Could this really be so? Ordinary objects never moved so fast as to make their lengths and masses show any measurable change. What about subatomic particles, however, which moved at tens of thousands of kilometers per second? The German physicist Alfred Heinrich Bucherer (1863-1927) reported in 1908 that speeding electrons did gain in mass just the amount predicted by Einstein’s theory. The increased mass with energy has been confirmed with great precision in recent years. Einstein’s special theory of relativity has met many experimental tests exactly ever since and it is generally accepted by physicists today. Einstein’s theory gave rise to something else as well. Einstein deduced that mass was a form of energy. He worked out a relationship (the “mass-energy equivalence”) that is expressed as follows: _E_ = _mc_² where _E_ represents energy, _m_ is mass, and _c_ is the speed of light. If mass is measured in grams and the speed of light is measured in centimeters per second, then the equation will yield the energy in a unit called “ergs”. It turns out that 1 gram of mass is equal to 900,000,000,000,000,000,000 (900 billion billion) ergs of energy. The erg is a very small unit of energy, but 900 billion billion of them mount up. The energy equivalent of 1 gram of mass (and remember that a gram, in ordinary units, is only ¹/₂₈ of an ounce) would keep a 100-watt light bulb burning for 35,000 years. [Illustration: ENERGY CREATED compared to MATTER (OR MASS) DESTROYED] It is this vast difference between the tiny quantity of mass and the huge amount of energy to which it is equivalent that obscured the relationship over the years. When a chemical reaction liberates energy, the mass of the materials undergoing the reaction decreases slightly—but _very_ slightly. Suppose, for instance, a gallon of gasoline is burned. The gallon of gasoline has a mass of 2800 grams and combines with about 10,000 grams of oxygen to form carbon dioxide and water, yielding 1.35 million billion ergs. That’s a lot of energy and it will drive an automobile for some 25 to 30 kilometers. But by Einstein’s equation all that energy is equivalent to only a little over a millionth of a gram. You start with 12,800 grams of reacting materials and you end with 12,800 grams minus a millionth of a gram or so that was given off as energy. No instrument known to the chemists of the 19th century could have detected so tiny a loss of mass in such a large total. No wonder, then, that from Lavoisier on, scientists thought that the law of conservation of mass held exactly. Radioactive changes gave off much more energy per atom than chemical changes did, and the percentage loss in mass was correspondingly greater. The loss of mass in radioactive changes was found to match the production of energy in just the way Einstein predicted.