The Principle of Relativity
In 1905, Einstein built special relativity on just two starting assumptions. The first sounds almost too simple to be radical: the laws of physics look exactly the same to every observer moving at constant velocity, however fast. Simple as it is, it rules out any experiment ever detecting your own smooth motion from the inside.
Einstein's first postulate
Einstein's special theory of relativity, published in 1905, is built on just two starting assumptions, or postulates — statements taken as true from the outset, from which everything else in the theory is derived. The first postulate states: the laws of physics are the same in all inertial frames of reference.
An inertial frame of reference is any frame of reference moving at constant velocity — constant speed, in a constant direction, with no acceleration at all. A laboratory sitting at rest on the ground is an inertial frame. So is a train moving perfectly smoothly along a straight, level track at a constant $80\,\text{km}\,\text{h}^{-1}$. So is an aircraft cruising at a constant altitude, constant speed, in a straight line. What makes all of these inertial is not that they're stationary — the train and the aircraft are clearly moving — but that their velocity isn't changing.
By contrast, a car accelerating away from traffic lights, a car braking, or a car turning a corner is not an inertial frame, because its velocity is changing in speed, direction, or both. The distinction matters enormously: the first postulate only makes its claim about inertial frames, and (as later lessons on general relativity-adjacent ideas would explore) accelerating frames behave differently.
There is no experiment — mechanical, electrical, optical, or otherwise — that could be performed entirely inside a smoothly moving laboratory to detect its own motion, without looking outside at some external reference point. Every inertial observer, whatever their constant velocity happens to be relative to anyone else, finds that exactly the same physical laws apply inside their own frame: the same laws of mechanics, the same laws of electromagnetism, the same value for every physical constant. There is no privileged "frame at rest" that physics treats differently from any other inertial frame — every inertial observer's point of view is equally valid.
A relatable example
Imagine dropping a ball while standing inside an aircraft cruising smoothly at constant altitude and constant velocity, with no turbulence. The ball falls straight down, lands at your feet, and behaves in every way exactly as it would if you dropped it while standing still on the ground — even though the aircraft (and you, and the ball) might be moving at $900\,\text{km}\,\text{h}^{-1}$ relative to the ground below. Nothing about the experiment of dropping the ball reveals that the aircraft is moving at all; you would get exactly the same result whether the plane were flying at $900\,\text{km}\,\text{h}^{-1}$ or sitting parked on the runway. Only by looking out of the window — bringing in information from outside the aircraft's own frame — could you tell the two situations apart.
This is precisely what the first postulate says must be true, for any physical law, in any inertial frame whatsoever: motion at constant velocity is, from the inside, physically undetectable. It's a principle with deep roots stretching back to Galileo, who made essentially the same point about ships sailing smoothly — but Einstein's radical step, covered in the next lesson, was to insist this principle must apply not just to mechanics, but to the behaviour of light itself.
Key takeaways
- Special relativity rests on two postulates; the first states that the laws of physics are the same in all inertial frames of reference.
- An inertial frame is one moving at constant velocity — constant speed and direction, with no acceleration.
- A smoothly moving train or cruising aircraft is an inertial frame; an accelerating, braking, or turning car is not.
- No experiment performed entirely inside a smoothly moving frame can detect that frame's own motion, without reference to something outside it.
- Dropping a ball inside a smoothly flying plane behaves exactly as it would on the ground — the plane's high speed relative to the ground is undetectable from inside.
- Every inertial observer, at whatever constant velocity, experiences exactly the same physical laws; no inertial frame is privileged over any other.