The Cracks in the Universe: Why are they lethal to some, but not to others?

Question

During the Doctor Who Season 5 story arc, the cracks in the universe absorb a number of creatures, erasing them completely from time:

• Amy Pond's mother and father.
• The military clerics Phillip, Crispin, Pedro, and Marco in "Flesh and Stone".
• The Weeping Angels in "Flesh and Stone".
• Rory's corpse in "Cold Blood".
• Also, apparently, the ducks in the duck pond. ;)

However, there are 5 characters/species that are able to be close to the cracks, in some cases for long periods of time, without getting erased from time:

• Amy Pond, who had a crack in her room for years but never got absorbed by it, even though her parents did.
• The Doctor, who reached into a crack to pull out a piece of the exploding TARDIS in "Cold Blood".
• Prisoner Zero, who was able to escape prison by fleeing into Amy's house through the crack in her wall.
• An Atraxi, who was peering at the Doctor and Amy through the crack in her wall in "The Eleventh Hour".
• The Saturnynians, who fled their damaged world for Earth through a crack in "The Vampires of Venice".

Are there any potential in-universe explanations for why the cracks in the universe are not lethal to these few characters?

Answer 1

The answer from abcooper makes a very good point that didn't dawn on me: that the cracks can separately exhibit spatial or temporal properties. It then reminded me of something Angel Bob said in the episode "Flesh and Stone":

There is a rupture in time. The Angels calculate that if you throw yourself into it, it will close and they will be saved.

This led me to a potential in-universe explanation:

• When a crack first appears, it connects that point in time and space directly to the time explosion that is destroying the universe. It behaves more like a temporal rift, absorbing objects around it and erasing them from time.
• Upon absorbing enough, the crack is disconnected from the time explosion, connecting to a different point in space (and possibly time). It behaves more like a spatial rift, allowing individuals to pass through unharmed.

The initial appearance of a crack as a temporal rift which then transitions to a spatial rift helps explain some of the apparent discrepancies:

• Being an exceedingly complicated space/time event (and a Lord of Time) makes the Doctor able to get near an "active" temporal rift for at least a short period of time without being absorbed by it, allowing him to reach into one in "Cold Blood", suffering some pain in the process. However, he is absorbed and erased from time when he ultimately plunges himself directly into the time explosion in "The Big Bang".
• When the crack first appeared in Amy's home, it absorbed her parents (and maybe other things) then went "dormant", transitioning to a spatial rift. This allowed Amy to remain near it without being erased from time, although small amounts of residual time energy may have been leaking through that affected her. Later, Prisoner Zero was able to escape through the crack, and the Doctor was able to force it open to communicate with the Atraxi on the other side.
• The crack that appears in the Byzantium seems much more powerful, requiring that it absorb more "complicated space/time events" to close it. After absorbing some soldiers and all of the Weeping Angels it transitions to a dormant state.

• In "The Vampires of Venice", the Saturnynian Rosanna Calvierri says:

  There were cracks. Some were tiny... some were as big as the sky. Through some we saw worlds and people and through others we saw silence... and the end of all things. We fled to an ocean like ours and the crack snapped shut behind us... and Saturnyne was lost.

  The cracks that appeared presumably absorbed and destroyed much of the planet, transitioning to spatial rifts which allowed a small group to flee through them to Earth.

Answer 2

There's some indication - Amy and the Doctor's ability to remember erased people, for example - that time travelers are immune to some of the effects of the crack, presumably because their personal timelines are already nonlinear, and so they are able to witness changes in the linear timeline from outside of it. Amy especially is immune to its effects because, the Doctor explains, she grew up with them.

My understanding (though I'm interested to see what others have to say) is that the crack in the universe is a crack in space-time, and the effects are not entirely regular. Prisoner Zero and his Atraxi guard are affected by the spacial properties of the rift, while the people who are erased are being affected by the temporal properties (they are being removed from the timeline, so that they were never born.)

Answer 3

There are a number of clues during the season that the entire explosion-in-space event is centered around Amy from the beginning. In other words, it wasn't simply a matter of Amy coincidentally living near a crack in time. Rather, the crack started with her life and erased things outward, until she was all that was left.

It's hard to tell since most of the show is told from Amy's perspective as the audience proxy, but we don't really hear or see any examples of people disappearing into the cracks other than people Amy knew, at least for a short time. And early on, The Doctor makes a connection between the date and time of the explosion being the same day as her wedding. He comments on several occasions that she is playing a crucial role in whatever disaster is about to happen, and it seems more than just being in the wrong place at the wrong time.

It seems likely (given Who's penchant for introducing paradoxical time loops) that Amy grew up near a crack in time because she was there when time actually exploded later on and because The Doctor relied on her growing up near a crack in time to fix time.

Given that possibility, it seems like the only people in real danger of being completely erased from time, initially, are those close to Amy (emotionally and/or physically). Her parents disappear first, before we ever meet her; as a young girl she still thinks she has an aunt but she seems to have already vanished. Amy is present as the angels and clerics vanish, which we learn only because she subsequently forgets them.

