Posts Tagged ‘GRAVITY

A New Exponential Gravity

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The square of Hubble parameter in our model comparing with ΛCDM, and its evolution

Qiang Xu, Bin Chen
We propose a new exponential f(R) gravity model with
and  n>3, 1≤λ, c>0 to explain late-time acceleration of the universe. At the high curvature region, the model behaves like the ΛCDM model. In the asymptotic future, it reaches a stable de-Sitter spacetime.
It is a cosmologically viable model and can evade the local gravity constraints easily. This model share many features with other f(R) dark energy models like Hu-Sawicki model and Exponential gravity model.
In it the dark energy equation of state is of an oscillating form and can cross phantom divide line ω<sub>de</sub>=-1. In particular, in the parameter range 3<n≤4, λ~1, the model is most distinguishable from other models.
For instance, when n=4, λ=1, the dark energy equation of state will cross -1 in the earlier future and has a stronger oscillating form than the other models, the dark energy density in asymptotical future is smaller than the one in the high curvature region. This new model can evade the local gravity tests easily when n>3 and λ>1….

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Written by physicsgg

April 2, 2012 at 2:30 pm


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Getting the Swing of Surface Gravity

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Brian C. Thomas & Matthew Quick
Sports are a popular and effective way to illustrate physics principles.
Baseball in particular presents a number of opportunities to motivate student interest and teach concepts.
Several articles have appeared in this journal on this topic, illustrating a wide variety of areas of physics.
In addition, several websites and an entire book are available.
In this paper we describe a student-designed project that illustrates the relative surface gravity on the Earth, Sun and other solar-system bodies using baseball.
We describe the project and its results here as an example of a simple, fun, and student-driven use of baseball to illustrate an important physics principle.
This project was completed to satisfy a course requirement in an introductory astronomy course at Washburn University (a Masters-level university) in Topeka, Kansas.
The assignment was an open-ended, independent project designed and executed by the student. The requirements were that the project must be self-designed and related to astronomy, with creativity emphasized.
The project described here asks the question “What would it be like to play baseball on other
planets?” Two quantities were chosen for comparison: “hang time” of the baseball and distance from home plate to the center field fence.
These values are affected by the surface gravity of the planet or other body.
Surface gravity means the gravitational acceleration at the surface of the body, which depends on both the body’s mass and the distance from the center to the surface.
We realize that one would not actually be able to stand (let alone play baseball!) on the surface of a planet such as Jupiter; the idea is to help students understand surface gravity. This concept may be difficult for some students, since it involves variation of two parameters simultaneously.
Therefore, we hope this exercise will be both engaging and useful in helping students understand the counter-intuitive fact that even a planet with greater mass than the Earth (for instance, Neptune) may have a smaller surface gravity, or vice versa…..

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Written by physicsgg

February 6, 2012 at 9:02 pm


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Flies walk on air in levitation experiment

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Scientists have used magnetic fields to ‘levitate’ flies in the first weightless tests conducted outside space
The technique, known as ”diamagnetic levitation”, allows water and organic based materials to become weightless.
Floating freely inside a plastic tube, the flies were observed closely to spot any changes in their behaviour.
The scientists confirmed effects previously seen in similar experiments in Earth orbit. The flies walked more quickly and more frequently while floating in zero gravity than they did on the ground.
Previously it was not clear whether the changing G-forces associated with space flight may have affected the flies.
The research is published today in the Journal of the Royal Society Interface.
Author Dr Richard Hill, from the University of Nottingham, and colleagues wrote: ”This study shows that the walking speed of fruit flies and their ‘activity’ is altered significantly by counteracting gravitational force.
”Diamagnetic levitation enabled us to maintain tight control over the experimental conditions of all the experimental subjects. This allowed us to identify, unambiguously, the alteration of effective gravity as the cause of the anomalous behaviour.
”Four billion years of evolution have equipped life on Earth to withstand the stresses generated by the ever-present pull of gravity. Here, we have shown that diamagnetic levitation can be used to investigate directly the influence of changing gravity on the locomotion of a complex multi-cellular organism, and that close comparison can be made with experiments performed in space.”
Magnetic fields have been used in previous studies to levitate organic materials, as well as small living organisms and even a live frog.
”Diamagnetic material is weakly repelled from magnetic fields, compared with the more commonly known ‘magnetic’…materials such as iron, which are strongly attracted to a magnetic field,” the scientists wrote. ”The diamagnetic force, balancing the weight of the levitating object, acts at the molecular level throughout the body of the object, just as the centrifugal force balances the gravitational force on an object in Earth orbit.”

