Einstein and Quantum Mechanics: The End of the Classical Worldview

Special Theory of Relativity

In 1905, twenty-six-year-old patent clerk Albert Einstein published four articles, each of which changed physics. The special theory of relativity was based on two postulates: the laws of nature are the same for all inertial reference frames; the speed of light in a vacuum is constant for all observers, regardless of the motion of the source or observer.

The consequences were revolutionary: time slows down for moving objects; length contracts; mass increases with speed; space and time are not absolute, but form a unified four-dimensional space-time. The famous $E=mc^2$ — energy and mass are equivalent, an enormous amount of energy is hidden in a small mass.

This contradicted Newtonian intuition: in Newton's theory, time is absolute (flows the same everywhere), space is absolute (does not change). Einstein showed: these intuitions are true for low velocities (which explains Newtonian mechanics as an approximation), but are violated at speeds comparable to the speed of light.

General Theory of Relativity

In 1915, Einstein completed the general theory of relativity — a theory of gravity. The key idea: gravity is not a force (as in Newton), but the curvature of space-time. Massive objects curve the surrounding space-time; other objects move along the curves of this space-time.

Image: stretch a bedsheet, place a heavy ball in the center — the bedsheet sags. A smaller ball, rolling on the bedsheet, moves along a curve. This is analogous to the motion of planets around the Sun.

The predictions of the general theory were confirmed: deflection of light near the Sun (measured by Eddington in 1919); precession of Mercury's perihelion; gravitational redshift; gravitational waves (detected by LIGO in 2015, a hundred years after their prediction).

Quantum Mechanics and Its Interpretations

Quantum mechanics — a theory describing the behavior of particles at the micro-level — emerged in the 1920s from the works of Bohr, Heisenberg, Schrödinger, Dirac. It discovered fundamentally new features of reality:

Uncertainty principle (Heisenberg): it is impossible to simultaneously measure exactly both the position and momentum of a particle. This is not a technical limitation — but a fundamental property of nature.

Wave-particle duality: the electron is both a particle and a wave. In the double-slit experiment, the electron passes through both slits simultaneously (as a wave) — but is registered at one point (as a particle). Observation “selects” one outcome.

Quantum superposition: prior to measurement, a particle is in a superposition of states. Schrödinger’s cat is both alive and dead at the same time, until the box is opened. This is not a metaphor — it is a literal consequence of quantum mechanics.

Einstein-Podolsky-Rosen paradox and entanglement: two entangled photons, separated by a great distance, instantly “know” each other’s state upon measurement. Einstein called this “spooky action at a distance” and believed it pointed to the incompleteness of quantum mechanics. Bell’s theorems and subsequent experiments showed: Einstein was wrong. Nature really is “spooky”.

Quantum mechanics is the most precisely tested theory in the history of science (predictions are confirmed with an accuracy up to $10^{-12}$). But its interpretation remains disputed: the Copenhagen interpretation, the many-worlds, the de Broglie-Bohm, relational — all describe the same predictions, but give fundamentally different answers to the question “what happens in reality.”