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Physicists have traced three of the four forces of nature the electromagnetic force and the strong and weak nuclear forces to their origins in quantum particles. But the fourth fundamental force, gravity, is different.

Our current framework for understanding gravity, devised a century ago by Albert Einstein, tells us that apples fall from trees and planets orbit stars because they move along curves in the space-time continuum. These curves are gravity. According to Einstein, gravity is a feature of the space-time medium; the other forces of nature play out on that stage.

But near the center of a black hole or in the first moments of the universe, Einsteins equations break. Physicists need a truer picture of gravity to accurately describe these extremes. This truer theory must make the same predictions Einsteins equations make everywhere else.

Physicists think that in this truer theory, gravity must have a quantum form, like the other forces of nature. Researchers have sought the quantum theory of gravity since the 1930s. Theyve found candidate ideas notably string theory, which says gravity and all other phenomena arise from minuscule vibrating strings but so far these possibilities remain conjectural and incompletely understood. A working quantum theory of gravity is perhaps the loftiest goal in physics today.

What is it that makes gravity unique? Whats different about the fourth force that prevents researchers from finding its underlying quantum description? We asked four different quantum gravity researchers. We got four different answers.

Claudia de Rham, a theoretical physicist at Imperial College London, has worked on theories of massive gravity, which posit that the quantized units of gravity are massive particles:

Einsteins general theory of relativity correctly describes the behavior of gravity over close to 30 orders of magnitude, from submillimeter scales all the way up to cosmological distances. No other force of nature has been described with such precision and over such a variety of scales. With such a level of impeccable agreement with experiments and observations, general relativity could seem to provide the ultimate description of gravity. Yet general relativity is remarkable in that it predicts its very own fall.

General relativity yields the predictions of black holes and the Big Bang at the origin of our universe. Yet the singularities in these places, mysterious points where the curvature of space-time seems to become infinite, act as flags that signal the breakdown of general relativity. As one approaches the singularity at the center of a black hole, or the Big Bang singularity, the predictions inferred from general relativity stop providing the correct answers. A more fundamental, underlying description of space and time ought to take over. If we uncover this new layer of physics, we may be able to achieve a new understanding of space and time themselves.

If gravity were any other force of nature, we could hope to probe it more deeply by engineering experiments capable of reaching ever-greater energies and smaller distances. But gravity is no ordinary force. Try to push it into unveiling its secrets past a certain point, and the experimental apparatus itself will collapse into a black hole.

Daniel Harlow, a quantum gravity theorist at the Massachusetts Institute of Technology, is known for applying quantum information theory to the study of gravity and black holes:

Black holes are the reason its difficult to combine gravity with quantum mechanics. Black holes can only be a consequence of gravity because gravity is the only force that is felt by all kinds of matter.If there were any type of particle that did not feel gravity, we could use that particle to send out a message from the inside of the black hole, so it wouldnt actually be black.

The fact that all matter feels gravity introduces a constraint on the kinds of experiments that are possible: Whatever apparatus you construct, no matter what its made of, it cant be too heavy, or it will necessarily gravitationally collapse into a black hole.This constraint is not relevant in everyday situations, but it becomes essential if you try to construct an experiment to measure the quantum mechanical properties of gravity.

Our understanding of the other forces of nature is built on the principle of locality, which says that the variables that describe whats going on at each point in space such as the strength of the electric field there can all change independently. Moreover, these variables, which we call degrees of freedom, can only directly influence their immediate neighbors. Locality is important to the way we currently describe particles and their interactions because it preserves causal relationships: If the degrees of freedom here in Cambridge, Massachusetts, depended on the degrees of freedom in San Francisco, we may be able to use this dependence to achieve instantaneous communication between the two cities or even to send information backward in time, leading to possible violations of causality.

