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to shoot his earlier self, then the gun will misfire or the bullet will simply miss. The cosmic censor, then,
ensures that any and every attempt by the time traveler to alter his personal history will be frustrated.
3. Parallel universes
This possibility is more complex and involves the quantum rules which govern the subatomic level of
the Universe. Put simply, when the time traveler kills the grandfather he immediately creates a new
quantum universe, in essence a parallel universe, where the young grandfather never existed and
where our time traveler is never born. The original universe still remains. Namely, if a time traveler
journeyed into the past, and introduced the change, then reality splits into two directions, with one fork
representing the change and other fork the original reality before change.
This fantastic view actually has scientific plausibility, because of the so-called Many-worlds
interpretation of quantum mechanics, pioneered by Hugh Everett in his 1957 Princeton doctoral
dissertation. Recently, Stephen Hawking by applying the quantum mechanics to the cosmology,
elaborated the similar idea, and he believes he can explain the origin of our Universe in a variation of
this parallel-worlds theme. This brings us to the subject of Multiverse theory.
Multiverse Theory
Quantum cosmology: At first, it is contradiction in terms. World quantum applies to the infinitesimally
small world of atomic and subatomic particles, while cosmology signifies the almost limitless
expansion of outer space. However, Hawking and many other believe that the ultimate questions of
cosmology can be answered by quantum theory.
The starting point of quantum theory is wave function that describes all the possible states of a
particle. As superbly illustrated in the Michio Kaku’s book “Hyperspace”, imagine a large irregular
thundercloud that fills up the sky. The darker the cloud, the greater the concentration of water vapor
and dust in the point. Therefore, simply by looking into the cloud, we can rapidly estimate the
probability of finding large concentrations of water and dust in certain regions of the sky. The
thundercloud can be compared to a single electron’s wave function. It fills up the space. Likewise, the
greater its value in the point, the greater probability of finding the electron there.
Hawking’s new idea is to treat the entire Universe as a single particle. We begin with a wave function
describing the set of all possible universes. According to this picture the wave function of the universe
spread over all possible universes. Therefore, it is more appropriate to speak of Multiverse as the
entity consisting of all universes.
Anthropic Principle and Philosophical Implications
Multiverse theory raises philosophical and perhaps even religious questions. Over the centuries
scientists have learned that the Universe is largely independent of human bias. Historically, early
scientists often committed the fallacy of anthropomorphism, which assumes that the objects and
animals have humanlike characteristics. However, there is some scientific merit to the debate over the
anthropic principle which revolves over the indisputable fact that the physical constants in our
Universe are tunned so precisely that they lead to the existence of (intelligent) life. If they were altered
only by a smallest amount, life in the Universe would be impossible.
Is this just a fortunate coincidence or does it show the work o Supreme Being? If Multiverse theory is
correct, there are infinite number of parallel universes with different physical constants. On one such
parallel universe (ours) the laws of physics are compatible with the life as we know it. If this is true,
than perhaps the Supreme Being does not have to be evoked to explain why life, precious as it is, is
possible in our Universe. Bluntly, according to Multiverse theory, life is statistically possible, or even
inevitable in at least one of the parallel universes.
What is the Price of a Time Travel Ticket?
In principle, physics does allow time travel via closed timelike curves. But, traveling through time has
been proved to be incredibly difficult. Princeton University physicists J. Richard Gott and Li-Xin Li at
Princeton have speculated on whether space-time could both allow travel into the past and insure a
consistent chronology (the traveler must remember shaking hands with his older self as he sets off).
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