Life in the Universe
What 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 = R* fP nPM fl fi L
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
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 temperature
N = R* fP nPM fl fi L
Too cold
nPM = number of suitable
planets and moons
= 0.1 - 1
Just right
Too hot
Getting Life Started
N = 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 Civilization
N = 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 Alive
N = 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
= 0 - billions
•Intelligent life could be rare
•Intelligent life could be common
R*= 1/yr
fP = 0.1 – 0.5
nPM = 0.1 - 1
fl = ?? – 1
fi = ?? – 0.5
L = 100 – 109 yr
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
P2 = a3
(M + m)P2 = a3
F = ma
GMm
F 2
r
  l
sin   
 2  2d
c = lf c = 3 
108 m/s
vrad
shift  rest
E = hf

rest
c
P = knT
lmaxT = 2900
Km
4 H + 2e -  He + 2 neutrino + energy
3.26 ly
d
p
L = 4d2B
4
L T   R 
  

L
T   R 
v = H0d H0 = 22 km/s/Mly
2
End of Material for Test 4
Questions?