Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 28/12/2014 02:20:01
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Can it be said that gravity exerts a negative stress?
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Can it be said that gravity exerts a negative stress?
Yes. gravity can exert both positive and negative stress in a body at any point in a body. Remember that stress is a tensor quantity. See: http://en.wikipedia.org/wiki/Stress_(mechanics)
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Can it be said that gravity exerts a negative stress?
I'd say not. You could reasonably say that the tidal force exerts a negative stress or a tension, which in extremis would result in spaghettification (http://en.wikipedia.org/wiki/Spaghettification). And you could say that the tidal force is associated with Riemann curvature which is the "defining feature" of a gravitational field. But the force of gravity isn't the tidal force. Also, take a look at the the stress-energy-momentum tensor (http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor) which "describes the density and flux of energy and momentum in spacetime". Note the energy-pressure diagonal? You'll be aware that stress is directional pressure. A gravitational field is a bit like pressing space outwards, like you're creating a pressure gradient in space. A positive stress rather than a negative stress.
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Can it be said that gravity exerts a negative stress?
I'd say not. You could reasonably say that the tidal force exerts a negative stress or a tension, which in extremis would result in spaghettification (http://en.wikipedia.org/wiki/Spaghettification). And you could say that the tidal force is associated with Riemann curvature which is the "defining feature" of a gravitational field. But the force of gravity isn't the tidal force. Also, take a look at the the stress-energy-momentum tensor (http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor) which "describes the density and flux of energy and momentum in spacetime". Note the energy-pressure diagonal? You'll be aware that stress is directional pressure. A gravitational field is a bit like pressing space outwards, like you're creating a pressure gradient in space. A positive stress rather than a negative stress.
You can't curve time just like you can't curve space. You can curve spacetime as a function of both distance and time. What exactly does this mean though? Something causes this curvature. If it is called the gravitational field then surely it is gravity causing the curvature. It's not called the spacey timey field.
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I'd say not. You could reasonably say that the tidal force exerts a negative stress or a tension, which in extremis would result in spaghettification (http://en.wikipedia.org/wiki/Spaghettification).
That's not the only way that gravity can exert negative stress, i.e., tension, is the radial forces or stretches the body is in falling inward.
And you could say that the tidal force is associated with Riemann curvature which is the "defining feature" of a gravitational field. But the force of gravity isn't the tidal force.
So what? Its defined in terms of gravitational forces just like tension is caused in an elastic band when we try to stretch it.
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tension, is the radial forces or stretches the body is in falling inward.
Not sure I understand that, Pete.
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tension, is the radial forces or stretches the body is in falling inward.
Not sure I understand that, Pete.
Yeah. I screwed that up when I posted it. When a system of charges falls then then the ones which are closer to the center of the system fall faster and the ones that are that are further away fall slower. As you can easily see this means that if a rod was placed along a radial line then it would be stretched by gravity. Stretching is the same thing as negative as negative tension.
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You can't curve time just like you can't curve space. You can curve spacetime as a function of both distance and time. What exactly does this mean though?
It means your plot of your measurements of distance and time made using say the motion of light, is curved. It doesn't mean space is curved. Instead space is inhomogeneous.
Something causes this curvature.
A concentration of energy causes it. A massive star is made of matter which is made of energy. So it's a concentration of energy.
If it is called the gravitational field then surely it is gravity causing the curvature. It's not called the spacey timey field.
The energy "conditions" the surrounding space, the effect diminishing with distance in line with the inverse square rule. Gravity is the result of this. Not the cause.
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That's not the only way that gravity can exert negative stress, i.e., tension, is the radial forces or stretches the body is in falling inward.
Go and look up spaghettification (http://en.wikipedia.org/wiki/Spaghettification).
So what? Its defined in terms of gravitational forces just like tension is caused in an elastic band when we try to stretch it.
Curved spacetime is associated with the tidal force, whilst the force of gravity is associated with "tilted" spacetime.
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You can't curve time just like you can't curve space. You can curve spacetime as a function of both distance and time. What exactly does this mean though?
It means your plot of your measurements of distance and time made using say the motion of light, is curved. It doesn't mean space is curved. Instead space is inhomogeneous.
It was a rhetorical question.
Something causes this curvature.
A concentration of energy causes it. A massive star is made of matter which is made of energy. So it's a concentration of energy.
OK then. Where does this energy come from? You aren't going to say the electromagnetic field are you?
If it is called the gravitational field then surely it is gravity causing the curvature. It's not called the spacey timey field.
The energy "conditions" the surrounding space, the effect diminishing with distance in line with the inverse square rule. Gravity is the result of this. Not the cause.
Space is not the issue here. You just agreed earlier that you can't curve space. So what exactly is this conditioning? It must indicate a change. That change is in a field, not space. The field EXISTS in space but curvature is a mathematical model of the behavior of the field, not space. The curvature is in the rate of change of the force as it moves away from the source. Since we have a time element then this field can be nothing like the magnetic field or even the electric field. Yet the magnetic field can induce a very short range acceleration.
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You can't curve time just like you can't curve space.
Where did you get this idea from, Jeff? It's wrong. It's quite possible to do those things.
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You can't curve time just like you can't curve space.
Where did you get this idea from, Jeff? It's wrong. It's quite possible to do those things.
I meant as individual entities. You have to consider both together. Sorry I did not make that clear.
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I meant as individual entities. You have to consider both together. Sorry I did not make that clear.
That's not true. There's such a thing for all of them. I.e. there is such a thing as
a) space curvature
b) time curvature
c) spacetime curvature
It's not one or the other but all of them working together. This is true in a Schwarzschild spacetime