Question: Is there a reason why energy can't be destroyed, i am aware of the principle of the law of conservation but it doesn't explain WHY.

Pierre Lasorak answered on 18 Nov 2014:

Hi
Good question, energy is NOT always conserved! In our standard model of cosmology we have something called the dark energy. This energy fills the vacuum and as the universe is expanding there is more vacuum and therefore more energy! So you create energy out of… nothing.
Why is energy conserved? Because except few exceptions, like big bang or universe, it is in effect conserved for all the known processes! We could never find something in our lab that violates this very strong principle! If you find it, Nobel prize assured.
One other comment: while referring to environment and sustainability we usually say stuffs like: “you’re wasting energy,” “save more energy…” How can that be possible if energy is conserved?? There is an (unfortunate) misunderstanding. It is the following, when we say energy we are actually referring to some more complex form of energy. There is “good” and “bad” energy. The quantity that account for that is what is called entropy. It basically measures the ordering of things: if there is a lot of energy not organized, then it is “bad” energy (difficult to harvest) and the entropy is high. Likewise for good energy and low entropy.
So if you take the elevator instead of climbing the staircase, you will transform electric energy in potential energy, but this potential energy will never go back into the electric system and you will never be able to use it again for other purposes.