universe splits into all the possible outcomes that could happen. I’m not even going to get into what counts as an “observation”! Some people say it has to involve a conscious or sentient observer (like a human being); others argue that any measuring instrument will do. It’s complicated. But the underlying message of Everett’s theory is that any time an event (such as a collision between particles) is watched, the universe splits in various ways to take account of all the possible outcomes. Needless to say, this gets pretty crazy pretty fast! If every outcome of every minuscule watched event gives rise to an entirely new universe, then the total number of universes in this quantum physical view of the multiverse is beyond mind-boggling.

So there are these two different multiverse scenarios – the one of the cosmologist and the one of the quantum physicist. And they aren’t mutually exclusive. They could both be right. Trying to figure out the consequences if both types of multiverse are real and co-existent very quickly overwhelms my brain’s paltry (and diminishing) collection of neurons. But let’s just focus on a couple of particulars. In the quantum physicist’s multiverse, there are bound to be a lot of universes that are very similar to the one we live in. In fact there are going to be a lot of universes with other you’s – some of them only very slightly different from the one that we’re in right now. The cosmologist’s multiverse also allows for a vast number of universes, but the chances of almost exact copies of you is more remote. Instead, the cosmologist’s multiverse is populated by an incredible variety of bubbles of space-time in which the laws and basic constants are expected to vary widely. Probably very few are capable of supporting life.

Another distinction between the two types of multiverse is their fundamental nature. The cosmologist’s multiverse is a bit easier to grasp. Put it this way, if there were a parallel you in a bubble-universe that was the product of chaotic inflation then this other you would exist in the familiar three dimensions of space. But an alternative you in Everett’s many-worlds picture is a much more esoteric affair: a creature living in a different quantum branch of something called Hilbert space. Not being a mathematician, I won’t try to explain what Hilbert space is (Google it, if you’re interested). Suffice it to say, it’s an extremely important concept in quantum mechanics – but far from easy to visualize.

Now the exciting thing is, physicists are getting close to being able to test if the multiverse is real. If there are other universes beyond our own, then it’s likely we may have bumped into them in the past, resulting in the cosmic equivalent of fender benders. The impacts ought to show up as dents in the cosmic microwave background – the now much-cooled afterglow of the Big Bang. For some time, the European Space Agency’s Planck satellite has been mapping the microwave background to an unprecedented level of precision. The results will be out soon and may confirm the multiverse theory.

On a different front, theorists have made a discovery that goes to the very heart of quantum mechanics. They’ve shown its very likely that something called the wavefunction – the most important concept in the physics of the very small – isn’t a mere wave of probability as previously supposed, but a real physical object. The most far-reaching conclusion of this is that Everett’s many-worlds interpretation is correct and the quantum physicist’s multiverse is also a fact.

So get ready for expanding horizons. Just a few centuries ago, people thought there was only one sun. Then it turned out the stars were suns too. Then we discovered that our Milky Way Galaxy, with its hundreds of billions of stars, was just one among many galaxies. Then it turned out that galaxies were arranged in clusters, which in turned formed superclusters. Now it seems our universe is just one of an unbelievable number of other universes. Who’s to say the hierarchy doesn’t extend beyond the multiverse?

Images: David Darling, Life Everywhere; bubble universes, Sally Bensusen/SciencePhotoLibrary.

25 Responses to “Spacetime ‘Branes: The Multiverse”

  1. Dylan Fox says:

    What’s always confused me about Everett’s multiverse theory–the one which seems to have seeped into popular culture–is the sheer number of near-identical universes that will almost inevitably result. Surely the numbers involve suppose a near-infinite amount of… I dunno, ‘stuff’, energy I suppose, for universes to be created from. Is that possible?

    A theory which occurred to me and I still quite like is the idea that, if two universes are similar enough, then they merge and become one universe. Thus a near-infinite number of universes becomes finite and the system continues to tend towards entropy. I failed my A Level Physics, so I’m in no way trying to speak from a position of authority. Is my little pet-theory as ridiculous as I’ve always assumed it is?

  2. Athena says:

    I’d say that the logical extreme of Everett’s theory is reductio ad absurdum… plus, merging two universes will likely require and produce ginormous amounts of energy. So although that’s a terrific idea for SF, I don’t see it easily happening in reality (Elizabeth Bear toyed with a variant of this in Undertow, which I recommend).

  3. Caliban says:

    Interestingly, I have a story just coming out in the January 2012 issue of Asimov’s SF Magazine which toys with the idea of the multiverse.

    But I must disagree with the statement that “a lot of quantum scientists” believe in Everett’s many-worlds hypothesis. Of course this depends on what you mean by “a lot” and especially how one defines “quantum scientist.” If you take physicists like mewho daily use quantum mechanics to calculate the behavior of subatomic systems, then, no, only a very small fraction believe in the many-worlds hypothesis. If you instead define this to mean “scientists who focus on foundational issues of quantum mechanics,” then, again, no, very few believe in many-worlds; most of them think it is something like decoherence or consistent quantum mechanics and so on. The only group that has a large proportion of believers in many-worlds is string theorists–and string theorists comprise only a fraction of the physics community (I recently calculated that less than 0.5% of the NSF physics budget goes to string theory). But string theory is not synonymous with quantum mechanics by any means.

