‘Symmetry’ has a special meaning in physics. A picture is said to be symmetrical if one side is somehow the same as the other side. Precisely, a thing is symmetrical if one can subject it to a certain operation and it appears exactly the same after the operation.

**For example**, if we look at a base that is left and right symmetrical, then turn it 180° around the vertical axis it looks the same.

Newton’s laws of motion do not alter when the position coordinates are altered, that is, they are moved (linearly) from one place to other. This is equally true for almost all other physical laws. Therefore, we can say that (almost all) **laws of physics** are symmetrical for linear displacements. The same is true for rotational displacement.

Not only Newton’s law, as said, all the other laws of physics known so far are symmetric under translation and rotation of axes. Using these concepts of symmetry, new mathematical techniques has been developed for writing and using physical laws, for example **Tensor Analysis**.

**Remark: **We use many other different terms for **symmetry** whenever needed, for example in high-schools we use the term **conservation **and at grad-level it turns out to be **invariance, invariant **or **symmetry,** itself.

There are following main symmetry (conservation) operations in physical laws:

• Symmetry in Matter and Energy **or ***Conservation of Mass (Matter) and Conservation of Energy*

• Conservation of Momentum

• Conservation of Angular Momentum

• Conservation of Electric Charge

• Conservation of Baryon Number

• Conservation of Lepton Number

• Conservation of Strangeness

• Conservation of Hypercharge

• Conservation of Iso-spin

• Conservation of Charge Conjugation

• Conservation of Parity.

## Conservation of Mass & Energy

This conservation involves the following two different definitions and one hypothesis by Einstein.

### Definition.1

#### Conservation of Mass / Matter

Matter can never be created or destroyed, but it can convert itself into several other forms of either matter or energy or both.

### Definition. 2

#### Conservation of Energy

Energy can never be created or destroyed, but it can convert itself to other forms of matter & energy.

## Hypothesis

In practical, we see that if we burn a coal, it emits heat & remains ash. Scientifically, the coal (matter) is converted into heat (energy) and precipitate (matter). This is a balance conversion in which matter converts into energy. Similarly, we can generate a lot of energy after nuclear fission, in which also matter is converted directly into energy. We have also seen, energies forming different kind of **unstable **matters in nature. Physics’ famous equation $E=mc^2$ given by **Einstein** also says the same : $E$ (energy) is directly related to $m$ (mass). Matter (mass) and Energy both are conserved with their inner-conversions and the total value of **mass + energy** is a constant, since origin of universe. The complete hypothesis was by Albert Einstein.

**After combining the two definitions and the hypothesis we have,**

The mass and energy can neither be produced nor destroyed — but they can be converted from one form to another.

## Conservation of Momentum

The linear momentum of a system is constant if there are no external forces acting on the system of physical bodies.

## Conservation of Angular Momentum

The angular momentum of a system remains constant if there are no external (angular) torques acting on the system.

## Conservation of Electric Charge

The electric charge can neither be created nor destroyed. The net algebraic sum of positive and negative electric charges is constant.

## Conservation of Baryon Number

In any nuclear reaction, the number of baryon particles must remain the same, at least ] until the reaction completes.

## Conservation of Lepton Number

The lepton number, i.e.; the algebraic sum of number of leptons and anti-leptons, remains constant throughout a nuclear reaction.

## Conservation of Strangeness

The algebraic sum of the number of **kaons **and **hyperons **, called strangeness, remains constant in electromagnetic and strong interactions.

## Conservation of Hypercharge

The flavor of quarks remains the same throughout an internuclear interaction.

## Conservation of Isospin

The i*sospin* of hadrons is constant in strong interactions.

## Conservation of Charge Conjugation

**Remark: **Charge conjugation C is the operation of changing a fundamental particle into its anti-particle. It’s something like applying inverse function to any value.

For example, let C be the charge conjugation operator

• $C (\pi^+) = \pi^- $ (i.e., $\pi$-mesons being converted into their antiparticles, and;

• $C (x^2-3x+5) = 3x-5-x^2 \Box$

The charge conjugation operator is conserved in strong and electromagnetic interactions.

## Conservation of Parity

**Remark: **The parity operation, $P$ is reflection of all coordinates through the origin. That is, in two dimensions co-ordinate system X-Y, $P(x, y) = (-x, -y)$ or $P(\mathbf{r})=-\mathbf{r}$.

The parity of any wave function describing an elementary particle is conserved.

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