1
00:00:02,000 --> 00:00:07,200
Just two months ago, a major
scientific discovery was announced.

2
00:00:07,200 --> 00:00:12,400
Ladies and gentlemen, we have
detected gravitational waves.

3
00:00:12,400 --> 00:00:14,280
APPLAUSE
We did it!

4
00:00:16,920 --> 00:00:21,360
This detection was solid evidence of
something Albert Einstein predicted

5
00:00:21,360 --> 00:00:23,520
100 years ago.

6
00:00:23,520 --> 00:00:26,640
But it was also
the most direct observation ever

7
00:00:26,640 --> 00:00:28,920
of black holes.

8
00:00:28,920 --> 00:00:32,640
The discovery of gravitational waves
has launched a new era

9
00:00:32,640 --> 00:00:34,960
in the study of perhaps
the most captivating

10
00:00:34,960 --> 00:00:38,240
and powerful objects
in the universe.

11
00:00:38,240 --> 00:00:42,800
In science fiction, spaceships
often go through a black hole

12
00:00:42,800 --> 00:00:44,640
to another universe,

13
00:00:44,640 --> 00:00:47,160
or another part of our universe.

14
00:00:48,520 --> 00:00:51,320
But black holes are stranger
than anything

15
00:00:51,320 --> 00:00:53,560
dreamt up
by science-fiction writers.

16
00:00:54,880 --> 00:00:57,560
Tonight, we're joined
by Stephen Hawking

17
00:00:57,560 --> 00:01:02,200
to take a mind-blowing journey into
the enigmatic world of black holes.

18
00:01:02,200 --> 00:01:05,240
We're going to find out
how the astonishing discovery

19
00:01:05,240 --> 00:01:08,240
of gravitational waves,
made just a few weeks ago,

20
00:01:08,240 --> 00:01:11,960
is already helping us understand
the fundamental nature of them.

21
00:01:41,280 --> 00:01:44,880
In a truly extraordinary life,
Professor Stephen Hawking

22
00:01:44,880 --> 00:01:47,880
has become the world's
most celebrated scientist.

23
00:01:49,560 --> 00:01:54,320
And for over 40 years, he's wrestled
with the toughest of questions.

24
00:01:54,320 --> 00:01:57,440
What happens inside a black hole?

25
00:01:57,440 --> 00:02:01,640
Black holes are formed
by the collapse of massive stars

26
00:02:01,640 --> 00:02:04,480
when they have exhausted
their nuclear fuel...

27
00:02:08,400 --> 00:02:11,000
..and can no longer
support themselves against

28
00:02:11,000 --> 00:02:12,560
their own gravity.

29
00:02:17,720 --> 00:02:22,360
They are quite literally holes
in space that stuff can fall into,

30
00:02:22,360 --> 00:02:24,760
but not get out of.

31
00:02:24,760 --> 00:02:27,800
They are places where
the gravitational field is

32
00:02:27,800 --> 00:02:32,800
so strong that nothing,
not even light, can get away.

33
00:02:34,240 --> 00:02:38,440
This basic picture of black holes
is very well-known.

34
00:02:38,440 --> 00:02:41,200
But, unfortunately,
it is also far from complete.

35
00:02:42,560 --> 00:02:44,920
At the heart of the mystery
of black holes

36
00:02:44,920 --> 00:02:47,440
lie two fundamental problems.

37
00:02:47,440 --> 00:02:50,760
Firstly, theorists have found it
hard to understand what's happening

38
00:02:50,760 --> 00:02:54,080
inside a black hole. In fact,
the more they crunch the numbers,

39
00:02:54,080 --> 00:02:57,720
the more it seems that black holes
defy the laws of physics.

40
00:02:57,720 --> 00:03:00,960
In particular, it throws up
a mind-boggling conundrum

41
00:03:00,960 --> 00:03:03,760
known as the information paradox.

42
00:03:03,760 --> 00:03:07,320
Secondly, astronomers trying
to observe these unusual objects

43
00:03:07,320 --> 00:03:10,560
and find physical evidence with
which to test these theories,

44
00:03:10,560 --> 00:03:13,120
hit an apparently
insurmountable problem -

45
00:03:13,120 --> 00:03:15,360
you can't see inside a black hole.

46
00:03:15,360 --> 00:03:17,480
The maths don't add up,
and no-one's ever

47
00:03:17,480 --> 00:03:21,000
seen inside a black hole,
so how do we know they even exist?

48
00:03:21,000 --> 00:03:23,840
Also, what kind of discovery
can hope to answer

49
00:03:23,840 --> 00:03:25,480
all of these mysteries?

50
00:03:25,480 --> 00:03:27,520
Well, with the help of
Professor Hawking,

51
00:03:27,520 --> 00:03:29,920
we're going to try
and answer all of those questions.

52
00:03:29,920 --> 00:03:32,680
So, we've come here to
the University of Cambridge,

53
00:03:32,680 --> 00:03:34,720
home of Stephen Hawking
and, arguably,

54
00:03:34,720 --> 00:03:36,840
the birthplace of
black hole science.

55
00:03:38,720 --> 00:03:43,120
In tonight's programme, we'll
hear from Stephen Hawking himself,

56
00:03:43,120 --> 00:03:46,000
how he transformed our view of
black holes.

57
00:03:47,320 --> 00:03:51,120
Black holes were thought
to be completely black,

58
00:03:51,120 --> 00:03:54,520
until I discovered that they
glow like hot bodies.

59
00:03:57,600 --> 00:04:00,600
I'll be finding out
from cosmologist Andrew Pontzen

60
00:04:00,600 --> 00:04:03,480
why Hawking's black hole theories
created one of the

61
00:04:03,480 --> 00:04:05,680
biggest conundrums in astrophysics.

