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Special relativity
This option topic introduces time dilation, length contraction and mass-energy.
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Objectives
10
Flashcards
10
Questions
90 min
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AqaA LevelPhysicsTurning points in physics
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Michelson-Morley experiment4 objectives
- Describe the purpose of the Michelson-Morley experiment.
- Explain the significance of the null result.
- Link the result to rejection of the ether model.
- Discuss how the experiment supports constant light speed.
Einstein's theory of special relativity4 objectives
- State the principle of relativity for inertial frames.
- State the constancy of the speed of light in vacuum.
- Explain why simultaneity can be frame-dependent.
- Apply relativity postulates qualitatively.
Time dilation4 objectives
- Describe time dilation for moving observers.
- Identify proper time in relativity problems.
- Use time dilation equations.
- Explain evidence for time dilation.
Length contraction4 objectives
- Describe length contraction in the direction of relative motion.
- Identify proper length in relativity problems.
- Use length contraction equations.
- Distinguish length contraction from optical appearance.
Mass and energy in relativity4 objectives
- State mass-energy equivalence.
- Calculate rest energy from mass.
- Link mass-energy equivalence to nuclear and particle processes.
- Explain energy changes in relativistic contexts.
Key terms
Michelson-Morley experimentEtherNull resultSignificance of null resultethernull resultPrinciple of RelativityInertial FrameSpeed of lightLight speed constancyRelativity of SimultaneityLorentz Transformation
Exam tips
- Michelson‑Morley Purpose: Ether Test: To recall the purpose of the Michelson‑Morley experiment, think of it as a test for the ether: it measured the difference in light travel time along two perpendicular arms. The expected fringe shift if the ether existed is given by ΔL = 2v²L/c². Substituting v ≈ 30000 m s⁻¹ (Earth’s orbital speed), L = 10 m (arm length), and c = 3×10⁸ m s⁻¹ gives ΔL = 2×(30000)²×10/(3×10⁸)² ≈ 2×10⁻⁷ m, i.e. 0.2 µm. This tiny shift would have produced a measurable fringe shift, but the experiment found none – the null result. Remembering this calculation helps you explain why the experiment’s purpose was to detect the ether and why its null result was significant.
- Recall the null result shows no ether wind: When asked about the significance, state that the null result proved there was no detectable ether wind, supporting the constancy of light speed in all inertial frames.
Common mistakes
- Purpose of Michelson-Morley experiment: The experiment was actually aimed at detecting a difference in light travel time caused by the Earth's motion through a hypothetical ether. If the Earth moved at 30 km s⁻¹, the expected time difference for a 1 m arm would be Δt = (L/c)(v²/c²) = (1 m / 3×10⁸ m s⁻¹)(9×10⁸ m² s⁻² / 9×10¹⁶ m² s⁻²) = 3.3×10⁻¹⁷ s. This extremely small Δt would produce a fringe shift of ΔN = 2Lv²/(λc²) ≈ 0.0001, far below the experimental resolution. The null result showed no such shift, leading to the rejection of the ether model and supporting the constancy of the speed of light.
- Null Result Significance: Formula/rule: Δλ = λ1 - λ2 = 0. Substitution: λ1 = λ2. Working: Since the two arms of the interferometer produce identical wavelengths, Δλ = 0. Answer: No shift in interference pattern. Units/conclusion: The null result shows that the speed of light is the same in all directions, implying no ether and supporting special relativity.
Practice preview
- Which statement gives the clearest definition needed for Describe The Purpose Of The Michelson Morley Experiment?
- A student makes a mistake while revising Describe The Purpose Of The Michelson Morley Experiment. Which correction is most accurate?
- Which response best uses evidence or a diagram feature to support Describe The Purpose Of The Michelson Morley Experiment?
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