Will quantum computers be able to simulate the universe?

Answer

Dear Guy,

At the moment, the practical quantum computers are still very limited in their capabilities (for example, the IBM Q System One only contains 20 qubits to work with, your laptop counts in Giga (billion) or even Tera (trillion) bits… admittedly classic bits) but they will undoubtedly grow over the years. Now to your question there is a catch. Despite the fact that a kantum computer gives access to algorithms far more powerful than what is possible with a classical computer, you are asking here to simulate a system that is itself quantum mechanical (and relativistic). Let’s make it a little simpler. Instead of the entire universe, let’s think about what we need to study 1 particle that has only 2 quantum mechanical states (you could use that as a qubit, because it can hold the same amount of information). Now how many qubits do you need to simulate 1 qubit? At least 1. And how long would a simulation describing the behavior of your particle for 1 second take? At least 1 second. To simulate the entire universe, you would therefore need a quantum computer that is at least as big as the universe itself (and then you have to assume that you run this quantum computer “outside” our universe), and that has to run at least as long. as the time span you want to study. So you are sabotaged here by the fact that you can see the universe and its evolution as the quantum computer simulating itself. As soon as you “make that simulation more efficient”, you start approaching, and you can say that you are no longer simulating the universe.

Does this mean that there are no ideas about “whether” and if so what existed before the big bang? Cosmologists do not need a quantum computer for this, and have been working on this for a long time. One idea is that our universe originated as a bubble on the edge of another universe. They believe they see evidence of this in the cosmic background radiation. Or how about the idea that our universe is actually the outlet of a white hole (a speculative anti-black hole), which is connected to a black hole in another universe?

Now, could we find out through a simulation of the proposed type (disregarding for a moment the practical feasibility of creating the necessary (quantum) computer) what happened before the Big Bang? It all depends on the “correctness” of the model and our confidence in that model. There are already several models in circulation, which have experimental support to a greater or lesser extent. In this case, the fact that we cannot go and see for ourselves forces us to use indirect information/experiments/observation. It is therefore difficult to estimate to what extent the proposed simulation would do better. The assumption that the laws of nature before and after the singularity (= the Big Bang) are (must) be the same is also a source of uncertainty.

All in all, a more complex question than you might expect at first sight.