The engineering that goes into this is really incredible. Each magnet must be at exactly the right position and the right strength to get the protons to move in a circle. You have to oscillate an electric field at exactly the right frequency to give the protons a kick each time they pass by to speed them up (if you mistime the kick, you slow them down instead!) Oh, and did I mention that all these magnets spread over the 27km ring must be cooled to almost absolute zero with liquid helium? With all these variables, it is very difficult to keep a beam going so don't be surprised if the first beams are short lived!
When these particles collide, that's where the discoveries are made. Several huge detectors have been built at collision points. These detectors will record millions of collisions per second and generate petabytes of data.
What will we discover? Well, hopefully the Higgs Boson. The Higgs Boson is what gives everything mass. We could find new supersymetric particles that are predicted by string theory. It's possible to create mini-black holes. And of course, who knows what else. Some people will ask what is this all good for, but they asked the same thing about the electron and it turned out to be kind of useful in the long run.
You can find some interesting pictures here and the vital stats here. And click below to watch physicist Brian Cox (made of awesomeness) give a 15 minute talk for the lay audience on the LHC.
Some people have raised concerns that it could create a black hole that devours the Earth or turn everything into a strangelet. Lots of smart people have done a lot of work to be sure this won't happen. The most compelling argument that it won't happen is that, well, it hasn't happened. Cosmic rays hit Earth's atmosphere every day, 24/7/365 with MILLIONS of times more energy than the collisions in the LHC. These collisions have been happening every day for 4.5 billion years and the Earth has been devoured by a black hole exactly zero times and turned into a strangelet exactly zero times. The same people claimed that the Relativisitic Heavy Ion Collider would do the same thing. It has been operating since 2000.
The LHC is an awe inspiring achievement. Thousands of people from dozens of countries, united by a common vision and thirst for knowledge. People overcoming huge obstacles including political, financial, engineering and design. Some people have dedicated over 20 years of their lives to see this become a reality. It takes vision, imagination, creativity, and extremely intelligent people to make this happen. Some say science is a cold heartless pursuit. The people who have worked on this show that nothing could be further from the truth.
Fermilab is hosting a pajama party to watch results from CERN as the first beam goes around at 1:30am CDT September 10th. If I still worked at Fermilab, I would be staying up all night with them to watch!