# What would be the effects on a planet if gravity turned off and on again? [duplicate]

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Gravity mostly works as normal, but it stops affecting the world for up to a few minutes at a time at least once every other day. There is a human population, but they don't have the technology to leave the world. They are stuck here. Are there any significant complications they would need to solve other than keeping themselves and their things stuck to the ground when the gravity turns off?

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• "Turning off" fundamental forces is tricky in a science-based world. It kinda doesn't work that way, so you would need to define precisely how this effect happens. If I may offer a phrasing, since you say they can't leave the world, we can say that we are modeling the force of gravity as a constant (g) and thus gravitational potential energy is (mgh), like we learn in high school. You're saying that, at some point in the day, g is set to 0, and all of the gravitational potential energy is magically transferred "elsewhere?" Then, a few minutes later, – Cort Ammon Mar 30 '17 at 22:07
• Perhaps whatever messed up the gravity would be impossible and therefore viewed as magical, but the aftereffects would not be and that is what I am looking for. – Azohn Mar 30 '17 at 22:08
• g is set back to normal, and every object is provided an amount of gravitational potential energy corresponding to where the object was when gravity switched back on? I phrase it to point out that there's a huge energy gap in the system, and it will be abused for free unlimited energy by any human population. – Cort Ammon Mar 30 '17 at 22:08
• Would the force of the spinning planet without gravity eject the oceans into space? – Twelfth Mar 30 '17 at 22:15
• @Twelfth , not so much. They would be propelled outwards at around 3 cm/s^2 (which in three minutes builds up to about 480 m as Schwern noted), then they would fall down again as mega-tsunamis. Something similar would also happen to tectonic plates and all the nice red glowing stuff that's beneath. This gives a new meaning to the phrase "a world of hurt". – LSerni Mar 30 '17 at 23:57

## 4 Answers

Forget the puny humans, the whole planet is in trouble, assuming it's spinning, which just about every heavenly body does.

The material at the surface of the Earth's equator is going at 465 m/s at a radius of 6378 km from the center (at 45N or 45S it's 329 m/s: cos(deg of latitude) * 465 m/s). That's like being spun at the end of a 6378 km tether very fast. When gravity is turned off, that tether is cut and everything goes whizzing out into space at 465 m/s tangent to the surface of the Earth. People. Cars. Buildings. Rocks. Dirt. Water. Air. Everything. See you, space cowboy. (This is why we launch rockets as near the equator as possible, you get a free velocity boost)

Let's say "a few minutes" is 3 minutes or 180 seconds. At 465 m/s that's about 84 km (no air drag, the air is moving with you). That means all the stuff on the surface of the Earth flies along in a straight line (not straight relative to the curved surface, actually straight) for 84 km.

If you want to see what this effect looks like, take a ball, wet it, and spin it; though in this case the tendrils don't get very far relative to the size of the ball/planet.

This doesn't mean you'll be 84 km up from where the surface was, far less. We can figure that out with the equation to draw a circle:

$$x^2 + y^2 = r^2$$

You start with $r = 6378 km, x = 0, y = r$. After 84 km of flight now we have $x = 84 km, r = 6378 km$. Solve for y...

$$y^2 = r^2 - x^2$$ $$y = \sqrt{r^2 - x^2}$$ $$y = \sqrt{6378^2 - 84^2}$$ $$y = 6377.45 km$$

Then the distance up is $r - y$ which is 0.55 km. It's so small because 84 km is so much smaller than the 6378 km radius of the Earth, a bit over 1/100th. At this scale the Earth is almost flat.

Everything is "only" about 500 meters "up" when gravity is switched back on. It all comes crashing back down at 9.8 m/s2 taking about 10 seconds. I don't think I have to do the math on that, you can imagine the carnage. Needless to say, every macroscopic organism dies, and everything is rubble. On the surface. Underground. Everything.

Having all that stuff suddenly be 500 meters up when gravity turns back on also introduces a tremendous amount of gravitational potential energy which turns into kinetic energy (ie. falling), and then heat as it crashes down. This will start to heat up the Earth and over time return it to a molten state it hasn't seen since it formed.

BTW If you could turn gravity on and off, this would produce a perpetual motion machine.

