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© Copyright 1999, Jim Loy
See the addendum below, for a correction of this article.
I was reading a book on archeology. The author, while trying to compare archeology to physics, said that Einstein was responsible for explaining that mass is converted to energy when something burns. That is a fairly amazing misconception, especially from a scientist. Combustion has nothing whatsoever to do with converting mass to energy. Combustion is a simple chemical process, which means that it is the simple conversion of one form of energy (chemical energy) into another form of energy (heat). Chemistry and most of physics is involved in the simple conversion of energy into energy. This has been known for a few hundred years.
Over the centuries, it became known that energy is conserved. In other words, energy is not gained or lost, it merely changes form. This is the First Law of Thermodynamics, conservation of energy. Chemical energy may be converted to heat or vice versa. You don't lose energy, but it may end up in a less usable form. In fact, that is the Second Law of Thermodynamics, that energy tends to go from a usable form to a less usable form, from available energy to unavailable energy.
Converting mass to energy, often involves nuclear reactions (reactions involving the nucleus of the atom). These are fission and fusion and natural radioactivity. Or it may involve matter/antimatter interaction. When an electron and a positron meet, they destroy each other and form energy. Converting energy to mass may involve nuclear processes or it may result from accelerating matter to near the speed of light. It all involves Einstein's famous equation E=mc². This famous equation merely shows that matter is a very compact form of energy. Matter can be converted to energy, and back again. But this is not a normal process, like combustion or mechanical work.
Nuclear processes (and other processes mentioned in the previous paragraph) involve conversion between mass and energy. Energy goes away (or is created). Does this violate the conservation of energy? No. It just means that mass must be included in the law, as another form of energy, just like chemical energy or heat.
Addendum:
A couple of people have emailed, saying that when an electron in an atom absorbes a photon, it increases in mass, that the entire energy of the photon was converted to mass (not a lot of mass, by the way). That's not what I had heard, and I am trying to check that out. So far, I have read Feynman's amazing book QED (in an effort to clear this up), and have not found any mention of this energy-mass conversion. I will keep searching.
When an electron in an atom absorbes a photon, just how is this energy stored? Is it an increase in mass, as these people tell me? Or is it a different position within an electric field, similar to a car partway up a hill in a gravitational field, as some books seem to imply?
OK, it's both position and mass, all people mentioned above were right. The position idea works well on a larger scale. In particular, the archeologist was right, and in that respect, I was wrong. See Mass and Energy by Michael Fowler for a particularly clear account of this.