62
00:04:05,680 --> 00:04:08,960
With Hawking radiation,
very slowly, a black hole

63
00:04:08,960 --> 00:04:11,160
gradually shrinks away
until it's completely gone.

64
00:04:11,160 --> 00:04:13,440
There's no trace of the Earth,
no evidence

65
00:04:13,440 --> 00:04:16,840
that it ever really existed,
let alone what it was made up of.

66
00:04:18,320 --> 00:04:21,800
And I'll be meeting one of the team
behind the wonderful discovery

67
00:04:21,800 --> 00:04:25,200
of gravitational waves,
exploring what they tell us

68
00:04:25,200 --> 00:04:26,240
about black holes.

69
00:04:27,480 --> 00:04:30,800
We know its mass,
we also know that it's spinning.

70
00:04:30,800 --> 00:04:32,960
It's spinning about 100 times
a second.

71
00:04:32,960 --> 00:04:35,200
We know it's about the size
of Iceland,

72
00:04:35,200 --> 00:04:38,520
it's not spherical,
it's actually an ovoid.

73
00:04:41,400 --> 00:04:44,280
But we'll start with that
astonishing discovery of

74
00:04:44,280 --> 00:04:45,560
gravitational waves.

75
00:04:46,920 --> 00:04:49,440
Unravelling the secrets
of the universe,

76
00:04:49,440 --> 00:04:53,280
the most important scientific
discovery for a generation.

77
00:04:53,280 --> 00:04:56,320
Scientists in the United States have
announced they have discovered

78
00:04:56,320 --> 00:04:57,880
gravitational waves.

79
00:04:57,880 --> 00:04:59,960
Einstein was right after all.

80
00:04:59,960 --> 00:05:03,200
Gravitational waves ripple
through space and time.

81
00:05:07,400 --> 00:05:11,560
The discovery of these elusive waves
is the end of a search

82
00:05:11,560 --> 00:05:15,200
that began with Einstein's work
100 years ago.

83
00:05:17,280 --> 00:05:20,400
But it's also the beginning
of a new way of seeing

84
00:05:20,400 --> 00:05:23,440
what had previously been
invisible in the universe,

85
00:05:23,440 --> 00:05:26,640
revealing remarkable objects,
like black holes.

86
00:05:29,200 --> 00:05:32,040
The idea that black holes
might exist was suggested

87
00:05:32,040 --> 00:05:35,320
here in Cambridge as far back
as 1784.

88
00:05:36,680 --> 00:05:38,840
The Queen's College Don,
John Michell,

89
00:05:38,840 --> 00:05:42,840
was fascinated by the idea of
extreme gravity,

90
00:05:42,840 --> 00:05:45,920
and he imagined that
a big enough star might generate

91
00:05:45,920 --> 00:05:48,720
a gravitational pull
that was so intense,

92
00:05:48,720 --> 00:05:50,480
not even light could escape.

93
00:05:54,000 --> 00:05:57,040
John Michell called these objects
"dark stars"

94
00:05:57,040 --> 00:05:59,960
and he realised that we couldn't
ever see them directly.

95
00:05:59,960 --> 00:06:02,240
But he thought
we might be able to detect them

96
00:06:02,240 --> 00:06:05,560
by watching for their gravitational
effects on objects around them.

97
00:06:05,560 --> 00:06:09,440
Sadly, his 18th-century colleagues
ignored him and dark stars

98
00:06:09,440 --> 00:06:12,480
were lost to the world for
the best part of 200 years.

99
00:06:14,200 --> 00:06:17,720
That is until the arrival of
Hawking and a new wave

100
00:06:17,720 --> 00:06:19,360
of black hole physicists.

101
00:06:20,560 --> 00:06:24,200
They were intent on understanding
some of the surprising implications

102
00:06:24,200 --> 00:06:27,320
of Einstein's theory of
general relativity.

103
00:06:28,800 --> 00:06:31,680
When physicists began to explore
Einstein's theory of

104
00:06:31,680 --> 00:06:34,840
general relativity, they found
that it made all sorts of

105
00:06:34,840 --> 00:06:37,360
bizarre predictions,
and one of the weirdest

106
00:06:37,360 --> 00:06:39,240
was the existence of
black holes.

107
00:06:40,400 --> 00:06:43,760
To see how black holes were
predicted by general relativity,

108
00:06:43,760 --> 00:06:46,640
you need get to grips with
the concept at the heart

109
00:06:46,640 --> 00:06:49,720
of Einstein's theory - space-time.

110
00:06:52,000 --> 00:06:53,720
Einstein insisted that time

111
00:06:53,720 --> 00:06:57,720
and three-dimensional space
weren't actually separate at all,

112
00:06:57,720 --> 00:07:01,960
but they were woven together into
the four dimensions of space-time.

113
00:07:01,960 --> 00:07:06,680
What's more, space-time was
distorted by mass, just as this ball

114
00:07:06,680 --> 00:07:10,800
distorts my sheet,
causing a curved dip.

115
00:07:12,320 --> 00:07:16,680
Just as our dip causes a
second ball to roll towards it,

116
00:07:16,680 --> 00:07:20,600
distortions in space-time cause
objects to fall together.

117
00:07:20,600 --> 00:07:23,440
And that is what we feel
as the pull of gravity.

118
00:07:26,640 --> 00:07:30,160
This curving of space-time
elegantly explains how planets

119
00:07:30,160 --> 00:07:32,920
and spaceships orbit
around a large mass.

