0:00:15.448,0:00:18.123
On a typical day at school,

0:00:18.147,0:00:22.731
endless hours are spent learning[br]the answers to questions,

0:00:22.755,0:00:25.051
but right now, we'll do the opposite.

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We're going to focus on questions[br]where you can't learn the answers

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because they're unknown.

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I used to puzzle about a lot of things[br]as a boy, for example:

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What would it feel like to be a dog?

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Do fish feel pain?

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How about insects?

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Was the Big Bang just an accident?

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And is there a God?

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And if so, how are we so sure[br]that it's a He and not a She?

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Why do so many innocent people[br]and animals suffer terrible things?

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Is there really a plan for my life?

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Is the future yet to be written,

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or is it already written[br]and we just can't see it?

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But then, do I have free will?[br]I mean, who am I anyway?

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Am I just a biological machine?

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But then, why am I conscious?[br]What is consciousness?

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Will robots become conscious one day?

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I mean, I kind of assumed that some day

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I would be told the answers[br]to all these questions.

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Someone must know, right?

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Guess what? No one knows.

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Most of those questions[br]puzzle me more now than ever.

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But diving into them is exciting

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because it takes you[br]to the edge of knowledge,

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and you never know what you'll find there.

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So, two questions that no one[br]on Earth knows the answer to.

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(Music)

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[How many universes are there?]

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Sometimes when I'm on a long plane flight,

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I gaze out at all those[br]mountains and deserts

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and try to get my head[br]around how vast our Earth is.

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And then I remember[br]that there's an object we see every day

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that would literally fit[br]one million Earths inside it:

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the Sun.

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It seems impossibly big.

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But in the great scheme[br]of things, it's a pinprick,

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one of about 400 billion stars[br]in the Milky Way galaxy,

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which you can see on a clear night

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as a pale white mist[br]stretched across the sky.

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And it gets worse.

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There are maybe 100 billion galaxies[br]detectable by our telescopes.

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So if each star was the size[br]of a single grain of sand,

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just the Milky Way has enough stars

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to fill a 30-foot by 30-foot[br]stretch of beach

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three feet deep with sand.

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And the entire Earth[br]doesn't have enough beaches

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to represent the stars[br]in the overall universe.

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Such a beach would continue for literally[br]hundreds of millions of miles.

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Holy Stephen Hawking,[br]that is a lot of stars.

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But he and other physicists[br]now believe in a reality

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that is unimaginably bigger still.

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I mean, first of all,[br]the 100 billion galaxies

0:03:08.208,0:03:09.716
within range of our telescopes

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are probably a minuscule[br]fraction of the total.

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Space itself is expanding[br]at an accelerating pace.

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The vast majority of the galaxies

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are separating from us so fast[br]that light from them may never reach us.

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Still, our physical reality here on Earth

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is intimately connected[br]to those distant, invisible galaxies.

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We can think of them[br]as part of our universe.

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They make up a single, giant edifice

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obeying the same physical laws[br]and all made from the same types of atoms,

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electrons, protons, quarks, neutrinos,[br]that make up you and me.

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However, recent theories in physics,[br]including one called string theory,

0:03:46.993,0:03:49.976
are now telling us there could be[br]countless other universes

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built on different types of particles,

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with different properties,[br]obeying different laws.

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Most of these universes[br]could never support life,

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and might flash in and out[br]of existence in a nanosecond.

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But nonetheless, combined,[br]they make up a vast multiverse

0:04:04.584,0:04:07.622
of possible universes[br]in up to 11 dimensions,

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featuring wonders[br]beyond our wildest imagination.

0:04:12.206,0:04:15.079
The leading version of string theory[br]predicts a multiverse

0:04:15.103,0:04:16.976
made up of 10 to the 500 universes.

0:04:17.000,0:04:20.869
That's a one followed by 500 zeros,

0:04:20.892,0:04:23.294
a number so vast that if every atom

0:04:23.318,0:04:27.424
in our observable universe[br]had its own universe,

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and all of the atoms[br]in all those universes each had

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their own universe,

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and you repeated that for two more cycles,

0:04:34.346,0:04:37.747
you'd still be at a tiny[br]fraction of the total,

0:04:37.771,0:04:41.248
namely, one trillion trillion trillion[br]trillion trillion trillion trillion

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trillion trillion trillion trillion[br]trillion trillion trillion trillionth.

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(Laughter)

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But even that number[br]is minuscule compared to another number:

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infinity.

0:04:54.488,0:04:57.679
Some physicists think the space-time[br]continuum is literally infinite

0:04:57.703,0:05:01.080
and that it contains an infinite number[br]of so-called pocket universes

0:05:01.104,0:05:02.247
with varying properties.

