Friday, November 29, The challenge of interstellar travel

Transcription

1 The challenge of interstellar travel

2 The challenge of interstellar travel Interstellar travel - travel between star systems - presents one overarching challenge: The distances between stars are enormous compared with the distances which our current spacecraft have travelled Voyager I is the most distant spacecraft, and is just over 100 AU from the Earth The closest star system (Alpha Centauri) is 270,000 AU away! Also, the speed of light imposes a strict upper limit to how fast a spacecraft can travel (300,000 km/s) in reality, only light can travel this fast

3 How long does it take to travel to Alpha Centauri?

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5 Rocket Equation Escape velocity from Earth is about 11 km/s (25,000 miles/hr) To reach this, M_init/M_fin = 39 rocket equation Chemical rockets v (final) v (gas expelled out the back) = ln M (initial) M (final)

6 Rocket Equation Best rockets have M_init/M_fin = 15 Thus, use multi stages Example, Saturn V --> used to send astronauts to the Moon (old technology) M_init/M_fin = 62 TOTAL but as a three stage rocket, each required M_init/M_fin = 3.4 (its a non-linear equation!) Thus, we can launch from Earth! Chemical rockets

7 If we want to send out a colony, how much mass do we need to launch? Would need a colony to reach and live on an extrasolar planet. Titanic example, need 18,000 kg per person Total mass for our Starship Enterprise is then ~100 million kg if we can reach 10% speed of light (thus 40 years to Alpha Centauri) then energy required is another formula, E = 1/2 mv x W/s, which is ~ 100x world s current annual energy use. double it to slow down the ship on arrival at 10 /kilowatt-hour, cost is $2.5 million trillion.

8 Propulsion Chemical rockets

9 Propulsion E = m c 2 fission (0.07% mass to Energy) fusion (0.7%, e.g., H to He) e.g., Project Orion (p.444) Nuclear drive

10 Propulsion Radiation Pressure from the Sun Force drops as (1/distance) 2 sail needs to be kilometers in size. or use a laser to propel it (but laser E and mirror size too large) Solar sail

11 Propulsion Ion drive

12 GOCE ion thruster Gravity field and steadystate Ocean Circulation Explorer (GOCE) NASA & ESA collaboration NASA press release (April 2009) The GOCE engines can provide 20 millinewtons of thrust - for a one-ton satellite, that's an acceleration of less than the width of a human hair per second squared, which is less than impressive. Unless you keep it on for a month, say, and end up moving at four kilometers a second - and with a little work, you can refuel anywhere there's an atmosphere.

13 Are faster speeds possible? None of these propulsion systems are capable of speeds faster than a few percent of the speed of light More speculative ideas may allow for speeds closer to the speed of light

14 Outside the Box Interstellar Ramjets Matter- Antimatter

15 Relativity and Wormholes?

16 Chapter 13, p Try Q 2-11, 32-35, Review Questions Science or Nonscience? Q21, Q23, Q29 Q23: Human colonization of the moons of Saturn occurs using spaceships powered by dropping nuclear bombs out the back of the ships. Q29: Aliens arrive on Earth but virtually ignore humans, finding the diversity of bacteria on Earth to be much more scientifically interesting.

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18 The stars look so close - isn t there someway to travel there? 18

19 Blackholes and Wormholes

20 The paradoxes of relativity The observations you make of the world around you depend upon the relative velocity between you and the thing being observed. because the speed of light is constant at c = 3 x 10 5 km/s

21 The paradoxes of relativity Time dilation: moving clocks run slower, T obs = γ x T rest Length contraction: moving rulers become shorter, L obs = L rest / γ

22 The Lorentz factor γ = 1 1 v2 c 2

23 The paradoxes of relativity

24 Einstein thought: - gravity causes falling objects to accelerate - but from special relativity, rulers & clocks moving with the object are also affected Thus, gravity affects the shape of space (rulers) and the flow of time (clocks) near massive objects. Thus, the mass of an object alters properties of space and time around it. These principles are the basis of the theory of General Relativity Can replace flat space-time with well diagrams & this eliminates the need for a force of gravity

25 Testing General Relativity: 1. Einstein himself calculated the effects on the orbits of the planets if the Sun is warping space-time. --> only Mercury is close enough to be within the potential well of the Sun. --> slightly elliptical orbit means it plunges in & climbs out of the well --> causes a slight precession (0.43 /yr)

26 Galactic Centre ~ 1 arcsecond sq ~1000 AU sq SO-16 elliptical orbit brought it within 45 AU of Sag A* The Galactic Centre has ~10 10 solar masses within 45 AU!! (1.5x distance between the Sun and Neptune). At closest approach it was travelling at v = 0.04c!!

27 Testing General Relativity: 2. Deflection of star light from a straight path by the Sun --> images during a total solar eclipse --> compared to images before or after --> closest stars to Sun show offset positions relative to background stars

28 Abell 2218: Orange, E z=0.7 Blue, SF z=1-2.5

29 Back to the future Einstein s theory of special relativity says that time slows down when you travel very fast This time dilation makes it possible to greatly reduce interstellar travel times For a spaceship travelling at speed v, its time will slow down by the Lorentz factor.

30 Back to the future Einstein s theory of special relativity says that time slows down when you travel very fast This time dilation makes it possible to greatly reduce interstellar travel times For a spaceship travelling at speed v, its time will slow down by the Lorentz factor.

31 Journey into a Black Hole: You & a partner orbit a 10 solar mass black hole Synchronize your watches One of you jumps out with a laser - agreeing to signal every 10 seconds So what happens next? Image of an accretion disk around a blackhole. The magnetic field can funnel energy & mass.

32 What about a trip through a wormhole? New Scientist TV 13 March 2012

33 It would be the trip of a lifetime: thanks to an animation by astrophysicist Andrew Hamilton from University of Colorado at Boulder. First, you free fall through the outer horizon of a black hole. Once you reach its inner horizon, you see an infinitely-energetic flash of light from the outside world containing an image of the entire history of the universe. In a real black hole you would be vaporised by the burst, but the visualisation assumes you have superpowers to survive it. As you emerge from the black hole, you enter a wormhole where the flow of space turns around and you start to accelerate back outward. The wormhole ends at the entrance to a white hole, which is a time-reversed version of a black hole. Instead of falling inward, space falls outwards at a speed faster than light. Soon you experience another flash of radiation, this time containing a picture of the entire future of the universe. Moving through the white hole, you see a third flash of light as you reach its outer horizon. This time, a new universe appears, containing an image of its entire past. As the camera turns around, you can see the white hole from which you emerged and an image of the old universe. This is as close as you'll get to a wormhole journey at the moment, unless new theories of gravity can make such trips possible in the future.

34 Wormholes, Warp Drive, Colonies

35 Astrobiology in the 21st Century