NSCI 314

LIFE IN THE COSMOS18 – INTERSTELLAR SPACE TRAVEL

(CONCLUSION), UFOs, AND

THE FERMI PARADOX

Dr. Karen Kolehmainen Department of Physics, CSUSB

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

METHODS OF PROPULSION(IN ORDER OF INCREASING EFFICIENCY)

CHEMICAL ROCKETS

ION ROCKETS

NUCLEAR-POWERED SPACECRAFT

MATTER-ANTIMATTER ANNIHILATION

NOTE: ALL OF THESE ARE “ACTIVE” PROPULSION METHODS, i.e., THE SPACECRAFT CARRIES ITS FUEL WITH IT

PAYLOAD MASS RATIOFOR 5 LY TRIP DESCRIBED EARLIER

(10 LY ROUND TRIP):

EVEN WITH PROPULSION VIA MATTER-ANTIMATTER ANNIHILATION (MOST EFFICIENT METHOD),

WE NEED 400 POUNDS OF FUELFOR EACH POUND OF PAYLOAD.

THIS IS 30 TONS OF FUEL PER HUMAN(ENVIRONMENT NOT INCLUDED).

THE PROBLEM IS EVEN WORSE FOR LESS EFFICIENT METHODS OF PROPULSION!

PASSIVE PROPULSION METHODS ADVANTAGE: NO NEED TO CARRY FUEL LIGHT SAIL

– LARGE SAIL PROPELS SHIP VIA PRESSURE EXERTED BY PHOTONS FROM STAR

– VERY SLOW– SAIL MUST BE VERY LIGHT, THUS FRAGILE– ONLY WORKS WELL NEAR A STAR

VARIATION ON THIS– USE PHOTON PRESSURE OF LASER BEAM FROM

EARTH (OR SPACE STATION)– SHIP WOULD BE DEPENDENT ON LASER BEAM FROM

EARTH– PROBLEM: HOW WOULD THE SHIP SLOW DOWN WHEN

APPROACHING ITS DESTINATION?

PASSIVE PROPULSION METHODS BUSSARD RAMJET

– COLLECT INTERSTELLAR GAS (HYDROGEN) FOR USE AS FUEL

– HYDROGEN IS FUEL FOR NUCLEAR FUSION– WOULD HAVE TO COLLECT FUEL FROM A HUGE

REGION OF SPACE– NEED HUGE “SCOOP” (SIZE OF A PLANET), OR USE

MAGNETIC FIELDS TO COLLECT HYDROGEN (MORE LIKELY)

– MUST HAVE ENOUGH ENERGY TO MAINTAIN MAGNETIC FIELDS

WARP DRIVE?? SCIENCE FICTION “TRICK” FOR GOING FASTER

THAN SPEED OF LIGHT (c). THE IDEA IS TO BEND OR WARP SPACE SO AS TO

MAKE A “SHORT CUT” BETWEEN DISTANT OBJECTS.

WE KNOW THAT MASSIVE OBJECTS BEND SPACE AROUND THEM:– STARS DO THIS A LITTLE– BLACK HOLES DO IT A LOT (THEY WARP

SPACE SUFFICIENTLY TO “PINCH OFF” THE INTERIOR OF THE BLACK HOLE FROM THE REST OF THE UNIVERSE)

WARP DRIVE?? HOWEVER, WE CAN’T VERY WELL CARRY ALONG A BLACK

HOLE WITH OUR SPACESHIP.– THE NEAREST BLACK HOLES ARE HUNDREDS OF LIGHT YEARS AWAY. – THE BLACK HOLE WOULD BE MUCH MORE MASSIVE THAN THE SHIP

ITSELF. – WE COULDN’T SURVIVE NEAR A BLACK HOLE. (TIDAL FORCES WOULD

RIP US APART.) WORMHOLE: HYPOTHETICAL CONNECTION BETWEEN

BLACK HOLES (“SHORT-CUT” TO PATH THROUGH NORMAL SPACE)– IF THESE EXIST, PERHAPS WE COULD USE EXISTING ONES.– GETTING TO AND FROM THEM WOULD STILL BE A PROBLEM (NEAREST

ONES HUNDREDS OF LIGHT YEARS AWAY).– TIDAL FORCES WOULD STILL BE A PROBLEM.

WARP DRIVE?? IF WARP DRIVE IS POSSIBLE, IT WILL

DEPEND ON BASIC LAWS OF PHYSICS THAT HAVEN'T BEEN DISCOVERED YET!–IT'S NOT JUST A MATTER OF

IMPROVING TECHNOLOGY.–IT'S NOT ANALOGOUS TO

“BREAKING THE SOUND BARRIER.”

INTERSTELLAR HAZARDSBECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY,

A 1 gm MASS MOVING AT 90% c HAS AN ENERGY EQUAL TO THAT OF A 30 KILOTON BOMB!

