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20 years ago, Stephen Hawking,
a young research student | 0:00:38 | 0:00:42 | |
at Cambridge University,
began to show the first symptoms | 0:00:42 | 0:00:44 | |
of an incurable disease
at he was told might kill him | 0:00:44 | 0:00:47 | |
within a few years. | 0:00:47 | 0:00:48 | |
Amyotrophic lateral sclerosis. | 0:00:48 | 0:00:52 | |
Undeterred, he married,
had three children, | 0:00:52 | 0:00:55 | |
and became a great scientist. | 0:00:55 | 0:01:00 | |
His subject is cosmology,
the study of the universe. | 0:01:00 | 0:01:08 | |
STUDENTS CHATTER. | 0:02:00 | 0:02:08 | |
You could pull that down. | 0:02:16 | 0:02:19 | |
Could you shut the door, please? | 0:02:27 | 0:02:30 | |
This week's seminar is given
by Paul Todd from Oxford. | 0:02:30 | 0:02:37 | |
What I was going to talk about is
some applications of Penrose's | 0:02:39 | 0:02:43 | |
quasi-local mass construction. | 0:02:43 | 0:02:47 | |
I'll remind you to begin with,
what that construction is, | 0:02:47 | 0:02:51 | |
because it's something of a novelty. | 0:02:51 | 0:02:55 | |
It's been around for about a year,
and the way the construction works, | 0:02:55 | 0:02:58 | |
or the way the definition works
is... | 0:02:58 | 0:03:00 | |
The Monday seminar is more or less
compulsory for all the relativity | 0:03:00 | 0:03:03 | |
group, especially the students. | 0:03:03 | 0:03:05 | |
Chris is in his third year. | 0:03:05 | 0:03:08 | |
He's working on
supersymmetric theories. | 0:03:08 | 0:03:11 | |
Bruce is studying the early
universe, and like Chris, | 0:03:11 | 0:03:13 | |
he's writing up his Ph.D. | 0:03:13 | 0:03:18 | |
Then you contract the... | 0:03:18 | 0:03:22 | |
Wayne concentrates more on the
mathematical side than the physics. | 0:03:22 | 0:03:25 | |
He is a second-year like me, Julian. | 0:03:25 | 0:03:27 | |
I'm working on quantum gravity. | 0:03:27 | 0:03:35 | |
It's like picking out some specific
bits of spherical harmonic. | 0:03:35 | 0:03:37 | |
Whereas in special relativity... | 0:03:37 | 0:03:38 | |
There are three more
students, all first years. | 0:03:38 | 0:03:42 | |
To get into this group,
you need a good advanced degree, | 0:03:42 | 0:03:45 | |
so most of us are in our early 20s,
except for Simon, who's only 17. | 0:03:45 | 0:03:51 | |
So you get the same answer
integrating it | 0:03:51 | 0:03:53 | |
over any hyper surface. | 0:03:53 | 0:03:59 | |
So if we integrate it over
a hyper surface, like so, | 0:03:59 | 0:04:02 | |
that's a number which depends
on which particular killing vector | 0:04:02 | 0:04:04 | |
I picked and if I picked one
of the translation killing vectors, | 0:04:04 | 0:04:07 | |
that's a component
of the total momentum. | 0:04:07 | 0:04:09 | |
If I picked one of the rotational
killing vectors, it's a component... | 0:04:09 | 0:04:13 | |
Most of us students are under
the charge of Stephen Hawking, | 0:04:13 | 0:04:16 | |
who is the Lucasian Professor
of Mathematics, and head | 0:04:16 | 0:04:19 | |
of the relativity group as well. | 0:04:19 | 0:04:25 | |
That's special relativity. | 0:04:25 | 0:04:26 | |
Now if you consider linearised
general relativity, | 0:04:26 | 0:04:28 | |
if this is the source,
it gives rise to a gravitational | 0:04:28 | 0:04:35 | |
field, so you should be able
to spot that momentum | 0:04:35 | 0:04:37 | |
in the gravitational field. | 0:04:37 | 0:04:38 | |
INAUDIBLE. | 0:04:38 | 0:04:39 | |
Sorry? | 0:04:39 | 0:04:44 | |
What do the self dual
rotations correspond to? | 0:04:44 | 0:04:48 | |
Oh, well I... | 0:04:48 | 0:04:54 | |
The combination of boost
and special rotation. | 0:04:54 | 0:04:56 | |
Yes, that's right. | 0:04:56 | 0:04:58 | |
If it is, if you think of it
as a rotation of the X Y plane, | 0:04:58 | 0:05:02 | |
plus I times a boost and the TZ
plane, they are just one | 0:05:02 | 0:05:05 | |
of the rotations, naturally. | 0:05:05 | 0:05:12 | |
Are they real? | 0:05:12 | 0:05:15 | |
