When you delete data, you make it impossible to retrieve for all times in the future, assuming you have done it right.

However, the data still existed in the past. In a world set in the 22nd century, where time travel has been achieved with negative mass tachyons, people can simply go back in time and get the data before it was ever deleted.

Similarly, if a credit chip is destroyed, one can simply go back to the time before it was destroyed and get the information off of it. Most people have access to negative mass tachyons if they are middle class or higher.

How could a citizen securely delete data and destroy sensitive papers when a criminal could go back in time and get it before it was deleted/destroyed?

  • 20
    $\begingroup$ Congratulations, you've reinvented the grandfather paradox. Also, welcome to Worlbuilding! Please be sure to take our tour and read over the FAQ to understand how this system works. You should also edit the question to break the "wall of text" and make it easier for people who'd like to help you to read it. $\endgroup$
    – Zeiss Ikon
    Commented Oct 11, 2022 at 14:05
  • 2
    $\begingroup$ en.wikipedia.org/wiki/Temporal_paradox $\endgroup$
    – Zeiss Ikon
    Commented Oct 11, 2022 at 14:06
  • 13
    $\begingroup$ What are the rules for time travel you use? If I go back and steal my grandmother's cake recipe, will that affect my own timeline? Does that mean that I never had my grandmother's cake as a child? Or do I retain the memories of it yet nobody else knows about the cake? Or did all the cakes still happen, I just went and stole the recipe from another version of my grandmother but not the one from my own past? Or what? $\endgroup$
    – VLAZ
    Commented Oct 11, 2022 at 15:45
  • 3
    $\begingroup$ The question is like asking “I have a magic spell which can kill anyone, anywhere, anytime, how do I protect against it?”. If you introduce impossible physics in your world you’ll just have to introduce more to mitigate it. Something like an unbreakable wall in time or a device which destroys things in all of time. $\endgroup$
    – Michael
    Commented Oct 12, 2022 at 11:15
  • 3
    $\begingroup$ @Guy IF your rule is that paradoxes are resolved because a new set of timelines are created then who would care about protecting their data? A criminal from our timeline who goes back in time to steal my data would end up in a completely different timeline to me, which means I only need to worry about time travellers arriving in the present rather than the past. $\endgroup$ Commented Oct 12, 2022 at 16:48

15 Answers 15


Go back in time, and prevent the data from being created in the first place. If it never existed, it cannot be retrieved.

If you wanted to make sure you have a private copy without creating a paradox, then before you travel back in time, make a copy of it, encrypt the crap out of it with a key that only you can know, take it back in time with you to the just before the information is created: As soon as it is created, destroy it, by whatever means necessary. It never gets out, but it was still created (no paradox) and you have that encrypted copy you can hide where only you will find it, and only you know how to decrypt it.

When you return to the future, retrieve the data from your hiding place and decrypt it. Thus the information was still created, and still existed the whole time, but you have the one and only copy.

If the future was changed by your actions and the information was re-discovered, just repeat the process.

Edit: Some comments are concerned about "causality". In any time-travel story, causality is out the window from the start: It is 2022 as I write, and I know a great deal I did not know 50 years ago, in 1972. If I travel, as I am, to 1972, I have broken causality just by existing. The myriad causes that caused me to change in 50 years have not happened. My brain and my knowledge exist without cause in 1972, and anything I know about their future is impossible for me to know.

A time travel story must be written, from the start, with a severely liberal take on "causality." The root of the problem is the time travel itself, events from the future (the invention of time travel) have already caused changes in the past: The existence of the time traveler and the knowledge from the future they have, including the certainty of what is about to happen, that nobody else, anywhere, may know. For example, they may lie in wait for Oswald (and any other assassins) and prevent Kennedy's assassination without Kennedy ever suspecting there even was a plan to assassinate him.

Precisely how one redefines "causality" in the fiction is up to the imagination of the author, but pretending that the definition of causality is unchanged is particularly unsatisfying, at least in my view.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Oct 13, 2022 at 15:34

The accepted situation (in the right circles, at least) is that if paradoxes exist, time travel is not possible, and if time travel is possible, paradoxes cannot exist.

In your case, you've described a paradox -- if you delete data retroactively to its creation, no one can retrieve it by traveling to a time before you deleted it, but if they can prevent you deleting it, they can still retrieve the data.