120
00:07:36,760 --> 00:07:39,680
It also reveals they can only
overcome that pull

121
00:07:39,680 --> 00:07:42,880
and escape from orbit
if they can travel fast enough

122
00:07:42,880 --> 00:07:45,280
to make it all the way up
the side of the hole.

123
00:07:47,040 --> 00:07:49,400
Pretty straightforward so far.

124
00:07:49,400 --> 00:07:53,520
But things get interesting when
you consider really dense objects -

125
00:07:53,520 --> 00:07:56,080
the sort of thing that cosmologists
consider result from

126
00:07:56,080 --> 00:07:57,720
the death of a giant star.

127
00:08:00,360 --> 00:08:04,000
If it was massive enough,
a star like this would collapse

128
00:08:04,000 --> 00:08:08,040
under its own gravity and,
according to general relativity,

129
00:08:08,040 --> 00:08:09,880
form a singularity.

130
00:08:11,720 --> 00:08:13,720
A speck of infinite density

131
00:08:13,720 --> 00:08:16,600
at the centre of a
bottomless pit in space-time.

132
00:08:20,040 --> 00:08:23,920
In order to escape the steeply
sided curve of a hole like this,

133
00:08:23,920 --> 00:08:27,160
you needed to be travelling faster
than the speed of light.

134
00:08:27,160 --> 00:08:30,680
Now, nothing can travel that fast,
so nothing - not even light -

135
00:08:30,680 --> 00:08:33,360
can escape a hole like this...

136
00:08:33,360 --> 00:08:34,800
a black hole.

137
00:08:39,160 --> 00:08:42,680
But even though general relativity
implied that black holes should

138
00:08:42,680 --> 00:08:46,640
exist in theory, most physicists
didn't much like the idea.

139
00:08:49,720 --> 00:08:51,960
It raised awkward questions,

140
00:08:51,960 --> 00:08:55,120
like - what would happen to things
once they'd fallen in?

141
00:08:58,040 --> 00:09:01,880
And besides, the sort of dense,
collapsed star that you need to

142
00:09:01,880 --> 00:09:04,400
produce a black hole
had never been observed.

143
00:09:06,400 --> 00:09:10,280
But then in 1967,
astronomers here in Cambridge,

144
00:09:10,280 --> 00:09:13,680
discovered the first pulsar -
an object powered by

145
00:09:13,680 --> 00:09:17,400
a collapsed, dense star,
the first hint that such objects

146
00:09:17,400 --> 00:09:21,680
really existed and, suddenly,
black holes were back on the agenda.

147
00:09:24,840 --> 00:09:28,320
Here at Gonville and Caius College,
a young Stephen Hawking took up

148
00:09:28,320 --> 00:09:31,880
the challenge to take a fresh look
at black hole theory...

149
00:09:33,400 --> 00:09:36,840
..and right from the start,
he managed to make waves.

150
00:09:38,480 --> 00:09:41,920
Black holes were thought
to be completely black,

151
00:09:41,920 --> 00:09:45,640
until I discovered that
they glow like hot bodies,

152
00:09:45,640 --> 00:09:49,800
with a temperature that is higher
the smaller the black hole.

153
00:09:51,040 --> 00:09:55,200
This points to a deep and unexpected
connection between black holes

154
00:09:55,200 --> 00:09:58,440
and thermodynamics,
the science of heat.

155
00:09:59,840 --> 00:10:03,600
The trouble was, how could heat
be coming from a black hole?

156
00:10:03,600 --> 00:10:06,120
Nothing should be able
to escape a black hole.

157
00:10:08,400 --> 00:10:11,680
Hawking suggested a new form
of radiation,

158
00:10:11,680 --> 00:10:15,920
consisting of strange particles
that exist in the bizarre world

159
00:10:15,920 --> 00:10:17,320
of quantum theory.

160
00:10:18,600 --> 00:10:22,720
Quantum mechanics implies that
the whole of space is filled with

161
00:10:22,720 --> 00:10:26,440
pairs of virtual particles
and antiparticles,

162
00:10:26,440 --> 00:10:29,640
that are constantly materialising
in pairs,

163
00:10:29,640 --> 00:10:32,320
and then annihilating each other.

164
00:10:32,320 --> 00:10:34,960
Now in the presence of a black hole,

165
00:10:34,960 --> 00:10:38,360
one member of a pair
may fall into the hole.

166
00:10:38,360 --> 00:10:41,320
The other particle may fall
after its partner,

167
00:10:41,320 --> 00:10:44,080
but it may also escape to infinity,

168
00:10:44,080 --> 00:10:48,400
where it appears to be radiation
emitted by the black hole.

169
00:10:48,400 --> 00:10:53,800
This mind-boggling concept is
now known as Hawking radiation.

170
00:10:53,800 --> 00:10:57,120
And with it, Stephen Hawking
had solved the problem

171
00:10:57,120 --> 00:11:01,360
of how energy can escape
from a black hole.

172
00:11:01,360 --> 00:11:05,360
The trouble is, that in solving
one problem, it created another,

173
00:11:05,360 --> 00:11:07,680
bigger one -
the information paradox.

174
00:11:10,880 --> 00:11:14,120
In Einstein's most famous equation,
E = mc2,

175
00:11:14,120 --> 00:11:17,920
he showed that energy and mass
are intertwined.

176
00:11:19,040 --> 00:11:22,880
So a black hole losing energy
must also be losing mass,

177
00:11:22,880 --> 00:11:24,680
albeit very slowly.

178
00:11:26,000 --> 00:11:30,440
The radiation will carry away
energy from the black hole.

179
00:11:30,440 --> 00:11:35,080
The black hole will lose mass
and eventually disappear.

