[quote]Prove to me that time travel is dangerous.[/quote] The Weeping Angels "are as old as the universe (or very nearly), but no one really knows where they come from." They have been described as "the deadliest, most powerful, most malevolent life-form ever produced." Weeping Angels are unusual as predators in that they neither kill nor directly parasitize their prey. Their usual mode of feeding is to make use of time paradoxes - with a single touch, a Weeping Angel can send a person into the past to a point before his/her own birth, and can then feed off the "potential energy" of the years which that victim would have lived in the present. The Doctor describes the Angels as "the only psychopaths in the universe to kill you nicely" because their victims are otherwise uninjured and may live out their lifespans in the past. Weeping Angels prefer to take their energy from live victims, but if required they can drain other forms such as that from electric lights (as seen in "Blink") or other electronics. In "Blink," the Angels attempted to steal the Doctor's TARDIS after trapping him in the past. The engine of the vehicle contained enough time energy to feed them forever, but The Doctor stated that the possible damage they could cause "could switch off the sun". So, this species is not only drawn to power sources, but are like moths to a flame when it comes to a TT Type 40, Mark 3 of Gallifreyan origin, powered by a harnessed singularity. Their capture of one could devastate a star system and even rip the fabric of spacetime. That's one example of a lifeform that is a danger for any being/technology capable of time travel. Another example are the Reapers, who detect, seek out and destroy paradoxes as well as the existence any time/matter/energy associated with the paradoxical event. A pretty harsh Quality Assurance program if ever I heard of one.