from
Nature Website
that physics will not be complete until it can explain not just the behavior of space and time,
but where these entities come from.
But for Mark 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,
That would make our Universe, with its
three spatial dimensions, a kind of hologram, projected from a
substrate that exists only in lower dimensions.
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.
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 spacetime beyond all recognition, or how researchers can unify atomiclevel quantum theory and planetlevel general relativity  a project that has resisted theorists' efforts for generations.
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.
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.
And so it is with black holes:
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
twodimensional hologram encodes a threedimensional image.
From that, he found, the mathematics yielded Einstein's equations of general relativity  but using only thermodynamic concepts, not the idea of bending spacetime.^{1}
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.
Padmanabhan is currently extending the thermodynamic approach in an effort to explain the origin and magnitude of dark energy:
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 nanometer  estimates suggest that
spacetime will look continuous all the way down to the Planck
scale: roughly 10^{35} meters, or some 20 orders of
magnitude smaller than a proton.
One oftenmentioned way to test whether spacetime is made of discrete constituents is to look for delays as highenergy photons travel to Earth from distant cosmic events such as supernovae and γray bursts. In effect, the shortestwavelength 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 AmelinoCamelia, a quantumgravity researcher at the University of Rome, and his colleagues have found ^{4} hints of just such delays in the photons from a γray burst recorded in April.
The results are not definitive, says AmelinoCamelia, but the group plans to expand its search to look at the travel times of highenergy neutrinos produced by cosmic events.
He says that if theories cannot be tested,
Other physicists are looking at laboratory tests.
In 2012, for example, researchers from the University of Vienna and Imperial College London proposed^{ 5} a tabletop experiment in which a microscopic mirror would be moved around with lasers.
They argued that Planckscale granularities in spacetime would produce detectable changes in the light reflected from the mirror (see Nature, Single photon could detect quantumscale black holes).
If spacetime 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 mid1980s by Abhay Ashtekar and others, describes the fabric of spacetime as an evolving spider's web of strands that carry information about the quantized areas and volumes of the regions they pass through. ^{6}
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 betterknown strings of string
theory. The latter move around in spacetime, whereas strands
actually are spacetime: the information they carry defines the
shape of the spacetime fabric in their vicinity.
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 spacetime.
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
centers of
black holes.
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.
They found that an observer travelling deep into the heart of a black hole would encounter not a singularity, but a thin spacetime tunnel leading to another part of space.
...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.
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 condensedmatter 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 spacetime.
Pioneered by Rafael Sorkin, a physicist at the Perimeter Institute in Waterloo, Canada, the theory postulates that the building blocks of spacetime are simple mathematical points that are connected by links, with each link pointing from past to future. Such a link is a barebones 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 spacetime.
In the late 1980s, Sorkin used this framework to estimate ^{9} 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.
Causal dynamical
triangulations
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 spacetime caught up in a
roiling sea of quantum fluctuations, and to follow how these chunks
spontaneously glue themselves together into larger structures.
The spacetime building blocks were simple hyperpyramids  fourdimensional counterparts to threedimensional 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.
After all, says Loll, the dimension of time is not quite like the three dimensions of space.
So the team changed its simulations to
ensure that effects could not come before their cause  and found
that the spacetime chunks started consistently assembling
themselves into smooth fourdimensional universes with properties
similar to our own.^{10}
Others have shown that a twodimensional
phase in the early Universe would create patterns similar to those
already seen in the cosmic microwave background.
Holography
Inspired by the hologramlike 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 ^{11} his influential model of a holographic universe in 1998.
In that model, the threedimensional
interior of the universe contains strings and black holes governed
only by gravity, whereas its twodimensional boundary contains
elementary particles and fields that obey ordinary quantum laws
without gravity.
But that does not affect the
mathematics: anything happening in the threedimensional universe
can be described equally well by equations in the twodimensional
boundary, and vice versa.
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 threedimensional 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 threedimensional space again and again, while the twodimensional boundary stays connected.
So, in effect, Van Raamsdonk concluded,
the threedimensional universe is being held together by quantum
entanglement on the boundary  which means that in some sense,
quantum entanglement and spacetime are the same thing.
References
