Humans like to be at the center of things.

The early Greeks knew the Earth was round, but most of them could not imagine that the land they walked on was anything but the dead center of reality. Maimonides, the medieval Spanish-Egyptian Jewish philosopher, took that geocentrism to heart, arguing that even the ancient Hebrew Bible described a world where everything revolved around our planet — a position that Rabbi Menachem Mendel Schneerson, the Lubavitcher Rebbe, defended using Albert Einstein's theory of relativity as recently as 1975. It took more than 350 years for the Catholic Church to apologize (in 1992!) for imprisoning the great heliocentrist astronomer Galileo Galilei and forcing him to recant his description of the solar system.

In the modern era, no serious thinker argues that the Earth has some special physical centrality in the universe. (Schneerson's paper claimed only that the Earth could be seen as the center of the universe from a particular reference frame.) All the evidence of the great telescopes has shown that Earth is just another small, rocky world orbiting a smallish sun in a far-flung region of a medium-size galaxy.

But there's another idea out there, popular among some of the greatest scientists alive, that centers humans (and creatures like us) to an extent that the ancient philosophers couldn't have imagined. It's so outlandish that Maimonides would likely have considered it a heresy, a violation of his principle that God and only God willed the universe into being. [Creationism vs. Evolution: 6 Big Battles]

Here's how it goes:

The universe is perfect — eerily, uncannily perfect— as a setting for creating life. All sorts of physical constants — the speed of light, the charge of an electron, the ratios of the four fundamental forces (gravity, electromagnetism, weak and strong) — seem fine-tuned to create a universe where life as we know it could emerge.

Here's how the writer Anil Ananthaswamy explained one example for PBS:

"[The neutron] is 1.00137841870 times heavier than the proton [a bare hydrogen nucleus], which is what allows it [a neutron] to decay into a proton, electron and neutrino — a process that determined the relative abundances of hydrogen and helium after the Big Bang and gave us a universe dominated by hydrogen. If the neutron-to-proton mass ratio were even slightly different, we would be living in a very different universe: one, perhaps, with far too much helium, in which stars would have burned out too quickly for life to evolve, or one in which protons decayed into neutrons rather than the other way around, leaving the universe without atoms. So, in fact, we wouldn't be living here at all — we wouldn't exist."

That is, even as tiny a number as the mass of a neutron — the subatomic particle inside all atomic nuclei except that of hydrogen — is perfectly calibrated to allow worlds like Earth to emerge and survive over long spans. This, the thinking goes, is evidence that our universe exists only because there are thinking beings here to observe it.

The idea has some relation to a basic principle of the world of the very small: According to quantum mechanics, a particle takes on a particular speed or a particular location only because someone observed it. Before it was observed, the particle just had a range of possible speeds or locations in space.

Perhaps a universe pops into full existence only when its physical constants are just such that they might be observed?

It's a strange and radical way of thinking about this vast space and our place in it. But it's not a fringe idea.

"The remarkable fact is that the values of [fundamental physics] numbers seem to have been very finely adjusted to make possible the development of life," the physicist Stephen Hawking wrote in his 1988 book "A Brief History of Time." [8 Shocking Things We Learned from Stephen Hawking's Book]

"For example," he went on, "if the electric charge of the electron had been only slightly different, stars either would have been unable to burn hydrogen and helium, or else they would not have exploded. Of course, there might be other forms of intelligent life, not dreamed of even by writers of science fiction, that did not require the light of a star like the sun or the heavier chemical elements that are made in stars and are flung back into space when the stars explode.

"Nevertheless, it seems clear that there are relatively few ranges of values for the numbers that would allow the development of any form of intelligent life. Most sets of values would give rise to universes that, although they might be very beautiful, would contain no one able to wonder at that beauty."

The universe might very well exist only so that we, and creatures like us, might live to see it. Even Hawking suggests the possibility.

Got the weak force?

But not everyone is convinced.

In a new paper made available Jan. 18 at the preprint website arXiv.org, a team of University of Michigan astronomers and physicists made the case that even a vastly different universe might support life.

Starting from physical principles, the researchers worked out how a universe might develop with one of its fundamental forces amputated entirely.

Remember the weak force mentioned above?

It's got the least impressive name of the four fundamentals, but it by no means played a minor part in how our universe came together. As Live Science previously reported, weak is the force of decay. When big particles fall apart into small particles, it's not because the strong force holding them together has failed. Rather, the weak force has forced them apart.

"I would say that the weak force is most important in the sun [and other stars]," said Evan Grohs, one of the authors of the arXiv paper.