The hypothesis of locality has been tested very well in ordinary settings, and it may seem natural to assume that it extends to the very short distances that are relevant for quantum gravity (these distances are small because gravity is so much weaker than the other forces).To confirm that locality persists at those distance scales, we need to build an apparatus capable of testing the independence of degrees of freedom separated by such small distances. A simple calculation shows, however, that an apparatus thats heavy enough to avoid large quantum fluctuations in its position, which would ruin the experiment, will also necessarily be heavy enough to collapse into a black hole!Therefore, experiments confirming locality at this scale are not possible. And quantum gravity therefore has no need to respect locality at such length scales.

Indeed, our understanding of black holes so far suggests that any theory of quantum gravity should have substantially fewer degrees of freedom than we would expect based on experience with the other forces. This idea is codified in the holographic principle, which says, roughly speaking, that the number of degrees of freedom in a spatial region is proportional to its surface area instead of its volume.

Juan Maldacena, a quantum gravity theorist at the Institute for Advanced Study in Princeton, New Jersey, is best known for discovering a hologram-like relationship between gravity and quantum mechanics:

Particles can display many interesting and surprising phenomena. We can have spontaneous particle creation, entanglement between the states of particles that are far apart, and particles in a superposition of existence in multiple locations.

In quantum gravity, space-time itself behaves in novel ways. Instead of the creation of particles, we have the creation of universes. Entanglement is thought to create connections between distant regions of space-time. We have superpositions of universes with different space-time geometries.

Furthermore, from the perspective of particle physics, the vacuum of space is a complex object. We can picture many entities called fieldssuperimposed on top of one another and extending throughout space. The value of each field is constantly fluctuating at short distances.Out of thesefluctuating fieldsand their interactions, the vacuum state emerges. Particles are disturbances in this vacuum state. We can picture them as small defects in the structure of the vacuum.

When we consider gravity, we find that the expansion of the universe appears to produce more of this vacuum stuff out of nothing. When space-time is created, it just happens to be in the state that corresponds to the vacuum without any defects. How the vacuum appears in precisely the right arrangement is one of the main questions we need to answer to obtain a consistent quantum description of black holes and cosmology. In both of these cases there is a kind of stretching of space-time that results in the creation of more of the vacuum substance.

Sera Cremonini, a theoretical physicist at Lehigh University, works on string theory, quantum gravity and cosmology:

There are many reasons why gravity is special. Let me focus on one aspect, the idea that the quantum version of Einsteins general relativity is nonrenormalizable. This has implications for the behavior of gravity at high energies.

In quantum theories, infinite terms appear when you try to calculate how very energetic particles scatter off each other and interact. In theories that are renormalizable which include the theories describing all the forces of nature other than gravity we can remove these infinities in a rigorous way by appropriately adding other quantities that effectively cancel them, so-called counterterms. This renormalization process leads to physically sensible answers that agree with experiments to a very high degree of accuracy.

The problem with a quantum version of general relativity is that the calculations that would describe interactions of very energetic gravitons the quantized units of gravity would have infinitely many infinite terms. You would need to add infinitely many counterterms in a never-ending process. Renormalization would fail. Because of this, a quantum version of Einsteins general relativity is not a good description of gravity at very high energies. It must be missing some of gravitys key features and ingredients.

However, we can still have a perfectly good approximate description of gravity at lower energies using the standard quantum techniques that work for the other interactions in nature. The crucial point is that this approximate description of gravity will break down at some energy scale or equivalently, below some length.

Above this energy scale, or below the associated length scale, we expect to find new degrees of freedom and new symmetries. To capture these features accurately we need a new theoretical framework. This is precisely where string theory or some suitable generalization comes in: According to string theory, at very short distances, we would see that gravitons and other particles are extended objects, called strings. Studying this possibility can teach us valuable lessons about the quantum behavior of gravity.

See the article here:

Why Gravity Is Not Like the Other Forces - Quanta Magazine

The atmosphere of Mars has a distinct green glow, just like Earth's.

