A deep connection exists between the microscopic world of atoms that holds together small molecules and the features of the macroscopic world around us, typified by complex structures, the likes of which include life, stars, and galaxies. The properties of the universe at large that have allowed biological life to evolve are determined by the fundamental forces of nature and the contrastive effects that these forces impose at the diverse scales that govern both worlds. Electric and magnetic forces act between atoms and control the chemical processes that take place in our bodies. The atomic nucleus exhibits a fine balance between the nuclear forces and electric forces within it, which makes possible the existence of stable elements that underpin the complex molecules that characterize biological life.

On the scale of the atom, the electrostatic interactions between subatomic particles happen to be 10³⁶  times weaker than the gravitational forces acting between the particles. Due to this difference, the effects of gravitation on the microscopic scale are treated as negligible. At the macroscopic level, however, when large masses are concerned, the effects of gravitation arise conspicuously. Such a contradistinctive interplay emerges out of the inherently differing properties that both electric and gravitational forces exhibit. A gravitational force acting between two particles is always attractive. Electric forces, on the other hand, are repulsive when like particles are concerned but attractive when unlike particles are concerned. The fact that gravitational charges are always the same sign and thus cannot cancel each other out leads to the effects of gravitation manifesting greatly as masses increase. Effectively, large masses end up having great gravitational attractions, but negligible electrical forces, between them and the electrical balance between protons and neutrons in these large objects results in their electrical neutrality. But, what happen if the force of gravity were any different? In his book Just Six Numbers, the Astronomer Royal Martin Rees remarks:

“What would happen if it [gravity] weren’t quite so weak? Imagine for instance, a universe where gravity was ‘only’ 10³⁰, rather than 10³⁶ feebler than electric forces. Atoms and molecules would behave just as in our actual universe, but objects would not need to be so large before gravity became competitive with the other forces. The number of atoms needed to make a star (a gravitationally bound fusion reactor) would be a billion times less in this imagined universe. Planet masses would also be scaled down by a billion. Irrespective of whether these planets could retain steady orbits, the strength of gravity would stunt the evolutionary potential on them. In an imaginary strong gravity world, even insects would need thick legs to support them, and no animals could get much larger.”

Our own Sun which powers life on Earth is made up of approximately 10⁵⁷ atoms of hydrogen. This appears to be the consequence of the strength of gravity being so weak. As the Astronomer Royal Martin Rees explains:

“Gravity starts off, on the atomic scale, with a handicap of thirty-six powers of
ten; but it gains two powers of ten (in other words 100) for every three
powers (factors of 1000) in mass. So gravity will have caught up for
the fifty-fourth object (54 = 36 × 3/2), which has about Jupiter’s mass.
In any still heavier lump more massive than Jupiter, gravity is so strong
that it overwhelms the forces that hold solids together. […] It is
because gravity is so weak that a typical star like the Sun is so massive.
In any lesser aggregate, gravity could not compete with the pressure, nor
squeeze the material hot and dense enough to make it shine.” 

As previously mentioned, there are other delicate balances that also appear to unfold on the subatomic scale. Indeed, if the nuclear forces were any stronger to bind two protons in a nuclei such that the electromagnetic forces would not be able to rip it apart, the di-proton would exist as a stable element. Thermonuclear fusion would take place too rapidly to sustain the evolution of biological life. As a result, all the hydrogen nuclei in the universe would form di-protons during the early stages of the Big Bang and the universe would be composed entirely of helium. Hydrogen, and therefore us, would not exist. On the other hand, if the nuclear forces were any weaker, only hydrogen would exist as a stable element. The deuteron would be unstable and no hydrogen would burn at all. Life would still not be possible.

All of these reflections may evoke some sense of a feeling that the universe is deliberately calibrated for our kind of life. But, indeed, it is not surprising that we find ourselves in a universe where the conditions are just right for our kind of life to exist. The very notion of observing a universe and remarking upon the coincidences requires that the universe that is observed must be fine-tuned anyhow. The conditions happened to exist for us to evolve. But, they, certainly, did not have to be this way. A universe that has allowed our kind of life is an interesting universe but not a necessary one.

Bibliography 

Rees, M. J. (2000). Just six numbers: The deep forces that shape the universe. New York: Basic Books.