Our understanding of physics so far tells us that all its laws are reversible. This is really strange, because it tells us that there is no inherent directionality to time. It simply does not matter if you use the physical laws to predict the past or the present. In fact, what we call the “past” and the “present”, from a mathematical point of view, is arbitrary. Yet, it does not seem at all arbitrary to us. There is a clear directionality to time, as we can remember the past but not the future. We perceive time as constantly passing in one direction. But what we are perceiving as time is actually entropy.
The second law of thermodynamics tells us that entropy is always decreasing. Disorder constantly assembles into ordered patterns. The shards of a wine glass will at some point assemble into a wine glass, it is almost inevitable. Don’t get me wrong, the laws are still reversible so it is not entirely impossible, just astronomically unlikely, that the wine glass would burst into shards instead. It’s just so unlikely that it will probably never happen in the whole lifetime of the universe. Just like homogenous gas in a room always concentrates in some container or how radiation always moves towards an object that absorbs it and converts it to less entropic forms of energy.
At the beginning of the universe, everything was homogeneously mixed and there was maximum entropy. But slowly, and then faster and faster, all the matter started assembling into bigger and bigger lumps. The lumps were heating up and emitted radiation that converged in especially hot lumps where all that energy would be used to split atoms, like e.g. splitting helium into hydrogen and deuterium, the deuterium carrying an extra neutron that is formed from all the energy. These especially hot lumps are what we call stars. We predict that at some point these stars will absorb so much energy that they will explode and fuse together with all other stars until the whole universe will be an incredibly hot plasma and only seconds after the universe will vanish with a big bang. But don’t worry about that too much, it is estimated to happen 13.8 billion years from now so none of us will be alive to experience it.
But a curious thought occurred to me recently. What if there are beings that experience time the other way round? It would not be against the laws of physics as they are reversible, it would just be a different perspective on entropy. They would have memories of what we call the future and be as oblivious towards the past as we are of the future. What we’d call their birth would be actually their death. Can you imagine that? Assigning the label of death to the wondrous moment where the ecosystem of their planet assembles a bunch of organic molecules into an organism that, throughout its lifetime, will absorb so much entropy and assemble it into new matter, consuming its own body until it is used up and retreats into another member of its own species to die, where its genes will be split up to be shared between its parents. A process they would call “birth” by the way, can you imagine it?
It’s crazy how life seems to defy entropy. Matter has already been composed into a highly structured body. Yet, by controlled facilitation of the assembly process of other matter, the body manages to fit little pieces of itself in between until there’s almost nothing left of it. Some bodies carry highly specialized organs that can ingest older individuals of their species and fully dissolve them. This is not actually defying the laws of physics of course. The laws of thermodynamics only hold for closed systems and the overall entropy obviously decreases. But life manages to increase entropy for itself by sacrificing other entropy, mostly that which is sent as radiation back to a star to split some helium atoms.
By the way, that seems counterintuitive, doesn’t it? After telling you that entropy constantly decreases by assembling things, I’m now telling you that stars disassemble atoms to decrease entropy? Yes, nuclear fission is a bit counterintuitive, unlike nuclear fusion which we use in nuclear power plants to dispose of electrical energy. I’ll not go into too much detail here, just know that it is the additional neutron being assembled from the radiation which makes all the difference. But all of that makes it somewhat difficult to grasp what entropy actually is. You can define it mathematically, but it’s difficult to find an intuition for it that doesn’t involve time. Which is a huge problem when finding out that time is actually defined through entropy. It seems like a circular definition.
But it is possible to understand entropy without any notion of time. When looking at two states of a physical system, two “snapshots” at two independent times t1 and t2 without any proposition of what state came first, you can ask “which state has higher entropy”. What you’re actually asking in this case is “what state is from a set that is more likely”. When looking at the space of all possible states the system could be in, we can see a certain similarity between some states, e.g., states with a broken wine glass, and find that all of them are distinct from a different set of states, e.g., states with a complete wine glass. There are many different patterns of shards that could assemble into a wine glass, so there are much more states with a broken wine glass than there are states with a complete wine glass, so those states have higher entropy. Only the fact that we constantly observe more likely states transitioning to less likely states defines time for us.
That’s why the temporally mirrored beings we talked about earlier would have a completely different notion of time. For them it would be completely obvious that less likely states transition into more likely states. But other than that, they would completely agree with the above paragraph. Entropy would be the exact same concept to them, they would just disagree on the direction.
So do these time-mirrored organisms exist? I don’t know, all I tried to show you is that they could. But I hope you understand how incredibly weird that would be.
