NSCI 314

LIFE IN THE COSMOS

19 - THE FERMI PARADOX

Dr. Karen Kolehmainen Department of Physics, CSUSB

http://physics.csusb.edu/~karen/

THE FERMI PARADOX THE DRAKE EQUATION LEADS US TO BELIEVE

THAT THERE IS LIKELY TO BE A LARGE NUMBER OF TECHNOLOGICALLY ADVANCED CIVILIZATIONS IN OUR GALAXY (AND EVEN MORE IN THE UNIVERSE AS A WHOLE).

BUT WE HAVE RECEIVED NO SIGNALS FROM

THEM NOR HAD ANY KNOWN VISITS BY THEM.

THEREFORE “WHERE ARE THEY?” (ASKED BY ENRICO FERMI, 1950)

CHRONOLOGY OF THE UNIVERSE 14 BYA BIG BANG

12 - 13 BYAMW AND OTHER GALAXIES FORM

4.6 BYA FORMATION OF EARTH

4.0 BYA LIFE BEGINS ON EARTH

600 MYA MULTICELLULAR LIFE

FEW MYA FIRST HOMINIDS

OTHER CIVILIZATIONS COULD HAVE A HEAD START OF SEVERAL BILLION YEARS ON US!

COMPRESS 14 BILLION YEAR HISTORY OF UNIVERSE INTO ONE YEAR:

JANUARY 1 – BIG BANG EARLY FEBRUARY – MILKY WAY AND OTHER

GALAXIES FORM MID-AUGUST – OUR SOLAR SYSTEM FORMS EARLY SEPTEMBER – LIFE STARTS ON EARTH MID-DECEMBER – CAMBRIAN EXPLOSION DEC 31 EVENING – FIRST HOMINIDS DEC 31 HALF MINUTE BEFORE MIDNIGHT –

HUMAN CIVILIZATION BEGINS

TECHNOLOGICAL LADDERARE ALIEN TECHNOLOGIES LIKELY TO BE MORE OR LESS ADVANCED THAN WE ARE?

1. DEFINE “TECHNOLOGICAL” AS HAVING THE ABILITY TO COMMUNICATE OVER INTERSTELLAR DISTANCES (AS IN OUR DISCUSSION OF THE DRAKE EQUATION).

2. WE HAVE BEEN TECHNOLOGICAL FOR ABOUT 50 YEARS.

3. ESTIMATE L (AS IN THE DRAKE EQUATION) – THE AVERAGE LIFETIME OF A TECHNOLOGICAL CIVILIZATION.

4. THE “AVERAGE” CIVILIZATION WILL BE APPROXIMATELY HALFWAY THROUGH THE LIFETIME L, WITH APPROXIMATELY HALF OF ALL CIVILIZATIONS MORE ADVANCED AND APPROXIMATELY HALF OF ALL CIVILIZATIONS LESS ADVANCED.

5. COMPARE THE TWO TIMES: OUR AGE AS A TECHNOLOGICAL CIVILIZATION (50 YEARS) AND THE AVERAGE AGE OF A CIVILIZATION(1/2 OF L).

TECHNOLOGICAL LADDER

RESULT:

- IF L IS MORE THAN A FEW HUNDRED YEARS, THEN MOST CIVILIZATIONS WILL BE MORE TECHNOLOGICALLY ADVANCED THAN WE ARE, PERHAPS BY A LARGE AMOUNT

- THE ONLY WAY OUR LEVEL OF TECHNOLOGY COULD BE AVERAGE IS FOR L TO BE ONLY 100 YEARS OR SO! IF L IS THIS SMALL, OUR CIVILIZATION PROBABLY WON’T LAST MUCH LONGER!

THE FERMI PARADOXREASONING AND ASSUMPTIONS LEADING TO

THE QUESTION:

THE DRAKE EQUATION LEADS US TO BELIEVE THAT THERE IS PROBABLY A LARGE NUMBER OF TECHNOLOGICAL CIVILIZATIONS IN OUR GALAXY.

UNLESS CIVILIZATIONS ARE VERY SHORT-LIVED, WE EXPECT MOST OF THESE CIVILIZATIONS TO BE MORE TECHNOLOGICALLY ADVANCED THAN WE ARE (PERHAPS BY MILLIONS OR BILLIONS OF YEARS).

SOME FORM OF INTERSTELLAR SPACE TRAVEL IS PROBABLY INEVITABLE FOR A SUFFICIENTLY LONG-LIVED AND ADVANCED CIVILIZATION.

