Archive for Zel’dovich

Flame Academy

Posted in Biographical, The Universe and Stuff with tags , , , , , , , on September 2, 2009 by telescoper

I heard on the radio this morning from that nice Mr Cowan that today is the anniversary of the start of the Great Fire of London which burned for four days in 1666. That provides for a bit of delayed synchronicity with yesterday’s post about the dreadful fires in the outskirts of Los Angeles and a similar conflagration in Athens (which now thankfully appears to be under control).

Fires are of course terrifying phenomena, and it must be among most people’s nightmares to be caught in one. The cambridge physicist Steve Gull experienced this at first hand when his boat exploded and caught fire recently. I’ll take this opportunity to wish him a speedy recovery from his injuries.

But frightening as such happenings are, a flame (the visible, light emitting part of a fire) can also be a very beautiful and fascinating spectacle. Flames are stable long-lived phenomena involving combustion in which a “fuel”, often some kind of hydrocarbon, reacts with an oxidizing element which, in the case of natural wildfires at any rate, is usually oxygen. However, along the way, many intermediate radicals are generated and the self-sustaining nature of the flame is maintained by intricate reaction kinetics.

The shape and colour of a flame is determined not just by its temperature but also, in a complicated way, by diffusion, convection and gravity. In a diffusion flame, the fuel and the oxidizing agent diffuse into each other and the rate of diffusion consequently limits the rate at which the flame spreads. Usually combustion takes place only at the edge of the flame: the interior contains unburnt fuel. A candle flame is usually relatively quiescent because the flow of material in it is predominantly laminar. However, at higher speeds you can find turbulent flames, like in the picture below!

Sometimes convection carries some of the combustion products away from the source of the flame. In a candle flame, for example, incomplete combustion forms soot particles which are convected upwards and then incandesce inside the flame giving it a yellow colour. Gravity limits the motion of heavier products away from the source. In a microgravity environment, flames look very different!

All this stuff about flames also gives me the opportunity to mention the great Russian physicist Yakov Borisovich Zel’dovich. To us cosmologists he is best known for his work on the large-scale structure of the Universe, but he only started to work on that subject relatively late in his career during the 1960s.  He in fact began his career as a physical chemist and arguably his greatest contribution to science was that he developed the first completely physically based theory of flame propagation (together with Frank-Kamenetskii). No doubt he used insights gained from this work, together with his studies of detonation and shock waves, in the Soviet nuclear bomb programme in which he was a central figure.

But one thing even Zel’dovich couldn’t explain is why fires are such fascinating things to look at. I remember years ago having a fire in my back garden to get rid of garden rubbish. The more it burned the more things  I wanted to throw on it,  to see how well they would burn rather than to get rid of them. I ended up spending hours finding things to burn, building up a huge inferno, before finally retiring indoors, blackened with soot.

I let the fire die down, but it smouldered for three days.

Cosmology Explained

Posted in Biographical, The Universe and Stuff with tags , , on September 29, 2008 by telescoper

I’ve always avoided describing myself as an astronomer, because most people seem to think that involves star signs and horoscopes. Anyone can tell I’m not an astrologer anyway, because I’m not rich. Astrophysicist sounds more impressive, but perhaps a bit too scary. That’s why I settled on “Cosmologist”. Grandiose, but at the same time somehow cuddly.

I had an inkling that this choice was going to be a mistake at the start of my first ever visit to the United States, which was to attend a conference in memory of the great physicist Yacov Borisovich Zel’dovich, who died in 1989. The meeting was held in Lawrence, Kansas, home of the University of Kansas, in May 1990. This event was notable for many reasons, including the fact that the effective ban on Russian physicists visiting the USA had been lifted after the arrival of glasnost to the Soviet Union. Many prominent scientists from there were going to be attending. I had also been invited to give a talk, the only connection with Zel’dovich that I could figure out was that the very first paper I wrote was cited in the very last paper to be written by the great man.

I think I flew in to Detroit from London and had to clear customs there in order to transfer to an internal flight to Kansas. On arriving at the customs area in the airport, the guy at the desk peered at my passport and asked me what was the purpose of my visit. I said “I’m attending a Conference”. He eyed me suspiciously and asked me my line of work. “Cosmologist,” I proudly announced. He frowned and asked me to open my bags. He looked in my suitcase, and his frown deepened. He looked at me accusingly and said “Where are your samples?”

