Theory of Relativity Explained: Einstein's Two Big Ideas
Space, time, and gravity at extreme speeds or scales
Einstein published two theories of relativity that permanently altered our understanding of space, time, and gravity. Special relativity (1905) showed that the speed of light is constant for all observers, that time slows down at high speeds, and that mass and energy are equivalent (E=mc²). General relativity (1915) went further: gravity is not a force but a curvature of spacetime caused by mass. The predictions of both theories have been confirmed repeatedly with extraordinary precision — from GPS satellites to gravitational waves detected a century after Einstein predicted them.
Physics: First direct gravitational wave detection
2015: First direct gravitational wave detection Gravitational waves are waves of spacetime curvature that propagate at the speed of light and are produced by the relative motion of gravitating masses.
Read commentary →Physics: Gravitational wave
Gravitational wave Gravitational waves are waves of spacetime curvature that propagate at the speed of light and are produced by the relative motion of gravitating masses.
Read commentary →Physics: Karl Schwarzschild
Karl Schwarzschild (1873) Karl Schwarzschild (German: [kaʁl ˈʃvaʁtsʃɪlt] ; 9 October 1873 – 11 May 1916) was a German physicist and astronomer.
Read commentary →Physics: General relativity
General relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in May 1916 and is the accepted description of the gravitation of macroscopic objects in modern physics.
Read commentary →Physics: Einstein completes general relativity
1915: Einstein completes general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in May 1916 and is the accepted description of the gravitation of macroscopic objects in modern physics.
Read commentary →Physics: Maxwell's equations
Maxwell's equations Maxwell's equations are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, electric and magnetic circuits.
Read commentary →Physics: Equivalence principle
Equivalence principle The equivalence principle is the hypothesis that the observed equivalence of gravitational and inertial mass is a consequence of nature.
Read commentary →Physics: Mass-energy equivalence
Mass-energy equivalence Form: E = mc² In physics, mass–energy equivalence is the relationship between mass and energy in a system's rest frame.
Read commentary →Physics: Newton publishes Principia Mathematica
1687: Newton publishes Principia Mathematica Philosophiæ Naturalis Principia Mathematica (English: The Mathematical Principles of Natural Philosophy), often called simply the Principia (), is a book by Sir Isaac Newton that expounds Newton's laws of motion and his law of universal gravitation.
Read commentary →Physics: 2001 Nobel Prize in Physics
2001 Nobel Prize in Physics Awarded to: Eric A. Cornell, Wolfgang Ketterle, Carl E. Wieman The achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates.
Read commentary →Physics: Hendrik Lorentz
Hendrik Lorentz (1853) Hendrik Antoon Lorentz (18 July 1853 – 4 February 1928) was a Dutch theoretical physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for their discovery and theoretical explanation of the Zeeman effect.
Read commentary →Physics: Maxwell unifies electricity, magnetism, and light
1864: Maxwell unifies electricity, magnetism, and light Maxwell's equations are a set of coupled partial differential equations that describe how electric and magnetic fields are generated by electric charges and currents.
Read commentary →Physics: Lens (optics)
Lens (optics) A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction.
Read commentary →Physics: Mach number
Mach number The Mach number (M or Ma), often only Mach (; German: [max]), is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound.
Read commentary →Physics: First observation of gravitational waves
First observation of gravitational waves By: LIGO (2015) The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016.
Read commentary →Physics: 2017 Nobel Prize in Physics
2017 Nobel Prize in Physics Awarded to: Rainer Weiss, Barry C. Barish, Kip S. Thorne Decisive contributions to the LIGO detector and the observation of gravitational waves.
Read commentary →Physics: Black hole
Black hole A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping.
Read commentary →Physics: Lorentz transformation
Lorentz transformation In physics, the Lorentz transformations are a six-parameter family of linear transformations from a coordinate frame in spacetime to another frame that moves at a constant velocity relative to the former.
