New paper reviews research on the grain of space-time

Smooth" or grainy? Is space-time continuous or is it made up of very fine (10-35 metres on the "Planck scale") but discrete grains, if we look at it very close up ?

If the latter were true, scientists think, this would lead to deviations from the theory of special relativity formulated by Albert Einstein more than 100 years ago.

In some theoretical scenarios, the "non-continuity" of space-time implies violations to the invariance of the physical laws under the so-called Lorentz transformations (which establish that physical laws are the same for all inertial reference frames that are at the basis of special relativity).

Since the 90s physicists have devised several methods (often based on phenomena connected to high-energy astrophysics) to test these deviations from standard physics.

Stefano Liberati

Stefano Liberati, coordinator of the Astroparticle Physics group of the International School for Advanced Studies (SISSA) of Trieste, recently published a systematic review to present the state of the art in this field and the constraints that can be placed on the various models that predict violations to Special Relativity.

The paper is an invited Topic Review published in the journal Classical and Quantum Gravity.

This journal periodically asks leading world experts to "sum up" what is known in a specific field of study.

The review has now been selected as one of the journal's Highlight papers for 2013.

"Physicists have been wondering about the nature of space-time for years. We've been asking ourselves whether it is continuous at all scales, as we perceive it in our daily experience, or whether at very small sizes it presents an irregular grain that we, in our direct experience, are unable to perceive", explains Liberati.

"Imagine looking at a slab of marble from some distance: it will probably seem to have a uniform texture.

However, on closer inspection, for example using a powerful microscope, you can see that the marble is porous and irregular".

"In a certain sense physicists have been trying to do something similar with space-time: to find something that acts as a microscope to find out whether at very small length scales there is indeed some irregularity.

In my paper I presented a systematic overview of the experiments and observations that can be exploited to investigate the existence of these irregularities.

Special relativity is one of the cornerstones of modern physics and as such it is very important to test its validity, insofar as current observations allow us".

More information: 'Tests of Lorentz invariance: a 2013 update' S Liberati 2013 Class. Quantum Grav. 30 133001 doi:10.1088/0264-9381/30/13/133001

Time machine will study the early universe

With MOSFIRE, it will now become much easier to identify faint galaxies, "families of galaxies" and merging galaxies. The instrument also will enable detailed observations of planets orbiting nearby stars, star formation within our own galaxy, the distribution of dark matter in the universe and much more.

A new scientific instrument, a "time machine" of sorts, built by UCLA astronomers and colleagues, will allow scientists to study the earliest galaxies in the universe, which could never be studied before.

The five-ton instrument, the most advanced and sophisticated of its kind in the world, goes by the name MOSFIRE (Multi-Object Spectrometer for Infra-Red Exploration) and has been installed in the Keck I Telescope at the W.M. Keck Observatory atop Mauna Kea in Hawaii.

MOSFIRE gathers light in infrared wavelengths - invisible to the human eye - allowing it to penetrate cosmic dust and see distant objects whose light has been stretched or "redshifted" to the infrared by the expansion of the universe.

"The instrument was designed to study the most distant, faintest galaxies," said UCLA physics and astronomy professor Ian S. McLean, project leader on MOSFIRE and director of UCLA's Infrared Laboratory for Astrophysics.

"When we look at the most distant galaxies, we see them not as they are now but as they were when the light left them that is just now arriving here. Some of the galaxies that we are studying were formed some 10 billion years ago - only a few billion years after the Big Bang. We are looking back in time to the era of the formation of some of the very first galaxies, which are small and very faint. That is an era that we need to study if we are going to understand the large-scale structure of the universe."

With MOSFIRE, it will now become much easier to identify faint galaxies, "families of galaxies" and merging galaxies. The instrument also will enable detailed observations of planets orbiting nearby stars, star formation within our own galaxy, the distribution of dark matter in the universe and much more.

"We would like to study the environment of those early galaxies," said McLean, who built the instrument with colleagues from UCLA, the California Institute of Technology and UC Santa Cruz, along with industrial sub-contractors. "Sometimes there are large clusters with thousands of galaxies, sometimes small clusters. Often, black holes formed in the centers of galaxies."

Light collected by the Keck I Telescope was fed into MOSFIRE for the first time on April 4, producing an astronomical image. Astronomers are expected to start using MOSFIRE by September, following testing and evaluation in May and June.

MOSFIRE allows astronomers to take an infrared image of a field and to study 46 galaxies simultaneously, providing the infrared spectrum for each galaxy. Currently, it can take three hours or longer to obtain a good spectrum of just one galaxy, McLean noted.

