In mainline continuity, is it possible for a Green Lantern ring to be removed without the wearer's consent (e.g. by slight of hand or brute force)?
The reason I bring this up is that an odd situation of this occurred in All Star Batman and Robin, the Boy Wonder #9 (infamously remembered as the issue where Batman and Robin painted their bodies and house yellow). Robin manages to steal Hal Jordan's Green Lantern ring without him realizing it. This seems a bit strange to me, as I would have figured that this would be a pretty big weakness for a Green Lantern to be able to have their ring removed so easily, much less without the wielder even being aware that it was removed.
Since the series is outside of the normal DC Universe continuity, I'm not sure if this is something that was invented for the series or if it is an established weakness. Are there instances in mainline continuity in which the Green Lantern ring has been removed without the wearer's consent? Or is this something that was unique to the All Star series?
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It would depend on the Green Lantern, their state of mind, the environment the Lantern was in and whether the power ring had been instructed to maintain it's physical connection to the wearer.
Power Ring Removal
We have seen Green Lanterns rendered unconscious in safe environments and their rings have been removed allowing them to be taken hostage.
We have seen slight of hand remove a ring from an active Green Lantern due to a lapse in concentration. The ring may not have been instructed to be unable to be removed, though this IS within its programmed abilities.
When the power rings were vulnerable to anything that was the color yellow (due to what was, at the time, designated a necessary impurity in the ring's design) it was possible for a person completely covered in yellow to remove the ring as the ring was powerless to resist anything coated completely in yellow.
Considered, arguably, the most powerful weapon in the Universe
The Oan power ring has numerous powers and abilities in addition to the skills, training and species knowledge of its wearer. Power rings are considered one of the DC Universe's most power weapon/technologies in its mainstream universe. Created by the Guardians and energized by the all-powerful Oan Power Battery, they are capable of creating constructs of light or sound capable of interacting with the physical universe with devastating results.
Unfortunately, power rings are rarely utilized as effectively as they could be by their users, even after a considerable amount of training.
Power rings are semi-autonomous computational artificial intelligences, capable of independent action, depending on the user parameters of the ring bearer.
When a ring takes a new user, some of these capacities are explained by the ring but most new users will experiment with the will-power construct aspects of the ring, completely ignoring the very capable computer interface. This tends to be remedied when they are inducted into training on Oa and the full capacity of the ring are explained in greater detail.
Most Green Lanterns establish parameters with their ring after they have received training on Oa. Deeper programming of the rings can only be done by the Guardians. These parameters included:
Whether to use the active voice control of the ring's inner database or to rely on the willpower and innate knowledge of the user.
Safety defaults, such as what to do when encountering hostile environments; rings are designed to automatically protect users from any environment which may be hostile or life-threatening.
Safety and medical procedures in case of unconsciousness or near-fatal injuries.
In environments hostile to the wearer of a Green Lantern ring, the ring is preprogrammed to sustain the life support necessary to keep the wearer alive. This overrides any attempt to remove the ring even by the wearer. The wearer is forced to override the ring to take it off in a hostile environment such as deep space.
Removing a power ring from a Green Lantern does not remove control of the ring. Within a certain distance (up to 20 light years) the ring can act as if it were still physically in contact with the user.
It is possible to remove a ring from a wearer who is not concentrating. When the yellow impurity limitation was removed, the rings were even more strongly linked to the willpower of the user. This means if a ring wearer's mind is distracted, manipulated or if the wearer is physically disabled, the ring's efficacy is reduced and can even be taken or destroyed if the force is great enough.
After winning the fight, Lobo had the idea of taking Chance's power ring before he left 'Hellhole', however, the ring could not be removed from its wielder's finger. Consequently, Lobo took the whole finger. As Lobo walked off, the ring informed him that, due to the specific deal the Guardians had made with Chance, it would not work beyond the parameters of Garnet's atmosphere. Disappointed, Lobo tossed the finger and ring, and left the planet. The ring and finger then made their way back to Chance and reattached themselves.