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Author Topic: Is there zero zero gravity?  (Read 16000 times)

Offline Soul Surfer

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Is there zero zero gravity?
« Reply #25 on: 15/12/2009 23:31:11 »
To come back to the original question.  The correct way of expressing the state is "free fall" that is, a state where the object is moving freely under the effect of the local gravitational field and the only forces experienced on an extended semi rigid body are those caused by the local gravitational gradients which in the case of normal stars and planets are very small for objects of the order of size of a human being.  In recent years the term "zero gravity" seems to have replaced the expression "free fall" which in effect is much more accurate because as you have pointed out many places have quite significant gravitational potentials.
 

Offline LeeE

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Is there zero zero gravity?
« Reply #26 on: 15/12/2009 23:43:24 »
So basically talking about zero gravity is a waist of time as it is nonexistent in our universe.   Yet it’s a term we hear a lot. 

Where did it come from?

It came from sloppy thinking.
 

Offline Geezer

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Is there zero zero gravity?
« Reply #27 on: 16/12/2009 01:46:32 »
which in the case of normal stars and planets are very small for objects of the order of size of a human being. 


The gravitational force exerted by a planet on an object is proportional to the mass of the object. However, it takes more force to accelerate an object with greater mass, and therefore, it does not matter whether it's an ant or an elephant; from a particular point in space, both would experience identical acceleration towards the planet.

Hopefully this is obvious to all our readers  :D
 

Offline Mr. Scientist

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Is there zero zero gravity?
« Reply #28 on: 16/12/2009 09:15:41 »
It's more a lack of them being bothered to understand the concepts they speak about. I'm not overly concerned about new conjectures and suggestions, but if it goes against physics that we understand with quite a great degree of certainty, then there is no reason why they spout it off as a true maintsream statement - it annoys me at best.

I just wish that people like Geezer (and obviously bored chemist whose not a scientist or a physics student) would not argue necesserily with the points of physics which are really actually very basic. It doesn't take too long to learn some of these things. The length to understand gravity on the basic level is actually very easy if one has a certain ineterest in learning that subject.

Einstinean is greately more difficult to understand, but i've been learning on it for years now and i just wish that the given posters exampled would accept what i say as being gospal, because i know for sure they wouldn't understand the math - even i fail to understand all the mathematical conjectures today which involve relativity, but i know quite a bit about it; there is no such thing as a stupid question - but if you are provided the mainstream answer, you should just accept it, instead of trying to flout it with impunity based on the fact you don't really like the character you are communicating with. I know this be the case especially in Geezers behalf. Sad thing is, i actually don't have any problems with either of them personally apart from their ignorance.

Mr Scientist,

If you persist in making personal attacks on other forum members I will have no choice but to ban you. Bored Chemist and I have patiently tried to fill in obvious gaps in your education. If you don't like that, I suggest you move to another forum.

Geezer - Moderator

The cheak. So one rule for one and no rules for moderators.

Huh.. i should have known better. As i said, i have little time for this place now. You personally have been nothing but a pain - i was dealing with serious problems the other day, and you, and your collective group hounded me intentionally - but that fact can hardly be surpressed.

The hyposcrisy of the site is enough for me never to indulge in the science part again. I hope this is to your enjoyment.

Mr Scientist.

Mod Edit: As Mr Scientist has made it very clear that he has no intention of obeying the site rules, he is now banned.
« Last Edit: 16/12/2009 16:59:38 by Geezer »
 

Offline Soul Surfer

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Is there zero zero gravity?
« Reply #29 on: 16/12/2009 10:35:18 »
Geezer you seem to have missed my point.  The forces experienced by the body of an object in free fall are only those due to the GRAVITATINAL GRADIENT.  The acceleration of an object either falling or in an orbit are of course the same on every atom except for the local gravitiational gradient and only measurable from the ponit of view of that body by observing how the body is moving with respect to other bodies.
« Last Edit: 16/12/2009 10:37:47 by Soul Surfer »
 

Offline lightarrow

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Is there zero zero gravity?
« Reply #30 on: 16/12/2009 11:52:38 »
but you can easily achieve quasi-zero gravity: inside a free-falling lift; you don't have to consider which planet or sun is near you, the lift authomatically accelerates the exact amount it's required to canceal gravity inside of it.

