Multi-univereses

DominO123 · July 16, 2003

Here I will be posting articles about multiple-Universes, and show that it does not come from my toilette.

http://www.enn.com/news/wire-stories/2001/...verse_42706.asp

Our universe not alone, say scientists

Sunday, March 25, 2001

By United Press International

Spanish and American astrophysicists claim the universe we inhabit contains an infinite number of other universes like our own, called O-regions, that we will someday be able to contact.

Jaume Garriga, of the University of Barcelona, and Alexander Vilenkin, of Tufts University, call the concept "many worlds in one."

As they explained to United Press International, these universes are likely similar to our own — share similar life forms, for instance — because they share a key feature with our world: a finite number of distinct histories. A history is the way something has evolved in time and will continue to evolve. Until now, physicists have never been able to make such an assertion.

The team will publish their hypothesis this fall in the journal Gravitation and Quantum Cosmology.

Are these ideas far-fetched? Alan Guth says no.

The Massachusetts Institute of Technology physicist told UPI, "Do I think that the ideas are viable? Definitely yes. In fact, I very much admire the precision with which the ideas are expressed. I consider the work of Alex Vilenkin and his collaborators to be the leading work in this field."

He explained, "What is new in their article is the realization that the total number of possible histories for each universe is finite."

An infinite number of universes like our own is a concept Guth — one of the foremost authorities on a concept called the inflationary universe — finds startling.

"That's a pretty mind-boggling conclusion," Guth told UPI from his Cambridge, Mass. office.

The idea that an infinite number of universes exist gives rise to some interesting, and troubling, implications.

"Whenever a thought crosses your mind that a terrible calamity might have happened," Vilenkin told UPI, "you can be assured that it has happened in some of the other O-regions."

Furthermore, since some O-regions have histories identical or nearly identical to our own, "if you nearly escaped an accident here, then you were not so lucky in some of the O-regions with the same prior history," he said

The worlds Garriga and Vilenkin imagine are not entirely calamitous, and may even be amusing.

"Distant copies of ourselves play all sorts of different roles" in these other worlds, Vilenkin said. In fact, "there are infinitely many O-regions where Al Gore is president and — yes — Elvis Presley is still alive!"

According to Vilenkin, Guth's work gave rise to the idea of many worlds in the first place. Guth formulated the now well-accepted idea that the universe is expanding — or inflating — and published the best-selling book "The Inflationary Universe" in 1997.

As the visible universe expands, or inflates, it gives birth to new universes. Since inflation is eternal, new universe creation is also eternal.

"In an eternally inflating universe, anything that can happen will happen; in fact, it will happen an infinite number of times," Guth told UPI.

Guth also believes the many-worlds hypothesis has profound philosophical implications.

"We already know that our planet is merely a tiny speck in a vast cosmos, but now we are being told that we do not even hold a unique copyright on our own identities," Guth told UPI. "Instead, each of us is actually only a single copy of an infinite number of beings that are completely identical to ourselves."

Edited July 16, 2003 by Fadix

DominO123 · July 17, 2003

http://www.space.com/scienceastronomy/gene...s_020205-1.html

Are There Other Universes?

By Andrew Chaikin

The irresistible, mind-boggling fantasy comes to just about everyone, sooner or later: What if everything we knew, our whole universe, was just a speck of dust on someone's shoulder?

Of course, that's not an idea astronomers take seriously. But many cosmologists are giving serious thought to a more scientific question: Do other universes exist?

At first glance, you can't help but wonder how anyone could have the chutzpah to ask a question like that. We can barely figure out this universe, and now we're wondering about others?

Believe it or not, theorists have an answer. And the answer appears to be, Yes.

To understand why, you have to go back to the Big Bang, that mysterious, mother-of-all-explosions that most astronomers believe spawned our universe. One second, according to theory, there was nothingness. The next, our cosmos sprang into existence. Nature seems to have pulled off the feat of getting something -- in fact, everything -- for nothing.

As unimaginable as that sounds, it comes straight out of the theory of quantum mechanics, a set of mathematical rules that describe how the universe works on the smallest scales, inside atoms. Quantum mechanics says that matter and energy can appear spontaneously out of the vacuum of space, thanks to something called a quantum fluctuation, a sort of hiccup in the energy field thought to pervade the cosmos.

Cosmologists say that a quantum fluctuation gave rise to the Big Bang. And the thing about quantum fluctuations is that they can happen anywhere, any time. And if our universe was born out of a quantum fluctuation, say theorists, then it's possible that other quantum fluctuations could have spawned other universes.

There's a reason some theorists want other universes to exist: They believe it's the only way to explain why our own universe, whose physical laws are just right to allow life, happens to exist. According to the so-called anthropic principle, there are perhaps an infinite number of universes, each with its own set of physical laws. And one of them happens to be ours. That's much easier to believe, say the anthropic advocates, than a single universe "fine-tuned" for our existence.

But there's a problem. If these other universes exist, there's no way for us to detect them.

Of course, as University of Arizona astronomer Chris Impey points out, there are parts of our own cosmos that we can't observe, because light from those extremely distant realms hasn't had time to reach us. "We know that our own physical universe is substantially, maybe enormously larger, than the visible universe," Impey says.

That doesn't mean, however, that Impey is prepared to accept the idea of other universes. For one thing, he says, cosmologists don't really understand the nature of quantum fluctuations, "because we don’t have any quantum gravity theory yet." That means the idea of multiple fluctuations, and multiple universes, is "truly speculative."

Other astronomers are even more forceful in their resistance to the idea.

"It's not a testable idea," says Paul Steinhardt of Princeton University. Because the different universes would not be detectable by one another, he says, "You can't really prove it exists or doesn't exist." When you talk about multiple universes, Steinhardt says, you're not talking about science anymore. "In my view, you're into metaphysics."

Not everyone rejects the multiple-universe idea out of hand. At the University of California, Virginia Trimble is more accepting. "I find it neat. In much the same way that I think it would be neat if there were reincarnation."

Not exactly a resounding scientific endorsement.

But Andreas Albrecht, a cosmologist at the University of California at Davis, says the question isn't open for debate. Why? You can't argue with quantum mechanics. "As far as we can tell," Albrecht says, "that's the fundamental language that Nature speaks. Nature doesn't answer questions for certain; it answers questions by giving probabilities."

And in quantum mechanics, "There's a possibility that almost anything happens." Including other universes. And if cosmologists are queasy about that, they don't have a choice. "It comes out of the mathematics," Albrecht explains. "It's forced down our throats."

"Quantum mechanics will not give up these other alternatives on its own," says Albrecht. "And we really don't know what to make about that. On one hand it sounds totally metaphysical. On the other hand, it's all we have to work with at the moment."

So, how would Albrecht answer Impey, and Steinhardt, and others who challenge the idea of a multiple universe?

"I would say, 'Your instincts are great, but do something with them. Give me a theory that doesn't have multiple universes.' And they'd be stuck."

That's not to say Albrecht agrees with the anthropic advocates. All the "fine-tuning" which they believe is too good to be true, is only true for life as we know it. Of the advocates, Albrecht says, "They don't know a thing about life. They don't know what it takes to have life in the universe. There could be forms of life out there that we haven't even thought of! It's really stupid."

There is no shortage of published scientific papers, articles, and books on the anthropic theory, including some by experienced cosmologists. And there's some good science involved in these studies, Albrecht says, if you take out the stuff about the anthropic principle.

"People get into these studies because they are intrigued because there are real scientific issues to address. Then they say, 'Oh, this looks familiar, I'd better call it anthropic,' and they start using all these buzz words. But they [alienate] a large part of the scientific community when they do that. It's a kind of sloppiness."

"Science is about trying not to be sloppy, but it's hard because we're human. This is an example of our humanity creeping in and getting in the way of sheer rationality. I think with time it will get much better."

"In the end," Albrecht says, "we may understand that there are a bunch of other universes, but it won't be the way the anthropic guys want…. There's a lot of room between the total smorgasbord that the anthropic people want, and just having a few extra universes around."

But if these other universes do exist, are we really destined never to detect them? Some theorists have speculated that gravitational energy from other universes might leak into ours, and that someday we might figure out how to detect it. But even the most open-minded cosmologists say that's a long shot at best.

"That is also pure speculation," says Impey. "It’s maybe reasonable speculation, but it’s speculation in a very similar vein to the speculation of someone like Kip Thorne about wormholes and time travel and white holes and black holes. It’s very careful speculation by a highly trained theoretical physicist who knows what the boundary of the current theory is."

It wouldn't be the first time that a wild idea turned out to be right.

A bit more than 100 years ago, in the second half of the 19th century, Albrecht says, most scientists didn't accept the idea that matter was composed of atoms -- an idea supported not by direct observation, but by inferences based on theories of temperature, heat, and viscosity.

"Atomic theory had some great things to say about that, and seemed to give a consistent, unified picture," Albrecht says, but "the majority of physicists at that time didn't really believe atoms existed; they thought it was just some flight of fancy."

Like quantum mechanics, Albrecht ponts out, atomic theory was a construction that went way beyond what anyone could see 100 years ago. And if it's a challenge for scientists now to embrace wild ideas like other universes, he says, that just comes with the territory.

