Why is particle physics called high energy physics? [duplicate]

Question

The highest energy accelerator till date is the LHC which operates at an energy scale of perhaps 10-100 TeV. In SI units this is about $\sim 10^{-6}-10^{-5}$ Joule which is several orders of magnitude smaller than the energy scales we are used to in daily life. For example, the work done and heat produced by thermodynamic engines are hundreds of Joules.

Why is then particle physics regarded as the 'high energy physics'? 'High' with respect to what? I feel this has something to do with very small masses of the elementary particles. Incidentally, why is it difficult to reach energies as high as hundreds of Joules for elementary particles?

Answer 1

'High' with respect to what?

High with respect to the number of particles involved when they mention the energy value. The TeV energy of particle accelerators is related to the average energy of each accelerated particle.

You are comparing the energy of a single particle with the average energy of a system of particles.

As you mention, in our daily lives we talk about energies of several joules. However, we also talk about systems (like the steam engine) composed of several moles of particles, i.e., a number of particles of the order of $10^{23}$. Therefore, the average energy of the particles (molecules) in a steam engine is of the order of $\rm 10^{-23} J$.

A system formed of a single mole of particles accelerated in LHC would have an average energy of $\rm \bar E \sim 10^{-6} \cdot 10^{23} \sim 10^{17} J$. Now compare that to the hundreds of Joules of the steam engine.

Yes, that’s high energy physics indeed.

Answer 2

You are right in saying that these energy scales are several orders of magnitude smaller than the ones that we have in daily life.

But let's note that a 10 TeV collision of two protons at the LHC results in sharing that 10 TeV between two protons (in the most shallow sense). In daily life, on the other hand, we have light sources -- take a generic lightbulb. The energy a lightbulb emits is shared between $1.8 x 10^{20}$ photons per second. A basic calculation reveals how a very huge a amount of energy -that is impossible to achieve with our daily life objects- is concentrated in a few particles.

So it's not an issue that directly concerns the energy scales themselves. We are interested in energies concentrated in the components - the particles, in that case.

This all, in the end, motivates us to use high energy physics synonymously for particle physics. Fun fact: If you were to accelerate a ball at the LHC, you would need a fantastic amount of energy which would almost correspond to $ 10^{21}$ J (to do the order-of-magnitude estimate on your own, consider the approximate number of nucleons in the ball).