Geodes...

(Just_Super/iStock)

Topics: Black Holes, Cosmology, Dark Energy, Einstein, General Relativity, Gravity

A fifty-year-old hypothesis predicting the existence of bodies dubbed Generic Objects of Dark Energy (GEODEs) is getting a second look in light of a proposed correction to assumptions we use to model the way our Universe expands.

If this new version of a classic cosmological model is correct, some black holes could hide cores of pure dark energy, pushing our Universe apart at the seams.

University of Hawaii astrophysicist Kevin Croker and mathematician Joel Weiner teamed up to challenge the broadly accepted notion that when it comes to the Universe's growing waistline, its contents are largely irrelevant.

"For 80 years, we've generally operated under the assumption that the Universe, in broad strokes, was not affected by the particular details of any small region," said Croker.

"It is now clear that general relativity can observably connect collapsed stars – regions the size of Honolulu – to the behavior of the Universe as a whole, over a thousand billion billion times larger."

Not only could this alternative interpretation of fundamental physics change how we understand the Universe's expansion, but we might need to also consider how that growth might affect compact objects like the cores of collapsing stars.

Black Holes May Hide Cores of Pure Dark Energy That Keep The Universe Expanding
Mike McCrae, Science Alert

Milky Way and Einstein...

This artist’s rendition shows the orbits of stars circling the supermassive black hole (blue halo) at the Milky Way’s center. A close analysis suggests the stars’ orbits are showing subtle effects predicted by Einstein’s theory of general relativity. Credit: ESO/M. Parsa/L. Calçada

Topics: Astrophysics, Black Holes, Einstein, General Relativity, Gravity

A giant star near the center of our galaxy hints, once again, that Albert Einstein was correct about gravity.

A group of astronomers in Germany and the Czech Republic observed three stars in a cluster near the supermassive black hole at the center of the Milky Way galaxy. Using data from the Very Large Telescope in Chile, among others, the researchers tracked how the stars moved as they went around the monster black hole.

One of the stars, called S2, showed slight deviations in its orbit that might indicate relativistic effects, scientists said. If the observations are confirmed, then it shows that Einstein's theory of general relativity holds even under extreme conditions — in gravity fields produced by objects like the galactic center's black hole, which contains the mass of 4 million suns. General relativity says that massive objects bend the space around them, causing other objects to deviate from straight lines they would follow absent any forces on them.

Closest Supermassive Black Hole Tests Einstein’s Relativity, Jesse Emspak, SPACE.com and Scientific American

Gravity on a Chip...

Diagram showing how a BEC (red dot) is created at the top of the chip. It then falls and is split into two BECs that take separate paths to the bottom of the chip. (Courtesy: S Abend/Phys. Rev. Lett.)

Topics: Bose-Einstein Condensate, Gravity, Nanotechnology, Quantum Mechanics, Semiconductor Technology

A new sensor that measures the local acceleration due to gravity using a Bose–Einstein condensate (BEC) of ultracold atoms has been made by physicists in Germany, the US and Canada. While the prototype device is not as accurate as commercial gravimeters, its makers say it could be made much smaller and much more accurate than existing devices.

Atoms can be used to measure the acceleration due to gravity by cooling a gas of them to near absolute zero and then dropping them along two different paths in an interferometer. The quantum interference that occurs when the paths converge at a detector provides a very good measure of gravity, with commercial atom interferometers able to measure the acceleration to within one part in 108. Such measurements are invaluable for geological exploration because the presence of certain minerals can be spotted by seeking tiny variations in gravity at the Earth's surface.

While these ultracold atom gravimeters are on a par with conventional absolute gravimeters based on macroscopic falling masses, their accuracy could be improved a lot by using a BEC. In a conventional atomic gravimeter, the ultracold atoms form a diffuse gas roughly a millimetre in size and a major cause of uncertainty is that the laser pulses used to control the atoms are not spatially uniform on that length scale. A BEC – formed by cooling a gas of atoms with integer spin until they condense into a single quantum state – reduces this uncertainty because it squeezes the atoms into a region that is about 100 times smaller.

Physics World: Gravity measured using a Bose–Einstein condensate on a chip
Hamish Johnston

Postage Stamp Gravimeter...

