Einstein's 1905 Kick-off of E=mc2: Definition?

Einstein kicks off modern physics in the last of the five articles from his Annus Mirabilis 1905 as a short note starting with the question:

• DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT?

and ending with the answer:

• If a body gives off the energy L in the form of radiation, its mass diminishes by $\frac{L}{c^2}$. The fact that the energy withdrawn from the body becomes energy of radiation evidently makes no difference, so that we are led to the more general conclusion that
• The mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by $\frac{L}{9}\times 10^{20}$, the energy being measured in ergs, and the mass in grammes.
• It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be successfully put to the test.
• If the theory corresponds to the facts, radiation conveys inertia between the emitting and absorbing bodies.

This is nothing but $E=mc^2$, which thus to Einstein was only a loose hypothesis in 1905 (If the theory corresponds to the facts), but became a truth after proclaimed experimental conformation to a very high precision, much welcomed by Einstein as you can hear above: 

• It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing -- a somewhat unfamiliar conception for the average mind. Furthermore, the equation E is equal to m c-squared, in which energy is put equal to mass, multiplied by the square of the velocity of light, showed that very small amounts of mass may be converted into a very large amount of energy and vice versa. The mass and energy were in fact equivalent, according to the formula mentioned above. This was demonstrated by Cockcroft and Walton in 1932, experimentally.

As a 100 year celebration of Einstein's $E=mc^2$ a team at MIT including David Pritchard presented an experimental conformation with improved accuracy of one part of a million, with the following caveats by Pritchard:

• In spite of widespread acceptance of this equation as gospel, we should remember that it is a theory. It can be trusted only to the extent that it is tested with experiments.
• Determining the mass difference requires the individual masses to be measured with the incredible accuracy of one part in 100 billion -- equivalent to measuring the distance from Boston to Los Angeles to within the width of a human hair!
• This doesn't mean it has been proven to be completely correct. Future physicists will undoubtedly subject it to even more precise tests because more accurate checks imply that our theory of the world is in fact more and more complete.

So we still cannot be sure. We should also be aware that if a physical law is confirmed to an extreme precision, then it may be that the law is not something created by Nature, but rather a logical necessity created by human minds as simply a definition. The equivalence of energy and mass may well be true by definition and so exactly true and then there is no wonder if experiments can confirm with extreme precision.

 

It is like experimental confirmation to a very high precision that there 100 centimeters on meter, which certainly can attract funding, although rather meaningless...

The equivalence of inertial and gravitational mass comes with the same ambiguity as definition or physical fact. Why should Nature play with two different notions of mass? Compare with Many-Minds Relativity.

Feynman prides himself of having predicted using his theory of QED, the anomalous magnetic dipole moment of the electron to a precision of better than a part in a billion. Again too precise to be credible as an agreement with real physical fact, rather than definition?

Experimental evidence

Here a neat little experiments which you can do with a pot of water and a kitchen scale: Heat the water allowing it to gain heat energy and check if its mass increases, and report back!

Another is to climb 10 stores and check if your mass has increased, or descend instead if you want loose weight:

Easy way loose body mass, if needed.