1. NSCI 314 LIFE IN THE COSMOS 17 - INTERSTELLAR SPACE TRAVEL Dr. Karen Kolehmainen Department of Physics, CSUSB http://physics.csusb.edu/~karen/

2. THE PROBLEM WITH INTERSTELLAR SPACE TRAVEL S TARS ARE VERY VERY FAR AWAY! –THE NEAREST STAR BEYOND THE SUN (ALPHA CENTAURI) IS ABOUT 4 LIGHT YEARS AWAY (100 MILLION TIMES FARTHER AWAY THAN THE MOON). THIS MEANS THAT IT WOULD TAKE 4 YEARS TO GET THERE IF YOU COULD TRAVEL AT THE SPEED OF LIGHT. A SPACESHIP CAN’T TRAVEL FASTER THAN THE SPEED OF LIGHT (c = 300,000 km/sec), OR EVEN AT THE SPEED OF LIGHT. IN FACT, NOTHING CAN TRAVEL FASTER THAN c. THIS IS AN ISSUE OF FUNDAMENTAL PHYSICS, NOT TECHNOLOGY!

3. INTERSTELLAR TRAVEL EXAMPLE CONSIDER A STAR 5 LIGHT YEARS AWAY. (THIS IS A VERY NEARBY STAR!)‏ AT THE SPEED OF LIGHT, IT WOULD TAKE 5 YEARS TO TRAVEL THIS DISTANCE. AT THE SPEED OF A SPACESHIP THAT WE CAN BUILD WITH CURRENT TECHNOLOGY (10 KM/SEC), IT WOULD TAKE 150,000 YEARS TO TRAVEL 5 LY.

4. COMPARISON OF TRAVEL TIMES METHODSPEED (KM/S)TIME (IN YEARS) NEEDED TO TRAVEL A DISTANCE OF 1 LY CAR0.0310,000,000 JET PLANE0.31,000,000 VOYAGER1225,000 SPACECRAFT FUSION 3000100 SPACECRAFT (DOESN’T YET EXIST)‏ PHOTONS300,0001

5. INTERSTELLAR TRAVEL BECAUSE DISTANCES ARE SO LARGE, LARGE SPEEDS ARE NEEDED TO KEEP THE TRIP TIME REASONABLE. RELATIVITY TELLS US THAT A SPACESHIP CAN'T TRAVEL AT OR NEAR THE SPEED OF LIGHT (MORE ON WHY THIS IS TRUE LATER). THIS IS A FUNDAMENTAL LIMITATION, NOT A STATEMENT ABOUT CURRENT (OR EVEN IMAGINABLE ) TECHNOLOGY. FOR NOW, LET'S ACCEPT THIS LIMITATION, AND CONTEMPLATE TRIPS THAT ARE POSSIBLE (ASSUMING THAT RELATIVITY IS CORRECT). NOTE: IT'S INCONCEIVABLE THAT RELATIVITY COULD BE “WRONG.” IT HAS BEEN TESTED THOROUGHLY VIA EXPERIMENTS. IT MAY BE SUPERSEDED BY A MORE COMPLETE THEORY SOMETIME IN THE FUTURE, BUT IT'S VERY UNLIKELY THAT THE MORE COMPLETE THEORY WILL DISAGREE WITH RELATIVITY ON SUCH A FUNDAMENTAL ISSUE.

6. INTERSTELLAR TRAVEL BECAUSE DISTANCES ARE SO LARGE, LARGE SPEEDS ARE NEEDED TO KEEP THE TRIP TIME REASONABLE. WE MUST ACCELERATE TO REACH LARGE SPEEDS. ACCELERATION: A CHANGE IN SPEED (OR DIRECTION OF MOTION) OVER TIME SOMETIMES “DECELERATION” IS USED TO MEAN LOSING SPEED, BUT THE PHYSICS DEFINITION OF “ACCELERATION” INCLUDES GAINING SPEED, LOSING SPEED, AND/OR CHANGING DIRECTION. HOW QUICKLY CAN (OR SHOULD) WE PICK UP (OR LOSE) SPEED, i.e., HOW LARGE WILL OUR ACCELERATION BE?