The only way time travel can get around this kind of "grandfather paradox" is if the many-worlds hypothesis is true: you go back in time and change something, but you don't change your own past, you just create a new timeline in which, in this case, the data you deleted never existed. It's still there where/when you came from, but when you return to your origin time, you'll be going up the new line.

This gets confusing very, very rapidly -- recommended reading is The Man Who Folded Himself by David Gerrold or All you Zombies and The Door Into Summer by Robert A. Heinlein.

  • 2
    $\begingroup$ Changing the past was only to to erase the data, I know that it is impossible. However, one doesn't need to change the past in order to observe the past, and criminals would observe and record the data. Thus, the criminals still get the data. $\endgroup$
    – user98873
    Commented Oct 11, 2022 at 14:41
  • $\begingroup$ Which is why you would need to change the past to delete data so criminals can't retrieve it before it was deleted. $\endgroup$
    – Zeiss Ikon
    Commented Oct 11, 2022 at 14:43
  • 1
    $\begingroup$ If the data ever existed, even for a nanosecond, the criminals can copy it in that nanosecond. $\endgroup$
    – Zeiss Ikon
    Commented Oct 11, 2022 at 14:45
  • 4
    $\begingroup$ "So then how could there be security of information?" Exactly. $\endgroup$
    – Zeiss Ikon
    Commented Oct 11, 2022 at 15:40
  • 1
    $\begingroup$ The Light of Other Days by Stephen Baxter (and Arthur C. Clarke) is another good read; in that, there is no actual time travel possible (hence a single timeline and no paradoxes), but information can be observed (almost undetectably) from the past via wormholes, which inevitably leads to complete failure of secrecy and privacy. $\endgroup$
    – Miral
    Commented Oct 13, 2022 at 4:10


Here’s one fascinatingly ridiculous idea: tachyonic magnetic propagation via an Alcubierre drive. We use a modified Alcubierre drive moving faster than light speed to transport an enclosed space-time field that contains a concerted high-power magnetic pulse backwards through time. The space only has to be as big as the storage device we want to destroy, and stay entangled to the hard drive’s “quantum identity signature” as it travels backwards through time and space. This TSEMP, or Time-space electromagnetic pulse, will magnetically wipe the drive across all instances of time. If you know exactly when the drive was created, stick a reverse timer on the TSMP and you’re good to go.

Encrypt It

That said, there’s no need to destroy it if it is encrypted properly and only you have the key and no one else can get it. Heavily encrypted data is not viewable in a reasonable time frame (until massively parallel quantum computers are a thing, anyway) because, while modern computers can technically crack a well-hashed bit of data, it would take a while, in some cases 200 years or more.


Interesting aspect of the ability to travel backwards in time: you could take a computer back far enough in time to make progress decrypting data, then travel back to the future to read it when it’s had many decades to decrypt the hash. Granted, you’d need a computer that can survive that kind of power use for decades AND travel back to a time with power and secure location, but those things can all be hand-waved if we’re traveling back in time to begin with.

  • $\begingroup$ You could easily modify a computer power supply to run on Edison's 100V DC power from the 1890s. Ought to give you plenty of working time... $\endgroup$
    – Zeiss Ikon
    Commented Oct 11, 2022 at 16:13
  • $\begingroup$ This answer solves my problem. I will just handwave the paradoxes and encrypt using a 100000000000000000 qubit encryption.That ought to keep it locked for a while. $\endgroup$
    – user98873
    Commented Oct 11, 2022 at 16:16
  • 7
    $\begingroup$ In a world with time travel, "a reasonable time frame" for decryption is virtually infinite. It doesn't really matter if it takes a 1000 years to get the password, it's not like you need to wait for the duration. If you can't leave the decryption machine running anywhere for 1000 years straight, just take 1000 one-year trips and restart where you left off each time. Decryption time isn't a barrier when there's time travel. $\endgroup$ Commented Oct 11, 2022 at 16:58
  • $\begingroup$ I agree, ergo the caveat the section of my answer. $\endgroup$
    – Tmartin
    Commented Oct 11, 2022 at 17:18
  • 1
    $\begingroup$ Encryption wouldn't help much. The encryption would have needed to be happened at some time, just travel to a time where it wasn't encrypted yet and observe it there $\endgroup$
    – Ivo
    Commented Oct 12, 2022 at 6:17

Store the data using quantum entanglement.

