The force of Gravity - evidence of fine tuning
https://reasonandscience.catsboard.com/t1366-the-force-of-gravity-evidence-of-fine-tuning
ROBIN COLLINS The Teleological Argument: An Exploration of the Fine-Tuning of the Universe 2009
Gravity is a long-range attractive force between all material objects, whose strength increases in proportion to the masses of the objects and falls off with the inverse square of the distance between them. Consider what would happen if there were no universal, long-range attractive force between material objects, but all the other fundamental laws remained (as much as possible) the same. If no such force existed, then there would be no stars, since the force of gravity is what holds the matter in stars together against the outward forces caused by the high internal temperatures inside the stars. This means that there would be no long-term energy sources to sustain the evolution (or even existence) of highly complex life. Moreover, there probably would be no planets, since there would be nothing to bring material particles together, and even if there were planets (say because planet-sized objects always existed in the universe and were held together by cohesion), any beings of significant size could not move around without floating off the planet with no way of returning. This means that physical life could not exist. For all these reasons, a universal attractive force such as gravity is required for life.
The constants of physics are fundamental numbers that, when plugged into the laws of physics, determine the basic structure of the universe. An example of a fundamental constant is Newton’s gravitational constant G, which determines the strength of gravity via Newton’s law. We will say that a constant is fine-tuned if the width of its life-permitting range, Wr, is very small in comparison to the width, WR, of some properly chosen comparison range: that is, Wr/WR << 1.
Fine-tuning of gravity
Using a standard measure of force strengths – which turns out to be roughly the relative strength of the various forces between two protons in a nucleus – gravity is the weakest of the forces, and the strong nuclear force is the strongest, being a factor of 10^40 – or 10 thousand billion, billion, billion, billion times stronger than gravity. Now if we increased the strength of gravity a billionfold, for instance, the force of gravity on a planet with the mass and size of the Earth would be so great that organisms anywhere near the size of human beings, whether land-based or aquatic, would be crushed. (The strength of materials depends on the electromagnetic force via the fine-structure constant, which would not be affected by a change in gravity.) Even a much smaller planet of only 40 ft in diameter – which is not large enough to sustain organisms of our size – would have a gravitational pull of 1,000 times that of Earth, still too strong for organisms of our size to exist. As astrophysicist Martin Rees notes, “In an imaginary strong gravity world, even insects would need thick legs to support them, and no animals could get much larger”. Consequently, such an increase in the strength of gravity would render the existence of embodied life virtually impossible and thus would not be life-permitting in the sense that we defined. Of course, a billionfold increase in the strength of gravity is a lot, but compared with the total range of the strengths of the forces in nature (which span a range of 10^40), it is very small, being one part in 10 thousand, billion, billion, billion. Indeed, other calculations show that stars with lifetimes of more than a billion years, as compared with our Sun’s lifetime of 10 billion years, could not exist if gravity were increased by more than a factor of 3,000. This would significantly inhibit the occurrence of embodied life. The case of fine-tuning of gravity described is relative to the strength of the electromagnetic force, since it is this force that determines the strength of materials – for example, how much weight an insect leg can hold; it is also indirectly relative to other constants – such as the speed of light, the electron and proton mass, and the like – which help determine the properties of matter. There is, however, a fine-tuning of gravity relative to other parameters. One of these is the fine-tuning of gravity relative to the density of mass-energy in the early universe and other factors determining the expansion rate of the Big Bang – such as the value of the Hubble constant and the value of the cosmological constant. Holding these other parameters constant, if the strength of gravity were smaller or larger by an estimated one part in 10^60 of its current value, the universe would have either exploded too quickly for galaxies and stars to form, or collapsed back on itself too quickly for life to evolve. The lesson here is that a single parameter, such as gravity, participates in several different fine-tunings relative to other parameters.
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.696.63&rep=rep1&type=pdf
CALUM MILLER Defence of the fine-tuning argument JULY 25, 2017
The gravitational constant
Gravity is a relatively weak force, just 1/1040 of the strength of the strong nuclear force. And it turns out that this relative weakness is crucial for life. Consider an increase in its strength by a factor of 109: in this kind of world, any organism close to our size would be crushed. Compare then, Astronomer Royal Martin Rees’ statement that “In an imaginary strong gravity world, even insects would need thick legs to support them, and no animals could get much larger”. If the force of gravity were this strong, a planet which had a gravitational pull one thousand times the size of Earth’s would only be twelve metres in diameter – and it is inconceivable that even this kind of planet could sustain life, let alone a planet any bigger.
Now, a billion-fold increase seems like a large increase – indeed it is, compared to the actual value of the gravitational constant. But there are two points to be noted here. Firstly, that the upper life-permitting bound for the gravitational constant is likely to be much lower than 109 times the current value. Indeed, it is extraordinarily unlikely that the relevant kind of life, viz. embodied moral agents, could exist with the strength of gravity being any more than 3,000 times its current value, since this would prohibit stars from lasting longer than a billion years (compared with our sun’s current age of 4.5 billion years). Further, relative to other parameters, such as the Hubble constant and cosmological constant, it has been argued that a change in gravity’s strength by “one part in 10^60 of its current value” would mean that “the universe would have either exploded too quickly for galaxies and stars to form, or collapsed back in on itself too quickly for life to evolve.” But secondly, and more pertinently, both these increases are minute compared with the total range of force strengths in nature – the maximum known being that of the strong nuclear force. This does not seem to be any consistency in supposing that gravity could have been this strong; this seems like a natural upper bound to the potential strength of forces in nature. But compared to this, even a billion-fold increase in the force of gravity would represent just one part in 1031 of the possible increases.
