Transcript 34_alone
Are we alone? Lecture Thirty-Four, April 25, 2003 Drake Equation • • • • • • • • Nc=Rs fp n fl fi fc L Rs=rate of (suitable) star formation fp=fraction of stars with planets n=number of “Earthlike” planets per solar system fl=fraction of “Earths” with life fi=fraction that is intelligent fc=fraction that develops a civilization L=lifetime of civilization Drake Equation Ns = A ¢ B ¢ C A = Astronomy = Rs¢ fp ¢ n B = Biology = fl¢ fi C = Civilization = fc¢ L Are we alone? Here’s to Us • Who is there like us? • Darn few and there all dead! –Scottish toast Lifetime of Civilization • Many civilizations on earth come and go: Egypt, Greece, Rome,… • We face many man made and natural dangers. • Vast majority of species that every lived on earth are now extinct. Total Extinction • How close have we ever come to the total extinction of all life on earth? • A very large impact could kill all life. – It would have to be so large that the heat boiled away all the oceans and filled the atmosphere with molten rock! – Perhaps this happened very early on and life had to start more than once. – We don’t think there has been an impact this large since. Death of the dinosaurs • A 10 km rock strikes earth about once every 100 million years. • Does this place an upper limit on civilization lifetime of around 100 million years? • Impact did not kill all life. Indeed it “helped” mammals evolve. • Perhaps we can now deflect/ destroy such a rock and, very likely, will be able to do so in the future. Impact Basins on Moon Great Impacts of 3.8 Billion Years Ago Could Have Killed All Life. • Possible to have an impact large enough to boil all the oceans and fill atmosphere with molten rock! • These impacts carved great basins on Moon. • However, most large objects now gone from solar system (or in very stable orbits). • Impacts this large are now very very rare (> 1 billion years between impacts). • So rare they don’t limit our lifespan. Painting of Moon just after Imbrium impact showing sea of melted rock. Permian Mass Extinction • Perhaps closet we have come to total extinction. • 245 million years ago. • Of order 96% of all species died! • But important animals made it through such as reptiles that evolved to become dinosaurs and the first mammals. Cambrian Explosion of Life • Around 580 million years ago there was a great explosion in the diversity and complexity of life. • Organisms develop hard parts such as bones that can leave clear fossils. • Hard to read earlier fossil record to track major advances and possible mass extinctions. • Indeed Precambrian in the past meant before life. Although we now know this to be very far from true. Snowball Earth • We have evidence that about 700 million years ago earth was covered in a global and very deep ice age. • Originally it was thought such an ice age would be disastrous with no way out. – Snow and ice reflect sun’s heat and this would keep earth cold with no way to melt the ice. – Apparently there was a way out. CO2 Cycle Controls Temp. • Volcanoes continue to put CO2 into atmosphere even in ice world. • However, ice covers rock. Thus no way to capture CO2 back into the rock to make new carbonates. • Greenhouse CO2 continues to build up in atmosphere. This finally melts ice and converts earth from frozen to very warm. • With ice gone, eventually high CO2 in atmosphere recaptured into rocks. Snowball earth and life • Ice killed much life. However, some small pockets survived perhaps in volcanically heated pools. • This was before life had evolved hard parts (such as bones) that left clear fossils. • Great explosion of life with great increase in complexity and diversity occurred just after the snowball. Other Extraterrestrial Dangers • Nearby supernova. Massive explosion of star could kill all nearby life. • No known defense except to move far away. • Low density of stars near sun suggest that a very close supernova is very rare! • Also worry about gamma ray burst (collapse of very massive star to form black hole). • Gamma Ray bursts would kill anything that happened to be in narrow cone of two jets. • Chance that burst is pointing at us is rare. Massive star Eta Carinae Evolution of Star • When sun becomes red giant this will certainly kill all earth life. • Even before this, sun is becoming hotter as it burns and in about 1 billion years it will become so hot that the oceans will boil away and all life on earth will end. • Thus one optimistic estimate of a lifetime is of order one billion years. Sun as a red giant Space colonization and Lifetime • If we colonize Mars say this might protect some life against many dangers such as a single large impact or a disease. • Still depend on the same sun so the sun going red giant will kill both earth and mars. • Colonizing other stars may prevent total extinction of humans and increase L. Number in Galaxy • Provides an estimate of distance between civilizations thus how far one must communicate. • Represent galaxy as a disk about 50000 Lt-years in radius. • Area of galaxy= R2 = 7.85e9 (Lt-Y)2. • Density of civilizations =N/A Example N=106 • =1e6/7.85e9 = 1.3e-4 civilizations per square Lt-Y. • Average distance to next civilization L • ¼ 1/L2 assume uniform grid of civilizations each L apart. • Or L¼ [1/]1/2. • L¼ 90 Lt-Years. What makes Earth Special? Is this a physics/ astronomy question? Or a biological question? Or a religious /philosophical question? What are some of the physics that goes into Earth having a stable climate for five billion years? Are We Alone? • Yes! • But how can that be. All those worlds, all those Suns in all those Galaxies. • Why only us? • How could the Universe be so big and so so lonely? • What is our destiny? If we kill ourselves, what has it all been for? • All those worlds we can not detect. What are they for? • “It would be a great waste of space”. We are not alone! • The Universe may team with intelligent life! • Man has been here for only a few million years. Less then one one thousandth the age of the Universe. It is very likely that other intelligences are much older and wiser then we. • We are but an insignificant new comer in a vast crowd. – What do we matter? – What will the older and wiser races do with us? Either Way it’s Scary • How can we be so utterly alone! • Or • How can we be so insignificant. Just one among so many countless millions. Primitive Life • Primitive life may be common in the Universe. – Organic compounds may be easy to make, [Review Miller Exp.] , and they may be common [Amino acids found in meteorites, spectral lines from some compounds found in space and in the atmosphere of Titan…]. – Life appears to have started quickly on Earth. [What happened quickly may be highly probable to happen elsewhere.] • Life is extraordinarily complicated. We don’t understand very much about how it got started on Earth.