"So far, everything we've done to try to understand the universe has pulled us out of our shell, so to speak, and made us think about things that are way beyond what we see, and way beyond what we'll see in the foreseeable future. So we're just stuck with that… Unfortunately, it's part of the nature of always being at the frontier of what we understand."

DominO123 · July 17, 2003

http://www.sciam.com/print_version.cfm?art...EA5809EC5880000

April 14, 2003

Parallel Universes

Not just a staple of science fiction, other universes are a direct implication of cosmological observations

By Max Tegmark

Is there a copy of you reading this article? A person who is not you but who lives on a planet called Earth, with misty mountains, fertile fields and sprawling cities, in a solar system with eight other planets? The life of this person has been identical to yours in every respect. But perhaps he or she now decides to put down this article without finishing it, while you read on.

The idea of such an alter ego seems strange and implausible, but it looks as if we will just have to live with it, because it is supported by astronomical observations. The simplest and most popular cosmological model today predicts that you have a twin in a galaxy about 10 to the 1028 meters from here. This distance is so large that it is beyond astronomical, but that does not make your doppelgänger any less real. The estimate is derived from elementary probability and does not even assume speculative modern physics, merely that space is infinite (or at least sufficiently large) in size and almost uniformly filled with matter, as observations indicate. In infinite space, even the most unlikely events must take place somewhere. There are infinitely many other inhabited planets, including not just one but infinitely many that have people with the same appearance, name and memories as you, who play out every possible permutation of your life choices.

You will probably never see your other selves. The farthest you can observe is the distance that light has been able to travel during the 14 billion years since the big bang expansion began. The most distant visible objects are now about 4 X 1026 meters away--a distance that defines our observable universe, also called our Hubble volume, our horizon volume or simply our universe. Likewise, the universes of your other selves are spheres of the same size centered on their planets. They are the most straightforward example of parallel universes. Each universe is merely a small part of a larger "multiverse."

By this very definition of "universe," one might expect the notion of a multiverse to be forever in the domain of metaphysics. Yet the borderline between physics and metaphysics is defined by whether a theory is experimentally testable, not by whether it is weird or involves unobservable entities. The frontiers of physics have gradually expanded to incorporate ever more abstract (and once metaphysical) concepts such as a round Earth, invisible electromagnetic fields, time slowdown at high speeds, quantum superpositions, curved space, and black holes. Over the past several years the concept of a multiverse has joined this list. It is grounded in well-tested theories such as relativity and quantum mechanics, and it fulfills both of the basic criteria of an empirical science: it makes predictions, and it can be falsified. Scientists have discussed as many as four distinct types of parallel universes. The key question is not whether the multiverse exists but rather how many levels it has.

Level I: Beyond Our Cosmic Horizon

The parallel universes of your alter egos constitute the Level I multiverse. It is the least controversial type. We all accept the existence of things that we cannot see but could see if we moved to a different vantage point or merely waited, like people watching for ships to come over the horizon. Objects beyond the cosmic horizon have a similar status. The observable universe grows by a light-year every year as light from farther away has time to reach us. An infinity lies out there, waiting to be seen. You will probably die long before your alter egos come into view, but in principle, and if cosmic expansion cooperates, your descendants could observe them through a sufficiently powerful telescope.

If anything, the Level I multiverse sounds trivially obvious. How could space not be infinite? Is there a sign somewhere saying "Space Ends Here--Mind the Gap"? If so, what lies beyond it? In fact, Einstein's theory of gravity calls this intuition into question. Space could be finite if it has a convex curvature or an unusual topology (that is, interconnectedness). A spherical, doughnut-shaped or pretzel-shaped universe would have a limited volume and no edges. The cosmic microwave background radiation allows sensitive tests of such scenarios [see "Is Space Finite?" by Jean-Pierre Luminet, Glenn D. Starkman and Jeffrey R. Weeks; Scientific American, April 1999]. So far, however, the evidence is against them. Infinite models fit the data, and strong limits have been placed on the alternatives.

Another possibility is that space is infinite but matter is confined to a finite region around us--the historically popular "island universe" model. In a variant on this model, matter thins out on large scales in a fractal pattern. In both cases, almost all universes in the Level I multiverse would be empty and dead. But recent observations of the three-dimensional galaxy distribution and the microwave background have shown that the arrangement of matter gives way to dull uniformity on large scales, with no coherent structures larger than about 1024 meters. Assuming that this pattern continues, space beyond our observable universe teems with galaxies, stars and planets.

Observers living in Level I parallel universes experience the same laws of physics as we do but with different initial conditions. According to current theories, processes early in the big bang spread matter around with a degree of randomness, generating all possible arrangements with nonzero probability. Cosmologists assume that our universe, with an almost uniform distribution of matter and initial density fluctuations of one part in 100,000, is a fairly typical one (at least among those that contain observers). That assumption underlies the estimate that your closest identical copy is 10 to the 1028 meters away. About 10 to the 1092 meters away, there should be a sphere of radius 100 light-years identical to the one centered here, so all perceptions that we have during the next century will be identical to those of our counterparts over there. About 10 to the 10118 meters away should be an entire Hubble volume identical to ours.

These are extremely conservative estimates, derived simply by counting all possible quantum states that a Hubble volume can have if it is no hotter than 108 kelvins. One way to do the calculation is to ask how many protons could be packed into a Hubble volume at that temperature. The answer is 10118 protons. Each of those particles may or may not, in fact, be present, which makes for 2 to the 10118 possible arrangements of protons. A box containing that many Hubble volumes exhausts all the possibilities. If you round off the numbers, such a box is about 10 to the 10118 meters across. Beyond that box, universes--including ours--must repeat. Roughly the same number could be derived by using thermodynamic or quantum-gravitational estimates of the total information content of the universe.

Your nearest doppelgänger is most likely to be much closer than these numbers suggest, given the processes of planet formation and biological evolution that tip the odds in your favor. Astronomers suspect that our Hubble volume has at least 1020 habitable planets; some might well look like Earth.

The Level I multiverse framework is used routinely to evaluate theories in modern cosmology, although this procedure is rarely spelled out explicitly. For instance, consider how cosmologists used the microwave background to rule out a finite spherical geometry. Hot and cold spots in microwave background maps have a characteristic size that depends on the curvature of space, and the observed spots appear too small to be consistent with a spherical shape. But it is important to be statistically rigorous. The average spot size varies randomly from one Hubble volume to another, so it is possible that our universe is fooling us--it could be spherical but happen to have abnormally small spots. When cosmologists say they have ruled out the spherical model with 99.9 percent confidence, they really mean that if this model were true, fewer than one in 1,000 Hubble volumes would show spots as small as those we observe.

The lesson is that the multiverse theory can be tested and falsified even though we cannot see the other universes. The key is to predict what the ensemble of parallel universes is and to specify a probability distribution, or what mathematicians call a "measure," over that ensemble. Our universe should emerge as one of the most probable. If not--if, according to the multiverse theory, we live in an improbable universe--then the theory is in trouble. As I will discuss later, this measure problem can become quite challenging.

Level II: Other Postinflation Bubbles

If the level i multiverse was hard to stomach, try imagining an infinite set of distinct Level I multiverses, some perhaps with different spacetime dimensionality and different physical constants. Those other multiverses--which constitute a Level II multiverse--are predicted by the currently popular theory of chaotic eternal inflation.

Inflation is an extension of the big bang theory and ties up many of the loose ends of that theory, such as why the universe is so big, so uniform and so flat. A rapid stretching of space long ago can explain all these and other attributes in one fell swoop [see "The Inflationary Universe," by Alan H. Guth and Paul J. Steinhard; Scientific American, May 1984; and "The Self-Reproducing Inflationary Universe," by Andrei Linde, November 1994]. Such stretching is predicted by a wide class of theories of elementary particles, and all available evidence bears it out. The phrase "chaotic eternal" refers to what happens on the very largest scales. Space as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles, like gas pockets in a loaf of rising bread. Infinitely many such bubbles emerge. Each is an embryonic Level I multiverse: infinite in size and filled with matter deposited by the energy field that drove inflation.

Those bubbles are more than infinitely far away from Earth, in the sense that you would never get there even if you traveled at the speed of light forever. The reason is that the space between our bubble and its neighbors is expanding faster than you could travel through it. Your descendants will never see their doppelgängers elsewhere in Level II. For the same reason, if cosmic expansion is accelerating, as observations now suggest, they might not see their alter egos even in Level I.

The Level II multiverse is far more diverse than the Level I multiverse. The bubbles vary not only in their initial conditions but also in seemingly immutable aspects of nature. The prevailing view in physics today is that the dimensionality of spacetime, the qualities of elementary particles and many of the so-called physical constants are not built into physical laws but are the outcome of processes known as symmetry breaking. For instance, theorists think that the space in our universe once had nine dimensions, all on an equal footing. Early in cosmic history, three of them partook in the cosmic expansion and became the three dimensions we now observe. The other six are now unobservable, either because they have stayed microscopic with a doughnutlike topology or because all matter is confined to a three-dimensional surface (a membrane, or simply "brane") in the nine-dimensional space.

