PERSPECTIVE FROM A PHOTON

Every model and, even, every theory before and after of general relativity theory considers space and time or spacetime an existing concept/s, physically or mathematically, for all observers. UNST or Nature Mechanics is considering a phys-math Nature individuality. Moreover, not for all observers spacetime is an existing concept. In fact spacetime is connected with locality and, recently, a new experiment "almost" demonstrates non locality of our Universe's Nature across the Bell's theorem free of loopholes proving, empirically, quantum entanglement free of any "spookiness". So far spacetime is a very efficient "construct" to calculate, as a very good approximation, especially for cosmological scales, but it is not the Nature of our Universe at all. Maybe it is a bit shocking for the most of physicist community, but even Einstein realized about it. According to his own words:

"It is neither the point in space, nor the instant in time, at which something happens that has physical reality, but only the event itself".

Ahead, the metaphysician Immanuel Kant said that neither space nor time can be empirically perceived—they are elements of a systematic framework that humans use to structure all experiences. Kant referred to "space" in his Critique of Pure Reason as being a subjective "pure a priori form of intuition", hence it is an unavoidable contribution of our human faculties.

However for a photon, according to the general relativity theory, spacetime is not exist at all. It just an emission and absorption pair that a photon "feels". The photon is not traveling across spacetime from its perspective. It just our perception from our inertial system that photons moves at the speed of light "c". Celeritas is just an illusion. An "relativistic illusion". Also spacetime is another "general relativity illusion".

In spite of the paper linked at the end of this post is claiming new constraints on quantum gravity from X-Rays & Gamma Rays observations, it is not possible to interpret that constraints from the point of view of quantum foaminess of the spacetime. The simple reason is that photons don't suffer any perturbation from the quantum fluctuations of the space time because they are not traveling across it. They are "out" of spacetime. This is harder conjecture than the Wheeler's conjecture (see a bit later in this post).

Spacetime & quantum gravity

Even at the minute scales of distance and duration examined with increasingly discriminating instruments, spacetime still appears to be smooth and structureless. However, a variety of models of quantum gravity posit that spacetime is, on Planck scales, subject to quantum fluctuations. As such, the effect of quantum gravity on light propagation (if detected) can possibly reveal a cou- pling to vacuum states postulated by Inflation and String Theories. In particular, models (e.g., Ng 2003) consistent with the “Holographic Principle” (’tHooft 1993; Susskind 1995; Aharony et al. 2000) predict that space-time foam may be detectable via intensity-degraded or blurred im- ages of distant objects. While these models are not a di- rect test of the Holographic Principle itself, the success or failure of such models may provide important clues to connect black hole physics with quantum gravity and information theory (Hawking 1975).

The fundamental idea is that, if probed at a small enough scale, spacetime will appear complicated – some- thing akin in complexity to a turbulent froth that Wheeler (1963) has dubbed “quantum foam,” also known as “spacetime foam.” In models of quantum gravity, the foaminess of spacetime is a consequence of the Energy Uncertainty Principle connecting the Planck mass and Planck time. Thus, the detection of spacetime foam is important for constraining models of quantum gravity. If a foamy structure is found, it would require that space- time itself has a probabilistic, rather than deterministic nature. As a result, the phases of photons emitted by a distant source would acquire a random component which increases with distance. Furthermore, the recent discov- ery of polarization in the cosmic microwave background by BICEP2 (Ade et al. 2014), if confirmed, also provides evidence for imprints of quantum gravitational effects from the inflationary era appearing on the microwave background. Although these effects originate from an epoch vastly different than the present time, they may be associated with a theoretically expected chaotic (e.g., foamy) inflation of space-time (for a recent review, see Linde 2014 and references therein). Therefore, searching for evidence of quantum foam in the present era, which is actually slowly inflating because of dark energy, may also be helpful in providing observational support for theories of quantum gravity’s role in inflation.

A number of prior studies have explored the possible image degradation of distant astronomical objects due to the effects of spacetime foam (Lieu & Hillman 2003; Ng et al. 2003; Ragazzoni et al. 2003; Christiansen et al. 2006; Christiansen et al. 2011; and Perlman et al. 2011). In particular, most of these focus on possible image blur- ring of distant astronomical objects. We demonstrate that this previous approach was incomplete, and take a different approach, examining the possibility that space- time foam might actually prevent the appearance of im- ages altogether at sufficiently short wavelengths. We con- centrate particularly on observations with the Chandra X-ray Observatory in the keV range (a possibility we con- sidered unfeasible in Christiansen et al. 2011 and Perl- man et al. 2011, but now reconsider), the Fermi Obser- vatory in the GeV range, and ground-based Cherenkov telescopes in the TeV range. Short-wavelength observations are particularly useful in constraining quantum gravity models.

