What is the Universe made of?
What are our bodies and all the things that we see made of? In olden times, it was believed that everything is made of four elements: Earth, Water, Air and Fire. Indians added a fifth element, Sky.
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After the growth of Science, it was discovered that everything is made of chemical elements. The Russian scientist Dmitri Mendelief ordered more than 60 elements that were known then, according to their properties and prepared what became known as the Periodic Table of Elements. This was an important milestone in the history of science. Now this table contains more than 120 elements including the 92 naturally occurring elements and the artificially made elements.
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Every element is made of atoms. A few scientists such as Ernest Mach did not believe in atoms. It was Ludwig Boltzman who proved the reality of atoms. Using the idea that heat is the energy of motion of atoms, he developed the subject of thermodynamics.
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In the year 1897, J J Thomson discovered the electron. In 1911, Ernest Rutherford discovered that every atom has a nucleus surrounded by electrons. After Rutherford's Student Chadwick discovered the neutron, it became clear that atomic nuclei are made of protons and neutrons.
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At the next stage, it was discovered that protons and neutrons are not elementary, but are made of elementary particles called quarks. These quarks are of two types, u quark and d quark. Proton is made two u quarks and one d quark while neutron is made of two d quarks and one u quark.
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In radioactive decay of nuclei, neutrino is emitted. Electron is in all atoms.
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These 4 elementary particles u, d, e, seem to be enough to make the whole Universe. But, in Nature, this quartet is repeated twice more. So there are 12 elementary particles in total:
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Quantum Mechanics is needed to understand the structure of the atom, and the atomic nucleus and quantum mechanics was discovered and developed during 1900-1924. Quantum Mechanics and Einstein's Theory of Relativity are the important revolutions of twentieth century.
Can we now conclude that the Universe is made of the elementary particles mentioned above?
We cannot. We have to mention another important feature of reality, the field. It was the electromagnetic field that was discovered first. In the middle of the 19th century, Michael Faraday performed many experiments in electricity and magnetism. The idea that appeared in Faraday's mind as a result of these experiments is electromagnetic field.
Einstein said:
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"The most important concept discovered by humans is the concept of the electromagnetic field."
James Clerk Maxwell who came after Faraday discovered the mathematical equations that govern the dynamics of the electromagnetic field. From these equations, he proved the existence of electromagnetic waves and that light is an electromagnetic wave.
Quantum Mechanics converted the field into quantum field. The important consequence of that was that the energy of the field is not continuous but is in discrete packets. These packets are called quanta. The quantum of the electromagneic field is the particle of light, called photon.
That is not all. All the elementary particles that we mentioned above are also quanta of quantum fields. Electron is the quantum of the electron field; quarks are the quanta of the quark fields. The twelve elementary particles are the quanta of twelve quantum fields.
These elementary particles interact with one another. Atom and atomic nucleus are built out of these interaction forces. Among these it is the electromagnetic interaction that was discovered first. Two interactions were discovered after the discovery of the atomic nucleus.
They are the strong interaction and weak interaction. Atom is governed by the electomagnetic force; nucleus is governed by the strong force.
Weak interaction is responsible for the decay of elementary particles.
Just as the electromagnetic field causes the electromagnetic interaction, there are 8 fields for strong interaction and 3 fields for the weak interaction. So there is a total of 12 fields for electromagnetic, strong and weak interactions.
If we convert all of these into quantum fields we will get 12 quantas; photon is one of them.
The theory that includes the electromagnetic, strong and weak forces is called the Standard Model which has been experimentally proved to be the correct theory and is now regarded as the basic theory of ALL of Physics. This contains the 12 quantum fields of the electromagnetic, strong and weak forces and the 12 quantum fields of the elementary particles such as the electron and the quark.
There is another important quantum field in the Standard Model, the Higgs field. Its quantum was captured in experiment only in 2012.
Standard Model was complete only after that.
Therefore, from the idea that the Universe is made of elementary particles, we have come to the idea that:
Universe is made of Quantum Fields.
So far we have not talked about a field, the gravitational field.
In Einstein's General Theory of Relativity, space-time,the four- dimensional union of three-dimentional space and one-dimensional time,becomes the gravitational field! Curvature of space-time manifests as gravitational force.
But Einstein's General Theory of Relativity does not combine with Quantum Mechanics. There is a big contradiction between between General Relativity and Quantum Mechanics. This problem has become an important challenge in Fundamental Physics.
The answer to this challenge is String Theory which contains Quantum Gravity in itself. But String Theory has not yet passed experimental tests.
Only 5 percent of what we see in the Universe is made of things that we have discussed. 70 percent is made of dark energy and
25 percent is made of dark matter. It is still not known what is the nature of this 95 percent. Research on these is going on.