# Forces of Nature

If asked how many types of forces exist, many people start counting on their fingers, and when the count exceeds ten, they answer plenty of. Indeed, there are gravitational forces , electrical, magnetic, elastic, frictional forces, and also forces of wind, of expanding steam, of contracting muscles, etc.
If, however, we analyze the root causes of all these forces, we can reduce their number to just a few fundamental forces (or fundamental interactions, as physicists say).
For example, the elastic force of a stretched rubber cord is due to the attraction between the molecules that the rubber is made of. Looking deeper, we find that the molecules attract each other because of the electromagnetic attraction between the electrons of one molecule and nuclei of the other. Similarly, if we depress a piece of rubber, it resists because the molecules refuse to approach each other too close due to the electric repulsion of the nuclei. Therefore the elasticity of rubber has the electromagnetic origin.
Any other force in the human world can be analyzed in the same manner. After doing this, we will find that all forces that we see around us (in the macroworld), are either of gravitational or electromagnetic nature. As we also know, in the microworld there are two other types of forces: The strong (nuclear) forces that act between all hadrons, and the weak forces that are responsible for changing the quark flavors.
Therefore, all interactions in the Universe are governed by only four fundamental forces: Strong, electromagnetic, weak and gravitational. These forces are very different in strength and range. Their relative strengths are given in Table 15.5. The most strong is the nuclear interaction. The strength of the electromagnetic forces is one hundred times lower. The weak forces are nine orders of magnitude weaker than the nuclear forces, and the gravity is 38 orders of magnitude weaker! It is amazing that this subtle interaction governs the cosmic processes. The reason is that the gravitational forces are of long range and always attractive. There is no such thing as negative mass that would screen the gravitational field, like negative electrons screen the field of positive nuclei.
Table: 15.5 Four fundamental forces and their relative strengths. :
| Force | Relative Strength | Range |
| Strong | 1 | Short |
| Electromagnetic | 0.0073 | Long |
| Weak | $10^{-9}$ | Very Short |
| Gravitational | $10^{-38}$ | Long |

## Towards the unified force

Physicists always try to simplify things. Since there are only four fundamental forces, it is tempting to ask "If only four, then why not only one?". Can it be that all interactions are just different faces of one master force?
The first who started the quest for unification of forces was Einstein. After completing his general theory of relativity, he spent 30 years in unsuccessful attempts to unify the electromagnetic and gravity forces. At that time, it seemed logical because both of them were infinite in range and obeyed the same inverse square law. Einstein failed because the unification should be done on the basis of quantum laws, but he tried to do it using the classical concepts.

## Electro-weak unification

Now it is known that despite the similarities in form of the gravity and electromagnetic forces, the gravity will be the last to yield to unification. The more implausible unification of the electromagnetic and weak forces turned out to be the first successful step towards the unified interaction.
In 1979, the Nobel prize was awarded to Weinberg, Salam, and Glashow, who developed a unified theory of electromagnetic and weak interactions. According to that theory, the electromagnetic and weak forces converge to one electro-weak interaction at very high collision energies. The theory also predicted the existence of heavy particles, the $W$ and $Z$ (vector bosons), with masses around 80000MeV and 90000MeV, respectively. These particles were discovered in 1983, which brought experimental verification to the new theory.

## Grand unification

The next step was to try to combine the electro-weak theory with the theory of the strong interactions (i.e. quark theory) in a single theory. This work was called the grand unification. Currently, physicists discuss versions of such theory that predicts the convergence of the three forces at awfully high energies $\sim 10^{17}$MeV. The quarks and leptons in this theory, are the unified leptoquarks.
The grand unification is not that successful as the electro-weak theory. It has the problem of mathematical consistency and contradicts to at least one experiment. The matter is that it predicts the proton decay,
$p\,\longrightarrow \,e^{+}+\pi ^{0}\$,
that does not conserve both the baryon and lepton numbers, with the lifetime of $\sim 10^{29}$years. The measurements show, however, that the lifetime of the proton is at least $10^{32}$ years.

## Theory Of Everything

Some people believe that the grand unification has an inherent principal flaw. According to them, one cannot unify the forces step by step (leaving the gravity out), and the correct way is to combine all four forces in the so-called theory of everything.
There are few different approaches to unifying everything. One of them suggests that all fundamental particles (quarks and leptons) are just vibrating modes of string loops in multidimensional space. The electron is a string vibrating one way, the up-quark is a string vibrating another way, and so on. The other approach introduces a new level of fundamental particles, the preons, that could be constituent parts of quarks and leptons. The quest goes on.
Everyone agrees that constructing the theory of everything would in no way mean that biology, geology, chemistry, or even physics had been solved. The universe is so rich and complex that the discovery of the fundamental theory would not mean the end of science. The ultimate theory of everything would provide an unshakable pillar of coherence forever assuring us that the universe is a comprehensible place.