Postulates of relativity
Einstein's two ideas: the laws of physics are the same for everyone moving at constant velocity, and light in a vacuum has the same speed c for all observers.
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Analogy
Two people on trains measuring the same beam of light and getting the same speed.
Example
c = 3.00e8 m/sSame for every observer.Nothing with mass reaches c.
In practice
Nobody, however fast they chase a light beam, measures it moving slower than 3.00e8 m/s.
Practice
Flashcards
- What are Einstein's two postulates?Same laws for all constant-velocity observers, and the speed of light is the same for all.
- Why can't an object with mass reach light speed?Its energy would grow without limit as v approaches c.
Interview questions
- Describe time dilation and calculate the Earth-time for a trip that takes 3 years on a spaceship at 0.8c.
- gamma = 1.67
- Moving clocks run slow
- 3 years x 1.67 = 5 years on Earth
Questions people ask
Why can nothing go faster than light?
As an object with mass speeds up, its energy grows as gamma times mc squared. This becomes infinite as speed approaches c. No finite push can supply infinite energy. Light itself has no mass and always moves at exactly c in a vacuum.
Was Newton wrong?
Newton's laws are an excellent approximation at everyday speeds. They fail only near light speed, where relativity takes over. At 300 m/s, the difference is about one part in a trillion. Relativity includes Newton's laws as the limit of slow motion.