7. ACCELERATION TYPICAL CAR ACCELERATES AT 1 OR 2 m/s 2 (PICKS UP 1 OR 2 m/s OF SPEED PER SECOND)‏ GRAVITY ACCELERATES A FALLING OBJECT AT ABOUT 10 m/s 2 = 1 g (g = ACCELERATION DUE TO GRAVITY)‏ A COMFORTABLE ACCELERATION FOR HUMANS IN A SPACESHIP IS 1 g = 10 m/s 2 (THIS WILL FEEL LIKE NORMAL EARTH GRAVITY TO TRAVELERS IN THE SHIP)‏

8. EXAMPLE: ROUND TRIP TO A STAR 5 LY AWAY ASSUME WE ACCELERATE AT 1 g FOR 2.5 LY (UNTIL WE’RE HALFWAY THERE), REACHING A FINAL SPEED OF 90% c. NOW WE MUST DECELERATE AT 1 g FOR 2.5 LY SO THAT WE CAN STOP AT OUR DESTINATION. THEN REPEAT THE ENTIRE PROCESS ON THE WAY BACK. ELAPSED TIME (EARTH CLOCK) = 20 YRS. ELAPSED TIME (SHIP CLOCK) = 8 YRS. WHAT HAPPENED?? INTERSTELLAR TRAVEL

9. RELATIVITY RELATIVITY DOES NOT MEAN THAT “EVERYTHING IS RELATIVE.” IMAGINE TWO OBSERVERS (PEOPLE), LET'S CALL THEM A AND B, WHO ARE MOVING RELATIVE TO EACH OTHER. (FOR EXAMPLE, A IS ON EARTH, AND B IS ON A SPACESHIP PASSING BY.) BOTH A AND B MEASURE A CERTAIN PHYSICAL QUANTITY (e.g., LENGTH OF AN OBJECT, TIME INTERVAL BETWEEN TWO EVENTS, VELOCITY OF A MOVING OBJECT, ETC.). HOW ARE THE VALUES THAT OBSERVERS A AND B MEASURE RELATED TO EACH OTHER? (ARE THEY NUMERICALLY EQUAL, RELATED BY A CERTAIN MATHEMATICAL EQUATION, ETC.?)‏

10. RELATIVITY THE ANSWER DEPENDS ON: 1. WHICH PHYSICAL QUANTITY IT IS: (a) OBSERVERS A AND B WILL MEASURE THE SAME NUMERICAL VALUE (300,000 KM/SEC) FOR THE SPEED OF LIGHT IN A VACUUM. (b) OBSERVERS A AND B WILL MEASURE DIFFERENT ANSWERS FOR THE LENGTH OF AN OBJECT, THE TIME INTERVAL BETWEEN TWO EVENTS, OR THE SPEED OF A MOVING OBJECT (OTHER THAN A PHOTON). 2. HOW A AND B ARE MOVING RELATIVE TO EACH OTHER: (a) IF A IS MOVING AT A CONSTANT VELOCITY RELATIVE TO B (OR VICE VERSA), THEN THE RULES RELATING WHAT A MEASURES AND WHAT B MEASURES ARE GIVEN BY SPECIAL RELATIVITY. (b) IF ONE OBSERVER IS ACCELERATING (CHANGING VELOCITY) RELATIVE TO THE OTHER, THEN THE RULES RELATING WHAT A MEASURES AND WHAT B MEASURES ARE GIVEN BY GENERAL RELATIVITY.

11. RELATIVITY CONSIDER THE CASE OF MEASURING TIME. OBSERVERS A (ON EARTH) AND B (ON SPACESHIP) BOTH MEASURE THE INTERVAL OF TIME BETWEEN TWO CERTAIN EVENTS. EXAMPLES OF TIME INTERVAL BETWEEN TWO EVENTS: - TIME BETWEEN PERSON B’S DEPARTURE FROM EARTH AND PERSON B’S ARRIVAL BACK AT EARTH AFTER A ROUND TRIP VOYAGE - TIME BETWEEN TWO “TICKS” OF A CLOCK LOCATED ON THE SPACESHIP (B’S CLOCK)‏ - TIME BETWEEN TWO “TICKS” OF A CLOCK LOCATED ON THE EARTH (A’S CLOCK)‏

