Einstein’s three most important (among others) legacies

2. Einstein’s three most important (among others) legacies • A revolution in our concept of time. • The possibility to convert mass into energy and vice versa. • Gravity as curved spacetime.

3. Time is the only “thing” we everybody all share! It’s like a river. We all swim in it. • River graph. Sharing means the time duration of an event you measure is the same as what I do. Time is absolute, independent of its observers.

4. Time measurement.Periodic motion. • Pendulum • Precise clock. • Atomic clock.

5. It could be done with more and more sophistication and accuracy. Atomic clock would error less than one second in 3 million years.

6. In the beginning of 20th century, it was a great plan to put clocks at grid points in a cityto provide or sell accurate time to city people.Commercialization and commoditization of time.

7. How to Synchronize all the clocks? Use light signal from a central facility to synchronize all the clocks For this to work, we need to subtract the time light travels from the central to the clock. Does the light travel at constant speed?

9. 相對性原則

12. 伽利略變換

13. Physicists long believe if the rule is correct relative to one observer at rest, it should also be true relative to other observers moving in constant speed. You can’t tell whether you are truly at rest or moving. You can only say you are at rest or moving relative to other observers. This is called Relativity Principle.

14. This is a kind of symmetry. Symmetric objects are beautiful in nature. So people also say physics is beautiful. That’s actually why Einstein insisted to himself that light speed is always the same

15. The earth is moving in the universe like we run in the air or take a train. It seems highly unlikely that the speed of light will always be the same all the time. So Maxwell’s rule and calculation couldn’t be right for all the observers.

17. ？

18. 直覺與經驗：光速與觀察者的運動狀態相關 The earth is moving in the universe like we run in the air or take a train. It seems highly unlikely that the speed of light will always be the same all the time. So Maxwell’s rule and calculation couldn’t be right for all the observers.

19. 理論思索：光速與觀察者的運動狀態無關 1860 Maxwell showed that light is a kind of electromagnetic signal, call EM wave, which can be generated by a antenna. He also calculated the speed of EM wave and light is m/s But ………. If we measure from a train…..

20. Michelson and Morley (1887) Fig 39-4, p.1249

21. The speed of light is always the same no matter how the observer moves.

22. Relativity of simultaneity It was Einstein who first realized how the constant light speed implies a total revolution of our concept of time The two explosions are simultaneous for Sam but not simultaneous for Sally!!!!! Time is no smooth river.

25. The time observed from rest observer is longer than what it is designed to be. A moving clock looks slowed down from an observer at rest !!!!!

27. Test of time dilation 1977 Hafele and Keating flew four atomic clocks twice around the globe. Relativity prediction is test within 10%. A few years later, another team flew round and round above Chesapeake bay for 15H. For a sonic jet, the dilation is about 10-8s. Predictions are good within 1%.

28. This is so revolutionary that Albert felt really sorry towards Newton: “Newton, forgive me, you found the only way which, in your age, was just about possible for a man of highest thought, and --creative power.” 1949 The idea of relative time is so mysterious that Einstein felt the need to defend for the god: The subtle is the lord, but malicious he is not.

29. 星際大戰 B A I II

30. 仔細分析事件1對齊及事件2發砲的時間差 對齊與發砲是在 I座標系中同時 故還未對齊，已經發砲 B -v A O’ I II O

31. 停車問題

32. II I I早於II II早於I 時間先後也是相對的

33. 4-vector

34. 座標軸旋轉 羅倫茲轉換 不變量 4-vector vector

35. Proper Time τ 它是一個羅倫茲變換下的不變量 Proper time 的小變化

36. 動量

37. 牛頓版動量守恆 u u u’ ？ u 完全非彈性碰撞 v=u O’ O 牛頓版的動量守恆遵守伽利略變換下的相對性原則

38. 但？ u u u’ ？ u v=u O’ O 牛頓的定義使動量守恆不遵守相對性原則

39. 新的定義 以Proper Time取代時間 在O’座標系中的動量 如果在O座標系中，Px及Pt都守恆，在O’座標系中，P’x自然保證守恆 動量P是4-vector

40. u u u’ u v=u O’ O

41. 新定義與舊定義的關係 接近牛頓的定義

42. P0是什麼？ 可以猜 動量守恆與能量守恆便整合成一個定律。 動量與能量的分別只是表面的

43. E=mc2 Using the relativity principle, Einstein is able to derive that the energy of an object can be written as For v=c, the energy is infinite. Hence you are never able to push an object faster than the speed of light! Even at rest, an object still contain an energy due to its mass: It opens up the possibility to convert mass into energy or vice versa.

44. Even at rest, an object still contain an energy due to its mass: It opens up the possibility to convert mass into energy or vice versa. 質量是能量的一種形式，能量守恆蘊含質量可以轉換為其他形式的能量，其他形式的能量亦可轉換為質量，質量不再守恆。

45. u u 質量不守恆，動能轉換成質量。

46. 原子能

47. If we can smash a coin and destroy it………. This is more than a day’s output of a power plant. Smash a coin and generate power plant.

48. Nuclei

49. 束縛態