Travel Recall Argument Against Travel Communication is much cheaper than travel Energy needed for Mass M at speed v E 1 2 Mv2 if v much less than c e g travel to nearest star 4 ly in 40 yr v 0 1 c E 4 1 10 9 ergs for M M electron h 6 6 10 27 frequency 6 6 10 18 ergs if 109 Hz Ratio 109 and photon gets there in 4 yrs Photon E h 100 M watt transmitter 1 yr 40 106 Spacecraft to nearest star 5 1016 some recent analysis questions this conclusion Why Consider Travel Reasons for Interstellar Travel Reasons 1 Communication if searches fail 2 Exploration of other planetary systems Planetary Science Exobiology many bacterial planets Nl R fpnefl Ll Ll 3 109 yr on Earth Birth Rate Ll Nl Happy Feller 50 3 109 1 5 1011 Angela Angst 7 5 10 4 3 109 2 3 106 Average Guy 2 2 3 109 6 7 109 Other Reasons Colonization Species Immortality Could survive the end of life on Earth The explorer s urge to boldly go Pattern of Solar System Exploration 1 2 3 4 Ground based observations telescopes Fly by missions Radio back results Orbit or land Radio back results Mission with human beings Return to Earth Moon Only 5 Permanent Base Not Yet Expect similar for Interstellar Except No Round Trips Distances to Stars Light Years Time Distance Ly 4 Ly 40 yr Speed Ly y 0 1 c Round trip 80 y Project Daedalus Design study for Fly by Barnard s Star 6 ly away Inertial confinement fusion Ignite pellets of hydrogen w lasers particles 4He n Terrestrial fusion 2H 3H deuterium tritium 2H Daedalus E 4 Problem 10 3 3He 3He mc2 4He causes problems p charged control with mag Field rare Mine Jupiter Design v 0 12c travel for 50 yrs 500 ton payload 54 000 tons at takeoff 50 000 tons of fuel ve 104 km s 1 RM 12 or 100 payload s i 106 sec Faster Travel If we could travel close to speed of light Time Slows down Could travel more light years than years on the space ship clock Though not on the clock on the home planet The Twin Paradox Twin one Alpha Centauri Twin two Earth Twin two V 866 C On board clock 5 years 2 5 years Earth clock Twin one 10 yrs older Twin two 5 yrs older Time measured by Twin two Gray Hair Earth Time measured by Twin one t t0 1 1 v c 2 Problems with fast travel Mass ratio RM increases rapidly with v M Mo at v 0 99c best possible fuel RM 14 You have to take fuel to slow down Fuel is payload on the way out RM 14 x 14 196 To return you need all this fuel as payload RM 196 x 196 40 000 And you need antimatter matter for RM 14 No Propellant Bussard RamJet Scoop up fuel as you go Problems Very diffuse need huge scoop Hydrogen is low grade fuel You want rare 2H 3He Future Fantasies Consider now some things that are outside physics as we know it but might be possible Wormholes General relativity A Einstein Matter warps space time This warp is gravity e g Black holes pinch off a piece of the Universe even light cannot escape Rotating black hole wormhole Wormholes Unlikely to form when a star collapses If it forms it is unstable Traveler probably cannot pass through Loophole stabilize it somehow Exotic Matter View entering a wormhole artist s conception Warp Drives Faster than light travel Not possible for ordinary matter physics M M0 M as v c Loophole c is a speed limit for motion in space time not of space time e g The space time of the universe expanded faster than c during very early inflationary expansion The Universe now seems to have a lot of dark energy Source unknown Acts like antigravity on large scales Could we ever control this Alcubierre Warp 1994 Miguel Alcubierre suggested use of exotic matter to surf a space time distortion Contract space in front expand behind Does exotic matter negative mass exist Can we control it Energy requirements Originally thought to exceed that available in entire universe Later calculations are less extreme Back to Reality Hard to decide if very advanced civilizations might develop such schemes Use only laws of physics as constraint apply to all civilizations no matter how advanced their technology Colonization Assume Daedalus technology t d v 4 ly 0 1 40 yrs Multi generational travel space colony propulsion How long to colonize galaxy v 0 1 c tgal rgal vexp vexp 2d t e g Colonization t d treg v v 0 1 c vexp 4 ly x 2 treg 500 yr 0 015 c 540 If rgal 80 103 ly tgal 80 103 1 5 10 2 50 105 tgal age of galaxy 5 106 yr Colonization With reasonable assumption t colonization t Galaxy How likely How many civilizations ever developed Time available 10 109 5 109 5 109 Colonization Birthrate age of Galaxy N ever Happy Feller Angela Angst Average Guy 50 7 5 10 8 0 93 5 109 2 5 1011 5 109 375 5 109 4 7 109 If even one of these decided to colonize it should already have happened Possible consequences 1 Galactic community 2 Solar system has been visited 3 Solar system being monitored 4 Solar system leakage radiation detected Hart Hypothesis Fact There are no intelligent beings from outer Space on the Earth now Only 5 possible explanations 1 Space travel is not feasible 2 Civilizations chose not to colonize 3 Not enough time to colonize galaxy 4 The Earth was visited but they did not colonize 5 There are no other advanced civilizations Answers to the Hart Hypothesis 1 Colonization may be much slower 106 yr regeneration tcolon 1010 yrs 2 Nomads explorers make trips not colonists 3 May be harder to adapt to life on a new planet than we think We need 20 essential amino acids 4 Optimist s time scale for colonization t for biological evolution Maybe 5 Possible development of ecological ethic Do not interfere 6 They are here UFO s
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