Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45NSCI 314LIFE IN THE COSMOS17 - INTERSTELLAR SPACE TRAVELDr. Karen KolehmainenDepartment of Physics, CSUSB http://physics.csusb.edu/~karen/THE PROBLEM WITH INTERSTELLAR SPACE TRAVELSTARS 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!INTERSTELLAR TRAVELEXAMPLECONSIDER 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.COMPARISON OF TRAVEL TIMESMETHOD SPEED (KM/S) TIME (IN YEARS) NEEDED TO TRAVEL A DISTANCE OF 1 LYCAR 0.03 10,000,000JET PLANE 0.3 1,000,000VOYAGER 12 25,000SPACECRAFTFUSION 3000 100SPACECRAFT(DOESN’T YET EXIST)PHOTONS 300,000 1INTERSTELLAR TRAVELBECAUSE 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 IT MAY BE SUPERSEDED BY A MORE COMPLETE THEORY SOMETIME IN THE FUTURE, BUT IT'S VERY UNLIKELY THAT THE MORE COMPLETE THEORY WILL DISAGREE WITH RELATIVITY ON SUCH A FUNDAMENTAL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.INTERSTELLAR TRAVELBECAUSE 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 TIMESOMETIMES “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?ACCELERATIONTYPICAL CAR ACCELERATES AT 1 OR 2 m/s2(PICKS UP 1 OR 2 m/s OF SPEED PER SECOND)GRAVITY ACCELERATES A FALLING OBJECT AT ABOUT 10 m/s2 = 1 g (g = ACCELERATION DUE TO GRAVITY)A COMFORTABLE ACCELERATION FOR HUMANS IN A SPACESHIP IS 1 g = 10 m/s2(THIS WILL FEEL LIKE NORMAL EARTH GRAVITY TO TRAVELERS IN THE SHIP)EXAMPLE: ROUND TRIP TO A STAR 5 LY AWAYASSUME 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 TRAVELRELATIVITYRELATIVITY 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.?)RELATIVITYTHE 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.RELATIVITYCONSIDER 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)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?SPECIAL RELATIVITYTIME 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!SPECIAL RELATIVITY – TIME DILATIONWHICH 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