Image Source: Link Below

Topics: Electrical Engineering, Geophysics, Gravitational Waves, Gravity, MEMS

UK researchers have built a small device that measures tiny fluctuations in gravity, and could be used to monitor volcanoes or search for oil.

Such gravimeters already exist but compared to this postage stamp-sized gadget, they are bulky and pricey.

The new design is based on the little accelerometers found in smartphones.

To begin with, the team - from the University of Glasgow - tested it by measuring the Earth's tides over a period of several days.

Tidal forces, caused by the interacting pull of the Sun and Moon, not only drag the oceans up and down but slightly squash the Earth's diameter.

"It's not a very big squeeze, but it means that essentially Glasgow - or anywhere else on the Earth's crust - goes up and down by about 40cm over the course of 12-13 hours," said Richard Middlemiss, the PhD student who made the new instrument.

"That means that we get a change in gravitational acceleration - so that's what we've been able to measure."

Like most gravimeters, the heart of the new instrument is a weight hanging from a spring. Unlike all other gravimeters thus far, this one is a MEMS: a "microelectromechanical system".

The whole sensor is carved from a sheet of silicon 0.2mm thick; the "weight" is a small slab of that silicon and the "spring" consists of several thin shafts that hold it in place.

BBC Science and Environment: Small, cheap gravity gadget to peer underground
Jonathan Webb

Killing Schrödinger's Cat...

Topics: Einstein, Gravity, General Relativity, Modern Physics, Quantum Mechanics, Schrödinger's Cat

If the cat in Erwin Schrödinger's famous thought-experiment behaved according to quantum theory, it would be able to exist in multiple states at once: both dead and alive. Physicists' common explanation for why we don’t see such quantum superpositions—in cats or any other aspect of the everyday world—is interference from the environment. As soon as a quantum object interacts with a stray particle or a passing field, it picks just one state, collapsing into our classical, everyday view.

But even if physicists could completely isolate a large object in a quantum superposition, according to researchers at the University of Vienna, it would still collapse into one state—on Earth's surface, at least. “Somewhere in interstellar space it could be that the cat has a chance to preserve quantum coherence, but on Earth, or near any planet, there's little hope of that,” says Igor Pikovski. The reason, he asserts, is gravity.

Cinema-goers who saw the film Interstellar are already familiar with the basic principle behind the Vienna team’s work. Einstein’s theory of general relativity states that an extremely massive object causes clocks near it to run more slowly because its strong gravitational field stretches the fabric of space-time (which is why a character in the film aged only an hour near a black hole, while seven years passed on Earth). On a subtler scale, a molecule placed nearer the Earth’s surface experiences a slightly slower clock than one placed slightly further away.

Because of gravity’s effect on space-time, Pikovski’s team realised that variance in a molecule’s position will also influence its internal energy—the vibrations of particles within the molecule, which evolve over time. If a molecule were put in a quantum superposition of two places, the correlation between position and internal energy would soon cause the duality to 'decohere' to the molecule taking just one path, they suggest. “In most situations decoherence is due to something external; here it’s as though the internal jiggling is interacting with the motion of the molecule itself,” adds Pikovski.

Scientific American: Gravity Kills Schrödinger's Cat, Elizabeth Gibney and Nature magazine

General Relativity...

Image Source: MIT Open Course Ware - General Relativity

Topics: Black Holes, Einstein, Special Relativity, GPS, Gravity, General Relativity, Spacetime, Wormholes

The 100th anniversary of the General Theory of Relativity also happens to have coincided with Einstein's birthday and the American Nerd-inspired Pi Day last Saturday (I say American, because it works when you use the dating sequence 3-14-15, and breaks down if you use military or European dating formats: e.g. 14 March 15; 14.3.15). Star Trek abused the word "warp" ad nauseum to get their astronauts from one side of the galaxy to the other in record time to solve galactic issues before the ending credits. Space is still vast, and getting to even our own solar system's planets in a human lifetime will take something more than conventional chemical rockets and Newtonian momentum, hence NASA's concentration on breakthrough propulsion technologies up to and inclusive of warp drive. Quoting one of the articles whose link I give below:

In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers. This was the theory of special relativity. It introduced a new framework for all of physics and proposed new concepts of space and time.

