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See also the full list of LHC FAQs at CERN's public website

What need is there for such large dimensions of your particle accelerator as you cannot accelerate particles beyond speed of light?

For the answer to this we have to turn Einstein's special theory of relativity.

Basically the relativistic mass of a particle increases with velocity and tends to infinity as the velocity approaches the speed of light. 

In practical terms our protons are moving a very small fraction below the speed of light. As we increase the energy (and momentum) they only get a very small fraction closer to the velocity of light - never reaching it. However, their energy and momentum do increase considerably.

For a given momentum, our magnets need to provide a force necessary to bend the beam around in the 27 km.  The increase in momentum is exactly reflected in the increased force we have to apply with these magnets as we increase the energy of the beam.

The size of the LHC is basically determined by the maximum strength of our dipole magnets. If the ring were smaller they would have to be a lot stronger.

For a more detailed discussion see:

http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

or any basic relativity text.

IS IT SAFE?

Hawking says,
" What happens when the mass of the black hole eventually becomes extremely small is not quite clear, but the most reasonable guess is that it would disappear completely in a tremendous final burst of emission, equivalent to the explosion of millions of H-bombs."

"there might be primordial black holes with a very much smaller mass that were made by the collapse of irregularities in the very early stages of the universe. Such black holes would have a much higher temperature and would be emitting radiation at a much greater rate."

Why does this not apply to the LHC?
How do you *know* it is safe?

Two main reasons:

1. Theory - Hawking himself recognized that black holes radiate. Given the energy available in the LHC, if a black hole was created it would necessarily be a very small one - a micro black hole - the energy available in the collision of two LHC protons is not a lot on a cosmological scale. The black hole would evaporate almost immediately into a shower of particles.

2. Cosmic rays - Extremely high energy particles (orders of magnitude above the LHC) coming from outer space are incident on upper atmosphere where they collide with the nuclei of gas molecules. We see the showers resulting from these collisions at sea level. This is appears to be safe, so we can be confident that the LHC will be.

How much do the protons weigh in the LHC at 7Tev?

The energy of a proton is 7 TeV. Via E = mc2 the mass is simply 7 TeV/c2 - and these are the units usually used.

7 TeV/c2 divided by the rest mass .938272029 GeV/c2 gives us 7460.52 times the rest mass

Working in SI units we can do the same thing more explicitly:

At 7 TeV:
Energy = 7 *1012 *1.60206 *10-19 Joules
c= 2.99793 108 m/s
m = Energy/c2 = 1.2477-23 Kg


At rest (rest mass proton = mp):
Energy = mp c2 = 0.938272029 *109*1.60206*10-19 Joules (or just say mp = 0.938272029 GeV/c2 )
mp = Energy/c2 = 1.672009-27 Kg

m/mp = 7460.52 as before

This number is gamma i.e. 1/square root( 1- v2/c2) - from which you can easily calculate the velocity.

COST?

The cost of the accelerator only (without experiments and computing) but including manpower and material is 4.7 Billion CHF (that's around 3.03 billion euros)

Is there a mechanism at CERN that would prevent the LHC from even turning on if someone was in the tunnel, making the chances of that effectively impossible? (For example, someone at Brookhaven National Laboratory told me that once they're tunnel is locked down it's only accessible by a key, and that key is also required to start the machine.)

The LHC using ID cards and biometrics to make sure the access system knows exactly who is in the tunnel at any given time. We carefully patrol the tunnel before closing for beam to make sure that no one is down there. (Plus sirens lights etc. as a safeguard.)

You mentioned that the radiation levels in the tunnel away from experiments would be 10 Gray per year. Any idea as to what the level would be NEAR the experiments, or in the event of a beam loss, or directly in-contact with the beam?

Some places (e.g. the absorber of neutral particles) down stream from the experiments will see 1000 Gy/yr  on the outside of the absorber - a lot more that the arcs. Radiation levels of 0.1 mSv/hr outside of the shielding. You pick up a year's permitted dose in a day.

How long does it take for a proton to go from zero to 14 TeV ?

When a proton leaves the source, it crosses the linac and reaches the PSB in a few microseconds. In the PSB it is accelerated from 50 MeV to 1.4 GeV in 530 ms,
then after less than a microsecond it is injected in the PS where it can either:
- be accelerated/manipulated/extracted in 1025 ms
- or wait for 1.2 more seconds before being accelerated, if it's part of the first PSB batch to the PS.

Then it is sent to the SPS where it waits for 10.8, 7.2, 3.6, or zero seconds whether it's part of the first, second, third, or fourth PS batch to the SPS.
The SPS accelerates it to 450 GeV in 4.3 seconds, and sends it to the LHC.

So the time it takes from the source to the exit of the SPS is between
0.53 + 1.025 + 4.3 = 5.86 seconds
and
0.53 + 1.2 + 1.025 + 10.8 + 4.3 = 17.86 seconds

Then our proton has to wait up to 20 minutes on the LHC 450 GeV injection plateau before the 25 minutes ramp to high energy, and these 45 minutes dominates the transit time.

Multiply by the speed of light to get the distance traveled!