## Physics for People Who Know Nothing About Physics

Curious about physics?

Since antiquity, humans have pondered the composition of the material world around us. Aristotle claimed that everything is a combination of earth, water, air and fire. Yet others claimed that everything is made up of indivisible units called atoms. These views remained speculation until very recently.

The period between the Renaissance and the seventeenth century launched the Scientific Revolution. A new theory of the heavenly spheres placed the sun rather than the earth at the center. An astronomer named Kepler formulated a set of laws for how these bodies behaved. After Kepler, Isaac Newton combined the laws of the planets with laws about objects on earth. Newton's unification of the heavenly realm with the earthly led to renewed confidence that humans could probe nature.

This renewed confidence ushered in the age of Enlightenment, which emphasized the power of reason to improve knowledge and the human condition. These developments placed physics at the forefront of human knowledge.

Since then, physics has undergone acute developments that play a part in every aspect of the world around us. Electricity, computers, and cars would not be possible without it.

Below you will find a guide to some of the central ideas in physics, the way they have evolved, and current mysteries science cannot explain. You will also find library books to help you expand your knowledge in all these areas.

### Classical Mechanics

The physical science that flowered following Newton is known as classical mechanics. Classical mechanics remains the intuitive picture of physics in popular awareness. It is also still used in engineering and astronomy.

The main idea behind classical mechanics is that physical bodies obey certain rules that can be predicted with mathematical precision, if we have the right information. These include Newton's three laws of motion that incorporated Kepler's laws of planetary motion.

For example, the famous first law, known as the law of inertia, states that all bodies tend to stay in uniform motion unless acted upon by an external force. Forces include things we are familiar with like push, pull and friction. Newton described force as the product of mass and acceleration in a system of bodies. He also described a special type of force called gravity that's unlike the forces we find intuitive. Gravity takes observable effect between large bodies in the form of an invisible pull. Newton wrote a mathematical formula, known as the universal law of gravitation, that quantifies the gravitational force of large bodies like stars and planets. This way Newton explained why the earth revolved around the sun. Because the sun is the more massive object, it pulls the earth in an elliptical orbit around it. The closer the earth is to the sun the faster it travels, and the farther away it is, the slower.

Other physicists later expanded Newton's theory to include quantities like energy and work. Yet, the description of physical bodies relied on assumptions that began to be challenged at the end of the 19th century.

One of these assumptions was that time and space are separate variables and absolute in their measurements. According to Newton, my clock ticks exactly at the same rate as any clock anywhere in the universe. By the same token, an arbitrary unit of distance is equal everywhere in the universe. These assumptions imply that the measurement of velocities is relative to observers. If I start running inside a train, my velocity for someone in the platform is the sum of my speed and the train's. This may be intuitive to your or I, but it led to contradictions in experimental evidence that scientists could not explain.

The famous Michelson-Morley experiment tried to detect the relative motion of light travelling through a hypothesized medium called the aether. If the aether existed, then light would measure at different velocities at different angles. Yet, they found the opposite. The speed of light appeared to remain constant.

At this time, advances in the relationship between electricity and magnetism showed that something was wrong with the classical assumptions. In the decades that followed, Einstein developed a new theory of space, time and gravity, while particle physics showed that very small particles did not behave in the predictable manner that Newton and his followers presumed.

Learn more about Newton's mechanics and the scientific revolution:

### Books

The Scientific Revolution: A Very Short Introduction by Lawrence Principe

Physics: A Very Short Introduction by Sidney Perkowitz

Classical Mechanics Illustrated by Modern Physics by David Guery-Odelin

### The Theory of Relativity

In the early 20th century Einstein turned classical mechanics on its head by proposing two equivalences that had escaped the brightest minds. The equivalence of mass to energy and gravity to acceleration. Before Einstein formulated his special and general theories of relativity, experimental evidence against classical mechanical predictions of light speed was recorded but discounted on grounds of faulty experiments.

In 1905, Einstein published his theory of special relativity. In it he worked out his famous equation of E=mc^{2}. In this simplified form, the equation states that energy and mass are convertible into each other. In other words, the two are equivalent: mass has energy, and energy has mass. There's only one catch: the universe has a fundamental speed limit, the speed of light, which can never be at rest or surpassed. To be able to see a beam of light at rest, you'd have to be travelling at light speed. But Einstein's equations show that it would need an infinite amount energy for that to happen. The total amount of energy in the universe could not supply it.

The mass and energy equivalence has the counterintuitive effect that the coordinates of time and distance are not the same for all observers. Rather, they measure different according to their moving frames of reference. For example, if I were to travel near the speed of light relative to you, and you are at rest here on earth, assuming we are both moving at uniform velocity, then my clock measures time slower than your clock. In other words, time has dilated, and by the same token, the unit of distance (any arbitrary segment) has contracted. Of course, in our everyday lives our relative speeds are much smaller than that of light, so Newton's equations work fine.

A limitation of special relativity is that it only applies to inertial frames of reference, or bodies moving at uniform velocity. Yet, most of everything around us is always changing its velocity, from trains, planes and people jogging, to the planets in the solar system. In other words, most of the time things are either accelerating or slowing down.

Einstein went back to the drafting table and came out in 1915 with a theory that generalized his special relativity to accelerated frames of reference. If you think back to Newton, one of the forces responsible for acceleration is gravity. Einstein took it a step further and stated that all gravitational effects are equivalent to accelerated frames of reference.

