# Understanding gravity

#### Quarinteen

Hello all. I was thinking about anti gravity and how it worked. I started to research then thought it would probably be best if I understood exactly how gravity worked. So I googled it, and it seems no one really knows how gravity works? I mean at a super low level. I would think if you knew exactly how gravity worked then it shouldn’t be hard to figure out how anti gravity works.

I keep reading it’s all about mass and distance, but I always assumed gravity was a bi product of our atmosphere. that the pressure held with in simply was just pushing harder. That’s why on the moon there is a lot less gravity because there is not enough pressure in the exosphere. I am curious if there has ever been a study on the atomic weight of the elements on the periodic table. Weigh them at the bottom of say Mount Everest then again at the top. Then I read that there were places on the planet where gravity isn’t consistent and wondered if anyone studied the same thing of weight and also maybe the amount of pressure with the electromagnetic field in the area? Can’t find any information about it.

#### Quarinteen

My thought on the last part is if anyone can figure out exactly how the gravity works in those areas it should make it easier to figure out how anti gravity would work

#### Hartmann352

According to Einstein's theory of General Relativity, gravity is a natural consequence of a mass’s influence on space.

I. Einstein, General Relativity and Gravity

Because mass and energy distort the shape of spacetime, the Euclidean geometry of standard textbooks cannot accurately describe it. Einstein’s general relativity uses more complicated math built on the non-Euclidean geometry devised in the 19th century by Bernhard Riemann. With help from his friend Marcel Grossmann, Einstein adopted further advances by the mathematicians Gregorio Ricci-Curbastro, Tullio Levi-Civita and Elwin Christoffel to describe spacetime geometry in terms of mathematical expressions called tensors. Tensors are similar to vectors — quantities, such as velocity, composed of two components (in velocity’s case, speed and direction). Tensors are similar but can encompass more than just two components. Einstein used tensors to develop his equation describing the gravitational field, known as the Einstein field equation.

Gμν = 8πTμν
On the left side of Einstein's Field Equation is a tensor describing the geometry of spacetime — the gravitational field. On the right is the tensor describing the matter and energy density — the source of the gravitational field. The equation shows that spacetime geometry equals mass-energy density when adjusted with the proper units and numerical constants. (Actually, the equation stands for a set of multiple equations owing to the complexity of tensors. So experts usually speak of the Einstein field equations, plural.)

Gμν + Λgμν = 8πTμν
When Einstein applied his Field Equation to the entire universe, Einstein found that the universe would be unstable, easily disturbed into a state in which spacetime would be either expanding or collapsing. So he added a term that came to be called the cosmological constant, symbolized by the Greek letter lambda. It represents a specific and constant amount of energy density throughout space that would supposedly keep the universe stable and changeless. Later, evidence that the universe was indeed expanding led Einstein to renounce lambda. But it has been revived by modern cosmologists to explain the apparent increase in the universe’s rate of expansion that was discovered in the late 1990s by Saul Perlmutter and his team.

In mid-1915 Einstein saw that there was a way to make relativity truly general. Rather than imposing energy-momentum conservation on the equations, he worked on devising equations that would impose the conservation law on the universe.

Einstein quickly realized that his new theory of gravity was really a theory of the cosmos. In 1917, he wrote a famous paper applying general relativity to the universe as a whole. Today that paper stands as the foundation for modern cosmology. But at the time, Einstein was troubled — his equations implied an unstable universe, either growing or collapsing. In those days, the universe was supposed to be eternal, everlasting and changeless. So Einstein altered his equation, adding a factor called the cosmological constant, representing a constant energy density in space that kept the universe static.

Others were not so sure. Alexander Friedmann, a Russian mathematician, developed a description of an expanding or contracting universe from Einstein’s original equations. Einstein first thought Friedmann to be in error, but then relented, although still viewing the “expanding universe” as of mathematical interest only. When Edwin Hubble’s analysis of light from distant galaxies confirmed the universe’s expansion, Einstein finally agreed to the principle. Despite his own reluctance to accept it, Einstein’s general relativity math did in fact imply what John A. Wheeler of Princeton later called the “most dramatic prediction that science has ever made” — the expansion of the universe.

For weak gravitational fields, the results of general relativity do not differ significantly from Newton’s law of gravitation. But for intense gravitational fields, the results diverge, and general relativity has been shown to predict the correct results. Even in our Sun’s relatively weak gravitational field at the distance of Mercury’s orbit, we can observe the effect. Starting in the mid-1800s, Mercury’s elliptical orbit has been carefully measured. However, although it is elliptical, its motion is complicated by the fact that the perihelion position of the ellipse slowly advances. Most of the advance is due to the gravitational pull of other planets, but a small portion of that advancement could not be accounted for by Newton’s law. At one time, there was even a search for a “companion” planet that would explain the discrepancy. But general relativity correctly predicts the measurements. Since then, many measurements, such as the deflection of light of distant objects by the Sun, have verified that general relativity correctly predicts the observations.

Rather than preventing the universe from collapse, the vacuum energy Einstein describes can explain why the universe now expands at an accelerating pace. General relativity, and it's cosmological constant, today forms the core science for analyzing the history of the universe and for forecasting its future.

But apart from its use in cosmology, general relativity was not widely applied to scientific problems in its first four decades. For the most part, general relativity languished in departments of mathematics, rarely studied in physics.

Shortly after Einstein introduced general relativity, Karl Schwarzschild calculated its implications for the gravity of a massive sphere. Schwarzschild determined that for any given mass there existed a “critical radius” — a limit, he believed, to how small that amount of mass could be compressed. In 1939, Einstein concluded that mass could not be compressed to within that “Schwarzschild radius.” But in the same year, J. Robert Oppenheimer, later to lead America's Manhattan Project for the development of the atomic bombs used against Japan during WWII, and Hartland Snyder, of Northwestern University and later Brookhaven Labs, calculated otherwise, claiming that a sufficiently massive object could indeed collapse within that radius, disappearing from view and leaving only its gravitational field behind.

At the time, nobody paid any attention. But in the 1960s, newfound astrophysical anomalies suggested that gravitational collapse was at work in the cosmos, and Oppenheimer and Snyder’s idea was revived as what came to be known as black holes. Famous for swallowing anything they encounter and allowing nothing to escape, black holes are probably the most bizarre astrophysical consequences of general relativity and gravity. Small black holes have been detected throughout space and supermassive black holes reside in the cores of most galaxies. And we now suspect that smaller black holes, even orbiting pairs of black holes, circle super massive black holes at the center of galaxies.

More recently black holes (schematic of one shown) have been used as thought-experiment laboratories for investigating several outstanding mysteries about the nature of space, time and gravity.

In creating general relativity, Einstein’s had to envision physical processes governing matter, space and time, while at the same time formulating abstract mathematical expressions corresponding to that reality. As a student, Einstein testified, he neglected mathematics. His intuition was not strong enough to guide him to the most profound of math’s many subfields. But in the physical realm of natural phenomena, “I soon learned to scent out that which was able to lead to fundamentals and to turn aside from … the multitude of things which clutter up the mind and divert it from the essential.” At first he didn’t realize that “a more profound knowledge of the basic principles of physics is tied up with the most intricate mathematical methods.” He learned that from his pursuit of general relativity.

