# Is a small tidal force on the body harmful over the long term?

• Medical
Gold Member
Summary:
In a page (source given below), it was stated that a small difference between force on the feet and force on the head (similar to a tidal force) would lead to harmful and/or unpleasant light-headedness. obviously this depends on what "small" is, but would a difference in acceleration of a few cm/s^2 make a difference? What difference would be significant?
In the following page (which, despite examining a question that comes up in science fiction, raises valid scientific questions):

http://pages.erau.edu/~andrewsa/sci...Game Eilts/Enders Game Eilts/index_Eilts.html

I encountered the following in the context of simulating gravity by means of centrifugal "force":

"This force simulates gravity, but due to natural inner ear function, humans become dizzy and light headed when spun at speeds required to get anywhere near Earth's gravity. This is due to blood pooling in the person's feet rather than making it to the brain."

Leave aside the fact that the author seems to attribute two causes to being light-headed -- one, the imbalance in the inner ear, and two, the force gradient between the brain and the feet -- and jotting down a couple of numbers,
---say for a tall person (a few centimeters above 2 meters) and
---acceleration=distance*(angular acceleration)^2,
then someone in a space station/centrifuge
--- rotating at one revolution per minute and
--- her feet at about 894 meters from the center, and her head at 2 meters less (about 892 meters) from the center,
this gives ordinary earth acceleration at the feet and about 2 cm/s^2 less at the feet. Is this significant?

If not, how far up before it becomes significant? If so, how far down before it stops become significant?

While I am at it, would the rotation (1 rev/min at around a kilometer from the center) be fast enough to cause dizziness from the inner ear? Again, if so, how far down (roughly) can it go, and if not, how far up (roughly) would it have to go?

jim mcnamara
Mentor
You have two choices:
A. micro-gravity plus slow physiological deterioration.

B: artificial gravity

https://pubmed.ncbi.nlm.nih.gov/11538748/ asserts B is required for lengthy stays in space. For short stays it makes no difference - depending on definitions of short stays.

So, no, the statement made in the OP is not relevant. - Not applicable for long stays out there - There is no sane choice but B.

In other words you would die far sooner in micro-gravity, sort of death-by-jellyfish. So why worry about artificial gravity effects assuming you actually must spend long periods out there somewhere?

DaveC426913
Gold Member
You have two choices:
While this may be technically true, the study of minute tidal forces would still be relevant to option B.

For example, if it turned out that small tidal forces did have deleterious effects, it would bias designs toward structures with larger circumference, with an eye toward lessening the tidal effect.

Gold Member
Thank you for your responses, DaveC426913 and jim mcnamara.

As DaveC426913 pointed out, jim mcnamara's reply does not seem to reply to the question: the linked abstract (I do not have access to the whole article) seems to only refer to the either/or situation, microgravity or full (possibly simulated) gravity: a choice between 0.000 and 1.000, whereby I am referring to a case of 0.998, i.e., almost, but not quite full (simulated) gravity. Or is this addressed in the full article?

if it turned out that small tidal forces did have deleterious effects

From the above 'if" answer, should I gather that the answer is not known?
it would bias designs toward structures with larger circumference
Indeed, both possible deleterious effects would -- the small tidal force and the alleged dizziness. Should I also assume, from the lack of response to this secondary question, that the ear/dizziness question also has no answer as of yet?

should I gather that the answer is not known?
Since there was no experiment so far it is hard to get an answer. Kind of an educated guess is, that people would simply adapt and negative effects would not last long, but this also depends on the radius applied for the centrifuge.

Bystander
Homework Helper
Gold Member
What sort of activities? Tumbling/gymnastics? Sedentary/recumbent?

Gold Member
Kind of an educated guess is, that people would simply adapt and negative effects would not last long, but this also depends on the radius applied for the centrifuge.
Thanks, Rive. Any educated guess for the types of radii usually bandied about for future space stations (starting at a radius of a third of a kilometer on up, whereby the centrifuge is at speeds to simulate earth gravity)? (I gave a numerical example in the original post) Assume longer stays when considering health effects.

What sort of activities?
Sorry, I didn't understand the question. Do you mean the activities performed in a centrifuge for which a person would have to adapt? In the case of the space station (the original question), all normal activities, including the ones you mentioned.

Bystander
Homework Helper
Gold Member
Sorry, I didn't understand the question.
Are you changing your physical orientation with respect to the gradient? Bedsores develop in "normal" gravity given a lack of activity; regular re-orientations ameliorate the effect.

TeethWhitener
Gold Member
Wikipedia seems to give a decent explanation of what's going on:
https://en.wikipedia.org/wiki/Artificial_gravity
It claims that rotations of 2 rpm or less (224 m radius for 1 g) are generally well-tolerated by humans. The Coriolis effect would be weird (things going in odd directions when thrown in the air from the point of view of a co-rotating observer), but the tidal force probably wouldn't be significant unless the rotation were fast/radius were small. What "small" and "fast" mean in this context, I'm not sure.

DaveC426913
Gold Member
I think it is safe to say that any symptoms due to tidal forces will be vastly outstripped by symptoms from the effects of rotation.

Any scenario in which there is some significant degree of tidal force is predicated on a rate of rotation that will be orders of magnitude more apparent.

It is left as an exercise for the reader to pick a few scenarios with varying rotation rate and radius to come up with predicted subjective symptoms.

Laroxe
Just some thoughts, Your body is perfectly capable of rapidly compensating for physiological changes induced by acceleration, thats why you can stand up rapidly without fainting. Unless the astronauts were elderly, I wonder if things like blood pooling reflect reality. Our body is very good at redistribution of blood and controlling blood pressure throughout our body. As I understand it the effects of the sideways acceleration on our inner ear would be a function of speed and the perception of speed. The larger the rotating circle, the lower the speed, people use visual cues to check their perceived movement, it would be easy to control what the person see's to reduce the perception of sideways movement. If the rotation was steady, it's likely that they would quickly adapt to the movement, in the same way sailors adapt to the ship's movement.

Most of the physiological harms seen in space travel tend to be associated with the lack of the stress caused by gravity, many of our physiological processes evolved to function under the pressure of gravity. Even bed rest leads to muscle wasting and bone demineralisation, our body doesn't like to waste resources unnecessarily. I see no reason as to why artificial gravity wouldn't provide the same stressors that would help maintain our body structure. Currently, these stressors are provided by long periods of exercise, but this is at best a partial solution but we have no real experience of the effects of really long periods in space or the effects of artificial gravity, I expect there is a lot of issues we haven't seen or even predicted.