Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: johnspannenburg on 07/01/2013 08:00:08
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So does anyone know just what gravity is??
Gravitons? are they a real thing? If spacetime really is curved (the marble on a sheet of rubber analogy) there isn't any need for gravitons as the marble just sits at the lowest point.
An object orbiting another object just goes along the curved path that result in net zero 'force'.
Curvature of spacetime makes orbiting object want to move towards the object being orbited and its 'straight line' motion makes it want to move away.. they balance and it goes around - why do we need gravitons at all.?
Alternatively if spacetime isn't curved or you just cant analogise this way then how do gravitons (theoretically) work?
As I understand it if we detect gravitational waves one day it will not be gravitons we are detecting but rather we will notice effects on other particles (photons say or variations in pulsar timings etc) from which we can infer the existence of gravitational waves as opposed to a 'direct' detection.
So what would it be about gravitons that makes the earth want to move towards the sun for example?
Anyone?
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Physicists think they understand gravity, and they think they understand the quantum theory of other subatomic particles, but they don't understand both of them together (which would be necessary to understand Hawking radiation around black holes in detail, for example).
Supporters of String Theory say they can produce something that looks like Gravitons, but String Theory also produces a lot of other stuff that doesn't look like the familiar electrons and protons. So the jury is still out on whether String Theory potentially unites quantum theory and gravity.
As I understand it, space is filled with various "Fields", and there are "Particles" which represent "Vibrations" of these fields:
- Vibrations of the electromagnetic field show up as the photon particle/wave, which are easily detected because we use them naturally for vision (and our technology uses them for lasers, X-Rays and radio/TV). Charged particles attract or repel each other via photons.
- Vibrations of the Higgs field show up as the Higgs particle/wave. These are not easily detectable because they require the massive energy levels of the LHC to produce them in reasonable numbers. Massive particles and objects interact with the Higgs field via Higgs particles, giving them inertia.
- Vibrations of the gravitational field show up as the graviton particle/wave. Massive objects (like Sun and Earth) attract each other via gravitons. Gravitons are not easily detectable because an individual graviton carries so little energy that we would have trouble detecting them. See: http://en.wikipedia.org/wiki/Graviton#Experimental_observation
While we can't see an individual photon with our eyes, we can see a laser beam in the visible spectrum - a laser is a coherent beam of many photons.
Similarly, we can't detect individual gravitons with even the the most sensitive detectors currently conceivable, but we still hold out hopes of that some sensitive detectors like LIGO might be able to detect gravity waves, which are coherent beams of gravitons produced by dramatic events such as coalescing black holes.
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The concept of Gravitons explains the impossibility of shielding against gravitational attraction if as is postulated the Graviton is much lighter than the Neutrino which would require a lead shield light years thick to stop the impossibility of shielding against Gravitons becomes apparent.
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johnspannenburg, I think Evan covered most of the details in his excellent post. But I'll address your question from a bit more philosophical side. You ask
Gravitons? are they a real thing? If spacetime really is curved (the marble on a sheet of rubber analogy) there isn't any need for gravitons as the marble just sits at the lowest point.
Remember that science deals in models, and no model is expected to be 100% accurate. When physicists say something like "space-time is curved" they generally mean that this model (general relativity) describes it as curved. Left unsaid is that the model is expected to sometimes be inaccurate. In particular, general relativity has problems dealing with very small, hot or dense things, such as the singularities of black holes or the very early universe. The equations predict infinities, which most physicists view as a sign that the theory is inaccurate at those scales.
It's natural to turn to quantum field theory, as Evan described, which is our best theory at describing very small things, as a way of describing gravity on those scales, and that leads to the idea of gravitons. (Actually coming up a consistent, testable theory is a completely different matter, and one of much ongoing research...)
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In flat space-time, SOMETHING causes masses to accelerate toward one another. In flat space-time, photons also accelerate toward masses; to preserve momentum, the masses must also accelerate (very slightly) toward the photons. Minkowski space-time is not flat because it eliminates the acceleration of photons by defining the path of light as a straight 4D line. Units of distance and time are defined by a certain emission from a cesium atom; that emission's wavelength and frequency are defined as constant in meters and seconds, regardless of where the cesium atom is. By definition, a cesium atom at the event horizon of a black hole emits the same wavelength in meters and frequency in seconds as a cesium atom isolated from all other masses. Defining meters and seconds in that way, and defining space-time as a 4D grid of meters and seconds, makes that space-time warped.
