Posts Tagged ‘GRAVITY

Hunting Gravitational Waves with Lasers

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Written by physicsgg

April 11, 2014 at 6:17 pm

Humans Running in Place on Water …

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… at Simulated Reduced Gravity


On Earth only a few legged species, such as water strider insects, some aquatic birds and lizards, can run on water. For most other species, including humans, this is precluded by body size and proportions, lack of appropriate appendages, and limited muscle power. However, if gravity is reduced to less than Earth’s gravity, running on water should require less muscle power. Here we use a hydrodynamic model to predict the gravity levels at which humans should be able to run on water. We test these predictions in the laboratory using a reduced gravity simulator.

Methodology/Principal Findings
We adapted a model equation, previously used by Glasheen and McMahon to explain the dynamics of Basilisk lizard, to predict the body mass, stride frequency and gravity necessary for a person to run on water. Progressive body-weight unloading of a person running in place on a wading pool confirmed the theoretical predictions that a person could run on water, at lunar (or lower) gravity levels using relatively small rigid fins. Three-dimensional motion capture of reflective markers on major joint centers showed that humans, similarly to the Basilisk Lizard and to the Western Grebe, keep the head-trunk segment at a nearly constant height, despite the high stride frequency and the intensive locomotor effort. Trunk stabilization at a nearly constant height differentiates running on water from other, more usual human gaits.

The results showed that a hydrodynamic model of lizards running on water can also be applied to humans, despite the enormous difference in body size and morphology.

Citation: Minetti AE, Ivanenko YP, Cappellini G, Dominici N, Lacquaniti F (2012) Humans Running in Place on Water at Simulated Reduced Gravity. PLoS ONE 7(7): e37300.


Written by physicsgg

September 13, 2013 at 9:34 am

Posted in Fluid Dynamics, HUMOR, PHYSICS

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Strength of gravity shifts – and this time it’s serious

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The gravitational constant might not be that... constant (Image: Noël Gaspard/Millennium Images)

The gravitational constant might not be that… constant (Image: Noël Gaspard/Millennium Images)

by Katia Moskvitch
Did gravity, the force that pins us to Earth’s surface and holds stars together, just shift? Maybe, just maybe. The latest measurement of G, the so-called constant that puts a figure on the gravitational attraction between two objects, has come up higher than the current official value.

Measurements of G are notoriously unreliable, so the constant is in permanent flux and the official value is an average. However, the recent deviation is particularly puzzling, as it is at once starkly different to the official value and yet very similar to a measurement made back in 2001, not what you would expect if the discrepancy was due to random experimental errors.

It’s possible that both experiments suffer from a hidden, persistent error, but the result is also prompting serious consideration of a weirder possibility: that G itself can change. That’s a pretty radical option, but if correct, it would take us a step closer to tackling one very big mystery – dark energy, the unknown entity accelerating the expansion of the universe.

“If G has changed by this tiny amount then we would expect that G depends on a new field,” says cosmologist Tony Padilla of the University of Nottingham, UK. “One could imagine a scenario in which this field plays a role in dark energy.”

Twisting wires

According to Isaac Newton, the gravitational attraction between two objects is proportional to their masses and inversely proportional to the square of distance between them. G puts an absolute value on the attraction.

It was first measured in a laboratory in 1798 by British scientist Henry Cavendish using a device that determines the twisting of a wire due to the gravitational attraction of two pairs of precisely known masses.

Since then, other methods have produced a multitude of different values. This is assumed to be due to various experimental errors and the official value of G is routinely updated to reflect this, with the assumption that the values will eventually converge.

Now a team led by Terry Quinn of the International Bureau of Weights and Measures (BIPM) in Paris, France, and Clive Speake of the University of Birmingham, UK, has measured G using two methods: a modern version of the Cavendish experiment and one that relies on electrostatics. The resulting value for G is 240 parts per million bigger than the official one, set in 2010.

Violets in springtime

The figure alone is not the weird part – one recent measurement came up 290 ppm below today’s official value. The strange thing about the latest one is that it is just 21 ppm off the value Quinn’s team got using the same set-ups in 2001. Since the team took care this time around to remove every source of error that might have been at play back then, you would not expect the two results to be identical.

