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Posted 23 February 2008

The Multiverse

by Frikkie de Bruyn



When we apply the quantum theory to the universe, we are then fored to admit the possibilty that the universe exists simultaneously in many states. In other words, once we open the door to applying quantum fluctuations to the universe, we are almost forced to admit the possibiltyof parallel universes. It seems we have little choice. - Michio Kaku

Quantum weirdness
We are all familiar with Schrodinger's thought experiment in which a cat is in a box
with a bottle of poison gas connected to a hammer, which in turn is connected to a
Geiger counter placed near radio active uranium which will activate the hammer
when the radio active uranium decays, smashing the bottle of poison, killing the cat.
How can you determine if the cat is alive or dead? Open the box and you can solve
the mystery. This is how it works in the macroscopic world. In the quantum world,
however, the cat is both alive and dead before we open the box. This sounds absurd.
To solve the mystery we must add the wave function of the cat in all possible states
(alive, dead, running, sleeping, eating, etc.) until we can do a measurement. One
interpretation, known as the Copenhagen interpretation, is that the act of observation
causes the wave function of the cat to collapse and the quantum indeterminacy to
cease. In terms of this interpretation we ask the question “in what state is the cat
before we open the box?” It appears that the cat is both dead and alvive until our
observation causes the wave function to collapse. Is this some kind of joke you may
ask? The answer is in the quantum world both probabilties are equally real.
There was another radical interpretation pioneered by Hugh Everett, a student of the
legendary physicist, John Wheeler, in which the cat is both alive and dead because
the universe has split into two. In one universe the cat is alive and in another the cat is
dead. Each time there is a quantum possibility, the universe splits in half. The
splitting of the universe is never ending. All universes are possible in this scenario,
each as real as the other. People living in each universe regard their own as real. In
this interpretation all these parallel universes are real with objects and events as real
as any other. This interpretation has the advantage that we can drop the collapse of
the wave function. According to this interpretation the cat's wave function never
collapse, it is forever splitting into other wave functions, into an entire new universe.

The Many Worlds Theory
Everett's interpretation of the quantum measurement problem is known as the many
worlds theory. There is a price to pay because we have universes that continually split
into millions of branches. Is it possible to keep track of all these universes?
Theoretically at least we can do it by tracing the evolution of the wave function
which enables one to immediately find the numerous branches of the wave. If we
accept this interpretation then your body coexists with the wave functions of the first
people who entered this part of Africa. Those parallel universes are all in the room
with you. The problem is we can no longer make contact with them because they
have decohered from us, that is, the crests of their wavefunctions are no longer
reinforced by the crest of our wavefunction. The Nobel laureate Stephen Weinberg
explain the multiple universe theory by referring to different radio waves
broadcasting from different stations. Your room or car is full of these radio waves but
you can listen to only one frequency at a time. The other frequencies have decohered
and are not in phase with each other. Each frquency have a different energy content
(according to its wavelength) and therefore a different frequency. You can listen to
one frequency at a time. Our universe is tuned into the frequency corresponding to
our reality. Although we cannot tune into the other parallel universes existing in the
room because they have different energies, they are as real as our own universe. In
one universe Schrodinger's cat is alive and in a parallel universe the cat is dead. This
interpretation is getting more and more support among scientists, particularly
theorists researhing the M theory.

The Quantum Multiverse
Another idea of a limitless number of universes stems from the quantum origin of the
universe. The universe was born from a quantum object far smaller than an atom and
the laws of quantum physics ruled supreme. The origin of many universes is
described in Alan Guth's inflationary universe theory which is a quantum theory. The
inflaton field (an inflaton field is a scalar field which created anti-gravity and which
made the universe expand exponentially), is subject to Heisenberg's uncertainty
principle and its strength will randomly fluctuate. Certain spots in the field will
undergo a stronger rate of inflation than others. It must be stressed that the regions of
inflation are rather rare because the field is unstable and will mosly decay wanting to
bring inflation to a stop. Regions that experience inflation are surrounded by regions
that have ceased inflating and the expanding regions will evolve into conventional
expanding universes (conventional expansion meaning at a decelerating rate). The
inflation from the quantum fluctuations is unstoppable and new universes are
constantly created; therefore the multiverse itself is embedded in a never ending
inflating space. Guth refer to his theory as 'an infinite chain reaction of creation and
self-reproduction which has no end and which may have no beginning'. In the
inflationary universe theory what we have been calling 'the universe' is a very small
part of a single bubble, also called pocket universe, set amid an infinite number of
universes which we call a multiverse while the multiverse itself is embedded in
inflating space with no end. The well known theoretical physicist, Leonard Susskind,
called it 'the bubble bath universe'.

