The idea of the multiverse has at least two conceptually distinct sources in theoretical physics: quantum mechanics and cosmology. The many worlds of quantum mechanics are very different in terms of their nature and origin from cosmology’s multiverse. However, physicists have reason to believe that ultimately, these two distinct multiverses are in fact one and the same, writes David Wallace.
In big budget science-fiction and fantasy franchises, the “multiverse” is a collection of universes – some quite like our own, some differing from ours only in the way some historical event played out or some person’s life unfolded, some vastly different and filled with strange wonders. But in the drier and more disciplined world of modern physics, “multiverse” means… well, pretty much the same, only without the prospect of easily moving from one universe to the next. The multiverse of physics is revealed more subtly, by hints hidden in our observations and our theories.
Or rather: the multiverses of physics are revealed more subtly. For remarkably, physics gives us not one but three different multiverses, and reasons to accept all three.
To see the evidence for the first multiverse for yourself, just look into the sky on a clear night. Points of light scattered across the sky are pretty, no doubt, but they make little difference to life here on Earth. And yet, astronomers tell us, the only way to explain those points of light, and the subtler patterns we see in the sky when we survey it with tools more sensitive than the naked eye, is to suppose that they are suns just like our sun, many with planets of their own, and that we are adrift in a Galaxy of a hundred billion stars, itself only one of a trillion galaxies in the observable universe. If our world, our solar system, is our universe, astronomy reveals a multiverse.
Consider that the observable universe – the part of the universe from which light can reach us – is only a small part of a yet larger, constantly expanding cosmos.
Does that seem like a cheat? If so, it is only because we are complacent about the astonishing size of the astronomical universe, and the status of our Solar System as just one among countless worlds. (The 17th century Catholic church was not complacent: when Giordano Bruno suggested that the stars were other suns, they burned him at the stake.) But to make it seem more like a multiverse, consider that the observable universe – the part of the universe from which light can reach us – is only a small part of a yet larger, constantly expanding cosmos. While you have been reading this article, a star will have shone its last light upon Earth and moved so far away that no traveler, no signal, from Earth can ever reach it, even in principle. We don’t know how many other stars there are so far away that we can never see them – far more than we can see, for sure; infinitely many, perhaps. We have no direct evidence for those stars, and never will; we are confident they exist nonetheless because our best theories, supported by powerful evidence, tell us so.
This first multiverse, though, lacks some of the science-fictional features we associate with multiverses: where are the branching possibilities, the other versions of you and me just a little different from us? To find evidence for that kind of multiverse in physics, we need to look not up but in: to the microscopic world, as revealed by quantum physics.
In that world, it seems, a particle that might be doing one thing and might be doing another can somehow be doing both things at once – so that it might be both here and there, moving both this way and that, at the same time. And the laws of quantum mechanics – those same laws whose enormous experimental success apparently compels us to accept these strange claims in the first place – seem to predict that when a microscopic system is measured, that both-things-at-once, that superposition in the language of physics, gets magnified up to the measurement device itself, so that a device used to measure the location of a particle which is at once here and there will, after the measurement, read both “here” and ‘there”. To borrow Schrödinger’s striking example: if we build our measurement device so that a cat is killed if the particle is here and not if it is there, then after the measurement the cat is at once alive and dead.
This seems not just weird but absurd, and more importantly, in contradiction with our observations, and there are various ways to try to get around it. But all have their problems – in one way or another, all seem to commit us to reconceptualizing and/or redoing successful physics – and so increasingly many physicists and philosophers have been attracted to the strategy advocated by Hugh Everett III in 1957: take the absurdity seriously; accept that indeed the cat is alive and dead at the same time; reconcile this with observation by noting that the live-cat and dead-cat bits of physical reality, while coinciding, do not interfere with one another, so that we can treat them as separate worlds: an ordinary world in which the cat lives, another in which it dies.
This is the Many-Worlds interpretation of quantum mechanics: not a modification of physics to add a quantum multiverse, but a recognition that the equations of quantum theory already described such a multiverse, once we looked carefully. Our second multiverse is an emergent multiverse: the quantum universe is fundamentally a realm of superposed quantum stuff, but when looked at on the scales of cats and people, the separate worlds emerge and are found not to interact with one another. (Just as stars and planets are not part of the fundamental stuff of physics, but emerge as higher-level agglomerates.)
But the worlds of our second, quantum, multiverse are not that different from our own world.
