… 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.
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.
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.”
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.
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, doi.org/nqw
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….
Read more at www.physicstoday.org or scitation.aip.org
Description and first application of a new technique to measure the gravitational mass of antihydrogen
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….
Read more: http://www.nature.com/ncomms/journal/v4/n4/full/ncomms2787.html