GALACTIC COLONIZATIONAT v = c, COULD CROSS THE GALAXY IN 100,000 YEARS

v = 0.1c 1 MILLION YEARS v = 0.001c 100 MILLION YEARS

THEREFORE, AN ADVANCED TECHNOLOGICAL CIVILIZATION SHOULD BE ABLE TO COLONIZE THE GALAXY IN A SHORT AMOUNT OF TIME COMPARED TO THE AGE OF THE GALAXY (10 BILLION YEARS).

COLONIZATION: HOME PLANET ESTABLISHES COLONIES ON SUITABLE PLANETS IN NEARBY SOLAR SYSTEMS. AFTER A FEW HUNDRED TO A FEW THOUSAND YEARS TO “GET ON THEIR FEET,” THESE COLONIES ESTABLISH MORE COLONIES. SUCH A STRATEGY WOULD LEAD TO COLONIZATION OF THE ENTIRE GALAXY WITHIN A PERIOD OF A FEW MILLION YEARS TO A FEW HUNDRED MILLION YEARS.

GALACTIC COLONIZATION MAYBE ALIEN CIVILIZATIONS AREN’T INTERESTED IN

COLONIZATION AND CONQUEST. IF HUMANS ARE TYPICAL, THIS ISN’T TRUE. WE’RE PRETTY

EXPANSIONIST. BIOLOGICAL EVOLUTION MAY NATURALLY PRODUCE

CREATURES THAT ARE SOMEWHAT AGGRESSIVE (ALTHOUGH THERE ALSO SEEM TO BE SOME EVOLUTIONARY ADVANTAGES TO BEING COOPERATIVE).

MAYBE SPECIES THAT ARE OVERLY AGGRESSIVE TEND TO DESTROY THEMSELVES VIA INTERNAL WARS BEFORE DEVELOPING INTERSTELLAR SPACE TRAVEL. IN ORDER TO DEVELOP A SOCIETY THAT LIVES LONG ENOUGH TO DEVELOP INTERSTELLAR SPACE TRAVEL, MAYBE A SPECIES NEEDS TO LEARN TO CURB ITS AGGRESSIVE TENDENCIES.

BUT EVEN IF THEY DON’T WANT TO COLONIZE, WOULDN’T THEY STILL WANT TO EXPLORE? CURIOSITY SEEMS TO GO ALONG WITH INTELLIGENCE.

GALACTIC COLONIZATION BUT EVEN IF THEY DON’T WANT TO COLONIZE,

WOULDN’T THEY STILL WANT TO EXPLORE?– MAYBE THEY WOULD AVOID COLONIZING PLANETS THAT

ALREADY HAVE LIFE, IN ORDER NOT TO CONTAMINATE IT OR INTERFERE WITH IT.

– THEY COULD STILL SET UP OUTPOSTS ON LESS SUITABLE PLANETS THAT COULD THEN (PERHAPS WITH A LONGER TIME BETWEEN EXPANSIONS) SET UP ADDITIONAL OUTPOSTS.

THEY’LL EVENTUALLY GO EXTINCT UNLESS THEY CAN MOVE TO ANOTHER SOLAR SYSTEM.– AS THEIR HOME STAR BRIGHTENS WITH AGE, ITS

HABITABLE ZONE WILL MOVE OUTWARD, AND THEIR PLANET WILL BECOME TOO HOT.

– EVENTUALLY THEIR HOME STAR WILL EVOLVE INTO A RED GIANT, MOST LIKELY VAPORIZING THEIT HOME PLANET.

– THERE MAY NOT BE ANOTHER SUITABLE PLANET IN THEIR HOME SOLAR SYSTEM. (EVEN IF THERE IS, IT'S ONLY A TEMPORARY SOLUTION.)

SO, “WHERE ARE THEY?”

WE DON'T KNOW, BUT LET'S EXAMINE SOME POSSIBLE ANSWERS.

SOME OF THESE ANSWERS ARE UNLIKELY, WHEREAS OTHERS ARE PLAUSIBLE.

RESPONSES TO “WHERE ARE THEY?”

THERE IS NO EXTRATERRESTRIAL LIFE AT ALL.

– THIS SEEMS UNLIKELY.

– WE THINK SUITABLE PLANETS ARE RELATIVELY COMMON, AND THAT LIFE ARISES EASILY WHEN THE CONDITIONS ARE APPROPRIATE. IT DID SO QUICKLY ON THE EARTH.

RESPONSES TO “WHERE ARE THEY?”