They are real. | 0:05:15 | 0:05:23 | |
So they are real in the Euclidian
space, but they're not | 0:05:23 | 0:05:26 | |
going to be real here. | 0:05:26 | 0:05:28 | |
This is what one might refer
to as old-fashioned relativity, | 0:05:28 | 0:05:36 | |
with plus minus minus minus. | 0:05:36 | 0:05:38 | |
LAUGHTER. | 0:05:38 | 0:05:46 | |
We gave that up ten years ago. | 0:05:53 | 0:05:55 | |
LAUGHTER. | 0:05:55 | 0:06:03 | |
A guy called Stewart
Lowther at Manchester... | 0:06:13 | 0:06:18 | |
INAUDIBLE. | 0:06:18 | 0:06:21 | |
There are two different... | 0:06:21 | 0:06:25 | |
In equivalent? | 0:06:25 | 0:06:30 | |
Ah-ha. | 0:06:30 | 0:06:36 | |
That equal something of the form
like this, then Chi. | 0:06:41 | 0:06:43 | |
It's going to be... | 0:06:43 | 0:06:49 | |
Self dual, then that's
got to be true. | 0:06:49 | 0:06:53 | |
Which if you stick
that in, gives you... | 0:06:53 | 0:06:57 | |
People tend to congregate
from the relativity group, and, | 0:06:57 | 0:07:01 | |
at the time, you find that you learn
almost as much as you do sitting | 0:07:01 | 0:07:06 | |
in your office working or reading. | 0:07:06 | 0:07:14 | |
And that equals something
of the form like this. | 0:07:16 | 0:07:20 | |
Right, so that one just corresponds? | 0:07:20 | 0:07:23 | |
That is what I was wondering. | 0:07:23 | 0:07:27 | |
So it doesn't work with one or two. | 0:07:27 | 0:07:32 | |
You can do a certain amount
of research and creative thinking | 0:07:32 | 0:07:35 | |
each day and then what's really
helpful is discussing with other | 0:07:35 | 0:07:38 | |
people, so that your
ideas are clarified. | 0:07:38 | 0:07:46 | |
Why doesn't it work
with five, for example? | 0:07:46 | 0:07:51 | |
The group is quite close. | 0:07:51 | 0:07:55 | |
We get to talk to each other
about problems and have | 0:07:55 | 0:08:00 | |
discussions all the time,
which is very good in a social | 0:08:00 | 0:08:02 | |
sense, and also in the sense
that there are people here who, | 0:08:02 | 0:08:06 | |
if you asked them a question
about any particular subject, | 0:08:06 | 0:08:08 | |
there's bound to be someone who can
find an answer to it, | 0:08:08 | 0:08:12 | |
and so you don't have to wait
for very long to find an answer! | 0:08:12 | 0:08:19 | |
Because the one... | 0:08:19 | 0:08:25 | |
Because you have much
greater contact with | 0:08:33 | 0:08:35 | |
Stephen as a supervisor,
because he needs your help | 0:08:35 | 0:08:37 | |
all the time, he's always available
to answer questions and to help | 0:08:37 | 0:08:40 | |
you with things you don't
understand, and he's also very, | 0:08:40 | 0:08:42 | |
very clear in the way
he explains it. | 0:08:42 | 0:08:47 | |
And since he knows all his research
students as friends, | 0:08:47 | 0:08:51 | |
he seems much more relaxed and much
less an academic physicist. | 0:08:51 | 0:08:58 | |
He most definitely is number one
an academic physicist. | 0:08:58 | 0:09:03 | |
The most famous of all academic
physicists is Albert Einstein. | 0:09:03 | 0:09:08 | |
The source of his fame,
the general theory of relativity, | 0:09:08 | 0:09:11 | |
burst on the world of
physics in 1915. | 0:09:11 | 0:09:16 | |
But after an initial rush
of enthusiasm, few other academic | 0:09:16 | 0:09:19 | |
physicists took up his theory,
and developed further. | 0:09:19 | 0:09:26 | |
General relativity passed out
of fashion for about 40 years. | 0:09:26 | 0:09:31 | |
The initial? | 0:09:31 | 0:09:38 | |
Sorry? | 0:09:55 | 0:09:58 | |
Stephen Hawking was one of a group
of scientists who resurrected | 0:09:58 | 0:10:02 | |
interest in Einstein's general
theory of relativity | 0:10:02 | 0:10:04 | |
during the 50s and 60s. | 0:10:04 | 0:10:10 | |
Stephen worked on mathematical
theorems in general relativity | 0:10:10 | 0:10:15 | |
which proved the necessity for a big
bang at the beginning | 0:10:15 | 0:10:17 | |
of the universe. | 0:10:17 | 0:10:21 | |
He also investigated
many of the attributes of a bizarre | 0:10:21 | 0:10:23 | |
