Archive for George Ellis

George Ellis – Are there multiple universes?

Posted in The Universe and Stuff with tags , , , on July 18, 2016 by telescoper

So, back to Brighton and a sweltering office on Sussex University Campus. I made it back to pick up the list of names I’ll be reading out at tomorrow afternoon’s graduation ceremony in time to give me a few hours’ practice tonight. On the train back from Cardiff I remembered a discussion I had at the conference last week, especially about the various views about cosmology, especially the idea that we might live in a multiverse. I did a bit of a dig around and found this nice video of esteemed cosmologist  (and erstwhile co-author of mine) George Ellis talking about this, and also about his favourite kind of universe (namely one with a compact topology).

 

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R.I.P. Jonah Lomu

Posted in Biographical, Rugby with tags , , , , , , , , on November 18, 2015 by telescoper

At the end of the 2015 Rugby World Cup, I wrote a post recalling the World Cup of 1995, which was held in South Africa while I was visiting there. I had the privilege of seeing the great Jonah Lomu demolishing the England defence that day. Today I learned with greant sadness that he has passed away, aged just 40. Since Jonah Lomu played such a central role in one of the most amazing sporting experiences of my life, which lives in my memory as if it happened yesterday, I wanted to take the opportunity to pay tribute to the awesome sportsman that he was by sharing that memory again.

In 1995 was visiting George Ellis at the University of Cape Town to work on a book, which was published in 1997. The book is now rather out of date, but I think it turned out rather well and it was certainly a lot of fun working on it. Of course it was a complete coincidence that I timed my trip to Cape Town exactly to cover the period of the Rugby Word Cup. Well, perhaps not a complete coincidence. In fact I was lucky enough to get a ticket for the semi-final of that tournament between England and New Zealand at Newlands, in Cape Town. I was in the stand at one end of the ground, and saw New Zealand – spearheaded by the incredible Jonah Lomu – score try after try in the distance at the far end during the first half. Here is the first, very soon after the kickoff when Andrew Mehrtens wrong-footed England by kicking to the other side of the field than where the forwards were lined up. The scrambling defence conceded a scrum which led to a ruck, from which this happened:

Jonah Lomu was unstoppable that day. One of the All Blacks later quipped that “Rugby is a team game. Fourteen players all know that their job is to give the ball to Jonah”.

It was one-way traffic in the first half but England played much better in the second, with the result that all the action was again at the far end of the pitch. However, right at the end of the match Jonah Lomu scored another try, this time at the end I was standing. I’ll never forget the sight of that enormous man sprinting towards me and am glad it wasn’t my job to try to stop him, especially have seen what happened to Underwood, Catt and Carling when they tried to bring him down. Lomu scored four tries in that game, in one of the most memorable performances by any sportsman in any sport. It’s so sad that he has gone. It’s especially hard to believe that such a phenomenal athlete could be taken at such a young age. My thoughts are with his family and friends.

Rest in Peace, Jonah Lomu (1975-2015)

1995 World Cup Memories

Posted in Biographical, Rugby with tags , , , , , , , , on October 28, 2015 by telescoper

So, the 2015 Rugby World Cup final takes place this weekend. It’s been an interesting tournament with some memorable games (and some notable disappointments). Anyway, I suddenly remembered that in 1995 I was in South Africa during the Rugby World Cup. In fact I was visiting George Ellis at the University of Cape Town to work on a book, which was published in 1997. The book is now rather out of date, but I think it turned out rather well and it was certainly a lot of fun working on it!

Was that really twenty years ago?

Of course it was a complete coincidence that I timed my trip to Cape Town exactly to cover the period of the Rugby Word Cup. Well, perhaps not a complete coincidence. In fact I was lucky enough to get a ticket for the semi-final of that tournament between England and New Zealand at Newlands, in Cape Town. I was in the stand at one end of the ground, and saw New Zealand – spearheaded by the incredible Jonah Lomu – score try after try in the distance at the far end during the first half. Here is the first, very soon after the kickoff when Andrew Mehrtens wrong-footed England by kicking to the other side of the field than where the forwards were lined up. The scrambling defence conceded a scrum which led to a ruck, from which this happened:

Even more impressively I had a very good view when Zinzan Brooke scored at the same end with a drop-goal off the back of a scrum. Not many No. 8 forwards have the skill to do that!

It was one-way traffic in the first half but in the second half England played much better, with the result that all the action was again at the far end of the pitch. However, right at the end of the match Jonah Lomu scored another try, this time at the end I was standing. I’ll never forget the sight of that enormous man sprinting towards me and am glad it wasn’t my job to try to stop him, especially have seen what happened to Underwood, Catt and Carling when they tried to bring him down.

Anyway, I hope it’s a good final on Saturday. For what it’s worth, I did pick the two finalists correctly before the tournament. I’m expecting the All Blacks to beat Australia comfortably, but am not going to bet on the result!

