NSCI 314 LIFE IN THE COSMOS 15 - THE DRAKE EQUATION: HOW COMMON ARE EXTRATERRESTRIAL CIVILIZATIONS THAT ARE SUFFICIENTLY TECHNOLOGICALLY ADVANCED THAT WE MIGHT BE ABLE TO COMMUNICATE WITH THEM? Dr. Karen Kolehmainen Department of Physics,CSUSB

THE DRAKE EQUATION THIS EQUATION IS USED TO ESTIMATE THE NUMBER OF “TECHNOLOGICAL” CIVILIZATIONS IN THE MILKY WAY GALAXY. –WE DEFINE A “TECHNOLOGICAL” CIVILIZATION AS ONE THAT IS CAPABLE OF (AND INTERESTED IN) ENGAGING IN INTERSTELLAR COMMUNICATIONS WITH OTHER CIVILIZATIONS. –NOTE: WE ARE ONLY MAKING THIS ESTIMATE FOR OUR GALAXY, BUT THE NUMBER SHOULD BE SIMILAR FOR ANY SIMILAR SPIRAL GALAXY. THIS IS THE NUMBER OF CIVILIZATIONS THAT COULD BE SENDING OUT RADIO (OR OTHER) SIGNALS THAT WE MIGHT BE ABLE TO RECEIVE.

THE DRAKE EQUATION WHY TRY TO ESTIMATE THE NUMBER OF TECHNOLOGICAL CIVILIZATIONS? –IF THE ESTIMATED NUMBER IS VERY SMALL, SEARCHES FOR SIGNALS FROM ALIEN CIVILIZATIONS MIGHT NOT BE WORTH THE TIME, EFFORT, AND EXPENSE. –IF THE ESTIMATED NUMBER IS LARGE, SEARCHES ARE MORE LIKELY TO BE SUCCESSFUL. THEREFORE IT’S EASIER TO ARGUE THAT THE TIME, MONEY, AND EFFORT ARE WORTH IT. KEEP IN MIND THAT: – WE CAN’T MAKE AN EXACT CALCULATION OF THE NUMBER OF CIVILIZATIONS, ONLY A VERY ROUGH ESTIMATE. –OUR ESTIMATE WILL APPLY ONLY TO LIFE THAT IS SIMILAR TO TERRESTRIAL LIFE. IF EXOTIC LIFE EXISTS, CIVILIZATIONS MAY BE MORE COMMON.

DRAKE EQUATION N = N * f s n p f l f i f c f L N = Number of civilizations in the MW galaxy capable of communication (what we'd like to find) N * = Number of stars in the MW galaxy = approximately 400 billion

SUITABLE STARS N = N * f s n p f l f i f c f L f s = fraction of stars that are suitable stars (result of N * f s is number of suitable stars in MW galaxy) f s = 0.1 = 1/10 (optimistic case) f s = = 1/1000 (pessimistic case) f s = 0.05 = 1/20 (my best estimate)

SUITABLE PLANETS Drake Equation: N = N * f s n p f l f i f c f L n p =average number of planets that are suitable for life per each suitable star n p = 2 (optimistic case) n p = 0.1 = 1/10 (pessimistic case) n p = 0.5 = 1/2 (my best estimate)

DEVELOPMENT OF LIFE Drake Equation: N = N * f s n p f l f i f c f L f l =fraction of suitable planets on which life actually originates f l = 1 (optimistic case - life will always arise if the planet is suitable) f l = = 1/200 (pessimistic case) f l = 1 (my best estimate)

DEVELOPMENT OF INTELLIGENCE Drake Equation: N = N * f s n p f l f i f c f L f i = fraction of planets with life on which intelligent life evolves f i = 1 (optimistic case) f i = = 1/1000 (pessimistic case) f i = 0.01 = 1/100 (my best estimate)

DEVELOPMENT OF TECHNOLOGY Drake Equation: N = N * f s n p f l f i f c f L f c =fraction of planets with intelligent life on which technology sufficient for interstellar communication develops f c =1 (optimistic case) f c = 0.01 = 1/100 (pessimistic case) f c = 0.5 = 1/2 (my best estimate)

DO THEY EXIST NOW? Drake Equation: N = N * f s n p f l f i f c f L f L = Probability that they’re around NOW (as opposed to civilizations that existed in the past, but don’t exist any more) = L/t t = Age of MW galaxy = 10 billion years L = Average lifetime of a technological civilization (measured in years) = Average lifetime of civilization with ability and desire to communicate

LIFETIMES OF CIVILIZATIONS L = Average lifetime of a technological civilization L = 10 billion years (optimistic case) = Age of galaxy L = 100 years (pessimistic case) Civilizations destroy themselves or lose interest in communication! NOTE: L is the least well-known factor in the Drake equation!

