NSCI 314 LIFE IN THE COSMOS 18 INTERSTELLAR

NSCI 314 LIFE IN THE COSMOS 18  INTERSTELLAR

NSCI 314 LIFE IN THE COSMOS 18 INTERSTELLAR SPACE TRAVEL (CONTINUED), UNIDENTIFIED FLYING OBJECTS (UFOs), AND THE FERMI PARADOX Dr. Karen Kolehmainen Department of Physics, CSUSB http://physics.csusb.edu/~karen/ 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 IN THE

DISTANT FUTURE (MATTER-ANTIMATTER ANNIHILATION): - WE CANT GO FASTER THAN THE SPEED OF LIGHT (c) - IT WOULD TAKE YEARS TO REACH EVEN THE STAR BEYOND THE SUN NEAREST - FUEL REQUIREMENTS ARE ENORMOUS -INTERSTELLAR GAS, DUST, AND PHOTONS WOULD BE HAZARDOUS INTERSTELLAR HAZARDS 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!

(THE HIROSHIMA BOMB WAS 12 KILOTONS AND THE NAGASAKI BOMB WAS 22 KILOTONS.) COLLISIONS WITH INTERSTELLAR GAS AND DUST WOULD BE SUFFICIENT TO HEAT A SPACECRAFT TO DANGEROUSLY HIGH TEMPERATURES AND PUNCH HOLES IN IT. COLLISIONS WITH HIGH ENERGY PHOTONS WOULD DAMAGE OCCUPANTS AND ELECTRONICS. SHIELD TECHNOLOGY IS NOT GOOD ENOUGH TO PROTECT AGAINST THIS. 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

FUEL REQUIREMENTS FOR 10 LY ROUND TRIP DESCRIBED LAST TIME: EVEN WITH PROPULSION VIA MATTER-ANTIMATTER ANNIHILATION (MOST EFFICIENT METHOD), WE NEED 400 POUNDS OF FUEL FOR 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! WE'D NEED 1000 TIMES MORE FUEL FOR NUCLEAR PROPULSION, AND 1,000,000 TIMES MORE FUEL FOR CHEMICAL ROCKETS PASSIVE PROPULSION METHODS

ADVANTAGE: NO NEED TO CARRY FUEL LIGHT SAIL OR SOLAR SAIL LARGE SAIL PROPELS SHIP VIA PRESSURE EXERTED BY PHOTONS FROM STAR SAIL MUST BE VERY LIGHT, THUS FRAGILE ACCELERATION IS VERY SMALL WORKS BEST (LARGEST ACCELERATION) NEAR A STAR COULD GET TO NEARBY STARS WITHIN A CENTURY PASSIVE PROPULSION METHODS

VARIATION ON LIGHT SAIL USE PHOTON PRESSURE OF LASER BEAM FROM EARTH (OR SPACE STATION) INSTEAD OF LIGHT FROM A STAR COULD REACH NEARBY STARS IN A FEW DECADES ACCELERATING A SHIP TO HALF THE SPEED OF LIGHT WOULD TAKE 1000 x AS MUCH POWER AS CURRENT TOTAL HUMAN POWER CONSUMPTION SHIP WOULD BE DEPENDENT ON LASER BEAM FROM EARTH HOW WOULD THE SHIP SLOW DOWN WHEN APPROACHING ITS DESTINATION? WOULD NEED A LASER BEAM FROM DESTINATION PUSHING BACK ON SHIP POSSIBLE ANSWER: LASER IS USED TO HEAT PROPELLANT ON BOARD, PROPELLANT IS THEN

EJECTED OUT IN FRONT OF THE SHIP 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 WAY BEYOND CURRENT TECHNOLOGY

INTERSTELLAR SPACE TRAVEL PROCEDURE: - ACCELERATE AT 1 g UNTIL WERE HALFWAY TO OUR DESTINATION AND TRAVELING AT ALMOST c - THEN DECELERATE THE REST OF THE WAY THERE - THEN REPEAT THE ENTIRE PROCESS ON THE WAY BACK THE FARTHER THE DESTINATION, 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) - FUEL REQUIREMENTS ARE EVEN LARGER BECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY - PHOTONS AND INTERSTELLAR DUST BECOMES HAZARDOUS BECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN

