Chapter XVIII: The Special and General Principle of Relativity

What is the special principle of relativity?

It states that the laws of nature are the same in all uniformly moving (inertial) reference frames.

What is the difference between relative motion and the special principle of relativity?

• Relative motion simply means that motion can be described relative to different reference frames

• The special principle asserts that the form of physical laws remains unchanged between uniformly moving frames.

What does Einstein mean by “neither of the reference bodies is specially marked out”?

No inertial reference frame is privileged; all are equally valid for describing physical laws.

What is a Galileian reference body?

A reference frame in which a free particle moves in a straight line at constant speed

i.e., an inertial frame.

What limitation does the special principle of relativity have?

It applies only to inertial (non-accelerating) reference frames, not to those undergoing acceleration.

What is the general principle of relativity?

It states that the laws of nature are the same in all reference frames, including those in non-uniform (accelerated) motion.

What problem arises when trying to apply relativity to non-uniform motion?

Accelerated motion causes detectable effects (like feeling a jerk when brakes are applied), suggesting a difference in the laws of mechanics.

Why does a passenger in a braking train feel a jerk forward?

Because the non-uniform motion causes mechanical effects not present in uniform motion, challenging the idea that all frames are equivalent.

How does this observation of feeling the jerk when braking initially challenge the general principle of relativity?

It seems to imply that non-uniform motion has absolute physical effects, violating the idea that all frames are equally valid.