From New Scientist...

Was the cosmos made for us?

For millennia, we humans have thought of ourselves as a pretty big deal. Then along came science and taught us how insignificant we are. Or so we thought

Is the universe ours? (Image: Sam Chivers)

For much of our existence on Earth, we humans thought of ourselves as a pretty big deal. Then along came science and taught us how utterly insignificant we are. We aren't the centre of the universe. We aren't special. We are just a species of ape living on a smallish planet orbiting an unremarkable star in one galaxy among billions in a universe that had been around for 13.8 billion years without us.

But maybe we were too hasty to write ourselves off. There is a sense in which we are still the centre of the universe.

Science also teaches us that the laws of physics are ridiculously, almost unbelievably, "fine-tuned" for you and me. Take the electromagnetic force. It has a value that is perfectly set for getting stars to bind protons and neutron to create carbon – the building block of life as we know it. Or the strong nuclear force, which binds the insides of protons and neutrons. If it were even a tiny bit stronger, the whole world would be made of hydrogen; if it were weaker, there would be no hydrogen at all. In either case, life as we know it wouldn't be possible. Even the amount of energy contained in empty space seems perfectly set to allow intelligent life to flourish. That's not all. All told, about 12 parameters have been identified as being just right for life.

Why is the universe so perfect? Most physicists now argue that in some sense, it could not have been otherwise. That reasoning has given rise to several different answers known as "anthropic principles".

One end of the spectrum puts us truly back in the centre. This extreme anthropic principle posits that the universe is so perfect that it must have been made for us, either by an intelligent creator or, more likely, because of some fundamental feature of the cosmos that drives it towards intelligent life. In his book The Goldilocks Enigma, Paul Davies, a cosmologist at Arizona State University, tentatively suggests the possibility that life could be a law of nature. He calls it the "life principle", although he admits it is verging on the theological.

Most physicists have no time for ideas like this. "To say that this is all for us? That is just completely bizarre," says Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena.

Perhaps, then, it is the other way round: according to another formulation of the anthropic principle, the universe only exists because we do. We conjure it up with our consciousness (see "Does consciousness create reality?").

Some less mind-bending versions of the principle are also available. They try to explain why the universe would appear perfectly made for us, even if it isn't. In a nutshell, this weak anthropic principle says that given that we are around to observe the universe, it simply has to allow for our existence.

This is often taken to imply that there are other regions of the universe – or even other universes – where physical laws and constants are different. So asking why things are this way amounts to asking why we are in this region. In that case, the anthropic principle is merely pointing out that places that are hospitable to our kind of life are the only places we can possibly be.

This line of reasoning has been bolstered by the possible existence of a multiverse, versions of which emerge from both quantum mechanics and standard cosmology. With lots of other universes, each with their own physical constants and laws, the mystery of our own fine-tuned universe evaporates. We can only be in one that is fine-tuned for carbon-based life.

But even if this version of the anthropic principle explains fine-tuning, it still restores us to some of our former (self-appointed) glory by putting human observers firmly back into our description of the cosmos. "If you want to explain the universe that we see, the very fact that we are seeing it is part of that explanation," says Davies. "It is a bit of a U-turn in the history of science that has been removing the observer from the picture altogether." Welcome back.