SPEED AND KINETIC ENERGY OTHER APPROACHES ACCEPT THAT INTERSTELLAR TRAVEL WILL BE SLOW. ADVANTAGES: DON'T NEED HUGE ENERGY REQUIREMENTS TO REACH HIGH SPEED INTERSTELLAR HAZARDS NOT EXACERBATED BY HIGH SPEED HOW DO WE DEAL WITH THE LONG TIMES NEEDED FOR VOYAGES? PUT CREW INTO HIBERNATION WE DON'T YET KNOW HOW TO DO THIS

HAVE GENERATIONS THAT ARE BORN AND DIE ON BOARD SHIP EN ROUTE TO DESTINATION CREW MAY FIGHT AMONG THEMSELVES, LOSE INTEREST IN MISSION, OR LOSE TECHNICAL EXPERTISE TO COMPLETE MISSION. ALSO, IS IT MORALLY CORRECT TO REQUIRE PEOPLE TO SERVE IN THIS WAY IF THEY DIDN'T CHOOSE TO DO SO? 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 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. 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. UNIDENTIFIED FLYING OBJECTS Thousands are reported annually, in all parts of the world. The vast majority of scientists are not convinced that these objects are alien spacecraft. WHY? No physical evidence of an alien visit has even

been produced. Photographs of alleged alien spaceships are either faked or else too vague (small, blurry, etc.) to be sure what the object is Sightings of alleged alien spacecraft depend on eyewitness accounts, which are unverifiable. UNIDENTIFIED FLYING OBJECTS If UFOs aren't alien spacecraft, how can so many people who claim to see them be wrong? (Many of these people are honest, sane, and intelligent.) 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 and other natural phenomena, and may therefore misidentify MOST UFO REPORTS INVOLVE: LIGHTS SEEN IN THE SKY AT NIGHT, OR DISKS (FLYING SAUCERS) USUALLY SEEN IN DAYTIME OBJECTS OFTEN MISIDENTIFIED AS ALIEN SPACECRAFT

NOCTURNAL LIGHTS PLANETS BRIGHT STARS METEORS AIRPLANES SATELLITES DISKS

CLOUDS BLIMPS WEATHER BALLOONS UNIDENTIFIED FLYING OBJECTS Certain facts 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 OBJECTS 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.

Many reports contain Inconsistencies (which are usually ignored by UFO enthusiasts). Failure to find a normal explanation is not evidence for alien visits. UNIDENTIFIED FLYING OBJECTS The high number of UFO sightings actually 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. UFOS UNIDENTIFIED 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! ROSWELL INCIDENT

A RANCHER FOUND DEBRIS FROM A CRASHED OBJECT IN 1947 NEAR ROSWELL, NEW MEXICO. MILITARY PERSONNEL CAME AND TOOK AWAY THE DEBRIS, AND TOLD REPORTERS THAT IT WAS FROM A FLYING DISK. THE NEXT DAY, A MILITARY OFFICIAL SAID THAT THE DEBRIS WAS FROM A

CRASHED WEATHER BALLOON. NOTHING MORE HAPPENED FOR MORE THAN 30 YEARS. ROSWELL INCIDENT IN 1978, A UFO ENTHUSIAST CLAIMED THAT THE DEBRIS WAS FROM AN ALIEN SPACECRAFT, THAT ALIEN CORPSES WERE RECOVERED AS WELL, AND THAT THE GOVERNMENT WAS ENGAGED IN A

COVER-UP. HIS CLAIMS WERE PICKED UP BY OTHERS AND PUBLICIZED. EYEWITNESSES WHO WERE THEN INTERVIEWED CONTRADICTED EACH OTHER. NO PHYSICAL EVIDENCE HAS EVER BEEN PRODUCED. ROSWELL INCIDENT DE-CLASSIFIED GOVERNMENT DOCUMENTS SAY: PROJECT MOGUL INVOLVED PUTTING BALLOONS NEAR THE BORDER OF THE SOVIET UNION TO LISTEN FOR SECRET