180
00:11:37,600 --> 00:11:39,680
This creates a paradox,

181
00:11:39,680 --> 00:11:43,600
because the information about
what fell into the black hole

182
00:11:43,600 --> 00:11:45,360
appears to be lost,

183
00:11:45,360 --> 00:11:49,680
but the laws of physics say that
information can never be lost.

184
00:11:52,080 --> 00:11:55,800
"Information" is tricky - it's not
things like names and stories

185
00:11:55,800 --> 00:11:57,600
that can't be lost.

186
00:11:57,600 --> 00:12:00,920
Maggie met up with cosmologist
Andrew Pontzen to find out

187
00:12:00,920 --> 00:12:04,480
what kind of information
does cause the paradox.

188
00:12:04,480 --> 00:12:06,720
Information is
a critical thing in physics.

189
00:12:06,720 --> 00:12:09,720
It's telling us where we are
and how we are sitting,

190
00:12:09,720 --> 00:12:10,840
what we're made out of.

191
00:12:10,840 --> 00:12:13,360
We think of all that stuff
as being information.

192
00:12:13,360 --> 00:12:16,120
So these are the parameters
governing the atoms of the universe?

193
00:12:16,120 --> 00:12:19,600
Exactly. The idea is that if you
know everything about the universe

194
00:12:19,600 --> 00:12:22,800
today then you should be able
to predict what will

195
00:12:22,800 --> 00:12:25,600
happen in the future, or work
through the equations backwards

196
00:12:25,600 --> 00:12:28,920
and predict what happened
in the past, but if you want to be

197
00:12:28,920 --> 00:12:33,160
able to do that, then you need this
idea of preserving information.

198
00:12:33,160 --> 00:12:36,600
The problem is, if you imagine
something falling into a black hole,

199
00:12:36,600 --> 00:12:39,480
you or me, or maybe just put
the whole Earth into a black hole -

200
00:12:39,480 --> 00:12:42,680
in fact, I can do that for you now,
if I take my...

201
00:12:42,680 --> 00:12:44,600
Phew, it's just a piece of paper!

202
00:12:44,600 --> 00:12:47,160
Not the real Earth,
just a piece of paper.

203
00:12:47,160 --> 00:12:50,800
Pop it into my black hole over here,
for which I'm using a shredder.

204
00:12:54,200 --> 00:12:57,160
Now, in principle, if you looked
carefully enough inside

205
00:12:57,160 --> 00:12:59,880
the black hole, you can imagine,
you've got some bits and pieces

206
00:12:59,880 --> 00:13:02,680
in there... So we could recreate
this, and put it back together,

207
00:13:02,680 --> 00:13:04,640
with a long time
and a lot of Sellotape.

208
00:13:04,640 --> 00:13:07,080
It would be boring
but you could do it in principle,

209
00:13:07,080 --> 00:13:08,840
so the information is still there.

210
00:13:08,840 --> 00:13:11,040
However, with Hawking radiation,

211
00:13:11,040 --> 00:13:16,400
very slowly, a black hole reduces
its mass so over time, it gradually

212
00:13:16,400 --> 00:13:18,560
shrinks away until
it's completely gone -

213
00:13:18,560 --> 00:13:21,280
there's no trace of the Earth,
no evidence that it ever existed,

214
00:13:21,280 --> 00:13:22,920
let alone what it's made up of.

215
00:13:22,920 --> 00:13:25,760
And you can't retrieve
that information in any way?

216
00:13:25,760 --> 00:13:27,000
Exactly.

217
00:13:27,000 --> 00:13:30,280
It seems like you wouldn't be able
to get that information at all,

218
00:13:30,280 --> 00:13:32,120
so there is no way you
could work backwards

219
00:13:32,120 --> 00:13:36,240
and work out that the Earth used
to exist, it's just gone and lost.

220
00:13:36,240 --> 00:13:38,320
So is there any way to resolve this?

221
00:13:38,320 --> 00:13:43,240
We hope there is a way but right
now, it's fair to say nobody knows.

222
00:13:43,240 --> 00:13:46,640
People are trying to resolve it
in lots of different ways.

223
00:13:46,640 --> 00:13:48,800
Despite 40 years of effort,

224
00:13:48,800 --> 00:13:52,280
the information paradox
is still unresolved.

225
00:13:54,400 --> 00:13:56,440
If Hawking radiation exists,

226
00:13:56,440 --> 00:13:59,280
then we have to solve
the information paradox -

227
00:13:59,280 --> 00:14:02,000
we have to work out what happens
to the information as it falls

228
00:14:02,000 --> 00:14:06,040
into a black hole, and how they can
evaporate without destroying it.

229
00:14:06,040 --> 00:14:08,880
If on the other hand
Hawking radiation doesn't exist,

230
00:14:08,880 --> 00:14:11,880
then something's fundamentally
wrong with our understanding

231
00:14:11,880 --> 00:14:14,920
of black holes
and perhaps even quantum theory.

232
00:14:14,920 --> 00:14:18,200
The problem is that theorists
have raced ahead with these ideas

233
00:14:18,200 --> 00:14:20,440
about what
black holes MIGHT be like,

234
00:14:20,440 --> 00:14:23,120
but how do we observe
that they exist at all?

235
00:14:23,120 --> 00:14:26,000
The trouble is,
we can never see them directly.

236
00:14:29,800 --> 00:14:33,200
What we can see is some
of the evidence that suggests

237
00:14:33,200 --> 00:14:35,160
black holes are lurking out there.

238
00:14:36,240 --> 00:14:40,800
In fact, some of it is surprisingly
easy to spot with a telescope.