Bayes, Bridge and the Brain

Posted in Books, Talks and Reviews, The Universe and Stuff with tags , , , on April 12, 2012 by telescoper

I was having a chat over coffee yesterday with some members of the Mathematics Department here at the University of Cape Town, one of whom happens to be an expert at Bridge, actually representing South Africa in international competitions. That’s a much higher level than I could ever aspire to so I was a bit nervous about mentioning my interest in the game, but in the end I explained that I have in the past used Bridge (and other card games) to describe how Bayesian probability works; see this rather lengthy post for more details. The point is that as cards are played, one’s calculation of the probabilities of where the important cards lie changes in the light of information revealed. It makes much more sense to play Bridge according to a Bayesian interpretation, in which probability represents one’s state of knowledge, rather than what would happen over an ensemble of “random” realisations.

This particular topic – and Bayesian inference in general – is also discussed in my book From Cosmos to Chaos (which is, incidentally, now available in paperback). On my arrival in Cape Town I gave a copy of this book to my genial host, George Ellis, and our discussion of Bridge prompted him to say that he thought I had missed a trick in the book by not mentioning the connections between Bayesian probability and neuroscience. I hadn’t written about this because I didn’t know anything about it, so George happily enlightened me by sending a few review articles, such as this:

I can’t post it all, for fear of copyright infringement, but you get the idea. Here’s another one:

And another…

Nature Reviews Neuroscience 11, 605 (August 2010) | doi:10.1038/nrn2787-c1

A neurocentric approach to Bayesian inference    Christopher D. Fiorillo

Abstract A primary function of the brain is to infer the state of the world in order to determine which motor behaviours will best promote adaptive fitness. Bayesian probability theory formally describes how rational inferences ought to be made, and it has been used with great success in recent years to explain a range of perceptual and sensorimotor phenomena.

As a non-expert in neuroscience, I find these very interesting. I’ve long been convinced that from the point of view of formal reasoning, the Bayesian approach to probability is the only way that makes sense, but until reading these I’ve not been aware that there was serious work being done on the possibility that it also describes how the brain works in situations where there is insufficient information to be sure what is the correct approach. Except, of course, for players of Bridge who know it very well.

There’s just a chance that I may have readers out there who know more about this Bayes-Brain connection. If so, please enlighten me further through the comments box!

My Friend Erdös..

Posted in Biographical with tags , , , , , , on March 28, 2010 by telescoper

After one of my  lectures a few weeks ago, a student came up to me and asked whether I had an Erdős number and, if so, what it was.  I didn’t actually know what he was talking about but was yesterday reminded of it, so tried to find out.

In case you didn’t know, Paul Erdős (who died in 1996) was an eccentric Hungarian mathematician who wrote more than 1000 mathematical papers during his life but never settled in one place for any length of time. He travelled between colleagues and conference, mostly living out of a suitcase, and showed no interest at all in property or possessions. His story is a fascinating one, and his contributions to mathematics were immense and wide-ranging.  The Erdős number is a tiny part of his legacy, but one that seems to have taken hold. Some mathematicians appear to take it very seriously, but most treat it with tongue firmly in cheek, as I certainly do.

So what is the Erdős number?

It’s actually quite simple to define. First, Erdős himself is assigned an Erdős number of zero. Anyone who co-authored a paper with Erdős has an Erdős number of 1. Then anyone who wrote a paper with someone who wrote a paper with Erdős has an Erdős number of 2, and so on. The Erdős number is thus a measure of “collaborative distance”, with lower numbers representing closer connections.

I say it’s quite easy to define, but it’s rather harder to calculate. Or it would be were it not for modern bibliographic databases. In fact there’s a website run by the American Mathematical Society which allows you to calculate your Erdős number as well as a similar measure of collaborative distance with respect to any other mathematician.

A list of individuals with very low Erdős numbers (1, 2 or 3) can be found here.

Given that Erdős was basically a pure mathematician, I didn’t expect first to show up as having any Erdős number at all, since I’m not really a mathematician and I’m certainly not very pure. However, his influence is clearly felt very strongly in  physics and a surprisingly large number of physicists (and astronomers) have a surprisingly small Erdős number. According to the AMS website, mine is 5 – much lower than I would have expected. The path from me to Erdős in this case goes through G.F.R. Ellis, a renowned expert in the mathematics of general relativity (as well as a ridiculous number of other things!). I wrote a paper and a book with George Ellis some time ago.

However, looking at the list I realise that I have another route to Erdős, through the great Russian mathematician Vladimir Arnold, who has an Erdős number of 3. Arnold wrote a paper with Sergei Shandarin with whom I wrote a paper some time ago. That gives me another route to an Erdős number of 5, but I can’t find any paths  shorter than that.