We do not have a comparable estimate for the lower life-permitting bound, but we do know that there must be some positive gravitational force, as demonstrated above. Setting a lower bound of 0 is even more generous to fine tuning detractors than the billion-fold upper limit, but even these give us an exceptionally small value for Wr/WR, in the order of 1/10^31.
https://calumsblog.com/2017/07/25/full-defence-of-the-fine-tuning-argument-part-4/
Leonard Susskind The Cosmic Landscape: String Theory and the Illusion of Intelligent Design 2006, page 184
Another essential requirement for life is that gravity be extremely weak. In ordinary life gravity hardly seems weak. Indeed, as we age, the daily prospect of fighting gravity gets more and more daunting. I can still hear my grandmother saying, “Oy vey, I feel like a thousand pounds.” But I don’t ever recall hearing her complain about electric forces or nuclear forces. Nonetheless, if you compare the electric force between the nucleus and an atomic electron with the gravitational force, you would find the electric force is about 10^41 times larger. Where did such a huge ratio come from? Physicists have some ideas, but the truth is that we really don’t know the origin of this humongous discrepancy between electricity and gravity despite the fact that it is so central to our existence. But we can ask what would have happened if gravity had been a little stronger than it is. The answer again is that we would not be here to talk about it. The increased pressure due to stronger gravity would cause stars to burn much too fast— so fast that life would have no chance to evolve. Even worse, black holes would have consumed everything, dooming life long before it began. The large gravitational pull might even have aborted the Hubble expansion and caused a big crunch very shortly after the Big Bang.
https://3lib.net/book/2472017/1d5be1
Brad Lemley Why is There Life? November 01, 2000
N, the ratio of the electromagnetic force to the gravitational force between a pair of protons, is approximately 1036. According to Rees, if it were significantly smaller, only a small and short-lived universe could exist
N, equal to 1,000,000,000,000,000,000,000,000,000,000,000,000. The number measures the strength of the forces that hold atoms together divided by the force of gravity between them. It means that gravity is vastly weaker than intra-atomic attraction. If the number were smaller than this vast amount, "only a short-lived, miniature universe could exist," says Rees.
Omega, which measures the density of material in the universe— including galaxies, diffuse gas, and dark matter. The number reveals the relative importance of gravity in an expanding universe. If gravity were too strong, the universe would have collapsed long before life could have evolved. Had it been too weak, no galaxies or stars could have formed.
https://web.archive.org/web/20140722210250/http://discovermagazine.com/2000/nov/cover/
Luke A. Barnes The Fine-Tuning of the Universe for Intelligent Life 7 Jun 2012
If gravity were repulsive rather than attractive, then matter wouldn’t clump into complex structures. Remember: your density, thank gravity, is 10^30 times greater than the average density of the universe.
https://arxiv.org/abs/1112.4647
Kelly James Clark: Religion and the Sciences of Origins: Historical and Contemporary Discussions 2014
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 10^36 of the total range of forces”. 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.
https://3lib.net/book/2383970/12b021
Guillermo Gonzalez, Jay W. Richards: The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery 2004 page 216
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 underway.” 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
Stephen C. Meyer: The return of the God hypothesis, page 173
The ratio of the electromagnetic force to gravity must be accurate to 1 part in 10^40 . Were this ratio a bit higher, the gravitational attraction would be too strong in comparison to the contravening force of electromagnetism pushing nuclei apart. In that case, stars would, again, burn too quickly and unevenly to allow for the formation of long-lived stars and stable solar systems. Were this ratio a bit lower, gravitational attraction would be too weak in comparison to electromagnetism. That would have prevented stars from burning hot enough to produce the heavier elements needed for life.
Mario Livio Fine-Tuning, Complexity, and Life in the Multiverse 2018
Constraints on Gravity
As numerical simulations of structure formation in the universe have demonstrated, gravity enhances density fluctuations. In our universe, gravity caused the denser regions to lag behind the cosmic expansion and to form the sponge-like structure that characterizes the universe on its largest scales. Eventually, gravity led to the formation of galaxies at the density peaks, of stars, and of planets. Stellar evolution also represents one continuous battle with gravity, the latter pushing the stellar central densities and temperatures to higher and higher values. On the surface of planets gravity played crucial roles in keeping an atmosphere bound and in bringing different elements into contact to initiate the chemical reactions that eventually led to life. But gravity in our universe is a very weak force — the ratio of the repulsive electric force between two protons to their gravitational mutual attraction is e2/Gm2 p ⁓ 1036. The reason gravity becomes important on the scale of large asteroids and higher, is that large objects have a net electric charge that is close to zero, so gravity wins once sufficiently many atoms are packed together. Figure 1 allows us to make a first attempt to examine what would happen in a universe in which the values of some “constants of nature” are different. How would Figure 1 be different if gravity were not so weak? The general structure of the diagram would remain the same, but there would be fewer powers of ten between the subatomic and the cosmic scales. Stars, which effectively are gravitationally bound nuclear fusion reactors, would be smaller in such a universe and would have shorter lives. If gravity were much stronger, then even small solid bodies (such as rocks) might be gravitationally crushed. If gravity’s strength were such that it would still have allowed tiny planets to exist, life forms the size of humans would be crushed on the planetary surface. Overall, the universe would be much smaller and there would be less time for complexity to emerge. In other words, to have what we may call an “interesting” universe (in the sense of complexity), we must have many powers of ten between the microscale and the cosmic scale, and this requires gravity to be very weak. It is important to note, however, that gravity does not need to be fine-tuned for complexity to emerge. In fact, a universe in which gravity is ten times weaker than in our universe, may be even more “interesting” in that it would allow bigger stars and planes, and more time for life to emerge and evolve.
https://arxiv.org/ftp/arxiv/papers/1801/1801.06944.pdf
Dr. Walter L. Bradley: Is There Scientific Evidence for the Existence of God? How the Recent Discoveries Support a Designed Universe 20 August 2010
Balancing Gravity and Electromagnetism Forces - Fine Tuning Our Star and Its Radiation
The electromagnetic force is 1038 times stronger than the gravity force. Gravity draws hydrogen into stars, creating a high temperature plasma. The protons in the plasma must overcome their electromagnetic repulsion to fuse. Thus the relative strength of the gravity force to the electromagnetic force determines the rate at which stars "burn" by fusion. If this ratio of strengths were altered to1032 instead of 1038 (i.e., if gravity were much stronger), stars would be a billion times less massive and would burn a million times faster.
https://web.archive.org/web/20110805203154/http://www.leaderu.com/real/ri9403/evidence.html#ref21
New Scientist: Gravity mysteries: Why does gravity only pull? 10 June 2009
All the other forces in nature have opposites. In the case of the electromagnetic force, for example, it can attract or repel, depending on the charges of the bodies involved. So what makes gravity different?
