Class events: week 14

1. Class events: week 14 Today’s goals: Interstellar travel! Theoretical considerations- Different blueprints, from mundane to insane!

2. Three obstacles to interstellar travel Obstacle #1: Extraordinary distances Recall from week #1… If we scaled the Sun’s radius to 17 cm (a grapefruit), the (ant-sized) Earth would be at 15 m, and the nearest star would be in New York City (4000 km distant). Space is vastly huge, and is filled with mysteries. How have we fared so far?

3. Three obstacles to interstellar travel Obstacle #1: Extraordinary distances We have launched four planetary probes which are destined to be interstellar probes—Pioneer 10, 11; Voyager 1, 2. Voyager 1 Our most distant probe; 110 a.u. from the Sun (0.0017 LY); It is travelling at 17 km/s (0.000056 c); 1/2500 the distance to α Centauri system; Would reach α Cen in 77,000 years—if it were heading that way!

4. Three obstacles to interstellar travel Obstacle #1: Extraordinary distances Interstellar arks could be a work-around for the huge distances. Deal with the long travel time by creating enormous, multigenerational ships; What would happen to the enclosed culture over time? Theoretical “sleeper ships” could involve suspended animation?

5. Interstellar travel Obstacle #2: Energy concerns The speed of light (“c”) is a good comparison—to accelerate a spacecraft to velocities near c would require incredible energies: K.E. = ½mv2 = ½ (18,000 kg/passenger) × 5000 passengers × (0.1c)2= 4.5 × 1022 J = 100 × world usage/year

6. Interstellar travel Obstacle #3: Special Relativity and c Einstein’s relativity tells us that to accelerate a particle (with mass=m) to the speed of light would take not just K.E. = ½mc2 , it would take an INFINITE amount of energy! You cannot generate an infinite amount of energy, therefore nothing can reach the speed of light. This is why the speed of light is a cosmic speed limit. Even if you were traveling at nearly the speed of light, your space ship traversing the enormity of interstellar space would take 4.3 years to reach the nearest star. You simply cannot cut down those huge travel times from star to star! I ask you… Would aliens really go to all the effort of visiting us, where such a journey took dozens of years, just to probe our farmers?

7. Interstellar travel Obstacle #3: Special Relativity and c Astonishingly, because of a relativistic effect called time dilation, space voyagers traveling at high speeds age more slowly than non-travelers: Consider a journey of 4.3 LY; how long would it take to occur? V Ttraveler/Tpotato Observed Experienced 0.1c 0.995 43 years 4.28 years 0.5c 0.866 8.6 years 7.45 years 0.9c 0.436 4.8 years 2.08 years 0.99c 0.045 4.3 years 2.35 months Near-light travelers would reach the stars after aging only a few months—but their families back home would age normally. Would you visit a star 10 LY away, if (after a trip lasting 11 months at 0.999c) your family would be 20 years older upon your return?

8. How rockets work • Rockets DO NOT push against the ground; • Rockets DO launch matter away at high velocity; • By their nature, rockets lose mass during operation. Optimizing thrust • The faster the ejected matter, the more thrust; • The more matter ejected, the more thrust. Interstellar propulsion

9. Chemical rocketry Chemical energy trivia Liquid oxygen and ethanol (V-2); Liquid oxygen and kerosene or liquid H2 (Saturn V); Liquid O2 and liquid H2(Shuttle External tank); Ammonium perchlorate (oxidizer) and aluminum (Shuttle Solid Rocket Boosters). Interstellar applications Chemical rockets are barely useful for interplanetary travel; Chemical rockets are useless for interstellar work.

10. Nuclear fission thermal rockets Nuclear fission heats a propellant gas (hydrogen); Project Rover intended power a Saturn V, for Martian mission (1955-1972), with Kiwi, Phoebus, Pewee engines; Russian RD-0410 thermal rocket in manned 1994 Mars proposal. Development would violate the Nuclear Test Ban Treaty, and would also violate the Comprehensive Nuclear Test Ban (currently signed but not ratified by the USA).

11. Nuclear fusion rockets Orion: up to 2400× as massive as a Saturn V; “Pusher plate” absorbs blast shock with hydraulics and airbags; With millions of bombs, Orion could reach 0.1c! The British Project Daedalus explored theory of continual fusion via pellets; These are all currently beyond our technology.

12. Ion engines Accelerates charged particles to high velocities; The high velocity compensates for low particle mass, thus generating useful thrust; Suitable for long missions, but not for landings; Many current uses—Hayabusa (Japan) visited asteroid Itokawa in 2005.

13. Solar sails Solar sails surf sunlight (assisted by lasers?); For high speed, it would be launched near the sun; Would it slow by the radiation from its target star? Lightsail 1 (Planetary Society) is being tested in 2011. Ikaros (Japan) is the first solar sail satellite that has flown. A square 20 m in diagonal size, it flew to Venus in 2010.

14. Realms of fantasy Ramjets An enormous leading scoop gathers hydrogen for fusion; The scoop would be comparable to California in size. Matter-antimatter Nuclear fission: 0.07% mass conversion; Nuclear fusion: 0.7% mass conversion; Matter-antimatter: 100% mass conversion; Antimatter takes energy to create—it is not free energy! Antimatter is not fiction: Was predicted in 1928 by Paul Dirac; Positrons were created in 1930; Antiprotons were discovered in 1955; Antihydrogen was made at CERN and Fermilab in 1995; Anti-helium (He3) was created in 2003.

15. Wormholes Wormholes in hyperspace provide a way to travel at sub-light speeds, but by taking a short cut. These Schwarzschild wormholes (or Einstein-Rosen Bridges) are not prohibited by science, as far as we can tell. However, wormholes are inherently unstable—not even light can pass through them before they fall apart.

16. Stable wormholes In the 1980s, astrobiologist Carl Sagan was writing Contact, and needed a way for aliens to communicate with humans. Work with black hole theoretician Kip Thorne led to the concept of a of stable wormhole. Sagan’s book/movie portrayed our galaxy as being filled with a network of wormholes, created by an ancient Type III galactic civilization. Theoretically possible, traversable wormholes would have to be stabilized by exotic matter with attributes (such as negative mass) that will probably be laughed at by future physicists….probably.

17. Really weird stuff Tachyons Entirely fictional particles that travel faster than light. Tachyons would have remarkable properties—for example, they would experience negative time. In 2011, CERN results indicate that neutrinos might be travelling at speeds greater than the speed of light. This was a bogus detection due to a faulty fiber optic cable. This is the realm (and love) of science fiction. Nuff said.

18. Really weird stuff Alcubierre metric In the 1990s, by studying Einstein’s equations, physicist Alcubierre discovered that space could be warped in a strange way. He developed a warped bubble of spacetime, in which spacetime is contracted in front of an object, and shrunken behind it. The warp bubble and its enclosed object could move at arbitrarily high speeds without violating physics. This is strictly hypothetical. Furthermore, exotic matter is needed to stabilize this structure, just as in wormholes.