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QLDPhysicsQuick questions
Unit 4: Revolutions in modern physics
Quick questions on Special relativity: postulates and time dilation (QCE Physics Unit 4)
10short 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?Show answer
Time dilation is not an optical illusion. Direct experimental confirmations:
What is postulate 1: The principle of relativity?Show answer
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?Show answer
The speed of light in vacuum is $c = 3 \times 10^8$ m s$^{-1}$ in every inertial frame, regardless of the motion of the source or the observer.
What is earth-frame round-trip time?Show answer
Distance $= 8$ light-years; speed $= 0.80 c$; time $= 8 / 0.80 = 10$ years.
What is astronaut-frame round-trip time?Show answer
Apply time dilation. The astronaut's clock is the proper time; the Earth's clock is dilated. So $t_{\text{Earth}} = \gamma t_{\text{astronaut}}$, giving $t_{\text{astronaut}} = 10 / 1.667 \approx 6$ years.
What is confusing proper time with dilated time?Show answer
Proper time is the shorter time, measured in the frame where the two events occur at the same place.
What is wrong formula direction?Show answer
$t = \gamma t_0$. The dilated time is larger by a factor $\gamma$.
What is treating relativity as applicable only to extreme speeds?Show answer
Even at $v / c = 0.1$, $\gamma \approx 1.005$, a 0.5 percent effect. The effects are real at all speeds; they are just very small at non-relativistic speeds.
What is confusing $\gamma$ with $1/\gamma$?Show answer
Length contraction has factor $1/\gamma$ (length shortens); time dilation has factor $\gamma$ (time lengthens).
What is mixing inertial and non-inertial frames?Show answer
Special relativity strictly applies only to inertial (non-accelerating) frames. Accelerating frames (like Twin B during turnaround) require care or general relativity.