12. SPECIAL RELATIVITY TIME DILATION: CONSIDER TWO OBSERVERS MOVING AT A CONSTANT VELOCITY RELATIVE TO EACH OTHER. THE TWO OBSERVERS WILL MEASURE DIFFERENT ANSWERS FOR THE INTERVAL OF TIME BETWEEN TWO EVENTS, i.e., TIME PASSES AT DIFFERENT RATES ON CLOCKS MOVING AT DIFFERENT SPEEDS. EXAMPLE: A SPACESHIP FLIES BY THE EARTH AT A CONSTANT VELOCITY CLOSE TO THE SPEED OF LIGHT. THERE ARE CLOCKS ON EARTH AND ON BOARD THE SHIP. BOTH THE PERSON ON THE SHIP (B) AND THE PERSON ON EARTH (A) CAN WATCH EACH OTHER’S CLOCKS (e.g., THROUGH TELESCOPES). WHAT DO THEY FIND?

13. SPECIAL RELATIVITY TIME DILATION: EACH OBSERVER SEES THE OTHER OBSERVER’S CLOCK AS RUNNING SLOW COMPARED TO HIS/HER OWN, e.g., THE OTHER PERSON’S CLOCK MOVES FORWARD ONLY 3 MINUTES WHILE YOUR OWN MOVES FORWARD 5 MINUTES. OBSERVER A SEES HIS OWN CLOCK AS “NORMAL” AND OBSERVER B’S CLOCK AS “SLOW.” OBSERVER B SEES HER OWN CLOCK AS “NORMAL” AND OBSERVER A’S CLOCK AS “SLOW.” EACH OF THEM SEES HIS/HER OWN CLOCK AS NORMAL AND THE OTHER PERSON’S CLOCK AS RUNNING SLOW, i.e., THEY DON’T AGREE ON WHICH CLOCK IS FAST AND WHICH CLOCK IS SLOW!

14. SPECIAL RELATIVITY – TIME DILATION WHICH OF THE OBSERVERS IS “CORRECT?” BOTH! TIME IS NOT AN “ABSOLUTE” UPON WHICH ALL OBSERVERS CAN AGREE! THIS IS A REAL EFFECT. - IT IS NOT SOME SORT OF OPTICAL ILLUSION! - IT CAN’T BE EXPLAINED BY THE FACT THAT THE LIGHT FROM A CLOCK TAKES A CERTAIN AMOUNT OF TIME TO REACH A DISTANT OBSERVER!

15. SPECIAL RELATIVITY – TIME DILATION TIME DILATION WORKS FOR ALL “CLOCKS” - MECHANICAL, ELECTRONIC, ATOMIC, BIOLOGICAL, ETC. THIS MEANS THAT PERSON A (ON THE EARTH) SEES ALL OF PERSON B’S BIOLOGICAL PROCESSES (INCLUDING AGING) OCCURRING IN “SLOW MOTION”. SIMILARLY, PERSON B SEES PERSON A’S BIOLOGICAL PROCESSES OCCURRING IN SLOW MOTION.

16. SPECIAL RELATIVITY – TIME DILATION THE FASTER THE RELATIVE SPEED OF THE SHIP AND THE EARTH, THE BIGGER THE DIFFERENCE IN THE TIME INTERVALS MEASURED BY THE TWO OBSERVERS. THE EFECT ISN’T DRAMATIC UNLESS THE RELATIVE SPEED IS NEAR THE SPEED OF LIGHT.