The most prominent models discussed in the literature of the ArXiv link at the end of this post are based in a free parameter "α" to be fixed by observation in the basic phase fluctuation model:

1. The random walk model (Amelino-Camelia 1999; Diosi & Lukacs 1989). In this model, the effects grow like a random walk, corresponding to α = 1/2.

2. The holographic model (Karolyhazy 1966; Ng & van Dam 1994, 1995), so-called because it is con- sistent (Ng 2003) with the holographic principle (‘tHooft 1993; Susskind 1995). (We explain the meaning of “consistent” below.) In this model, the information content in any three-dimensional re- gion of space can be encoded on a two-dimensional surface surrounding the region of interest, like a hologram. (This is the restricted form of the holo- graphic principle that we are referring to.) The holographic model corresponds to a value of α = 2/3 (Christiansen et al. 2011).

3. The original Wheeler conjecture, which in this con- text means that the distance fluctuations are anti- correlated with successive fluctuations (Misner et al. 1973), in which case there are no cumulative ef- fects, so that the distance fluctuation remains sim- ply the Planck length. This corresponds to α = 1 and spacetime foam is virtually undetectable by astronomical means.

While all three of the above models are tested by the techniques discussed in the ArXiv link below and their researches devote most of our attention in this paper to the holographic model (# 2 above) because it is most directly connected to theories of quantum gravity via the holographic principle, I will require your attention to the Wheeler conjecture (# 3 above) because it is most directly connected to theories as UNST (Universal Nature States Theory) or Nature Mechanics. Please read again the first paragraphs of this post to grasps their extreme deepness.

Many researchers believe that physics will not be complete until it can explain not just the behaviour of space and time, but where these entities come from.

"Imagine waking up one day and realizing that you actually live inside a computer game,” says Mark Van Raamsdonk, describing what sounds like a pitch for a science-fiction film. But for Van Raamsdonk, a physicist at the University of British Columbia in Vancouver, Canada, this scenario is a way to think about reality. If it is true, he says, “everything around us — the whole three-dimensional physical world — is an illusion born from information encoded elsewhere, on a two-dimensional chip”. That would make our Universe, with its three spatial dimensions, a kind of hologram, projected from a substrate that exists only in lower dimensions.
This 'holographic principle' is strange even by the usual standards of theoretical physics. But Van Raamsdonk is one of a small band of researchers who think that the usual ideas are not yet strange enough. If nothing else, they say, neither of the two great pillars of modern physics — general relativity, which describes gravity as a curvature of space and time, and quantum mechanics, which governs the atomic realm — gives any account for the existence of space and time. Neither does string theory, which describes elementary threads of energy.

Van Raamsdonk and his colleagues are convinced that physics will not be complete until it can explain how space and time emerge from something more fundamental — a project that will require concepts at least as audacious as holography. They argue that such a radical reconceptualization of reality is the only way to explain what happens when the infinitely dense 'singularity' at the core of a black hole distorts the fabric of space-time beyond all recognition, or how researchers can unify atomic-level quantum theory and planet-level general relativity — a project that has resisted theorists' efforts for generations.

“All our experiences tell us we shouldn't have two dramatically different conceptions of reality — there must be one huge overarching theory,” says Abhay Ashtekar, a physicist at Pennsylvania State University in University Park.

Finding that one huge theory is a daunting challenge. Here, Nature explores some promising lines of attack — as well as some of the emerging ideas about how to test these concepts (see 'The fabric of reality').

NIK SPENCER/NATURE; Panel 4 adapted from Budd, T. & Loll, R. Phys. Rev. D 88, 024015 (2013)
Gravity as thermodynamics

One of the most obvious questions to ask is whether this endeavour is a fool's errand. Where is the evidence that there actually is anything more fundamental than space and time?

A provocative hint comes from a series of startling discoveries made in the early 1970s, when it became clear that quantum mechanics and gravity were intimately intertwined with thermodynamics, the science of heat.

In 1974, most famously, Stephen Hawking of the University of Cambridge, UK, showed that quantum effects in the space around a black hole will cause it to spew out radiation as if it was hot. Other physicists quickly determined that this phenomenon was quite general. Even in completely empty space, they found, an astronaut undergoing acceleration would perceive that he or she was surrounded by a heat bath. The effect would be too small to be perceptible for any acceleration achievable by rockets, but it seemed to be fundamental. If quantum theory and general relativity are correct — and both have been abundantly corroborated by experiment — then the existence of Hawking radiation seemed inescapable.

