STEPHEN HAWKING: WE ALL HAVE QUESTIONS... BIG QUESTIONS.

WILL: DO ALIENS EXIST?

HAWKING: IT'S PART OF WHAT IT MEANS TO BE HUMAN.

LINDSAY: HOW WOULD WE FIND THEM?

LISTEN TO THAT!

HAWKING: MY NAME IS STEPHEN HAWKING.

AND I BELIEVE THAT ANYONE CAN ANSWER BIG QUESTIONS FOR THEMSELVES.

LINDSAY: WHOA!

HAWKING: SO WITH THE HELP OF SOME ORDINARY PEOPLE... SIAN: OH, MY GOD!

HAWKING: AND A TEAM OF EXPERTS... DAVID CHARBONNEAU: THE FIRST TIME I SAW THAT, IT BLEW MY MIND.

HAWKING: WE ARE GOING ON THE ULTIMATE VOYAGE-- A QUEST TO ANSWER THE GREATEST MYSTERIES OF THE UNIVERSE... SIAN: I CAN HEAR INTELLIGENT LIFE!

HAWKING: USING THE POWER OF THE HUMAN MIND.

SIAN: RIGHT THERE!

HAWKING: BECAUSE ANYONE CAN THINK LIKE A GENIUS.

"ARE WE ALONE?"

HAWKING: HAVE YOU EVER LOOKED UP AT THE NIGHT SKY AND WONDERED IF THERE IS ANYONE OUT THERE... ALIEN CREATURES, PERHAPS LIVING ON A PLANET ORBITING SOME FAR-AWAY STAR?

IF YOU CONTEMPLATE THESE THINGS, YOU ARE IN GOOD COMPANY.

"ARE WE ALONE?"

IS ONE OF HUMANITY'S MOST IMPORTANT QUESTIONS, CONTEMPLATED BY SOME OF THE SMARTEST MINDS WHO EVER LIVED.

[OVERLAPPING TRANSMISSIONS] YET I BELIEVE ANYONE CAN PRETTY MUCH ANSWER IT FOR THEMSELVES.

LET'S SEE IF I'M RIGHT.

I'VE ASKED 3 CURIOUS MINDS TO JOIN ME ON A JOURNEY OF DISCOVERY.

A SERIES OF EXCITING CHALLENGES AWAITS THEM.

THEY WILL BE GIVEN TOOLS AND EQUIPMENT.

CAN THEY FOLLOW THE GREATEST THINKERS IN HISTORY TO GRASP HOW LIKELY IT IS THAT WE ARE ALONE?

ARE WE ALONE?

ARE WE ALONE?

I THINK THAT'S A BIG QUESTION.

THE HONEST ANSWER IS I DON'T KNOW.

I HOPE, MAYBE BELIEVE, THAT WE ARE NOT ALONE.

ON THE OTHER HAND, I WOULDN'T MIND BELIEVING THAT WE ARE SO SPECIAL.

I'D SAY, "BEAM ME UP!"

I WANT TO GO, I WANT TO EXPLORE, I WANT TO SEE WHAT'S OUT THERE.

HAWKING: THE FIRST STEP TO FINDING OUT THE LIKELIHOOD OF ALIENS IS TO ASK, WHAT ULTIMATELY MAKES LIFE POSSIBLE?

SIAN: HOW DID LIFE START HERE ON EARTH?

OK, WE HAVE LOADS OF DIFFERENT TYPES OF LIFE ON EARTH.

HOW DID THAT HAPPEN?

I THINK A SOURCE OF ENERGY IS ESSES-- ESSENTIAL.

HAWKING: CONSIDER WHAT POWERS LIFE ON OUR LUSH PLANET, THE EARTH.

IT IS ENERGY FROM OUR STAR, THE SUN.

SO PERHAPS OTHER STARS COULD BE POWERING LIFE ON OTHER WORLDS IN OUR GALAXY--THE MILKY WAY.

TO BEGIN TO WORK OUT HOW LIKELY THAT IS, THE FIRST THING THE VOLUNTEERS MUST DO IS UNDERSTAND JUST HOW MANY STARS THERE ARE IN OUR GALAXY.

WHAT IF THIS TINY GRAIN OF SAND REPRESENTS A SINGLE STAR IN THE MILKY WAY?

AND ON THIS SPOON ARE 50,000 GRAINS OF SAND.

SO THAT WOULD BE 50,000 STARS.

USING THIS AS A STARTING POINT, CAN MY VOLUNTEERS TAKE A GUESS AT THE NUMBER OF STARS IN OUR GALAXY AND FIND A WAY TO VISUALIZE IT?

A SPOON OF SAND.

HMM.

ALL RIGHT.

THIS IS 50,000 STARS.

WOW.

SIAN: AND IT FITS ON A SPOON.

I KNOW.

THAT'S CRAZY.

THAT'S CRAZY.

SO WHAT CAN WE DO FROM HERE?

CAN YOU GUESS HOW MANY STARS WE HAVE IN THE GALAXY?

UM, WELL, THAT'S A GOOD QUESTION.

YEAH.

I KNOW THAT THERE ARE BILLIONS FOR SURE, BUT I DON'T WANT TO SAY... WILL: BILLIONS?

I'M THINKING A HUNDRED BILLION.

I THINK I HEARD THAT SOMEWHERE.

WILL: ONE BILLION.

SAY--TRY WITH THAT.

IT WILL BE AN EASY FIGURE.

ONE BILLION?

NO?

YOU WANT MORE?

OK. OK.

SOME MORE.

I'M LEANING TOWARDS BIGGER.

WILL: BIGGER THAN 100 BILLION?

LINDSAY: YEAH.

NO.

I--I LIKE 100 BILLION, BUT I DON'T KNOW IF I LIKE ONE BILLION.

LINDSAY: I THINK IT'S SOMEWHERE BETWEEN 90 BILLION AND 100 BILLION.

SIAN: WHY DON'T WE WEIGH THIS AND FIND OUT HOW MUCH THIS WEIGHS, AND THEN WE CAN SCALE UP TO THE 100 BILLION?

SIAN: THERE GOES ALL OF OUR STARS.

WILL: 50,000 STARS WEIGHED... 70 GRAMS.

70 GRAMS.

SIAN: 70 GRAMS.

LINDSAY: OK.

SO LET'S DO THE MATH.

100.

140,000 KILOGRAMS.

SIAN: 140,000 KILOGRAMS... HAWKING: THEY'VE CORRECTLY CALCULATED THEY'LL NEED 140,000 KILOGRAMS OF SAND TO REPRESENT THEIR GUESS OF 100 BILLION STARS.

THAT'S A LOT OF SAND.

WHY DON'T WE WEIGH A BUCKET AND SEE HOW MANY BUCKETS WE NEED?

SIAN: THAT SOUNDS GREAT.

WILL: OK. GOT IT?

UGH!

SIAN: ALL RIGHT.

WHAT DO WE HAVE?

LINDSAY: ALL RIGHT.

SO LOOKS LIKE IT'S ABOUT 14 KILOGRAMS.

THAT IS 10,000 BUCKETS.

MY GOSH... YEAH, THAT'S GONNA BE CRAZY.

SO WE CAN TRY PILING UP UP TO A BILLION STARS.

AND THEN A HUNDRED TIMES OF THAT WOULD GIVE US AN IDEA OF HOW BIG OUR GALAXY IS... WILL: WE ESTIMATE IT.

BASED ON ESTIMATION.

LET'S DO IT.

OK. LINDSAY: ALL RIGHT.

IT'S HARD WORK SHOVELING BUCKETS OF STARS.

HAWKING: FINALLY, WE ARE BEGINNING TO GRASP THE SCALE OF OUR GALAXY.

IT'S A SCIENTIFIC JOURNEY THAT BEGAN LONG AGO...

WHEN MYTHS TOLD THAT THE MILKY WAY WAS A CELESTIAL PATHWAY MADE FROM MILK.

THEN 500 YEARS AGO, A REVOLUTIONARY MIND REVEALED WHAT IT REALLY IS FOR THE FIRST TIME.

FRANCISCO DIEGO: GALILEO GALILEI IS PERHAPS THE FOUNDER OF MODERN SCIENCE.

HE COMES, UH, IN THE LATE 1500s, EARLY 1600s WITH A NEW WAY OF LOOKING AT THE WORLD.

THIS NEW WAY IS EXPERIMENT.

HAWKING: GALILEO'S BIG EXPERIMENT WAS TO TAKE A TECHNOLOGY DEVELOPED FOR NAVIGATION AT SEA AND TURN IT TO THE SKIES.

HIS TELESCOPE REVEALED A BEAUTIFUL TRUTH.

