The questions I would like to talk about are:

one, where did we come from?

How did the universe come into being?

Are we alone in the universe?

Is there alien life out there?

What is the future of the human race?

Up until the 1920s,

everyone thought the universe was essentially static

and unchanging in time.

Then it was discovered that the universe was expanding.

Distant galaxies were moving away from us.

This meant they must have been closer together in the past.

If we extrapolate back,

we find we must have all been on top of each other

about 15 billion years ago.

This was the Big Bang, the beginning of the universe.

But was there anything before the Big Bang?

If not, what created the universe?

Why did the universe emerge from the Big Bang the way it did?

We used to think that the theory of the universe

could be divided into two parts.

First, there were the laws

like Maxwell's equations and general relativity

that determined the evolution of the universe,

given its state over all of space at one time.

And second, there was no question

of the initial state of the universe.

We have made good progress on the first part,

and now have the knowledge of the laws of evolution

in all but the most extreme conditions.

But until recently, we have had little idea

about the initial conditions for the universe.

However, this division into laws of evolution and initial conditions

depends on time and space being separate and distinct.

Under extreme conditions, general relativity and quantum theory

allow time to behave like another dimension of space.

This removes the distinction between time and space,

and means the laws of evolution can also determine the initial state.

The universe can spontaneously create itself out of nothing.

Moreover, we can calculate a probability that the universe

was created in different states.

These predictions are in excellent agreement

with observations by the WMAP satellite

of the cosmic microwave background,

which is an imprint of the very early universe.

We think we have solved the mystery of creation.

Maybe we should patent the universe

and charge everyone royalties for their existence.

I now turn to the second big question:

are we alone, or is there other life in the universe?

We believe that life arose spontaneously on the Earth,

so it must be possible for life to appear on other suitable planets,

of which there seem to be a large number in the galaxy.

But we don't know how life first appeared.

We have two pieces of observational evidence

on the probability of life appearing.

The first is that we have fossils of algae

from 3.5 billion years ago.

The Earth was formed 4.6 billion years ago

and was probably too hot for about the first half billion years.

So life appeared on Earth

within half a billion years of it being possible,

which is short compared to the 10-billion-year lifetime

of a planet of Earth type.

This suggests that a probability of life appearing is reasonably high.

If it was very low, one would have expected it

to take most of the ten billion years available.

On the other hand, we don't seem to have been visited by aliens.

I am discounting the reports of UFOs.

Why would they appear only to cranks and weirdoes?

If there is a government conspiracy to suppress the reports

and keep for itself the scientific knowledge the aliens bring,

it seems to have been a singularly ineffective policy so far.

Furthermore, despite an extensive search by the SETI project,

we haven't heard any alien television quiz shows.

This probably indicates that there are no alien civilizations

at our stage of development

within a radius of a few hundred light years.

Issuing an insurance policy

against abduction by aliens seems a pretty safe bet.

This brings me to the last of the big questions:

the future of the human race.

If we are the only intelligent beings in the galaxy,

we should make sure we survive and continue.

But we are entering an increasingly dangerous period of our history.

Our population and our use of the finite resources of planet Earth

are growing exponentially, along with our technical ability

to change the environment for good or ill.

But our genetic code

still carries the selfish and aggressive instincts

that were of survival advantage in the past.

It will be difficult enough to avoid disaster

in the next hundred years,

let alone the next thousand or million.

Our only chance of long-term survival

is not to remain lurking on planet Earth,

but to spread out into space.

The answers to these big questions

show that we have made remarkable progress in the last hundred years.

But if we want to continue beyond the next hundred years,

our future is in space.

That is why I am in favor of manned --

or should I say, personed -- space flight.

All of my life I have sought to understand the universe

and find answers to these questions.

I have been very lucky

that my disability has not been a serious handicap.

Indeed, it has probably given me more time than most people

to pursue the quest for knowledge.

The ultimate goal is a complete theory of the universe,

and we are making good progress.

Thank you for listening.

Chris Anderson: Professor, if you had to guess either way,

do you now believe that it is more likely than not

that we are alone in the Milky Way,

as a civilization of our level of intelligence or higher?

This answer took seven minutes, and really gave me an insight

into the incredible act of generosity this whole talk was for TED.

Stephen Hawking: I think it quite likely that we are the only civilization

within several hundred light years;

otherwise we would have heard radio waves.

The alternative is that civilizations don't last very long,

but destroy themselves.

CA: Professor Hawking, thank you for that answer.

We will take it as a salutary warning, I think,

for the rest of our conference this week.

Professor, we really thank you for the extraordinary effort you made

to share your questions with us today.

Thank you very much indeed.

(Applause)