17. INTERSTELLAR TRAVEL BUT SPECIAL RELATIVITY (AND THEREFORE TIME DILATION) ONLY ADDRESSES THE CASE WHERE THE SPACESHIP MOVES RELATIVE TO EARTH AT A CONSTANT VELOCITY. IN ORDER TO COMPLETE A ROUND TRIP VOYAGE TO A DISTANT STAR, THE SHIP MUST ACCELERATE ALONG ITS WAY. EVEN IF MOST OF THE TRIP OCCUES AT A CONSTANT SPEED, ACCELERATIONS ARE NEEDED: –UPON TAKE-OFF FROM EARTH –CHANGING DIRECTION AND PERHAPS SPEED WHILE TURNING AROUND AT THE DESTINATION –DECELERATION TO LAND BACK ON EARTH THEREFORE WE NEED GENERAL RELATIVITY! NOTE: THIS POINT IS NOT EXPLAINED WELL IN THE TEXTBOOK!

18. THE TWIN PARADOX JANE AND JOE ARE TWINS. JANE TAKES OFF ON A ROUND TRIP INTERSTELLAR SPACE VOYAGE AT HIGH SPEED. JOE REMAINS ON EARTH. WHEN JANE RETURNS TO EARTH, WHICH TWIN IS OLDER? –TIME DILATION  EACH WOULD SAY THE OTHER IS YOUNGER! –THIS CAN’T BE TRUE, HENCE THE “PARADOX!”

19. EXPLANATION OF THE TWIN PARADOX SPECIAL RELATIVITY DOESN’T APPLY, BECAUSE JANE MUST ACCELERATE DURING HER TRIP. THEREFORE WE CAN’T USE SPECIAL RELATIVISTIC TIME DILATION. GENERAL RELATIVITY  A CLOCK RUNS MORE SLOWLY IF IT’S ACCELERATING. EVERYONE AGREES ON THIS! THEREFORE JANE IS YOUNGER!

20. EXAMPLE: ROUND TRIP TO A STAR 5 LY AWAY ASSUME WE ACCELERATE AT 1 g FOR 2.5 LY (UNTIL WE’RE HALFWAY THERE), REACHING A FINAL SPEED OF 90% c. NOW WE MUST DECELERATE AT 1 g FOR 2.5 LY SO THAT WE CAN STOP AT OUR DESTINATION. THEN REPEAT THE ENTIRE PROCESS ON THE WAY BACK. ELAPSED TIME (EARTH CLOCKS) = 20 YRS. ELAPSED TIME (SHIP CLOCKS) = 8 YRS. WHAT HAPPENED?? THE TWIN PARADOX! INTERSTELLAR TRAVEL

21. BECAUSE OF GENERAL RELATIVITY AND THE TWIN “PARADOX,” A TRAVELER IN A SHIP CAN ACCELERATE TO NEAR THE SPEED OF LIGHT AND MAKE A ROUND TRIP VOYAGE TO A DISTANT DESTINATION IN A REASONABLE TIME (A FEW YEARS FOR A NEARBY STAR), BUT… UPON HER RETURN, MORE TIME WILL HAVE ELAPSED ON EARTH THAN FOR HER.

22. INTERSTELLAR TRAVEL THE FARTHER THE DESTINATION... THE HIGHER THE SPEED THAT IS REACHED BY THE HALFWAY POINT... THE LARGER THE EFFECTS OF RELATIVITY... THE LARGER THE DIFFERENCE IN ELAPSED TIME FOR THE TRAVELER VS. ON EARTH IN AN EXTREME CASE, THE ASTRONAUT MAY RETURN TO FIND THAT HER FRIENDS AND RELATIVES MAY ALL BE DEAD, SOCIETY MAY HAVE CHANGED DRASTICALLY, ETC.

23. RELATIVITY “CLASSICAL” (PRE-RELATIVITY) RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY IS NOT CORRECT AT SPEEDS APPROACHING THE SPEED OF LIGHT (c). KE = (1/2) M v 2 IS VALID ONLY IF v << c KE = KINETIC ENERGY M = MASS v = SPEED (OR VELOCITY)‏ RELATIVITY TELLS US THAT: KE INCREASES MUCH MORE QUICKLY WITH v THAN THE ABOVE EQUATION PREDICTS. THIS IS SOMETIMES DESCRIBED AS AN INCREASE IN MASS AS v INCREASES. AS v  c, KE  INFINITY