A second key discovery was closely related. In standard thermodynamics, an object can radiate heat only by decreasing its entropy, a measure of the number of quantum states inside it. And so it is with black holes: even before Hawking's 1974 paper, Jacob Bekenstein, now at the Hebrew University of Jerusalem, had shown that black holes possess entropy. But there was a difference. In most objects, the entropy is proportional to the number of atoms the object contains, and thus to its volume. But a black hole's entropy turned out to be proportional to the surface area of its event horizon — the boundary out of which not even light can escape. It was as if that surface somehow encoded information about what was inside, just as a two-dimensional hologram encodes a three-dimensional image.

In 1995, Ted Jacobson, a physicist at the University of Maryland in College Park, combined these two findings, and postulated that every point in space lies on a tiny 'black-hole horizon' that also obeys the entropy–area relationship. From that, he found, the mathematics yielded Einstein's equations of general relativity — but using only thermodynamic concepts, not the idea of bending space-time.

“This seemed to say something deep about the origins of gravity,” says Jacobson. In particular, the laws of thermodynamics are statistical in nature — a macroscopic average over the motions of myriad atoms and molecules — so his result suggested that gravity is also statistical, a macroscopic approximation to the unseen constituents of space and time.

In 2010, this idea was taken a step further by Erik Verlinde, a string theorist at the University of Amsterdam, who showed that the statistical thermodynamics of the space-time constituents — whatever they turned out to be — could automatically generate Newton's law of gravitational attraction.

And in separate work, Thanu Padmanabhan, a cosmologist at the Inter-University Centre for Astronomy and Astrophysics in Pune, India, showed that Einstein's equations can be rewritten in a form that makes them identical to the laws of thermodynamics — as can many alternative theories of gravity. Padmanabhan is currently extending the thermodynamic approach in an effort to explain the origin and magnitude of dark energy: a mysterious cosmic force that is accelerating the Universe's expansion.

Testing such ideas empirically will be extremely difficult. In the same way that water looks perfectly smooth and fluid until it is observed on the scale of its molecules — a fraction of a nanometre — estimates suggest that space-time will look continuous all the way down to the Planck scale: roughly 10−35metres, or some 20 orders of magnitude smaller than a proton.

But it may not be impossible. One often-mentioned way to test whether space-time is made of discrete constituents is to look for delays as high-energy photons travel to Earth from distant cosmic events such as supernovae and γ-ray bursts. In effect, the shortest-wavelength photons would sense the discreteness as a subtle bumpiness in the road they had to travel, which would slow them down ever so slightly. Giovanni Amelino-Camelia, a quantum-gravity researcher at the University of Rome, and his colleagues have found hints of just such delays in the photons from a γ-ray burst recorded in April. The results are not definitive, says Amelino-Camelia, but the group plans to expand its search to look at the travel times of high-energy neutrinos produced by cosmic events. He says that if theories cannot be tested, “then to me, they are not science. They are just religious beliefs, and they hold no interest for me.”

Other physicists are looking at laboratory tests. In 2012, for example, researchers from the University of Vienna and Imperial College London proposed a tabletop experiment in which a microscopic mirror would be moved around with lasers. They argued that Planck-scale granularities in space-time would produce detectable changes in the light reflected from the mirror (see Naturehttp://doi.org/njf; 2012).

Loop quantum gravity

Even if it is correct, the thermodynamic approach says nothing about what the fundamental constituents of space and time might be. If space-time is a fabric, so to speak, then what are its threads?

One possible answer is quite literal. The theory of loop quantum gravity, which has been under development since the mid-1980s by Ashtekar and others, describes the fabric of space-time as an evolving spider's web of strands that carry information about the quantized areas and volumes of the regions they pass through. The individual strands of the web must eventually join their ends to form loops — hence the theory's name — but have nothing to do with the much better-known strings of string theory. The latter move around in space-time, whereas strands actually are space-time: the information they carry defines the shape of the space-time fabric in their vicinity.

Because the loops are quantum objects, however, they also define a minimum unit of area in much the same way that ordinary quantum mechanics defines a minimum ground-state energy for an electron in a hydrogen atom. This quantum of area is a patch roughly one Planck scale on a side. Try to insert an extra strand that carries less area, and it will simply disconnect from the rest of the web. It will not be able to link to anything else, and will effectively drop out of space-time.

One welcome consequence of a minimum area is that loop quantum gravity cannot squeeze an infinite amount of curvature onto an infinitesimal point. This means that it cannot produce the kind of singularities that cause Einstein's equations of general relativity to break down at the instant of the Big Bang and at the centres of black holes.

