What will be the physiological and behavioral effect on humans if there is a change in the gravitational field on earth?

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    $\begingroup$ Welcome to Stack Exchange. Your question is currently impossible to answer, because you haven't described what kind of change you're thinking about. Stronger? Weaker? A change in direction? Some idea of why it has changed would also help to produce good answers. You can edit your question to clarify these points. $\endgroup$ – John Dallman Oct 5 '16 at 4:50
  • $\begingroup$ I am not sure whether it is strong or weak and the direction. So ,it is a general question $\endgroup$ – RABINDRANATH Oct 5 '16 at 4:53
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    $\begingroup$ Unfortunately, the answer depends on the details. If Earth's gravity gets 100x stronger, humans will all die immediately, crushed by their own weight. If it gets 0.01% weaker, there will be very little effect. You see the problem? $\endgroup$ – John Dallman Oct 5 '16 at 4:56
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    $\begingroup$ Hello, Rabindranath, please be specific about the change. As John Dallman said the effect on humans depends on the change in gravity. Some changes will have big effects, others none at all. Here questions need to be precise enough to answer. Questions that are too broad are either put on old or closed. Your participation is welcome. Please help us help you. $\endgroup$ – a4android Oct 5 '16 at 5:38
  • $\begingroup$ Any general answer is bound to be incomplete. Have you seen the other questions around the topic? They should give some context to start with. $\endgroup$ – Theraot Oct 5 '16 at 9:44

I need to go to bed, so this is going to be short and probably miss some stuff.

Low Gravity

If the gravity on Earth goes to zero, a tiny jump sends us all into space and we all die. Even if we're tethered down, all the atmosphere runs away into space and we all die. The only hope is to make little bubbles of atmosphere with green vegetation and animals and whatever. In this case, you're basically looking at a generational ship with a giant asteroid in tow.

If gravity is near zero, we have "microgravity". Humans are designed for standing up in a 1g environment, so we start to atrophy if we don't take special precautions. We end up with weak muscles, which don't really matter unless someone turns the gravity back on. We also end up with a lot of bone loss which might kill us. This article covers a lot of cool stuff here.

Of note, we have no idea if humans could survive indefinitely in microgravity. So we might have to invent new technologies pretty fast. Like centrifuges where you have to "stand up" against the edge of the wall several hours a day to not die. It would likely be doable, but we don't know for sure.

Normal-ish Gravity

As you get more and more gravity, things will become more and more normal. At 1g (normal Earth gravity), you obviously won't notice a difference. At, say, 0.8g, you'd get some bone/muscle loss, but not nearly as bad as at 0.0001g or something.

Also, the closer we get to normal gravity, the thicker our atmosphere is. There's going to be some critical point where the air is just thick enough to breath normally. This Physics question suggests we'd need the escape velocity to be about 6 times the gas's mean speed. That answer says 0.51 $km\over s$ is about the highest mean speed, so we'd need an escape velocity of 3.1 $km\over s$ or so.

From here, escape velocity is given by:

$v_e = \sqrt{2GM\over r}$

$v_e^2 = {2GM\over r}$

${v_e^2\cdot r\over 2G}=M$

Plugging this into WolframAlpha, we get:

${(3.1 {km\over s})^2\cdot(\text{Earth radius})\over 2\cdot G}=0.077\cdot (\text{Earth mass})$

Surface acceleration is directly proportional to mass (all else being equal, which they are here), which tells us that about 0.077g could keep an atmosphere, just barely.

If we have an atmosphere, we have plant life, and at least some animals will survive. Some species will die out, but Earth will survive, and humans will adapt.

Ideally though, we want breathable atmosphere. According to this Biology question, we need about 0.35 atm pressure to survive with oxygen masks. From the same question, we need about 0.12 atm to survive with 100% oxygen. From this PhysicsForum thread, it looks like minimum pressure at 20% oxygen (normal) is about 37.6 kPa, compared to normal pressure of 101 kPa, which is about 0.37 atm.

From this Wikipedia article, pressure is proportional to the weight of the air. This is proportional to gravity. So to keep 0.37 atm, we need at least 0.37g gravity (roughly). Realistically, the oxygen content will change, so the required gravity will change, but that's way past my expertise limits.

High Gravity

At really high gravity, like 100g, we'll all be crushed by our own weight, and either suffocate because we can't open our lungs, or outright go splat just from falling to the ground. Insects will survive better, but there's some point where nothing living survives. Even bacteria die at some point.

Between 1g at splatg gravity, there will be problems. At mildly higher gravity, we'll just get stronger to compensate, but we'll notice nobody, even athletes, can go as far or as fast as they used to because more energy is used just existing.

As it gets even higher, we'll start to develop health problems. Bones will deform earlier, and bone-related problems will come up at earlier ages. Joint pain, muscle problems, etc. will all have similar results. Likewise, our hearts will be working overtime and we'll have much higher rates of heart failure. Other organs will suffer, but I don't know how much.

Eventually, the force will be enough to cause mental issues from our brains crushing against themselves. No idea if that's before or after we suffocate from collapsed lungs.

Answers from this Worldbuilding question suggest that 3g forces will likely be around the limit for survival, and that's just the fittest. There's no way of knowing the long term effects of gravity on a population, but "nature finds a way" probably applies here as long as we survive long enough to birth children.

  • $\begingroup$ A note on your microgravity points. The ISS is a microgravity environment that's been permanently inhabited for 16 years, with examples of astronauts living there in excess of one year. Astronauts exercise for two or three hours a day, but that's mostly in order to survive the return trip to normal gravity. The effect of microgravity on the human body is actually a fairly well-documented (though still thoroughly active) area of research. $\endgroup$ – ktyldev Oct 5 '16 at 16:26

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