McLean built the world's first infrared camera for wide use by astronomers in 1986 and since then has built eight increasingly sophisticated infrared cameras and spectrometers - which split light into its component colors - as well as helping on a few others.

McLean and Charles Steidel, the Lee A. DuBridge Professor of Astronomy at the California Institute of Technology, led the project to build MOSFIRE from scratch over seven years. Harland Epps, a UC Santa Cruz professor of astronomy and astrophysics, designed the optics for the instrument.

A team of nearly two dozen people helped, including Kristin Kulas and Gregory Mace, UCLA graduate students in physics and astronomy who work in McLean's laboratory; Keith Matthews, an instrument designer from Caltech; and Sean Adkins, an engineer who is the instrument program manager for the Keck Observatory in Hawaii.

Most of the mechanical parts for MOSFIRE were built at UCLA and Caltech. The slit unit that enables 46 objects to be isolated was manufactured in Switzerland. The computer programming was led by UCLA.

"My father, who was an engineer, called me an astronomer by inclination, a physicist by training and an engineer by default," McLean said. "I'm an applied physicist and an astronomer."

MOSFIRE cost $14 million and likely would have cost at least twice as much if the scientists had not built it themselves, McLean estimates.

MOSFIRE was federally funded by the National Science Foundation (through the Telescope System Instrumentation program), and by Gordon and Betty Moore. Gordon Moore is co-founder, former chairman and chief executive officer, and chairman emeritus of Intel Corp.

"He is a wonderful man with a penetrating intellect," McLean said of Moore. "We are deeply indebted to him and hope to be able to show him MOSFIRE this summer."

"We had an outstanding team," he added, "with four institutions involved and many industrial partners. It was a fantastic team effort."

In the late 1990s, McLean delivered an infrared spectrometer called NIRSPEC to the Keck Observatory in Hawaii, which housed the world's largest optical and infrared telescope at the time and which contains what had been the most powerful infrared spectrometer in the world. NIRSPEC is still on the Keck II Telescope.

While NIRSPEC's camera has one megapixel, MOSFIRE has four megapixels. MOSFIRE's detectors are approximately five times more sensitive than those on NIRSPEC and about 100 times more sensitive than those from McLean's 1986 infrared camera. In addition, the digital imaging devices available today are far superior to those of 15 years ago. The result is that MOSFIRE is much more sensitive to faint objects.

Discoveries made with NIRSPEC include the detection of water on comets, insights into the stars orbiting the enormous black hole at the center of the Milky Way galaxy, and the discovery of the chemical composition of brown dwarfs.

Physicists continue on work to abolish Time as the Fourth Dimension of Space

Light clocks A and B moving horizontally through space. According to length contraction, clock A should tick faster than clock B.

In a new study, scientists argue that there is no length contraction, and both clocks should tick at the same rate in accordance with special relativity. 

Image credit: Sorli and Fiscaletti.

Philosophers have debated the nature of time long before Einstein and modern physics but in the 106 years since Einstein, the prevailing view in physics has been that time serves as the fourth dimension of space, an arena represented mathematically as 4D Minkowski spacetime.

However, some scientists, including Amrit Sorli and Davide Fiscaletti, founders of the Space Life Institute in Slovenia, argue that time exists completely independent from space.

In a new study, Sorli and Fiscaletti have shown that two phenomena of special relativity - time dilation and length contraction, can be better described within the framework of a 3D space with time as the quantity used to measure change (i.e., photon motion) in this space.

The scientists have published their article in a recent issue of Physics Essays. The work builds on their previous articles, in which they have investigated the definition of time as a “numerical order of material change.”

Space-Time Continuum

The main concepts of special relativity - that the speed of light is the same in all inertial reference frames, and that there is no absolute reference frame - are traditionally formulated within the framework of Minkowski spacetime.

In this framework, the three spatial dimensions are intuitively visualized, while the time dimension is mathematically represented by an imaginary coordinate, and cannot be visualized in a concrete way.

In their paper, Sorli and Fiscaletti argue that, while the concepts of special relativity are sound, the introduction of 4D Minkowski spacetime has created a century-long misunderstanding of time as the fourth dimension of space that lacks any experimental support.

They argue that well-known time dilation experiments, such as those demonstrating that clocks do in fact run slower in high-speed airplanes than at rest, support special relativity and time dilation but not necessarily Minkowski spacetime or length contraction.

According to the conventional view, clocks run slower at high speeds due to the nature of Minkowski spacetime itself as a result of both time dilation and length contraction.

But Sorli and Fiscaletti argue that the slow clocks can better be described by the relative velocity between the two reference frames, which the clocks measure, not which the clocks are apart of. In this view, space and time are two separate entities.