Em, I don't think so. Gravity is not cancelled. Weight is.
But according to equivalence principle of general relativity, a gravitational field is equivalent to an inertial field (= accelerating frame of reference).
 

Offline Geezer

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« Reply #31 on: 16/12/2009 16:45:10 »
which in the case of normal stars and planets are very small for objects of the order of size of a human being. 


The gravitational force exerted by a planet on an object is proportional to the mass of the object. However, it takes more force to accelerate an object with greater mass, and therefore, it does not matter whether it's an ant or an elephant; from a particular point in space, both would experience identical acceleration towards the planet.

Hopefully this is obvious to all our readers  :D
Geezer you seem to have missed my point.  The forces experienced by the body of an object in free fall are only those due to the GRAVITATINAL GRADIENT.  The acceleration of an object either falling or in an orbit are of course the same on every atom except for the local gravitiational gradient and only measurable from the ponit of view of that body by observing how the body is moving with respect to other bodies.

I'm glad we agree about the acceleration. You are correct. I do not understand what your point is. Are you saying this is a case where there are more than two bodies exerting gravitational force on each other?
 

Offline Geezer

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Is there zero zero gravity?
« Reply #32 on: 16/12/2009 16:55:33 »
but you can easily achieve quasi-zero gravity: inside a free-falling lift; you don't have to consider which planet or sun is near you, the lift authomatically accelerates the exact amount it's required to canceal gravity inside of it.

Em, I don't think so. Gravity is not cancelled. Weight is.
But according to equivalence principle of general relativity, a gravitational field is equivalent to an inertial field (= accelerating frame of reference).

Lightarrow, I'm in no position to argue with you about that  :D

I take your word for it that it is equivalent, but does that mean it's the same thing? From my rather "traditional" perspective, gravity applies a force to the body in the lift at all times, regardless of the motion of the lift. That's why I'm making the point that gravity is not cancelled. On the other hand, if the body was standing on a weighing device (spring type of course) we would observe that the weight of the body changes dramatically, and that the weight might even be zero if the elevator accelerates towards the earth at g m/s/s
 

Offline lightarrow

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Is there zero zero gravity?
« Reply #33 on: 16/12/2009 21:54:08 »
Lightarrow, I'm in no position to argue with you about that  :D
Too kind...Maybe I don't deserve it.

Quote
I take your word for it that it is equivalent, but does that mean it's the same thing? From my rather "traditional" perspective, gravity applies a force to the body in the lift at all times, regardless of the motion of the lift.
I know you intended that, but physics can be quite complicated, or...simpler than what we think, sometimes. Imagine to be inside a little laboratory, very far from planets or stars or other massive object, and to measure an acceleration inside of it. How would you prove that the acceleration is due to the presence of gravity and not to something else?
Remember that, in GR, gravitational forces don't exists, there is only spacetime curvature.
Anyway, I think most physicists would agree with you...
« Last Edit: 16/12/2009 21:58:08 by lightarrow »
 

Offline Soul Surfer

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Is there zero zero gravity?
« Reply #34 on: 16/12/2009 23:36:36 »
Gravitational fields are seldom absolutely uniform so the gravitational forces on any extended body are likely to be smaller or larger at different points in the body due to the gradient of the local gravitational field.  This applies even when only one other gravitational body is causing the acceleration because of the inverse square law.  In general these forces are very small but they have to be taken into account in the design of satellites which have to maintain a particular orientation in order to use narrow beam communications for example.

In the case of extreme gravitational fields like those close to the event horizon of small (a few solar masses) black holes the gradient fields can be very large.

for example  at the event horizon of a five solar mass black hole which is about 15 Km across  the gravitational gradient is about 40,000,000 g per meter.  well enough to rip anything apart and spaghettify it.

Mind you  for a 5 billion solar mass black hole which is about the size of the orbit of Uranus the gradient is minute and a decent sized star could drop through it without being disrupted.
 

Offline Geezer

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Is there zero zero gravity?
« Reply #35 on: 17/12/2009 00:32:15 »
SS - Thanks. I'm with you now. I was assuming single point masses for the earth and the body in orbit which, as you point out, is only an approximation.
 

Offline Geezer

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« Reply #36 on: 17/12/2009 08:38:04 »
Lightarrow, I'm in no position to argue with you about that  :D
Too kind...Maybe I don't deserve it.