In 2006, Ashtekar and his colleagues reported a series of simulations that took advantage of that fact, using the loop quantum gravity version of Einstein's equations to run the clock backwards and visualize what happened before the Big Bang. The reversed cosmos contracted towards the Big Bang, as expected. But as it approached the fundamental size limit dictated by loop quantum gravity, a repulsive force kicked in and kept the singularity open, turning it into a tunnel to a cosmos that preceded our own.

This year, physicists Rodolfo Gambini at the Uruguayan University of the Republic in Montevideo and Jorge Pullin at Louisiana State University in Baton Rouge reported a similar simulation for a black hole. They found that an observer travelling deep into the heart of a black hole would encounter not a singularity, but a thin space-time tunnel leading to another part of space. “Getting rid of the singularity problem is a significant achievement,” says Ashtekar, who is working with other researchers to identify signatures that would have been left by a bounce, rather than a bang, on the cosmic microwave background — the radiation left over from the Universe's massive expansion in its infant moments.

Loop quantum gravity is not a complete unified theory, because it does not include any other forces. Furthermore, physicists have yet to show how ordinary space-time would emerge from such a web of information. But Daniele Oriti, a physicist at the Max Planck Institute for Gravitational Physics in Golm, Germany, is hoping to find inspiration in the work of condensed-matter physicists, who have produced exotic phases of matter that undergo transitions described by quantum field theory. Oriti and his colleagues are searching for formulae to describe how the Universe might similarly change phase, transitioning from a set of discrete loops to a smooth and continuous space-time. “It is early days and our job is hard because we are fishes swimming in the fluid at the same time as trying to understand it,” says Oriti.

Causal sets

Such frustrations have led some investigators to pursue a minimalist programme known as causal set theory. Pioneered by Rafael Sorkin, a physicist at the Perimeter Institute in Waterloo, Canada, the theory postulates that the building blocks of space-time are simple mathematical points that are connected by links, with each link pointing from past to future. Such a link is a bare-bones representation of causality, meaning that an earlier point can affect a later one, but not vice versa. The resulting network is like a growing tree that gradually builds up into space-time. “You can think of space emerging from points in a similar way to temperature emerging from atoms,” says Sorkin. “It doesn't make sense to ask, 'What's the temperature of a single atom?' You need a collection for the concept to have meaning.”

In the late 1980s, Sorkin used this framework to estimate the number of points that the observable Universe should contain, and reasoned that they should give rise to a small intrinsic energy that causes the Universe to accelerate its expansion. A few years later, the discovery of dark energy confirmed his guess. “People often think that quantum gravity cannot make testable predictions, but here's a case where it did,” says Joe Henson, a quantum-gravity researcher at Imperial College London. “If the value of dark energy had been larger, or zero, causal set theory would have been ruled out.”

Causal dynamical triangulations

That hardly constituted proof, however, and causal set theory has offered few other predictions that could be tested. Some physicists have found it much more fruitful to use computer simulations. The idea, which dates back to the early 1990s, is to approximate the unknown fundamental constituents with tiny chunks of ordinary space-time caught up in a roiling sea of quantum fluctuations, and to follow how these chunks spontaneously glue themselves together into larger structures.

The earliest efforts were disappointing, says Renate Loll, a physicist now at Radboud University in Nijmegen, the Netherlands. The space-time building blocks were simple hyper-pyramids — four-dimensional counterparts to three-dimensional tetrahedrons — and the simulation's gluing rules allowed them to combine freely. The result was a series of bizarre 'universes' that had far too many dimensions (or too few), and that folded back on themselves or broke into pieces. “It was a free-for-all that gave back nothing that resembles what we see around us,” says Loll.

But, like Sorkin, Loll and her colleagues found that adding causality changed everything. After all, says Loll, the dimension of time is not quite like the three dimensions of space. “We cannot travel back and forth in time,” she says. So the team changed its simulations to ensure that effects could not come before their cause — and found that the space-time chunks started consistently assembling themselves into smooth four-dimensional universes with properties similar to our own.

Intriguingly, the simulations also hint that soon after the Big Bang, the Universe went through an infant phase with only two dimensions — one of space and one of time. This prediction has also been made independently by others attempting to derive equations of quantum gravity, and even some who suggest that the appearance of dark energy is a sign that our Universe is now growing a fourth spatial dimension. Others have shown that a two-dimensional phase in the early Universe would create patterns similar to those already seen in the cosmic microwave background.

