# String Theory Describes Matter as Physical and Energy-Filled

String theory is currently the most likely candidate for a successful theory of quantum gravity. Because gravity is an interaction between objects made up of matter, understanding matter is crucial to understanding why physicists need a theory of quantum gravity.

Einstein helped to revolutionize our ideas about the composition of matter as much as he did about space, time, and gravity. Thanks to Einstein, scientists realize that mass — and therefore matter itself — is a form of energy. This realization is at the heart of modern physics.

## Viewing matter classically: Chunks of stuff

The study of matter is one of the oldest physics disciplines, because philosophers tried to understand what made up objects. Even fairly recently, a physical understanding of matter was elusive, as physicists debated the existence of atoms — tiny, indivisible chunks of matter that couldn’t be broken up anymore.

One key physics principle was that matter could be neither created nor destroyed, but could only change from one form to another. This principle is known as the conservation of mass.

Though it can’t be created or destroyed, matter can be broken, which led to the question of whether there was a smallest chunk of matter, the atom, as the ancient Greeks had proposed — a question that, throughout the 1800s, seemed to point toward an affirmative answer.

As an understanding of thermodynamics — the study of heat and energy, which made things like the steam engine (and the Industrial Revolution) possible — grew, physicists began to realize that heat could be explained as the motion of tiny particles.

The atom had returned, though the findings of 20th-century quantum physics would reveal that the atom wasn’t indivisible as everyone thought.

## Viewing matter at a quantum scale: Chunks of energy

With the rise of modern physics in the 20th century, two key facts about matter became clear:

• As Einstein had proposed with his famous E = mc2 equation, matter and energy are, in a sense, interchangeable.

• Matter was incredibly complex, made up of an array of bizarre and unexpected types of particles that joined together to form other types of particles.

The atom, it turned out, was composed of a nucleus surrounded by electrons. The nucleus was made up of protons and neutrons, which were, in turn, made up of strange new particles called quarks! As soon as physicists thought they had reached a fundamental unit of matter, they seemed to discover that it could be broken open and still smaller units could be pulled out.

Not only that, but even these fundamental particles didn’t seem to be enough. It turned out that there were three families of particles, some of which only appeared at significantly higher energies than scientists had previously explored.

Today, the Standard Model of particle physics contains 18 distinct fundamental particles, 17 of which have been observed experimentally. (Physicists are still waiting on the Higgs boson.)