A Reddit user asks the community: What is time?
A user who goes by the name of Amadiro replies:
Einstein showed us that space and time are inextricably linked, and really form one four-dimensional framework he called space-time. The laws of physics do not really say anything about direction in either space or time — they are symmetric, and events happen the same, regardless whether time runs forwards or backwards, or in which manner the events are oriented in space. Yet time does not seem to be symmetric — it seems to flow into one direction. We can remember the past, but we cannot remember the future. This phenomena is called a "time arrow", and we can think of (at least) 3 such arrows existing:
- The cosmological arrow, i.e. the direction the universe is expanding (the other direction would be contracting)
- The thermodynamic arrow (Entropy always increases)
- The psychological arrow (we remember yesterday but not tomorrow)
Lets start with explaining the thermodynamic arrow, and why the psychological arrow and the thermodynamic arrow really are the same:
The thermodynamic arrow says that order always decreases, such that the net energy inside a system that is available for work decreases. One can think of a cup to be in a fairly ordered state, whereas a bunch of broken shards are in a more unordered state, so the entropy was increased. Even though the laws of physics are symmetric, this is how we could easily distinguish between a film played forwards or backwards; if we see a film of a cup falling down a table and breaking into many pieces, we know immediately the film is played forwards, but if we see a bunch of pieces gathering together and suddenly jumping up the table, we know the film plays backwards, as such an occurrence never happens in real life. The key to understanding why this works in one direction, but not the other, is to understand that the other way around isn’t impossible; it’s just exceedingly unlikely. If one considers a bunch of atoms in a particular configuration, and then randomizes the system, there are many many states the system can end up in, but there are always much much more states in which the system is unordered rather than ordered. So it is simply extremely unlikely that a system would ever end up in a state more ordered, as there are always infinitely (?) more states that are unordered for the system to fall into. One can compare this to a solved puzzle inside its box; there is one and only one configuration of the puzzle pieces in which they form an image, but many many states in which they just appear unordered. So one can imagine this to be like a string of states with events in the middle, and the state on one end is ordered, whereas the state on the other end is almost surely unordered.
Now for the psychological arrow. As stated earlier, the psychological arrow (how we perceive time to flow into one direction, but never the other) is really just the same as the thermodynamic arrow. The thermodynamical error tells us that when changes are applied to a system, the system will usually be ordered on one side of the event, and unordered on the other side, but why do we perceive it to go from ordered to unordered, and not the other way around? To explain this, we can reason about a computer, which, arguably works roughly the same as the human memory (at least in terms of the psychological arrow; just as a human brain, a computer cannot remember the future), and, to make it simpler, we consider a simple abacus. Assuming the abacus starts in an unordered state, and we want to store some numbers on the abacus, we will need to move some of the pearls around, and after that transformation, the abacus will be in a state of higher order. However, to move the pearls around, we need to expend work, which, in term, increases the net entropy in the universe around us by many many magnitudes compared to the small decrease in entropy we applied to the abacus. This is pretty much dictated to be so by the thermodynamic arrow, which says that entropy always has to increase, so it is not possible to remember something the other way around, because to remember something, we have to increase the net entropy of the universe, and that only works in one direction, essentially. This is the reason we can remember what happened yesterday but not tomorrow, and because we can remember what happened in time "before" now, we psychologically perceive time to be "flowing" in one direction (but one could argue that it is not any more flowing in one direction as space is). So whichever way the arrow of entropy points, our perception of time also has to point. One could imagine another universe, where the arrow of thermodynamics is reversed, and broken shards would suddenly jump together and form a cup sitting on a table, and people would remember tomorrow rather than yesterday — but then, their psychological arrow of time would be the other way around, and they would perceive their world to be normal, just as we do.
That still leaves the question of the cosmological arrow. Why does the universe expand in the same direction the thermodynamical arrow points? Could it possibly be the other way around? The only way I have of answering this, is based on the anthropic principle: if it was the other way around, according to our understanding of modern physics, the fundamental preconditions for atoms to exist would likely not be given, and thus no life could evolve, and nobody could be there to ask the question "why is the cosmological arrow pointing in the same direction as the thermodynamical arrow?". Thus we can reason that we could not have found this to be any different than we have, because all life that could possibly evolve in the universe would evolve during the period of expansion, not contraction (given there is such a phase).
Of course this leaves many questions still unanswered, but I hope this gives you some perspective on what time is, and it sheds some light on common questions like "what came before the big bang?" (if time is really just another dimension, and the dimensions where created by the big-bang, asking the question of what came before is of course meaningless.)
I can recommend reading Stephen Hawkings "A brief history of time" if you want a more in-depth explanation. Many of the examples and principles I listed here are directly or indirectly taken from his work, and he manages to convey complicated concepts quite simple and convincingly.