239
00:14:40,800 --> 00:14:44,280
Pete Lawrence has spent a night
on the hunt for black holes.

240
00:14:49,160 --> 00:14:52,280
Various clues have been spotted
over the years that seem to

241
00:14:52,280 --> 00:14:54,280
signal the presence of black holes.

242
00:14:55,920 --> 00:14:58,520
You just need to know where to look.

243
00:14:58,520 --> 00:15:03,240
One area of interest lies in the
constellation of Cygnus, the swan.

244
00:15:03,240 --> 00:15:06,120
Here, there are several pieces
of evidence which may suggest

245
00:15:06,120 --> 00:15:07,760
the presence of black holes.

246
00:15:07,760 --> 00:15:11,240
You can currently find Cygnus low
in the north-east part of the sky

247
00:15:11,240 --> 00:15:12,640
at about 1am.

248
00:15:18,440 --> 00:15:22,160
In 1964, one of the earliest
space telescopes -

249
00:15:22,160 --> 00:15:25,080
a suborbital rocket fitted
with a Geiger counter -

250
00:15:25,080 --> 00:15:29,480
detected a flood of radiation
coming from this part of the sky.

251
00:15:29,480 --> 00:15:34,000
They narrowed the source down
to about there...

252
00:15:34,000 --> 00:15:36,200
and called it "Cygnus X-1".

253
00:15:40,200 --> 00:15:41,720
On closer inspection,

254
00:15:41,720 --> 00:15:44,720
it appeared that this
was something incredibly small

255
00:15:44,720 --> 00:15:48,720
and compact, producing an
extraordinary amount of energy.

256
00:15:50,880 --> 00:15:54,800
Astronomers put forward a theory
to explain what they were seeing.

257
00:15:54,800 --> 00:15:58,160
What initially looked like a single
star actually turned out to be

258
00:15:58,160 --> 00:16:02,760
a binary system - a large blue star
with an invisible companion.

259
00:16:04,680 --> 00:16:06,400
Could this be a black hole?

260
00:16:09,040 --> 00:16:12,240
One explanation for all that energy
being released,

261
00:16:12,240 --> 00:16:14,120
was that gas from the visible star

262
00:16:14,120 --> 00:16:18,400
was being sucked towards a black
hole, creating immense friction

263
00:16:18,400 --> 00:16:21,600
as it spiralled into what is called
an accretion disk.

264
00:16:23,280 --> 00:16:27,760
As it's heated, that gas would
release a colossal amount of energy

265
00:16:27,760 --> 00:16:29,840
and even bright X-ray flashes.

266
00:16:32,760 --> 00:16:36,360
There are signs of much bigger
black holes to look for too.

267
00:16:38,600 --> 00:16:42,400
If we look back at Cygnus,
there's an unremarkable patch of sky

268
00:16:42,400 --> 00:16:44,520
in the western wing of the swan.

269
00:16:44,520 --> 00:16:47,280
There's not much to see
here visually.

270
00:16:47,280 --> 00:16:51,120
But astronomers were astounded
to see a bright radio source

271
00:16:51,120 --> 00:16:52,960
emanating from this region.

272
00:16:54,040 --> 00:16:58,280
The bright signals came from
two jets of material spewing out

273
00:16:58,280 --> 00:17:02,400
from either side of a distant
galaxy, at tremendous speeds.

274
00:17:03,920 --> 00:17:07,800
Such jets should take huge amounts
of energy to produce -

275
00:17:07,800 --> 00:17:12,600
like converting a million times
the mass of the sun to pure energy,

276
00:17:12,600 --> 00:17:16,800
more than the nuclear fusion that
drives stars could ever produce.

277
00:17:18,240 --> 00:17:21,800
The only known phenomenon
that could convert matter to energy

278
00:17:21,800 --> 00:17:24,480
that efficiently was what
astronomers had seen

279
00:17:24,480 --> 00:17:28,160
in the Cygnus X-1 system -
accretion.

280
00:17:28,160 --> 00:17:31,320
But this must be on
a much, much larger scale -

281
00:17:31,320 --> 00:17:34,800
so there must
a massive, unseen black hole

282
00:17:34,800 --> 00:17:36,640
at the centre of the galaxy.

283
00:17:37,960 --> 00:17:41,720
Bright jets and accretion disks
give us enough evidence

284
00:17:41,720 --> 00:17:45,160
to suggest similar
"supermassive" black holes exist

285
00:17:45,160 --> 00:17:47,520
at the centre of almost
all galaxies.

286
00:17:50,840 --> 00:17:54,440
But if we look towards
the heart of our own galaxy,

287
00:17:54,440 --> 00:17:56,560
there's even more
compelling evidence.

288
00:17:59,160 --> 00:18:01,280
The centre of the Milky Way galaxy

289
00:18:01,280 --> 00:18:03,720
is in the constellation
of Sagittarius,

290
00:18:03,720 --> 00:18:06,920
which, at the moment,
is more or less behind me.

291
00:18:06,920 --> 00:18:09,800
If you're out on
a clear summer evening,

292
00:18:09,800 --> 00:18:13,680
then this region of sky can be
seen low down in the south.

293
00:18:15,080 --> 00:18:18,080
Now, of course, we can't see
the supermassive black hole

294
00:18:18,080 --> 00:18:19,440
at the centre of our galaxy,

295
00:18:19,440 --> 00:18:22,920
but we can see the effect
it's having on the stars around it.

296
00:18:24,400 --> 00:18:28,280
For 25 years, astronomers have
tracked the motion of stars

297
00:18:28,280 --> 00:18:30,360
at the heart of the galaxy.

