nsci 314 life in the cosmos 15 - the drake equation: how common are extraterrestrial civilizations...

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 KolehmainenDepartment of Physics,CSUSB

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

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 EQUATIONN = N*fsnpflfifcfL

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 STARSN = N*fsnpflfifcfL

fs = fraction of stars that are suitable stars

(result of N*fs is number of suitable

stars in MW galaxy)

fs = 0.1 = 1/10 (optimistic case)

fs = 0.001 = 1/1000 (pessimistic case)

fs = 0.05 = 1/20 (my best estimate)

SUITABLE PLANETS

Drake Equation: N = N*fsnpflfifcfL

np = average number of planets that are suitable for life per each suitable star

np = 2 (optimistic case)

np = 0.1 = 1/10 (pessimistic case)

np = 0.5 = 1/2 (my best estimate)

DEVELOPMENT OF LIFE


fl = fraction of suitable planets on which life actually originates

fl = 1 (optimistic case - life will always arise if the planet is suitable)

fl = 0.005 = 1/200 (pessimistic case)

fl = 1 (my best estimate)

DEVELOPMENT OF INTELLIGENCE


fi = fraction of planets with life on which intelligent life evolves

fi = 1 (optimistic case)

fi = 0.001 = 1/1000 (pessimistic case)

fi = 0.01 = 1/100 (my best estimate)

DEVELOPMENT OF TECHNOLOGY


fc= fraction of planets with intelligent life on which technology sufficient for interstellar communication develops

fc = 1 (optimistic case)

fc = 0.01 = 1/100 (pessimistic case)

fc = 0.5 = 1/2 (my best estimate)

DO THEY EXIST NOW?


fL= 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 = 0.005 L = L/200


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 0.001 x 0.1 x 0.005 x 0.001 x 0.01 x L/10 billion

RESULT: N = 0.0000000002 L = 2 X 10-10 L


IF L = 100 YEARS (pessimistic case for L), THEN N = 0.00000002

IF L = 10 BILLION YRS (optimistic case for L) THEN N = 2

DRAKE EQUATIONWE KNOW THAT N MUST BE AT LEAST 1

BECAUSE WE EXIST!

THEREFORE:

IF THE EXTREME PESSIMISTIC CASE IS CORRECT (N = 2 X 10-10 L),

WE WOULD CONCLUDE THAT

L > 5 BILLION YEARS.

THIS WOULD MEAN THAT CIVILIZATIONS ARE LONG-LIVED!

DRAKE EQUATIONWE 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 = 0.005 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 EQUATIONN = # OF CIVILIZATIONS IN MW GALAXY CAPABLE OF

INTERSTELLAR COMMUNICATION

L = AVERAGE LIFETIME OF SUCH A CIVILIZATION IN YEARS

RESULT: N ~ LVERY 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

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WHY ISN’T THIS REALISTIC?

*

MILKY WAY GALAXY


N=10

THIS IS US

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RANDOM DISTRIBUTION

*

MILKY WAY GALAXY


N=50

*

THIS IS US

*

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*

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*

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*

*

*

HOW HAVE THE DISTANCES BETWEEN CIVILIZATIONS CHANGED FROM N=10?

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*

*

*

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*

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MILKY WAY GALAXYDISTRIBUTION 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 EQUATIONWITH N = 2 MILLION:

IF N ~ 10 L, THEN L ~ 200,000 YEARS, TIME FOR 100 ROUND-TRIP CONVERSATIONS.

IF N ~ 0.001 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 EQUATIONWHAT IF SETI DOES NOT FIND ANY EVIDENCE OF EXTRATERRESTRIAL CIVILIZATIONS WITHIN 1,000 LY? THEN N < 2000.

IF N ~ 10 L, THEN L < 200 YEARS, NO TIME FOR ANY CONVERSATIONS.

IF N ~ 0.001 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 50-60 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 WAVESELECTROMAGNETIC WAVES

ELECTROMAGNETIC SPECTRUM

SPEED OF LIGHT c = 300,000 km/sc = 1 light year / year

FREQUENCY = NUMBER OF WAVES PER SECOND

1 Hertz = 1 wave per second1 kHz = 1,000 Hz1 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

SIZE OF ATOM

VIRUS

BACTERIA

DUST

1 cm

1 m

100 m

FREQUENCY

VERY HIGH

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 1940. 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.

nsci 314 life in the cosmos 15 - the drake equation: how common are extraterrestrial civilizations...

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