Seven Key Ideas of Modern Physics
Modern Physics has introduced several new concepts and ideas in our lives, concepts that changed irreversibly the way we view and understand the world. These ideas replaced older ones in our way of thinking and it is certain that they will be replaced by other, better, more complete and more fundamental ones in the future; because that is the fate of ideas, to evolve like everything else. Below we list some of the most important ones in hope that they might inspire us.
1. The idea of Universality: the laws of physics are the same everywhere
A. Einstein: "the miracle is not what are the laws of the Universe but that it has such laws"
The idea is a simple one: The laws of physics are the same everywhere; here on Earth, in a-Centauri, near the center of a Galaxy, or next to a black hole. They are therefore independent of space (location) but also time (yesterday's laws are valid today too).
One might ask why this is so. The explanation is not simple. Let us say that this is the way Nature is. This fact makes our lives easier; imagine a Universe with laws that change from one place to next...a nightmare for those trying to study and understand it.
The law of Universality is behind many modern scientific endeavors. We won't have being able to go to the Moon if we did not know for sure that the laws of gravity are identical beyond Earth. We will not look for exoplanets or search for the E.T. if life obeyed different laws out there.
When Newton conceived the Law of Gravity what he realized was not that the apple falls because of Earth's gravitational attraction; everybody knows that. He realized that the same force that acts on the apple holds the Moon on its trajectory around Earth and Earth around the Sun. He realized the Universality of the law of gravity thus revealing the dynamics behind celestial motion.
2. The idea of Unification: there is strong evidence that ultimately all the basic elements of the Cosmos (particles, forces and the space-time canvas) are inseparable and have a common origin.
The basic element of this idea is that the multiplicity of all fundamental entities in subatomic physics has a common origin, that are nothing but facets of the same ultimate manifestation. The nature of that entity still remains to be determined.
It all started with the Maxwell's equations that unified electricity and magnetism into a single force, the electromagnetic force. It continued with the unification of the radioactive (weak) force with the electromagnetic one...this is were we stand today in, the so called "standard model". Beyond that there is a plethora of attempts to go beyond and unify the other two remaining forces, the nuclear (strong) force and gravity.The theories that try to achieve the grant unification of all particles and forces into a single "something" are called Theories Of Everything (TOE). Physicists in their attempts uncover unexpected beauty and surprises.
The most popular and controversial modern TOE is the 'string' theory. It belongs to the class of theories called 'super-symmetric' and the basic premise is that the elementary particles in physics, like the electron and the quarks, are nothing else than modes of vibrations, notes played on a string or membrane vibrating in a multi-dimensional space. A version of this theory suggests that there is only one force, gravity, in higher dimensions and the observed nuclear force is nothing but the result of a holographic projection in three dimensions of the gravitational interactions in that space.
There are also extreme, pure abstract hypotheses where the fundamental entity is just a 'number', therefore the ultimate physics TOE reduces inevitably to number theory, but whatever the nature of that fundamental 'something' might be it is still hidden from us.
3. Symmetries and Conservation Laws: Every time nature is indifferent to a choice, i.e. every time we have the freedom to make an arbitrary choice and the law is not affected, then something, a physics quantity, remains the same all the time, it is conserved.
This is an intriguing feature of nature, that is the fact that there are certain symmetries in the physical laws. If these symmetries are broken then we get differentiation and manifestation. If we were in the fundamental state of unbroken symmetry, there will be no way to separate particles from forces, electrons from quarks, you from me. The fact that there is diversity proves that the nature entered an imperfect state, that several symmetries are broken.
A symmetry in the laws of physics means some kind of freedom, some indifference of the nature to our choices. For example, when I study a system, I can do it here or there (freedom of place, "space-translations", space uniformity), today or tomorrow (freedom to pickup the starting time, "time-translations", time uniformity), orient myself in this or that direction (space isotropy) etc. This might sound trivial but every time I have such a choice some physics quantity stays "conserved", unchanged over time. For example, the freedom to pick up a starting time leads to energy conservation, the freedom to pick up a place to momentum conservation, the freedom of direction to angular momentum conservation and so on. This latter one [angular momentum conservation] makes a spinning top stay up, in an apparent defiance of the law of gravity.
But this relationship between freedom of choice and conservation is not always apparent. We know of several subatomic quantities that are conserved that have no obvious symmetry associated with them, so for the time being we call them "hidden" symmetries.
4. The BIG BANG Cosmology: The Universe was born and it is currently expanding. It is not static, infinite or eternal.
Details about the BIG BANG theory and the observations that support are given in this article here: [The Big Bang theory]
5. The strange world of Quantum Mechanics: The Dual-nature of entities, their Omni-presence, the death of "determinism" and the birth of "probable", the vacuum that is not empty, are just some examples.
The atomic and subatomic particles that make up this world have a dual nature; when we try to detect their presence they appear as solid objects with certain features (mass, charge etc) but when they are free, their omnipresent, wave-nature takes over and move in a land without specific paths...or, as Richard Feynman put it, they move on all possible paths. This etheric part of their nature has a very important role in real life since it defines the probability to find/detect them here or there, and this is quantum mechanics. Without it there is not way to understand how an electron can 'pass' through two holes simultaneously on its way to the detection screen. Without it there is no way to understand how light, like a billiard ball, can kick an electron sideways. We will expand on all these intriguing ideas of quantum physics in a separate article.
6. The Theory of Relativity: Space, Time, Mass, Energy lost their absolute nature; their flow can be modified. They depend on the "point of view", they are "relative". At the same time the speed of light becomes the speed-limit of any object with mass and the only stable quantity in the Cosmos.
We thought that the space-time canvas where the play of life takes place is solid and given. We thought the mass, the solidity of the objects around us is something to rely upon, to build upon. Alas, Einstein sent these ideas to oblivion. If anything, the only secure and stable "thing" in this world is just light, the rest is fluid, a matter of relative perception.
7. The concept of Entropy and the Arrow of Time.
All processes in physics are reversible; you can play them backwards and still be valid processes. Well, this is not true with real life where some processes are irreversible. For example, the breaking of a piece of glass or an egg, aging etc. It relates to the tendency of things to become chaotic, if left alone, rather than create order. This tendency is called entropy and it accounts for the amount of order or randomness (sort of chaoticity) in a system or organism. The physics law says that entropy cannot decrease globally and that leads to the prediction of the thermal death of the Universe, despite the struggle of life to create order. Entropy has many uses these days in many different disciplines, from particle physics to information theory where it is related to information; more information equals to negative amounts of entropy. In any case, entropy introduces this biological ticking of time, the inevitable direction of the arrow of time...or so it seems.