Einstein’s theory of special relativity (AQA A-Level Physics): Revision Notes

12.3.2 Einstein's Theory of Special Relativity

Inertial Frames of Reference:

An inertial frame of reference is a frame that moves at a constant velocity relative to other frames. In an inertial frame:

• Objects either remain at rest or move at a constant speed in a straight line unless acted upon by an external force.
• A frame that is accelerating or rotating cannot be classified as an inertial frame because it would experience changes in velocity, leading to non-uniform motion.

Einstein's Theory of Special Relativity:

Einstein's theory of special relativity is a foundational concept in modern physics that describes how motion affects the observed properties of objects, such as length, time, and mass, particularly as they approach the speed of light. This theory is restricted to inertial frames of reference and is based on two key postulates:

1. The Speed of Light is Invariant:

• The speed of light in a vacuum is always the same, regardless of the relative motion between the observer and the light source.
• This means that, unlike other objects whose speeds add or subtract based on relative motion, light's speed remains constant at approximately 3.00 × 10⁸ m/s for all observers.

2. The Laws of Physics are the Same in All Inertial Frames:

• The fundamental laws governing physics apply equally in all inertial frames of reference.
• This postulate implies that there is no preferred frame of reference in the universe; physics operates identically whether one is on a moving train, in a stationary laboratory, or on a spacecraft moving at high speed.

Implications of Einstein's Theory:

These two postulates lead to several counterintuitive but experimentally verified phenomena, including:

• Time Dilation: Time moves slower for an observer in motion relative to a stationary observer.
• Length Contraction: Objects moving at high speeds appear contracted in the direction of motion to a stationary observer.
• Relativistic Mass Increase: As an object moves faster, its mass effectively increases, making it harder to accelerate as it approaches the speed of light.