Quantum entanglement has a neat property: entangled data can only be read once, and doing so instantly turns it into random noise. And by instantly I mean literally instantly, not delayed by the speed of light. Anything faster than the speed of light can be seen for some reference frames to be going back in time, so though we can't currently test this without access to time travel, it's not implausible to speculate that reading quantum-entangled data erases it across all of time. This means you're set! Once you retrieve the data in the future, by doing so you erase that data from the past.

  • $\begingroup$ So someone else discovers you've read the data, goes back in time to a point at which you had not yet read the data (and therefore it cannot have been random noise yet), and reads it before you. The data you 'read' becomes random noise, but it wasn't yet random noise when you read it 'first', so your solution creates a paradox. $\endgroup$ Commented Oct 14, 2022 at 21:06

Remove, Falsify, then Actively Defend the Data:

This answer assumes you and the others trying to obtain the data DON'T want to create paradoxes. If paradoxes are okay, I don't think you have a problem. Copy and destroy the data at its creation. Then kill the creator before it was created.

That's not very satisfying. Not to mention that the data could still exist in a parallel dimension if you create a paradox.

While the ability to time travel is amazing, it doesn't make the traveler god-like. Someone has to go back in time and obtain/observe the data. Find out all you can about the handling of the data, then falsify the records of it to throw off rivals trying to get it. They don't simply get to pluck it from mid-air. If they don't know what the data is first (or even if it exists), they can't know they are being fed false data.

So obtain, then delete the data just after it's created. Without causing paradoxes, this will be hard. Defend the creation site/time until you know no other time traveler has the data. Kill rival time travelers if they show up and make sure there is no record of your theft so time travelers don't know why their friends went to steal the data and never returned. Replace the data with fraudulent information so thieves from the future don't know they are being tricked.

Now you need to know each time the data was accessed historically. Use the same steps to go, add the data back long enough to allow it to fulfill history, then delete it again. If the data was openly circulated, then the only way is to disrupt the timeline. But if it was a private file, then these steps mean you are present to protect the data when it is exposed but don't need to guard it throughout history to assure no one else has access.

  • $\begingroup$ Huh. I never thought of that before. I guess the problems are the paradoxes and not the time travel. So if I make the rules of time travel be that there are no paradoxes... $\endgroup$
    – user98873
    Commented Oct 11, 2022 at 16:50
  • $\begingroup$ @Guy I fleshed the answer out a bit. If you like it, upvote it! Site etiquette, however, is to wait 24 hours to formally accept an answer so people around the world have a chance to read and answer the question. $\endgroup$
    – DWKraus
    Commented Oct 11, 2022 at 16:57
  • $\begingroup$ Oh ok. +1 then : ) $\endgroup$
    – user98873
    Commented Oct 11, 2022 at 16:57
  • $\begingroup$ The 'fun' part of this answer is that the time-traveler writing it presumes he isn't already a victim of the very technique he proposes. $\endgroup$ Commented Oct 14, 2022 at 21:03

Welcome to the real world.

You don't need to have time travel in order to get the problems you describe.

The important thing is, data travels to the future by itself.

In order to have particular data secret, you need to guard it from the act of the data creation all the way to the data destruction.

A single failure - and the data is copied somewhere and the control is lost.

What the time travel brings is that the future may offer a different, more efficient methods of data extraction from encrypted, physically locked or otherwise guarded storage.

But this is not a big deal, really. What you have on your side is the technology progress and technology obsolescence.

Have you ever tried to read a 2001-made HDD in 2022? A tape from 1978? A punch card? 20 or 40 years passed and it is already hard - even if the media is in quite a good state.

A book from 19th century is hard because the language is already not the same.


Generate and store the data securely.

So long as the data is generated in a place that's inaccessible to others it's much harder to steal. Have facilities that have anti time travel defenses so people can't spy on them or get inside easily. Have guns so if someone time travels outside you can shoot them. This ensures the data is secure. Make all your key data here, and store it here as well. The site will be extensively monitored well before building the facility to ensure no one time travels before the building and digs under it or something.

The site will be airgapped, and have lots of heavy steel doors to go through to generate data, of course, along with security measures designed to resist future technology attacks.

Have heavy explosives near all sensitive data.