Thus, the original symmetry among the dimensions broke. The quantum fluctuations that drive chaotic inflation could cause different symmetry breaking in different bubbles. Some might become four-dimensional, others could contain only two rather than three generations of quarks, and still others might have a stronger cosmological constant than our universe does.

Another way to produce a Level II multiverse might be through a cycle of birth and destruction of universes. In a scientific context, this idea was introduced by physicist Richard C. Tolman in the 1930s and recently elaborated on by Paul J. Steinhardt of Princeton University and Neil Turok of the University of Cambridge. The Steinhardt and Turok proposal and related models involve a second three-dimensional brane that is quite literally parallel to ours, merely offset in a higher dimension [see "Been There, Done That," by George Musser; News Scan, Scientific American, March 2002]. This parallel universe is not really a separate universe, because it interacts with ours. But the ensemble of universes--past, present and future--that these branes create would form a multiverse, arguably with a diversity similar to that produced by chaotic inflation. An idea proposed by physicist Lee Smolin of the Perimeter Institute in Waterloo, Ontario, involves yet another multiverse comparable in diversity to that of Level II but mutating and sprouting new universes through black holes rather than through brane physics.

Although we cannot interact with other Level II parallel universes, cosmologists can infer their presence indirectly, because their existence can account for unexplained coincidences in our universe. To give an analogy, suppose you check into a hotel, are assigned room 1967 and note that this is the year you were born. What a coincidence, you say. After a moment of reflection, however, you conclude that this is not so surprising after all. The hotel has hundreds of rooms, and you would not have been having these thoughts in the first place if you had been assigned one with a number that meant nothing to you. The lesson is that even if you knew nothing about hotels, you could infer the existence of other hotel rooms to explain the coincidence.

As a more pertinent example, consider the mass of the sun. The mass of a star determines its luminosity, and using basic physics, one can compute that life as we know it on Earth is possible only if the sun's mass falls into the narrow range between 1.6 X 1030 and 2.4 X 1030 kilograms. Otherwise Earth's climate would be colder than that of present-day Mars or hotter than that of present-day Venus. The measured solar mass is 2.0 X 1030 kilograms. At first glance, this apparent coincidence of the habitable and observed mass values appears to be a wild stroke of luck. Stellar masses run from 1029 to 1032 kilograms, so if the sun acquired its mass at random, it had only a small chance of falling into the habitable range. But just as in the hotel example, one can explain this apparent coincidence by postulating an ensemble (in this case, a number of planetary systems) and a selection effect (the fact that we must find ourselves living on a habitable planet). Such observer-related selection effects are referred to as "anthropic," and although the "A-word" is notorious for triggering controversy, physicists broadly agree that these selection effects cannot be neglected when testing fundamental theories.

What applies to hotel rooms and planetary systems applies to parallel universes. Most, if not all, of the attributes set by symmetry breaking appear to be fine-tuned. Changing their values by modest amounts would have resulted in a qualitatively different universe--one in which we probably would not exist. If protons were 0.2 percent heavier, they could decay into neutrons, destabilizing atoms. If the electromagnetic force were 4 percent weaker, there would be no hydrogen and no normal stars. If the weak interaction were much weaker, hydrogen would not exist; if it were much stronger, supernovae would fail to seed interstellar space with heavy elements. If the cosmological constant were much larger, the universe would have blown itself apart before galaxies could form.

Although the degree of fine-tuning is still debated, these examples suggest the existence of parallel universes with other values of the physical constants [see "Exploring Our Universe and Others," by Martin Rees; Scientific American, December 1999]. The Level II multiverse theory predicts that physicists will never be able to determine the values of these constants from first principles. They will merely compute probability distributions for what they should expect to find, taking selection effects into account. The result should be as generic as is consistent with our existence.

Level III: Quantum Many Worlds

The Level I and Level II multiverses involve parallel worlds that are far away, beyond the domain even of astronomers. But the next level of multiverse is right around you. It arises from the famous, and famously controversial, many-worlds interpretation of quantum mechanics--the idea that random quantum processes cause the universe to branch into multiple copies, one for each possible outcome.

In the early 20th century the theory of quantum mechanics revolutionized physics by explaining the atomic realm, which does not abide by the classical rules of Newtonian mechanics. Despite the obvious successes of the theory, a heated debate rages about what it really means. The theory specifies the state of the universe not in classical terms, such as the positions and velocities of all particles, but in terms of a mathematical object called a wave function. According to the Schrödinger equation, this state evolves over time in a fashion that mathematicians term "unitary," meaning that the wave function rotates in an abstract infinite-dimensional space called Hilbert space. Although quantum mechanics is often described as inherently random and uncertain, the wave function evolves in a deterministic way. There is nothing random or uncertain about it.

The sticky part is how to connect this wave function with what we observe. Many legitimate wave functions correspond to counterintuitive situations, such as a cat being dead and alive at the same time in a so-called superposition. In the 1920s physicists explained away this weirdness by postulating that the wave function "collapsed" into some definite classical outcome whenever someone made an observation. This add-on had the virtue of explaining observations, but it turned an elegant, unitary theory into a kludgy, nonunitary one. The intrinsic randomness commonly ascribed to quantum mechanics is the result of this postulate.

Over the years many physicists have abandoned this view in favor of one developed in 1957 by Princeton graduate student Hugh Everett III. He showed that the collapse postulate is unnecessary. Unadulterated quantum theory does not, in fact, pose any contradictions. Although it predicts that one classical reality gradually splits into superpositions of many such realities, observers subjectively experience this splitting merely as a slight randomness, with probabilities in exact agreement with those from the old collapse postulate. This superposition of classical worlds is the Level III multiverse.

Everett's many-worlds interpretation has been boggling minds inside and outside physics for more than four decades. But the theory becomes easier to grasp when one distinguishes between two ways of viewing a physical theory: the outside view of a physicist studying its mathematical equations, like a bird surveying a landscape from high above it, and the inside view of an observer living in the world described by the equations, like a frog living in the landscape surveyed by the bird.

From the bird perspective, the Level III multiverse is simple. There is only one wave function. It evolves smoothly and deterministically over time without any kind of splitting or parallelism. The abstract quantum world described by this evolving wave function contains within it a vast number of parallel classical story lines, continuously splitting and merging, as well as a number of quantum phenomena that lack a classical description. From their frog perspective, observers perceive only a tiny fraction of this full reality. They can view their own Level I universe, but a process called decoherence--which mimics wave function collapse while preserving unitarity--prevents them from seeing Level III parallel copies of themselves.

Whenever observers are asked a question, make a snap decision and give an answer, quantum effects in their brains lead to a superposition of outcomes, such as "Continue reading the article" and "Put down the article." From the bird perspective, the act of making a decision causes a person to split into multiple copies: one who keeps on reading and one who doesn't. From their frog perspective, however, each of these alter egos is unaware of the others and notices the branching merely as a slight randomness: a certain probability of continuing to read or not.

As strange as this may sound, the exact same situation occurs even in the Level I multiverse. You have evidently decided to keep on reading the article, but one of your alter egos in a distant galaxy put down the magazine after the first paragraph. The only difference between Level I and Level III is where your doppelgängers reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional Hilbert space.

The existence of Level III depends on one crucial assumption: that the time evolution of the wave function is unitary. So far experimenters have encountered no departures from unitarity. In the past few decades they have confirmed unitarity for ever larger systems, including carbon 60 buckyball molecules and kilometer-long optical fibers. On the theoretical side, the case for unitarity has been bolstered by the discovery of decoherence [see "100 Years of Quantum Mysteries," by Max Tegmark and John Archibald Wheeler; Scientific American, February 2001]. Some theorists who work on quantum gravity have questioned unitarity; one concern is that evaporating black holes might destroy information, which would be a nonunitary process. But a recent breakthrough in string theory known as AdS/CFT correspondence suggests that even quantum gravity is unitary. If so, black holes do not destroy information but merely transmit it elsewhere. [Editors' note: An upcoming article will discuss this correspondence in greater detail.]

If physics is unitary, then the standard picture of how quantum fluctuations operated early in the big bang must change. These fluctuations did not generate initial conditions at random. Rather they generated a quantum superposition of all possible initial conditions, which coexisted simultaneously. Decoherence then caused these initial conditions to behave classically in separate quantum branches. Here is the crucial point: the distribution of outcomes on different quantum branches in a given Hubble volume (Level III) is identical to the distribution of outcomes in different Hubble volumes within a single quantum branch (Level I). This property of the quantum fluctuations is known in statistical mechanics as ergodicity.

The same reasoning applies to Level II. The process of symmetry breaking did not produce a unique outcome but rather a superposition of all outcomes, which rapidly went their separate ways. So if physical constants, spacetime dimensionality and so on can vary among parallel quantum branches at Level III, then they will also vary among parallel universes at Level II.

In other words, the Level III multiverse adds nothing new beyond Level I and Level II, just more indistinguishable copies of the same universes--the same old story lines playing out again and again in other quantum branches. The passionate debate about Everett's theory therefore seems to be ending in a grand anticlimax, with the discovery of less controversial multiverses (Levels I and II) that are equally large.

