Gravity, like the scale of the universe, is also finely tuned. This force is represented by the gravitational constant, G. If G had been weaker, it would not have had the strength to overcome the initial explosive forces of the Big Bang and bring particles in the universe together, forming stars and planets. If G had been slightly weaker, stars would have been too cool for nuclear fusion, and, as a result, many of the elements needed for life chemistry would never have formed. On the other hand, if G were stronger, the universe would have collapsed in on itself too quickly for life to evolve. Had it been slightly stronger, stars would have been too hot and would have burned too rapidly to produce the chemicals necessary for the creation of life; our life-prospects would have gone up in smoke. According to the philosopher of physics Bradley Monton, “the range of life-permitting gravitational forces is only about one part in 1036 of the total range of forces” (Monton, 2009: 79). You can see why scientists have been so impressed. The odds of gravity falling within that range are incredible. Thus, gravity is precisely fine-tuned for the formation of stars, galaxies, and planets. If we held constant all the other fundamental laws of the universe, any change in G would have had devastating consequences for the development of life.
Gravity is the least important force at small scales but the most important at large scales. It is only because the minuscule gravitational forces of individual particles add up in large bodies that gravity can overwhelm the other forces. Gravity, like the other forces, must also be fine-tuned for life. Think of its role in stars. A star is in a state of temporary balance between gravity and pressure provided by hot gas (which, in turn, depends on the electromagnetic force). A star forms from a parcel of gas when gravity overcomes the pressure forces and turbulence and causes the gas to coalesce and contract. As the gas becomes more concentrated, it eventually becomes so hot that its nuclei begin to fuse, releasing radiation, which itself heats the gas. What would happen to stars if the force of gravity were a million times stronger? Martin Rees, Britain’s Astronomer Royale, surmises, “The number of atoms needed to make a star (a gravitationally bound fusion reactor) would be a billion times less . . . in this hypothetical strong-gravity world, stellar lifetimes would be a million times shorter. Instead of living for ten billion years, a typical star would live for about ten thousand years. A mini- Sun would burn faster, and would have exhausted its energy before even the first steps in organic evolution had got under way.” Such a star would be about one-thousandth the luminosity, three times the surface temperature, and one-twentieth the density of the Sun. For life, such a mini-Sun is a mere “shooting star,” burning too hot and too quickly. A universe in which gravity was weaker would have the opposite problem.
Gravity would alter the cosmos as a whole. For example, the expansion of the universe must be carefully balanced with the deceleration caused by gravity. Too much expansion energy and the atoms would fly apart before stars and galaxies could form; too little, and the universe would collapse before stars and galaxies could form. The density fluctuations of the universe when the cosmic microwave background was formed also must be a certain magnitude for gravity to coalesce them into galaxies later and for us to be able to detect them.26 Our ability to measure the cosmic microwave background radiation is bound to the habitability of the universe; had these fluctuations been significantly smaller, we wouldn’t be here.
Cosmologist Brandon Carter first noticed the interesting coincidence that mid-range mass stars are near the dividing line between convective and radiative energy transport. This dividing line is another razor’s edge, a teetering balance between gravity and electromagnetism. If it were shifted one way or the other, main-sequence stars would be either all blue or all red (convection resulting in red stars). Either way, stars in the main sequence with the Sun’s surface temperature and luminosity would be rare or nonexistent.
What about planets? A stronger gravity would result in a stronger surface gravity for a planet the mass of Earth, and would also boost the planet’s self-compression, increasing the surface gravity even more. Martin Rees notes that a strong-gravity terrestrial planet would prevent organisms from growing very large. Such a planet would also suffer more frequent and higher-velocity impacts from comets and asteroids. Perhaps such a planet also would retain more heat, possibly leading to too much volcanic activity. Of course, these problems could be avoided by having a smaller planet with a surface gravity comparable to Earth’s. But a smaller planet would lose its internal heat much faster, preventing long-lived plate tectonics.
