George Ellis

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Response to Part IV: The Debate on Top-Down Causation and Emergence

In this response, George Ellis comments on the publications of Part IV. He responds first to Jame... more

Response to Part II: The View from Physics

In this response, George Ellis comments on the publications of part II. He responds first to Barb... more

Causal structures in cosmology

This article reviews the properties and limitations associated with the existence of particle, vi... more This article reviews the properties and limitations associated with the existence of particle, visual, and event horizons in cosmology in general and in inflationary universes in particular, carefully distinguishing them from `Hubble horizons'. It explores to what extent one might be able to probe conditions beyond the visual horizon (which is close in size to the present Hubble radius), thereby showing that visual horizons place major limits on what are observationally testable aspects of a multiverse, if such exists. Indeed these limits largely prevent us from observationally proving a multiverse either does or does not exist. We emphasize that event horizons play no role at all in observational cosmology, even in the multiverse context, despite some claims to the contrary in the literature.

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Theoretical Cosmology

We review current theoretical cosmology, including fundamental and mathematical cosmology and phy... more

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Cosmology and Verifiability

Physical Sciences and History of Physics, 1984

Relativistic Cosmology aims to determine the structure of the Universe from a fusion of the resul... more Relativistic Cosmology aims to determine the structure of the Universe from a fusion of the results of astronomical observations with knowledge derived from local physical experiments. The problem of determining this structure1 is centred on the fact that there is only one universe to be observed, and that we effectively can only observe it from one space-time point. Because it is a unique object, we cannot infer its probable nature by comparing it with similar objects; and (on the scale we are considering) we are unable to choose the time or position from which we view it. Our predicament is analogous to that of a premaritime man living on a small island in an ocean, who observes around him a host of other small islands apparently scattered at random on a seemingly limitless sea. Unable to move from his island, his theory of the world in which he lives can only be based on this partial view.

The Covariant and Gauge Invariant Approach to Perturbations in Cosmology

These lectures look at the gauge problem in studying cosmological perturbations, and how this pro... more These lectures look at the gauge problem in studying cosmological perturbations, and how this problem can be solved by use of a covariant and gauge-invariant approach in which the variables used have a transparent physical and geometric meaning. A kinetic-theory version of this approach can be used to study anisotropies in the Cosmic Background Radiation (CBR), and in particular to generalise the Ehlers-Geren-Sachs theorem which is the basis of the best available approach to proving spatial homogeneity of the universe. The extension of this approach to Sachs-Wolfe style analyses is not yet complete, and a series of issues to do with averaging scales and with modelling the real, lumpy universe by means of simpler, smoothed out models remain to be resolved.

The different nature of cosmology

Astronomy & Geophysics, 1999

is the study of the large-scale structure of the universe, where "the universe" means all that ex... more is the study of the large-scale structure of the universe, where "the universe" means all that exists in a physical sense to be distinguished from the observable universe, namely that part of the universe containing matter accessible to our astronomical observations. Thus cosmology considers the vast domain of galaxies, clusters of galaxies, quasistellar objects etc, and their origins. Observational cosmology aims to determine the largescale geometry of the observable universe and the distribution of matter in it; physical cos-

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Issues in the study of inhomogeneity

ASIC, 1993

Paper for the 1992 Pont d’Oye NATO workshop on growth of structure in the early universe: a broad... more

Multiverses, Science, and Ultimate Causation

Astrophysics and Space Science Library, 2012

This chapter the motivation and evidence for the various types of multiverses that have been prop... more This chapter the motivation and evidence for the various types of multiverses that have been proposed. A key problem is their lack of testability, because of the existence of cosmic horizons; nevertheless they are claimed to be a scientific hypothesis. I review the arguments in their favour, and suggest none is conclusive, although there is one case where they could be disproved (the small universe case) and one that would indeed be quite convincing circumstantial evidence (circles in the CMB sky associated with variation of fundamental constants).

The evolving block universe and the meshing together of times

Annals of the New York Academy of Sciences, 2014

It has been proposed that spacetime should be regarded as an evolving block universe, bounded to ... more It has been proposed that spacetime should be regarded as an evolving block universe, bounded to the future by the present time, which continually extends to the future. This future boundary is defined at each time by measuring proper time along Ricci eigenlines from the start of the universe. A key point, then, is that physical reality can be represented at many different scales: hence, the passage of time may be seen as different at different scales, with quantum gravity determining the evolution of spacetime itself at the Planck scale, but quantum field theory and classical physics determining the evolution of events within spacetime at larger scales. The fundamental issue then arises as to how the effective times at different scales mesh together, leading to the concepts of global and local times.

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Top-down causation and emergence: some comments on mechanisms

Interface focus, Jan 6, 2012

Both bottom-up and top-down causation occur in the hierarchy of structure and causation. A key fe... more Both bottom-up and top-down causation occur in the hierarchy of structure and causation. A key feature is multiple realizability of higher level functions, and consequent existence of equivalence classes of lower level variables that correspond to the same higher level state. Five essentially different classes of top-down influence can be identified, and their existence demonstrated by many real-world examples. They are: algorithmic top-down causation; top-down causation via non-adaptive information control, top-down causation via adaptive selection, top-down causation via adaptive information control and intelligent top-down causation (the effect of the human mind on the physical world). Through the mind, abstract entities such as mathematical structures have causal power. The causal slack enabling top-down action to take place lies in the structuring of the system so as to attain higher level functions; in the way the nature of lower level elements is changed by context, and in mi...

