Announcements

1. Announcements • Two more `real' classes left • Last class: available for doing presentations, some final discussions. • Marks-to-date available again • 8 people who are currently not passing • Still time to do bonus credit work • Double-check info

2. Last Week: Finding Extra-solar Planets • Techniques • Direct(ish) measuring of planet • Indirect measurement – effect on star • Results of search so far • `Hot Jupiters' • Implications • Can life be found in these systems? - Maybe • Are most systems like this? - Probably not • Migration vs. Direct Formation

3. This Week: Interstellar Travel, Interstellar Communication • Interstellar Travel • Force, Acceleration, Velocities • Fuel Requirements for Travel • Fuel stops? • Time to nearby stars • How fast can we go? • Special Relativity

4. Force, Acceleration, and Velocities • To talk about the requirements of space travel, need to understand three concepts: • Velocity • Acceleration • Force

5. Velocity • Like speed, but includes direction • e.g., 65 mph due north • Measure of how much distance you are covering over time • 65 mph due north: in one hour, will travel 65 miles due north.

6. Acceleration • Change in velocity over time • Can be any change in speed • Can also be a change in direction, even if speed stays the same

7. Force • Forces accelerate objects • Force = Mass x Acceleration • More mass: takes more force to accelerate • Larger force: more acceleration

8. Force • Any time there is an acceleration, there must be forces acting • Gravity • Friction • Electric • Magnetic • Pressure...

9. Force • No acceleration means steady (possibly zero) velocity • Either no forces acting on body or balanced forces (forces add up to zero)

10. Force • Force is a two-way street • If Sun pulls Earth with a given gravitational force, then Earth pulls Sun with the exact same force • Two balanced forces: why is there motion?

11. Newton's Laws of Motion • A body remains at rest, or moves in a straight line (at a constant velocity), unless acted upon by a net outside force. • The acceleration of an object is proportional to the force acting upon it. (F = m x a) • For every action, there is an equal and opposite reaction.

12. Rockets • Have to exert force to overcome that of gravity • Reactions from some sort of fuel • Chemical • Electrical... • Propel exhaust downwards • By Newton's 3rd law, propel rocket upwards Net Force -> acceleration Gravitational Force Force exerted by exhaust

13. Easy to accelerate upwards • Hard to keep from falling back down! • Can either: • Accelerate very quickly to escape vel (25,000 mph) and coast up • Gravity will keep decellerating you but never quite pull you back • Or accelerate slowly throught ascent • Luckily, further up you get, weaker force from Earth's gravity becomes Net Grav Force exhaust

14. Rockets: Fuel • Takes a lot of fuel to move something into Earth's orbit or further • Would take about as much fuel to launch me into orbit as it takes to heat a Chicago home through an entire winter • Unlike a car trip, fuel starts weighing a lot, even compared to rocket • Shuttle launch: • Empty Shuttle: 230,000 lb • Fuel : 2,700,000 lb

15. Rockets: Fuel • Once far enough away from Earth and in vacuum, may not need to keep burning fuel • Why?

16. Rockets: Fuel • Will stay in motion without accelleration • But may want to pick up speed • Voyager: 26,000 mph • 1g of acceleration for 20 min • Any acceleration will require fuel, which will require still more fuel to get it to this point • Any includes slowing down to stop at destination • How much fuel is required depends on efficiency of engines

17. Rockets: Fuel • Most efficient engines concievable • Turn matter directly into energy • All energy directed to motion with 100% efficiency • Even so, a trip accelerating to 99% of speed of light takes 14x mass of spacecraft to accelerate, and 14x to decellerate • 196 times as much fuel as craft! • 40,000 times if round trip!

18. Rockets: Fuel • Most efficient engines concievable • Turn matter directly into energy • All energy directed to motion with 100% efficiency • Even so, a trip accelerating to 99% of speed of light takes 14x mass of spacecraft to accelerate, and 14x to decellerate • 196 times as much fuel as craft! • 40,000 times if round trip!

19. Fuel along the way? • Interstellar medium VERY tenuous • Sprinkled with hydrogen • Could it be collected and then burned (nuclear fusion?) • Hard to see how • Drag on ship • Power to magnetic fields • But would solve enormous fuel problem

20. Time to nearby stars • How long would it take to get to the nearest stars? • Alpha Centauri • Jet aircraft speeds: • 3 Million years • Speed of Voyager 1,2: • 81,000 years • But if fuel were not a problem, could we accelerate endlessly, to any speed we wanted?

