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WHISTLING

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00:00:13,000 --> 00:00:15,000
Ah, excellent, there you are.

3
00:00:15,000 --> 00:00:20,000
# Things can only get better... #

4
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You're a bit late. Or early.

5
00:00:22,000 --> 00:00:24,000
Possibly both, hard to keep track
of time, Prof.

6
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Or shall we just go with "Bri"?

7
00:00:26,000 --> 00:00:28,000
What is this place? Amazing.

8
00:00:28,000 --> 00:00:30,000
Hey, come on, man, be cool.
You're supposed to be a physicist.

9
00:00:30,000 --> 00:00:32,000
And put a tie on.

10
00:00:32,000 --> 00:00:35,000
You're not the make-up artist.

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Sharp. I can see why you got
that fellowship. Where am I?

12
00:00:37,000 --> 00:00:38,000
Bit complicated.

13
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Sort of a spaceship/time machine/
swimming pool.

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Optional hat stand.
I need five minutes of your time,

15
00:00:43,000 --> 00:00:45,000
and when I say five minutes,
I'm lying.

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I'm just going to give a lecture.
I know, I've just seen it.

17
00:00:47,000 --> 00:00:50,000
It's great.
But I haven't given it yet.

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00:00:50,000 --> 00:00:52,000
Tricky to explain - seen it anyway.

19
00:00:52,000 --> 00:00:54,000
You've seen it
and you think it's great?

20
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Did I say great? I meant lousy.

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You need a spot of help with that.
That's why I'm here.

22
00:00:58,000 --> 00:00:59,000
I've bought you a gift.

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00:00:59,000 --> 00:01:01,000
Actually, I say bought.
More like pinched.

24
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But it's the thought that counts.
I couldn't find any paper.

25
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Is this what I think it is?

26
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Yeah. Unless you think
it's a hat, or a banana,

27
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in which case Manchester Uni
needs a re-boot.

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This is over 250 years old.
About a week old, actually.

29
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I picked it up last Saturday
tea-time.

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No, no, no, that is impossible

31
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Naughty word, Brian.

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2p in the swear box, please.

33
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Space and time. Time and space.

34
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Locked in an intricate
dance across the cosmos,

35
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and if you know the tune...
anything is possible.

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I was going for poetry.
Forgot you were a physicist.

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Right, hold on to something.
Probably your sanity. Ready?

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Usually it just twirls around.

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It's probably this. Shut up, Brian.

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SONG: "Doctor Who Theme"

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APPLAUSE

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I have absolutely no idea what
Mr Cox has in store for us tonight.

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He's an enigma.
It could be anything.

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He might just start singing,
for all I know.

45
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It might also be something to
do with science.

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It might be something to do
with time travel.

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I've no idea.

48
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And you're going to have a
crash helmet on

49
00:02:29,000 --> 00:02:31,000
to protect your beautiful
shiny head.

50
00:02:32,000 --> 00:02:36,000
I'm looking to Brian to prove
that everything that happens

51
00:02:36,000 --> 00:02:38,000
in Doctor Who could actually happen.
Is cast iron fact.

52
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All right, there we are. Sorry.

53
00:02:41,000 --> 00:02:43,000
Is this how you're going to
collapse my mass?

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LAUGHTER

55
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Just some bloke with a really
tight backpack on.

56
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Well, knowing Brian,
it will be mind-blowing.

57
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Hopefully we'll all go home
knowing how to make a TARDIS.

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I'm really looking forward to it, to
see what he's got in store for us.

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CLOCK TICKS

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If I could borrow the TARDIS
just for one day,

61
00:03:31,000 --> 00:03:35,000
of all the places I would travel
through space and time,

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I'd choose here
on the 27th of December, 1860.

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On that day,
Michael Faraday stood on this stage

64
00:03:44,000 --> 00:03:48,000
and delivered his
Royal Institution Christmas Lecture

65
00:03:48,000 --> 00:03:50,000
on the chemical history
of the candle.

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These are his original
lecture notes,

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singed by the burning candle
he used to illuminate them

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00:03:58,000 --> 00:04:02,000
during the dark winter
nights of Victorian London.

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Faraday was one of the greatest
scientists in the world.

70
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He laid the foundations of our
modern understanding

71
00:04:08,000 --> 00:04:12,000
of electricity and magnetism,
but the route he took

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through time and space to change
the world was unusual.

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The son of a Yorkshire blacksmith,

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he left school at 13 to become
an apprentice bookbinder.

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An ordinary young man,
but someone who loved to think.

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He was curious about the world.

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His life changed in 1812 when he
attended a series of lectures

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00:04:32,000 --> 00:04:36,000
by another of the great scientific
ghosts that haunt this place

79
00:04:36,000 --> 00:04:39,000
on Albemarle Street in London -
Humphry Davy,

80
00:04:39,000 --> 00:04:43,000
the charismatic professor of
chemistry at the Royal Institution

81
00:04:43,000 --> 00:04:47,000
and a passionate believer in the
power and possibilities of science.

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00:04:47,000 --> 00:04:52,000
Faraday diligently transcribed
the lectures and gave them to Davy,

83
00:04:52,000 --> 00:04:54,000
who was so impressed that he
appointed the young man

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as his scientific assistant.

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The rest, as they say, is history.

86
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So this building,
this lecture theatre, has a past

87
00:05:03,000 --> 00:05:07,000
that is inextricably bound up
with our present and our future,

88
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not only through the
great discoveries

89
00:05:10,000 --> 00:05:12,000
that have shaped our
scientific civilisation,

90
00:05:12,000 --> 00:05:15,000
but also through the
countless generations

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of children and adults alike
who have been inspired

92
00:05:19,000 --> 00:05:23,000
by lectures given in this theatre
to explore nature

93
00:05:23,000 --> 00:05:27,000
and, to echo Humphry Davy,
to find new worlds to conquer.

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00:05:27,000 --> 00:05:30,000
Tonight, I want to explore if,
just like the Doctor,

95
00:05:30,000 --> 00:05:35,000
if we could one day conquer time,
allowing me to travel to that night

96
00:05:35,000 --> 00:05:38,000
in this room at Christmas 1860.

97
00:05:38,000 --> 00:05:42,000
Will that be forever impossible?

98
00:05:42,000 --> 00:05:45,000
Are the doors to the past
firmly closed?

99
00:05:47,000 --> 00:05:52,000
Well, this object is known
unromantically as H4.

100
00:05:52,000 --> 00:06:00,000
It's a maritime timekeeper built
over 250 years ago by John Harrison.

101
00:06:00,000 --> 00:06:02,000
At the time of its design,

102
00:06:02,000 --> 00:06:06,000
this was the most accurate portable
means of telling the time

103
00:06:06,000 --> 00:06:07,000
ever invented.

104
00:06:07,000 --> 00:06:11,000
It was built to navigate,
to map the world.

105
00:06:11,000 --> 00:06:13,000
Listen, it still works.

106
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H4 TICKS

107
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Can you hear that?

108
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Beautiful thing.

109
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Time, as the Doctor knows,
is the key to exploration.

110
00:06:28,000 --> 00:06:33,000
If you divide our planet into strips
by lines of longitude,

111
00:06:33,000 --> 00:06:38,000
marching east to west, then,
for every 15 degrees you travel,

112
00:06:38,000 --> 00:06:41,000
noon, that's the time that the sun
reaches its highest point

113
00:06:41,000 --> 00:06:44,000
in the sky, shifts by one hour.

114
00:06:44,000 --> 00:06:47,000
So, if you have a clock
that keeps perfect time,

115
00:06:47,000 --> 00:06:52,000
synchronised, let's say, with
noon at Greenwich, which is here,

116
00:06:52,000 --> 00:06:56,000
then you can simply
read off your longitude.

117
00:06:56,000 --> 00:07:00,000
For example, if I left Greenwich
with my H4 and travel west

118
00:07:00,000 --> 00:07:06,000
on my ship, then H4 would read
5pm Greenwich Mean Time

119
00:07:06,000 --> 00:07:10,000
when the sun is directly
overhead at my position.

120
00:07:10,000 --> 00:07:16,000
Then I know that I'm at 15 times 5,
equals 75 degrees west of Greenwich,

121
00:07:16,000 --> 00:07:22,000
which is roughly on the line here
that passes through New York City.

122
00:07:22,000 --> 00:07:27,000
Harrison's H4 was the first watch
that could keep time near-perfectly

123
00:07:27,000 --> 00:07:29,000
through the rigours
of an ocean voyage.

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00:07:29,000 --> 00:07:31,000
It changed the fortunes of Britain,

125
00:07:31,000 --> 00:07:34,000
and it changed
the fortunes of the world.

126
00:07:34,000 --> 00:07:37,000
With the help of the design
of this watch,

127
00:07:37,000 --> 00:07:40,000
the Earth was systematically
explored and mapped.

128
00:07:40,000 --> 00:07:42,000
Trade and travel were
no longer a lottery.

129
00:07:42,000 --> 00:07:48,000
We knew, for the first time, our
place on the surface of our planet.

130
00:07:48,000 --> 00:07:53,000
So time can be used to
determine our position in space.

131
00:07:53,000 --> 00:07:57,000
But space and time still feel as
if they're separate things.

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00:07:57,000 --> 00:08:00,000
Time marches inexorably on,

133
00:08:00,000 --> 00:08:06,000
marked out for 250 years by the
relentless ticking of H4.

134
00:08:06,000 --> 00:08:08,000
This is not the world
the Doctor inhabits.

135
00:08:08,000 --> 00:08:12,000
He has freedom of movement
through space and time,

136
00:08:12,000 --> 00:08:17,000
and, surprisingly, it's not the
world we inhabit either.

137
00:08:17,000 --> 00:08:22,000
I'm going to show you that we
too are ALMOST free

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00:08:22,000 --> 00:08:23,000
to wander through time.

139
00:08:23,000 --> 00:08:27,000
During the late years
of the 19th century, physicists,

140
00:08:27,000 --> 00:08:29,000
and in particular Albert Einstein,

141
00:08:29,000 --> 00:08:33,000
were forced to re-examine our
intuitive picture of space and time,

142
00:08:33,000 --> 00:08:38,000
and halfway through the 20th
century, Einstein's colleague

143
00:08:38,000 --> 00:08:41,000
and tutor Hermann Minkowski
was compelled to write

144
00:08:41,000 --> 00:08:47,000
his now-famous obituary for the
simple tick-tock of the clock.

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00:08:47,000 --> 00:08:52,000
"From henceforth, space by itself
and time by itself

146
00:08:52,000 --> 00:08:55,000
"have vanished into the merest
shadows

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00:08:55,000 --> 00:09:00,000
"and only a kind of blend of the two
exists in its own right."

148
00:09:00,000 --> 00:09:05,000
I'm tempted to say, as the Doctor
would, "Wibbly-wobbly, timey-wimey"

149
00:09:05,000 --> 00:09:07,000
But I won't.

150
00:09:07,000 --> 00:09:08,000
HE CHUCKLES

151
00:09:08,000 --> 00:09:09,000
I just did.

152
00:09:10,000 --> 00:09:14,000
What could Minkowski have meant?
Well, let me draw a map.

153
00:09:14,000 --> 00:09:15,000
A map like no other.

154
00:09:15,000 --> 00:09:19,000
It's a map that contains
the entirety of the known universe.

