Advanced Physics → Modern Physics → Relativity (Foundations)
Core idea
The laws of physics are the same for all inertial observers.
The laws of physics are the same for all inertial observers.
1. Why Relativity Was Needed
Classical physics assumes absolute space and universal time. Experiments involving light revealed contradictions in this view.
2. Inertial Frames and Relativity
An inertial frame is one moving at constant velocity. No inertial frame is physically preferred over another.
3. Relativity of Simultaneity
Events simultaneous in one frame need not be simultaneous in another.
Time itself becomes frame-dependent.
4. Time Dilation
Moving clocks run slower compared to stationary ones. This effect is not mechanical—it arises from spacetime geometry.
5. Length Contraction
Objects moving relative to an observer appear shorter along the direction of motion.
6. Spacetime and Worldlines
Events are best described in spacetime diagrams. Motion becomes geometry.
7. Mass–Energy Equivalence
Mass and energy are two forms of the same physical quantity.
Conceptual Problems
Why can no inertial frame be considered “at rest”?
Answer
All inertial frames obey the same physical laws.
Why is time dilation not caused by mechanical slowing?
Answer
It arises from spacetime geometry, not physical resistance.
Why does relativity not contradict everyday experience?