# How does the Doctor’s screwdriver run a calculation whilst destroyed?

In The Almost People, and The Eleventh Hour, the Doctor's screwdriver is destroyed and later regenerated. However, in The Day of the Doctor, the Doctor(s), run a calculation running over 400 years.

How is this possible, if the screwdriver was destroyed between the start and finish?

• They follow best practices and make off-site backups of all important data every day, including all the intermediate results of that calculation. Nov 29, 2013 at 17:07

This is technically trivial. Anyone running a very long computation (and some real computations can last for weeks) will regularly save the state of the computation on some reliable medium that does not need power, so that the computation can be restarted after any kind of failure. Actually there can even be several levels of back-up, balancing reliability and cost.

The computation does not need to be restarted with exactly the same software, as long as the intermediate results that have been saved can be interpreted and reused by the new software. Actually this can even be a way to improve the software (or the hardware) while the computation is on-going. This is standard technology, even for people who are not time-lords.

The screwdriver could simply save its state on the Tardis whenever there are close enough to communicate by whatever means. It could also be saved in various places known to be stable in time by someone who travels through time.

If the screwdriver is destroyed, only that part of the computation done since the last back-up is lost and must be redone.

By the way, since the Doctor travels through time, he could well organize the computation and back-up to get a lot more than 400 years worth of computation, depending on the structure of the algorithm and the computational capacity of the screwdriver.

It is to be expected that the computation does not take all the computational power of the screwdriver. If a computational thread actually leaves a lot of untapped computational power, it can be parallelized with a future fragment of the same thread by getting from the future the starting state for the later thread. Time travel extends the possibilities for parallelizing computations.

• It might seem like the Doctor could have left his screwdriver running a calcuation somewhere safe, and jumped forward in time with the Tadis to complete any arbitary number of years instantly. However, as we know whenever he travels in the Tadis he encounters some alien plot to destabilise something, and so its very likely his screwdriver would have been stolen / turned into an evil sonic beast if left unattended...
– Nick
Nov 29, 2013 at 17:53
• If left unattended for aeons, the screwdriver could run into problems at any time, with nothing to save the computation state. Furthermore, it seems that it is implicit that the Doctor will never travel without his screwdriver if he can help it. Hence I made the assumption that the screwdriver age is the same as the doctor's age, about 400 years in the episode I saw when the current doctor meets the (first ?) one in existence when Gallifrey is about to be destroyed. Still, that may leave room for longer computations. Other point is I am not sure the screwdriver can work without the Doctor (?). Nov 30, 2013 at 14:52
• @babou i really like the point that the screwdriver may need the doctor to operate. thank you very much! Dec 1, 2013 at 2:11
• I would like to point out that in The Big Bang the Doctor leaves his screwdriver in Amy's pocket for about 2000 years, and it was running the computation the entire time. Dec 1, 2013 at 17:57
• @childcat15: In fairness, they did then reboot the entire universe. I'm not sure there are many backup strategies designed to maintain data integrity in that scenario. Dec 2, 2013 at 13:58

Like Rose said in The Day of The Doctor

" Same Software different case"

Therefore the Doctor's screwdriver was not completely destroyed, only the case containing it was; the software was safe making it possible for the TARDIS to repair the case and give the Doctor a seemingly "new" screwdriver with the computation still intact.

In "Let's Kill Hitler", it's explained that instead of having settings, this version operates through a telepathic interface, linked to the Doctor. Hence he can use it to do such a varied number of things without needing to continually change settings on it.

WARNING GUESSWORK*

Therefore the internal state of the screwdriver is probably stored in the Tardis (on the cloud) as this is telepathically linked to the Dr. too. When it is regenerates it just syncs and carries on without minor disruption.

• please cite a source for the first bit. Dec 1, 2013 at 2:12
• In "Let's Kill Hitler", it's explained that instead of having settings, this version operates through a psychic interface, basically doing whatever the user thinks of while pointing and holding down the button. Dec 1, 2013 at 20:25