Extra-dimensional beings are the same size and speed as human (fast walking). They walk in random direction- they have no idea that they came to Earth (accident on their world) and arrived at random locations anywhere over Earth. They are immune to all Earth-based energy or matter effects- all transferred to the other world. They destroy everything they contact on any side. They make an atom thick vacuum around them and are essentially voids. They walk on water and of course go through everything. How many and how long would it take for them to absolutely decimate civilisation- assuming they don't destroy the biosphere? They are bound by gravity and other forces in their own world. Their planet is geophysically and astrophysically the same as Earth- just not hydrophysically- they "apparently" walk on water on their own world- hence it solves all your movement issues. They don't tire because they absorb some energy that passes through them. Imagine one day you are on your world and an accident suddenly spread some of your population all other the place and your world is changed unrecognisable- that is their situation. Humans don't know they are extra-dimensional beings since they just appear and they will seem to be meteorological anomalies expect powerful winds that make observation extremely hard. Answers need to consider the number of being say 1 billion beings.
closed as primarily opinion-based by Mołot, Elmy, Blade Wraith, Alexander von Wernherr, Renan Oct 17 '18 at 11:17
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To the simplified mathmobile!
Total surface area of Earth is about 510000000 $km^2$ (197 million sq mi). Average human take 0,066 square metres. For future calculations we will calculate that this earth surface is rectangular with a width of 25 cm and 2040000000000 metres in length. We ignore everything that is above ground level as we need to vacuum the surface not anything that hang above.
Now, average human speed is 1,4m/s but as people can walk even 2,5 m/s we assume that this alien walks with exactly 2 m/s which give us 283333333,3333333 hours which roughly translate to 32433 years which translate to
welp, that's a nice anomaly, but it's nothing bad so why should we care?
And it's all BEFORE we add his randomness in movement and the requirement for him to cover ALL surface.
If you add two of them you won't lower the amount of years by two as their random movement will result in standing on the same surface thrice or even more. The randomness can be calculated as this "earth stripe" can be divided into cells (6800000000000 to be exact) and then you can calculate given time when all cell will be visited at least once.
The easy method would be to calculate how many those being fit into average hurricane surface, lower the speed of the hurricane to this of walking speed and then calculate how much time it would need to cover all earth, give it +25% for the randomness and then you will have time and amount of aliens needed to destroy things.
You have to keep in mind that if the time is larger than few years the destruction could affect smaller countries or civilization but won't affect countries like US (which by now is visited by hurricanes every years in semirandom pattern)
Once people would realize they are safe under water and in underground bunkers, as well as in the air, they would simply adjust their living conditions to avoid anything permanent on the surface.