dibly lucky that it's as small as it is. However, if we admit the possibility of a multiverse containing countless universes, then most of these universes would have a cosmological constant way too large for life to exist, but a few, including our own, would be habitable.

A fourth reason for believing in the multiverse comes from string/M theory. These are theories which attempt to unify all the forces and in principle be capable of explaining everything. According to string theory, the universe has 10 dimensions and according to M theory it has 11. So where are these extra dimensions? According to these theories, we do not observe the extra dimensions because they are compactified (curled up) very tightly. It turns out that there are some 10^500 ways to curl up the extra dimensions, but our universe chose only one way. Which one? Some string

There is a recent trend in science to believe in the existence of a multiverse, a vast collection of parallel universes, each with its own physical constants and possibly even with distinct physical laws, our universe being just one member. There are several reasons for this new idea. One comes from the many worlds interpretation of quantum mechanics as formulated by Hugh Everett III in 1957. Another comes from the idea of eternal inflation, put forth by Andre Linde in 1986. A third reason is in order to explain the anthropic principle, by which the conditions of our universe seem to be just right for the formation of intelligent life, such as ours. A fourth reason comes from trying to make sense out of string theory and M theory, which seem to predict the existence of countless ways to compactify the six or seven unseen dimensions of the universe.

Anyone who has studied quantum mechanics in any detail must admit that it's quite baffling. In fact, Richard Feynman once said that if quantum mechanics doesn't confuse you, then you don't really understand it. Among the bizarre predictions of quantum mechanics, which have all been demonstrated, are wave-particle duality, the uncertainty principle, and tunneling. Now one of the most popular interpretations of quantum mechanics is the many worlds hypothesis, which asserts that every time a quantum mechanical system is observed, the universe splits into two or more mutually unobservable branches. For instance, in the famous double-slit experiment, in which a photon appears to pass through two slits at once, the many worlds hypothesis states that in one universe it passes through the left slit and in another universe it passes through the right slit, and which slit is passes through in our universe depends on the result of our observation of which slit it passes through. Since we make countless observations, and in fact, every particle interaction may be regarded as an "observation" of some sort, according to this hypothesis, there are essentially infinitely many universes, each of which involves different histories.

A second reason for believing in a multiverse comes from cosmology. In 1980, Alan Guth devised a theory of cosmic inflation, whereby the very early universe (from about 10^-35 to 10^-32 seconds after the big bang) underwent a tremendous expansion known as inflation, involving an increase in size by a factor of 10^50 or so. In 1986, Andre Linde proposed a theory in which inflation is still going on in vastly distant regions of the universe, each with their own big bang. According to this theory, our universe is but a bubble, or "pocket universe", among an unimaginably vast collection of other pocket universes, each with its own big bang and possibly its own physics.

A third reason comes from attempting to explain the anthropic principle. It has been shown that if the values of several physical constants had differed by a slight amount, then life would not have been possible. The most dramatic example of this is the value of the cosmological constant, which determines how fast the rate of universal expansion is accelerating or decelerating. According to quantum field theory, its value should be approximately 10^120 times larger than its actual value! But if its value were just ten times larger, then galaxies would not have been able to form and life as we know it would not be possible, so we appear to be incredibly lucky that it's as small as it is. However, if we admit the possibility of a multiverse containing countless universes, then most of these universes would have a cosmological constant way too large for life to exist, but a few, including our own, would be habitable.

A fourth reason for believing in the multiverse comes from string/M theory. These are theories which attempt to unify all the forces and in principle be capable of explaining everything. According to string theory, the universe has 10 dimensions and according to M theory it has 11. So where are these extra dimensions? According to these theories, we do not observe the extra dimensions because they are compactified (curled up) very tightly. It turns out that there are some 10^500 ways to curl up the extra dimensions, but our universe chose only one way. Which one? Some string/

Anyone who has studied quantum mechanics in any detail must admit that it's quite baffling. In fact, Richard Feynman once said that if quantum mechanics doesn't confuse you, then you don't really understand it. Among the bizarre predictions of quantum mechanics, which have all been demonstrated, are wave-particle duality, the uncertainty principle, and tunneling. Now one of the most popular interpretations of quantum mechanics is the many worlds hypothesis, which asserts that every time a quantum mechanical system is observed, the universe splits into two or more mutually unobservable branches. For instance, in the famous double-slit experiment, in which a photon appears to pass through two slits at once, the many worlds hypothesis states that in one universe it passes through the left slit and in another universe it passes through the right slit, and which slit is passes through in our universe depends on the result of our observation of which slit it passes through. Since we make countless observations, and in fact, every particle interaction may be regarded as an "observation" of some sort, according to this hypothesis, there are essentially infinitely many universes, each of which involves different histories.

A second reason for believing in a multiverse comes from cosmology. In 1980, Alan Guth devised a theory of cosmic inflation, whereby the very early universe (from about 10^-35 to 10^-32 seconds after the big bang) underwent a tremendous expansion known as inflation, involving an increase in size by a factor of 10^50 or so. In 1986, Andre Linde proposed a theory in which inflation is still going on in vastly distant regions of the universe, each with their own big bang. According to this theory, our universe is but a bubble, or "pocket universe", among an unimaginably vast collection of other pocket universes, each with its own big bang and possibly its own physics.