Read also: Levitating fruit flies help scientists understand how astronauts are affected by zero gravity

Written by physicsgg

January 4, 2012 at 4:39 pm

Falling atoms measure the Earth’s rotation

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A diagram showing the Stanford set-up, which included two atom interferometers side by side. The parabolic trajectory of the atoms is shown in blue and the laser light in orange

A new type of gyroscope based on interfering atoms has been developed that can determine the latitude where the instrument is located – and also measure true north and the Earth’s rate of rotation. The device has been developed by physicists in the US, who hope to scale it up so that it can test Einstein’s general theory of relativity. They also want to miniaturize the technology so it can be used in portable navigation systems.

The gyroscope has been built by a team led by Mark Kasevich at Stanford University in California. It works by firing a cloud of atoms upwards at a slight angle to the vertical so that the atoms follow a parabolic trajectory as gravity pulls them down. A series of laser pulses is then fired at the cloud while in flight, which separates the atoms into a number of different bunches that follow different trajectories. The pulses are carefully selected so that two of these trajectories cross paths at a detector….. Read the rest of this entry »

Written by physicsgg

October 7, 2011 at 5:09 pm


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Solar Interior May Reveal Modifications to Gravity

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We know lots about gravity in a vacuum but very little about gravity inside extremely massive objects. But astrophysicists say the Sun could reveal all Einstein’s theory of general relativity is one of the cornerstones of modern physics. As such, it is unquestionably a towering achievement. And yet it also raises uncomfortable questions for physicists. The most widely discussed is the conflict between relativity and quantum mechanics.

But there are other less well known problems. Einstein’s equations tell us about gravity in a vacuum at some distance from a massive body. This distortion of spacetime by a massive body is famously analogous to a heavy ball on a rubber sheet.

But what of gravity inside a massive body such the Sun or a neutron star? In a weak gravitational field, like that inside the Earth, Einstein’s equations reduce to their Newtonian equivalent and are well understood.

But in a much stronger field, the answer is not so clear cut because nobody knows how the distortion of spacetime occurs inside matter.

This coupling is a mystery and various theorists have proposed modifications to the theory that make little or no difference to gravity in a vacuum but have important implications for gravitational fields inside big, massive things.

That’s important for the way astrophysicists think about things like neutron stars, since the strength of gravity inside the star determines its internal structure. But since these effects only come into play in extreme fields, nobody has thought of a way to distinguish them from plain vanilla gravity.

Until now. Today, Jordi Casanellas at the Technical University of Lisbon in Portugal and a few pals say that these small modifications to gravity should influence the internal structure of the Sun and that we ought to be able to spot them with measurements we can make today (or at last constrain how big they can be).

The workings of the Sun are reasonably well understood. Astrophysicists assume that our star is in hydrostatic equilibrium–the force of gravity exactly balances the hydrostatic pressure generated by the fusion of hydrogen to form helium.

This assumes an entirely Newtonian gravitational field inside the Sun. So any small modifications should have a significant effect. “Any corrections would affect the thermal balance and, in turn, the temperature profile inside the star, leaving potentially observable signatures,” say Casanellas and co.

Perhaps the best window into the solar interior comes from the flux of neutrinos it emits. That’s because the rate of neutrino production is highly sensitive to temperature. In particular, the neutrinos produced during the decay of boron-8 are very sensitive to temperature and so a good litmus test of the internal temperature profile.

As it turns out, neutrino telescopes on Earth have accurately measured the flux of neutrinos from this reaction. So this has the potential to place strong constraints on the size of any modification to gravity.

Another way of probing the Sun’s interior is to look at the way it vibrates. Again, solar physicists can see the Sun ringing like a bell and this tells them about the internal density and temperature profile.

