What do you feel in orbit around a massive object?

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What do you feel in orbit around a massive object?

Postby aleph_one » Wed Sep 14, 2011 5:07 pm UTC

Suppose you are in a circular orbit around a massive body, at a distance so that the gravitational acceleration is much greater than 1g (that on the surface of the Earth). My question is: do you perceive the crushing gravitational force? For instance, would there be adverse effects on your body? Also, what if you're in freefall instead of orbit?

This question came from thinking about why fighter jet pilots suffer harmful physiological effects from large acceleration. My first thought was that it's because a force is being applied non-uniformly throughout their body, causing parts of the body to be pulled relative to each other. Their seat pushes against the back, pushing it into their front, squishing them. Moreover, the skeleton is relatively rigid and forced forward, causing it to pull along fleshy internal organs attached to it. My intuition is that it is differences in acceleration throughout the body that cause the perception and effects of fast acceleration.

But would these feelings and effects happen if a force is applied uniformly to the entire body (so each differential element of the body is acted on by force proportional to its mass in a fixed direction)? This is exactly the situation for gravitational force. While my intuition tells me that a uniform force should be indistinguishable from no force, equivalence of inertial frames doesn't let us ignore uniform accelerations as it does uniform velocities.

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Re: What do you feel in orbit around a massive object?

Postby eSOANEM » Wed Sep 14, 2011 5:14 pm UTC

Look into the equivalence principle.

Essentially, it says that, if you're sitting in a perfectly sealed box, and you feel some gravitational force, there is no way you can tell whether you're stationary in a gravitational field or whether you're accelerating. The most important side effect of this is that in free fall, the acceleration creates and opposite pseudo-gravitational force in your reference frame which exacts counteracts the one you feel from gravity as such, in free fall, you feel weightless.

An orbit is still a type of free fall and so you'll be weightless. Tidal forces on the other hand will still apply because you're bound together and so some bits of you are not quite in free fall. It is these tidal forces that lead to spaghetification around black holes (where the field is really strong).
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Re: What do you feel in orbit around a massive object?

Postby Aiwendil » Wed Sep 14, 2011 5:25 pm UTC

My question is: do you perceive the crushing gravitational force? For instance, would there be adverse effects on your body? Also, what if you're in freefall instead of orbit?


Orbit is free-fall. Or, to think of it another way, a 'non-orbital' free-fall is just an orbit that happens to intersect with the ground. An orbit and a fall, from the point of view of the person doing the orbiting/falling, would feel exactly the same (well, until one hits the ground).

If you're close enough to a very massive body you'll feel tidal effects. This is essentially because the parts of you pointing toward the body at any given moment are feeling slightly more gravitational force (because they're closer to it) than the parts pointing away from the body. The magnitude of this effect depends on the mass of the body, the size of the object orbiting, and the distance between the two. At worst (e.g. very close to a black hole), you'll be ripped apart.

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Re: What do you feel in orbit around a massive object?

Postby aleph_one » Wed Sep 14, 2011 6:32 pm UTC

Thanks for the answers! If I understand correctly, orbit, freefall, and uniform motion are perceived the same, because after a non-inertial change into coordinates where one is at rest, the situations become equivalent, as the virtual forces cancel out the actual ones.

Riddle me this: I recall reading that a barrier to interstellar travel is that the cosmonaut's bodies can tolerate only some amount of acceleration (say 4g), making travelling a distance d require ~sqrt(d) time. But could one safely overcome this by having the travelers be accelerated arbitrarily fast, but completely uniformly throughout their bodies?

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Re: What do you feel in orbit around a massive object?

Postby douglasm » Wed Sep 14, 2011 6:38 pm UTC

aleph_one wrote:Riddle me this: I recall reading that a barrier to interstellar travel is that the cosmonaut's bodies can tolerate only some amount of acceleration (say 4g), making travelling a distance d require ~sqrt(d) time. But could one safely overcome this by having the travelers be accelerated arbitrarily fast, but completely uniformly throughout their bodies?