Quinn has arranged a special conference on G at the Royal Society in London in February to discuss the problem.

“This meeting is going to be very exciting,” says James Hough, an experimental physicist from the University of Glasgow, UK. But he suggests carrying out the experiment a third time. “My own view is that the BIPM experiment needs to be copied exactly in another laboratory on a different continent by different experimenters initially to see if the same result is obtained,” he says.

However, James Faller of the University of Colorado at Boulder, who tested G in 2010, is holding out for an error: “Errors are like violets in the springtime: they can spring up in any group’s experiment,” he says.

Fifth force

But the latest result could also be evidence that gravity itself may be changing.

“Logically, either some of the experiments are wrong, or G is not constant,” says Mark Kasevich of Stanford University.

An oscillating G could be evidence for a particular theory that relates dark energy to a fifth, hypothetical fundamental force, in addition to the four we know – gravity, electromagnetism, and the two nuclear forces. This force might also cause the strength of gravity to oscillate, says Padilla. “This result is indeed very intriguing.”

A further, less radical option is that G is still a constant but that Quinn’s team has hit upon its true value. That would mean the actual value of G is higher than the official figure, which is interesting in itself, says cosmologist Clare Burrage of Nottingham University.

“If the value of G is slightly bigger, then we have to go back and redo all the calculations,” she says. “Stars would burn up quicker than we previously thought because it takes more energy to push against a stronger gravitational force.”

Journal reference: Physical Review Letters,

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Written by physicsgg

September 12, 2013 at 9:38 am


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Video: Lagrange Points

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Sixty Symbols

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June 8, 2013 at 6:05 pm


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Anatomy of a fall: Giovanni Battista Riccioli and the story of g

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The Asinelli Tower,which Giovanni Riccioli considered to be “as commodious as possible” to falling-body experiments, stands nearly 100 m above the heart of Bologna, Italy. (a) Riccioli’s sketch illustrates his experimental findings: A ball dropped from the tower’s summit, point O, reaches points C, Q, R, S, and T in times corresponding to 5, 10, 15, 20, and 25 pendulum strokes, respectively. (Image from ref. 14.) (b) A photo shows the tower as seen today.

The Asinelli Tower,which Giovanni Riccioli considered to be “as commodious as possible” to falling-body experiments, stands nearly 100 m above the heart of Bologna, Italy.
(a) Riccioli’s sketch illustrates his experimental findings: A ball dropped from the tower’s summit, point O, reaches points C, Q, R, S, and T in times corresponding to 5, 10, 15, 20, and 25 pendulum strokes, respectively.
(b) A photo shows the tower as seen today.

Christopher M. Graney
Every physics student learns about falling bodies and g, the acceleration due to Earth’s gravitational field. But few physicists learn the story of the first experiments—now more than three centuries old—to measure g.
That story begins in earnest with the famed Italian astronomer Galileo Galilei. In his 1632 tome, Dialogue Concerning the Two Chief World Systems, Galileo writes that the acceleration of straight motion in heavy [falling] bodies proceeds according to the odd numbers beginning from one.
That is, marking off whatever equal times you wish . . . if the moving body leaving a state of rest shall have passed during the first time such a space as, say, an ell, then in the second time it will go three ells; in the third, five; in the fourth, seven, and it will continue thus according to the successive odd numbers.
In sum, this is the same as to say that the spaces passed over by the body starting from rest have to each other the ratios of the squares of the times in which such spaces were traversed.
To Giovanni Battista Riccioli—an astronomer, Jesuit priest, and fellow Italian—Galileo’s claims were dubious, especially the assertion that an iron ball dropped from a height of 100 cubits took five seconds to reach the ground.
The ball seemed too heavy, and the time of fall too long, to be plausible. Plus, Galileo had provided few details about his experimental procedure.
So Riccioli conducted his own free-fall study. His experiments, which for the most part vindicated Galileo’s theory, have come to be regarded by historians as the first precise measurements of g.
Although historians of science have discussed the experiments in some detail, Riccioli’s own report has yet to be fully translated into a modern language. That remains the physics world’s loss, for Riccioli’s report on falling bodies tells the story of a remarkable experiment performed by a remarkable scientist….
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May 5, 2013 at 8:54 am

