# Question #72b9c

Nov 22, 2017

Relativity Principle: Laws of physics are the same in all inertial frames.

#### Explanation:

Relativity Principle: Laws of physics are the same in all inertial frames.

What does that mean?:
Suppose you are inside a train with all the windows closed. The train is an inertial frame if it is either at rest or smoothly gliding at a constant velocity. The relativity principle states that it is absolutely impossible to know if the train is at rest or moving at a uniform velocity, by performing an experiment. But the moment the driver slams the brake, the train is no longer inertial and it will betray the state of motion.

Galileo Galilei was the first to reason this out and state. But electrodynamics seemed to violate this. So its validity was questioned for a while before Albert Einstein established it back to its original status. In that process he dislodged common sense notions about space and time.

The common sense notion was that space is absolute (distance between two points in space is the same for all inertial observers) and time is absolute (time interval between two events is the same for all inertial observers). These assumptions were not true. Space is relative and Time is relative. All counterintuitive things in Special Relativity arise from these facts.