Einsteins three most important among others legacies A

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.

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.

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. 時間是絕對的 時間如一條河流，大家一起共有，穩 定而不能改變流速

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

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?

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.

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

How to Synchronize all the clocks? Use light signal from a central facility to synchronize all the clocks Does the light travel at constant speed? 地球的運動早晚不同，光速是否就不同？

Supernova 1994 D in the outskirt of the galaxy NGC 4526

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

Michelson and Morley (1887)

伽利略變換錯在哪裡？ It was Einstein who first realized how the constant light speed implies a total revolution of our concept of time Relativity of Simultaneity 同時性 The two explosions are simultaneous for Sam but not simultaneous for Sally!!!!! Time is no smooth river.

從Sally的觀點來看 Sally Sam The two explosions are simultaneous for Sam but not simultaneous for Sally!!!!! 相同兩事件是否同時是相對的，與觀察者運動狀態有關！

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 !!!!! 移動中的鐘看起來較靜止時走得慢！ time dilation 時間延遲

Test of time dilation 時間延遲 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 15 H. For a sonic jet, the dilation is about 10 -8 s. Predictions are good within 1%.

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.

Time Dilation

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