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By Root 923 0

provocative statement sounds superficially profound in the way that Zenlike utterances from fortune-tellers often do. In this case, however, it does turn out to be a great insight; Galileo deserves his reputation. To see why, let’s say that we want to establish whether Aristotle’s grid, which would allow us to judge whether something is in absolute motion, is useful from a scientific perspective. Useful in a scientific sense means that the idea has observable consequences. That means it has some kind of effect that can be detected by carrying out an experiment. By “experiment,” we mean any measurement of anything at all; the swing of a pendulum, the color of light emitted by a burning candle flame, or the collisions of subatomic particles in the Large Hadron Collider at CERN (we’ll come back to this experiment later on). If there are no observable consequences of an idea, then the idea is not necessary to understand the workings of the universe, although it might have some sort of chimerical value in making us feel better.

This is a very powerful way of sorting out the wheat from the chaff in a world full of diverse ideas and opinions. In his china teapot analogy, the philosopher Bertrand Russell illustrates the futility of holding on to concepts that have no observable consequences. Russell asserts that he believes there is a small china teapot orbiting between Earth and Mars, which is too small to be discovered by the most powerful telescopes in existence. If a larger telescope is constructed and, after an exhaustive and time-consuming survey of the entire sky, finds no evidence of the teapot, Russell will claim that the teapot is slightly smaller than expected but still there. This is commonly known as “moving the goalposts.” Although the teapot may never be observed, it is an “intolerable presumption,” says Russell, on the part of the human race to doubt its existence. Indeed, the rest of the human race should respect his position, no matter how preposterous it appears. Russell’s point is not to assert his right to be left alone to his personal delusions, but that devising a theory that cannot be proved or disproved by observation is pointless in the sense that it teaches you nothing, irrespective of how passionately you may believe in it. You can invent any object or idea you like, but if there is no way of observing it or its consequences, you haven’t made a contribution to the scientific understanding of the universe. Likewise, the idea of absolute motion will mean something in a scientific context only if we can devise an experiment to detect it. For example, we could set up a physics laboratory in an aircraft and carry out high-precision measurements on every conceivable physical phenomenon, in a last valiant attempt to detect our movement. We could swing a pendulum and measure the time it takes to tick, we could conduct electrical experiments with batteries, electric generators, and motors, or we could watch nuclear reactions take place and make measurements on the emitted radiation. In principle, with a big enough aircraft, we could carry out pretty much any and every experiment that has ever been conducted in human history. The key point that underpins this entire book and forms one of the very cornerstones of modern physics is that, provided the aircraft is not accelerating or decelerating, none of these experiments will reveal that we are in motion. Even looking out the window doesn’t tell us this, because it is equally correct to say that the ground is flying past beneath us at six hundred miles per hour and that we are standing still. The best we can do is to say, “we are stationary relative to the aircraft,” or “we are moving relative to the ground.” This is Galileo’s principle of relativity; there is no such thing as absolute motion, because it cannot be detected experimentally. This probably won’t come as much of a shock, because we really do know it already at an intuitive level. A good example is the experience of sitting on a stationary train as the train on the next platform slowly pulls out of