When the hot mass of a burning star forces two protons — bare hydrogen nuclei — together, Grohs told Live Science, they fuse into a hydrogen isotope called a deuteron (along with some spare particles). This is a weak force interaction. The deuteron then fuses with another free proton to form a nucleus of two protons and one neutron (which is also known as helium-3). That's an electromagnetic interaction. Finally, the strong force brings that helium-3 particle together with another helium 3, forming a helium-4 nucleus and two free protons. Without the weak force, that chain of events couldn't happen, and the sun would quickly burn itself out.

Similarly, the weak force is responsible for the abundance of water in the universe, Grohs said, a feature generally thought necessary for life.

During and shortly after the Big Bang, the weak force caused free neutrons to decay into single protons — loose hydrogen nuclei floating free in the universe. Just about all the hydrogen around today is a result of those weak-force interactions during the Big Bang era, Grohs said. And their abundance is necessary for the formation of water, with its two hydrogen atoms to each oxygen atom.

If a universe formed that was otherwise entirely like ours, but missing the weak force, just about all the free neutrons and protons would fuse together into helium in the few moments after the universe emerged, according to Grohs.

A vast, dim sun across an oxygen-rich sky

But Grohs and his colleagues, in their paper, imagined a "weakless" universe with some other key parameters changed. Their universe, they showed, would still seem to meet all the known requirements for life. [Top 5 Reasons We May Live in a Multiverse]

First, their universe would begin with way more photons (that is, light) than matter particles screaming into space — reducing the ratio of starting matter to energy by a factor of at least 100 compared to our universe, the researchers said. Out of that high-energy, low-matter particle cloud, they calculated, would emerge a mix of protons, free neutrons, deuterium (another hydrogen isotope) and helium similar to the one in our universe.

And then, for a long time, whatever alien god created this weakless place could just sit back and wait. The weak force acts on tiny scales, affecting the behaviors of elementary particles. So, in this other universe, with the large-scale forces of gravity and electromagnetism intact, clouds of matter would still form galactic discs and condense into stars, the researchers showed. There would be some differences, the scientists found — most importantly, an unusual abundance of deuterium resulting from all those free protons and neutrons floating around. However, nothing would upset the basic structure of space.

Finally, when it came time to light up the stars, the alien god should look closely. Without a weak force in this oddball universe, hydrogen wouldn't fuse into helium. But there would be a lot of deuterium there, and deuterium lights up the darkness in its own way.

Smash a free proton into deuterium, and the strong force will bind the two particles together in a flash of energy, leaving behind the heavy helium isotope helium-3.

This deuterium fusion burns less brightly than the weak-force process that occurs in our sun. Most of the stars in the alternate universe would form into something like our red giants: big and dim and gone in just a short span of time.

But some stars that would burn longer, some more than a billion years. And that's critical.

"We don't have any other examples of life besides this planet," Grohs said, and on this planet, life took about a billion years to form. There's no reason, Grohs said, to assume it would take any more (or less) time in his weakless other place. That means you would likely need these long-lasting stars for life to take root, he said.

So, what would it be like to walk around on a planet orbiting in weakless space?

"I think one thing you would notice is that you probably wouldn't have as many solid structures, because you're not going to have those heavy Earth elements like you have on our planet," Grohs told Live Science.

In the weakless universe, as in ours, stars would be chemical factories. As the stars aged, they would fuse more and more protons onto their heaviest particles, building heavier elements. In our universe, this process goes pretty far, building plenty of oxygen and carbon, but also heavy iron and even a significant amount of superheavy radioactive elements like uranium.

But in the weakless universe, without neutron decay, strong-force fusion would mostly run out of steam at around the level of nickel, a relatively light element, with just 28 protons. Heavier atoms — like iron, gold, iodine and xenon — might still emerge, but in much smaller quantities, Grohs said.

Lighter chemicals, like oxygen and carbon, Grohs said, would be much more abundant.

Still, he added, "I think if you were on a planet in a weakless universe, it would be fairly similar. The stars might be a little larger if you looked into the sky, because in order to have a star that burns deuterium for billions of years, it needs to actually physically have a larger radius than an equivalent star in our universe, and in addition, it doesn't shine as brightly."

So, a life-supporting planet in a weakless universe would likely be much closer to its much-larger star, a big, unusually dim disc taking up a large fraction of the sky.

Grohs acknowledged that the research is fundamentally speculative.

"This is all theoretical," he said. "We don't have any evidence to suggest that there are other universes beyond what we can see."

And the questions he and his colleagues answer — whether an alien universe could have water or structure or long-lasting stars — might not be an exhaustive list of factors necessary to produce life, he said. And a weakless universe might not even be the best candidate for an alternative universe that might produce life.

Still, Grohs said, this paper throws a wrench in the argument that there's something special or necessary about the life-giving physical constants of our universe. And it raises the real possibility that our perception just isn't at the center of things at all.

Originally published on Live Science.