# Are all physics theories reliant on "the speed of light"?

1. Feb 1, 2015

### ttvlr

Are all physics theories based on "the equivalence of energy E and mass m always reliant on the speed of light c?
Are there any alternative theories related to the equivalence of energy and mass that are not reliant on "the speed of light"?
I would appreciate your attention,
ttvlr

2. Feb 1, 2015

### DrStupid

That depends on what you accept as "alternative theories". In classical mechanics there is no such equivalence..

3. Feb 1, 2015

### Staff: Mentor

Yes. The units require a constant with units of speed^2. Any such constant can always be expressed as some multiple of c^2. The c^2 isn't a big thing with deep meaning, it is just a question of units.

Last edited: Feb 1, 2015
4. Feb 1, 2015

### ttvlr

Thanks for your response. I appreciate it. However I just want to be sure that this means that all existent energy theories present use the speed of light as a point of constant reference thus eliminating any other constant as a point of reference to c?

5. Feb 1, 2015

### Staff: Mentor

Did you even read my response?

Any possible constant that could be used would always be expressible as some multiple of c^2. The idea of "eliminating any other constant" doesn't make sense because any other constant would simply be a multiple of c^2 anyway.

6. Feb 1, 2015

### Drakkith

Staff Emeritus
There's a few key things to understand. The equivalence of mass and energy is an observed phenomena. We observe it all the time in particle colliders. To date, the LHC has looked at over 1.5 trillion collision events, and I assume that the total number is far more when you factor in all the other colliders that have been in operation over the years.

So, since it is an observed phenomena, any theoretical explanation has to fit it. It turns out that c2 is a good unit to use when converting from units of energy to units of mass. In the end, that's all the equation is really stating anyways. If you know how much total energy a system has, you can find its mass using the equation, and vice-versa.

Any new theory that uses joules and kilograms as its units for energy and mass will always use this equation just as it is. But say you don't want to convert from joules to kilograms. A common unit of energy is the electron volt, or eV. One eV is equal to 1.602176565(35)×10−19 joules. So now we can't use the equation as it is. It won't be correct. We have to change something else in the equation. We can either change the units of mass, or we can change what c2 is equal to. A common method is to set c2 equal to 1 and use eV as both a unit of mass and energy. (So c2 no longer represents the speed of light in this equation) This has the advantage of making certain calculations easier, but you lose some of the context and clarity since setting c2 equal to 1 allows you to remove it from the equation. Those who aren't familiar with the equation may not realize how it actually works.

What we've done with c2 in the equation e=mc2 (using joules and kilograms) is we've set up our units around the speed of light.

Note that einstein says in his original paper on the subject, that "if the energy (of a body) changes by L, the mass changes in the same sense by L/9x1020, if the energy is measured in ergs and the mass in grams". He initially uses the speed of light only because of how he develops the theory using relativity and light waves, but then he gives two specific units, neither of which can be used with c2 as the conversion factor if c is the speed of light.

Ref: http://einsteinpapers.press.princeton.edu/vol2-trans/186

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