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What is Energy? Is Energy Conserved?
"Emmy Noether in 1915. Noether proved that if you have a system whose equations do no change in time then this system has a conserved quantity. Physicists would say, such a system has time-translation invariance. Energy is then by definition the quantity that is conserved in a system with time-translation invariance."
By Sabine Hossenfelder
http://backreaction.blogspot.com/2020/10/what-is-energy-is-energy-conserved.html