The answer seems to lie with quantum field theory. The particles that transmit the strong, weak and electromagnetic forces have various types of charge, such as electric or colour charge. “Those charges can be either positive or negative, leading to different possibilities for the sign of the force,” says Frank Wilczek of the Massachusetts Institute of Technology. This is not the case with gravitons, the hypothetical particles that quantum field theory says should transmit gravity. “Gravitons respond to energy density, which is always positive,” says Wilczek. Or are we assuming too much here? “We don’t know that gravity is strictly an attractive force,” cautions Paul Wesson of the University of Waterloo in Ontario, Canada. He points to the “dark energy” that seems to be accelerating the expansion of the universe, and suggests it may indicate that gravity can work both ways. Some physicists speculate that dark energy could be a repulsive gravitational force that only acts over large scales. “There is precedent for such behaviour in a fundamental force,” Wesson says. “The strong nuclear force is attractive at some distances and repulsive at others.”
Either way, the apparent difference between gravity and the other fundamental forces poses a problem for physicists.
https://www.newscientist.com/article/mg20227122-800-gravity-mysteries-why-does-gravity-only-pull/
Natalie Wolchover Why Gravity Is Not Like the Other Forces June 15, 2020
Physicists have traced three of the four forces of nature — the electromagnetic force and the strong and weak nuclear forces — to their origins in quantum particles. But the fourth fundamental force, gravity, is different.
Our current framework for understanding gravity, devised a century ago by Albert Einstein, tells us that apples fall from trees and planets orbit stars because they move along curves in the space-time continuum. These curves are gravity. According to Einstein, gravity is a feature of the space-time medium; the other forces of nature play out on that stage. But near the center of a black hole or in the first moments of the universe, Einstein’s equations break. Physicists need a truer picture of gravity to accurately describe these extremes. This truer theory must make the same predictions Einstein’s equations make everywhere else. Physicists think that in this truer theory, gravity must have a quantum form, like the other forces of nature. Researchers have sought the quantum theory of gravity since the 1930s. They’ve found candidate ideas — notably string theory, which says gravity and all other phenomena arise from minuscule vibrating strings — but so far these possibilities remain conjectural and incompletely understood. A working quantum theory of gravity is perhaps the loftiest goal in physics today. What is it that makes gravity unique? What’s different about the fourth force that prevents researchers from finding its underlying quantum description? We asked four different quantum gravity researchers. We got four different answers.
Gravity Breeds Singularities
Claudia de Rham, a theoretical physicist at Imperial College London, has worked on theories of massive gravity, which posit that the quantized units of gravity are massive particles:
Einstein’s general theory of relativity correctly describes the behavior of gravity over close to 30 orders of magnitude, from submillimeter scales all the way up to cosmological distances. No other force of nature has been described with such precision and over such a variety of scales. With such a level of impeccable agreement with experiments and observations, general relativity could seem to provide the ultimate description of gravity. Yet general relativity is remarkable in that it predicts its very own fall. General relativity yields the predictions of black holes and the Big Bang at the origin of our universe. Yet the “singularities” in these places, mysterious points where the curvature of space-time seems to become infinite, act as flags that signal the breakdown of general relativity. As one approaches the singularity at the center of a black hole, or the Big Bang singularity, the predictions inferred from general relativity stop providing the correct answers. A more fundamental, underlying description of space and time ought to take over. If we uncover this new layer of physics, we may be able to achieve a new understanding of space and time themselves. If gravity were any other force of nature, we could hope to probe it more deeply by engineering experiments capable of reaching ever-greater energies and smaller distances. But gravity is no ordinary force. Try to push it into unveiling its secrets past a certain point, and the experimental apparatus itself will collapse into a black hole.
Gravity Leads to Black Holes
Daniel Harlow, a quantum gravity theorist at the Massachusetts Institute of Technology, is known for applying quantum information theory to the study of gravity and black holes:
Black holes are the reason it’s difficult to combine gravity with quantum mechanics. Black holes can only be a consequence of gravity because gravity is the only force that is felt by all kinds of matter. If there were any type of particle that did not feel gravity, we could use that particle to send out a message from the inside of the black hole, so it wouldn’t actually be black. The fact that all matter feels gravity introduces a constraint on the kinds of experiments that are possible: Whatever apparatus you construct, no matter what it’s made of, it can’t be too heavy, or it will necessarily gravitationally collapse into a black hole. This constraint is not relevant in everyday situations, but it becomes essential if you try to construct an experiment to measure the quantum mechanical properties of gravity.
Our understanding of the other forces of nature is built on the principle of locality, which says that the variables that describe what’s going on at each point in space — such as the strength of the electric field there — can all change independently. Moreover, these variables, which we call “degrees of freedom,” can only directly influence their immediate neighbors. Locality is important to the way we currently describe particles and their interactions because it preserves causal relationships: If the degrees of freedom here in Cambridge, Massachusetts, depended on the degrees of freedom in San Francisco, we may be able to use this dependence to achieve instantaneous communication between the two cities or even to send information backward in time, leading to possible violations of causality.
The hypothesis of locality has been tested very well in ordinary settings, and it may seem natural to assume that it extends to the very short distances that are relevant for quantum gravity (these distances are small because gravity is so much weaker than the other forces). To confirm that locality persists at those distance scales, we need to build an apparatus capable of testing the independence of degrees of freedom separated by such small distances. A simple calculation shows, however, that an apparatus that’s heavy enough to avoid large quantum fluctuations in its position, which would ruin the experiment, will also necessarily be heavy enough to collapse into a black hole! Therefore, experiments confirming locality at this scale are not possible. And quantum gravity therefore has no need to respect locality at such length scales.
Indeed, our understanding of black holes so far suggests that any theory of quantum gravity should have substantially fewer degrees of freedom than we would expect based on experience with the other forces. This idea is codified in the “holographic principle,” which says, roughly speaking, that the number of degrees of freedom in a spatial region is proportional to its surface area instead of its volume.
Gravity Creates Something From Nothing
Juan Maldacena, a quantum gravity theorist at the Institute for Advanced Study in Princeton, New Jersey, is best known for discovering a hologram-like relationship between gravity and quantum mechanics:
Particles can display many interesting and surprising phenomena. We can have spontaneous particle creation, entanglement between the states of particles that are far apart, and particles in a superposition of existence in multiple locations. In quantum gravity, space-time itself behaves in novel ways. Instead of the creation of particles, we have the creation of universes. Entanglement is thought to create connections between distant regions of space-time. We have superpositions of universes with different space-time geometries.
