Why does time only move forward?

Time moves forward because you can’t unspill milk or unshatter a glass; in other words, time obeys the law of entropy.

One of the great conundrums of physics is explaining why time’s arrow points forward. The arrow of time is an idea devised in 1927 by British physicist Arthur Eddington, who used it to describe the way time seems to move in only one direction. You can move from the past into the future, but not back the other way. If space and time together create four dimensions, time is the only dimension in which you can only travel in one direction.

All of the equations that physicists use to describe the universe, such as ones dealing with gravity and electromagnetism, work perfectly well in either direction. They are said to be symmetrical, and they don’t seem to be affected by the direction of time’s arrow. Yet time does have an arrow.

The answer to this mystery is the second law of thermodynamics, which states that entropy always increases. Entropy translates to disorder, or unusable energy. In a closed system, where you are not allowed to add any energy to what you started with, disorder will always increase. In real life, this means that if you spill milk from a jug, the milk cannot leap back into the jug, or if you drop a glass and it shatters, it will not reassemble itself and leap back into your hand. If you ran a film of either of these events, you would be able to tell whether it was running backward or forward, unlike the symmetrical equations mentioned above. This increase of entropy means that time can run in only one direction.

The irreversible nature of the second law of thermodynamics is probably linked to the origins of the universe itself, which started off incredibly small and dense, and has been expanding ever since. Immediately after the Big Bang, the universe had a low degree of entropy, and as time passed, its entropy increased and continues to increase. This is known as the cosmological arrow of time.

A possible fate for the universe is that it will eventually become completely disordered with maximum entropy, which means that all matter and energy will become evenly distributed. The available energy would end up spread thinly across the entire universe, so it would be very cold everywhere. This fate is sometimes called the heat death of the universe. An alternative fate is that the universe will stop expanding and go into reverse, a scenario called the Big Crunch. According to the premise of the Big Crunch, entropy might decrease and time might change direction!