Fantastic physical phenomena were first discovered not through the lenses of telescopes, but within the squiggles Einstein had scratched out on paper to make the world make sense — to him. And now physical nature makes sense to modern science only because of Einstein’s insights through thought experiments.

“Einstein’s ideas,” his friend the physicist Max Born wrote over half a century ago, “have given the physical sciences the impetus which has liberated them from outdated philosophical doctrine, and made them one of the decisive factors in the modern world of man.”

See:
University Physics Volume 1, Einstein's Theory of Gravity, Rice University.
"Einstein's genius changed science's perception pf gravity" by Tom Siegfried, 10/4/2015, ScienceNews
"An improved test of the strong equivalence principle with the pulsar in a triple star system⋆"
G. Voisin (1,2), I. Cognard (3,4), P. C. C. Freire (5), N. Wex (5), L. Guillemot (3,4), G. Desvignes (6,5), M. Kramer (5,1), and G. Theureau (2,3,4).
1. Jodrell Bank Centre for Astrophysics, The University of Manchester, Manchester, UK
e-mail: guillaume.voisin@manchester.ac.uk, astro.guillaume.voisin@gmail.com
2. LUTH, Observatoire de Paris, PSL Research University, Meudon, France
3. Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, Université d’Orléans, 18330 Nançay, France
4. Laboratoire de Physique et Chimie de l’Environnement, CNRS, 3A Avenue de la Recherche Scientifique,
45071 Orléans Cedex 2, Franc
5. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
6. LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 Place Jules Janssen, 92195 Meudon, France
Received 6 April 2020 / Accepted 28 April 2020

II. Is Gravity Quantized?

In 2015, however, theoretical physicist James Quach, now at the University of Adelaide in Australia, suggested a way to detect gravitons by taking advantage of their quantum nature. Quantum mechanics suggests the universe is inherently fuzzy—for instance, one can never absolutely know a particle's position and momentum at the same time. One consequence of this uncertainty is that a vacuum is never completely empty, but instead buzzes with a “quantum foam” of so-called virtual particles that constantly pop in and out of existence. These ghostly entities may be any kind of quanta, including gravitons.

Decades ago, scientists found that virtual particles can generate detectable forces. For example, the Casimir effect is the attraction or repulsion seen between two mirrors placed close together in vacuum. These highly reflective surfaces move due to the force generated by virtual photons winking in and out of existence. Previous research suggested that superconductors might reflect gravitons more strongly than normal matter, so Quach calculated that looking for interactions between two thin superconducting sheets in vacuum could reveal a gravitational Casimir effect. The resulting force could be roughly 10 times stronger than that expected from the standard virtual-photon-based Casimir effect.

Recently, Richard Norte of Delft University of Technology in the Netherlands and his colleagues developed a microchip to perform this experiment. This chip held two microscopic aluminum-coated plates that were cooled almost to absolute zero so that they became superconducting. One plate was attached to a movable mirror and a laser was fired at that mirror. If the plates moved because of a gravitational Casimir effect, the frequency of light reflecting off the mirror would measurably shift akin to the red shift detected from far off celestial objects due to expanding space. As detailed online July 20 in Physical Review Letters, the scientists failed to see any gravitational Casimir effect. This null result does not necessarily rule out the existence of gravitons—and thus gravity’s quantum nature. It may simply mean that gravitons do not interact with superconductors as strongly as prior theories and work postulated, says quantum physicist and Nobel laureate Frank Wilczek of the Massachusetts Institute of Technology, who did not participate in this study and was unsurprised by its null results. Even so, James Quach says, this “was a courageous attempt to detect gravitons.”

Although Norte’s microchip did not discover whether gravity is quantum, other scientists are pursuing a variety of approaches to find gravitational quantum effects. For example, in 2017 two independent studies suggested that if gravity is quantum it could generate a link known as “entanglement” between particles, so that one particle instantaneously influences another no matter where either is located in the cosmos. A tabletop experiment using laser beams and microscopic diamonds might help search for such gravity-based entanglement. The crystals would be kept in a vacuum to avoid collisions with atoms, so they would interact with one another through gravity alone. Scientists would let these diamonds fall at the same time, and if gravity is quantum the gravitational pull each crystal exerts on the other could entangle them together.

The researchers would seek out entanglement by shining lasers into each diamond’s heart after the drop. If particles in the crystals’ centers spin one way, they would fluoresce, but they would not if they spin the other way. If the spins in both crystals are in sync more often than chance would predict, this would suggest entanglement. “Experimentalists all over the world are curious to take the challenge up,” says quantum gravity researcher Anupam Mazumdar of the University of Groningen in the Netherlands, co-author of one of the entanglement studies, which was written by Ryan J. Marshman, Peter F. Barker and Sougato Bose (University College London, UK), Gavin W. Morley (University of Warwick, UK) and Steven Hoekstra (University of Groningen, the Netherlands). Instead of the current multi-kilometer-sized LIGO and VIRGO detectors, the physicists working in the UK and in the Netherlands proposed a table-top detector. This device would be sensitive to lower frequencies than the current detectors and it would be easy to point them to specific parts of the sky – in contrast, the current detectors only see a fixed part.

Another strategy to find evidence for quantum gravity is to look at the cosmic microwave background radiation, the faint afterglow of the big bang, says cosmologist Alan Guth of M.I.T. Quanta such as gravitons fluctuate like waves, and the shortest wavelengths would have the most intense fluctuations. When the cosmos expanded staggeringly in size within a sliver of a second after the big bang, according to Guth’s widely supported cosmological model known as inflation, these short wavelengths would have stretched to longer scales across the universe. This evidence of quantum gravity could be visible as swirls in the polarization, or alignment, of photons from the cosmic microwave background radiation.

However, the intensity of these patterns of swirls, known as B-modes, depends very much on the exact energy and timing of inflation. “Some versions of inflation predict that these B-modes should be found soon, while other versions predict that the B-modes are so weak that there will never be any hope of detecting them,” Guth says. “But if they are found, and the properties match the expectations from inflation, it would be very strong evidence that gravity is quantized.”

Another way to find out whether gravity is quantum is to look directly for quantum fluctuations in gravitational waves, which are thought to be made up of gravitons that were generated shortly after the big bang. The Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected gravitational waves in 2016, but it is not sensitive enough to detect the fluctuating gravitational waves in the early universe that inflation stretched to cosmic scales, Guth says. A gravitational-wave observatory in space, such as the Laser Interferometer Space Antenna (LISA), could potentially detect these waves, Wilczek adds.

In a paper recently accepted by the journal Classical and Quantum Gravity, however, astrophysicist Richard Lieu of the University of Alabama, Huntsville, argues that LIGO should already have detected gravitons if they carry as much energy as some current models of particle physics suggest. It might be that the graviton just packs less energy than expected, but Lieu suggests it might also mean the graviton does not exist. “If the graviton does not exist at all, it will be good news to most physicists, since we have been having such a horrid time in developing a theory of quantum gravity,” Lieu says.