• Unless you have cavorite... – L.Dutch - Reinstate Monica Mar 31 '17 at 6:47
• Is there a version which is acceptable? Say .5 gravity, curving off and on (as opposed to abrupt)? – Martijn Mar 31 '17 at 7:26
• @Martijn The magnitude of the effect is a combination of angular velocity and gravity. The slower the planet's spin, and the smaller its radius, the slower everything will be flung, and the shorter it will have to fall. The lighter the gravity, the softer it falls. But it's all bad. Imagine dropping a house, its foundation, all the city's plumbing and utilities from 100 meters up? 10 meters? 1 meter? It's all bad. None of this changes @LSerni's answer, so the effects are going to be very bad regardless. – Schwern Mar 31 '17 at 17:25

# Fastest killer: Hyper-Quakes

The whole of the planet is continuously subjected to a strong compressive force due to its mass.

Should gravity compression abruptly cease, the inner layers of the planet would be subjected to an expansion that, while slight in proportion (the molten core of a planet is next to incompressible), would wreak havoc on the surface.

Specifically, most of the matter in the planet's mantle is compressed to a density at least 5% greater than normal. Think of it as an omnidirectional spring, kept compressed by gravity. Then gravity turns off, and the spring is released; the whole volume expands by at least 5% and does so at the speed of sound in the material, which is in the order of kilometers per second.

So after very few seconds, before people has had the time of floating off too far due to centrifugal force or a chance to asphyxiate (see below), the whole crust of the planet explodes outwards; some places faster, some slower, causing the tectonic plates to rip open and start spewing molten lava, setting up for a subsequent nuclear winter. The real killer, though, is the piston effect of the surface itself, equivalent to a crash against the floor at a speed in excess of twenty-two hundred miles per hour (it is actually the floor coming to get you at around 1000 m/s, depending on the speed of sound in the local type of ground). The whole planet turns into an enormous captive bolt gun, and the entire population is slaughtered.

It does not need to travel upwards too much (even a 1% would probably be enough to kill everything up to the stratosphere).

# Second place: Asphyxiation

At the same time, the atmosphere is kept dense enough to breathe by gravity too. Turn off gravity somehow, and the whole lower atmosphere expands abruptly (as soon as the enormous compression seen above relents. It will probably bounce back, amplifying the decompression effect). This would also imply that it will cool very, very rapidly. Also, it will do so asymmetrically, and on that scale this means hurricanes of terrifying force (for a few minutes at least, until the air is too thin to breathe or support a hurricane). But by the time the air is too thin to breathe, all life-forms larger than an insect have been smashed.

• I don't think that it would be too abrupt. There's just too much of it. The lower earth/air/water would have to overcome the inertia of what's above it first. – ShadoCat Mar 30 '17 at 22:09
• which it could easily do, since that material itself is also going to move outwards, You are more likely to see the planet explode like the mother of all grenades. I mean the earth is under so much compressive force from gravity that it most of the planet behaves like a plastic and a large portion has liquified. It would be like instantly disappearing the the metal tank from a tank of compressed gas. – John Mar 30 '17 at 22:28
• Everyone Dies ™ – Hugh Nolan Mar 30 '17 at 22:32
• The answer I came here to write. +1. – ApproachingDarknessFish Mar 31 '17 at 1:31
• academic.oup.com/gji/article/84/3/561/599877/… Density at the inner core: 13g/cm^3. That's close to twice what it is at Earth's surface. Turn off the gravity and the core volume close to doubles. – Loren Pechtel Mar 31 '17 at 4:37

### Bye, bye Moon

Each time this happens, everything in orbit moves farther away than its immediate previous orbit. The orbits probably won't be circular, but both the highest point and the average distance would tend to increase. So either the Moon would fall out of orbit (because the lowest point dipped into the atmosphere) or it would eventually fly off.

The nature of orbits is that they happen when gravity and velocity are balanced. Take away the gravity, and the object will move to a place where that velocity won't be balanced by the restored gravity.

Same thing with satellites. In particular, geosynchronous satellites will no longer be geosynchronous. Over time, that will kill GPS, Google Maps, cable television, and other things that use satellites.