DRAKE EQUATION EXTREME OPTIMISTIC CASE (Use optimistic values of all factors except L) N = 400 billion x 0.1 x 2 x 1 x 1 x 1 x L/10 billion RESULT: N = 8 L Now look at different values of L: IF L = 100 YEARS (pessimistic case for L), THEN N = 800 IF L = 10 BILLION YRS (optimistic case for L), THEN N = 80 BILLION

DRAKE EQUATION MY BEST ESTIMATE N = 400 billion x 0.05 x 0.5 x 1 x 0.01 x 0.5 x L/10 billion RESULT: N = L = L/200 Now look at different values of L: IF L = 100 YEARS (pessimistic case for L), THEN N = 0.5 IF L = 10 BILLION YRS (optimistic case for L), THEN N = 50 MILLION

DRAKE EQUATION EXTREME PESSIMISTIC CASE (Use pessimistic values of all factors except L) N = 400 billion x x 0.1 x x x 0.01 x L/10 billion RESULT: N = L = 2 X L Now look at different values of L: IF L = 100 YEARS (pessimistic case for L), THEN N = IF L = 10 BILLION YRS (optimistic case for L) THEN N = 2

DRAKE EQUATION WE KNOW THAT N MUST BE AT LEAST 1 BECAUSE WE EXIST! THEREFORE: IF THE EXTREME PESSIMISTIC CASE IS CORRECT (N = 2 X L), WE WOULD CONCLUDE THAT L > 5 BILLION YEARS. THIS WOULD MEAN THAT CIVILIZATIONS ARE LONG-LIVED!

DRAKE EQUATION WE KNOW THAT N MUST BE AT LEAST 1 BECAUSE WE EXIST! THEREFORE: IF THE EXTREME OPTIMISTIC CASE IS CORRECT (N = 8L), WE CONCLUDE THAT L > 1/8 YEAR. BUT WE ALREADY KNOW THIS! (WE’VE HAD THE RELEVANT TECHNOLOGY FOR ABOUT 50 YEARS SO FAR.)

DRAKE EQUATION WE KNOW THAT N MUST BE AT LEAST 1 BECAUSE WE EXIST! THEREFORE: IF MY BEST ESTIMATE IS CORRECT (N = L), WE CONCLUDE THAT L > 200 YEARS.

DRAKE EQUATION CONCLUSIONS BASED ON THE FACT THAT WE EXIST 1.EITHER N = L IS VERY ROUGHLY CORRECT (TO WITHIN A FACTOR OF A FEW HUNDRED OR A FEW THOUSAND), AS IN THE EXTREMELY OPTIMISTIC CASE OR MY BEST ESTIMATE OR 2. IF THE EXTREMELY PESSIMISTIC VALUES OF VARIOUS FACTORS ARE CLOSE TO CORRECT, THEN L MUST BE VERY LARGE

DRAKE EQUATION BUT WE SUSPECT FROM HUMAN EXPERIENCE THAT L COULD EASILY BE SMALL! (MORE ON THIS LATER) THEREFORE WE CAN PROBABLY EXCLUDE THE EXTREMELY PESSIMISTIC CASE. REALITY IS PROBABLY CLOSER TO THE OPTIMISTIC CASE (N ~ L) OR TO MY BEST ESTIMATE (N ~ L/200).

DRAKE EQUATION N = # OF CIVILIZATIONS IN MW GALAXY CAPABLE OF INTERSTELLAR COMMUNICATION L = AVERAGE LIFETIME OF SUCH A CIVILIZATION IN YEARS RESULT: N ~ L VERY ROUGHLY, (TO WITHIN A FACTOR OF A FEW 100 OR FEW 1000) BUT HOW LARGE IS L?? (BIGGEST SOURCE OF UNCERTAINTY)

MILKY WAY GALAXY N=1 * THIS IS US

MILKY WAY GALAXY DISTRIBUTION OF CIVILIZATIONS N=10 * THIS IS US * * * * * * * * WHY ISN’T THIS REALISTIC? *

MILKY WAY GALAXY DISTRIBUTION OF CIVILIZATIONS N=10 THIS IS US * * * * * * * * * RANDOM DISTRIBUTION *

MILKY WAY GALAXY DISTRIBUTION OF CIVILIZATIONS N=50 * THIS IS US * * * * * * * * * * HOW HAVE THE DISTANCES BETWEEN CIVILIZATIONS CHANGED FROM N=10? * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

MILKY WAY GALAXY DISTRIBUTION OF CIVILIZATIONS THE LARGER THE NUMBER OF CIVILIZATIONS, THE SMALLER THE AVERAGE DISTANCE BETWEEN THEM, THE MORE FEASIBLE INTERSTELLAR COMMUNICATION BECOMES.

