# Why does time flow forward?

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• TheQuestionGuy14
TheQuestionGuy14
What makes time flow forward? Why does it never flow backward? Some people say entropy, but if Closed Timelike Curves exist, wouldn't it violate entropy? As you can go back in time to a universe of lower entropy and more order?

Staff Emeritus
The flow of time is both mysterious and not very mysterious.

The not-very-mysterious explanation. Let's take a situation in which you have an ice cube sitting in a puddle of water on a hot slab of concrete. As far as the laws of physics, the following two futures are both possible:
1. The ice cube gets smaller and the puddle of water gets larger and the concrete gets cooler.
2. The ice cube gets larger and the puddle of water gets smaller and the concrete gets warmer.
So why is it that we only see #1 and not #2? That's actually not mysterious. There are an enormous number of ways you can assign velocities to the water molecules in the ice and puddle. The vast majority of those choices will lead to the ice melting in the future. So it is almost certain that the ice will melt.

So that's not mysterious. But back up a little further, and ask why was there a block of ice sitting on a slab of concrete, in the first place? THAT seems enormously unlikely, as well.

So that's the mysterious and non-mysterious sides of the question. Given that the universe is the way that it is, with hydrogen to burn in stars, and with plants to absorb light to grow, and with creatures to eat the plants, etc., it's very likely that events will unfold in one direction only: the direction of increasing entropy. So that's not mysterious. But the fact that the universe started out having a very low entropy is mysterious. I don't think there is a consensus about why it should be that way, other than: that's just the way it is.

Mentor
Some people say entropy, but if Closed Timelike Curves exist,
Entropy has been observed. Closed timelike curves have not.

Gold Member
When I think about it, if you took a wormhole or timewarp to a previous time, you could encounter the younger you. However, supposedly there can only be one true version of each person so that might be what makes it not possible.

Mentor
supposedly there can only be one true version of each person

Where does this "supposition" come from?

Gold Member
I've never been replicated or cloned. And if I went back in time, my younger self would exist along with the present self. That would be quite a conundrum.

Mentor
2022 Award
I've never been replicated or cloned. And if I went back in time, my younger self would exist along with the present self. That would be quite a conundrum.
It would, but this is no scientific reason it cannot happen under the assumption that traveling back in time is possible at all. Thus it is not suited to falsify this hypothesis.

Mentor
if I went back in time, my younger self would exist along with the present self

Yes, but not because you were replicated or cloned; because your path through spacetime would have a loop in it, so it could return to the same small region of spacetime twice. That doesn't require duplicating anything.