Furthermore, from the perspective of particle physics, the vacuum of space is a complex object. We can picture many entities called fields superimposed on top of one another and extending throughout space. The value of each field is constantly fluctuating at short distances. Out of these fluctuating fields and their interactions, the vacuum state emerges. Particles are disturbances in this vacuum state. We can picture them as small defects in the structure of the vacuum.
When we consider gravity, we find that the expansion of the universe appears to produce more of this vacuum stuff out of nothing. When space-time is created, it just happens to be in the state that corresponds to the vacuum without any defects. How the vacuum appears in precisely the right arrangement is one of the main questions we need to answer to obtain a consistent quantum description of black holes and cosmology. In both of these cases there is a kind of stretching of space-time that results in the creation of more of the vacuum substance.
Gravity Can’t Be Calculated
Sera Cremonini, a theoretical physicist at Lehigh University, works on string theory, quantum gravity and cosmology: There are many reasons why gravity is special. Let me focus on one aspect, the idea that the quantum version of Einstein’s general relativity is “nonrenormalizable.” This has implications for the behavior of gravity at high energies. In quantum theories, infinite terms appear when you try to calculate how very energetic particles scatter off each other and interact. In theories that are renormalizable — which include the theories describing all the forces of nature other than gravity — we can remove these infinities in a rigorous way by appropriately adding other quantities that effectively cancel them, so-called counterterms. This renormalization process leads to physically sensible answers that agree with experiments to a very high degree of accuracy. The problem with a quantum version of general relativity is that the calculations that would describe interactions of very energetic gravitons — the quantized units of gravity — would have infinitely many infinite terms. You would need to add infinitely many counterterms in a never-ending process. Renormalization would fail. Because of this, a quantum version of Einstein’s general relativity is not a good description of gravity at very high energies. It must be missing some of gravity’s key features and ingredients.
However, we can still have a perfectly good approximate description of gravity at lower energies using the standard quantum techniques that work for the other interactions in nature. The crucial point is that this approximate description of gravity will break down at some energy scale — or equivalently, below some length. Above this energy scale, or below the associated length scale, we expect to find new degrees of freedom and new symmetries. To capture these features accurately we need a new theoretical framework. This is precisely where string theory or some suitable generalization comes in: According to string theory, at very short distances, we would see that gravitons and other particles are extended objects, called strings. Studying this possibility can teach us valuable lessons about the quantum behavior of gravity.
https://www.quantamagazine.org/why-gravity-is-not-like-the-other-forces-20200615/
Question: Why is the gravitational force always attractive? Of course, if gravity was repulsive then no large bodies like stars or planets could have formed - and consequently nu human would have been around to ask this question.
Answer: gravity is always attractive because nothing can have a negative mass value.
https://www.quora.com/Why-is-gravity-force-an-attractive-force-rather-than-repulsive
Gravity can be either attractive or repulsive, as explained in following paper:
Imanol Albarran What if gravity becomes really repulsive in the future? 24 March 2018
The Universe is evolving. Hubble's discovery was based on observing that the spectrum of far-away galaxies was red-shifted which implied that those galaxies were moving away from us. He even measured the galaxies radial outward velocities and realised that it followed a rule: (1) the velocities were proportional to the distances at which the galaxies were located from us and (2) the proportionality factor was a constant, the Hubble constant. About 70 years later, two independent teams realized that by measuring further objects, SNeIa, the Hubble constant was not quite constant, as was already expected. The issue was that the deviation from the constancy was not in the anticipated direction. It was no longer enough to invoke only matter to explain those observations. A new dark component had to be invoked, interacting as far as we know only gravitationally, and named dark energy. This component started recently fuelling a second inflationary era of the visible Universe. Of course, all these observations, and subsequent ones, are telling us how gravity behaves at cosmological scales through the kinematic expansion of our Universe .
This kinematic description is linked to the dynamical expansion through the gravitational laws of Einstein's theory. To a very good approximation, we may assume that our Universe is homogeneous and isotropic on large scales and that it is filled with matter (standard and dark) and dark energy.
Summarising, what we have shown is that after all gravity might behave the other way around in the future and, rather than the apple falling from the tree, the apple may fly from the earth surface to the branches of the tree, if dark energy is repulsive enough, as could already be indicated by current observations.
https://link.springer.com/article/10.1140/epjc/s10052-018-5728-x
Gravity is mediated by a spin 2 particle. Electromagnetism by spin 1.
even and odd spin do differ in that they require a product of charges with different signs to get attraction or repulsion:
Fermion or Boson? The Spin-Statistics Theorem
https://www.youtube.com/watch?v=I7EYDv7vbfg
Belt Trick
https://www.youtube.com/watch?v=JaIR-cWk_-o
spin even:
q1q2>0q1q2>0: attractive
q1q2<0q1q2<0: repulsive
spin odd:
q1q2<0q1q2<0: attractive
q1q2>0q1q2>0: repulsive
In the case of gravity, mediated by spin 2 particles, charge is mass, which is always positive. Thus, q1q2q1q2 is always greater than zero, and gravity is always attractive. For spin 0 force mediators, however, there is no restriction on the charges and you can very well have repulsive forces. A better rephrasing of the question is: "Why do particles of odd spin generate repulsive forces between like charges, while particles of even spin generate attractive forces between like charges?"
https://physics.stackexchange.com/questions/11542/why-is-the-gravitational-force-always-attractive
Quantum Field Theory: Why do particles of odd integer spin generate forces which can be both attractive and repulsive, whereas particles of even integer spin only attract?
