Physics I For Dummies
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The principle of symmetry is crucial to the study of physics and has special implications for string theory in particular. Symmetry exists when you can take something, transform it in some way, and nothing seems to change about the situation. When a transformation to the system causes a change in the situation, scientists say that it represents a broken symmetry.

This is obvious in geometry. Take a circle and draw a line through its center, as in this figure.


Now, imagine flipping the circle around that line. The resulting image is identical to the original image when flipped about the line. This is linear or reflection symmetry. If you were to spin the figure 180 degrees, you’d end up with the same image again. This is rotational symmetry.

The trapezoid, on the other hand, has asymmetry (or lacks symmetry) because no rotation or reflection of the shape will yield the original shape.

The most fundamental form of symmetry in physics is the idea of translational symmetry, which is where you take an object and move it from one location in space to another. If you move from one location to another, the laws of physics should be the same in both places. This principle is how scientists use laws discovered on Earth to study the distant universe.

In physics, though, symmetry means way more than just taking an object and flipping, spinning, or sliding it through space.

The most detailed studies of energy in the universe indicate that, no matter which direction you look, space is basically the same in all directions. The universe itself seems to have been symmetric from the very beginning.

The laws of physics don’t change over time (at least according to most physicists and certainly not on short timescales, like a human lifetime or the entire age of the United States of America). If you perform an experiment today and perform the same experiment tomorrow, you’ll get essentially the same result.

The laws of physics possess a basic symmetry with respect to time. Changing the time of something doesn’t change the behavior of the system.

These and other symmetries are seen as central to the study of science, and in fact, many physicists have stated that symmetry is the single most important concept for physics to grasp.

The truth is that while physicists often speak of the elegance of symmetry in the universe, the string theorist Leonard Susskind is quite right when he points out that things get interesting when the symmetry breaks.

In fact, the 2008 Nobel Prize in Physics was awarded to three physicists — Yoichiro Nambu, Makoto Kobayashi, and Toshihide Maskawa — for work in broken symmetry performed decades ago.

Without broken symmetry, everything would be absolutely uniform everywhere. The very fact that we have a chemistry that allows us to exist is proof that some aspects of symmetry don’t hold up in the universe.

Many theoretical physicists believe that a symmetry exists between the four fundamental forces (gravity, electromagnetism, weak nuclear force, strong nuclear force), a symmetry that broke early in the universe’s formation and causes the differences we see today. String theory is the primary (if not the only) means of understanding that broken symmetry, if it does (or did) indeed exist.

This broken symmetry may be closely linked to supersymmetry, which is necessary for string theory to become viable. Supersymmetry has been investigated in many areas of theoretical physics, even though there’s no direct experimental evidence for it, because it ensures that the theory includes many desirable properties.

Supersymmetry and the unification of forces are at the heart of the string theory story. As you read more about string theory, it’s up to you to determine whether the lack of experimental evidence condemns it from the start.

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