DIEGO: ONCE GALILEO POINTS HIS TELESCOPE TO THE MILKY WAY, HE REALIZES, HAVING THE-- THE AID OF THE TELESCOPE THAT THE MILKY WAY IS MADE OUT OF STARS.

I CANNOT IMAGINE THE--THE EXCITEMENT OF--OF GALILEO AT THAT POINT.

IT'S A MAJOR, MAJOR DISCOVERY.

HAWKING: GALILEO HAD TAKEN A MAJOR STEP IN THE SCIENTIFIC QUEST TO QUANTIFY OUR GALAXY... A JOURNEY I HOPE MY VOLUNTEERS WILL FOLLOW.

TODAY WE KNOW THE MILKY WAY IS A VAST SPIRAL OF STARS, PLANETS, GAS, AND INTERSTELLAR DUST.

BUT EVEN SO, WE CAN ONLY ESTIMATE THE AMAZING NUMBER OF STARS IT CONTAINS, WHICH IS ALSO WHAT THE VOLUNTEERS ARE TRYING TO DO.

WILL: THAT'S IT.

ONE BILLION STARS.

SIAN: THAT'S A BILLION STARS.

I THINK MY ORIGINAL ESTIMATE OF 100 BILLION WAS TOO BIG.

LINDSAY: LET'S SEE.

100 PILES... SIAN: THAT'S A LOT.

LINDSAY: THAT'S A LOT.

WELL, MAYBE IT'S ONLY A BILLION STARS.

HAWKING: SCIENTISTS NOW ESTIMATE THE NUMBER OF STARS IN OUR GALAXY WITH A COMPLEX MATHEMATICAL EQUATION.

AND THE ANSWER IS QUITE EXTRAORDINARY.

AN ESTIMATED 300 BILLION STARS.

THAT'S 3 TIMES THEIR FIRST GUESS AND 300 TIMES THEIR REVISED ESTIMATE.

HOW WRONG CAN YOU BE?

OHH!

OHH, MY GOSH.

OH, MY GOSH.

WOW.

300 BILLION.

I MEAN, THAT'S JUST UNBELIEVABLE.

I MEAN, IT'S JUST SO BIG.

I THINK WE'RE GONNA NEED SOME MORE SAND.

LINDSAY: THAT IS A LOT.

WHOA!

OH, MY GOD.

LINDSAY: WOW!

SIAN: THAT'S CRAZY.

WILL: NOW THAT'S A GALAXY.

OH, YEAH.

WOW... WHOO!

HAWKING: NOW THEY'VE GOT THE EQUIPMENT TO MATCH THE SCALE OF THE JOB.

HOW MUCH SAND WILL THEY NEED?

LINDSAY: THAT'S 8 TONS.

WE NEED A TOTAL OF 416 TONS, AND WE HAVE 40 SO FAR.

SIAN: 362 TO GO.

MOVING THE SHOVELS AND, YOU KNOW, DIRECTING THE TRUCKS JUST MADE ME THINK, LIKE, HOW-- REALLY HOW BIG THE GALAXY IS.

61 TONS SO FAR.

SIAN: MIND-BOGGLING.

HAWKING: I THINK THIS IS THE ONLY WAY TO REALLY GRASP THE NUMBER OF STARS IN JUST OUR GALAXY.

EACH 8-TON SHOVEL REPRESENTS 6 BILLION STARS... EACH ONE A POSSIBLE ENERGY SOURCE FOR AN ALIEN WORLD.

SIAN: WE COULD HAVE NEVER BUILT THIS PILE ON OUR OWN.

LINDSAY: NO WAY.

I WOULD BE DEAD BY NOW.

SIAN: I GOT TO SAY, THAT'S A HECK OF A LOT OF STARS WE GOT GOING ON THERE.

LINDSAY: WOW.

OK, WE NEED 15 MORE TONS.

SHOVEL NUMBER ONE, YOU READY TO DO THE LAST ONE?

LAST ONE FINALLY!

WHOO-HOO!

SIAN: I NEVER WOULD HAVE GOT IT UNTIL I SAW IT.

HAWKING: SO THIS IS A GALAXY'S WORTH OF STARS-- A PILE THAT WEIGHS 416 TONS.

ABOUT 300 BILLION INDIVIDUAL GRAINS OF SAND.

THIS I GET.

SIAN, VOICE-OVER: TO THINK OF THE MILKY WAY BEING THAT BIG WITH THAT MANY STARS, LIKE, HOW IS THAT EVEN POSSIBLE?

I THINK NEXT TIME I'M GONNA BE ABLE TO SEE THE STARS, IT'S JUST GONNA CRUSH ME A BIT TO THINK.

IT'S ABOUT 300 BILLION.

THAT'S OUR GALAXY.

ALL OF IT JUST MADE OF TINY, TINY, LITTLE GRAIN OF SAND.

AND IF YOU GET JUST ONE RIGHT HERE, THAT'S OUR SUN.

NOW WHEN I SEE THAT, IT'S NOT EVEN PROBABLE ANYMORE.

THERE--THERE HAS TO BE LIFE SOMEWHERE.

SIAN: WHEN I LOOK UP AT THE NIGHT SKY AND I SEE ALL THOSE STARS, IT ADDS ANOTHER LAYER OF MEANING TO WHY WE ARE HERE AND WHERE WE'RE GOING.

LINDSAY: IT IS MAKING ME THINK ABOUT ACTUALLY WHAT THE MEANING OF LIFE IS.

HAWKING: WITH A BIT OF HELP, MY BUDDING GENIUSES HAVE GRASPED THE REMARKABLE NUMBER OF STARS IN OUR GALAXY-- POTENTIAL POWERHOUSES FOR LITTLE GREEN MEN AND WOMEN.

HAWKING: THE NEXT STEP IN FINDING OUT IF WE ARE ALONE IS TO ASK, WHAT ELSE WOULD AN ALIEN LIFE FORM PROBABLY NEED?

SO WHAT'S THE NEXT STEP?

WELL, WE ARE A PLANET AROUND OUR STAR.

AND PERHAPS THE CLOSEST THING WE CAN LOOK FOR ARE PLANETS THAT ORBIT AROUND THESE BILLIONS OF STARS.

SIAN, VOICE-OVER: SO WHAT I FIND INTERESTING IS THAT YOU HAVE ALL OF THESE STARS IN OUR MILKY WAY, BUT THAT DOESN'T MEAN ANYTHING UNLESS YOU HAVE PLANETS.

HAWKING: THE EXISTENCE OF FAR-AWAY PLANETS IS A SURPRISINGLY OLD IDEA.

IN 1584, ITALIAN PHILOSOPHER GIORDANO BRUNO PROPOSED THE STARS WERE SURROUNDED BY PLANETS.

IT WAS THOUGHT THESE PLANETS COULD BE POPULATED BY ALIENS.

UNFORTUNATELY, BRUNO WAS BURNED AT THE STAKE FOR WHAT WAS THEN AN OUTRAGEOUS IDEA.

I AM LUCKY THAT NOW IS A SAFER TIME TO BE A COSMOLOGIST.

TODAY IT SEEMS LOGICAL THAT IF LIFE STARTED HERE ON OUR PLANET, IT COULD ALSO BEGIN ON OTHER PLANETS ORBITING OTHER STARS.

BUT HOW DO WE FIND THEM?

LINDSAY: WHOA.

WOW.

SIAN: WHA--WHAT THE...?

HAWKING: FINDING PLANETS IN THE VASTNESS OF SPACE IS NOT EASY.

[SIAN, LINDSAY, AND WILL SPEAKING INDISTINCTLY] HAWKING: HERE ARE SOME SCALE MODELS OF PLANETS.

USING THESE SMALL SPHERES, CAN THE VOLUNTEERS WORK OUT WHAT THE SOLUTION IS?

SIAN: MY GOD, THAT IS CRAZY.

LINDSAY: THIS-- THIS IS A THEATER.

WHAT THE HECK?

I LOVE IT!

OH, WOW.

LINDSAY: IT'S GREAT!

SIAN: THAT'S COOL.

THAT'S REALLY NICE.

LINDSAY: WOW!

WE'RE ON STAGE.

LINDSAY, VOICE-OVER: WE SEE THIS BRIGHT SPOTLIGHT, AND I JUST IMMEDIATELY CONNECTED, THAT'S PROBABLY A STAR THAT, YOU KNOW-- THAT'S THE ANALOGY WE ARE WORKING WITH.

SIAN: OH, WOW.

LINDSAY: RIGHT THERE, RIGHT?

WILL: OH, DEFINITELY.

OH, THAT'S COOL.

