**1.1 Fundamental Interactions**

When we observe our universe, we see that everything interacts with one another. As a result of these interactions, forces arise. These interactions and the corresponding forces can be in many many different forms, but in physics we can classify them under the title of fundamental interactions into four groups, namely gravitational interactions, let’s abbreviate this with G.I.; electromagnetic interactions, strong nuclear interactions, and finally weak nuclear interactions.

If we just quickly browse over these interactions, gravitational interactions are the ones that you have already been introduced, and you have seen that if we consider two point masses m1 and m2 separated by a distance of r always attract each other with a force such that its magnitude is proportional to the product of the masses and it is inversely proportional to the square of the distance separating these two masses.

Once can find a proportionality constant to be able to express these relationships in the form of an equation. Then the magnitude of this force becomes equal to proportionality constant g, which is known as universal gravitation constant, times the product of the masses divided by the square of the distance separating these two masses, which you have seen this as the Universal Law of Gravitation.

And from the expression, one can easily see that the magnitude of this force goes to zero as the distance between the masses r approaches to infinity, indicating that this is a long-range interaction and the masses do not need to be in contact with one another. And the range of these interactions basically covers the whole space between zero and infinity.

Strong nuclear interactions and as well as the weak nuclear interactions are the ones that we observe inside of the nucleus of the atom such that the forces associated with strong nuclear interactions hold the sub-nuclear particles together to be able to make a specific nucleus, whereas the forces associated with the weak nuclear interactions are responsible for radioactivity.

These two interactions dominate only inside of the nucleus of the atom, therefore their range is extremely small. It is basically as big as the size of the nucleus of an atom, which is basically in the region of between 0 and 10 to the minus 14 meters.

So, once we leave this region, then the gravitational and electromagnetic interactions dominate the whole universe, but inside of the nucleus, the forces associated with the nuclei interactions are much bigger than the forces associated with the other two interactions.

Therefore, if you make a simple chart to be able to represent the range of these interactions starting from zero and going towards infinity, between 0 and 10 to the minus 14 meter region, we observe mainly strong nuclear interactions and weak nuclear interactions, and the effect diminishes once we leave the nucleus. Then the gravitation interactions and the electromagnetic interactions dominate the remaining part of the universe.