- The Arrows of Time - A Debate in Cosmology | Laura Mersini-Houghton | Springer
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Indian philosophers two millennia ago were faced with the same paradox as modern physicists: how can an unchanging reality hold within it observers that undergo change? Once we do so, then from our individual perspective, experiencing reality as a separate being, the rest of the Universe starts to tick, evolving in time relative to each human being as an observer.
So far, so mystical.
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Here, the very laws of quantum physics hold that some particles are connected together no matter how far they are pulled apart; indeed, in experiment after experiment, measurements carried out on one always instantaneously influences the properties of its entangled mate. Page and Wootters pondered what would happen if you took the whole unchanging Universe and chopped it into two entangled pieces.
They calculated that an observer, a human consciousness, say, or maybe even an inanimate recording device, sitting in one entangled part would monitor the other part of the Universe evolving relative to its own. The crucial insight was that the presence of an observer on one side starts the clock running on the other side. The wavefunction of the entire Universe would remain timeless, just as DeWitt had predicted, solving the problem of how an unchanging Universe can house time.
Though this was just a mathematical speculation, it has since been tested in the lab, in an extremely pared-down version of the Universe, containing a meagre two particles — not a complex enough model system for anything too exciting to happen, perhaps, but with just enough pieces to test the theoretical claim. In , the quantum physicist Marco Genovese at the Istituto Nazionale di Ricerca Metrologica in Italy and colleagues used two photons to represent the two sides of a divided microcosmos.
The photons were both polarised, meaning that each one vibrated along its length. The team entangled the pair of photons in such a way that, if the polarisation of the first photon was measured to be vibrating up and down, its entangled partner would instantaneously be forced to vibrate from side to side. The photons also served as mini clocks because, in addition to being polarised, they also each literally rotated at a constant rate, like the hands on a watch. The team could thus, in principle, measure how time passed within each half — if time did indeed pass — by monitoring how far the photon in that half had rotated.
From this vantage point, they could then monitor how the second photon — the second half of the Universe — evolved, by measuring how far it had rotated, relative to the first photon.
The Arrows of Time - A Debate in Cosmology | Laura Mersini-Houghton | Springer
In that case, the team could not allow themselves to become entangled with either photon; they were allowed only to measure the joint state of both photons, taken together as a pair. That meant that they could no longer see any relative rotation between the two photons, or the passage of time. All they could do was confirm that the two photons were permanently polarised in opposing directions — up-and-down and side-to-side — with this eternal embrace never changing.
Research confirmed that when viewed from outside, their two-photon Universe, as a whole, was frozen in time. This duality between you and the Universe is part of the whole package.
N obody is suggesting that progress in physics will be found by mining ancient Hindu scriptures directly for inspiration. Nor, indeed, that scholars of the Advaita Vedanta had some privileged insight into scientific truths. More so than Linde, Ashtekar has spent many years practising meditation, and he is unabashed about the interplay between his scientific thinking and his spirituality — the parallels between his two worlds are poetic and profound.
With colleagues, he has proposed an alternative to the conventional picture, in which time is created in the explosion of the Big Bang, arguing instead that the cosmos is eternal, and removing the need for those pesky infinitely small and dense singularities that physicists have spent decades struggling to explain.
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But he has also thought about ways to bring the two modes of thought — spiritual and scientific — together more explicitly, when considering the nature of consciousness. Ashtekar was raised a Jain — an Indian religion that eschews the idea of a deity, and places emphasis on avoiding cruelty to humans and animals, as the soul moves through cycles of reincarnation. His childhood travels also exposed him to a variety of communities, and he read voraciously on Hinduism, Buddhism and Chinese Taoism, alongside his scientific studies.
While enrolled in graduate study in physics at the University of Texas in Austin, Ashtekar also took a year of classes with the renowned Indian philosopher Raja Rao, debating the merits of Hinduism compared with Buddhism. For Ashtekar, it was not enough to just accept that the Universe is pervaded by a four-dimensional spacetime fabric.
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He wanted to know how that fabric was stitched together, believing that the answer held the key to explaining how general relativity and quantum theory can come together on the tiniest scales. Ashtekar was wearing a grey shirt and started to pull at its threads to illustrate his thinking. He remarked that when it is viewed from afar, the shirt appears to be cut from one continuous smooth material; viewed up close, however, you can see the threads from which it is woven.
