Why is the square root of a negative number not a real number?

Because there is no real number which you can square, which will result in a negative real number. So they came up with imaginary numbers, and denoted the letter i to represent the square root of negative one. At first, they were thought to be just that - imaginary - nonexistent, whose only purpose was to fill in and make equations solvable. But now these numbers are useful in solving equations which govern electrical waves and other types of wave motion.