Cosmology may be on the cusp of a revolution.
That’s not in itself a problem. Cosmology is used to revolutions. It’s already had three paradigm-changing ones. The first was when we realised that the Earth went round the Sun, not vice versa. Then, in the 20th century, we recognised that our Sun was just one of a hundred billion stars in our galaxy, the Milky Way. Then we discovered that there are billions of other galaxies, that have all emerged from a hot dense beginning, the “big bang”.
Now, we may be about to see another. It is possible that what we’ve traditionally called “the universe” – the aftermath of “our” big bang – may be just one patch of space and time, just one island in a perhaps infinite archipelago. There may have been many big bangs, not just one. Our current concept of physical reality could be as constricted, in relation to the whole, as the perspective of the Earth available to plankton whose “universe” is a spoonful of water.
Note that word “may”. These scenarios are still on the speculative fringe of cosmology. But they are serious scientific questions – not metaphysics – even though the answers may elude us for many decades, or perhaps even be beyond the capacity of human minds.
There are a few commentators who deride the multiverse (1), and its theoretical basis, as self-indulgent mathematical creations, too far from validation (or even constraint by data) to be worth pursuing – but I personally feel we should cheer on the bold efforts of theorists tackling these challenges, and not disparage them.
Some years ago, I was on a panel at Stanford University where we were asked by the chairman: “On the scale, ‘would you bet your goldfish, your dog, or your life,’ how confident are you about this multiverse concept”? I said that I was nearly at the dog level. Andrei Linde, a Russian cosmologist who had spent 25 years promoting a variant of this idea, said he’d almost bet his life. Later, on being told of this debate, the eminent theorist Steven Weinberg said he’d happily bet Martin Rees’s dog and Andrei Linde’s life.
Andrei Linde, my dog, and I will all be dead before this is settled. And it is just one of the speculations being currently explored by physicists. Of these the best known – and most intensively studied – is “string theory” (2), which conceives that all the particles of nature, and the forces that govern them, are manifestations of ultra-convoluted geometrical “tangling” of space itself, on a scale a trillion trillion times smaller than an atom.
Crucial “unfinished business” for physics is to unify our understanding of the microworld (where quantum effects are key) and the large-scale cosmos (where gravity, described by Einstein’s general relativity, is the dominant force). Such a unification could be a prerequisite for convincingly describing the ultra-early stages of cosmic expansion – the first trillionth of a trillionth of a trillionth of a second – when the precursors of entire galaxies would have been squeezed to microscopic size and could have shaken by quantum fluctuations. It would also be necessary to corroborate or refute multiverse theories like Linde’s.
Admittedly string theory has its detractors – and its difficulties. But I think it’s too soon to give up. We should cheer on and not disparage those who devote their efforts to this “Everest problem” – to seek a testable theory from many perspectives. But we must be open-minded about the possibility that the true theory may simply be too difficult for unaided human brains ever to fully grasp.
The stumbling block is that the mathematics is so intricate that it’s not been possible to calculate anything that can be checked against things we know, such as the properties of electrons and protons, and the strengths of the forces governing them. But maybe AI will be able to help with such calculations.
But we shouldn’t be too surprised that progress is so elusive – indeed it’s remarkable how much we have understood. Human intuition evolved to cope with the conditions our remote ancestors had to cope with on the African savanna. Our brains haven’t changed much since that time, so it is remarkable that in the last few decades we’ve managed to grasp the counterintuitive behaviours of the quantum micro-world and the cosmos.
As the frontiers of our knowledge are extended, new mysteries, just beyond the frontiers, come into sharper focus. In every subject there will, at every stage, be “unknown unknowns”. (Donald Rumsfeld (3) was mocked for saying this in a different context – but of course he was right, and it might have been better for the world had he become a philosopher.)
The challenges of string theory and the multiverse may not be the most daunting. Less than 1 per cent of scientists work in those fields. There are other “Everest” challenges in science – understanding the human brain being supreme among them – where AI may help, but may still be insufficient.
Indeed, odd though it may seem, some of the best-understood phenomena are far away in the cosmos. Astronomers can convincingly attribute tiny vibrations in a gravitational wave detector to a “crash” between two black holes more than a billion light years from Earth.
In contrast, our grasp of some everyday matters that interest us all – diet and childcare for instance – is still so meagre that “expert” advice changes from year to year. And there is still no cure for many of the commonest ailments.
But it actually isn’t paradoxical that we’ve achieved confident understanding of arcane and remote cosmic phenomena while being flummoxed by everyday things. It’s because astronomy deals with phenomena far less complex than the biological and human sciences. The smallest insect is harder to understand than a star or a galaxy.
Our everyday environment is too complicated to be predicted, or even fully described, in detail. But much of its essence can nonetheless be captured by a few key insights. Our perspective has been transformed by great unifying ideas. Darwin’s insight – evolution via natural selection – reveals the overarching unity of the entire web of life on this planet. And the double helix of DNA reveals the universal basis for heredity.
We are of course already being aided by computational power. In the “virtual world” inside a computer astronomers can mimic galaxy formation (4); meteorologists can simulate the atmosphere. Just as video games get more elaborate as their consoles get more powerful, so, as computer power grows, these “virtual” experiments become more realistic and useful.
Computers will enable discoveries that have eluded unaided human brains. Searches for the optimal chemical composition for new drugs, of for materials that conduct electricity better, will increasingly be done by computers rather than by real experiments, just as aeronautical engineers now simulate air-flow over wings by computer calculations rather than doing wind-tunnel experiments.
Exploring the cosmos and microworld will require hugely sophisticated instruments – and we can expect to construct telescopes and accelerators (5) even more powerful than those we have today. Indeed the European Southern Observatory is building a telescope with a mirror 39m across, and just this week Cern in Geneva announced plans for an accelerator four times more powerful then its existing machine. But it may be too optimistic to expect that a full understanding of physical reality – life, intelligence, and the grand scheme of the cosmos – is within humanity’s grasp, and that no enigmas will remain to challenge our posthuman descendants.
Martin Rees is the Astronomer Royal and emeritus professor of cosmology and astrophysics at the University of Cambridge. His new book On the Future: Prospects for Humanity is published by Princeton University Press
Animations by James Wood