Holography

Meanwhile, Van Raamsdonk has proposed a very different idea about the emergence of space-time, based on the holographic principle. Inspired by the hologram-like way that black holes store all their entropy at the surface, this principle was first given an explicit mathematical form by Juan Maldacena, a string theorist at the Institute of Advanced Study in Princeton, New Jersey, who published his influential model of a holographic universe in 1998. In that model, the three-dimensional interior of the universe contains strings and black holes governed only by gravity, whereas its two-dimensional boundary contains elementary particles and fields that obey ordinary quantum laws without gravity.

Hypothetical residents of the three-dimensional space would never see this boundary, because it would be infinitely far away. But that does not affect the mathematics: anything happening in the three-dimensional universe can be described equally well by equations in the two-dimensional boundary, and vice versa.

In 2010, Van Raamsdonk studied what that means when quantum particles on the boundary are 'entangled' — meaning that measurements made on one inevitably affect the other. He discovered that if every particle entanglement between two separate regions of the boundary is steadily reduced to zero, so that the quantum links between the two disappear, the three-dimensional space responds by gradually dividing itself like a splitting cell, until the last, thin connection between the two halves snaps. Repeating that process will subdivide the three-dimensional space again and again, while the two-dimensional boundary stays connected. So, in effect, Van Raamsdonk concluded, the three-dimensional universe is being held together by quantum entanglement on the boundary — which means that in some sense, quantum entanglement and space-time are the same thing.

Or, as Maldacena puts it: “This suggests that quantum is the most fundamental, and space-time emerges from it.”

A new study combining data from NASA's Chandra X-ray Observatory and Fermi Gamma-ray Telescope, and the Very Energetic Radiation Imaging Telescope Array (VERITAS) in Arizona is helping scientists set limits on the quantum nature of space-time on extremely tiny scales. Credit: NASA/CXC/FIT/E.Perlman et al, Illustration: NASA/CXC/M.Weiss
A team of scientists has used X-ray and gamma-ray observations of some of the most distant objects in the universe to better understand the nature of space and time. Their results set limits on the quantum nature, or "foaminess," of spacetime at extremely tiny scales.

This study combines data from NASA's Chandra X-ray Observatory and Fermi Gamma-ray Space Telescope along with ground-based gamma-ray observations from the Very Energetic Radiation Imaging Telescope Array (VERITAS).

At the smallest scales of distance and duration that we can measure, spacetime—that is, the three dimensions of space plus time—appears to be smooth and structureless. However, certain aspects of quantum mechanics, the highly successful theory scientists have developed to explain the physics of atoms and subatomic particles, predict that spacetime would not be smooth. Rather, it would have a foamy, jittery nature and would consist of many small, ever-changing, regions for which space and time are no longer definite, but fluctuate.

"One way to think of spacetime foam is if you are flying over the ocean in the airplane, it looks completely smooth. However, if you get low enough you see the waves, and closer still, foam, with tiny bubbles that are constantly fluctuating" said lead author Eric Perlman of the Florida Institute of Technology in Melbourne. "Even stranger, the bubbles are so tiny that even on atomic scales we're trying to observe them from a very high-flying airplane."

The predicted scale of spacetime foam is about ten times a billionth of the diameter of a hydrogen atom's nucleus, so it cannot be detected directly. However, If spacetime does have a foamy structure there are limitations on the accuracy with which distances can be measured because the size of the many quantum bubbles through which light travels will fluctuate. Depending on what model of spacetime is used, these distance uncertainties should accumulate at different rates as light travels over the large cosmic distances.

The researchers used observations of X-rays and gamma-rays from very distant quasars—luminous sources produced by matter falling towards supermassive black holes—to test models of spacetime foam. The authors predicted that the accumulation of distance uncertainties for light traveling across billions of light-years would cause the image quality to degrade so much that the objects would become undetectable. The wavelength where the image disappears should depend on the model of spacetime foam used.

Chandra's X-ray detection of quasars at distances of billions of light-years rules out one model, according to which photons diffuse randomly through spacetime foam in a manner similar to light diffusing through fog. Detections of distant quasars at shorter, gamma-ray wavelengths with Fermi and even shorter wavelengths with VERITAS demonstrate that a second, so-called holographic model with less diffusion does not work.

"We find that our data can rule out two different models for spacetime foam," said co-author Jack Ng of the University of North Carolina in Chapel Hill. "We can conclude that spacetime is less foamy that some models predict."

The X-ray and gamma-ray data show that spacetime is smooth down to distances 1,000 times smaller than the nucleus of a hydrogen atom.

These results appear in the May 20th issue of The Astrophysical Journal but by now you could find more information I the following link:

http://arxiv.org/pdf/1411.7262v5.pdf