Still, devising theories that eliminate the graviton may be no easier than devising theories that keep it. “From a theoretical point of view, it is very hard to imagine how gravity could avoid being quantized,” Guth says. “I am not aware of any sensible theory of how classical gravity could interact with quantum matter, and I can't imagine how such a theory might work.”

See:
"Is Gravity Quantum" By Charles Q. Choi on August 14, 2018, Scientific American
"A stepping stone for measuring quantum gravity" University of Groningen, 18 August 2020.

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David J Franks

#### Hayseed

An asymmetric oscillation weakly attracts other asymmetric oscillations. But which asymmetry is the cause? There is asymmetry in the physical size of the poles. (the poles of a dipole bond) There is asymmetry to the absolute, and the angular energy of the poles. There is asymmetry of pole movement of the oscillation. There is asymmetry of the pole electric density. There is asymmetry of the pole masses.

I would look for an asymmetric dynamic cause, because gravity is one way. It might be a combination of asymmetric affects/effects.

What ever turns out to be the cause, we will not find it until we understand mass, for what it really is. And you will not do that with the strategy of QM. One hundred years and no one knows what an electron looks like......they will even deny it has a structure.

#### Hartmann352

An asymmetric oscillation weakly attracts other asymmetric oscillations. But which asymmetry is the cause? There is asymmetry in the physical size of the poles. (the poles of a dipole bond) There is asymmetry to the absolute, and the angular energy of the poles. There is asymmetry of pole movement of the oscillation. There is asymmetry of the pole electric density. There is asymmetry of the pole masses.

I would look for an asymmetric dynamic cause, because gravity is one way. It might be a combination of asymmetric affects/effects.

What ever turns out to be the cause, we will not find it until we understand mass, for what it really is. And you will not do that with the strategy of QM. One hundred years and no one knows what an electron looks like......they will even deny it has a structure.
"...we will not find it until we understand mass, for what it really is."

Einstein formulated E = mc2 and it tells us that energy and mass are, effectively, the same thing, and it also tells us how much energy is contained in a given mass, or vice versa. In other words, mass can be thought of as very tightly packed energy. Converting one into the other doesn’t therefore violate either of the two conservation laws. Both quantities are conserved, although the state of the mass/energy may have changed. Each atom of a substance can be thought of as a little ball of tightly packed energy that can be released under certain circumstances. Likewise, we can take energy (such as quanta of light, called photons) and turn it into matter.

#### David J Franks

"...we will not find it until we understand mass, for what it really is."

Einstein formulated E = mc2 and it tells us that energy and mass are, effectively, the same thing, and it also tells us how much energy is contained in a given mass, or vice versa. In other words, mass can be thought of as very tightly packed energy. Converting one into the other doesn’t therefore violate either of the two conservation laws. Both quantities are conserved, although the state of the mass/energy may have changed. Each atom of a substance can be thought of as a little ball of tightly packed energy that can be released under certain circumstances. Likewise, we can take energy (such as quanta of light, called photons) and turn it into matter.
Hi, can you elaborate a little more, please? I read that particles are not the most fundamental thing and that each particle has an associated quantum field with it, which is fundamental. that's all I know. Are you able to shed any more light on that, please?

#### Hayseed

"...we will not find it until we understand mass, for what it really is."

Einstein formulated E = mc2 and it tells us that energy and mass are, effectively, the same thing, and it also tells us how much energy is contained in a given mass, or vice versa. In other words, mass can be thought of as very tightly packed energy. Converting one into the other doesn’t therefore violate either of the two conservation laws. Both quantities are conserved, although the state of the mass/energy may have changed. Each atom of a substance can be thought of as a little ball of tightly packed energy that can be released under certain circumstances. Likewise, we can take energy (such as quanta of light, called photons) and turn it into matter.

Mass = energy. That's it? After one hundred years and all we get is mass = energy? I suggest you review the classical model of matter. Parson's Magneton. It not only explains matter.......but explains emission and how wrong modern science is about it. Understanding light is the key. For it disproves local and space-time.

Then one may understand gravity.

And "photons" will never make matter. Mass is much more than just energy. Energy just moves the stuff of mass. Mass is a stuff. This stuff can only be moved with energy at certain rates..........because of the structure of the stuff. A physical structure sets the constants of the quantum. For 13 billion years.......no randomness or probability. It's the most solid thing there is.

#### jinn

Hartmann 352
Wouldn’t you say that Einstein’s theory of gravity is more or less an exact clone of Newton’s theory of gravity? Einstein just substituted a gravitational potential field that was based on the mass of objects for Newton’s gravitational force field based on the mass of objects. In both cases how the field manifests is hypothetical; in other words it could act either through an aether type of medium in Newton’s case and a gravitational potential field in Einstein’s case, neither of which have factual existence. Yet, it is Newton who is castigated on the grounds that his theory of gravity is in error and requires action at a distance to work. In the same way, Newton’s greatest achievement, namely discovering the difference between mass and weigh was also hi-jacked by Einstein who claimed that it was impossible to tell the difference between gravity and acceleration. Surely, the two concepts are identical, booth represent a force and manifest as the weight of a mass. Whatever, the case may be, one would have to admit that it was very convenient for Einstein that his theory meshed closely with Newton’s. One can say that Einstein used gravity as an explanation of itself! Further Einstein’s General Relativity is a continuation of his theory of Special Relativity, which today with improved calculators and understanding is easily proved to be wrong.

Make no mistake Einstein WAS probably one of the greatest scientists of the day. He proved, in his explanation of Brownian motion
that atoms and molecules almost certainly existed. He proved that mass and energy were equivalent, although this was the culmination of the work done by many scientists. He proved the existence of photons through the photo-electric effect. In short modern science would not be what it is today without his contributions. Yet Einstein was also a great individualist and an eccentric. The way to look at special relativity is as a marvellous fantasy world, where every question has answers but that had/has no relation to reality.

Yet, it is also true that zany theories like time dilation and length contraction were accepted, not by the scientists but by the Government itself, after the exploding of the atom bomb. All of Einstein’s words were treated with reverence, no one was allowed to oppose anything he said. Even scientists of the status of Robert Millikan were almost fired because of their opposition to some of his ideas. That is still the situation today, and it doesn’t look as if it will change.

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#### bolide

Hello all. I was thinking about anti gravity and how it worked. I started to research then thought it would probably be best if I understood exactly how gravity worked. So I googled it, and it seems no one really knows how gravity works? I mean at a super low level. I would think if you knew exactly how gravity worked then it shouldn’t be hard to figure out how anti gravity works.

