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Unit 4: Revolutions in modern physics

Quick questions on Special relativity: postulates and time dilation (QCE Physics Unit 4)

9short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is time dilation is real?
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Time dilation is not an optical illusion. Direct experimental confirmations:
What is postulate 1: The principle of relativity?
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The laws of physics are the same in all inertial (non-accelerating) reference frames. No experiment can detect uniform motion.
What is postulate 2: The constancy of the speed of light?
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The speed of light in vacuum is c=3×108c = 3 \times 10^8 m s−1^{-1} in every inertial frame, regardless of the motion of the source or the observer.
What is earth-frame round-trip time?
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Distance =8= 8 light-years; speed =0.80c= 0.80 c; time =8/0.80=10= 8 / 0.80 = 10 years.
What is astronaut-frame round-trip time?
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Apply time dilation. The astronaut's clock is the proper time; the Earth's clock is dilated. So tEarth=γtastronautt_{\text{Earth}} = \gamma t_{\text{astronaut}}, giving tastronaut=10/1.667≈6t_{\text{astronaut}} = 10 / 1.667 \approx 6 years.
What is wrong formula direction?
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t=γt0t = \gamma t_0. The dilated time is larger by a factor γ\gamma.
What is q1?
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State Einstein's two postulates. [2 marks]
What is q2?
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Calculate the time-dilated half-life of a particle with rest half-life 1.5 ns1.5 \text{ ns} at v=0.95cv = 0.95c. [3 marks]
What is q3?
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A cosmic-ray muon at 0.99c0.99c has proper lifetime 2.2 microseconds2.2 \text{ microseconds}. (a) Calculate γ\gamma and the Earth-frame lifetime. (b) Determine the distance travelled before decay.

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