Is a parallel universe another dimension

Doppelganger and shadow creatures

“Listen: There's a hell of a universe next door; let's go! ”wrote the American poet Edward Estling Cummings in 1944. Broadening horizons, escapism or - intellectual - seduction: more and more scientists seem to follow this invitation. A revolution in the physical worldview is looming, the scope of which is incalculable: our universe could only be one among innumerable.

Perhaps nature has realized everything that is possible at all - in the physical or even mathematical-logical sense. Then there would also be perfect doppelgangers from each of us, as well as other I's with different lives as well as universes with completely unknown laws of nature, more or less spatial dimensions, perhaps two time directions, faster-than-light particles or ghostly shadow beings.

What has long been a confusing and fascinating topic in myths, philosophy and literature has now also met with a surprising response in physics and cosmology - not to the delight of all researchers. “The idea of ​​parallel universes was very suspect to scientists as a kind of province of mystics, charlatans and weirdos. Every physicist who researched parallel universes was the subject of ridicule and endangered his career, because not even today there is the slightest experimental evidence of their existence, ”admits Michio Kaku. "But in the meantime the climate has changed drastically and the most intelligent minds on our planet are working diligently on this topic," writes the professor of theoretical physics at the City University of New York in his book "Parallel Worlds", which was published a few months ago. The entirety of the universes has now been given a name, which is not entirely unproblematic: multiverse. The assumption of the existence of other universes is therefore also called the multiverse or M-hypothesis for short.

“Our universe is not alone, but just one in an infinity of others that flow like bubbles in the flow of time,” the British astrophysicist and science reporter Marcus Chown is convinced. "Out there, far beyond the furthest limits of our telescopes, there are universes that dance to the tones of every imaginable mathematical equation," he writes in his book "The Universe Next Door".

These are not extravagant individual opinions. Even highly respected researchers like Martin Rees, professor at Cambridge University and Astronomer Royal, the Royal Astronomer of Great Britain, are convinced: “What is usually called the universe” could only be one in a whole ensemble. Countless others could exist in which the laws of nature are very different. The universe in which we were created belongs to the unusual subset that allows complexity and consciousness to arise. "

And Max Tegmark, one of the most creative cosmologists of today, states: "Nature tells us from many different directions that our universe is only one of a huge number of other universes." The professor at the Massachusetts Institute of Technology admits: " Right now we're not able to see how the pieces fit together to make a big picture. ”And he admits,“ A lot of people find the idea despicable, and that's why a lot of scientists are against it. But that's an emotional reaction - people just don't like the rubbish of all these dead universes. ”Tegmark found out for himself how strong the resistance is when he first published his ideas on the Internet - because he believed that nobody wanted to print them - , then in journals and finally in "Scientific American". There was a hail of bad letters and even subscription cancellations.

Quantum physics was the first to suspect that there had to be other universes. The “Many Worlds” interpretation assumes that there is a ghostly shadowy realm beyond our reality, a superposition of all physically possible alternative developments that are no less “real” than those we observe. These parallel worlds are not actually spatially separated from our branch of reality, but are present in a ghostly but unobservable way. Although many physicists now accept this bizarre interpretation, the common sense, which has not been trained in quantum phenomena in evolution, is opposed to it.

Just as hard to grasp with the hands, physically more speculative, but in principle much easier to understand are the other universes with which cosmologists grapple - at least in their theories. Some universes are only a hair's breadth away - but in a dimension inaccessible to us - while others are separated from us by the abysses of time or spatial infinities. The M-Hypothesis is nevertheless not an unfounded fantasy, but is based primarily on developments in modern cosmology and fundamental physics.

• On the one hand, attempts to explain the observable space, which was probably formed with a Big Bang 13.7 billion years ago, suggest that there are other universes.

• On the other hand, the results of string theory - currently the best candidate for a "world formula" or "Theory of Everything", which describes all natural forces and elementary particles as well as space and time in a uniform manner - demand the existence of a breathtaking number of parallel universes.

