“Everyone carries a piece of the puzzle. Nobody comes into your life by mere coincidence. Trust your instincts. Do the unexpected. Find the others.” -Timothy Leary
Here we are, on planet Earth, the product of generations of civilization-building, maybe four-billion years of evolution on our world, around 13.7 billion years into the existence of our observable Universe.
The Universe, quite possibly, didn’t have to be exactly the way it is. It didn’t have to have the laws of physics be exactly what they are, with the masses and charges of particles having the specific values they’re observed to have, with the forces having the exact coupling constants that they do.
And yet, you may wonder if there’s some significance to these constants of nature having the values they do, and whether they have those values out of some necessity.
When you think about the Universe, there are some plain, indisputable scientific truths that stare you right in the face. Stars and galaxies exist, held together by the force of gravity. Here on Earth, a huge variety of elements — found to have the relative abundances they have — are present. And humans, for all of our flaws and fallibilities, have, in fact, come to exist.
All of these things can be taken together, and a very simple logical conclusion can be drawn about the whole shebang: The Universe, with everything that exists in it, must be governed by physical laws that at least make it possible for things to turn out the way they have been observed to have happened.
For example, we cannot live in a Universe where the existence of matter is forbidden; because it is observed to exist, the Universe must exist in such a way that the creation of matter was possible at some point in the Universe’s past. Because the Universe is observed to be 13.7 billion years old, any model of the Universe where the lifetime of that Universe is smaller than 13.7 billion years is ruled out.
These examples may seem so simple and self-evident that they appear to be not even worth mentioning, and yet this method of reasoning can not only be incredibly powerful, this is sometimes the only method of reasoning available to us. In the absence of other physical information about the Universe, it must be true that the laws of nature allow the history of the Universe to unfold as it has already done so. This method of reasoning has a special name: The Anthropic Principle.*
Historically, when it comes to cosmology, there are two (and, as far as I know, only two) instances where this has been applied correctly when there were no other avenues of evidence to consider to move our understanding of the Universe forward.
The first one came from the realization that stars are made up of mostly hydrogen, and nearly all of them are powered by nuclear fusion of hydrogen into helium, which occurs through a chain reaction known as the proton-proton chain.
This produces Helium-4 (a nucleus with two protons and two neutrons bound together) in great abundance; this is, for instance, how our Sun works. But where, then, would all the heavier elements — the other 88 naturally occurring elements here on Earth — come from? They must have been created somehow, but there’s a problem: you can’t add a proton to a Helium-4 nucleus (because there’s no stable mass-5 nucleus), and you can’t add another Helium-4 nucleus to it (because there’s no stable mass-8 nucleus, either). You might want to add an intermediate element to Helium-4, but temperatures in the core of a star are so high that any mass-6 or mass-7 nucleus would be immediately destroyed as well. So how do you make heavier elements?
It’s a good question, and the answer came from perhaps the greatest scientist never to be awarded a Nobel Prize: Fred Hoyle.
Because carbon is abundant, Hoyle reasoned, there must be a way to combine three helium nuclei together at once to form Carbon-12 from that. Beryllium-8, what you’d get if you combined two Helium-4 atoms, is so unstable that it decays after some 10-17 seconds. But if you could get a third Helium-4 in there quickly enough, it’s conceivable that you’d be able to form carbon. This process, in theory, is known as the triple-alpha process, since a Helium-4 nucleus is also known as an alpha particle.
The only problem with this reasoning was that the mass of Carbon-12 didn’t match the mass of three Helium-4 atoms; it was significantly lighter. In perhaps the greatest leap ever made using the Anthropic Principle, Hoyle went out on a limb to predict that an excited state of Carbon-12 — with the right energy to allow its creation via the triple-alpha process — must exist.
The discovery of this state with exactly Hoyle’s predicted properties, now named the Hoyle State, is the greatest scientific achievement ever made by use of the Anthropic Principle. In perhaps second place, we can note that spacetime itself — empty space — could have any intrinsic amount of energy to it you can imagine; there are no constraints placed on it by the laws of physics.
If we were to make an order-of-magnitude estimate of what this value should be, based on our fundamental constants (Planck’s constant, the gravitational constant, and the speed of light), we get a number that’s unreasonably large. As in, if it had that value, either positive or negative, the Universe would have either expanded into oblivion or recollapsed in some tiny fraction of the first femtosecond. Therefore, it was reasoned, if the value of this cosmological constant (or vacuum energy) was non-zero, it must be less than 10-120 times the “expected” value in magnitude.
This prediction, made in 1987 by Steven Weinberg, was verified in 1998 when the value was finally measured to be about 10-122 times that value, and to be positive after all.
That’s what the Anthropic Principle has been good for in the past: where the only data is that we are here in this Universe as-is, we can still say intelligible things about what the Universe must allow.
But it doesn’t tell you why or how; the Anthropic Principle is what we use to guide us only when there is no better data or observation to point us in a better direction.
Using the Anthropic Principle as a justification for the Universe to be the way it is is no way forward scientifically. It is a bridge, to be used only to constrain what is possible, and to tell us where to look for a way through our problems. What one mustn’t do is to use the Anthropic Principle as a replacement for dynamics, or the question of how/why the laws of physics brought forth some aspect of our Universe. [When you hear people use it as respects the string landscape, they succumb to exactly this pitfall. If string theory is correct, and the landscape is real, and if physics is ever to understand why our cosmological constant has the value it does, the Anthropic argument will shed no light on that problem. I see it as a craven excuse to avoid asking a tough -- and perhaps unanswerable -- question.]
As far as science goes, our existence in this Universe the way it is remains a self-evident scientific truth. We can learn some amazing things about it just by realizing that the Universe must exist in such a way that it has allowed our existence to be possible. (Not inevitable, as some mistakenly claim.) But the science we can extract from that fact is very limited, and still must hold up to the scrutiny of observations and experimentation. We must be careful not to fall prey to thinking that the possibility we think of first is the only way it could have actually happened; the Universe is full of phenomena that have surprised us before, and while the Anthropic Principle is often the beginning of an investigation, it’s never the end!
* — More technically, this is called the Weak Anthropic Principle. Some people adhere to a variant of this known as the Strong Anthropic Principle, which states that the Universe “must be such as to admit the creation of [intelligent] observers within it at some stage.” This is (IMO) a clear logical fallacy and an uninteresting, unproductive line of thought, as just because the Universe did, in fact, result in our being here, it by no means had to result in the creation of us (or any other intelligent life). But you may disagree; your mileage may vary.