May 08 2018

## Stephen Hawking’s Parting Shot

In the excellent series, Rick and Morty, the scientist Rick Sanchez invented a portal gun that allows him to jump into any of the infinite number of universes. This is a great plot device that allows for many funny and absurd scenarios. There are also parts of this idea that are not implausible, according to cosmologists.

Stephen Hawking, with coauthor Thomas Hertog, had something to say about the multi-universe theory in his final paper published 10 days prior to his death. The paper, 20 years in the making, reverses some of Hawking’s earlier positions and also, if ultimately viable, still leaves much work to be done.

This is one of those scientific arguments that is incomprehensible in its language and math outside of a small group of experts. I have no hope of reading and understanding the original paper. But I will do my best to pull together translations of the basic concepts.

In this paper Hawking and Hertog are reversing one idea introduced by Hawking many years ago, that the universe is finite but unbound in time. This is literally impossible to imagine, but his analogy is to think of how being unbound but finite in space works. Imagine the surface of a ball. You can walk around the surface and never reach an end, but that surface is finite because it is curved back in on itself. What if time worked the same way? The life of our universe is a finite time loop, with no edge.

This appeared to solve one problem for cosmology – where did the universe come from? Hawking essentially used quantum mechanics to argue that the Big Bang represented the universe emerging from a quantum state to the semiclassical state it is in now – it emerged from quantum nothingness (which is not really nothing, but this is a potentially long side discussion).

This model implied the finite but unbound universe. It also implied an infinite number of universes, because if one universe can emerge from the quantum foam, then there is nothing to stop an infinite number of universes from doing so and a cartoon scientist from hopping among them. But this model also created what Hawking considered a problem – there was also nothing to constrain the physical laws of those universes. Other universes might have completely different laws of physics.

Why is this a “problem”? Because if any laws of physics could exist, than any theory about why the laws of physics are the way they are could be compatible with this multi-universe model. Some universe out there could potentially have the laws of physics that emerge out of any such cosmological theory. When a theory is compatible with any possible outcome, scientists get cranky because that means the theory is not testable. You can’t ever prove it wrong by finding an outcome that is incompatible with the theory. If the theory is not testable, than scientists have nothing to do.

“Hawking was not satisfied with this state of affairs,” Hertog told Live Science in March. “‘Let’s try to tame the multiverse,’ he told me a year ago. So, we set out to develop a method to transform the idea of a multiverse into a coherent, testable scientific framework.”

The result is the current paper. In it Hawking and Hertog rework their theory with several effects. First, the Big Bang becomes a boundary for the time of the universe. The universe is no longer unbound in the temporal dimension, it has one boundary at the beginning, but remains unbound into the future.

Second, this reduces the infinite universes to a finite number, and also constrains the range of the laws of physics that might be present in those universes. Here is one of the more understandable paragraphs in the paper (really, most are complete jargon):

Our conjecture strengthens the intuition that holographic cosmology implies a significant reduction of the multiverse to a much more limited set of possible universes. This has important implications for anthropic reasoning. In a significantly constrained multiverse discrete parameters are determined by the theory. Anthropic arguments apply only to a subset of continuously varying parameters, such as the amount of slow roll inflation.

By anthropic arguments I assume they mean – why does our universe have laws of physics compatible with the existence of life and humans? In their new model, the laws of physics are constrained. They have to be what they are, or at least very similar. This at least creates the potential of further exploring why the laws of physics are the way they are.

The holographic universe idea is that all of the information that makes up our three-dimensional universe can be represented on a two-dimensional surface at the boundary of the universe.

One point on which I am still not clear – does their model imply a finite number of universes, or only a finite range of types of universes? Perhaps there are an infinite number of universes, but all with the same or similar laws of physics.

In any case, in order to progress their work further a number of fundamental issues in cosmology need to be worked out. This includes developing a working theory of quantum gravity – unifying quantum mechanics and general relativity. This is one of the Holy Grails of theoretical physics. We also have no real idea how close we are to any solution – whether string theory, loop quantum gravity, or something else not yet proposed.

Most intriguing in Hawking and Hertog’s new ideas is that we may be able to investigate other universes by their effect on the cosmic background radiation. That was the whole idea of their new approach – assume the multiverse can be understood and tested, and then develop a testable model. Hertog, however, does not think we will ever be able to hop from our universe to another, Rick and Morty style (although, honestly, how can we say anything like this with any confidence at this point?).

One final point on Rick and Morty – I never liked the logic that because there are an infinite number of universes then everything exists. Anything you can possibly imagine must exist in some universe somewhere. Again – this is a great and humorous plot device, and I know that is as far as it goes on the show, but as a thought experiment, does the logic hold? I don’t think so.

If there are an infinite number of possibilities, then even with an infinite number of universes there would be an infinite number of unrealized possibilities. For example, you can have an infinite series of integers. But between every two integers there are an infinite number of non-integers that are excluded from the infinite series. It seems to me that even with infinite universes, only plausible things will occur, and there is an infinite number of implausible outcomes that are excluded.

So there probably isn’t a universe out there in which sentient corn has duplicated Earth culture in every detail, just in corn.