The theories behind the big bang experiment

PROF BOB CYWINSKI, Dean of Applied Sciences at the University of Huddersfield, looks at superconductivity

CERN’s Large Hadron Collider, or LHC, has been back in the news again this week.

This giant particle accelerator buried deep beneath Geneva has just reached a new milestone, successfully accelerating sub-atomic particles to the highest energies ever achieved in the laboratory – 3.5 million million electron volts.

Physicists are now eagerly awaiting the next stage of development in which two proton beams circulating the LHC in opposite directions at these enormous energies will be brought into collision, releasing a myriad of even more fundamental particles.

It is expected that the information obtained from these collisions will provide answers to some of the most compelling questions in science, such as what actually happened in the big bang that created the Universe, why does matter have mass, and why are there not equal amounts of matter and anti-matter in the Universe?

However, you may remember that shortly after the LHC experiment was first switched on in September, 2008, it was abruptly shut down again, and did not restart until the end of last year.

This was not because of any safety issues or the concerns of the prophets of doom that the LHC would destroy the world, but because of a simple technical fault associated with some of the 1,200 superconducting magnets that bend and steer the hadron beams around their 27 kilometre circular path.

We are all familiar with the magnets we use to stick our childrens’ artwork to the fridge, and some of us have come across electromagnets, the much more powerful systems in which an electrical current circulating in a coil of wire generates a high magnetic field.

But what exactly is a superconducting magnet, and why have those that make up the LHC proved problematic?

Superconductivity was first discovered in 1911 by Kamerlingh-Onnes in the Netherlands during his first attempts to create temperatures close to absolute zero.

He noticed that the metal mercury lost all of its electrical resistance, that is it became super-conducting, at temperatures below 4 degrees above absolute zero (or 4 degrees Kelvin).

At first superconductivity was considered to be just a curiosity, but the importance of a material that could carry electricity without resistance was very soon realised: not only would the material not heat up as large currents flowed, but any current injected into a loop of superconductor would essentially flow for ever.

Almost magically a superconducting loop, once energised, would continue to generate a magnetic field even after the supply of electricity was disconnected, as long as the superconductor remained below its “transition temperature”.

This is the basis of the superconducting magnet — and the forerunner of the giant magnets used in today’s Medical Resonance Imaging (MRI) facilities in most hospitals, in scientific research and of course in the LHC accelerator.

Using superconducting materials such as niobium-titanium and niobium-tin alloys, with transition temperatures of 10 and 18 degrees Kelvin, magnetic fields of almost a million times stronger than the Earth’s magnetic field can be generated.

The problem is that such superconducting magnets have to be cooled by liquid helium to temperatures of 4 degrees Kelvin, or even lower.

Unfortunately, liquid helium is an expensive cryogenic coolant which can only be contained in very complex vacuum vessels, or cryostats — and here lies the problem with the LHC.

Slight movement of the coils within a superconducting magnet can cause local heating, which rapidly boils off the cryogenic helium and takes the superconductor back into a normal resistive state, which in turn heats the coil and could damage the magnet.

The limitation of having to work at such low temperatures has led physicists, chemists and materials scientists to search for new and better superconductors with higher transition temperatures.

Surprisingly, superconductivity has been found to be a relatively common phenomenon in metals, alloys, compounds, ceramics and even organic material.

The highest temperatures achieved so far has been in a family of exotic copper oxides in which transitions above 130 degrees Kelvin have been recorded.

The goal is, of course, to discover materials that superconduct above room temperature (which is 293 degrees Kelvin).

The technological advantages that such a discovery would bring cannot be overestimated: lossless electricity transmission and storage, ultra high speed electronics, frictionless motors and levitating trains on frictionless tracks would all be possible.

Intriguingly, although we know that superconductivity is an entirely different state of matter and one that is entirely dependent on the complex laws of quantum mechanics, we still do not fully understand its origins.

Therefore, even after a hundred years, superconductivity itself poses just as big a scientific question as those the LHC is trying to answer.