>> We're entering the realm of mathematical cosmology.

It's always been puzzling, even mysterious to physicists,

that mathematics describes so beautifully the universe.

This was a point made by Einstein in his general theory of relativity.

A beautiful set of second order partial differential equations

that seems to describe the behavior of the entire universe.

It's also true in the multidimensional geometry that underlies string theory.

Beautiful mathematics, in its own right, whether or

not it has any application to the real world.

We can imagine, therefore,

levels of reality that we might be able to inspect with science.

The first is the fact that the model tells us there are regions in the Big Bang model

that we cannot see.

The second level, the multiverse, is that if the universe was a quantum event,

there may be other universes,

bubbles of space-time if you like, that are unobservable by us.

All beyond our horizon.

The third level of this is the unpredictable nature

of quantum genesis and the other possible universes.

Since they're governed by the indeterminacy of the quantum theory,

very hard for us to predict.

Perhaps an ensemble of behaviors can be predicted, but

not the behavior of any individual quantum event.

The final level is pure mathematics, the description of space-time, perhaps in

multi-dimensions beyond the three of space and one of time that we know,

the pristine beauty of mathematics that may or may not apply to the real universe.

String theory is the working canvas for the multiverse model, because string

theory predicts that the vacuum is rich with Byzantine possibilities.

Quantum events can occur in string theory

with enormous range in their physical properties.

When string theorists calculate the number of possible states of the vacuum,

they come up with a prodigious number, ten to the power of 500.

That's an unimaginable number, and

even if almost all of those vacuum states don't ever emerge into a real universe or

something that could be observed or has macroscopic size.

If only a tiny fraction do, then it's a rich set of possibilities for

a multiverse model.

Inflation is something that we believe happened to our universe

in the first tiny fraction of a second.

But in the eternal inflation idea,

similar things could have happened to other bubbles of space-time and

could have inflated them into large universes like ours.

So, we have a speculative physical basis for

an ensemble of universes with randomly different properties.

By different, I really mean different.

The laws of physics would be different in these space-times.

Some similar to our laws of physics, others different.

Perhaps some universes with the arrow of time was not discernible and

events move forward or backward with equal facility.

Perhaps universes with equal amounts of matter and antimatter.

Perhaps universes that live and die in a microsecond or

universes that are more eternal than ours.

We don't know.

But what's clear is that the set of properties in our universe that enables

life to exist, biology, humans, is quite special, because we need long-lived

stable stars, molecules, chemistry, and then biochemistry to develop.

If you look through all the possible parameters and

laws of physics that could exist, only a small range of these laws of physics

will accommodate our kind of universe, the one that holds life.

This logic is called the Anthropic Principle and

says that our universe does has special properties, in an ensemble of possible

universes, because the vast majority of them will be stillborn,

in the sense of not accommodating complexity as exemplified by biology.

In the theory, time loses its meaning.

We have a sense of the age of our universe and the clock stops there.

In fact, at the plank era, time has no meaning, so neither space and

time are definable at the genesis event of our universe.

In this backdrop theory for the multi verse, time also has no meaning.

Time may be an emergent property in some of the universes that develop, but

it may not exist in many of them, and it's not necessary for the overall construct.

[MUSIC]

>> Brian, I am about to show you something that's going to blow your mind.

Do you know what the Big Bang theory is?

>> Yeah, the theory that the universe started with a massive outward explosion

from a singularity of infinite mass and infinite density.

>> Check out the big brain on Brian.

Good. Now, take a look.