Here are the most important scenarios of the M-Hypothesis:

The most conservative argument for the existence of parallel universes assumes that the observable space is only a tiny section of a larger, possibly infinite universe. This cannot be proven, but all astronomical data are compatible with it. We cannot decide whether the same natural laws, constants and so on apply everywhere. Because an infinite number of regions of such a gigantic universe have never been in causal contact with an infinite number of other regions - including ours - so far. So it cannot be ruled out that completely different conditions prevail elsewhere.

According to the well-established principle of mediocrity, we do not occupy a special position, but our place in space is average. As a result, all - or at least many - areas beyond our observation horizon should be by and large similar, i.e. consist of glowing groups, clusters and superclusters of galaxies as well as enormous empty spaces in between. This has an adventurous consequence if this structure continues indefinitely - or at least sufficiently far. According to quantum theory, differences in nature cannot become arbitrarily small - that would contradict Heisenberg's uncertainty principle. The cosmology professor Alexander Vilenkin from Tufts University in Medford, Massachusetts, therefore razor-sharply concluded that all possible configurations then exist within the framework of the physical conditions - and indeed infinitely often. But the number of different stories - physical processes related to one another through cause and effect - since the Big Bang is finite even in an infinite universe with the same laws. Thus every single story repeats itself infinitely often. Even more: All kinds of similar stories happen - and also infinitely often. Each person would only be unique in our limited region of the universe. In an infinite number of other regions, our doppelgangers would also shake their heads in disbelief or think in horror of the infinite number of tax returns and editorial conferences out there. Vilenkin, who characterizes his hypothesis as a “metaphysical exercise” because it cannot be directly checked, compares the situation with the Copernican Revolution: “At first people believed that the earth was at the center of the universe. Then it became increasingly clear that our position in the universe is more or less the same as that of other planets. It's hard to swallow that we're nothing special. ”Of course, the doppelganger worlds are separated from one another by astronomical distances. Max Tegmark roughly estimated the distances. The next doppelganger of yours, dear reader, would be statistically 1010 meters away, the next doppelganger volume of a sphere with a radius of 100 light years even 1010 meters and the next doppelganger universe, which is as big as our observable space, an unimaginable 1010 meters. So it is very unlikely that you will ever shake hands with an alter ego ...

Many cosmologists today are convinced that our universe went through a phase of exponential expansion at the beginning: a space expansion faster than light. This is the basic idea behind the Cosmic Inflation Hypothesis. Vilenkin and other cosmologists have recognized that inflation only stops in individual places, but not as a whole. Where it ends, bubble universes form, which are separated from one another by increasingly inflating spaces. These bubble universes - the observable universe is only a tiny part of such a bubble - can differ drastically from one another in their natural laws, and the whole cosmos, as it were, constantly reproduces itself by constantly forming new bubbles (see next article "Mr. Universe") .

Even more curious: daughter universes could even split off from the individual universes. Various scenarios are conceivable for this. According to Alexander Vilenkin, due to the energy density of the vacuum, a new phase of cosmic inflation can spontaneously, albeit rarely, begin and form daughter universes. The exponential expansion of space does not run into the mother universe and annihilates it, but turns out from it, as it were, and turns itself off, similar to the shimmering soap bubbles that children like to blow into the air. "Each of these exponentially expanding bubbles is developing into a universe with its own eternal inflation," says Vilenkin. “From this, too, new inflationary bubbles can split off, from these again some and so on.” Vilenkin coined the term “recycling universe” for this endless chain of reproduction. According to other scenarios, the replenishment universes are created by wormholes or whenever matter collapses into a black hole. If an entire universe collapses in a "final bang," it could be a transition to a new big bang. In this respect, even an oscillating sequence of big and final bang universes is possible. Even our own universe could once have been thrown into existence as a baby universe from an earlier or larger universe and quickly become large and strong as it expanded. The Big Bang would then not have been the beginning of everything, just a transition and a local birth event among myriads.

According to quantum cosmology, universes borrow their existence from a long-lived quantum fluctuation. Or they arise through a kind of tunnel effect from the almost nothingness of the quantum vacuum - the physical ground state. Even the hypothesis of a time loop has been proposed, according to which the universe would have created itself by going back to the past, that is to say, it would be its own mother.