ABUNDANCE OF LIFE IN THE GALAXY (ASSUMING N = L) CASE ABUNDANT SCARCE RARE L(YEARS) 1 billion 2 million 2000 N 1 billion 2 million 2000 CASE ABUNDANT SCARCE RARE Average Distance 15 LY 100 LY 1000 LY Number of 2-Way Conversations 30 million 10,000 1 NUMBER OF CONVERSATIONS: NUMBER POSSIBLE WITHIN TIME L, BASED ON THE ASSUMPTION THAT SIGNALS TRAVEL BACK AND FORTH AT THE SPEED OF LIGHT

SOLVING THE DRAKE EQUATION “ONCE SETI FINDS THE FIRST ONE, IT’S JUST STATISTICS.” PHILLIP MORRISON SETI = SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (METHODS TO BE DISCUSSED NEXT TIME) WHAT DOES THIS STATEMENT MEAN? ONCE WE FIND THE FIRST EXTRATERRESTRIAL CIVILIZATION, WE'LL KNOW AN APPROXIMATE VALUE FOR N, AND THUS WE'LL HAVE A BETTER ESTIMATE OF L ALSO. LET'S SEE HOW THIS WORKS.

SOLVING THE DRAKE EQUATION SUPPOSE SETI FINDS A SIGNAL, AND ASTRONOMERS DETERMINE THE DISTANCE TO THAT CIVILIZATION TO BE 100 LY. ASSUMING THIS DISTANCE IS ALSO THE AVERAGE DISTANCE BETWEEN NEAREST- NEIGHBOR CIVILIZATIONS (STATISTICALLY A GOOD ASSUMPTION), THEN N = 2 MILLION.

SOLVING THE DRAKE EQUATION WITH N = 2 MILLION: IF N ~ 10 L, THEN L ~ 200,000 YEARS, TIME FOR 100 ROUND-TRIP CONVERSATIONS. IF N ~ L, THEN L ~ 2 BILLION YEARS, TIME FOR 10 MILLION ROUND-TRIP CONVERSATIONS. THEN WE CAN FEEL CONFIDENT THAT THERE ARE A LOT OF LONG-LIVED CIVILIZATIONS, AND THAT COMMUNICATION WITH THEM IS FEASIBLE.

SOLVING THE DRAKE EQUATION WHAT IF SETI DOES NOT FIND ANY EVIDENCE OF EXTRATERRESTRIAL CIVILIZATIONS WITHIN 1,000 LY? THEN N < IF N ~ 10 L, THEN L < 200 YEARS, NO TIME FOR ANY CONVERSATIONS. IF N ~ L, THEN L < 2,000,000 YEARS, TIME FOR <1000 ROUND TRIP CONVERSATIONS. THEN DEPENDING ON WHAT WE ASSUME ABOUT THE DRAKE EQUATION, CIVILIZATIONS MAY BE SO FEW AND FAR BETWEEN THAT COMMUNICATION WITH THEM MAY NOT BE FEASIBLE.

HOW TO SEARCH FOR LIFE IN OTHER SOLAR SYSTEMS: TRAVEL OR COMMUNICATION? INSTEAD OF SPACESHIPS, USE PHOTONS ADVANTAGES OF PHOTONS (LIGHT): –TRAVEL AT SPEED c (FASTER THAN ANY SHIP) –REQUIRE LESS ENERGY THAN SPACESHIPS –CHEAPER –CAN CARRY JUST AS MUCH INFORMATION –RISK OF BIOLOGICAL CONTAMINATION IS ELIMINATED

TYPES OF SIGNALS DELIBERATE SIGNALS: SENT IN HOPES THAT ANOTHER CIVILIZATION WILL RECEIVE THEM ACCIDENTAL SIGNALS: USED BY THE CIVILIZATION FOR OTHER PURPOSES, BUT ESCAPE INTO SPACE INADVERTANTLY WHAT HAVE WE SENT? –SO FAR, WE HAVE SENT ONLY ONE SHORT (3 MINUTE DURATION) DELIBERATE SIGNAL –HOWEVER, WE ARE SENDING OUT LOTS OF ACCIDENTAL SIGNALS WHAT ARE WE TRYING TO RECEIVE? –SETI = SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (ONGOING) –SEARCHING FOR EITHER DELIBERATE OR ACCIDENTAL SIGNALS