298
00:18:30,360 --> 00:18:34,640
And it's quite clear
they're orbiting around something.

299
00:18:34,640 --> 00:18:38,160
That something must be
over four million times

300
00:18:38,160 --> 00:18:42,880
the mass of our sun, squeezed
into just 17 times its size.

301
00:18:44,680 --> 00:18:48,520
And that's what we now call
"Sagittarius A Star" -

302
00:18:48,520 --> 00:18:50,680
our very own
supermassive black hole.

303
00:18:53,400 --> 00:18:56,080
With all the evidence
we've seen over the years,

304
00:18:56,080 --> 00:18:59,920
we're pretty convinced that
black holes do actually exist.

305
00:18:59,920 --> 00:19:03,560
But the next challenge
would be to detect one directly.

306
00:19:07,400 --> 00:19:09,760
Until this year,
astronomers have relied

307
00:19:09,760 --> 00:19:12,240
on this circumstantial evidence
to deduce

308
00:19:12,240 --> 00:19:15,120
almost everything that
we know about black holes.

309
00:19:15,120 --> 00:19:18,200
But all that changed
just a few weeks ago,

310
00:19:18,200 --> 00:19:23,280
with the first direct detection
of not one, but two black holes.

311
00:19:23,280 --> 00:19:25,800
It was these black holes
that caused

312
00:19:25,800 --> 00:19:29,040
the gravitational waves in
February's big announcement.

313
00:19:31,920 --> 00:19:36,440
'Professor Sheila Rowan is one
of the team behind the discovery.

314
00:19:36,440 --> 00:19:38,120
'I joined her to find out more

315
00:19:38,120 --> 00:19:41,440
'and to see what it all means
for black hole science.'

316
00:19:42,600 --> 00:19:45,400
It is a very exciting time
to be talking about

317
00:19:45,400 --> 00:19:48,040
gravitational waves,
but what exactly have we seen?

318
00:19:48,040 --> 00:19:54,120
What we have seen is space
vibrating, space shaking,

319
00:19:54,120 --> 00:19:58,720
as picked up by the LIGO
observatories in the United States

320
00:19:58,720 --> 00:20:01,480
with a device that is
about 4km long,

321
00:20:01,480 --> 00:20:05,120
measuring motions
1/10,000th of the size

322
00:20:05,120 --> 00:20:07,960
of a proton in the nucleus
of an atom, so...

323
00:20:07,960 --> 00:20:11,200
Quite phenomenal.
Wow. Incredibly small.

324
00:20:11,200 --> 00:20:13,920
This is the signal
that we observed,

325
00:20:13,920 --> 00:20:19,080
and what we saw was those vibrations
speeding up, vibrating faster

326
00:20:19,080 --> 00:20:23,920
and faster to a peak, and then
the vibrations died down slightly.

327
00:20:23,920 --> 00:20:28,040
And all of this detail, all of this
vibration happens very quickly.

328
00:20:28,040 --> 00:20:32,240
It does, it happens in about
0.2 of a second, but amazingly,

329
00:20:32,240 --> 00:20:35,840
encoded in the vibrations,
in that 0.2 of a second,

330
00:20:35,840 --> 00:20:39,480
is information about the source
of those vibrations.

331
00:20:39,480 --> 00:20:45,000
Two black holes colliding about
1.3 billion light years away,

332
00:20:45,000 --> 00:20:46,440
so 1.3 billion years ago,

333
00:20:46,440 --> 00:20:49,560
and arriving with us here
on Earth last September.

334
00:20:49,560 --> 00:20:52,440
And that speeding up is actually
the two black holes

335
00:20:52,440 --> 00:20:56,280
spiralling in faster and faster
until they eventually collide

336
00:20:56,280 --> 00:21:00,240
to form a new black hole that then
wobbles at a particular frequency,

337
00:21:00,240 --> 00:21:03,000
and we can use that again
to tell us about the properties

338
00:21:03,000 --> 00:21:05,440
of the new black hole
that has been formed.

339
00:21:05,440 --> 00:21:07,200
See, this is, to me,
as an astronomer,

340
00:21:07,200 --> 00:21:10,040
the really exciting thing,
to be able to make an observation

341
00:21:10,040 --> 00:21:13,160
that tells you about the properties
of the black hole itself.

342
00:21:13,160 --> 00:21:16,640
Not about stuff falling into it,
but actually about the black hole.

343
00:21:16,640 --> 00:21:19,080
So what do we know about the two
that merged

344
00:21:19,080 --> 00:21:21,640
and what do we know about
the state of the system now?

345
00:21:21,640 --> 00:21:24,800
We can tell the masses of
the two black holes that merged,

346
00:21:24,800 --> 00:21:28,560
and one of them was about 36 times
the mass of our sun.

347
00:21:28,560 --> 00:21:31,360
The other one was about 29 times
the mass of our sun.

348
00:21:31,360 --> 00:21:36,680
The mass of that final black hole is
about 62 times the mass of our sun.

349
00:21:36,680 --> 00:21:40,280
If you add up the initial masses
of the two black holes

350
00:21:40,280 --> 00:21:43,520
that merged, in that final mass,
you will discover there is

351
00:21:43,520 --> 00:21:46,640
energy equivalent to three times
the mass of our sun

352
00:21:46,640 --> 00:21:48,280
that has gone somewhere,

353
00:21:48,280 --> 00:21:51,960
and where it has gone is
into gravitational waves. Wow.

354
00:21:51,960 --> 00:21:54,880
In a short period,
a huge amount of energy is produced,

355
00:21:54,880 --> 00:21:58,200
more than the luminosity,
the light power, of all the stars

356
00:21:58,200 --> 00:22:00,640
and galaxies in the
observable universe.

