Particle

A particle is an object that occupies a localized region of spacetime and possesses certain intrinsic attributes, such as mass, electric charge or Spin.

Particles are typically used as an idealization within a physical model in order to simplify treatment. For instance, in statistical mechanics an ideal gas is described as being composed of numerous particles, irrespective of the shape, size and specifics of the actual atoms and molecules that make up the gas, in order to treat them as a large set of equivalent objects. In general, a particle is assumed to be an object that is much smaller than the scale of the environment that it is in.

In particle physics#

In particle physics, the term refers to objects smaller than an atom, so-called subatomic particles. These may be of two kinds:

• Elementary particles have no internal components and are not made of other particles. Examples include the electron, any neutrino or the Photon.
• Composite particles are composed of other particles, usually elementary ones. Examples include the proton and the neutron, both made of three quarks each.

Whereas we are aware of a large number of composite particles, there are only few elementary ones. Our current understanding of elementary particles is formalized in the Standard Model of particle physics, which is a model containing all 17 known elementary particles, their known properties and how they behave with respect to electromagnetism, the weak interaction and the strong interaction.

Proof of elementarity#

The elementarity of a particle of size¹ $d$ can be proven experimentally. This is generally done by shining a strong light onto the particle and analyzing its response. More formally, the particle is illuminated with an electromagnetic wave whose wavelength $\lambda$ satisfies

as in, the wavelength is smaller than the size scale of the object. This allows probing the internal structure of the particle, if there is any. Note the challenge with this: smaller wavelengths are associated with higher frequencies. The Planck formula says $E=\hbar\omega$, so higher frequencies mean higher energies and thus the tinier the object to investigate, the more energy will need to be channeled into the wave to study. This poses a not insignificant technical challenge since subatomic particles are generally minuscule, in the range of $10^{-15}\text{ m}$ or even less, so the energies can easily reach

the gigaelectronvolt range.