Gravity's Role in the Cosmic Balance: A Delicate Dance of Forces
Paragraph 1
Two of the four main physical forces are familiar ones: electromagnetism and gravity. Even though it is the most obvious force we experience in everyday life, gravity is by far the weakest of the four. The reason why it is so important to us is that our weight is caused by the pull of the entire Earth, almost six million billion kilograms of matter (6 × 1024 kg), acting together. It takes the gravitational pull of all that mass put together to hold us down on the surface of the Earth with the weight we feel. This can be put in perspective by comparing the strength of gravity with the strength of the electromagnetic force, or with one aspect of electromagnetism: the electric force. The electric force of repulsion between two protons is 1036 times stronger than the strength of the gravitational attraction between the same two protons the same distance apart.
Paragraph 2
On the nuclear and atomic scales, gravity is utterly insignificant, and molecules are held together by electric forces without any complications caused by the gravitational interactions between atoms. These electric forces can, of course, produce attraction, not just repulsion, which is what holds electrons and nuclei together in atoms, and holds atoms together to make molecules. On the surface of the Earth, there is constant competition between electric forces holding things together and gravitational forces tending to break things apart. Because of this smaller bodies can survive more easily if they suffer a fall. But a large animal is likely to suffer broken limbs even by falling over, let alone in a fall from a tree or over a cliff. We humans are close to the limit of how big an active animal can be and survive on Earth. In order to be much larger than a human being, you have to be sturdy and ponderous, like an elephant, or live in the sea, like a whale, where the water offers support. Roughly speaking, the rule of thumb is that the volume of a body (and therefore its mass) is proportional to the cube of its linear size (its height), but the strength of its bones is only proportional to its cross-section, which depends on the square of the linear size. Since mass is proportional to volume, and the force of gravity pulling on a body (its weight) is proportional to its mass, as bodies get bigger the forces operating when they fall increase more than the ability of their bones to withstand a fall.
Paragraph 3
This puts the seemingly incredible weakness of gravity in a different perspective. Suppose gravity were a million times stronger (which would still leave it 1030 times weaker than the electric force). This would not be enough to affect atomic and molecular processes, so everything on the scale of atoms and molecules – in particular, chemistry – would operate the way it does in our Universe. But because of the volume rule, anything living on the surface of a planet in such a Universe would also have to be very small, in order not to break apart when it fell over. There could not be anything as large as us, and nothing with the same sort of complexity as us. Most important of all, in this high-gravity universe, the stars would live for only about 10 thousand years before they had used up all their fuel, instead of living for about 10 billion years, as stars like the Sun do in our Universe. Since the chemistry in such a universe would be no different from that in our Universe, there would be no time for evolution even to begin. Gravity has to be as weak as it is for us to exist. A truly cosmic coincidence!
