Questions: Einstein's Relativity Revolutions: Special and General Theory

Question 1

What was the 'Michelson-Morley experiment' (1887), and why did its result create a crisis for classical physics?

Model answer: The Michelson-Morley experiment attempted to detect the 'luminiferous aether' -- the medium through which light waves were thought to propagate, analogous to air for sound. If the aether existed and Earth moved through it, light sent in different directions should show slightly different speeds due to the 'aether wind.' The experiment used an interferometer to detect this difference with high precision. It found no difference -- light speed appeared the same in all directions. This was deeply troubling because the aether was considered necessary for electromagnetic wave propagation. Various patches were proposed (Lorentz contraction, aether drag), but they were ad hoc. Einstein's special relativity dissolved the problem by abandoning the aether entirely: light has a constant speed in all inertial reference frames by postulate.

The Michelson-Morley result did not directly cause special relativity -- Einstein later said he was barely aware of it. But it illustrates the empirical pressure that made the aether problematic and shows how conceptual revolutions often respond to accumulated anomalies.

Question 2

Special relativity's most famous equation, E=mc2, implies what physical relationship?

• A. Energy and mass are different forms of the same thing -- mass can be converted to energy and vice versa, with c2 as the conversion factor
• B. An object moving at the speed of light would have infinite kinetic energy
• C. The energy of a photon equals its mass times the square of its frequency
• D. Matter and antimatter annihilate according to conservation of mass

Answer: (A)

E=mc2 states that mass is a form of energy and energy has equivalent mass. The factor c2 (about 9x1016 m2/s2) means a tiny amount of mass corresponds to enormous energy. This is why nuclear reactions release so much energy: a small fraction of nuclear mass converts to energy. It also means any object at rest has 'rest energy' proportional to its mass. The equation unified mass and energy as aspects of a single quantity, overturning classical mechanics' assumption that they were independently conserved.

Question 3

How did Einstein's general relativity reconceive gravity compared to Newton's theory?

Model answer: Newton conceived gravity as a force acting at a distance -- massive objects pull each other instantaneously across space. General relativity (1916) replaced this: massive objects curve spacetime geometry, and what we perceive as gravitational attraction is actually objects following the straightest possible paths (geodesics) through curved spacetime. The Earth orbits the Sun not because the Sun pulls it but because the Sun's mass curves spacetime, and Earth's orbit is the straightest path through that geometry. This reconception made gravity a geometric property of spacetime rather than a force, unified gravity with the structure of space and time, and predicted phenomena impossible in Newtonian gravity: gravitational lensing of light, gravitational time dilation, gravitational waves, and black holes.

General relativity's conceptual departure from Newton is profound: gravity is not a force but the curvature of the arena in which forces act. This required 10 years of mathematical development after special relativity, involving entirely new mathematical tools (tensor calculus, Riemannian geometry).

Question 4

Arthur Eddington's 1919 solar eclipse expedition confirmed general relativity by observing that light from distant stars was bent by the Sun's gravity.

• T. True
• F. False

Answer: True

General relativity predicted that light passing near a massive object would be deflected by twice the amount predicted by Newtonian gravity. During a solar eclipse, stars near the Sun become visible; their apparent positions (shifted by the Sun's gravitational lensing) can be measured. Eddington's 1919 expedition to Principe Island measured this deflection and found results consistent with Einstein's prediction. The result was announced publicly in London, making Einstein internationally famous. Later analysis has found Eddington's data had significant uncertainties, but subsequent eclipse expeditions confirmed the prediction. The 1919 result was nevertheless scientifically important and symbolically transformative.

Question 5

What phenomena predicted by general relativity have been experimentally confirmed in the 21st century?

Model answer: General relativity has been confirmed repeatedly with increasing precision. Gravitational lensing (light deflection) has been extensively observed and used as a tool for mapping dark matter. Gravitational time dilation -- clocks run slower in stronger gravitational fields -- is large enough to affect GPS satellites, which must correct for it to maintain accuracy. Gravitational waves -- ripples in spacetime caused by accelerating masses -- were directly detected by LIGO in 2015 from colliding black holes, confirmed the existence of binary black hole systems, and opened gravitational wave astronomy. The Event Horizon Telescope imaged the shadow of the black hole in M87 in 2019 and Sagittarius A* (the Milky Way's central black hole) in 2022, confirming the existence of black holes predicted by general relativity.

LIGO's detection of gravitational waves was among the most precise measurements in scientific history -- it detected a change in detector arm length of 10-18 meters (smaller than a proton). This technological achievement validated a prediction Einstein himself doubted would ever be testable.