Human physiology is adapted for life at 1G, but planets colonised by future humans will likely have different surface gravities.
What is the range of values for surface gravity that humans can comfortably tolerate long-term?
Note: I am not interested in the absolute maximum gravity that a human could live through, but rather the gravity that would be suitable for a permanent colony.
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One way to narrow down the range of acceptable and safe surface gravities would be to read Habitable Planets for Man, Stephen Dole, 1964, 2007.
As I remember, Stephen Dole suggested a maximum safe surface gravity for a planet to be colonized in the 1964 edition, no doubt based on space medicine experiments and tests previously done. I doubt whether any later scientific experiments, tests, and theories have broadened any of Dole's standards for habitability since then, and it is more likely that present day scientists have more restricted standards for habitability.
So if Dole says that Xgs are the upper limit for habitability, it is possible that Xgs is correct, but it is also possible that a lesser number of gs would be the correct higher limit for surface gravity.
Chapter 2 discusses human requirements for habitability. On page 12 it is staid that:
"On the basis of the available data, one might conclude that few people would choose to live on a planet where the surface gravity was higher than 1.25 or 1.50g."
https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf1
And see this question at space exploration stack exchange:
https://space.stackexchange.com/questions/6154/maximum-survivable-long-term-g-forces2
And this one:
https://www.reddit.com/r/TheExpanse/comments/8arlck/how_many_gs_can_the_human_body_sustain_over/3
It is said that:
Human volunteers have tolerated 1.5g for seven days with no apparent ill effects. However, after just twenty-four hours at 2g, evidence of significant fluid imbalance is detectable. At 3g to 4g fatigue is limiting, and above 4g cardiovascular factors limit g tolerance.
https://books.google.fr/books?hl=fr&id=Lqz-4XU5m28C&q=Human%20volunteers%20have%20tolerated#v=onepage&q&f=false4
And here is a link to a NASA study:
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930020462.pdf5
It doesn't seem to cover g forces higher than 1 g sustained for months, years, or decades.
At the present time there is no evidence that a mixed human population with different states of health such as in a colony could survive and thrive while experiencing surface gravity higher than the 1.25 to 1.50g suggested by Dole, and there doesn't seem to be any experimental proof that anyone could survive 1.50g for more than 7 days without adverse health effects.
Obviously the majority of people should have no problems living their lives in 1.01g or 1.02g, but it seems reasonable, based on current evidence, for science fiction writers to say that, in a space opera setting with many colonized extra solar planets, experience has shown that the highest safe surface gravity for a colony planet is somewhere in the range of about 1.25g to 1.5g.
If a science fiction writer claims that the higher safe limit for colony planet surface gravity is about 1.25g to 1.50g, and/or limits the surface gravity of fictional colony planets to less than that, no one should accused him of being unrealistic - in that aspect of his story at least.
Added 01-11-2019. As far as I know the only worlds in our solar system with higher surface gravities than Earth are Jupiter, Saturn, and Neptune, and they don't have solid surfaces.
So the three ways in this solar system to test human long term tolerance of higher gravity are:
1) Put people in balloons at various levels in the atmospheres of giant planets for long periods.
2) Test people in centrifuges for long periods.
3) Put humans in rockets that can accelerate and decelerate at higher than 1g for long periods of time.
Only the second method is plausible in the immediate future so it is possible that the answer to the question might not be known with certainty for decades or centuries in the future.
There's plenty of ways to create a sustainable long term colony in high G environment. It would be very inefficient to ignore colonizing otherwise suitable planets with >1.5G just because of their gravity only.
Spend most of your time in a pool filled with mercury
Colonists would spend most of their time in giant pools filled with a high buoyancy liquid when not wearing their g-suit (described below) to rest/research/eat etc...
The buoyancy of that liquid would counteract the gravity and help the colonist with the high G. Mercury seems to be a good choice since it is very dense and not easily absorbed or metabolized. Mercury is 13x denser than our bodies.
https://nerdist.com/float-mercury-video-science/
A rough calculation would indicate that you would be able to live on a 12G planet but with an apparent specific gravity of 1G.
Of course this will still have limits but we would certainly be able to colonize planets with >1.5G.
Use G-suits when you want to work outside the pool
If we have the technology to travel to other solar systems, we would certainly be able to craft some sort of advanced g suit to offset the negative consequences of >1.5G environments for short times (a few hours).
It already exists for current jet fighter pilots.
https://en.wikipedia.org/wiki/G-suit
So all you need is some sort of motorized exoskeleton to help you move around in high G environment and some system to put pressure on the bottom of your body to relieve your cardiovascular system.
Or you could just send drones to do all the outside work and stay in your mercury pool forever.