Needless to say, the implications are profound, and physicists are only beginning to explore them. For instance, consider the ramifications of the answer to a long-standing question: Does the number of universes exponentially increase over time? The surprising answer is no. From the bird perspective, there is of course only one quantum universe. From the frog perspective, what matters is the number of universes that are distinguishable at a given instant--that is, the number of noticeably different Hubble volumes. Imagine moving planets to random new locations, imagine having married someone else, and so on. At the quantum level, there are 10 to the 10118 universes with temperatures below 108 kelvins. That is a vast number, but a finite one.

From the frog perspective, the evolution of the wave function corresponds to a never-ending sliding from one of these 10 to the 10118 states to another. Now you are in universe A, the one in which you are reading this sentence. Now you are in universe B, the one in which you are reading this other sentence. Put differently, universe B has an observer identical to one in universe A, except with an extra instant of memories. All possible states exist at every instant, so the passage of time may be in the eye of the beholder--an idea explored in Greg Egan's 1994 science-fiction novel Permutation City and developed by physicist David Deutsch of the University of Oxford, independent physicist Julian Barbour, and others. The multiverse framework may thus prove essential to understanding the nature of time.

Level IV: Other Mathematical Structures

The initial conditions and physical constants in the Level I, Level II and Level III multiverses can vary, but the fundamental laws that govern nature remain the same. Why stop there? Why not allow the laws themselves to vary? How about a universe that obeys the laws of classical physics, with no quantum effects? How about time that comes in discrete steps, as for computers, instead of being continuous? How about a universe that is simply an empty dodecahedron? In the Level IV multiverse, all these alternative realities actually exist.

A hint that such a multiverse might not be just some beer-fueled speculation is the tight correspondence between the worlds of abstract reasoning and of observed reality. Equations and, more generally, mathematical structures such as numbers, vectors and geometric objects describe the world with remarkable verisimilitude. In a famous 1959 lecture, physicist Eugene P. Wigner argued that "the enormous usefulness of mathematics in the natural sciences is something bordering on the mysterious." Conversely, mathematical structures have an eerily real feel to them. They satisfy a central criterion of objective existence: they are the same no matter who studies them. A theorem is true regardless of whether it is proved by a human, a computer or an intelligent dolphin. Contemplative alien civilizations would find the same mathematical structures as we have. Accordingly, mathematicians commonly say that they discover mathematical structures rather than create them.

There are two tenable but diametrically opposed paradigms for understanding the correspondence between mathematics and physics, a dichotomy that arguably goes as far back as Plato and Aristotle. According to the Aristotelian paradigm, physical reality is fundamental and mathematical language is merely a useful approximation. According to the Platonic paradigm, the mathematical structure is the true reality and observers perceive it imperfectly. In other words, the two paradigms disagree on which is more basic, the frog perspective of the observer or the bird perspective of the physical laws. The Aristotelian paradigm prefers the frog perspective, whereas the Platonic paradigm prefers the bird perspective.

As children, long before we had even heard of mathematics, we were all indoctrinated with the Aristotelian paradigm. The Platonic view is an acquired taste. Modern theoretical physicists tend to be Platonists, suspecting that mathematics describes the universe so well because the universe is inherently mathematical. Then all of physics is ultimately a mathematics problem: a mathematician with unlimited intelligence and resources could in principle compute the frog perspective--that is, compute what self-aware observers the universe contains, what they perceive, and what languages they invent to describe their perceptions to one another.

A mathematical structure is an abstract, immutable entity existing outside of space and time. If history were a movie, the structure would correspond not to a single frame of it but to the entire videotape. Consider, for example, a world made up of pointlike particles moving around in three-dimensional space. In four-dimensional spacetime--the bird perspective--these particle trajectories resemble a tangle of spaghetti. If the frog sees a particle moving with constant velocity, the bird sees a straight strand of uncooked spaghetti. If the frog sees a pair of orbiting particles, the bird sees two spaghetti strands intertwined like a double helix. To the frog, the world is described by Newton's laws of motion and gravitation. To the bird, it is described by the geometry of the pasta--a mathematical structure. The frog itself is merely a thick bundle of pasta, whose highly complex intertwining corresponds to a cluster of particles that store and process information. Our universe is far more complicated than this example, and scientists do not yet know to what, if any, mathematical structure it corresponds.

The Platonic paradigm raises the question of why the universe is the way it is. To an Aristotelian, this is a meaningless question: the universe just is. But a Platonist cannot help but wonder why it could not have been different. If the universe is inherently mathematical, then why was only one of the many mathematical structures singled out to describe a universe? A fundamental asymmetry appears to be built into the very heart of reality.

As a way out of this conundrum, I have suggested that complete mathematical symmetry holds: that all mathematical structures exist physically as well. Every mathematical structure corresponds to a parallel universe. The elements of this multiverse do not reside in the same space but exist outside of space and time. Most of them are probably devoid of observers. This hypothesis can be viewed as a form of radical Platonism, asserting that the mathematical structures in Plato's realm of ideas or the "mindscape" of mathematician Rudy Rucker of San Jose State University exist in a physical sense. It is akin to what cosmologist John D. Barrow of the University of Cambridge refers to as "π in the sky," what the late Harvard University philosopher Robert Nozick called the principle of fecundity and what the late Princeton philosopher David K. Lewis called modal realism. Level IV brings closure to the hierarchy of multiverses, because any self-consistent fundamental physical theory can be phrased as some kind of mathematical structure.

The Level IV multiverse hypothesis makes testable predictions. As with Level II, it involves an ensemble (in this case, the full range of mathematical structures) and selection effects. As mathematicians continue to categorize mathematical structures, they should find that the structure describing our world is the most generic one consistent with our observations. Similarly, our future observations should be the most generic ones that are consistent with our past observations, and our past observations should be the most generic ones that are consistent with our existence.

Quantifying what "generic" means is a severe problem, and this investigation is only now beginning. But one striking and encouraging feature of mathematical structures is that the symmetry and invariance properties that are responsible for the simplicity and orderliness of our universe tend to be generic, more the rule than the exception. Mathematical structures tend to have them by default, and complicated additional axioms must be added to make them go away.

What Says Occam?

The scientific theories of parallel universes, therefore, form a four-level hierarchy, in which universes become progressively more different from ours. They might have different initial conditions (Level I); different physical constants and particles (Level II); or different physical laws (Level IV). It is ironic that Level III is the one that has drawn the most fire in the past decades, because it is the only one that adds no qualitatively new types of universes.

In the coming decade, dramatically improved cosmological measurements of the microwave background and the large-scale matter distribution will support or refute Level I by further pinning down the curvature and topology of space. These measurements will also probe Level II by testing the theory of chaotic eternal inflation. Progress in both astrophysics and high-energy physics should also clarify the extent to which physical constants are fine-tuned, thereby weakening or strengthening the case for Level II.

If current efforts to build quantum computers succeed, they will provide further evidence for Level III, as they would, in essence, be exploiting the parallelism of the Level III multiverse for parallel computation. Experimenters are also looking for evidence of unitarity violation, which would rule out Level III. Finally, success or failure in the grand challenge of modern physics--unifying general relativity and quantum field theory--will sway opinions on Level IV. Either we will find a mathematical structure that exactly matches our universe, or we will bump up against a limit to the unreasonable effectiveness of mathematics and have to abandon that level.

So should you believe in parallel universes? The principal arguments against them are that they are wasteful and that they are weird. The first argument is that multiverse theories are vulnerable to Occam's razor because they postulate the existence of other worlds that we can never observe. Why should nature be so wasteful and indulge in such opulence as an infinity of different worlds? Yet this argument can be turned around to argue for a multiverse. What precisely would nature be wasting? Certainly not space, mass or atoms--the uncontroversial Level I multiverse already contains an infinite amount of all three, so who cares if nature wastes some more? The real issue here is the apparent reduction in simplicity. A skeptic worries about all the information necessary to specify all those unseen worlds.

But an entire ensemble is often much simpler than one of its members. This principle can be stated more formally using the notion of algorithmic information content. The algorithmic information content in a number is, roughly speaking, the length of the shortest computer program that will produce that number as output. For example, consider the set of all integers. Which is simpler, the whole set or just one number? Naively, you might think that a single number is simpler, but the entire set can be generated by quite a trivial computer program, whereas a single number can be hugely long. Therefore, the whole set is actually simpler.

Similarly, the set of all solutions to Einstein's field equations is simpler than a specific solution. The former is described by a few equations, whereas the latter requires the specification of vast amounts of initial data on some hypersurface. The lesson is that complexity increases when we restrict our attention to one particular element in an ensemble, thereby losing the symmetry and simplicity that were inherent in the totality of all the elements taken together.

In this sense, the higher-level multiverses are simpler. Going from our universe to the Level I multiverse eliminates the need to specify initial conditions, upgrading to Level II eliminates the need to specify physical constants, and the Level IV multiverse eliminates the need to specify anything at all. The opulence of complexity is all in the subjective perceptions of observers--the frog perspective. From the bird perspective, the multiverse could hardly be any simpler.

The complaint about weirdness is aesthetic rather than scientific, and it really makes sense only in the Aristotelian worldview. Yet what did we expect? When we ask a profound question about the nature of reality, do we not expect an answer that sounds strange? Evolution provided us with intuition for the everyday physics that had survival value for our distant ancestors, so whenever we venture beyond the everyday world, we should expect it to seem bizarre.

