Relativity

Formulated by Albert Einstein from 1905, the **theory of relativity** explains the behavior of objects in space and time, and it can be used to predict things such as the existence of black holes, light bending due to gravity and the behavior of planets in their orbits.

In 1905 his theory of **special relativity** explained how speed affects mass, time and space. The theory formulates how the speed of light defines the relationship between energy and matter and the mass-energy equivalence defined by his famous equation *E* = *mc*2. As an object approaches the speed of light, the object's mass becomes infinite and so does the energy required to move it. That means it is impossible for any matter to go faster than light with the old laws of physics.

**General relativity** was developed by Einstein between 1907 and 1915, and this says that the observed gravitational effect between masses results from their warping of spacetime. Prior to this, Newton's law of universal gravitation said that gravity was the result of an attractive force between massive objects. However, Einstein's theory then accounted for gravitational effects unexplained by Newton's law, such as minute anomalies in the orbits of planets, gravitational waves, gravitational lensing and gravitational time dilation.

"Relativity" means that every time you measure an object's velocity, its momentum or how it experiences time, it's always in relation to something else, and there is no "absolute" frame of reference, and the speed of light is constant. This means that an astronaut going very fast relative to Earth will measure the seconds ticking by more slowly than an Earthbound observer will. Time essentially slows down for the astronaut — a phenomenon called time dilation. An object such as a spaceship in a big gravity field of a black hole will accelerate, so it also experiences gravitational time dilation. The astronaut's spaceship experiences length contraction and appears "squished", but to observers all would seem normal.

### Impact and examples

#### Nature

Magnetism and light would not exist, because relativity requires that changes in an electromagnetic field move at a finite speed instead of instantaneously. If changes in electric fields were communicated instantaneously, both magnetism and light would be unnecessary.

Without *E* = *mc*2, the sun and the rest of the stars wouldn't shine. Intense temperatures and pressures constantly squeeze four separate hydrogen atoms into a single helium atom, and the extra mass gets directly converted into energy, which shows up as sunlight on our planet.

#### Magnetism

Magnetism is a relativistic effect. If you take a wire and move it through a magnetic field, you generate an electric current. The charged particles in the wire are affected by the changing magnetic field, which forces some of them to move, and this creates the current. If the wire is at rest, the current still flows. This is the principle behind transformers and electric generators, so anyone who uses electricity is experiencing the effects of relativity. Electromagnets work via relativity as well.

#### GPS navigation

For GPS navigation to function accurately, satellites have to consider relativistic effects. The satellites are orbiting fast and sending signals to ground stations on Earth. These stations (and the GPS technology in a car or smartphone) are all experiencing higher accelerations due to gravity than the satellites in orbit. This creates a relativistic time dilation that adds microseconds each day. If no relativistic effects were accounted for, a GPS unit would be off by an entire factor.

#### Gold

Gold is a heavy element, and the electrons in its atoms jump fast enough between different energy levels, or "orbitals, so that the relativistic mass increase and the length contraction are significant. This results in yellow, orange and red reflected light having longer wavelengths than blue light, and gold appears yellowish.

The relativistic effect on gold's electrons is the reason it doesn't corrode. Because the electrons in gold are "heavier" than they should be, since they are moving near the speed of light, increasing their mass, they are held closer to the atomic nucleus. This means that the outermost electron isn't likely to be where it can react with anything at all.

#### CRT TV

Until about the early 2000s, most televisions and monitors had cathode ray tube screens. These work by firing electrons at a surface with a big magnet. Each electron makes a lighted pixel when it hits the back of the screen, and the electrons fire out to make the picture move at up to 30% the speed of light. Relativistic effects are noticeable, and when manufacturers shaped the magnets, they had to consider those effects.