A third reason comes from attempting to explain the anthropic principle. It has been shown that if the values of several physical constants had differed by a slight amount, then life would not have been possible. The most dramatic example of this is the value of the cosmological constant, which determines how fast the rate of universal expansion is accelerating or decelerating. According to quantum field theory, its value should be approximately 10^120 times larger than its actual value! But if its value were just ten times larger, then galaxies would not have been able to form and life as we know it would not be possible, so we appear to be incredibly lucky that it's as small as it is. However, if we admit the possibility of a multiverse containing countless universes, then most of these universes would have a cosmological constant way too large for life to exist, but a few, including our own, would be habitable.

A fourth reason for believing in the multiverse comes from string/M theory. These are theories which attempt to unify all the forces and in principle be capable of explaining everything. According to string theory, the universe has 10 dimensions and according to M theory it has 11. So where are these extra dimensions? According to these theories, we do not observe the extra dimensions because they are compactified (curled up) very tightly. It turns out that there are some 10^500 ways to curl up the extra dimensions, but our universe chose only one way. Which one? Some string

slit, and which slit is passes through in our universe depends on the result of our observation of which slit it passes through. Since we make countless observations, and in fact, every particle interaction may be regarded as an "observation" of some sort, according to this hypothesis, there are essentially infinitely many universes, each of which involves different histories.

A second reason for believing in a multiverse comes from cosmology. In 1980, Alan Guth devised a theory of cosmic inflation, whereby the very early universe (from about 10^-35 to 10^-32 seconds after the big bang) underwent a tremendous expansion known as inflation, involving an increase in size by a factor of 10^50 or so. In 1986, Andre Linde proposed a theory in which inflation is still going on in vastly distant regions of the universe, each with their own big bang. According to this theory, our universe is but a bubble, or "pocket universe", among an unimaginably vast collection of other pocket universes, each with its own big bang and possibly its own physics.

A third reason comes from attempting to explain the anthropic principle. It has been shown that if the values of several physical constants had differed by a slight amount, then life would not have been possible. The most dramatic example of this is the value of the cosmological constant, which determines how fast the rate of universal expansion is accelerating or decelerating. According to quantum field theory, its value should be approximately 10^120 times larger than its actual value! But if its value were just ten times larger, then galaxies would not have been able to form and life as we know it would not be possible, so we appear to be incredibly lucky that it's as small as it is. However, if we admit the possibility of a multiverse containing countless universes, then most of these universes would have a cosmological constant way too large for life to exist, but a few, including our own, would be habitable.

A fourth reason for believing in the multiverse comes from string/M theory. These are theories which attempt to unify all the forces and in principle be capable of explaining everything. According to string theory, the universe has 10 dimensions and according to M theory it has 11. So where are these extra dimensions? According to these theories, we do not observe the extra dimensions because they are compactified (curled up) very tightly. It turns out that there are some 10^500 ways to curl up the extra dimensions, but our universe chose only one way. Which one? Some string

According to this theory, our universe is but a bubble, or "pocket universe", among an unimaginably vast collection of other pocket universes, each with its own big bang and possibly its own physics.

A third reason comes from attempting to explain the anthropic principle. It has been shown that if the values of several physical constants had differed by a slight amount, then life would not have been possible. The most dramatic example of this is the value of the cosmological constant, which determines how fast the rate of universal expansion is accelerating or decelerating. According to quantum field theory, its value should be approximately 10^120 times larger than its actual value! But if its value were just ten times larger, then galaxies would not have been able to form and life as we know it would not be possible, so we appear to be incredibly lucky that it's as small as it is. However, if we admit the possibility of a multiverse containing countless universes, then most of these universes would have a cosmological constant way too large for life to exist, but a few, including our own, would be habitable.

A fourth reason for believing in the multiverse comes from string/M theory. These are theories which attempt to unify all the forces and in principle be capable of explaining everything. According to string theory, the universe has 10 dimensions and according to M theory it has 11. So where are these extra dimensions? According to these theories, we do not observe the extra dimensions because they are compactified (curled up) very tightly. It turns out that there are some 10^500 ways to curl up the extra dimensions, but our universe chose only one way. Which one? Some string

dibly lucky that it's as small as it is. However, if we admit the possibility of a multiverse containing countless universes, then most of these universes would have a cosmological constant way too large for life to exist, but a few, including our own, would be habitable.

A fourth reason for believing in the multiverse comes from string/M theory. These are theories which attempt to unify all the forces and in principle be capable of explaining everything. According to string theory, the universe has 10 dimensions and according to M theory it has 11. So where are these extra dimensions? According to these theories, we do not observe the extra dimensions because they are compactified (curled up) very tightly. It turns out that there are some 10^500 ways to curl up the extra dimensions, but our universe chose only one way. Which one? Some string/M theorists now believe that there exists a multiverse consisting of separate universe with each type of compactification, though others insist that there's only one compactification and that the theory will ultimately predict which is the correct one, i.e. the one corresponding to our universe.

The multiverse is a fascinating idea with potentially mind boggling consequences. For instance, if the multiverse consists of infinitely many universes, it must contain infinitely many with life, and in fact, infinitely many with life very much like our own. This would mean that somewhere in the multiverse, there are countless copies of you and me in which circumstances turned out only slightly differently. To make matters worse, it has been shown that if the multiverse idea is correct, then the vast majority of universes with intelligent life are in fact fakes, i.e. virtual reality programs being run on a huge computer. So is our universe a fake? I certainly hope not! In any case, the multiverse idea is quite fascinating and it will be interesting to see how it develops, once we learn more.

An excellent reference on the subject, which is the basis for most of this article, is "Cosmic Jackpot" by the well-renowned cosmologist Paul Davies.