1. Late 1800’s: Physics was triumphant!
Newton’s gravity, Maxwell’s equations of electricity and magnetism, Boltzmann’s thermodynamics An old Kirchhoff (of circuit laws fame) said to young Max Planck: “Why do you want to go into physics, where everything is already understood?”

2. One little problem
We didn’t understand details of blackbody radiation. Solution: photons and quantum mechanics
(Planck made a good career choice.)

3. And a second little problem
One observer sees electric fields (E) from charged particles at rest. A second observer (moving to the left vs the first) see the charges moving to the right, and so sees E and B fields! Electricity and magnetism equations didn’t work right for different observers. Solution: special relativity

4. “Modern” Physics breakthroughs
Relativity
1905: special relativity: time, length and the speed of light
1915: general relativity: how gravity and acceleration affect time and space
Quantum mechanics
1913: Bohr proposes model for H atom
1924: deBroglie proposes QM waves
1925: Schrodinger’s wave equation

5. Classical “relativity”
The observers see different x-velocities and different Dx for the ball…
…but they agree on the time the ball was in the air (Dt). Time was considered universal.

6. Einstein’s musing: Bouncing light in a train
Sally sees a pulse of light go up to the ceiling and back to the laser. She sees the events at the same x.
Mike sees the ceiling and floor moving, and the path is longer. He sees the events at different x’s.
If we assume (as people did in 1900) that they both measure the same time (Dt) for the light to travel, who must measure the fastest speed of light?

7. Einstein’s correct guess (postulate)
There is no universal time. The speed of light c in vacuum is universal: the same in every reference frame.
P1. If they must agree on the speed of light, who measures the longest time for the pulse to hit the laser again? a) Mike b) Sally c) both the same

8. Mike’s time vs Sally’s time

9. Time dilation
Proper time interval for an event: (Hess:) measured by an observer who sees the beginning and end occur at the same point in his reference frame. (Text): measured by an observer at rest relative to the event being observed.
This is always the shortest time measured.
All other observers see a longer or “dilated” time interval.

10. Reference frames
Two observers are in different reference frames moving at a constant v with respect to each other.
Motion is relative. No experiment can define which observer is “moving” and which is not. They both move relative to each other. (But we can detect accelerations ( and forces) with experiments.)

11. Time dilation
Joe and Mary each measure their own heartbeat to be 100 beats/min. A heart monitor emits pulses of light from their spaceships. Joe sees Mary’s ship passing by him going very fast, and measures Mary’s heart rate to be 80 beats/min. Mary measure’s Joe’s heartbeat to be 80 beats/min. Who is measuring the proper time for Mary’s heartbeat? a) Mary b) Joe
How fast are they moving vs each other? Working in units of c

13. Length contraction
Proper length: the length of an object (or distance between points) measured by someone who sees it at rest, and is the greatest length observed. Someone moving will see shorter (contracted) lengths and distances, but only along the direction of relative motion.
This is how Mary sees her ship and Joe’s ship

14. Length contraction
A woman on the Empire State Building sees superman go by very fast as shown How does superman look to the woman? How does the tower look to superman?

15. Implications for space travel
Planets X and Y are at rest with respect to each other. Mary is on planet X. Joe is traveling from X to Y P2. Who will measure the proper length or distance (always the longest) of the trip from X to Y? A. Mary B. Joe

16. Implications for space travel
Planets X and Y are at rest with respect to each other. Mary is on planet X. Joe is traveling from X to Y. P3. Which is the proper time (always the shortest) for the trip (in which frame do the beginning and happen at the same position)? A. The time interval Mary measured for Joe’s rocket to pass X then pass Y B. The time interval between Joe’s seeing planet X pass his window, then seeing planet Y pass the window.

17. Implications for space travel
By reflecting light off planet Y, Mary measures the distance to Y to be 1 ly. Joe is traveling from X to Y at 0.9 c (hence g = 2.0). P4. What distance between planets would Joe measure in his frame, as he moves toward Y? A. 1/4 ly B. ½ ly C. 1 ly D. 2 ly E. 4 ly So Joe measures a time of ____ yr to get to Planet Y. And Mary measures a time of ____ yr for Joe to go to Planet Y.

18. Implications for space travel
A space ship can never go faster than the speed of light, but you can get to a planet “one light year away”…“in less than a year”! But measured by who?

19. Videos
Time and relativity Start at 0:25. Correction: This theory says nothing about time travel! And the expert’s comment that you can “alter time if you move fast enough through space” is wrong. Each observer moves vs the other, and each has his own time, just different. Einstein's postulates