In a week of Google searches that I'm hoping what will not result in a rendition flight, I've been researching the protection of the president after reading an odd news story about some WWII era war planes being escorted out of the skies by F-16 fighter jets.
When the president travels around the country he is protected by temporary flight restrictions that clear the skies above him. In the case of his visit to Emmitsburg, MD last week, the president was afforded a zone of 30 nautical miles with a ceiling of 18000 feet above ground, the height at which commercial jets are allowed to fly. This invisible line in the sky forms the uppermost boundary of a multi-layered security "bubble," the crowning achievement of the US Secret Service (USSS).
Expanding in both time and space, this bubble around the president is meant to provide security by preemptively removing threats and making undetected attacks difficult. Using sensor and surveillance technology in advance of the president's arrival, teams establish a baseline of security and then maintain observation to ensure that this baseline is sustained. Once hidden threats (buried, camouflaged) have been removed, protection is a problem of geometry.
Insofar as present threats must achieve at least an approximate line of sight to issue an attack, a combination of restrictive security and active deterrents make this geometrically impossible. The outermost layers of protection such as flight restrictions and traffic routing operate restrictively in a passive manner: they keep things at a non-threatening distance and are enforced only when breeched. Active deterrence is provided by armed agents who may pre-empt attackers trying to gain geometrical access to the president while also acting as a clear signifier of a larger security apparatus. That Secret Service figures are enshrined as part of the White House at Lego Land is proof enough that the bubble has fused with the thing it is protecting.
We cannot escape bubbles. That the material reality of the USSS' protection plan- the accumulation of people, weapons, barriers, etc- is so distant from any real, literal bubble except at the most local scale is quite an accomplishment of design. By operating in a mode of active vigilance the bubble proposes a radical (if expensive) alternative to the protection of a passive defense. By mobilizing substantial effort, the USSS applies intelligence (of both meanings) to create a zone of protection that is still highly transparent. This is no fortress. It's not even a Pope Mobile.
Applying the bubble model to a building requires a move away from the default safety of a defensive fortress. The line of defense must be thickened and activated, allowed to react to threats as they develop and, importantly, retreat when they have been mitigated. This implies a system of variability along the lines of a WWI-era trench where the 'ownership' of a specific trench moved back and forth between sides as these lines were captured, lost, and re-captured. Protected by manned guns, these trenches represent a commitment of both budget line-item and constant human attention. If there is hope in the trench it's that someday it may be filled in and forgotten.