Have you ever wondered why time seems to flow in only one direction? This apparent irreversibility of time has long puzzled physicists, and in this article I will explore this mystery.
Our everyday world is full of irreversible processes.
For example, imagine a glass of water on a table that gets knocked over. Naturally, the glass shatters into pieces and the water spills. Conversely, we never see the reverse: shards of glass reassembling into an intact cup and water flowing back into it.
This everyday distinction between past and future seems obvious. However, when we examine time from a physical standpoint, the laws of physics themselves make no such distinction.
At the fundamental level, physical laws are symmetric in time—they do not prefer past over future.
For instance, consider two billiard balls colliding: they approach, collide, and then separate. If we record this event and play it in reverse, the motion still obeys the laws of physics. Most physical theories are time-reversal symmetric.
Although time-reversal violation has been predicted for K-meson decays, it remains experimentally unconfirmed, and even if real, such violations are far too subtle to explain the macroscopic arrow of time we experience.
Thermodynamics introduces the second law, which states that entropy—a measure of disorder—tends to increase over time. Some consider the growth of entropy to be the source of time’s asymmetry. However, why entropy increases remains an open question.
Why does time flow only toward the future? One way to picture this world is as a movie tape, with the Big Bang at one end and an infinite extension in the other. But why is the tape asymmetric?
Here I propose how the Many-Worlds Interpretation might explain the arrow of time.
In this interpretation, worlds at different moments branch into a larger multiverse. Each world does not follow a single, linear history like a movie tape; instead, histories branch out like a spider’s web. There is no universal time label for each world—only the relationships among them.
For example, consider all worlds whose internal clocks read time t. From each such world, many possible successor worlds branch out. While most successors have clocks showing t+1, some may branch to clocks reading t−1.
However, we perceive time as moving forward because the vast majority of branches lead to worlds with clocks at t+1. In this view, the arrow of time emerges because there are far more high-entropy branches than low-entropy ones, making transitions toward higher entropy overwhelmingly more likely.
Nevertheless, if a branch reached a state of maximum entropy, we might no longer observe time progressing, since time loses meaning when entropy is maximal. In such a world, internal clocks would no longer function reliably.
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