Einstein's two postulates state that the laws of physics are identical in all inertial reference frames and that every inertial observer measures the same speed of light in a vacuum. This is an IB Physics HL-only concept in A.5 Galilean and special relativity.
| Postulate | Meaning |
|---|---|
| Principle of relativity | The laws of physics have the same form in every inertial reference frame. No experiment performed entirely within an inertial frame can establish that the frame is in absolute motion. |
| Constancy of the speed of light | The speed of light in a vacuum is always for every inertial observer, regardless of the motion of the light source or observer. |
An inertial reference frame is one that is not accelerating. Newton's laws apply in their standard form within such a frame. The first postulate therefore rejects the existence of a preferred or absolutely stationary inertial frame.
The second postulate conflicts with Galilean velocity addition. If a spacecraft moves at speed and emits light forward, Galilean reasoning would predict that a stationary observer measures . Special relativity instead requires both the spacecraft observer and the stationary observer to measure the light's speed as exactly .
For both postulates to remain true, different inertial observers cannot agree universally on measurements of space and time. The Lorentz transformations replace Galilean transformations, leading to consequences including time dilation, length contraction, and the relativity of simultaneity. These are consequences of the postulates, not additional postulates.
A common misconception is that the second postulate applies only when the light source is stationary. It applies regardless of the relative motion between the inertial observer and the source.
Exam technique: For a State question, give both postulates precisely without explaining their consequences. For an Explain question, connect the constant value of to the failure of Galilean transformations and the need for Lorentz transformations.