0:05:03.107,0:05:04.236
How's your brain doing?

0:05:05.061,0:05:06.976
Quantum theory adds a whole new wrinkle.

0:05:07.000,0:05:09.652
I mean, the theory's been proven[br]true beyond all doubt,

0:05:09.676,0:05:11.885
but interpreting it is baffling,

0:05:11.909,0:05:15.006
and some physicists think[br]you can only un-baffle it

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if you imagine that huge numbers[br]of parallel universes

0:05:17.656,0:05:20.237
are being spawned every moment,

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and many of these universes would actually[br]be very like the world we're in,

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would include multiple copies of you.

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In one such universe,[br]you'd graduate with honors

0:05:28.515,0:05:32.567
and marry the person of your dreams,[br]and in another, not so much.

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Well, there are still some scientists[br]who would say, hogwash.

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The only meaningful answer to the question[br]of how many universes there are is one.

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Only one universe.

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And a few philosophers[br]and mystics might argue

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that even our own universe is an illusion.

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So, as you can see, right now

0:05:51.077,0:05:54.748
there is no agreement[br]on this question, not even close.

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All we know is the answer is somewhere[br]between zero and infinity.

0:05:59.173,0:06:01.147
Well, I guess we know one other thing.

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This is a pretty cool time[br]to be studying physics.

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We just might be undergoing[br]the biggest paradigm shift in knowledge

0:06:08.677,0:06:10.488
that humanity has ever seen.

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(Music)

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[Why can't we see evidence of alien life?]

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Somewhere out there in that vast universe

0:06:20.877,0:06:23.937
there must surely be countless[br]other planets teeming with life.

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But why don't we see any evidence of it?

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Well, this is the famous question[br]asked by Enrico Fermi in 1950:

0:06:30.289,0:06:31.767
Where is everybody?

0:06:33.006,0:06:36.820
Conspiracy theorists claim that UFOs[br]are visiting all the time

0:06:36.844,0:06:38.990
and the reports are just being covered up,

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but honestly, they aren't very convincing.

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But that leaves a real riddle.

0:06:44.330,0:06:47.020
In the past year,[br]the Kepler space observatory

0:06:47.044,0:06:49.925
has found hundreds of planets[br]just around nearby stars.

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And if you extrapolate that data,

0:06:52.335,0:06:55.697
it looks like there could[br]be half a trillion planets

0:06:55.721,0:06:57.086
just in our own galaxy.

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If any one in 10,000 has conditions

0:07:00.378,0:07:02.976
that might support a form of life,

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that's still 50 million possible[br]life-harboring planets

0:07:06.000,0:07:07.515
right here in the Milky Way.

0:07:08.551,0:07:10.239
So here's the riddle:

0:07:10.263,0:07:11.899
our Earth didn't form

0:07:11.923,0:07:14.516
until about nine billion years[br]after the Big Bang.

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Countless other planets in our galaxy[br]should have formed earlier,

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and given life a chance to get underway

0:07:21.459,0:07:26.260
billions, or certainly many millions[br]of years earlier than happened on Earth.

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If just a few of them[br]had spawned intelligent life

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and started creating technologies,

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those technologies would have[br]had millions of years

0:07:35.833,0:07:37.837
to grow in complexity and power.

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On Earth,

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we've seen how dramatically[br]technology can accelerate

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in just 100 years.

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In millions of years,[br]an intelligent alien civilization

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could easily have spread out[br]across the galaxy,

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perhaps creating giant[br]energy-harvesting artifacts

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or fleets of colonizing spaceships

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or glorious works of art[br]that fill the night sky.

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At the very least, you'd think[br]they'd be revealing their presence,

0:08:06.153,0:08:07.483
deliberately or otherwise,

0:08:07.507,0:08:10.460
through electromagnetic signals[br]of one kind or another.

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And yet we see no convincing[br]evidence of any of it.

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Why?

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Well, there are numerous possible answers,[br]some of them quite dark.

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Maybe a single,[br]superintelligent civilization

0:08:24.000,0:08:27.101
has indeed taken over the galaxy

0:08:27.125,0:08:29.647
and has imposed strict radio silence

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because it's paranoid[br]of any potential competitors.

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It's just sitting there[br]ready to obliterate

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anything that becomes a threat.

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Or maybe they're not that intelligent,

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or perhaps the evolution[br]of an intelligence

0:08:44.975,0:08:47.023
capable of creating[br]sophisticated technology

0:08:47.047,0:08:49.017
is far rarer than we've assumed.

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After all, it's only happened once[br]on Earth in four billion years.

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Maybe even that was incredibly lucky.

