Relativity Advanced Quiz

Mass-energy equivalence means mass and energy are interchangeable. When uranium fissions, the resulting fragments plus released neutrons have slightly less total mass than the original nucleus. That mass deficit — typically about 0.1% of the original mass — appears as kinetic energy of the fragments and radiation. The energy is enormous because the conversion factor is c², roughly 9 × 10¹⁶ joules per kilogram. Chemical reactions rearrange electrons and convert a negligible fraction of mass; nuclear reactions rearrange nucleons and access energy six orders of magnitude deeper.

Einstein's central insight is that gravity is not a force at all. Mass and energy curve the geometry of spacetime, and objects in freefall follow geodesics — the straightest possible paths through that curved geometry. An orbiting planet is not being "pulled" by the Sun. It is moving in a straight line through spacetime that the Sun's mass has curved into an orbit. Newton's gravitational force is the low-speed, weak-field approximation of this geometric reality. The curvature framework eliminates action-at-a-distance entirely: spacetime is curved locally by local mass-energy, and objects respond to their local geometry.

The event horizon is not a physical surface, not the singularity, and not where gravity becomes infinite. It is a causal boundary: the set of spacetime points from which outward-directed light rays can never escape to distant observers. An astronaut crossing the event horizon of a sufficiently large black hole would notice nothing locally — spacetime is smooth there, tidal forces can be mild, and local physics is normal. But once past the horizon, every possible future trajectory leads further inward. The horizon is defined by its causal property: it is the point of no return for information, the boundary beyond which events are permanently disconnected from the external universe.

LIGO splits a laser beam and sends the halves down two perpendicular 4-kilometer arms. The beams reflect off mirrors and recombine. When both arms are exactly the same length, the beams destructively interfere — no signal. A gravitational wave stretches space along one arm while compressing it along the other, creating a tiny path-length difference that shifts the interference pattern. The sensitivity required is extraordinary: LIGO detects length changes of 10⁻¹⁸ meters, one-thousandth the diameter of a proton. Two detectors separated by 3,000 km provide correlation to distinguish astrophysical signals from local noise — the same principle as requiring consensus across distributed witnesses.

The cosmic horizon exists because space itself is expanding. Beyond a certain distance, the rate of spatial expansion exceeds the speed of light. Light emitted from those regions will never reach us — not because it has not had enough time, but because the distance it must cross is growing faster than it can travel. This is a permanent partition, not a temporary one. No future technology, no amount of waiting, will ever bring those regions into causal contact with us. Option C captures part of the story (the observable universe is finite) but misses the crucial mechanism: it is not a time limitation but a causal disconnection caused by the expansion rate exceeding c.