# Physics and Reality

> Explaining how the physical world works.


## Posts

- [How does complexity arise?](https://physicsandreality.com/complexity/): The universe around us is extraordinarily complex.
- [Do wormholes exist?](https://physicsandreality.com/wormhole/): Wormholes are mentioned abundantly in popular culture, to the extent that they have acquired an undeserved status as established objects of modern physics.
- [What is a particle?](https://physicsandreality.com/particle/): Classically, a particle is envisioned as a point-like object characterized by a position and a velocity, or equivalently a momentum.
- [Is “spooky action at a distance” real?](https://physicsandreality.com/spooky-action/): Quantum entanglement is often described as “spooky action at a distance.” The phrase suggests that something is being transmitted instantaneously between distant objects, as if one system could influence another without anything passing between them.
- [How did the Solar System form?](https://physicsandreality.com/solar-system/): The Solar System appears stable and orderly.
- [What is a star?](https://physicsandreality.com/star/): A star is often described as a glowing ball of gas. While that is not wrong, it misses the essential point. A star is not simply a hot object.
- [What is a black hole?](https://physicsandreality.com/black-hole/): As discussed previously, gravity is not just a force between objects, but a dynamical structure created by energy and governing how matter and radiation move.
- [What is gravity?](https://physicsandreality.com/gravity/): Gravity, also known as General Relativity, is often introduced as a geometrical theory. That is a useful description, but it is not the only one.
- [Why is the sky blue, the Sun yellow, and sunset red?](https://physicsandreality.com/blue-sky/): These three facts are closely connected. The sky is blue, the Sun often looks yellow, and the sunrise or sunset appear orange or red.
- [Why do we have seasons?](https://physicsandreality.com/seasons/): Seasons are among the most familiar features of life on Earth, and yet the reason for them is often misunderstood.
- [Why do things have color?](https://physicsandreality.com/color/): Color seems like one of the most immediate things in the world. Grass is green, the sky is blue, blood is red, snow is white.
- [What is time?](https://physicsandreality.com/time/): Time is one of the most fascinating concepts in physics.
- [Is there room for free will in a quantum world?](https://physicsandreality.com/free-will/): Classical determinism says that, given the full state of the universe at one moment and the laws governing it, the future is fixed.
- [Why the quantum world looks classical](https://physicsandreality.com/quantum-classicality/): Quantum physics is the description we currently have of nature at its most fundamental level.
- [Did quantum physics end determinism?](https://physicsandreality.com/quantum-determinism/): Classical determinism says that, given the full state of a system at one moment and the laws governing it, its future is fixed.
- [How we observe the quantum universe](https://physicsandreality.com/quantum-observation/): Given that quantum physics, and quantum field theory in particular, seems to be the language spoken by the universe, it is essential to understand what it implies for the way we perceive reality.

## Pages

- [What quantum physics changed](https://physicsandreality.com/quantum-physics/): There was a time when classical physics was thought to be the language spoken by the universe.
- [What classical determinism means](https://physicsandreality.com/classical-determinism/): The laws of nature determine how systems evolve in time.
- [How we know the Sun is still burning](https://physicsandreality.com/burning-sun/): Sunlight reaches us in about eight minutes, and it is natural to think that this means we are seeing what is happening in the Sun almost immediately.
- [How stars forged heavy elements](https://physicsandreality.com/forged-in-stars/): The early universe did not produce the full periodic table.
- [Tornados in emptiness](https://physicsandreality.com/tornados-in-emptiness/): We tend to imagine absolute emptiness as something unique.
- [About](https://physicsandreality.com/about/): This website, Physics and Reality, is written by a theoretical physicist with a PhD in physics and nearly two decades of research experience at leading institutions.
- [The states the universe could be in](https://physicsandreality.com/possible-states/): Physics aims to describe the state the universe actually finds itself in.
- [When the universe became transparent](https://physicsandreality.com/transparent-universe/): In the previous post, we saw how the first nuclei formed.
- [How the first elements formed](https://physicsandreality.com/first-elements/): In the previous post, we looked at the universe before atoms existed. It was a hot soup of a few particle species, all constantly interacting with one another.
- [What came before atoms?](https://physicsandreality.com/before-atoms/): There is a point in the history of the universe beyond which our reconstruction becomes increasingly uncertain and model dependent.
- [How particles interact](https://physicsandreality.com/how-particles-interact/): The universe is made of particles, and its evolution is dictated by the interactions among them.
- [What is dark energy?](https://physicsandreality.com/dark-energy/): All familiar ingredients of the universe — visible matter, radiation, and even dark matter — share a common feature: as the universe expands, their density decreases.
- [What is dark matter?](https://physicsandreality.com/dark-matter/): We have discussed the visible content of the universe.
- [What is the visible universe made of?](https://physicsandreality.com/visible-universe/): So far, we have discussed how the universe expands and how structure emerges from small perturbations.
- [How did structure form in the universe?](https://physicsandreality.com/structure-formation/): In the previous post, we constructed the universe by placing particles into the vacuum one at a time, building a homogeneous expanding state from simple ingredients.
- [What is the quantum structure of the universe?](https://physicsandreality.com/quantum-structure/): In order to fully grasp the fundamental quantum nature of the universe, let us begin its construction from elementary ingredients by placing particles into the vacuum one at a time.
- [What does it mean that the universe is expanding?](https://physicsandreality.com/expanding-universe/): The expansion of the universe is often described as the stretching of the fabric of spacetime.
- [What is empty space?](https://physicsandreality.com/vacuum/): We tend to think of empty space as something that contains nothing. That intuition is surprisingly accurate.
- [Home](https://physicsandreality.com/): How does complexity arise?
- [What is a theory in physics?](https://physicsandreality.com/theory-in-physics/): In the previous post, we argued that natural phenomena need to be understood within a language, and that meaning only exists inside a well-defined framework.
- [What is the language of the universe?](https://physicsandreality.com/language-of-the-universe/): The phrase is appealing — but it is not quite accurate.

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