Energy

A reader trained in physics may feel that a carrier has been found after all. Energy is conserved. It changes form, chemical into heat, heat into motion, mass into light, while the total stays the same. Surely energy is the something that persists through every change: the modern successor to substance.

Physics itself does not treat it so. Energy is not a fluid poured from one container into another. It is a quantity, a number computed in different ways for different forms, whose total for a closed system comes out the same before and after. Richard Feynman made the point in his lectures: physics has no picture of energy as blobs of anything, only a calculation that keeps balancing.

Three further facts confirm that energy is no substrate. Its amount depends on the frame of reference. A moving ball has kinetic energy relative to the ground and none relative to a runner keeping pace with it, so no amount belongs to the ball by itself. Its conservation follows, by Noether's theorem, from a symmetry: the laws of physics do not change over time. It is a feature of how regularities hang together, not a store of stuff. And in an expanding universe, where light is stretched and loses energy, there is no well-defined total that is conserved at all. A true substance could not simply fail to add up.

From the side of this book, "where did the energy come from, and where did it go?" is the extreme of coming and going asked of a number. When a match burns, chemical energy becomes heat and light. The sentence is correct, and the regularity it records is among the most reliable conventions there are. But nothing is found leaving the match head and travelling into the flame. Conservation is a designated regularity of the highest reliability. It is not a passenger.