LINDSAY: STAR AND THE PLANETS.

SIAN: LOOK AT THIS.

LINDSAY: LOOKIT HERE.

SIAN: THERE IS A SHADOW.

LINDSAY: MAKES A SHADOW.

RIGHT.

SIAN: COULD YOU ACTUALLY SEE THE SHADOW OF A PLANET ON--IT'D HAVE TO BE A BIG PLANET.

WILL: WELL, I THINK THAT'S WHAT YOU'RE LOOKING FOR.

YOU KNOW, YOU'RE LOOKING AT YOUR STAR.

AND IF IN A REGULAR-- YOU KNOW, EVERY MONTH, EVERY 5 YEAR, EVERY 10 YEARS, YOU SEE A SHADOW COMING IN OR A CHANGE OF YOUR ENERGY LEVELS.

YES!

IF THE STAR HAS PLANETS, IT WOULD BE ORBITING AROUND THE STAR.

AND WHEN IT PASSED THROUGH THE--THE LINE BETWEEN THE SUN AND THE EARTH, WE WOULD PROBABLY SEE A DIM IN THE LIGHT JUST BECAUSE IT BLOCKS THE LIGHT.

AND SO THAT'S WHAT WE WANT TO LOOK FOR.

THE PROBLEM WAS WE HAD THIS HUGE DISTANCE BETWEEN THE STAR AND WHERE WE HAD SET UP THE PLANETS.

SO DO YOU THINK THAT THE-- THE DISTANCE BETWEEN THE PLANETS AND THE STAR SHOULD BE MUCH CLOSER THAN THE DISTANCE BETWEEN THE OBSERVER FROM EARTH AND THE PLANETS, RIGHT?

ABSOLUTELY.

AND WE ONLY HAVE THAT MUCH SPACE TO GO BACK.

'CAUSE THAT'S A HUGE SPACE, SO MAYBE WE SHOULD ACTUALLY GO UP TO THE BALCONY.

HAWKING: THAT'S GOOD.

THEY'RE CORRECTLY REPLICATING THE LAYOUT OF A DISTANT SOLAR SYSTEM AND ANTICIPATING A PROBLEM.

THE PLANETS ARE COMPARATIVELY TINY, AND THEIR STAR IS VERY BRIGHT.

I THINK WE'RE LOOKING FOR A FLICKER AS IT PASSES IN FRONT.

YEAH.

SIAN: HEY, LINDSAY, YOU UP THERE?

LINDSAY: I'M UP HERE.

OK, LET'S GO WITH THE FIRST--UM, NUMBER ONE-- THE SMALLEST PLANET.

LET'S SEE IF WE CAN SEE THAT, OK?

WILL: I THINK IT'S TOO SMALL.

ALL RIGHT.

OK. UM, LET'S TRY NUMBER TWO.

LINDSAY: COULD YOU SEE IT?

SIAN: WE COULDN'T SEE ANYTHING.

OH, WAIT.

WHOA.

LINDSAY: THAT WAS MY ARM.

[LAUGHTER] NOW, WE COULD SEE THAT.

SO THAT WORKED OUT JUST FINE.

I'VE GOT MY GLASSES, SO I'M GONNA PUT MY GLASSES ON AND SEE WHAT I CAN SEE 'CAUSE THIS IS CRAZY.

WILL: I'LL JUST FOLLOW YOU UP.

OK. ALL RIGHT.

WAIT.

LET'S GO TO NUMBER THREE AND SEE WHAT'S GOING ON.

LET'S TRY THE BIGGER ONE, OK?

ALL RIGHT.

I HAVE PLANET NUMBER THREE RIGHT IN THE MIDDLE.

CAN YOU SEE ANYTHING?

STILL NOTHING FOR ME.

I GOT NOTHING.

UH-UH.

I CAN'T SEE ANYTHING.

EVEN IF WE KNOW WE HAVE THE PLANET RIGHT IN FRONT OF US, THE LIGHT IS SO BRIGHT THAT YOU--YOU CAN'T SEE THE SHADOW OF THE PLANET.

ALL RIGHT.

WELL, WHAT DO YOU THINK?

BECAUSE WE CAN'T SEE IT.

WE CAN'T SPOT THE LIGHT, WE CAN'T SPOT THE FLICKER, THE SHADOW... HAWKING: THEY MAY HAVE A PROBLEM, BUT THEY'RE ON THE RIGHT TRACK.

ANY PLANET ORBITING A DISTANT STAR IS TOO SMALL TO SEE DIRECTLY FROM EARTH.

IT IS LOST IN THE GLARE OF ITS STAR.

BUT SCIENTISTS REALIZED THEY MAY BE ABLE TO SPOT DISTANT PLANETS ANOTHER WAY.

AS A PLANET ORBITS BETWEEN ITS STAR AND US ON EARTH, ASTRONOMERS HYPOTHESIZED IT WOULD FRACTIONALLY DECREASE THE AMOUNT OF STARLIGHT REACHING US.

THE EFFECT IS RATHER LIKE THE STAR WINKING.

BUT IT WAS JUST A THEORY UNTIL 1999, WHEN A TEAM OF SCIENTISTS USING JUST A SIMPLE TELESCOPE WITH A LIGHT METER ATTEMPTED TO DETECT THIS EFFECT ON A DISTANT STAR FOR REAL.

CHARBONNEAU: IN 1999 AS A GRADUATE STUDENT, I HAD THE VERY SPECIAL PRIVILEGE OF BEING THE FIRST HUMAN TO SEE A PLANET PASS IN FRONT OF ANOTHER STAR.

THE FIRST TIME I SAW THAT PLOT, IT BLEW MY MIND.

THIS WAS EXACTLY THE SIGNAL THAT I WAS HOPING TO SEE, AND THERE IT WAS ON THE COMPUTER IN FRONT OF ME.

HAWKING: 150 LIGHT YEARS AWAY, PLANET OSIRIS, AS IT WAS UNOFFICIALLY NAMED, IS HUGE-- A GAS GIANT ABOUT 220 TIMES THE MASS OF PLANET EARTH.

CONFIRMING THE EXISTENCE OF SUCH A WORLD WAS A MONUMENTAL ACHIEVEMENT.

BUT IS IT POSSIBLE TO FIND MUCH SMALLER PLANETS MORE LIKE THE EARTH... PLANETS THAT COULD BE POPULATED BY ALIENS?

IN THEIR SEARCH FOR ALIEN LIFE, SCIENTISTS HAVE SHOWN THERE IS A WAY TO DETECT PLANETS OUTSIDE OUR SOLAR SYSTEM.

LINDSAY, ARE YOU IN POSITION... HAWKING: WITH A NEW SET OF TOOLS, CAN MY VOLUNTEERS WORK OUT HOW THE SCIENTISTS MADE THEIR BREAKTHROUGH?

SIAN: OH, MY GOD.

WILL: OH, NICE.

SIAN: THAT'S A TELESCOPE.

OH, MY GOD.

WILL: DO YOU KNOW HOW TO MAKE IT WORK?

SIAN: UM, NO.

BUT IT CAN'T BE THAT HARD, I DON'T THINK.

I THINK THAT'S LIKE A SENSOR.

SO YOU PUT IT ON THE TELESCOPE AND IT--TELLS YOU THE INTENSITY OF LIGHT.

YEAH.

I THINK WE CAN ACTUALLY MEASURE THE LUMENS, THE CHANGE IN LUMENS.

SO WE MEASURE HOW MUCH WE GET TO THAT.

SIAN: ALL RIGHT.

I'M TOTALLY EXCITED ABOUT THIS.

WILL: OH, WOW.

SIAN: OH, MY GOD!

OK.

SO THAT'S WHAT WE PICKED FROM THE STAR.

SIAN: SO THAT'S OUR BASELINE.

YEAH.

YOU WANT TO MARK THAT?

NOW, THAT'S OUR UPPER ZERO.

THAT'S WITH NO PLANET.

OK, SOUNDS GOOD.

HAWKING: THEIR LIGHT METER, WHICH IS MUCH MORE SENSITIVE THAN THE HUMAN EYE, WILL REACT TO EVEN THE TINIEST CHANGE IN LIGHT INTENSITY.

SIAN: ALL RIGHT, LINDSAY, WE'RE GOING TO TRY PUTTING THE PLANET ACROSS AGAIN.

LET'S START WITH THE SMALLEST ONE.

WILL: I CAN FEEL THE TENSION.

SIAN: I KNOW!

SIAN: OK, I DON'T SEE ANYTHING.

WILL: OH, OH, OH.

SIAN: OH, MY GOD!

OK, WE TOTALLY SAW SOMETHING CHANGE.

