METHODS OF PROPULSION IN ORDER OF INCREASING EFFICIENCY

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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

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FOR 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!
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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?

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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

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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)

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WARP DRIVE??

• HOWEVER, WE CANT 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 COULDNT 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.

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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.

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BECAUSE 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.
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INTERSTELLAR SPACE TRAVEL
THE BASIC LAWS OF PHYSICS DONT 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 CANT 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.
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INTERSTELLAR SPACE TRAVEL

• PROCEDURE
• - ACCELERATE AT 1 g UNTIL WERE 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.

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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?

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MOST UFO REPORTS INVOLVE

• LIGHTS SEEN IN THE SKY AT NIGHT, OR
• DISKS (FLYING SAUCERS) USUALLY SEEN IN DAYTIME,
  OR
• UNIDENTIFIED RADAR SIGHTINGS

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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

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• 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.

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• Certain 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)

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• 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).

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• 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.

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• 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.)

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• Some 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.

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The 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.
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• 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!

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• If 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 arent 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?

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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 DOESNT
  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

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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.

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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

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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)

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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!

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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

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ARE 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).
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THE FERMI PARADOX

• REASONING 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.

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GALACTIC COLONIZATION

• AT 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 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.

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GALACTIC COLONIZATION

• MAYBE ALIEN CIVILIZATIONS ARENT INTERESTED IN
  COLONIZATION AND CONQUEST.
• IF HUMANS ARE TYPICAL, THIS ISNT TRUE. WERE
  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 DONT WANT TO COLONIZE, WOULDNT
  THEY STILL WANT TO EXPLORE?
• WHEN THEIR STAR DIES, THEYLL PROBABLY GO EXTINCT
  UNLESS THEY CAN MOVE TO ANOTHER SOLAR SYSTEM.

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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.

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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.

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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.

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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.

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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.

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RESPONSES TO WHERE ARE THEY?

• ALIEN CIVILIZATIONS EXIST, BUT THEYVE 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.

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RESPONSES TO WHERE ARE THEY?

• ALIENS ARE ENGAGING IN INTERSTELLAR EXPLORATION,
  BUT THEY HAVENT 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.

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RESPONSES TO WHERE ARE THEY?

• THEY KNOW WERE HERE, BUT THEY DONT 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.

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RESPONSES TO WHERE ARE THEY?

• THEY HAVE VISITED EARTH, BUT THEIR VISITS ARE
  VERY INFREQUENT AND THEY HAVENT 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.

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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?

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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?

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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.