Thursday, 29 October 2009

Speed limit for computer processors - serial vs parallel

This news item from Nature about this PRL talks about how computer processors are going to hit a speed limit due to the speed at which a system can make transitions between quantum states.

There are two independent bounds on this minimum time — one based on the average energy of the quantum system, the other based on the uncertainty in the system's energy. In their calculations, Levitin and Toffoli unify the bounds and show there is an absolute limit to the number of operations that can be achieved per second by a computer system of a given energy.

I'm not an expert in quantum information so all I can say is that it looks interesting. There are implications for myself because most of my work is pretty intensive computer simulation. Some of what I do simply needs fast processors, there are sections of my Monte Carlo simulations that cannot be parallelised (fancy cluster algorithms being one). So for these, in principle, it limits what could ever be done.

However, mostly my limit is on what statistics I can collect and that can be solved by using more and more processors. The move from single core being standard, to eight these days, has been a revolution in terms of what I can now get done in a reasonable time scale.

In fact one very interesting development is using standard computer graphics cards to perform molecular dynamics (MD) simulations. I've only read the abstract of this paper I'm afraid but they've apparently done this. Graphics cards designed for games have many little processors on them (GPUs) and they can all work on the problem more efficiently than one super powered CPU trying to do it on its own.

So next time you say that computer games are a waste of time think of this...

Monday, 19 October 2009

There's more to the LHC than bloody black holes

The LHC is cold again. This is very exciting, and also it can't come soon enough. In the absence of any actual science going on an endless stream of bollocks seems to have been coming out about the collider. The latest being this drivel about things coming from the future to... oh God I can't be bothered. It rather upsets me that the only things people really know about the LHC are that it might make a black hole and maybe something is coming through time to sabotage it. So I thought I'd talk about why this machine is ridiculously fantastic and complicated (the more likely cause of breakage).

One of the features of synchrotrons that I've always thought is amazing is the way they cool the beams. By cool I'm not talking about temperature around the beam pipe (although that's bloody cold too so that the magnets work). I'll quickly describe what it is and how people solve it, although I'm still not 100% sure how they've solved it at the LHC.

Our general collider accelerates particles around a ring using strong electric fields. The particles are bent into a circle by bending magnets and they are kept in a beam by the focussing quadropole magnets. The effect of these magnets is that if a particle is heading sideways out of the beam then they push it back in in the opposite direction. In this way the particles kind of snake around the course never straying too far out of line. The task of cooling the beam is to reduce this snaking as much as possible so that we have a really dense, straight running beam.