Relativity explains how mass-energy and speed curve spacetime resulting in time, space, and motion (but not light speed) being relative to an observer’s frame of reference.

Relativity refers to Einstein’s theories of general and special relativity, Galileo’s Galilean relativity, Sir Issac Newton’s Newtonian relativity, and the work of all the other physicists who helped complete these theories (like Maxwell and Lorentz).

Galileo and Newton (and many others) showed how physics applied to earth (an inertial frame), and Einstein (and many others) showed how it applied to the universe and at high speeds (an accelerated frame).

In a overly-simple nutshell, Galileo found that motion was relative, Newton found that force was relative, and Einstein showed how time and space were relative to speed and gravity (relative to the constant the nature of mass-energy, in which mass and acceleration result in “spacetime” curvature, which warps space and time, and thus needs to be accounted for when we calibrate instruments).

All types of relativity depend on frame of reference, or “point of view”. As from any point of view an observer will always measure the physical constants, including the speed of light, Planck’s constant, and Newtonian gravity, the same. This is why the phrase “relative to the observer’s reference frame” is used and also why the physical constants are important to physics.

See PBS’s SpaceTime series for a great video series on relativity (highly recommended and liberally featured on FactMyth.com), or simply check out our list of facts and myths on relativity below.

## A Planck Unit Represents the Smallest Measurable UnitFact

Planck units, based on the reduced Planck constant (ℏ) and Planck constant (h), represent the smallest measurable units in the physical universe.

## All Particles are Made from Massless ParticlesFact

All known particles are either massless energy particles, or are composite particles with mass that are made from massless energy particles.

## Einstein Wrote ‘E=mc^2’ in His Famous PaperMyth

Einstein’s 1905 paper on mass-energy equivalence doesn’t actually say ‘E=mc^2‘ it says, roughly, ‘m=L/c^2‘.

## Everything Depends on Frame of ReferenceFact

Everything we perceive depends on our frame of reference. What we observe is relative to our point of view. In other words, “it is all a matter of perspective”.

## Nothing Can Travel Faster Than the Speed of LightFact

No “thing” (including particles) can travel faster than light speed, but some “non-things” can. In both ways “nothing travels faster than the speed of light”.

## There are Different Ways to Measure MassFact

There are different ways to measure mass, but all of them are related to rest-mass (invariant mass) the “true” inertial mass of an object at rest.

## E=mc^2 (Mass and Energy are Equivalent)Fact

Einstein’s mass-energy equivalence equation (E=mc2) shows that mass and energy are equivalent (but not “exactly the same”) properties of a physical system.

## Energy is Neither Created or DestroyedFact

Energy can’t be created or destroyed and neither can mass. Although energy can change forms, all energy in a closed system must remain constant.

## Time is RelativeFact

Time is relative to speed and gravity (time dilation), and so is space (length contraction). Light speed is constant for all observers, so time and space can’t be.

## The Physical Constants Explained

The physical constants are measurable properties of the physical universe that don’t change, everything else is relative to these constants.