A common feature of all four multiverse levels is that the simplest and arguably most elegant theory involves parallel universes by default. To deny the existence of those universes, one needs to complicate the theory by adding experimentally unsupported processes and ad hoc postulates: finite space, wave function collapse and ontological asymmetry. Our judgment therefore comes down to which we find more wasteful and inelegant: many worlds or many words. Perhaps we will gradually get used to the weird ways of our cosmos and find its strangeness to be part of its charm.

DominO123 · July 17, 2003

This one is really worth reading.

http://www.bbc.co.uk/science/horizon/2001/...lunitrans.shtml

Parallel Universes

NARRATOR (DILLY BARLOW): Imagine you could find an explanation for everything in the Universe, from the smallest events possible to the biggest. This is the dream which has captivated the most brilliant scientists since Einstein. Now they think they may have found it. The theory is breathtaking and it has an extraordinary conclusion: that the Universe we live in is not the only one.

MICHIO KAKU (City University of New York): That there could be an infinite number of universes each with a different law of physics. Our Universe could be just one bubble floating in an ocean of other bubbles.

NARRATOR: Everything you are about to hear is true, at least in this Universe it is. For almost a hundred years science has been haunted by a dark secret: that there might be mysterious hidden worlds beyond our human senses. Mystics had long claimed there were such places. They were, they said, full of ghosts and spirits. The last thing science wanted was to be associated with such superstition, but ever since the 1920s physicists have been trying to make sense of an uncomfortable discovery. When they tried to pinpoint the exact location of atomic particles like electrons they found it was utterly impossible. They had no single location.

ALAN GUTH (Massachusetts Institute of Technology): When one studies the properties of atoms one found that the reality is far stranger than anybody would have invented in the form of fiction. Particles really do have the possibility of, in some sense, being in more than one place at one time.

NARRATOR: The only explanation which anyone could come up with is that the particles don't just exist in our Universe. They flit into existence in other universes, too and there are an infinite number of these parallel universes, all of them slightly different. In effect, there's a parallel universe in which Napoleon won the Battle of Waterloo. In another the British Empire held on to its American colony. In one you were never born.

ALAN GUTH: Essentially anything that can happen does happen in one of the alternatives which means that superimposed on top of the Universe that we know of is an alternative universe where Al Gore is President and Elvis Presley is still alive.

NARRATOR: This idea was so uncomfortable that for decades scientists dismissed it, but in time parallel universes would make a spectacular comeback. This time they'd be different, they'd be even stranger than Elvis being alive. There's an old proverb that says: be careful what you wish for in case your wish comes true. The most fervent wish of physics has long been that it could find a single elegant theory which would sum up everything in our Universe. It was this dream which would lead unwittingly to the rediscovery of parallel universes. It's a dream which has driven the work of almost every physicist.

MICHIO KAKU: On the ice rink I am communing with the fundamental laws of physics. At the instant of creation we believe that the Universe was symmetrical, it was pure, it was elegant. Without friction Newtonian laws are laid bare, simple, elegant and beautiful, pure, noble, elemental, just like it was at the beginning of time. When I was a child of eight my elementary school teacher came in the room and announced that a great scientist had just died and on the evening news that night everyone was flashing pictures of his desk with the unfinished manuscript of his greatest work. I wanted to know what was in that manuscript. Years later I found out that it was the attempt of Albert Einstein to create a Theory of Everything, a theory of the Universe and I wanted to be part of that quest.

NARRATOR: Einstein never achieved his goal of a Theory of Everything, but again and again others have thought they were on the brink of this ultimate achievement. This was always wishful thinking - until recently. A revolution occurred in the 1980s. In universities across the world new ideas in science streamed forth. Finally, it seemed, everything in the Universe was about to be explained. In Britain the famous physicist Stephen Hawking, was even so confident he claimed physics was ready to read the mind of God. There would soon be no big scientific problems left. One idea was the most revolutionary of all. It seemed a sure fire Theory of Everything and captured the imagination of scientists like Burt Ovrut. It was all to do with string.

BURT OVRUT (University of Pennsylvania): It has been thought since physics began that matter was made up of particles. We had changed that point of view now. We now think that matter is made up of little strings.

NARRATOR: For years it had been an article of faith that all the matter in the Universe was made of tiny, invisible particles. Now suddenly the particle physicists discovered they'd been studying the wrong thing. The particles were really tiny, invisible strings. The theory was called String Theory and it maintained that matter emanated from these tiny strings like music.

BURT OVRUT: You can think of it as a violin string or a guitar string. If you pluck it in a certain way you get a certain frequency, but if you pluck it a different way you can get more frequencies on this string and in fact you have different notes. Nature is made of all the little notes, the musical notes, that are played on these super-strings.

MICHIO KAKU: All of a sudden we realised the Universe is a symphony and the laws of physics are harmonies of a super-string.

NARRATOR: String Theory was so provocative and downright weird that it immediately began to sound like a perfect Theory of Everything.

BURT OVRUT: It certainly did sweep us all by storm. It's a beautiful, elegant and simple theory and a number of people said well if it's so elegant and simple why don't we try to use it as the basic unifying principle for nature.

NARRATOR: But if String Theory was to become Einstein's missing Theory of Everything it would have to pass one test. It would have to explain a rather special event: the birth of the Universe. The origins of the Universe had always been the special subject of the cosmologists who studied the big world of stars and galaxies. They, too, felt they were on the verge of a great triumph, a complete understanding of how the world had begun. They'd long known things had started with a giant explosion - the Big Bang - but by now cosmologists had refined the idea. They had worked backwards in time from the present day, closer and closer to the instant of the Big Bang. Their work was incredibly precise.

PAUL STEINHARDT (Princeton University): We have confidence in extrapolating back from the present to when the first stars and galaxies formed and the Universe was only a billion years old, or extrapolating back farther to when the first atoms were formed, when the Universe was a few hundred thousand years old, or when the first nuclei formed when the Universe was only a few seconds old.

ALAN GUTH: Physics was now actually ready to talk about these bizarre sounding events in the Universe, fractions of a second and even billionths and billionths and billionths of a second, 10-35 seconds after the instant of the Big Bang. Absolutely fantastic.

NARRATOR: If everything in the Universe was to be explained then String Theory and the Big Bang would now seamlessly merge and they'd complement each other perfectly. After all, one concerned the birth of the Universe and the other all the matter in it. It was surely a foregone conclusion. Physics seemed to be on the edge of glory, but it all went terribly wrong. Try as they might they just couldn't get the two ideas to merge and then, after 10 years of struggling, something even worse happened: their two pet theories now began to self-destruct. The first problem appeared with the Big Bang. The cosmologists had assumed that as they worked backwards in time they would eventually work their way back all the way to the beginning of the Big Bang. There would be no awkward gaps, but after years of end-less refinement there was one gap which refused to disappear, the most important one of all.

ALAN GUTH: In spite of the fact that we call it the Big Bang Theory it really says absolutely nothing about the Big Bang. It doesn't tell us what banged, why it banged, what caused it to bang. It doesn't even describe, doesn't really allow us to predict what the conditions are immediately after this bang.

MICHIO KAKU: The fundamental problem of cosmology is that the laws of physics as we know them break down at the instant of the Big Bang. Well some people say what's wrong with that, what's wrong with having the laws of physics collapse? Well for a physicist this is a disaster. All our lives we've dedicated to the proposition that the Universe obeys knowable laws, laws that can be written down in the language of mathematics and here we have the centrepiece of the Universe itself, a missing piece beyond physical law.

NARRATOR: The very beginning of the Big Bang was the single biggest mystery in all of cosmology. It was called the singularity.

PAUL STEINHARDT: When you extrapolate Einstein's general Theory of Relativity back to the beginning you discover what we call a singularity, a cosmic singularity, which is to say that the equations blow up.

NARRATOR: But the problem with the Big Bang was soon overshadowed. The strings were in trouble, too. The hope had been that String Theory would evolve into the single definitive explanation for the Universe, but as more and more people worked on it something puzzling happened. The physicists found a second version of it and then a third. Soon they had found five different String Theories. That wasn't single and it didn't sound very definitive.

BURT OVRUT: Five, even though it's not a very large number, is too large for us because we would like to have a more unique theory than that and this definitely was a problem, was a great crisis, so a lot of time was spent studying those individual five theories, but in the back of our minds always was why are there five of these things, shouldn't there only be one?

NARRATOR: String Theory had begun to unravel. It seemed as if the dream of a Theory of Everything was as far away as ever.

MICHIO KAKU: Cynics began to come out and say that String Theory is too hard, it's a dead end, it's simply not the way to go and it's not the Theory of Everything, it's the theory of nothing.

NARRATOR: But just as the scientists were about to give up hope, a new and startling discovery would be made. This would inspire them to begin their quest again and force them at last to confront their least popular ides: parallel universes. When String Theory fell apart, not everyone was distraught. Some people even seemed to relish the fact.

MICHAEL DUFF (University of Michigan): If String Theory really was this so-called Theory of Everything five theories of everything seems like an embarrassment of riches.

NARRATOR: Michael Duff had been the rising star of an earlier idea called super gravity. String Theory had displaced it and almost destroyed Duff's career.