ALTHOUGH THERE MAY BE EXTRATERRESTRIAL LIFE, THERE ARE NO ALIEN CIVILIZATIONS. THE FACTORS ALLOWING COMPLEX, INTELLIGENT, OR TECHNOLOGICALLY-DEVELOPED LIFE FORMS TO EVOLVE ARE UNIQUE TO EARTH.

– THIS COULD BE TRUE. SUPPORTING ARGUMENTS:• COMPLEX AND INTELLIGENT LIFE TOOK A LONG TIME

TO EVOLVE ON EARTH.

• SPECIAL FEATURES OF EARTH (PRESENCE OF A LARGE MOON, SPECIFIC CLIMATE CHANGES, ETC.) MAY HAVE BEEN CRUCIAL IN STIMULATING THE EVOLUTION OF COMPLEX AND/OR INTELLIGENT LIFE FORMS ON EARTH.

RESPONSES TO “WHERE ARE THEY?”ALIEN CIVILIZATIONS HAVE DEVELOPED, BUT THEY

TEND TO DESTROY THEMSELVES (OR ARE DESTROYED BY NATURAL DISASTERS) BEFORE DEVELOPING INTERSTELLAR SPACE TRAVEL OR COMMUNICATION.

– EVEN IF L IS VERY SHORT, REMEMBER THAT L IS THE AVERAGE LIFETIME OF A TECHNOLOGICAL CIVILIZATION. SOME CIVILIZATIONS WILL BE SHORTER-LIVED THAN THE AVERAGE, AND SOME WILL BE LONGER-LIVED. IF THERE ARE AS MANY CIVILIZATIONS AS THE DRAKE EQUATION LEADS US TO BELIEVE, THERE SHOULD BE A FEW OF THEM THAT ARE SUFFICIENTLY LONG-LIVED TO EXPLORE THE UNIVERSE AND VISIT THE EARTH.

RESPONSES TO “WHERE ARE THEY?”ALIEN CIVILIZATIONS EXIST, BUT THEY ARE ALL LESS

TECHNOLOGICALLY ADVANCED THAN WE ARE. • MAYBE!

• ON THE ONE HAND, THIS SEEMS UNLIKELY GIVEN THAT THERE ARE OTHER APPARENTLY SUITABLE STARS SEVERAL BILLION YEARS OLDER THAN OUR SUN.

• BUT ON THE OTHER HAND, PERHAPS LIFE ON EARTH DEVELOPED UNUSUALLY QUICKLY.

• OR PERHAPS THE “THRESHOLD” ABUNDANCES OF HEAVY ELEMENTS NEEDED FOR PLANETARY FORMATION AND/OR FOR LIFE AND/OR FOR TECHNOLOGY IS HIGHER THAN WE THINK, AND STARS THAT FORMED TOO MUCH EARLIER THAN OUR SUN DON'T HAVE ENOUGH HEAVY ELEMENTS TO SUPPORT LIFE.

• SEE “REFUGES FOR LIFE IN A HOSTILE UNIVERSE” IN SCIENTIFIC AMERICAN, OCTOBER 2001.

RESPONSES TO “WHERE ARE THEY?”

ALIEN CIVILIZATIONS EXIST, BUT THEY’VE DECIDED THAT INTERSTELLAR TRAVEL IS NOT WORTH THE EXPENDITURE OF TIME AND RESOURCES.

• THIS SEEMS UNLIKELY.

• IF THERE ARE AS MANY CIVILIZATIONS AS WE THINK, IT SEEMS UNLIKELY THAT THEY WOULD ALL DECIDE THIS.

•CURIOSITY IS LIKELY TO BE A COMMON FEATURE OF ALL INTELLIGENT LIFE FORMS.

• SUCH AN ATTITUDE WOULD DOOM THEM TO EXTINCTION WHEN THEIR STAR DIES.

RESPONSES TO “WHERE ARE THEY?”ALIENS ARE ENGAGING IN INTERSTELLAR EXPLORATION, BUT THEY HAVEN’T DISCOVERED US YET.

• THIS SEEMS UNLIKELY.

• RECALL THAT A CIVILIZATION COULD THOROUGHLY EXPLORE OUR MW GALAXY IN A FEW MILLION YEARS TO A FEW HUNDRED MILLION YEARS. IF THERE ARE AS MANY CIVILIZATIONS AS THE DRAKE EQUATION LEADS US TO BELIEVE, THERE SHOULD BE CIVILIZATIONS WITH “HEAD STARTS” OF A FEW BILLION YEARS ON US, SO THEY WOULD HAVE HAD PLENTY OF TIME TO FIND US.