(COMPARISON: HIROSHIMA BOMB WAS 12 KILOTONS AND NAGASAKI BOMB WAS 22 KILOTONS.)

COLLISIONS WITH INTERSTELLAR GAS AND DUST WOULD BE SUFFICIENT TO HEAT A FAST-MOVING SPACECRAFT TO DANGEROUSLY HIGH TEMPERATURES AND PUNCH HOLES IN IT.

COLLISIONS WITH HIGH ENERGY PHOTONS WOULD DAMAGE OCCUPANTS AND ELECTRONICS.

“SHIELD” TECHNOLOGY IS NOWHERE GOOD ENOUGH TO PROTECT AGAINST THIS.

THE BASIC LAWS OF PHYSICS DON’T MAKE IT IMPOSSIBLE, BUT THEY MAKE IT VERY DIFFICULT.

CURRENT TECHNOLOGY IS NOWHERE NEAR ADEQUATE.

EVEN WITH THE BEST TECHNOLOGY WE CAN IMAGINE BEING ABLE TO BUILD (MATTER-ANTIMATTER ANNIHILATION):

- WE CAN’T GO FASTER THAN THE SPEED OF LIGHT (c).

- IT WOULD TAKE YEARS TO REACH EVEN THE NEAREST STAR BEYOND THE SUN.

- FUEL REQUIREMENTS ARE ENORMOUS.

INTERSTELLAR SPACE TRAVEL

INTERSTELLAR SPACE TRAVELPROCEDURE:

- ACCELERATE AT 1 g UNTIL WE’RE HALFWAY TO OUR DESTINATION AND TRAVELING AT ALMOST THE SPEED OF LIGHT

- THEN DECELERATE THE REST OF THE WAY THERE - THEN REPEAT THE ENTIRE PROCESS ON THE WAY BACK

THE FARTHER THE DESTINATION STAR, THE CLOSER TO c OUR MAXIMUM SPEED GETS, AND THE LARGER THE EFFECTS OF RELATIVITY BECOME.

- THE ELAPSED TIME ON THE SHIP IS LESS THAN THE ELAPSED TIME ON THE EARTH (THE TWIN PARADOX).

- HUGE FUEL REQUIREMENTS BECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY.

- PHOTONS AND INTERSTELLAR DUST PARTICLES BECOME HAZARDOUS BECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY.

ANOTHER WAY? WHAT IF WE TRAVEL AT SPEEDS MUCH

LOWER THAN THE SPEED OF LIGHT?– ADVANTAGE: FUEL REQUIREMENTS ARE LESS

IT WILL TAKE MANY YEARS TO COMPLETE THE TRIP, PROBABLY LONGER THAN A HUMAN LIFETIME.– GENERATIONS OF PEOPLE WILL BE BORN AND

LIVE THEIR ENTIRE LIVES ON BOARD THE SHIP.– WE CAN'T BE SURE PEOPLE BORN ABOARD THE

SHIP WILL BE INTERESTED IN CARRYING OUT THE MISSION. IS IT FAIR TO EXPECT THEM TO DO SO, GIVEN THAT THEY DIDN'T MAKE THE CHOICE TO DO SO?

MOST UFO REPORTS INVOLVE: LIGHTS SEEN IN THE SKY AT NIGHT, OR

DISKS (“FLYING SAUCERS”) USUALLY SEEN IN DAYTIME, OR

UNIDENTIFIED RADAR SIGHTINGS

OBJECTS OFTEN MISIDENTIFIED AS ALIEN SPACECRAFT

NOCTURNAL LIGHTS– PLANETS OR BRIGHT STARS– METEORS– AIRPLANES– SATELLITES

DISKS– CLOUDS– BLIMPS– WEATHER BALLOONS

RADAR SIGHTINGS– FLOCKS OF BIRDS– DENSITY FLUCTUATIONS IN THE AIR– FALSE RADAR ECHOES ARE COMMON

UNIDENTIFIED FLYING OBJECTSThousands are reported annually, in all parts

of the world.Why are so many UFOs seen?

• Most people spend most of their time indoors.

• Air and light pollution prevent regular clear views of the sky.

• People are increasingly unfamiliar with astronomical phenomena, and may therefore misidentify objects.

UNIDENTIFIED FLYING OBJECTSCertain “facts” given in UFO reports are almost certainly erroneous.

• Witnesses often claim large sizes and high speeds. However, to estimate the size and speed of an object, its distance must be known. It is very difficult to estimate the distance to an object seen in the sky.

• Witnesses often report hearing a sound at the same moment that they see something happen (e.g., a sound from the engines at the same time there is a change in the object's motion). However, sound travels much more slowly than light. Thus if a distant object makes a noise at the same time it visibly does something, you'll see the action before you'll hear the sound. (example: lightning and thunder)

UNIDENTIFIED FLYING OBJECTS

• VERY STRANGE CASES (SUCH AS ALLEGED ABDUCTIONS) ARE NEVER WELL DOCUMENTED (E.G., ONE WITNESS OF QUESTIONABLE RELIABILITY).

