Qualitative Approach to Time
Watch: "The Mind‑Bending Physics of Time"- One of the most noticeable features of time is that there is a direction to it – there is a difference between the past and the future and the present. Humans tend to think of the past as what has happened, the current as what is happening, and the future as what is yet to happen.
- However, our best theories of physics tend to suggest that there is no distinguishment between the past, present, and the future – this, however, seems mind‑bending. One way to think about it is by comparison to space: there is no special direction in space, yet space still exists. Similarly, time need not have a built‑in “arrow” to exist.
- But what about real life? It seems that things happen in an order (dropping a cup of coffee leads to the coffee falling, etc.). We seem to perceive the flow of time because we live in the aftermath of the Big Bang. This brings us to the topic of entropy.
- Entropy is a measure of disorder – less disorder means lower entropy, and more disorder means higher entropy. The Second Law of Thermodynamics says that lower entropy tends to evolve into higher entropy. This raises the question: why does entropy increase over time?
- The past‑hypothesis suggests today’s entropy is lower than tomorrow’s because it was even lower yesterday – tracing back to the universe’s beginning in a very low‑entropy state (the Big Bang). Philosopher of physics David Albert popularized the name.
- Without entropy increasing, there’s no causal leverage on the future – the universe would sit in thermal equilibrium, maximally boring, with nothing interesting happening. So why do complicated structures appear at all? If entropy were already maxed out, they wouldn’t; increasing entropy is necessary. Sean Carroll often describes us as “passing events” that occur as entropy rises.
- We register change in time via change in entropy – without changes in entropy, it would seem as if nothing were happening, and we’d have no reference against which to form a linear sense of time.
Watch: "Kurzgesagt: Did the Future Already Happen? The Paradox of Time"
- It is possible that the past, future, and present exist at the same time - someone reading this sentence could be reading it as they are born and as they lie on their deathbed at the same time.
- We tend to observe of the universe as a group of "pictures" or "snapshots" that show everything happening at a single moment. As we stack these snapshots together, we get a pile, almost like a movie, that moves in a linear fashion from past to present to future. Some theories of physics, including Relativity, suggest a different persepctive - they instead suggeset that the "pile" has already been completed - instead of layering the snapshots on together one by one, it has already been layed together and created as a "block universe"
- But in this block, the past, present, and future are equally as real and coexist at the exact same moment. This is contrary to what we perceive - we only seem to perceive the "now". According to the theory of relativity, time and space are not separated, but one connected spacetime. We are moving through space, and also time.
- However, there is a very important distinguishment between your now and a different now. All of them are equally real. As an example, consider three spaceships a million light years away. The first is stationary - if we had a "magic connection" that instantaneously connected us with them, our now would also be their now. The second is flying away from us three times as fast as a rocket. It's now is different from your now. In fact, if there was the "magic connection", you would notice that the aliens on the second spaceship would be talking with humans from the year 1924! The third spaceships moves in the opposite angle to the second ship, moving towards us at 30 km/s. It's now is also different from ours - they would talk to humans in the year 2124!
- Which of these is the correct "now"? According to Relativity, there is absolutely zero distinguishment- they are all equally valid according to the principle of cosmic democracy.