https://www.quora.com/Quantum-Field-Theory/Quantum-Field-Theory-Why-do-particles-of-odd-integer-spin-generate-forces-which-can-be-both-attractive-and-repulsive-whereas-particles-of-even-integer-spin-only-attract
http://www.answersingenesis.org/articles/cm/v22/n3/gravity
http://www.focus.org.uk/gravity.php
http://evolutionfacts.com/Ev-V1/1evlch04.htm
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
https://reasonandscience.catsboard.com/t1366-the-force-of-gravity-evidence-of-fine-tuning
ROBIN COLLINS The Teleological Argument: An Exploration of the Fine-Tuning of the Universe 2009
Gravity is a long-range attractive force between all material objects, whose strength increases in proportion to the masses of the objects and falls off with the inverse square of the distance between them. Consider what would happen if there were no universal, long-range attractive force between material objects, but all the other fundamental laws remained (as much as possible) the same. If no such force existed, then there would be no stars, since the force of gravity is what holds the matter in stars together against the outward forces caused by the high internal temperatures inside the stars. This means that there would be no long-term energy sources to sustain the evolution (or even existence) of highly complex life. Moreover, there probably would be no planets, since there would be nothing to bring material particles together, and even if there were planets (say because planet-sized objects always existed in the universe and were held together by cohesion), any beings of significant size could not move around without floating off the planet with no way of returning. This means that physical life could not exist. For all these reasons, a universal attractive force such as gravity is required for life.
The constants of physics are fundamental numbers that, when plugged into the laws of physics, determine the basic structure of the universe. An example of a fundamental constant is Newton’s gravitational constant G, which determines the strength of gravity via Newton’s law. We will say that a constant is fine-tuned if the width of its life-permitting range, Wr, is very small in comparison to the width, WR, of some properly chosen comparison range: that is, Wr/WR << 1.
Fine-tuning of gravity
Using a standard measure of force strengths – which turns out to be roughly the relative strength of the various forces between two protons in a nucleus – gravity is the weakest of the forces, and the strong nuclear force is the strongest, being a factor of 10^40 – or 10 thousand billion, billion, billion, billion times stronger than gravity. Now if we increased the strength of gravity a billionfold, for instance, the force of gravity on a planet with the mass and size of the Earth would be so great that organisms anywhere near the size of human beings, whether land-based or aquatic, would be crushed. (The strength of materials depends on the electromagnetic force via the fine-structure constant, which would not be affected by a change in gravity.) Even a much smaller planet of only 40 ft in diameter – which is not large enough to sustain organisms of our size – would have a gravitational pull of 1,000 times that of Earth, still too strong for organisms of our size to exist. As astrophysicist Martin Rees notes, “In an imaginary strong gravity world, even insects would need thick legs to support them, and no animals could get much larger”. Consequently, such an increase in the strength of gravity would render the existence of embodied life virtually impossible and thus would not be life-permitting in the sense that we defined. Of course, a billionfold increase in the strength of gravity is a lot, but compared with the total range of the strengths of the forces in nature (which span a range of 10^40), it is very small, being one part in 10 thousand, billion, billion, billion. Indeed, other calculations show that stars with lifetimes of more than a billion years, as compared with our Sun’s lifetime of 10 billion years, could not exist if gravity were increased by more than a factor of 3,000. This would significantly inhibit the occurrence of embodied life. The case of fine-tuning of gravity described is relative to the strength of the electromagnetic force, since it is this force that determines the strength of materials – for example, how much weight an insect leg can hold; it is also indirectly relative to other constants – such as the speed of light, the electron and proton mass, and the like – which help determine the properties of matter. There is, however, a fine-tuning of gravity relative to other parameters. One of these is the fine-tuning of gravity relative to the density of mass-energy in the early universe and other factors determining the expansion rate of the Big Bang – such as the value of the Hubble constant and the value of the cosmological constant. Holding these other parameters constant, if the strength of gravity were smaller or larger by an estimated one part in 10^60 of its current value, the universe would have either exploded too quickly for galaxies and stars to form, or collapsed back on itself too quickly for life to evolve. The lesson here is that a single parameter, such as gravity, participates in several different fine-tunings relative to other parameters.
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.696.63&rep=rep1&type=pdf
CALUM MILLER Defence of the fine-tuning argument JULY 25, 2017
The gravitational constant
Gravity is a relatively weak force, just 1/1040 of the strength of the strong nuclear force. And it turns out that this relative weakness is crucial for life. Consider an increase in its strength by a factor of 109: in this kind of world, any organism close to our size would be crushed. Compare then, Astronomer Royal Martin Rees’ statement that “In an imaginary strong gravity world, even insects would need thick legs to support them, and no animals could get much larger”. If the force of gravity were this strong, a planet which had a gravitational pull one thousand times the size of Earth’s would only be twelve metres in diameter – and it is inconceivable that even this kind of planet could sustain life, let alone a planet any bigger.
Now, a billion-fold increase seems like a large increase – indeed it is, compared to the actual value of the gravitational constant. But there are two points to be noted here. Firstly, that the upper life-permitting bound for the gravitational constant is likely to be much lower than 109 times the current value. Indeed, it is extraordinarily unlikely that the relevant kind of life, viz. embodied moral agents, could exist with the strength of gravity being any more than 3,000 times its current value, since this would prohibit stars from lasting longer than a billion years (compared with our sun’s current age of 4.5 billion years). Further, relative to other parameters, such as the Hubble constant and cosmological constant, it has been argued that a change in gravity’s strength by “one part in 10^60 of its current value” would mean that “the universe would have either exploded too quickly for galaxies and stars to form, or collapsed back in on itself too quickly for life to evolve.” But secondly, and more pertinently, both these increases are minute compared with the total range of force strengths in nature – the maximum known being that of the strong nuclear force. This does not seem to be any consistency in supposing that gravity could have been this strong; this seems like a natural upper bound to the potential strength of forces in nature. But compared to this, even a billion-fold increase in the force of gravity would represent just one part in 1031 of the possible increases.