class of objects whose properties
are predicted by Einstein's theory. | 0:10:23 | 0:10:26 | |
Black holes. | 0:10:26 | 0:10:32 | |
Massive as they are,
black holes are not things | 0:10:33 | 0:10:36 | |
you can actually see,
because a black hole | 0:10:36 | 0:10:38 | |
doesn't emit any light. | 0:10:38 | 0:10:43 | |
But if a black hole passes in front
of a background of stars, | 0:10:43 | 0:10:46 | |
the stars appear to move away
from their real positions, | 0:10:46 | 0:10:54 | |
just as if the black hole
were a giant lens. | 0:10:54 | 0:10:56 | |
In fact, the light from
the background stars is bent | 0:10:56 | 0:10:59 | |
round and round the black hole,
by its intense gravitational field, | 0:10:59 | 0:11:02 | |
so you can see several images
of each star at once. | 0:11:02 | 0:11:08 | |
If you are a long way
away from a black hole, | 0:11:08 | 0:11:12 | |
you are quite safe! | 0:11:12 | 0:11:16 | |
If you're a long way
away from a black hole, | 0:11:16 | 0:11:18 | |
you are quite safe. | 0:11:18 | 0:11:26 | |
If our sun were to become a black
hole, | 0:11:32 | 0:11:35 | |
we would continue to orbit
around it just as we do | 0:11:35 | 0:11:38 | |
at the moment. | 0:11:38 | 0:11:41 | |
In fact, it wouldn't make any
difference to our orbit. | 0:11:41 | 0:11:49 | |
Of course, we would get rather cold! | 0:11:50 | 0:11:58 | |
But if you go close up to a black
hole, | 0:12:02 | 0:12:08 | |
then the gravitational
field becomes stronger. | 0:12:08 | 0:12:15 | |
And at a certain point, | 0:12:15 | 0:12:21 | |
the gravitational field
reaches critical strength. | 0:12:21 | 0:12:29 | |
And if you go beyond there,
you can't get out at all again. | 0:12:38 | 0:12:43 | |
Stephen says that a black hole
is rather like a whirlpool. | 0:12:43 | 0:12:46 | |
Imagine you have a whirlpool, and
you have some little boats nearby. | 0:12:46 | 0:12:53 | |
Far away, there are quite safe,
but if they get within a certain | 0:12:53 | 0:12:56 | |
critical distance of the world pool,
then even if they try to motor | 0:12:56 | 0:12:59 | |
directly away from it,
they will get sucked | 0:12:59 | 0:13:01 | |
in by the current which is much
faster than they are. | 0:13:01 | 0:13:05 | |
From within this critical radius,
nothing, whether little boats, | 0:13:05 | 0:13:08 | |
rays of light or spacecraft
can ever return. | 0:13:08 | 0:13:15 | |
If it was a black hole
with the mass of the sun... | 0:13:21 | 0:13:28 | |
Then you would be torn apart
by tidal forces before you got | 0:13:35 | 0:13:38 | |
inside the black hole. | 0:13:38 | 0:13:41 | |
But if it was a very
large black hole... | 0:13:41 | 0:13:49 | |
Such as we believe may
occur in the centre | 0:13:51 | 0:13:55 | |
of our galaxy, or in quasis... | 0:13:55 | 0:14:03 | |
Then you wouldn't see anything
special | 0:14:20 | 0:14:28 | |
if you passed inside the black hole. | 0:14:29 | 0:14:31 | |
But once you pass a certain
critical point... | 0:14:31 | 0:14:39 | |
Then you would never be
able to get out again, | 0:14:39 | 0:14:41 | |
no matter how much rocket
power you used. | 0:14:41 | 0:14:49 | |
Moreover, we assume you would run
into a singularity... | 0:14:58 | 0:15:06 | |
You would be doomed to run
into a singularity. | 0:15:06 | 0:15:08 | |
In a fairly short time. | 0:15:08 | 0:15:16 | |
Like a few hours. | 0:15:16 | 0:15:21 | |
So far, even though astronomers have
been busily looking for black holes, | 0:15:21 | 0:15:24 | |
none have been definitely
identified, although there | 0:15:24 | 0:15:26 | |
are some strong candidates. | 0:15:26 | 0:15:30 | |
So the properties of black holes
have had to be entirely worked | 0:15:30 | 0:15:33 | |
out using mathematics. | 0:15:33 | 0:15:35 | |
If Einstein's general theory
of relativity is true, | 0:15:35 | 0:15:39 | |
then inside the radius
from which nothing can escape, | 0:15:39 | 0:15:44 | |
called the event horizon,
and at the centre of the black hole | 0:15:44 | 0:15:47 | |
