I have a city on a 3km-high mountain-top with a population of perhaps 50,000 humans, at a technological level of late renaissance or early steam. Below the city is a coniferous forest with wildlife typical to such forests in Europe, and the base of the mountain is surrounded by a medieval-style curtain wall.

This city has something of a night-life that might be considered unusual for a city in this time-period, but midnight is considered the latest that those not on shift-work might want to stay awake (the city gets a lot of visitors who must leave by midnight or be trapped in the city at least until dawn, which will be no less than 6 hours later, and typically must then make their way home over a considerably greater distance than that travelled to reach the city). Most visitors are typically out of the city by 11-11:30 PM, as they must travel to the base of the mountain in order to leave, and most businesses will close during this time.

For magical reasons, a couple of minutes after midnight on one particular night, the mountain unexpectedly goes into zero-gravity for a period of one minute. For the same aforementioned magical reasons, this is not a localised nullification of gravity, nor does the city move relative to the surrounding air, so there are no atmospheric effects that might otherwise occur if gravity was neutralised - think of this as a city in a bubble being dropped in a vacuum.

The city, its mountain and the surrounding air magically moves from place to place, departing at midnight and arriving at its new location at dawn, and this movement takes the observable form of being lowered into the ground and being raised up in a different place. Normally this does not cause any significant change in local gravity. However, on this rare occasion, one of the individuals who has power over the movement of the mountain delays its departure by a minute, and when the mountain is free to move on, it drops in an event similar to freefall (though into a fourth physical/magical dimension) until it catches up with its intended position in the magical dimension, thus causing this event.

At the start of the event, gravity effectively drops from 1g to 0g over the course of 2-3 seconds, and at the end of the event, about one minute later, gravity ramps back up from 0g to 1g over the same period of time. During this time, there are no vibrations or shocks transmitted through the mountain; the only effect is the reduction of apparent gravity effectively to zero. Since the mountain's surrounding air is magically contained and carried along with the mountain, as if in a dome perhaps 4km high, there will be no loss-of-atmosphere event such as might occur if gravity on a spherical world was locally neutralised.

What would be the effect of this rare event on the city and its inhabitants, as well as the forest and its wildlife be? Would we expect any casualties amongst the human/humanoid population of the city or amongst the wildlife in the forest below?

Despite this event being caused magically, I'm after science-based answers, as few people have magical powers that could affect the situation during this time.

This is a very rare event, having occurred no more than twice in the hundreds of years the mountain has been moving around, and it has not occurred previously in the last hundred years either.

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    $\begingroup$ Strap down your fine china, things are going to get ugly. $\endgroup$ – Xandar The Zenon Apr 10 '16 at 23:19
  • $\begingroup$ @XandarTheZenon, This isn't a regular occurrence. $\endgroup$ – Monty Wild Apr 10 '16 at 23:50
  • $\begingroup$ Please don't use the terms "freefall" and "micro-gravity" interchangeably. $\endgroup$ – iAdjunct Apr 11 '16 at 0:18
  • $\begingroup$ @XandarTheZenon, I've edited the question to provide more detail. The event itself is "rare", and so far, has only happened once or perhaps twice in hundreds of years. It would certainly be unanticipated. $\endgroup$ – Monty Wild Apr 11 '16 at 1:03

Structural issues

As others mentioned, depending on the architecture used and the different stress levels in architecture under gravity there could be some serious implications for some large structures. Archways for example are stable purely through force of gravity - remove this and the stones can float apart (some kinds of masonry techniques do not even use mortar for this).

However, worst case I see some buildings collapsing and major damage to some lager structures like the wall. All in all not too catastrophic.

Your city will burn to the ground.

This is by fare the biggest problem i see. The lack of gravity will also almost surely lead to fires all over the city. Embers and coals will float out of their fireplaces... Candles, lanterns, torches will float away and catch on reed roofing, curtains or other flammable things...

Worst of all, it will be impossible to react to these events: with everybody suddenly floating around in 0-g it will be quite unthinkable that people can offer any kind of reaction to the threat of fire. For most people it will be too much to ask to even stop their lonely candle from setting fire to their living room. Let alone organizing the high-manpower reaction you would need to essentially fight a fire in a city which started in hundreds of places simultaneously.