But while the cracks are only erasing people from Amy's timeline, they are still very much cracks in space and time; the universe is "unraveling", making it possible for people to hijack the cracks for their own purposes. This is how the various creatures can use the cracks as a means of egress. But as we get closer to the date of the explosion, the damage gets worse, starting to erase things further and further out from Amy herself.

Answer 4

Okay in order to explain the seemingly inconsistant vanishings you have to look at this from a different point in time. Start with the Explosion of the TARDIS. This happened when River Song was piloting it which then closed her into a time loop. Also the planet nearest the explosion is Earth which means it is the last place in the Universe to be erased from existance according to http://tardis.wikia.com/wiki/Total_Event_Collapse

So following the logic of the event (Total Event Collapse) happened so it is now a fixed point in time this is what makes Amy basically exempt from being erased. In order for it to happen River must pilot the TARDIS. Since River was piloting in order for River to exist Amy must exist. This protects Amy but what about Rory? Well Rory was only erased when he had died. Since he had grown up with and often visited Amy's room it seems he also was safe from being erased until he died. His death created a paradox of sorts that allowed for him to be erased without changing the event. The Doctor is mostly safe because he is a Time Lord and as a species is the best suited to survive any sort of time or space paradoxes. Also it's been noted that the crack was not glowing until after the Doctor reached into it which does seem to have a difference on the effect that the crack radiates (erasure or another point in space/time).

As it states for the Saturnynians http://tardis.wikia.com/wiki/Saturnynian there were different cracks that they observed one of which showed Earth and they escaped through it. The same thing happened to Prisoner Zero. He was in an extra dimensional prison too which might have added in his escape through a crack. Even though the crack he escaped through was the one in Amy Pond's room It's strength at the time was not enough to erase an alien being of his strength. The Atraxi were in a space ship which likely helped to protect them as well. Also remember the Earth is the last point in the universe to be erased which means many cracks will lead to it as it is closest to the point of orgin.

Amy's parents were erased after Amy was a child able to somewhat take care of herself. The clerics and Angels dissappeared on a world that was being swallowed by a crack. The ducks were a lesser unimportant item and were erased easily. Really only the Doctor missed the ducks.

So to clarify there is an event, anything or anyone directly tied to the making of the event is safe until total event collaspe. Amy, River, The Doctor and Rory are safe. Rory's death creates a minor paradox that allows him to be erased. Anything close to Earth is safer than anything further away in the universe. The more complex and powerful the lifeform the safer it is until it's closer to the event's happening. Also the Pandorica opening seems to be another point of event collapse. Because it's not a linear event it is destorying reality space and time it is not the easiest of concepts to explain everything without writing a complex paper but I believe this explanation makes for a good in-universe answer to the question.

Answer 5

Here is all we end up learning the cracks in time (at the end I'll summarize what it all means):

• In The Vampires in Venice the Saturnyn queen tells The Doctor about how the cracks in time started to open and "through some we saw other worlds, and through some silence, emptiness and the end of all things."

• In Flesh and Stone The Doctor says "it would take a complex space-time event to seal the crack" and once all the Angles get fed into it, it seals.

• In Flesh and Stone The Doctor also tells Amy "That since you're a time-traveller now you won't perceive time other people." This is a rule that applies a lot throughout the "Whoniverse", laid out plainly the rule is: What you experience as a time traveler you remember, even if it never really happened as far as the current time-line/universe is concerned. (Another example of this: The Doctor's rant in A Good Man Goes To War where he ends up figuring out that technically River was conceived on Amy and Rory's wedding night.)

• Throughout the whole season The Doctor keeps hinting to Amy that:

  1) Anything that is remembered can be brought back.

  2) She is special because she grew up with a crack in her wall.

My conclusion from all this (and the bunch of stuff that was too subtle and boring to go into in detail) is:

1. The cracks were caused by The Doctor's TARDIS exploding.
2. Since the TARDIS travels in space-time when it blew up the explosion effected time.
3. This explosion, like any other, went in all directions (not just forward but also into the time before the explosion itself happened).
4. Instead of fire and smoke this explosion caused cracks.

Here is what I know about the cracks (now that we know what they were and where they came from):

1. The cracks first appear and swallow all that they can.
2. The more complex a space-time event they swallow the quicker they are to settle.
3. If you touch a crack you get sucked in.
4. Not all cracks lead to nowhere, however traveling through any crack is dangerous and not every species can survive.
5. The cracks eat away at the more central parts of a persons life.

What those things mean to the story:

1. This is why they swallowed everyone who got swallowed.
2. Which is why the Angles caused the crack to close.
3. That is why The Doctor uses a handkerchief when he reaches in to the crack.
4. This is how Prisoner Zero escaped, and why the Vampires in Venice needed the girls to become brides (because all their girls died). It also explains why the Atraxi chose to track The Doctor instead of jumping through the crack.
5. This is why Amy forgot Rory but not the Clerics/Soldiers.

And Never for get this rule: What you experience as a time-traveler you remember, even if it never really happened as far as the current time-line/universe is concerned.

(There is always the short answer which is, “artistic license”, but Steven Moffat [current head writer for Doctor Who] tends to work out his plots pretty well throughout all his work.)