In Hitler’s Germany, a handful of physicists bristled at the mere mention of quantum theory. The troubling uncertainties of Einsteinian relativity and other physical exotica were viewed as “Jewish science” inimical to German nationhood and the Newtonian mechanics of Deutsche Physik. “German physics” (sometimes called “Aryan physics”) failed to make inroads in 1930s Germany because its champions were so plainly deluded. To forward-looking German physicists such as Max Planck and Werner Heisenberg, the idea that relativity was a “Jewish fraud” was manifest nonsense. Albert Einstein was indeed Jewish, but had he masterminded a “world crisis” in physics, as the anti-relativity lobby insisted? Hardly.
Nobody said that relativity theory was easy. Einstein’s notion that time and space are essentially one (the concept of curved “spacetime”) is the stuff of abstract poetry. Fortunately, the Italian physicist Carlo Rovelli writes of “warped time” and other tentative physics with incisive clarity. Known for his work on loop quantum gravity theory and the pre-Socratic Greek philosopher Anaximander, Rovelli is one of our great scientific explicators. His poetic essay collection Seven Brief Lessons on Physics sold more than a million copies in English translation in 2017 and remains one of the fastest-selling science books ever.
The Order of Time, a deeper, more abstruse meditation, elucidates some of the key developments in the philosophy and physics of time. Fortified with quotations from Proust, Anaximander and the Grateful Dead (Rovelli has a hippyish past), the book continues a tradition of jargon-free scientific writing from Galileo to Darwin that disappeared in the academic specialisation of the last century.
Clock time, said Einstein, is an illusion. In his general theory of relativity (published over a century ago in 1915) he predicted that time passes more quickly “high up” than below, nearer to the Earth. So if a man who has lived at sea-level meets his twin who has lived in the mountains, he will find that his sibling is slightly older. Analogously, a clock placed on the floor runs a little more slowly than one on a table. So which of the two tells the real time? The question runs through Rovelli’s book.
What is real? What exists? Einstein’s observation that time passes at different speeds in different places unsettled not just the anti-Jewish physicists of the Third Reich but the Roman Catholic church. Can it really be a sin to know? In the 17th century, curiosity-driven researchers such as Galileo had dared to put divine laws to the test. (“For in much wisdom is much grief”: Ecclesiastes.) In tasting of the tree of knowledge, Einstein quite as much as Galileo had offended against the established order of things, according to Rovelli.
Beautifully translated by Erica Segre and Simon Carnell, The Order of Time is an expression of the scientific desire to know and understand the world. “Physicists are not immune to talking nonsense,” says Rovelli, but physicists are said to go deeper than other scientists into the mystery of existence. The laws of physics – gravity, energy, motion, time – underpin those of chemistry, astrophysics and meteorology combined. So an understanding of the workings of time requires some understanding of physics.
Rovelli’s book opens with a discussion of Newton’s idea of absolute “true time”, ticking relentlessly across the universe. This is how most of us still imagine time, though Einstein showed that there is no single “now” but rather a multitude of “nows”. Rovelli goes on to consider Aristotle’s belief that what we call “time” is simply the measurement of change: if nothing changed, time would not exist. Newton chose to disagree. If the universe was to be frozen, time would tick on regardless.
Impishly, Einstein asserted that both Aristotle and Newton were right. Aristotle correctly explained that time flows in relation to a before and after; and Newton’s absolute time does indeed exist – but as a special case in Einstein’s “spacetime” theory of gravity, which treated space and time as one and the same.
In Rovelli’s own elucidation, the Earth moves round the sun because of the distortion of “spacetime” by the sun’s greater mass. An analogy presents “spacetime” as a rubber sheet distorted by a heavy ball representing the sun; a smaller ball rolling by, representing a planet, will tend to fall into this depression, apparently attracted. In Einstein’s universe, this is what is known as gravity. Time runs slower wherever gravity is strongest, and this is because gravity warps or curves “spacetime”.
The riddle of time may ultimately be beyond our “blurred”, Earth-bound comprehension, says Rovelli. All the same, in lucid pages, he manages to bring difficult ideas down a level. Not since the late Stephen Hawking’s A Brief History of Time has there been so genial an integration of physics and philosophy.
• The Order of Time by Carlo Rovelli is published by Allen Lane (£12.99). To order a copy for £9.75 go to guardianboookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99