AQA A LEVEL Physics Special relativity

📚 1. Classical (Newtonian) Physics Assumptions Time and space are absolute — the same for all observers. Velocities add linearly (Galilean transformations). Light was thought to travel through a medium called the aether. These ideas held until the late 19th century. 🌠 2. The Aether Theory Maxwell's equations predicted that light is a wave, needing a medium → the luminiferous aether. Aether was believed to be stationary and fill all space. Earth’s movement through it should create an “aether wind”, affecting the speed of light. ⚗️ 3. Michelson-Morley Experiment (1887) Aimed to detect the aether wind by measuring changes in the speed of light in different directions using an interferometer. 🧪 Result: No difference detected in the speed of light in any direction. Contradicted the aether theory. Suggested that the speed of light is constant in all directions. 🧠 4. Einstein’s Postulates of Special Relativity (1905) Einstein proposed two postulates that form the basis of special relativity: Laws of physics are the same in all inertial frames (no preferred frame). The speed of light (c) is constant for all observers, regardless of motion. 🕓 5. Key Predictions of Special Relativity ⏳ Time Dilation Moving clocks run slower as observed from a stationary frame. t = t₀ / √(1 − v²/c²) where: t₀ = proper time (in moving frame) v = relative speed c = speed of light 📏 Length Contraction Moving objects appear shorter in the direction of motion. L = L₀ √(1 − v²/c²) 🧲 Relativity of Simultaneity Events that are simultaneous in one frame may not be simultaneous in another moving frame. ⚡ Mass-Energy Equivalence Mass and energy are interchangeable: E = mc² → A small amount of mass can produce enormous energy. 🚀 6. Experimental Evidence for Special Relativity Muons from cosmic rays: More reach Earth than expected due to time dilation. Particle accelerators: Particles gain mass and experience time dilation at near-light speeds. GPS systems: Must correct for relativistic time effects to remain accurate. 🧪 7. Significance of Special Relativity Replaced Newton’s absolute space and time with a space-time framework. Changed our understanding of reality at high speeds. Foundations for modern physics including particle physics, cosmology, and quantum field theory.

What was the aim of the Michelson-Morley Experiment?

To detect the presence of ether and measure the speed of light as it changed with different rotations in respect to the ether.

What was the expectation of the Michelson-Morley experiment and how did it differ from the result?

It was expected that when rotating the blocks and mirrors by 90 degrees, the interference pattern should change as the speed of light should be changing, due to the ether.

There was no change in the interference pattern, so the speed of light was determined to be constant and ether did not exist.

State Einstein’s two postulates of special relativity.

1. The laws of physics are the same in all inertial frames of reference.

2. The speed of light in a vacuum is constant and independent of the motion of the source or observer.

What is time dilation?

Time runs slower for an observer when observing a moving object than it does for the object itself.

This effect increases greatly as you approach relativistic speeds.

Why is time dilation a consequence of special relativity?

Since the speed of light is the same for all refernece frames. the time experienced by an observer will differ depending on their relative velocities and frames of reference.

Define proper time

Time observed by an object, or an observer that is stationary with respect to the moving object.

How does muon decay provide evidence for time dilation?

Intensity of muons with distance decreases less than expected. This is because the half life to human observers is much larger than the actual half life of the muon due to time dilation.

How does special relativity effect lengths (Use a rod as an example)?

AN observer parallel to an moving object will observe a shorter length than an observer at rest relative to the rod.