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Special Relativity

Time dilation, length contraction, and mass-energy equivalence.

Jun 12, 2026··Fork (sign in)Last verified: Dec 17, 2025
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Einstein's 1905 postulates and their consequences: time dilation, length contraction, E=mc^2.

#physics#relativity#einstein#spacetime
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Special Relativity

The Two Postulates

  1. The laws of physics are the same in all inertial reference frames.
  2. The speed of light in vacuum is c≈3×108 m/sc \approx 3\times 10^8\,\text{m/s}c≈3×108m/s for all observers.

Lorentz Factor

γ=11−v2/c2\gamma = \frac{1}{\sqrt{1 - v^2/c^2}}γ=
1−v2/c2​
1
​

Time Dilation and Length Contraction

Δt′=γ ΔtL′=L/γ\Delta t' = \gamma\,\Delta t \qquad L' = L/\gammaΔt′=γΔtL′=L/γ

Mass-Energy Equivalence

E=mc2E2=(pc)2+(mc2)2E = mc^2 \qquad E^2 = (pc)^2 + (mc^2)^2E=mc2E2=(pc)2+(mc2)2

See also article-general-relativity for the curved-spacetime extension.

The pendulum animation above demonstrates classical mechanics intuition before we break it with relativity.

Related events

  • Einstein's Annus Mirabilis
    Jun 30, 1905Physics History