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Top-Down Causation and the Human Brain

Understanding Complex Systems, 2009

A reliable understanding of the nature of causation is the core feature of science. In this paper... more A reliable understanding of the nature of causation is the core feature of science. In this paper the concept of top-down causation in the hierarchy of structure and causation is examined in depth. Five different classes of top-down causation are identified and illustrated with real-world examples. They are (1) al gorithmic top-down causation; (2) top-down causation via nonadaptive information control;

Relativistic Cosmology

Cosmology has been transformed by dramatic progress in high-precision observations and theoretica... more Cosmology has been transformed by dramatic progress in high-precision observations and theoretical modelling. This book surveys key developments and open issues for graduate students and researchers. Using a relativistic geometric approach, it focuses on the general concepts and relations that underpin the standard model of the Universe. Part I covers foundations of relativistic cosmology whilst Part II develops the dynamical and observational relations for all models of the Universe based on general relativity. Part III focuses on the standard model of cosmology, including inflation, dark matter, dark energy, perturbation theory, the cosmic microwave background, structure formation and gravitational lensing. It also examines modified gravity and inhomogeneity as possible alternatives to dark energy. Anisotropic and inhomogeneous models are described in Part IV, and Part V reviews deeper issues, such as quantum cosmology, the start of the universe and the multiverse proposal. Colour...

On the nature of causation in complex systems

Transactions of the Royal Society of South Africa, 2008

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Cosmology: The untestable multiverse

Nature, 2011

Ellis reminds us that Brian Greene's beguiling book on parallel worlds is more theory than fact. ... more Ellis reminds us that Brian Greene's beguiling book on parallel worlds is more theory than fact. osmology must seem odd to scientists in other fields. More and more accurate data about the distant Universe are being generated by high-tech observational techniques, giving rise to an era of 'precision cosmology' -but to explain these impressive data, cosmologists are increasingly turning to untestable theories. In The Hidden Reality, theoretical physicist Brian Greene explores one of the strangest proposals: that we live in a multiverse.

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The shape of the Universe

Nature, 2003

Therefore, the most interesting result of our paper is not that the universe appears to be curved... more Therefore, the most interesting result of our paper is not that the universe appears to be curved rather than flat, but the fact that it may force us to rearrange the pieces of the cosmic puzzle in a completely different way. [32] Now physicists at MIT, Kenyon College, and elsewhere have simulated in detail an intermediary phase of the early universe that may have bridged cosmic inflation with the Big Bang.

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Time and Spacetime: The Crystallizing Block Universe

International Journal of Theoretical Physics, 2010

The nature of the future is completely different from the nature of the past. When quantum effect... more The nature of the future is completely different from the nature of the past. When quantum effects are significant, the future shows all the signs of quantum weirdness, including duality, uncertainty, and entanglement. With the passage of time, after the time-irreversible process of state-vector reduction has taken place, the past emerges, with the previous quantum uncertainty replaced by the classical certainty of definite particle identities and states. The present time is where this transition largely takes place, but the process does not take place uniformly: Evidence from delayed choice and related experiments shows that isolated patches of quantum indeterminacy remain, and that their transition from probability to certainty only takes place later. Thus, when quantum effects are significant, the picture of a classical Evolving Block Universe ('EBU') cedes place to one of a Crystallizing Block Universe ('CBU'), which reflects this quantum transition from indeterminacy to certainty, while nevertheless resembling the EBU on large enough scales. Keywords: Space-time, quantum uncertainty, arrow of time, block universe. 1 The Nature of Space Time: Emergent universes According to the spacetime view associated with both special and general relativity, time, in a real sense, is little more than an illusion. Given data at an arbitrary instant, we assume that everything occurring at a later or earlier time is determined, evolved according to time-reversible local physics. Consequently, nothing is, or can be, special about any particular moment. The standard spacetime diagrams used in relativity enforce such a view: no special status is accorded to the present and indeed the "now" is not usually even denoted on the diagram. Rather, all possible "presents" are simultaneously represented on an equal footing. In a few instances, cosmology takes into account

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Life at the end of the universe?

General Relativity and Gravitation, 1994

Tipler has claimed in a recent scientific paper [1] that life could exist until the very end of a... more Tipler has claimed in a recent scientific paper [1] that life could exist until the very end of a universe that recollapses to a final fireball in the future—including the Planck era when quantum gravity dominates. His article is one of a number that have excited general interest because of some strong claims made on the basis of this type of argument about life in the future (indeed it has even been claimed [2,3] that this type of mathematical-physical argument can prove that Resurrection of the Dead will take place!). We point out that the underlying argument of Tipler's paper runs into problems on two quite different grounds. Firstly, it assumes a flat space information theory result and makes other technical assumptions about information processing, which invalidate the immediate technical argument. However at least as important is the fact that the argument is unsustainable because physical mechanisms required to make coherent information processing possible would not be able to operate under the extreme physical conditions that would occur towards the end of the universe.

Editorial note to: Brandon Carter, Large number coincidences and the anthropic principle in cosmology

General Relativity and Gravitation, 2011

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The history of the Universe: cosmology up to 1995

Cosmology is the study of the Universe on the grandest scale : what exists on the largest scales,... more Cosmology is the study of the Universe on the grandest scale : what exists on the largest scales, what it is doing, and how it got there. These topics have been the subject of speculation for thousands of years, but it is only in the last 90 years that we have attained a solid scientific understanding of the nature of the cosmos. This progress has been based on a parallel growth of our understanding of physical processes on the one hand, and the development of observational instruments and techniques that have given us ever better images and data about astronomical objects on the other.

George Ellis

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