“With clocks we measure the numerical order of motion in 3D space,” Sorli reported. “Time is 'separated' from space in a sense that time is not a fourth dimension of space.

Instead, time as a numerical order of change exists in a 3D space. Our model on space and time is founded on measurement and corresponds better to physical reality.”

To illustrate the difference between the two views of time, Sorli and Fiscaletti consider an experiment involving two light clocks.

Each clock's ticking mechanism consists of a photon being reflected back and forth between two mirrors, so that a photon's path from one mirror to the other represents one tick of the clock.

The clocks are arranged perpendicular to each other on a platform, with clock A oriented horizontally and clock B vertically.

When the platform is moved horizontally at a high speed, then according to the length contraction phenomenon in 4D spacetime, clock A should shrink so that its photon has a shorter path to travel, causing it to tick faster than clock B.

But Sorli and Fiscaletti argue that the length contraction of clock A and subsequent difference in the ticking rates of clocks A and B do not agree with special relativity, which postulates that the speed of light is constant in all inertial reference frames.

They say that, keeping the photon speed the same for both clocks, both clocks should tick at the same rate with no length contraction for clock A.

They mathematically demonstrate how to resolve the problem in this way by replacing Minkowski 4D spacetime with a 3D space involving Galilean transformations for three spatial coordinates X, Y, and Z, and a mathematical equation (Selleri's formalism) for the transformation of the velocity of material change, which is completely independent of the spatial coordinates.

Video of Mock Discussion on Time and Space

Sorli explained that this idea that both photon clocks tick at the same rate is not at odds with the experiments with flying clocks and other tests that have measured time dilation. This difference, he says, is due to a difference between photon clocks and atom-based clocks.

“The rate of photon clocks in faster inertial systems will not slow down with regard to the photon clocks in a rest inertial system because the speed of light is constant in all inertial systems,” he said.

“The rate of atom clocks will slow down because the 'relativity' of physical phenomena starts at the scale of pi mesons.”

He also explained that, without length contraction, time dilation exists but in a different way than usually thought.

“Time dilatation exists not in the sense that time as a fourth dimension of space dilates and as a result the clock rate is slower,” he explained.

“Time dilatation simply means that, in a faster inertial system, the velocity of change slows down and this is valid for all observers.

GPS confirms that clocks in orbit stations have different rates from the clocks on the surface of the planet, and this difference is valid for observers that are on the orbit station and on the surface of the planet.

So interpreted, 'time dilatation' does not require 'length contraction,' which as we show in our paper leads to a contradiction by the light clocks differently positioned in a moving inertial system.”

He added that the alternative definition of time also agrees with the notion of time held by the mathematician and philosopher Kurt Gödel.

“The definition of time as a numerical order of change in space is replacing the 106-year-old concept of time as a physical dimension in which change runs,” Sorli said.

“We consider time being only a mathematical quantity of change that we measure with clocks. This is in accord with a Gödel view of time.

By 1949, Gödel had produced a remarkable proof: 'In any universe described by the theory of relativity, time cannot exist.'

Our research confirms Gödel's vision: time is not a physical dimension of space through which one could travel into the past or future.”

In the future, Sorli and Fiscaletti plan to investigate how this view of time fits with the broader surroundings.

They note that other researchers have investigated abolishing the idea of spacetime in favour of separate space and time entities, but often suggest that this perspective is best formulated within the framework of an ether, a physical medium permeating all of space.

In contrast, Sorli and Fiscaletti think that the idea can be better modeled within the framework of a 3D quantum vacuum.

Rather than viewing space as a medium that carries light, light's propagation is governed by the electromagnetic properties (the permeability and permittivity) of the quantum vacuum.

“We are developing a mathematical model where gravity is a result of the diminished energy density of a 3D quantum vacuum caused by the presence of a given stellar object or material body,” Sorli said.

"Inertial mass and gravitational mass have the same origin: diminished energy density of a quantum vacuum."

"This model gives exact calculations for the Mercury perihelion precession as calculations of the general theory of relativity.”

More information: Amrit Sorli and Davide Fiscaletti. “Special theory of relativity in a three-dimensional Euclidean space.” Physics Essays: March 2012, Vol. 25, No. 1, pp. 141-143. DOI: 10.4006/0836-1398-25.1.141

Proposed nuclear clock (ACES) may keep time with the Universe

The exquisite accuracy of atomic clocks is widely used in applications ranging from GPS navigation systems and high-bandwidth data transfer to tests of fundamental physics and system synchronisation in particle accelerators.