Where does the big bang feature in this endless sea of inflating universes? Each
pocket universe will be born in a tremendous burst of heat when inflation occurs in
that bubble. Thus, the 'big bangs are occurring endlessly and each new born universe
will enjoy a life cycle like our own depending on the laws of physics in the new born
universe. There is no reason why the laws of physics should be the same as in our
bubble universe. If gravity is very strong in such a universe there will be no
expansion and the universe will never develop. If, on the other hand, gravity is very
weak, the expansion will be so fast that no stars or galaxies can develop. Where can
we find these other universes? This is very interesting. Unlike the parallel universes
we were introduced to above, the universes described here are incredibly far away.
Paul Davies estimated that our universe is embedded in a region about
10^10,000,000,000 km across. Compare this with the size of our own observable
universe of only 10^23 km.

The idea of eternal inflation marked an important shift in cosmology. Suddenly our
universe is one of an infinite number of universes, the multiverse. Predictably, many
scientists and philosophers rejected the idea, despite its widespread appeal. I will very
briefly deal with the reasons for acceptance and rejection in the scientific community.
Inflationary universe explained the assumptions in cosmology of homogeneity
(matter is uniformly spread throughout space), isotropy (at large scales the universe
looks the same in every direction) and the cosmological principle (every observer in
every galaxy sees the same general features of the universe), which the Big Bang
Theory alone could not. It further explains why the early universe was very smooth,
(if we ignore the quantum fluctuations), which was a prerequisite for the second law
of thermodynamics and the apparent one-directional cosmology time of which we are
all aware. The reader should note that this is not what we understand as 'time' when
we look at our watches. Why are the parallel universes in our rooms while the
quantum multiverses from the birth of the universe are so far away from us? Parallel
universes stem from a quantum interpretation of the 'measurement problem, i.e. How
to determine if the cat is alive or dead without having to open the box. The quantum
multiverses on the other hand is an interpretation of our universe and others on a
cosmological scale. In quantum language, both interpretations are equally valid.
Prominent scientists like Paul Steinhardt, Albert Einstein Professor at Princeton
University finds the concept distateful and declared: “This is a dangerous idea that I
am simply unwilling to contemplate”. Why such condemnation? The answer is
simply that scientists like Steinhardt are theoretical physicists hard at work to
formulate a final theory, a theory of everything. They regard the multiverse concept
as 'cheap'. These physicists are trying to find deep reasons why the universe is the
way it is while the multiverse concept declares that we observe what we observe
because it is observable. The randomness and the observer selection strike many
physicists as ugly compared to an all-encompassing mathematical theory describing
the world with quantative preision.

Can the multiverse theory be tested?
The critcism that the multiverse theory is not science because it cannot be tested has
some truth. We have seen that such universes cannot be observed because they have
decohered from our universe and the quantum multiverses are so far away that it is
impossible to prove their existence. Indirectly, however, if you have confidence in a
theory, you may trust its predictions, such as general relativity and the inside of black
holes. If the M theory is successful in proving the existence of other universes there is
a distant prospect that the existence of multiverses might be proved.

Frikkie de Bruyn

 


Suggested further reading.
Gell-Mann, M. (1994). The Quark and the Jaguar. W.H. Freeman, New York.
Greene, B. (2004). The Fabric of the Cosmos. Space, Time and the Texture of
Reality. Allen Lane, London.
Greene, B. (1999). The Elegant Universe. Random House, London.
Hawking, S.W. (2001). The Universe in a Nutshell. Bantam, New York.
Paul. D. (1995). About Time. Simon and Schuster, New York.
Penrose, R. (2004). The Road to Reality. A Complete Guide of the Laws of the
Universe. Vintage Books, London.
Rees, M. (1997). Before the Beginning. Addison-Wesley, Reading, Mass.



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