The question of why time constantly moves forward has puzzled physicists for over a century, and some of the brightest minds in the world have tried wrapping their heads around it. One new theory, which sounds straight out of science fiction, is that the Big Bang created a mirror universe where time runs backwards!
Over at PBS Nova, they get into the theory:
Two leading theories propose to explain the direction of time by way of the relatively uniform conditions of the Big Bang. At the very start, what is now the universe was homogeneously hot, so much so that matter didn’t really exist. It was all just a superheated soup. But as the universe expanded and cooled, stars, galaxies, planets, and other celestial bodies formed, birthing the universe’s irregular structure and raising its entropy.
One theory, proposed in 2004 by Sean Carroll, now a professor at Caltech, and Jennifer Chen, then his graduate student, says that time moves forward because of the contrast in entropy between then and now, with an emphasis on the fact that the future universe will so much more disordered than the past. That movement toward high entropy gives time its direction.
To test the idea, the theory’s proponents assembled a simple model with nothing more than 1,000 particles and the physics of Newtonian gravity. Here’s Lee Billings, reporting for Scientific American:
The system’s complexity is at its lowest when all the particles come together in a densely packed cloud, a state of minimum size and maximum uniformity roughly analogous to the big bang. The team’s analysis showed that essentially every configuration of particles, regardless of their number and scale, would evolve into this low-complexity state. Thus, the sheer force of gravity sets the stage for the system’s expansion and the origin of time’s arrow, all without any delicate fine-tuning to first establish a low-entropy initial condition.
For more on the science behind this fascinating phenomena, check out the PBS video below.