Like all major civil engineering projects, the construction of the LHC has its risks, but these are the risks that you would find on any project involving heights and heavy objects, and great care is taken to avoid accidents. The accelerator and detectors will operate deep underground, and their operation will not pose any risk to the public.
The possibility of more exotic risks due to possible new physics processes which might occur at the very high energies explored at the LHC was carefully investigated, and the report, which established that these posed no conceivable danger, is available here. In particular, the report addresses the possibilities (which have received much attention in popular media) of stable strange matter ("strangelets") and formation of tiny black holes. Very tiny black holes may indeed be produced at the LHC, if there exist additional dimensions beyond the three space and one time dimensions we know, and if these additional dimensions have certain properties - micro-black hole production is thus, of course, only an interesting theoretical possibility, and is far from being considered probable. Black holes formed this way would be microscopic, and so short-lived that they could pose no risk. In the unlikely event that they did not decay as expected, the gravitational attraction exerted by a black hole with the mass of a thousand hydrogen atoms is identical to the gravitational attraction exerted by a thousand hydrogen atoms: negligible by many orders of magnitude compared with the pull of the tiniest speck of dust. Moreover, if they are produced at the LHC, micro-black holes must also be produced by the much more energetic cosmic rays which occur naturally and bombard the Earth continually. This point is very important: the physics which we are studying at the LHC does occur in Nature, but it is rare, and therefore difficult to observe.
More information about LHC safety is available on the CERN Press site and CERN LHC Safety site.
ATLAS-Canada is supported by grants from NSERC.
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