is a singularity. | 0:15:47 | 0:15:49 | |
A place where gravity is infinite,
and space and time come to an end. | 0:15:49 | 0:15:57 | |
It would be a very nice idea. | 0:16:02 | 0:16:09 | |
If one could fall into a black
hole and then come out | 0:16:09 | 0:16:11 | |
of another universe. | 0:16:11 | 0:16:19 | |
And there are some solutions
to the Einstein field equations | 0:16:27 | 0:16:30 | |
which have this property that
you can come out | 0:16:30 | 0:16:33 | |
in another universe. | 0:16:33 | 0:16:41 | |
But all the evidence we have shows
these solutions are very unstable. | 0:16:44 | 0:16:52 | |
So that is, if you disturb them
slightly, for example by falling | 0:17:06 | 0:17:09 | |
into the black hole... | 0:17:09 | 0:17:17 | |
Than a passage which takes
you through to the other universe | 0:17:24 | 0:17:29 | |
gets closed off and you run right
into the singularity. | 0:17:29 | 0:17:32 | |
We all came out of a singularity. | 0:17:32 | 0:17:38 | |
The Big Bang singularity
at the beginning of the universe. | 0:17:38 | 0:17:46 | |
So it wouldn't be that
unnatural if we ended up | 0:17:58 | 0:18:00 | |
in another singularity. | 0:18:00 | 0:18:04 | |
Either a singularity in a black
hole or the collapse | 0:18:04 | 0:18:07 | |
of the whole universe. | 0:18:07 | 0:18:15 | |
You could say, dust to dust
and ashes to ashes, | 0:18:21 | 0:18:25 | |
and singularity to singularity. | 0:18:25 | 0:18:33 | |
It can be very frustrating
when you're working at something | 0:18:55 | 0:18:58 | |
and banging your head
against a wall and never | 0:18:58 | 0:19:02 | |
getting anywhere day in,
day out, but then suddenly it clicks | 0:19:02 | 0:19:05 | |
and everything works
fine for a few days. | 0:19:05 | 0:19:09 | |
An answer comes out. | 0:19:09 | 0:19:12 | |
Whether it's what you want
or what you don't want, | 0:19:12 | 0:19:17 | |
you have to work out later. | 0:19:17 | 0:19:19 | |
Part of it's just a search for
beauty and prettiness in physics. | 0:19:19 | 0:19:27 | |
So what do you want it for? | 0:19:28 | 0:19:32 | |
Just to check on how he did. | 0:19:32 | 0:19:38 | |
I tend to be more on
the mathematical side, | 0:19:38 | 0:19:43 | |
if you are looking at equations
for mathematical consistency rather, | 0:19:43 | 0:19:45 | |
then a physical relevance. | 0:19:45 | 0:19:48 | |
I certainly wouldn't mind doing
relativity all day, or mathematics, | 0:19:48 | 0:19:51 | |
or anything like that. | 0:19:51 | 0:19:53 | |
It just interests you. | 0:19:53 | 0:19:55 | |
I don't think you could explain it,
you would have to ask | 0:19:55 | 0:19:58 | |
a psychologist about that. | 0:19:58 | 0:20:04 | |
And there's certainly
no monetary reward. | 0:20:04 | 0:20:07 | |
Well, there's a bit of monetary
reward, but not much, I could get | 0:20:07 | 0:20:10 | |
more on the outside. | 0:20:10 | 0:20:11 | |
But it's very comfortable. | 0:20:11 | 0:20:12 | |
You can do what you want to do
all your life, if you get | 0:20:12 | 0:20:15 | |
to do it all your life. | 0:20:15 | 0:20:18 | |
You're playing games all your life. | 0:20:18 | 0:20:20 | |
It's pretty good. | 0:20:20 | 0:20:23 | |
All of theoretical physics
is formulated in mathematical terms. | 0:20:27 | 0:20:33 | |
The theory of physics is really
a mathematical model of the world. | 0:20:33 | 0:20:39 | |
But being good at mathematics isn't
enough, one also needs what one | 0:20:39 | 0:20:43 | |
calls physical intuition. | 0:20:43 | 0:20:51 | |
You can't deduce physics purely
deductibly from a set | 0:20:52 | 0:20:54 | |
of basic principles,
you have to make certain intuitive | 0:20:54 | 0:20:57 | |
leaps to introduce new models. | 0:20:57 | 0:21:05 | |
The ability to make these intuitive
leaps is what characterises a good | 0:21:05 | 0:21:08 | |
theoretical physicist. | 0:21:08 | 0:21:15 | |
Stephen is lucky that he chose
one of the few fields | 0:21:27 | 0:21:30 | |
in which his disability is not
a serious handicap. | 0:21:30 | 0:21:36 | |
