[The Scientist (OP)]

I understand that there is a law that says energy cannot be destoryed but converted. WHY is this so? Please give your answer as detailed as possible. Thanks!
 

[Andrew P]

It's a great question.

Energy conservation is one example of a deep idea in physics, that any symmetry will give rise to conservation of some quantity. This is known as Noether's theorem. The particular example of energy corresponds to time symmetry (the fact that a ball kicked today behaves today much as it did when kicked yesterday, for instance).

In a sense, it follows that energy can't be destroyed by humans because we don't have enough control over the flow of time.

The idea is discussed in my posts in this thread [nofollow], although that thread does focus on an example where energy is not conserved!