155
00:09:19,000 --> 00:09:23,000
Our past, our present
and our future.

156
00:09:23,000 --> 00:09:25,000
So, this line, here...

157
00:09:27,000 --> 00:09:28,000
..represents space.

158
00:09:31,000 --> 00:09:35,000
This dot represents
our place in space,

159
00:09:35,000 --> 00:09:39,000
here, at the Royal Institution.

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00:09:39,000 --> 00:09:41,000
Now, let me add time to the map.

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00:09:49,000 --> 00:09:52,000
So, this is our future.

162
00:09:52,000 --> 00:09:56,000
This is all the time that is
yet to come, if you like,

163
00:09:56,000 --> 00:10:00,000
and this is our past.

164
00:10:00,000 --> 00:10:04,000
Now, the dot represents what
physicists call an event.

165
00:10:04,000 --> 00:10:08,000
That's this lecture room,
our place in space,

166
00:10:08,000 --> 00:10:11,000
tonight, our place in time.

167
00:10:11,000 --> 00:10:13,000
So this is a strange kind of map.

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00:10:13,000 --> 00:10:15,000
It's a map of infinite size,

169
00:10:15,000 --> 00:10:20,000
and this line, space, here,
represents our now.

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00:10:20,000 --> 00:10:23,000
The Doctor has complete freedom of
movement around this map

171
00:10:23,000 --> 00:10:25,000
in the TARDIS.

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00:10:25,000 --> 00:10:27,000
He can visit any event he likes -

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00:10:27,000 --> 00:10:30,000
any place in space,
any place in time.

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00:10:30,000 --> 00:10:33,000
Now, we, of course,
don't have that freedom,

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00:10:33,000 --> 00:10:38,000
although, as we'll see, we have more
freedom than you might think.

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00:10:38,000 --> 00:10:41,000
Now, let me explain what
I mean by that cryptic statement.

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00:10:41,000 --> 00:10:44,000
Let's travel back down the timeline.

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00:10:44,000 --> 00:10:49,000
Let's travel back to,
let's say, 1830,

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00:10:49,000 --> 00:10:51,000
a point in the past.

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00:10:51,000 --> 00:10:55,000
Same place, here in
Albemarle Street, different time.

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00:10:55,000 --> 00:10:59,000
Our event is Michael Faraday
conducting experiments

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00:10:59,000 --> 00:11:02,000
in his laboratory, just a
few feet away from us here.

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00:11:02,000 --> 00:11:05,000
And I need a volunteer to
re-create one of his experiments,

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00:11:05,000 --> 00:11:08,000
and, because I can, I'm going to
choose Dallas Campbell.

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00:11:08,000 --> 00:11:09,000
Where's Dallas?

186
00:11:16,000 --> 00:11:20,000
Thank you, Dallas. Thank you.
I know you're a man of science...

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00:11:20,000 --> 00:11:22,000
Well, yes... ..and engineering.
I try, I try.

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00:11:22,000 --> 00:11:25,000
So, if we wheel this experiment
forward...

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00:11:25,000 --> 00:11:29,000
what I want you to do is re-create
one of Faraday's famous experiments.

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00:11:29,000 --> 00:11:31,000
There's a bit of danger involved.

191
00:11:31,000 --> 00:11:33,000
I-I thought there must be
I was, expecting it.

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00:11:33,000 --> 00:11:37,000
These will save you in the event of,
er, an explosion. OK.

193
00:11:37,000 --> 00:11:39,000
It looks quite Doctor Who.

194
00:11:39,000 --> 00:11:41,000
It does look quite Doctor Who,
you're right.

195
00:11:41,000 --> 00:11:44,000
What it is, is a series of coils
of wire.

196
00:11:44,000 --> 00:11:47,000
So, I've got coil of wire,
coil of wire, coil of wire.

197
00:11:47,000 --> 00:11:51,000
And on this pole
there are a series of magnets. Yes.

198
00:11:51,000 --> 00:11:54,000
We've got magnets... Yes.
..inside a coil of wire.

199
00:11:54,000 --> 00:11:57,000
And to make it more televisual,
an explosive, of course.

200
00:11:59,000 --> 00:12:02,000
So, what I'm going to do is
move over here.

201
00:12:03,000 --> 00:12:05,000
What I'm going to do is ask you

202
00:12:05,000 --> 00:12:08,000
to move the magnets in and out
of the coils of wire.

203
00:12:08,000 --> 00:12:11,000
Now, you may need to be
relatively vigorous.

204
00:12:15,000 --> 00:12:18,000
Stand back, stand back, here we go.
Ready?

205
00:12:18,000 --> 00:12:19,000
LAUGHTER AND APPLAUSE

206
00:12:23,000 --> 00:12:26,000
And that's how science works.

207
00:12:26,000 --> 00:12:28,000
OK, was I too vigorous? Yeah.

208
00:12:28,000 --> 00:12:29,000
Years of practice.

209
00:12:29,000 --> 00:12:31,000
Yes, exactly.

210
00:12:31,000 --> 00:12:33,000
OK, here we go. I can't do this.

211
00:12:33,000 --> 00:12:35,000
You can.

212
00:12:35,000 --> 00:12:36,000
You're turning the lights on.

213
00:12:36,000 --> 00:12:38,000
Hooray. Keep going. Come on, Dallas.

214
00:12:38,000 --> 00:12:40,000
Oh!

215
00:12:40,000 --> 00:12:43,000
CHEERING

216
00:12:43,000 --> 00:12:45,000
A spectacular demonstration.

217
00:12:53,000 --> 00:12:57,000
So, just by moving these magnets
in and out of coiled wire,

218
00:12:57,000 --> 00:13:01,000
Dallas created electricity,
enough to light up two light bulbs

219
00:13:01,000 --> 00:13:04,000
and ignite that electric match.

220
00:13:04,000 --> 00:13:06,000
So, what does this mean?

221
00:13:06,000 --> 00:13:10,000
Well, the answer is that electricity
and magnetism

222
00:13:10,000 --> 00:13:12,000
are in some way linked.

223
00:13:12,000 --> 00:13:15,000
Now, Faraday and his colleagues
were intrigued.

224
00:13:15,000 --> 00:13:17,000
How can a moving magnet,

225
00:13:17,000 --> 00:13:20,000
which seems physically unconnected
to the electric wire,

226
00:13:20,000 --> 00:13:23,000
cause an electric current to flow?

227
00:13:23,000 --> 00:13:26,000
Well, the elegant answer was
provided in 1861,

228
00:13:26,000 --> 00:13:29,000
30 years after Faraday's
experiments,

229
00:13:29,000 --> 00:13:33,000
by the great Scottish physicist,
James Clerk Maxwell.

230
00:13:36,000 --> 00:13:37,000
These...

231
00:13:39,000 --> 00:13:41,000
..are Maxwell's wave equations.

232
00:13:41,000 --> 00:13:45,000
Now, Maxwell's genius was to
discover these equations

233
00:13:45,000 --> 00:13:48,000
by bringing the whole of electricity
and magnetism together

234
00:13:48,000 --> 00:13:51,000
into a single framework.

235
00:13:51,000 --> 00:13:54,000
They describe electric
and magnetic fields.

236
00:13:54,000 --> 00:13:57,000
This is the electric field here

237
00:13:57,000 --> 00:13:59,000
and this is the magnetic field here.

238
00:13:59,000 --> 00:14:04,000
But they described fields driving
themselves through space as waves.

239
00:14:04,000 --> 00:14:06,000
Electromagnetic waves.

240
00:14:06,000 --> 00:14:11,000
Now, in itself this is a remarkable
thing, a fascinating discovery.

241
00:14:11,000 --> 00:14:15,000
But even more remarkable is the
prediction from Maxwell's equations

242
00:14:15,000 --> 00:14:19,000
that these waves travel
at a very particular speed.

243
00:14:19,000 --> 00:14:24,000
Now, the speed enters as the ratio

244
00:14:24,000 --> 00:14:27,000
of the strengths of the electric
and magnetic forces.

245
00:14:27,000 --> 00:14:30,000
It was something he'd seen before.

246
00:14:30,000 --> 00:14:36,000
A number measured as far back as
1676 by an astronomer called Romer.

247
00:14:36,000 --> 00:14:40,000
It was, magically,
the speed of light.

248
00:14:42,000 --> 00:14:43,000
This is a tremendous discovery.

249
00:14:43,000 --> 00:14:47,000
Maxwell had found an explanation
for the nature of light itself.

250
00:14:47,000 --> 00:14:51,000
Light is a wave,
electric and magnetic fields,

251
00:14:51,000 --> 00:14:55,000
sloshing energy between them and
propelling themselves through space

252
00:14:55,000 --> 00:14:57,000
at this specific speed.

253
00:14:57,000 --> 00:15:01,000
Very beautiful.
But puzzling as well,

254
00:15:01,000 --> 00:15:04,000
because the speed of light
appears in Maxwell's equations

255
00:15:04,000 --> 00:15:06,000
as a constant.

256
00:15:06,000 --> 00:15:08,000
It is always, in modern units,

257
00:15:08,000 --> 00:15:15,000
precisely
299,792,458 metres per second.

258
00:15:16,000 --> 00:15:21,000
The crucial point is that Maxwell's
equations don't say that this speed

259
00:15:21,000 --> 00:15:24,000
is measured in relation
to something.

260
00:15:24,000 --> 00:15:26,000
They're not measured relative to
anything at all.

261
00:15:26,000 --> 00:15:31,000
It just states the speed of light,
of electrometric waves,

262
00:15:31,000 --> 00:15:38,000
is 299,792,458 metres per second.

263
00:15:38,000 --> 00:15:39,000
Everybody!

264
00:15:39,000 --> 00:15:41,000
LAUGHTER

265
00:15:41,000 --> 00:15:43,000
Just feels like I should say that!

266
00:15:43,000 --> 00:15:48,000
299,792,458 metres per second.

267
00:15:48,000 --> 00:15:51,000
That is stranger than it sounds.

268
00:15:51,000 --> 00:15:54,000
To understand the consequences
of this,

269
00:15:54,000 --> 00:15:57,000
let's return to the beautiful
timepiece, the H4,

270
00:15:57,000 --> 00:16:02,000
which usually resides, actually, at
the Royal Observatory in Greenwich.

271
00:16:02,000 --> 00:16:03,000
Is it moving?

272
00:16:03,000 --> 00:16:06,000
Well, easy. No, it isn't.

273
00:16:06,000 --> 00:16:11,000
Except of course, that it IS moving,
after a fashion.

274
00:16:11,000 --> 00:16:13,000
Remember,
the Earth is spinning on its axis.

275
00:16:13,000 --> 00:16:17,000
It's about 650mph at this
latitude.

276
00:16:17,000 --> 00:16:20,000
So, as well as being stationary,
relative to this lecture theatre,

277
00:16:20,000 --> 00:16:24,000
the clock is also moving
at 650mph

278
00:16:24,000 --> 00:16:28,000
relative to, let's say,
the Doctor in his TARDIS,

279
00:16:28,000 --> 00:16:31,000
looking down on the
Earth from space.

280
00:16:31,000 --> 00:16:37,000
So, for the watch and everything
else, speed is relative.

281
00:16:37,000 --> 00:16:41,000
The watch is stationary,
relative to this lecture theatre,

282
00:16:41,000 --> 00:16:45,000
but according to Maxwell,
light doesn't play by these rules.

