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Could you survive high g's if your whole body was accelerated uniformly (not just by the seat of a rocket pushing on your back)?

Your body is really only affected by gravity if you are touching something else such as the ground or the air.

Are there strategies for mitigating physiologic effects of very high acceleration?

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  • $\begingroup$ Interesting, question. I'm really curious to see what other have to say! $\endgroup$ Oct 28, 2021 at 1:33
  • $\begingroup$ this might be better suited for physics.stackexchange.com $\endgroup$ Oct 28, 2021 at 1:59
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    $\begingroup$ Sure, but how do you do that without magic? $\endgroup$
    – PM 2Ring
    Oct 28, 2021 at 2:41
  • $\begingroup$ To a (finite) extent, isn't this the idea behind immersing astronauts in breathable liquid? Hasn't actually been done but I know there's been research into it though I don't have any references handy. $\endgroup$
    – Nathan
    Oct 28, 2021 at 5:05
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    $\begingroup$ @Nathan To some extent, yes.The problem is that bones, muscles, and organs have different densities, so suspending a human body in a breathable liquid won't quite work against extremely high non-gravitational accelerations. $\endgroup$ Oct 31, 2021 at 21:28

3 Answers 3

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Of course.
(if you replace that pesky "infinite" with "arbitrarily large", physics really really hates infinite forces)

Who said you (and everything around you), is not currently being accelerated at a million g' in some direction? It is impossible to prove this false.

As a lesser example: When orbiting the Earth in a 400km LEO, you are constantly being accelerated towards the Earth with an acceleration of about 8.682m/s2 (.885 g)
Yet, because this acceleration is (very nearly almost *) uniformly applied over your whole body and spacecraft, you feel as if you are in perfect zero-g.

If you could somehow apply a uniform acceleration to every particle in your body and spaceship equally, you and the spaceship would accelerate relative to the cosmos, but subjectively you would feel as if you were just floating in space with not even enough gravity to keep your cornflakes in their bowl.

* only very nearly almost, because the gravity field is decreasing as you get further from the Earth, and when facing the Earth your nose is closer than the back of your head, thus in a stronger gravity field. Your body will be experiencing a 'tidal' force trying to pull it apart. However, in a gravity gradient as gentle as that of Earth, this effect is miniscule and not likely observable using only human senses.

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  • $\begingroup$ A query, not a criticism. From where did you get 8.682m/s2 as the value of gravitational acceleration 400 km above the Earth? If it's an equation I'd like to compare it to the one I use. $\endgroup$
    – Fred
    Oct 28, 2021 at 10:43
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    $\begingroup$ @Fred Probably by calculating $9.80665 * (6371/(6371+400))^2$. Using $(GM)/(6371\,\text{km}+400\,\text{km})^2$ where $GM=398600.4418\,\text{km}^3/\text{s}^2$ yields a slightly different value of $8.694\,\text{m}/\text{s}^2$. $\endgroup$ Oct 28, 2021 at 11:36
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    $\begingroup$ To everyone who responded to my query, thank you. Many years ago I found an equation that calculated gravity for a specified latitude & elevation above sea level. It's was so long ago I don't even know which website I got the equation from. It gave a value of 8.5 something. It just makes me question the applicability of the equation I've been using for more than a decade. $\endgroup$
    – Fred
    Oct 28, 2021 at 15:40
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    $\begingroup$ @Fred You probably found the formula for the free air anomaly, which says to subtract 0.3086 mGal for every meter of altitude above sea level (or 0.3086 cm/s^2 for every kilometer above sea level). This yields a value of 8.57 m/s^2 for an altitude of 400 km. However, this is formula is only approximately correct, and is not very good for altitudes above a few tens of kilometers. $\endgroup$ Oct 31, 2021 at 14:40
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    $\begingroup$ @Fred That linearization is much better than assuming gravitational acceleration remains constant, regardless of altitude, but it implies gravitational acceleration toward the Earth becomes negative at altitudes above 3177.8 km. That of course does not make sense. $\endgroup$ Oct 31, 2021 at 21:08
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To just answer your question, Einstein says “yes”. All uniformly accelerating frames of reference are the same.

A sort of related topic used in SF plots is flotation tanks used to protect crew from acceleration and tidal forces during extreme maneuvers. Orbital insertion around a binary neutron star sort of a thing.

This flotation strategy is used in the Libelle G-suit which keeps pilots functioning over 10G.

This works-up to a point. Maybe 20G. The problem is due to the different densities of tissues.

If all tissues had the same density as the flotation fluid, it would work really well. First step is to get the lungs full of breathable liquid. Like perflurocarbons. But they have a density of about 2.0 so they don’t do much good. If you came up with a substitute breathing liquid with a s.g. of normal saline, what would the next limiting tissue be?

Calcium containing tissues like bone are higher density than saline. Bone s.g. tops out at about 2.0. This will generate differential acceleration forces between water density tissues and bones. But bones are strong and well tied together, so who cares? Your ossicles (middle ear bones) care. They are very tiny and they are surrounded by air, not saline… unless you did a very thorough perflurocarbon flush up your Eustachian tubes. Something like waterboarding.

Another organ that will care about acceleration is the otolith organs (for linear and rotational acceleration sensing) in your inner ear. There is a microscopic blob of gel containing bits of calcium carbonate to increase the s.g. of the gel. Acceleration causes this blob of gel to move relative to the saline it is floating in and this movement is picked Up by microscopic hairs also embedded in the gel. Violent acceleration can damage this delicate organ, producing violent prolonged motion sickness.

Say you don’t care about you balance and go for more G’s? The tissues inside your skull (blood, grey matter, cerebrospinal fluid) have slightly different s.g. Violent acceleration can cause relative motion, especially shearing motion. This is particularly bad for long, delicate structures like nerve axons. Acceleration can also cause contra-coupe injury via cavitation https://aip.scitation.org/doi/10.1063/5.0041139

So what is the maximum G-force a human can withstand if protected by immersion and breathing liquid? I don’t know. But chimpanzee experiments done in the 1970’s replaced the back half of their sculls with a plexiglass dome to get high speed cines of cavitation formation during experimental contra-coupe injuries. I remember they were using accelerations of about 300G.

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This is not impossible. Gravity accelerates this way.

This is just free fall condition, and, yes, by itself should be ok.

The gravity field should be close to uniform, like when falling into big black hole from large distance. The gravity field gets stronger when approaching a black hole, and when close enough, the difference of the gravity forces between front and aft of the approaching spacecraft is enough to break it.

As long as the gravity field does not change much in space, it can change at any speed in time.

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  • $\begingroup$ I think, WARP does it. $\endgroup$
    – Robotex
    Oct 29, 2021 at 7:42
  • $\begingroup$ Freefall isn't safe if the gravitational field strength changes rapidly as you approach the gravity source, eg near a neutron star or small black hole. Please see physics.stackexchange.com/a/631427/123208 $\endgroup$
    – PM 2Ring
    Oct 29, 2021 at 7:55
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    $\begingroup$ The gravity field must be uniform. $\endgroup$
    – Nightrider
    Oct 29, 2021 at 8:38
  • $\begingroup$ A uniform gravity field would produce no acceleration. It is the field gradient that defines the direction of acceleration. But I'm way out of my depth here.... $\endgroup$
    – Woody
    Oct 29, 2021 at 15:17
  • $\begingroup$ @Bruce A uniform gravity field does produce acceleration. Such a field is not physically realisable, but in (Newtonian) theory, a homogeneous disc of infinite radius produces a uniform gravitational field. $\endgroup$
    – PM 2Ring
    Oct 31, 2021 at 15:36

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