Why On Earth...

The chances of living in a universe suited for life

Why Here?

Among the great philosophical questions of science sits this one: Of all the trillions of galaxies in the known universe, of all the billions of stars each of those galaxies contain, of all the planets orbiting those stars, how is it that we humans happen to find ourselves on a planet so perfectly fine-tuned for life? We could have landed anywhere, but we ended up on Earth, where oxygen is abundant, where water cycles through a perfectly closed system, where the temperature band is ideal (for now)? What are the chances?

Or one can ask it like this: There are fundamental laws of physics that govern our universe. Stars can only form because the gravitational constant is what it is. Atoms can only stay together because of the relative strengths of the fundamental forces. So how is it that of all the possible numbers these key elements could have taken, they just so happened to be those which allow matter and energy to exist as we know them?

There is no answer. Or perhaps there is. It depends on how you define answer.

In the 1950s, physicists developed one line of thinking called the Anthropic Principle.

In response to the question, “How do we happen to live in a place that is so perfectly suited for life?” the Anthropic Principle essentially replies, “Well, if you were anywhere else, you wouldn’t be alive to know it.”

The only way we living things can observe a universe which supports life is to live in a universe which supports life.

The Anthropic Principle, a kind of dragon eating its own tail


The Anthropic Principle’s logic goes something like this: Were we to find ourselves on any other planet, in any other universe, we would never have existed at all, and therefore would have been incapable of perceiving life in that life-suppressing environment.

It is an attempt to answer the question of “Why here?” by pointing at the statistical concept of survivorship bias. Just as history is written by the victors, because they’re the ones around to write it, our perfectly tuned universe is analyzed by us, the observers living within it.

This is both a mind-bending and mind-numbing explanation. And many scientists dislike the Anthropic Principle, because, frankly, it doesn’t really explain much. Yet I find it to be an oddly satisfying answer to the unknown mysteries of life.

“Why here?” you ask.

“Where else?” it answers.

Trees in Forests

The Anthropic Principle carries echoes of the old thought experiment, “If a tree falls in a forest and no one is around to hear it, does it make a sound?”

If there existed a universe unable to sustain life, thereby never having anyone or anything observing it, would it matter? Would it matter in the sense of existing in any real sense of the word? Existence requires observation. And if we (or some other living life form) would never be created within it, this universe might as well have never been formed at all. For what would have been the point?

So to the question of “If a tree falls in the forest…” one might respond, “It depends on whether or not anyone will ever come by and see the tree on the ground.”

If the future will contain an observer, then the concept of causality requires that yes, the tree did fall, and yes, it did make a sound. It existed in the past, because otherwise a future observer would never have been able to stumble upon it in the future.

By that logic, our universe was there before we evolved within it. The Big Bang did occur even though no life yet existed, because it paved the way for the cause and effect chain that did ultimately lead to life. (Check out my article on the discovery of the Big Bang here).

Of course, what none of this illuminates is the answer to what happens if the tree falls and no one will ever come around to see it. What if the Big Bang happens in a universe that will never support life because it cannot ever support life? Might it look into the future and recognize its futility? Might we say such a universe has never existed and will never exist, because no one will ever come along to observe it?

Or do things exist independently of whomever might be there to appreciate them?