I keep reading it’s all about mass and distance, but I always assumed gravity was a bi product of our atmosphere. that the pressure held with in simply was just pushing harder. That’s why on the moon there is a lot less gravity because there is not enough pressure in the exosphere. I am curious if there has ever been a study on the atomic weight of the elements on the periodic table. Weigh them at the bottom of say Mount Everest then again at the top. Then I read that there were places on the planet where gravity isn’t consistent and wondered if anyone studied the same thing of weight and also maybe the amount of pressure with the electromagnetic field in the area? Can’t find any information about it.
Of course all the people rushed in here to argue about Einstein, but it sounds like you need a more basic explanation than that. Let's just start with Newton.
Gravity is a force that exists between any two material objects. It's proportional to the masses of the objects, and inversely to the square of the distance between them. Actually it is the square of the distance between their centers, which is why Earth's gravity is indeed less at the top of Mt Everest than at sea level, and an object will weigh less there. There are also gravitational anomalies at different locations on Earth, because Earth's mass is not uniformly distributed.
The atmosphere has nothing to do with it. An object suspended in a fluid (whether an atmosphere of air, or an ocean of water) will experience a force of buoyancy, in the opposite direction from gravity. That's an entirely separate force, although in the end it depends on gravity, because it depends on the density (weight/volume) of the fluid vs. the density of the object. So in that way gravity causes buoyancy, not the other way around.
Electromagnetic fields likewise have nothing to do with it, as far as we know.
Objects weigh less on the Moon than on Earth, simply because the Moon's mass is so much less than Earth's.
As for how gravity really works, in the sense that you asked, nobody knows yet. We don't have a worked-out theory of gravitational fields, the way we do for electromagnetic fields, for example. We don't have a gravitational equivalent of photons, or of the double-slit experiment. Einstein says it is something completely different--a curvature of space-time. As yet, there's no more evidence that "anti-gravity" is possible, than there is for time travel.

#### robshed134

According to Einstein's theory of General Relativity, gravity is a natural consequence of a mass’s influence on space.

I. Einstein, General Relativity and Gravity

Because mass and energy distort the shape of spacetime, the Euclidean geometry of standard textbooks cannot accurately describe it. Einstein’s general relativity uses more complicated math built on the non-Euclidean geometry devised in the 19th century by Bernhard Riemann. With help from his friend Marcel Grossmann, Einstein adopted further advances by the mathematicians Gregorio Ricci-Curbastro, Tullio Levi-Civita and Elwin Christoffel to describe spacetime geometry in terms of mathematical expressions called tensors. Tensors are similar to vectors — quantities, such as velocity, composed of two components (in velocity’s case, speed and direction). Tensors are similar but can encompass more than just two components. Einstein used tensors to develop his equation describing the gravitational field, known as the Einstein field equation.

Gμν = 8πTμν
On the left side of Einstein's Field Equation is a tensor describing the geometry of spacetime — the gravitational field. On the right is the tensor describing the matter and energy density — the source of the gravitational field. The equation shows that spacetime geometry equals mass-energy density when adjusted with the proper units and numerical constants. (Actually, the equation stands for a set of multiple equations owing to the complexity of tensors. So experts usually speak of the Einstein field equations, plural.)

Gμν + Λgμν = 8πTμν
When Einstein applied his Field Equation to the entire universe, Einstein found that the universe would be unstable, easily disturbed into a state in which spacetime would be either expanding or collapsing. So he added a term that came to be called the cosmological constant, symbolized by the Greek letter lambda. It represents a specific and constant amount of energy density throughout space that would supposedly keep the universe stable and changeless. Later, evidence that the universe was indeed expanding led Einstein to renounce lambda. But it has been revived by modern cosmologists to explain the apparent increase in the universe’s rate of expansion that was discovered in the late 1990s by Saul Perlmutter and his team.

In mid-1915 Einstein saw that there was a way to make relativity truly general. Rather than imposing energy-momentum conservation on the equations, he worked on devising equations that would impose the conservation law on the universe.

Einstein quickly realized that his new theory of gravity was really a theory of the cosmos. In 1917, he wrote a famous paper applying general relativity to the universe as a whole. Today that paper stands as the foundation for modern cosmology. But at the time, Einstein was troubled — his equations implied an unstable universe, either growing or collapsing. In those days, the universe was supposed to be eternal, everlasting and changeless. So Einstein altered his equation, adding a factor called the cosmological constant, representing a constant energy density in space that kept the universe static.

Others were not so sure. Alexander Friedmann, a Russian mathematician, developed a description of an expanding or contracting universe from Einstein’s original equations. Einstein first thought Friedmann to be in error, but then relented, although still viewing the “expanding universe” as of mathematical interest only. When Edwin Hubble’s analysis of light from distant galaxies confirmed the universe’s expansion, Einstein finally agreed to the principle. Despite his own reluctance to accept it, Einstein’s general relativity math did in fact imply what John A. Wheeler of Princeton later called the “most dramatic prediction that science has ever made” — the expansion of the universe.

For weak gravitational fields, the results of general relativity do not differ significantly from Newton’s law of gravitation. But for intense gravitational fields, the results diverge, and general relativity has been shown to predict the correct results. Even in our Sun’s relatively weak gravitational field at the distance of Mercury’s orbit, we can observe the effect. Starting in the mid-1800s, Mercury’s elliptical orbit has been carefully measured. However, although it is elliptical, its motion is complicated by the fact that the perihelion position of the ellipse slowly advances. Most of the advance is due to the gravitational pull of other planets, but a small portion of that advancement could not be accounted for by Newton’s law. At one time, there was even a search for a “companion” planet that would explain the discrepancy. But general relativity correctly predicts the measurements. Since then, many measurements, such as the deflection of light of distant objects by the Sun, have verified that general relativity correctly predicts the observations.

Rather than preventing the universe from collapse, the vacuum energy Einstein describes can explain why the universe now expands at an accelerating pace. General relativity, and it's cosmological constant, today forms the core science for analyzing the history of the universe and for forecasting its future.

But apart from its use in cosmology, general relativity was not widely applied to scientific problems in its first four decades. For the most part, general relativity languished in departments of mathematics, rarely studied in physics.

Shortly after Einstein introduced general relativity, Karl Schwarzschild calculated its implications for the gravity of a massive sphere. Schwarzschild determined that for any given mass there existed a “critical radius” — a limit, he believed, to how small that amount of mass could be compressed. In 1939, Einstein concluded that mass could not be compressed to within that “Schwarzschild radius.” But in the same year, J. Robert Oppenheimer, later to lead America's Manhattan Project for the development of the atomic bombs used against Japan during WWII, and Hartland Snyder, of Northwestern University and later Brookhaven Labs, calculated otherwise, claiming that a sufficiently massive object could indeed collapse within that radius, disappearing from view and leaving only its gravitational field behind.

At the time, nobody paid any attention. But in the 1960s, newfound astrophysical anomalies suggested that gravitational collapse was at work in the cosmos, and Oppenheimer and Snyder’s idea was revived as what came to be known as black holes. Famous for swallowing anything they encounter and allowing nothing to escape, black holes are probably the most bizarre astrophysical consequences of general relativity and gravity. Small black holes have been detected throughout space and supermassive black holes reside in the cores of most galaxies. And we now suspect that smaller black holes, even orbiting pairs of black holes, circle super massive black holes at the center of galaxies.

More recently black holes (schematic of one shown) have been used as thought-experiment laboratories for investigating several outstanding mysteries about the nature of space, time and gravity.

