# The Naked Scientists Forum

### Author Topic: Gravity and stress  (Read 5493 times)

#### jeffreyH

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##### Gravity and stress
« on: 28/12/2014 02:20:01 »
Can it be said that gravity exerts a negative stress?

#### PmbPhy

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##### Re: Gravity and stress
« Reply #1 on: 28/12/2014 08:01:27 »
Quote from: jeffreyH
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)

#### JohnDuffield

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##### Re: Gravity and stress
« Reply #2 on: 29/12/2014 17:05:10 »
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. 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 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.

#### jeffreyH

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##### Re: Gravity and stress
« Reply #3 on: 29/12/2014 18:44:04 »
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. 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 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.

#### PmbPhy

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##### Re: Gravity and stress
« Reply #4 on: 30/12/2014 07:21:30 »
Quote from: JohnDuffield
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.
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.

Quote from: JohnDuffield
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.

#### Bill S

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##### Re: Gravity and stress
« Reply #5 on: 30/12/2014 16:20:10 »
Quote from: Pete
tension, is the radial forces or stretches the body is in falling inward.

Not sure I understand that, Pete.

#### PmbPhy

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##### Re: Gravity and stress
« Reply #6 on: 31/12/2014 16:34:59 »
Quote from: Pete
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.

#### JohnDuffield

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##### Re: Gravity and stress
« Reply #7 on: 31/12/2014 17:04:41 »
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.

#### JohnDuffield

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##### Re: Gravity and stress
« Reply #8 on: 31/12/2014 17:09:22 »
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.

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.

#### jeffreyH

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##### Re: Gravity and stress
« Reply #9 on: 31/12/2014 21:07:03 »
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.

#### PmbPhy

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##### Re: Gravity and stress
« Reply #10 on: 01/01/2015 12:08:17 »
Quote from: jeffreyH
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.

#### jeffreyH

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##### Re: Gravity and stress
« Reply #11 on: 01/01/2015 17:04:56 »
Quote from: jeffreyH
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.

#### PmbPhy

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##### Re: Gravity and stress
« Reply #12 on: 02/01/2015 13:22:59 »
Quote from: jeffreyH
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
« Last Edit: 02/01/2015 13:28:44 by PmbPhy »

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##### Re: Gravity and stress
« Reply #12 on: 02/01/2015 13:22:59 »