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I have an idea about a story where gravity suddenly changed and became about 10-20% weaker (lower). Humanity now needs to adapt to the new situation.

What disasters and catastrophes would this sudden change cause on Earth?

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closed as too broad by dot_Sp0T, Ash, Gryphon - Reinstate Monica, RonJohn, StephenG Sep 2 '18 at 16:48

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  • $\begingroup$ None that we would be aware of; since we would all be dead. $\endgroup$ – steverino Sep 2 '18 at 14:14
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Gravity, according to the well known general formula given by Newton, is given by

$ g = const \times $$m \over distance^2$

Therefore a sudden decrease in the gravitational attraction produced by Earth can only be due to either:

  • sudden decrease of its mass
  • sudden increase of the distance from its center of mass

The first one means a big chunk of Earth is gone all of a sudden, while the second one means a big chunk of (light) material is added all of a sudden.

In both cases that is a disaster on its own: it means that some massive object struck Earth and got either splatted on it or broke it apart.

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  • $\begingroup$ How? Can't it just reduce without any explanation? $\endgroup$ – SovereignSun Sep 2 '18 at 13:07
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    $\begingroup$ @SovereignSun, no if you want to stay within the realm of physics. $\endgroup$ – L.Dutch - Reinstate Monica Sep 2 '18 at 13:16
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    $\begingroup$ @SovereignSun - this question has the 'Reality-Check' tag. This answer is appropriate (excellent) for that tag. If you wish for answers that ignore the cause, remove the tag. $\endgroup$ – user535733 Sep 2 '18 at 13:23
  • $\begingroup$ Or, under really strange conditions if $G$ is changed. But that would be total collapse of everything existing. $\endgroup$ – rus9384 Sep 2 '18 at 18:37
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I just want to expand the answer of @L.Dutch.

$$g = G_{constant} \times \frac{mass}{radius^2}$$

Which in Earth is:

$$(6.6740831 \times 10^{-11} \times \frac{N \times m^2}{kg^2}) \times \frac{5,9736 \times 10^{24}kg}{6,378,100\ m^2} = 9.8004\ m/s^2$$

To reduce gravity decreasing mass, we will need this equation:

$$G \times \frac{(5.9736-m_{loss}) \times 10^{24}\ kg}{6,378,100\ m^2} = 9.8004 \ m/s^2 - (1-g_{\text{loss%}})$$

  • To reduce gravity to 8.82 m/s2 (a 10% less) our parameter will be $g_{\text{loss%}} = 0.1$.
    Using Wolframalpha equation solver our mass loss must be $m_{loss} = 0.597376 \times 10^{24}\ kg$.
    That is around 8 times the weight of the Moon.
    Which is the same as $549.62\ kg/m^3$ of density loss.
  • To reduce gravity to 7,84 m/s2 (a 20% less) our parameter will be $g_{\text{loss%}} = 0.2$.
    Using Wolframalpha equation solver our mass loss must be $m_{loss} = 1.19473 \times 10^{24}\ kg$.
    That is around 16 times the weight of the Moon.
    Which is the same as $1,099.26\ kg/m^3$ of density loss.

To reduce gravity increasing radius, we will need this equation:

$$G \times \frac{5.9736 \times 10^{24}\ kg}{(6,378,100 + r_{gain})\ m^2} = 9.8004 \ m/s^2 - (1-g_{\text{loss%}})$$

  • To reduce gravity to 8.82 m/s2 (a 10% less) our parameter will be $g_{\text{loss%}} = 0.1$.
    Using Wolframalpha equation solver our radius loss must be $r_{loss} = 345,018\ m$.
    Equivalent to $53,810,219\ km^2$ of surface, which is a bit more than America.
  • To reduce gravity to 7,84 m/s2 (a 10% less) our parameter will be $g_{\text{loss%}} = 0.2$.
    Using Wolframalpha equation solver our radius loss must be $r_{loss} = 752,843\ m$.
    Equivalent to $113,557,808\ km^2$ of surface, which is a bit more than 2 Americas.

Both scenarios are catastrophes:

  • A sudden loss of mass could mean:
    • A big chunk of Earth has been "vaporized" or flew away from Earth. That mass isn't enough for a whole continent, but an important part... millions of persons who lives there will die. Also, the mass loss is heavier than the mass of all the water and air of the world, so drying the whole oceans and vacuum the atmosphere isn't enough. This may cause massive earthquakes.
    • A density of Earth has dropped an important amount. That means all the heavy elements of Earth has disappeared to reduce density: iron, lead, uranium, silver, gold, copper, etc. We will have a lack of minerals.
  • A sudden increase in size will also increase the surface. That mean:
    • The space between all continents increases. All the cables underwater (you know that internet use cables over the oceans, right?) will break apart. The distance between continents increases as flight and travel time does. The water level lowers because the available space increase.
    • The size of the continent increases. All the houses, roads, cities, maybe even mountains break apart. The land will break apart (new rivers and lakes?).
  • A new continental and tectonic plate raise.
  • Obviously, all of this causes massive earthquakes that can erase Quote needed all the life on Earth
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  • $\begingroup$ I think "massive earthquakes" might even be an understatement. Increasing the sizes of the continents and the oceans between them could be done by expanding the plates, which would put massive stress on them, or by just moving them apart and leaving gaps between them which would be quickly filled by upwelling magma. Either way, it sounds more like a new Hadean Eon than like just a Michael Bay movie. $\endgroup$ – abarnert Sep 2 '18 at 16:52

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