Understanding time is one of the big open questions of physics, and it has puzzled philosophers throughout history. What is time? Why does it appear to have a direction? The concept is defined as the “arrow of time,” which is used to indicate that time is asymmetric – even though most laws of the universe are perfectly symmetric.
A potential explanation for this has now been put forward. Physicist Sean Carroll from CalTech and cosmologist Alan Guth from MIT created a simulation that shows that arrows of time can arise naturally from a perfectly symmetric system of equations.
The arrow of time comes from observing that time does indeed seem to pass for us and that the direction of time is consistent with the increase in entropy in the universe. Entropy is the measure of the disorder of the world; an intact egg has less entropy than a broken one, and if we see a broken egg, we know that it used to be unbroken. Our experience tells us that broken eggs don’t jump back together, that ice cubes melt, and that tidying up a room requires a lot more energy than making it messy.
Carroll and Guth’s work is still unpublished but they discuss it at length in New Scientist. Their simulation includes a large number of particles interacting under gravity and moving in random directions. Some particles naturally assemble into a clump, an area of low entropy and then separate and expand in a specific time direction. Surprisingly mirroring the entire system, the entropy still increases, which shows that both directions of time are a viable solution.
This is not the first time something like this has been shown. In 2014, an international group of physicists developed a simple model that shows you can have symmetric laws of physics and an only “apparent” arrow of time. Their findings were published in Physical Review Letters. They noticed that there was an arrow of time, but only from the point of view of the particle in the system – for an external observer, there was no special direction in time.
What the scientists have announced will definitely become hotly debated once the research is published. The model offers intriguing ideas, but it doesn’t get us closer to answering what time is; if anything, it increases the questions we have about its nature and complexity. In this situation, I subscribe to the Douglas Adams’ interpretation: “Time is an illusion, lunchtime doubly so.”