The central idea of String Theory is that all the quanta we know of are described by tiny strings that vibrate in various ways on minuscule scales: far below what’s ever been probed. String theory is an attempt at a framework for quantum gravity, and arguably the only viable candidate for finding out what’s real in the Universe on trans-Planckian scales. (Credit: Berkeley Center for Cosmological Physics)
There are limits to where physics makes meaningful predictions: beyond the Planck length, time, or energy. Here’s why we can’t go further.
At all times and locations, the laws of physics endure.
At the start of the hot Big Bang, the Universe was rapidly expanding and filled with high-energy, very densely packed, ultra-relativistic quanta. An early stage of radiation domination gave way to several later stages where radiation was sub-dominant, but never went away completely, while matter then clumped into gas clouds, stars, star clusters, galaxies, and even richer structures over time, all while the Universe continues expanding. The laws of physics, as known, apply at all times and locations to this picture. (Credit: CfA/M. Weiss)
Our Universe contains the Standard Model particles, plus whatever dark matter and dark energy are.
This diagram displays the structure of the Standard Model (in a way that displays the key relationships and patterns more completely, and less misleadingly, than in the more familiar image based on a 4×4 square of particles). In particular, this diagram depicts all of the particles in the Standard Model (including their letter names, masses, spins, handedness, charges, and interactions with the gauge bosons: i.e., with the strong and electroweak forces). It also depicts the role of the Higgs boson, and the structure of electroweak symmetry breaking, indicating how the Higgs vacuum expectation value breaks electroweak symmetry and how the properties of the remaining particles change as a consequence. Neutrino masses remain unexplained. (Credit: Latham Boyle and Mardus/Wikimedia Commons)
They interact via the four fundamental forces: gravity, electromagnetism, plus the two nuclear forces.
The idea of unification holds that all three of the Standard Model forces, and perhaps even gravity at higher energies, are unified together in a single framework. This idea, although it remains popular and mathematically compelling, does not have any direct evidence in support of its relevance to reality. Only electroweak unification, among all the unified possibilities, has been established. (Credit: ABCC Australia, 2015)
