The geopolitical influence that Einstein’s paper would have could never have been suspected at the time — not even by him. In fact, it’s hard to imagine a more nondescript journal article, given that it could easily fit on a single page and was named quite blandly (as is traditional in physics) Does the Inertia of a Body Depend on its Energy Content? [2]. Really, this paper can be seen as a mere addendum to his previous paper on special relativity, which we covered in the last edition of Scientific. But its key result identifies the foundational principle behind the nuclear age and has become perhaps the most famous equation on Earth:

E = mc²

As the paper’s brevity indicates, the derivation of this equation is not particularly convoluted and more or less comes directly from an equation Einstein presented in his previous paper (albeit, not one we covered last time). So, it is not the origin of this equation that we will consider; its significance is far more interesting.

To understand E = mc², we should begin by defining its terms. The E stands for energy. An object’s energy is its ability to perform work; to make something move, lift it up, change its state, or some such thing. We often speak of ‘kinetic energy’: the energy stored in motion (an object has more kinetic energy the faster it moves). There is also ‘potential energy’, which is energy stored in interactions between different things. For example, an Acme anvil held stationary above Wile E. Coyote has no kinetic energy, but due to its interaction with the Earth (via gravity), it has substantial potential energy that can be converted into kinetic energy when the anvil is released. Your body stores considerable amounts of potential energy in chemical bonds — interactions between atoms. On the other side of the equation, m represents the mass of an object (also known as its inertia, hence the title of Einstein’s paper), while c is the speed of light.

The equation, when put together, leads us to a simple but remarkable conclusion: the more energy an object intrinsically possesses, the greater its mass will become. When you take the elevator from the ground floor to your luxury penthouse apartment, the increase in your gravitational potential energy will make you heavier by E/c².

Importantly, the speed of light is a very big number (approximately 300,000,000 m/s). Even a moderately large change in your energy will result in an immeasurably small variation in your mass. Conversely, though, if enough energy were released to produce a noticeable change in mass, then you will have released a very large amount of energy indeed. This is exactly the idea behind nuclear power and The Bomb. Nuclear reactions (far more than chemical reactions) can involve non-negligible changes in the reactants’ masses, and thus release extraordinary amounts of energy. When controlled, such reactions power entire cities, but uncontrolled, they level cities to the ground.

Before concluding, we should clarify one final point. Some people describe E = mc² as being about converting mass into energy, as though it meant you were using mass as fuel to burn and bring about energy in its stead, but that is not the case. Rather, the principle of mass-energy equivalence is exactly that: equivalence. The existence of mass implies the existence of energy; wherever you find mass, you already have energy. Mass disappearing and energy coming out is not a conversion process — the energy has left, therefore that mass has also left.

The history of nuclear power and the Cold War involves far more stories than Einstein’s, of course. The pioneering work of Ernest Rutherford (New Zealand’s greatest physicist), Marie Curie (one of the greatest scientists of all time), and other atomic scientists played a major role. However, mass-energy equivalence provided a fitting capstone to a year of miracles. E = mc2 has become the equation most deeply associated with Einstein’s legacy — and fair enough, given its significance — but 1905 must be known for more than just that one equation. In a single year, Albert Einstein kickstarted the quantum revolution, brought atoms out of the realm of speculation, invented special relativity, and then used it to demonstrate a fundamental truth which would define the 20th century. Physics has never been the same since. Einstein would go on to make substantial contributions to the fields he helped invent — most significantly by generalising special relativity to form our modern theory of gravity. However, he was never able to match 1905. Perhaps no one ever has. Fortunately for us, though, there is still a great deal about our universe we do not understand. It’s about time we got another Einstein to have another crack at it. Any volunteers?