Let me start off with the alleged threats first.
1. Black Holes. It is feared that the collisions to be made on October 21st will result in black holes. Black holes are theoretical regions in space that have an extremely powerful gravitational field that can pull any particle inside it, even light particles.
2. Strangelets. These are also theoretical sorts of exotic matter that if comes in contact with Earth, it will reduce all the atoms of Earth to strange matter as well, transforming the planet into a hot, large lump of strange matter. How strange is it? Do your own research, but it's strange enough to end the world.
3. Vacuum Bubbles: They are a so-called stable state of the universe. It is feared that perturbations caused by the LHC would cause it, turning our planet to an inhabitable one.
4. Magnetic Monopoles: Yet another theoretical particles that have not been observed by science. If they form, they cause protons to decay.
Why you shouldn't worry about the Large Hadron Collider:
1 The main argument against the claimed threats I mentioned before is the existence of cosmic rays. Cosmic rays are constantly bombarding the surface of our planet (and most are penetrating), causing particle collisions similar to that to-be-conducted in the LHC. The Safety Assessment Group writes:
Nature has already conducted the equivalent of about a hundred thousand LHC experimental programmes on Earth – and the planet still exists.2 Cern says that the black holes are expected to form, but they will be too tiny to even absorb other particles, and will decay immediately. This is due to the very small scale of the experiment that will greatly minimize its dangers.
3 Strangelets, the exotic matter as explained above, are also produced by cosmic rays. If cosmic rays that are much powerful than those produced in the LHC cannot produce strangelets, then the LHC will not produce strangelets. In the same way that strangelets reduce Earth's atoms into similar strangelets, they, too, are transformed into ordinary matter within thousand-millionth of a second, as the conditions of the experiment are not favourable for the production of strangelets.
4 Vacuum bubbles are yet another sort of matter that has not been produced by cosmic rays in the universe, since they're only theoretical. LHC rays are not capable of producing vacuum bubbles if cosmic rays do not.
5 Magnetic monopoles cannot be produced due the conditions of the experiment, as they are too heavy to even form up. Once again, the fact that cosmic rays cannot produce proton-eating magnetic monopoles is enough to rule out any possibility that the LHC could produce one.
Now wouldn't it be cool if we collide the particles and get the experiment going?
"That's the essence of experimental particle physics: You smash stuff together and see what other stuff comes out." - Nat GeoBesides, if the experiment fails, it's gonna be quick and painless that you won't even know about it. Plus, I wouldn't have to sit for my economics exam. Anyway, we have one less thing to worry about! That still doesn't mean that the world is safe, though. We still got global warming, possible nuclear war, etc. So don't go "Hooray", but stay depressed and sit in the corner of your room. :D