IT WAS A RELIEF TO ACTUALLY SEE THAT IT WORKED BECAUSE I CAME TO THAT POINT WHERE I WAS LIKE, ARE WE ACTUALLY GOING TO SPOT ANYTHING?

WE MANAGED TO SPOT PLANETS GOING IN FRONT OF OUR STAR.

HIGH FIVE.

HA HA!

HAWKING: NOW THEY ARE REALLY ON TO SOMETHING.

SIAN: NOW WE'RE GONNA TRY NUMBER TWO, OK?

LINDSAY: OK... HAWKING: WITH EVERY NEW PLANET THEY TRY, THE STARLIGHT DIMS TO A DIFFERENT DEGREE.

CAN THE VOLUNTEERS WORK OUT WHY THIS EFFECT IS SO IMPORTANT?

NOW LET'S TRY PLANET NUMBER THREE.

WILL: WELL, A TINY BIT FURTHER.

DEPENDING ON THE SIZE OF THE PLANET, YOU'LL GET A BIGGER DEFLECTION.

OK. SIAN: THERE IT GOES.

THERE IT GOES.

SIAN, VOICE-OVER: AND SO YOU CAN START TO GET INTO DETERMINING SIZE POTENTIALLY OF THE PLANETS ORBITING STARS FAR OFF IN THE MILKY WAY.

IT MEANS THAT WE'RE GETTING SO MUCH CLOSER TO THE POSSIBILITY OF FINDING INTELLIGENT LIFE BECAUSE WE KNOW WHERE TO LOOK.

AND, I MEAN, WHEN YOU'RE DEALING WITH 300 BILLION STARS, YOU KNOW, YOU GOT TO KNOW WHERE TO LOOK.

HAWKING: BY GRASPING HOW TO DETECT ALIEN WORLDS, MY VOLUNTEERS HAVE TAKEN THE NEXT STEP TOWARD ANSWERING THE QUESTION "ARE WE ALONE?"

SCIENTISTS USE THE SAME METHOD TO HUNT FOR SOLAR SYSTEMS LIKE OUR OWN THROUGHOUT OUR GALAXY, ONLY THEY USE MORE SOPHISTICATED INSTRUMENTS.

CHARBONNEAU: WHAT WE NEEDED TO DO WAS TO GET ABOVE THE EARTH'S ATMOSPHERE WITH A LARGE TELESCOPE IN SPACE.

THAT TELESCOPE WAS TO BECOME THE NASA KEPLER MISSION.

HAWKING: THE KEPLER TELESCOPE SIMULTANEOUSLY MEASURES THE BRIGHTNESS OF 100,000 STARS.

ITS OBJECTIVE IS TO SEEK OUT SOLAR SYSTEMS WITH SMALL ROCKY PLANETS, LIKE EARTH.

AND ITS RESULTS ARE ASTONISHING.

CHARBONNEAU: WE COULD SEE, UNAMBIGUOUSLY, DOZENS OF PLANETS IN THE KEPLER DATA.

AND THEN WITH EACH NEW DOWNLOAD FROM THE SPACECRAFT, MORE PLANETS POURED IN.

CURRENTLY THE TALLY STANDS AT 4,000.

WE'VE LEARNED THAT WHEN WE LOOK OUT AT STARS IN THE NIGHT SKY, WE ARE LOOKING AT BROTHERS AND SISTERS-- OTHER SYSTEMS OF PLANETS THAT ONE DAY WE MIGHT HOPE TO INTERACT WITH.

MAYBE WE LIVE IN THE "STAR TREK" UNIVERSE, WHERE EVERY SINGLE STAR HOSTS A SYSTEM OF PLANETS WITH DIFFERENT CIVILIZATIONS FOR US TO GO AND EXPLORE.

HAWKING: YET WHAT KEPLER HAS DISCOVERED SO FAR IS ONLY PART OF THE STORY.

KEPLER CAN ONLY DETECT PLANETS THAT COME DIRECTLY BETWEEN IT AND THE STAR THEY ORBIT.

SO THERE MUST BE MANY MORE PLANETS OUTSIDE KEPLER'S DIRECT LINE OF SIGHT.

WHAT'S MORE, ITS FIELD OF VIEW IS JUST 1/4 OF ONE PERCENT OF THE SKY.

SO SCIENTISTS EXTRAPOLATED KEPLER'S DATA FOR THE ENTIRE GALAXY.

THEIR CALCULATIONS REVEALED AT LEAST 100 BILLION PLANETS... OF WHICH, 50 BILLION ARE LIKELY TO BE ROCKY WORLDS IN SOME WAY SIMILAR TO OUR OWN.

SIAN: I FIND ALL OF THESE NUMBERS FASCINATING BECAUSE IT--THEY'RE SO LARGE, LARGER THAN I WOULD HAVE EVER IMAGINED.

IF YOU HAVE IN THE GALAXY ABOUT 50 BILLIONS MORE CHANCE TO HAVE IT HAPPEN, WE CAN'T BE THE ONLY ONE.

ARE WE REALLY THAT SPECIAL, RIGHT, OUT OF THE 50 BILLION ROCKY PLANETS?

TO ME THAT JUST GIVES ME ENOUGH HOPE TO SAY, YEAH, IT'S OUT THERE.

HAWKING: IN THE SEARCH FOR EXTRATERRESTRIAL LIFE, SCIENTISTS HAVE DISCOVERED THAT ROCKY PLANETS ARE A COMMON FEATURE OF OUR GALAXY.

THEIR NEXT STEP IS TO CONSIDER WHAT COULD MAKE SOME OF THESE PLANETS SUITABLE FOR LIFE.

WELL, OF COURSE, ON THIS PLANET, LIFE MOST ASSUREDLY GOT STARTED IN A WATERY ENVIRONMENT.

WATER'S THE ESSENTIAL LIQUID.

WATER HAS CERTAIN ADVANTAGES.

TO BEGIN WITH, THERE'S A LOT OF WATER ALL OVER THE PLACE.

HYDROGEN IS THE MOST ABUNDANT ELEMENT IN THE COSMOS.

THE THIRD MOST ABUNDANT ELEMENT IN THE COSMOS IS OXYGEN.

HYDROGEN, OXYGEN: H2O.

THE OTHER ADVANTAGE OF WATER IS THAT IT'S LIQUID OVER A VERY WIDE RANGE OF TEMPERATURES... AND CAN INTERACT WITH OTHER MOLECULES IN WAYS THAT FACILITATE CHEMISTRY.

[THUNDER] HAWKING: LIFE AS WE KNOW IT NEEDS WATER TO BE IN ITS LIQUID FORM.

THIS MEANS WE CAN NARROW OUR SEARCH FOR HABITABLE PLANETS STILL FURTHER.

THE DISTANCE FROM A ROCKY PLANET'S ORBIT FROM ITS STAR IS CRITICAL TO WHETHER THERE CAN BE LIQUID WATER.

TOO CLOSE TO ITS HEAT SOURCE AND ANY WATER WOULD BOIL INTO STEAM AND LIKELY DISAPPEAR.

TOO FAR AWAY, AND IT WOULD FREEZE INTO ICE.

BUT IN BETWEEN, THE PLANET'S TEMPERATURE WOULD BE JUST RIGHT... RATHER LIKE THE PORRIDGE IN THE STORY OF THE THREE BEARS.

SO SCIENTISTS NAME THIS HABITABLE REGION THE GOLDILOCKS ZONE.

SCIENTISTS NOW ESTIMATE THERE COULD BE AROUND 500 MILLION ROCKY PLANETS WITH LIQUID WATER IN GOLDILOCKS ZONES.

IT'S VERY SIMPLE.

UH, IF YOU HAVE A PLANET THERE, THEN IT COULD HAVE LIQUID OCEANS, RIGHT?

SO IN OTHER WORDS, IT'S CONCEIVABLE THAT THERE'S SOME LIFE THERE SIMPLY ON THE BASIS OF THE AVAILABILITY OF LIQUID WATER.

HAWKING: FOR MANY YEARS, SCIENTISTS BELIEVED GOLDILOCKS ZONES WERE THE ONLY PLACE WITH LIQUID WATER AND SO THE ONLY PLACE WORTH LOOKING FOR ALIENS.

BUT A RECENT DISCOVERY POINTS TO THERE BEING AN ALTERNATIVE SOURCE OF ENERGY FOR LIFE, A SOURCE FROM DEEP WITHIN CELESTIAL BODIES.

IT'S A CONCEPT I HOPE MY VOLUNTEERS CAN GRASP IN THEIR NEXT CHALLENGE.