The popular belief is that the warp of Minkowski space-time is the CAUSE of gravity. That's putting the cart before the horse. The definition of meters and seconds is the cause of the warp, and the question of what causes gravity is swept aside as a frivolous pursuit of ontology. The mainstream of science isn't interested in ontology; they just want formulas that predict accurately. As long as GR's predictions are sufficiently accurate, there will be no incentive for the mainstream to investigate the real cause of gravity.
Ontological gravity theories are anathema to mainstream science, so they are not permitted on this board. You must go the New Theories board to discuss such theories.
Another popular belief is that a field is a real entity. A field is nothing but a mathematical description of how an effect depends on relative locations. The field is not the cause of the effect. For analogy, you could describe a football game as a set of force fields surrounding the ball and the players. That might result in a winning strategy for betting on the outcome of games, but it would not explain the motivation and strategy behind a single play. A mainstream football scientist only cares about winning bets. A football fanatic wants to understand the minute details of every player and every play. Fanatics don't always make good gamblers, but occasionally, a fanatic may discover a way to improve the field theory of football.
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So does anyone know just what gravity is??
We’ve always known what it is. It’s what makes things fall. When Newton came along he had a theory to describe it. When Einstein came along he had a theory to describe it from a different viewpoint and was more accurate. As yet there is no quantum theory of gravity.
Curved spacetime is merely a description of the nature of spacetime and is only an analogy so as to be able to visualize the math and physics better.
But nobody has ever come up with a mechanism as of yet
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But nobody has ever come up with a mechanism as of yet
Mechanisms have been proposed, like the Fatio/Lesage model, but no one has come up with a mechanism that isn't seriously flawed. At least no such model has yet been accepted by the mainstream as free of flaws. Browse the New Theories board, and you might find a model that is destined to be accepted by the mainstream, some day. Just don't expect to find it on this board because it will be removed until such time as the mainstream accepts it.
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Hmmm it would appear that, in fact, no one does really 'how' gravity works. As Phracticality says... we use the term 'field' which is nothing but a mathematical description and yet evan_au talks about fields 'vibrating' which seems to imply a real thing.
It seems 'gravity' and 'field' and 'graviton' etc are terms like 'dark energy' - place holder words almost - which don't have any 'real' meaning at all. I guess the predictive power of a theory is virtually (or perhaps really ;-)) paramount. I guess this is true in much of physics where terms ( in relation to particles) are used such as colour / spin / up / down etc which don't actually mean what you might assume from the normal meaning of the words. Again they are placeholder words for some property that I guess has no analogy to anything we can relate to in a real / meaningful way.
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It seems 'gravity' and 'field' and 'graviton' etc are terms like 'dark energy' - place holder words almost - which don't have any 'real' meaning at all.
Why stop there, on fundamental levels... What's a particle? what is charge? ect ect
On another thread somewhere on these boards I once mentioned the model of particles being waves or oscillations in fundamental fields, and said it’s fields all the way down.
Well, you could say of science and its study of the physical world… it’s models all the way down. How and When will we know we have the truth of anything fundamental and not just a ‘very’ good model?
And just how do you link to posts on other threads..
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It seems 'gravity' and 'field' and 'graviton' etc are terms like 'dark energy' - place holder words almost - which don't have any 'real' meaning at all.
Why stop there, on fundamental levels... What's a particle? what is charge? ect ect
On another thread somewhere on these boards I once mentioned the model of particles being waves or oscillations in fundamental fields, and said it’s fields all the way down.
Well, you could say of science and its study of the physical world… it’s models all the way down. How and When will we know we have the truth of anything fundamental and not just a ‘very’ good model?
And just how do you link to posts on other threads..
Particles made of waves in a medium made of particles made of waves... all the way down.
Each post has a message number, but I don't see any way to get that number except to find the post with a search and copy the link from the search result. That's how I obtained a link to this post of yours (http://www.thenakedscientists.com/forum/index.php?topic=45151.msg394557#msg394557) in the thread Can gravity exist without matter? There really should be a link icon on each post. (Linking to your own new theory, at least on the mainstream board, is frowned upon.)
