General Relativity and Quantum Mechanics

The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory — Brian Greene— (Chapter 1: Tied Up with String)         There are two foundational pillars upon which modern physics rests.       One is Albert Einstein's general relativity, which provides a theoretical framework for understanding the universe on the largest of scales: stars, galaxies, clusters of galaxies, and beyond to immense expanse of the universe itself.       The other is quantum mechanics, which provides a theoretical framework for understanding the universe on the smallest of scales: molecules, atoms, and all the way down to subatomic particles like electrons and quarks.       Through years of research, physicists have experimentally confirmed to almost unimaginable accuracy virtually all predictions made by each of these theories.         But: as they are currently formulated, general relativity and quantum mechanics cannot both be right.         The two theories underlying the tremendous progress of physics during the last hundred years are mutually incompatible.         In the central depths of a black hole an enormous mass is crushed to a minuscule size.       At the moment of big bang the whole of the universe erupted from a microscopic nugget whose size makes a grain of sand look colossal.              There are realms that are tiny and yet incredibly massive, therefore requiring that both quantum mechanics and general relativity simultaneously be brought to bear.         Well-posed physical questions elicit nonsensical answers from the unhappy amalgam of these two theories.         Can it really be that the universe at its most fundamental level is divided, requiring one set of laws when things are large and a different, incompatible set when things are small?         Superstring theory: this new approach to describing matter at its most fundamental level resolves the tension between general relativity and quantum mechanics. Within this new framework, general relativity and quantum mechanics require one another for the theory to make sense.

Galaxies

Turn Left at Orion: A Hundred Night Sky Objects to See in a Small Telescope —and How to Find Them   —Guy Consolmagno, Dan M. Davis, Karen Kotash Sepp, Anne Drogin, Mary Lynn Skirvin— (Seasonal Objects: Spring)       Galaxies are the basic units of the universe.       After the universe was created it seems to have fragmented into discrete lumps of matter, each with enough mass to make billions of stars.       A lump would condense into a galaxy, with a cloud of globular clusters swarming erratically around the galactic center, and a disk of stars orbiting the center.         Galaxies come in three general forms: elliptical galaxies, irregular galaxies, spiral galaxies.                Elliptical galaxies look like large, somewhat flattened globular clusters (a collection of tens of thousands of stars in a ball within the galaxy).         In spiral galaxies, stars are organized into two or more arms that twist around their galactic center. Spirals are the most beautiful of galaxies.           The Whirlpool Galaxy, and our own Milky Way are examples of spiral galaxies. The Whirlpool's companion may be an elliptical galaxy.         Galaxies are observed to be clumped together into clusters, which may be anywhere from a dozen to hundreds of galaxies, each tied by the gravity of others into a cloud moving together through space.                Andromeda and its companions, the Triangulum Galaxy, and the Milky Way and its companions (the Magellanic Clouds) are all part of the Local Group. The galaxies in Ursa Major and in Leo are examples of members of other groups.          All the clusters appear to be moving away from each other, implying that they are the fragments of the Big Bang that took place some twelve to fifteen thousand billion years ago.       The clusters themselves are associated into clusters of clusters called superclusters.       We don't know whether these superclusters are independent entities, like raisins in a pudding, or connected together like the stuff in a sponge around “bubbles” of empty space.