What did Einstein do different? Newton's gravitational law described gravity as instantaneous action-at-a-distance. While this was successful in describing planetary orbits, it left the question unanswered how did massy objects create such an effect. Einstein answered that question by changing the mathematical meaning of gravity. Instead of a force exerted by very large objects, gravity became a geometric curvature those objects created in 3 dimensions of space and 1 of time. When a smaller object falls into a larger object's orbit, it accelerates proportional to the curvature of spacetime created by the larger object. This way, gravity obeys the speed of light limit, is less mysterious than action-at-a-distance, and describes relativistic effects in cosmology that Newton's gravitational law cannot.

Learn more about the theory of relativity:

### Books

Relativity: The Special and General Theory by Albert Einstein

Space and Time in Contemporary Physics: an introduction to the theory of relativity and gravitation by Moritz Schlick

Six Not-So-Easy Pieces: Einstein's Relativity, Symmetry and Space-Time by Richard Feynman

### Quantum Mechanics

At roughly the same time that Einstein showed that classical mechanics was not good at very high velocities, particle physics was negating Newton at very small scales.

In classical mechanics all you need are the positions and velocities of a set of finite particles in a system to predict the complete evolution of the system in time. The smartest people had assumed that this general framework would hold true no matter how small we divide particles. Yet, the 19th century had expanded the Newtonian picture of the world with a deeper understanding of light and electricity.

While electricity was harnessed to build motors and batteries, and light was thought to be a wave, their true nature was not well understood until James Clerk Maxwell. He showed that light is a strange wave of oscillating magnetic and electric fields that radiate from electrically charged objects. Until this point, the furniture of the world consisted of matter and an attraction force that described how very large objects affected each other called gravity. Atomic changes were thought to explain electrical change. But what about this strange wave that radiated from matter in different frequencies? Most other waves are not "real", but vibrations in regular matter like sound. Electromagnetism appeared to propagate within its own field, leading to the view that "energy fields" are as real as matter.

Moreover, toward the end of the 19th and early 20th century, evidence of subatomic particles led to a better understanding of atoms.

The atom consists of negatively charged particles called electrons orbiting a nucleus of positively charged protons and uncharged neutrons. These hold together by something called the strong force. Deficits and surpluses of electrons explain electricity, but when we apply Newton's laws to predict the behaviour of electrons, they fail. Electrons have mass and momentum like all other particles. Yet when we try to calculate their positions, we appear to change their momenta and vice versa. This is because of another strange symmetry or equivalence that scientists discovered during the first half of the 20th century.

Just as light behaves like a particle under certain conditions, matter behaves like a wave in others. In other words, both light and matter are wave and particle. Both also share a property foreign to our Newtonian intuitions. The electron can only occupy certain discrete energy states inside the atom, not just any continuous energy quantity. At the same time, light waves also carry discrete total energy states without any in-between values. The quantization of energy at very small scales gives quantum mechanics its name.

The bizarre nature of subatomic physics can be best described by something called a superposition. A superposition refers to the fact that an electron can exist in two or more states at once. In fact, our best mathematical theory depicts a single electron as a smear of energy whose position can only be described as a probability distribution. The strange thing is that when we make a measurement, the electron takes on a definite state. This is known as the measurement problem and its explanation remains unresolved in physics.

### Books

Quantum Mechanics: The Theoretical Minimum by Leonard Susskind and Art Friedman

The Amazing Story of Quantum Mechanics: A Math-Free Exploration of the Science that Made our World by James Kakalios

### Beyond The Standard Model

Where does this leave us today? Despite its bizarre, probabilistic nature, quantum mechanics stands as the most successful physical theory to date.

Particle physics describes the most fundamental components of matter and the forces through which they interact. These forces are the electromagnetic force, which explains light. The strong force, which explains what keeps protons and neutrons bound together. And lastly, the weak force, which explains radioactive decay in the atomic nucleus. These forces describe the interactions of the most elementary particles grouped into quarks and leptons. Together they form a theory known as the standard model. One of the standard model's predictions, the existence of a particle known as the Higgs Boson and a corresponding field known as the Higgs field, came true in 2012 when the Large Hadron Collider in Geneva, Switzerland found confirming evidence.

While its discovery showed that the standard model is on the right track, it remains far from a complete theory of physical phenomena. The standard model cannot explain gravity, for which we rely on Einstein's theory of general relativity. The standard model also cannot explain the gravitational integrity of galaxies or the acceleration of the expansion of the universe. Lastly, the standard model cannot explain why the observable universe has more matter than anti-matter. Anti-matter is matter with the opposite properties of matter.

The gap between the standard model and gravity, as well as the inability to predict the evolution of the universe, have set scientists on a quest for a better and possibly unified theory. This quest has yet to be successful, but some of its fruits include string theory, loop quantum gravity and supersymmetry.

Find books about the standard model and the current state of physics research.

### Unresolved Questions

Many unresolved questions remain in physics. Albert Einstein once remarked: "The more I learn, the more I realize how much I don't know". This appears to apply to physics on the whole. As our models and theories get more successful, greater gaps in our knowledge become apparent. Here are some questions that remain unresolved by current science and can be food for thought for any contemplating mind out there:

- What causes the wave function to collapse in quantum states?
- Can our theory of gravity be reconciled with quantum theory?
- Why are the physical constants the way they are?
- Why did the evolution of the universe favour matter over anti-matter?
- Why does time have a direction?
- What are dark matter and dark energy if they are real?
- What is the shape of the universe? Is it infinite or finite? Does it have a flat, open, or closed curvature?

### Books

The Physics Book: 250 Milestones in the History of Physics by Clifford A. Pickover

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