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http://www.laboratoryequipment.com/news/2013/01/scientists-find-evidence-speed-gravity
This says nothing about what gravity is, but it is related and interesting.
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http://www.laboratoryequipment.com/news/2013/01/scientists-find-evidence-speed-gravity (http://www.laboratoryequipment.com/news/2013/01/scientists-find-evidence-speed-gravity)
This says nothing about what gravity is, but it is related and interesting.
I'm still skeptical; I think gravity propagates many times faster than light. If gravity propagates at light speed, the Sun's gravity should accelerate Earth in the direction where we see the Sun, not the direction where the Sun is now. If you put that "correction" into Kepler's laws of planetary motion, the planets would all be dragged into higher and higher orbits until they escape from the Sun's gravity, all together.
Here's that Chinese article (http://link.springer.com/article/10.1007%2Fs11434-012-5603-3) at SpringerLink, and here's another interesting article (http://link.springer.com/article/10.1007%2Fs10701-006-9059-7) which also claims to have demonstrated that the speed of gravity is the speed of light. It may take me forever to wade thru them in search of the fallacies.
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I'm still skeptical; I think gravity propagates many times faster than light. If gravity propagates at light speed, the Sun's gravity should accelerate Earth in the direction where we see the Sun, not the direction where the Sun is now. If you put that "correction" into Kepler's laws of planetary motion, the planets would all be dragged into higher and higher orbits until they escape from the Sun's gravity, all together.
Of course, the moon is receding from the Earth at a rate of about 4cm per year.
And, I'm seeing estimates that the Earth is also receding from the sun at a rate of about 15cm per year.
However, are you correct that the planets would be dragged into a higher orbit?
If we had "slow gravity", then the pull of gravity would always be behind the planets. For example, consider that the Earth is about 8.3 light minutes from the sun. During this period, the Earth travels about 14,871 km. So, if the gravity was pulling to where the Sun-Earth relationship was 8 minutes earlier, it should always be behind the Earth.
Even if you thought of gravity as being a static field around the sun. It would still be like the Earth was always moving uphill on the gravity gradient as everything in space is moving.
And, thus slow gravity should decrease the orbital energy, and send the planets into a lower orbit.
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The popular belief is that the warp of Minkowski space-time is the CAUSE of gravity. That's putting the cart before the horse. The definition of meters and seconds is the cause of the warp, and the question of what causes gravity is swept aside as a frivolous pursuit of ontology.
Could you here or somewhere esle start a thread to explain how a rock, from interstellar space, when nearing the sun changes its path just because of ''The definition of meters and seconds is the cause of the warp'', in what way does such a warp caused by the definition of meters and seconds make the rock alter its path when nearing the sun. How does the rock 'know' it's in the vicinity of the sun to start this acceleration of change in path/direction ? what's special about just being near the sun to make a change possible? Why didn't it make the change of path when it was in interstellar space? A big sorry Phractality if I have misunderstood you here and that is not your idea. Kisses. Remember we are told there is no force in GR, so that's out the window here.
Ps. I've given up on making a llnk...my life is only so long :)
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Even if you thought of gravity as being a static field around the sun. It would still be like the Earth was always moving uphill on the gravity gradient as everything in space is moving.
Would this also apply if you thought of gravity as being just a distortion of spacetime?
In that case would it not be that the major distortion occupied a fixed position relative to the sun; with a smaller distortion in place around the Earth?
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No, nobody know 'what' gravity is yet. There are theories and there are deductive math describing effects and how it may 'hang together' but to go from that to saying 'we know what gravity is' sounds rather presumptuous to me.
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And, by the way. I do not agree on a sheer 'wave universe', just as I won't agree on a sheer 'particle universe'. What we see is waves, and particles, presenting a duality. And they come to be under a specific direction that we call the arrow of time, pointing one way for us. What lies under those expressions may be a field of some sort, obeying that direction. And I also think that the field should be considered as static, ignoring that arrow.
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Now that can be considered very simplistic :) But consider the implications. What such a view do is to lift forward the arrow, to become something in its own right. And that is the most interesting thing about it, what that arrow would be, if so?
