One Dimension Over: Earth and People

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Columbia University physicist Brian Greene describes the idea as the notion that "our universe is one of potentially numerous 'slabs' floating in a higher-dimensional space, much like a slice of bread within a grander cosmic loaf," in his book "The Hidden Reality" Vintage Books, A further wrinkle on this theory suggests these brane universes aren't always parallel and out of reach. Sometimes, they might slam into each other, causing repeated Big Bangs that reset the universes over and over again.

The theory of quantum mechanics, which reigns over the tiny world of subatomic particles, suggests another way multiple universes might arise. Quantum mechanics describes the world in terms of probabilities, rather than definite outcomes. And the mathematics of this theory might suggest that all possible outcomes of a situation do occur — in their own separate universes. For example, if you reach a crossroads where you can go right or left, the present universe gives rise to two daughter universes: one in which you go right, and one in which you go left.

Scientists have debated whether mathematics is simply a useful tool for describing the universe, or whether math itself is the fundamental reality, and our observations of the universe are just imperfect perceptions of its true mathematical nature. If the latter is the case, then perhaps the particular mathematical structure that makes up our universe isn't the only option, and in fact all possible mathematical structures exist as their own separate universes.

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Our universe may be one of many, physicists say. Here are the five most plausible scientific theories suggesting we live in a multiverse: 1.

Infinite Universes Scientists can't be sure what the shape of space-time is, but most likely, it's flat as opposed to spherical or even donut-shape and stretches out infinitely. If so, then everything in our universe is bound to repeat at some point, creating a patchwork quilt of infinite universes. Our universe may live in one bubble that is sitting in a network of bubble universes in space. This has made some people suspect the hand of God.

Yet an inflationary multiverse, in which all conceivable physical laws operate somewhere, offers an alternative explanation. In every universe set up in this life-friendly way, the argument goes, intelligent beings will be scratching their heads trying to understand their luck. In the far more numerous universes that are set up differently, there is no one to ask the question.

How Many Dimensions Does The Universe Have?

This is an example of the "anthropic principle", which says that things have to be the way we find them: if they were not, we would not be here and the question would never arise. For many physicists and philosophers, this argument is a cheat: a way to evade rather than explain the fine-tuning problem. How can we test these assertions, they ask? Surely it is defeatist to accept that there is no reason why the laws of nature are what they are, and simply say that in other universes they are different?

The trouble is, unless you have some other explanation for fine-tuning, someone will assert that God must have set things up this way. The astrophysicist Bernard Carr has put it bluntly: " If you don't want God, you'd better have a multiverse ". Another kind of multiverse avoids what some see as the slipperiness of this reasoning, offering a solution to the fine-tuning problem without invoking the anthropic principle. In he proposed that universes might reproduce and evolve rather like living things do.

On Earth, natural selection favours the emergence of "useful" traits such as fast running or opposable thumbs. In the multiverse, Smolin argues, there might be some pressure that favours universes like ours. He calls this "cosmological natural selection".


Smolin's idea is that a "mother" universe can give birth to "baby" universes, which form inside it. The mother universe can do this if it contains black holes. A black hole forms when a huge star collapses under the pull of its own gravity, crushing all the atoms together until they reach infinite density. This suggested to Smolin that a black hole could become a Big Bang, spawning an entire new universe within itself.

If that is so, then the new universe might have slightly different physical properties from the one that made the black hole. This is like the random genetic mutations that mean baby organisms are different from their parents. If a baby universe has physical laws that permit the formation of atoms, stars and life, it will also inevitably contain black holes. That will mean it can have more baby universes of its own. Over time, universes like this will become more common than those without black holes, which cannot reproduce.

View image of Could one universe create others? It is a neat idea, because our Universe then does not have to be the product of pure chance.

Will humans ever travel to different dimensions?

If a fine-tuned universe arose at random, surrounded by many other universes that were not fine-tuned, cosmic natural selection would mean that fine-tuned universes subsequently became the norm. The details of the idea are a little woolly, but Smolin points out that it has one big advantage: we can test it. For example, if Smolin is right we should expect our Universe to be especially suited to making black holes.

This is a rather more demanding criterion than simply saying it should support the existence of atoms. But so far, there is no evidence that this is the case — let alone proof that a black hole really can spawn an entirely new universe. When Albert Einstein's theory of general relativity began to come to public attention in the s, many people speculated about the "fourth dimension" that Einstein had allegedly invoked. What might be in there? A hidden universe, maybe? This was nonsense. Einstein was not proposing a new dimension.

What he was saying was that time is a dimension, similar to the three dimensions of space. All four are woven into a single fabric called space-time, which matter distorts to produce gravity. Even so, other physicists were already starting to speculate about genuinely new dimensions in space.

The first intimation of hidden dimensions began with the work of the theoretical physicist Theodor Kaluza. In a paper Kaluza showed that, by adding an extra dimension to the equations of Einstein's theory of general relativity, he could obtain an extra equation that seemed to predict the existence of light. The Swedish physicist Oskar Klein offered an answer in Perhaps the fifth dimension was curled up into an unimaginably small distance: about a billion-trillion-trillionth of a centimetre.