283
00:16:45,000 --> 00:16:50,000
Instead, everyone will measure
the speed of light to be the same.

284
00:16:50,000 --> 00:16:53,000
This is a profoundly
strange concept.

285
00:16:53,000 --> 00:16:57,000
This is the way the universe is
built, and it has consequences.

286
00:16:57,000 --> 00:17:01,000
Now, to explore these,
I need a professor of physics,

287
00:17:01,000 --> 00:17:04,000
so I'm going to choose
Jim Al-Khalili.

288
00:17:04,000 --> 00:17:05,000
Where are you Jim?

289
00:17:10,000 --> 00:17:12,000
Now, Einstein did famous
experiments.

290
00:17:12,000 --> 00:17:15,000
He used to do things called
thought experiments,

291
00:17:15,000 --> 00:17:17,000
and we're going to re-create
one of those tonight.

292
00:17:17,000 --> 00:17:18,000
He also had very good hair.

293
00:17:18,000 --> 00:17:21,000
Not as good as yours,
but he had very good hair.

294
00:17:21,000 --> 00:17:25,000
Anyone with any hair has very good
hair, as far as I'm concerned.

295
00:17:25,000 --> 00:17:26,000
I get these
jokes in before Brian does.

296
00:17:26,000 --> 00:17:27,000
I wouldn't dare to comment.

297
00:17:27,000 --> 00:17:30,000
You're going to get me to do
something silly, aren't you?

298
00:17:30,000 --> 00:17:33,000
Please have a seat. Yes. Right.
Notice that I'm in control of this.

299
00:17:33,000 --> 00:17:35,000
You're on a wheeled contraption.
Yes, yes.

300
00:17:35,000 --> 00:17:37,000
And you're going to have a
crash helmet on.

301
00:17:37,000 --> 00:17:39,000
Jolly good.

302
00:17:39,000 --> 00:17:41,000
To protect your beautiful
shiny head.

303
00:17:41,000 --> 00:17:43,000
LAUGHTER

304
00:17:43,000 --> 00:17:45,000
There we are.

305
00:17:45,000 --> 00:17:47,000
Now Jim's going to help me
demonstrate

306
00:17:47,000 --> 00:17:50,000
one of Einstein's most
famous thought experiments.

307
00:17:50,000 --> 00:17:52,000
This will vividly illustrate

308
00:17:52,000 --> 00:17:55,000
the consequences of taking
Maxwell's equations,

309
00:17:55,000 --> 00:17:59,000
Maxwell's constant speed of light,
seriously.

310
00:17:59,000 --> 00:18:02,000
I should explain what's happened.
It's not just for fun, this.

311
00:18:02,000 --> 00:18:05,000
So, what Jim's got on his head
is a video camera.

312
00:18:05,000 --> 00:18:06,000
That's why he's got this
crash helmet on.

313
00:18:06,000 --> 00:18:10,000
So that's going to enable us to see
the world from Jim's perspective -

314
00:18:10,000 --> 00:18:12,000
and we, of course,
are looking at Jim,

315
00:18:12,000 --> 00:18:15,000
so we can see the world
from our perspective.

316
00:18:15,000 --> 00:18:18,000
Now, Einstein's light clock
thought experiment

317
00:18:18,000 --> 00:18:20,000
is essentially a very simple idea.

318
00:18:20,000 --> 00:18:24,000
Einstein just imagined a clock
made of two mirrors

319
00:18:24,000 --> 00:18:28,000
with a beam of light
bouncing between the mirrors.

320
00:18:28,000 --> 00:18:32,000
So, Jim can simulate that with this
torch here, this little bulb,

321
00:18:32,000 --> 00:18:34,000
by moving it up and down.

322
00:18:34,000 --> 00:18:36,000
So, Einstein's clock is
essentially...

323
00:18:36,000 --> 00:18:38,000
LAUGHTER
Shall I do that now?

324
00:18:38,000 --> 00:18:41,000
It's not quite as powerful
as Dallas's...

325
00:18:42,000 --> 00:18:44,000
So, Einstein's light clock
worked like this -

326
00:18:44,000 --> 00:18:46,000
so, if you bounce that beam
of light...

327
00:18:46,000 --> 00:18:50,000
So, my two hands there are the
mirrors, and what you can see

328
00:18:50,000 --> 00:18:54,000
is that you could construct a clock
out of this sort of arrangement.

329
00:18:54,000 --> 00:18:58,000
Essentially, one tick,
which would be, like...

330
00:18:58,000 --> 00:18:59,000
tick...

331
00:19:00,000 --> 00:19:03,000
..tick...

332
00:19:03,000 --> 00:19:05,000
tick - so that's like the pendulum,

333
00:19:05,000 --> 00:19:07,000
the beam of light bouncing
between the mirrors,

334
00:19:07,000 --> 00:19:11,000
and you could use that, actually,
to build a very accurate clock.

335
00:19:11,000 --> 00:19:15,000
Then, Einstein imagined what that
clock would look like

336
00:19:15,000 --> 00:19:18,000
if it were moving relative to us.

337
00:19:18,000 --> 00:19:22,000
So, what I'm going to have happen
is, Jim is going to be moved...

338
00:19:22,000 --> 00:19:24,000
along the stage...

339
00:19:24,000 --> 00:19:26,000
Keep moving the clock.

340
00:19:26,000 --> 00:19:29,000
And then we can dim the light,
so we can see what that looks like

341
00:19:29,000 --> 00:19:32,000
from our perspective,
stationary relative to Jim.

342
00:19:32,000 --> 00:19:36,000
And we've also got... So there's a
little box there, you can see.

343
00:19:36,000 --> 00:19:37,000
That's Jim's head camera,

344
00:19:37,000 --> 00:19:41,000
so Jim is seeing, of course,
the clock in exactly the way

345
00:19:41,000 --> 00:19:44,000
that we pictured it when it was
stationary, relative to us.

346
00:19:44,000 --> 00:19:46,000
The light beam is bouncing up
and down between the mirrors.

347
00:19:46,000 --> 00:19:49,000
But if you look, and we've got a
little video effect on there

348
00:19:49,000 --> 00:19:51,000
so you can see the trail,

349
00:19:51,000 --> 00:19:55,000
you can see that the beam of light
that we see is tracing out

350
00:19:55,000 --> 00:19:58,000
a triangular pattern across
the stage.

351
00:20:01,000 --> 00:20:02,000
Beautiful.

352
00:20:03,000 --> 00:20:04,000
Thank you.

353
00:20:05,000 --> 00:20:09,000
Can I get off now? I'm feeling
a bit sick. You can, yes!

354
00:20:09,000 --> 00:20:11,000
Thank you, Jim.
It was a bit fast. Thank you.

355
00:20:20,000 --> 00:20:23,000
What a great use of that
wonderful intellect.

356
00:20:24,000 --> 00:20:26,000
But it was beautifully demonstrated.

357
00:20:26,000 --> 00:20:31,000
What we saw there was,
if I sketch it out again,

358
00:20:31,000 --> 00:20:33,000
from our perspective now,

359
00:20:33,000 --> 00:20:36,000
from the audience's perspective,
is that here are all those mirrors,

360
00:20:36,000 --> 00:20:39,000
so this is the light clock
that Jim was carrying

361
00:20:39,000 --> 00:20:44,000
but you saw that from your
perspective, watching Jim move,

362
00:20:44,000 --> 00:20:47,000
the light took a kind of
triangular path

363
00:20:47,000 --> 00:20:50,000
as it bounced across the stage
between the mirrors.

364
00:20:53,000 --> 00:20:59,000
Here is what Einstein's postulate,
if you like, Einstein's suggestion

365
00:20:59,000 --> 00:21:03,000
that the speed of light is constant
for all observers, implies.

366
00:21:03,000 --> 00:21:09,000
See, this path is obviously longer
than this path.

367
00:21:09,000 --> 00:21:13,000
So, if we all agree
on the speed of light,

368
00:21:13,000 --> 00:21:18,000
then it is obvious that it must
take the light longer

369
00:21:18,000 --> 00:21:24,000
to tick for the moving clock than
it does for the stationary clock.

370
00:21:25,000 --> 00:21:30,000
Moving clocks run slowly.

371
00:21:30,000 --> 00:21:32,000
This is true.

372
00:21:32,000 --> 00:21:35,000
Time really did pass
at a different rate for Jim.

373
00:21:35,000 --> 00:21:37,000
It passed at a different
rate for him

374
00:21:37,000 --> 00:21:41,000
than it did for you in the audience,
watching Jim move.

375
00:21:41,000 --> 00:21:43,000
There's no sleight of hand here.

376
00:21:43,000 --> 00:21:46,000
Jim really is a time traveller.

377
00:21:46,000 --> 00:21:48,000
Yes!

378
00:21:50,000 --> 00:21:53,000
Our time is personal to us.

379
00:21:53,000 --> 00:21:55,000
This is what Einstein
had discovered.

380
00:21:55,000 --> 00:21:57,000
There's no such
thing as absolute time.

381
00:21:57,000 --> 00:22:00,000
Now, why don't we notice this
in everyday life?

382
00:22:00,000 --> 00:22:04,000
It's because the amount by which
time slowed down for Jim

383
00:22:04,000 --> 00:22:10,000
was minuscule, because the speed he
was travelling was so small

384
00:22:10,000 --> 00:22:12,000
compared to the speed of light.

385
00:22:12,000 --> 00:22:16,000
But if we'd have sent Jim off
in a rocket...

386
00:22:16,000 --> 00:22:18,000
Would you like that?

387
00:22:18,000 --> 00:22:19,000
A rocket?

388
00:22:19,000 --> 00:22:20,000
Flying out into space.

389
00:22:20,000 --> 00:22:26,000
Let's say that we catapulted Jim off
at 99.94% of the speed of light

390
00:22:26,000 --> 00:22:30,000
for five years according
to his watch.

391
00:22:30,000 --> 00:22:33,000
Then we tell Jim to turn
around and come back.

392
00:22:33,000 --> 00:22:36,000
It takes another five years
to get back to the Earth.

393
00:22:36,000 --> 00:22:40,000
So, for him, the journey
would take ten years.

394
00:22:40,000 --> 00:22:43,000
But for us, with our watches ticking
faster than Jim's,

395
00:22:43,000 --> 00:22:46,000
29 years would have passed.

396
00:22:46,000 --> 00:22:53,000
Jim would return in 2042
having aged only ten years.

397
00:22:53,000 --> 00:22:56,000
It's a real effect,
he'd be a time traveller.

398
00:22:56,000 --> 00:22:59,000
Time travel into the future
is possible.

399
00:22:59,000 --> 00:23:04,000
In fact it's an intrinsic part
of the way the universe is built.

400
00:23:04,000 --> 00:23:09,000
We're all time travellers
in our own small way.

401
00:23:09,000 --> 00:23:11,000
APPLAUSE

402
00:23:16,000 --> 00:23:18,000
What on Earth?

403
00:23:25,000 --> 00:23:27,000
Oh, hello. Get your tally out.

404
00:23:27,000 --> 00:23:30,000
That's a Silent. You've got to
admire a monster that puts on a tie.

405
00:23:30,000 --> 00:23:33,000
It's amazing. Yeah.
Shunned by the rest of the galaxy.

406
00:23:33,000 --> 00:23:34,000
They'd be vastly more popular

407
00:23:34,000 --> 00:23:37,000
if they laundered their shirts
every now and then.

