Gravity

The facts of the matter Physics

May the mass times acceleration be with you

Electromagnetism Quantum mechanics Relativity Thermodynamics

Let's get physical!

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Atoms trying to understand atoms

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Gravity is a fundamental force of nature that has been speculated about since at least 800 BCE, albeit inconclusively till Sir Isaac Newton began his scientific career in the late 1600s. Today, Albert Einstein's theory of General Relativity is commonly considered the most accurate description of how objects move under gravity, although there are still unsolved complexities with the integration of relativity with quantum mechanics.

Because Einstein's theory of gravity is quite complicated and hard to understand, various other theories have been proposed, and the production of alternative explanations is still a cottage industry for kooks and cranks. Meanwhile more serious physicists are still trying to iron out some problems with alternative theories.

General Relativity

From Newton to Einstein

Since his day, Newton's Law of Gravity has proven sufficiently accurate to enable the prediction of the location of the planet Neptune. In the late 19^th century, small irregularities in the orbit of the innermost planet, Mercury, could not be explained through Newtonian Gravity Theory, and it began to be questioned. Einstein's General Theory of Relativity solved the problem.

Everyone will be familiar with the visual and physical representation of gravitation as described by the General Theory, i.e., a stretched elastic membrane deflected by the weight of objects placed on it. This is a difficult visualisation of the mathematical idea of curvature in spacetime, requiring that the four dimensions of spacetime be visualised as a two dimensional elastic sheet which is distorting in some other dimension. Once this mental leap has been performed it is easy to see how varying masses have varying effects on the curvature of spacetime; the larger the mass, the greater the distortion and the further reaching its effect. This is most often shown by placing a large, dense object (a bowling ball or cannonball) on the membrane and then rolling other balls past it at varying distances. As a thought experiment one can envisage fairly well the effects of the two objects on each other. The trouble with this demonstration is, of course, friction with the membrane.

Law of Universal Gravitation

The Law of Universal Gravitation, published in 1687 by Newton in his Principia Mathematica, is a theory which seeks to explain the force of attraction between all bodies of the universe. It is a theory, not fact, and as such the contents of this article are to be approached critically.

Mathematically, the law applied to two bodies is:

$F=G{\frac {m_{1}m_{2}}{r^{2}}},$

In which:

• F is the magnitude of the force between the bodies.
• G is the gravitational constant. In our universe, G = 6.67 × 10^-11 N m² kg^-2
• m₁ and m₂ are the masses of the bodies.
• r is the distance between the centres of the bodies, not the radius, don't forget that.

In other words, the law states that the gravitational force between two objects is proportional to the mass of each object and inversely proportional to the square of the distance between them.

More advanced versions

The law of universal gravitation fails when confronted with situations involving strong gravitational effects. For example, it does not account for small perturbations in Mercury's orbit around the Sun, or for gravitational lensing. General Relativity, which interprets the gravitational force as a consequence of warped spacetime, can accurately account for these observations, and has therefore surpassed Newton's theory as the preferred explanation for gravitation. However, the above equation is adequate for almost all practical calculations.

Effects of general relativity

General relativity predicts many phenomena that are unfamiliar to everyday experience. Many of these effects are extremely small, and require cutting-edge scientific instrumentation to detect.

General relativity predicts that space is curved around a mass, and allows one to calculate how light, which follows the curvature of space, bends around massive objects. The best-known detection of this effect was Eddington's detection of the apparent change of position of stars near the Sun measured during a solar eclipse. However, even more massive objects such as galaxy clusters have been shown to act as "magnifying glasses" that distort the images of galaxies behind them.

Gravitational redshift is an effect that has been measured with high precision in the Pound-Rebka experiment and its improvements. Briefly, a variation of Mössbauer gamma-ray spectroscopy is used to detect the change of wavelength of a gamma ray as it falls from a tower or from an airplane. This is considered a precise test of general relativity. Any competing theory should be able to predict the same effect.