I thought about pointing out that there was indeed a sample of the Universe in my bag but that it was way too small to be regarded as representative. Fortunately, I thought better of it. Eventually I realised he thought cosmologist was something to do with cosmetics, and was expecting me to be carrying little bottles of shampoo or make-up to a sales conference or something like that. I explained that I was a scientist, and showed him the poster for the conference I was going to attend. He seemed satisfied. As I gathered up my possessions thinking the formalities were over, he carried on looking through my passport. As I moved off he suddenly spoke again. “Is this your first visit to the States, son?”. My passport had no other entry stamps to the USA in it. “Yes,” I said. He was incredulous. “And you’re going to Kansas?”

This little confrontation turned out to be a forerunner of a more dramatic incident involving the same lexicographical confusion. One evening during the Zel’dovich meeting there was a reception held by the University of Kansas, to which the conference participants, local celebrities (including the famous writer William Burroughs, who lived nearby) and various (small) TV companies were invited. Clearly this meeting was big news for Lawrence. It was all organized by the University of Kansas and there was a charming lady called Eunice largely running the show. I got talking to her near the end of the party. As we chatted, the proceedings were clearly winding down and she suggested we go into Kansas to go dancing. I’ve always been up for a boogie, Lawrence didn’t seem to be offering much in the way of nightlife, and my attempts to talk to William Burroughs were repelled by the bevy of handsome young men who formed his entourage, so off we went in her car.

It takes over an hour to drive into Kansas City from Lawrence but we got there safely enough. We went to several fun places and had a good time until well after midnight. We were about to drive back when Eunice suddenly remembered there was another nightclub she had heard of that had just opened. However, she didn’t really know where it was and we spent quite a while looking for it. We ended up on the State Line, a freeway that separates Kansas City Kansas from Kansas City Missouri, the main downtown area of Kansas City actually being for some reason in the state of Missouri. After only a few moments on the freeway a police car appeared behind us with its lights blazing and siren screeching, and ushered us off the road into a kind of parking lot.

Eunice stopped the car and we waited while a young cop got out of his car and approached us. I was surprised to see he was on his own. I always thought the police always went around in pairs, like low comedians. He asked for Eunice’s driver’s license, which she gave him. He then asked for mine. I don’t drive and don’t have a driver’s license, and explained this to the policeman. He found it difficult to comprehend. I then realised I hadn’t brought my passport along, so I had no ID at all.

I forgot to mention that Eunice was black and that her car had Alabama license plates.

I don’t know what particular thing caused this young cop to panic, but he dashed back to his car and got onto his radio to call for backup. Soon, another squad car arrived, drove part way into the entrance of the parking lot and stopped there, presumably so as to block any attempted escape. The doors of the second car opened and two policemen got out, kneeled down and and aimed pump-action shotguns at us as they hid behind the car doors which partly shielded them from view and presumably from gunfire. The rookie who had stopped us did the same thing from his car, but he only had a handgun.

“Put your hands on your heads. Get out of the car. Slowly. No sudden movements.” This was just like the movies.

We did as we were told. Eventually we both ended up with our hands on the roof of Eunice’s car being frisked by a large cop sporting an impressive walrus moustache. He reminded me of one of the Village People, although his uniform was not made of leather. I thought it unwise to point out the resemblance to him. Declaring us “clean”, he signalled to the other policemen to put their guns away. They had been covering him as he searched us.

I suddenly realised how terrified I was. It’s not nice having guns pointed at you.

Mr Walrus had found a packet of French cigarettes (Gauloises) in my coat pocket. I clearly looked scared so he handed them to me and suggested I have a smoke. I lit up, and offered him one (which he declined). Meanwhile the first cop was running the details of Eunice’s car through the vehicle check system, clearly thinking it must have been stolen. As he did this, the moustachioed policeman, who was by now very relaxed about the situation, started a conversation which I’ll never forget.

Policeman: “You’re not from around these parts, are you?” (Honestly, that’s exactly what he said.)

Me: “No, I’m from England.”

Policeman: “I see. What are you doing in Kansas?”

Me: “I’m attending a conference, in Lawrence..”

Policeman: “Oh yes? What kind of Conference?”

Me: “It’s about cosmology”

At this point, Mr Walrus nodded and walked slowly to the first car where the much younger cop was still fiddling with the computer.

“Son,” he said, “there’s no need to call for backup when all you got to deal with is a limey hairdresser.”

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