Quote
I take your word for it that it is equivalent, but does that mean it's the same thing? From my rather "traditional" perspective, gravity applies a force to the body in the lift at all times, regardless of the motion of the lift.
I know you intended that, but physics can be quite complicated, or...simpler than what we think, sometimes. Imagine to be inside a little laboratory, very far from planets or stars or other massive object, and to measure an acceleration inside of it. How would you prove that the acceleration is due to the presence of gravity and not to something else?
Remember that, in GR, gravitational forces don't exists, there is only spacetime curvature.
Anyway, I think most physicists would agree with you...


I agree. If we cannot observe what is happening "outside" the elevator then we can only rely on the effects that we observe inside the elevator, so, if I understand correctly, this would prove "equivalence". However, in this particular case we can observe both inside and outside, so is it necessary to rely on equivalence?

Regarding GR, I don't have any problem accepting that the observed forces are really only a manifestation of how matter affects spacetime. Actually, I rather like it, but I'm afraid of believing in it too much in case someone blows up that theory as well!

I do tend to use Newtonian methods to describe interactions because, in many situations, those models are sufficiently accurate to describe the behaviour of massive bodies, and I find them a lot simpler to understand.

 

Offline lightarrow

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Is there zero zero gravity?
« Reply #37 on: 17/12/2009 14:17:00 »
I do tend to use Newtonian methods to describe interactions because, in many situations, those models are sufficiently accurate to describe the behaviour of massive bodies, and I find them a lot simpler to understand.
Then you are forced to ascribe a negative mass to the gravitational field: it's easy to prove it in newtonian mechanics...

http://www.negative-mass.com/
« Last Edit: 17/12/2009 14:18:31 by lightarrow »
 

Offline yor_on

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Is there zero zero gravity?
« Reply #38 on: 17/12/2009 23:48:14 »
LeeE please expand on what you mean when you say that a Langranian point can be both Gravitationaly equipotential, and at the same time not?

If you are there you will be 'immobilized', yes?

:)

Otherway I think gravity exist.
 

Offline Geezer

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Is there zero zero gravity?
« Reply #39 on: 18/12/2009 04:46:42 »
I do tend to use Newtonian methods to describe interactions because, in many situations, those models are sufficiently accurate to describe the behaviour of massive bodies, and I find them a lot simpler to understand.
Then you are forced to ascribe a negative mass to the gravitational field: it's easy to prove it in newtonian mechanics...

http://www.negative-mass.com/

Which demonstrates that it is possible to prove just about anything with math  ;D
 

Offline lightarrow

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« Reply #40 on: 18/12/2009 13:10:36 »
Which demonstrates that it is possible to prove just about anything with math  ;D
Have you tried to compute the energy of the gravitational field?
 

Offline Geezer

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« Reply #41 on: 18/12/2009 19:13:45 »
Which demonstrates that it is possible to prove just about anything with math  ;D
Have you tried to compute the energy of the gravitational field?

Uh-oh! I can see I'm heading for a beating here ;D I didn't think the gravitational field had any energy. Do you mean the energy of the objects in the gravitational field?
 

Offline LeeE

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Is there zero zero gravity?
« Reply #42 on: 18/12/2009 22:41:09 »
LeeE please expand on what you mean when you say that a Langranian point can be both Gravitationaly equipotential, and at the same time not?

If you are there you will be 'immobilized', yes?

:)

Otherway I think gravity exist.

I said that the Lagrangian points are in equipotential relative to us, but not in absolute equipotential.

Imagine a plot of gravitational strength over distance from two bodies, one in the +ve x-axis direction and the other in the -ve x-axis direction.  The y-axis represents the gravitational strength.

If you plot the gravitational strength over distance for each of the two bodies their curves will intersect at some point between them and it is at this point that gravitational equipotential exists.  Either side of that point is 'up hill'.  That point though, because gravity appears to have infinite range, will always be above zero in the y-axis and is thus not equal to zero gravity, which must be at zero on the y-axis.

Even with just two bodies, you can't have zero gravity, but I was also trying to show how the equipotential was also relative and not absolute due to us not being at a point of equipotential relative to the local supercluster, the effect of which is to apply an offset to the entire plot.
 

