Einstein’s theory of relativity reveals that there is no such thing as absolute time. The universe is not a clock with a universal tick. Rather, every infinitesimal point has its own clock and its own time that it keeps.
Nevertheless, on the scale of the universe, time evolves more or less as one, as if the universe were just a big clock. The variations in the rates at which time flows at individual points is negligible.
From a certain point of view, this makes sense. The universe started at a single time and so continued to evolve in a very homogeneous way, one moment after another.
But, that is a very naive way to look at a universe that is, fundamentally, just a collection of events. Every point in space and time is an event, something either happens or it does not. And all those events are occurring independently of one another. There is no conductor keeping the orchestra in sync as Isaac Newton supposed. No. Each player is simply watching the ones around it.
From this point of view, it seems astounding that time does not flow at different rates in different parts of the universe. Even with light crossing vast distances to reach us, surely the impact of causation from one place on another weakens with distance.
Why is it not that when one point in the universe gets out of sync with its neighbors, does that not grow and spread so that large regions get out of sync?
It seems to me that the universe must be very good at synchronizing itself despite being decentralized, like a flock of birds or a swarm of bees, or a bunch of pendulums on a swaying bridge.
Unlike birds and bees, points of spacetime are not aware of their surroundings. How could they be?
I hit upon the idea that they stay together in time not by watching one another or some distant conductor but rather through a phenomenon so common that you can recognize it with your feet.
Resonance.
Each point in spacetime oscillates, say, at some frequency and that oscillation is the local “tick”. Rates of oscillation are always the same locally, say the Planck time, the smallest measurable time, but differ from place to place because spacetime can stretch and compress. Yet, because spacetime points are all connected to one another, these oscillations end up damping out variations for the most part.
I’m talking about very small variations, of course, that are kept, by the phenomenon of resonance, from becoming larger. Large variations, like the warping from black holes and stars, takes a long time to dampen. This dampening is produced by phenomena like Hawking radiation, which is the conversion of curved spacetime energy into radiation, as well as gravitational radiation, which carries energy and spacetime curvature away from a localized source and spreads it around the universe.
Many of us have experienced resonance when walking on a bridge that is free to sway. As many feet walk across it, the bridge sways at a certain frequency. In order to adapt to the sway, the feet begin to walk in a kind of lock step. This causes the bridge to sway even more. The result is nausea. This actually happened with the Millennium bridge across the Thames, which made quite a few tourists and locals ill. The bridge had to be modified to prevent the swaying.
A more catastrophic example of resonance was the collapse of the Tacoma Narrows suspension bridge, which, because of how it resonated with the wind whipping across it, eventually twisted and swayed so much that it collapsed into the narrows below.
Christian Huygens, a brilliant 17th century Dutch physicist, astronomer, and inventor, was one of the first to discover pendulum resonance. Indeed, he was the inventor of the pendulum clock. In 1665 he performed an experiment that gave a result that surprised him. He guessed that two pendulum clocks should keep different time, but, when put on a single beam, he found that the clocks would synchronize with one another. It turned out that the beam itself carried vibrations that would keep the clocks in sync. When one pendulum was too fast, its fast tick would be carried to the other pendulum, speeding it up and carrying energy away from the other, slowing it down. This experimental result is so important that scientists are still studying it today 350 years later.
Getting back to spacetime, we can imagine that spacetime acts as both a medium, like the beam, and an oscillator, like the pendulum. As points in spacetime go through shocks and distortions from, e.g., quantum vacuum fluctuations, that cause time to start to deviate in its local rate, those shocks and distortions communicate to nearby points, which then provide their own anti-shocks and anti-distortions that smooth things out, returning the points to a sympathetic vibration. That means that while at the smallest levels the universe is a boiling froth of energy, the phenomenon of resonance keeps it all under wraps at larger scales.
The nature of time itself, then, is both local oscillation, which provides the tick that moves events forward like the pendulum, and sympathetic vibration, which keeps all those local clocks synchronized with one another even in the presence of quantum foam at the smallest scales.
This resolves at least for me how a universe with no universal clock can nevertheless behave as if it did have one.