ElShamah - Reason & Science: Defending ID and the Christian Worldview
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ElShamah - Reason & Science: Defending ID and the Christian Worldview

Otangelo Grasso: This is my library, where I collect information and present arguments developed by myself that lead, in my view, to the Christian faith, creationism, and Intelligent Design as the best explanation for the origin of the physical world.

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The force of Gravity - evidence of fine tuning

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

Belt Trick

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?"

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?


2. ibid 197

Last edited by Otangelo on Sun Aug 01, 2021 12:08 pm; edited 32 times in total




Ethan Siegel The Greatest Unsolved Problem In Theoretical Physics  Dec 18, 2015
How the “hierarchy problem,” or why gravity is so much weaker than everything else, might be the key to the entire Universe.

“I just think too many nice things have happened in string theory for it to be all wrong. Humans do not understand it very well, but I just don’t believe there is a big cosmic conspiracy that created this incredible thing that has nothing to do with the real world.” –Ed Witten

Our standard model of elementary particles and forces has recently become as close to “complete” as we could conceivably ask for. Every single one of the elementary particles — in all their different conceivable incarnations — has been created in the lab, measured, and had its properties determined. The last holdouts, the top quark and antiquark, the tau neutrino and antineutrino, and finally the Higgs boson, have all fallen prey to our detection capabilities at last. That last one, in particular — the Higgs — solved a long-standing problem in physics: finally, we can confidently explain where these elementary particles each get their rest mass from!

The force of Gravity - evidence of fine tuning 0*qwiwzr3iUXGoN7Wj

That’s great and all, but it’s not like science ends now that we’ve finished that part of the puzzle. Rather, there are important follow-up questions, and one that we can always ask is, “what comes next?” When it comes to the standard model, we still don’t have everything figured out. One thing in particular stands out to most physicists: to find it, I’d like you to consider the following property of the standard model.

On the one hand, the weak, electromagnetic, and strong forces can all be quite important, depending on the energy and distance scales of the interaction in question.
But gravitation? Not so much.
If you’ve ever had the opportunity to read this fabulous book by Lisa Randall, she writes at great length about this puzzle, which I would call the greatest unsolved problem in theoretical physics: the Hierarchy Problem.

The force of Gravity - evidence of fine tuning 0*RApUgf83jrBq5VO3

What we can do is take any two fundamental particles — of any mass and any of the forces through which they interact — and find that gravity is literally forty orders of magnitude weaker than all the other known forces in the Universe. That means the gravitational force is a factor of 10⁴⁰ weaker than the other three forces. For example, even though they’re not fundamental, if you placed two protons a single meter apart, the electromagnetic repulsion between them would be approximately 10⁴⁰ times stronger than the gravitational attraction. Or, and I’ll write it out just this once, we’d need to increase the force of gravity’s strength by 10,000,000,000,000,000,000,000,000,000,000,000,000,000 in order to have its strength be comparable to the other known forces.
You can’t just “make” a proton weigh 10^²⁰ times as much as it would normally; that’s what it would take to make gravity bring two protons together, overcoming the electromagnetic force. Instead, if you want to make a reaction like the one above happen spontaneously, where protons do overcome their electromagnetic repulsion, you need something like 10⁵⁶ protons all together. Only by collecting that many of them, under their combined force of gravity, can you overcome electromagnetism and bring these particles together. As it turns out, 10⁵⁶ protons is approximately the minimum mass of a successful star. That’s a description of the way our Universe works, but we don’t understand why. Why is gravity so much weaker than all the other forces? Why is the “gravitational charge” (i.e., mass) so much weaker than the electric or color charge, or even than the weak charge, for that matter?
That’s what the Hierarchy Problem is, and that problem is by many measures the greatest unsolved problem in physics. We don’t know the answer, but we’re not completely in the dark on this. Theoretically, we have some good ideas as to what the solution might be, and a tool to help us investigate whether any of these possibilities could be correct.

