Our world works on four basic fundamental forces. They are the strong force, the weak force, electromagnetism and gravitation. Each force has a different nature, strength and range. Every force has a corresponding mediating particle that mediates the force. This is called the Gauge Boson of the Force.
The strong force is a short range force that acts only at nuclear levels. It is the strongest of all four fundamental forces. The strong force holds quarks together to form protons, neutrons and other barons, and mediates the color force between hadrons. The theory that explains working of the strong force is Quantum Chromo Dynamics. The gauge boson of strong force is a gluon.
The electromagnetic force is the next strongest force, which governs attraction and repulsion due to electric and magnetic fields and charges. It has an infinite range. Quantum Electrodynamics is the theory that describes the electromagnetic force. The electromagnetic forces are mediated by photons, which are also quanta of electromagnetic radiation.
The weak force comes up next. It governs weak interactions between fermions, and is responsible for radioactivity. Like the strong force, it is also a short range force. The weak force has three gauge bosons – W+, W– and Z. The theory governing the weak interaction is the Quantum Flavour Dynamics; or better known as the electroweak theory.
Gravity is the last force we discuss here. Gravitation is the weakest of all fundamental forces but the only one we can see on a day to day basis. Gravitation is also an infinite force. Einstein’s General Theory of Relativity is the most accurate description of gravitation, whereas Newton’s Law of Gravitation is a fair approximation. The gauge boson of gravity is the Graviton, which has been hypothesized but never seen in experiments. Gravity is an attractive force that acts on all particles, even photons.
Gravity is the oldest force known of all, with Newton’s Law being more than 300 years old. While strong force, weak force and electromagnetic force have observable effects on quantum scales, the mass of small particles is so miniscule that the gravitational attraction between them is negligible. On a large scale, strong force and weak force do not act. Also, in larger bodies, the net positive charge is balanced out by the negative charge. However, mass is always additive, and the more particles a body has, the more massive it gets. Thus, gravitation is the most dominant force on larger scales and has the most observable effects.
According to modern day physics, electromagnetic and the weak force can be unified into one – the Electroweak Force. The exact description of what happens in complicated and requires knowledge of QFT, the simple way of explanation is that above 1015 Kelvin, electromagnetic and weak force behave the same way. At lower temperatures, the electroweak force splits into electromagnetic and weak force. This is analogous to Maxwell’s unification of electric and magnetic forces into a single force.
While the electroweak unification has been experimentally proven, there is another unification that happens at higher temperatures. The strong force unifies with the electroweak force at 1016 Kelvin. The resulting theory which incorporates all three forces is the Grand Unified Theory.
There has been a hunt for a theory of everything, which unifies gravitation with all other forces. However, experimental issues aside, this theory is not even theoretically feasible.
Gravitation behaves very weirdly as compared to the other four forces. While all other forces can be attractive or repulsive, gravitation is always attractive – there is no negative mass. Also, there is no consistent quantum theory of gravitation yet, unlike other forces. The graviton has also never been experimentally verified yet, while the other gauge bosons have been. The grand unified theory deals with physics at smaller scales, whereas gravitation deals with large scale physics. This makes their unification a bit complicated, to say the least.
While all four forces are relativistically covariant, only Maxwell’s equations work in the General Theory of Relativity. Dark Matter and Dark Energy further complicate formulation of such a theory. The discrepancy at explaining the value of Einstein’s cosmological constant between GTR and QFT is of the order of 10120, implying that there is something seriously wrong with our current approach.
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Research is going on to establish a quantum theory of gravity that will satisfy GTR at larger scales. There are also other approaches, such as the widely popular string theory. The basic fundamental assumption of string theory is that all elementary particles are made of small, vibrating strings and membranes. String theory also requires that the Universe have 10 dimensions, or more.
All of this sounds fun, but it will be long before we can experimentally verify any of them. Quantum physics was born in the 1900’s and in such a short time, has unified three of the four fundamental forces of nature. However, it surely will take more time to add gravity to the GUT, and give us the true Theory of Everything.