    This is not meant to say that it is out of bounds to consider the many-worlds hypothesis, only that it is misleading and by any sensible definition wrong to say that “a lot of quantum scientists” believe in the many-worlds. In truth, only a few do.

  4. Dylan Fox says:

    Hm, shamefully I hadn’t thought about the energy… Even fusion on an atomic level requires quite stupid amounts of energy and creates even more stupid amounts of energy. Mind you, you do get stars… Still, yeah, probably best left to SF. Now to add Undertow to my Amazon list!

  5. Athena says:

    We’ll announce that and if you can/want to, you can write an article discussing this further. As I just discussed with Dylan, the requirements for most multiverse variations are prohibitive, except perhaps a few femtoseconds after the Big Bang (and that’s just one restriction). What are your thoughts on the results that are interpreted to suggest the possible existence of more than one universe?

  6. Caliban says:

    I don’t want to spoil my story, but the science idea is the opposite of Dylan’s–the splitting of universes has an observable effect.

    I’m vaguely aware of the suggestion looking for evidence for cosmological multiuniverses (which I agree are different from quantum multi-universes, and to my mind much more plausible–of course, we’re taking a plausibility of say 1% compared to 0.01%). While it’s always good and laudable to propose and look for measurable effects, I think the evidence is still verrryyyy weak.

    The basic idea, as I understand it, is that if there are “nearby” baby universes, they may somehow impact each other, and so one should find artifacts in the cosmic microwave background–the dents that Darling refers to. I guess the question is, are these imbedded in branes in a higher dimensional universe, or what? It’s not a stupid idea, but is still highly speculative so far.

  7. Athena says:

    Indeed, the cosmological version is far more plausible — and makes testable predictions that are within our technological reach.

  8. Walden2 says:

    Very nice job, David. Especially attempting to explain so many complex and vast subjects.

    Folks here may find this of interest:

    The Ultimate Guide to the Multiverse

    Posted on November 28, 2011 by woit

    Yet another cover story about the Multiverse can be found this week at New Scientist, which calls it The Ultimate Guide to the Multiverse. As just one more in a long line of such stories over the last decade, a trend that shows no signs of slowing down, one can be pretty sure that this is not the yet the “ultimate” one, nor even the penultimate one.

    Full article and numerous good links here:

    Two quantum physicists are vacationing in Las Vegas playing craps. One of the scientists keeps throwing the dice and losing time after time. His companion becomes rather concerned about their limited funds and asks his friend why he keeps playing this game and notes that he keeps losing again and again.

    “Ah yes, I am losing here” replied the physicist. “But in an alternate universe, I am making a fortune!”

    Cue drum beat.

  9. intrigued_scribe says:

    Excellent post on an infinitely interesting subject! Admittedly, the theory of two universes fusing is one I hadn’t thought of, having speculated chances of enough similarity between them might be too remote. Likewise, Everett’s many-worlds theory also strikes me as one that couldn’t happen easily.

  10. Athena says:

    That’s a terrific link, Larry! I check Woit’s blog periodically — always a refreshing visit. His point about hypotheses making predictions that are testable is crucial (for string theory as well). I enjoyed Brian Greene’s first book on the subject, even while I was aware he was wildly overselling his thesis. I have a sneaking fondness for string theory, but realize that right now it’s all pretty hand gestures.

    Heather, indeed! The QM variation of the multiverse is really off the table, whether string theory is correct or not. The cosmological variation is the one that’s still viable, pending some kind of verification.

  11. Walden2 says:

    Wednesday, November 30, 2011

    Microphysics and Cosmophysics in the 1930s

    Full article here:

  12. Walden2 says:

    Multiverses where there are infinite versions of us brings up an ethical question:

    If no matter how good we are in this Universe, we end up having alternate versions of us who do bad things due to the idea that every possible action splits off into its own universe every moment, then what good does it do to be good in this universe when we and everyone else are rotten bastards in other realities causing misery and worse? What does this also say about free will?

  13. Athena says:

    Shades of the Star Trek mirror universe… not that there’s any real likelihood of this, but just to play along: how about the alternate versions of everyone who also chose a better path of action than their counterparts in this universe did? In the end, it will average out per universe. As for free will, it gets exercised just before each split.

  14. Walden2 says:

    Well, there go my plans to be bad so that at least some of my alternate selves can go on to lead better lives.

  15. Mephane says:

    Are both ideas really mutually exclusive? What if we are in a cosmological multiverse while at the same time there being vast numbers of parallel multiverses – a multi-multiverse, so to say.

    Also, on the notion of observation itself: you (Athena) said yourself that basically, any measurement can be considered an observation; yet a measurement is just some kind physical interaction in the end. So any interaction could be regarded as a measurement, and ultimately there would be no distinct notion of ‘observation’ at all.

