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UT AST 309L - Travel

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TravelRecall Argument Against TravelCommunication is much cheaper than travelEnergy needed for Mass (M) at speed (v)E = 1/2 Mv2 if v much less than ce.g., travel to nearest star (4 ly) in 40 yr⇒ v = 0.1 c ⇒ E = 4.1 × 10–9 ergsfor M = M (electron)Photon E = hν h = 6.6 × 10–27ν = frequency= 6.6 × 10–18 ergs if ν = 109 HzRatio ~ 109 (and photon gets there in 4 yrs)100 M watt transmitter - 1 yr$ 40 × 106Spacecraft to nearest star~ $ 5 × 1016(some recent analysis questions this conclusion)Why Consider Travel?Reasons for Interstellar TravelReasons:1. Communication if searches fail2. Exploration of other planetary systemsPlanetary ScienceExobiology (many bacterial planets)Nl = R fpnefl LlLl > 3 × 109 yr on Earth Birth Rate Ll NlHappy Feller 50 3 × 109 1.5 × 1011Angela Angst 7.5 × 10-4 3 × 109 2.3 × 106Average 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 Exploration1. Ground-based observations (telescopes)2. Fly-by missions, Radio back results3. Orbit or land, Radio back results4. Mission with human beingsReturn to Earth (Moon Only)5. Permanent Base(Not Yet)Expect similar for InterstellarExceptNo Round TripsDistances to Stars ~ Light YearsTime = Distance (Ly) ~ 4 Ly = 40 yr Speed (Ly/y) 0.1 cRound trip = 80 yProject DaedalusDesign study for Fly-byBarnard’s Star 6 ly awayInertial confinement fusion(Ignite pellets of hydrogen w/lasers, particles)Terrestrial fusion: 2H + 3H 4He + n deuterium tritium causes problemsDaedalus:2H + 3He 4He + p charged, control with mag. FieldProblem: 3He rare ⇒ Mine Jupiter?Δ E = 4 × 10–3 mc2Design: v = 0.12c travel for 50 yrs500 ton payload 54,000 tons at takeoff50,000 tons of fuelve =104 km s–1RM = 12 or 100(payload)s.i. = 106 secFaster Travel?• If we could travel close to speed of light• Time Slows down• Could travel more light years than years onthe space ship clock• Though not on the clock on the home planete.g. Twin ParadoxEarthTwin oneTwin twoV = .866 CEarth5 yearsclockTwin two*Alpha Centauri2.5 yearsOn-board clockEarthTwin one(10 yrs older) Gray HairTwin two(5 yrs older) t = γ t0Time measured byTwin oneTime measured by Twin two**γ = 1√ 1– (v/c)2Problems with fast travel• Mass ratio (RM) increases rapidly with v– M = Mo γ– 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=14No Propellant?• Bussard RamJet– Scoop up fuel as you go– Problems• Very diffuse (need huge scoop)• Hydrogen is low-grade fuel• You want rare 2H + 3HeFuture Fantasies?Consider now some things that are outsidephysics as we know it, but might be possible.WormholesGeneral relativity: A. EinsteinMatter warps space-timeThis warp is gravitye.g. Black holes pinch off a piece of theUniverse - even light cannot escapeRotating black hole wormholeWormholesUnlikely to form when a star collapsesIf it forms, it is unstableTraveler probably cannot pass throughLoophole - stabilize it somehow?Exotic Matter?View entering a wormholeartist’s conceptionWarp DrivesFaster than light travel?Not possible for ordinary matter, physicsM = γ M0 ⇒ M → ∞ as v → cLoophole:c is a speed limit for motion in space-timenot of space-timee.g. The space-time of the universe expandedfaster than c during very early inflationaryexpansionThe Universe now seems to have a lot of“dark energy”Source unknownActs like antigravity on large scalesCould we ever control this?Alcubierre Warp~ 1994 Miguel Alcubierre suggested use of“exotic matter” to surf a space-time distortionContract space in front, expand behindDoes exotic matter (negative mass) exist?Can we control it?Energy requirementsOriginally thought to exceed that available in entireuniverseLater calculations are less extremeBack to Reality• Hard to decide if very advanced civilizationsmight develop such schemes• Use only laws of physics as constraint - apply to all civilizations, no matter how advanced their technologyColonizationAssume Daedalus technology (v = 0.1 c)t = d ~ 4 ly = 40 yrs v 0.1Multi-generational travel(space colony + propulsion)How long to colonize galaxy?*Colonizationtgal= rgalvexpvexp = 2d*tt = d * + treg ve.g. v = 0.1 c treg = 500 yr ⇒ vexp = 4 ly x 2 ~ 0.015 c540If rgal = 80 × 103 ly tgal = 80 × 1031.5 × 10-2 ~ 50 × 105 tgal << age of galaxy 5 × 106 yr*****ColonizationWith “reasonable” assumption,t colonization < < t GalaxyHow likely?How many civilizations ever developed?(Time available: 10 × 109 - 5 × 109 ~ 5 × 109)ColonizationBirthrate × age of Galaxy = N(ever)Happy Feller 50 × 5 × 109 = 2.5 × 1011Angela Angst 7.5 × 10–8 × 5 × 109 = 375Average Guy 0.93 × 5 × 109 = 4.7 × 109If even one of these decided to colonize, it shouldalready have happened!Possible consequences:1. Galactic community2. Solar-system has been visited3. Solar-system being monitored4. Solar-system leakage radiation detected?Hart HypothesisFact: There are no intelligent beings from outerSpace on the Earth now.Only 5 possible explanations:1. Space travel is not feasible2. Civilizations chose not to colonize3. Not enough time to colonize galaxy4. The Earth was visited but they did not colonize5. There are no other advanced civilizations“Answers” to the Hart Hypothesis1. Colonization may be much slower106 yr regeneration → tcolon > 1010 yrs2. 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 acids4. Optimist’ s time scale for colonization > t for biological evolutionMaybe >>5. Possible development of “ecological ethic”Do not interfere6. They are here!


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UT AST 309L - Travel

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