357
00:22:00,640 --> 00:22:02,000
Wow.
It's kind of amazing. OK.

358
00:22:02,000 --> 00:22:05,360
So what else do we know about
this newly-formed black hole?

359
00:22:05,360 --> 00:22:07,400
We also know that it is spinning.

360
00:22:07,400 --> 00:22:10,240
It's spinning about 100 times
a second.

361
00:22:10,240 --> 00:22:13,160
We know it is about
the size of Iceland.

362
00:22:13,160 --> 00:22:16,160
It is not spherical,
it is actually an ovoid,

363
00:22:16,160 --> 00:22:18,280
it's kind of squished
in one direction

364
00:22:18,280 --> 00:22:20,280
and stretched in
the other slightly,

365
00:22:20,280 --> 00:22:24,680
and spinning away,
and all of that we can get from

366
00:22:24,680 --> 00:22:27,200
this signal that we detect
in gravitational waves.

367
00:22:27,200 --> 00:22:30,560
And this is the first time we have
been able to do anything like that.

368
00:22:30,560 --> 00:22:33,800
I think the most remarkable thing
is not just the signal,

369
00:22:33,800 --> 00:22:35,440
but the fact that it represents

370
00:22:35,440 --> 00:22:37,480
a huge amount of effort
by a lot of people,

371
00:22:37,480 --> 00:22:41,160
most of whom laboured for years
without ever detecting anything.

372
00:22:41,160 --> 00:22:44,160
How does it feel to be sitting here
describing a real signal?

373
00:22:44,160 --> 00:22:46,520
It feels amazing,
and it is the first time

374
00:22:46,520 --> 00:22:48,840
we have seen these binary systems.

375
00:22:48,840 --> 00:22:50,520
They might never have existed.

376
00:22:50,520 --> 00:22:55,040
And the black holes merging is
the birth of a new black hole,

377
00:22:55,040 --> 00:22:58,160
that is a unique signature
that we see,

378
00:22:58,160 --> 00:23:00,640
that we really couldn't see
any other way.

379
00:23:00,640 --> 00:23:02,480
So that is really fantastic.

380
00:23:02,480 --> 00:23:05,800
This is incredibly exciting,
but it is just one detection,

381
00:23:05,800 --> 00:23:08,520
so what is next?
It is just one detection,

382
00:23:08,520 --> 00:23:14,240
but we can combine that with
our models for how the universe is,

383
00:23:14,240 --> 00:23:19,560
and that lets us calculate
possible rates of these events,

384
00:23:19,560 --> 00:23:21,680
how many we might expect.

385
00:23:21,680 --> 00:23:24,320
It could be anything from
a few a month to one a day,

386
00:23:24,320 --> 00:23:27,120
and so that is
phenomenally exciting,

387
00:23:27,120 --> 00:23:30,480
that we may see a whole population
of these events out in the universe.

388
00:23:30,480 --> 00:23:32,520
And that's just from these
two detectors.

389
00:23:32,520 --> 00:23:35,240
That's just from these
two detectors, that's right.

390
00:23:35,240 --> 00:23:39,000
These are just part of
a whole potentially new astronomy

391
00:23:39,000 --> 00:23:42,000
using a whole set of
different instruments to detect

392
00:23:42,000 --> 00:23:44,920
gravitational waves across
a wide range of frequencies,

393
00:23:44,920 --> 00:23:46,520
and that is very exciting.

394
00:23:46,520 --> 00:23:48,600
So more data coming?
More data coming.

395
00:23:49,760 --> 00:23:54,360
2016 is going to go down in history
as a big year for black holes.

396
00:23:54,360 --> 00:23:58,120
But now we know that they exist
and can even detect them directly,

397
00:23:58,120 --> 00:24:00,720
what does that mean for
the information paradox

398
00:24:00,720 --> 00:24:02,160
and Hawking radiation?

399
00:24:03,720 --> 00:24:06,560
Critically, it's an opportunity
for Stephen Hawking

400
00:24:06,560 --> 00:24:08,360
to test his theories at last.

401
00:24:09,920 --> 00:24:13,480
Especially his extraordinary idea
that when black holes combine,

402
00:24:13,480 --> 00:24:16,360
they make one new one,
with more surface area

403
00:24:16,360 --> 00:24:18,160
than the first two put together.

404
00:24:20,000 --> 00:24:23,520
The signal LIGO detected
came from the collision

405
00:24:23,520 --> 00:24:28,640
and merger of two black holes
in a black hole binary.

406
00:24:28,640 --> 00:24:31,600
This should make it possible
to experimentally test

407
00:24:31,600 --> 00:24:36,080
my prediction the area of the
horizon of the final black hole

408
00:24:36,080 --> 00:24:41,280
is greater than the sum of the areas
of the original holes.

409
00:24:41,280 --> 00:24:42,920
This prediction is crucial

410
00:24:42,920 --> 00:24:47,640
to our understanding of the
thermodynamics of black holes.

411
00:24:47,640 --> 00:24:50,160
By making sense
of their thermodynamics,

412
00:24:50,160 --> 00:24:53,960
LIGO and its successors
could provide the first

413
00:24:53,960 --> 00:24:56,560
experimental evidence
that black holes DO glow

414
00:24:56,560 --> 00:24:58,640
with Hawking radiation.

415
00:24:58,640 --> 00:25:00,880
But could there be
more direct evidence,

416
00:25:00,880 --> 00:25:04,160
out at the very edge
of the observable universe?

417
00:25:04,160 --> 00:25:05,880
The cosmological horizon.

