Life in the UniverseWhat We Want to Know

•There is intelligent life on Earth•Could it have happened elsewhere?

•Alien Spacecraft/UFO’s?•Evidence is unconvincing

•Can we calculate probabilities?•No•Drake’s Equation is a way to estimate how many intelligent civilizations (besides ours) are in the galaxy

Drake’s Equation

N = number of civilizations in our galaxy

R* = rate at which solar-type stars form

fP = fraction of those stars with planets

nPM= number of suitable planets and moons

fl = probability of getting life

fi = probability of life evolving intelligence

L = lifetime of intelligent species

N = R* fP nPM fl fi L

Stars, Planets, and Moons

N = R* fP nPM fl fi L

R* = rate at which solar-type stars form = 1/year

fP = fraction of those stars with planets = 0.1 – 0.5

The right temperatureN = R* fP nPM fl fi L

nPM = number of suitable planets and moons

= 0.1 - 1 Too hot

Too cold

Just right

Getting Life StartedN = R* fP nPM fl fi L

fl = probability of getting life started = ?? – 1

•The basic building blocks of life (elements, amino acids) are common•Life started fairly quickly on Earth

•But even very simple life is extremely complicated/unlikely

Getting Advanced CivilizationN = R* fP nPM fl fi L

fi = probability of life evolving intelligence = ?? – 0.5

•Evolution follows naturally from physical laws

•Evolution naturally leads to some complexity•Intelligence is a very useful kind of complexity

•Catastrophes could wipe out advanced life•Evolution may sometimes lead to dead ends

Keeping Civilization AliveN = R* fP nPM fl fi L

L = Mean lifetime of intelligent life = 100 – 109 years

•We have the ability to destroy civilization•We are also damaging our environment•We are using up non-renewable resources

•Civilizations may “mature” – some evidence•Sustainable civilizations is technically possible

Are We Alone?N = R* fP nPM fl fi L

•Intelligent life could be rare•Intelligent life could be common

R*= 1/yr fP = 0.1 – 0.5nPM = 0.1 - 1 fl = ?? – 1 fi = ?? – 0.5 L = 100 – 109 yr

= 0 - billions

My opinion:•We are first in the galaxy, and possibly the visible universe•Ultimately, this is an experimental question

Searching Stars for Life

How can we know if intelligent life exists around other stars?•Visitors from outer space?

•Fermi paradox – If they exist, why aren’t they here?•Travel to distant stars?•Search for signals from space?

Traveling Between the Stars

How Best to do it?•Best chemical rockets: 10-4c – 10-3c

•Millenia to nearest star•Nuclear rockets, solar sails, ion engines

•10-2c – 10-1c•Decades – centuries to nearest star

•Solutions:•Unmanned missions•Generation ships – parents have children•Long life span/hibernation

Signals From Space

•We can send/receive electromagnetic signals throughout the universe at c

•The universe is a noisy place•Dust/gas/etc. blocks many wavelengths•Some radiation can penetrate gas and dust•In radio waves, the Earth outshines the Sun

Signals From Space•We send accidental and intentional signals into space•We also look for signals from alien civilizations

•Nothing found so far

Final Exam Equations

sin2 2

l

d

P2 = a3

F = ma

2

GMmF

r

(M + m)P2 = a3 c = f c = m/s

shift rest rad

rest

v

c

E = hfP = knT

maxT = 2900 Km

4 H + 2e - He + 2 neutrino + energy

L = 4d2B

3.26 lyd

p

4 2L T R

L T R

v = H0d H0 = 22 km/s/Mly

End of Material for Test 4

Questions?