So far, the Large Hadron Collider — the highest-energy particle collider ever developed — has reached unprecedented energies under laboratory conditions here on Earth, collecting huge amounts of data and reconstructing exactly what took place at the collision points. This includes the creation of new, never-before-seen particles (like the Higgs, which the LHC discovered), our old, familiar standard model particles (quarks, leptons, and gauge bosons), and it can — if they exist — produce any other particles that may exist beyond the standard model. There are four conceivable ways — i.e., four good ideas — that I am aware of to solve the hierarchy problem. The good news for experiment is that if any of these solutions are the one that nature has chosen, the LHC should find it! (And if not, we’ll need to keep searching.) Other than the single Higgs boson whose discovery was announced three years ago now, no new fundamental particles have been found at the LHC. (Not only that, but there are no compelling new candidate particles that have emerged, either.) Furthermore, the particle that was found was completely consistent with the standard model Higgs; there is no statistically significant result that strongly suggests any new physics has been observed beyond the standard model. Not for a composite Higgs, not for multiple Higgs particles, not for un-standard-model-like decays, not anything of that sort.

But we've begun taking data at even higher energies — up to 13/14 TeV from just half that — to try and find out even more. With this in mind, what are the possible, reasonable solutions to the hierarchy problem that we’re poised to explore?

One of the striking things about electromagnetism and gravitation is the vast disparity in their relative strengths. In a normal hydrogen atom, the single electron is bound to the single proton by electrical attraction. But another source of attraction is at work here too—gravitation. It is easy to calculate the relative strengths of these two forms of attraction. It turns out that the electrical force is about 10^40 times stronger than the gravitational force. Clearly, then, gravity is extraordinarily weak compared with electromagnetism.

Years ago, Brandon Carter found an amazing relationship between the unexplained ratio 10^40 and the properties of stars. Every star must transport heat from the nuclear furnace in its core to the surface, where it radiates into space. Heat can flow in two ways: by radiation, in which photons convey the energy, and by convection, in which hot gas from deep down rises to the surface, bringing heat with it. Our sun has a convective outer layer, and through a telescope, its surface looks like a boiling maelstrom.Larger stars rely on radiative transfer of heat rather than convection, and this is thought to be important in creating the conditions that lead to supernova explosions. Because both planets and supernovas are a major part of the life story, it is important for the universe to contain a selection of both radiative and convective stars. Carter discovered from the theory of stellar structure that to get both sorts of stars, the ratio of the strengths of the electromagnetic and gravitational forces needs to be very close to the observed value of 10^40. If gravity were a bit stronger, all stars would be radiative and planets might not form; if gravity were somewhat weaker, all stars would be convective and supernovas might never happen. Either way, the prospects for life would be diminished.




Gravity: The Cosmic Architect

Gravity, the subtlest yet most pervasive force in the universe, stitches the fabric of the cosmos together, maintaining from the orbits of planets to the grand structures of galaxies. As described by Einstein's General Theory of Relativity, gravity is more than a force; it's the curvature of spacetime itself, dictated by the mass and energy within it. While \( E = mc^2 \) introduces us to the relationship between mass and energy, the true essence of gravity lies in the field equations of General Relativity, which reveal the interaction between mass energy and the geometry of spacetime. This gravitational force, although the weakest among the fundamental forces, holds the cosmos in its embrace. Without it, the architectural marvels of the universe, from the tiniest satellites orbiting Earth to the vast Milky Way, would not exist. Gravity ensures that everything, from a grain of sand to the Moon, remains anchored, preventing the celestial and terrestrial from drifting into the void. 

Envision a universe where gravity's pull is magnified, a world where its gentle caress turns into an unyielding grasp. Such a universe would be starkly different from our own, with stars burning fiercely and fleeting, their lives too short to foster complex life on surrounding planets. The very act of standing on a planet like Earth would become an insurmountable task, as the increased gravitational force would bind all life forms tightly to the surface, rendering movements like leaping or flying fantastical. This alternate reality underscores the fine-tuning of gravity in our universe. It's a delicate balance that allows stars to shine, planets to form, and life to flourish. The gravitational constant, G, is a testament to this precision. Even a slight increase could render life impossible, crushing the potential for complexity under the weight of an overwhelming force. Astrophysicist Martin Rees's reflections on a high-gravity world, where even insects would need substantial support to bear their own weight, highlight the fine line between existence and extinction. This fine-tuning extends beyond the comparison with the electromagnetic force; it is intricately woven with the very birth of the universe. The early universe's mass-energy density, the Hubble constant, and the cosmological constant all play roles in this cosmic symphony. A minuscule deviation in gravity's strength could either halt the universe's expansion prematurely or prevent it altogether, stifling the emergence of life before it begins. In this delicate cosmic balance, gravity emerges not merely as a force but as a cornerstone of existence. 

