If you hear an astrophysicist or some other bearded type suggest that there are multiple universes, you may quite reasonably wonder what possible reason they could have for speculating such a weird notion.
Well, the answer is that logic pretty much leads you to the conclusion that there must be many many universes (a ‘multiverse’), of which our universe is but one.
A fine-tuned universe
The problems stem from what is known as ‘the anthropic principle’. Proponents of the principle note that our universe appears to be incredibly ‘fine tuned’ for life.
That is, the twenty or so physical constants that describe how the universe is physically put together – and which were decided at the Big Bang – are so well balanced that if any one of them was even slightly different, life as we know it would be impossible.
The physical constants include such things as the Newtonian constant of gravity, the elementary charge of an electron, the speed of light in a vacuum and the mass of a neutron.
If any of these things was different, sometimes by a factor of a trillionth of a percentage point, then we or any other life could not exist.
Given this fact, and given how easy it is to imagine things being different, we’re confronted with a devilish philosophical puzzle.
An intelligent designer?
It seems so unlikely that we could have just ‘got lucky’ with the values of the physical constants and the fine tuning, that some people think that the Universe must have been set up to allow for the existence of life. This is the anthropic principle.
The anthropic principle is sometimes used as evidence by people who believe in an Intelligent Designer, or God, who created the universe just so, in order that life, and specifically humans, could exist.
Most scientists and philosophers tend not to like this essentially religious answer to the problem. It’s too easy a get-out (well, we don’t know so we’ll just assume someone made it). And it leads to further questions: why would he make us so small in the scale of the universe? How could he exist outside of physical/time laws and yet interact with those laws? Where did the designer come from? Who made the designer, and who made the designer’s designer, and so on to infinity.
Brute fact, mathematics or multiverse?
So what are the more scientific responses to the problem? There are basically three ways of answering it (so far).
The first is just to accept that the fine tuning is a brute fact. Just like you accept a roll of double-six in a game of ludo. It’s just dumb luck and that’s that, and we couldn’t ask the question otherwise.
But the odds are so remote of the one universe having these properties just so to trillions and trillions of degrees, that most people don’t like that answer.
The second is to suggest that there are sound mathematical reasons why the physical constants had to be as they are, so it’s not luck but necessity.
The problem is that we haven’t found these laws yet. And you’ve still got the problem that it seems remarkable that the laws just happen to dictate for an arrangement that allows for life.
The third way is to suggest that this is not the only universe, and that there must be many many universes, the vast majority of which are not fine-tuned for life.
If you imagine this, the anthropic principle vanishes at a stroke. If there are trillions of universes, it’s not surprising that one or two will be suitable for life. It’s no longer a question of luck, but of probability.
So we of course happen to be in one of the universes that is suitable for life. (That’s not incredible luck – we couldn’t ask the question if we weren’t.)