# Rest mass

Rest mass, also known as invariant mass, intrinsic mass, proper mass, or simply mass in the case of bound systems, is a fundamental property of a particle or a system of particles that remains constant regardless of the observer’s frame of reference.

• For a single particle, its invariant mass equals its energy at rest divided by the square of the speed of light.
• Photons, which have zero rest mass, still possess an invariant mass calculated using their energy and momentum, related by the equation E=pc, where E is energy, p is momentum, and c is the speed of light.
• Invariant mass is a crucial concept in particle physics because it’s conserved in all interactions, including those involving the conversion of mass into energy and vice versa.
• The conservation of invariant mass stems from the conservation of energy and momentum, fundamental principles in physics.
###### References
• Rest mass, also known as invariant mass, intrinsic mass, proper mass (or simply mass in the case of bound systems,) is a fundamental property of a particle or a system of particles that remains constant regardless of the observer’s frame of reference.
• For a single particle, its invariant mass equals its energy at rest divided by the square of the speed of light.
• Photons, which have zero rest mass, still possess an invariant mass calculated using their energy and momentum, related by the equation E=pc, where E is energy, p is momentum, and c is the speed of light.
• Invariant mass is a crucial concept in particle physics because it’s conserved in all interactions, including those involving the conversion of mass into energy and vice versa.
• The conservation of invariant mass stems from the conservation of energy and momentum, fundamental principles in physics.