WHAT TO SEARCH FOR? LET’S SEARCH FOR A SIGNAL (DELIBERATE OR ACCIDENTAL) PRODUCED BY AN EXTRATERRESTRIAL CIVILIZATION SETI: SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE TECHNOLOGICALLY DEVELOPED: HAVING THE ABILITY TO COMMUNICATE OVER INTERSTELLAR DISTANCES WHY THIS LIMITATION? WE DON’T HAVE THE TECHNOLOGY TO SEARCH FOR MORE “PRIMITIVE” LIFE FORMS AT INTERSTELLAR DISTANCES WE WILL LIMIT THE SEARCH TO INTELLIGENT, TECHNOLOGICALLY DEVELOPED LIFE FORMS

THINGS TO KEEP IN MIND WE HAVE BEEN A TECHNOLOGICAL CIVILIZATION FOR YEARS. IF L (THE AVERAGE LIFETIME OF A TECHNOLOGICAL CIVILIZATION) IS MORE THAN A FEW HUNDRED YEARS, THEN MOST CIVILIZATIONS WILL BE MORE ADVANCED THAN US. IF AN ADVANCED CIVILIZATION WANTS TO KEEP THEIR EXISTENCE SECRET FROM US, THEY WOULD EASILY BE ABLE TO DO SO.

ARE WE A TECHNOLOGICALLY DEVELOPED CIVILIZATION? ARECIBO RADIO TELESCOPE (WORLD’S LARGEST) COULD SEND DELIBERATE SIGNALS DETECTABLE BY A SIMILAR RADIO TELESCOPE AT A DISTANCE OF UP TO 100,000 LY (ANYWHERE IN THE MILKY WAY GALAXY) SO HOW DO WE SEARCH? LOOK IN DIRECTION OF A SUITABLE STAR AND SEARCH FOR ENERGY (AS SEEN FROM INTERSTELLAR DISTANCES, A PLANET IS CLOSE ENOUGH TO THE STAR IT ORBITS THAT WE WILL BE ABLE TO DETECT A SUFFICIENTLY STRONG SIGNAL FROM THE PLANET BY POINTING OUR TELESCOPE AT THE STAR) ENERGY----- IN WHAT FORM? ELECTROMAGNETIC WAVES

ELECTROMAGNETIC SPECTRUM SPEED OF LIGHT c = 300,000 km/s c = 1 light year / year FREQUENCY = NUMBER OF WAVES PER SECOND 1 Hertz = 1 wave per second 1 kHz = 1,000 Hz 1 MHz = 1,000 kHz = 1,000,000 Hz SPEED = FREQUENCY x WAVELENGTH

LONG WAVELENGTH LOW FREQUENCY LOW ENERGY SHORT WAVELENGTH HIGH FREQUENCY HIGH ENERGY TYPE GAMMA RAYS X- RAYS ULTRAVIOLET VISIBLE LIGHT INFRARED MICROWAVE TV & FM RADIO AM RADIO WAVELENGTH SIZE OF ATOM VIRUS BACTERIA DUST 1 cm 1 m 100 m FREQUENCY VERY HIGH HIGH BILLION MHz MILLION MHz 1000 MHz 100MHz 1000 kHz

RADIO COMMUNICATION ADVANTAGES OF RADIO WAVES: NOT AFFECTED MUCH BY INTERSTELLAR DUST LOW ENERGY COSTS LOW NOISE BACKGROUND HIGH INFORMATION CAPACITY TRAVEL AT THE SPEED OF LIGHT (LIKE ALL FORMS OF ELECTROMAGNETIC RADIATION)

HOW COULD SOMEONE ELSE DETECT US? TV AND FM RADIO TRANSMISSIONS HAVE BEEN LEAVING EARTH SINCE ABOUT THEY HAVE MADE IT A LITTLE OVER 60 LY IN THAT TIME (REACHED 2500 STARS). ANYONE WITH OUR LEVEL OF DETECTION TECHNOLOGY COULD DETECT US IF THEY ARE WITHIN 30 TO 40 LY. SIGNAL STRENGTH WOULD VARY AS DIFFERENT PARTS OF THE EARTH CAME INTO AND OUT OF VIEW. DOPPLER SHIFTS IN SIGNALS WOULD RESULT FROM THE EARTH’S ROTATION AND ITS ORBIT AROUND SUN. WE HAVEN’T DETECTED ANYTHING LIKE THIS FROM NEARBY STARS.

BUT WILL WE CONTINUE TO BROADCAST MUCH LONGER? A “BROADCAST” SENDS OUT A SIGNAL IN ALL DIRECTIONS (ALTHOUGH IT IS NOT EQUALLY STRONG IN ALL DIRECTIONS). CABLE AND OTHER FORMS OF DIRECTED DELIVERY MAY BE REPLACING BROADCASTS. IF MOST CIVILIZATIONS PROCEED THIS WAY, THE FRACTION OF THE CIVILIZATION’S LIFETIME SPENT BROADCASTING MAY BE RELATIVELY SHORT.