### Bye, bye atmosphere

Each time this happens, some of the atmosphere will escape. That actually happens now, but slowly. This will make it happen faster during and just after the event.

Lower atmospheric pressure will lead to more water evaporation. So the oceans are also in trouble. Some of that water vapor will migrate higher in the atmosphere over time and escape.

This could still take a long time to have an impact. It makes this happen faster, not instantaneously.

### Bye, bye crops

Currently waves stop at the ocean's edge. In this situation, instead of following the Earth's curvature, they'd travel in a straight line above "sea level". So a great deal of water would end up inland and some would flow further inland after it came down. But this isn't fresh rain water. This is salt water (and fish and stuff). So it would tend to kill off the plants on which it landed. And cause weird flooding. Iowa corn would be fine, but Florida oranges would be in trouble.

A lot of people live in coastal areas, so the population impact would be larger than the geographic impact.

Rivers would also flow out of their channels. Some of the water would eventually make its way back. But some would end up soaking into the ground or evaporating. This could cause trouble in areas that rely on rivers for their water supply. Less drinking water and irrigation water. And of course the rivers might stop being navigable. New channels might get cut to support the new flow patterns.

Lakes would also be affected but probably less so that oceans and rivers. Less motion means that they won't move as far. They'll probably expand and crash a bit larger than the lake. More of the water will flow back.

### Bye, bye zebras

Livestock will be problematic. How do you tell them to move inside where they can be strapped down? How many will break legs anyway when the gravity returns?

Wild animals are even worse. Most animals that don't climb, burrow, fly, or perhaps swim will be extinct outside zoos rather quickly.

### Hello heat

When things crash back to ground, they will turn the potential energy they gained into heat. That will reinforce the atmospheric losses, the loss of water in the oceans and rivers, and plant deaths.

### Hello volcanoes

During the event, this will relieve pressure on the continental plates. They'll drift up. When gravity returns, they will fall down again. Just not quite in the same places. Volcanic activity, earthquakes, etc. On the bright side, volcanic activity has a cooling effect as less light reaches the surface.

### Hello shortages

If things can't be transported (because they may fly off into space), then there will a lot of shortages. Sure, we could change trains to grip the rails (particularly a monorail), but how long will that take? Most freight transport is by a combination of ships, trains, and trucks. None of that works well.

### Period matters

If this happens regularly, say three minutes every third day, then it will affect things differently than if it happens randomly. Random action can accentuate other effects. For example, three minutes off followed by three minutes of gravity followed by three minutes off. Or if it's usually from zero to five minutes but one day is a whole hour.

Well, no one would fly off into space. Gravity wouldn't be off long enough but there might be injuries from short falls.

Air loss would be a factor since gravity is what holds it to the planet. Granted that there's a lot of air but when gravity turned back on, some small amount will have gotten too far away to fall back down.

Satellites would have a problem. Every time gravity shut off, they'd head off in a tangent. Gravity would soon start pulling them back but without thrust to correct it their orbit would get less circular each time making their position hard to track and limiting their lifetime before they, likely, get close enough to the planet to catch enough air to slow them down causing them to spiral in.

• Actually the satellites are gaining potential energy if they started in a circular orbit. They would fly out of earths gravity well. (barring resonant effects) – Donald Hobson Mar 30 '17 at 22:28
• Not necessarily, Unless they spend thrust to circularize their orbits, their orbits will become more and more oblong. Yes, the farthest point becomes farther out but the nearest point can get closer. To move to an outer orbit, you must thrust to speed up (to move out), thrust to slow down (to stop moving out), and thrust to circularize the orbit. Only the first two are covered by gravity turning off then on. – ShadoCat Mar 30 '17 at 22:35
• You'd need "off" periods of well under a second for there to be merely "injuries from short falls". One second of "off" on a spinning Earth corresponds to about a fall of about 2.5 meters. – Mark Mar 31 '17 at 6:06
• @ShadoCat their orbits will become more and more oblong .... Uh, were you going for "oval" there? – Grimm The Opiner Mar 31 '17 at 7:06
• @GrimmTheOpiner, Actually, I my brain glitched on the word: elliptical. – ShadoCat Mar 31 '17 at 17:05