Because most of his work
is really just thinking. | 0:21:36 | 0:21:43 | |
And his disabilities don't
stop him doing that. | 0:21:43 | 0:21:50 | |
In a way, they give him
more time to think. | 0:21:50 | 0:21:57 | |
In 1973, Stephen started
a new line of research | 0:22:14 | 0:22:16 | |
that was eventually to make him
famous, with the discovery | 0:22:16 | 0:22:19 | |
of Hawking radiation. | 0:22:19 | 0:22:23 | |
Up until then, his work
on black holes was concerned | 0:22:23 | 0:22:26 | |
only with large ones,
with the mass of the sun or bigger. | 0:22:26 | 0:22:30 | |
But then, he began to think
that there might also be very | 0:22:30 | 0:22:36 | |
very small black holes. | 0:22:36 | 0:22:38 | |
Stephen realised, in order
to an designed them, | 0:22:38 | 0:22:41 | |
Einstein's general relativity
would not be enough. | 0:22:41 | 0:22:43 | |
He needed to use a completely
different branch of physics | 0:22:43 | 0:22:45 | |
called quantum mechanics. | 0:22:45 | 0:22:47 | |
Quantum mechanics was formulated
by Werner Heisenberg | 0:22:47 | 0:22:49 | |
and Erwin Schrodinger
in the mid-1920s. | 0:22:49 | 0:22:53 | |
Theirs is a theory of very
small things, like atoms. | 0:22:53 | 0:22:58 | |
Quantum mechanics is the greatest
achievement in physics this century, | 0:22:58 | 0:23:01 | |
even greater than Einstein's general
theory of relativity. | 0:23:01 | 0:23:06 | |
It implies that what we normally
think of as empty space isn't | 0:23:06 | 0:23:09 | |
really empty at all,
but is filled with pairs | 0:23:09 | 0:23:11 | |
of particles and antiparticles. | 0:23:11 | 0:23:15 | |
These appear together at some point
in space, move apart, | 0:23:15 | 0:23:18 | |
and then come together again,
annihilating each other. | 0:23:18 | 0:23:22 | |
They are called virtual particles
because you can't directly measure | 0:23:22 | 0:23:24 | |
them with a particle detector. | 0:23:24 | 0:23:32 | |
According to Hawking, if there's
a small black hole present, | 0:23:32 | 0:23:35 | |
one of the members of these pairs
might fall into it. | 0:23:35 | 0:23:38 | |
Of course, the other
one might fall in, too, | 0:23:38 | 0:23:42 | |
but it's also possible for one
of them to escape, and in that case, | 0:23:42 | 0:23:45 | |
it would appear to be a particle
emitted from the black hole. | 0:23:45 | 0:23:53 | |
In fact, to an observer a long way
away, it appears that the black | 0:24:00 | 0:24:04 | |
hole is emitting particles
and radiation as if | 0:24:04 | 0:24:06 | |
it was a hot body. | 0:24:06 | 0:24:07 | |
Very small black holes
aren't black at all, | 0:24:07 | 0:24:09 | |
they shine with Hawking radiation. | 0:24:09 | 0:24:12 | |
If you have a black hole
with the mass of the sun, | 0:24:12 | 0:24:20 | |
then its temperature is only one
10,000,000th of a degree | 0:24:22 | 0:24:24 | |
above absolute zero. | 0:24:24 | 0:24:31 | |
And the amount of radiation would be
absolutely insignificant. | 0:24:31 | 0:24:39 | |
But if you have one of these small
black holes, then the temperature | 0:24:44 | 0:24:48 | |
would be much higher,
and it would emit | 0:24:48 | 0:24:49 | |
a lot of radiation. | 0:24:49 | 0:24:57 | |
In fact, the most interesting
mass of a black hole | 0:25:01 | 0:25:05 | |
is about a thousand million tonnes,
which is about the mass | 0:25:05 | 0:25:08 | |
of a mountain. | 0:25:08 | 0:25:16 | |
But the actual size of such a black
hole would only be that | 0:25:23 | 0:25:27 | |
of the nucleus of an atom. | 0:25:27 | 0:25:32 | |
But it would emit a lot
of radiation and energy. | 0:25:32 | 0:25:40 | |
Equivalent to about six
nuclear power stations. | 0:25:43 | 0:25:51 | |
So if you could find
such a small black hole, | 0:25:59 | 0:26:04 | |
and if we could harness it properly,
then we would really solve | 0:26:04 | 0:26:07 | |
all our energy problems. | 0:26:07 | 0:26:15 | |
However, we have been
looking for radiation | 0:26:26 | 0:26:29 | |
from the black holes like this,