I HAVE GIVEN THEM AN IRON BAR AND SOME ICE CUBES.

CAN THEY WORK OUT HOW LIQUID WATER COULD BE PRESENT, FAR FROM ANY STAR?

SIAN: OHH.

OH, MY GOD.

WHAT HAVE WE GOT GOING ON HERE?

LINDSAY: OK. OH, WOW.

SIAN, VOICE-OVER: AND I'M THINKING, WHAT THE HECK CAN WE DO WITH THIS?

LINDSAY: WHAT IS THIS THING?

LINDSAY, VOICE-OVER: AND THE CHALLENGE IS TO GENERATE HEAT WITHOUT A HEAT-- HEAT SOURCE.

WILL: SO WE KNOW WE HAVE TO MELT THAT.

WE HAVE TO GO FROM ICE TO WATER.

HOW DO WE HEAT UP THE ICE WITHOUT A HEAT SOURCE?

SIAN: WELL WE CAN GENERATE OUR OWN HEAT.

RIGHT?

PROBABLY WE HAVE TO USE OUR HANDS TO CREATE SOME SORT OF MECHANICAL FORCE, KINETIC ENERGY TO BE TRANSFORMED INTO HEAT.

LINDSAY: I THINK WE NEED TO RUB THIS ROD--AND TO HEAT IT UP AND THEN PUT THE ROD IN THERE.

BUT THE WHOLE IDEA IS ENERGY.

YOU NEED A SOURCE OF ENERGY.

I MEAN, IF YOU WANT TO RUB IT, I THINK IT'LL BE EASIER TO RUB IT AGAINST THE OTHER METAL.

YEAH.

FIRST OF ALL, IT WAS THAT PROCESS OF, WELL, COULD WE USE FRICTION?

SIAN: KEEP GOING.

WILL: LET'S TRY IT.

TRY--TRY TO PUT--PUT THE ICE CUBE ON.

LINDSAY: OK.

SO THAT--OH, YEAH.

LINDSAY: OK.

THE PROBLEM WE HAVE AT THAT MOMENT THAT WE'RE NOT GENERATING ENOUGH HEAT TO MELT THE ICE PROPERLY.

SIAN: IF WE TRY TO FLEX THE IRON UP AND DOWN-- BUT WAIT.

IT HAS TO BE REALLY TIGHT.

YEAH.

AND THEN WE'LL GENERATE HEAT.

SO THE NEXT STEP WAS JUST TO ACTUALLY TWIST IT AND BEND IT.

SIAN: SO WE'RE TRYING TO GET THIS AREA TO HEAT UP.

I DIDN'T THINK, ONCE AGAIN, THAT WE WOULD GENERATE ENOUGH ENERGY.

SIAN: AND THEN... BY GOING BACK AND FORTH... OH, MY GOD.

WHOA!

IT'S ACTUALLY STARTING TO BREAK AND BEND... OH, MY GOD.

WOW!

AND WEAKEN... OH, MY GOD.

ACTUALLY GENERATE MAYBE ENOUGH HEAT.

WOW.

OH, MY GOD.

YOU GUYS ARE GOING TO HAVE TO HELP ME.

THAT'S AMAZING!

OH, MY GOD.

SIAN: OK, WE'RE GOING TO ROTATE IN AND OUT.

YOU'RE ACTUALLY BREAKING IT...

THE NEXT PERSON READY TO GO?

WILL: UH, IN TWO SECONDS, YOU HAVE ANOTHER 10 SECONDS.

KEEP GOING.

SIAN: ALL RIGHT.

SOMETIMES, YOU KNOW, YOU JUST GOT TO SIT BACK AND SUPERVISE... SIAN: IT'S GETTING HOT.

KEEP GOING, FURTHER.

AND MOTIVATE.

OH, YEAH.

KEEP GOING, KEEP GOING.

AND I FEEL LIKE I'M A GOOD MOTIVATOR.

I KNOW IT'S TOUGH.

JUST KEEP CONCENTRATING ON IT.

YEAH!

KEEP GOING.

KEEP GOING.

BACK AND FORTH, BACK AND FORTH.

WILL: NEED TO PUT THE ICE ON.

LINDSAY: WOW.

BUT I WAS WATCHING THAT IRON BEND.

HAWKING: THE ALTERNATING STRETCHING AND COMPRESSION PRODUCES INTERNAL FRICTION AND HEAT INSIDE THE ROD.

KEEP GOING.

KEEP GOING.

YOU'RE STARTING TO GET IT.

YOU'RE STARTING TO GENERATE IT.

AND IT WAS GETTING HOT.

HAWKING: BUT CAN THEY GENERATE ENOUGH HEAT TO MELT THE ICE?

SIAN: KEEP GOING.

ALL RIGHT.

LOOK OUT, LOOK OUT, LOOK OUT.

LINDSAY: ALL RIGHT.

LET'S TRY THE ICE.

LINDSAY AND SIAN: OH, WOW.

LINDSAY: NICE.

SIAN: OH, HO HO!

THAT'S AWESOME.

LOOK AT THAT GO.

THIS IS COOL.

LIKE, WHEW!

SIAN: THAT IS FANTASTIC.

HA HA!

GOOD JOB, YOU GUYS.

LINDSAY: GOOD JOB.

SIAN: HA HA!

I DIDN'T EXPECT, UM, THAT MUCH HEAT, YOU KNOW.

UM, ACTUALLY IT WAS STEAMING, SO IT WAS VERY AMAZING.

HAWKING: HAVE MY VOLUNTEERS GRASPED WHY SCIENTISTS NOW THINK LIFE COULD EXIST FAR FROM THE HEAT SOURCE OF STARS?

LINDSAY: YOU KNOW, WHEN WE'RE LOOKING FOR LIFE, WE'RE LOOKING FOR THE PLANETS SURROUNDING A STAR, BECAUSE WE NEED A HEAT SOURCE.

AND THE HEAT COMES FROM THE STAR, BUT IT SEEMS LIKE THE HEAT THAT MIGHT BE NEEDED FO--FOR LIFE CAN ALSO BE FROM MECHANICAL FORCES.

HAWKING: THIS IS A PROCESS SCIENTISTS THINK IS HAPPENING RIGHT NOW IN OUR OWN SOLAR SYSTEM.

484 MILLION MILES FROM OUR SUN IS THE MAGNIFICENT GAS GIANT, THE PLANET JUPITER.

IT HAS 67 MOONS.

ONE OF THEM, EUROPA, IS ABOUT THE SIZE OF OUR OWN MOON, BUT IT'S A VERY DIFFERENT PLACE.

CHARBONNEAU: THE SURFACE OF EUROPA IS A BONE-CHILLING MINUS 150 DEGREES CENTIGRADE.

WE KNOW THAT EUROPA HAS A LOT OF WATER, BUT EUROPA IS SO FAR FROM THE SUN, THERE'S NO QUESTION THAT THE WATER IS ABSOLUTELY ROCK SOLID AND NOT A GOOD PLACE FOR LIFE.

HAWKING: BUT EUROPA DOES NOT BEHAVE AS YOU'D EXPECT BECAUSE ITS ORBIT AROUND JUPITER ISN'T A PERFECT CIRCLE.

INSTEAD, IT HAS AN ELLIPTICAL PATH.

CHARBONNEAU: AS A RESULT, THAT SLIGHTLY ELLIPTICAL ORBIT-- ABOUT A 1% DEVIATION FROM A PERFECT CIRCLE-- MEANS THAT EUROPA'S DISTANCE FROM JUPITER CHANGES AS IT GOES AROUND IN ITS ORBIT, SOMETIMES SLIGHTLY FARTHER AWAY AND SOMETIMES EVER SO SLIGHTLY CLOSER.

HAWKING: THIS MEANS THE STRENGTH OF JUPITER'S GRAVITATIONAL PULL ON EUROPA CHANGES, TOO.

AND THAT MAKES THE ENTIRE MOON STRETCH ONE WAY AND THEN THE OTHER.

CHARBONNEAU: THAT CONSTANT STRETCHING AND SQUISHING AND STRETCHING AND SQUISHING HEATS THE INSIDE OF EUROPA AND ALLOWS THE ICE OUT OF WHICH EUROPA IS MADE TO MELT.

WHAT EUROPA TELLS US IS THERE'S ANOTHER WAY TO HEAT A PLANET, TO KEEP THE WATER LIQUID, AND ULTIMATELY TO PROVIDE A HABITABLE ENVIRONMENT WHERE LIFE MIGHT THRIVE.

HAWKING: NASA IS PROPOSING A MISSION TO EXPLORE THIS MYSTERIOUS WORLD.

