Physics, Astronomy & Cosmology / Re: How are particles monitored for position and velocity in the LHC?« on: 27/07/2020 12:32:40 »
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how are particles monitored for position and velocity in the LHC?In the LHC, protons are accelerated up to the desired speed and energy (and maintained at that speed and energy) by applying carefully-synchronised microwave signals as the protons travel around the LHC.
- If a particle is traveling a bit slow, it sees a voltage that encourages it to accelerate slightly
- If a particle is traveling a bit fast, it sees a voltage that encourages it to decelerate slightly
- If a particle is traveling at the right speed, it sees a voltage that provides just enough energy to overcome the energy lost as the protons maintain a circular path (this energy loss is called synchrotron radiation)
- So, rather than measuring the individual position and velocity of 100 billion protons, the LHC applies voltages and magnetic fields that shepherd all these protons into a compact bunch.
- One of the rules of the uncertainty principle is that you can get a quite accurate measurement, if you are prepared to take a long time about it - and the LHC takes hours to get the particles to the right energy (and can keep them there for many hours). In quantum mechanics, even 1 second is considered a "long time". [Oops - crossover with Alan's Policeman analogy]
The LHC does measure the position and timing of the proton bunches as they travel around the circuit.
- Effectively, the proton bunches are little pulses of electrical current, which can be measured by a current transformer, which gives a little pulse of current every time a proton bunch passes.
- This is a very simple application of "weak measurements": Rather than try to get a precise position for all 100 billion individual protons, you just look for an average over many particles. So although you can't measure an individual particle with great precision, you can get a very accurate measure of the average position of 100 billion particles, without disturbing any one of them too much.
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