If such a manner of acceleration were possible, then yes. With sufficient power behind it, that kind of acceleration could take someone from rest to 99% the speed of light in half a second without the traveler feeling anything. Figuring out how to do that kind of acceleration in meaningful quantity in arbitrary directions, well, that's a bit tricky.

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Re: What do you feel in orbit around a massive object?

Postby Tass » Wed Sep 14, 2011 7:37 pm UTC

douglasm wrote:
aleph_one wrote:Riddle me this: I recall reading that a barrier to interstellar travel is that the cosmonaut's bodies can tolerate only some amount of acceleration (say 4g), making travelling a distance d require ~sqrt(d) time. But could one safely overcome this by having the travelers be accelerated arbitrarily fast, but completely uniformly throughout their bodies?

If such a manner of acceleration were possible, then yes. With sufficient power behind it, that kind of acceleration could take someone from rest to 99% the speed of light in half a second without the traveler feeling anything. Figuring out how to do that kind of acceleration in meaningful quantity in arbitrary directions, well, that's a bit tricky.


Gravitomagnetism is one theoretical way to do it. However, the engineering of getting sufficient mass in the right place and moving fast enough in the right way is, to use Douglasms words, "a bit tricky".

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Re: What do you feel in orbit around a massive object?

Postby obfpen » Wed Sep 14, 2011 7:45 pm UTC

So much for the physics; what about the biology?

aleph_one wrote:This question came from thinking about why fighter jet pilots suffer harmful physiological effects from large acceleration. My first thought was that it's because a force is being applied non-uniformly throughout their body, causing parts of the body to be pulled relative to each other. Their seat pushes against the back, pushing it into their front, squishing them. Moreover, the skeleton is relatively rigid and forced forward, causing it to pull along fleshy internal organs attached to it. My intuition is that it is differences in acceleration throughout the body that cause the perception and effects of fast acceleration.

But would these feelings and effects happen if a force is applied uniformly to the entire body (so each differential element of the body is acted on by force proportional to its mass in a fixed direction)?

[...]

I recall reading that a barrier to interstellar travel is that the cosmonaut's bodies can tolerate only some amount of acceleration (say 4g) [...] But could one safely overcome this by having the travelers be accelerated arbitrarily fast, but completely uniformly throughout their bodies?

While sudden large accelerations (positive or negative) can certainly cause problems physiologically - if they didn't, leaps from heights would be a much less popular approach to suicide - and tidal forces can cause deformations (as has been mentioned) these aren't really the main concerns for fighter pilots, or their flight suits.

In high-g maneouvres, the body, and everything in it - including the blood - weighs much more. The heart has a great ability to increase cardiac output several times above what is needed in normal resting conditions, but it has its limits. Experiencing an increase in g-force is one way of testing them.

As g-force increases, the heart has a harder time pumping blood back up the body against "gravity", critically to the brain, which leaves it starved of oxygen. An early symptom of this is greying of vision; not long after follows loss of consciousness. Obviously this is a condition someone operating a plane wants to avoid. (The people who've paid millions for the plane and to train the pilot don't like it much either.)

To combat the effects of high-g, fighter pilots wear flight suits and learn some exercises that compress the lower body, and maintain blood in the upper body, where it's more useful to survival.

Unaided, the human body can only cope with a few g; with an anti-g suit it can survive a few more. Uniform acceleration or not, travel much beyond that you get and you get into splattery fatal territory as the structures of the body fail under their own vastly increased weight.

So if survival important, you need to neutralize the acceleration. Simply making it uniform across the body isn't going to help much.
Edit: Misinterpreted "uniform" for the last bit (thanks, mfb), but the pilot physiology holds.
Last edited by obfpen on Wed Sep 14, 2011 9:59 pm UTC, edited 1 time in total.

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Re: What do you feel in orbit around a massive object?

Postby mfb » Wed Sep 14, 2011 8:29 pm UTC

obfpen wrote:So if survival important, you need to neutralize the acceleration. Simply making it uniform across the body isn't going to help much.

It would help. But it has to be uniform across the volume, not only the surface. For pilots, the force is only applied on their surface, that is the real problem here. If you find a way to accelerate blood and everything else at the same time and with the same amount, the pilot would be unable to feel it at all.