I THINK THEY MAY FIND AN ENTIRE ECOSYSTEM, LIKE THE DEEP SEA ECOSYSTEMS WE HAVE HERE ON EARTH.

THAT WOULD BE TRULY AMAZING.

WITH A VAST NUMBER OF WATERY ROCKY WORLDS TO CONSIDER, IT IS LIKELY THERE ARE COUNTLESS PLACES WHERE WATER-BASED LIFE COULD BE THRIVING AT THIS VERY MOMENT.

AND WITH NEW DISCOVERIES, LIKE WATER ON MARS AND PLUTO, I BELIEVE WE WILL ENCOUNTER ALIEN ORGANISMS IN OUR LIFETIME.

THE MATHEMATICS PREDICTS IT.

LIFE IS UNDOUBTEDLY OUT THERE.

NOW HERE'S A PROSPECT I FIND EVEN MORE TANTALIZING.

GIVEN THAT OUR GALAXY IS OVER 13 BILLION YEARS OLD, THERE HAS BEEN AMPLE TIME FOR SIMPLE LIFE TO EVOLVE INTO COMPLEX ORGANISMS AND PERHAPS INTO INTELLIGENT CIVILIZATIONS JUST AS WE HAVE ON EARTH.

AND IF ADVANCED LIFE HAS EVOLVED ON ONLY A TINY PERCENTAGE OF THESE PLANETS, THERE MIGHT STILL BE A GOOD NUMBER OF ALIEN CIVILIZATIONS IN OUR GALAXY, WITH THE TECHNOLOGY TO COMMUNICATE WITH US.

MAYBE SOME ARE ALREADY TRYING.

TO FIND OUT, ALL THE SCIENTISTS HAVE TO DO IS LISTEN.

BUT THIS ISN'T AS STRAIGHTFORWARD AS IT FIRST SEEMS.

I HAVE SENT MY VOLUNTEERS TO WHERE SCIENTISTS PROBE FOR SIGNALS FROM DEEP SPACE.

OH, MY GOD!

LINDSAY: WOW!

THIS IS AMAZING.

SIAN: THIS IS UN-FREAKIN'-BELIEVABLE.

LINDSAY: THAT IS CRAZY.

[SIAN LAUGHING] SIAN: THIS-- THIS FOR ME IS A SIGN OF INTELLIGENT LIFE RIGHT HERE-- WITH ADVANCED TECHNOLOGY.

HAWKING: NOW LET'S FIND OUT WHAT ASTRONOMERS ARE PICKING UP AND HOW IT MAY HELP THEM TRACK DOWN INTELLIGENT EXTRATERRESTRIALS.

LINDSAY: OH, WOW... SIAN: THIS IS COOL.

WILL: I'M GONNA PLAY WITH THAT.

HOW ABOUT THIS ONE... SIAN: OH, LOOK AT THIS.

WAIT.

[HISSING STATIC-LIKE SOUND] LISTEN TO THAT.

SIAN: WE SEE THIS WAVE GOING ACROSS THE SCREEN, BUT NOT ONLY THAT.

YOU CAN HEAR THE SOUND.

LIKE, YOU KNOW, WEIRD KIND OF-- [IMITATES UNDULATING NOISE] YOU KNOW?

WILL: I WAS THINKING THAT THIS SIGNAL THAT WE TAKE FROM THE DISH, IS WHAT WE SEE HERE.

HAWKING: THAT'S RIGHT.

WHEREVER THIS RADIO TELESCOPE POINTS, IT RECEIVES ELECTROMAGNETIC VIBRATIONS FROM SPACE.

BECAUSE DISTANT STARS AND PLANETS-- IN FACT, EVERYTHING IN OUR GALAXY-- EMITS RADIO WAVES ALL AT ONCE AND ALL THE TIME.

HAWKING: THIS WHITE NOISE IS THE COMBINED SOUND OF THE UNIVERSE, CONVERTED INTO SOMETHING THAT WE CAN HEAR WITH OUR OWN EARS.

AND SOMEWHERE IN THERE MAY BE A MESSAGE FROM INTELLIGENT LIFE.

SO IN THE SEARCH FOR EXTRATERRESTRIAL LIFE, WE FACE A PROBLEM.

HOW IS IT POSSIBLE TO HEAR AN ALIEN BROADCAST AMONG SO MUCH BACKGROUND NOISE?

WILL: ALL RIGHT.

SIAN: OH, MY GOD.

THAT'S AWESOME.

LINDSAY: OH, OK. WOW.

GREAT.

HAWKING: WITH THE EQUIPMENT I'VE LAID OUT, CAN MY VOLUNTEERS WORK OUT THE SOLUTION?

SIAN, VOICE-OVER: IT'S AN INTERESTING CHALLENGE BECAUSE SPACE IS NOISY.

AND IF SOMEBODY OUT THERE IS INTELLIGENT AND BROADCASTING TO US, HOW ARE WE GOING TO HEAR THEM?

HOW ARE WE GOING TO FIND THEM?

LINDSAY: THIS LOOKS FAMILIAR.

WE KNOW ALREADY THAT'S--THAT'S SPACE WHITE NOISE.

WE'RE LISTENING TO SPACE.

LINDSAY: THE WHITE NOISE IS COMING FROM THESE SPEAKERS SURROUNDING US.

SIAN: IT LOOKS LIKE WE HAVE AN EQUALIZER HERE.

WILL: YEAH.

DO YOU THINK WE CAN MIX--LIKE, MIX OUT AND TRY TO FIGURE OUT WHAT WE WANT TO HEAR?

HAWKING: THE VOLUME OF THE WHITE NOISE IS FIXED.

SO USING THE EQUALIZER AND MEGAPHONE ALONE, HOW WILL MY VOLUNTEERS GO ABOUT SIMULATING A MESSAGE FROM DEEP SPACE?

SIAN: YOU'RE GONNA BE INTELLIGENT LIFE... WILL: YEAH.

OF THE UNIVERSE.

I LIKE THAT.

WE MADE WILL AN ALIEN, AND HIS JOB WAS TO BROADCAST AS AN ALIEN.

SIAN: GO, WILL.

GO!

SO THEY SEND ME TO BE THIS TEST ALIEN SOMEWHERE IN SPACE.

SO WE WANT TO SEE IF WE CAN HEAR WILL... YEAH.

WHO IS OUR ALIEN IN DEEP SPACE.

LINDSAY: RIGHT.

AND SO...

BUT THE BIG PROBLEM IS IT'S SO NOISY OVER HERE THAT EVEN IF THERE IS AN INTELLIGENT SIGNAL, WE WOULDN'T BE ABLE TO PICK IT UP.

WE'RE DROWNED IN THIS NOISE.

WILL: I AM AN ALIEN.

SIAN: I CAN'T HEAR HIM.

I AM AN ALIEN.

SIAN: WHAT CAN WE DO?

IF WE HAVE THIS HERE...

I AM AN ALIEN.

HE WAS CREATING ARTIFICIAL NOISE.

I AM AN ALIEN.

WE--WE CAN'T HEAR ANYTHING, THOUGH HE WAS SHOUTING AT US.

I AM AN ALIEN.

I AM AN ALIEN.

I AM... HAWKING: THE CHALLENGE OF DETECTING ALIEN BROADCASTS WAS CONSIDERED BY SCIENTISTS BACK IN THE 1950s.

GIUSEPPE COCCONI AND PHILIP MORRISON PROPOSED THAT INTELLIGENT ALIENS WOULD MAKE USE OF ONE PECULIAR FEATURE OF THE UNIVERSE.

THERE IS ONE BAND OF FREQUENCIES WHERE THERE ARE VERY FEW VIBRATIONS, AND SO IT IS EERILY QUIET.

AND WITHIN THIS BAND IS A UNIQUE FREQUENCY THAT IS IDEAL FOR LONG-DISTANCE COMMUNICATION.

SETH SHOSTACK: THERE'S A CERTAIN FREQUENCY CALLED THE HYDROGEN LINE FREQUENCY.

IT'S AT 1,420 MEGAHERTZ.

THAT'S WAY ABOVE TELEVISION/RADIO.

AND THAT FREQUENCY IS ONE THAT'S VERY IMPORTANT FOR ASTRONOMY.

HAWKING: BECAUSE THIS FREQUENCY BELONGS TO THE MOST COMMON ELEMENT IN THE UNIVERSE, HYDROGEN, SCIENTISTS FIGURED ALIEN ASTRONOMERS WOULD UNDOUBTEDLY BE AWARE OF IT.

SHOSTACK: TUNING TO THIS MAGIC FREQUENCY, IF YOU WILL.

