As a quantum theory of gravity, loop quantum gravity could potentially solve one of the biggest problems in physics: reconciling general relativity and quantum mechanics. But like all tentative theories of quantum gravity, loop quantum gravity has never been experimentally tested. Now in a new study, scientists have found that, when black holes evaporate, the radiation they emit could potentially reveal “footprints” of loop quantum gravity, distinct from the usual Hawking radiation that black holes are expected to emit.

In this way, evaporating black holes could enable the first ever experimental test for any theory of quantum gravity. However, the proposed test would not be easy, since scientists have not yet been able to detect any kind of radiation from an evaporating black hole.

The scientists, from institutions in France and the US, have published their study called “Probing Loop Quantum Gravity with Evaporating Black Holes” in a recent issue of Physical Review Letters.

“For decades, Planck-scale physics has been thought to be untestable,” coauthor Aurélien Barrau of the French National Institute of Nuclear and Particle Physics (IN2P3) told PhysOrg.com. “Nowadays, it seems that it might enter the realm of experimental physics! This is very exciting, especially in the appealing framework of loop quantum gravity.”

In their study, the scientists have used algorithms to show that primordial black holes are expected to reveal two distinct loop quantum gravity signatures, while larger black holes are expected to reveal one distinct signature. These signatures refer to features in the black hole’s energy spectrum, such as broad peaks at certain energy levels.

Using Monte Carlo simulations, the scientists estimated the circumstances under which they could discriminate the predicted signatures of loop quantum gravity and those of the Hawking radiation that black holes are expected to emit with or without loop quantum gravity. They found that a discrimination is possible as long as there are enough black holes or a relatively small error on the energy reconstruction.

While the scientists have shown that an analysis of black hole evaporation could possibly serve as a probe for loop quantum gravity, they note that one of the biggest challenges will be simply detecting evaporating black holes.

“We should be honest: this detection will be difficult,” Barrau said. “But it is far from being impossible.”

(via Physicists propose test for loop quantum gravity)

Experiments Show Gravity Is Not an Emergent Phenomenon

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.

(via Technology Review)

I’m going to guess that this has something to do with wormholes. Maybe?

(via eirizu)

TOBA is a-swingin’, looking for gravity waves

A few months ago, we reported on a theoretical paper that discussed the potential advantages of a gravity wave detector based on a torsion bar, which the creators called TOBA. In the intervening time, the team has not been idle, as they have a small-scale test bar up and running. Deep in the night of August 15, 2009, they performed a test run to look for gravity waves—not gravity induced pressure fluctuations in Earth’s atmospheric pressure, but stretching space-time.

The good thing about the TOBA experiment is that it fills an important spectral gap in the current generation of gravity wave detectors. Cosmological and astronomical observations can be used to look for extremely low frequency (10^-6Hz) gravitational waves, while the laser interferometer detector (LIGO), and other Earth-bound instruments are used to look for gravity waves with frequencies in the 100Hz plus range. TOBA is designed to operate in the 0.1-1Hz range.

This is important because of something called the cosmic gravitational wave background. You may have heard of cosmic background radiation. This radiation is the oldest in the Universe. It originates in that time interval when the hot dense Universe went through a phase transition from a plasma (consisting of unbound electrons and ions) to neutral atomic species. Before this time, radiation was scattered an awful lot, meaning that its history was rapidly lost. After the Universe became neutral, the scattering was very much reduced, allowing this light to retain its history. The remnant that we measure is the unscattered light from that transition but, optically, that moment in time is like a wall that we simply cannot see through.

So, all the physics of the Universe from times before the transition must be tested rather indirectly, based on what the physics implies about the phase transition. That doesn’t mean it’s all guesswork, but more direct observational data would always be welcome.

This is where gravitational waves can play an important role. Gravity waves don’t care about electromagnetic charge, so this phase transition is unimportant as far as they are concerned. Furthermore, the early Universe is thought to have rung like a bell, with space-time stretching and contracting rhythmically. The frequency and directionality of these waves will tell us about the symmetry properties and, as a result, the physics of the early Universe. These waves are spread across the low frequency end of the gravity wave spectrum, including the range covered by TOBA.

(via Ars Technica)

Earth’s gravity revealed in unprecedented detail

After just two years in orbit, ESA’s GOCE satellite has gathered enough data to map Earth’s gravity with unrivalled precision. Scientists now have access to the most accurate model of the ‘geoid’ ever produced to further our understanding of how Earth works.
 
The new geoid was unveiled today at the Fourth International GOCE User Workshop hosted at the Technische Universität München in Munich, Germany. Media representatives and scientists from around the world have been treated to the best view yet of global gravity.

The geoid is the surface of an ideal global ocean in the absence of tides and currents, shaped only by gravity. It is a crucial reference for measuring ocean circulation, sea-level change and ice dynamics – all affected by climate change.

