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.
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 the early big bang period of the universe’s history
Inside black holes
These subjects are covered in more detail in Chapter 9, but both situations involve a density of matter (a lot of matter in a small space) that’s enough to cause problems with the smooth space-time geometry that relativity depends on.
Some believe that this problem can be solved by altering Einstein’s theory of gravity (as you see in Chapter 20). String theorists don’t usually want to modify gravity (at least at the energy levels scientists normally look at); they just want to create a framework that allows gravity to work without running into these mathematical (and physical) infinities.
Quantum jitters: Space-time under a quantum microscope
A second type of infinity, proposed by John Wheeler in 1955, is the quantum foam or, as it’s called by string theorist and best-selling author Brian Greene, the quantum jitters. Quantum effects mean that space-time at very tiny distance scales (called the Planck length) is a chaotic sea of virtual particles being created and destroyed. At these levels, space-time is certainly not smooth, as relativity suggests, but is a tangled web of extreme and random energy fluctuations, as Figure 2-1 shows.
FIGURE 2-1: If you zoom in on space-time enough, you may see a chaotic “quantum foam.”
The basis for the quantum jitters is the uncertainty principle, one of the key (and most unusual) features of quantum physics. This is explained in more detail in Chapter 7, but the key component of the uncertainty principle is that certain pairs of quantities — for example, position and velocity, or time and energy — are linked together, so that the more precisely one quantity is measured, the more uncertain the other quantity is. This isn’t just a statement about measurement, though; it’s fundamental uncertainty in nature!
One example of the blurriness comes in the form of virtual particles. According to quantum field theory (a field theory is one where each point in space has a certain value, similar to a gravitational field or an electromagnetic field), even the empty void of space has a slight energy associated with it. This energy can be used to, very briefly, bring a pair of particles — a particle and its antiparticle, to be precise — into existence. The particles exist for only a moment, and then they destroy each other. It’s as if they borrowed enough energy from the universe to exist for just a few fractions
