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The paradox of Schrödinger’s cat is so bizarre that one is often reminded of how Alice reacted to the vanishing of the Cheshire cat in Lewis Carroll’s fable: “‘You’ll see me there,’ said the Cat, and vanished. Alice was not much surprised at this, she was getting so well used to queer things happening.” Over the years, physicists, too, have gotten used to “queer” things happening in quantum mechanics.

There are at least three major ways that physicists deal with this complexity. First, we can assume that God exists. Because all “observations” imply an observer, then there must be some “consciousness” in the universe. Some physicists, like Nobel laureate Eugene Wigner, have insisted that quantum theory proves the existence of some sort of universal cosmic consciousness in the universe.

The second way of dealing with the paradox is favored by the vast majority of working physicists—to ignore the problem. Most physicists, pointing out that a camera without any consciousness can also make measurements, simply wish that this sticky, but unavoidable, problem would go away.

The physicist Richard Feynman once said, “I think it is safe to say that no one understands quantum mechanics. Do not keep saying to yourself, if you can possibly avoid it, ‘But how can it be like that?’ because you will go ‘down the drain’ into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.”
⁹
In fact, it is often stated that of all the theories proposed in this century, the silliest is quantum theory. Some say that the only thing that quantum theory has going for it, in fact, is that it is unquestionably correct.

However, there is a third way of dealing with this paradox, called the
many-worlds theory
. This theory (like the anthropic principle) fell out of favor in the past decades, but is being revived again by Hawking’s wave function of the universe.

In 1957, physicist Hugh Everett raised the possibility that during the evolution of the universe, it continually “split” in half, like a fork in a road. In one universe, the uranium atom did not disintegrate and the cat was not shot. In the other, the uranium atom did disintegrate and the cat was shot. If Everett is correct, there are an infinite number of universes. Each universe is linked to every other through the network of forks in the road. Or, as the Argentinian writer Jorge Luis Borges wrote in
The Garden of Forking Paths
, “time forks perpetually toward innumerable futures.”

Physicist Bryce DeWitt, one of the proponents of the many-worlds theory, describes the lasting impact it made on him: “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 of itself. I still recall vividly the shock I experienced on first encountering this multiworld concept.”
¹⁰
The many-worlds theory postulates that
all
possible quantum worlds exist. In some worlds, humans exist as the dominant life form on earth. In other worlds, subatomic events took place that prevented humans from ever evolving on this planet.

As physicist Frank Wilczek noted,

It is said that the history of the world would be entirely different if Helen of Troy had had a wart at the tip of her nose. Well, warts can arise from mutations in single cells, often triggered by exposure to the ultraviolet rays
of the sun. Conclusion: there are many, many worlds in which Helen of Troy
did
have a wart at the tip of her nose.
¹¹

Actually, the idea that there may be multiple universes is an old one. The philosopher St. Albertus Magnus once wrote, “Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of Nature.” However, the new twist on this ancient idea is that these many worlds resolve the Schrödinger cat paradox. In one universe, the cat may be dead; in another, the cat is alive.

As strange as Everett’s many-worlds theory seems, one can show that it is mathematically equivalent to the usual interpretations of quantum theory. But traditionally, Everett’s many-worlds theory has not been popular among physicists. Although it cannot be ruled out, the idea of an
infinite
number of equally valid universes, each fissioning in half at every instant in time, poses a philosophical nightmare for physicists, who love simplicity. There is a principle of physics called Occam’s razor, which states that we should always take the simplest possible path and ignore more clumsy alternatives, especially if the alternatives can never be measured. (Thus Occam’s razor dismisses the old “aether” theory, which stated that a mysterious gas once pervaded the entire universe. The aether theory provided a convenient answer to an embarrassing question: If light is a wave, and light can travel in a vacuum, then what is waving? The answer was that aether, like a fluid, was vibrating even in a vacuum. Einstein showed that the aether was unnecessary. However, he never said that the aether didn’t exist. He merely said it was irrelevant. Thus by Occam’s razor, physicists don’t refer to the aether anymore.)

One can show that communication between Everett’s many worlds is not possible. Therefore, each universe is unaware of the existence of the others. If experiments cannot test for the existence of these worlds, we should, by Occam’s razor, eliminate them.

Somewhat in the same vein, physicists do not say categorically that angels and miracles cannot exist. Perhaps they do. But miracles, almost by definition, are not repeatable and therefore not measurable by experiment. Therefore, by Occam’s razor, we must dismiss them (unless, of course, we can find a reproducible, measurable miracle or angel). One of the developers of the many-worlds theory, Everett’s mentor John Wheeler, reluctantly rejected it because “it required too much metaphysical baggage to carry around.”
¹²

The unpopularity of the many-worlds theory, however, may subside as Hawking’s wave function of the universe gains popularity. Everett’s
theory was based on single particles, with no possibility of communication between different universes as they fissioned. However, Hawking’s theory, although related, goes much further: It is based on an infinite number of self-contained universes (and not just particles) and postulates the possibility of tunneling (via wormholes) between them.

Hawking has even undertaken the daunting task of calculating the solution to the wave function of the universe. He is confident that his approach is correct partly because the theory is well defined (if, as we mentioned, the theory is ultimately defined in ten dimensions). His goal is to show that the wave function of the universe assumes a large value near a universe that looks like ours. Thus our universe is the most likely universe, but certainly not the only one.

By now, there have been a number of international conferences on the wave function of the universe. However, as before, the mathematics involved in the wave function of the universe is beyond the calculational ability of any human on this planet, and we may have to wait years before any enterprising individual can find a rigorous solution to Hawking’s equations.