and we haven't found any so far. | 0:26:29 | 0:26:37 | |
In a way, that's rather
disappointing for Stephen. | 0:26:42 | 0:26:50 | |
Because, had we found one, Stephen
would have got a Nobel Prize! | 0:27:00 | 0:27:03 | |
No biscuit? | 0:27:04 | 0:27:04 | |
Er, yes, please. | 0:27:04 | 0:27:12 | |
I should go and buy some coffee. | 0:27:33 | 0:27:37 | |
Oh, that's right,
it's quarter to 11. | 0:27:37 | 0:27:39 | |
Should I buy some instant? | 0:27:39 | 0:27:40 | |
Try eight ounces. | 0:27:40 | 0:27:41 | |
Eight ounces. | 0:27:41 | 0:27:45 | |
There'll be crowds pouring
into Stephen's lecture. | 0:27:45 | 0:27:46 | |
I mean, we didn't have enough last
week, and it was getting | 0:27:46 | 0:27:49 | |
a bit weak at the end. | 0:27:49 | 0:27:51 | |
Hello. | 0:27:51 | 0:27:52 | |
Tea? | 0:27:52 | 0:27:54 | |
Some of us are mathematicians,
and others physicists, | 0:27:57 | 0:27:59 | |
and we're all working
on different problems. | 0:27:59 | 0:28:02 | |
Do you want the Omega
for the embedding in the cylinder? | 0:28:03 | 0:28:08 | |
These problems are usually either
suggested or allocated by Stephen. | 0:28:08 | 0:28:16 | |
These problems are very
different from each other, | 0:28:18 | 0:28:23 | |
but are basically connected,
in that they are | 0:28:23 | 0:28:25 | |
all trying to unearth
the fundamentals of the universe. | 0:28:25 | 0:28:28 | |
So that's supposed to have... | 0:28:28 | 0:28:29 | |
Does it? | 0:28:29 | 0:28:37 | |
No, it won't, will it? | 0:28:39 | 0:28:44 | |
It is certainly an ambitious task. | 0:28:44 | 0:28:47 | |
People have been working on it for,
I'd say, 50 years or longer, | 0:28:47 | 0:28:54 | |
so I guess it's even more ambitious,
since we don't even know if | 0:28:54 | 0:28:57 | |
the answer is that it can be done. | 0:28:57 | 0:29:00 | |
A zero on the horizon,
on the boundary? | 0:29:00 | 0:29:04 | |
Yes, because r is half pi there. | 0:29:04 | 0:29:11 | |
So it's not right? | 0:29:11 | 0:29:13 | |
Yes. | 0:29:13 | 0:29:15 | |
In that case...! | 0:29:15 | 0:29:18 | |
OK. | 0:29:18 | 0:29:21 | |
Morning, Chris. | 0:29:49 | 0:29:52 | |
Right here! | 0:29:52 | 0:29:58 | |
The boundary partitions... | 0:30:07 | 0:30:15 | |
We are trying to unify
many of the modern ideas of physics. | 0:30:25 | 0:30:29 | |
I'm interested in the almost
philosophical, or even religious, | 0:30:29 | 0:30:33 | |
quest for what actually makes
the universe work. | 0:30:33 | 0:30:41 | |
I mean, certainly the conformal
group corresponding to flat | 0:30:43 | 0:30:49 | |
three space would be 041. | 0:30:49 | 0:30:57 | |
There's an embarrassing
inconsistency at the heart | 0:31:02 | 0:31:03 | |
of modern physics. | 0:31:03 | 0:31:04 | |
Einstein's general theory
of relativity, which describes | 0:31:04 | 0:31:06 | |
the nature of very big things,
disagrees with the theory of very | 0:31:06 | 0:31:09 | |
small things, quantum mechanics,
in apparently unresolvable ways, | 0:31:09 | 0:31:11 | |
even though no one has managed
to prove either theory untrue. | 0:31:11 | 0:31:13 | |
So perhaps the way out
of this dilemma is to find | 0:31:13 | 0:31:16 | |
a more profound theory,
which incorporates both. | 0:31:16 | 0:31:23 | |
Two outstanding partial theories
have been discovered this century. | 0:31:31 | 0:31:36 | |
They are general relativity
and quantum mechanics. | 0:31:36 | 0:31:44 | |
Ultimately we have to find
one consistent theory, | 0:31:57 | 0:32:01 | |
which will describe everything... | 0:32:01 | 0:32:09 | |
Not only general relativity
and quantum mechanics, | 0:32:11 | 0:32:14 | |
but all the other interactions
in physics, as well. | 0:32:14 | 0:32:22 | |
We have had quite
a success recently... | 0:32:31 | 0:32:39 | |
In that we've developed
a theory which unifies | 0:32:42 | 0:32:44 | |
electromagnetism in
the weak nuclear force. | 0:32:44 | 0:32:52 | |
Now we want to go on to unify these
interactions with gravity, | 0:33:00 | 0:33:03 | |
and also unify gravity