In creating general relativity, Einstein’s had to envision physical processes governing matter, space and time, while at the same time formulating abstract mathematical expressions corresponding to that reality. As a student, Einstein testified, he neglected mathematics. His intuition was not strong enough to guide him to the most profound of math’s many subfields. But in the physical realm of natural phenomena, “I soon learned to scent out that which was able to lead to fundamentals and to turn aside from … the multitude of things which clutter up the mind and divert it from the essential.” At first he didn’t realize that “a more profound knowledge of the basic principles of physics is tied up with the most intricate mathematical methods.” He learned that from his pursuit of general relativity.

Fantastic physical phenomena were first discovered not through the lenses of telescopes, but within the squiggles Einstein had scratched out on paper to make the world make sense — to him. And now physical nature makes sense to modern science only because of Einstein’s insights through thought experiments.

“Einstein’s ideas,” his friend the physicist Max Born wrote over half a century ago, “have given the physical sciences the impetus which has liberated them from outdated philosophical doctrine, and made them one of the decisive factors in the modern world of man.”

See:
University Physics Volume 1, Einstein's Theory of Gravity, Rice University.
"Einstein's genius changed science's perception pf gravity" by Tom Siegfried, 10/4/2015, ScienceNews
"An improved test of the strong equivalence principle with the pulsar in a triple star system⋆"
G. Voisin (1,2), I. Cognard (3,4), P. C. C. Freire (5), N. Wex (5), L. Guillemot (3,4), G. Desvignes (6,5), M. Kramer (5,1), and G. Theureau (2,3,4).
1. Jodrell Bank Centre for Astrophysics, The University of Manchester, Manchester, UK
e-mail: guillaume.voisin@manchester.ac.uk, astro.guillaume.voisin@gmail.com
2. LUTH, Observatoire de Paris, PSL Research University, Meudon, France
3. Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, Université d’Orléans, 18330 Nançay, France
4. Laboratoire de Physique et Chimie de l’Environnement, CNRS, 3A Avenue de la Recherche Scientifique,
45071 Orléans Cedex 2, Franc
5. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
6. LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 Place Jules Janssen, 92195 Meudon, France
Received 6 April 2020 / Accepted 28 April 2020

II. Is Gravity Quantized?

In 2015, however, theoretical physicist James Quach, now at the University of Adelaide in Australia, suggested a way to detect gravitons by taking advantage of their quantum nature. Quantum mechanics suggests the universe is inherently fuzzy—for instance, one can never absolutely know a particle's position and momentum at the same time. One consequence of this uncertainty is that a vacuum is never completely empty, but instead buzzes with a “quantum foam” of so-called virtual particles that constantly pop in and out of existence. These ghostly entities may be any kind of quanta, including gravitons.

Decades ago, scientists found that virtual particles can generate detectable forces. For example, the Casimir effect is the attraction or repulsion seen between two mirrors placed close together in vacuum. These highly reflective surfaces move due to the force generated by virtual photons winking in and out of existence. Previous research suggested that superconductors might reflect gravitons more strongly than normal matter, so Quach calculated that looking for interactions between two thin superconducting sheets in vacuum could reveal a gravitational Casimir effect. The resulting force could be roughly 10 times stronger than that expected from the standard virtual-photon-based Casimir effect.

Recently, Richard Norte of Delft University of Technology in the Netherlands and his colleagues developed a microchip to perform this experiment. This chip held two microscopic aluminum-coated plates that were cooled almost to absolute zero so that they became superconducting. One plate was attached to a movable mirror and a laser was fired at that mirror. If the plates moved because of a gravitational Casimir effect, the frequency of light reflecting off the mirror would measurably shift akin to the red shift detected from far off celestial objects due to expanding space. As detailed online July 20 in Physical Review Letters, the scientists failed to see any gravitational Casimir effect. This null result does not necessarily rule out the existence of gravitons—and thus gravity’s quantum nature. It may simply mean that gravitons do not interact with superconductors as strongly as prior theories and work postulated, says quantum physicist and Nobel laureate Frank Wilczek of the Massachusetts Institute of Technology, who did not participate in this study and was unsurprised by its null results. Even so, James Quach says, this “was a courageous attempt to detect gravitons.”

Although Norte’s microchip did not discover whether gravity is quantum, other scientists are pursuing a variety of approaches to find gravitational quantum effects. For example, in 2017 two independent studies suggested that if gravity is quantum it could generate a link known as “entanglement” between particles, so that one particle instantaneously influences another no matter where either is located in the cosmos. A tabletop experiment using laser beams and microscopic diamonds might help search for such gravity-based entanglement. The crystals would be kept in a vacuum to avoid collisions with atoms, so they would interact with one another through gravity alone. Scientists would let these diamonds fall at the same time, and if gravity is quantum the gravitational pull each crystal exerts on the other could entangle them together.

The researchers would seek out entanglement by shining lasers into each diamond’s heart after the drop. If particles in the crystals’ centers spin one way, they would fluoresce, but they would not if they spin the other way. If the spins in both crystals are in sync more often than chance would predict, this would suggest entanglement. “Experimentalists all over the world are curious to take the challenge up,” says quantum gravity researcher Anupam Mazumdar of the University of Groningen in the Netherlands, co-author of one of the entanglement studies, which was written by Ryan J. Marshman, Peter F. Barker and Sougato Bose (University College London, UK), Gavin W. Morley (University of Warwick, UK) and Steven Hoekstra (University of Groningen, the Netherlands). Instead of the current multi-kilometer-sized LIGO and VIRGO detectors, the physicists working in the UK and in the Netherlands proposed a table-top detector. This device would be sensitive to lower frequencies than the current detectors and it would be easy to point them to specific parts of the sky – in contrast, the current detectors only see a fixed part.

Another strategy to find evidence for quantum gravity is to look at the cosmic microwave background radiation, the faint afterglow of the big bang, says cosmologist Alan Guth of M.I.T. Quanta such as gravitons fluctuate like waves, and the shortest wavelengths would have the most intense fluctuations. When the cosmos expanded staggeringly in size within a sliver of a second after the big bang, according to Guth’s widely supported cosmological model known as inflation, these short wavelengths would have stretched to longer scales across the universe. This evidence of quantum gravity could be visible as swirls in the polarization, or alignment, of photons from the cosmic microwave background radiation.

However, the intensity of these patterns of swirls, known as B-modes, depends very much on the exact energy and timing of inflation. “Some versions of inflation predict that these B-modes should be found soon, while other versions predict that the B-modes are so weak that there will never be any hope of detecting them,” Guth says. “But if they are found, and the properties match the expectations from inflation, it would be very strong evidence that gravity is quantized.”

Another way to find out whether gravity is quantum is to look directly for quantum fluctuations in gravitational waves, which are thought to be made up of gravitons that were generated shortly after the big bang. The Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected gravitational waves in 2016, but it is not sensitive enough to detect the fluctuating gravitational waves in the early universe that inflation stretched to cosmic scales, Guth says. A gravitational-wave observatory in space, such as the Laser Interferometer Space Antenna (LISA), could potentially detect these waves, Wilczek adds.

In a paper recently accepted by the journal Classical and Quantum Gravity, however, astrophysicist Richard Lieu of the University of Alabama, Huntsville, argues that LIGO should already have detected gravitons if they carry as much energy as some current models of particle physics suggest. It might be that the graviton just packs less energy than expected, but Lieu suggests it might also mean the graviton does not exist. “If the graviton does not exist at all, it will be good news to most physicists, since we have been having such a horrid time in developing a theory of quantum gravity,” Lieu says.