The worlds of the quantum multiverse are not separated from our world in ordinary space: they coexist with ours. And they are constantly being created: whenever a quantum measurement is made, whenever a chaotic natural process magnifies quantum superpositions to the macroscopic scale, new worlds are born.
But the worlds of our second, quantum, multiverse are not that different from our own world. They differ in matters of contingency – whether a particle decayed, whether a wedding was rained off, whether a leader succumbed to disease – but the basic laws of physics, which particles and fields there are and how they interact, do not change from one quantum world to the next, any more than they change from one galaxy to the next. Which brings us to our third multiverse.
In the very earliest moments of time – we believe – the universe was expanding at an enormous rate in a process we call cosmic inflation, and that expansion stopped at different times in different parts of the universe. Indeed, in many parts (far beyond our sight) it has yet to stop, and cosmic inflation continues to this day. This process, called ‘eternal inflation’, is not directly confirmed by observations but is extremely hard to eliminate in our models without failing to account for those observations. If it is true, the ordinary spatial universe is larger by far than even astronomy would suggest.
So far, this would just be the first multiverse again, just on a still larger scale. But there are good – though speculative – reasons to think that the way inflation stops in a given corner of the universe is connected to how the apparently ‘fundamental’ laws turn out. The best known reasons arise from string theory, the leading contender for a final theory of physics: in string theory, our three-dimensional world emerges from a fundamental world of much higher dimension through a process called ‘compactification’, and the details of that compactification determine what particles there are and how they interact. Since there are a vast number of ways this compactification can occur, there is a correspondingly vast number of possibilities for physical law. If this is true, different worlds in the multiverse of eternal inflation will end up differing from one another not just in contingencies like the positions of stars and planets, but in matters as basic as the numbers of particles, their charges and masses, and perhaps even whether space is three-dimensional or not.
For this reason, the inflationary multiverse (sometimes called the `string theory landscape’) is often invoked to explain so-called ‘cosmic fine tuning’, the supposed fact that certain constants of nature in our universe are such that small changes in them would make our universe inhospitable to life. The explanation is simply that all values of those constants occur, in different worlds in the inflationary multiverse, and it is no more surprising that our particular universe supports life than it is that we live on the one planet in the Solar System that does. Notice that neither the astronomical nor the quantum multiverse can explain cosmic fine-tuning in this way: it requires not just contingent facts but the laws of physics to vary across the multiverse.
These, then, are our three multiverses: the astronomical multiverse, the quantum multiverse, the inflationary multiverse. Their contents differ widely from one another, as do the arguments why they might exist and, arguably, the level of confidence we should have in them. The existence of the astronomical multiverse looks unassailable. The quantum multiverse is based on extant and well-evidenced physics but relies on a controversial philosophical interpretation of that physics. The inflationary multiverse seems to emerge directly from certain models in physics with little intervening philosophy, but the theories underpinning those models – eternal inflation, string theory – are themselves controversial, and as yet unsupported by evidence. To me, the philosophical arguments from quantum theory to the quantum multiverse look compelling, so the empirical evidence for quantum theory is evidence for the quantum multiverse, whereas the inflationary multiverse is plausible speculation but speculation nonetheless. But I know excellent physicists who come to the opposite conclusion, rejecting the quantum multiverse as unscientific philosophizing while embracing the inflationary multiverse.
It is then plausible to speculate, and several physicists have speculated, that really the quantum multiverse and the inflationary multiverse are after all one and the same.
There is one more twist in the tale. I have stressed that the worlds of the quantum multiverse have the same basic physics as one another, but that only applies if we apply the Many-Worlds interpretation to extant, well-evidenced physics. Yet inflation and string theory are parts of quantum theory and the logic of Many-Worlds applies to them as surely as to any other part. If they are true, and if the Many-Worlds interpretation is correct, then the compactifications that lead to different particles and different laws for those particles occur not just at different regions of space in the inflationary multiverse, but in different worlds within the quantum multiverse. And so the multiverse of quantum mechanics and the inflationary multiverse perhaps do not after all look so different.
It is then plausible to speculate, and several physicists have speculated, that really the quantum multiverse and the inflationary multiverse are after all one and the same. And maybe they are – but the physics we have today does not imply any such unification. Some future advance in physics, or perhaps its philosophy, may imply it, and if so our counterparts in distant galaxies, or different quantum worlds, or faraway in a different corner of the eternally inflating multiverse, already have it; but in this world, we still have work to do.