Outer Limits of Reason (54 page)

The theory of a multiverse comes in many different flavors. Over the years, scientists have developed various theories explaining why there is a multiverse rather than a universe. Recently Brian Greene published an excellent book,
The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos
, where he describes nine different variations of the multiverse theme. I will briefly describe some of them in the coming paragraphs. A word of caution is in order. Some of these ideas are totally insane, to put it mildly. They take science fiction to new levels and come to very strange conclusions.

We first met the concept of a multiverse in 
section 7.2
when we discussed Hugh Everett's theory that every time there is a measurement, the universe splits into different universes. For every measurement, there are different possible outcomes and the universe splits into that many exact copies with each daughter universe having that particular outcome. Since there are millions of observations every second, there are going to be billions and billions of universes. It is not clear how these universes differ in their physical laws. Nor is it clear how these universes can have nonintelligent life if only observers can make measurements. Nevertheless, this was the first theory of a multiverse.

We met string theory several times on these pages. As we have seen, the idea that the universe is made out of very small strings solves many problems in physics. It turns out that string theory also predicts the existence of a multiverse. The strings wiggle around in different multidimensional spaces called
branes
(short for membranes) or
D-branes
. String theorists believe that there are many different branes and they can bang into each other and cause new universes with many different types of properties. There are so many types of universes that the multiverse is called the
string-theory landscape.
With so many universes, it is not surprising that some parts of this landscape have life in them.

The above two explanations for a multiverse posit many different universes, but all of them will be as improbable as ours. While the hypothesized multiverses answer the questions we asked, things seem a bit too random. Lee Smolin in his book
The Life of the Cosmos
has proposed an interesting model of the multiverse where universes with intelligent life are more probable than universes without life. He puts forward the idea that one universe can emerge from a collapsed black hole of another universe. This new universe can have many of its own black holes and in turn many baby universes. He further postulates that the laws of physics for the new daughter universes will be only slightly different than the laws for the parent universe. This mechanism gives the universe a certain aura of natural selection. The universes that have more black holes will have more daughter universes and are more likely to survive. Since black holes come from heavy suns where stellar nucleosynthesis can occur, there will be more universes with such heavy suns. In Smolin's multiverse, universes are evolutionary, coming closer and closer to being life-permitting.

Max Tegmark of MIT has a very interesting notion of a multiverse. He takes Plato and Pythagoras to an extreme and believes that the only thing that really exists is mathematics. Every type of mathematical structure that can exist, does exist. If the structure is coherent and within reason, then it exists. Some of those systems describe life-sustaining universes. Some mathematical structures even describe life forms that have some mental processes that we might call intelligent. And there are even mathematical structures that describe creatures that can ponder their own existence. We happen to find ourselves in a universe of the Tegmark multiverse where the mathematics is sophisticated enough for human beings. Tegmark explains why we don't see mathematics but we see trees, flowers, and humans. On the one hand, this theory clearly violates Occam's simplicity-of-ontology rule since
everything
exists. On the other hand, since there is no choosing what does and does not exist, there are actually fewer rules in such a multiverse. In other words, Tegmark's multiverse does satisfy the simplicity-of-hypothesis criterion. This intriguing idea deserves much more thought but is beyond the scope of this book.

To me, the most interesting versions of the multiverse are a consequence of some of the ideas we met in
section 7.2
with our short exploration of quantum mechanics. One of the main ideas was that a property of an object is in a superposition of values until it is observed by a conscious being, and then it collapses to a single value. John Wheeler applies this concept to the entire universe. When the universes came into being, there were no human observers and so everything that existed was in a hazy superposition of values. But rather than thinking of this as a hazy superposition, think of it as a form of multiverse within our universe. Objects had many possible values in our single universe.
⁵³
Each of these superpositions followed the laws of physics (or perhaps many different laws of physics) and continued on that path. The theory goes on to say that one of the many possible superpositions developed a complicated life or consciousness with the capacity to observe the surrounding universe, as in
figure 8.8
.
⁵⁴
This observation (symbolized by the eye in the figure) collapsed the entire superposition into the one universe we know and love. The superposition that brought along consciousness caused the superposition to collapse. This theory is called the
participatory anthropic principle
. The observer participated in the creation of the universe and permits there to be observers. All the other superpositions that did not spawn sentient beings (they are without eyes in the figure) simply collapsed away. It is important to realize that if the participatory anthropic principle is true, not only would it be a legitimate explanation for the weak anthropic principle but it would actually satisfy the strong anthropic principle. The universe would stay in a hazy superposition unless there was an intelligent observer. Wheeler goes further with this theory. We saw with the delayed-choice quantum eraser experiment in
section 7.2
that outcomes of experiments can change the past. More exactly, the outcome of an experiment depends on the total experiment. Perhaps we can say that the universe's past depends on the existence and observation of human observers.
⁵⁵

We saw how a multiverse would help us answer the questions about why intelligent life exists in the universe. We also glimpsed several different types of multiverses and how they work. However, the idea of the multiverse is not without its critics. The most obvious objection to a multiverse is that there is no empirical evidence for any multiverse. We live in one universe and we only see one universe. This book is only being published in this universe (at least I have not received any royalties from any other universe), and there is no empirical evidence of any other universe. If they do exist where are they? What do they look like? We have simply postulated them because they help explain the existence of intelligent life, they help us with determinism (Everett's many-worlds interpretation), or their math says it is so (string theory), but that does not make it so. Yes, they lend support to the anthropic principle, but that does not make their existence an actual fact. Your financial problems would be solved if you won the lottery, but that does not mean that you, in fact, won the lottery.