Still, devising theories that eliminate the graviton may be no easier than devising theories that keep it. “From a theoretical point of view, it is very hard to imagine how gravity could avoid being quantized,” Guth says. “I am not aware of any sensible theory of how classical gravity could interact with quantum matter, and I can't imagine how such a theory might work.”

See:
"Is Gravity Quantum" By Charles Q. Choi on August 14, 2018, Scientific American
"A stepping stone for measuring quantum gravity" University of Groningen, 18 August 2020.
big G ...in field equations.....
what in nature causes it to have the value it has
Cavendish measured it modern physics confirms and slightly improves the accuracy of the value.
yet we do not know what causes it

#### jinn

Apart from the question of gravity: if energy is examined e = mc^2, the answer that is received is that nothing can move faster than light. If an object tries to exceed the speed of light then, it would, according to the equation for relativistic energy, exceed the total energy it is capable of containing and thus reach a mass equivalent to infinity. Using classical notation the mass/energy equivalence can never be exceeded although it comes close. For instance:

A mass of 1 kg will be used to keep things simple. With 1 kg of mass:

E = mc^2

E = 1 x 9 x 10^16 m/s^2

E = 9 x 10^ 16 J

Which is a lot of energy

According to the relativistic equation for energy:

E = mc^2/ sqrt(1 – (v^2/c^2) = infinity

Leaving this aside for the moment, it is interesting to note that this point: That the mass of an object varies according to the force that it is subjected to, is the very heart of Newtonian gravity, although Einstein tried to appropriate this idea as his own by claiming that it was impossible to tell the difference between the forces exerted on an object by gravity or by acceleration. This is absolutely irrelevant: In the final analysis the weight of an object is due to the exertion of some force on it; be it gravity or some other force. Take an object in deep space far from any source of gravity, if it subjected to an acceleration of 2 G’s it will feel a force of gravity equivalent to twice the gravitational force experienced on earth. This is the whole point, how was it possible to calculate the weight of objects on the moon or on Venus, if the force acting on the object was not known? Einstein claims that general relativity predicts that mass bends space, but in order to do this there must exist some hypothetical gravitational potential or gradient. This is in fact no better an explanation than Newton’s was of what gravity is, although both Einstein and Newton explain how gravity works. All of the work done in space so far has used Newtonian gravity.

#### bolide

okay. time for a rhetorical question: why is it that all this blather is occurring, under a post that makes, clear, the poster's extreme LACK of knowledge. (gravity related to ATMOSPHERE!!!) OMg.
You and I seem to be the only people here who have noticed that (see my answer).

#### wpro153

Good morning.

I'm Italian and I use the automatic translator. The maximum gravity on earth is measured on the surface. At the center the gravity is zero because what is above attracts what is underneath and vice versa. Obviously I assumed a homogeneity of the planet, a spherical form and eliminating external gravitational influences. Now the problem is that we can imagine the central zero as the sum of two positive terms and the other negative. The term negative in a geometric representation of time space such as relativity cannot be described, revealing a dangerous inconsistency in theory. In the absence of a convincing theory of gravity it is permissible to think in the same way even in extreme environments such as black holes and BB. And, I'm afraid, it's the end of the chance to imagine a "point singularity" for BKs. The motion of galaxies and explanations that postulate exotic elements such as "dark matter" are explained much easier to assume maximum gravity in the periphery than in the center.

I report the text in Italian because I fear errors of translation on my part. Have a good day and excuse the intrusion.

wpro.

Buongiorno.

Sono italiano ed uso il traduttore automatico. Il massimo della gravità sulla terra lo si misura in superfice. Al centro la gravità è pari a zero perché ciò che è sopra attrae ciò che vi è sotto e viceversa. Ovviamente assumento una omogeneità del pianeta, una forma sferica ed eliminando le influenze gravitazionali esterne. Ora il problema è che possiamo immaginare lo zero centrale come la somma di due termini uno positivo e l'altro negativo. Il termine negativo in una rappresentazione geometrica della spazio tempo quale la relatività non può essere descritto, rivelando una pericolosa inconsistenza nella teoria. In mancanza di una teoria convincente della gravità è lecito pensare allo stesso modo anche in ambienti estremi come per esempio i buchi neri ed il BB. E, temo, sia la fine della possibilità di immaginare una "singolarità puntiforme" per i BK. Il moto delle galassie e le spiegazioni che postulano elementi esotici quali la "materia oscura" si spiegano molto più facilemnte assumento il massimo della gravità in periferia piuttosto che al centro.

Riporto il testo in italiano perchè temo errori di traduzione da parte mia. Buona giornata e scusate l'intrusione.

wpro

#### jinn

wpro153 : The maximum gravity on earth is measured on the surface. At the center the gravity is zero because what is above attracts what is underneath and vice versa. Obviously I assumed a homogeneity of the planet, a spherical form and eliminating external gravitational influences. Now the problem is that we can imagine the central zero as the sum of two positive terms and the other negative.
It is an interesting point of view that gravity is stronger at the periphery than at the centre, it would explain the manner in which Galaxies move. But consider for a moment Newton’s explanation of gravity as emanating from the centre of all things. It is, if you think about it, a much more cogent and well put together explanation than that mass bends space time. In fact one cannot use both explanations, Newton’s and Einstein’s , simultaneously. It is not possible to mix up the two theories into an amalgam as your post seems to indicate.

Consider the stars in our Universe; accumulations of hydrogen gas that are so massive, that gravity creates huge pressures and densities at their centres, which results in fusion taking place. The Gestalt Aether Theory of Gravity posted at this forum gives an introduction to a new theory of gravity. The theory continues seamlessly to explain how Black Holes are formed. That is in a different paper that has yet to be posted.

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#### Gringoz

BANNED
According to Einstein's theory of General Relativity, gravity is a natural consequence of a mass’s influence on space.

I. Einstein, General Relativity and Gravity

Because mass and energy distort the shape of spacetime, the Euclidean geometry of standard textbooks cannot accurately describe it. Einstein’s general relativity uses more complicated math built on the non-Euclidean geometry devised in the 19th century by Bernhard Riemann. With help from his friend Marcel Grossmann, Einstein adopted further advances by the mathematicians Gregorio Ricci-Curbastro, Tullio Levi-Civita and Elwin Christoffel to describe spacetime geometry in terms of mathematical expressions called tensors. Tensors are similar to vectors — quantities, such as velocity, composed of two components (in velocity’s case, speed and direction). Tensors are similar but can encompass more than just two components. Einstein used tensors to develop his equation describing the gravitational field, known as the Einstein field equation.

Gμν = 8πTμν
On the left side of Einstein's Field Equation is a tensor describing the geometry of spacetime — the gravitational field. On the right is the tensor describing the matter and energy density — the source of the gravitational field. The equation shows that spacetime geometry equals mass-energy density when adjusted with the proper units and numerical constants. (Actually, the equation stands for a set of multiple equations owing to the complexity of tensors. So experts usually speak of the Einstein field equations, plural.)