RESPONSES TO “WHERE ARE THEY?”

THEY KNOW WE’RE HERE, BUT THEY DON’T CARE. IF THEY ARE SO MUCH MORE ADVANCED THAN US, WE MAY NOT SEEM PARTICULARLY INTERESTING.

• THIS IS POSSIBLE, ESPECIALLY IF COMPLEX LIFE FORMS ARE VERY COMMON IN THE UNIVERSE. WE MAY NOT SEEM LIKE ANYTHING SPECIAL.

• HOWEVER, IF COMPLEX LIFE FORMS ARE NOT COMMON, WE'RE PROBABLY UNUSUAL ENOUGH TO BE INTERESTING.

RESPONSES TO “WHERE ARE THEY?”THEY HAVE VISITED EARTH, BUT THEIR VISITS ARE VERY INFREQUENT AND THEY HAVEN’T BEEN HERE FOR A LONG TIME.

• THIS IS POSSIBLE.

• THE UNIVERSE IS A BIG PLACE, AND INTERSTELLAR SPACE TRAVEL IS DIFFICULT, COSTLY, AND TIME-CONSUMING.

• MAYBE THEY HAVE IDENTIFIED THE EARTH AS A POTENTIALLY INTERESTING PLANET, AND SO THEY CHECK UP ON IT EVERY FEW MILLIONS OF YEARS. •MAYBE THE LAST TIME THEY WERE HERE, HUMANS HADN'T EVOLVED YET.

RESPONSES TO “WHERE ARE THEY?”

THEY ARE HERE, BASED ON UFO EVIDENCE.

- THIS SEEMS UNLIKELY.

- THE ARGUMENTS THAT UFOs ARE ALIEN SPACESHIPS ARE NOT PARTICULARLY COMPELLING.

- IF THEY WANTED TO CONTACT US, WHY WOULDN'T THEY DO SO MORE DIRECTLY?

RESPONSES TO “WHERE ARE THEY?”

ZOO HYPOTHESIS - THEY ARE HERE, SECRETLY OBSERVING US (OR MAYBE EVEN CONTROLLING US). WE ARE LIKE ANIMALS IN A ZOO.

- MAYBE, ALTHOUGH IT SOUNDS A BIT PARANOID AND EXAGGERATES OUR LIKELY IMPORTANCE.

- WHAT MAKES US SO WORTHY OF OBSERVATION? THIS WOULD MAKE SENSE ONLY IF CIVILIZATIONS ARE RARE.

- WHY BOTHER CONTROLLING US? WHAT DO THEY HAVE TO GAIN BY DOING SO?

RESPONSES TO “WHERE ARE THEY?”

GALACTIC QUARANTINE HYPOTHESIS - THEY ARE HERE, OR HAVE BEEN HERE, BUT THEY HAVE CHOSEN NOT TO MAKE THEIR PRESENCE KNOWN TO US.

- MAYBE THEY'VE DECIDED THAT WE'RE TOO VIOLENT AND DANGEROUS.

- MAYBE THEY DON'T WANT TO INTERFERE WITH DEVELOPING CIVILIZATIONS (E.G., “PRIME DIRECTIVE” ON STAR TREK TV SERIES).

RESPONSES TO “WHERE ARE THEY?” SENTINEL HYPOTHESIS (VARIATION ON GALACTIC

QUARANTINE HYPOTHESIS) - MAYBE WE MUST ACHIEVE A CERTAIN LEVEL OF TECHNOLOGY OR PASS SOME OTHER TYPE OF “TEST” BEFORE THEY ARE WILLING TO MAKE THEIR PRESENCE KNOWN TO US.

- NOTE: THIS IS THE ANSWER THAT I'D LIKE TO BELIEVE!

- HOWEVER, IF THERE ARE AS MANY CIVILIZATIONS AS WE THINK, IT MAY BE UNLIKELY THAT THEY WOULD ALL FEEL THIS WAY.

- ON THE OTHER HAND, MAYBE A SUFFICIENTLY ADVANCED CIVILIZATION WILL INEVITABLY DEVELOP THE WISDOM TO FEEL THIS WAY (OR DESTROY ITSELF FIRST). OR MAYBE THERE IS A GALACTIC CIVILIZATION (OR ONE THAT COVERS THE WHOLE UNIVERSE) THAT ENFORCES THIS BASIC RULE.