MICHAEL DUFF: Physics tend to be dictated by fad and fashion. There are the gurus who dictate the direction in which new ideas grow. It was a very lonely time in many ways. When I tried to get graduate students interested many of them would say well look, you may be right and you may be wrong, but if I work in super gravity I'm not going to find a job.

NARRATOR: What made the experience of the super gravity guys so galling was that their theory wasn't so very different from String Theory to begin with. In fact, the main disagreement between them was a point of detail which, to outsiders, could seem like nitpicking. It was about the number of dimensions in the Universe. We normally think of ourselves as living in a three-dimensional world. We can move in three ways: left or right, up or down, and forwards and backwards, but physics liked adding extra dimensions. Einstein suggested time should be a fourth dimension. Then someone suggested a fifth special dimension and then a sixth. The numbers just kept growing. The extra dimensions were spaces in the Universe which we could never perceive. Most were microscopically small, but scientists believed they were really there. String Theory had been convinced there were in total exactly 10 dimensions.

BURT OVRUT: Now if you have a little oscillating string it has to have enough room to oscillate properly and when one works this out mathematically you find it, it just got a very clear answer. It had to be in 10 dimensional space.

MICHIO KAKU: Ten dimensions.

BURT OVRUT: Nine spatial dimensions and one time.

NARRATOR: Super gravity though had been convinced there were exactly eleven dimensions.

MICHAEL DUFF: The equations of super gravity took their simplest and most elegant form when written in this 11 dimensional framework.

MICHIO KAKU: There was a war between the tenth dimension and the eleventh dimension. In the 10-dimensional bandwagon we had string theorists, hundreds of them, working to tease out all the properties of the known universe from one framework: a vibrating string and then we had this small band of outcasts, outlaws, working in the eleventh dimension.

NARRATOR: While String Theory was in its ascendancy, few took seriously the eleventh dimension, but the super gravity guys never gave up hope.

MICHAEL DUFF: I did at bottom always feel convinced that eventually 11 dimensions would have its day. I wasn't sure when and I wasn't sure how, but I felt convinced that sooner or later 11 dimensions would be seen to be at the heart of things.

NARRATOR: But by now the boot was on the other foot. String Theory was in trouble. Its five different versions meant it couldn't be the all embracing theory physics was looking for. Everything, it seemed, had been tried to save String Theory. Well, almost everything.

MICHAEL DUFF: An astonishing announcement was made.

MICHIO KAKU: It was yet another shockwave that revolutionised the whole landscape.

NARRATOR: In a final desperate move the string theorists tried adding one last thing to their cherished idea. They added the very thing they had spent a decade rubbishing: the eleventh dimension. Now something almost magical happened to the five competing String Theories.

BURT OVRUT: The answer turned out to be - and it really was absolutely remarkable, I mean it really is remarkable - it turns out that they were all the same. These five String Theories turned out to be simply different manifestations of a more fundamental theory, precisely this theory which we had discarded back in the early 1980s.

MICHIO KAKU: In 11 dimensions looking from the mountain-top, looking down you could see String Theory as being part of a much larger reality, reality of the eleventh dimension.

MICHAEL DUFF: Well it was a wonderful feeling to think that all those years spent in the eleventh dimension were not completely wasted.

NARRATOR: The two camps had been absolutely certain the other was wrong. Now, suddenly, they realised their ideas complemented each other perfectly. With the addition of one extra dimension String Theory made sense again, but it had become a very different kind of theory.

BURT OVRUT: What happened to the string?

NARRATOR: The tiny invisible strings of String Theory was supposed to be the fundamental building blocks of all the matter in the Universe, but now, with the addition of the eleventh dimension, they changed. They stretched and they combined. The astonishing conclusion was that all the matter in the Universe was connected to one vast structure: a membrane. In effect our entire Universe is a membrane. The quest to explain everything in the Universe could begin again and at its heart would be this new theory. It was dubbed Membrane Theory, or M Theory, but so enigmatic and profound did the idea seem that some thought M should stand for other things.

BURT OVRUT: M Theory.

MICHAEL DUFF: Where M stands for magic, mystery or membrane.

BURT OVRUT: M theory.

PAUL STEINHARDT: Physicists get kind of dreamy-eyed when they talk about M Theory.

BURT OVRUT: M Theory.

MICHIO KAKU: Maybe M stands for mother, the mother of all strings. Maybe it's magic. Maybe it's the majesty, the majesty of a comprehensive theory of the Universe.

BURT OVRUT: M Theory.

NEIL TUROK (Cambridge University): Magical mystery, madness.

BURT OVRUT: M Theory.

NARRATOR: With M Theory it seemed at last there was a theory which might explain everything in the Universe, but before they could decide if this was true the scientists needed to know more about this new eleventh dimension. It quickly became clear it was a place where all the normal rules of common-sense have been abandoned. For one thing it is both infinitely long, but only a very small distance across.

PAUL STEINHARDT: That eleventh dimension will, at its maximum size, could be something like a trillionth of a millimetre.

BURT OVRUT: Well this is 10 to the -20 of a millimetre. That's taking a millimetre and dividing it by 10 with 20 zeros after it, so that's very, very small.

NARRATOR: That means that it exists only one trillionth of a millimetre from every point in our three-dimensional world. It's closer than your clothes to your body and yet we can't sense it. In this mysterious space our membrane Universe is floating. At first no one could imagine how that worked. Then some suggested it might float like a thin rubber sheet. Others that it might be more like a bubble which vibrated as it was blown aimlessly across hyperspace. If all this wasn't surreal enough, it was then proposed that there might be another membrane universe pulsating at the opposite end of the eleventh dimension. At first this idea wasn't taken very seriously, but eventually it would be re-examined for physics was about to ask whether our Universe was really alone. It began with Lisa Randall.

LISA RANDALL (Harvard University): People look at rock climbing and it's of course very physical, but you also find that you can concentrate on one little thing. I like solving problems, I like games, I like figuring things out.

NARRATOR: Randall had been fascinated by an apparently inexplicable phenomenon: the weakness of gravity.

LISA RANDALL: There are various forces we see in nature. Most of them we understand at some level and then there's gravity which seems very different. The gravitational force is extremely weak in comparison with the other forces. Now you might look around and say gravity doesn't seem weak, but if you think about it you have the entire Earth pulling on you and yet you can manage to pick things up.

NIMA ARKANI-HAMED (Harvard University): Gravity certainly does not look weak in everyday life. It's responsible for keeping our feet on the ground and keeping Earth spinning around the Sun and so on, but actually gravity is incredibly weak compared to the, to the other forces. This is easy to appreciate if you take an ordinary refrigerator magnet and stick it on top of a metal pin. We all know this fridge magnet will actually pick that pin up off the table, so that sort of dramatically illustrates how feeble gravity is compared even to the magnetic force of a tiny fridge magnet.

LISA RANDALL: It turns out that there are very new ideas on how to explain the weakness of gravity if we have extra dimensions.

NARRATOR: When M Theory emerged, Randall and her colleagues wondered if it might provide the explanation. Could gravity be leaking from our Universe into the empty space of the eleventh dimensions?

NIMA ARKANI-HAMED: Gravity might only appear to be weak even though it's fundamentally just as strong as everything else because it dilutes its strength out in all these extra dimensions that we can't see.

NARRATOR: Randall tried to calculate how gravity could leak from our membrane Universe into empty space, but she couldn't make it work. Then she heard the theory that there might be another membrane in the eleventh dimension. Now she had a really strange thought. What if gravity wasn't leaking from our Universe but to it? What if it came from that other universe? On that membrane, or brane, gravity would be as strong as the other forces, but by the time it reached us it would only be a faint signal. Now when she reworked her calculations everything fitted exactly.

LISA RANDALL: If you were to imagine that there are two membranes. Say there's one in which we sit and one in which if there's other stuff it sits there, but not our particles, not the stuff that we're made of and not the stuff that we see forces associated with. If we live anywhere else in the extra dimension we would see gravity as very weak because it's mostly spending its time near the other brane. We only see the tail end of gravity.

NARRATOR: The weakness of gravity could at last be explained, but only by introducing the idea of a parallel universe. Randall's idea opened a Pandora's Box. Now suddenly physicists all over the world piled into the eleventh dimension trying to solve age-old problems and every time it seemed the perfect explanation was another parallel universe. Everywhere they looked it seemed they began to find more and more of them. From every corner of the eleventh dimension parallel universes came crawling out of the woodwork. Some took the form of three-dimensional membranes, like our own Universe. Others were merely sheets of energy. Then there were cylindrical and even looped membranes. Within no time at all the eleventh dimension seemed to be jam-packed full of membranes.

MICHIO KAKU: We began to ask ourselves the question: who lives in the eleventh dimension? We have intersecting membranes, we have membranes with holes in them, we have membranes that look like doughnuts or have many different kinds of doughnut holes. We're just littered with different kinds of membranes.

MICHAEL DUFF: This eleventh dimension not only had the membrane which was the bubble-like or sheet-like object, but it had a whole wealth of different branes of varying dimensions, unfortunately called pea branes.

NARRATOR: Each of these membranes was a possible other universe. M Theory had unwittingly made the idea of parallel universes respectable again.

