String Theory For Dummies. Andrew Zimmerman Jones

to form other types of particles.

      

The atom, it turns out, is composed of a nucleus surrounded by electrons. The nucleus is made up of protons and neutrons, which are in turn made up of strange new particles called quarks! As soon as physicists thought they had reached a fundamental unit of matter, they seemed to discover that it could be broken open and still smaller units could be pulled out.

      Not only that, but even these fundamental particles didn’t seem to be enough. It turns out that there are three families of particles, some of which only appear at significantly higher energies than scientists had previously explored.

      Today, the Standard Model of particle physics contains 25 distinct fundamental particles, all of which have been experimentally detected (often decades after theoreticians proposed them).

Grasping for the Fundamental Forces of Physics

      Even while the types of particles identified by scientists became more bizarre and complex, the ways those objects interacted turned out to be surprisingly straightforward. In the 20th century, scientists discovered that objects in the universe experience only four fundamental types of interactions:

       Electromagnetism

       Strong nuclear force

       Weak nuclear force

       Gravity

      Physicists have discovered profound connections between these forces — except for gravity, which seems to stand apart from the others for reasons that physicists still aren’t completely certain about. Trying to incorporate gravity with all the other forces — to discover how the fundamental forces are related to each other — is a key insight that many physicists hope a theory of quantum gravity will offer.

      Electromagnetism: Super-speedy energy waves

      Discovered in the 19th century, the electromagnetic force (or electromagnetism) is a unification of the electrostatic force and the magnetic force. In the mid-20th century, this force was explained in a framework of quantum mechanics called quantum electrodynamics, or QED. In this framework, the electromagnetic force is transferred by particles of light, called photons.

      The relationship between electricity and magnetism is covered in Chapter 5, but the basic relationship comes down to electrical charge and its motion. The electrostatic force causes charges to exert forces on each other in a relationship that’s similar to (but more powerful than) gravity — an inverse square law. This time, though, the strength is based not on the mass of the objects but on the electrical charge.

      The electron is a particle that contains a negative electrical charge, while the proton in the atomic nucleus has a positive electrical charge. Traditionally, electricity is seen as the flow of electrons (negative charge) through a wire. This flow of electrons is called an electrical current.

      A wire with an electrical current flowing through it creates a magnetic field. Alternately, when a magnet is moved near a wire, it causes a current to flow. (This is the basis of most electric power generators.)

      This is the way in which electricity and magnetism are related. In the 1800s, physicist James Clerk Maxwell unified the two concepts into one theory, called electromagnetism, which depicted the electromagnetic force as waves of energy moving through space.

      One key component of Maxwell’s unification was a discovery that the electromagnetic force moved at the speed of light. In other words, the electromagnetic waves that Maxwell predicted from his theory were a form of light waves.

Quantum electrodynamics retains this relationship between electromagnetism and light because, in QED, the information about the force is transferred between two charged particles (or magnetic particles) by another particle — a photon. (Physicists say that the electromagnetic force is mediated by a photon.)

      Nuclear forces: What the strong force joins, the weak force tears apart

      In addition to gravity and electromagnetism, 20th-century physicists identified two nuclear forces called the strong nuclear force and the weak nuclear force. These forces are also mediated by particles. The strong force is mediated by a type of particle called a gluon — there are eight gluons, distinguished by their color charge. This has nothing to do with actual colors, it’s just a name that physicists invented. This is why the theory of strong interactions is called quantum chromodynamics (chroma is Greek for color).

      The weak force is mediated by three particles: Z, W⁺, and W^– bosons. These are actually closely related to the photon, the particle that mediate electromagnetic interactions. (You can read more about these particles in Chapter 8.)

      The strong nuclear force holds quarks together to form protons and neutrons, but it also holds the protons and neutrons together inside the atom’s nucleus.

      The weak nuclear force, on the other hand, is responsible for radioactive decay, such as when a neutron decays into a proton. The processes governed by the weak nuclear force are responsible for the burning of stars and the formation of heavy elements inside stars.

Infinities: Why Einstein and the Quanta Don’t Get Along

      Einstein’s theory of general relativity, which explains gravity, does an excellent job of explaining the universe on the scale of the cosmos. Quantum physics does an excellent job of explaining the universe on the scale of an atom or smaller. In between those scales, good old-fashioned classical physics usually rules.

Unfortunately, some problems bring general relativity and quantum physics into conflict, resulting in mathematical infinities in the equations. (Infinity is essentially an abstract number that’s larger than any other numbers. Though certain cartoon characters like to go “To infinity and beyond,” scientists don’t like to see infinities come up in mathematical equations.) Infinities arise in quantum physics, but physicists have developed mathematical techniques to tame them in many cases so the results match the experiments. In some cases, however, these techniques don’t apply. Because physicists never observe real infinities in nature, these troublesome problems motivate the search for quantum gravity.

      Each of the theories works fine on its own, but when you get into areas where both have something specific to say about the same thing — such as what’s going on at the border of a black hole — things get very complicated. The quantum fluctuations make the distinction between the inside and outside of the black hole kind of fuzzy, and general relativity needs that distinction to work properly. Neither theory by itself can fully explain what’s going on in these specific cases.

      

This is the heart of why physicists need a theory of quantum gravity. With the current theories, you get situations that don’t look like they make sense. Physicists don’t see infinities, but both relativity and quantum physics indicate that they should exist. Reconciling this bizarre region in the middle, where neither theory can fully describe what’s going on, is the goal of quantum gravity.

      Singularities: Bending gravity to the breaking point

      Because matter causes a bending of space-time, cramming a lot of matter into a very small space causes a lot of bending of space-time. In fact, some solutions to Einstein’s general relativity equations show situations where space-time bends an infinite amount — called a singularity. Specifically, a space-time singularity shows up in the mathematical equations of general relativity in the following two situations:

       During