Gμν + Λgμν = 8πTμν
When Einstein applied his Field Equation to the entire universe, Einstein found that the universe would be unstable, easily disturbed into a state in which spacetime would be either expanding or collapsing. So he added a term that came to be called the cosmological constant, symbolized by the Greek letter lambda. It represents a specific and constant amount of energy density throughout space that would supposedly keep the universe stable and changeless. Later, evidence that the universe was indeed expanding led Einstein to renounce lambda. But it has been revived by modern cosmologists to explain the apparent increase in the universe’s rate of expansion that was discovered in the late 1990s by Saul Perlmutter and his team.

In mid-1915 Einstein saw that there was a way to make relativity truly general. Rather than imposing energy-momentum conservation on the equations, he worked on devising equations that would impose the conservation law on the universe.

Einstein quickly realized that his new theory of gravity was really a theory of the cosmos. In 1917, he wrote a famous paper applying general relativity to the universe as a whole. Today that paper stands as the foundation for modern cosmology. But at the time, Einstein was troubled — his equations implied an unstable universe, either growing or collapsing. In those days, the universe was supposed to be eternal, everlasting and changeless. So Einstein altered his equation, adding a factor called the cosmological constant, representing a constant energy density in space that kept the universe static.

Others were not so sure. Alexander Friedmann, a Russian mathematician, developed a description of an expanding or contracting universe from Einstein’s original equations. Einstein first thought Friedmann to be in error, but then relented, although still viewing the “expanding universe” as of mathematical interest only. When Edwin Hubble’s analysis of light from distant galaxies confirmed the universe’s expansion, Einstein finally agreed to the principle. Despite his own reluctance to accept it, Einstein’s general relativity math did in fact imply what John A. Wheeler of Princeton later called the “most dramatic prediction that science has ever made” — the expansion of the universe.

For weak gravitational fields, the results of general relativity do not differ significantly from Newton’s law of gravitation. But for intense gravitational fields, the results diverge, and general relativity has been shown to predict the correct results. Even in our Sun’s relatively weak gravitational field at the distance of Mercury’s orbit, we can observe the effect. Starting in the mid-1800s, Mercury’s elliptical orbit has been carefully measured. However, although it is elliptical, its motion is complicated by the fact that the perihelion position of the ellipse slowly advances. Most of the advance is due to the gravitational pull of other planets, but a small portion of that advancement could not be accounted for by Newton’s law. At one time, there was even a search for a “companion” planet that would explain the discrepancy. But general relativity correctly predicts the measurements. Since then, many measurements, such as the deflection of light of distant objects by the Sun, have verified that general relativity correctly predicts the observations.

Rather than preventing the universe from collapse, the vacuum energy Einstein describes can explain why the universe now expands at an accelerating pace. General relativity, and it's cosmological constant, today forms the core science for analyzing the history of the universe and for forecasting its future.

But apart from its use in cosmology, general relativity was not widely applied to scientific problems in its first four decades. For the most part, general relativity languished in departments of mathematics, rarely studied in physics.

Shortly after Einstein introduced general relativity, Karl Schwarzschild calculated its implications for the gravity of a massive sphere. Schwarzschild determined that for any given mass there existed a “critical radius” — a limit, he believed, to how small that amount of mass could be compressed. In 1939, Einstein concluded that mass could not be compressed to within that “Schwarzschild radius.” But in the same year, J. Robert Oppenheimer, later to lead America's Manhattan Project for the development of the atomic bombs used against Japan during WWII, and Hartland Snyder, of Northwestern University and later Brookhaven Labs, calculated otherwise, claiming that a sufficiently massive object could indeed collapse within that radius, disappearing from view and leaving only its gravitational field behind.

At the time, nobody paid any attention. But in the 1960s, newfound astrophysical anomalies suggested that gravitational collapse was at work in the cosmos, and Oppenheimer and Snyder’s idea was revived as what came to be known as black holes. Famous for swallowing anything they encounter and allowing nothing to escape, black holes are probably the most bizarre astrophysical consequences of general relativity and gravity. Small black holes have been detected throughout space and supermassive black holes reside in the cores of most galaxies. And we now suspect that smaller black holes, even orbiting pairs of black holes, circle super massive black holes at the center of galaxies.

More recently black holes (schematic of one shown) have been used as thought-experiment laboratories for investigating several outstanding mysteries about the nature of space, time and gravity.

In creating general relativity, Einstein’s had to envision physical processes governing matter, space and time, while at the same time formulating abstract mathematical expressions corresponding to that reality. As a student, Einstein testified, he neglected mathematics. His intuition was not strong enough to guide him to the most profound of math’s many subfields. But in the physical realm of natural phenomena, “I soon learned to scent out that which was able to lead to fundamentals and to turn aside from … the multitude of things which clutter up the mind and divert it from the essential.” At first he didn’t realize that “a more profound knowledge of the basic principles of physics is tied up with the most intricate mathematical methods.” He learned that from his pursuit of general relativity.

Fantastic physical phenomena were first discovered not through the lenses of telescopes, but within the squiggles Einstein had scratched out on paper to make the world make sense — to him. And now physical nature makes sense to modern science only because of Einstein’s insights through thought experiments.

“Einstein’s ideas,” his friend the physicist Max Born wrote over half a century ago, “have given the physical sciences the impetus which has liberated them from outdated philosophical doctrine, and made them one of the decisive factors in the modern world of man.”

See:
University Physics Volume 1, Einstein's Theory of Gravity, Rice University.
"Einstein's genius changed science's perception pf gravity" by Tom Siegfried, 10/4/2015, ScienceNews
"An improved test of the strong equivalence principle with the pulsar in a triple star system⋆"
G. Voisin (1,2), I. Cognard (3,4), P. C. C. Freire (5), N. Wex (5), L. Guillemot (3,4), G. Desvignes (6,5), M. Kramer (5,1), and G. Theureau (2,3,4).
1. Jodrell Bank Centre for Astrophysics, The University of Manchester, Manchester, UK
e-mail: guillaume.voisin@manchester.ac.uk, astro.guillaume.voisin@gmail.com
2. LUTH, Observatoire de Paris, PSL Research University, Meudon, France
3. Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, Université d’Orléans, 18330 Nançay, France
4. Laboratoire de Physique et Chimie de l’Environnement, CNRS, 3A Avenue de la Recherche Scientifique,
45071 Orléans Cedex 2, Franc
5. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
6. LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 Place Jules Janssen, 92195 Meudon, France
Received 6 April 2020 / Accepted 28 April 2020

II. Is Gravity Quantized?

In 2015, however, theoretical physicist James Quach, now at the University of Adelaide in Australia, suggested a way to detect gravitons by taking advantage of their quantum nature. Quantum mechanics suggests the universe is inherently fuzzy—for instance, one can never absolutely know a particle's position and momentum at the same time. One consequence of this uncertainty is that a vacuum is never completely empty, but instead buzzes with a “quantum foam” of so-called virtual particles that constantly pop in and out of existence. These ghostly entities may be any kind of quanta, including gravitons.