Einstein then spent ten years trying to include acceleration in the theory and published his theory of general relativity in 1915. In it, he determined that massive objects cause a distortion in space-time, which is felt as gravity. [1]

It is our current, best description in modern physics of gravity, and along with its effects, our Global Positioning Systems in our cars and smart phones; the evolution of stars into Brown Dwarfs; White Dwarfs, Black Holes and the theoretical possibility of Wormholes. It has outlived Einstein and proven its usefulness time and again.

1. Space.com: Einstein's Theory of General Relativity, Nola Taylor Redd
2. Einstein-Online: General Relativity
3. Princeton University Press: Relativity: The Special and the General Theory
100th Anniversary edition, Edited by Hanoch Gutfreund & Jürgen Renn
4. Physics Central: Einstein's Relativity and Everyday Life, Clifford M. Will (think GPS)

DSR and Gravity's Rainbow...

Dr. Stephen Hawking of Cambridge University alongside illustrations of a black hole and an event horizon with Hawking Radiation. He continues to engage his grey matter to uncover the secrets of the Universe while others attempt to confirm his existing theories. Credit: Photo: BBC, Illus.: T.Reyes

Topics: Big Bang, Black Holes, Einstein, DSR, Gravity, Spacetime, Special Relativity

We've come a long way in 13.8 billion years; but despite our impressively extensive understanding of the Universe, there are still a few strings left untied. For one, there is the oft-cited disconnect between general relativity, the physics of the very large, and quantum mechanics, the physics of the very small. Then there is problematic fate of a particle's intrinsic information after it falls into a black hole. Now, a new interpretation of fundamental physics attempts to solve both of these conundrums by making a daring claim: at certain scales, space and time simply do not exist.

Let's start with something that is not in question. Thanks to Einstein's theory of special relativity, we can all agree that the speed of light is constant for all observers. We can also agree that, if you're not a photon, approaching light speed comes with some pretty funky rules – namely, anyone watching you will see your length compress and your watch slow down.

But the slowing of time also occurs near gravitationally potent objects, which are described by general relativity. So if you happen to be sight-seeing in the center of the Milky Way and you make the regrettable decision to get too close to our supermassive black hole's event horizon (more sinisterly known as its point-of-no-return), anyone observing you will also see your watch slow down. In fact, he or she will witness your motion toward the event horizon slow dramatically over an infinite amount of time; that is, from your now-traumatized friend's perspective, you never actually cross the event horizon. You, however, will feel no difference in the progression of time as you fall past this invisible barrier, soon to be spaghettified by the black hole's immense gravity.

So, who is "correct"? Relativity dictates that each observer's point of view is equally valid; but in this situation, you can't both be right. Do you face your demise in the heart of a black hole, or don't you? (Note: This isn't strictly a paradox, but intuitively, it feels a little sticky.)

And there is an additional, bigger problem. A black hole's event horizon is thought to give rise to Hawking radiation, a kind of escaping energy that will eventually lead to both the evaporation of the black hole and the destruction of all of the matter and energy that was once held inside of it. This concept has black hole physicists scratching their heads. Because according to the laws of physics, all of the intrinsic information about a particle or system (namely, the quantum wavefunction) must be conserved. It cannot just disappear.

Why all of these bizarre paradoxes? Because black holes exist in the nebulous space where a singularity meets general relativity – fertile, yet untapped ground for the elusive theory of everything.

Enter two interesting, yet controversial concepts: doubly special relativity and gravity's rainbow.

Phys.org:
Space-time theory may reconcile black hole conundrum
Vanessa Janek, Universe Today

Climate and Gravity...

Source: ESA GOCE

GOCE stands for "Gravity Field and Steady-State Ocean Circulation Explorer." It's not an acronym I know off the top of my head, either.

The physics definition: "the force that attracts a body toward the center of the earth, or toward any other physical body having mass. For most purposes Newton's laws of gravity apply, with minor modifications to take the general theory of relativity into account."

Penguins are not flying off into orbit, and I hope the news doesn't do its usual bit of sensationalism for ratings - why, for the most part I've given up looking too much at the news and prefer to read summaries off the Internet - from other countries.