String cosmology also leads to the assumption of a monstrous number of universes: Because there are myriads of ways in which the extra dimensions postulated by this theory can roll in and out, there are at least as many different basic states or string vacuums - i.e. types of universes with different natural laws. String theorists like Leonard Susskind of Stanford University discuss numbers between 10100 and 101500 and assume that the Cosmic Inflation scenario provides a physical mechanism to incubate the myriads of string vacuums - in the form of myriads of different bubble universes.

String theory also opens up the possibility that other universes are not even a hair's breadth away from us - in the direction of other dimensions of space. “Extra dimensions have changed the way physicists think about the universe,” writes Lisa Randall in her recently published book “Warped Passages”. The young physics professor at Harvard University caused a great response in the professional world with her multi-dimensional ideas. “Extra dimensions could even have consequences for the world we see,” she is convinced. Perhaps the only reason gravity is so weak - 1037 times weaker than electromagnetic interaction - is that most of it escapes into an extra dimension. This could even be infinitely extended, but curved so that hardly anything has access to it. But perhaps the Large Hadron Collider, which is due to go into operation at the European Nuclear Research Center near Geneva in 2007, will open a little door. The particle accelerator could generate certain particles called Kaluza Klein modes, which disappear into the extra dimensions, but which still leave traces in our world beforehand. And if he even creates black mini-holes, they would also have to be higher-dimensional and would disintegrate in a characteristic way. Of course, this is all speculative. But for Lisa Randall such speculations are inevitable for the progress of knowledge. "Even if it turns out that many details do not match reality, a new theoretical idea can shed light on the physical principles that make up the true theory of the cosmos."

According to another variant of string cosmology, parallel universes even exist in the most geometrically exact sense. According to her, there is a five-dimensional space-time that is bounded by two four-dimensional universes - similar to how two sheets of paper hung in parallel enclose the space between them. The structure is not rigid, but dynamic like two hands that clap applause: According to the model, the universes - one of which is ours - move towards each other, collide, move away again and attract each other again. Every collision in the fifth dimension corresponds to a new big bang.

Even opponents of the Big Bang theory, who argue for an eternal, quasi-stationary universe, which repeatedly creates matter in strong gravitational fields, assume different regions with different physical properties (e.g. varying particle masses) that are largely isolated from one another.

When scientists speak of other universes or the multiverse, very different things can be meant. However, all variants of the multiverse or “M” hypothesis have in common that our universe represents a whole (Latin “universus” = total), but despite its name it is not unique, not unique (Latin “unus” = the one ).

Critics reject the M-Hypothesis as speculative, extravagant, not verifiable or even unfounded. One reason for this is the inaccessibility of other universes. But why should it be impossible in principle to extrapolate hypotheses about what lies beyond our observation horizon from known laws or facts? It is presumptuous to say that these universes do not exist simply because we can never know anything about them. It would therefore be premature to reject the M-Hypothesis from the outset as a transgression of our speculating reason. The accusation that the M-Hypothesis is extravagant is nonsense - especially since quantum theory with its many bizarre statements has long since blown “common sense”.

Another objection is that the M-hypothesis cannot be tested and is therefore useless. But it does not necessarily claim the complete separation of the individual universes. Therefore, it is not in principle outside of all testability.

It is also criticized that the M-hypothesis violates the economic principle of science. It can be countered by the fact that the M-Hypothesis deals very generously with worlds, but not with different entities, principles and restrictions. And everything stays within the framework of physics.

With the sentence “Everything that is not forbidden is mandatory” from Terence H. White's Arthurian novel “The Once and Future King”, physicists like to assume that everything that is not expressly prohibited by physics also exists. The British cosmologist Dennis William Sciama has argued for this. According to this attempt to reverse the burden of proof, one would have to show that other universes cannot (!) Exist. "According to the traditional view of a unique universe, we must assume that it has been decided that all but one logically possible universe should not exist," wrote Sciama.“But it is completely obscure how such a decision could have been made. If there were only one universe, the problem would arise of understanding why some possible universes do not really exist, while a certain one does. ”And if there are many different universes with possibly completely different laws and constants of nature, then we don't need each other either to wonder why our universe is so "life-friendly" - in the "stillborn" there would simply be no one who could wonder about it.

