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What are the units for which e=mc2 is true?

What are the units for which e=mc2 is true? Since units for energy, mass and speed already existed before Einstein, it has always seemed to me an extraordinary coincidence that this equation does not require any additional constant. Or does Einstein just mean that e/m is extremely large?
Tony Wren from Greater London (Age 55+)

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2 Responses

  1. The units for this equation do work out. The units of e, the energy, are Joules, which can be written as kg m2 s-2 in the fundamental SI unit (sorry the editor doens’t let me do superscripts..). On the other side of the equation we have mass, kg, and the velocity of light, m s-1. The product mc2 has units of kg m2 s-2: the units on either side of the equal sign are the same.

    You would use any untis you like for this equation – for example the mass could be specified using my favourite units of Hippos, and the velocity of light in furlongs per fortnight, giving rather unorthodox units for e. Assuming you get the correct numerical values for m and c the value for e will be correct.

  2. I suppose it is true in any consistent set of units. But to take the standard SI units, energy will be in Joules (Newton metres, where a Newton, the unit of force, is kilograms metres per second squared), mass will be in kilograms, and c will be in metres per second. e/m is large in a numerical sense using these units.

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