24. RELATIVITY BECAUSE OF THIS, IN ORDER TO ACCELERATE AN OBJECT (WITH NONZERO MASS) TO THE SPEED OF LIGHT, AN INFINITE AMOUNT OF ENERGY WOULD BE NEEDED! THIS IS WHY A SPACESHIP CAN’T TRAVEL AT THE SPEED OF LIGHT! TO ACCELERATE TO EVEN NEAR THE SPEED OF LIGHT REQUIRES AN INCREDIBLY LARGE AMOUNT OF ENERGY. ASIDE: AN OBJECT WITH ZERO MASS CAN TRAVEL AT THE SPEED OF LIGHT (WITHOUT HAVING AN INFINITE AMOUNT OF ENERGY). IN FACT, IT ISN’T ALLOWED TO TRAVEL AT ANY OTHER SPEED. EXAMPLE: PHOTON

25. INTERSTELLAR TRAVEL CURRENT TECHNOLOGY: TOP SPEED FOR A SPACESHIP IS ABOUT 0.00004 c. AT THIS SPEED, RELATIVISTIC EFFECTS ARE NOT IMPORTANT. ASIDE: WE CAN ACCELERATE INDIVIDUAL PARTICLES, LIKE ELECTRONS AND PROTONS, TO NEARLY c (AND WE HAVE THEREBY VERIFIED THAT RELATIVITY REALLY IS TRUE). HOWEVER, WE HAVE SEEN THAT A SPACESHIP ACCELERATING AT 1g FOR SEVERAL YEARS WILL REACH A SPEED NEAR THE SPEED OF LIGHT, AT WHICH POINT RELATIVISTIC EFFECTS BECOME IMPORTANT. HOWEVER, TO ACCELERATE THE SHIP TO SUCH HIGH SPEEDS, AN ENORMOUS AMOUNT OF ENERGY IS REQUIRED.

26. PAYLOAD-MASS RATIO THE MORE MASSIVE THE PAYLOAD (PASSENGERS, CARGO, ETC.), THE MORE ENERGY (FUEL) IT TAKES TO MOVE IT. THE FASTER THE PAYLOAD IS MOVED, THE MORE ENERGY (FUEL) IT TAKES TO MOVE IT. THIS IS TRUE EVEN CLASSICALLY (I.E., WITHOUT RELATIVITY), BUT RELATIVITY MEANS WE NEED EVEN MORE ENERGY (FUEL). THE FUEL MUST ALSO BE MOVED! AS A RESULT, THE TOTAL MASS OF THE SPACESHIP INCLUDING FUEL (AT THE BEGINNING OF ITS TRIP) IS MUCH GREATER THAN THE MASS OF THE PAYLOAD! THEREFORE, WE NEED TO USE A VERY EFFICIENT FUEL SOURCE.

27. INTERSTELLAR HAZARDS BECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY, A 1 gm MASS MOVING AT 90% c HAS AN ENERGY EQUAL TO THAT OF A 30 KILOTON BOMB! (THE HIROSHIMA BOMB WAS 12 KILOTONS AND THE NAGASAKI BOMB WAS 22 KILOTONS.)‏ COLLISIONS WITH INTERSTELLAR GAS AND DUST WOULD BE SUFFICIENT TO HEAT A SPACECRAFT TO DANGEROUSLY HIGH TEMPERATURES AND PUNCH HOLES IN IT. COLLISIONS WITH HIGH ENERGY PHOTONS WOULD DAMAGE OCCUPANTS AND ELECTRONICS. “SHIELD” TECHNOLOGY IS NOT GOOD ENOUGH TO PROTECT AGAINST THIS.

28. THE BASIC LAWS OF PHYSICS DON’T MAKE IT IMPOSSIBLE, BUT THEY MAKE IT VERY DIFFICULT. CURRENT TECHNOLOGY IS NOWHERE NEAR ADEQUATE. EVEN WITH THE BEST TECHNOLOGY WE CAN IMAGINE BEING ABLE TO BUILD (MATTER- ANTIMATTER ANNIHILATION): - WE CAN’T GO FASTER THAN THE SPEED OF LIGHT (c)‏ - IT WOULD TAKE YEARS TO REACH EVEN THE NEAREST STAR BEYOND THE SUN - FUEL REQUIREMENTS ARE ENORMOUS INTERSTELLAR SPACE TRAVEL

