jloughry

That exists: look for 'trusted path'. It was a feature of compartmented mode workstation (CMW) operating systems like Trusted Solaris and lives on in the requirement to use Ctrl-Alt-Del to call up the Windows login prompt. In Trusted Solaris (TSOL) it was a dedicated area of the screen—along the bottom—where no user mode process was allowed to write; the OS displayed a special symbol there (sort of like the padlock in a web browser) when the user was interacting directly with the OS. Some CMW systems even implemented that functionality in hardware, electronically compositing windows from different physical frame buffers to the video display. Ctrl-Alt-Del is actually in hardware, too (or it used to be); the keyboard interface on the first IBM PC detected that specific key combination and toggled the reset line on the CPU (or maybe it was an interrupt; I forget). Every subsequent PC-compatible machine, to this day, has the same functionality built in to the hardware, on the A20 reset line. It's mostly vestigial today.

Also, you don't want to breathe it; xenon is an anesthetic gas and will put you to sleep.

This is one global-scale geoengineering project I can rally get behind. It has the smallest chance of catastrophic second-order effects of any of the several methods I've heard of.

Mother's maiden name is documented to have been in use for authentication at least as far back as 1882, and even then it was known to be a weakness [1, §3].

[1] Stephen M. Bellovin. 'Frank Miller: Inventor of the One-Time Pad'. Cryptologia 35(3), pp. 203–222, 2011. DOI: 10.1080/01611194.2011.583711

It could be a 12V AC adapter. Not too common, but they do exist. Four of the myriad diodes would be connected in a bridge rectifier to convert the AC to DC. (I don't see any smoothing capacitors though....)

Depending on how creative you wanted to get, a clock might be designed to run directly on a pulsating rectified DC supply. That would give you a sort of baseband embedded clock signal at 120 Hz. Or use two half-wave rectifiers and design the whole clock based on trinary logic, +12V/0/-12V.

Realistically, since the mains connection is hidden behind the frame, it would be easy to tap off internally a connection to the 12V AC signal from the secondary of the transformer, before it gets rectified and filtered to DC.

Build one near the Equator.

Yes:

Interestingly, the audio chipsets in modern motherboards and sound cards include an option to change the function of an audio port at the software level, a type of audio port programming sometimes referred to as ’jack retasking’. This option is available on most audio chipsets (e.g., Realtek’s audio chipsets) integrated into PC motherboards today. Jack retasking, although documented in the technical specifications, is not well-known [34]. For an in-depth technical discussion on malicious retasking of an audio jack, from the hardware to the operating system level, we refer the interested reader to the following previous work [29].

References:

Mordechai Guri and Yosef Solwicz and Andrey Daidakulov and Yuval Elovici. 'MOSQUITO: Covert Ultrasonic Transmissions between Two Air-Gapped Computers using Speaker-to-Speaker Communication'. arXiv preprint 1803.03422v1 [cs.CR], 9th March 2018.

Mordechai Guri and Yosef Solewicz and Andrey Daidakulov and Yuval Elovici. 'Speake(a)r: Turn speakers to microphones for fun and profit'. 11th USENIX Workshop on Offensive Technologies (WOOT 17). USENIX Association, 2017.

Lightsail. You carry a laser and a lightsail. You shoot the laser at the black hole and the laser light gravitationally slingshots around the black hole, returning to you with more energy than you gave the laser. The returning laser light pushes your lightsail. The black hole needs to be moving for this to work, although it doesn't really matter what direction the black hole is moving.

From The Martian by Andy Weir:

"They say once you grow crops somewhere, you have officially 'colonized' it. So technically, I colonized Mars."

The laser doesn't illuminate the road directly; it excites a phosphor puck that lights up white and is then focused and aimed like a regular incandescent filament or gas discharge arc. Just like white LEDs, only much more efficient.

TIL tetrode has a special meaning in biology: a four-conductor microelectrode for probing neurons that gives essentially a direction reading along with amplitudes from each of the four electrode components.