Editing Macroscopic scale (section)

== High energy physics compared to low energy physics ==
[[Particle physics]], dealing with the smallest physical systems, is also known as ''high energy physics''. Physics of larger [[length]] scales, including the macroscopic scale, is also known as ''low energy physics''. Intuitively, it might seem incorrect to associate "high energy" with the physics of very small, ''low'' [[mass–energy equivalence|mass–energy]] systems, like subatomic particles. By comparison, one [[gram]] of [[hydrogen]], a macroscopic system, has ~ {{val|6|e=23|u=}} times<ref>[http://physics.nist.gov/cgi-bin/cuu/Value?na "CODATA Value: Avogadro constant"]. The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 13 December 2016.</ref> the mass–energy of a single [[proton]], a central object of study in high energy physics. Even an entire [[particle beam|beam]] of protons circulated in the [[Large Hadron Collider]], a high energy physics experiment, contains ~ {{val|3.23|e=14}} protons,<ref name="lhcbeam">{{cite web|title=Beam Requirements and Fundamental Choices |url=http://edms.cern.ch/ui/file/445762/3/Vol3_Chap2_v4.pdf|publisher=CERN Engineering & Equipment Data Management Service (EDMS)|access-date=10 December 2016}}</ref> each with {{val|6.5|e=12|u=[[electronvolt|eV]]}} of energy, for a total beam energy of ~ {{val|2.1|e=27|u=eV}} or ~ 336.4&nbsp;[[megajoule|MJ]], which is still ~ {{val|2.7|e=5|u=}} times lower than the mass–energy of a single gram of hydrogen. Yet, the macroscopic realm is "low energy physics", while that of quantum particles is "high energy physics".

The reason for this is that the "high energy" refers to energy ''at the quantum particle level''. While macroscopic systems indeed have a larger total energy content than any of their constituent quantum particles, there can be no experiment or other [[observation]] of this total energy without extracting the respective amount of energy from each of the quantum particles – which is exactly the domain of high energy physics. Daily experiences of matter and the Universe are characterized by very low energy. For example, the [[photon energy]] of [[visible light]] is about 1.8 to 3.2&nbsp;eV. Similarly, the [[bond-dissociation energy]] of a [[carbon-carbon bond]] is about 3.6&nbsp;eV. This is the energy scale manifesting at the macroscopic level, such as in [[chemical reaction]]s. Even photons with far higher energy, [[gamma ray]]s of the kind produced in [[radioactive decay]], have photon energy that is almost always between {{val|e=5|u=eV}} and {{val|e=7|u=eV}} – still two [[order of magnitude|orders of magnitude]] lower than the mass–energy of a single proton. Radioactive decay gamma rays are considered as part of [[nuclear physics]], rather than high energy physics.

Finally, when reaching the quantum particle level, the high energy domain is revealed. The proton has a mass–energy of ~ {{val|9.4|e=8|u=eV}}; some other massive quantum particles, both elementary and [[hadron]]ic, have yet higher mass–energies. Quantum particles with lower mass–energies are also part of high energy physics; they also have a mass–energy that is far higher than that at the macroscopic scale (such as [[electron]]s), or are equally involved in reactions at the particle level (such as [[neutrino]]s). [[Mass in special relativity|Relativistic effects]], as in particle accelerators and [[cosmic ray]]s, can further increase the accelerated particles' energy by many orders of magnitude, as well as the total energy of the particles emanating from their [[event (particle physics)|collision]] and [[annihilation]].