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 of a second.

      The problem is that when you look at space-time at very small scales, the effects of these virtual particles become very important. The energy fluctuations predicted by the uncertainty principle take on massive proportions. Without a quantum theory of gravity, there’s no way to really figure out what’s going on at sizes that small.

Unifying the Forces

      The attempt to unite gravity with the other three forces, as well as with quantum physics, was one of the driving forces of physics throughout the 20th century (and it still is). In a way, these sorts of unifications of different ideas are the major discoveries in science throughout the ages.

      Quantum electrodynamics successfully created a quantum theory of electromagnetism. Later, the electroweak theory unified this theory with the weak nuclear force. The strong nuclear force is explained by quantum chromodynamics. The current model of physics that explains all three of these forces is called the Standard Model of particle physics, which is covered in much more detail in Chapter 8.

      Unifying gravity with the other forces would create a new version of the Standard Model and would explain how gravity works on the quantum level. Many physicists hope that string theory will ultimately prove to be this theory.

      Einstein’s failed quest to explain everything

      After Einstein successfully worked the major kinks out of his theory of general relativity, he turned his attention toward trying to unify this theory of gravity with electromagnetism as well as with quantum physics. In fact, he would spend most of the rest of his life trying to develop this unified theory but would die unsuccessful.

      Throughout this quest, Einstein looked at almost any theory he could think of. One of these ideas was to add an extra space dimension and roll it up into a very small size. This approach, called Kaluza-Klein theory after the men who proposed it, is addressed in Chapter 6. This same approach would eventually be used by string theorists to deal with the pesky extra dimensions that arose in their own theories.

      Ultimately, none of Einstein’s attempts bore fruit. To the day of his death, he worked feverishly on completing his unified field theory in a manner that many physicists consider a sad end to such a great career.

      Today, however, some of the most intense theoretical physics work is in the search for a theory to unify gravity and the rest of physics, mainly in the form of string theory.

      A particle of gravity: The graviton

      The Standard Model of particle physics explains electromagnetism, the strong nuclear force, and the weak nuclear force as fields that follow the rules of gauge theory. Gauge theory is based heavily on mathematical symmetries. Because these forces are quantum theories, the gauge fields come in discrete units (quantum means “how much” in Latin) — and these units actually turn out to be particles in their own right, called gauge bosons. The forces described by a gauge theory are carried, or mediated, by these gauge bosons.

      For example, the electromagnetic force is mediated by the photon. When gravity is written in the form of a gauge theory, the gauge boson for gravity is called the graviton. (If you’re confused about gauge theories, don’t worry too much — just remember that the graviton is what makes gravity work and you’ll know everything you need to know to understand their application to string theory.)

      Physicists have identified some features of the theoretical graviton so that, if it exists, it can be recognized. For one thing, the particle is massless, which means it has no rest mass — a graviton is always in motion, and that probably means it travels at the speed of light (although in Chapter 20 you find out about a theory of modified gravity in which gravity and light move through space at different speeds).

Another feature of the graviton is that it has a spin of 2. (Spin is a quantum number indicating an inherent property of a particle that acts kind of like angular momentum. Fundamental particles have an inherent spin, meaning they interact with other particles like they’re spinning even when they aren’t.)

      A graviton also has no electrical charge. It’s