# General Relativity and the Human Body

by Kindayr
Tags: body, human, relativity
 P: 161 In SPH 4U1, (Ontario Grade 12 Physics) we learn the attempts to stop the affects that the human body experiences when in prolonged periods of time in space; such as muscle decay, loss of bone density, etc. So we learn of the attempts to use centripetal force and angular motion to cause a 9.8 m/s acceleration towards the center of the ideal circular space ship. This is easily dictated through the equation of a=v^2/r I wonder, if explained though terms of General Relativity, that if gravity is truely the curving of space-time. If this is true, would that not mean that our bodies are not developed to experience a maintained 9.8 m/s pull towards the center of the earth, that we're actually build to exists in the curved space-time that earth produces for us. I propose that the circular space ships and use of centripetal force would not stop the decay of muscle or loss of bone density. That in fact, the human body needs curved space-time to function, not simply an outside force. Obviously I can't prove this, but it has always been something that has made me wonder since we were proposed the idea of centripetal force in space earlier in the semester. What if the dysfunctionality is actually our bodies straightening out?
 Mentor P: 15,201 You are misinterpreting general relativity here. I suggest you read up on Einstein's elevator though experiment. Whether what you are experiencing is gravity pulling you in one direction or acceleration pulling you in the opposite direction are completely distinguishable. There is no way to tell which is which on a small enough scale. This concept is embodied in the equivalence principle, which is one of the most accurately tested concepts in all of physics. Some reading: http://www.pbs.org/wgbh/nova/einstein/relativity http://www.astronomynotes.com/relativity/s3.htm http://physicsworld.com/cws/article/print/21148
 P: 161 I realize the equivilance principle, but what I'm wondering is if the constitution of the human body doesn't only recognize the force acting upon it, but we're built to function within specific curved space the earth creates. Einstein's elevator simple force and the circular spaceships use of centripetal force don't shape space-time like gravity does as general relativity states.
 Mentor P: 17,529 General Relativity and the Human Body Tidal effects are completely negligible at the scale of a human body on the surface of the earth. (about 600 micro newtons for a 2 meter tall, 100 kilogram adult)
P: 801
 Quote by Kindayr I realize the equivilance principle, but what I'm wondering is if the constitution of the human body doesn't only recognize the force acting upon it, but we're built to function within specific curved space the earth creates. Einstein's elevator simple force and the circular spaceships use of centripetal force don't shape space-time like gravity does as general relativity states.
A body's resistance to acceleration would maintain muscle and bone mass, even if the acceleration is not identical to earth's. At a minimum, any proper acceleration at all would be better than freefall.

And it would be possible to build a spinning ship that would approximate the space time curvature at earth's surface reasonable closely. At least better than holding a person's body down on a treadmill with bungee cords. It's not like we have a high standard to beat there.

Al
PF Gold
P: 1,164
 Quote by Kindayr I realize the equivilance principle, but what I'm wondering is if the constitution of the human body doesn't only recognize the force acting upon it, but we're built to function within specific curved space the earth creates. Einstein's elevator simple force and the circular spaceships use of centripetal force don't shape space-time like gravity does as general relativity states.
From what I remember (from many years ago), experiments and simulations have already been done to determine the sensitivity of the human sense of balance and orientation to the effects of using rotation to simulate gravity, and as expected it would be necessary for a circular rotating spaceship to be quite large (as in a Clarke space station) to provide a reasonably comfortable duplication of a 1g environment. There is also a problem that the rotating structure has to be fairly massive in order to avoid uncomfortable bouncing and wobbling when people and objects are moving around in it.

This is all calculated and simulated using Newtonian gravity theory; the corrections for General Relativity are far too small for humans to be able to sense them at all.

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