Historical context

The story behind English physicist Isaac Newton's (1642-1727) discovery of gravitational force is one of the most fascinating in all of science. It begins in ancient Greece, in the period from the sixth to the third century BC. During this time, a number of Greek philosophers attempted to explain common observations from the natural world, such as the fact that most objects fall to the ground if they are not lifted in some way. Among the explanations developed for this trend was offered by the Greek philosopher Aristotle (384-322 BC). Aristotle developed a grand scheme of natural philosophy asserting that all objects "belonged" to some place or other. Heat belonged to the atmosphere because it originally came from the Sun (as Aristotle taught). For this reason, the heat increases. Objects fall toward the Earth's surface, Aristotle said, because that's where "terrestrial" objects belong. Aristotle's philosophy was an attempt to explain why objects fall. Aristotle's philosophy dominated the thinking of European scholars for nearly 2,000 years. Then, in the 16th century, the Italian physicist Galileo Galilei (1564-1642) suggested another way of answering questions in science. Scientists shouldn't bother trying to understand why things happen in the natural world, Galileo said. Instead, they should focus solely on describing how things occur. Galileo also taught that the way to learn more about the natural world is not just to think logically about it, but to carry out experiments that produce measurable results. One of the most famous experiments attributed to Galileo was the one he performed at the Leaning Tower of Pisa. He is said to have dropped two balls from the top of the tower and discovered that they took the same amount of time to hit the ground.  Galileo's greatest achievements were not in defining the true nature of gravity, but in laying the groundwork for the work of Isaac Newton, who was born the year Galileo died. Newton's achievements in the field of gravity are also associated with a famous story. Legend has it that Newton was hit in the head by an apple falling from a tree. This event made him question the force between two objects on Earth (the apple and the earth) and the force between two objects in the universe (the force between a planet and the Sun). Gravity on Earth and in the heavens. The connection between the gravitational forces on Earth and in the heavens is very important. Measuring the strength of gravity on Earth is very difficult for a simple reason. Suppose we want to measure what happens when an object falls to Earth. In terms of gravity, what actually happens is that the object and planet Earth attract each other. The object moves downward toward the Earth, and the Earth moves upward toward the object. The problem is that the Earth is so much larger than the object that it is impossible to see any movement on the planet's part. The situation is quite different in the skies. The reason planets travel in orbit around the Sun, Newton said, is that they are responding to two forces. A force is caused simply by its movement through the heavens. Just imagine that at some point in the past, someone grabbed Mars and threw it towards the Sun. Mars would be traveling through space, because of the initial speed given to it. But Mars wouldn't travel in a straight line. It moves in a circle (or nearly a circle) around the Sun What changes Mars' motion from a straight line to a curve, Newton wondered. The answer he proposed was gravity. The gravitational force between the Sun and Mars causes the planet to move out of a straight line and toward the Sun. The combination of linear motion and the force of gravity then represents the shape of Mars' orbit.