• WELL DOCUMENTED CASES (THOSE WITH NUMEROUS RELIABLE WITNESSES) ARE NEVER REALLY STRANGE (E.G., JUST AN UNEXPLAINED LIGHT IN THE SKY).

PSYCHOLOGICAL FACTORS• Whenever a case is publicized, there is an

increase in reports of similar cases soon afterwards. The more well-publicized the case is, the larger the number of similar subsequent reports.

• Descriptions of aliens do not reflect the expected biological and cultural diversity of life elsewhere, but they do reflect psychological biases of witnesses.• Descriptions of aliens are almost always humanoid (human-like in appearance).• Descriptions of aliens are usually male. • Descriptions of aliens often reflect biases associated with the culture and time period of the witness.

PSYCHOLOGICAL FACTORS• Eyewitness accounts are notoriously

unreliable (especially years after the fact). This has been established by psychologists who study witnesses in legal cases.

• Humans seem to have a psychological need to believe in superior beings. (Some might argue that this leads to the existence of religion.)

UNIDENTIFIED FLYING OBJECTSSome sightings of UFOs are difficult to explain.

• Sightings must be explained “after the fact” without complete information about what happened.

• Eyewitness reports are known to be unreliable.

• Inconsistencies are often ignored.

• Failure to find a “normal” explanation is not evidence for alien visits.

UNIDENTIFIED FLYING OBJECTSThe high number of UFO sightings argues against

their being alien spacecraft.Remember the enormity of interstellar distances,

the extreme difficulties of interstellar space travel, and the number of planets to explore.

Why would there be so many alien spacecraft visiting the earth constantly? There are a lot of other planets to check out. What makes us so interesting? We should not overestimate our significance.

If every planet were visited as often as some people claim the earth is visited, there would have to be a huge number of spacecraft exploring the universe.

UFOSUNIDENTIFIED FLYING OBJECTS

Ideally, large numbers of experienced observers would watch the skies on a nightly basis and report true UFOs.

This is being done:

• Amateur astronomers• Automated asteroid searches• Automated nova and supernova searches

A UFO has never been seen in such circumstances!

UNIDENTIFIED FLYING OBJECTSIf UFOs are alien spacecraft, what would be the

motives of the aliens on board?

• If aliens wanted to observe us without being detected, we would not be able to detect them. Surely a civilization that is capable of interstellar space travel has the technology to avoid being detected by us.

• If aliens aren’t trying to keep their existence secret, why not meet us directly and openly? Why wouldn't they contact official bodies such as governments, or groups of scientists, or at least large groups of people all at once?

GOVERNMENT CONSPIRACY? MANY GOVERNMENTS WORLDWIDE

WOULD HAVE TO BE IN ON IT.– THIS SEEMS UNLIKELY GIVEN THEIR

NUMEROUS DISAGREEMENTS ON OTHER MATTERS.

EVEN IF A “COVER-UP” EXISTS, THAT DOESN’T NECESSARILY MEAN THAT ALIENS ARE INVOLVED.– MAY BE TRYING TO KEEP MILITARY

PROJECTS SECRET (E.G., ROSWELL)– MAY BE MERELY COVERING UP FOR

IMCOMPETENCE OR LAZINESS

ALIEN INTERVENTION IN HUMAN HISTORY?

Some people have suggested that aliens guided the development of human civilization (e.g., showed us how to build the pyramids).

Such claims are an insult to human ingenuity and creativity! Humans are capable of impressive accomplishments without help.

Accomplishments such as the pyramids were possible with the technology of the time. Explanations of the details exist.

SUMMARY THE VAST MAJORITY OF

SCIENTISTS DO NOT BELIEVE THAT UFOs ARE ALIEN SPACECRAFT.

WE CAN'T PROVE THAT ALIEN SPACECRAFT NEVER HAVE VISITED THE EARTH.

BUT IF THEY HAVE, MOST LIKELY THE VISITS HAVE BEEN–INFREQUENT–DONE IN SECRECY

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 BYA MW 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.

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 YEARSTHEREFORE, AN ADVANCED TECHNOLOGICAL

CIVILIZATION SHOULD BE ABLE TO EXPLORE OR COLONIZE THE GALAXY IN A SHORT AMOUNT OF TIME (COMPARED TO THE AGE OF THE GALAXY).

COLONIZATION: HOME PLANET ESTABLISHES COLONIES ON A FEW NEARBY SUITABLE PLANETS. 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 CURIOUS AND AGGRESSIVE.

IN ORDER TO DEVELOP A SOCIETY THAT IS CAPABLE OF 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?

WHEN THEIR STAR DIES, THEY’LL PROBABLY GO EXTINCT UNLESS THEY CAN MOVE TO ANOTHER SOLAR SYSTEM.

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 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 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 (E.G., “PRIME DIRECTIVE” ON STAR TREK TV SERIES).

- 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.