21. Time to nearby stars • No.

22. Special Relativity • Einstein: • Physics is the same in all inertial frames of reference • Speed of light in a vacuum is a fundamental physical constant of the Universe

23. Special Relativity • This means that weird things happen as we approach the speed of light • (`Weird' because our intuition is based entirely on much-slower-than-light motion) • Connection between time and space becomes more apparent

24. Special Relativity • Imagine two observers who measure time by bouncing light off of distant mirrors.

25. Special Relativity • Imagine two observers who measure time by bouncing light off of distant mirrors.

26. Special Relativity • Imagine two observers who measure time by bouncing light off of distant mirrors.

27. Special Relativity • Imagine two observers who measure time by bouncing light off of distant mirrors. • Tick

28. Special Relativity • Imagine two observers who measure time by bouncing light off of distant mirrors. • Tick

29. Special Relativity • Imagine two observers who measure time by bouncing light off of distant mirrors. • Tick, Tock.

30. Special Relativity • Now imagine one was moving at a constant speed compared to the other.

31. Special Relativity • Now imagine one was moving at a constant speed compared to the other.

32. Special Relativity • Now imagine one was moving at a constant speed compared to the other.

33. Special Relativity • Now imagine one was moving at a constant speed compared to the other. • From Blue's point of view, Red's `tick tock' took longer • Light had to travel further • Light has const speed 1. Tock 2. ..Tock

34. Special Relativity • But from Red's point of view, the opposite • Light traveled regular amount • Blue's light travelled further, takes longer • Measurements of time, distance are changed when you travel at speeds near light speed! 1. Tock 2. ..Tock

35. Special Relativity • How much must it change? • Figure out from Pythagorean theorem • Measurements must change depending on ratio of velocity to speed of light • Small velocities – no effect • Voyager – 26,000 mph – effect is in 9th decimal place 1

36. Special Relativity • But for higher velocities, can be significant! • Astronaut goes to Alpha Centauri and back at 95% of speed of light • Astronaut ages 3 years, people back home 9 • At closer and closer to speed of light, effect gets bigger and bigger.

37. Special Relativity • Speed of light becomes moving target • Astronaut can put more and more energy into travelling faster • But because can never pass light (light must always travel at same velocity!) can never pass speed of light • Takes infinite amount of energy to even get to speed of light

38. Special Relativity • Starts to become meaningful to talk of space and time together • Light cone: • Time along vertical axis • Space along others • `Cone' traced out by all possible light rays from origin • Cannot leave light cone

39. Special Relativity • These are extremely strong claims • What evidence is there for them?

40. Special Relativity • These are extremely strong claims • What evidence is there for them? • Particle accelerators: • Accelerate particles to near speed of light • Unstable particles have built-in clocks: decay time • Very high speed particles decay after much longer time: depends on speed exactly as predicted by special relativity

41. Special Relativity • These are extremely strong claims • What evidence is there for them? • Particle accelerators: • Timing of radio signals • GPS satellites require nanosecond accuracy • High motions: lose 7,000 nanoseconds/day due to SR effects • Don't take into account SR: navigation would be off by 1 mile/day! • (General Relativity effects even larger)

42. Special Relativity • Unfortunately, stuck with limit of speed of light • Good news: from astronauts point of view, can travel large distances in short time • Bad news: from observer on Earth, 1000 ly journey will take at least 1000 years each way.

43. Difficulties of Sending People • Safety • Cost • If can't travel near speed of light, will need `generational' ships • Colonies which would have future generations make the rest of voyage • Either way, original crew will never see Earth-bound family again

44. Automated Probes? • High-tech Voyagers or Pioneers • Aim towards nearby stars • Enough fuel to accelerate • Enough smarts to navigate toward system • Get solar power once near star • Send message • To nearby planets • To us

45. Automated Probes? • Advantage? • Cheaper • Faster? (could accelerate more without humans) • Could send out many • Once found something, could communicate back and forth much faster than travel

46. Travel Difficult • Communication much simpler than Transportation.

47. This Week: Interstellar Travel, Interstellar Communication • Interstellar Communication • Easier to send messages then people • Where do we aim? • What frequencies do we use? • Meaningful signals • SETI@home

48. Messages • Its a lot easier sending signals than things • Messages • Have no mass • Don't require fuel • Don't require food/provisions for long journey • Cheap to produce • Travel at speed of light

49. What frequencies to use? • Two choices for long-distance forces: • Gravity (difficult) • Electromagnetic • But there's an essentially infinite range of frequencies to examine • Radio waves: • Easy/cheap to generate, focus

50. What frequencies to use? • Fairly broad window of frequencies with relatively little background noise • Any civilization that does any radio astronomy at all will examine 21cm (1. 4 Ghz) • An important line of Hydrogen • Used to examine galactic structure • Put messages near there to be noticed?