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Maybe we are the first[br]such civilization in our galaxy.

0:09:01.042,0:09:06.500
Or, perhaps civilization carries with it[br]the seeds of its own destruction

0:09:06.524,0:09:09.382
through the inability to control[br]the technologies it creates.

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But there are numerous[br]more hopeful answers.

0:09:13.624,0:09:15.621
For a start, we're not looking that hard,

0:09:15.645,0:09:18.026
and we're spending[br]a pitiful amount of money on it.

0:09:18.547,0:09:21.847
Only a tiny fraction[br]of the stars in our galaxy

0:09:21.871,0:09:25.062
have really been looked at closely[br]for signs of interesting signals.

0:09:26.144,0:09:28.262
And perhaps we're not looking[br]the right way.

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Maybe as civilizations develop,

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they quickly discover[br]communication technologies

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far more sophisticated and useful[br]than electromagnetic waves.

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Maybe all the action takes place[br]inside the mysterious

0:09:41.507,0:09:43.524
recently discovered dark matter,

0:09:43.548,0:09:47.337
or dark energy, that appear to account[br]for most of the universe's mass.

0:09:49.348,0:09:51.976
Or, maybe we're looking[br]at the wrong scale.

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Perhaps intelligent[br]civilizations come to realize

0:09:55.000,0:09:57.810
that life is ultimately[br]just complex patterns of information

0:09:57.834,0:10:00.038
interacting with each other[br]in a beautiful way,

0:10:00.062,0:10:02.839
and that that can happen more[br]efficiently at a small scale.

0:10:03.379,0:10:05.976
So, just as on Earth,[br]clunky stereo systems have shrunk

0:10:06.000,0:10:08.976
to beautiful, tiny iPods,[br]maybe intelligent life itself,

0:10:09.000,0:10:11.724
in order to reduce its footprint[br]on the environment,

0:10:11.748,0:10:13.691
has turned itself microscopic.

0:10:13.715,0:10:16.024
So the Solar System[br]might be teeming with aliens,

0:10:16.048,0:10:17.653
and we're just not noticing them.

0:10:17.677,0:10:20.676
Maybe the very ideas in our heads[br]are a form of alien life.

0:10:21.580,0:10:24.250
Well, okay, that's a crazy thought.

0:10:24.274,0:10:25.599
The aliens made me say it.

0:10:26.445,0:10:30.223
But it is cool that ideas do seem[br]to have a life all of their own

0:10:30.247,0:10:32.718
and that they outlive their creators.

0:10:33.724,0:10:37.047
Maybe biological life[br]is just a passing phase.

0:10:39.459,0:10:40.976
Well, within the next 15 years,

0:10:41.000,0:10:43.976
we could start seeing[br]real spectroscopic information

0:10:44.000,0:10:45.810
from promising nearby planets

0:10:45.834,0:10:48.370
that will reveal just[br]how life-friendly they might be.

0:10:48.394,0:10:51.976
And meanwhile, SETI, the Search[br]for Extraterrestrial Intelligence,

0:10:52.000,0:10:53.976
is now releasing its data to the public

0:10:54.000,0:10:56.976
so that millions of citizen scientists,[br]maybe including you,

0:10:57.000,0:10:59.477
can bring the power of the crowd[br]to join the search.

0:11:00.582,0:11:02.596
And here on Earth, amazing experiments

0:11:02.620,0:11:04.976
are being done to try[br]to create life from scratch,

0:11:05.000,0:11:08.694
life that might be very different[br]from the DNA forms we know.

0:11:09.320,0:11:13.976
All of this will help us understand[br]whether the universe is teeming with life

0:11:14.000,0:11:17.828
or whether, indeed, it's just us.

0:11:19.823,0:11:22.308
Either answer, in its own way,

0:11:22.332,0:11:24.178
is awe-inspiring,

0:11:25.407,0:11:26.976
because even if we are alone,

0:11:27.000,0:11:31.531
the fact that we think and dream[br]and ask these questions

0:11:31.555,0:11:33.147
might yet turn out to be

0:11:33.171,0:11:35.552
one of the most important facts[br]about the universe.

0:11:36.347,0:11:39.081
And I have one more piece[br]of good news for you.

0:11:39.105,0:11:41.931
The quest for knowledge[br]and understanding never gets dull.

0:11:41.955,0:11:43.976
It doesn't. It's actually the opposite.

0:11:44.000,0:11:46.976
The more you know,[br]the more amazing the world seems.

0:11:47.758,0:11:51.191
And it's the crazy possibilities,[br]the unanswered questions,

0:11:51.215,0:11:52.452
that pull us forward.

0:11:52.476,0:11:55.329
So stay curious.