When physicists say, for example, that gravity is “fine-tuned” for life, what they usually mean is that if the gravitational force had even a slightly different value, life would not have been possible. If gravity were slightly weaker, the expansion after the Big Bang would have dispersed matter too rapidly, preventing the formation of galaxies, planets, and astronomers. If it were slightly stronger, the universe would have collapsed in on itself, retreating into oblivion like the groundhog returning to his hole on a wintry day. In either case, the universe would not be compatible with the sort of stable, ordered complexity required by living organisms. Specifically, physicists normally refer to the value of, say, gravity relative to other forces, like electromagnetism or the strong nuclear force. In this case, the ratio of gravity to electromagnetism must be just so if complex life as we know it is to exist. If we were just to pick these values at random, we would almost never find a combination compatible with life or anything like it. Given the prevailing assumptions of nineteenth- and twentieth-century science, discovering that the universe is fine-tuned was a surprise. Underlying the astonishment is the implication that the range of uninhabitable (theoretical) universes vastly exceeds the range of universes, like our own, that are hospitable to life. Thrown to the winds of chance, an uninhabitable universe is an astronomically more likely state of affairs. 2
It is now known that if the force of gravity were any weaker, stars would not have compacted tight enough together so that nuclear fusion would occur. Fusion is necessary to produce the heavier elements upon which life depends (such as carbon, nitrogen and oxygen) ---and without fusion, there would only be hydrogen and helium in all the universe. On the other hand, if gravity were any stronger, stars would burn so hot that they would burn up in about one year or so (ref. G. Easterbrook, cited, p.26). As it is, the gravitational force is so finely tuned, that the average star is capable of burning in a stable fashion for about 80 billion years (ref. H. Ross, cited, p.60).
How finely tuned is gravity? -- Well, the strength of gravity could be at any one of 14 billion billion billion settings, but there is only one setting which is adequate (and optimal) for a universe with intelligent life to exist.
-- To illustrate: This is as if you had a measuring tape with one-inch sections stretched across the known universe, it would be 14 billion billion billion inches long, and only one or two of those inches in the middle is the optimal strength-setting for gravity. If you moved the strength-setting to the right or left just a couple of inches, then intelligent life could not exist (though bacterial life might survive with gravity stronger or weaker by one setting up or down).
THE PROBABILITY: Although the force of gravity could obviously have attained a large number of wrong magnitude-ranges, the chance of it being correct for intelligent life to exist, is one chance out of 14 billion billion billion. --Thus, we can conservatively say that it was about one chance out of 1,000,000,000,000,000,000,000 (or 1 out of 10^21, or 1 out of a billion trillions) that the force of gravity might have randomly attained such an advantageous strength for the making of life-necessary elements in the stars.
In a strong-gravity universe, there would not be plants and animals anything like the size of human beings; galaxies, stars and planets would all be much smaller; planets would be more frequently pulled out of their orbits by passing stars, and stars would burn for much less time than they do in our universe. All in all, the prospects for complicated life like ours would not look promising:
Though we perceive gravity to be a ‘strong’ force (because we are close to a very massive body) it is actually incredibly weak in comparison with the electrostatic forces that control atomic structures and, for example, cause protons to repel each other. The factor is of order ~ 10-36. Let us suppose gravity was stronger by a factor of a million. On the small scale, that of atoms and molecules, there would be no difference, but it would be vastly easier to make a gravitationally bound object such as the Sun and planets but whose sizes would be about a billion times smaller. Any galaxies formed in the universe would be very small with tightly packed stars whose interactions would prevent the formation of stable planetary orbits. The tiny stars would burn up their fuel rapidly allowing no time for life to evolve even if there were suitable places for it to arise. Our intelligent life could not have arisen here on Earth if this ration had been even slightly smaller than its observed value. (Morison 2008:327)
Gravity. Gravity is the weakest force in the universe, yet it is in perfect balance. If gravity were any stronger, the smaller stars could not form, and if it were any smaller, the bigger stars could not form and no heavy elements could exist. Only "red dwarf" stars would exist, and these would radiate too feebly to support life on a planet.
All masses are found to attract one another with a force that varies inversely as the square of the separation distance between the masses. That, in brief, is the law of gravity. But where did that "2" [square] come from? Why is the equation exactly "separation distance squared"? Why is it not 1.87, 1.95, 2.001, or 3.378; why is it exactly 2? Every test reveals the force of gravity to be keyed precisely to that 2. Any value other than 2 would lead to an eventual decay of orbits, and the entire universe would destroy itself!
Kepler’s three empirical laws served as the foundation of Isaac Newton’s more general physical laws of motion and gravity, which became the foundation for Einstein’s General Theory of Relativity two centuries later. The planets may have inspired Kepler, but the Moon inspired Newton to apply his Earthly laws to the broader universe. Without the Earth-centered motion of the Moon, the conceptual leap from falling bodies on Earth’s surface to the motions of the Sun-centered planets would have been much more difficult. By linking the motions of the Moon and planets to experiments on Earth’s surface, Newton gave a physical basis to Kepler’s Third Law. Otherwise, the Third Law would have remained a mathematical curiosity, more an indication of the cleverness of a mathematician with too much time on his hands than of a deep truth about the universe. As it is, astronomy gave birth to physics. 1
1. Guillermo Gonzalez and Jay W. Richards THE PRIVILEGED PLANET HOW OUR PLACE IN THE COSMOS IS DESIGNED FOR DISCOVERY page 104
2. ibid 197
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