Somewhere out there in that vast universe

there must surely be countless other planets teeming with life,

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

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

"Where is everybody?"

Conspiracy theorists claim that UFOs are visiting all the time

and the reports are just being covered up,

but honestly, they aren't very convincing.

But that leaves a real riddle.

In the past year, the Kepler space observatory has found

hundreds of planets just around nearby stars,

and if you extrapolate that data,

it looks like there could be half a trillions planets

just in our own galaxy.

If any one in 10,000 has conditions

that might support a form of life, that's still

50 million possible life-harboring planets

right here in the Milky Way.

So here's the riddle.

Our Earth didn't form until about 9 billion years after the Big Bang.

Countless other planets in our galaxy

should have formed earlier and given life a chance to get underway

billions, or certainly many millions, of years

earlier than happened on Earth.

If just a few of them had spawned intelligent life

and started creating technologies,

those technologies would have had millions of years

to grow in complexity and power.

On Earth,

we've seen how dramatically

technology can accelerate in just 100 years.

In millions of years, an intelligent alien civilization

could easily have spread out across the galaxy,

perhaps creating giant energy-harvesting artifacts

or fleets of colonizing spaceships

or glorious works of art that fill the night sky.

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

deliberately or otherwise,

through electromagnetic signals

of one kind or another. And yet we see no convincing evidence of any of it.

Why?

Well, there are numerous possible answers,

some of them quite dark.

Maybe a single, super-intelligent civilization

has indeed taken over the galaxy,

and has imposed strict radio silence

because it's paranoid of any potential competitors.

It's just sitting there ready to obliterate

anything that becomes a threat.

Or maybe they're not that intelligent,

or perhaps the evolution of an intelligence

capable of creating sophisticated technology

is far rarer than we've assumed.

After all, it's only happened once on Earth in 4 billion years.

Maybe even that was incredibly lucky.

Maybe we are the first such civilization in our galaxy.

Or perhaps civilization carries with it

the seeds of its own destruction

through the inability to control the technologies it creates.

But there are numerous more hopeful answers.

I mean, for a start, we're not looking that hard. And we're spending a pitiful amount of money on it.

Only a tiny fraction of the stars in our galaxy

have really been looked at closely for signs of interesting signals.

And perhaps we're not looking the right way.

Maybe as civilizations develop,

they quickly discover communication technologies

far more sophisticated and useful than electromagnetic waves.

Maybe all the action takes place

inside the mysterious recently discovered dark matter,

or dark energy, that appear to account for most of the universe's mass.

Or maybe we're looking at the wrong scale.

Perhaps intelligent civilizations come to realize

that life is ultimately just complex patterns of information

interacting with each other in a beautiful way,

and that that can happen more efficiently at a small scale.

So, just as on Earth clunky stereo systems

have shrunk to beautiful, tiny iPods, maybe intelligent life itself,

in order to reduce its footprint on the environment,

has turned itself microscopic,

so the Solar System might be teeming with aliens, and we're just not noticing them.

Maybe the very ideas in our heads are a form of alien life.

Well, okay, that's a crazy thought.

The aliens made me say it.

But it is cool that ideas do seem to have a life all of their own

and that they outlive their creators.

Maybe biological life is just a passing phase.

Well, within the next 15 years,

we could start seeing real spectroscopic information

from promising nearby planets that will reveal just how life-ready they might be.

And meanwhile SETI, the Search for Extra-Terrestrial Intelligence,

is now releasing its data to the public

so that millions of citizen scientists, maybe including you,

can bring the power of the crowd to join the search.

And here on Earth, amazing experiments are being done

to try to create life from scratch,

life that might be very different from the DNA forms we know.

All of this will help us understand

whether the universe is teeming with life

or whether, indeed, it's just us.

Either answer, in its own way,

is awe-inspiring,

because even if we are alone,

the fact that we think and dream and ask these questions

might yet turn out to be one of the most important facts about the universe.

And I have one more piece of good news for you.

The quest for knowledge and understanding never gets dull.

It doesn't. It's actually the opposite. The more you know,

the more amazing the world seems.

And it's the crazy possibilities, the unanswered questions,

that pull us forward. So stay curious.