With frogs and strong magnetic fields, it is possible to manage an acceleration of 1g (the frogs levitate). If you can scale that up and find some mysterious way to increase the field strength even further, maybe higher accelerations for humans are possible. The human will continue to stay in the magnetic field if you accelerate the whole structure. However, you have to be very careful with these fields - they can affect (and induce) electric currents inside the body.

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Re: What do you feel in orbit around a massive object?

Postby eSOANEM » Wed Sep 14, 2011 9:09 pm UTC

aleph_one wrote:Thanks for the answers! If I understand correctly, orbit, freefall, and uniform motion are perceived the same, because after a non-inertial change into coordinates where one is at rest, the situations become equivalent, as the virtual forces cancel out the actual ones.


Only if the uniform motion is in the absence of a gravitational field. The important thing is not that they are all perceived the same, but that they all belong to the same class of paths and that they are all examples of free-fall.
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Re: What do you feel in orbit around a massive object?

Postby obfpen » Wed Sep 14, 2011 9:31 pm UTC

mfb wrote:it has to be uniform across the volume, not only the surface. For pilots, the force is only applied on their surface, that is the real problem here. If you find a way to accelerate blood and everything else at the same time and with the same amount, the pilot would be unable to feel it at all.

Yes, I suppose I was limiting my thinking to the surface. (Please forgive the Earth-bound physiologist in the room - it's been a while since A-level Mechanics modules.) But is it possible to create sufficiently uniform acceleration throughout the volume of any number of human bodies without hampering their viability? Just how large is the variation throughout the volume of a jet pilot flying in Earth's gravitational field?
Given the low tolerances to acceleration in non-uniform conditions, surely a device that counteracts the acceleration (rather than trying to render it perfectly uniform) would be less deathy if its effectiveness fluctuated for any reason?

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Re: What do you feel in orbit around a massive object?

Postby Soralin » Thu Sep 15, 2011 1:11 am UTC

aleph_one wrote:Riddle me this: I recall reading that a barrier to interstellar travel is that the cosmonaut's bodies can tolerate only some amount of acceleration (say 4g), making travelling a distance d require ~sqrt(d) time. But could one safely overcome this by having the travelers be accelerated arbitrarily fast, but completely uniformly throughout their bodies?

That's not really a barrier to interstellar travel. A rocket that can produce even 1g for interstellar periods of time would be absurdly powerful and require enormous energy. I mean, even if you're just going to alpha centauri, if you go 1g the whole way (with a flip in the middle), and using a 100% efficient matter-antimatter rocket, you need 38x the mass of the rest of the rocket in fuel, to burn at 1g the whole way. (http://math.ucr.edu/home/baez/physics/R ... ocket.html) And further distances, burning at 1g become even more absurd, Vega, at 27 ly, would take about 886x the mass of everything else in matter-antimatter, and the center of the galaxy at 30k ly would take about 995 million times the mass of the rest of the rocket in fuel. :)

If you want something that goes a bit slower, one of the variants of Valkyrie, would accelerate up to 92% of the speed of light, and back down to rest again, and that has a mass ratio of 22, of matter-antimatter fuel, and doesn't assume perfect efficiency. :) If you're looking at merely nuclear fission or fusion, you might need to increase that by an order of magnitude or more. So acceleration is definitely not a limiting factor of any sort on interstellar travel, fuel/propellent is, and you'd require absurd or impossible amounts of it for multiple-g accelerations over interstellar distances.

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Re: What do you feel in orbit around a massive object?

Postby Diadem » Thu Sep 15, 2011 4:29 am UTC

This thread has made me wonder.

You can't feel gravity when you are in free fall, no matter how strong the field. But what about jerk (change in acceleration)?

Imagine you're freely falling in an immensely strong uniform gravitational field. So you have a huge acceleration, but since you are in free fall you don't notice. Suddenly, someone switches off the field completely, so that suddenly your acceleration is 0. You don't feel this new situations either. For you the new situations is no different than the old. But what about the transition? Do you feel the massive jerk?

