The most famous equation in the history of science,
Einstein’s “energy
(E) equals mass (m) times the speed of light
(c) squared” changed our view of matter and energy—and
consequently changed our fundamental understanding of the universe.
Rather than seeing mass and energy as separate things, we now see
them as different aspects of a single thing: You can think of mass
as a particular kind of energy that can be converted into other
kinds of energy such as heat, light, and motion.
The equation E = mc^{2} describes matter at rest. (A more
complicated version of the equation includes matter in motion.)
Essentially, it says that the mass of an object at rest can be
converted to energy, and energy can be converted to the rest mass
of an object.
To find out exactly how much energy an object at rest contains,
we multiply the mass by c^{2}. Because mass is measured in
kilograms and energy is measured in joules, c^{2} is used
as a conversion factor so that the energy of the object can be
expressed in joules. (If scientists long ago had understood the
equivalence of mass and energy, we would undoubtedly use the same
unit for both.)
You may wonder why the conversion factor involves the speed of
light. It may help to think of c—299 792 kilometers
per second—as the “ultimate speed limit” of the
universe, which makes it a very basic number in special relativity
that affects far more than the speed of light. Einstein didn’t
pluck c^{2} out of the air, though. He did some
complicated mathematics to work it out.
Einstein thought that the massenergy equation was the most important
outcome of special relativity—even though, in the early 1900s,
there was no way to test it. In fact, it wasn’t demonstrated
experimentally until 1932. Today we know that massenergy conversions
take place inside our sun, in nuclear power plants, in particle
accelerators, and in a host of other processes, and we can easily
measure all but the smallest conversions.
Try this!
You’ve probably heard the saying that, in terms of the chemicals
in your body, you’re worth only a few U.S. dollars. But how
much are you worth in terms of energy? Use the calculator
to find out how much energy is stored in the mass of your body—and
its value, based on the price of electricity.
If you don't know how many kilograms your mass is, you can divide
your weight in pounds by 2.2 to find out. Mass and weight are different
quantities, but they're directly proportional, so we can convert
from one to the other. At the surface of Earth, an object with
the mass of 1 kilogram weighs 2.2 pounds.
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