One obvious way the government could get secure data is sending an order into the past repeatedly to send an army to the location to kill everyone and take it. Have a tachyon detector to detect if you're in the main timeline. If you're not, automatically set off the explosives. If you detect a large army, detonate the explosives. This ensures people can't use time travel to access your data. These explosives will be set up well before the facility is made with a particular timeline, to ensure people can't butterfly them away.

The longer the better. Over time the facility will get more secure as it will be harder to break the timeline explosives by going back.

To delete the data, delete it.

The data is stored in a reliably accessible way only in the main timeline. In other timelines it explodes. If you delete the data in the present there's no easy way to access it.


Self Healing Paradox
Go back in time to the birth of the universe and stir violently using the large scale weapon of your choice. Vanish in a puff of logical conflict, well satisfied that any chance of "your" universe existing is gone. This includes the invention of time travel.


So a lot of answers here talk about paradoxes and the need to avoid them etc.

But that really depends on the rules of time travel in your universe.

You could have the act of departing one time and arriving in another become two decoupled events. So if you travel back in time, someone killing you before you travel back in time wouldn't stop you appearing in the past.

Now you appearing in the past might then seem like a paradox, but it's no greater paradox than time travel itself. If you accept time travel you have to also accept the ability to influence events before your influencing act takes place. It seems reasonable to me to say removing the initial act of travelling wouldn't undo the change it caused in the previous timeframe.

Taking all of that means you could travel back in time and prevent the data from being created, so long as your very appearance caused the dataloss (so there's no window of time to stop you).

  1. You time travel on top of the father of the person that creates the data, killing them in the process (Harry potter splinching-esque).
  2. Nobody can stop you from travelling back, because the events are decoupled.
  3. Nobody can usefully prevent you from killing the father, because even moving him out of the way likely means the conditions that led to the creation of his exact same child are lost (different sperm reaches the egg, child has a different life, etc)
  4. To be extra certain you could go back a couple more generations and splinch the great-great grandfather instead. Ensuring the ripple effects are so huge that nobody could repair them.

This form of time travel also allows you to jump back and forth in the same timeline without much headache, means you can have multiple versions of the time traveller in the same timeline, and avoid the grandfather paradox.

I suppose this could be called 'lossy' time travel. Like an MP3, where all the information that leads to a specific output is lost, but the final result is the same (don't @ me hi-fi purists; I know, I know.)


Keep an Access Log

If you just go back in time to delete the data you no longer need, then you will keep yourself from being able to use it in the past when you needed it. So instead of deleting it from the whole timeline, you just delete it from most of the timeline.

As the name implies, an access log is a file or data table of some sort that you associate with data to keep a record of every time the data was accessed. So, let's say you have a user record that has been accessed 7 times in the past 5 years since it was created, you now know the exact 7 moments in time you need this data to be there. So, using the log file you just delete everything around those 7 moments (including the log record itself), so that way if a time hacker tried to go back in time to access the record, he could not without knowing the exact microsecond that this information will pop in and out of existence... which he can't know because the only place to look that information up is in these 7 microseconds spread out across the 5 years this data existed. This means any random scan of the hard drive in the past would only have a 2.3e13 to 1 chance of having the data on it.

Furthermore, while this technology makes data breaches seem more dangerous, they actually make them less dangerous than they are in our world. In our world, hacks and spyware are nearly always identified and addressed eventually, but the damage that is done in the past in immutable. However, in your world, if you ever find malware on your server you not only have the power to delete it, but you can delete the malware from every point in history such that the data was never compromised and hackers who get caught don't just get locked out, but they lose everything they ever gained from thier hack. Not getting caught for now is a lot easier to do as a hacker than not getting caught ever.

Reality Check

Most people have access to negative mass tachyons if they are middle class or higher.

I find this a very unlikely event at any point in the future no matter how advanced society becomes. Today, nuclear weapons are a nearly 80 year old technology that are disturbingly easy to make with modern manufacturing technologies compared to all the smart phones, nanotechnologies, and other high-end capabilities our society has access to. But the average middle class citizen can not nor will he ever be given the freedom to own nuclear weapons because that is automatic suicide for your entire civilization, and probably the world.