408
00:23:37,000 --> 00:23:40,000
An intelligent bipedal life form.
That's a near-impossibility.

409
00:23:40,000 --> 00:23:43,000
Oh, no, don't look away.

410
00:23:43,000 --> 00:23:44,000
What on Earth?

411
00:23:44,000 --> 00:23:46,000
That's a Silent.
Keep staring at it, would you?

412
00:23:46,000 --> 00:23:49,000
I haven't got time right
now to keep introducing it.

413
00:23:49,000 --> 00:23:52,000
I want more aliens. Where can we go?

414
00:23:52,000 --> 00:23:55,000
Oh, you're applying for the job,
then? Job?

415
00:23:55,000 --> 00:23:57,000
My assistant. What does it involve?

416
00:23:57,000 --> 00:23:59,000
Oh, you know, getting captured,
dying occasionally.

417
00:23:59,000 --> 00:24:01,000
The benefits are obviously
the travel.

418
00:24:01,000 --> 00:24:04,000
I mean, Earth people need to get out
more, Brian. Spread your wings,

419
00:24:04,000 --> 00:24:06,000
meet the neighbours.
I mean, what year is this?

420
00:24:06,000 --> 00:24:08,000
From your hair I'd say the sixties.
It looks like an upside down mop.

421
00:24:08,000 --> 00:24:12,000
Yes, the moon is nice,
but come on, my man, have a wander,

422
00:24:12,000 --> 00:24:16,000
stop loitering around your own solar
system like a sulky teenager.

423
00:24:17,000 --> 00:24:20,000
What on Earth? Shut up, Brian.

424
00:24:36,000 --> 00:24:39,000
Are we alone in the universe?

425
00:24:39,000 --> 00:24:42,000
Well, I'd say this is one of the
most important questions

426
00:24:42,000 --> 00:24:43,000
in modern science.

427
00:24:43,000 --> 00:24:47,000
In Doctor Who,
the answer is an emphatic no.

428
00:24:47,000 --> 00:24:49,000
The universe is filled with aliens,

429
00:24:49,000 --> 00:24:53,000
many with technology far
more advanced than our own.

430
00:24:53,000 --> 00:24:56,000
Science fiction's replete
with aliens,

431
00:24:56,000 --> 00:25:00,000
partly, I think, because we
desperately want them to exist.

432
00:25:00,000 --> 00:25:04,000
The alternative, that we're alone
in a possibly infinite universe

433
00:25:04,000 --> 00:25:06,000
is a frightening concept.

434
00:25:06,000 --> 00:25:09,000
But what do we know
about the possibility

435
00:25:09,000 --> 00:25:11,000
of finding the alien life,
and, in particular,

436
00:25:11,000 --> 00:25:14,000
intelligent life
somewhere beyond our solar system?

437
00:25:14,000 --> 00:25:19,000
Well, in 1950, the great Italian
physicist Enrico Fermi

438
00:25:19,000 --> 00:25:24,000
took this question and rephrased it,
he turned it into a paradox,

439
00:25:24,000 --> 00:25:28,000
highlighting, in the process,
one of the great mysteries.

440
00:25:28,000 --> 00:25:31,000
Our sun and its system
of eight planets

441
00:25:31,000 --> 00:25:35,000
is one star out of an estimated
400 billion

442
00:25:35,000 --> 00:25:38,000
that form our home galaxy,
the Milky Way.

443
00:25:38,000 --> 00:25:42,000
Fermi argued that with
so many worlds

444
00:25:42,000 --> 00:25:44,000
and such vast expanses of time

445
00:25:44,000 --> 00:25:46,000
stretching back over
12 billion years

446
00:25:46,000 --> 00:25:50,000
to the formation of our galaxy,
there must be planets out there

447
00:25:50,000 --> 00:25:54,000
with civilisations
far in advance of our own.

448
00:25:54,000 --> 00:25:58,000
So, our universe should be
like Doctor Who.

449
00:25:58,000 --> 00:26:01,000
We should expect,
just on statistical grounds,

450
00:26:01,000 --> 00:26:06,000
to have caught some glimpse of those
spacefaring civilisations

451
00:26:06,000 --> 00:26:13,000
out there amongst the stars and yet
we have seen no evidence of anyone.

452
00:26:13,000 --> 00:26:15,000
This is known as the Fermi paradox.

453
00:26:15,000 --> 00:26:18,000
If they were out there,
we should see them.

454
00:26:20,000 --> 00:26:21,000
The problem, of course,

455
00:26:21,000 --> 00:26:24,000
is that to send a space probe
to even the nearest star

456
00:26:24,000 --> 00:26:28,000
would take many thousands of years
with our current technology,

457
00:26:28,000 --> 00:26:30,000
so the search must proceed

458
00:26:30,000 --> 00:26:33,000
without physically travelling
beyond our solar system,

459
00:26:33,000 --> 00:26:36,000
at least for the foreseeable future.
And there is a way.

460
00:26:36,000 --> 00:26:40,000
The most ancient way of observing
the sky at night.

461
00:26:40,000 --> 00:26:42,000
Astronomy.

462
00:26:42,000 --> 00:26:44,000
By capturing light from distant
star systems,

463
00:26:44,000 --> 00:26:48,000
using an array of telescopes
both on the ground and in orbit,

464
00:26:48,000 --> 00:26:53,000
we've found 992 exoplanets,

465
00:26:53,000 --> 00:26:56,000
and we can now begin to characterise
those planets,

466
00:26:56,000 --> 00:26:58,000
to search for signs of life

467
00:26:58,000 --> 00:27:01,000
encoded in the faint light
from these distant worlds.

468
00:27:01,000 --> 00:27:04,000
So far, one of the best
candidates for life

469
00:27:04,000 --> 00:27:07,000
orbits around one
of the stars in this constellation -

470
00:27:07,000 --> 00:27:09,000
in the constellation of Lyra.

471
00:27:09,000 --> 00:27:12,000
It's a planet called Kepler 62E,

472
00:27:12,000 --> 00:27:16,000
after the recently-retired Kepler
telescope that first identified it.

473
00:27:16,000 --> 00:27:21,000
It seems to be just the right size
and mass to make it a rocky planet

474
00:27:21,000 --> 00:27:23,000
and in just the right orbit
to give it a chance

475
00:27:23,000 --> 00:27:26,000
of possessing liquid water
on its surface.

476
00:27:26,000 --> 00:27:29,000
But, remarkably, we can do better

477
00:27:29,000 --> 00:27:32,000
than simply estimating what these
planets are made of.

478
00:27:32,000 --> 00:27:35,000
See, we're on the verge of being
able to look directly

479
00:27:35,000 --> 00:27:38,000
into the atmospheres
of these planets

480
00:27:38,000 --> 00:27:41,000
and search for the
tell-tale fingerprints of life.

481
00:27:41,000 --> 00:27:45,000
And I'm going to ask
Charles Dance to come down

482
00:27:45,000 --> 00:27:46,000
and help me show you how.

483
00:28:00,000 --> 00:28:01,000
I've got a coat, have I?

484
00:28:01,000 --> 00:28:06,000
Yeah, er, I think its fireproof.
Thank you very much, thank you.

485
00:28:06,000 --> 00:28:08,000
So, what we're going to do, here...

486
00:28:08,000 --> 00:28:12,000
You just want me to clean this
trolley, don't you, really?

487
00:28:12,000 --> 00:28:15,000
Give you a mop! Yeah. Yes.

488
00:28:15,000 --> 00:28:17,000
What we're going to do is,

489
00:28:17,000 --> 00:28:20,000
we're going to demonstrate the
technique that astronomers use

490
00:28:20,000 --> 00:28:23,000
to identify...
Should I be standing where you are?

491
00:28:23,000 --> 00:28:25,000
..the presence of chemicals
in the atmosphere.

492
00:28:25,000 --> 00:28:29,000
You can stand wherever you want,
it's not going to help you at all.

493
00:28:30,000 --> 00:28:33,000
So, er... What I'm going
to ask you to do is,

494
00:28:33,000 --> 00:28:36,000
we've got a selection of four
chemical elements...

495
00:28:36,000 --> 00:28:40,000
Right.
..dissolved in these solutions,

496
00:28:40,000 --> 00:28:42,000
and I want you to spray them
through the Bunsen flame.

497
00:28:42,000 --> 00:28:44,000
Which direction would you like me
to spray them?

498
00:28:44,000 --> 00:28:47,000
I think, actually, probably
sort of just upwards and...

499
00:28:47,000 --> 00:28:48,000
Really, are you sure? Er, yeah.

500
00:28:48,000 --> 00:28:50,000
In any particular order?

501
00:28:50,000 --> 00:28:51,000
No, no, just -
let's see what happens.

502
00:28:51,000 --> 00:28:55,000
So, you can see, apart from this
one, they're all colourless liquids,

503
00:28:55,000 --> 00:28:58,000
but they've got chemical elements
dissolved... OK. ..in the solution.

504
00:28:58,000 --> 00:28:59,000
All right.
So, let's have a go at that.

505
00:28:59,000 --> 00:29:02,000
We could dim the lights a bit,
actually, perhaps.

506
00:29:02,000 --> 00:29:05,000
Go on. Spray that one through.
Let's see what that does.

507
00:29:05,000 --> 00:29:06,000
OK.

508
00:29:06,000 --> 00:29:08,000
There it goes.

509
00:29:08,000 --> 00:29:09,000
Shall I do that one again?

510
00:29:09,000 --> 00:29:11,000
Go again. Beautiful green colour.

511
00:29:11,000 --> 00:29:13,000
Bright green colour.

512
00:29:13,000 --> 00:29:15,000
So, now let's try that one.

513
00:29:15,000 --> 00:29:18,000
Same thing? Yeah.

514
00:29:18,000 --> 00:29:19,000
AUDIENCE: Ooh! Oh, I like that.

515
00:29:19,000 --> 00:29:21,000
A bright red.

516
00:29:21,000 --> 00:29:23,000
This takes me back.

517
00:29:23,000 --> 00:29:24,000
I know!

518
00:29:24,000 --> 00:29:26,000
To school chemistry lessons?

519
00:29:26,000 --> 00:29:29,000
No, to early psychedelic
rock concerts.

520
00:29:29,000 --> 00:29:31,000
APPLAUSE

521
00:29:36,000 --> 00:29:38,000
I quite like that one, actually.

522
00:29:38,000 --> 00:29:40,000
Oh, where's the red?
What if we do two together?

523
00:29:40,000 --> 00:29:43,000
Go on, let's do it, let's go for it.
Shall we do three together?

524
00:29:48,000 --> 00:29:49,000
Oh, dear.

525
00:29:49,000 --> 00:29:51,000
Thank you very much.

526
00:29:57,000 --> 00:30:02,000
Now, the reason that those chemical
elements behaved in different ways

527
00:30:02,000 --> 00:30:05,000
is down to the structure
of the elements themselves.

528
00:30:05,000 --> 00:30:07,000
See, what happens
when you burn that element,

529
00:30:07,000 --> 00:30:11,000
when you heat it up,
is the electrons jump around

530
00:30:11,000 --> 00:30:14,000
between different orbits around the
atomic nucleus

531
00:30:14,000 --> 00:30:18,000
and then fall back down again
and emit light,

532
00:30:18,000 --> 00:30:19,000
and so what we're seeing there

533
00:30:19,000 --> 00:30:22,000
is the structure of
the atoms themselves

534
00:30:22,000 --> 00:30:24,000
that make up the chemical elements.