Read commentary →Physics: Einstein publishes special relativity
1905: Einstein publishes special relativity In physics, the special theory of relativity, or simply special relativity, is a scientific theory of the relationship between space and time.
Read commentary →Physics: Bose–Einstein condensate
Bose–Einstein condensate In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low densities is cooled to temperatures very close to absolute zero, i.
Read commentary →Physics: Albert Einstein
Albert Einstein (1879) Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist best known for developing the theory of relativity.
Read commentary →Physics: Planck-Einstein relation
Planck-Einstein relation Form: E = hf The Planck constant, or Planck's constant, denoted by h {\displaystyle h} , is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and a particle's momentum is equal to the wavenumber of the associated matter w
Read commentary →Physics: Special relativity
Special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between space and time.
Read commentary →Physics: Arthur Eddington
Arthur Eddington (1882) Sir Arthur Stanley Eddington, (28 December 1882 – 22 November 1944) was an English astrophysicist and mathematician.
Read commentary →Physics: Spacetime
Spacetime In physics, spacetime, also called the space-time continuum, is a mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum.
Read commentary →Physics: Mach's principle
Mach's principle In theoretical physics, particularly in discussions of gravitation theories, Mach's principle (or Mach's conjecture) is the name given by Albert Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach.
Read commentary →Physics: Gravity Probe B
Gravity Probe B By: NASA / Stanford (2004) Gravity Probe B (GP-B) was a satellite-based experiment whose objective was to test two previously-unverified predictions of general relativity: the geodetic effect and frame-dragging.
Read commentary →Physics: John Wheeler
John Wheeler (1911) John Archibald Wheeler (July 9, 1911 – April 13, 2008) was an American theoretical physicist.
Read commentary →Physics: Time dilation
Time dilation Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity, a consequence of special relativity, or a difference in gravitational potential between their locations due to gravitational time dilation.
Read commentary →Physics: Eddington experiment
Eddington experiment By: Arthur Eddington (1919) The Eddington experiment was an observational test of general relativity, organised by the British astronomers Frank Watson Dyson and Arthur Stanley Eddington in 1919.
Read commentary →Physics: Pieter Zeeman
Pieter Zeeman (1865) Pieter Zeeman (25 May 1865 – 9 October 1943) was a Dutch experimental physicist who shared the 1902 Nobel Prize in Physics with Hendrik Lorentz for their discovery and theoretical explanation of the Zeeman effect.
Read commentary →Physics: 2024 Nobel Prize in Physics
2024 Nobel Prize in Physics Awarded to: John J. Hopfield, Geoffrey Hinton Foundational discoveries and inventions that enable machine learning with artificial neural networks.
Read commentary →Physics: Eddington expedition confirms light deflection
1919: Eddington expedition confirms light deflection The Eddington experiment was an observational test of general relativity, organised by the British astronomers Frank Watson Dyson and Arthur Stanley Eddington in 1919.
Read commentary →Physics: Pound–Rebka experiment
Pound–Rebka experiment By: Pound and Rebka (1959) The Pound–Rebka experiment monitored frequency shifts in gamma rays as they rose and fell in the gravitational field of the Earth.
Read commentary →Physics: 2020 Nobel Prize in Physics
2020 Nobel Prize in Physics Awarded to: Roger Penrose, Reinhard Genzel, Andrea Ghez The discovery that black hole formation is a robust prediction of the general theory of relativity / for the discovery of a supermassive compact object at the centre of our galaxy.
Read commentary →Get Relativity Daily
Receive one physics of the day entry each morning with commentary. Free, no spam, unsubscribe anytime.
Start Free Daily EmailFrequently Asked Questions
What is Einstein's theory of relativity in simple terms?
Special relativity has two key ideas: the laws of physics are the same for all observers moving at constant speed, and the speed of light is always the same regardless of the observer's motion. This means that time and space are not absolute — they change depending on how fast you're moving relative to something else. General relativity adds that mass curves spacetime, and this curvature is what we experience as gravity.