- Every conservation principle corresponds to symmetry in nature
- A symmetry of a particular kind exists when a certain operation leaves something unchanged.
- There is an intimate connection between symmetry and so called conserved quantities.
- Well known conserved quantity is energy and corresponding symmetry in this case is time translation.

**Momentum Conservation**

- Holds for all type of interactions
- Related to the invariance of physical laws under translation in space.
- Thus laws of interaction do not depend on the place of measurement so the space is homogeneous.
- This transnational uniformity of space leads to the conservation of linear momentum.
- Particle at rest have no momentum. If it decays into two less massive particles , momentum conservation requires that the two particles travel away in exactly opposite directions.

**Conservation of Energy**

- Holds for all type of interactions.
- related to the invariance of physical laws under translations along the time axis i.e., homogeneity of time.
- laws of interaction do not depend on the time of measurement

**Angular momentum conservation**

- In addition to transnational symmetry , space also has a rotational symmetry.
- This symmetry of space gives rise to another conserved quantity , angular momentum.
- This law is also of general validity for all types of interactions.
- It is related to the invariance of the physical laws under rotation (isotropy of space).
- The orbital and spin angular momentum may be separately conserved.

**Parity Conservation**

- Holds for strong, nuclear and electromagnetic interactions but is violated in case of week interactions.
- related to the invariance of the physical laws under inversion of space co-ordinates. x,y,z are replaced by -x.-y,-z.
- is equivalent to combined reflection and rotation.
- physical laws do not depend on the right handedness of co-ordinate system.
- Parity operation symmetry represents discrete symmetry (reflection and rotation through 180 degree)
- Every particle with non zero mass has an intrinsic parity
π which can either be +1(even) or -1 (odd). Thus total parity of a system of n particles is the product of their intrinsic parities and the orbital parity (-1)
^{l}.
Thus, π - Intrinsic parity of pions is odd.

_{tot}=π

_{1}π

_{2}π

_{3}.......π

_{n}(-1)

^{l}

**Conservation of charge**

- Conservation of electric charge is related to gauge transformations which are shifts in the zeros of the scalar and vector electromagnetic potentials V and A
- Gauge transformations leave E and B unaffected since the latter are obtained by differentiating potentials , and this invariance leads to charge conservation.
- Charge and baryon number are conserved in all interactions.

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