The Gravitational Constant and the Fabric of Life

In the vast expanse of the cosmos, gravity's subtle influence extends from the atomic to the astronomical scale. Despite being the weakest of the fundamental forces—over 40 orders of magnitude weaker than the strong nuclear force—gravity's role is paramount in shaping the universe and fostering the conditions necessary for life. The electromagnetic force, which binds atoms together, dwarfs gravitational attraction by a staggering factor of approximately 10^{36}. This immense disparity ensures that atomic structures remain stable and resilient against the comparatively gentle pull of gravity. Astronomer Royal Martin Rees highlights the delicate balance of forces, noting that a significant decrease in this ratio would confine the universe to a fleeting and diminutive state, incapable of supporting complex structures or life as we know it. The parameter Omega offers another perspective on the cosmic balance, measuring the universe's material density, including galaxies, diffuse gas, and dark matter. The precise calibration of gravitational strength ensures that the universe's expansion neither halts prematurely in a catastrophic collapse nor dissipates too swiftly for the formation of stars and galaxies. Gravity's fine-tuning is evident in its ability to orchestrate the cosmos's evolution, from the fiery aftermath of the Big Bang to the serene glow of starlight. The range of gravitational forces conducive to life is a mere sliver, one part in 10^{36}, of the spectrum of possible forces. This narrow window allows for the formation of stars, galaxies, and planetary systems, setting the stage for the chemistry of life. The gravitational constant, G, embodies this fine-tuning. This intricate balance underscores the exceptional nature of our universe, where gravity, despite its relative weakness, plays a crucial role in life's cosmic ballet. The interplay of forces, so finely adjusted, invites reflection on the origins of such precise tuning and the remarkable emergence of life within the vastness of space.

How fine-tuned is Gravity?

To analyze the fine-tuning of the gravitational force in the context of an observationally allowed parameter space, we need to consider the constraints imposed by various cosmological observations and measurements. These observations essentially define a narrow range or "window" within which the strength of gravity must lie for the universe to be consistent with our current understanding and observations. The key observational constraints that define this allowed parameter space for the strength of gravity (or the gravitational constant G) come from:

Cosmic Microwave Background (CMB) observations: The precise measurements of the temperature anisotropies and polarization patterns in the CMB put stringent limits on the value of G during the early universe and its impact on the formation of large-scale structures.
Big Bang Nucleosynthesis (BBN): The abundances of light elements (such as hydrogen, helium, and lithium) produced in the first few minutes after the Big Bang are sensitive to the strength of gravity, as it affects the expansion rate and nuclear reaction rates during that epoch.
Large-Scale Structure formation: The growth and evolution of galaxies, clusters, and the overall matter distribution in the universe are influenced by the strength of gravity, which governs the gravitational collapse and clustering of matter.
Precision tests of General Relativity: Various experiments and observations in the Solar System, such as the precession of Mercury's orbit, the deflection of light by the Sun, and the Shapiro time delay, place constraints on deviations from General Relativity and, consequently, on the value of G.

G, has its observed value measured with incredible precision as 6.67430 × 10^-11 N m^2 kg^-2. This value is extraordinarily special in the context of an unbounded parameter space, where G could theoretically take any real value, positive or negative, across an infinite range of possibilities. Even the most minuscule deviations from this value would have profound and catastrophic consequences for the universe, likely precluding the existence of intelligent life and the cosmic structures we observe. Detailed studies indicate that for a life-permitting universe, G must lie within an astonishingly narrow range around its observed value, no more than one part in 10^36 above or below. If G were larger than its current value by merely one part in 10^36, the gravitational force would be too strong, causing stars to be ripped apart by tidal forces before nuclear fusion could occur. If G were smaller by one part in 10^36, the gravitational pull would be too feeble for galaxies and planetary systems to form, as the attractive force would be overwhelmed by other forces.

When we say the allowable deviation is one part in 10^36, we are referring to the precision needed relative to the value itself, not to a specific range. This means that the deviation must be extremely small compared to the value set.

Concrete Example: Let's use a simplified example involving a physical constant, like the gravitational constant  G:

Target Value: Suppose the target value for G  is   6.67430 × 10^-11 N m^2 kg^-2
Required Precision:  The required precision is one part in 10^36.
Calculating the Allowable Deviation: The permitted deviations for the gravitational constant (G) from its observed value of 6.67430 × 10^-11 N m^2 kg^-2 are within one part in 10^36 above or below this value.
In extended form, the numerical value of one part in 10^36 can be written as: 0.0000000000000000000000000000000000001

Therefore, the permitted range for G in a life-permitting universe is: 6.67430000000000000000000000000000000000001 × 10^-11 N m^2 kg^-2 to 6.67429999999999999999999999999999999999999 × 10^-11 N m^2 kg^-2
Any deviation from this incredibly narrow range, even by the smallest amount, would have catastrophic consequences for the formation and existence of cosmic structures and intelligent life.