We do not have a comparable estimate for the lower life-permitting bound, but we do know that there must be some positive gravitational force, as demonstrated above. Setting a lower bound of 0 is even more generous to fine tuning detractors than the billion-fold upper limit, but even these give us an exceptionally small value for Wr/WR, in the order of 1/10^31.
https://calumsblog.com/2017/07/25/full-defence-of-the-fine-tuning-argument-part-4/
Leonard Susskind The Cosmic Landscape: String Theory and the Illusion of Intelligent Design 2006, page 184
Another essential requirement for life is that gravity be extremely weak. In ordinary life gravity hardly seems weak. Indeed, as we age, the daily prospect of fighting gravity gets more and more daunting. I can still hear my grandmother saying, “Oy vey, I feel like a thousand pounds.” But I don’t ever recall hearing her complain about electric forces or nuclear forces. Nonetheless, if you compare the electric force between the nucleus and an atomic electron with the gravitational force, you would find the electric force is about 10^41 times larger. Where did such a huge ratio come from? Physicists have some ideas, but the truth is that we really don’t know the origin of this humongous discrepancy between electricity and gravity despite the fact that it is so central to our existence. But we can ask what would have happened if gravity had been a little stronger than it is. The answer again is that we would not be here to talk about it. The increased pressure due to stronger gravity would cause stars to burn much too fast— so fast that life would have no chance to evolve. Even worse, black holes would have consumed everything, dooming life long before it began. The large gravitational pull might even have aborted the Hubble expansion and caused a big crunch very shortly after the Big Bang.
https://3lib.net/book/2472017/1d5be1
Brad Lemley Why is There Life? November 01, 2000
N, the ratio of the electromagnetic force to the gravitational force between a pair of protons, is approximately 1036. According to Rees, if it were significantly smaller, only a small and short-lived universe could exist
N, equal to 1,000,000,000,000,000,000,000,000,000,000,000,000. The number measures the strength of the forces that hold atoms together divided by the force of gravity between them. It means that gravity is vastly weaker than intra-atomic attraction. If the number were smaller than this vast amount, "only a short-lived, miniature universe could exist," says Rees.
Omega, which measures the density of material in the universe— including galaxies, diffuse gas, and dark matter. The number reveals the relative importance of gravity in an expanding universe. If gravity were too strong, the universe would have collapsed long before life could have evolved. Had it been too weak, no galaxies or stars could have formed.
https://web.archive.org/web/20140722210250/http://discovermagazine.com/2000/nov/cover/
Luke A. Barnes The Fine-Tuning of the Universe for Intelligent Life 7 Jun 2012
If gravity were repulsive rather than attractive, then matter wouldn’t clump into complex structures. Remember: your density, thank gravity, is 10^30 times greater than the average density of the universe.
https://arxiv.org/abs/1112.4647
Kelly James Clark: Religion and the Sciences of Origins: Historical and Contemporary Discussions 2014
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 10^36 of the total range of forces”. 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.
https://3lib.net/book/2383970/12b021
Guillermo Gonzalez, Jay W. Richards: The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery 2004 page 216
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 underway.” 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
Stephen C. Meyer: The return of the God hypothesis, page 173
The ratio of the electromagnetic force to gravity must be accurate to 1 part in 10^40 . Were this ratio a bit higher, the gravitational attraction would be too strong in comparison to the contravening force of electromagnetism pushing nuclei apart. In that case, stars would, again, burn too quickly and unevenly to allow for the formation of long-lived stars and stable solar systems. Were this ratio a bit lower, gravitational attraction would be too weak in comparison to electromagnetism. That would have prevented stars from burning hot enough to produce the heavier elements needed for life.
Mario Livio Fine-Tuning, Complexity, and Life in the Multiverse 2018
Constraints on Gravity
As numerical simulations of structure formation in the universe have demonstrated, gravity enhances density fluctuations. In our universe, gravity caused the denser regions to lag behind the cosmic expansion and to form the sponge-like structure that characterizes the universe on its largest scales. Eventually, gravity led to the formation of galaxies at the density peaks, of stars, and of planets. Stellar evolution also represents one continuous battle with gravity, the latter pushing the stellar central densities and temperatures to higher and higher values. On the surface of planets gravity played crucial roles in keeping an atmosphere bound and in bringing different elements into contact to initiate the chemical reactions that eventually led to life. But gravity in our universe is a very weak force — the ratio of the repulsive electric force between two protons to their gravitational mutual attraction is e2/Gm2 p ⁓ 1036. The reason gravity becomes important on the scale of large asteroids and higher, is that large objects have a net electric charge that is close to zero, so gravity wins once sufficiently many atoms are packed together. Figure 1 allows us to make a first attempt to examine what would happen in a universe in which the values of some “constants of nature” are different. How would Figure 1 be different if gravity were not so weak? The general structure of the diagram would remain the same, but there would be fewer powers of ten between the subatomic and the cosmic scales. Stars, which effectively are gravitationally bound nuclear fusion reactors, would be smaller in such a universe and would have shorter lives. If gravity were much stronger, then even small solid bodies (such as rocks) might be gravitationally crushed. If gravity’s strength were such that it would still have allowed tiny planets to exist, life forms the size of humans would be crushed on the planetary surface. Overall, the universe would be much smaller and there would be less time for complexity to emerge. In other words, to have what we may call an “interesting” universe (in the sense of complexity), we must have many powers of ten between the microscale and the cosmic scale, and this requires gravity to be very weak. It is important to note, however, that gravity does not need to be fine-tuned for complexity to emerge. In fact, a universe in which gravity is ten times weaker than in our universe, may be even more “interesting” in that it would allow bigger stars and planes, and more time for life to emerge and evolve.
https://arxiv.org/ftp/arxiv/papers/1801/1801.06944.pdf
Dr. Walter L. Bradley: Is There Scientific Evidence for the Existence of God? How the Recent Discoveries Support a Designed Universe 20 August 2010
Balancing Gravity and Electromagnetism Forces - Fine Tuning Our Star and Its Radiation
The electromagnetic force is 1038 times stronger than the gravity force. Gravity draws hydrogen into stars, creating a high temperature plasma. The protons in the plasma must overcome their electromagnetic repulsion to fuse. Thus the relative strength of the gravity force to the electromagnetic force determines the rate at which stars "burn" by fusion. If this ratio of strengths were altered to1032 instead of 1038 (i.e., if gravity were much stronger), stars would be a billion times less massive and would burn a million times faster.
https://web.archive.org/web/20110805203154/http://www.leaderu.com/real/ri9403/evidence.html#ref21
New Scientist: Gravity mysteries: Why does gravity only pull? 10 June 2009
All the other forces in nature have opposites. In the case of the electromagnetic force, for example, it can attract or repel, depending on the charges of the bodies involved. So what makes gravity different?
