|The Equivalence of Mass and Energy
Before Einstein finished formulating his ideas about special relativity and the dynamics of motion he
turned his attention to two of the fundamental principles that underpinned physics and chemistry.
Einstein united these principles into a single equation which brought chemistry and physics closer
together than they had ever been previously.
The first of these conventions was the conservation of mass in chemistry. Regardless of the number of
chemical transformations that are made, the mass of a substance or compound is always conserved-
meaning that it never varies. No matter how you change the molecular bonds between atoms of
different elements, once the reaction is complete, the mass that you started with is exactly equal to the
mass in the end product.
The second convention or principle, which always holds in physics, is the conservation of energy. After
any physical process, the energy that you put into a system will be precisely equal to the energy put out.
For example, most of the energy that you put into the pedals of your bike goes towards propelling you
down the street. I said most, because not all of it is turned into your energy of movement (kinetic
energy). Some of it will be transferred into the sounds that your tires and chain make, some of it will be
transferred into the concrete in the form of heat, and more energy still will go into slightly warping the
frame of your bike. But, all of the energy that you expend will be transferred into the environment without
any loss of energy.
Einstein’s insight was that mass and energy are interchangeable, like different currencies, and either
mass or energy will always be conserved. Of course you can’t make this transformation at the bank,
but it is possible to do here on earth. To change energy into mass you need a particle accelerator (a
huge structure that uses magnetic fields to accelerate elementary particles to very high speeds). To
change mass into energy you need an atomic bomb or a nuclear reactor. This energy can be found,
not in the molecular (chemical) bonds between atoms, but in the bonds between protons and neutrons
in the nuclei of atoms. Breaking these bonds, even in very small amounts, can unleash incredibly large
amounts of energy. Also, no matter which of the two transformations you make, mass and energy are
conserved in set proportions. That proportion is actually expressed in the equation E=MC^2 (where E
is energy, M is mass and C is the speed of light).
Einstein first realized that mass and energy were interchangeable by adding the concept of mass to his
relativity thought experiment (and then afterwards he did all of the necessary math). He assumed that
an object’s mass represents the energy locked up inside of it. He knew that a massive object that is
moving has more energy than the same massive object at rest. From this he was able to determine
that the energy of movement is represented as added mass. Special relativity tells us that time slows
down for an object that is increasing its speed, and now we know that mass also increases with speed.
Calculations indicate that at 40% of the speed of light, an object will get 10% heavier. At 99% of c, the
mass will increase sevenfold, and at the speed of light any object with mass will be infinitely heavy.
Clearly infinite mass is not physically possible, so light speed for a massive object is impossible as
well. From this logic (and the accompanying math and physics) physicists have determined that light
has no mass- it is pure energy. This is why it sets the speed limit for the universe.
To find out about Einstein's theory of general relativity, click here.
The maintenance of a physical quantity, such as mass or energy, throughout a physical or chemical
Kinetic Energy: noun
The energy that an object has due to its motion. A value equal to one half the mass of an object
multiplied by the square of its speed.
Potential Energy: noun
The energy that an object has due to its position relative to other objects, or due to its condition or
arrangement of parts. A coiled spring, a copper battery and a suspended weight all have potential