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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.

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Although we experience time in one direction—we all get older, we have records of the past but not the future—there’s nothing in the laws of physics that insists time must move forward.

In trying to solve the puzzle of why time moves in a certain direction, many physicists have settled on entropy, the level of molecular disorder in a system, which continually increases. But two separate groups of prominent physicists are working on models that examine the initial conditions that might have created the arrow of time, and both seem to show time moving in two different directions.

When the Big Bang created our universe, these physicists believe it also created an inverse mirror universe where time moves in the opposite direction. From our perspective, time in the parallel universe moves backward. But anyone in the parallel universe would perceive our universe’s time as moving backward.

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