Question to reconcile 2 seemingly contradictory theories

[nixon]

The first is Einsteins "Energy is neither created nor destroyed"

second is entropy.

How are the 2 compatible? If not, which is the wrong one?

[robto]

No contradiction.

Energy is conserved.

Entropy is a completely different quantity. Very roughly, the law of increase of entropy says that energy will always spread itself out as much as possible.

But the total energy will always remain the same.

[CanoeMan]

What he said.

[theyeti]

Einstein didn't come up with that stuff, dudes during the 19th century did.

Entropy can be thought of as the order of a system. Two systems can have the same energy but vastly different amounts of entropy.

theyeti

[Barbarian]

Or yet another interpretation: entropy measures how much of the energy of a system has taken a form which is impossible to use for doing work with. The energy is there, just not available for conversion into other forms of work. Like when you have two objects, one cold, one hot, and you use them to alternatively heat and chill a container with gas, and have the gas move a piston up/down/up/down, but only until the two objects reach the same temperature. The total energy of the system stays where it was, but it ends up in form of heat with no temperature differences, and also as investment in some broken bonds (friction of the piston ...), so it cannot be used to do more work with. The original setup has a lower entropy than the final one, on account of having a less uniform heat/energy distribution. The more uniform the energy distribution, the higher the entropy - but that was already said above.

[jeffevnz]

Conservation of energy is a very old idea. It started with Newton I, and has been extended and generalized over the centuries.

Entropy is a 19th-century idea, and it really pertains to the availability of energy, rather than its existence. Roughly speaking, the idea is that, over time, a macroscopic system will approach its most likely configuration, which is the most uniform. (It's almost purely a statistical argument, which is why it's considered one of the most unassailable laws of physics.) Now, energy can't be created or destroyed as the system evolves. However, in order for the system to do anything useful, like drive an engine, you need some kind of nonuniformity, like a temperature difference. Energy, or mass, or something, needs to be clustered somehow, or nothing interesting will happen. Since the distribution of mass and energy tends to become more uniform over time, the ability of the system to do useful work diminishes.

[nixon]

Thanks you all much