Thursday, December 23, 2004

The anthropic principle and the multiverse

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.

Here’s why.

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).

Brute fact
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.

Mathematical law
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 multiverse
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.)

8 comments:

martpol said...

Andrew, you should consider a career as a popular explainer of science (I mean a genuinely populist one, not a Stephen Hawking too-damn-intelligent-to-really-spell-it-out-for-all-and-sundry one).

One interesting point is the anthropic principle, which is essentially what I, as a religious believer, tend to rely on when atheists accuse "all Christians" of believing in God creating the universe and all that's in it in 7 days.

I don't think that you're right that this principle is an easy 'get-out', though. Rather, it's a way for more open-minded religious believers to square these beliefs with some sort of scientific logic. I'd guess that few religious people gain their beliefs after rationally deciding that God must exist as a consequence of the universe being an amazing place; this leaves the spiritual side of religion out in the cold. Instead (taking me as an example) we tend to believe in God first, then consider how this might fit with established science such as evolution, the Big Bang etc.

Questions of how God could exist outside the laws of space-time he 'created' seem to be irrelevant. If God is omnicompetent (as most religions assume), why should he/she/it (but for these purposes, he) need to exist within this framework? If God exists, surely he's capable of existing/doing things beyond our comprehension, otherwise what would mark him out as God (rather than just a very bright human)?

Also, believing in God doesn't necessarily go alongside assuming that he made the universe purely for humans. Many would simply say that God is worshipped by cultures wherever they exist in the universe, just through different forms of religion: God is the same wherever you are.

Lastly, many of the most accomplished scientists throughout history have been strong believers in God (Einstein, Hawking, Newton, Kepler etc.): proof, if nothing else, that having a good head for scientific theory doesn't deny the importance of spirituality.

I'm sure I'm being ridiculously simplistic, but still, it's all fascinating stuff.

Brit said...

Martin:

If you took all the shades of belief which are not completely atheistic, you could represent them by drawing a curve which ran from extreme fundamentalism (eg. a belief in the literal truth of Genesis) at one end, to a very weak statement such as “the existence of a Higher Intelligence existing outside the physical bounds of the universe is not logically impossible” at the other.

Now while of course not all astrophysicists, philosophers and scientists are atheists, there’s no doubt that if you placed the believers among them on the curve, you’d see a dramatic cluster towards the latter ‘weak belief’ end, and a lot of empty space at the fundamentalist end.

Science and philosophy do not necessarily eliminate Faith, but they tend to erode unquestioning belief in the things our religious forbears held as received truths. That’s because science is about positing testable theories, then testing them, rejecting those that fail the tests, and believing nothing much unless you have a compelling reason to do so, or no better theory.

I would call the approach "we tend to believe in God first, then consider how this might fit with established science such as evolution, the Big Bang etc" fundamentally 'unscientific'.

Scientists generally prefer to assume nothing much except their observations, and will always favour the minimal theory that explains the observations without contradiction.

Assuming an Intelligent Designer is not a minimal theory because it opens an infinity of further unanswerable questions: eg. Where did the Designer come from?

Of course, many scientists have Faith. But I would say that the multiverse is a more ‘scientific’ answer than the Intelligent Designer explanation, because it is more minimal.

But I do appreciate that we’re dealing here with the furthest, weirdest, most speculative and least provable outreaches of science – if it even deserves the name ‘science’ rather than ‘philosophy’.

Duck said...

One way to eliminate the need for multiverse or fine tuning is to say that no tuning is possible. The question assumes that, prior to the Big Bang, these constants needed to be "set" as one tunes a radio dial or a thermostat. Tuning is a metaphor laid on top of the whole structure which may have no revelance at all. Just assume that all the constants are "baked in", and no variablility is possible. That is what "constant" means after all, isn't it? Of course that opens up who the "Baker" is.

The answer is beyond us all, because our universe must be embedded in a larger "meta-verse" or reality that itself is beyond our concepts of three spatial dimensions and linear time. Human metaphors, which are ways of explaining it within the confines of these four dimensions, cannot possibly describe it. God, or the "Creator", "Tuner", or "Baker", is just such a metaphor, a way to explain events within the linear time dimension of causality which we know breaks down once you move above our universe in the meta-physical "protocol stack" (a programming metaphor).

Brit said...

Duck:

Thanks for your comment, and welcome to the blog.

I think your solution is similar to the one I've called 'mathematical law' above.

If we could show that the constants must necessarily be as they are, we don't need the multiverse. The problems with this solution are: 1) so far we haven't found any rules that mean the constants must be as they are (which doesn't mean we won't_; and 2) even if we did, there's still the spectre of, as you call it, the 'baker'.

I think when physicists speak of a 'fine-tuned' universe they don't necessarily mean it must have been 'set' like this. They just observe that it is easy to imagine changing any of the constants - sometimes by a tiny fraction - and rendering life impossible.

I suspect that you're right about the answer being 'beyond us all.' Other universes are probably by definition the kind of things we will never be able to verify, other than by rational inference. Having said that, looking at such wonders as DNA, nanotechnology and the Google search engine, it seems to me that science and human thought have developed so far and so rapidly that I would be hesitant to rule out anything much per se.

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Chris Hunt said...

Just coming to this post from a random search, hence the late comment...

I think you've missed another possible explanation. If any tweak to the rules of the universe would make life-as-we-know-it impossible, maybe life-as-we-know it evolved to suit those rules.

A different universe could be teeming with life-as-we-don't-know-it, evolved to fit its particular properties.

Could it be that it's us that exactly fit in with the rules of the universe rather than the other way round?

Chris Hunt said...

Just coming to this post from a random search, hence the late comment...

I think you've missed another possible explanation. If any tweak to the rules of the universe would make life-as-we-know-it impossible, maybe life-as-we-know it evolved to suit those rules.

A different universe could be teeming with life-as-we-don't-know-it, evolved to fit its particular properties.

Could it be that it's us that exactly fit in with the rules of the universe rather than the other way round?