535
00:30:24,000 --> 00:30:28,000
Each element will have a different
signature of light that it emits

536
00:30:28,000 --> 00:30:33,000
when heated, because it has a
different configuration of electrons

537
00:30:33,000 --> 00:30:35,000
around the nucleus.

538
00:30:35,000 --> 00:30:38,000
Now, as well as emitting light
when heated,

539
00:30:38,000 --> 00:30:42,000
elements also absorb light
of exactly the same colour

540
00:30:42,000 --> 00:30:46,000
if they're present in the atmosphere
of a star or a planet.

541
00:30:46,000 --> 00:30:50,000
Here, for example, is a spectrum
of light from the sun.

542
00:30:50,000 --> 00:30:54,000
So, this is sunlight split up into
all the colours of the rainbow,

543
00:30:54,000 --> 00:30:56,000
by a prism, for example.

544
00:30:56,000 --> 00:31:00,000
And you can see
that it is covered in black lines,

545
00:31:00,000 --> 00:31:02,000
all over, in every colour.

546
00:31:02,000 --> 00:31:05,000
These are the fingerprints
of chemical elements,

547
00:31:05,000 --> 00:31:09,000
in the same way that we saw Charles
show us the beautiful colours,

548
00:31:09,000 --> 00:31:12,000
the fingerprint of the element
in those bottles.

549
00:31:12,000 --> 00:31:16,000
Now, we're on the verge of launching
telescopes and detectors

550
00:31:16,000 --> 00:31:20,000
so sensitive that we can analyse
the light not only from stars,

551
00:31:20,000 --> 00:31:25,000
like the sun, but also
the light reflected and absorbed

552
00:31:25,000 --> 00:31:28,000
by the atmospheres of planets
around those stars.

553
00:31:28,000 --> 00:31:31,000
This will allow us to look
for the fingerprints of molecules

554
00:31:31,000 --> 00:31:35,000
such as water, methane,
and even organic molecules,

555
00:31:35,000 --> 00:31:40,000
the fingerprints of life
in the atmospheres of alien worlds.

556
00:31:40,000 --> 00:31:44,000
These techniques might prove
the first direct evidence

557
00:31:44,000 --> 00:31:47,000
that we're not alone
in the universe.

558
00:31:47,000 --> 00:31:50,000
But they still won't allow
us to resolve Fermi's paradox,

559
00:31:50,000 --> 00:31:54,000
because these chemical fingerprints
won't differentiate

560
00:31:54,000 --> 00:31:57,000
between simple,
single-celled organisms

561
00:31:57,000 --> 00:32:02,000
and the complex multi-cellular life
that is surely a prerequisite

562
00:32:02,000 --> 00:32:05,000
for the existence of
a civilisation like our own.

563
00:32:05,000 --> 00:32:08,000
But there is just a possibility

564
00:32:08,000 --> 00:32:11,000
that we can look for signatures
of intelligent civilisations.

565
00:32:11,000 --> 00:32:14,000
See, as a civilisation gets more
and more advanced,

566
00:32:14,000 --> 00:32:17,000
its energy consumption rises
dramatically.

567
00:32:17,000 --> 00:32:20,000
With every new machine we create
here on Earth,

568
00:32:20,000 --> 00:32:23,000
from the tiniest mobile phone
to the largest power station,

569
00:32:23,000 --> 00:32:26,000
we produce more heat.

570
00:32:26,000 --> 00:32:27,000
I'll show you what I mean.

571
00:32:27,000 --> 00:32:30,000
Here is an infrared camera.

572
00:32:30,000 --> 00:32:34,000
So, this is measuring not the light
from you, the audience,

573
00:32:34,000 --> 00:32:36,000
but the heat from the audience,

574
00:32:36,000 --> 00:32:39,000
because those colours are
representing the amount of heat

575
00:32:39,000 --> 00:32:41,000
that you are putting out.

576
00:32:41,000 --> 00:32:43,000
Yeah, give us a wave.

577
00:32:44,000 --> 00:32:47,000
I can see exactly what you're
doing at the back.

578
00:32:49,000 --> 00:32:52,000
That's because you are
biological machines.

579
00:32:52,000 --> 00:32:56,000
Every machine, no matter how
sophisticated or efficient,

580
00:32:56,000 --> 00:32:57,000
must do this.

581
00:32:57,000 --> 00:33:01,000
It must leave a tell-tale heat
signature behind

582
00:33:01,000 --> 00:33:03,000
as it goes about its business.

583
00:33:03,000 --> 00:33:08,000
Now, a group of researchers at
Penn State University

584
00:33:08,000 --> 00:33:11,000
are attempting to exploit this
fundamental universal law,

585
00:33:11,000 --> 00:33:15,000
using infrared cameras to
search the stars

586
00:33:15,000 --> 00:33:18,000
and even to search for
entire galaxies

587
00:33:18,000 --> 00:33:21,000
to see if they can see
hot spots,

588
00:33:21,000 --> 00:33:24,000
systems that are giving out more
heat in the infrared spectrum

589
00:33:24,000 --> 00:33:28,000
than you would expect
from purely natural processes.

590
00:33:28,000 --> 00:33:30,000
If they sift through all their data,

591
00:33:30,000 --> 00:33:34,000
and actually find a star,
a planet or even a galaxy

592
00:33:34,000 --> 00:33:37,000
with this characteristic
infrared signature,

593
00:33:37,000 --> 00:33:40,000
then they could claim evidence,
not only for complex life

594
00:33:40,000 --> 00:33:42,000
but for a machine-building,
star-harnessing,

595
00:33:42,000 --> 00:33:46,000
transgalactic civilisation.

596
00:33:46,000 --> 00:33:48,000
Doctor Who from afar.

597
00:33:48,000 --> 00:33:51,000
Far-fetched? Yeah, of course it is.

598
00:33:51,000 --> 00:33:54,000
But the simple act of looking,
of observing nature,

599
00:33:54,000 --> 00:33:58,000
is the key to science, and we
shouldn't take anything for granted.

600
00:33:58,000 --> 00:34:01,000
And it's worth noting, finally,

601
00:34:01,000 --> 00:34:05,000
that we may already inadvertently
have made contact.

602
00:34:05,000 --> 00:34:07,000
The first episode of Doctor Who

603
00:34:07,000 --> 00:34:11,000
was broadcast on the 23rd of
November, 1963.

604
00:34:11,000 --> 00:34:15,000
The programme was
encoded in beams of radio waves,

605
00:34:15,000 --> 00:34:19,000
as beams of light that were
broadcast to the nation's TVs.

606
00:34:19,000 --> 00:34:24,000
These radio waves didn't simply
hang around floating above the UK,

607
00:34:24,000 --> 00:34:25,000
they left our atmosphere,

608
00:34:25,000 --> 00:34:30,000
expanding in spheres just
like the light from Faraday's candle

609
00:34:30,000 --> 00:34:32,000
and began their journey
out into space.

610
00:34:32,000 --> 00:34:36,000
Today, that signal will have reached
50 light years from this planet.

611
00:34:36,000 --> 00:34:40,000
SONG: "Doctor Who Theme"

612
00:34:40,000 --> 00:34:42,000
What would an alien
civilisation think

613
00:34:42,000 --> 00:34:46,000
if their first experience of
our civilisation

614
00:34:46,000 --> 00:34:49,000
was the adventures of the
time-travelling doctor?

615
00:35:03,000 --> 00:35:05,000
Oi, Cox, no.

616
00:35:05,000 --> 00:35:07,000
Hands off. Complicated.

617
00:35:07,000 --> 00:35:08,000
Ish.

618
00:35:08,000 --> 00:35:10,000
Ish?! Hah! Don't you "ish" me.

619
00:35:10,000 --> 00:35:11,000
Beyond human understanding.

620
00:35:11,000 --> 00:35:14,000
Relative internal spatial
co-ordinates are completely at odds

621
00:35:14,000 --> 00:35:17,000
with externally observed
dimensions. So, nur.

622
00:35:17,000 --> 00:35:19,000
Bigger on the inside
than the outside

623
00:35:19,000 --> 00:35:20,000
doesn't seem too complicated to me.

624
00:35:22,000 --> 00:35:24,000
Don't listen to him. Cover your ears.

625
00:35:24,000 --> 00:35:26,000
Where exactly are your ears?

626
00:35:26,000 --> 00:35:28,000
Listen,
how do you fuel something like this?

627
00:35:28,000 --> 00:35:30,000
The power requirements must be
immense.

628
00:35:30,000 --> 00:35:32,000
Oh, yeah? Yeah, I use a black hole.

629
00:35:32,000 --> 00:35:34,000
A black hole?
Little bit of Time Lord engineering,

630
00:35:34,000 --> 00:35:37,000
siphon off the energy. Powering this
thing is like falling off a log.

631
00:35:37,000 --> 00:35:39,000
A very big log,
an n-dimensional log.

632
00:35:39,000 --> 00:35:40,000
Read some Einstein.

633
00:35:40,000 --> 00:35:43,000
The tidal forces on a black hole
in there would rip it to bits.

634
00:35:43,000 --> 00:35:44,000
Hah! Yeah, I know that.

635
00:35:44,000 --> 00:35:48,000
Nice chap, Einstein.
Bow tie wearer. Always gets my vote.

636
00:35:48,000 --> 00:35:51,000
Wicked hair.
But he's behind the times, Coxy.

637
00:35:51,000 --> 00:35:52,000
You want to see my black hole?

638
00:35:52,000 --> 00:35:54,000
I keep it down there,
in the basement.

639
00:36:14,000 --> 00:36:17,000
So, the Doctor's world
is closer to our own

640
00:36:17,000 --> 00:36:18,000
than you might have imagined.

641
00:36:18,000 --> 00:36:20,000
We're all time travellers,

642
00:36:20,000 --> 00:36:23,000
and we've reached out
and touched alien worlds.

643
00:36:23,000 --> 00:36:27,000
But I'm drawn back to these notes.

644
00:36:27,000 --> 00:36:31,000
To December 1860, and
Michael Faraday's Christmas Lecture

645
00:36:31,000 --> 00:36:35,000
when he inspired a generation
of children to become scientists,

646
00:36:35,000 --> 00:36:39,000
using the simple but magical candle.

647
00:36:39,000 --> 00:36:43,000
What about my dream to return to
that moment in time?

648
00:36:43,000 --> 00:36:47,000
So, let's take
a look at our map again.

649
00:36:49,000 --> 00:36:54,000
Now, we have everything in the past
that has ever happened down there,

650
00:36:54,000 --> 00:36:59,000
and we have everything that ever
could happen in the future up here.

651
00:36:59,000 --> 00:37:02,000
The Doctor has complete
freedom of movement on the map.

652
00:37:02,000 --> 00:37:04,000
He can go anywhere.

653
00:37:04,000 --> 00:37:08,000
But what Einstein realised is that
we can't have freedom of movement,

654
00:37:08,000 --> 00:37:10,000
otherwise we'd run into trouble.

655
00:37:10,000 --> 00:37:13,000
So, he discovered a limit.

656
00:37:13,000 --> 00:37:15,000
He built it into his theory.

