Inspired by comment under this question

"Darling, does this dress make me look fat?


"Uhm ... no?"


We all dreamed about having possibility to redo something in real life. There are movies around it but they seem to be all based in pure fiction.

So, Is there some possibility to have undo for real? And how to make it reality?


  • Stay inside current science knowledge
  • If you need to go to the future, such future should be plausible given current science knowledge
  • Any form of magic, or all-powerful being appearing from nowhere (a.k.a The God) is not allowed
  • Does not matter how pricey it is. You have all money on Earth
  • The same applies to energy consumption.
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    $\begingroup$ A certain kind of mistakes e.g. the given example could be undone by simply clearing that other persons short term memories using surgery, drugs or a blunt blow to the head. (There might however by certain undesirable side effects). $\endgroup$ – Ghanima Dec 8 '14 at 13:08
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    $\begingroup$ Reminds me of The Girl Who Lept Through Time $\endgroup$ – Ajedi32 Dec 8 '14 at 14:40
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    $\begingroup$ Me. ten characters $\endgroup$ – Undo Dec 9 '14 at 0:35
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    $\begingroup$ I had a great idea for an answer, but then someone activated the Omega 13 and it was gone. $\endgroup$ – celtschk Dec 9 '14 at 21:16
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    $\begingroup$ Well, that might be a downer, but if you have a real-life "undo", you should be able revert death. Which is (as far as I know) impossible at the current level of technology. $\endgroup$ – user8808 Nov 19 '15 at 11:37

Can do with current technology, but it will require all the money in the world... Send me all the money in the world and I will let you in on the trick!

No seriously, a current day totalitarian state could mandate that all citizens attend conditioning classes as part of their juvenile education system. In those conditioning classes, hypnosis and subliminal messaging could be used to "program" each citizen's mind with a command phrase. High level officials within the state would know the command phrases such they would finally have the absolute control that Leaders have always dreamed of.

For example:

"Does this dress make me look fat?"

"No Dear, the donuts you constantly stuff in your maw are what make you look fat!"

...wife glares furiously, then raises butcher knife.


...wife's eyes glaze over, lowers knife.

"put away the knife. forget everything you or I have said since you put on that dress. then wake up."

...wife complies, then turning to her husband asks,

"Does this dress make me look fat?"

  • $\begingroup$ Seems like (fascinating, mind you) conjecture from popular fiction, do you have examples? $\endgroup$ – Smithers Dec 8 '14 at 20:51
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    $\begingroup$ The first brainwashing story I ever read was <u>The Manchurian Candidate</u> by Richard Condon. In that story the trigger phrase was a playing card, but the concept was the same. Several Dean Koontz novels, most notably <u>Strangers</u> also use the idea to high effect. As for non-fictional instances, nothing so powerful can be proven, but hypnotherapy is commonly used in addiction and aggression management treatments. $\endgroup$ – Henry Taylor Dec 8 '14 at 22:04
  • $\begingroup$ Don't you mean... "эта курам на смех"? ;) $\endgroup$ – Lightness Races with Monica Dec 9 '14 at 15:07

Fork a software copy of your wife (she pre-approved such copies) and a copy of yourself and run them both in a simulation space that runs much faster than real time. Have the simulated you answer your simulated wife over and over, repeating the copying and answering procedure until the simulated you produces an answer that pleases both simulated you and your simulated wife. Have the umpteenth copy of you report back the optimum response, whereupon you halt execution of all the copies and answer your wife in real life.

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    $\begingroup$ Not with a sufficiently fast simulation. This is actually almost possible in theory - if we were able to simulate humans well enough. $\endgroup$ – Tim B Dec 9 '14 at 18:13
  • $\begingroup$ I have pondered as a premise of a story something similar - take a perfect simulation, run of 'now', and run it forwards in fast-time to predict the future. Not too dissimilar to how weather prediction works, just at a much more detailed level. $\endgroup$ – Sobrique Nov 20 '15 at 13:07
  • $\begingroup$ @Sobrique See Accidentals Gods. I've read it; surprisingly enough the author missed your angle altogether, so you should take a swing at it. $\endgroup$ – Kyle Jones Nov 20 '15 at 21:10

No - not without limits of physics. Time flows in direction of increasing entropy. Undo will decrease the global entropy, negating thermodynamics. Time means causality. Reverting time reverts causality. Do you really want to dive that deep?