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Would this also apply if you thought of gravity as being just a distortion of spacetime?
The distortions in spacetime that you mention here are tidal gradients in the gravitational field. Since its concievable to have gravitational fiels without tidal gradients it wouldn't make much sens to think of gravity as a distortion in spacetime.
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The distortions in spacetime that you mention here are tidal gradients in the gravitational field. Since its concievable to have gravitational fiels without tidal gradients it wouldn't make much sens to think of gravity as a distortion in spacetime.
Pmb, I would like to understand you but you are a little over my head sometimes.
Sorry to put it like this but I'm puzzled. I'm certainly not trying to catch you out, i'm trying to lean ( yes, I know I shouldn't expect to lean on forums).
Do you agree that mass/energy alter space in their vicinity ? Yes or no
Or, do you agree that mass/energy alter spacetime in their vicinity ? Yes or no
Would an interstellar rock that by chance enters the solar system and starts to accelerate, would you say that acceleration has something to do with the sun's mass/energy Yes or No?
There are no forces in GR, so how does the sun 'transmit' this effect of its mass/energy to the nearby passing rock? what is it about the sun's vicinity that allows the rock to start accelerating?
A quickie... Are you saying gravity distorts space or spacetime and not the other way round i.e. distorted space or spacetime gives rise to gravity?
Sorry to put it this way.. Here's an apple sir.
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Pmb, I would like to understand you but you are a little over my head sometimes.
Oops! Sorry. My bad. I don’t mean to post in a manner that is over anybody’s head, really I don’t. However it’s hard to know how to explain something when you don’t know whom your audience is. The hope is that if I post something that is over someone’s head that they’ll be courageous enough to ask. Thanks for your courage. It’s both admirable and appreciated.
Sorry to put it like this but I'm puzzled. I'm certainly not trying to catch you out, i'm trying to lean ( yes, I know I shouldn't expect to lean on forums).
No worries my friend. That’s why I’m here.
Do you agree that mass/energy alter space in their vicinity ? Yes or no
I’m sorry but that does not have a yes/no answer to it. The answer is that matter “can” alter change space near it but it need not. Whether it does or not depends on the particular distribution of matter. But just because space is altered near an object it doesn’t mean that the spacetime is curved near the matter.
Or, do you agree that mass/energy alter spacetime in their vicinity? Yes or no
Again, this does not have not a yes/no answer to it. Some distributions of matter curve the spacetime near then and other don’t, it all depends on the distribution of matter.
Would an interstellar rock that by chance enters the solar system and starts to accelerate, would you say that acceleration has something to do with the sun's mass/energy
Yes, of course. Gravitational acceleration is caused by gravity. With the sun there is a gravitational field that has tidal gradients in it. The acceleration is caused by the gravitational acceleration, not due to the fact that the acceleration varies from place to place.
Yes or No?
There are no forces in GR, …
That is a common misconception. In GR there most certainly are gravitational forces. It’s just that the gravitational force is of a different nature than other kinds of forces. Non-gravitational forces are represented by 4-vectors. Gravitational forces are what’s called “inertial forces” and as such are not represented by 4-vectors. Inertial forces have the unique property that they can be made to vanish by an appropriate change of coordinates.
so how does the sun 'transmit' this effect of its mass/energy to the nearby passing rock?
Just like all interactions in physics. Objects generate a field and that field exerts forces on objects in the field
A quickie... Are you saying gravity distorts space or spacetime and not the other way round i.e. distorted space or spacetime gives rise to gravity?
Sorry to put it this way.. Here's an apple sir.
Let me put it like this. You’ve heard of tidal forces, right? Well in GR they still exist. They just have a different name. What Newton called tidal forces Einstein called spacetime curvature. Let’s consider an example of matter which generates a gravitational field in which there is no spacetime curvature.
Take a look at this web page that I created to explain this kind of thing. It’s at
http://home.comcast.net/~peter.m.brown/gr/grav_cavity.htm
If you have a spherical body which uniform mass density and cut out a spherical cavity centered at the origin of the sphere then there will be no gravitational field inside. However if instead of centering the cavity at the origin of the sphere you center of off of the origin. In this case there will be a gravitational field inside the cavity. The field inside the cavity will be uniform, i.e. there will be no tidal gradients in it. This is an example of a gravitational field with no spacetime curvature. The matter distribution generates the field inside the cavity but that field has zero spacetime curvature.
