Transcript Quantum Well Electron Gain Structures and Infrared

Interstellar Travel
Search for Extraterrestrial
Intelligence
Stephen Eikenberry
17 April 2013
AST 2037
1
Interstellar Travel: Why Bother?
• It is already hard enough just to send messages to other
civilizations!
• Why go to the hassle of flying there?
• Soft, fuzzy answers:
• Humans have always felt the need to explore in
person
• Even today, world leaders forgo telecoms for
important deals – travel in person instead for faceto-face meeting (face-to-tentacles here?)
• Real answer: If you can’t actually GO there, it is
pretty hard to kill them and take their stuff!
2
Current Rocket Travel Time
3
Our Friend: Time Dilation
• So … at v = 0.5c, we on Earth see a trip to  Cen take 6
years each way
• But, the astronaut on the ship sees it take about 5 years each
way (!)
• Now, try v=0.99c
• We see it takes 3 years each way
• Astronaut sees it take about 6 weeks each way (!)
• Twin paradox: one twin on Earth ages 6 years during the
round-trip, the other twin ages only 3 months (!!)
4
Light Speed: Rocket Trouble
• Energy of flying mass is mc2
• To deliver payload mass m1 , need enough fuel to slow-down
and stop it
• For nuclear fusion (VERY efficient), this is 0.007 mfuel c2
• So, mass of fuel needed to stop m1 is mfuel = m1/0.007
• This means you need about 3500x as much fuel as payload,
just to stop (not to mention the fuel to get started in the first
place!!!)
5
Light Speed: Antimatter Rockets
• Even more efficient than nuclear fusion is matter/anti-matter
annihilation; Converts 100% of mass into energy
• For argument’s sake, let’s assume that we can be 100%
efficient in harnessing the energy released for space travel 
a “perfect rocket” (100% of fuel rest mass converted into
usable energy)
• For the return trip, still need mc2 of stopping energy, so
mfuel = m1 ; total return trip in-flight mass is m2 = mfuel + m1
= (+1) m1 – or about 26 times the payload mass
• But, need fuel to start return trip of (+1) m2 = (+1)2 m1 – or
about 650 times the payload mass
• Similar thing for the way there  even a perfect rocket
needs about 400,000 times as much fuel mass as payload
mass (!!)
• So … rockets are not a great idea for interstellar travel; need
to carry HUGE amounts of fuel with you
6
Light Speed: Other Techniques?
• Other ideas: solar sails
• Capture light pressure from
star to accelerate the
payload
• No fuel mass, only extra sail
mass (could be very small)
• Problems:
• Once you leave the
vicinity of Sun/star, not
much acceleration
• As you get closer to the
speed of light, redshift
decreases efficiency
• So … nice idea, but looks
tricky
7
Light Speed: Other Techniques?
• Other ideas:
• Bussard Ramjets
8
How did Luke Skywalker do it?
• Hyperdrive
• “Folding” space via wormholes
9
Summary
• Stars are VERY far apart
• Travel times with current rockets are CRAZY long!
• Even at light speed, travel times are HUGE (though not for
the traveler)
• Rockets are very mass-inefficient for these trips, though –
even matter/anti-matter drive
• Best bet – find a way to tunnel through space via wormholes!
(probably not as easy as it sounds – and it sounds hard!)
10
What Else Are We Missing?
• Search for Extraterrestrial Intelligence (SETI)
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What Else Are We Missing?
• UFOs
12
Exam Review
• Life on Mars: environment, Viking, Mars Rovers,
ALH84001, water on Mars, future prospects
• Life on Venus/Mercury: none found – why not?
• Life on Giant Planets & Their Moons: environment(s); weird
possibilities; unexpected warm/watery spots, future
prospects
• Habitable Zones: Definition; how do we determine location;
different star types and “best” ones for life
• ExtraSolar planets: how many, what kinds, implications for
life
• How Common Is Life: Drake equation and factors
• Extra-terrestrial Intelligence: More Drake Equation, more
factors
• Interstellar Travel: this week!
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