Special relativity key terms

Michelson-Morley experiment

An 1887 interferometer experiment that attempted to detect Earth's motion through the luminiferous ether by measuring differences in light speed along perpendicular arms.

Ether

A hypothetical, all-pervading medium once thought necessary for light wave propagation, whose existence was tested by the Michelson-Morley experiment.

Null result

The observation that the Michelson-Morley experiment detected no difference in the speed of light due to Earth's motion, indicating that the ether does not exist.

Significance of null result

It demonstrates that light speed is constant in all inertial frames, supporting special relativity and refuting the ether hypothesis.

ether

A hypothetical, all-pervading medium once thought necessary for the propagation of light waves.

null result

An outcome where no difference in light speed is detected between perpendicular paths, indicating no preferred reference frame.

Michelson-Morley experiment

A 1887 interferometer experiment designed to detect the relative motion of Earth through the luminiferous ether by measuring differences in the speed of light along perpendicular arms.

null result

The observation that no difference in light speed was detected between the two arms, indicating that the speed of light is constant regardless of Earth's motion.

Principle of Relativity

Principle of Relativity means All physical laws are the same in all inertial reference frames. In Einstein's theory of special relativity, use this term to connect State the principle of relativity for inertial frames. to evidence, observation and model change.

Inertial Frame

A reference frame that is either at rest or moving at constant velocity, where Newton’s first law holds.

Speed of light

The speed of light in vacuum is constant and equal to approximately 3.00×10^8 m s⁻¹ for all inertial observers.

Light speed constancy

The principle that the speed of light in vacuum remains the same for all observers, regardless of their relative motion.

Relativity of Simultaneity

Cause: Observers moving relative to one another. Mechanism: The Lorentz transformation shows that time coordinates depend on relative velocity, mixing space and time because the speed of light is constant. Effect: Events that are simultaneous in one inertial frame are not simultaneous in another. Consequence: The ordering of events becomes frame‑dependent, influencing the interpretation of experiments such as the Michelson‑Morley experiment.

Lorentz Transformation

Cause: The need to relate space and time coordinates between inertial frames moving at constant velocity. Mechanism: The Lorentz equations incorporate the invariant speed of light and mix time and space coordinates. Effect: Time intervals and spatial separations are not invariant; simultaneity depends on relative motion. Consequence: Different observers assign different times to the same event, making simultaneity frame‑dependent.

Principle of Relativity

Principle of Relativity means All physical laws are the same in all inertial frames of reference. In Einstein's theory of special relativity, use this term to connect Apply relativity postulates qualitatively. to evidence, observation and model change.

Constancy of the Speed of Light

The speed of light in vacuum is the same for all observers, regardless of their relative motion.

Time dilation

The phenomenon where a moving clock runs slower relative to a stationary observer, as predicted by special relativity.

Proper time

The time interval measured by a clock that is at rest relative to the events being timed.

Proper time

The time interval measured by a clock that travels with the object, i.e., the time experienced by the moving observer.

Coordinate time

The time interval measured by a stationary observer in a given inertial frame, used to compare events at different positions.

Lorentz factor

The factor γ = 1/√(1−v²/c²) that quantifies time dilation and length contraction in special relativity.

Proper time

The time interval measured by a clock moving with the event, denoted τ, which is the shortest possible time between two events.

time dilation

The phenomenon where a clock moving relative to an observer runs slower than a stationary clock, as predicted by special relativity.

muon lifetime

The experimentally observed increase in the decay time of high‑velocity muons compared with their rest‑frame lifetime, providing evidence for time dilation.

Length contraction

The shortening of an object's length measured in the direction of its relative motion, as predicted by special relativity.

Lorentz factor

The factor γ = 1/√(1−v²/c²) that quantifies time dilation, length contraction and relativistic mass increase.

proper length

The length of an object measured in its rest frame, i.e., the length it would have if it were not moving relative to the observer.

length contraction

The reduction in the measured length of an object in the direction of its motion relative to an observer, given by L = L₀/γ.

Length contraction

The reduction in the measured length of an object moving relative to an observer, occurring only along the direction of motion, as predicted by special relativity.

Optical appearance

The visual impression of an object's size or shape that can be altered by light propagation effects such as refraction, reflection, or perspective, without any physical change in the object's dimensions.