Offline lightarrow

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« Reply #43 on: 19/12/2009 11:05:58 »
Which demonstrates that it is possible to prove just about anything with math  ;D
Have you tried to compute the energy of the gravitational field?

Uh-oh! I can see I'm heading for a beating here ;D I didn't think the gravitational field had any energy. Do you mean the energy of the objects in the gravitational field?
Ok, let's start from before: do you know that electrostatic field has an energy? How do you compute it? Then do the same with gravitational field and tell me your result.
 

Offline yor_on

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Is there zero zero gravity?
« Reply #44 on: 19/12/2009 14:37:18 »
LeeE please expand on what you mean when you say that a Langranian point can be both Gravitationaly equipotential, and at the same time not?

If you are there you will be 'immobilized', yes?

:)

Otherway I think gravity exist.

I said that the Lagrangian points are in equipotential relative to us, but not in absolute equipotential.

Imagine a plot of gravitational strength over distance from two bodies, one in the +ve x-axis direction and the other in the -ve x-axis direction.  The y-axis represents the gravitational strength.

If you plot the gravitational strength over distance for each of the two bodies their curves will intersect at some point between them and it is at this point that gravitational equipotential exists.  Either side of that point is 'up hill'.  That point though, because gravity appears to have infinite range, will always be above zero in the y-axis and is thus not equal to zero gravity, which must be at zero on the y-axis.

Even with just two bodies, you can't have zero gravity, but I was also trying to show how the equipotential was also relative and not absolute due to us not being at a point of equipotential relative to the local supercluster, the effect of which is to apply an offset to the entire plot.

Okay, I think I see your point.
 

Offline Geezer

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« Reply #45 on: 20/12/2009 07:26:32 »
Which demonstrates that it is possible to prove just about anything with math  ;D
Have you tried to compute the energy of the gravitational field?

Uh-oh! I can see I'm heading for a beating here ;D I didn't think the gravitational field had any energy. Do you mean the energy of the objects in the gravitational field?
Ok, let's start from before: do you know that electrostatic field has an energy? How do you compute it? Then do the same with gravitational field and tell me your result.

I don't think I could. But can you help me understand why a gravitational field has energy? I can understand why matter in the field has energy by virtue of the field, but to me at least, that does not suggest the field has energy.
 

Offline lightarrow

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Is there zero zero gravity?
« Reply #46 on: 20/12/2009 13:34:37 »
Ok, let's start from before: do you know that electrostatic field has an energy? How do you compute it? Then do the same with gravitational field and tell me your result.
I don't think I could. But can you help me understand why a gravitational field has energy? I can understand why matter in the field has energy by virtue of the field, but to me at least, that does not suggest the field has energy.
It has energy for the same reason an electrostatic field has. If this last fact is not clear, then we have to discuss this first.
 

Offline Farsight

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Is there zero zero gravity?
« Reply #47 on: 20/12/2009 16:53:58 »
Geezer: an easier way to think about it to say that where there's a gravitational field, the properties of space have been affected by the central matter/energy of a planet. This is what Einstein said:

"According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν).."

The important point is that in a gravitational field, space is not homogeneous, and because space has "vacuum energy", this energy is not uniform. Ergo the space where a gravitational field is, has energy above and beyond that of free space.

NB: people often describe the gravitational field as negative energy, but this is an error related to "immersive scale-change" and whether two bodies falling towards each other truly gain energy.   
 

Offline Geezer

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Is there zero zero gravity?
« Reply #48 on: 21/12/2009 01:21:37 »
Uh oh! Thanks Lightarrow and Farsight. I think I'm going to have to do some studying.

I understand the concept that matter distorts space to produce the phenomenon that we call gravity. Is it correct to say the energy in space is a function of the degree of distortion of space? With physical objects it is not difficult to understand that they can store energy when they are stressed (as I recall that is true of dielectric stress also.) Or is this a different form of energy altogether?
 
 

Offline lightarrow

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Is there zero zero gravity?
« Reply #49 on: 21/12/2009 13:41:30 »
[...]
The important point is that in a gravitational field, space is not homogeneous, and because space has "vacuum energy", this energy is not uniform. Ergo the space where a gravitational field is, has energy above and beyond that of free space.
Can you expand this concept? I didn't understand very much.
Thanks.
 

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