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 = 0.2 – 0.9

The Right Temperature N = R* fP nPM fl fi L Too cold Just right 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 Q. 118: Duration of Intelligent Life 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 R*= 1/yr L = 100 – 109 yr N = R* fP nPM fl fi L = 0 - billions Intelligent life could be rare Intelligent life could be common 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? Q. 119: Spreading Civilization

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 Q. 120: How to Communicate We can send/receive electromagnetic signals throughout the universe at c The universe is a noisy place Dust/gas/etc. blocks many wavelengths Q. 120: How to Communicate 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

End of Material for Test 4 Outline of Final: 66 questions total 39 new material 18 = 63 review questions Mostly a bit easier (I hope) 9 = 33 questions from this semester’s exams, word for word Not necessarily easier Final Info Online Final Exam Friday May 3 2 – 4 PM Lab Makeups: Talk to me or TA if no opportunities left

Final Exam Equations c = lf P2 = a3 (M + m)P2 = a3 c = 3  108 m/s F = ma E = hf P = knT lmaxT = 2900 Km 4 H + 2e -  He + 2 neutrino + energy L = 4d2B v = H0d H0 = 21 km/s/Mly