Why Does E=mc2_ - Brian Cox [59]
The process of converting mass into energy and energy into mass is therefore absolutely fundamental to the workings of nature; it really is an everyday occurrence. For anything to happen at all in the universe, energy and mass must be continually sloshing back and forth. How on earth did anyone manage to explain anything involving energy before we knew this seemingly most basic of facts about the workings of nature? It’s worth remembering that Einstein first wrote down E = mc2 in 1905 in a world that was far from primitive. The first intercity passenger railway, powered by coal-burning steam locomotives, was opened in 1830 between Liverpool and Manchester. Coal-burning ocean liners had been crossing the Atlantic for almost seventy years, and the golden age of steam was in full swing with advanced steam-turbine-powered liners, such as the Mauretania and Titanic , about to enter service. The Victorians certainly knew how to burn coal efficiently and to spectacular effect, but how did the scientists of the day think of the physics behind a burning fire before Einstein? A nineteenth-century engineer would have said the coal has latent energy stored within it (rather like the energy stored in lots of miniature mousetraps) and the chemical reactions that burn the coal spring the traps and liberate that energy. This picture works, and allows calculations to be made with the accuracy required to design a beautiful machine like an ocean liner or an express steam locomotive. The post-Einstein view does not disagree with this picture but rather it adds to it. That is to say, we now understand that latent energy is irrevocably intertwined with the concept of mass. The more latent energy something has, the more massive it is. It would not have occurred to scientists before Einstein that there was a link between mass and energy, because they had not been forced to think in that way. Their view of nature was accurate enough to explain the world they observed and to solve the problems they encountered, because the changes in mass were so tiny that they never needed to know them.
Here lies another insight into science. With each new level of understanding, a more accurate worldview emerges. The current worldview is never claimed to be correct, in the very important sense that there are no absolute truths in science. The body of scientific knowledge at any point in history, including now, is simply the collection of theories and views of the world that have not yet been shown to be wrong.
All of the examples we just looked at lead to very tiny fractional changes in mass, but of course the release of the corresponding energy can be very significant. A fire keeps us warm and a hot pie is much tastier than a cold one. In the case of burning coal, the stored energy is chemical in origin. The molecules that make up the coal get rearranged and turn to ash as a result of a chemical chain reaction initiated by a lighted match. As the bonds between the molecules snap and reform and atoms recombine with atoms to make new molecules, energy is released and the mass reduces. Chemical energy has its origins in the structure of atoms. The simplest example is a single hydrogen atom, which is a single electron in orbit around a single proton. It is simple enough that physicists can use the quantum theory to calculate how the mass of the atom should change as the electron moves around. There is a smallest value for the mass of a hydrogen atom. It is an utterly miniscule 0.00000000000000000000000000000000002 kilograms less than the combined mass of an electron and a proton that are far apart. Nevertheless, that difference, when converted into energy, is a very big deal. Ask any chemist or experience its effect yourself sitting in front of that nice coal fire.
Because particle physicists are as lazy as the next guy, they don’t like writing very small numbers down with lots of zeros and