29. PROCEDURE: - ACCELERATE AT 1 g UNTIL WE’RE HALFWAY TO OUR DESTINATION AND TRAVELING AT ALMOST c - THEN DECELERATE THE REST OF THE WAY THERE - THEN REPEAT THE ENTIRE PROCESS ON THE WAY BACK THE FARTHER THE DESTINATION, THE CLOSER TO c OUR MAXIMUM SPEED GETS, AND THE LARGER THE EFFECTS OF RELATIVITY BECOME. - THE ELAPSED TIME ON THE SHIP IS LESS THAN THE ELAPSED TIME ON THE EARTH (THE TWIN PARADOX)‏ - FUEL REQUIREMENTS ARE EVEN LARGER BECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY - PHOTONS AND INTERSTELLAR DUST BECOMES HAZARDOUS BECAUSE OF THE RELATIVISTIC RELATIONSHIP BETWEEN SPEED AND KINETIC ENERGY

30. METHODS OF PROPULSION (IN ORDER OF INCREASING EFFICIENCY)‏ CHEMICAL ROCKETS ION ROCKETS NUCLEAR-POWERED SPACECRAFT MATTER-ANTIMATTER ANNIHILATION NOTE: ALL OF THESE ARE “ACTIVE” PROPULSION METHODS, i.e., THE SPACECRAFT CARRIES ITS FUEL WITH IT

31. CHEMICAL ROCKETS CURRENT TECHNOLOGY FUEL IS BURNED (CHEMICAL REACTION)‏ EXHAUST (RESULT OF CHEMICAL REACTION) IS PUSHED OUT THE BACK REACTION TO THIS PUSHES ROCKET FORWARD (LIKE RECOIL OF A GUN WHEN FIRED)‏ MOST USE STAGES –SEVERAL ROCKETS STACKED ON TOP OF EACH OTHER –WHEN THE FUEL IN EACH STAGE IS USED UP, THE EMPTY FUEL TANK IS EJECTED (SO THAT YOU DON'T HAVE TO CARRY THE MASS OF EMPTY FUEL TANKS)‏

32. ION ROCKETS A FEW EXPERIMENTAL ONES HAVE BEEN TESTED ELECTRICALLY CHARGED PARTICLES (IONS) ARE PUSHED OUT THE BACK REACTION TO THIS PUSHES ROCKET FORWARD (LIKE RECOIL OF A GUN WHEN FIRED)‏ AMOUNT OF MATERIAL EJECTED PER SECOND IS LESS THAN FOR A CHEMICAL ROCKET, SO FUEL WILL LAST LONGER AFTER FIRING ENGINES FOR A LONG TIME, IT COULD REACH APPROXIMATELY 1% OF SPEED OF LIGHT

33. NUCLEAR ROCKETS COULD USE EITHER FISSION OR FUSION –FISSION IS BREAKING APART OF A LARGE NUCLEUS INTO SEVERAL SMALLER NUCLEI. –FUSION IS COMBINING OF SEVERAL SMALL NUCLEI TO MAKE A LARGER NUCLEUS. –BOTH FUSION AND FISSION BOMBS EXIST. –BUT WE ONLY HAVE THE TECHNOLOGY TO DO FISSION (NOT FUSION) IN A CONTROLLED WAY, i.e., IN A REACTOR. –WE MAY DEVELOP THE TECHNOLOGY FOR FUSION REACTORS WITHIN A FEW DECADES.