  16. Athena says:

    Theoretically, that’s true in a very scholastic way. However, if a universe split off each time a measurement was taken, we would see concrete repercussions. Generally, when quantum-effect logic is applied to the macro world, absurdities pop up, starting with Zeno’s paradoxes.

  17. Mephane says:

    Well, what is a measurement? It’s only the human notion of processes that lead to us receiving a point of data. An abstract concept, albeit a quite useful one. Just like, for example, anger is but a human concept, physically it is nothing more but electromagnetic interactions as part of chemical reactions.

    The thing is, from a scientific viewpoint, there is nothing suggesting that consciousness were anything special. So, from a physical standpoint, while a human performing a measurement (with whatever technical apparatus possibly used) is obviously regarded as an observation, the entire process still boils down to a long series of interactions between atoms, electrons, photons etc. But the actual event of ‘measurement’ is the initial interaction, like a photon being absorbed for example. After that, it is already over, and whatever happens as a consequence, is not part of that event any more. So when should the universe split in this scenario, if not at the initial interaction itself? And since that in the other hand is in no way special from all other subsequent interactions, all of them would have to trigger a split, too.

    So for me it boils down to this: the entire notion of an observation being something special seems absurd to me. Either the universe splits at each and every physical interaction, or it never does. And since, as you suggest, if that were true we would have the evidence, my conclusion is that the parallel universe theory of quantum physics can be ruled out, and only the cosmological multiverse may be viable.

  18. Athena says:

    I agree with your conclusion (though what constitutes an “interaction” and whether consciousness is special or not depends on definition and what scale you consider). As I said to Dylan earlier, I think the rapid devolution of the QM variant to reductio ad absurdum effectively rules that version out. If there is a multiverse, it is almost certainly of the cosmological kind, and it most likely happened very early.

  19. Caliban says:

    Most quantum mechanics (like myself) would agree that the idea of *conscious* observation being special is, to put it mildly, unlikely.

    One popular (among physicists) approach to the measurement problem is “decoherence”–the idea is that when a quantum system comes in contact with a large, messy system, such as happens in most measurements, thermal fluctuations cause a smearing of quantum phases and leads to classical behavior and what looks like “collapse” of the wavefunction. In this view the process of measurement with macroscopic instruments has an effect, but it’s not consciousness at all but thermal fluctuations that is the source.

  20. Athena says:

    Exactly. Those who keep babbling about “quantum consciousness” forget that QM phenomena are several orders of magnitude smaller than thermal fluctuations. What I meant (and of course stated way too briefly) in my previous comment is that consciousness is “special”, if at all, at large scales — organisms and above. Within the physical/chemical domain, it’s like everything else and has no magical properties that, for example, allow conscious beings to bend spoons with “the power of their mind”.

  21. Walden2 says:

    The latest from Not Even Wrong on the multiverse concept. The comments to this piece are also quite enlightening:

  22. Athena says:

    I agree; it’s particularly useful that they separate what many people conflate routinely.

  23. Walden2 says:

    Not Even Wrong aka Peter Woit comments on Brian Greene’s new article about the Multiverse in Newsweek magazine:

    To quote:

    The article is pretty uniformly a promotional piece for multiverse mania, although buried fairly deep in the piece is something a bit more skeptical:

    Because the proposal is unquestionably tentative, we must approach it with healthy skepticism and invoke its explanatory framework judiciously.

    Imagine that when the apple fell on Newton’s head, he wasn’t inspired to develop the law of gravity, but instead reasoned that some apples fall down, others fall up, and we observe the downward variety simply because the upward ones have long since departed for outer space. The example is facetious but the point serious: used indiscriminately, the multiverse can be a cop-out that diverts scientists from seeking deeper explanations. On the other hand, failure to consider the multiverse can place scientists on a Keplerian treadmill in which they furiously chase answers to unanswerable questions.

    Which is all just to say that the multiverse falls squarely in the domain of high-risk science. There are numerous developments that could weaken the motivation for considering it, from scientists finally calculating the correct dark-energy value, or confirming a version of inflationary cosmology that only yields a single universe, or discovering that string theory no longer supports a cornucopia of possible universes. And so on.

  24. Walden2 says:

    Say whatever else you want about the Multiverse concept, it has brought us some very memorable and entertaining episodes of Star Trek across its various incarnations starting with “Mirror, Mirror”.

    That episode brought us the inside SF joke about everyone who is an alternate version of our Universe has to wear a Spock-style goatee. A recent episode of Community had a parallel universe episode where the bad versions of our main characters all had to wear those beards, male and female. :^)

    And Sliders was pretty good in its first few seasons, too, as far as mainstream network SF goes.

  25. Athena says:

    I agree on both Greene’s work and Star Trek. The problem with most multiverse theories is that they don’t seem to be amenable to experimental confirmation. If there were any type of “leakage” between them, to the effect of influencing exotic matter of energy, that would be a different story.