The standard model

If you are asked, how is it that we are built or constituted? Your answer most likely will be “of atoms”, and even though this is a correct answer, it partially describe how all things we know are formed. Is the standard model the one that describes in detail how matter is constituted.

 

The atomic model

How matter is formed in general is a knowledge we get in our school years, where we are taught that everything that surrounds us, as well as ourselves, are made up of atoms; the atom being the fundamental component of matter. But to get to the atomic model we all know, a series of discoveries and preliminary models of how it was constituted were necessary, beginning with the model that J.J. Thomson proposed in 1904, considering that the atom was formed of electrons (that he discovered in 1897) , floating in a positively charged “soup or pudding” to balance the electron’s negative charge. A model that was later redefined by Ernest Rutherford in 1911 when, after his experimental discoveries he proposed the atom model with a very small and massive nucleus, surrounded by electrons orbiting it. A model that was again perfected by Niels Bohr in 1913; where, following a quantum model, he established the atom model as we know it now, with its electrons circulating around the nucleus in specific orbits restricted by quantum rules. But this model, even when it was very convincing, presented some contradictions, such as the fact of having a tight cluster of protons with positive charge, which should repel each other. Finally, in 1932 this was resolved with the Neutron discovery achieved by James Chadwick; being this a constituent part of the atomic nucleus, a particle without electric or neutral charge, hence its name.

Atom_history

Atom history (compoundchem.org)

This model with which we are familiar, a model that resembles a planetary system, having a nucleus formed of neutrons and protons, and with a series of electrons orbiting around in specific orbits, is quite approximate to what actually happens in the atom. With this model, depending on the number of protons, neutrons and electrons, the atom obtains a series of specific properties (which are properly described in the periodic table of the elements). It is the combination of different atoms or elements, which creates the diversity of compounds that form everything that surrounds us, from the molecules in our body to the galaxies and stars that we see in the firmament.

 

Particle physics

Even after having an atomic model, new experimentation and research made it evident that probably these weren’t the fundamental components of the matter. With the work of Paul Dirac and phenomenal theoretical physicist , who made the first association of quantum mechanics and special relativity and among its results, predicted the existence of antimatter (as in the case of positrons, with the same properties of the electron but with a positive charge), followed in the early 40’s, with the different experiments of the new Cyclotrons, the first particle accelerators that gave rise to the so-called “particle explosion”, the 50’s were a time of great proliferation of new sub particle discoveries, which made us question whether the components of the atom were really fundamental. But after studying and formulating a mathematical model of the properties on these new sub particles, physicists began to identify patterns in them, identifying new groups like those of the leptons, baryons and hadrons.

 

The new model

It was already in the 60’s when a new model of particle physics was proposed, called “Standard Model”, where the matter was constituted by “Fermiones” (term defined by Pul Dirac, based on the name of the Italian physicist Enrico Fermi), which were the particles that follow the “Fermi-Dirac statistical properties”, which are what make up all the matter we know, including protons and neutrons (formed of quarks), and electrons (part of the group of fermions called “Leptons”)

It was also identified that fermions are presented in three groups, with different energy levels; those of ordinary matter, which form the atoms we all know and appreciate, followed by exotic matter, with higher energy levels, and extremely exotic matter, with even higher energy levels; although these types of “exotic” fermions have only been observed experimentally, and are not detected in nature.

The Quarks, particles proposed in 1964 by Murray Gell-Mann and George Zweig in their so-called model of Quarks, which are the components of baryons, formed by three quarks, and are part of the group of hadrons.

The name of quarks, chosen by Gell-Mann, is based on a passage from the book “Finnegans Wake” by James Joyce:

Three quarks for Muster Mark! , he hasn’t got much of a bark …

He considered this passage due to the fact that quarks are always kept in groups of three; these have a very interesting feature, as they have partial electric charge.

