The End of the Universe

I have been thinking about time inversion and looked more carefully into arrow of time reversal experiments like the one published in Nature in 2019.

This was a groundbreaking experiment. It showed that if you have two ions, one hot and the other cold, their local experience of time changed direction depending on how they were correlated. If their quantum states were uncorrelated, heat would go from hot to cold and both ions would end up in equilibrium, both warm. If, on the other hand, their quantum states started out correlated, then the arrow of time could be reversed, cold would become colder and hot would become hotter.

The arrow of time is the direction of time we experience. Time moves forward and not backward for both us and also any system that is moderately complex and interactive. Entropy always increases. Disorder always increases. Information always increases. That is, as long as the system is closed, meaning there aren’t outside influences.

Coffee cups fall and smash on the floor. They never leap up and reassemble themselves even though that is physically possible. Heat transfers from hot to cold. 

People remember the past and not the future.

Likewise, computers, notches on trees in the forest, Egyptian hieroglyphs, and the remnants of natural processes like lava flows and human habitations all contain information about the past, but never the future.

Our knowledge of the future is based not on some memory or evidence of what will happen but an extrapolation, an inference about what will happen based only on information we have from the past. Science exists because the universe follows rules and those rules do not change with time.

These arrow of time reversal experiments, however, show that the arrow of time is not always true everywhere, all at once. It is possible to reverse it in some small corner of the universe, the very tiny corner where two quantum entangled atoms live.

At the time of the hot dense early universe, a new paper proposes that the level of quantum entanglement was very low and that the arrow of time emerges from the growth of entanglement. This is called the “Entanglement Past Hypothesis” and coupled with the experiment above it makes a lot of sense.

Entanglement is related to entropy because when you entangle two particles you increase their entropy.

This all fits quite nicely together:

1. Entropy increases in time.
2. Entanglement creates entropy.
3. Therefore, entanglement increases in time.
4. Entropy was low in the past.
5. Therefore, entanglement was low in the past.
6. We can reverse entropy (the arrow of time) by preparing particles that are already entangled.

The conclusion (number 6), however, is a bit startling, because we can’t do this with classical mixed systems. If you start with a system at maximal entropy, it will stay at maximal entropy. That turns out to be false for quantum-correlated systems, i.e., systems like in the experience that are non-classical.

This implies that the arrow of time is not directly connected to entropy but rather to entanglement. Entropy is a byproduct.

I spent a lot of time thinking about the implications of this and whether that meant that, by reversing or inverting time as in the movie Tenet, we could send messages into the past.

I concluded that this is not possible. From our perspective, when we prepare the two correlated ions, we are preparing its initial state but from its perspective, we are preparing its final state. 

As it evolves, the two-qubit system loses entropy and therefore information. Once it reaches its final state from our perspective, it is at its initial state from its perspective. The system evolves backward in time from our perspective, but we already knew how it “started” (as much as it is possible to know quantum states). We cannot impart information onto its final state and have that information show up magically at its initial state.

This will become clearer with the following thought experiment:

Suppose I prepare an AI using many correlated qubits and set it up so that it evolves backward in time. Would the AI be able to tell me about my future? No! All it can tell me is the information I gave to it. To preserve its arrow of time, I cannot interact with the AI while it is evolving or I might destroy its correlations. Anything I want to tell it must be imparted at its creation.

All the AI’s experiences are self-contained, so it could tell me about what happened to it in its past of course, but that past would have nothing to do with me. It would all be information I would already know.

This is a bit like playing God because, like God, I know everything about this AI. Everything it knows, I gave to it. It doesn’t know, however, that I created it at the end of its world, not the beginning.

And now we get to the subject of this article which is: what if the universe was created not at the Big Bang but at the Gnab Gib (the end of time). At the end of time all information will exist. Our creation will impart everything that ever happened in this universe at that point and let ’er rip back to the Big Bang. (If quantum mechanics is non-deterministic, this wouldn’t necessarily mean there is no free will but that is a whole separate discussion.)

From my Judeo-Christian perspective, therefore, it is more important that the universe has an end than a beginning, a Revelation rather than a Genesis. Our creation lies in the future, not the past.

Timothy Andersen

Principal Research Scientist at Georgia Tech

Dr. Tim Andersen is a Principal Research Scientist at Georgia Tech. Dr. Andersen is author of The Infinite Universe (2020) and writes about science and philosophy for The Infinite Universe on medium.com. He earned his Doctorate in Mathematics from Rensselaer Polytechnic Institute. He lives and works remotely from Wisconsin with his wife and three children