The ability to go back in time and edit it, is FAR more dangerous than any nuke. While a nuke can end a civilization, a rando with a gun could go back in time to prevent a civilization from ever being born causing far more damage with far less resources; so, for this setting to make any since, the technology to edit data in the past (pushing around a couple of electrons here and there) has to be distinctly different than being able to actually send a person into the past, and even the ability to manipulate past records would have to be VERY strictly regulated or preventable. If the same technology that a private citizen can use to protect thier privacy could also be used to manipulate the outcome of the 45th presidential election of the United States, then the benefits would outweigh the risk of allowing private ownership, and all time travel would become the exclusive prevue of a small number of superpower governments.

So to limit the destructive eventuality of these devices, there would need to be one of 2 solutions. Either:

A) An effective means of locking off significant parts of the timeline to time travel... unfortunately, making this whole technology a moot point. Or B) This technology needs to be a closely guarded secrete controlled only by a small handful of individuals to make sure no sociopath ever gets ahold of it and decides to start playing God over billions of people's lives.



The data is important, so it must be stored on a mainframe disconnected from the internet. Even tax organizations have systems where people have two seperate computers with one connected to the internal servers and one with internet access.

So important information is stored on such a disconnected computer or server, and you simply have security. Even if someone travels back in time they still have to deal with said security to get in. If necessary you send someone back with the newest security upgrade tech to improve it every now and then.

After all, the information was secure for some time before someone timetraveled right?


As others have already posted technically this is impossible.

The way around it is instead of permanently deleting the data (Which is technically impossible). You permanently protect it. You will have to say that a special machine exists inside of this world (For now we will call it the black box).

There can be multiple black boxes. A person enters in the data they want to permanently protect into the black box, including what time the data was first created, the black box essentially runs a program so that it is constantly detecting if anyone touches/accesses/ or sees that data in the past.

If someone does the black box jumps back further than that person and does something to keep the data protected.


Limit the time the information exists.

The premise that a time traveler can "just go back in time and take it" supposes there's a point in time when the information is easy to steal, and, to a lesser degree, that no one else is trying to do the same thing at the same time.

If you know a piece of information is sensitive, you can schedule to have it destroyed quickly. For the short time that it exists, it can be actively guarded against all except authorized time travelers. Anyone who needs to access the information, near-future or otherwise, will need to visit that short window and clear security.


Ensure it doesn't come into existence

The simplest way to secure your data is to guarantee it doesn't exist in the first place. This answer tries to ensure that it happens.

Let us say you need your secured bank statements to do your taxes. We shall assume all data is safe before you ask for it, and the transfer of the data can't reasonably be intercepted.

  1. Ask for your data, in whatever way you want.
  2. Fill out the tax forms
  3. Take the tax forms to a past you, before you asked for the data.
  4. Hand over the tax forms, and do not ask for your data.

This could ensure your data technically isn't stealable. It does come with its own problems though.

The way around

Ever heard of save scumming? It is the act of going back to an older save with new information, you gathered to beat RNG or try for a different outcome. Now, mister X likes to do something similar. He simply tortures people to get the data he wants, knowing he only needs to keep it secret for as long as it takes to obtain the data. Then he copies said data. Afterwards, he brings the data back to just before the point he started the whole operation, erasing the torture timeline while keeping the data.

This breaks any security you might create. Ever. Now try to keep people out of the stock market...

  • 3
    $\begingroup$ In cyber security, we call that a pipe-wrench vulnerability. Queue the appropriate XKCD reference: xkcd.com/538 $\endgroup$
    – Nosajimiki
    Commented Oct 12, 2022 at 15:42

Police and Lawsuits

You said this is a Universe where time travel is easy. I think it goes without saying then that it requires policing, in order to prevent paradoxes like the ones above.

This doesn't have to be government policing. A company that specialises in data protection may have as part of their service a kind of temporal enforcement to prevent data from entering the future from the past. Or they may simply bring out a lawsuit against anyone who has managed to gain access to said data and use it to affect the future.

The entire question over the legality of accessing data in the past without necessarily having it in the future would become an interesting philosophical discussion inspired by such a world, potentially even a hinge of the plot.

  • $\begingroup$ But then the question becomes: how does the 'tempolice' prevent data from entering the future? $\endgroup$
    – Joachim
    Commented Oct 13, 2022 at 6:02
  • $\begingroup$ Monitoring time travel phenomena and apprehending individuals who misuse it. The same way they stop any crime or online hacker. $\endgroup$ Commented Oct 14, 2022 at 10:58

You must log in to answer this question.