This IS a data point, and an important one. It means climate change can be measured not just in temperature or change in weather patterns.

From one of the articles (GOCE):

Scientists are now armed with the most accurate gravity model ever produced. This is leading to a much better understanding of many facets of our planet – from the boundary between Earth’s crust and upper mantle to the density of the upper atmosphere.

The strength of gravity at Earth’s surface varies subtly from place to place owing to factors such as the planet’s rotation and the position of mountains and ocean trenches.

Changes in the mass of large ice sheets can also cause small local variations in gravity. [1]

It will be useful information we can use in the exploration of other worlds, some in our own solar system's backyard.

Discovery ended on a somber note:

The news doesn't get much better for Antarctica. Earlier this year, two studies were released that indicated the West Antarctic Ice Sheet is headed for an irreversible collapse in roughly 200 years. Should the ice sheet completely collapse, scientists believe it could raise sea level by more than 10 feet. [2]

Bye-bye Florida in the 23rd century. Considering it's where we have traditionally launched space vehicles, that would be fairly unfortunate, especially for the Floridians!

1. GOCE: GOCE reveals gravity dip from ice loss
2. Discover: Antarctic Ice Melt is Changing Earth’s Gravity, Carl Engelking

Rivers and GRACE...

Source: Science Magazine; US Army Corp of Engineers

When ground water saturates a river basin, the risk for flooding goes up. So does the strength of Earth’s gravity in that region, ever so slightly, because of the extra mass of the underground water. By using tiny variations in gravity detected from space, researchers report online today in Nature Geoscience that they can identify basins that are primed for flooding if additional rains come—sometimes with several months' warning. As a test case, the scientists looked at the gravity signals leading up to catastrophic floods in 2011 on the Missouri River (pictured above). They used data from NASA’s Gravity Recovery and Climate Experiment (GRACE), a pair of orbiting satellites that get tugged around the Earth faster in places where gravity is slightly stronger.

Science Magazine: Gravity measurements can predict river flooding, Eric Hand
UT Austin Climate Science Research: GRACE
NASA: GRACE - Earth Missions

Dark Matter and Dino...

Artist's impression of a comet striking the Earth. (Courtesy: iStock/PaulPaladin)

Two theoretical physicists in the US have made a surprising connection between dinosaur extinction and dark matter. Lisa Randall and Matthew Reece of Harvard University believe that some of this mysterious invisible matter – which makes up 85% of all matter in the universe – could exist in a special form that affects the rate at which comets strike our planet. A comet crashing into Earth about 66 million years ago is one possible reason why these giant creatures died off.

Comets have smashed into Earth throughout its history, creating huge craters and possibly causing mass extinctions, such as that which befell the dinosaurs. Many of these comets come from the Oort cloud, which is a huge halo of small icy objects that surrounds the Sun, out to a distance of about one light year. But rather than being entirely random, there is some evidence that the frequency of comet impacts oscillates on a timescale of about 35 million years.

Although this oscillation is not certain, if it is true, there could be something on that timescale that affects the rate at which comets from the Oort cloud are sent towards Earth. Two possible explanations have been proposed so far. One – dubbed the "nemesis hypothesis" – involves the gravitational pull of an as-yet-undiscovered distant companion star to the Sun. The other involves the oscillating pull of the dense galactic disc as the solar system crosses and re-crosses the plane of the Milky Way.

Physics World: Did dark matter help kill off the dinosaurs?

The Pendulum and Dimensions...

Discover Magazine

While most of us take gravity for granted, physicists have a big problem with it. Their beef: As forces go, gravity is implausibly feeble. (Try asking a physicist why a kitchen magnet can pick up a paper clip even though the gravitational force of the entire Earth is pulling the clip down.) In 1999 University of Washington physicist Eric Adelberger heard a lecturer offer an intriguing explanation: Perhaps gravity only appears weak, because it operates in additional spatial dimensions beyond length, width, and height. These extra dimensions would be imperceptible in our macro world but might have a detectable influence on gravity at scales of less than the width of a hair.

Discover Magazine:

Tools of the Trade: The Pendulum That Detects Extra Dimensions