 

The atom and ordinary matter

To exemplify how the atoms are constituted, we know that the atom’s nucleus has protons and neutrons, which are part of the low energy Fermions and are formed by three Quarks called “Up” charge +2/3 and “Down” with charge of -1/3. The proton consists of two quarks “Up” and a “Down” with a final charge of +1 (+4/3 & -1/3), and neutrons formed by two quarks “Down” and one “Up”, and resulting neutral charge (-2/3 & +2/3), both being part of the atom nucleus. We also have electrons, which are low-energy leptons and, as commented, are the ones circulating around the nucleus, and finally the electron-neutrino, this is another type of lepton that has no electrical charge and that acts in the processes of radioactive decay (or beta decay), complementary part of the nuclear fission and part of the decay effect of the Neutron in Proton, which releases an electron and an electron-neutrino; this decay happens only on free neutrons, with an average decay time of fifteen minutes. Neutrons in the atom’s nucleus remain stable for millions of years

Beta_decay

Beta decay (nuclear-power.net)

In the same way, for the high energy exotic matter, we have the quarks “Strange” (+2/3), and “Charm” ( -1/3), the Muon (equivalent to the electron), and its corresponding Muon-neutrino. And for the extremely exotic matter, with very high energy, are the quarks “Top” (+2/3), and “Bottom” (-1/3), as well as the Tau and its corresponding Tau-neutrino.

 

The force and its particles

But to completely describe the image of how everything that surrounds us is formed, we cannot discard the forces that act on matter, three of which are described in the mentioned standard model. Each force has also a particle that transmits it. These particles are called “Bosons” and are those that interact with the Fermions (the aforementioned particles of matter) and obey the “Bose-Einstein statistics properties”. These particles form the “Fields” where these forces act (like the electromagnetic field)

As we saw in “May the forces be with you“, there are four fundamental forces in nature:
– The electromagnetic force and its associated boson, the “photon”, which is a mass-less particle and acts at a great distance.
– The strong nuclear force, exerted by the “gluon”, which holds the quarks together in protons and neutrons and acts only in very short distance, such as the nucleus of the atom.
– The weak nuclear force, exerted by the intermediate bosons (W & Z); They have mass and are the ones that act in radioactive decay, also acting at very close range.

And finally, the force responsible for the mass in all the particles, the Higgs Boson, a particle whose existence was experimentally confirmed by the Atlas and CMS experiments of the Large Hadron Collider (LHC), in 2012; experiments carried out by the CERN in Switzerland. And it is the particle that controls the Higgs mechanism, that generates the mass.

Standard_model

Standard Model (physik.uzh.ch)

Problems with the model

Even though this model is the one that best describes how nature works, there are still questions that remain unanswered; for example, the absence of the force of gravity, fundamental force in nature, which is not part of the standard model, but which obviously we know that it is present, and we experience it on a daily basis. The unification of forces, the “theory of everything” or “unified theory” is one of the lines of more research in modern physics, which already has candidates to solve this problem, such as the particle supersymmetry, and the inclusion of the Graviton, the boson that according to this theory, would carry the force of gravity.

One of the major difficulties with gravity is how weak this force is; the electromagnetic force is 36 orders of magnitude (that is, “one followed by 36 zeros”), stronger than gravity. While the strong nuclear force is only 140 times stronger than the electromagnetism. Gravity, just like the electromagnetic force acts at a great distance.

To compare the abysmal difference between the numbers of the electromagnetic force and that of gravity, making a comparison with the size of the observable universe, which is considered to be 46.5 billion light-years in any direction, this gives us a diameter of the universe of approximately 93 billion light-years, which converted to kilometers is 5.5 x 1023 miles, or 5.5 followed by 23 zeros, a number that is still a thousand billion times smaller than that of the difference between the forces of gravity and electromagnetism. Currently, particle supersymmetry is still a bold theory that has many detractors, and it is very difficult to prove given this problem with the difference in gravity, which makes it difficult to experimentally prove its integration in the standard model. But certainly, the search for answers continues.

Regards, Alex – ScienceKindle.

 

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