Two observers in different inertial frames moving at constant velocity relative to each other must necessarily disagree about which of the following?
AThe speed of light in vacuum
BThe form of Maxwell's equations for electromagnetism
CWhether two spatially separated events occurred simultaneously
DThe outcome of a local mechanical experiment
The first postulate guarantees that the laws of physics — including electromagnetism and mechanics — are identical in all inertial frames, ruling out options B and D. The second postulate guarantees both observers measure the same c, ruling out option A. But simultaneity of spatially separated events is frame-dependent: events simultaneous in one inertial frame are generally not simultaneous in another moving relative to it. This relativity of simultaneity is forced by the constancy of c.
Question 2 True / False
Special relativity's corrections to Newtonian mechanics are unmeasurable at everyday speeds, so Newtonian mechanics is exactly correct for objects moving slowly compared to light.
TTrue
FFalse
Answer: False
Special relativity applies at all speeds without exception — Newtonian mechanics is an approximation that becomes increasingly accurate as v/c → 0, but it is never exactly correct. GPS satellites orbit at roughly 14,000 km/h (~0.0013% of c) yet require relativistic corrections to maintain centimeter-level positioning accuracy. 'Exactly correct' is too strong a claim; relativity is always the more fundamental theory.
Question 3 Short Answer
What experimental and theoretical evidence motivated Einstein's two postulates? Give at least one example of each.
Think about your answer, then reveal below.
Model answer: Experimental: The Michelson-Morley experiment (1887) failed to detect any variation in the speed of light due to Earth's motion through the supposed ether, providing direct evidence that c is the same in all directions regardless of the observer's motion. Theoretical: Maxwell's equations of electromagnetism predict a fixed electromagnetic wave speed c with no dependence on the motion of the source or observer — inconsistent with Newtonian velocity addition.
Einstein's postulates were not invented arbitrarily — they formalized what the physics of the late 19th century was demanding. Maxwell's theory already implied a universal c; the Michelson-Morley null result confirmed that no preferred ether frame exists. The postulates resolved the contradiction between electromagnetism and Newtonian mechanics by revising mechanics rather than Maxwell's equations.