Accompanying this were all sorts of other effects. It could be shown by Einstein’s reasoning that no object possessing mass could move faster than the speed of light. What’s more, as an object moved faster and faster, its length in the direction of motion (as measured by a stationary observer) grew shorter and shorter, while its mass grew greater and greater. At 260,000 kilometers per second, its length in the direction of movement was only half what it was at rest, and its mass was twice what it was. As the speed of light was approached, its length would approach zero in the direction of motion, while its mass would approach the infinite.
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 = mc²
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.
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.