The Big Question: Is our understanding of the Universe about to be transformed?

Why are we asking this now?

Next Wednesday the biggest machine and international scientific experiment ever built will be switched on. Called the Large Hadron Collider (LHC), it is a giant $10bn "atom smasher" that has been constructed at the European centre for nuclear research (Cern) in Geneva.

It consists of an underground circular tunnel 27 kilometres in circumference, which is about the size of the Circle Line on the London Underground. At various points along the tunnel, four massive instruments have been positioned to act as sub-atomic microscopes for analysing the extremely high-energy collisions that will occur between two opposing beams of protons, the atomic nuclei of hydrogen atoms. The aim of the experiment is to understand the fundamental forces of nature and the sub-atomic particles that compose all matter in the Universe.

Why is it causing such excitement?

Although we have built "atom smashers" before, this one is different in terms of how much energy will be involved. Two beams of protons will be spun in opposite directions within the underground tunnel and will attain speeds just a fraction shy of the speed of light, meaning that they will make about 11,000 laps of the circuit every second.

When they are accelerated in this way to collide head-on with each other, the resulting impact between the two proton beams will generate about seven times the energy of the LHC's nearest rival machine, the Tevatron atom smasher in Batavia, Illinois. The LHC scientists hope to get up to energy levels of 14 teraelectron volts (TeV) and so in the process create conditions that last occurred less than a billionth of a second after the Big Bang, when the Universe was created some 13.7 billion years ago.

What's the point of all this?

In order to understand what things are made of, and the forces that hold them together, it is necessary to break apart the sub-atomic constituents of matter. It is only by breaking apart a proton that scientists are able to see what is going on within this infinitesimally small unit of matter. The answer comes down to even smaller particles, some of which are so small or elusive that they have so far escaped detection. So far we know of 12 subatomic particles and 4 forces, but this is just the start.

More importantly, scientists hope to resolve some of the biggest problems in physics. They hope for instance to one day unify all the disparate forces of nature, from the small-scale nuclear forces within an atomic nucleus to the force of gravity, which acts between planets and galaxies. They call this the "theory of everything" and there is hope that the LHC will make important contributions to our wider understanding of the biggest questions concerning creation, time and the nature of matter.

Isn't it risky to mess around with high-energy collisions?

There are some theorists who believe that the collisions may create "mini" black holes. But even if they do result from the experiment, they will be sub-microscopic in size and disappear within a fraction of second of coming into existence. Few if any sensible scientists believe that these minuscule black holes pose any threat, for instance by merging into a bigger black hole that could swallow up Geneva.

Some Russian scientists have also suggested that it may be possible for the LHC to create the conditions that could in theory allow time travel. They have rather fancifully painted a scenario where future time travellers come back to visit us through the LHC, but, as other theorists have pointed out, such time travellers would have to be atom-sized to pass through the tiny "worm holes" through time and space that the LHC may or may not create.

What exactly will happen when the experiment gets under way?

For the first time, scientists will attempt to put a beam of protons into the tunnel and to accelerate it around the entire circuit. Then, possibly later that day, or certainly in the days to follow, a second beam will be put into the tunnel and accelerated around the same tunnel but in the opposite direction. It is just possible, although unlikely, that the two beams might collide, which will cause the instruments to start registering readings. However, it is only when all the finer adjustments have been made that the two beams will reach the highest energy levels that could result in some very interesting discoveries.

What important findings might emerge?

The most interesting things are almost certainly going to be those that are least expected -- or even totally unpredicted. However, there is one sub-atomic particle that theorists have already predicted to exist.

Formally called the Higgs boson, but nicknamed the "God Particle", it could explain why matter has mass and hence lead to a greater understanding of the force of gravity. At the energy levels of the LHC, it is very likely that the first Higgs boson will be registered. Indeed, Prof Peter Higgs of Edinburgh University is 90 per cent confident that the particle named after him will be discovered by the LHC. How quickly the Higgs is found – assuming it exists – depends on how heavy it is, with a lighter Higgs being harder to detect than a heavier one.

But this is just one of many possible discoveries that the LHC could make. Physicists hope that the machine will also find the mysterious supersymmetry particles that are thought to have been created at the beginning of the Universe. The theory of supersymmetry says that all known particle have a heavier partner, but none has ever been detected. If the LHC finds evidence of supersymmetrical particles, it may have also found the reason why 90 per cent of the mass of the Universe exists as invisible "dark matter".

How difficult was it to build the LHC and its machines?

Very. The 27-km tunnel is aligned to better than a tenth of a millimetre and underground rivers had to be temporarily frozen to permit its construction. The giant magnets used to accelerate the proton beams have to be held together with a force that can resist 500 tons per square metre -– equivalent to one jumbo jet per square metre.

They are supercooled to 1.8 degrees above absolute zero (-273C), making the LHC the coldest place in the known universe, with enough freezing capacity to keep 140,000 domestic fridges at a temperature of -271.2C. The civil and mechanical engineering involved was almost as momentous as the science, which could account for why next week's switch on was originally scheduled for three years ago.

Is such a huge experiment worth it?

Yes...

* We need to know how the Universe is put together to understand our place in it

* The cost is trivial compared with that of not expanding on our existing knowledge

* There have been huge spin-offs from similar experiments, notably the internet

No...

* The science is too distant and abstruse for enough worthwhile benefits to humanity

* Particle physics is less important than, say, medicine and biology

* If scientists have misunderstood the physics there's a risk of creating a black hole

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