34. NUCLEAR ROCKETS SOME PLANNING DONE, BUT NEVER BUILT –PROJECT ROVER: FISSION USED TO HEAT GAS THAT IS PUSHED OUT THE BACK –PROJECT ORION: FUSION BOMBS RELEASED FROM BACK, THEY EXPLODE AND PUSH SHIP FORWARD –PROJECT DAEDALUS: FUSION-POWERED, BUT REACTOR TECHNOLOGY DOESN'T EXIST YET MIGHT BE ABLE TO REACH SPEEDS OF 10% OF SPEED OF LIGHT – WOULD TAKE DECADES TO REACH NEARBY STARS SAFETY ISSUES ARE A CONCERN (ESPECIALLY FOR FISSION)‏ WOULD VIOLATE EXISTING TREATIES

35. ANTIMATTER FOR EVERY TYPE OF PARTICLE, THERE EXISTS A TYPE OF ANTIPARTICLE. PARTICLE AND ANTIPARTICLE HAVE THE SAME MASS, BUT OPPOSITE ELECTRIC CHARGE. COULD MAKE ANTI-ATOMS OUT OF –ANTIPROTONS (NEGATIVE CHARGE)‏ –ANTINEUTRONS (NO CHARGE)‏ –POSITRONS OR ANTI-ELECTRONS (POSITIVE CHARGE)‏ THERE IS MORE MATTER THAN ANTIMATTER IN THE UNIVERSE!

36. MATTER-ANTIMATTER ANNIHILATION PARTICLE AND ANTIPARTICLE WILL BOTH ANNIHILATE (CEASE TO EXIST) WHEN THEY COLLIDE THE ENTIRE MASS OF BOTH IS CONVERTED TO ENERGY VIA E = m c 2 –FOR COMPARISON, APPROXIMATELY 1/100,000,000 OF THE MASS IS CONVERTED TO ENERGY IN A CHEMICAL REACTION, AND –APPROXIMATELY 1/1000 OF THE MASS IS CONVERTED TO ENERGY IN A NUCLEAR REACTION. ENERGY PRODUCED IS USUALLY IN THE FORM OF HIGH-ENERGY PHOTONS (GAMMA RAYS)‏

37. MATTER-ANTIMATTER ANNIHILATION PHOTONS PRODUCED WHEN PARTICLES AND ANTIPARTICLES ANNIHILATE CAN BE PUSHED OUT THE BACK, AND THE REACTION TO THIS ACCELERATES THE SHIP (AGAIN, LIKE THE RECOIL OF A GUN WHEN FIRED)‏ THIS IS WAY BEYOND CURRENT TECHNOLOGY! –ANTIMATTER IS VERY DIFFICULT AND EXPENSIVE TO PRODUCE, WOULD TAKE MORE ENERGY THAN HUMANS HAVE USED THROUGHOUT ALL HISTORY TO MAKE ENOUGH FOR A VOYAGE –ANNIHILATION WITH MATTER MAKES IT DIFFICULT TO WORK WITH ANTIMATTER CAN’T KEEP IT IN A CONTAINER MADE OF ORDINARY MATTER CAN MANIPULATE IT WITH MAGNETIC FIELDS

38. PAYLOAD MASS RATIO FOR 5 LY TRIP DESCRIBED EARLIER (10 LY ROUND TRIP): EVEN WITH PROPULSION VIA MATTER-ANTIMATTER ANNIHILATION (MOST EFFICIENT METHOD), WE NEED 400 POUNDS OF FUEL FOR EACH POUND OF PAYLOAD. THIS IS 30 TONS OF FUEL PER HUMAN (ENVIRONMENT NOT INCLUDED). THE PROBLEM IS EVEN WORSE FOR LESS EFFICIENT METHODS OF PROPULSION!

39. PASSIVE PROPULSION METHODS ADVANTAGE: NO NEED TO CARRY FUEL LIGHT SAIL OR SOLAR SAIL –LARGE “SAIL” PROPELS SHIP VIA PRESSURE EXERTED BY PHOTONS FROM STAR –SAIL MUST BE VERY LIGHT, THUS FRAGILE –ACCELERATION IS VERY SMALL –WORKS BEST (LARGEST ACCELERATION) NEAR A STAR –COULD GET TO NEARBY STARS WITHIN A CENTURY

