Postulates of special relativity
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Flip to reveal answersState Einstein's first postulate of special relativity.
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All 12 Flashcards — Postulates of special relativity
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Question
State Einstein's first postulate of special relativity.
Answer
The **laws of physics are the same in all inertial (non-accelerating) reference frames**.
Question
State Einstein's second postulate of special relativity.
Answer
The **speed of light in a vacuum is the same for all inertial observers**, regardless of the motion of the source or observer.
Question
What is the constant value of the speed of light?
Answer
$c = 3.00 \times 10^{8}$ m s⁻¹ — the same for every inertial observer.
Question
What is an inertial reference frame?
Answer
A frame moving at **constant velocity** — no acceleration (no speeding up, slowing down, or turning).
Question
A ship at 0.50c shines a torch forward. What speed does a planet observer measure for the light?
Answer
Exactly **c**, not 1.5c — by postulate 2 light's speed never adds on the source's speed.
Question
Classical vs relativistic: do speeds add for light?
Answer
Classically speeds add; **relativistically light always measures c** for everyone, so they do not add.
Question
Name the cosmic speed limit and why it exists.
Answer
**c** — the postulates make it impossible for anything with mass to reach or exceed the speed of light.
Question
What does 'simultaneity is relative' mean?
Answer
Whether two events happen **'at the same time' depends on the observer's motion** — observers in relative motion can disagree.
Question
Why can't two objects each at 0.90c have a relative speed of 1.80c?
Answer
Because **c is the speed limit**, so any relative speed must stay **below c**; velocities do not add the everyday way near c.
Question
Are space and time absolute in special relativity?
Answer
**No** — lengths and time intervals depend on the observer's motion; only the speed of light c is the same for all.
Question
How do you explain why moving observers disagree on timing?
Answer
Because **both measure light at the same speed c**, they are forced to disagree about **when** events happen.
Question
In the exam, how should you phrase postulate 2?
Answer
'The speed of light in a vacuum is the same for **all inertial observers, regardless of the motion of the source or observer**.'
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Galilean and special relativity (HL)
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