I know people have physiological limits to jerk, just like they have limits to acceleration. But what about uniform jerk? Do you even feel it? Man, I studied GR and I have no idea.
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Re: What do you feel in orbit around a massive object?

Postby eSOANEM » Thu Sep 15, 2011 6:35 am UTC

I would think rapid changes of the gravitational field would not cause any force to be felt (excepting the tidal forces which would change) because, if we model this "instant" change as just a very rapid change, at any point on this change in field, no force would be felt because you're in free-fall and the acceleration creates an equal and opposite force (in your reference frame). As the time taken to change goes to zero, this argument still holds so I'd say that, even for instant changes in the gravitational field, no force would be felt.
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Re: What do you feel in orbit around a massive object?

Postby idobox » Thu Sep 15, 2011 3:56 pm UTC

Since gravity travels at the speed of light, when you shut down the gravitational field, you will have a "wave" travelling at c through your body, causing a short, but strong, tidal effect.
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Re: What do you feel in orbit around a massive object?

Postby gmalivuk » Thu Sep 15, 2011 4:16 pm UTC

idobox wrote:Since gravity travels at the speed of light, when you shut down the gravitational field, you will have a "wave" travelling at c through your body, causing a short, but strong, tidal effect.
Which will be through your body pretty much before any atoms actually have time to move far in response to said force.
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Re: What do you feel in orbit around a massive object?

Postby Diadem » Thu Sep 15, 2011 5:17 pm UTC

Yeah, I don't think the time it takes for the gravitational wave to pass through your body is significant. And anyway I specified uniform jerk. So our magic Uniform Gravitational Field Generator is (it would appear) able to do so instantaneously, instead of relativisticly. In other words, I want to know about jerk, not tidal forces.

I can't find anything in the equations that would lead me to think the jerk would matter. But at the same time I know people don't like high jerks, so that seems to be a contradiction.
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Re: What do you feel in orbit around a massive object?

Postby gmalivuk » Thu Sep 15, 2011 5:48 pm UTC

People also don't like high accelerations, and yet that doesn't contradict the fact that under a *uniform* high acceleration there's no problem.

The problem with jerk is that it's a sudden change in acceleration that is felt non-uniformly by different parts of the body, and which happens too fast for us to compensate effectively.
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Re: What do you feel in orbit around a massive object?

Postby PM 2Ring » Thu Sep 15, 2011 6:30 pm UTC

Diadem wrote: But at the same time I know people don't like high jerks, so that seems to be a contradiction.
Eg, Charlie Sheen. :)

On a slightly more serious note, in Greg Bear's novel Anvil Of Stars, an alien race has spacecraft equipped with a system that applies acceleration evenly to all parts of the ship and its contents, so they don't have to worry about jerk and the effects that it causes when resisting acceleration. It works by dividing up the ship volume into small cells and using some kind of force field to give each cell the appropriate acceleration. This works well enough at lower accelerations, when the cell size is moderately large, but at very high accelerations the people in the ship are advised to minimize movement because it's not easy for the system to deal with the large number of small cells especially if those cells are moving relative to one another. Apart from this semi-magical field, most of the physics in the novel is fairly realistic, as it conforms to relativity.

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Re: What do you feel in orbit around a massive object?

Postby eSOANEM » Thu Sep 15, 2011 6:50 pm UTC

Diadem wrote:Yeah, I don't think the time it takes for the gravitational wave to pass through your body is significant. And anyway I specified uniform jerk. So our magic Uniform Gravitational Field Generator is (it would appear) able to do so instantaneously, instead of relativisticly. In other words, I want to know about jerk, not tidal forces.

I can't find anything in the equations that would lead me to think the jerk would matter. But at the same time I know people don't like high jerks, so that seems to be a contradiction.


The thing is, because acceleration is in a meaningful way the same as a gravitational field, a constant jerk would be the same as a ramping gravitational field. Because the changes in this apparent gravitational field propagate at the speed of light, their is a difference in the strength of this field across the body in the direction of acceleration and this is felt as a tidal force.
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