The answer seems to lie with quantum field theory. The particles that transmit the strong, weak and electromagnetic forces have various types of charge, such as electric or colour charge. “Those charges can be either positive or negative, leading to different possibilities for the sign of the force,” says Frank Wilczek of the Massachusetts Institute of Technology. This is not the case with gravitons, the hypothetical particles that quantum field theory says should transmit gravity. “Gravitons respond to energy density, which is always positive,” says Wilczek. Or are we assuming too much here? “We don’t know that gravity is strictly an attractive force,” cautions Paul Wesson of the University of Waterloo in Ontario, Canada. He points to the “dark energy” that seems to be accelerating the expansion of the universe, and suggests it may indicate that gravity can work both ways. Some physicists speculate that dark energy could be a repulsive gravitational force that only acts over large scales. “There is precedent for such behaviour in a fundamental force,” Wesson says. “The strong nuclear force is attractive at some distances and repulsive at others.”
Either way, the apparent difference between gravity and the other fundamental forces poses a problem for physicists.
https://www.newscientist.com/article/mg20227122-800-gravity-mysteries-why-does-gravity-only-pull/
Natalie Wolchover Why Gravity Is Not Like the Other Forces June 15, 2020
Physicists have traced three of the four forces of nature — the electromagnetic force and the strong and weak nuclear forces — to their origins in quantum particles. But the fourth fundamental force, gravity, is different.
Our current framework for understanding gravity, devised a century ago by Albert Einstein, tells us that apples fall from trees and planets orbit stars because they move along curves in the space-time continuum. These curves are gravity. According to Einstein, gravity is a feature of the space-time medium; the other forces of nature play out on that stage. But near the center of a black hole or in the first moments of the universe, Einstein’s equations break. Physicists need a truer picture of gravity to accurately describe these extremes. This truer theory must make the same predictions Einstein’s equations make everywhere else. Physicists think that in this truer theory, gravity must have a quantum form, like the other forces of nature. Researchers have sought the quantum theory of gravity since the 1930s. They’ve found candidate ideas — notably string theory, which says gravity and all other phenomena arise from minuscule vibrating strings — but so far these possibilities remain conjectural and incompletely understood. A working quantum theory of gravity is perhaps the loftiest goal in physics today. What is it that makes gravity unique? What’s different about the fourth force that prevents researchers from finding its underlying quantum description? We asked four different quantum gravity researchers. We got four different answers.
Gravity Breeds Singularities
Claudia de Rham, a theoretical physicist at Imperial College London, has worked on theories of massive gravity, which posit that the quantized units of gravity are massive particles:
Einstein’s general theory of relativity correctly describes the behavior of gravity over close to 30 orders of magnitude, from submillimeter scales all the way up to cosmological distances. No other force of nature has been described with such precision and over such a variety of scales. With such a level of impeccable agreement with experiments and observations, general relativity could seem to provide the ultimate description of gravity. Yet general relativity is remarkable in that it predicts its very own fall. General relativity yields the predictions of black holes and the Big Bang at the origin of our universe. Yet the “singularities” in these places, mysterious points where the curvature of space-time seems to become infinite, act as flags that signal the breakdown of general relativity. As one approaches the singularity at the center of a black hole, or the Big Bang singularity, the predictions inferred from general relativity stop providing the correct answers. A more fundamental, underlying description of space and time ought to take over. If we uncover this new layer of physics, we may be able to achieve a new understanding of space and time themselves. If gravity were any other force of nature, we could hope to probe it more deeply by engineering experiments capable of reaching ever-greater energies and smaller distances. But gravity is no ordinary force. Try to push it into unveiling its secrets past a certain point, and the experimental apparatus itself will collapse into a black hole.
Gravity Leads to Black Holes
Daniel Harlow, a quantum gravity theorist at the Massachusetts Institute of Technology, is known for applying quantum information theory to the study of gravity and black holes:
Black holes are the reason it’s difficult to combine gravity with quantum mechanics. Black holes can only be a consequence of gravity because gravity is the only force that is felt by all kinds of matter. If there were any type of particle that did not feel gravity, we could use that particle to send out a message from the inside of the black hole, so it wouldn’t actually be black. The fact that all matter feels gravity introduces a constraint on the kinds of experiments that are possible: Whatever apparatus you construct, no matter what it’s made of, it can’t be too heavy, or it will necessarily gravitationally collapse into a black hole. This constraint is not relevant in everyday situations, but it becomes essential if you try to construct an experiment to measure the quantum mechanical properties of gravity.
Our understanding of the other forces of nature is built on the principle of locality, which says that the variables that describe what’s going on at each point in space — such as the strength of the electric field there — can all change independently. Moreover, these variables, which we call “degrees of freedom,” can only directly influence their immediate neighbors. Locality is important to the way we currently describe particles and their interactions because it preserves causal relationships: If the degrees of freedom here in Cambridge, Massachusetts, depended on the degrees of freedom in San Francisco, we may be able to use this dependence to achieve instantaneous communication between the two cities or even to send information backward in time, leading to possible violations of causality.
The hypothesis of locality has been tested very well in ordinary settings, and it may seem natural to assume that it extends to the very short distances that are relevant for quantum gravity (these distances are small because gravity is so much weaker than the other forces). To confirm that locality persists at those distance scales, we need to build an apparatus capable of testing the independence of degrees of freedom separated by such small distances. A simple calculation shows, however, that an apparatus that’s heavy enough to avoid large quantum fluctuations in its position, which would ruin the experiment, will also necessarily be heavy enough to collapse into a black hole! Therefore, experiments confirming locality at this scale are not possible. And quantum gravity therefore has no need to respect locality at such length scales.
Indeed, our understanding of black holes so far suggests that any theory of quantum gravity should have substantially fewer degrees of freedom than we would expect based on experience with the other forces. This idea is codified in the “holographic principle,” which says, roughly speaking, that the number of degrees of freedom in a spatial region is proportional to its surface area instead of its volume.
Gravity Creates Something From Nothing
Juan Maldacena, a quantum gravity theorist at the Institute for Advanced Study in Princeton, New Jersey, is best known for discovering a hologram-like relationship between gravity and quantum mechanics:
Particles can display many interesting and surprising phenomena. We can have spontaneous particle creation, entanglement between the states of particles that are far apart, and particles in a superposition of existence in multiple locations. In quantum gravity, space-time itself behaves in novel ways. Instead of the creation of particles, we have the creation of universes. Entanglement is thought to create connections between distant regions of space-time. We have superpositions of universes with different space-time geometries.