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    $\begingroup$ Unless undo decreases only local entropy. $\endgroup$ – mouviciel Dec 8 '14 at 17:53
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    $\begingroup$ So your entropy reverting engine would have to work in different directions on the borders of "local undo". So time would need to flow in different directions. @mouviciel Can you explain how you can do that? $\endgroup$ – Peter M. - stands for Monica Dec 8 '14 at 18:10
  • $\begingroup$ I think of an entropy container where entropy increases whenever Undo is activated, so that global entropy stays within the limits of the second law of thermodynamics. Like the Sun/Earth pair: life on earth decreases local entropy at the expense of creating entropy in the Sun. $\endgroup$ – mouviciel Dec 9 '14 at 14:12
  • $\begingroup$ ... so entropy of the whole system increases. No way around it. And no way to contain it. $\endgroup$ – Peter M. - stands for Monica Dec 9 '14 at 15:38

There is an example in Science Fiction where this is done - which is The Void from Peter F Hamilton's Commonwealth Saga. It's an artificially constructed portion of the universe where the rules have been re-written on a quantum level. One of the consequences of this is the ability to rewind time. The Void actually remembers all previous states and is able to rewind the universe internally to the remembered state while not rewinding the traveller(s).

This seems to me like the only way in which something like that would be possible, you need something to remember the prior states and a way to reset everything back to that prior state.

Of course if everything goes computerized and virtual then this becomes easier, we already know how to rewind computer time. Of course that doesn't work when you are talking to a real human being who would still remember no matter how often you pressed the undo button... unless real-time memory editing was a thing.

  • $\begingroup$ I wanted to ask: "But would it be possible with all Earth money and all Earth scientist working together? And the answer was "yes" :) " $\endgroup$ – Pavel Janicek Dec 8 '14 at 13:31
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    $\begingroup$ @PavelJanicek Actually the answer is "no". It is entirely impossible with our current level of scientific knowledge. Sufficiently advanced science in the future may allow something similar but even that is requiring some pretty improbable engineering. $\endgroup$ – Tim B Dec 8 '14 at 14:03

This isn't possible without a huge uptick in technology. You certainly can't go back in time, because that would violate, among other things, causality. Also, going back in time wouldn't help, because you'd still have the past you screwing up the answers to those questions. The alternative, by the way, is to kill your past self - but then that would kill your current self, and so we enter yet another paradox.

So let's get creative. There has been talk of turning people's biological bodies into mechanical ones, and turning brains into computer hard drives. I think this is possible within, say, 50 years - for perhaps a handful of brain cells. On a scale as large as the human brain, it's going to take us a lot longer. However, if you devoted all the resources in the world to this task, you could speed up the process a lot. So let's say that all the relevant people in this scenario have computers storing their memories, emotions, and personalities. But they still, for some reason, have normal bodies. Creepy.

If you mess something up (although at this point, the question would be more like, "Honey, does this new chip make me look slower?"), there's an easy solution: Modify the hard drives storing the person's memories. Make them unconscious, then use some programming to turn that memory into whatever you want it to be. It won't change your actions for the rest of the world, but you can "change" your actions for all who observed it.


In theory, one possible outcome of "the singularity" will allow people to transfer their consciousness into a machine. A machine, then, that could significantly alter their perception of the "world", simulating the world individually for each human. Undo would simply rewind the simulation, and change the user's memory of the events as needed so humans could fully interact, and any one could rewind short things.

Would probably need to have limits built in, otherwise the world would be in a constant state of rewind.

For instance, tiny, localized rewind effects have low to no cost - oops, dropped my phone and it broke, let's go back a few seconds. No need to rewind the day for anyone else, and the user would just have to accept the "lost" time since the rest of the world kept going on.

Large area rewinds (The rainstorm caused too much damage), or rewinds that affect a lot of people (sports exhibition), or rewinds that pass through a lot of time (I wish I hadn't entered into this relationship) would require greater resources, such as "lifespan", or would have to require several people wanting the same exact rewind, etc.

Then there are human factors. "I wish I hadn't gotten married and had 2 kids" - what happens to the kids if this rewind is successful? "I wish I had killed modern-day-hitler when I had the chance"

These problems would be present in any undo system - not just a simulated human system.

But as far as a reasonably possible future, many of our time believe the "singularity" is inevitable.


Yes, it is possible (ish) with today's technology . . .

I am not entirely sure how a real life "undo" may be possible, but a prevention of "do" may be by exploiting quantum entanglement between two photons with temporal separation.