The European Space Agency's Trace Gas Orbiter (TGO) spotted an emerald glow in Mars' wispy atmosphere, marking the first time the phenomenon has been spotted on a world beyond Earth, a new study reports.

"One of the brightest emissions seen on Earth stems from night glow. More specifically, from oxygen atoms emitting a particular wavelength of light that has never been seen around another planet," study lead author Jean-Claude Grard, of the Universit de Lige in Belgium, said in a statement.

"However, this emission has been predicted to exist at Mars for around 40 years and, thanks to TGO, weve found it," Grard said.

Related: The 7 biggest mysteries of Mars

As Grard noted, the green emission is characteristic of oxygen. Skywatchers at high latitudes here on Earth can see this signature in the ethereal, multicolored displays known as the auroras, which are generated by charged particles from the sun slamming into molecules high up in the atmosphere.

But night glow is different. It's caused by the interaction of sunlight with atoms and molecules in the air, which generates a subtle but continuous light. This emission is hard to see, even here on Earth; observers often need an edge-on perspective to make it out, which is why some of the best images of our planet's green night glow come courtesy of astronauts aboard the International Space Station (ISS).

Day glow, the diurnal component of this constant emission, is even harder to spot. And it's driven by a slightly different mechanism.

"Night glow occurs as broken-apart molecules recombine, whereas day glow arises when the sun's light directly excites atoms and molecules such as nitrogen and oxygen," European Space Agency (ESA) officials wrote in the same statement.

Grard and his colleagues used TGO's Nadir and Occultation for Mars Discovery (NOMAD) instrument suite, which includes the Ultraviolet and Visible Spectrometer (UVIS), to study the Red Planet's air in a special observing mode from April through December of last year.

"Previous observations hadn't captured any kind of green glow at Mars, so we decided to reorient the UVIS nadir channel to point at the 'edge' of Mars, similar to the perspective you see in images of Earth taken from the ISS," study co-author and NOMAD principal investigator Ann Carine Vandaele, of the Institut Royal d'Aronomie Spatiale de Belgique in Belgium, said in the same statement.

The team scanned the Martian atmosphere at altitudes between 12 miles and 250 miles (20 to 400 kilometers). They found the green oxygen glow at all heights, though it was strongest around 50 miles (80 km) up and varied with the Red Planet's distance from the sun.

The researchers also performed modeling work to better understand what's driving the glow. Those calculations suggested the light is driven mainly by the breakup of carbon dioxide, which makes up 95% of Mars' thin atmosphere, into carbon monoxide and oxygen.

TGO saw these stripped oxygen atoms glowing in both visible and ultraviolet light, with the visible emission about 16.5 times more intense than the UV.

"The observations at Mars agree with previous theoretical models, but not with the actual glowing we've spotted around Earth, where the visible emission is far weaker," Grard said. "This suggests we have more to learn about how oxygen atoms behave, which is hugely important for our understanding of atomic and quantum physics."

TGO has been circling Mars since October 2016. The orbiter is part of the two-phase European-Russian ExoMars program, which plans to launch a life-hunting rover called Rosalind Franklin toward the Red Planet in 2022. (The Rosalind Franklin was originally supposed to lift off this summer, but technical issues with the spacecraft's parachute and other systems caused the mission to miss that window.)

The new TGO results, which were published online today (June 15) in the journal Nature Astronomy, will be helpful to the Rosalind Franklin team, ESA officials said.

"This type of remote-sensing observation, coupled with in situ measurements at higher altitudes, helps us to predict how the Martian atmosphere will respond to seasonal changes and variations in solar activity," Hkan Svedhem, ESA's TGO project scientist, said in the same statement.

"Predicting changes in atmospheric density is especially important for forthcoming missions, including the ExoMars 2022 mission that will send a rover and surface science platform to explore the surface of the Red Planet," said Svedhem, who is not a co-author of the new study.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.

Read more from the original source:

Weird green glow spotted in atmosphere of Mars - Space.com