MICHIO KAKU: In another universe the proton may be unstable, in which case atoms would dissolve and DNA cannot form and therefore there's no intelligent life in these universes. Perhaps it's a universe of electrons and electricity, perhaps a universe of lightening bolts and neutrinos, but no stable matter.

MICHAEL DUFF: The other universes are parallel to ours and may be quite close to ours, but of which we'd never be aware. They may be completely different with completely different laws of nature operating.

ALAN GUTH: It may not all have life, but some fraction of them will have life and whatever that fraction is if there's an infinite number of these universes there'll be an infinite number of universes that have living civilisations.

MICHIO KAKU: Some of these universes may look just like ours, except perhaps you're not there.

NARRATOR: M Theory was getting stranger and stranger, but could it really be a theory which explained everything in our Universe? To have any chance of that it would have to do something no other rival theory had ever been able to do. It would have to make sense of the baffling singularity at the beginning of the Big Bang. M Theory was about to come up with a suitably outrageous answer and parallel universes would be at the very heart of it.

BURT OVRUT: I was a teenager, I don't remember quite when it was and I don't remember precisely why my father and I were down in Manhattan at the harbour. One of the great ocean liners at the time was the Michelangelo and it was in harbour I remember on the west side of Manhattan. Must have been near 42nd Street. It was a remarkable sight. It's an enormous ship, let's say 150 or 200ft high, and the entire superstructure in the front of the ship, the entire bow had been just crushed by a wave which had blown out all of the windows in the forward bulkhead right up to the bridge. This is one of what they call a white wave, or a rogue wave which had hit the Michelangelo and done all this damage. What's interesting is that there are waves somewhat similar to this which inhabit the higher dimensions and then you can imagine if you had this huge rogue wave moving through the higher dimension if it slams into another wave you're going to have a tremendous cataclysmic collision.

NARRATOR: Waves had long fascinated Burt Ovrut. Now they were just about to turn M Theory upside down. At the beginning of 2001 the received wisdom was that the eleventh dimension was a tranquil place with membrane universes gently floating in it, but Burt suggested a much more exciting idea. Universes moving through the eleventh dimension like giant, turbulent waves.

BURT OVRUT: These things can move. They are not static, they're, you know, like everything else in the world they can move around and there's not much room for them to move in. In fact if they move they're very likely to bang into each other. In fact they either move away from each other, or they bang into each other, and one thing that had occurred to me very early on is what happens if they collide?

NARRATOR: To a new generation of cosmologists like Neil Turok Burt's vision of the eleventh dimension sounded intriguing, but he and his colleagues had other things on their mind. They were still wrestling with the big problems of cosmology.

NEIL TUROK: Was there a beginning? Did time continue before the Big Bang? Where did the Universe come from?

NARRATOR: Above all, they were still trying to solve the biggest problem of all: what caused the very start of the Big Bang, the singularity?

NEIL TUROK: Nobody has a solution for the singularity problem other than essentially by hand starting the Universe at a certain time and saying let's go from there and let's not worry about what happened before and that's very unsatisfactory. This is the deepest problem in cosmology. If you can get through the singularity you're on your way to a complete theory of the Universe.

NARRATOR: Most cosmologists have begun to think they might never find a solution. They'd almost given up completely, which is when Turok and his colleagues heard Burt explain his idea properly for the first time. At a conference in Cambridge pioneers of M Theory had been brought together to explore its implications. Burt was the star of the show. His vision of a violent eleventh dimension wowed the assembled physicists and caught the attention of the cosmologists.

PAUL STEINHARDT: We heard about a vast variety of ideas. The ideas that struck both Neil and myself most strongly were the ideas that Burt presented.

NARRATOR: On the last day of the conference Neil Turok, Paul Steinhardt and Burt decided to take time out. They went to see a play.

BURT OVRUT: We wanted to see the play Copenhagen which was being performed in London at the time and the three of us took the train down to London one evening and we had whatever it was, an hour or so on the train to sit and talk about these ideas.

NARRATOR: On the journey they began to throw ideas around. Three physicists, one train, and the biggest secret about our Universe: what caused the Big Bang.

PAUL STEINHARDT: I think people get the wrong impression about scientists in that they think in an orderly, rigid way from step 1 to step 2 to step 3. What really happens that often you make some imaginative leap which at the time may seem nonsensical. When you capture the field at those stages it looks like poetry in which you are imagining without yet proving.

NEIL TUROK: Paul, Burt and me were sitting together on the train and just free associating.

PAUL STEINHARDT: One of us, maybe it was me, began by saying oh well why can't we make a universe out of collision and Neil sort of pitching in and saying well, if you did that then you could create all the matter and radiation of the Universe, so we had this conversation, one of us completing the sentences of the other in which we kind of just, just let our imaginations go.

BURT OVRUT: And as we went along, at least I learned more and more about how it might be possible to have these brane collisions produce all of the effects of the early Universe and in particular it's just easy to do with my hands, when they collide you might have a Big Bang.

NEIL TUROK: And the Big Bang is the aftermath of some encounter between two parallel worlds.

NARRATOR: But how could such a collision go on to cause the world we know? The Universe we live in has vast clumps of matter we call stars and galaxies.

BURT OVRUT: We know that things are not smooth out in the Universe. In fact we have little clumps, we have stars, we have galaxies, we have quasars, we have clumps of matter.

NARRATOR: Now they had to explain how the collision of two parallel universes could go on to create these lumps of matter. Was there something about the membranes, or branes, which could explain it?

NEIL TUROK: People tended to think of branes as being flat, perfect sheets, geometrical plains, but I think to us it was clear that that picture could not be correct. It cannot be perfectly flat. It has to ripple.

PAUL STEINHARDT: What would happen as these branes approach that there are ripples in the surface of each brane and when they come together they don't hit at exactly the same time, same place, but in fact they hit at different points and at different times.

BURT OVRUT: We found that as the brane moves it literally ripples, so when the collision takes place it imparts those ripples into real matter.

NARRATOR: The parallel universes move through the eleventh dimension like waves and like any wave these would ripple. It was the ripples which went on to cause the clumps of matter after the Big Bang. They finally had their complete explanation of the birth of our Universe and now they could do something even more profound. They could take the laws of physics back in time to the moment of the Big Bang and through to the other side.

NEIL TUROK: The existence of branes before the singularity implies there was time before the Big Bang. Time could, can be followed through the initial singularity.

BURT OVRUT: You sort of go back and back and back until you get near the place where the expansion would have taken place and then it just sort of changes into another world. When the branes collide the collision of those can be explained within M Theory, so it just simply enters the realm of mathematics and science now rather than being a, an unknown point that exploded.

NARRATOR: The singularity had disappeared and it had taken them just under an hour.

PAUL STEINHARDT: Then we went to see the play.

NARRATOR: This idea is so new it's only begun to be discussed, but if it's accepted it will mean Einstein's missing theory has finally been found. M Theory may really be able to explain everything in the Universe, but the victory will be bittersweet, for at the end of its long quest, science has discovered that the Universe it's thought to explain may be nothing special. It is nothing more than one of an infinite number of membranes, just one of the many universes which make up the multiverse.

MICHIO KAKU: The latest understanding of the multiverse is that there could be an infinite number of universes each with a different law of physics. Big Bangs probably take place all the time. Our Universe co-exists with other membranes, other universes which are also in the process of expansion. Our Universe could be just one bubble floating in an ocean of other bubbles.

NARRATOR: But this isn't quite the end of the story. Now that the Theory of Everything may have been found some are keen to use it. Physics is preparing for the ultimate flight of fancy: to make a universe of its very own without any mysteries or unanswered questions at all.

ALAN GUTH: I in fact have worked with several other people for some period of time on the question of whether or not it's in principle possible to create a new universe in the laboratory. Whether or not it really works we don't know for sure. It looks like it probably would work. It's actually safe to create a universe in your basement. It would not displace the universe around it even though it would grow tremendously. It would actually create its own space as it grows and in fact in a very short fraction of a second it would splice itself off completely from our Universe and evolve as an isolated closed universe growing to cosmic proportions without displacing any of the territory that we currently lay claim to.

DominO123 · July 17, 2003

So Sasun, it sound that what I am proposing is making a strong come back, considering that it came from a toilette.

DominO123 · July 17, 2003

http://www.newscientist.com/hottopics/quan...jsp?id=22994400

Taming the multiverse

Parallel universes are no longer a figment of our imagination. They're so real that we can reach out and touch them, and even use them to change our world, says Marcus Chown

FLICKING through New Scientist, you stop at this page, think "that's interesting" and read these words. Another you thinks "what nonsense", and moves on. Yet another lets out a cry, keels over and dies.

Is this an insane vision? Not according to David Deutsch of the University of Oxford. Deutsch believes that our Universe is part of the multiverse, a domain of parallel universes that comprises ultimate reality.

Until now, the multiverse was a hazy, ill-defined concept-little more than a philosophical trick. But in a paper yet to be published, Deutsch has worked out the structure of the multiverse. With it, he claims, he has answered the last criticism of the sceptics. "For 70 years physicists have been hiding from it, but they can hide no longer." If he's right, the multiverse is no trick. It is real. So real that we can mould the fate of the universes and exploit them.