Furthermore, from the perspective of particle physics, the vacuum of space is a complex object. We can picture many entities called fields superimposed on top of one another and extending throughout space. The value of each field is constantly fluctuating at short distances. Out of these fluctuating fields and their interactions, the vacuum state emerges. Particles are disturbances in this vacuum state. We can picture them as small defects in the structure of the vacuum.
When we consider gravity, we find that the expansion of the universe appears to produce more of this vacuum stuff out of nothing. When space-time is created, it just happens to be in the state that corresponds to the vacuum without any defects. How the vacuum appears in precisely the right arrangement is one of the main questions we need to answer to obtain a consistent quantum description of black holes and cosmology. In both of these cases there is a kind of stretching of space-time that results in the creation of more of the vacuum substance.
Gravity Can’t Be Calculated
Sera Cremonini, a theoretical physicist at Lehigh University, works on string theory, quantum gravity and cosmology: There are many reasons why gravity is special. Let me focus on one aspect, the idea that the quantum version of Einstein’s general relativity is “nonrenormalizable.” This has implications for the behavior of gravity at high energies. In quantum theories, infinite terms appear when you try to calculate how very energetic particles scatter off each other and interact. In theories that are renormalizable — which include the theories describing all the forces of nature other than gravity — we can remove these infinities in a rigorous way by appropriately adding other quantities that effectively cancel them, so-called counterterms. This renormalization process leads to physically sensible answers that agree with experiments to a very high degree of accuracy. The problem with a quantum version of general relativity is that the calculations that would describe interactions of very energetic gravitons — the quantized units of gravity — would have infinitely many infinite terms. You would need to add infinitely many counterterms in a never-ending process. Renormalization would fail. Because of this, a quantum version of Einstein’s general relativity is not a good description of gravity at very high energies. It must be missing some of gravity’s key features and ingredients.
However, we can still have a perfectly good approximate description of gravity at lower energies using the standard quantum techniques that work for the other interactions in nature. The crucial point is that this approximate description of gravity will break down at some energy scale — or equivalently, below some length. Above this energy scale, or below the associated length scale, we expect to find new degrees of freedom and new symmetries. To capture these features accurately we need a new theoretical framework. This is precisely where string theory or some suitable generalization comes in: According to string theory, at very short distances, we would see that gravitons and other particles are extended objects, called strings. Studying this possibility can teach us valuable lessons about the quantum behavior of gravity.
https://www.quantamagazine.org/why-gravity-is-not-like-the-other-forces-20200615/
Question: Why is the gravitational force always attractive? Of course, if gravity was repulsive then no large bodies like stars or planets could have formed - and consequently nu human would have been around to ask this question.
Answer: gravity is always attractive because nothing can have a negative mass value.
https://www.quora.com/Why-is-gravity-force-an-attractive-force-rather-than-repulsive
Gravity can be either attractive or repulsive, as explained in following paper:
Imanol Albarran What if gravity becomes really repulsive in the future? 24 March 2018
The Universe is evolving. Hubble's discovery was based on observing that the spectrum of far-away galaxies was red-shifted which implied that those galaxies were moving away from us. He even measured the galaxies radial outward velocities and realised that it followed a rule: (1) the velocities were proportional to the distances at which the galaxies were located from us and (2) the proportionality factor was a constant, the Hubble constant. About 70 years later, two independent teams realized that by measuring further objects, SNeIa, the Hubble constant was not quite constant, as was already expected. The issue was that the deviation from the constancy was not in the anticipated direction. It was no longer enough to invoke only matter to explain those observations. A new dark component had to be invoked, interacting as far as we know only gravitationally, and named dark energy. This component started recently fuelling a second inflationary era of the visible Universe. Of course, all these observations, and subsequent ones, are telling us how gravity behaves at cosmological scales through the kinematic expansion of our Universe .
This kinematic description is linked to the dynamical expansion through the gravitational laws of Einstein's theory. To a very good approximation, we may assume that our Universe is homogeneous and isotropic on large scales and that it is filled with matter (standard and dark) and dark energy.
Summarising, what we have shown is that after all gravity might behave the other way around in the future and, rather than the apple falling from the tree, the apple may fly from the earth surface to the branches of the tree, if dark energy is repulsive enough, as could already be indicated by current observations.
https://link.springer.com/article/10.1140/epjc/s10052-018-5728-x
Gravity is mediated by a spin 2 particle. Electromagnetism by spin 1.
even and odd spin do differ in that they require a product of charges with different signs to get attraction or repulsion:
Fermion or Boson? The Spin-Statistics Theorem
https://www.youtube.com/watch?v=I7EYDv7vbfg
Belt Trick
https://www.youtube.com/watch?v=JaIR-cWk_-o
spin even:
q1q2>0q1q2>0: attractive
q1q2<0q1q2<0: repulsive
spin odd:
q1q2<0q1q2<0: attractive
q1q2>0q1q2>0: repulsive
In the case of gravity, mediated by spin 2 particles, charge is mass, which is always positive. Thus, q1q2q1q2 is always greater than zero, and gravity is always attractive. For spin 0 force mediators, however, there is no restriction on the charges and you can very well have repulsive forces. A better rephrasing of the question is: "Why do particles of odd spin generate repulsive forces between like charges, while particles of even spin generate attractive forces between like charges?"
https://physics.stackexchange.com/questions/11542/why-is-the-gravitational-force-always-attractive
Quantum Field Theory: Why do particles of odd integer spin generate forces which can be both attractive and repulsive, whereas particles of even integer spin only attract?
https://www.quora.com/Quantum-Field-Theory/Quantum-Field-Theory-Why-do-particles-of-odd-integer-spin-generate-forces-which-can-be-both-attractive-and-repulsive-whereas-particles-of-even-integer-spin-only-attract
http://www.answersingenesis.org/articles/cm/v22/n3/gravity
http://www.focus.org.uk/gravity.php
http://evolutionfacts.com/Ev-V1/1evlch04.htm
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
Last edited by Otangelo on Sun Aug 01, 2021 12:08 pm; edited 32 times in total
