1758!) and Galileo ordered not to repeat his ‘foolish and absurd’ conclusions. Galileo didn’t keep quiet, and he achieved his historical notoriety by being put under house arrest in 1633, where he stayed for the remainder of his life.
Many historians characterise Galileo as a bit of an egotistical social climber who brought it all on himself, which is partly true and yet also desperately unfair. He was undoubtedly a great scientist and a supremely talented astronomical observer. In particular, he was the first to clearly state the principle of relativity which lies at the heart of Newton’s laws of motion; namely that there is no such thing as absolute rest or absolute motion. This is why we don’t feel the movement of the Earth around the Sun, and why Aristotle et al. were misled into reading far too much into their stationary feelings. In the hands of Albert Einstein, the principle of relativity can be generalised to freely falling objects in a gravitational field, and this ultimately leads to modern cosmology and the Big Bang theory. But we are jumping ahead again. The purpose of recounting the story of Galileo is not to attack the easy target of the Inquisition (which nobody expects). Rather, it is to highlight the fact that the smallest and most modest of scientific observations can lead to great philosophical and theological shifts that in turn can have a tremendous impact on society. Galileo, by looking through a telescope, doing some drawings and thinking about what he saw, helped to undermine centuries of autocratic idiocy and woolly thinking. In doing so, he got himself locked up, but also bridged the gap between Copernicus and Kepler, and paved the way for Isaac Newton and ultimately Albert Einstein to construct a complete description of the universe and our place within it.
THE HAPPIEST THOUGHT OF MY LIFE
Scientific progress, then, is often triggered by rather innocuous discoveries or simple realisations. There is a terrible cliché about scientists exhibiting a ‘childlike’ fascination with nature, but I can’t think of a better way of putting it. The sense in which the cliché rings true is that children are occasionally in the habit of focusing on a very small thing and continuing to ask the question ‘Why?’ until they get an answer that satisfies their curiosity. Adults don’t seem to do this as much. Good scientists do, however, and if I have a thesis in this chapter then it is as follows: by focusing on tiny but interesting things with honesty and clarity, great and profound discoveries are made, often by flawed human beings who don’t initially realise the consequences of their investigations. The absolutely archetypal example of such an approach can be found at the beginning of Einstein’s quest to replace Newton’s Theory of Gravity.
Einstein is most famous for his equation E=mc2, which is contained within the special theory of relativity he published in 1905. At the heart of the theory is a very simple concept that dates all the way back to Galileo. Put simply, there is no way that you can tell whether you are moving or not. This sounds a bit abstract, but we all know it’s true. If you are sitting in a room at home reading this book, then it feels the same as if you are sitting in an aircraft reading this book, as long as there is no turbulence and the aircraft is in level flight. If you aren’t allowed to look out of the window, then nothing you can do in the room or on the plane will tell you whether or not you are ‘sitting still’ or moving. You might claim that your room is self-evidently not moving, whereas a plane obviously is because otherwise it wouldn’t take you from London to New York. But that’s not right, because your room is moving in orbit around the Sun, and indeed it is spinning around the Earth’s axis, and the Sun itself is in orbit around the galaxy, which is moving relative to other galaxies in the universe. Einstein discovered his famous equation E=mc2 by taking this seemingly pedantic reasoning seriously and asserting that NO experiment you can ever do, even in principle, using clocks, radioactive atoms, electrical circuits, pendulums, or any physical object at all, will tell you whether or not you are moving. Anyone has the absolute right to claim that they are at rest, as long as there is no net force acting on them causing them to accelerate. You are claiming it now, no doubt, if you are reading this book sitting comfortably on your sofa. Pedantry is very useful sometimes, because without Einstein’s theory of special relativity we wouldn’t have E=mc2, we wouldn’t really understand nuclear or particle physics, how the Sun shines or how radioactivity works. We wouldn’t understand the universe.
Something important bothered Einstein after he published his theory in 1905, however. Newton’s great achievement – the all-conquering Universal Law of Gravitation – did not fit within the framework of special relativity, and therefore one or the other required modification. Einstein’s response to this problem was typically Einsteinian: he thought about it very carefully, and, in November 1907, whilst sitting in his chair in the patent office in Bern, he found the right thread to pull. Looking back at the moment in an article written in 1920, Einstein described his idea with beautiful, and indeed child-like, simplicity.
‘Then there occurred to me the “glücklichste Gedanke meines Lebens”, the happiest thought of my life, in the following form. The gravitational field has only a relative existence in a way similar to the electric field generated by magnetoelectric induction. Because for an observer falling freely from the roof of a house there exists – at least in his immediate surroundings – no gravitational field [his italics]. Indeed, if the observer drops some bodies then these remain relative to him in a state of rest or of uniform motion, independent of their particular chemical or physical nature (in this consideration the air resistance is, of course, ignored). The observer therefore has a right to interpret his state as “at rest”.’
I am well aware that you might object quite strongly to this statement, because it appears to violate common sense. Surely an object falling under the action of the gravitational force is accelerating towards the ground, and therefore cannot be said to be ‘at rest’? Good, because if you think that then you are about to learn a valuable lesson. Common sense is completely worthless and irrelevant when trying to understand reality. This is probably why people who like to boast about their common sense tend to rail against the fact that they share a common ancestor with a monkey. How, then, to convince you that Einstein was, and indeed still is, correct?
Most of the time, books are better at conveying complex ideas than television. There are many reasons for this, some of which I’ll discuss in a future autobiography when my time on TV is long over. But when done well, television pictures can convey ideas with an elegance and economy unavailable in print. Human Universe contains, I hope, some of these moments, but there is one sequence in particular that I think fits into this category.
NASA’s Plum Brook Station in Ohio is home to the world’s largest vacuum chamber. It is 30 metres in diameter and 37 metres high, and was designed in the 1960s to test nuclear rockets in simulated space-like conditions. No nuclear rocket has ever been fired inside – the programme was cancelled before the facility was completed – but many spacecraft, from the Skylab nosecone to the airbags on Mars landers, have been tested inside this cathedral of aluminium. To my absolute delight, NASA agreed to conduct an experiment using their vacuum chamber to demonstrate precisely what motivated Einstein to his remarkable conclusion. The experiment involves pumping all the air out of the chamber and dropping a bunch of feathers and a bowling ball from a crane. Both Galileo and Newton knew the result, which is not in question. The feathers and the bowling ball both hit the ground at the same time. Newton’s explanation for this striking result is as follows. The gravitational force acting on a feather is proportional to its mass. We’ve already seen this written down in Newton’s Law of Gravitation. That gravitational force causes the feather to accelerate, according to Newton’s other equation, F=ma. This equation says that the more massive something is, the more force has to be applied to make it accelerate. Magically, the mass that appears in F=ma is precisely the same as the mass that appears in the Law of Gravitation, and so they precisely cancel each other out. In other words, the more massive something is, the stronger the gravitational force between it and the Earth, but the more massive it is, the larger this force has to be to get it moving. Everything cancels out, and so everything ends up falling at the