Why believe in something so extraordinary? Because it can explain one of the greatest mysteries of modern science: why the world of atoms behaves so very differently from the everyday world of trees and tables.

The theory that describes atoms and their constituents is quantum mechanics. It is hugely successful. It has led to computers, lasers and nuclear reactors, and it tells us why the Sun shines and why the ground beneath our feet is solid. But quantum theory also tells us something very disturbing about atoms and their like: they can be in many places at once. This isn't just a crazy theory-it has observable consequences (see "Interfering with the multiverse").

But how is it that atoms can be in many places at once whereas big things made out of atoms-tables, trees and pencils-apparently cannot? Reconciling the difference between the microscopic and the macroscopic is the central problem in quantum theory.

The many worlds interpretation is one way to do it. This idea was proposed by Princeton graduate student Hugh Everett III in 1957. According to many worlds, quantum theory doesn't just apply to atoms, says Deutsch. "The world of tables is exactly the same as the world of atoms."

But surely this means tables can be in many places at once. Right. But nobody has ever seen such a schizophrenic table. So what gives?

The idea is that if you observe a table that is in two places at once, there are also two versions of you-one that sees the table in one place and one that sees it in another place.

The consequences are remarkable. A universe must exist for every physical possibility. There are Earths where the Nazis prevailed in the Second World War, where Marilyn Monroe married Einstein, and where the dinosaurs survived and evolved into intelligent beings who read New Scientist.

However, many worlds is not the only interpretation of quantum theory. Physicists can choose between half a dozen interpretations, all of which predict identical outcomes for all conceivable experiments.

Deutsch dismisses them all. "Some are gibberish, like the Copenhagen interpretation," he says-and the rest are just variations on the many worlds theme.

For example, according to the Copenhagen interpretation, the act of observing is crucial. Observation forces an atom to make up its mind, and plump for being in only one place out of all the possible places it could be. But the Copenhagen interpretation is itself open to interpretation. What constitutes an observation? For some people, this only requires a large-scale object such as a particle detector. For others it means an interaction with some kind of conscious being.

Worse still, says Deutsch, is that in this type of interpretation you have to abandon the idea of reality. Before observation, the atom doesn't have a real position. To Deutsch, the whole thing is mysticism-throwing up our hands and saying there are some things we are not allowed to ask.

Some interpretations do try to give the microscopic world reality, but they are all disguised versions of the many worlds idea, says Deutsch. "Their proponents have fallen over backwards to talk about the many worlds in a way that makes it appear as if they are not."

In this category, Deutsch includes David Bohm's "pilot-wave" interpretation. Bohm's idea is that a quantum wave guides particles along their trajectories. Then the strange shape of the pilot wave can be used to explain all the odd quantum behaviours, such as interference patterns. In effect, says Deutsch, Bohm's single universe occupies one groove in an immensely complicated multi-dimensional wave function.

"The question that pilot-wave theorists must address is: what are the unoccupied grooves?" says Deutsch. "It is no good saying they are merely theoretical and do not exist physically, for they continually jostle each other and the occupied groove, affecting its trajectory. What's really being talked about here is parallel universes. Pilot-wave theories are parallel-universe theories in a state of chronic denial."

Back and forth

Another disguised many worlds theory, says Deutsch, is John Cramer's "transactional" interpretation in which information passes backwards and forwards through time. When you measure the position of an atom, it sends a message back to its earlier self to change its trajectory accordingly.

But as the system gets more complicated, the number of messages explodes. Soon, says Deutsch, it becomes vastly greater than the number of particles in the Universe. The full quantum evolution of a system as big as the Universe consists of an exponentially large number of classical processes, each of which contains the information to describe a whole universe. So Cramer's idea forces the multiverse on you, says Deutsch.

So do other interpretations, according to Deutsch. "Quantum theory leaves no doubt that other universes exist in exactly the same sense that the single Universe that we see exists," he says. "This is not a matter of interpretation. It is a logical consequence of quantum theory."

Yet many physicists still refuse to accept the multiverse. "People say the many worlds is simply too crazy, too wasteful, too mind-blowing," says Deutsch. "But this is an emotional not a scientific reaction. We have to take what nature gives us."

A much more legitimate objection is that many worlds is vague and has no firm mathematical basis. Proponents talk of a multiverse that is like a stack of parallel universes. The critics point out that it cannot be that simple-quantum phenomena occur precisely because the universes interact. "What is needed is a precise mathematical model of the multiverse," says Deutsch. And now he's made one.

The key to Deutsch's model sounds peculiar. He treats the multiverse as if it were a quantum computer. Quantum computers exploit the strangeness of quantum systems-their ability to be in many states at once-to do certain kinds of calculation at ludicrously high speed. For example, they could quickly search huge databases that would take an ordinary computer the lifetime of the Universe. Although the hardware is still at a very basic stage, the theory of how quantum computers process information is well advanced.

In 1985, Deutsch proved that such a machine can simulate any conceivable quantum system, and that includes the Universe itself. So to work out the basic structure of the multiverse, all you need to do is analyse a general quantum calculation. "The set of all programs that can be run on a quantum computer includes programs that would simulate the multiverse," says Deutsch. "So we don't have to include any details of stars and galaxies in the real Universe, we can just analyse quantum computers and look at how information flows inside them."

If information could flow freely from one part of the multiverse to another, we'd live in a chaotic world where all possibilities would overlap. We really would see two tables at once, and worse, everything imaginable would be happening everywhere at the same time.

Deutsch found that, almost all the time, information flows only within small pieces of the quantum calculation, and not in between those pieces. These pieces, he says, are separate universes. They feel separate and autonomous because all the information we receive through our senses has come from within one universe. As Oxford philosopher Michael Lockwood put it, "We cannot look sideways, through the multiverse, any more than we can look into the future."

Sometimes universes in Deutsch's model peel apart only locally and fleetingly, and then slap back together again. This is the cause of quantum interference, which is at the root of everything from the two-slit experiment to the basic structure of atoms.

Other physicists are still digesting what Deutsch has to say. Anton Zeilinger of the University of Vienna remains unconvinced. "The multiverse interpretation is not the only possible one, and it is not even the simplest," he says. Zeilinger instead uses information theory to come to very different conclusions. He thinks that quantum theory comes from limits on the information we get out of measurements (New Scientist, 17 February, p 26). As in the Copenhagen interpretation, there is no reality to what goes on before the measurement.

But Deutsch insists that his picture is more profound than Zeilinger's. "I hope he'll come round, and realise that the many worlds theory explains where the information in his measurements comes from."

Why are physicists reluctant to accept many worlds? Deutsch blames logical positivism, the idea that science should concern itself only with objects that can be observed. In the early 20th century, some logical positivists even denied the existence of atoms-until the evidence became overwhelming. The evidence for the multiverse, according to Deutsch, is equally overwhelming. "Admittedly, it's indirect," he says. "But then, we can detect pterodactyls and quarks only indirectly too. The evidence that other universes exist is at least as strong as the evidence for pterodactyls or quarks."

Perhaps the sceptics will be convinced by a practical demonstration of the multiverse. And Deutsch thinks he knows how. By building a quantum computer, he says, we can reach out and mould the multiverse.

"One day, a quantum computer will be built which does more simultaneous calculations than there are particles in the Universe," says Deutsch. "Since the Universe as we see it lacks the computational resources to do the calculations, where are they being done?" It can only be in other universes, he says. "Quantum computers share information with huge numbers of versions of themselves throughout the multiverse."

Imagine that you have a quantum PC and you set it a problem. What happens is that a huge number of versions of your PC split off from this Universe into their own separate, local universes, and work on parallel strands of the problem. A split second later, the pocket universes recombine into one, and those strands are pulled together to provide the answer that pops up on your screen. "Quantum computers are the first machines humans have ever built to exploit the multiverse directly," says Deutsch.

At the moment, even the biggest quantum computers can only work their magic on about 6 bits of information, which in Deutsch's view means they exploit copies of themselves in 26 universes-that's just 64 of them. Because the computational feats of such computers are puny, people can choose to ignore the multiverse. "But something will happen when the number of parallel calculations becomes very large," says Deutsch. "If the number is 64, people can shut their eyes but if it's 1064, they will no longer be able to pretend."

What would it mean for you and me to know there are inconceivably many yous and mes living out all possible histories? Surely, there is no point in making any choices for the better if all possible outcomes happen? We might as well stay in bed or commit suicide.

Deutsch does not agree. In fact, he thinks it could make real choice possible. In classical physics, he says, there is no such thing as "if"; the future is determined absolutely by the past. So there can be no free will. In the multiverse, however, there are alternatives; the quantum possibilities really happen. Free will might have a sensible definition, Deutsch thinks, because the alternatives don't have to occur within equally large slices of the multiverse. "By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives," he says. "When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen."

Let's hope that deciding to read this article was the right choice.

THOTH · July 17, 2003

So are you the Dasi that we know and love or his evil twin from the bad/reversed universe (a la Star trek)!...subjecting us to all of this...LOL. Actually I love this sort of stuff...but not after two glasses of wine...perhaps I'll check it out another time...

Sasun · August 5, 2003

Hey Domino, I found your multi-universes!!!

http://www.krishna.com/seva/ifolio/imageFo....jpg&img=32&tt=

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