Mass and Energy
المؤلف:
Franklin Potter and Christopher Jargodzki
المصدر:
Mad about Modern Physics
الجزء والصفحة:
p 41
13-10-2016
548
Mass and Energy
A symbol of the twentieth century is the famous Einstein relation between mass and energy. Here are four possible equations: (1) E0 = mc2 (2) E = mc2 (3) E0 = m0c2 (4) E = m0c2. In the equations c is the velocity of light, E is the total energy of a free body, E0 its rest energy, m0 its rest mass, and m its mass.
Which of these equations expresses one of the main consequences of the STR? Which equation was first written by Einstein and was considered by him a consequence of STR?
Answer
The answer to both questions is equation 1, although the majority of physicists seem to prefer equations 2 or 3! Their choices probably are caused by the confusing terminology widely used in the physics literature that says that a body at rest has a “proper mass” or “rest mass” m0, and a body in motion has a “relativistic mass” m = 
.
There is only one mass in physics, m, which does not depend on the reference frame. This mass m is the relativistic invariant quantity in E2 – p2c2 = m2c4, whereas the energy is different in different reference systems. There is no need to place the index 0 with the mass. However, the total energy E needs the 0 index if the particle has no momentum in that reference frame that is, E0 = mc2.
For a complete and stimulating discussion of these ideas and their history see the L. V. Okun reference below.
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