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Quiz Relativity: Einstein's special and general theories

From E = mc² to curved spacetime: special and general relativity, time dilation, black holes. Eleven verified questions with clear explanations.

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What is the exact speed of light in a vacuum?

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  1. Q1. What is the exact speed of light in a vacuum?

    • 186,000 m/s
    • 299,792,458 m/s
    • 1,000,000 km/s
    • It varies with observer
    Light's speed in vacuum is exactly 299,792,458 m/s, denoted c. Since 1983, the metre has actually been defined from this value. It's the cosmic speed limit for matter and information.

  2. Q2. In what year did Einstein publish special relativity?

    • 1879
    • 1905
    • 1915
    • 1925
    Special relativity appeared in his 'Annus Mirabilis' paper 'On the Electrodynamics of Moving Bodies' in 1905. The same year he also explained the photoelectric effect and Brownian motion.

  3. Q3. What does the equation E = mc² tell us?

    • Mass is zero
    • Energy equals mass times the speed of light squared
    • Light has no energy
    • Force equals mass times c
    Mass and energy are equivalent: a tiny mass corresponds to enormous energy because c² ≈ 9 × 10¹⁶ m²/s². This is the principle behind nuclear fission, fusion, and stellar power.

  4. Q4. What is time dilation?

    • Time stops in a vacuum
    • Moving clocks tick slower as seen from a stationary frame
    • Time speeds up underground
    • Time only depends on temperature
    A clock moving at speed v ticks slower by a factor 1/√(1−v²/c²). GPS satellites must correct for both this effect and gravitational time dilation; without it, positions would drift several km per day.

  5. Q5. When did Einstein publish general relativity?

    • 1905
    • 1909
    • 1915
    • 1921
    General relativity was finalized in November 1915. It generalizes special relativity to include gravity, describing it as the curvature of spacetime caused by mass and energy.

  6. Q6. In general relativity, gravity is interpreted as...

    • A magnetic force
    • Curvature of spacetime caused by mass and energy
    • A type of friction
    • An illusion
    Massive objects curve spacetime, and other objects move along the straightest possible paths (geodesics) in that curved geometry. John Wheeler summed it up: 'Matter tells spacetime how to curve; spacetime tells matter how to move.'

  7. Q7. What was the famous 1919 observation that confirmed general relativity?

    • Discovery of Pluto
    • Bending of starlight by the Sun during a solar eclipse
    • Detection of the photon
    • Splitting of the atom
    Arthur Eddington measured the deflection of starlight passing near the Sun during the May 29, 1919 eclipse. The angle matched Einstein's prediction (about 1.75 arcseconds), making him world-famous overnight.

  8. Q8. What is a black hole, in relativistic terms?

    • An empty region of space
    • A region where spacetime curvature is so extreme not even light escapes
    • A dark planet
    • A burned-out star with no gravity
    Inside a black hole's event horizon, escape velocity exceeds c. The Event Horizon Telescope released the first direct image of one (M87*) in April 2019, then of Sagittarius A* in 2022.

  9. Q9. What are gravitational waves?

    • Sound waves in space
    • Ripples in spacetime predicted by Einstein in 1916
    • Light reflecting off planets
    • Ocean waves caused by the Moon
    Gravitational waves are propagating distortions of spacetime caused by accelerating massive objects. LIGO detected them directly for the first time on Sept 14, 2015, from two merging black holes. Nobel Prize 2017.

  10. Q10. What is the twin paradox?

    • Identical twins always disagree
    • A travelling twin returns younger than the stay-at-home twin
    • Twins age twice as fast in space
    • Two photons annihilate
    Due to time dilation, a twin on a near-light-speed round trip ages less than the one on Earth. It's not really a paradox: only one twin actually accelerates, breaking the symmetry between frames.

  11. Q11. What does length contraction predict?

    • Objects expand at high speed
    • Moving objects appear shorter along their direction of motion
    • Mass becomes negative
    • Light bends in metals
    An object moving at speed v measures shorter along its motion direction by the same Lorentz factor 1/√(1−v²/c²). For everyday speeds the effect is utterly tiny, only noticeable near light-speed.

  12. Q12. What is gravitational time dilation?

    • Time runs at the same rate everywhere
    • Time runs slower in stronger gravitational fields
    • Gravity stops time
    • Time runs faster near Earth's surface than in space
    Clocks run slower deeper in a gravitational well. A clock at Earth's surface runs about 38 microseconds per day slower than one on a GPS satellite — a correction GPS must apply to stay accurate.

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