You would need to capture a photon in a device and have a system periodically read the quantum data on said photon.

Now, whenever an unsatisfactory event occurs, you would write the quantum data to that photon. The quantum data will inadvertently be present on the entangled photon in the past, the one captured in the device.

Once the device reads the quantum data and collapses the quantum state, you will "receive a message from yourself in the future" warning you of the event that you do not wish for yourself to perform.

There is no telling if this device will honestly work, for, if you think about it, many paradoxes are at play here, but this is possible with today's technology.

It would not require all of the money in the world, yet it would be very expensive. I also believe that if you had all of the right "know-how", you could build this device yourself.

  • $\begingroup$ So you would only need to monitor one photon to rewind the entire universe? $\endgroup$ – Smithers Dec 8 '14 at 21:17
  • $\begingroup$ Well, your never actually "rewinding the universe." Your just preventing the event from occurring by interfering with the past. But yes, you only need to monitor one photon and alter its quantum data, yet that photon would have to be entangled with itself in a time either before or after it. In this case, the photon is considered two different photons due to the temporal separation, yet is still technically one. $\endgroup$ – WebWanderer Dec 8 '14 at 21:37
  • $\begingroup$ Once the quantum data is written to the quantum system, it may only exist on the quantum system within the other realm of time for a very short period of time and need to be captured by the device reading the photon. Yet, for the system to function, the machine would have absolutely no way of detecting the change of quantum data on the photon. You would need to broadcast the quantum data for some time, and then have the machine read the quantum data from the system on a timed interval. I'm not exactly sure if it would work. I am very intrigued by Quantum Mechanics, but I am self taught... $\endgroup$ – WebWanderer Dec 8 '14 at 21:41
  • $\begingroup$ Works for me in Worldbuilding.SE... it's internally consistent! $\endgroup$ – Smithers Dec 8 '14 at 22:09
  • $\begingroup$ So, don't get me wrong for being a member here, but what is this WorldBuilding? It sounds so cool just from the neat conversations I've seen, thats why I joined, but I still don't know what it is. $\endgroup$ – WebWanderer Dec 10 '14 at 1:47

This also kinda falls under movies so may not be what you're looking for, but the book behind Edge of Tomorrow came up with a hypothetical science way of resetting time. I asked about it after seeing the movie (as it left all the theory out) and got a very interesting reply, so I'll copy and paste that. From my non university level physics, it actually sounds like something that could potentially work many many years into the future.

The way they did it is through theoretical particles called Tachyons. Tachyons are like normal particles, but instead of moving through space like a photon or any other particle we are familiar with, they move through time.

Every 30 hours the Mimics are able to place a "bookmark" or "save point" of sorts, which gives them another 30 hour grace period. Throughout those 30 hours the "original" Mimic server (the bookmark) will be receiving tachyon signals from its "parallel" server. If things do not go according to plan, the parallel server sends a "reset" signal and it all starts over from where the "bookmark" was. That "bookmark" server will be endowed with all of the knowledge of the 30 hours that are to come, and so can change their tactics.

They can do this as many times as they want, and that is why they have been so utterly unpredictable and deadly.

Think about it like playing a video game with statically placed obstacles and enemies. You can play a level, die, then jump back, this time having learned something new and now anticipating what you know will come. It isn't a "new" timeline, it is the same timeline. At most it is a very closely parallel timeline. They are just "rewinding" to their save point.

  • $\begingroup$ While that was an awesome movie, I don't think we're at the point scientifically of utilizing tachyons, especially since we still consider them theoretical. Then again, if discovered to actually exist, it would only take humans a few years to figure out how to manipulate them to, say, crash the market by sending encoded messages back in time on which stocks to buy or sell to cause said crash... $\endgroup$ – phyrfox Dec 9 '14 at 5:21
  • $\begingroup$ Yeah unfortunately while you are right the film did exactly present an "undo" there's a lot of hand waving and throwing in buzzwords in that description and not much with actual meaning. "Tachyons are to do with time, lets throw those and a few other scientific sounding words in and that will do". $\endgroup$ – Tim B Dec 9 '14 at 18:17

This is not going to work, assume I know the secret for undo. If you give me 100 $ I tell you how that works.

  • You give me money
  • I tell you the secret
  • You do undo to receive back 100$
  • Ops you also undo the knowledge of the secret

actually undo could be perfectly possible, but we cannot know that.


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