What is the smallest particle in the Universe?

This is a very difficult question to answer, not only because we may not have discovered the smallest particle yet, but because the fundamental particles we know do not have a measurable size with the technology that we have, nor apparently with any technology that we can currently imagine.

The most we can do is divide the known particles into their constituent particles until we reach the fundamental particles, although we are not able to know exactly which of them is the smallest, at least in size, although we can know which has the least mass. or energy.

Quarks and electrons

For a long time it was considered that the atom was the unit that constituted all matter. Later it was discovered that it was in turn composed of other smaller particles, electrons, protons and neutrons.

These were also thought to be the fundamental and indivisible particles of matter, but then it was discovered that protons and neutrons are in turn made up of different types of quarkseach proton and each neutron is formed by the interaction of three quarks.

Even though smaller particles have yet to be discovered, and even though they don't exist, you can't measure the size of electrons or quarksso we cannot know which of them is smaller.

Our concept of size when talking about elementary particles is not applicable. Not even the concept of a particle. When we think of a particle we imagine something similar to a tiny sphere, but in reality these elementary particles are energy concentrates in a spatial point without dimensions.

In the world of quantum mechanics, the definition of shape and size is very different from our daily experience in the macroscopic world. On a quantum scale, it is not even possible to determine exactly the existence of the particle in a region of space, the position is determined as a probability and not as an absolute value.

Even so, some calculations have been made of the radius that electrons and quarks would theoretically have using ratios of mass and energy. Most of these calculations have arrived at a value of 0.00000000000000001 cm for both types of particles. Electrons and quarks would have a size a trillion trillion times smaller than 1 cm.

However, it must always be borne in mind that the so-called Standard Model in particle physics does not describe the fundamental particles in terms of their size, but rather in terms of their energy.

Electrons have an energy of 0.000511 GeV, while quarks vary by type, from 4.5 GeV for the b (bottom) quark to 0.003 GeV for the u (up) quark. In any case, the electron has less energy than the quarks. But in the electron family of particles (the leptons), there is a particle with even less mass and energy: the neutrino. having an energy less than 0.000000001 GeV and a mass equivalent to one millionth of the mass of the electron.

But neutrinos are not part of atoms as such. They are formed in beta decay, a process of disintegration of neutrons and protons in which electrons are also formed. Electrons can strongly interact with atomic nuclei, but not neutrinos, which easily escape in the form of ionizing radiation. If they happen to collide with a nucleus, they can interact with it, transforming it into a different nucleus, but this does not happen easily; neutrinos are so small that they can easily pass through matter and are very difficult to detect.

Chords, Singularities, and the Planck Length

In many experiments, fundamental particles like the electron and the quark act as points of matter with no spatial distribution. A point, by definition, has no height or width, and this complicates the laws of physics, since it introduces the possibility of indeterminacies; for example, two point-type objects could approach their positions infinitely without touching, which would lead the forces acting on them to also increase infinitely; in these circumstances the laws of physics fail.

String theory solves this problem. Point-like particles are replaced by strings of energy in the form of loops or loops, although this theory has not yet been corroborated.

The point-like particle problem is also solved in quantum foam theory, also known as space-time foam. According to this theory, space-time is not continuous but is made up of discrete fragments, like bubbles of space-time similar to the pixels in a digital image. In this case, two particles could not approach infinitely either, since they would always be either together or separated by at least the space between these bubbles.

Singularities, like those that seem to exist at the center of black holes, also have the problem of infinity. The various physical theories calculate that the center of black holes also become dimensionless points of infinite density. However, there are those who believe that the holes are indeed very dense but not infinitely dense; they attribute the infinite density to the defects of the two prevailing physical theories: general relativity and quantum mechanics.

Be that as it may, both strings and singularities, and even possible quantum bubbles, seem to be ruled by a minimum measurable length, the so-called Planck lengthWhich is equivalent to 1.6 x 10^-35 meters, that is, a 16 preceded by 34 decimal zeros. Below the Planck length it is believed that space ceases to have a classical geometry and that, therefore, it would be the smallest distance that we could measure.

The Planck length has been postulated as the boundary between general relativity and quantum mechanics, and it may be that whatever is the smallest thing in the Universe would be of a size equivalent to the Planck length.

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