What is the time in space
Theoretical Physics: The Origin of Space and Time
In the late 1980s, based on these thoughts, Sorkin estimated the number of points the observable universe should contain. The points would create a weak internal energy that makes the universe expand at an accelerated rate, he speculated . A few years later, the discovery of dark energy confirmed his suspicion. "It's often assumed that quantum gravity doesn't make verifiable predictions - but here we have a case where it did just that," says Joe Henson of Imperial College London. "If the value of dark energy were greater or zero, we would have had to discard the causal set model."
However, this can hardly be taken as evidence. Furthermore, the causal set model made few other predictions that could be verified. Some physicists found it more promising to work with computer simulations. Since the early 1990s there has been the idea of approximating the unknown elementary building blocks with the help of tiny segments of ordinary space-time, trapped in a troubled sea of quantum fluctuations. Based on this, the aim was to follow how these segments come together to form larger structures by themselves.
The first attempts were disappointing, reports Renate Loll, currently at Radboud University Nijmegen in the Netherlands. Simple hyperpyramids - the four-dimensional counterparts of a three-dimensional tetrahedron - were used as the building blocks of spacetime, which were allowed to join together in the simulation as desired. The result was a series of bizarre "universes" that had far too many (or too few) dimensions and collapsed or fell apart. "It was a mess and didn't produce anything that reminded us of the world around us," says Loll.
"It was a mess and didn't produce anything that reminded of the world around us"
But like Sorkin, Loll and her colleagues found that adding causality changed everything. As a dimension, time does not behave quite like the three spatial dimensions, according to the physicist. "We can't travel back and forth in time," she explains. So the team made sure in their simulations that the effects could not occur before their cause. In fact, the spacetime pieces now consistently put themselves together in smooth, four-dimensional universes, with properties similar to ours .
Interestingly, the simulations suggest that shortly after the Big Bang, the universe went through a phase with only one space and one time dimension. Independently of this, other physicists made this prediction when they wanted to derive the equations of quantum gravity or assumed that the appearance of dark energy was a sign that our universe was just developing a fourth spatial dimension. A two-dimensional phase in the early universe could even produce patterns similar to those already observed in cosmic microwave background radiation, some researchers have shown.
Meanwhile, Van Raamsdonk pursues a thought about the origin of space-time based on the holographic principle. Spurred on by the way in which black holes store entropy on their surface, the string theorist Juan Maldacena at the Institute of Advanced Study in Princeton, New Jersey , developed an explicit mathematical form for this principle for the first time. In 1998 he published his pioneering model of a holographic universe. In this model, the three-dimensional inner workings of the universe contain both strings and black holes that are subject to gravity alone. Their two-dimensional interfaces, on the other hand, contain elementary particles and fields that obey the usual quantum laws - without gravity.
In 2010, Van Raamsdonk investigated what would happen if quantum particles were entangled with one another on the interface - measurements carried out on one particle inevitably also influence the other partners . His result: if the entanglement between particles in two separate regions of the interface is continuously reduced to zero, so that the quantum mechanical connection between them disappears, the three-dimensional space reacts by gradually dividing like a cell - until finally the last, thin bridge between the two halves breaks apart. If this process is repeated, the three-dimensional space is subdivided more and more, while the two-dimensional interface remains coherent. Van Raamsdonk concludes from this that the three-dimensional universe is held together by the quantum mechanical entanglement on the interface. So entanglement and space-time are in a sense one and the same thing.
The article appeared under the title "Theoretical physics: The origins of space and time" in Nature 500, pp. 516-519, 2013.
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