TESTS OF NUCLEAR WEAPONS. PROJECT MOGUL WAS BEING TESTED AT ROSWELL IN THE LATE 1940'S. THE CHIEF SCIENTIST FOR PROJECT MOGUL HAS VERIFIED THAT THE RECOVERED DEBRIS WAS FROM ONE OF THE PROJECT BALLOONS. PHOTOS OF THE ROSWELL DEBRIS LOOK LIKE THE PROJECT MOGUL BALLOONS. UNIDENTIFIED FLYING OBJECTS 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? GOVERNMENT CONSPIRACY? IS THE GOVERNMENT COMPETENT ENOUGH TO MASTERMIND SUCH AN EFFORT? LOTS OF SCIENTISTS AND OTHERS WOULD HAVE TO BE IN ON THE SECRET. IS IT

BELIEVABLE THAT THERE HAS NEVER BEEN A LEAK? UNLESS THE ALIENS HAD CONTACTED ONLY THE U.S. GOVERNMENT, MANY GOVERNMENTS WORLDWIDE WOULD HAVE TO BE IN ON THE SECRET. THIS SEEMS UNLIKELY, GIVEN THEIR NUMEROUS DISAGREEMENTS ON OTHER GOVERNMENT CONSPIRACY? EVEN IF A COVER-UP EXISTS, THAT DOESNT NECESSARILY MEAN THAT

ALIENS ARE INVOLVED. THE GOVERNMENT MAY BE TRYING TO KEEP MILITARY PROJECTS SECRET (E.G., ROSWELL) GOVERNMENT OFFICIALS MAY BE MERELY COVERING UP FOR INCOMPETENCE OR LAZINESS ALIEN INTERVENTION IN HUMAN HISTORY? Some people have suggested that aliens have constructed artifacts on the earth or guided the development of human civilization. Egyptian pyramids

Mayan pyramids Stonehenge and similar structures Nazca lines ALIEN INTERVENTION IN HUMAN HISTORY? Such claims are an insult to human ingenuity and creativity! Ancient humans are capable of impressive accomplishments without help. Accomplishments such as the pyramids were possible with the human

technology of the time. Archaeologists have provided explanations of the details. UFO SUMMARY THE VAST MAJORITY OF SCIENTISTS DO NOT BELIEVE THAT UFOs ARE ALIEN SPACECRAFT. HOWEVER, WE CAN'T PROVE THAT ALIEN SPACECRAFT NEVER HAVE VISITED THE EARTH. IF THEY HAVE, MOST LIKELY THE VISITS HAVE

BEEN INFREQUENT (GIVEN THE DIFFICULTY OF INTERSTELLAR SPACE TRAVEL AND THE NUMBER OF PLANETS TO EXPLORE) DONE IN SECRECY (IF ANY HAVE OCCURRED SINCE HUMANS HAVE BEEN AROUND) 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 LADDER ARE ALIEN TECHNOLOGIES LIKELY TO BE MORE OR LESS ADVANCED TECHNOLOGICALLY 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 A TECHNOLOGICAL CIVILIZATION 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 WONT LAST MUCH

LONGER! 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). INTERSTELLAR SPACE TRAVEL IS PROBABLY INEVITABLE FOR A SUFFICIENTLY LONG-LIVED AND ADVANCED CIVILIZATION. GALACTIC COLONIZATION AT v = c, COULD CROSS THE GALAXY IN 100,000 YEARS v = 0.1c v = 0.001c 1 MILLION YEARS 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 ARENT INTERESTED IN COLONIZATION OR CONQUEST. IF HUMANS ARE TYPICAL, THIS ISNT TRUE. WERE 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 DONT WANT TO COLONIZE, WOULDNT THEY STILL WANT TO EXPLORE? CURIOSITY SEEMS TO GO ALONG WITH INTELLIGENCE. GALACTIC COLONIZATION BUT EVEN IF THEY DONT WANT TO COLONIZE, WOULDNT 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. THEYLL 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 THEIR 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 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. 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. 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. 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 TIMECONSUMING. 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

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