WELL, THAT MAKES SENSE BECAUSE, YOU KNOW, THEY WILL KNOW THAT FREQUENCY.

AND IF THEY'RE DELIBERATELY TRYING TO GET IN TOUCH, MAYBE THEY'LL USE IT.

[OVERLAPPING TRANSMISSIONS] HAWKING: COCCONI AND MORRISON SUGGESTED THAT THIS QUIET SPOT ON THE RADIO DIAL WOULD BE THE IDEAL FREQUENCY FOR ALIEN CIVILIZATIONS TO COMMUNICATE ON.

WILL THE VOLUNTEERS REALIZE THEY HAVE TO CREATE THEIR OWN VERSION OF THE GALAXY'S QUIET FREQUENCY BAND?

SIAN: NO, NO.

THIS IS, UH, AMPLIFYING IT.

IT'S BRINGING IT DOWN.

I AM AN ALIEN.

WE'VE GOT ALL THESE FREQUENCIES, AND HE'S BROADCASTING ON JUST A SHORT FEW.

HE'S BROADCASTING IN A NARROW BAND OF FREQUENCIES.

WE FIGURED OUT THAT IF WE COULD ELIMINATE THE SIGNALS FROM THE BACKGROUND NOISE, THEN WE COULD HEAR HIM.

YEAH.

IF WE CAN FIGURE OUT WHAT THAT IS, THEN WE CAN ISOLATE IT.

JE SUIS UN EXTRATERRESTRE.

SIAN: HERE.

GET THESE ONES DOWN.

WILL: I AM AN ALIEN.

SIAN: SO WE CAN EITHER BOOST SIGNALS.

I AM AN ALIEN.

OOH.

I'M STARTING TO HEAR HIM.

I'M ACTUALLY STARTING TO HEAR HIM.

I AM AN ALIEN.

WILL, VOICE-OVER: EVENTUALLY THEY MANAGED TO PICK THE FREQUENCY I WAS ON BY LOWERING THE WHITE NOISE ON THIS SPECIAL FREQUENCY.

THEY MANAGED TO ISOLATE MY VOICE FROM THE NOISE.

I AM AN ALIEN... SIAN: I CAN HEAR HIM.

OH, MY GOD.

YEAH.

WILL: I AM AN ALIEN.

NOW WE CAN HEAR HIM.

I CAN TOTALLY HEAR HIM.

HAWKING: NOW CAN THEY MAKE THE SAME CONNECTION SCIENTISTS MADE?

IF YOU'RE AN INTELLIGENT SPECIES, YOU ACTUALLY WILL CHOOSE TO BROADCAST, "HEY, WE'RE HERE," YOU KNOW, IN A FREQUENCY THAT'S LESS NOISY SO THAT THE OTHER INTELLIGENT LIFE WOULD ACTUALLY FIND YOU AND HEAR YOU.

HE'S SAYING, "I AM AN ALIEN."

RIGHT!

CAN YOU HEAR ME?

SIAN: I CAN HEAR INTELLIGENT LIFE OUT THERE.

WILL: I AM AN ALIEN.

THAT IS SO AWESOME.

WILL: I AM AN ALIEN.

HAWKING: ASTRONOMERS HAVE BEEN LISTENING FOR ALIEN SIGNALS FOR REAL ON THE SWEET SPOT OF THE COSMIC DIAL FOR 30 YEARS.

SHOSTACK: FOR A LOT OF PEOPLE, THE OBVIOUS WAY TO FIND THE ALIENS IS JUST GO LOOK.

I MEAN, THAT'S WHAT THEY DO ON "STAR TREK," RIGHT?

THEY BOLDLY GO OUT INTO SPACE WITH A REALLY HIGH-POWERED CRAFT, AND THEY JUST LOOK FOR THEM.

WELL, I MEAN, THAT--THAT MAY BE MORE INTERESTING FOR TELEVISION, BUT THIS IS ACTUALLY FAR MORE PRACTICAL.

HAWKING: IT'S A PROJECT CALLED SETI-- THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE.

SHOSTACK: THE TOTAL NUMBER OF STAR SYSTEMS THAT WE'VE LOOKED AT CAREFULLY IS A FEW THOUSAND.

SO, YOU KNOW, IT'S VERY EARLY DAYS.

WE'VE JUST SCRATCHED THE SURFACE.

THERE COULD BE SIGNALS GOING THROUGH MY BODY AS I STAND HERE RIGHT NOW.

AND IF ONLY WE HAD THESE ANTENNAS POINTED THE RIGHT WAY, I'D FLY TO STOCKHOLM IN A WEEK AND COLLECT THE NOBEL PRIZE.

HAWKING: YET IN 3 DECADES OF SCANNING THE GALAXY, SCIENTISTS HAVE HEARD NOTHING.

IT HIGHLIGHTS A PARADOX REVEALED BY PHYSICIST ENRICO FERMI, WHEN HE ASKED, "WHERE IS EVERYBODY?"

FERMI WAS TALKING ABOUT A CONTRADICTION-- THE LARGE NUMBER OF ALIEN CIVILIZATIONS THE MATHEMATICS PREDICTS AND THE LACK OF EVIDENCE OF THOSE CIVILIZATIONS.

FERMI BELIEVED ASTRONOMERS OUGHT TO HAVE MADE CONTACT LONG AGO AND MANY TIMES OVER.

BUT BECAUSE WE DON'T SEEM TO HAVE BEEN VISITED BY ALIENS, PERHAPS THERE IS A PROBLEM THAT SCIENTISTS HADN'T CONSIDERED.

TO GET MY VOLUNTEERS THINKING ABOUT THIS PARADOX, HERE IS A SPECIAL MACHINE.

SIAN: OH, MY GOODNESS.

HAWKING: ITS DESIGN REFLECTS THE JOURNEY WE'VE TAKEN TO WORK OUT THE PROBABILITY OF ALIEN LIFE.

IT'S A DEMONSTRATION OF A PRINCIPLE CALLED THE GREAT FILTER.

CAN THEY FIGURE OUT WHAT IT REPRESENTS?

[MUSIC PLAYING ON SOUNDTRACK] I'M LOOKING AT THIS CONTRAPTION, AND THEN I'M LIKE, "WILL, PUT THE BUCKET OF BALLS IN," YOU KNOW?

"LET'S SEE WHAT GOES ON."

AND THEN I'M LIKE, I'M SWITCHING SWITCHES.

SIAN: ONE... WILL: OH!

SIAN: MY GOD, THAT'S AWESOME.

WILL: HOW NICE!

LOOK AT THAT.

LINDSAY: WHAT?

OK, NUMBER TWO.

A GOOD AMOUNT OF BALLS WOULD GO FROM THE FIRST COLUMN TO THE SECOND ONE.

FROM ONE TO TWO, EH, IT'S EASY.

FROM TWO TO THREE, HMM, NOT SO EASY.

LINDSAY: OH, RIGHT!

YES!

SIAN: OH, WE DID IT?

YEAH!

SIAN, VOICE-OVER: VERY FEW ARE MAKING IT IN--INTO THREE.

AND NONE AT THIS POINT ARE MAKING IT INTO FOUR.

WHEN YOU GO CLOSER AND YOU INSPECT IT, YOU'RE LIKE, "WAIT.

WHAT'S GOING ON HERE?"

TO ME THAT MACHINE LOOKS LIKE, YOU KNOW, IT COULD BE AN ANALOGY FOR A NUMBER OF THINGS.

WE'RE JUST KIND OF THROWING OUT DIFFERENT HYPOTHESIS.

WILL: I FEEL WE'VE MADE THEM MORE LIKE, THAT'S ALL YOUR PLANETS.

AND THEN THEY HAVE TO MEET CRITERIA TO GET LIFE.

LINDSAY: SO THERE--YOU'RE SAYING THERE ARE A LOT OF CRITERIA THAT NEEDS TO BE MET BEFORE LIFE HAPPENS AND THEN MORE CRITERIA FOR LIFE TO BE INTELLIGENT.

AND SO THIS IS LIKE THE... WILL: I THINK SO.

YEAH.

THE DIFFERENT PROCESS, AND IT GETS RARER AND RARER.

LINDSAY, VOICE-OVER: THERE ARE OBVIOUS BARRIERS, UM, TO THE EVOLUTION OF LIFE.

AND, YOU KNOW, THAT SEEMS KIND OF FITTING.

HAWKING: THEY'VE GOT IT.

AND THESE BARRIERS SEPARATE THE PARTS OF MY GREAT FILTER MACHINE.

THE WINDOWS IN THE BARRIERS GET SMALLER, SO IT BECOMES HARDER TO PROGRESS THROUGH THE MACHINE.