(via ESA Portal)

But how can…and…out of that? Okay. Okay.

fuckyeahmath:

sebseballade:

Zero Gravity Coffee Cup

Forgotten Soviet Moon Rover Beams Light Back to Earth

Rediscovering the 40-year-old robot could help astronomers put general relativity to the test

Sitting at his home computer on the evening of 22 April, Tom Murphy, an astrophysicist at the University of California, San Diego, logged into an observation session 1200 kilometers away, at Apache Point Observatory. From a pine-dotted ridge above the White Sands Missile Range, Russet McMillan, the on-site specialist, aimed the New Mexico observatory’s telescope at a small patch of dust near the edge of the moon’s face. Then, at Murphy’s go-ahead, she fired a stream of laser pulses into the night sky.

The pulses—20 per second—shot toward the moon and, after little more than a second, bathed the lunar dust patch in a pool of green light. Another second passed. Then Murphy saw a blip in the data on his screen. It suggested that an unusually large number of photons had returned from the moon and were being recorded by the telescope’s photodiode.

At first, Murphy thought the blip might just be an artifact of the instrumentation, a common disturbance caused by turning the detector on and off. But no matter how McMillan tweaked the instruments, the signal kept showing up. By the next morning after analyzing the data, he was sure the blip represented something much more significant: contact with the first robot to roam a surface beyond Earth. Until NASA’s Lunar Reconnaissance Orbiter snapped photographs of the robot’s tracks earlier that month, no one had been able to locate the Soviet rover Lunokhod 1 for nearly four decades.

But the discovery has turned out to be more than a just a fun bit of space archaeology. Now that Murphy has confirmed the location of Lunokhod 1, he plans on using the aging rover to help measure the moon’s movements and test theories of gravity with the greatest precision to date.

[Source: IEEE]

Waitaminutewaitaminute - what about Petr Hořava’s theory of gravity? Doesn’t it do away with the most of the handwaving that is Dark Energy/Dark Matter?

physicsphysics:

loveyourchaos:

heavycurses:space-and-time:

there is so much we don’t know

unknownskywalker:

Pushing and Pulling

Rather than being an unchanging disk of peaceful particles, the material that makes up Saturn’s rings is constantly pushed and pulled into spectacular shapes.

On the left of the image, the moon Daphnis affects material as it orbits in the A ring’s Keeler Gap. The moon’s orbit is inclined relative to the plane of Saturn’s rings. Daphnis’ gravitational pull perturbs the orbits of the particles forming the Keeler Gap’s edge. This sculpts the edge into waves having both horizontal and out-of-plane components. Material on the inner edge of the gap orbits faster than the moon so that the waves there lead the moon in its orbit. Material on the outer edge moves slower than the moon, so waves there trail the moon.

On the right, the material at the edge of the Encke Gap shows waves caused by Pan.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 3, 2010. The view was acquired at a distance of approximately 531,000 kilometers from Saturn. Image scale is 3 kilometers (2 miles) per pixel.

Image Credit: NASA JPL

unknownskywalker:

Gravity-like theories give insight into the strong force

A new computation of the constant that describes the strength of the force between the quarks in a proton may help theorists tackle one of the most challenging problems of physics: analytically solving the theory of QCD and determining its coupling strength at large distances.

Quantum Chromodynamics is the theory of the strong force, describing how quarks combine to make the protons and neutrons in the nucleus of the atom. While the strong force strength is known to be weak at small separation between quarks, its value and behavior at large distances is uncertain and hotly debated.

To tackle that problem, scientists computed the constant that describes the strength of the force between the quarks in a proton. They computed the constant using a novel approach: the Maldacena conjecture, a method that connects QCD-like theories in physical space to gravity-like theories in a mathematical five-dimensional space.

The calculation showed that the Maldacena conjecture provides an analytical way to solve QCD. Their analysis also clarifies why different earlier calculations have yielded different values for the constant, thus giving new insights into how to consistently define strong force coupling, as well as providing new non-trivial tests of QCD.

Image: The typical four-dimensional structure of gluon-field configurations averaged over in describing the vacuum properties of QCD. QCD induces chromo-electric and chromo-magnetic fields throughout space-time in its lowest energy state. After a few sweeps of smoothing the gluon field, a lumpy structure reminiscent of a lava lamp is revealed. This is the QCD Lava Lamp. [via]

Source: The paper is available online at Physical Review D. [via]

Proposed Spacetime Structure Could Provide Hints for Quantum Gravity Theory

Spacetime, which consists of three dimensions of space and one time dimension, is such a large, abstract concept that scientists have a very difficult time understanding and defining it. Moreover, different theories offer different, contradictory insights on spacetime’s structure. While general relativity describes spacetime as a continuous manifold, quantum field theories require spacetime to be made of discrete points. Unifying these two theories into one theory of quantum gravity is currently one of the biggest unsolved problems in physics.

(via PhysOrg)