There should be gravitational waves created (analogous to bursts of electromagnetic energy) when massive events, such as the collapse of a star into a neutron star or black hole, occur. In an attempt to detect or measure these, there are several devices in place and under construction on the surface of the Earth which rely on the compression or expansion of physical objects at angles to each other. These are normally kilometer-plus long tubes whose lengths are constantly monitored by laser reflection/interference. The anticipated difference in these lengths is of the order of 10^-10 cm or less. Scientists claim to have detected gravitational waves in the background radiation from the Big Bang, indirect evidence previously existed from two pulsars spiraling inwards and losing energy. The detection of gravitational waves is considered definitive evidence in favor of the existence of black holes. However, one may remain skeptical until more detection events are publicized. In fact, physicists are now highly confident they have detected two signals from two black hole binaries. The dawn of gravitational wave astronomy is upon us.

Quantum gravity

In order to discuss gravity and quantum field theory in the same mathematical environment, gravitation is theorised to be communicated by the exchange of virtual particles called gravitons between masses. So far, nobody has been able to formulate a consistent and sound theory of quantum gravity. String theory is currently thought to be the best hope for this, although it is not certain. To date, string theory, as well as its competitors, remain under development. And since none of them have garnered any support from experiments or observation, they strictly speaking should be called hypotheses. This is a legitimate mystery, and mystery attracts cranks: many cranks - that claim to prove Einstein wrong - from claiming to have solved quantum gravity.

Gravity is one of the basic forces of the Universe, and the gravitational constant is thought to have remained the same since a vanishingly small time (the Planck time) following the Big Bang. As with the other forces, any variation, however small, would result in a very different universe.

The above has been extracted loosely from Wikipedia's "Gravitation" article which is a good start for a more in-depth view of the subject.

Quirks and misconceptions

Gravity is weak

Of the four fundamental forces, gravity, the strong force, electromagnetism and the weak force, gravity is by far the weakest. It is significant on the large scale because it is it is the only one of the four that is universal—it never completely stops with distance—and affects all normal mass-energy (nothing has a gravitational charge of zero, not even light) and always attractively. (Compare electromagnetism, which may attract or repel, and as a consequence of that can be shielded by materials or masses of charged particles, or not affect a particle at all, e.g. a neutron or photon. The strong force dies-off completely at super-nuclear distances and doesn't affect leptons like electrons.) ("dark energy" might interact repulsively to cause the accelerated expansion of the universe, but this is not a settled matter).

Higgs boson

Gravity and the Higgs boson aren't directly connected. General relativity treats mass and energy as equivalent; therefore, if a particle has an energy, from afar it looks like it has mass. Most of the mass of ordinary objects comes from the field energy of the strong nuclear force that keeps protons and neutrons together. Unfortunately, this explains only 99% of their mass. Protons and neutrons consist of quarks, which retain their (relatively small) mass at all times, even when not affected by electromagnetic or other external fields. Particles that move slower than light have mass, and vice versa, particles that move at the speed of light don't have mass. So, you need an explanation for what is the energy that quarks and other massive particles still retain on their own right.

It's been experimentally confirmed that the bosons W+, W- and Z0 that transmit the weak nuclear force - the force responsible for radioactive decay - are massive. The Higgs mechanism is an explanation for this: it proposes the existence of a Higgs field, which is present everywhere in space. Unlike the familiar electromagnetic field, its value is not zero in empty space. Therefore, the bosons gain energy simply by standing still in space. From afar, this appears as mass. Fermions - like the quarks that constitute protons and neutrons, and so all ordinary matter - also interact with the Higgs field, although in a more complicated way than the weak bosons. In this way, they gain mass and cause gravitational attraction. So, the connection of gravity and Higgs is indirect. If you could switch off the Higgs field, the particles in the atoms we're made of would immediately disintegrate and fly away at the speed of light.

If there exists a Higgs field, there must also exist a single wave in this field, a quantum of the Higgs field, a "Higgs boson". The Higgs boson was detected by two experiments at LHC. Unfortunately, the omnipresence of the Higgs field has elicited claims that the Higgs is a "God particle", a term that the physicists involved hate. It's somewhat puzzling that this is something that scientists and creationists finally agree on: it's not a "God particle". The Higgs field is a simple scalar field, not a "god". From something that would be ordinarily a highly technical detail in particle physics research, a surprisingly amount of popular interest has been generated.

