Einstein’s former professor, was called space-time and came out of Einstein’s 1905 theory of special relativity.
As Einstein generalized this theory, creating the theory of general relativity in 1915, he was able to include gravity in his explanation of motion. In fact, the concept of space-time was crucial to it. The space-time coordinate system bent when matter was placed in it. As objects moved within space and time, they naturally tried to take the shortest path through the bent space-time.
Einstein’s relativity is covered in depth in Chapter 6, and the major implications of relativity to the evolution of the universe are covered in Chapter 9. The space-time dimensions are discussed in Chapter 15.
Describing Matter: Physical and Energy-Filled
Einstein helped to revolutionize our ideas about the composition of matter as much as he changed our understanding of 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. 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.
Viewing matter classically: Chunks of stuff
The study of matter is one of the oldest physics disciplines, going back to the ancient philosophers who tried to understand what made up the objects around them. Even fairly recently, a physical understanding of matter was elusive as physicists debated the existence of atoms — tiny, indivisible chunks of matter that can’t be broken up any further.
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 can be explained as the motion of tiny particles.
The atom had returned, though the findings of 20th-century quantum physics would reveal that it 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 is incredibly complex, made up of an array of bizarre and unexpected types of particles that join together to form other types of particles.
Not only that, but even these fundamental particles didn’t seem to be enough. It turns out that there are three families of particles, some of which only appear at significantly higher energies than scientists had previously explored.
Today, the Standard Model of particle physics contains 25 distinct fundamental particles, all of which have been experimentally detected (often decades after theoreticians proposed them).
Grasping for the Fundamental Forces of Physics
Even while the types of particles identified by scientists became more bizarre and complex, the ways those objects interacted turned out to be surprisingly straightforward. In the 20th century, scientists discovered that objects in the universe experience only four fundamental types of interactions:
Electromagnetism
Strong nuclear force
Weak nuclear force
Gravity
Physicists have discovered profound connections between these forces — except for gravity, which seems to stand apart from the others for reasons that physicists still aren’t completely certain about. Trying to incorporate gravity with all the other forces — to discover how the fundamental forces are related to each other — is a key insight that many physicists hope a theory of quantum gravity will offer.
Electromagnetism: Super-speedy energy waves
Discovered in the 19th century, the electromagnetic force (or electromagnetism) is a unification of the electrostatic force and the magnetic force. In the mid-20th century, this force was explained in a framework of quantum mechanics called quantum electrodynamics, or QED. In this framework, the electromagnetic force is transferred by particles of light, called photons.
The relationship between electricity and magnetism is covered in Chapter 5, but the basic relationship comes down to electrical charge and its motion. The electrostatic force causes charges to exert forces on each other in a relationship that’s similar to (but more powerful than) gravity — an inverse square law. This time, though, the strength is based not on the mass of the objects but on the electrical charge.
The electron is a particle that contains a negative electrical charge, while the proton in the atomic nucleus has a positive electrical charge. Traditionally, electricity is seen as the flow of electrons (negative charge) through a wire. This flow of electrons is called an electrical current.
A wire with an electrical current flowing through it creates a magnetic field. Alternately, when a magnet is moved near a wire, it causes a current to flow. (This is the basis of most electric power generators.)
This is the way in which electricity and magnetism are related. In the 1800s, physicist James Clerk Maxwell unified the two concepts into one theory, called electromagnetism, which depicted the electromagnetic force as waves of energy moving through space.
One key component of Maxwell’s unification was a discovery that the electromagnetic force moved at the speed of light. In other words, the electromagnetic waves that Maxwell predicted from his theory were a form of light waves.
Quantum