For instance, physicists have a quantum description for light, which states that light particles, or photons, are actually excited bundles of energy that rise up from a background electromagnetic field — like water waves swelling up from an otherwise still ocean. These loops then link together to create a web that weaves together spacetime. That, in turn, sets a minimum size below which spacetime, itself woven from loops, cannot be squeezed. This means that, according to the loop quantum gravity picture, the Universe could never have been squashed into a tiny singularity, even at its birth.
To find out what might have happened at the Big Bang, according to his loopy framework, Ashtekar and colleagues created a computer simulation of the Universe and then wound the clock back roughly 13 billion years, to the time when the Big Bang is thought to have occurred. At first, things proceeded in the conventional way: as time reversed, the cosmos became smaller and smaller. Ashtekar argues that this indicates that our cosmos had no beginning — no birth at a Big Bang singularity — but instead has always existed.
At some point, in the past, he says, the cosmos contracted, and then bounced outwards again, and we now live in that expanding phase. A shtekar says that the parallels between his theory of loops and the ancient scriptures — both describing a universe cycled through phases of creation and destruction — are merely coincidental, if pleasingly consonant.
PDF Download The Arrows of Time A Debate in Cosmology Fundamental Theories of Physics PDF Online
But there are other areas where he makes more explicit links between his physics and spirituality. Over the past decade or so, Ashtekar has become a more committed adherent of Buddhism; following the Vipassana school, he has taken part in intense day meditations, during which he is banned from speaking and reading. Ashtekar felt like a helpless child. Inspired by his meditative practice, Ashtekar is training a scientific eye on other aspects of Buddhist philosophy. The practice teaches of a cycle of personal reincarnation broken by reaching enlightenment, or nirvana.
Cosmology and the Arrow of Time (TEDxCaltech)
Ashtekar has been pondering whether it might be possible to develop a physical model of consciousness that chimes with this. Why is there such an arrow of time, closely related to the initial state of the universe, and why do the cosmic, thermodynamic and other arrows agree? These issues are discussed here by leading experts, and each offers a new perspective on the debate.
Their contributions delve into the most difficult research topic in physics, also describing the latest cutting edge research on the subject. The book also offers readers a comparison between the different outlooks of philosophy, physics and cosmology on the puzzle of time.
This volume is intended to be useful for research purposes, but most chapters are also accessible to a more general audience of scientifically educated readers looking for deeper insights. Passar bra ihop. Recensioner i media. Bloggat om The Arrows of Time.
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In this view, gravity causes matter to clump together, defining an arrow of time that aligns itself with growth of complexity, said Tim Koslowski , a physicist at the National Autonomous University of Mexico he described the idea in a paper co-authored by the British physicist Julian Barbour and Flavio Mercati , a physicist at Perimeter. As one moves away from that point, in either direction, complexity increases.
Naturally, we — complex creatures capable of making observations — can only evolve at some distance from the minimum. Still, wherever we happen to find ourselves in the history of the universe, we can point to an era of less complexity and call it the past, Koslowski said. The models are globally time-symmetric, but every observer will experience a local arrow of time. Rather, it emerges naturally from it.
Back in , Carroll, along with his graduate student Jennifer Chen, put forward a similar proposal based on eternal inflation , a relatively well-known model of the beginning of the universe. Carroll sees the work of Koslowski and his colleagues as a useful step, especially since they worked out the mathematical details of their model he and Chen did not. Still, he has some concerns. Increasing complexity, Koslowski said, has one crucial side effect: It leads to the formation of certain arrangements of matter that maintain their structure over time.
What all of these entities have in common is that they contain information about some earlier state of the universe. I asked Koslowski if memories stored in brains are another kind of record. Yes, he said. But perhaps time is even more fundamental than this. For George Ellis , a cosmologist at the University of Cape Town in South Africa, time is a more basic entity, one that can be understood by picturing the block universe as itself evolving. The surface of this volume can be thought of as the present moment. Although some colleagues disagree, Ellis stresses that the model is a modification, not a radical overhaul, of the standard view.
In this view, while the past is fixed and unchangeable, the future is open. Although quantum uncertainty, as Ellis pointed out, may be enough to sink such a deterministic view. At the conference, someone asked Ellis if, given enough information about the physics of a sphere of a certain radius centered on the British Midlands in early June, one could have predicted the result of the Brexit vote. Another approach that aims to reconcile the apparent passage of time with the block universe goes by the name of causal set theory.
First developed in the s as an approach to quantum gravity by the physicist Rafael Sorkin — who was also at the conference — the theory is based on the idea that space-time is discrete rather than continuous.