rel="nofollow" href="#udc291359-2de9-54ae-a36c-07cc15f63666">Chapter 13 for more on the holographic principle.)
Why Is String Theory So Important?
String theory yields many fascinating subjects for thought, but you may be wondering about the practical importance of it. For one thing, string theory is the next step in our growing understanding of the universe. If that’s not practical enough, then there’s this consideration: Your tax money goes to fund scientific research, and (a tiny fraction of) the people trying to get that money want to use it to study string theory (or its alternatives).
A completely honest string theorist would be forced to say that there are probably no practical applications for string theory, at least in the foreseeable future. This admission doesn’t look that great on either the cover of a book or at the top of a webpage, so it gets spiced up with talk about discovering parallel universes, extra time dimensions, and new fundamental symmetries of nature. They might exist, but the theory’s predictions make it seem that they’re unlikely to ever be particularly useful, so far as we know.
Better understanding the nature of the universe is a good goal in its own right — as old as humanity, some might say — but when you’re looking at funding multibillion-dollar particle accelerators or research satellite programs, you might want something tangible for your money. Unfortunately, there’s no reason to think that string theory is going to give you anything practical.
Does this mean that exploring string theory isn’t important? No, and it’s our hope that reading Part 2 of this book will help illuminate the key at the heart of the search for string theory, or any new scientific truth.
In 1905, when Albert Einstein first presented his famous equation E = mc2, he thought it was an intriguing relationship, but he had no idea that it would result in something as potent as the atomic bomb. He had no way of knowing that the corrections to time calculations demanded by special relativity and general relativity would someday be required to get the worldwide global positioning system (GPS) to operate accurately (more on GPS in Chapter 6).
Quantum physics, which on the surface is about as theoretical a study as they come, is the basis for the laser and the transistor, two pieces of technology that are at the heart of modern computers and communication systems.
Even though we don’t know what a purely theoretical concept like string theory may lead to, history has shown that it will almost certainly lead somewhere profound.
For an example of the unexpected nature of scientific progress, consider the discovery and study of electricity, which was originally seen as a mere parlor trick. To be sure, you could predict some technologies from the discovery of electricity, such as the lightbulb. But some of the most profound discoveries are things that may never have been predicted — radio and television, the computer, the internet, the cell phone, and so on.
The impact of science extends into culture as well. Another by-product of electricity is rock and roll music, which was created with the advent of electric guitars and other electric musical instruments.
If electricity can lead to rock and roll and the internet, then imagine what sort of unpredicted (and potentially unpredictable) cultural and technological advances string theory could lead to!
Chapter 2
The Physics Road Dead-Ends at Quantum Gravity
IN THIS CHAPTER
Physicists like to group concepts together into neat little boxes with labels, but sometimes the theories they try to put together just don’t want to get along. Right now, nature’s fundamental physical laws can fit into one of two boxes: general relativity or quantum physics. But concepts from one box just don’t work together well with concepts from the other box.
Any theory that can get these two physics concepts to work together would be called a theory of quantum gravity. String theory is currently the most likely candidate for a successful theory of quantum gravity.
In this chapter, we explain why scientists want (and need) a theory of quantum gravity. We begin by giving an overview of the scientific understanding of gravity, which is defined by Einstein’s theory of general relativity, and our understanding of matter and the other forces of nature, in terms of quantum mechanics. With these fundamental tools in place, we then explain the ways in which these two theories clash with each other, which provides the basis for quantum gravity. Finally, we outline various attempts to unify these theories and the forces of physics into one coherent system, and the failures they’ve run into.
Understanding Two Schools of Thought on Gravity
Physicists are searching for a theory of quantum gravity because the current laws governing gravity don’t work in all situations. Specifically, the theory of gravity seems to “break down” (that is, the equations become physically meaningless) in certain circumstances that we describe later in the chapter. To understand what this means, you must first understand a bit about what physicists know about gravity.
Gravity is an attractive force that binds objects together, seemingly across any amount of distance. The formulation of the classical theory of gravity by Sir Isaac Newton was one of the greatest achievements of physics. Two centuries later, the reinvention of gravity by Albert Einstein placed him in the pantheon of indisputably great scientific thinkers.
Unless you’re a physicist, you probably take gravity for granted. It’s an amazing force, able to hold the heavens together while being overcome by a 3-year-old on a swing — but not for long. At the scale of an atom, gravity is irrelevant compared to the electromagnetic force. In fact, a simple magnet can overcome the entire force of the planet Earth to pick up metallic objects, from paper clips to automobiles.
Newton’s law of gravity: Gravity as force
Sir Isaac Newton developed his theory of gravity in the late 1600s. This amazing theory involved bringing together an understanding of astronomy and the principles of motion (known as mechanics or kinematics) into one comprehensive framework that also required the invention of a new form of mathematics: calculus. In Newton’s gravitational theory, objects are drawn together by a physical force that spans vast distances of space.
The key is that gravity binds all objects together (much like the Force in Star Wars). The apple falling from a tree and the moon’s motion around Earth are two manifestations of the exact same fundamental force.
The relationship that Newton discovered was a mathematical relationship (he did, after all, have to invent calculus to get it all to work out), just like relativity, quantum mechanics, and string theory.
In Newton’s gravitational theory, the force between two objects is based on the product of their masses, divided by the square of the distance between them. In other words, the heavier