with quantum mechanics. | 0:33:03 | 0:33:11 | |
Unfortunately, this is a very
ambitious programme, | 0:33:12 | 0:33:20 | |
but there's quite a reasonable
chance of success. | 0:33:23 | 0:33:31 | |
Stephen would put
the chances at 50-50. | 0:33:35 | 0:33:43 | |
In that you could succeed in this
task by the end of the century. | 0:33:43 | 0:33:51 | |
Now we have a definite candidate... | 0:33:56 | 0:34:04 | |
For the complete unified theory,
which will describe everything, | 0:34:05 | 0:34:13 | |
and this candidate is called N=8
Supergravity. | 0:34:16 | 0:34:24 | |
If it doesn't work, then we have no
idea what will work. | 0:34:30 | 0:34:38 | |
We're working very
hard on this theory. | 0:34:52 | 0:35:00 | |
But at the moment it doesn't seem
to predict the kind of particles | 0:35:05 | 0:35:08 | |
that we actually observe. | 0:35:08 | 0:35:16 | |
But we're hoping that maybe
when we understand the theory | 0:35:18 | 0:35:22 | |
better... | 0:35:22 | 0:35:30 | |
Then we could construct
the particles we observe | 0:35:43 | 0:35:45 | |
out of smaller pieces,
which are the particles | 0:35:45 | 0:35:47 | |
in the N=8 Supergravity theory. | 0:35:47 | 0:35:55 | |
And, in that case, we could actually
say that theoretical physics | 0:36:10 | 0:36:13 | |
was over, we had a complete theory
of the whole universe. | 0:36:13 | 0:36:21 | |
If we had a complete theory
of the universe, we could, | 0:36:31 | 0:36:39 | |
in principle, predict everything. | 0:36:42 | 0:36:50 | |
But in practice, the computations
involved are very, very complicated, | 0:37:21 | 0:37:23 | |
so in effect, we can't predict
anything, apart from | 0:37:23 | 0:37:25 | |
the most simple situations. | 0:37:25 | 0:37:33 | |
In fact, we already know
all the laws which govern all normal | 0:37:37 | 0:37:39 | |
matter and all normal situations... | 0:37:39 | 0:37:47 | |
So, in principle, we can
predict everything that | 0:37:59 | 0:38:01 | |
happens on the earth. | 0:38:01 | 0:38:09 | |
But we haven't had much success
in predicting human behaviour | 0:38:11 | 0:38:13 | |
from mathematical equations. | 0:38:13 | 0:38:21 | |
It's mostly complicated,
a human being contains | 0:38:31 | 0:38:34 | |
about a million million million
million million particles. | 0:38:34 | 0:38:41 | |
I don't think we really
need a very big lunch, | 0:39:46 | 0:39:48 | |
because we felt ourselves up
with doughnuts this | 0:39:48 | 0:39:50 | |
morning, haven't we, Timmy? | 0:39:50 | 0:39:56 | |
She's gone out to lunch with Juliet,
and she's gone swimming. | 0:39:56 | 0:40:04 | |
Well, I can afford to buy
myself a small black... | 0:40:06 | 0:40:09 | |
And I can afford to buy
Stephen half of one. | 0:40:09 | 0:40:11 | |
Neither Mrs Hawking nor their son,
Timmy, are particularly | 0:40:11 | 0:40:13 | |
interested in mathematics,
so that when they come | 0:40:13 | 0:40:15 | |
to lunch we try not to talk
too much about work. | 0:40:15 | 0:40:21 | |
Oh, dear, look, Stephen,
that's a bit much! | 0:40:21 | 0:40:23 | |
White? | 0:40:23 | 0:40:24 | |
Black. | 0:40:24 | 0:40:25 | |
Two white... | 0:40:25 | 0:40:29 | |
How's that? | 0:40:29 | 0:40:36 | |
OK, what Stephen's going to say... | 0:40:55 | 0:40:59 | |
So Stephen will basically be
talking about infinity. | 0:40:59 | 0:41:07 | |
LAUGHTER. | 0:41:11 | 0:41:13 | |
Unfortunately, infinity's
rather hard to talk about, | 0:41:13 | 0:41:14 | |
because it's rather a long,
long way away! | 0:41:14 | 0:41:22 | |
So what Stephen's going to do
is he's going to try | 0:41:24 | 0:41:27 | |
and bring it a lot nearer. | 0:41:27 | 0:41:34 | |
In other words, Stephen's
going to conformally compactify | 0:41:34 | 0:41:36 | |
anti-de Sitter space. | 0:41:36 | 0:41:43 | |
The Einstein static universe
is topologically S3 cross of one... | 0:41:51 | 0:41:59 | |
Where S3 gives the spatial sections
and R1 gives the time. | 0:42:05 | 0:42:13 | |
So the Einstein static universe... | 0:42:14 | 0:42:22 | |
So it's really a sort of cylinder. | 0:42:22 | 0:42:27 | |
