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Home , Light cone

Four Dimensions Velocity and Light cone cTime v < c v = c • Space (Cartesian) coordinate : (x,y,z) A – Distance2 = (x interval)2+(y interval)2+(z interval)2 • Fourth dimension is v > c – More general coordinate = : (x,y,z,t) B – (Spacetime distance)2 Space = c2(t interval)2 – [(x interval)2+(y interval)2+(z interval)2] O • • The steeper the slope is, the smaller the velocity is. cTime – “Events” connected by “Worldlines” • The lines with 45 degrees tilt represent the “light cone” • Since nothing can travel faster than light… – O and A can communicate, but O and B cannot communicate Space – This diagram shows a “”

Spacetime Distances Acceleration and Deceleration Time v < c v = c Time A A B C C

v > c B Space O Space (Spacetime distance)2 O = c2(time interval)2 – (space interval)2 • OA : Decelerated OA 1. “Timelike” worldline : (Spacetime distance)2 > 0 • OB : Constant velocity OC 2. “Null” worldline : (Spacetime distance)2 = 0 • OC : Accelerated OB 3. “Spacelike” worldline : (Spacetime distance)2 < 0 Special Relativity (1905) Relativity of Space and Time A’s point of view B’s point of view • Two Invariants ct ct’ – , c – Spacetime distance 2 2 2 2 (ds =c dt -dx ) A B A B • Unification of space and time x x’ – No absolute space or time • A’s space coordinate, x, does not coincide with B’s, x’. exists: Relativity Rather, x is a combination of x’ and ct’. • does not • The same is true for time coordinate. include gravity – This means that simultaneous events in A’s coordinate would not appear simultaneous in B’s coordinate. (1879- • But, spacetime distance remains unchanged. 1955)

Time Dilation and Intuitive way to ct ct ct’ ct’ understand it A’s frame B’s frame • From your point of view, the ball appears to move faster; however, light x’ cannot travel faster! x x’ • Therefore, it must x take light more time • When A sees B moving, B’s time interval appears to be to come back down longer (clock ticks more slowly; ) ed and B’s to the laser length appears to be shorter (length contraction). And – Time Dilation vice versa. Relativistic Gamma Factor Mass Increase 1 • ! is always greater than 1. " = • A pushes B (whose mass at rest is m) by 2 v •As v approaches c, ! becomes large. applying a force F. 1! 2 c •When v=c, ! is infinite. – Acceleration is given by a=F/m. – Velocity acquired would be v=a dt=Fdt/m • B’s unit time in A’s frame equals A’s unit time in • When B is moving, the clock ticks more slowly A’s frame multiplied by !. (Hence time dilation) – B feels the force for a shorter time – Be careful! The time actually elapsed in B’s frame – v’=a dt’=Fdt’/m=Fdt/(m!) gets shorter because the unit time gets longer. • B’s unit length in A’s frame equals B’s unit • Thus, the mass of B appears to be bigger by !. length in B’s frame divided by !. (Hence length • Nothing can be accelerated to the speed of light, contraction) because the mass becomes infinite.

Twin Paradox Case 1 • A and B are at rest at the same place • There are twins, A and B Original Frame until 1. ct • B moves relative to A • Then A and B go on a trip on opposite – A’s point of view directions. C • B is moving at speed v 3 • A and B turn around and come back at • B’s clock ticks more slowly by !. events 2. • Therefore, B appears to be aging more slowly. 2 2 • A and B finally meet at event 3. – B’s point of view A B • In this case, A’s and B’s worldlines are • A is moving at speed v 1 • A’s clock ticks more slowly by !. symmetric. • Therefore, A appears to be aging more slowly. • A and B have traveled the same • So, which one is older, when they meet? x spacetime distance. – – Therefore, A and B have aged the same years. Case 1 (a different point of view) Case 2 • C’s point of view C’s Frame Original Frame • A remains at rest at all . – C is moving to the left with respect to the ct original frame ct • B leaves home at event 1, turns around at event 2, and finally meets A at event 3. • A and B are moving to the right together C C until event 1. 3 • In this case, A’s and B’s worldlines are 3 • Then A is at rest but B speeds up. not symmetric!

2 • A turns around earlier than B. A 2 2 • What happens? – Then B is at rest but A moves to the right faster A B B – The answer is that A has aged more than B. than before 1 1 • Why? • A and B finally meet at event 3. – B’s spacetime distance is shorter than A’s • In this case, A’s and B’s worldlines are still – Remember, ds2=c2dt2-dx2 x symmetric. x – A and B have traveled the same spacetime distance; thus, A and B have aged the same years.

Case 2 (a different point of view) So, what was it? • Motion of A and B remains completely relative C’s Frame • In C’s frame, A is moving to the only when both are moving at constant velocity. ct left at all times. B is initially moving to the left, together with A. – Motion has to be inertial for two frames to be C completely equivalent. • B becomes at rest at event 1, and • However, for two people to know their initial ages 3 then moves to the left faster than 2 before. and then meet later again, the motion cannot stay A B • A and B meet at event 3. inertial ! two frames are no longer equivalent. 1 They meet here, Non-inertial • In this case, A’s and B’s worldlines inertial but they don’t are still not symmetric! know each other’s initial age x – A has aged more than B. – B’s spacetime distance is still shorter.