Decades ago, scientists found that virtual particles can generate detectable forces. For example, the Casimir effect is the attraction or repulsion seen between two mirrors placed close together in vacuum. These highly reflective surfaces move due to the force generated by virtual photons winking in and out of existence. Previous research suggested that superconductors might reflect gravitons more strongly than normal matter, so Quach calculated that looking for interactions between two thin superconducting sheets in vacuum could reveal a gravitational Casimir effect. The resulting force could be roughly 10 times stronger than that expected from the standard virtual-photon-based Casimir effect.

Recently, Richard Norte of Delft University of Technology in the Netherlands and his colleagues developed a microchip to perform this experiment. This chip held two microscopic aluminum-coated plates that were cooled almost to absolute zero so that they became superconducting. One plate was attached to a movable mirror and a laser was fired at that mirror. If the plates moved because of a gravitational Casimir effect, the frequency of light reflecting off the mirror would measurably shift akin to the red shift detected from far off celestial objects due to expanding space. As detailed online July 20 in Physical Review Letters, the scientists failed to see any gravitational Casimir effect. This null result does not necessarily rule out the existence of gravitons—and thus gravity’s quantum nature. It may simply mean that gravitons do not interact with superconductors as strongly as prior theories and work postulated, says quantum physicist and Nobel laureate Frank Wilczek of the Massachusetts Institute of Technology, who did not participate in this study and was unsurprised by its null results. Even so, James Quach says, this “was a courageous attempt to detect gravitons.”

Although Norte’s microchip did not discover whether gravity is quantum, other scientists are pursuing a variety of approaches to find gravitational quantum effects. For example, in 2017 two independent studies suggested that if gravity is quantum it could generate a link known as “entanglement” between particles, so that one particle instantaneously influences another no matter where either is located in the cosmos. A tabletop experiment using laser beams and microscopic diamonds might help search for such gravity-based entanglement. The crystals would be kept in a vacuum to avoid collisions with atoms, so they would interact with one another through gravity alone. Scientists would let these diamonds fall at the same time, and if gravity is quantum the gravitational pull each crystal exerts on the other could entangle them together.

The researchers would seek out entanglement by shining lasers into each diamond’s heart after the drop. If particles in the crystals’ centers spin one way, they would fluoresce, but they would not if they spin the other way. If the spins in both crystals are in sync more often than chance would predict, this would suggest entanglement. “Experimentalists all over the world are curious to take the challenge up,” says quantum gravity researcher Anupam Mazumdar of the University of Groningen in the Netherlands, co-author of one of the entanglement studies, which was written by Ryan J. Marshman, Peter F. Barker and Sougato Bose (University College London, UK), Gavin W. Morley (University of Warwick, UK) and Steven Hoekstra (University of Groningen, the Netherlands). Instead of the current multi-kilometer-sized LIGO and VIRGO detectors, the physicists working in the UK and in the Netherlands proposed a table-top detector. This device would be sensitive to lower frequencies than the current detectors and it would be easy to point them to specific parts of the sky – in contrast, the current detectors only see a fixed part.

Another strategy to find evidence for quantum gravity is to look at the cosmic microwave background radiation, the faint afterglow of the big bang, says cosmologist Alan Guth of M.I.T. Quanta such as gravitons fluctuate like waves, and the shortest wavelengths would have the most intense fluctuations. When the cosmos expanded staggeringly in size within a sliver of a second after the big bang, according to Guth’s widely supported cosmological model known as inflation, these short wavelengths would have stretched to longer scales across the universe. This evidence of quantum gravity could be visible as swirls in the polarization, or alignment, of photons from the cosmic microwave background radiation.

However, the intensity of these patterns of swirls, known as B-modes, depends very much on the exact energy and timing of inflation. “Some versions of inflation predict that these B-modes should be found soon, while other versions predict that the B-modes are so weak that there will never be any hope of detecting them,” Guth says. “But if they are found, and the properties match the expectations from inflation, it would be very strong evidence that gravity is quantized.”

Another way to find out whether gravity is quantum is to look directly for quantum fluctuations in gravitational waves, which are thought to be made up of gravitons that were generated shortly after the big bang. The Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected gravitational waves in 2016, but it is not sensitive enough to detect the fluctuating gravitational waves in the early universe that inflation stretched to cosmic scales, Guth says. A gravitational-wave observatory in space, such as the Laser Interferometer Space Antenna (LISA), could potentially detect these waves, Wilczek adds.

In a paper recently accepted by the journal Classical and Quantum Gravity, however, astrophysicist Richard Lieu of the University of Alabama, Huntsville, argues that LIGO should already have detected gravitons if they carry as much energy as some current models of particle physics suggest. It might be that the graviton just packs less energy than expected, but Lieu suggests it might also mean the graviton does not exist. “If the graviton does not exist at all, it will be good news to most physicists, since we have been having such a horrid time in developing a theory of quantum gravity,” Lieu says.

Still, devising theories that eliminate the graviton may be no easier than devising theories that keep it. “From a theoretical point of view, it is very hard to imagine how gravity could avoid being quantized,” Guth says. “I am not aware of any sensible theory of how classical gravity could interact with quantum matter, and I can't imagine how such a theory might work.”

See:
"Is Gravity Quantum" By Charles Q. Choi on August 14, 2018, Scientific American
"A stepping stone for measuring quantum gravity" University of Groningen, 18 August 2020.
Except that Einsteins equations fail to allow for galaxies traveling faster than light requiring a universe 85 percent more massive than the one known

#### David J Franks

Mass = energy. That's it? After one hundred years and all we get is mass = energy? I suggest you review the classical model of matter. Parson's Magneton. It not only explains matter.......but explains emission and how wrong modern science is about it. Understanding light is the key. For it disproves local and space-time.

Then one may understand gravity.

And "photons" will never make matter. Mass is much more than just energy. Energy just moves the stuff of mass. Mass is a stuff. This stuff can only be moved with energy at certain rates..........because of the structure of the stuff. A physical structure sets the constants of the quantum. For 13 billion years.......no randomness or probability. It's the most solid thing there is.
Needs checking but I think recent experiments have turned photons into other particles, with mass. However, my main question to you is what is stuff?

#### David J Franks

Except that Einsteins equations fail to allow for galaxies traveling faster than light requiring a universe 85 percent more massive than the one known
Distant galaxies are moving away from us faster light because space is expanding faster than light beyond a certain distance from us (Hubble constant). Relativity allows this.

#### Gringoz

BANNED
Distant galaxies are moving away from us faster light because space is expanding faster than light beyond a certain distance from us (Hubble constant). Relativity allows this.
Relativity does not allow that or the cosmological constant would work instead of failing because there is 85 percent too little mass in the universe and as such mythical dark matter was invented to allow the equation to work. Try buying a house without 85 percent of the funds needed to pay your mortgage and let me know what the bank loans you

#### Gringoz

BANNED
Needs checking but I think recent experiments have turned photons into other particles, with mass. However, my main question to you is what is stuff?
So massless photons can be turned into mass, isn't that something from nothing?

#### Valentine Michael Smith

I thought energy equalled mass. This reminded me of a thought I had a couple weeks ago: /_ E = S , where all things consist of S.