The allowable deviation is extremely small compared to the target value itself. In the context of the gravitational constant, this means setting the value of  G so precisely that the deviation is smaller than 1 part in 10^36 of its value. The analogy of an infinite dial helps to illustrate that this precision is about the relative accuracy needed, not about being within a finite range.

This life-permitting range for G is infinitesimally small compared to the vast infinite range of values it could theoretically take. It is akin to threading an infinitely fine cosmic needle. Any slight deviation from the observed value would rapidly lead to a universe utterly antithetical to life, devoid of stars, galaxies, and the intricate chemistry upon which biology depends. Furthermore, the value of G is intimately linked to other fundamental constants and parameters governing matter, energy, and their interactions. Deviations would disrupt this delicate balance, preventing the existence of complex structures from subatomic particles to celestial objects. Within this context of an unbounded parameter space, the gravitational constant being precisely calibrated to its observed value of 6.67430 × 10^-11 N m^2 kg^-2 is an extraordinary and improbable coincidence. It is as if the value of G has been carefully dialed and finely tuned to be this one unique number that allows the cosmos to unfold in a life-permitting way.   This level of precise fine-tuning within an infinite parameter space defies simple chance, underscoring the astonishingly special nature of the fundamental constants that have given rise to a universe conducive to life as we know it. It is a testament to the remarkable and exquisite calibration of the parameters governing our universe.

This analysis underscores the extraordinary precision with which the strength of gravity appears to be calibrated, not just based on conceptual arguments but also on rigorous observational constraints from various cosmological probes. The fact that gravity must be finely tuned to such an incredible degree to permit the existence of intelligent life, as dictated by the observationally allowed parameter space, further emphasizes the remarkable fine-tuning of the fundamental forces and constants in our universe.

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.
Reply: Gravity is fundamentally an attractive force, a characteristic that arises from the nature of mass and the behavior of the gravitational field. This inherent attraction is rooted in the properties of mass and the particles that mediate gravitational interactions. In our universe, mass is always positive. Unlike electric charge, which can be positive or negative and gives rise to both attractive and repulsive forces in electromagnetism, mass does not have a negative counterpart. This positivity of mass ensures that the gravitational force between any two masses is always attractive. The concept that "nothing can have a negative mass value" is central to understanding why gravity cannot be repulsive under ordinary circumstances.

Gravity is mediated by hypothetical particles known as gravitons, which, according to theoretical physics, have a spin of 2. This is important because the spin of a particle influences how forces behave. For instance, electromagnetism is mediated by photons, which have a spin of 1. The rule of thumb is that particles with an even spin (like 2) result in forces that are always attractive when involving like charges (in this case, mass), while particles with an odd spin (like 1) can result in both attractive and repulsive forces depending on the signs of the charges involved. Since mass is always positive, and gravity is mediated by a spin-2 particle, the interaction between any two masses will always be attractive. This is because the product of two positive masses (charges, in this context) is always positive, leading to attraction.

While gravity itself is always attractive, the accelerated expansion of the universe introduces a nuanced aspect of how gravity operates on a cosmological scale. Observations of distant galaxies and supernovae have revealed that the universe is not just expanding, but doing so at an accelerating rate. This phenomenon is attributed to dark energy, a mysterious form of energy that permeates all of space and exerts a repulsive effect on the large-scale structure of the universe. Dark energy interacts gravitationally and is leading to a "second inflationary era" of the visible universe, causing galaxies to move away from each other more rapidly. This repulsive effect, however, is not a direct property of gravity itself but rather a consequence of the presence of dark energy within the framework of Einstein's theory of general relativity. In essence, while gravity remains an attractive force, dark energy introduces a repulsive component to the overall dynamics of the universe's expansion.


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