Einsteins three most important among others legacies A

Time is the only “thing” we everybody all share! It’s like a river. We

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

It could be done with more and more sophistication and accuracy. Atomic clock would

In the beginning of 20 th century, it was a great plan to put

How to Synchronize all the clocks? Use light signal from a central facility to

Physicists long believe if the rule is correct relative to one observer at rest,

This is a kind of symmetry. Symmetric objects are beautiful in nature. So people

The earth is moving in the universe like we run in the air or

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

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

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

• The speed of light is always the same no matter how the

伽利略變換錯在哪裡？ It was Einstein who first realized how the constant light speed implies a

The time observed from rest observer is longer than what it is designed to

Test of time dilation 1977 Hafele and Keating flew four atomic clocks twice around

This is so revolutionary that Albert felt really sorry towards Newton: “Newton, forgive me,

E=mc 2 Using the relativity principle, Einstein is able to derive that the energy

Even at rest, an object still contain an energy due to its mass: It

If we can smash a coin and destroy it………. This is more than a

Count the mass difference The mass total decreases and the difference turns into energy.

With the current rate of burning, the sun will last for another five billion

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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.

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

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

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

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.

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…. .

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.

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

• The speed of light is always the same no matter how the observer moves. Maxwell方程式在不同的慣性座標系中，型式不變。伽利略變換必須修正

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 The two explosions are simultaneous for Sam but not simultaneous for Sally!!!!! Time is no smooth river.

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 !!!!!

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 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

長度縮短效應 Length Contraction

4 -vector

u u u’ u v=u O O’

E=mc 2 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.

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。