Certainly the M-Hypothesis is the culmination and end point of a series of “cosmic expulsions” that began with the realization that the earth is not the center of space. The psychoanalyst Sigmund Freud had described this as the first great "offense" of humanity. The end of geocentrism - which was the expression and consequence of an anthropocentric midpoint madness - is relativized to the utmost with the M-hypothesis. But at the same time the philosophical and physical cosmology advanced to the greatest conceivable success story of science and testifies to the intellectual development of a being who was able to break the fetters of his necessarily provincial origin. From the narrow perspective of the terrestrial forests to an insight into a possibly infinite multiverse - what an amazing broadening of horizons! Rüdiger Vaas ■


"We are haunted by the ghost that an infinite number of minimally different copies of ourselves are living their parallel life and that every moment more duplicates come into existence and our many alternative futures enter," said the American Nobel laureate in physics, Frank Wilczek, once brought the Many- Worlds interpretation of quantum mechanics to the point. It goes back to the dissertation of the American physicist Hugh Everett III from 1957. According to him, the universe and all of its inhabitants are constantly “splitting up”, so that all possible alternatives become real and these worlds henceforth go through their own, almost independent development. How this increase occurs is of course controversial; Conservation laws are apparently not violated. Although the Many Worlds interpretation seems bizarre, it has found numerous followers, especially among quantum cosmologists. "Everett's interpretation is an inevitable consequence if one considers quantum theory to be universally valid," says H. Dieter Zeh from the University of Heidelberg with conviction. Likewise, David Deutsch from Oxford University: “The quantum theory of the parallel universes is not an annoying, optional interpretation that stems from mysterious theoretical considerations. It is the explanation - the only tenable one - of a remarkably counterintuitive reality. "


• The assumption that our universe is only one of - perhaps infinitely - many is gaining more and more supporters in modern physics and cosmology.

• There could be perfect doppelganger worlds, but also universes with completely different laws of nature and dimensions - or even everything that is mathematically possible at all.

• The multiverse hypothesis has numerous theoretical advantages, is in principle even verifiable and makes it understandable why we live in a life-friendly universe.


The term “multiverse” today means “multitude of universes”. It has become increasingly popular in recent years, but still competes with “Megaversum”, “Omniversum” and “Ultraversum”. Perhaps these should be kept in reserve as additional generic terms. It is conceivable that there is not only one multiverse made up of many universes - all of which are in contact with one another in a certain way, for example through cosmic inflation - but also many multiverses that are completely and in every respect isolated from one another. The entirety of these could then be called the “omniverse”, of which by definition there can only be one - as originally assumed by the “universe”.

Curiously, “multiverse” used to mean the opposite of today, namely a part or branch of the universe. Andy Nimmo of the Scottish Section of the British Interplanetary Society introduced the term in 1960 as "an apparent universe of which a multitude makes up the whole universe". That was intended with regard to the many-worlds interpretations of quantum physics, which continue to split up. The science fiction author Michael Moorcock then used "Multiversum" from 1962 as the totality of all universes in his "Eternal Champion" short stories and in the novel "The Blood-Red Game". Quantum physicist David Deutsch of Oxford University read this and introduced the term into quantum physics in the opposite sense from Nimmo.

Some critics still believe that there cannot be other universes by definition, because the very word implies uniqueness. But the term “universe” as it is currently used is ambiguous. Whether you want to speak of other universes, as in this issue, and designate their entirety as a multiverse, or whether you prefer to speak of world ensembles or partial universes because you want to reserve “universe” for the totality of (physical) being is one thing rather secondary terminological question. It depends what you mean.

It makes sense to speak of different universes if they are completely or at least largely separated from one another in space or time. They are separated in time when they arise and pass one after the other (like the cyclical worlds in Eastern thought and in Friedrich Nietzsche's “eternal return”). They are spatially separated if they exist in different dimensions and only interact with one another to a limited extent, or if they are in far-away regions that are outside of our observation horizon and are therefore not (yet) in causal contact with us. A kind of embedding is also conceivable: In the 17th century, for example, the French philosopher Blaise Pascal imagined universes inside the atoms of our universe. And recently it was suggested by James Daniel Bjorken, physics emeritus professor at the Stanford Linear Accelerator Center, that our universe is a kind of black hole in a larger universe and that its black holes contain other universes and so on - an endless series of “Russian dolls ".

January 17, 2006