So what do the current observations tell us about potential modifications to gravity? Casanellas and buddies say current observations place powerful constraints on the size of any new coupling between matter and gravity inside a mass.

But interestingly, the observations do not rule out the main modified theories.

Casanellas and pals take a positive view: “Our results show that the Sun is a very good testing ground to constrain generic modified theories of gravity.”

So there may yet be new gravitational physics to find by studying the Sun in more detail.

Ref: Testing Alternative Theories of Gravity Using the Sun

Written by physicsgg

September 6, 2011 at 8:20 am


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Testing black hole no-hair theorem with OJ287

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We examine the ability to test the black hole no-hair theorem at the 10% level in this decade using the binary black hole in OJ287. In the test we constrain the value of the dimensionless
parameter q that relates the scaled quadrupole moment and spin of the primary black hole: q2 = −q χ^2.  At the present we can say that q = 1 ± 0.3 (one σ), in agreement with General
Relativity and the no-hair theorems. We demonstrate that this result can be improved if more observational data is found in historical plate archives for the 1959 and 1971 outbursts. We also show that the predicted 2015 and 2019 outbursts will be crucial in improving the accuracy of the test. Space-based photometry is required in 2019 July due the proximity of OJ287 to the Sun at the time of the outburst. The best situation would be to carry out the photometry far from the Earth, from quite a different vantage point, in order to avoid the influence of the nearby Sun. We have considered in particular the STEREO space mission which would be ideal if it has a continuation in 2019 or LORRI on board the New Horizons mission to Pluto….
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Written by physicsgg

September 3, 2011 at 5:49 pm

Experiments Show Gravity Is Not an Emergent Phenomenon

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The way gravity effects quantum particles proves that it cannot be an emergent phenomenon, says physicist.

One of the most exciting ideas in modern physics is that gravity is not a traditional force, like electromagnetic or nuclear forces. Instead, it is an emergent phenomenon that merely looks like a traditional force.

This approach has been championed by Erik Verlinde at the University of Amsterdam who put forward the idea in 2010. He suggested that gravity is merely a manifestation of entropy in the Universe, which always increases according to the second law of thermodynamics. This causes matter distribute itself in a way that maximises entropy. And the effect of this redistribution looks like a force which we call gravity.

Much of the excitement over Verlinde’s idea is that it provides a way to reconcile the contradictions between gravity, which works on a large scale, and quantum mechanics, which works on a tiny scale.

The key idea is that gravity is essentially a statistical effect. As long as each particle is influenced by a statistically large number of other particles, gravity emerges. That’s why it’s a large-scale phenomenon.

But today, Archil Kobakhidze at The University of Melbourne in Australia points to a serious problem with this approach. He naturally asks how gravity can influence quantum particles.

Kobakhidze argues that since each quantum particle must be described by a large number of other particles, this leads to a particular equation that describes the effect of gravity.

But here’s the thing: the conventional view of gravity leads to a different equation.

In other words, the emergent and traditional views of gravity make different predictions about the gravitational force a quantum particle ought to experience. And that opens the way for an experimental test.

As it happens, physicists have been measuring the force of gravity on neutrons for ten yeas or so. And…wait for the drum roll… the results exactly match the predictions of traditional gravitational theory, says Kobakhidze.

“Experiments on gravitational bound states of neutrons unambiguously disprove the entropic origin of gravitation,” he says.

That’s an impressive piece of physics. It’ll be interesting to see how Verlinde and his supporters respond.

Ref: Once More: Gravity Is Not An Entropic Force
Read also: Entropic Gravity Getting Messy?

Written by physicsgg

August 24, 2011 at 10:47 pm


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52 Years and $750 Million Prove Einstein Was Right

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An artist's conception of Gravity Probe B orbiting Earth to measure space-time.

In a tour de force of technology and just plain stubbornness spanning half a century and costing more than $750 million, a team of experimenters from Stanford University reported on Wednesday that a set of orbiting gyroscopes had detected a slight sag and an even slighter twist in space-time. The finding confirms some of the weirdest of the many strange predictions — like black holes and the expanding universe — of Albert Einstein’s theory of gravity, general relativity…..Read more:

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May 7, 2011 at 8:35 am


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