AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
The two postulates of Einstein’s theory of special relativity: 1. physical…
Full specification wording
Einstein’s theory of special relativity (A-level only): The two postulates of Einstein’s theory of special relativity: 1. physical laws have the same form in all inertial frames 2. the speed of light in free space is invariant.
Einstein’s special relativity rests on two postulates.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Einstein’s special relativity rests on two postulates. First, the laws of physics take the same form in every inertial frame, so no inertial observer can perform an experiment that reveals absolute rest or absolute motion. Second, the speed of light in free space is invariant: every inertial observer measures the same value c, approximately 3.00 × 10⁸ m s⁻¹, regardless of the motion of the source or the observer. The second postulate is radical because it conflicts with everyday velocity addition; if a torch moves towards you, you still measure the light at c, not c plus the torch’s speed. Together the postulates force us to abandon absolute time and lead to time dilation, length contraction and the relativity of simultaneity.
What earns the marks
- States the first postulate: all physical laws have the same form in every inertial frame, so all inertial frames are equivalent.
- States the second postulate: the speed of light in free space is invariant and has the same value c for all inertial observers.
- Explains that the invariance applies to light in free space and is independent of the motion of the source or the observer.
- Contrasts the postulates with Galilean velocity addition, where relative velocities simply add or subtract.
- Uses a concrete example, such as a moving source emitting light, to show that the measured speed remains c.
- Connects the postulates to consequences such as time dilation and the relativity of simultaneity.
Common mistakes and how to put them right
Believing that the speed of light depends on the speed of the source.
Put it right: All inertial observers measure the same value c in free space.
Thinking the first postulate means all observers measure the same numerical values for every quantity.
Put it right: The laws have the same form, while measured lengths, times and simultaneity may differ.
Applying the postulates to light travelling through a medium such as glass or water.
Put it right: The invariant speed c refers specifically to light in free space.
Exam tips
- Quote both postulates precisely, using the phrase inertial frames in the first and free space in the second.
- When explaining a thought experiment, state which postulate is being used and what an observer in each inertial frame measures.
- Avoid saying that the speed of light is constant in all situations; specify free space and inertial observers.
Try the lesson
The start of the Einstein’s theory of special relativity (A-level only): The two postulates of Einstein’s theory of special relativity: 1. physical laws have the same form in all inertial frames 2. the speed of light in free space is invariant. lesson, made from checked teaching notes for this specification point.
Have a go first
According to Einstein's second postulate of special relativity, which quantity is invariant?
Listen
Einstein’s special relativity rests on two postulates. First, the laws of physics take the same form in every inertial frame, so no inertial observer can perform an experiment that reveals absolute rest or absolute motion. Second, the speed of light in free space is invariant: every inertial observer measures the same value c, approximately 3.00 × 10⁸ m s⁻¹, regardless of the motion of the source or the observer. The second postulate is radical because it conflicts with everyday velocity addition; if a torch moves towards you, you still measure the light at c, not c plus the torch’s speed. Together the postulates force us to abandon absolute time and lead to time dilation, length contraction and the relativity of simultaneity. A strong answer earns marks for these points. States the first postulate: all physical laws have the same form in every inertial frame, so all inertial frames are equivalent. States the second postulate: the speed of light in free space is invariant and has the same value c for all inertial observers. Explains that the invariance applies to light in free space and is independent of the motion of the source or the observer. Contrasts the postulates with Galilean velocity addition, where relative velocities simply add or subtract. A common slip: Believing that the speed of light depends on the speed of the source. Instead, all inertial observers measure the same value c in free space. Exam tip: Quote both postulates precisely, using the phrase inertial frames in the first and free space in the second.
Key words
Say what each word means before you turn the card.
Equivalence of all inertial frames
Try answering aloud or in your head before turning the card.
Invariance of the speed of light in free space
Try answering aloud or in your head before turning the card.
Rejection of absolute space and time
Try answering aloud or in your head before turning the card.
Consequences for simultaneity, time and length
Try answering aloud or in your head before turning the card.
See every definition
- Equivalence of all inertial frames
- Physical laws have the same form in all inertial frames, so no inertial observer can perform an experiment that reveals absolute rest or absolute motion.
- Invariance of the speed of light in free space
- Every inertial observer measures the same value c, regardless of the motion of the source or the observer.
- Rejection of absolute space and time
- Together the postulates force us to abandon absolute time.
- Consequences for simultaneity, time and length
- The postulates lead to time dilation, length contraction and the relativity of simultaneity.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Einstein’s theory of special relativity (A-level only): The two postulates of Einstein’s theory of special relativity: 1. physical laws have the same form in all inertial frames 2. the speed of light in free space is invariant.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge