“E=mc2” – David Bodanis

It’s easy to see how extraordinary a vision was the energy concept that Faraday’s work helped create. It’s as if when God created the universe, He has said “I’m going to put X amount of energy in this universe of mine. I will let stars grow and explode, and planets move in their orbits, and I will have people create great cities, and there will be battles which destroy those cities, and then I’ll let the survivors create new civilisations. There will be fires and horses and oxen pulling carts; there will be coal and steam engines and factories and even mighty locomotives. Yet throughout the whole sequence, even though the types of energy that people see will change, even though sometimes the energy will appear as heat of human or animal muscle, and sometimes it will appear as the gushing of waterfalls or the explosions of volcanoes; despite all those variations, the total amount of energy will remain the same. The amount I created at the beginning will not change. There will not be a millionth part less that what there was at the start.

With Lavoisier’s work, the conservation of mass was on its way to being established. He had played a central role in helping to show that there was a vast, interconnected world of physical objects around us. The substances that fill our universe can be burned, squeezed, shredded or hammered to bits, but they won’t disappear. The different sorts floating around just combine or recombine. The total amount of mass, however, remains the same. It would be the perfect match to what Faraday later found: that energy is conserved as well. …Breathing was more of the same, simply a means of shifting oxygen from the outer atmosphere to the inside of our bodies.

By the mid-1800s, scientists accepted the vision of energy and mass as being like two separate domed cities. One was composed of fire and crackling battery wires and flashes of light – this was the realm of energy. The other was composed of trees, rocks, and people and planets – the realm of mass.

Each one was a wondrous, magically balanced world; each was guaranteed in some unfathomable way to keep its total quantity unchanged, even though the forms in which it appeared could vary tremendously. If you tried to get rid of something within one of the realms, then something would pop always pop up to take its place.

Everyone thought that nothing connected the two realms, however. There were no tunnels or gaps to get between the blocking domes. This is what Einstein was taught in the 1890’s: that energy and mass were different topics; that they had nothing to do with each other.