THE BALLS IN THE FIRST COMPARTMENT REPRESENT THE NUMBER OF ROCKY PLANETS ORBITING STARS IN OUR GALAXY.

IN THE SECOND ARE THOSE ROCKY PLANETS WITH LIQUID WATER, WHICH HAVE DEVELOPED SIMPLE LIFE.

IN THE THIRD ARE SPECIES ON THOSE PLANETS THAT HAVE FURTHER EVOLVED INTO COMPLEX LIFE FORMS.

AND THE FEW BALLS THAT HAVE MADE IT TO THE LAST COMPARTMENT REPRESENT INTELLIGENT LIFE THAT HASN'T YET DEVELOPED TECHNOLOGY.

BUT THERE IS ONE FINAL STAGE REMAINING-- MAKING IT OUT OF THE COMPARTMENTS ALTOGETHER.

LINDSAY, VOICE-OVER: IT HAS THIS SPIRAL THING DOWN.

AND EVENTUALLY, BOOM.

OHH.

LOOK AT THAT.

OH, MY GOSH!

LINDSAY, VOICE-OVER: FINALLY, WE WERE VERY EXCITED TO SEE THAT THERE WAS ONE BALL THAT MADE IT, YOU KNOW, OUT THAT--THAT TUBE THING.

COMES DOWN, SLIDES DOWN.

HAWKING: THIS REPRESENTS INTELLIGENT BEINGS WHO HAVE DEVELOPED TECHNOLOGIES FOR COMMUNICATING WITH OTHER WORLDS.

SO IF THAT BALL REPRESENTS AN ALIEN LIFE THAT CAN, UM, HAVE THE TECHNOLOGY TO COMMUNICATE, IT IS VERY UNLIKELY AND VERY HARD TO GET TO THAT STAGE.

HAWKING: THAT ALIEN CIVILIZATION'S GOAL IS TO NOW ACHIEVE FIRST CONTACT BY GETTING TO THE BOX AT THE END OF MY MACHINE.

WE WERE ROOTING FOR IT TO GET TO THE BOX.

SIAN: OH, WELL, COME ON.

COME ON...

STEADY.

COME ON, COME ON.

AND THEN, OH, MY GOD.

AAH!

OH, MY GOD!

WHAT THE HECK?

SIAN: OH, MY GOD!

OUR BALL JUST GOT ANNIHILATED.

IT WAS JUST DISAPPOINTING ALMOST TO THINK IT'S BEEN THROUGH THE ENTIRE PROCESS AND, LAST MINUTE, IT GETS BLOWN AWAY.

HAWKING: SO WHAT DO THEY THINK THE FLAME SIGNIFIES?

SIAN: THE FLAME REPRESENTED THE FACT THAT BY THE TIME WE GET TO INTELLIGENT LIFE, THE PLANET OR THE SPECIES THAT IS INTELLIGENT, JUST, SOMETHING HAPPENS TO IT.

YOU CAN HAVE INTELLIGENT LIFE.

IT DOESN'T MEAN IT'S GOING TO SURVIVE.

ANYTHING CAN HAPPEN.

LINDSAY, VOICE-OVER: AND IT COULD BE NATURAL DISASTERS; THE PLANET COULD BE HIT BY AN ASTEROID; OR IT COULD BE WARS.

OR THERE COULD BE AN EPIDEMIC OF, YOU KNOW, DISEASE THAT CAN DESTROY.

SO, YOU KNOW, LIFE IS FRAGILE.

HAWKING: THE GREAT FILTER THEORY SUGGESTS THERE MAY BE ONE SIGNIFICANT STUMBLING BLOCK TO BECOMING AN ADVANCED EXTRATERRESTRIAL CIVILIZATION-- THAT ADVANCED ALIEN LIFE OCCURS SO RARELY OR BRIEFLY THAT WE ARE UNLIKELY TO MAKE CONTACT.

I, FOR ONE, AM MORE OPTIMISTIC THAN THAT.

BUT THE IDEA DELIVERS A STARK WARNING TO OUR OWN CIVILIZATION.

SIAN: I THINK HUMANS ARE IN A VERY DELICATE POSITION RIGHT NOW.

THE MACHINE IS ABSOLUTELY TELLING ME THAT HUMAN LIFE IS FRAGILE AND OUR EXISTENCE AS AN INTELLIGENT SPECIES IS EXTREMELY FRAGILE.

HAWKING: CONSIDER WHAT WE'VE ACHIEVED ON OUR JOURNEY.

OUR GALAXY CONTAINS AROUND 300 BILLION STARS... SIAN: OH, MY GOD.

LINDSAY: WHOO!

HAWKING: EACH ONE A POTENTIAL POWERHOUSE FOR ALIEN WORLDS.

SIAN: I NEVER WOULD HAVE GOT IT UNTIL I SAW IT.

HAWKING: ORBITING THOSE STARS ARE 100 BILLION PLANETS, 50 BILLION OF WHICH COULD BE ROCKY, LIKE OUR OWN.

SIAN: IT'S OFFICIAL.

WE KNOW HOW TO FIND PLANETS ORBITING STARS FAR AWAY.

HAWKING: WE KNOW 500 MILLION ROCKY WORLDS COULD HAVE THE VITAL MATERIAL NEEDED BY LIFE--LIQUID WATER.

SIMPLE LIFE COULD BE THRIVING ON COUNTLESS NUMBERS OF THESE WORLDS AT THIS VERY MOMENT.

AND ON SOME OF THESE, EVOLUTION HAS HAD THE TIME TO PRODUCE INTELLIGENT, TECHNOLOGICALLY ADVANCED LIFE.

CAN YOU HEAR ME?

SIAN: I CAN HEAR INTELLIGENT LIFE OUT THERE.

HAWKING: I BELIEVE HUMAN BEINGS CANNOT BE THE SINGULAR ADVANCED SPECIES IN OUR GALAXY...

THOUGH I THINK IT QUITE LIKELY THAT WE ARE THE ONLY CIVILIZATION AT OUR STAGE OF DEVELOPMENT WITHIN SEVERAL HUNDRED LIGHT YEARS.

SIAN: OH!

WILL: IT MADE IT.

IT MADE IT.

LINDSAY: OH, MY GOD!

HAWKING: SO IF I WERE YOU, I WOULDN'T LOSE TOO MUCH SLEEP WORRYING ABOUT BEING ABDUCTED BY ALIENS.

OUR JOURNEY HAS CERTAINLY THROWN UP SOME PRETTY BIG THOUGHTS.

SIAN: THIS JOURNEY ENABLED ME TO GO DEEPER, UM, TO-- TO COME TO REALIZATIONS THAT I DIDN'T HAVE BEFORE.

WILL: I DO HOPE WE'LL DISCOVER LIFE SOMEWHERE ELSE AND THAT WE'LL LEARN NEW THINGS FROM THAT.

I WON'T LOOK AT THE SKY IN THE SAME WAY.

THAT'S FOR SURE.

LINDSAY: THERE'S A SAYING THAT EVERY JOURNEY LEADS YOU BACK TO WHERE YOU START.

WHEN WE SEARCH OUTSIDE, WE OFTEN FIND SOMETHING ABOUT OUR--OURSELVES.

HAWKING: NOW, HAVING SO FAR EXPLORED THE POSSIBILITY FOR LIFE IN OUR COSMIC NEIGHBORHOOD-- THE MILKY WAY--WE CAN TURN AGAIN TO OUR GRAND QUESTION: "ARE WE ALONE IN THE UNIVERSE?"

CONSIDER THIS.

THE UNIVERSE AT ITS GRANDER SCALE IS A MAELSTROM OF GALAXIES.

SCIENTISTS ESTIMATE THAT THERE MAY BE MORE THAN 200 BILLION IN NUMBER.

THAT'S PRACTICALLY A GALAXY IN THE UNIVERSE FOR EVERY STAR IN OUR GALAXY.

EVEN IF EACH OF THESE GALAXIES IS HOME TO JUST ONE TECHNOLOGICALLY ADVANCED SPECIES LIKE US, THEN THERE WILL BE BILLIONS OF POSSIBILITIES FOR EXTRATERRESTRIAL CIVILIZATIONS.

AND THE UNIVERSE IS ABOUNDING WITH INTELLIGENT LIFE.

THE FACT WE CAN ALL UNDERSTAND THIS IS A REMARKABLE FEAT OF THE HUMAN MIND.

SO NOW I HOPE YOU ARE BEGINNING TO REALIZE THAT WITH A LITTLE BIT OF THINKING, YOU'VE HAD THE GENIUS TO FIGURE OUT THAT WE CANNOT BE ALONE.