418
00:25:07,560 --> 00:25:11,760
LIGO is not sensitive to
the wavelengths at which there is

419
00:25:11,760 --> 00:25:15,760
appreciable Hawking radiation
from black holes.

420
00:25:15,760 --> 00:25:19,120
However, there is likely
to be another kind of

421
00:25:19,120 --> 00:25:22,400
Hawking radiation
of much longer wavelength

422
00:25:22,400 --> 00:25:25,280
coming from
the cosmological horizon

423
00:25:25,280 --> 00:25:28,480
which might be detected
by radio telescopes.

424
00:25:30,320 --> 00:25:33,640
Longwave radiation like this would
have to come from a type

425
00:25:33,640 --> 00:25:37,120
of black hole formed right
after the big bang,

426
00:25:37,120 --> 00:25:39,920
in the early primordial universe.

427
00:25:39,920 --> 00:25:44,880
It would be a unique kind
of gravitational radiation.

428
00:25:46,640 --> 00:25:51,000
I hope that radio telescopes detect
primordial gravitational radiation

429
00:25:51,000 --> 00:25:55,200
from the cosmological horizon.

430
00:25:55,200 --> 00:25:59,520
That would mean black holes
almost certainly emit radiation

431
00:25:59,520 --> 00:26:01,800
and would get me a Nobel Prize.

432
00:26:04,240 --> 00:26:07,520
If Hawking radiation
can be proved to exist,

433
00:26:07,520 --> 00:26:10,800
then there must be a solution
to the information paradox.

434
00:26:10,800 --> 00:26:14,560
And Professor Hawking
thinks he has found that too.

435
00:26:14,560 --> 00:26:18,080
He believes that objects
falling in to a black hole

436
00:26:18,080 --> 00:26:20,440
leave the information
they carry behind.

437
00:26:20,440 --> 00:26:24,440
It's stored on the hole's surface,
which becomes turbulent,

438
00:26:24,440 --> 00:26:27,440
a process called
"super translation".

439
00:26:27,440 --> 00:26:31,320
Last year, I realised that
a black hole can store

440
00:26:31,320 --> 00:26:36,600
the information in what is called
super translations of the horizon.

441
00:26:36,600 --> 00:26:40,920
I am now working with my colleagues
Malcolm Perry at Cambridge

442
00:26:40,920 --> 00:26:43,920
and Andrew Strominger at Harvard

443
00:26:43,920 --> 00:26:46,640
on whether this can resolve
the paradox.

444
00:26:48,400 --> 00:26:49,600
If he is right,

445
00:26:49,600 --> 00:26:53,960
Stephen Hawking has solved one of
the greatest mysteries in cosmology.

446
00:26:53,960 --> 00:26:57,240
But does that mean you can escape
from a black hole after all,

447
00:26:57,240 --> 00:26:59,520
or would it still be bad news
to fall in one?

448
00:27:01,320 --> 00:27:03,360
Definitely bad news.

449
00:27:03,360 --> 00:27:05,800
If it were
a stellar-mass black hole,

450
00:27:05,800 --> 00:27:10,160
you would be made into spaghetti
before reaching the horizon.

451
00:27:10,160 --> 00:27:14,720
On the other hand, if it were
a supermassive black hole,

452
00:27:14,720 --> 00:27:17,240
you would cross
the horizon with ease,

453
00:27:17,240 --> 00:27:20,600
but be crushed out of existence
at the singularity.

454
00:27:22,320 --> 00:27:23,760
BLAST

455
00:27:27,920 --> 00:27:30,520
Well, there's no doubt in my mind
that with the detection

456
00:27:30,520 --> 00:27:34,840
of gravitational waves, the future
of black hole science looks bright.

457
00:27:34,840 --> 00:27:38,640
And after 50 years of theoretical
debates, Stephen Hawking might get

458
00:27:38,640 --> 00:27:40,720
the experimental evidence he wants

459
00:27:40,720 --> 00:27:42,960
to test his ideas
about black holes.

460
00:27:42,960 --> 00:27:46,000
And there is another exciting
project on the horizon too -

461
00:27:46,000 --> 00:27:47,960
the Event Horizon Telescope,

462
00:27:47,960 --> 00:27:51,480
a worldwide network of
radio telescopes that next year

463
00:27:51,480 --> 00:27:55,080
will team up to try and capture
the first image of the shadow cast

464
00:27:55,080 --> 00:27:58,760
by the enormous black hole
at the Milky Way's centre.

465
00:27:58,760 --> 00:28:00,400
This would have been unimaginable

466
00:28:00,400 --> 00:28:03,080
when Stephen Hawking started
grappling with black holes

467
00:28:03,080 --> 00:28:06,080
over 40 years ago, but it shows
this is another chapter

468
00:28:06,080 --> 00:28:09,240
in the history
of black hole science.

469
00:28:09,240 --> 00:28:10,880
Next month, we will be previewing

470
00:28:10,880 --> 00:28:12,640
one of the astronomical highlights

471
00:28:12,640 --> 00:28:15,240
of the year, the transit of Mercury,

472
00:28:15,240 --> 00:28:17,680
visible across the UK on May 9th.

473
00:28:19,800 --> 00:28:22,960
And we will be using the opportunity
to take a closer look

474
00:28:22,960 --> 00:28:26,440
at Mercury, one of the strangest
planets in the solar system.

475
00:28:28,000 --> 00:28:30,920
To find out how to catch
a glimpse of it before then,

476
00:28:30,920 --> 00:28:33,960
check out the website
for Pete's April star guide.

477
00:28:37,200 --> 00:28:41,680
In the meanwhile, get outside and...
get looking up. Good night.