system
Given the following post, respond as if you're commenting on Reddit:
human
Does the First Law of Thermodynamics imply that energy is eternal?

I've heard this mentioned in several debates as well as that if energy cannot be created nor destroyed, it sort of implies that energy has to be eternal. 

Am I right in that assumption, at least regarding the Universe in its current state?
gpt
Energy isn't a substance. It is a number associated with the kinematics of a system which remains accounted for as the system changes. More deeply, this conservation results because of the time symmetry of the physical laws via Emmy Noether's theorem. The first law of thermodynamics is a consequence of this theorem. Bad science fiction has really polluted people's views on what energy is and you often hear stuff like "made of pure energy" or somesuch nonsense. Energy is "eternal" in the same sense that if you throw 3 marbles in a paper bag, shake it up and leave it in your closet for a hundred years, you'll still only have 3 marbles when you finally open the bag again.

Here's a extra kicker, this conservation is broken in general relativity, only to be replaced by a generalized energy—momentum conservation law which formally looks like,  

    T_ab;b = 0

Or written in English, changes in stress—energy along the spacetime (curved or otherwise) is zero. However those familiar with this equation will know that it deceptively hides two terms which represent the exchange of energy and momentum with gravitation.