With the time being the axis. | 0:42:27 | 0:42:34 | |
Now it just so happens that we
happen to have the universe here. | 0:42:34 | 0:42:37 | |
LAUGHTER. | 0:42:37 | 0:42:44 | |
LAUGHTER. | 0:42:44 | 0:42:45 | |
Sorry! | 0:42:45 | 0:42:53 | |
Unfortunately, we were unable to get
the full 4-dimensional | 0:43:00 | 0:43:02 | |
universe in here today... | 0:43:02 | 0:43:10 | |
But the anti-de Sitter space
is a time-like hyper surface. | 0:43:17 | 0:43:20 | |
And that leads to an
important difference... | 0:43:20 | 0:43:22 | |
Crystal clear. | 0:43:22 | 0:43:25 | |
The way he thinks, he manages
to cut away all the dross, | 0:43:25 | 0:43:29 | |
cut away all the trees,
and just see down to the base | 0:43:29 | 0:43:34 | |
cut away all the trees,
and just see down to the basic | 0:43:34 | 0:43:37 | |
simple, central fact
that is necessary to consider. | 0:43:37 | 0:43:39 | |
And he makes everything
so crystal clear, simple. | 0:43:39 | 0:43:40 | |
It's quite astounding sometimes. | 0:43:40 | 0:43:48 | |
And because he can't write,
because he can't, he finds it | 0:43:49 | 0:43:51 | |
hard to read papers,
hard to read books, he tends to, | 0:43:51 | 0:43:54 | |
he thinks in terms of diagrams
all the time, he thinks very | 0:43:54 | 0:43:57 | |
clearly, and manages to make
everything very, very simple. | 0:43:57 | 0:44:04 | |
That equation is conforming... | 0:44:04 | 0:44:07 | |
It depends upon the taste
of the person in a way, | 0:44:07 | 0:44:12 | |
there is a certain taste people
have, in which they appreciate | 0:44:12 | 0:44:17 | |
mathematical beauty of the theory,
and it's sort of hard to describe, | 0:44:17 | 0:44:20 | |
but this is really one
of the reasons for doing physics, | 0:44:20 | 0:44:24 | |
that you find that there are just
a certain number of laws, | 0:44:24 | 0:44:32 | |
and they are very simple
when written out, mathematically, | 0:44:32 | 0:44:35 | |
and simplicity is quite beautiful,
and the fact it describes | 0:44:35 | 0:44:39 | |
what's happening around us
is quite amazing, really. | 0:44:39 | 0:44:43 | |
And the question is whether we can
keep on simplifying our laws | 0:44:43 | 0:44:48 | |
and postulates, and maybe derive
an ultimate law like that. | 0:44:48 | 0:44:56 | |
APPLAUSE. | 0:44:58 | 0:45:00 | |
Are there any questions? | 0:45:00 | 0:45:07 | |
Either everyone's understood
everything, or no one's | 0:45:14 | 0:45:15 | |
understood anything! | 0:45:15 | 0:45:18 | |
Shall we have a vote? | 0:45:18 | 0:45:20 | |
LAUGHTER. | 0:45:20 | 0:45:23 | |
In the tearoom, we have a number
of portraits of former | 0:45:28 | 0:45:32 | |
professors of mathematics... | 0:45:32 | 0:45:40 | |
And Stephen's not quite
sure what the criterion | 0:45:51 | 0:45:53 | |
are that should determine
whether you get your | 0:45:53 | 0:45:55 | |
portrait in the tearoom... | 0:45:55 | 0:46:03 | |
But one of them seems to be that
you've left the department. | 0:46:05 | 0:46:13 | |
On the wall that this
office is on... | 0:46:16 | 0:46:24 | |
There are the portraits of Stephen's
four immediate predecessors | 0:46:31 | 0:46:35 | |
in the Lucasian chair... | 0:46:35 | 0:46:41 | |
One of them was Paul Dirac... | 0:46:41 | 0:46:49 | |
Who is in fact still alive... | 0:46:54 | 0:46:56 | |
He was one of the founders
of quantum mathematics. | 0:46:56 | 0:47:03 | |
It was he who had
the idea of antimatter. | 0:47:03 | 0:47:11 | |
In the corner, there's
Sir George Gabriel Stokes... | 0:47:17 | 0:47:25 | |
Who was professor
for about 54 years... | 0:47:29 | 0:47:37 | |
Because in those days you didn't
have any retirement age... | 0:47:37 | 0:47:44 | |
And there's a space outside here... | 0:47:47 | 0:47:55 | |
In which it's fairly obvious
they'll put Stephen's | 0:48:06 | 0:48:08 | |
portrait if he leaves... | 0:48:08 | 0:48:16 | |
It gives him a rather
creepy feeling... | 0:48:19 | 0:48:25 | |
It's like seeing your own tombstone. | 0:48:25 | 0:48:33 |