A proposed new time-keeping system tied to the orbiting of a neutron around an atomic nucleus could have such unprecedented accuracy that it neither gains nor loses 1/20th of a second in 14 billion years - the age of the Universe.

"This is nearly 100 times more accurate than the best atomic clocks we have now," says one of the researchers, Scientia Professor Victor Flambaum, who is Head of Theoretical Physics in the UNSW School of Physics.

"It would allow scientists to test fundamental physical theories at unprecedented levels of precision and provide an unmatched tool for applied physics research."

In a paper to be published in the journal Physical Review Letters - with US researchers at the Georgia Institute of Technology and the University of Nevada - Flambaum and UNSW colleague Dr Vladimir Dzuba report that their proposed single-ion clock would be accurate to 19 decimal places.

The exquisite accuracy of atomic clocks is widely used in applications ranging from GPS navigation systems and high-bandwidth data transfer to tests of fundamental physics and system synchronization in particle accelerators.

"With these clocks currently pushing up against significant accuracy limitations, a next-generation system is desired to explore the realms of extreme measurement precision and further diversified applications unreachable by atomic clocks," says Professor Flambaum.

"Atomic clocks use the orbiting electrons of an atom as the clock pendulum. But we have shown that by using lasers to orient the electrons in a very specific way, one can use the orbiting neutron of an atomic nucleus as the clock pendulum, making a so-called nuclear clock with unparalleled accuracy."

Because the neutron is held so tightly to the nucleus, its oscillation rate is almost completely unaffected by any external perturbations, unlike those of an atomic clock's electrons, which are much more loosely bound.

ESA VEGA Payload: Italian satellite to help measure space-time warp

Payloads for Vega's first mission include LARES, ALMASat 1, and seven CubeSats from universities across Europe. Credit: ESA/CNES/Arianespace

Scheduled for launch from French Guiana on Monday, Europe's first lightweight Vega rocket is packed with nine small research satellites, including a unique Italian craft designed to help make an elusive accurate measurement of a central tenet of Albert Einstein's theory of general relativity.

The 98-foot-tall booster is due for liftoff in a two-hour launch window opening at 1000 GMT (5 a.m. EST) Monday.

The four-stage launcher, developed with Italian leadership, will make its first flight from the Guiana Space Center, a European-run spaceport in French Guiana.

The Vega rocket will deploy its nine payloads at different altitudes, first releasing the Laser Relativity Satellite about 55 minutes after launch in a circular 901 mile-high orbit with an inclination of 69.5 degrees.

Another firing by the Vega's fourth stage, powered by a Ukrainian liqiud-fueled engine, will reduce the altitude of the orbit's low point to 217 miles before deploying Italy's ALMASat 1 technology demonstration satellite and seven two-pound CubeSats built by learning institutions across Europe.

The Vega's mission will conclude 81 minutes after launch.

Officials selected LARES as the main passenger for the rocket's qualification flight, which aims to prove Vega's flight and ground systems before it is entrusted with more costly payloads on subsequent missions.

Carved out of a single ball of tungsten, LARES is covered 92 laser retroreflectors, allowing a network of ranging stations around the world to track the spherical satellite in orbit.

By bouncing laser signals off reflectors on LARES, scientists can precisely compute its position in space.

After comparing the actual location of LARES against predictions, researchers can measure the frame-dragging effect, part of Einstein's theory of general relativity which states that a rotating mass can distort space-time around it.

"Imagine the Earth as if it were immersed in honey. As the planet rotates, the honey around it would swirl, and it's the same with space and time," said Francis Everitt, a Standford University researcher who led the science team for NASA's Gravity Probe B mission, which confirmed the frame-dragging, or Lense-Thirring, effect at 37.2 milliarcseconds with a margin for error of about 19 percent.

Artist's concept of the frame-dragging effect around Earth. Credit: Stanford University

The frame-dragging effect manifests itself in minuscule changes in the orbits of satellites. Scientists observed two precursors to LARES, named LAGEOS 1 and LAGEOS 2, over several years to determine their orbital planes shifted 6 feet per year in the direction of Earth's rotation.

The LAGEOS tracking, coupled with a precise Earth gravity model, produced an estimation of the frame-dragging effect at 99 percent of the value predicted under general relativity.

The joint U.S.-Italian LAGEOS mission ultimately measured the effect with an accuracy of about 10 percent.

The objective of LARES is to refine the real effect of frame-dragging with an accuracy approaching 1 percent, according to the Italian space agency, which is funding the mission.

Better data on the frame-dragging effect around Earth will help astrophysicists study distant black holes, neutron stars and active galactic nuclei, whose immense gravity creates much stronger warping of space-time.