657
00:37:15,000 --> 00:37:18,000
Something that we can all agree on.

658
00:37:18,000 --> 00:37:20,000
The speed of light.

659
00:37:20,000 --> 00:37:24,000
Let's think about Faraday's
candle again.

660
00:37:24,000 --> 00:37:27,000
If there wasn't a roof on this
lecture theatre,

661
00:37:27,000 --> 00:37:31,000
then this would be sending out light
into the universe.

662
00:37:31,000 --> 00:37:34,000
An expanding sphere of light
travelling outwards

663
00:37:34,000 --> 00:37:37,000
at 300,000 kilometres per second.

664
00:37:37,000 --> 00:37:41,000
In one and a half seconds it would
have passed by the moon.

665
00:37:41,000 --> 00:37:45,000
In eight minutes it would speed
past the sun,

666
00:37:45,000 --> 00:37:48,000
and in around 100,000 years,

667
00:37:48,000 --> 00:37:52,000
it would completely clear
the Milky Way Galaxy.

668
00:37:52,000 --> 00:37:56,000
Now, I can draw this onto my map.

669
00:37:56,000 --> 00:38:00,000
So, this is here and now
in this lecture theatre

670
00:38:00,000 --> 00:38:01,000
at the Royal Institution.

671
00:38:01,000 --> 00:38:08,000
So, I can draw a line on my map
that represents the trajectory

672
00:38:08,000 --> 00:38:11,000
of a beam of light through
space-time.

673
00:38:11,000 --> 00:38:14,000
Of course it expands
in all directions,

674
00:38:14,000 --> 00:38:17,000
so I have another one of those
lines going out there.

675
00:38:17,000 --> 00:38:19,000
A pair of diagonal lines.

676
00:38:19,000 --> 00:38:22,000
Now, I could also draw lines
on this map

677
00:38:22,000 --> 00:38:25,000
which represent the paths of beams
of light from the past,

678
00:38:25,000 --> 00:38:30,000
if they arrived here, now,
in this lecture theatre.

679
00:38:30,000 --> 00:38:31,000
And here they'll be.

680
00:38:31,000 --> 00:38:36,000
They'll look the same, but they'll
extend out into the past.

681
00:38:36,000 --> 00:38:39,000
Now, we all agree on these lines

682
00:38:39,000 --> 00:38:41,000
because we all agree on the
speed of light,

683
00:38:41,000 --> 00:38:47,000
so they must be important
in some way. And they are.

684
00:38:47,000 --> 00:38:51,000
This is how Einstein protects
the past from the future.

685
00:38:51,000 --> 00:38:54,000
They limit how we can
move around on the map,

686
00:38:54,000 --> 00:38:59,000
because nothing can travel faster
than the speed of light.

687
00:38:59,000 --> 00:39:02,000
It is a universal speed limit.

688
00:39:02,000 --> 00:39:04,000
What does that mean?

689
00:39:04,000 --> 00:39:09,000
Well, imagine that there
is someone sat here, let's say,

690
00:39:09,000 --> 00:39:10,000
with a telescope.

691
00:39:10,000 --> 00:39:12,000
If I wanted some signal,

692
00:39:12,000 --> 00:39:15,000
some flash of light to get out to
that event there,

693
00:39:15,000 --> 00:39:18,000
which would be, let's say,
an alien in some distant galaxy,

694
00:39:18,000 --> 00:39:20,000
taking a telescope out
and looking at us,

695
00:39:20,000 --> 00:39:23,000
then it would have to travel -
the influence, the light -

696
00:39:23,000 --> 00:39:25,000
would have to travel
faster than the speed of light.

697
00:39:25,000 --> 00:39:27,000
It can't happen.

698
00:39:27,000 --> 00:39:32,000
So, this line seems to restrict
the movement of things.

699
00:39:32,000 --> 00:39:34,000
Things that travel slower than light

700
00:39:34,000 --> 00:39:39,000
are condemned to live inside
this area.

701
00:39:39,000 --> 00:39:42,000
This area is clearly important,
and it's got a name.

702
00:39:42,000 --> 00:39:44,000
It's called the future light cone.

703
00:39:44,000 --> 00:39:47,000
That encompasses all of our futures.

704
00:39:47,000 --> 00:39:50,000
Every event that's going to happen
to any of us in this audience

705
00:39:50,000 --> 00:39:55,000
or watching at home, that happens,
will happen in this region

706
00:39:55,000 --> 00:39:58,000
of space-time inside
the future light cone.

707
00:39:58,000 --> 00:40:00,000
It also applies to the past.

708
00:40:00,000 --> 00:40:02,000
So, this is a special region.

709
00:40:02,000 --> 00:40:05,000
It's called our past light cone.

710
00:40:05,000 --> 00:40:10,000
This is the region that contains
events in space and time

711
00:40:10,000 --> 00:40:14,000
that could in principle have
influenced us now,

712
00:40:14,000 --> 00:40:17,000
at this point, here, tonight.

713
00:40:17,000 --> 00:40:21,000
This is the geometry of space-time
as described by Einstein

714
00:40:21,000 --> 00:40:26,000
in his theory of special relativity
that he published in 1905.

715
00:40:26,000 --> 00:40:30,000
It allows me to trace my life
through these two regions.

716
00:40:30,000 --> 00:40:34,000
I can locate any event that
happened in my life on this map.

717
00:40:34,000 --> 00:40:42,000
So, I was born on
March the 3rd 1968,

718
00:40:42,000 --> 00:40:45,000
and the first picture I have of me
at Christmas

719
00:40:45,000 --> 00:40:49,000
was actually 1972 in Oldham.

720
00:40:49,000 --> 00:40:55,000
There I am, that's that event.
It's me in Oldham in Christmas 1972.

721
00:40:55,000 --> 00:40:58,000
Now, there are lots of things
that happened to me.

722
00:40:58,000 --> 00:41:01,000
I've got a very embarrassing picture
actually in 1989...

723
00:41:04,000 --> 00:41:08,000
What was I thinking?

724
00:41:08,000 --> 00:41:10,000
I-I...

725
00:41:10,000 --> 00:41:11,000
The kind of lifestyle I had.

726
00:41:11,000 --> 00:41:14,000
That was actually when I was on tour
with a rock band somewhere,

727
00:41:14,000 --> 00:41:17,000
I think I was somewhere in Europe.
So it could have been... Actually...

728
00:41:17,000 --> 00:41:20,000
Oh, where shall I put myself?
Over there, that would be 1989.

729
00:41:20,000 --> 00:41:23,000
That's another event,
me on a tour bus,

730
00:41:23,000 --> 00:41:28,000
um, drinking sensibly
in Europe in 1989.

731
00:41:28,000 --> 00:41:29,000
And so on.

732
00:41:29,000 --> 00:41:35,000
So, my life is a series of events
that I can plot on this diagram.

733
00:41:35,000 --> 00:41:39,000
I'm now here, of course,
the Royal Institution in 2013.

734
00:41:39,000 --> 00:41:43,000
So, we could imagine plotting every
event in my life on this diagram.

735
00:41:43,000 --> 00:41:49,000
That would make a line, and it's a
line known as a world line.

736
00:41:49,000 --> 00:41:53,000
And it can wander around in space,
cos I've been at different places,

737
00:41:53,000 --> 00:41:56,000
and, of course,
it wanders around in time

738
00:41:56,000 --> 00:42:02,000
from 1968 to 2013 there.

739
00:42:02,000 --> 00:42:06,000
And, of course, Faraday's Christmas
Lecture on the candle,

740
00:42:06,000 --> 00:42:09,000
the event I most want to
visit in space-time,

741
00:42:09,000 --> 00:42:15,000
is also sitting somewhere down here
in my past light cone.

742
00:42:15,000 --> 00:42:16,000
It's there.

743
00:42:16,000 --> 00:42:19,000
Christmas 1860.

744
00:42:19,000 --> 00:42:20,000
Why is it in my past light cone?

745
00:42:20,000 --> 00:42:23,000
It has to be
because it's influenced me.

746
00:42:23,000 --> 00:42:27,000
These lecture notes were present
at that event

747
00:42:27,000 --> 00:42:30,000
when Faraday stood here
and delivered his lecture,

748
00:42:30,000 --> 00:42:32,000
and they're present
in front of me now.

749
00:42:32,000 --> 00:42:36,000
So, I could draw the world line
at that note book on this diagram.

750
00:42:36,000 --> 00:42:39,000
And they've stayed
in the Royal Institution,

751
00:42:39,000 --> 00:42:42,000
the same place in space,
pretty much their whole life,

752
00:42:42,000 --> 00:42:49,000
because they began in 1860 and
they're here now with me in 2013.

753
00:42:49,000 --> 00:42:52,000
But according to Einstein's Theory
of Special Relativity,

754
00:42:52,000 --> 00:42:57,000
I can never visit Faraday,
because my future world line,

755
00:42:57,000 --> 00:42:59,000
the things I can experience,

756
00:42:59,000 --> 00:43:04,000
is restricted to stay inside
the future light cone.

757
00:43:04,000 --> 00:43:09,000
To get out, to escape into the past,
what would I have to do?

758
00:43:09,000 --> 00:43:12,000
Well, the first thing I'd have to do

759
00:43:12,000 --> 00:43:16,000
is travel faster than
the speed of light,

760
00:43:16,000 --> 00:43:20,000
even before I begin to consider
how I could possibly do that

761
00:43:20,000 --> 00:43:24,000
and loop round to 1860, and
the universe isn't built that way.

762
00:43:24,000 --> 00:43:28,000
The doors to the past,
unless we have a TARDIS,

763
00:43:28,000 --> 00:43:31,000
appear to be firmly closed.

764
00:43:31,000 --> 00:43:33,000
What if there's another way?

765
00:43:33,000 --> 00:43:38,000
What if I can change the direction
of my future light cone,

766
00:43:38,000 --> 00:43:42,000
change the direction
of my entire future,

767
00:43:42,000 --> 00:43:46,000
and perhaps begin to tilt it
towards the past?

768
00:43:46,000 --> 00:43:50,000
Well, there are objects
in our universe

769
00:43:50,000 --> 00:43:54,000
that can tilt light cones,
and if I could get close enough

770
00:43:54,000 --> 00:43:58,000
they'd affect the direction of my
future in a radical way.

771
00:43:58,000 --> 00:44:02,000
There's one at the heart
of the TARDIS, a black hole.

772
00:44:02,000 --> 00:44:07,000
The Eye of Harmony is described in
Doctor Who as a star,

773
00:44:07,000 --> 00:44:11,000
frozen at the point of collapse
into a black hole.

774
00:44:11,000 --> 00:44:17,000
It's a poetic line, but unusually,
it has to be said, for poetry,

775
00:44:17,000 --> 00:44:21,000
this one turns out
to be physically accurate.

776
00:44:21,000 --> 00:44:23,000
Black holes form
at the end of the lives

777
00:44:23,000 --> 00:44:26,000
of the most massive stars
in the universe.

778
00:44:26,000 --> 00:44:30,000
When such stars, at least 20 times
the mass of our sun,

779
00:44:30,000 --> 00:44:31,000
run out of fuel in their cores,

780
00:44:31,000 --> 00:44:35,000
no known force can overcome the
inward pull of gravity

781
00:44:35,000 --> 00:44:37,000
and prevent them from collapsing,

782
00:44:37,000 --> 00:44:41,000
as far as anyone knows,
to a single, infinitely dense point

783
00:44:41,000 --> 00:44:44,000
known as a singularity.