40. PASSIVE PROPULSION METHODS VARIATION ON LIGHT SAIL –USE PHOTON PRESSURE OF LASER BEAM FROM EARTH (OR SPACE STATION)‏ INSTEAD OF LIGHT FROM A STAR –COULD REACH NEARBY STARS IN A FEW DECADES –ACCELERATING A SHIP TO HALF THE SPEED OF LIGHT WOULD TAKE 1000 x AS MUCH POWER AS CURRENT TOTAL HUMAN POWER CONSUMPTION –SHIP WOULD BE DEPENDENT ON LASER BEAM FROM EARTH –HOW WOULD THE SHIP SLOW DOWN WHEN APPROACHING ITS DESTINATION? WOULD NEED A LASER BEAM FROM DESTINATION PUSHING BACK ON SHIP POSSIBLE ANSWER: LASER IS USED TO HEAT PROPELLANT ON BOARD, PROPELLANT IS THEN EJECTED OUT IN FRONT OF THE SHIP

41. PASSIVE PROPULSION METHODS BUSSARD RAMJET –COLLECT INTERSTELLAR GAS (HYDROGEN) FOR USE AS FUEL –HYDROGEN IS FUEL FOR NUCLEAR FUSION –WOULD HAVE TO COLLECT FUEL FROM A HUGE REGION OF SPACE –NEED HUGE “SCOOP” (SIZE OF A PLANET), OR USE MAGNETIC FIELDS TO COLLECT HYDROGEN (MORE LIKELY)‏ –MUST HAVE A LOT OF ENERGY TO MAINTAIN MAGNETIC FIELDS –WAY BEYOND CURRENT TECHNOLOGY

42. OTHER APPROACHES ACCEPT THAT INTERSTELLAR TRAVEL WILL BE SLOW. ADVANTAGES: –DON'T NEED HUGE ENERGY REQUIREMENTS TO REACH HIGH SPEED –INTERSTELLAR HAZARDS NOT EXACERBATED BY HIGH SPEED HOW DO WE DEAL WITH THE LONG TIMES NEEDED FOR VOYAGES? –PUT CREW INTO HIBERNATION DON'T YET KNOW HOW TO DO THIS –HAVE GENERATIONS THAT ARE BORN AND DIE ON BOARD SHIP EN ROUTE TO DESTINATION CREW MAY FIGHT AMONG THEMSELVES, LOSE INTEREST IN MISSION, OR LOSE TECHNICAL EXPERTISE TO COMPLETE MISSION

43. WARP DRIVE?? SCIENCE FICTION “TRICK” FOR GOING FASTER THAN SPEED OF LIGHT (c). THE IDEA IS TO BEND OR WARP SPACE SO AS TO MAKE A “SHORT CUT” BETWEEN DISTANT OBJECTS. WE KNOW THAT MASSIVE OBJECTS BEND SPACE AROUND THEM: –STARS DO THIS A LITTLE –BLACK HOLES DO IT A LOT (THEY WARP SPACE SUFFICIENTLY TO “PINCH OFF” THE INTERIOR OF THE BLACK HOLE FROM THE REST OF THE UNIVERSE)‏

44. WARP DRIVE?? HOWEVER, WE CAN’T VERY WELL CARRY ALONG A BLACK HOLE WITH OUR SPACESHIP. –THE NEAREST BLACK HOLES ARE HUNDREDS OF LIGHT YEARS AWAY. –THE BLACK HOLE WOULD BE MUCH MORE MASSIVE THAN THE SHIP ITSELF. –WE COULDN’T SURVIVE NEAR A BLACK HOLE. (TIDAL FORCES WOULD RIP US APART.)‏ WORMHOLE: HYPOTHETICAL CONNECTION BETWEEN BLACK HOLES (“SHORT-CUT” TO PATH THROUGH NORMAL SPACE)‏ –IF THESE EXIST, PERHAPS WE COULD USE EXISTING ONES. –GETTING TO AND FROM THEM WOULD STILL BE A PROBLEM (NEAREST ONES HUNDREDS OF LY AWAY). –TIDAL FORCES WOULD STILL BE A PROBLEM.

45. WARP DRIVE?? IF WARP DRIVE IS POSSIBLE, IT WILL DEPEND ON BASIC LAWS OF PHYSICS THAT HAVEN'T BEEN DISCOVERED YET! –IT'S NOT JUST A MATTER OF IMPROVING TECHNOLOGY. –IT'S NOT ANALOGOUS TO “BREAKING THE SOUND BARRIER.”