Introduction to our discussion of “Just Six Numbers”

Let me begin by discussing what the book is not about.

It is well known that our existence here on Earth would have been impossible, if our Solar System were even slightly different than it is.

For example, everyone knows that if the Earth were even a little (by astronomical terms) closer to the Sun than it is, then the surface of the planet would be too hot for us to live on it. And if the Earth were even a little further away from the Sun than it is, then the surface of the planet would be too cold for us to live on it. And if the orbit of the Earth were a little more elliptical (instead of the near-perfect circle that it is), then there would be times of the year when the planet is too hot for us to survive on it, or there would be times of the year when the planet is too cold for us to survive on it, or both.

If the Earth did not rotate, but kept one side always facing the Sun and one side facing away from the Sun (as one side of the Moon is always facing us, and its other side is always facing away from us), then again we could not exist, because the side of the planet facing the sun would be too hot to support life, while the side facing away from the sun would be too cold to support life. This is also the reason why the Sun cannot be colder than it is. If the Sun were colder than it is, but we were closer to it, so that the average temperature on Earth would be the same, then, at that distance from the Sun, the tidal forces would be greater, and the rotation of the Earth would stop after only a few hundred million years, with its rotational period equal to its year length (just as the Sun has slowed the rotation of Mercury, so that its rotational period is now equal to ⅔ of its year length). Half of the planet would be too hot to support life, and half would be too cold. And if the sun were hotter than it is (but we were further away from it, so that the average temperature on Earth would be the same), then it would not last long enough for life to evolve on Earth. Before the necessary billion of years or so for evolution to take place had elapsed, the sun would already have burnt out.

If the Earth were even a little smaller than it is (like, e.g., Mars) then its gravity would not be strong enough for it to hold on to its atmosphere, and we could not exist. If the Earth were larger than it is, than organisms of our size and complexity could not exist on land, they would be crushed by gravity, nor could there be the vascular plants on which we feed.

We need the iron-nickel core at the center of the Earth, even though it is 8000 miles away from anything that lives, and without it, we could not survive. The iron-nickel core produces the magnetic field that protects us from the charged particles that are constantly streaming out at us from the Sun (the “solar winds”) and pouring down on us from outside of our Solar system (the “cosmic rays”), which would otherwise be fatal to all life. This is also why the Earth cannot be smaller and denser than it is; if the Earth were smaller than it is, but denser, so that its surface gravity were the same, its surface area would be greater relative to its volume and its core would therefore cool more quickly. After a few billion years, it would no longer have a liquid outer core, and without a liquid outer core flowing around a solid inner core made of a suitable ferromagnetic material, there is no magnetic field, and no protection from the solar winds and cosmic rays. Earth would be a dead planet.

We need the Moon, exactly where it is, two hundred forty thousand miles away from us, and without it, we could not exist. Without the Moon to stabilize the Earth's axis, the Earth's axis would wobble much more than it does, and much more quickly than it does. Climates would change more rapidly, and the changes would be greater. There would be no time for organisms to evolve to adapt to their climate, because climates would not be stable long enough for that to happen. Maybe there could be chemosynthetic bacteria on the ocean floor near volcanic vents, where the conditions do not depend on the climate at the surface, but there could be no complex and delicately-balanced ecosystems on the surface, such as we now have, and such as produced our species.

We need the gas giants in our outer solar system, hundreds of millions of miles away from us, and without them, there could be no life on Earth. Jupiter and Saturn and Uranus and Neptune are our solar system's vacuum cleaners, pulling the debris of the solar system toward themselves instead of letting it fall on us. If we didn't have the gas giants in our outer solar system, asteroids and comets would be falling on the Earth every million years or so, periodically sterilizing our planet.

None of this even begins to discuss the complexity of life itself. The only thing that has been mentioned so far is the necessary but not sufficient conditions for life — the prerequisites, without with, life would be impossible. But even with those conditions, the arrival of life is not inevitable, quite the contrary, the complexity of a single procaryotic cell is beyond human understanding, and a pismire, to the poet, is miracle enough to stagger sextillions of infidels.

All of this, as I said, is well known, and it is interesting, but none of it is amazing. Maybe the odds of a solar system giving birth to complex life are a billion to one, or ten billion to one, or a hundred billion to one, or a trillion to one. But there are billions and billions of stars in our galaxy, and there are billions and billions of galaxies in the universe, so it is not amazing if there is life in one of them. It is like being amazed that someone won the lottery. True, the odds of winning the lottery may be a hundred million to one, but there are a hundred million people who bought tickets, so it does not amaze me that Shlomo Pipik won the lottery. If it had not been Shlomo Pipik, it would have been someone else.

(Footnote: in higher mathematics, “billions and billions” is a unit of quantity that can be abbreviated as “1 Sagan”. You can apply metric prefices to this term, so “millions and millions” is 1 milliSagan (mS), “thousands and thousands” is 1 microSagan (μS), and “trillions and trillions” is 1 kiloSagan (kS).)

In Just Six Numbers, the author discusses something else, and something far less well known. He does not discuss the conditions on and around Earth that made life possible. Everyone already knows that. Rather, he discusses the conditions of the universe that made the universe possible. That is a far different thing, and it is a far more amazing thing, because there are not billions and billions of universes, there is only one that we know of, and the laws of physics, the fundamental properties of the one universe that exists, are so well-tuned — they have to be so well-tuned — that it is utterly amazing that the universe exists, and the more we know, the more amazed we are.

Thus (and this is one that I knew about before I read the book), if the strong nuclear force were slightly, ever so slightly, stronger than it is, then a nucleus of two protons would be stable, and there would be no hydrogen. And if the strong nuclear force were only slightly weaker than it is, then even an alpha particle would not be stable, and there could be only hydrogen. There is no reason — none that we know of, anyway — why the strong nuclear force should be exactly as strong as it is, but if it were not, the universe as we know it could not exist.