784
00:44:44,000 --> 00:44:48,000
I can draw one of those on a
space-time diagram.

785
00:44:48,000 --> 00:44:52,000
So here is space, and here is time.

786
00:44:52,000 --> 00:44:56,000
And this is a diagram from the point
of view of the black hole,

787
00:44:56,000 --> 00:45:01,000
so that's the singularity ticking
forward in time, as it were.

788
00:45:01,000 --> 00:45:04,000
And these two lines,
which are very important,

789
00:45:04,000 --> 00:45:08,000
have the evocative names
of event horizons.

790
00:45:08,000 --> 00:45:12,000
These mark out a
region in space and time

791
00:45:12,000 --> 00:45:15,000
where the gravitational pull
is so strong

792
00:45:15,000 --> 00:45:17,000
that light itself cannot escape.

793
00:45:17,000 --> 00:45:21,000
In the vicinity of the event horizon
very strange things happen.

794
00:45:21,000 --> 00:45:26,000
And I need a very strange volunteer
to demonstrate that.

795
00:45:26,000 --> 00:45:29,000
So, Rufus Hound, where are you?

796
00:45:36,000 --> 00:45:38,000
That was perhaps a little unkind,
wasn't it?

797
00:45:38,000 --> 00:45:40,000
"A very strange volunteer."
No, it seems about right.

798
00:45:40,000 --> 00:45:41,000
Is it about right? Yeah.

799
00:45:41,000 --> 00:45:44,000
Um, so...
I thought that with my fourth brain.

800
00:45:44,000 --> 00:45:46,000
Did you?

801
00:45:46,000 --> 00:45:50,000
What I'd like to do is to throw
you into a black hole.

802
00:45:50,000 --> 00:45:52,000
You wouldn't be the first.

803
00:45:52,000 --> 00:45:54,000
In the name of physics, now...
You would be the first.

804
00:45:54,000 --> 00:45:56,000
I think it's going to mean

805
00:45:56,000 --> 00:45:59,000
that you're going to meet
a very noble end,

806
00:45:59,000 --> 00:46:01,000
a very wonderful
exit from this universe.

807
00:46:01,000 --> 00:46:05,000
But in order to observe you as you
exit our plane of existence,

808
00:46:05,000 --> 00:46:08,000
as it were, I want to kit you out
with two watches. OK.

809
00:46:08,000 --> 00:46:11,000
This one, which I want you to put
on your back,

810
00:46:11,000 --> 00:46:15,000
is going to be the one
that we can observe.

811
00:46:17,000 --> 00:46:19,000
All right, there we are. Sorry.

812
00:46:19,000 --> 00:46:22,000
Is this how you're going to
collapse my mass?

813
00:46:22,000 --> 00:46:24,000
Is that a bit...
Is that comfortable?

814
00:46:24,000 --> 00:46:26,000
You're going to do the straps up,
is that how black holes work?

815
00:46:26,000 --> 00:46:29,000
Just some bloke with a really
tight backpack on.

816
00:46:29,000 --> 00:46:30,000
There we go.

817
00:46:38,000 --> 00:46:40,000
I already feel implosiony.

818
00:46:41,000 --> 00:46:44,000
And I'd like to give you - well,
actually, have you got a watch?

819
00:46:44,000 --> 00:46:46,000
I've got a watch.
Oh, you've got a watch.

820
00:46:46,000 --> 00:46:48,000
And there's a second hand
ticking away. Yep.

821
00:46:48,000 --> 00:46:49,000
That's good.

822
00:46:49,000 --> 00:46:53,000
Right, so, what we're going to do,
is we're going to...

823
00:46:53,000 --> 00:46:56,000
Right - it's low voltage,
it's all right.

824
00:46:56,000 --> 00:46:59,000
I'm going to turn it...
Where are my safety goggles, Brian?

825
00:46:59,000 --> 00:47:02,000
If you just turn round...

826
00:47:02,000 --> 00:47:05,000
If it'll make you feel better
I can get some, but it won't help.

827
00:47:05,000 --> 00:47:06,000
No. Great, fine.

828
00:47:06,000 --> 00:47:09,000
If you turn around
so we can see this clock,

829
00:47:09,000 --> 00:47:12,000
and I'm going to turn the clock on,
and there it is.

830
00:47:12,000 --> 00:47:13,000
So it's whizzing forward in time.

831
00:47:13,000 --> 00:47:16,000
And I want you to face the
blackboard, the Eye of Harmony,

832
00:47:16,000 --> 00:47:18,000
that's the black hole, there.

833
00:47:18,000 --> 00:47:20,000
And what I've done is,
I've speeded time up

834
00:47:20,000 --> 00:47:22,000
just so we can
see it ticking along.

835
00:47:22,000 --> 00:47:25,000
This is the rate that time's
passing for us now,

836
00:47:25,000 --> 00:47:27,000
and it would be the same
on your watch here. Right.

837
00:47:27,000 --> 00:47:31,000
And I'm going to ask you to move
slowly towards the event horizon.

838
00:47:31,000 --> 00:47:33,000
Very slowly.

839
00:47:35,000 --> 00:47:37,000
That's it.

840
00:47:38,000 --> 00:47:39,000
How do you feel?

841
00:47:39,000 --> 00:47:41,000
Like this is slightly TOO slow.

842
00:47:44,000 --> 00:47:46,000
It's all right.

843
00:47:46,000 --> 00:47:48,000
But you see what's happening.

844
00:47:48,000 --> 00:47:51,000
If you stop there, you're
approaching the event horizon,

845
00:47:51,000 --> 00:47:55,000
and time on the watch that we're
looking at, attached to your back,

846
00:47:55,000 --> 00:47:58,000
is slowing down. How's the time,
though, on your watch?

847
00:47:58,000 --> 00:47:59,000
Exactly the same.

848
00:47:59,000 --> 00:48:03,000
It's ticking along at exactly
the same rate.

849
00:48:03,000 --> 00:48:06,000
Now, you might start to feel
a bit uncomfortable

850
00:48:06,000 --> 00:48:10,000
because for these sort of
stellar mass black holes,

851
00:48:10,000 --> 00:48:13,000
the gravitational force on your feet
would now be significantly stronger

852
00:48:13,000 --> 00:48:16,000
than the gravitation force
on your head.

853
00:48:16,000 --> 00:48:18,000
Now, this is called
spaghettification.

854
00:48:21,000 --> 00:48:23,000
Why?

855
00:48:23,000 --> 00:48:26,000
So, you're beginning to get
slightly taller. Right.

856
00:48:26,000 --> 00:48:28,000
And eventually, actually, as you
approach the event horizon

857
00:48:28,000 --> 00:48:30,000
I think, really, you'd get so tall

858
00:48:30,000 --> 00:48:33,000
that you'd just be a long line
of atoms, disassociated.

859
00:48:33,000 --> 00:48:36,000
But anyway, let's ignore that
for the moment. Carry on.

860
00:48:38,000 --> 00:48:41,000
So, you see... I don't know why I
feel slightly in awe of a picture.

861
00:48:43,000 --> 00:48:45,000
Right towards the black hole.

862
00:48:45,000 --> 00:48:48,000
And what we see - there, stop.

863
00:48:48,000 --> 00:48:50,000
That is on the event horizon

864
00:48:50,000 --> 00:48:54,000
and we would see Rufus' watch,
strapped to his back, freeze.

865
00:48:54,000 --> 00:48:57,000
It would stop,
but what does your watch look like?

866
00:48:57,000 --> 00:48:58,000
Still going.

867
00:48:58,000 --> 00:49:00,000
Still going at exactly
the same rate.

868
00:49:00,000 --> 00:49:04,000
This is precisely what
Einstein tells us would happen

869
00:49:04,000 --> 00:49:06,000
as Rufus fell into the black hole.

870
00:49:06,000 --> 00:49:09,000
We'd see time freeze.

871
00:49:09,000 --> 00:49:12,000
We would see an image of Rufus
just like that, actually,

872
00:49:12,000 --> 00:49:14,000
that's quite powerful.

873
00:49:14,000 --> 00:49:17,000
How long can you stand on one leg,
just like that?

874
00:49:17,000 --> 00:49:21,000
We'd see a frozen image of Rufus
on his way across the event horizon.

875
00:49:21,000 --> 00:49:24,000
Time would stop, that image
would still be there.

876
00:49:24,000 --> 00:49:26,000
It would be a sort of immortality,

877
00:49:26,000 --> 00:49:29,000
whereas from Rufus' perspective,
time would pass as normal,

878
00:49:29,000 --> 00:49:31,000
he would pass over the
event horizon,

879
00:49:31,000 --> 00:49:33,000
he would approach the singularity

880
00:49:33,000 --> 00:49:36,000
and be crushed to an infinitely
dense point.

881
00:49:36,000 --> 00:49:38,000
Thank you.

882
00:49:49,000 --> 00:49:50,000
Thanks Rufus.

883
00:49:50,000 --> 00:49:53,000
Um, let me explain what happened
to Rufus.

884
00:49:53,000 --> 00:49:57,000
So here is my space-time
diagram again.

885
00:49:57,000 --> 00:50:00,000
Remember that the black hole
is sat here, stationary.

886
00:50:00,000 --> 00:50:03,000
There's the singularity,
here are the event horizons.

887
00:50:03,000 --> 00:50:06,000
And what I'm going to do is,

888
00:50:06,000 --> 00:50:11,000
I'm going to superimpose
Rufus' world line...

889
00:50:13,000 --> 00:50:15,000
..onto this diagram.

890
00:50:17,000 --> 00:50:19,000
Now, we're looking at Rufus,
remember,

891
00:50:19,000 --> 00:50:21,000
from the point of view
of the black hole.

892
00:50:21,000 --> 00:50:23,000
So it's just sat there,
it's going nowhere,

893
00:50:23,000 --> 00:50:27,000
and Rufus is on a journey towards
the event horizon

894
00:50:27,000 --> 00:50:30,000
and beyond into oblivion.

895
00:50:30,000 --> 00:50:33,000
What I've also drawn are
Rufus' light cones,

896
00:50:33,000 --> 00:50:36,000
the various
points along his world line.

897
00:50:36,000 --> 00:50:41,000
These mark out Rufus'
accessible future.

898
00:50:41,000 --> 00:50:44,000
But look what happens
to these light cones

899
00:50:44,000 --> 00:50:46,000
as he approaches the event horizon.

900
00:50:46,000 --> 00:50:48,000
They're tilting.

901
00:50:48,000 --> 00:50:52,000
Now, this tilt,
according to Albert Einstein,

902
00:50:52,000 --> 00:50:55,000
is caused by the mass
of the black hole itself.

903
00:50:55,000 --> 00:50:58,000
It's a representation
of a central idea

904
00:50:58,000 --> 00:51:02,000
in Einstein's theory of gravity,
general relativity.

905
00:51:02,000 --> 00:51:07,000
The idea is this - mass and energy
curve space and time,

906
00:51:07,000 --> 00:51:10,000
the very fabric
of the universe itself.

907
00:51:10,000 --> 00:51:14,000
That curvature, the warping of space
and time, if you like,

908
00:51:14,000 --> 00:51:19,000
is what we're seeing in this diagram
as the tilting of light cones,

909
00:51:19,000 --> 00:51:24,000
the tilting of Rufus' future
towards the event horizon.