A major difference between Everett’s many-worlds theory and Hawking’s wave function of the universe is that Hawking’s theory places wormholes that connect these parallel universes at the center of his theory. However, there is no need to wonder whether you will someday walk home from work, open the door, enter a parallel universe, and discover that your family never heard of you. Instead of rushing to meet you after a hard day’s work, your family is thrown into a panic, scream about an intruder, and have you thrown in jail for illegal entry. This kind of scenario happens only on television or in the movies. In Hawking’s approach, the wormholes do, in fact, constantly connect our universe with billions upon billions of parallel universes, but the size of these wormholes, on the average, is extremely small, about the size of the Planck length (about a 100 billion billion times smaller than a proton, too small for human travel). Furthermore, since large quantum transitions between these universes are infrequent, we may have to wait a long time, longer than the lifetime of the universe, before such an event takes place.

Thus it is perfectly consistent with the laws of physics (although
highly
unlikely) that someone may enter a twin universe that is precisely like
our universe except for one small crucial difference, created at some point in time when the two universes split apart.

This type of parallel world was explored by John Wyndham in the story “Random Quest.” Colin Trafford, a British nuclear physicist, is almost killed in 1954 when a nuclear experiment blows up. Instead of winding up in the hospital, he wakes up, alone and unhurt, in a remote part of London. He is relieved that everything appears normal, but soon discovers that something is very wrong. The newspaper headlines are all impossible. World War II never took place. The atomic bomb was never discovered.

World history has been twisted. Furthermore, he glances at a store shelf and notices his own name, with his picture, as the author of a best-selling book. He is shocked. An exact counterpart of himself exists in this parallel world as an author instead of a nuclear physicist!

Is he dreaming all this? Years ago, he once thought of becoming a writer, but instead he chose to become a nuclear physicist. Apparently in this parallel universe, different choices were made in the past.

Trafford scans the London telephone book and finds his name listed, but the address is wrong. Shaking, he decides to visit “his” home.

Entering “his” apartment, he is shocked to meet “his” wife—someone he has never seen before—a beautiful woman who is bitter and angry over “his” numerous affairs with other women. She berates “him” for his extramarital indiscretions, but she notices that her husband seems confused. His counterpart, Trafford finds out, is a cad and a womanizer. However, he finds it difficult to argue with a beautiful stranger he has never seen before, even if she happens to be “his” wife. Apparently, he and his counterpart have switched universes.

He gradually finds himself falling in love with “his” own wife. He cannot understand how his counterpart could ever have treated his lovely wife in such a despicable manner. The next few weeks spent together are the best of their lives. He decides to undo all the harm his counterpart inflicted on his wife over the years. Then, just as the two are rediscovering each other, he is suddenly wrenched back into his own universe, leaving “his” love behind. Thrown back into his own universe against his will, he begins a frantic quest to find “his” wife. He has discovered that most, but not all, people in his universe have a counterpart in the other. Surely, he reasons, “his” wife must have a counterpart in his own world.

He becomes obsessed, tracking down all the clues that he remembers from the twin universe. Using all his knowledge of history and physics, he concludes that two worlds diverged from each other because of some
pivotal event in 1926 or 1927. A single event, he reasons, must have split the two universes apart.

He then meticulously traces the birth and death records of several families. He spends his remaining savings interviewing scores of people until he locates “his” wife’s family tree. Eventually, he succeeds in tracking down “his” wife in his own universe. In the end, he marries her.

One Harvard physicist who has jumped into the fray concerning wormholes is Sidney Coleman. Resembling a cross between Woody Allen and Albert Einstein, he shuffles through the corridors of Jefferson Hall, trying to convince the skeptics of his latest theory of wormholes. With his Chaplinesque moustache, his hair swept back like Einstein’s, and his oversize sweatshirt, Coleman stands out in any crowd. Now he claims to have solved the celebrated cosmological constant problem, which has puzzled physicists for the past 80 years.

His work even made the cover of
Discover Magazine
, with an article entitled “Parallel Universes: The New Reality—From Harvard’s Wildest Physicist.” He is also wild about science fiction; a serious science-fiction fan, he even co-founded Advent Publishers, which published books on science-fiction criticism.

At present, Coleman vigorously engages the critics who say that scientists won’t be able to verify wormhole theories within our lifetime. If we believe in Thorne’s wormholes, then we have to wait until someone discovers exotic matter or masters the Casimir effect. Until then, our time machines have no “engine” capable of shooting us into the past. Similarly, if we believe in Hawking’s wormholes, then we have to travel in “imaginary time” in order to travel between wormholes. Either way, it a very sad state of affairs for the average theoretical physicist, who feels frustrated by the inadequate, feeble technology of the twentieth century and who can only dream of harnessing the Planck energy.

This is where Coleman’s work comes in. He recently made the claim that the wormholes might yield a very tangible, very measurable result in the present, and not in some distant, unforeseeable future. As we pointed out earlier, Einstein’s equations state that the matter-energy content of an object determines the curvature of space-time surrounding it. Einstein wondered whether the pure vacuum of empty space could contain energy. Is pure emptiness devoid of energy? This vacuum energy is measured by something called the
cosmological constant;
in principle,
there is nothing to prevent a cosmological constant from appearing in the equations. Einstein thought this term was aesthetically ugly, but he could not rule it out on physical or mathematical grounds.

In the 1920s, when Einstein tried to solve his equations for the universe, he found, much to his chagrin, that the universe was expanding. Back then, the prevailing wisdom was that the universe was static and unchanging. In order to “fudge” his equations to prevent the expansion of the universe, Einstein inserted a tiny cosmological constant into this solution, chosen so it would just balance out the expansion, yielding a static universe by fiat. In 1929, when Hubble conclusively proved that the universe is indeed expanding, Einstein banished the cosmological constant and said it was the “greatest blunder of my life.”