# Physics explained

## Particles

The actual composition of particles is the domain of [quantum physics](science-physics-quantum.md), so physics itself is more about the relationships *between* the particles.

## Heat

Each atom can hold heat energy, which can be measured by how much the atoms vibrate. The more they vibrate, the less they hold together:

- Solids vibrate very little, so they maintain a set structure.
- Liquids vibrate enough to break their structure, but not enough to push away from each other.
- Gases vibrate so hard that they'll push away from each other if left unattended.
- Plasma vibrates so hard that the *atom* doesn't really stay together, and typically only naturally occurs in stars.

Heat transfers through 3 major methods:

- Conduction: a fast-moving hot particle collides with a slower-moving cold particle, and heat transfers on contact.
- Convection: in a fluid (e.g., air, water) heat flows upward through it and churns the cooler fluid downward, which can work as a cycle.
- Radiation: heat can travel over electromagnetic waves (e.g., a star's light energy).

## Gravity

All atoms exert an incredibly small attractive force toward each other, called "gravity". This is usually irrelevant for the sake of measuring anything, unless it's dealing with massive objects (like stars). However, a large enough object will pull on things to tweak that smaller object's existing motion to make the smaller object travel in an elliptical "orbit" around the larger one. When smaller things are *very* close to larger things (e.g., standing on a planet), the gravity makes things go downward very fast.

## Waves and kinetic energy

Atoms interact with other atoms by various forces:

- Inertia is the tendency for a force to stay in motion. There is no centrifugal force from rotation, but there *is* centripetal force.
- Gravity is an absurdly weak force that draws upon all other atoms.
- Friction is the tendency for atoms to slow down other atoms, often assisted by gravitational pull.

There's a *lot* of energy stored in atoms. By separating them (fission) or joining them together (fusion), they will often release this energy. People have already found ways to split atoms quickly (nuclear detonation) and slowly (nuclear power plant).

These atoms can bond together with other atoms (often via opposite ions), and that's what creates molecules, which then moves into the realm of chemistry.

Photon waves are complicated because they're both particles *and* waves, depending on how you measure them. This is the domain of [quantum physics](computers-quantum.md), and has very complicated [metaphysical implications](understanding.md) because the physical properties of particles are defined by whether we observe them or not.

Sound is a wave made from the vibration of molecules that ripple shockwaves across neighboring molecules. For this reason, sound travels fastest and farthest in dense liquid (e.g., water).

Wave effects, both with sound and with light, happen relative to the location of the thing that causes the wave. For this reason, an object moving fast enough toward or away from an observer will appear/sound different for the observer:

- Red shifts (like the longer light wavelength red) come from the distance *increasing* (such as a car driving away quickly)
- Blue shifts (like the shorter light wavelength blue) come from the distance *decreasing* (such as a car driving toward someone)
- The best example comes from observing a train honking its horn as it speeds past. First, the train sounds more high-pitched than if it were standing still, then shifts rapidly downward to a much lower pitch as it drives by.
- This red shift/blue shift also blends in with spacetime (proven partially by [GPS](logistics-navigation.md) calculations), so a person traveling near the speed of light will age slower than someone who stays still.

## Further reading

- [How does entropy work?](https://aatishb.com/entropy/)
- [How does sound work?](https://pudding.cool/2018/02/waveforms/)
- [How do lights work?](https://ciechanow.ski/lights-and-shadows/)
- [Physics: Susan Rigetti](https://www.susanrigetti.com/physics)

Obvious questions that remain unanswered:

- Where did the matter in the universe first come from?
- How long has the universe been around?
- Below the subatomic, what are atoms and the universe made from? Dark matter? Dark energy?
- Does a [creator of the universe](religion.md) exist?
- Why is ice, as a jagged crystal, slippery?
- Why does time only move forward? Is time travel possible?
- Is light a wave or a particle? If it's both, when does it change?
- What causes gravity?
- How does a bicycle stay up while in motion?