*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