Frank Tipler, who tries to prove the existence of God through physics, has claimed in this Omega Point cosmology that the universe could be made to collapse by annihilating the protons and neutrons in the universe, and other such extraordinary claims about Higgs, cosmology and gravity. None of this research has been accepted into a peer-reviewed journal. Physicist Lawrence M. Krauss referred to Tipler's The Physics of Christianity as "a collection of half-truths and exaggerations, I am tempted to describe Tipler's new book as nonsense—but that would be unfair to the concept of nonsense" in a New Scientist article.

Antigravity of antimatter?

Does matter attract or repel antimatter? (In other words: Does antimatter have negative "gravitational mass"? Does antimatter fall down or fall up in Earth's gravity?) It is difficult to measure. Not enough antimatter has been observable for long enough to do a simple free-fall experiment, but it is getting closer. In 2011 CERN's Antihydrogen Laser Physics Apparatus (ALPHA) trapped 309 antihydrogen atoms for up to 1000 seconds.

Alternative theories

Intelligent falling

See also intelligent falling.

There is no such thing as gravity: the earth sucks!

Also the Church of the Flying Spaghetti Monster has shown that gravity is only an illusion caused by the FSM in His goodness pushing everything downwards with His noodly appendage. From this it clearly follows that public schools should teach the controversy about gravity.

Some heretics, however, claim that everything is tied down by the hairs of the mane of the Invisible Pink Unicorn.

There really is no such thing as gravity!

Apparently it's just a phenomenon arising from other stuff.

“”Starting from first principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.

—Erik P. Verlinde 6 Jan '10

... and they said quantum mechanics was hard to comprehend! This is an attempt to solve the problem of the weakness of gravity by claiming it is not a true force, but only an apparent force. There are several such theories, but a common thread is the lack of experimental tests.

Legitimate alternative theories

Einstein's general relativity is considered the best theory of gravity as of today. It is possible to formulate, in principle, infinitely many theories that produce the same results as a given theory. It's the classical "fitting the elephant" problem: John Neumann said, "With four parameters I can fit an elephant, and with five I can make him wiggle his trunk." Therefore, even though a theory might have the exact same predictions as general relativity, that isn't any achievement at all unless: 1) the theory is simpler than relativity, or 2) the theory predicts something which is observed but which relativity predicts wrong.

Nordström's theory of gravity is highly related; however, it is inconsistent with data, but it remains in use as a "toy model" for exploration of the mathematics. Modified Newtonian dynamics is another, more up-to-date attempt at proving Einstein wrong. Generally, however, these theories suffer from having more free parameters than relativity, or the infeasibility of proving them right vs. relativity. Massive amounts of work have been done to detect a "Lorentz violation", in effect anything that moves faster than the speed of light and so contradicts relativity, with no Lorentz violations detected.

Of particular interest are apparent anomalies where objects under the influence of gravity do not behave exactly as expected by general relativity. The Pioneer anomaly was an apparent disparity in the acceleration of the space probes Pioneer 10 and Pioneer 11 which some people believed was due to an incorrect understanding of gravity, although it has been explained by the complex way spacecraft emit heat. The flyby anomaly is a still-unexplained anomaly in the behavior of spacecraft during planetary flybys. Another important discrepancy is in measured galaxy rotation speeds, which led to the hypothesis of dark matter (and other explanations).

Changes in earth's gravity

There are various fringe theories involving changes in the earth's gravity, often correlated with a change in the earth's size (if the earth expanded without gaining mass, gravity on its surface would be reduced). To some people, lower gravity in the past would explain why dinosaurs were so big. Changes in the earth's size such as an Expanding Earth or contraction from Geophysical global cooling have also been used to explain plate tectonics.

This is distinct from the issue of whether the gravitational constant G is changing; a disparity in measurements of G has caused some to speculate that it may not actually be a constant.

Quotations

“”Gravity is not a version of the truth. It is the truth. Anyone who doubts it is invited to jump out a tenth-story window.

—Richard Dawkins

“”Christians don't believe in gravity!

—Peter Griffin, Family Guy, after Mel Gibson fell off Mount Rushmore

For those of you in the mood, RationalWiki has a fun article about Gravitationalism.

See also

• Electromagnetism
• Intelligent falling

External links

• What you don't learn about mass (in grade school).