In the modern version of string theory, known as M-theory, there are up to seven hidden dimensions. The idea of a dimension being curled may seem strange, but it is actually a familiar phenomenon. A garden hose is a three-dimensional object, but from far enough away it looks like a one-dimensional line, because the other two dimensions are so small.

Similarly, it takes so little time to cross Klein's extra dimension that we do not notice it. Physicists have since taken Kaluza and Klein's ideas much further in string theory. This seeks to explain fundamental particles as the vibrations of even smaller entities called strings.

Space and Time Warps - Stephen Hawking

When string theory was developed in the s, it turned out that it could only work if there were extra dimensions. What's more, these dimensions need not be compact after all. They can be extended regions called branes short for "membranes" , which may be multi-dimensional. A brane might be a perfectly adequate hiding place for an entire universe.

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M-theory postulates a multiverse of branes of various dimensions, coexisting rather like a stack of papers. If this is true, there should be a new class of particles called Kaluza-Klein particles. In theory we could make them, perhaps in a particle accelerator like the Large Hadron Collider. They would have distinctive signatures, because some of their momentum is carried in the hidden dimensions. These brane worlds should remain quite distinct and separate from each other, because forces like gravity do not pass between them. But if branes collide, the results could be monumental.

Conceivably, such a collision could have triggered our own Big Bang. View image of Perhaps two branes collided Credit: Nicolle R. It has also been proposed that gravity, uniquely among the fundamental forces, might "leak" between branes. This leakage could explain why gravity is so weak compared to the other fundamental forces.

As Lisa Randall of Harvard University puts it: "if gravity is spread out over large extra dimensions, its force would be diluted. In , Randall and her colleague Raman Sundrum suggested that the branes do not just carry gravity, they produce it by curving space. In effect this means that a brane "concentrates" gravity, so that it looks weak in a second brane nearby. This could also explain why we could live on a brane with infinite extra dimensions without noticing them.

If their idea is true, there is an awful lot of space out there for other universes. The theory of quantum mechanics is one of the most successful in all of science. It explains the behaviour of very small objects, such as atoms and their constituent fundamental particles. It can predict all kinds of phenomena, from the shapes of molecules to the way light and matter interact, with phenomenal accuracy. Quantum mechanics treats particles as if they are waves, and describes them with a mathematical expression called a wave function.

Perhaps the strangest feature of a wave function is that it allows a quantum particle to exist in several states at once. This is called a superposition. But superpositions are generally destroyed as soon as we measure the object in any way. An observation "forces" the object to "choose" one particular state. This switch from a superposition to a single state, caused by measurement, is called "collapse of the wave function". The trouble is, it is not really described by quantum mechanics, so no one knows how or why it happens. In his doctoral thesis , the American physicist Hugh Everett suggested that we might stop fretting about the awkward nature of wave function collapse, and just do away with it.

Everett suggested that objects do not switch from multiple states to a single state when they are measured or observed.

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Instead, all the possibilities encoded in the wave function are equally real. When we make a measurement we only see one of those realities, but the others also exist. Everett was not very specific about where these other states actually exist. But in the s, the physicist Bryce DeWitt argued that each alternative outcome must exist in a parallel reality: another world. Suppose you conduct an experiment in which you measure the path of an electron. In this world it goes one way, but in another world it goes another way.

That requires a parallel apparatus for the electron to pass through. It also requires a parallel you to measure it. In fact you have to build an entire parallel universe around that one electron, identical in all respects except where the electron went. This picture really gets extravagant when you appreciate what a measurement is. In DeWitt's view, any interaction between two quantum entities, say a photon of light bouncing off an atom, can produce alternative outcomes and therefore parallel universes. The quantum multiverse must be in some sense real, because quantum theory demands it and quantum theory works.

As DeWitt put it, "every quantum transition taking place on every star, in every galaxy, in every remote corner of the Universe is splitting our local world on earth into myriads of copies. Not everyone sees Everett's many-worlds interpretation this way. Some say it is largely a mathematical convenience, and that we cannot say anything meaningful about the contents of those alternative universes. But others take seriously the idea that there are countless other "yous", created every time a quantum measurement is made.

The quantum multiverse must be in some sense real, they say, because quantum theory demands it and quantum theory works. You either buy that argument or you do not. But if you accept it, you must also accept something rather unsettling. Illingworth, D. Magee, P. The other kinds of parallel universes, such as those created by eternal inflation, are truly "other worlds". They exist somewhere else in space and time, or in other dimensions. They might contain exact copies of you, but those copies are separate, like a body double living on another continent.


In contrast, the other universes of the many-worlds interpretation do not exist in other dimensions or other regions of space. Instead, they are right here, superimposed on our Universe but invisible and inaccessible. The other selves they contain really are "you". In fact, there is no meaningful "you" at all.