Relativity
26 questions tagged Relativity — spanning every one of the seven Ws.
Why can’t anything travel faster than light in vacuum?
According to relativity, the speed of light is a cosmic speed limit built into spacetime. As an object approaches that speed, its energy requirement rises dramatically.
Why do black holes have an event horizon?
An event horizon forms because gravity becomes so strong that once you cross a boundary, even light cannot escape. For a non-rotating black hole, that boundary is called the Schwarzschild radius.
What is the equivalence principle?
The equivalence principle says that gravitational and inertial mass are equivalent, and that free fall is locally indistinguishable from weightlessness. It is a cornerstone of general relativity.
Where is the world's most famous laboratory for testing the equivalence principle?
Equivalence-principle tests are performed in precision physics labs and space missions, including experiments at Stanford and CERN-linked research groups. One famous modern test was the MICROSCOPE satellite mission in orbit around Earth.
Whose theory predicted the existence of black holes from relativity?
Karl Schwarzschild produced the first exact solution of Einstein’s field equations in 1916, implying a region from which light cannot escape. This became the basis for black-hole theory.
Whose equation says energy and mass are interchangeable?
Albert Einstein’s mass–energy equivalence, written as E=mc². It shows that a small amount of mass can correspond to a huge amount of energy.
Whose work established that the speed of light is the same for all observers in vacuum?
Albert Einstein’s special relativity. In 1905, he made the constancy of the speed of light a central postulate.
How does a gravitational lens magnify a distant galaxy?
A massive object bends light from something behind it, acting like a cosmic lens. This can stretch, brighten, or multiply the image of a distant galaxy or star.
How do black holes bend time near them?
Their extreme gravity makes time pass more slowly close to the black hole than far away. This effect is called gravitational time dilation.
How does gravity bend light?
Massive objects curve spacetime, and light follows that curved path. This effect is called gravitational lensing.
How does time slow down near a strong gravitational field?
In general relativity, clocks run more slowly deeper in a gravitational field. This effect is called gravitational time dilation.
Why do black holes bend light around them?
Black holes bend light because their gravity curves spacetime itself. Light follows the curved geometry, so its path appears bent to us.
What is a gravitational lens?
A gravitational lens is a massive object that bends light from a more distant source behind it. It can magnify, distort, or produce multiple images of the background object.
What is the equivalence principle in physics?
The equivalence principle says that locally, the effects of gravity are indistinguishable from acceleration. It is a cornerstone of Einstein’s theory of general relativity.
What is spacetime curvature?
Spacetime curvature is the warping of space and time caused by mass and energy. In general relativity, this curvature tells objects how to move and explains gravity.
Where is the Michelson–Morley experiment site that tested the ether hypothesis?
The famous 1887 experiment was conducted at the Case School of Applied Science in Cleveland, Ohio, now part of Case Western Reserve University. It used an interferometer to look for Earth’s motion through the luminiferous ether.
Whose theory first described black holes mathematically?
Karl Schwarzschild produced the first exact solution to Einstein’s equations that described what we now call a black hole. He did this in 1916, while serving in the First World War.
Whose equations first described black holes mathematically?
Karl Schwarzschild’s. In 1916 he found the first exact solution to Einstein’s equations for a black hole-like object.
Whose idea introduced the gravitational field equation of general relativity?
Albert Einstein published general relativity in 1915. It replaced Newton’s gravity with curved spacetime.
Whose equation described how particles behave at near-light speeds?
Paul Dirac wrote the Dirac equation in 1928. It combined quantum mechanics with special relativity for the electron.
Why does light have no mass but still carry momentum?
Photons are massless particles, but they still carry energy and momentum because of relativity. That is why light can push on matter, as in solar sails.
When was gravitational time dilation first experimentally confirmed?
It was confirmed in 1959 by the Pound-Rebka experiment.
Which force is the only one known to act on massless particles such as photons?
Gravity is the one fundamental force that can affect massless particles like photons. Light bends near massive objects because spacetime itself is curved.
Which force is described by Einstein's theory of general relativity?
Gravity is described by Einstein's general relativity. The theory, published in 1915, explains gravity as the curvature of spacetime.
How do astronomers use gravitational lensing to study distant galaxies?
A massive object between us and a distant galaxy bends the light like a lens. This can magnify, distort, or even create multiple images of the background galaxy.
How does gravity bend light around massive objects?
Massive objects curve spacetime, and light follows that curved path. This effect is called gravitational lensing and can magnify or distort distant galaxies.