Time Warp: Event-hiding 'temporal cloak' demonstrated

Last year researchers at Imperial College London proposed that along with being used to cloak physical objects metamaterials could also be used to cloak a singular event in time.

A year later, researchers from Cornell University have demonstrated a working "temporal cloak" that is able to conceal a burst of light as if it had never occurred.

In a research paper published in the Journal of Optics last year, Prof. Martin McCall and his team at Imperial College London said it should be theoretically possible to create a "Spacetime Cloak" by using metamaterials - a class of artificial materials engineered to have properties not be found in nature - to speed up the leading edge of light waves, while slowing down the trailing half.

This would create a "corridor" between the two halves, at which point their source wouldn't be observable.

To demonstrate the theory, a Cornell research team led by Moti Fridman sent a beam of light down an optical fiber and passed it through a split-time lens - a silicon device originally designed to speed up data transfer.

As the beam passes through the first lens it is compressed, leaving a dead zone or gap in the flow of light.

A similar lens further along the path reverses the velocity adjustments, decompressing the light wave so it appears that the light coming through the second lens is uninterrupted as if no distortion had occurred.

To test the temporal cloak's performance the researchers created pulses of light directly between the two lenses that repeated like clockwork at a rate of 41 kHz.

When the cloak was off, the researchers were able to detect a steady beat, but after switching on the cloak, which was synchronized with the light pulses, it appeared as if the pulses were erased from the data stream.

Rather than relying on the properties of metamaterials as was initially proposed by McCall, the temporal cloak demonstrated by the Cornell research team relies on the fundamental properties of light and how it behaves under highly constrained space and time conditions.

The length of the cloaked area is a mere six millimeters (0.2 in) long and the effect can only lasts for 110 nanoseconds.

The team says the best it can achieve will be 120 microseconds because longer durations would create turbulence in the system that would hint that an event had occurred.

To achieve any measurable macroscopic effects would require an experiment on planetary or even interplanetary scales, the researchers say.

The Cornell team will present their findings in a presentation "Demonstration of Temporal Cloaking" at the Optical Society's Annual Meeting, Frontiers in Optics (FiO) 2011, being held in San Jose, California, next week.

Einstein and the end of space-time

Physicists struggling to reconcile gravity with quantum mechanics have hailed a theory – inspired by pencil lead – that could make it all very simple

IT WAS a speech that changed the way we think of space and time. The year was 1908, and the German mathematician Hermann Minkowski had been trying to make sense of Albert Einstein's hot new idea - what we now know as special relativity - describing how things shrink as they move faster and time becomes distorted.

"Henceforth space by itself and time by itself are doomed to fade into the mere shadows," Minkowski proclaimed, "and only a union of the two will preserve an independent reality."

And so space-time - the malleable fabric whose geometry can be changed by the gravity of stars, planets and matter - was born. It is a concept that has served us well, but if physicist Petr Horava is right, it may be no more than a mirage.

Horava, who is at the University of California, Berkeley, wants to rip this fabric apart and set time and space free from one another in order to come up with a unified theory that reconciles the disparate worlds of quantum mechanics and gravity - one the most pressing challenges to modern physics.

Since Horava published his work in January 2009, it has received an astonishing amount of attention. Already, more than 250 papers have been written about it. Some researchers have started using it to explain away the twin cosmological mysteries of dark matter and dark energy.

Others are finding that black holes might not behave as we thought. If Horava's idea is right, it could forever change our conception of space and time and lead us to a "theory of everything", applicable to all matter and the forces that act on it.

For decades now, physicists have been stymied in their efforts to reconcile Einstein's general theory of relativity, which describes gravity, and quantum mechanics, which describes particles and forces (except gravity) on the smallest scales.

The stumbling block lies with their conflicting views of space and time. As seen by quantum theory, space and time are a static backdrop against which particles move. In Einstein's theories, by contrast, not only are space and time inextricably linked, but the resulting space-time is moulded by the bodies within it.

Part of the motivation behind the quest to marry relativity and quantum theory - to produce a theory of quantum gravity - is an aesthetic desire to unite all the forces of nature. But there is much more to it than that.

We also need such a theory to understand what happened immediately after the big bang or what's going on near black holes, where the gravitational fields are immense.

One area where the conflict between quantum theory and relativity comes to the fore is in the gravitational constant, G, the quantity that describes the strength of gravity. On large scales - at the scale of the solar system or of the universe itself - the equations of general relativity yield a value of G that tallies with observed behaviour.

But when you zoom in to very small distances, general relativity cannot ignore quantum fluctuations of space-time. Take them into account and any calculation of G gives ridiculous answers, making predictions impossible.

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