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    $\begingroup$ A lantern won’t burn in a low gravity since it depends on convection. A candle can burn only if has some airspeed. Moreover, some flammable things won’t catch fire in microgravity as easily as they can in the usual gravity. Massive fires will, likely, start immediately after switching the gravity on, not earlier. $\endgroup$ – Incnis Mrsi May 17 '16 at 11:17
  • $\begingroup$ Interesting point, I did not consider this. Have people done 0-g experiments with candles/lanterns? In any case, I think the started fires will be devastating even if started only after the 0-g phase. $\endgroup$ – fgysin reinstate Monica May 17 '16 at 11:29
  • $\begingroup$ Yes, they did. $\endgroup$ – Incnis Mrsi May 17 '16 at 11:38

@Draco18's answer is almost correct, but he is missing one important thing: only things that are under spring tension will start floating upwards, because when you are in zero gravity, you need to shove to actually get moving upwards.

You also added the condition that your event is gradual, over 2-3 seconds. This is longer than the dampening period of most vehicle suspensions.

So for the most part, things will stay near the ground.

The biggest problem will be vehicles in motion. They suddenly lose all traction. They cannot stop and they cannot turn. Even trains will suffer this since there is nothing keeping them from flipping over; at the first curve, the trains will get their wheels pulled out from under them and go flying along the tangent of the track in a slow tumbling motion. So the biggest, most noticeable, and devastating effect is that any vehicle in motion will careen out of control until it hits something, or someone. Traffic intersections are going to be a huge mess.

Also people that are in the path of this vehicle will not be able to jump out of the way. Any vehicle going up an incline with a peak — like a bridge — will keep going straight ahead after the peak and gain altitude.

All people that are in motion will suffer the same effect. Their first step at zero gravity will also give them a slight upward velocity. So anyone that is walking will keep moving forward and slightly upward until they hit something. People that were standing still or sitting down will lose their footing but for the most part not start floating upwards.


Something that other people haven't talked about yet is the air.

This loss of gravity affects a sphere with a radius of 4km, and you say it magically holds in all of the air (if you didn't, there would be even more problems). But 4km up, the air is at a lower pressure than the air lower down. Normally, this is only possible because of gravity.

When gravity is removed, there will be a bulk rushing of air upwards to equalize the pressure difference. The pressure should drop only ~10%, which would definitely be a pain (ears popping, maybe trouble breathing), but the rushing air would also be a force which causes everything to move upwards.

While other answers propose that you would just start floating gently, I think that most people (and cars, pebbles, animals, etc.) would be sucked upwards.

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    $\begingroup$ The city is at the top of the mountain, 2-3 km high. The dome is 4km high. If anything, the air pressure would increase in the city and decrease in the forest at the foot of the mountain. $\endgroup$ – Monty Wild Apr 11 '16 at 23:59
  • $\begingroup$ @MontyWild Is the dome centred in the city, or underneath it? $\endgroup$ – Lacklub Apr 12 '16 at 12:23
  • $\begingroup$ the dome is centred beneath the mountain peak, at the level of the base of the mountain. Since the mountain is ~3km high, there is around 1km above it before the dome. $\endgroup$ – Monty Wild Apr 12 '16 at 22:00
  • $\begingroup$ @MontyWild So air would still rush up, the pressure just wouldn't change that much for the citizens. $\endgroup$ – Lacklub Apr 13 '16 at 14:35

I see one big problem here but I don't know just how catastrophic it would be:

There will be many things in the city that are held together simply by gravity. Remove gravity and they'll come apart. For small stuff this isn't going to be an issue as there will be a tendency to stick together anyway and there won't be any great force trying to change that.

However, there will be big structures (the wall around the city if nothing else) where there is a lot of mass that has been pressing things down. Remove that mass and there will be a rebound effect--and now things aren't held down. Unless they mortared all the joints in that wall it's suddenly going to lob itself into the air.

