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 systems, like subatomic particles. By comparison, one gram of hydrogen, a macroscopic system, has ~ 6 × 10 times the mass–energy of a single proton, a central object of study in high energy physics. Even an entire beam of protons circulated in the Large Hadron Collider, a high energy physics experiment, contains ~ 3.23 × 10 protons, each with 6.5 × 10 eV of energy, for a total beam energy of ~ 2.1 × 10 eV or ~ 336.4 MJ, which is still ~ 2.7 × 10 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".