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Ever wonder why life doesn't have an undo button?

Because 6 billion people pushing the undo button at once would make the system crash.

I know my science fiction knowledge is a bit lacking, but from what I have gleaned from movies and books, if a general undo button was suddenly put into place now, it would result in one of two outcomes:

1) At any given moment on earth, there is probably someone regretting something that they would like undone (even if countries with dictators restrict access to this button, I'm sure that there would be enough regrets accumulating somewhere). Now if every time the button was pushed, time was rewinded to the point before the incident, that earlier point would likely coincide with another undo-seeking individual. etc. etc. etc. The world goes back to the beginning of mankind, where some greater being undoes the process of creating an undo button.

2) If the undo button did not rewind time, and just undid an incident, then some Einstein comes into play: for every action there is an equal and opposite reaction. For example if someone wanted to undo ever meeting someone, then one of the two individuals would likely have to be somewhere else at the original meeting time. This means that once the button was pushed, everyone who could be impacted by the change would suddenly be in a different present, having gone through a different course of past events. Like in the Simpsons episode where Homer uses the toaster as a time machine and goes back in time and usually steps on something. The end result is a different earth each time, meaning that everyone had to have accumulated different experiences in the interim.

So while I recognize that a general undo process would likely result in either going back to before I was born or completely changing the course of my life, I still wouldn't mind my own personal undo button.