Many moons ago, I wrote a short story inspired by a sentence Henry David Thoreau had written in his journal: “The cold has stopped the clock.” In my story, a married couple awakens to find that the winter freeze is so cold, it stopped time, all except in places where it had been warm at the moment time stopped: the kitchen where the stove was on, the bed where their bodies warmed the sheets. I concerned myself with making the logic within the story sound. I did not consider that Thoreau’s statement may have scientific merit.
Last week, I attended Carlo Rovelli’s only New York appearance for his new book, The Order of Time. He was in conversation Jim Holt, whose book When Einstein Walked with Gödel comes out today. In Rovelli’s book, he points out that physics equations almost never include time’s passing.
“Not Newton’s laws governing the mechanics of the world; not the equations for electricity and magnetism formulated by Maxwell. Not Einstein’s on relativistic gravity, nor those of quantum mechanics devised by Heisenberg, Shrodinger, and Dirac. Not those for elementary particles formulated by twentieth-century physicists. . . . Not one of these equations distinguishes the past from the future.”
The only basic law of physics that distinguishes the past from the future involves heat: “Every time a difference is manifested between the past and the future, heat is involved.” Heat can only go from hot things to cold things (also phrased as from ordered toward entropy), just as we can only go from being in the past to being in the future. This is the second principle of thermodynamics. Although the winter freeze isn’t going to stop time itself, there is a connection between heat and time.
During the conversation at NYPL, Holt praised Rovelli for having such a high level of liberal arts knowledge. If you read The Order of Time, you’ll see how seemlessly he connect physics to ancient philosophers, religions, and writers like Lewis Carroll (and also the Smurfs). I thought it was appropriate, therefore, to start this article with a reference to Thoreau. As much as you will learn from Rovelli’s book about thermal time and quantum mechanics, it is also a poetic and philosophical work, with lines like, “We are stories, contained within the twenty complicated centimeters behind our eyes,” or “Our fear of death seems to me to be an error of evolution” caused by “an exaggerated ability to predict the future.”
At one point during Rovelli and Holt’s conversation, Rovelli reached into his pocket and said, “Here is time.” He pulled out a long, red string and used it to illustrate his ideas about time. There is much more in his book than heat and entropy. We learn about the differences between perceived time and how time actually works. Time passes more quickly for those on a mountain than those sitting at sea level; time passes more slowly for those moving fast than those sitting still. We learn about black holes and the connection between space and time. We learn about quantum time and the block universe, as well as about our identities as constantly changing beings. And we learn about Kurt Gödel, Einstein’s last friend who he used to go on walks with.