I don't know how fast the pieces will be moving so I don't know how far away they'll get when the gravity returns and the come crashing down.

I'm sure there will be many other lesser examples in town, also. In many cases it doesn't take much to do big damage--think about the common design of a house with a crawlspace underneath--offset it a few inches and those supports won't be on the joists anymore.

I also expect many injuries and a few deaths from people who don't know that you must make gentle movements in such conditions--a panic reaction with your legs can easily leave you moving too fast to stop yourself with your arms. Half of people who were jogging or faster outside will die--if they were heading up at the moment gravity turned off they'll wind up way up in the air when it comes back. Likewise anyone who was going up outdoor stairs.

If the city has varied terrain walkers could find themselves in trouble, also--if the ground curves down (or merely less up) in front of them they'll end up in the air also.

You'll also have a serious fire problem: Any uncontained fire burning pieces of fuel (think of your typical wood-burning fireplace) will have an updraft in existence when the gravity turns off--burning bits will be carried into the air. The flames will be extinguished but the bigger bits will retain enough heat to relight--and such cities lack meaningful firefighting capability. If they're not totally made of stone they'll probably burn down.

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    $\begingroup$ The buildings are typically stone, all cemented together and cemented to the bedrock, including the wall around the base of the mountain. Heat is supplied by geothermally-heated water, and generally supplants fireplaces, though enclosed stoves and ovens exist for cooking. Since this takes place at midnight, most people will be in bed if they can help it. $\endgroup$ – Monty Wild Apr 11 '16 at 1:39
  • $\begingroup$ @MontyWild Why did they cement everything together when they made the town? That's not normal construction. $\endgroup$ – Loren Pechtel Apr 11 '16 at 3:50
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    $\begingroup$ It is normal in cold climates (which this is), where any gaps result in heat loss. The stone used is in place of brick, and the stones are no bigger than cinder blocks. $\endgroup$ – Monty Wild Apr 11 '16 at 4:00

Ignoring the potential concerns about a poorly worded question, the answer you're looking for is:

Bad Things.™

Removing 9.8 m/s$^{2}$ of downward acceleration is roughly equivalent to a sudden impulse of 9.8 m/s$^{2}$ of upward acceleration. (Funny thing about vectors: they add together...which means we can subtract one out, too).

Everything not strapped down is going to go flying up, anything that isn't secured in a way to suddenly be moving in the other direction isn't strapped down. The only stuff that won't be moving would be the stuff that was already falling.

Only thing I'm not sure about is how long that virtual acceleration would last (i.e. how quickly stuff will be moving away from the ground). But for sure that building foundations/structural supports will crack under the unexpected stress (most buildings are constructed to support the weight top down, not bottom up!) small objects will leave whatever surface they were resting on, larger objects might possibly float as well (would depend on several factors, including air resistance and mass). Cars would, probably, due to the mechanical resistance of the shocks and tire compression (getting an effective "jump" in the neutral gravity effect as the springs pushed against the ground without gravity pulling back). People would start floating as soon as they tried to walk, if they weren't already.

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    $\begingroup$ Why do you think things will go flying up??? $\endgroup$ – Loren Pechtel Apr 11 '16 at 3:50
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    $\begingroup$ So, you're saying that if I drop a building, there will be stress associated with no longer being under gravity? I don't think that things really work that way. Besides, I specified a ramp-down and ramp-up time over several seconds. $\endgroup$ – Monty Wild Apr 11 '16 at 4:05
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    $\begingroup$ Loren Pechtel : The Earth rotates. If you're not kept on the ground by gravity, this rotation would send you flying up :) So would the oceans, and quite everything. $\endgroup$ – Nico Apr 11 '16 at 8:00

Other answers are noting the effect on buildings, vehicles, and structures - though it is worth noting something about people in motion who find themselves floating away from the ground and out of reach of any handholds. It is possible to propel oneself in zero-g slightly by either A. throwing something (such as one's clothes) or B. blowing air. A swimming motion might also help. If a person is smart and resourceful, they might be able to navigate back to the ground before the gravity turns back on and they go splat.


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