The Sun rises every day. Water boils at 100°C. Apples fall to the ground. We live in a world in which objects behave the same given the same circumstances. We can imagine living in a different world: a world that constantly changes, a world in which the Sun does not rise every day, a world in which water one day boils at 50°C, and at 120°C another day, a world in which apples sometimes fall from trees and sometimes rise into the sky. Only because we live in a world that displays stable regularities are we able to reliably shape our environment and plan our lives.

We have an intuition that these regularities are due to laws of nature, but we normally do not interrogate what these laws are and how they work in any basic metaphysical sense. Instead, we assume that science not only provides these laws but also elucidates their structure and metaphysical status, even when the answers seem partial at best. In short, we assume that, thanks to science, there is a recipe of sorts for how the laws of nature work. You take the state of the Universe at a given moment – every single fact about every single aspect of it – and combine it with the laws of nature, then assume that these will reveal, or at least determine, the state of the Universe in the moment that comes next.

I refer to this as the layer-cake model of the Universe, which dates back to the 17th-century philosopher René Descartes. Not long after Descartes embraced the idea of a deterministic universe, Isaac Newton presented a mathematical law for gravitation, which gave the concept a powerful quantitative update. The gravitational force on one body at one time is determined by the location of all the bodies in the Universe at that time; the state of the Universe plus the law of gravitation tells you how all bodies will move: a layer-cake model, indeed.

The influence of Descartes and Newton on how we think about laws of nature is immense – and not without justification. It has helped to unify whole fields of physics, including mechanics, gravitation and electromagnetism. It is still so widespread in the scientific community, and it has such a distinguished pedigree, that scientists may not even realise that they subscribe to the layer-cake model at all.

But the uncomfortable truth is that there are many aspects of modern physics that seem to provide counterexamples to the layer-cake model. To date, some of these alternatives have occupied only a rogue niche in physics. But they should be studied more deeply and understood more widely because they pose major challenges to our fundamental understanding of the Universe – how it began, where it is going, and what kind of entity, if any, is driving it.

The first massive challenge to the layer-cake model, Albert Einstein’s theory of general relativity, appeared in the 20th century. The laws of nature that are core to the theory of general relativity, the Einstein field equations, do not immediately lend themselves to the layer-cake model at all.

The difference can be seen in the structure of the mathematics itself. An equation that adheres to the layer-cake model describes the changes that occur in space in terms of the underlying reasons for these changes. For example, Newton’s equation for his second law of motion describes the acceleration of physical bodies in terms of the underlying forces causing that acceleration. The Einstein field equations, on the other hand, describe the very structure of spacetime as the change agent for moving physical bodies; in fact, most of the solutions to the Einstein field equations yield a spacetime structure that is incompatible with the layer-cake model. When faced with this challenge, physicists do something highly revealing: they specifically search for solutions to the Einstein field equations that comport with the layer-cake model, and they rule out solutions that do not comport with the model as ‘unphysical’ – as artefacts of the mathematics that do not tell us anything about reality, or, at least, not the reality we live in.

Physics has many theories where the future seems to somehow influence the past

In the case of general relativity, there are good reasons for doing this, but in other cases the challenge to the layer-cake model becomes harder........

© Aeon