After hearing about the Moment for many episodes we finally learn details about it in ‘The Day Of The Doctor’. Stored within the Time Vault in Omega arsenal it is a galaxy eater, the final work of the ancients of Gallifrey. A weapon so powerful that it’s operating system became sentient.
When we first see it the Moment is a square wooden box with metal cogs inside. The War Doctor is not initially sure how he is supposed to operate it, starting the clockwork but not finding any input devices. It is capable of giving off enough heat to be hot to the touch.
By the conclusion it is a much larger pillar, with metal of steel and gold. It is activated by a glowing red jewel, that is suggested at least partially by the War Doctor’s desire for a big red button.
By the end of the episode we never actually see the weapon activated but we know what the Doctor expected to see. He believes he saw the destruction of his planet but a ‘galaxy eater’ should have had a much bigger effect.
A galaxy can consist of ten million to a hundred trillion stars. For a weapon to be capable of destroying all of them it would be incredibly powerful. This seems to be overkill when the Doctor only needed a stellar manipulator to detonate Gallifrey’s sun and take out the local star system (wiping out his own people and the Daleks).
The desert world he travels to might be outside of the galaxy that contains Gallifrey but he seems to believe that he won’t survive its activation. It is possible he just took it away from Gallifrey so he’d have time to activate it. If the world is outside of the weapons area of affect it would need to be an incredibly vast distance away.
The operating system manifests as Bad Wolf, taking Rose Tyler’s form. This would appear to be a psychic projection, since only the War Doctor is able to see it (the fact that his 10th and 11th incarnation can’t see her indicate that different incarnations have different brain waves). He does touch her at one point but this could be part of the sensory illusion.
In many ways the personality of the operating system shares a lot in common with the TARDIS (when it was personified in Idris in ‘The Doctor’s Wife’). It can be flirtatious, childish, wise and shares the same confusion about past and future.
It knows enough about the future to be able to observe the 10th and 11th Doctor. It chooses an incident in the War Doctor’s future that will give him inspiration that will change his mind about using the device.
Yet it states to the War Doctor that the future he is shown isn’t real yet (it is only real to the future Doctors) as he still has a choice. It is also surprised when the fez comes through, just after it opens up a time vortex (which could indicate that her actions change what should have happened and she has no way to view the outcome of her own actions).
As mentioned above the interface is able to open up swirling gold holes to different times and control the direction in which things that pass through travel. This is a much more advanced form of time travel that a TARDIS, especially as it can retrieve a traveller at any point (as it does with the War Doctor).
This might hint at how the Moment worked. It could be that rather than destroy a galaxy in the present it could extinguish it throughout time, consuming it from the past or maybe stopping stars and planets from forming in the first place. If it has access to the vortex its destructive power could come from the vortex, using time winds to erase a galaxy from existence.
Having a Time Lord weapon use time as the source of its destructive power makes sense and helpfully explains its capabilities it displays. Since this is the last work of the ancients it could be that there were previous weapons with similar capabilities on a smaller scale.
It isn’t clear if the operating system was intended to be sentient. This is treated as a drawback by the member of the war council but it could be the wisdom of the ancients that such a powerful weapon have a safe guard in place, just as Rassilion saw that immortality was a curse.
This sentient operating system judges the user but from what we see it doesn’t at any point prevent the war doctor from using the device. It is always his choice whether to activate it. If he had pushed the button then it would have destroyed Gallifrey as intended. In short the interface isn’t the one deciding if the user should be able to activate it.
The judgement instead takes the form of making the user appreciate the full extent of their actions. Not only by showing them all the lives they will sacrifice but showing them glimpses of the future so they can understand how will impact them.
Further expanding on the idea that the ancients intended the operating system to function in this manner it could be that is actually the purpose of the device. It could be that it isn’t a galaxy destroyer at all.
Rather it is an anchor point in time, allowing multiple incarnations of a Time Lord to come back to a specific moment. As was shown in this episode this gives them much more time to come up with alternative solution.
The 10th and 11th Doctor are surprised they were able to break through the time lock, as if something was guiding them. The involvement of all of his incarnation further supports the theory that the Moment suspends the laws of time that prevent a Time Lord from crossing his own time stream.
If the War Doctor had pushed the button maybe nothing would have happened at all and the ancients ruse would have been discovered. They knew that anyone willing to use a galaxy destroy would have been in a dire situation indeed and if the Moment didn’t exist for them to seek out they would have sought an alternative. By providing the perfect tool they could put in place safeguards to make sure that level of destruction was never used.
That theory could be wishful thinking and it could be that the Moment would indeed have wiped out a galaxy. Whether this would have impacted established history (since we know now that this is what did happen originally) is a matter of debate.
There is also the question whether the Moment or a prototype was ever used before. Is there a galaxy erased from the universe? What would have driven the Time Lords to destroy it?
The PCs could find evidence of this or even find themselves in the past, before the Moment is activated. If they prevent its activation how will the timeline be altered?