*Like any other science theoretical particle physics has its own language and its own terms. Even if some of these terms seem to be the same as in everyday life they have slightly other meanings in science. What can we do about this? The members of our group picked a list of the most common terms and give their best to explain them. The first one is Taushif Ahmed who describes what **“symmetry”** means in particle physics and why Nature likes to break it. Taushif is now a postdoc at the University of Turin. Check out his **personal homepage**.*

*“It is only slightly overstating the case to say that physics is the study of symmetry” (Philip Warren Anderson)*

*“It is only slightly overstating the case to say that physics is the study of symmetry” (Philip Warren Anderson)*

Look into a mirror, wave your hands and your reflection will wave back at you! It might be natural to take the effect of symmetry for granted in everyday life, however, symmetry plays a very crucial role in unraveling the principles on which our universe works. So, what do we mean by symmetry in physics? Well, it can be described by the action of a transformation that leaves the system unchanged, or in other words, it’s a change that makes no change.* *For example, all snowflakes show a hexagonal symmetry around an axis that runs perpendicular to their face: every one-sixth of a revolution around this axis produces a design identical to the original.

In physics there is a remarkable consequence of symmetry which was first noticed by Emmy Noether in 1918: for every continuous symmetry of the universe, there exists a conserved quantity! A long road in the middle of nowhere is continuously symmetric under spatial translation in the direction of the road. If two cars collide on that road, the sum of their momentum stays the same or in other words, the momentum is conserved.

**Nature likes to hide symmetry**

To our surprise, Nature does not always want to reveal the symmetry, it likes to hide it and this is called symmetry breaking. One of the most profound ways of hiding symmetry is the phenomena of spontaneous symmetry breaking** **where the laws of physics are symmetric but the state of the system is not! This is responsible for an enumerable number of remarkable phenomena that Nature exhibits. For instance, a metal consists of a lot of tiny magnets which are oriented arbitrarily and therefore looks the same from any direction, i.e.,** **it exhibits rotational symmetry. Once we bring the metal near a magnet, the arbitrarily oriented tiny magnets align themselves in a particular direction which not only breaks the rotational symmetry but also converts it into a magnet. Physicists believe the same phenomenon lies behind the existence of the different fundamental forces that we encounter in Nature.

**Symmetry dictates the structure of the fundamental forces**

When the universe was in its early stages and temperatures were extremely high, all of the forces were presumably not distinct, i.e., there was only a single fundamental force. As the universe started cooling down, it gave birth to different kinds of forces, such as strong, weak and electromagnetic forces. As physicists peered into the infinitesimal world of fundamental interactions, they started discovering some different kinds of symmetries - local gauge symmetries. These local symmetries are the cornerstone of the leading theory in modern physics - the **Standard Model**, which encapsulates the principles of the fundamental nongravitational forces governing the universe - electromagnetic, weak and strong interactions. This reminds me of a famous quote by Chen-Ning Yang, "Symmetry dictates interaction." It means that the structures of all the forces are intimately related to the concept of symmetry.

Albert Einstein employed the symmetry of space-time under the local transformation of space-time to determine the laws of gravity. As physicists try to explore Nature at higher and higher energy and thus reveal its behavior at shorter and shorter distances, they discover more and more symmetry: at the fundamental level Nature, for whatever reason, prefers Beauty and is marvelous in inventing new forms of Beauty. This symmetry is usually broken or hidden in everyday life. In short, symmetry provides us with an important tool for the exploration of Nature. If you want to search for new and more fundamental laws of Nature, look for new symmetries.