I guess many researchers will have links through their PhD supervisors, so I checked mine – John D. Barrow. It turns out he also has an Erdős number of 5 so a path through him doesn’t lower my number.

I used to work in the School of Mathematical Sciences at Queen Mary, University of London, and it is there that I found some people I know well who have lower Erdős numbers than me. Reza Tavakol, for example, has an Erdős number of 3 but although I’ve known him for 20 years, we’ve never written a paper together. If we did, I could reduce my Erdős number by one. You never know….

This means that anyone I’ve ever written a paper with has an Erdős number no greater than 6. I doubt if it’s very important, but it definitely qualifies as Quite Interesting.

Cranks Anonymous

Posted in Biographical, Books, Talks and Reviews, The Universe and Stuff with tags , , , , on September 22, 2009 by telescoper

Sean Carroll, blogger-in-chief at Cosmic Variance, has ventured abroad from his palatial Californian residence and is currently slumming it in a little town called Oxford where he is attending a small conference in celebration of the 70th birthday of George Ellis. In fact he’s been posting regular live commentaries on the proceedings which I’ve been following with great interest. It looks an interesting and unusual meeting because it involves both physicists and philosophers and it is based around a series of debates on topics of current interest. See Sean’s posts here, here and here for expert summaries of the three days of the meeting.

Today’s dispatches included an account of George’s own talk which appears to have involved delivering a polemic against the multiverse, something he has been known to do from time to time. I posted something on it myself, in fact. I don’t think I’m as fundamentally opposed as Geroge to the idea that we might live in a bit of space-time that may belong to some sort of larger collection in which other bits have different properties, but it does bother me how many physicists talk about the multiverse as if it were an established fact. There certainly isn’t any observational evidence that this is true and the theoretical arguments usually advanced are far from rigorous.The multiverse certainly is  a fun thing to think about, I just don’t think it’s really needed.

There is one red herring that regularly floats into arguments about the multiverse, and that concerns testability. Different bits of the multiverse can’t be observed directly by an observer in a particular place, so it is often said that the idea isn’t testable. I don’t think that’s the right way to look at it. If there is a compelling physical theory that can account convincingly for a realised multiverse then that theory really should have other necessary consequences that are testable, otherwise there’s no point. Test the theory in some other way and you test whether the  multiverse emanating from it is sound too.

However, that fairly obvious statement isn’t really the point of this piece. As I was reading Sean’s blog post for today you could have knocked me down with a feather when I saw my name crop up:

Orthodoxy is based on the beliefs held by elites. Consider the story of Peter Coles, who tried to claim back in the 1990’s that the matter density was only 30% of the critical density. He was threatened by a cosmological bigwig, who told him he’d be regarded as a crank if he kept it up. On a related note, we have to admit that even scientists base beliefs on philosophical agendas and rationalize after the fact. That’s often what’s going on when scientists invoke “beauty” as a criterion.

George was actually talking about a paper we co-wrote for Nature in which we went through the different arguments that had been used to estimate the average density of matter in the Universe, tried to weigh up which were the more reliable, and came to the conclusion that the answer was in the range 20 to 40 percent of the critical density. There was a considerable theoretical prejudice at the time, especially from adherents of  inflation, that the density should be very close to the critical value, so we were running against the crowd to some extent. I remember we got quite a lot of press coverage at the time and I was invited to go on Radio 4 to talk about it, so it was an interesting period for me. Working with George was a tremendous experience too.

I won’t name the “bigwig” George referred to, although I will say it was a theorist; it’s more fun for those working in the field to guess for themselves! Opinions among other astronomers and physicists were divided. One prominent observational cosmologist was furious that we had criticized his work (which had yielded a high value of the density). On the other hand, Martin Rees (now “Lord” but then just plain “Sir”) said that he thought we were pushing at an open door and was surprised at the fuss.

Later on, in 1996, we expanded the article into a book in which we covered the ground more deeply but came to the same conclusion as before.  The book and the article it was based on are now both very dated because of the huge advances in observational cosmology over the last decade. However, the intervening years have shown that we were right in our assessment: the standard cosmology has about 30% of the critical density.

Of course there was one major thing we didn’t anticipate which was the discovery in the late 1990s of dark energy which, to be fair, had been suggested by others more prescient than us as early as 1990. You can’t win ’em all.

So that’s the story of my emergence as a crank, a title to which I’ve tried my utmost to do justice since then. Actually, I would have liked to have had the chance to go to George’s meeting in Oxford, primarily to greet my ertswhile collaborator whom I haven’t seen for ages. But it was invitation-only. I can’t work out whether these days I’m too cranky or not cranky enough to get to go to such things. Looking at the reports of the talks, I rather think it could be the latter.

Now, anyone care to risk the libel laws and guess who Professor BigWig was?