Quantum Physics & Cosmology
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### Quantum Mechanics - **Quantum:** The smallest discrete unit of energy, light, or matter. For example, a photon is a quantum of light. - **Superposition:** A fundamental principle where a quantum system can exist in multiple states simultaneously until it is measured. - *Example:* Schrödinger's cat is both alive and dead until the box is opened. - **Quantum Entanglement:** Two or more quantum particles become linked in such a way that they share the same fate, regardless of the distance separating them. Measuring the state of one instantly reveals the state of the other. - *Key feature:* "Spooky action at a distance" (Einstein). ### Fundamental Theories - **String Theory:** A theoretical framework where the fundamental constituents of the universe are not point-like particles but one-dimensional "strings" vibrating at different frequencies. These vibrations correspond to different types of particles and forces. - *Goal:* To unify all fundamental forces (including gravity) into a single theory. - **Gravity:** A fundamental force of attraction between any two objects with mass or energy. - *Classical view (Newton):* An invisible force. - *Modern view (Einstein's General Relativity):* A curvature of spacetime caused by mass and energy. ### Spacetime and Dimensions - **Space:** The three-dimensional extent in which objects and events have relative position and direction. - **Time:** The fourth dimension of spacetime, which orders events and allows for causality. It's often described as a continuous flow. - **Dimensions:** - **Spatial:** Up/down, left/right, forward/backward (3 dimensions). - **Temporal:** Past/future (1 dimension, time). - **Extra Dimensions (String Theory):** String theory postulates additional compactified spatial dimensions, often 6 or 7, that are "curled up" and not directly observable. ### Matter and Energy - **Matter:** Anything that has mass and takes up space. Composed of elementary particles like quarks and leptons, which form protons, neutrons, and electrons. - **Energy:** The capacity to do work. According to Einstein's $E=mc^2$, mass and energy are interchangeable. ### Paradoxes - **Paradox:** A statement or proposition that, despite sound (or apparently sound) reasoning from acceptable premises, leads to a conclusion that seems logically unacceptable or self-contradictory. - *Examples in Physics:* - **Grandfather Paradox (Time Travel):** If you travel back in time and prevent your grandfather from meeting your grandmother, you wouldn't be born, thus you couldn't travel back in time. - **Fermi Paradox:** If the universe is so vast and old, and life is common, why haven't we found any evidence of alien civilizations? - **Black Hole Information Paradox:** Quantum mechanics states information cannot be destroyed, but general relativity suggests information falling into a black hole is lost.