There are other objections to a multiverse. All these concepts of a multiverse have laws that explain how the universes branch off each other and come into existence. These laws are not particular to one universe but are laws for the whole multiverse and are called
superlaws
or
metalaws.
Now we can ask a deeper question: Why are these superlaws set up so perfectly that some universes will produce intelligent life? The superlaws are created so that different universes will have different characteristics and some will be well suited for intelligent life. Why? We first asked why there is a structure in the universe that makes intelligent life possible. This was answered by positing the notion of a multiverse and saying that the reason there is intelligent life in our universe is because our universe is just one part of the vast multiverse. Now we are asking why there is a structure
in the multiverse
that can bring about intelligent life.
⁵⁶

Yet another criticism of a multiverse is that it is often invoked as an ad hoc alternative to the notion of an intelligent designer. In other words, an atheistic scientist would rather posit the existence of a multiverse than a deity of some sort. Neil Manson, a philosopher, called the concept of multiverse “the last resort for the desperate atheist.”
⁵⁷
In fact, the notion of a multiverse is just as unscientific as a deity. That does not mean they are equally possible or equally probable. It just means they are unobservable, unprovable, undeniable, and untestable. Many critics say that positing the existence of a multiverse demands the same leap of faith as most religions do.

Some scientists deny that the very concept of a multiverse is even science. A multiverse is neither empirical nor testable. If the different universes in a multiverse do not interact with each other, how can we even test if there are other universes? In other words, one of the deepest questions in all of science and perhaps existence is answered by many scientists with an answer that is, by definition, beyond the limits of science. But whether the concept of a multiverse is, in fact, science should not stop us from thinking about multiverses. We think about many topics that are not exactly science. They are interesting ideas and they just might explain our universe.

Symmetry

There is another appealing group of ideas that help explain the structure of the universe. The gist of these ideas is that any structure in the universe that we observe comes from the fact that we are observing the universe in a certain way.

In a sense, some of these ideas—like so many other ideas in philosophy—are a response to the problems raised by David Hume. In
section 8.1
we saw that Hume's problem of induction was extremely vexing. He called into doubt the very notion of cause and effect, which is central to all of science. Immanuel Kant took these problems as a wakeup call. He tried to address the problems by saying that humans do not look at phenomena through clear glasses. According to Kant, we look at the world through colored glasses. We have preconceived notions that are built into us and help us understand and categorize all the phenomena we see. Notions like space, time, and causality are ideas that are part of us, and we use these ideas to make sense of the universe. These notions preexist in us and do not come from experience. With these hardwired notions, the universe looks the way it does. Without these notions, we would not be able to see the structure that we do see. For Kant, our view of the universe is influenced by our own mind and we cannot observe what is really out there “in itself” without these built-in notions. This is a step away from the traditional view of the relationship between human beings and the universe. The traditional view is that the universe is the way it is and we are viewing it that way. Kant is advocating the notion that our view of the universe is dependent on our perspective on it.

Einstein was also interested in how we view different phenomena. As we saw in 
section 7.2
, he formulated relativity theory by insisting that the laws of physics should be the same regardless of how they are observed. Before Einstein, Galileo insisted that the laws of physics should remain the same as long as the observer is moving at a constant velocity. Einstein generalized this with special relativity, where he insisted that the laws of physics must remain fixed regardless of whether an observer is moving at a constant velocity or moving close to the speed of light. His general relativity further generalized this. He insisted that the laws should be the same even when changing velocity (i.e., accelerating or decelerating). This fact that the laws of physics must remain the same regardless of how the viewer observes them reflects a type of
symmetry
. Colloquially, we say that a room has symmetry if the room looks the same after the left and right sides are swapped. Scientists extend this notion of symmetry to the way laws of nature are described. The laws look the same if they are viewed from diverse perspectives. As science progresses, the notion of symmetry is playing an ever-increasing role.

Einstein actually did something more radical here. Before Einstein, most physicists would find and describe a law of physics and then go on and describe its properties such as its symmetries. In the case of relativity theory,
Einstein used symmetry to find the laws of physics
. He postulated that the law must satisfy these symmetries and then went on to describe that law that satisfies them. Anything that did not satisfy the symmetries could not be a law of physics. He was the first person to use symmetries as an important arbitrator or sieve that determines what is or is not a law of physics.