The Schwarzschild radius for an object is, simply put, the radius the object would have to be squeezed into to form a black hole. This can be calculated by determining the radius at which the escape velocity exceeds the speed of light. The formula for the Schwarzschild radius is $r=2GM/c^2$. It is named after physicist Karl Schwarzschild.

## Spacetime

Spacetime is, quite simply, space and time put together. We can think of spacetime as a four-dimensional continuum with three spatial dimensions and one time dimension.

## Special Relativity

Albert Einstein’s theory of Special Relativity replaced Isaac Newton’s classical laws of motion as the most accurate description of moving objects. Specifically, it helps us understand how objects behave when they are traveling near the speed of light. You can learn about some of the many fascinating consequences of Special Relativity in this video. (See also General Relativity.)

## Speed of Light

The speed of light is exactly 299,792,458 meters per second (roughly $3.00 \times 10^8 m/s$). It is abbreviated with the letter c for the Latin word “celer” (meaning speed). According to Special Relativity, nothing can travel faster than the speed of light.

## Standard Model

The Standard Model of particle physics records the properties of all of the fundamental particles that make up everything we see around us. The Standard Model organizes all particles into two categories: fermions and bosons.

## String Theory

String Theory is a field of study at the cutting edge of theoretical physics research. It proposes that all fundamental particles are actually composed of unimaginably tiny vibrating strings. The theory proposes that the possible vibrations of these strings are what cause the properties of different particles.

## Strong Force

The strong force (or strong interaction) is a fundamental interaction mediated by the gluon. It affects all particles with a strong charge, also called a color charge. There are three color charges: red, green, and blue (plus their anti-equivalents, antired, antigreen, and antiblue). All quarks can take color charges and thus interact with the strong force.

## Theory of Everything

An enticing goal of physics is to construct a single theory underlying all of our experiences. Such a “Theory of Everything” has not yet been developed, but with every new insight into the way our world works, physicists grow closer step by step. One central theme in physicists’ efforts to build a theory of everything is unification.

## Unification

By unifying separate theories into more general conglomerates, we may eventually combine all of our different models of the universe into a single universal theory. For example, James Clerk Maxwell unified the separate theories of electricity and magnetism into a single powerful theory of electromagnetism.

## Wave-Particle Duality

Wave-particle duality is the name for the fact that tiny particles of matter and energy behave like waves in some ways and like particles in others. This behavior is modeled in quantum mechanics by a mathematical construct called a wavefunction.