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CURATOR
A pinboard by
Endre Szvetnik

I cover science and tech news for Sparrho and work with Sparrho Heroes to curate, translate and disseminate scientific research to the wider public.

PINBOARD SUMMARY

Prof. Stephen Hawking has moved forward the understanding of the universe and popularised science.

Here we offer some of Stephen Hawking's important papers and research inspired by him and also have a brief look at his work trying to solve the Information Paradox around black holes.

In 10 seconds? The world’s best known scientist had furthered the theory about black holes – extremely dense objects in space with massive gravity - but sometimes contradicted himself.

How did Hawking contradict himself? Initially he followed Einstein’s theory of relativity – about how large objects bend space and time - and agreed that black holes never changed and were cold. But when he applied quantum theory – which explains the behaviour of particles - he proved that black holes radiated heat which would make them gradually disappear.

So was this the Information Paradox? We’re getting there! Hawking lent his name to the radiation phenomenon he discovered and now owned a problem. He used quantum physics and he stated that black holes vanished, which would mean information about the particles inside – their traits and what they do – would also disappear.

And what's the problem? That it contradicts the basic rule of quantum mechanics, stating that information cannot disappear. For example, from the current state of a particle we can deduct its previous state, something like from soap diluted in water we could reproduce the soap. This is the information paradox.

And how did he suggest to solve it? By claiming that black holes did not exist, or at least not as we knew them. In 2014, he broke with previous ideas rejecting the so-called 'event horizon'. (If you cross this perimeter near a black hole, there is no escape and you fall in.) Hawking said information would get out, but would be so jumbled that it could not be interpreted. (Read more)

And that was it? No! Although by this time he was more famous for his public appearances, he co-authored a late paper in 2016. He proposed a solution by saying that ‘zero-energy’ particles float around black holes, which he called ‘hair’, and this is where the information that escaped would be stored.

So, did he solve the paradox? By this time you’ve guessed he didn’t. But, he inspired researchers to carry on. There are a lot of competing ideas. Scientists are trying to bridge the gaps to come up with a unified ‘theory of everything’, that according to Stephen Hawking could finally explain the workings of the universe.


The black hole in Interstellar: a hard bargain between art and science

Stephen Hawking popularised black holes and one of his colleagues, Kip Thorne from Caltech, served as scientific advisor in the sci-fi film, Interstellar.

Its depiction of a black hole was hailed as ‘scientifically accurate’, but Thorne wanted a more realistic version.

Eventually, director Christopher Nolan used a spectacular and symmetrical ‘accretion disk’ – representing matter revolving around the black hole.

Thorne’s models took into account the position of the observer, their perception of colour and light from a vantage point near the black hole.

He later consoled himself by winning the Nobel prize for the discovery of gravitational waves.

13 ITEMS PINNED

Singularities and the geometry of spacetime

Abstract: The aim of this essay is to investigate certain aspects of the geometry of the spacetime manifold in the General Theory of Relativity with particular reference to the occurrence of singularities in cosmological solutions and their relation with other global properties. Section 2 gives a brief outline of Riemannian geometry. In Section 3, the General Theory of Relativity is presented in the form of two postulates and two requirements which are common to it and to the Special Theory of Relativity, and a third requirement, the Einstein field equations, which distinguish it from the Special Theory. There does not seem to be any alternative set of field equations which would not have some undeseriable features. Some exact solutions are described. In Section 4, the physical significance of curvature is investigated using the deviation equation for timelike and null curves. The Riemann tensor is decomposed into the Ricci tensor which represents the gravitational effect at a point of matter at that point and the Welyl tensor which represents the effect at a point of gravitational radiation and matter at other points. The two tensors are related by the Bianchi identities which are presented in a form analogous to the Maxwell equations. Some lemmas are given for the occurrence of conjugate points on timelike and null geodesics and their relation with the variation of timelike and null curves is established. Section 5 is concerned with properties of causal relations between points of spacetime. It is shown that these could be used to determine physically the manifold structure of spacetime if the strong causality assumption held. The concepts of a null horizon and a partial Cauchy surface are introduced and are used to prove a number of lemmas relating to the existence of a timelike curve of maximum length between two sets. In Section 6, the definition of a singularity of spacetime is given in terms of geodesic incompleteness. The various energy assumptions needed to prove the occurrence of singularities are discussed and then a number of theorems are presented which prove the occurrence of singularities in most cosmological solutions. A procedure is given which could be used to describe and classify the singularites and their expected nature is discussed. Sections 2 and 3 are reviews of standard work. In Section 4, the deviation equation is standard but the matrix method used to analyse it is the author’s own as is the decomposition given of the Bianchi identities (this was also obtained independently by Trümper). Variation of curves and conjugate points are standard in a positive-definite metric but this seems to be the first full account for timelike and null curves in a Lorentz metric. Except where otherwise indicated in the text, Sections 5 and 6 are the work of the author who, however, apologises if through ignorance or inadvertance he has failed to make acknowledgements where due. Some of this work has been described in [Hawking S.W. 1965b. Occurrence of singularities in open universes. Phys. Rev. Lett. 15: 689–690; Hawking S.W. and G.F.R. Ellis. 1965c. Singularities in homogeneous world models. Phys. Rev. Lett.17: 246–247; Hawking S.W. 1966a. Singularities in the universe. Phys. Rev. Lett. 17: 444–445; Hawking S.W. 1966c. The occurrence of singularities in cosmology. Proc. Roy. Soc. Lond. A294: 511–521]. Undoubtedly, the most important results are the theorems in Section 6 on the occurrence of singularities. These seem to imply either that the General Theory of Relativity breaks down or that there could be particles whose histories did not exist before (or after) a certain time. The author’s own opinion is that the theory probably does break down, but only when quantum gravitational effects become important. This would not be expected to happen until the radius of curvature of spacetime became about 10-14 cm.

Pub.: 10 Nov '14, Pinned: 14 Mar '18