When we think of any type of matter, a couple of unrelated images come up. First, we cannot go “through” matter: We cannot move through walls, and even wading in water, we encounter resistance. Second, all matter falls down, thanks to gravity.
Newton’s key insight was to understand that these are truly fundamental properties of all matter. The first property can be summarized by saying that all matter resists any attempt to put it into motion (we explain this in more detail later in this chapter in “Force, mass, and acceleration: Putting objects into motion”). This is called inertia, and the degree to which an object resists this change is its inertial mass.
The second property is that all matter is subject to the force of gravity. In fact, in Newton’s law of universal gravitation, the mass of an object plays the role of the charge of the gravitational attraction, just like the electric charge does for the electric force. Not that Newton could have known this: The electric force was discovered by Charles-Augustin de Coulomb a century after Newton. To emphasize that the mass is the source of the gravitational attraction, we talk of gravitational mass.
If you think that using two different words — inertial and gravitational — to refer to mass is a little odd, you are spot on: It turns out that the two concepts of mass are one and the same! This is absolutely not obvious at first look. It wasn’t until the early 1900s that a Hungarian gentleman named Loránd Eötvös came up with a clever experiment that convinced people that this was the right way to think about Newton’s discovery. This fact is a crucial ingredient in Einstein’s general relativity theory and in string theory.
Scientists discover that mass can’t be destroyed
Antoine-Laurent Lavoisier’s work in the 18th century provided physics with another great insight into matter. Lavoisier and his wife, Marie Anne, performed extensive experiments that indicated matter can’t be destroyed; it merely changes from one form to another. This principle is called the conservation of mass.
This isn’t an obvious property. If you burn a log, when you look at the pile of ash, it certainly looks like you have a lot less matter than you started with. But, indeed, Lavoisier found that if you’re extremely careful that you don’t misplace any of the pieces — including the pieces that normally float away during the act of burning — you end up with as much mass at the end of the burning as you started with.
Over and over again, Lavoisier showed this unexpected trait of matter to be the case, so much so that we now take it for granted as a familiar part of our universe. Water may boil from liquid into gas, but the particles of water continue to exist and can, if care is taken, be reconstituted back into liquid. Matter can change form, but can’t be destroyed This picture becomes more complicated when one starts looking at chemical and nuclear reactions, when one can trade a tiny amount of mass for quite a lot of energy. But this was not yet understood until well after Lavoiser’s time.
As the study of matter progresses through time, things grow stranger instead of more familiar. In Chapter 8, we discuss the modern understanding of matter, which is that we are composed mostly of tiny particles that are linked together with invisible forces across vast (from their scale) empty distances. In fact, as string theory suggests, it’s possible that even those tiny particles aren’t really there — at least not in the way we normally picture them.
Add a little energy: Why stuff happens
The matter in our universe would never do anything interesting if it weren’t for the addition of energy. There would be no change from hot to cold or from fast to slow. Energy is also conserved, as scientists discovered throughout the 1800s as they explored the laws of thermodynamics, but the story of energy’s conservation is more elusive than that of matter. You can see matter, but tracking energy proves to be trickier.
Kinetic energy is the energy involved when an object is in motion. Potential energy is the energy contained within an object, waiting to be turned into kinetic energy. It turns out that the total energy — kinetic energy plus potential energy — is conserved any time a physical system undergoes a change.
String theory makes new predictions about physical systems that contain a large amount of energy packed into a very small space. The energies needed for string theory predictions are so large that it may never be possible to construct a device able to generate that much energy and directly test the theory’s new predictions. String theory also works when the energies are small, in which case it reproduces the “usual” laws of gravity, as we expect.
The energy of motion: Kinetic energy
Kinetic energy is most obvious in large objects, but it’s present in objects of all sizes, down to molecules and atoms. Heat (or thermal energy) is really just a bunch of atoms moving rapidly, representing a form of kinetic energy. When water is heated, the particles accelerate until they break free of the bonds with other water molecules and become a gas. The motion of particles can cause energy to be emitted in different forms, such as when a burning piece of coal glows white hot.
Sound is another form of kinetic energy. If two billiard balls collide, the particles in the air will be forced to move, resulting in a noise carried by a wave. Normally this is a tiny amount of energy, but it can actually reach destructive levels when we consider the blast of an explosion or the waves in a fluid other than air — for instance, a tsunami in the ocean. It turns out that light and radiation are also described by waves, and they also carry energy, but they do so in a different way than sound waves and ocean waves do. We will come back to this in Chapter 7.
Stored energy: Potential energy
Potential energy is stored energy. Potential energy takes many more forms than kinetic energy and can be a bit trickier to understand.
A spring, for example, has potential energy when it’s stretched out or compressed. When the spring is released, the potential energy transforms into kinetic energy as the spring moves into its least energetic length.
Moving an object in a gravitational field changes the amount of potential energy stored in it. A penny held out from the top of the Empire State Building has a great deal of potential energy due to gravity, which turns into a great deal of kinetic energy when it’s dropped (although not, as demonstrated on an episode of MythBusters, enough to kill an unsuspecting pedestrian on impact).
It may sound a bit odd, talking about something having more or less energy just because of where it is, but the environment is part of the physical system described by the physics equations. These equations tell us exactly how much potential energy is stored in different physical systems, and they can be used to determine outcomes when the potential energy gets released.
Symmetry: Why some laws were made to be broken
A change in location or position that retains the properties of the system is called a geometric symmetry (or sometimes translational symmetry). Another form of symmetry is an internal symmetry, which is when something within the system can be swapped for something else and the system (as a whole) doesn’t change. When a symmetrical situation at high energy collapses into a lower energy ground state that is asymmetrical, it’s called spontaneous symmetry breaking. An example would be when a roulette wheel spins and slows into a “ground state.” The ball ultimately settles into one slot in the wheel — and the gambler either wins or loses.
String theory goes beyond the symmetries we observe to predict even more symmetries that aren’t observed in nature. It predicts a necessary symmetry that’s not observed in nature called supersymmetry. At the energies we observe, supersymmetry is an example of a broken symmetry, though physicists believe that