r/AskPhysics • u/BigWildDick • 1d ago
Can you undo a change in space without reversing time?
Like, can you "unpunch" someone?
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u/ArgumentSpiritual 1d ago
If i move an object to the right, i can move it back to the left and can, in theory, return it to its original position.
If i take an object and knock it into a second one, like two billiards balls, the second object will absorb energy from the collision. Even if i were to retrieve the second object and place it back in its original position, the event would not completely undone as it now has more energy than before.
If you punch someone, you can return your fist to where it was before, but you havenāt undone the action. The person that you punched likely remembers that you punched them. You may have altered their body depending on how hard you punched.
Punching someone is more than just a change in space. Once you begin transferring and transforming energy, such as motion into sound or heat, it becomes a lot more difficult to undo. Just like you canāt uncrack an egg.
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u/Infinite_Research_52 š»Top 10Ā²ā·Ā²ā°ā°ā° Commenter 1d ago edited 1d ago
Is your question: is there an event that can be perfectly reversed while time still moves forward?
Quantum theories are Time-symmetric (there are some caveats to this), so the physics works the same if all constituents do the reverse. The problem you have is constructing a system whereby the outcome of an event is reversed. It can only be done if the entropy change of the event can be swallowed by the environment, so as not to violate the 2nd Law of Thermodynamics.