Einstein’s Two Postulates
The Foundation of Special Relativity
Postulate 1
The laws of physics are the same in all inertial reference frames. That means no constant-velocity observer gets a special privilege in describing physical laws.
Postulate 2
The speed of light in vacuum is the same for all inertial observers, no matter how the source or observer moves.
What the Postulates Replace
Old intuition
- Time is universal
- Velocities add in the ordinary way
Einstein’s view
- Time can differ between observers
- Light speed stays fixed at
Why This Is Revolutionary
These postulates force us to abandon the idea that everyone shares one global clock and one universal notion of distance.
How the Logic Works
-
1
Assume the laws of physics should not depend on constant motion.
-
2
Accept that light’s speed is the same for all inertial observers.
-
3
Notice that ordinary ideas about time and distance cannot survive unchanged.
-
4
Build a new framework: special relativity.
Which statement is one of Einstein’s two postulates?
This is the first postulate of special relativity.
Correct answer: The laws of physics are the same in all inertial frames
What happens to ordinary ideas of space and time after accepting Einstein’s postulates?
The postulates make classical space and time incomplete.
Correct answer: They must change
Why can’t we keep ordinary velocity addition for light?
If speeds simply added classically, different observers would measure different light speeds, contradicting the second postulate.
Correct answer: Because it would make light’s speed change between observers