The Conservation of Mass-Energy

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There is a scientific law called the Law of Conservation of Mass, discovered by Antoine Lavoisier in 1785. In its most compact form, it states:

matter is neither created nor destroyed.

In 1842, Julius Robert Mayer discovered the Law of Conservation of Energy. In its most compact form, it it now called the First Law of Thermodynamics:

energy is neither created nor destroyed.

In 1907 (I think), Albert Einstein announced his discovery of the equation E = mc2 and, as a consequence, the two laws above were merged into the Law of Conservation of Mass-Energy:

the total amount of mass and energy in the universe is constant.

Generally, textbooks would add, as I am doing, that mass and energy can interconvert.

An interesting historical footnote: during the radioactive decay called beta decay, tremendous amounts of energy were being produced. This was expected, but what was not was that the energy amounts released varied widely for the exact same decay process. The amounts should always have been the same. This was very puzzling to the early researchers and I believe it was Niels Bohr who proposed that the Law of Conservation of Energy was being violated. Of course, this turned out to not be the case.

The correct answer was a new particle called the "neutrino," proposed about 1930 by Wolfgang Pauli. The neutrino was finally detected in 1952 (I think) and the discoverers were able to inform Pauli, then near death due to cancer. By the way, the neutrino is a very, very important particle in modern science. You may wish to research how neutrinos were useful in learning about Supernova 1987A.

The Law of Conservation of Mass is still a useful idea in chemistry. This is because the energy changes in a chemical reaction are so tiny that they did not affect any measurements. 100 kJ is a typical value for the energy involved in a chemical reaction and it is only about 10¯9 gram. Only recently has such a small amount been able to be accurately measured. The mass loss or gain due to energy loss or gain in a chemical reaction may someday be something that is routinely measured.

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