910
00:51:24,000 --> 00:51:26,000
And look what happens
here on the horizon.

911
00:51:26,000 --> 00:51:28,000
You see what's happened
to the light cone?

912
00:51:28,000 --> 00:51:34,000
It's tilted so much, space and time
are curved and warped so much,

913
00:51:34,000 --> 00:51:39,000
that all of Rufus' future is
pointing inwards,

914
00:51:39,000 --> 00:51:41,000
into the horizon,
into the black hole.

915
00:51:41,000 --> 00:51:45,000
His world line is heading towards
the singularity.

916
00:51:45,000 --> 00:51:47,000
There's no escape for Rufus

917
00:51:47,000 --> 00:51:52,000
because his entire future
is inside the black hole.

918
00:51:52,000 --> 00:51:55,000
He'd have to travel faster than
light to get out,

919
00:51:55,000 --> 00:51:59,000
and that is not
allowed in our universe.

920
00:51:59,000 --> 00:52:01,000
This diagram is very beautiful.

921
00:52:01,000 --> 00:52:03,000
It allows us to see something else,

922
00:52:03,000 --> 00:52:06,000
it also allows us to see what
happened to Rufus' clock

923
00:52:06,000 --> 00:52:09,000
as we watched it tick slower
and slower and slower

924
00:52:09,000 --> 00:52:12,000
as he approached the horizon.

925
00:52:12,000 --> 00:52:14,000
So, let's imagine...

926
00:52:14,000 --> 00:52:17,000
let's imagine that on each tick
of Rufus' clock,

927
00:52:17,000 --> 00:52:21,000
the one on his back,
a pulse of light was sent out

928
00:52:21,000 --> 00:52:24,000
and we detected that pulse of light
from our vantage point

929
00:52:24,000 --> 00:52:26,000
far away from the black hole.

930
00:52:28,000 --> 00:52:29,000
So, let me put them on.

931
00:52:30,000 --> 00:52:32,000
There.

932
00:52:33,000 --> 00:52:35,000
You see what happens.

933
00:52:35,000 --> 00:52:37,000
As the light cones pulse,

934
00:52:37,000 --> 00:52:43,000
then those pulses of light arrive
at us at later and later times.

935
00:52:43,000 --> 00:52:45,000
This is the ticking of the clock.

936
00:52:45,000 --> 00:52:48,000
As far as Rufus is concerned,
the clock's ticking away normally,

937
00:52:48,000 --> 00:52:51,000
one second, two seconds,
three seconds, four seconds.

938
00:52:51,000 --> 00:52:57,000
But, as we see it, the first second
is faster than the second second,

939
00:52:57,000 --> 00:52:59,000
which is faster
than the third second.

940
00:52:59,000 --> 00:53:01,000
Tick...

941
00:53:01,000 --> 00:53:02,000
tick...

942
00:53:03,000 --> 00:53:07,000
..tick. And here, on the horizon,

943
00:53:07,000 --> 00:53:11,000
the light pulse goes flying up the
side of the light cone,

944
00:53:11,000 --> 00:53:14,000
which is aligned along the event
horizon itself.

945
00:53:14,000 --> 00:53:20,000
This pulse never reaches us,
so time stops from our perspective.

946
00:53:20,000 --> 00:53:23,000
We see that frozen image of Rufus.

947
00:53:23,000 --> 00:53:27,000
He never makes it across the horizon
from our vantage point.

948
00:53:27,000 --> 00:53:30,000
According to him everything proceeds
quite normally -

949
00:53:30,000 --> 00:53:33,000
although he's getting spaghettified,
it has to be said -

950
00:53:33,000 --> 00:53:37,000
until he gets squashed
on the singularity.

951
00:53:37,000 --> 00:53:41,000
This image of Rufus is frozen
forever at the horizon.

952
00:53:41,000 --> 00:53:44,000
But here's the wonderful thing -

953
00:53:44,000 --> 00:53:48,000
the same is true
for the collapsing star itself.

954
00:53:48,000 --> 00:53:52,000
See, from the perspective of an
outside observer,

955
00:53:52,000 --> 00:53:57,000
time stops, so we'd never actually
see the star collapse,

956
00:53:57,000 --> 00:54:01,000
we'd see a frozen image fading away

957
00:54:01,000 --> 00:54:06,000
of the dying star forever frozen
in time at the moment of collapse,

958
00:54:06,000 --> 00:54:12,000
that is precisely the Eye of Harmony
as described in Doctor Who.

959
00:54:12,000 --> 00:54:14,000
How beautiful.

960
00:54:14,000 --> 00:54:17,000
But what of my ambition to get
back into the past

961
00:54:17,000 --> 00:54:21,000
and experience Michael Faraday
deliver his lecturer?

962
00:54:21,000 --> 00:54:24,000
Well, everything I've spoken about
so far in this lecture

963
00:54:24,000 --> 00:54:29,000
is science fact, including this
description of a frozen star.

964
00:54:31,000 --> 00:54:34,000
But now it's time to speculate
just a little,

965
00:54:34,000 --> 00:54:39,000
but still remain constrained
by the known laws of physics.

966
00:54:39,000 --> 00:54:42,000
Notice what the Eye of Harmony,
the black hole, did.

967
00:54:42,000 --> 00:54:46,000
It tilted light cones,
it changed the direction

968
00:54:46,000 --> 00:54:49,000
of the accessible future
in space-time.

969
00:54:49,000 --> 00:54:54,000
Now, could it be that we could dream
up some geometry of space-time,

970
00:54:54,000 --> 00:54:58,000
a distribution of matter and energy
that would tilt light cones

971
00:54:58,000 --> 00:55:00,000
all the way around?

972
00:55:01,000 --> 00:55:06,000
What I want to do
is tilt my future light cone

973
00:55:06,000 --> 00:55:11,000
in such a way that it gets me back
to Faraday's Christmas Lecture

974
00:55:11,000 --> 00:55:12,000
in 1860.

975
00:55:15,000 --> 00:55:16,000
Something like this.

976
00:55:18,000 --> 00:55:20,000
So, here...

977
00:55:20,000 --> 00:55:22,000
is a piece of space-time.

978
00:55:22,000 --> 00:55:25,000
It's meant to map directly onto this
diagram I drew here.

979
00:55:27,000 --> 00:55:33,000
Here's 1860, and here's me in 2013.

980
00:55:33,000 --> 00:55:39,000
Now, we've seen that a black hole
can tilt light cones like that.

981
00:55:39,000 --> 00:55:44,000
What if we could arrange the
geometry so that the light cone

982
00:55:44,000 --> 00:55:51,000
tilts around,
so it bends in some way

983
00:55:51,000 --> 00:55:53,000
so that...

984
00:55:53,000 --> 00:55:59,000
I can reattach space-time,
as it were, around into the past?

985
00:55:59,000 --> 00:56:03,000
I could curve space-time
in such a way that this area,

986
00:56:03,000 --> 00:56:08,000
my accessible future, ends up
pointing into my own past

987
00:56:08,000 --> 00:56:13,000
and specifically, in this case,
ends up pointing to this place,

988
00:56:13,000 --> 00:56:18,000
this event I want to visit,
Faraday's lecture in 1860.

989
00:56:18,000 --> 00:56:22,000
Could we design some configuration
of matter and energy

990
00:56:22,000 --> 00:56:24,000
that would curve
the light cones around

991
00:56:24,000 --> 00:56:29,000
so I could get back into my
own past?

992
00:56:29,000 --> 00:56:31,000
The answer is...

993
00:56:31,000 --> 00:56:32,000
we don't know.

994
00:56:35,000 --> 00:56:41,000
But nobody has been able to prove
that space-time geometries

995
00:56:41,000 --> 00:56:46,000
similar to this cannot exist,
at least in principle.

996
00:56:46,000 --> 00:56:52,000
Although most experts believe that
they must in some way be forbidden.

997
00:56:52,000 --> 00:56:56,000
But there's still the faintest
possibility,

998
00:56:56,000 --> 00:56:59,000
given the laws of physics as we
understand them today,

999
00:56:59,000 --> 00:57:04,000
that someone, someday -
maybe a young girl or a young boy -

1000
00:57:04,000 --> 00:57:06,000
will be inspired to try.

1001
00:57:06,000 --> 00:57:10,000
And, even if they fail,
by the very act of trying

1002
00:57:10,000 --> 00:57:15,000
they might just go on
to change the world.

1003
00:57:15,000 --> 00:57:17,000
APPLAUSE

1004
00:57:27,000 --> 00:57:29,000
Home!

1005
00:57:29,000 --> 00:57:31,000
Oh, I want to visit
more alien worlds.

1006
00:57:31,000 --> 00:57:32,000
No. Greedy, Brian. Can't be greedy.

1007
00:57:32,000 --> 00:57:36,000
You've got a lecture to give, people
to inspire, merchandise to sell.

1008
00:57:36,000 --> 00:57:38,000
Actually, that reminds me,
could you rustle me up a lunchbox?

1009
00:57:38,000 --> 00:57:40,000
Maybe a T-shirt, slim-fitting.

1010
00:57:40,000 --> 00:57:42,000
Oh, don't forget the gift I got you,
you'll need that.

1011
00:57:42,000 --> 00:57:44,000
So, what was this all about, then,

1012
00:57:44,000 --> 00:57:46,000
just taking me on a tour
of the wonders of the universe?

1013
00:57:46,000 --> 00:57:48,000
Ah!

1014
00:57:48,000 --> 00:57:51,000
Well, there's someone
in your audience today,

1015
00:57:51,000 --> 00:57:54,000
just an ordinary kid,
so high, sad eyes, look out for her,

1016
00:57:54,000 --> 00:57:57,000
someone who loves to think about
why the sky is blue

1017
00:57:57,000 --> 00:57:59,000
and how bees can hover like
helicopters,

1018
00:57:59,000 --> 00:58:04,000
but after today she
stops being ordinary,

1019
00:58:04,000 --> 00:58:08,000
she grows up to be extraordinary,
a woman who changes the world.

1020
00:58:09,000 --> 00:58:13,000
And all she needed was a nudge
from you, eh? Today, right now.

1021
00:58:13,000 --> 00:58:14,000
No pressure.

1022
00:58:14,000 --> 00:58:15,000
I do love humans.

1023
00:58:15,000 --> 00:58:19,000
They can be a bit defeatist.
You know, "Mustn't," "Can't"...

1024
00:58:19,000 --> 00:58:21,000
Sometimes you just need
a helping hand.

1025
00:58:21,000 --> 00:58:24,000
Every adventure starts with
a moment, a spark. Ooh!

1026
00:58:24,000 --> 00:58:26,000
Whilst I'm here...

1027
00:58:26,000 --> 00:58:29,000
Bit of anti-shine. You'll need that.

1028
00:58:29,000 --> 00:58:31,000
Ah.

1029
00:58:31,000 --> 00:58:33,000
Don't forget to twiddle
the size of the event horizon.

1030
00:58:33,000 --> 00:58:34,000
Shut up, Brian.

1031
00:58:41,000 --> 00:58:42,000
One more adventure before tea.

1032
00:58:49,000 --> 00:58:51,000
SONG: "Doctor Who Theme"

1033
00:58:54,000 --> 00:58:57,000
Subtitles by Red Bee Media Ltd