ALPHA weighs in on antimatter

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Description and first application of a new technique to measure the gravitational mass of antihydrogen

The Alpha experiment's antimatter chamber uses magnetic fields to sequester antihydrogen atoms

The Alpha experiment’s antimatter chamber uses magnetic fields to sequester antihydrogen atoms

Physicists have long wondered whether the gravitational interactions between matter and antimatter might be different from those between matter and itself. Although there are many indirect indications that no such differences exist and that the weak equivalence principle holds, there have been no direct, free-fall style, experimental tests of gravity on antimatter. Here we describe a novel direct test methodology; we search for a propensity for antihydrogen atoms to fall downward when released from the ALPHA antihydrogen trap. In the absence of systematic errors, we can reject ratios of the gravitational to inertial mass of antihydrogen >75 at a statistical significance level of 5%; worst-case systematic errors increase the minimum rejection ratio to 110. A similar search places somewhat tighter bounds on a negative gravitational mass, that is, on antigravity. This methodology, coupled with ongoing experimental improvements, should allow us to bound the ratio within the more interesting near equivalence regime….
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Read also: Antigravity gets first test at Cern’s Alpha experiment

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April 30, 2013 at 8:38 pm

Posted in High Energy Physics

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What if the Earth were Hollow?

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Written by physicsgg

August 28, 2012 at 5:33 pm

Posted in PHYSICS

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Probing gravity

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The Gravity Probe-B spacecraft. A series of new papers describes the key astronomical results pertaining to this mission which measured Einstein’s relativistic frame-dragging effect. Credit: NASA/Stanford

Einstein’s theory of relativity is remarkable not only because it is so successful in explaining seemingly bizarre observations (like the bending of starlight) or because it has assembled a coherent picture of nature. One would expect these results from any good theory. Relativity is also amazing because its has shown that the universe behaves in completely non-intuitive ways (at least to humans): time dilates, lengths contract, gravity warps space, and mass and energy are related by E=mc^2. Our so-called “common sense” is sometimes just plain wrong.
It is no wonder, therefore, that astronomers are constantly testing relativity to see whether all of its details are perfectly in order, or if some adjustment might be necessary that might also change our basic understanding of space and time. One of its more curious, non-intuitive predictions is that space is not only warped by the gravity of a massive body – it is also warped (though to a lesser degree) by the rotation of a body, the so-called “frame dragging effect.” This particular prediction of relativity is small and extremely hard to measure. How small? The axis of a precessing gyroscope traces a circle that is 360 degrees around. According to Einstein’s theory a gyroscope orbiting the Earth (as per the experiment described below) would, because of frame dragging, have its axis precess by 11 millionths of one degree per year — very tiny indeed. In 2004, NASA launched the Gravity Probe-B mission, an heroic experiment developed primarily at Stanford University, to test this minuscule but critically important prediction, and a team of CfA astronomers worked on the mission. In 2011, the NASA/Stanford team reported their conclusion: no disagreement with relativity. In a series of seven papers published in this month’s Astrophysical Journal Supplement, the many astronomical issues involved with the analysis are presented in detail. In the summary paper, CfA scientists Irwin Shapiro, Daniel Lebach, Michael Ratner, and four colleagues discuss the critical issue of how to measure the tiny predicted precession. Key to the experiment was a guide star that provided the absolute reference for the spacecraft and its four cryogenically cooled, superconducting “gyroscopes.” The experiment team in the planning stages chose the star IM Pegasi because it is bright at both optical and radio wavelengths and is located in a convenient part of the sky for the satellite. Using techniques of ground-based very long baseline radio interferometry referenced to distant quasars, the astronomers began an intensive multi-year program of study of this star’s motion in the sky, working from 1997 until 2005. All motions of the star would have to be taken into account in the analysis; an ancillary result would be the distance of the star from Earth.
In their new paper the team reports that the star is located 314. 4 light-years away with an uncertainty of about 2.2 light-years, and that it moves across the sky (“proper motion”) at a rate of 34.3 thousandths of an arc-second per year. The new series of papers, and the meticulous discussion of the many astronomical factors that had to be accounted for in the analyses, mark an important stage in the effort to probe Einstein’s theory at amazing new levels of precision.
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Written by physicsgg

July 30, 2012 at 3:05 pm


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