CourseCourseQ BankQuestion BankDocsDocuments

A.5.5 (HL)—Two postulates of special relativity

Syllabus
First assessment 2025
Objective
—
Level
HL

State the Two Relativity Postulates

HL only

Postulate 1: relativity

The laws of physics have the same form in all inertial reference frames. No inertial observer is privileged by uniform motion; an experiment performed entirely within a frame cannot reveal its constant velocity relative to another inertial frame.

Postulate 2: invariant light speed

Every inertial observer measures the same speed of light in vacuum, cc, independent of the motion of the source or observer. This replaces the Galilean expectation that measured velocities simply add.

Use the consequence

Together, the postulates require space and time coordinates to transform differently from the Galilean model. They lead to Lorentz transformations, time dilation, length contraction and relativity of simultaneity. The syllabus does not require deriving those equations.

Common trap

The second postulate does not say that every object moves at cc, or that light speed is the same in every material. It refers to light in vacuum measured by inertial observers.

A.5.5 (HL) Exam Analysis

HL only

Assessment in practice

1–3 marks
How it is assessed

The objective is assessed through short explanation or consequence questions in relativistic settings. Identify which postulate supplies the frame argument and which supplies the constant-light-speed argument before explaining the consequence.

Command terms

State / Explain

What earns marks

State both postulates separately. For the first, name all inertial frames and the same form of the laws of physics. For the second, state that all inertial observers measure the same vacuum light speed c, independent of source or observer motion. Do not replace either statement with a consequence such as time dilation.

Watch for

Giving only “nothing can travel faster than light” and omitting the two postulates or the condition of inertial observers.

Representative question

Question 1

[Maximum number: 3]

Explain, by reference to the equivalence principle, why the frequency of the photon measured at B will be larger than f0f_{0}.