Now let’s consider what is called a vacuum domain wall. The matter that makes up the wall has the shape of an infinite plane. The matter has non-zero mass-energy in it. There is a negative pressure inside the wall. In GR there are several sources of gravity. Here there is the mass-energy source as well as the pressure. Pressure is a source of gravity in GR. Now outside the wall there is a gravitational field on each side of it. It extends to infinity. Objects that are placed outside the wall will be accelerated away from the wall (negative pressure contributes to repulsive gravity). In this case there are no tidal gradients in the gravitational field. Therefore there is no spacetime curvature anywhere outside the source of gravity.
Now let’s examine an infinitely straight cosmic string. Such a string is incredibly thin, so thin you couldn’t see it with a strong microscope. There is mass-energy in the string plus negative pressure. In this case the contributions cancel out and objects place outside of the string will not accelerate anywhere. They remain at rest. Therefore there is no gravitational field generated by such an object. However it does alter the space around it. The string changes the topology from that of a place to that of a cone. What this means in practice is hard to describe without diagrams but basically its like a wedge of space is cut out of the space around it and along the string. If you were to place two parallel lines on opposite sides of the string then while on the original side the lines remain parallel as they go off into infinity. But on the other side the lines cross each other.
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Pmb
I’m sorry but that does not have a yes/no answer to it. The answer is that matter “can” alter change space near it but it need not. Whether it does or not depends on the particular distribution of matter. But just because space is altered near an object it doesn’t mean that the spacetime is curved near the matter.
PmbSome distributions of matter curve the spacetime near then and other don’t, it all depends on the distribution of matter.
So, in the above two quotes are you saying the sun does alter space or spacetime around its vicinity? Yes or No…
But it’s also possible in the event of an evenly distribution of matter for there to be a mutual cancelling-out and so no curvature results? Is that what’s happening here?
Sorry to put it like that almost repeating you, but I wanted to be specific.
PmbLet’s consider an example of matter which generates a gravitational field
What is matter or the sun doing to generate a gravitational field, are they altering space or spacetime around them, so that a smaller interstellar rock passing, by chance, close by would have its path altered?
Or, are we talking of a Newtonian force and nothing to do with space or spacetime around the sun?
so how does the sun 'transmit' this effect of its mass/energy to the nearby passing rock?
Just like all interactions in physics. Objects generate a field and that field exerts forces on objects in the field
So, your saying Newtonian force here and nothing to do with mass/energy altering space or spacetime in their surroudings?
PmbThat is a common misconception. In GR there most certainly are gravitational forces.
Again,do you mean like the force which Newton postulated or is it something to do with being in the space or spacetime near matter?
Will read the rest of your post when I have time, thanks for your link...even distribution of matter and cancelling out in a cavern in a sphere.
I
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So, in the above two quotes are you saying the sun does alter space or spacetime around its vicinity? Yes or No…
Yes.
But it’s also possible in the event of an evenly distribution of matter for there to be a mutual cancelling-out and so no curvature results? Is that what’s happening here?
What do you mean by “here”? In the case of the sun, no. In the case of the cavity that I explained, yes.
Sorry to put it like that almost repeating you, but I wanted to be specific.
No worries.
What is matter or the sun doing to generate a gravitational field, are they altering space or spacetime around them, so that a smaller interstellar rock passing, by chance, close by would have its path altered?
The sun is altering both space and spacetime as well as generating a gravitational field.
Or, are we talking of a Newtonian force and nothing to do with space or spacetime around the sun?
I don’t know what you mean by a “Newtonian force.” Do you mean a 3-force, i.e. F = dp/dt?
so how does the sun 'transmit' this effect of its mass/energy to the nearby passing rock?
Nobody knows. All physicists know is the description of gravity. They don’t know the mechanism of gravity.
Again,do you mean like the force which Newton postulated or is it something to do with being in the space or spacetime near matter?
Newton never postulated a force. Newton only postulated the relationship between forces and bodies. In his Principia Newton gave a relationship between force and momentum and that’s F = dp/dt. Nowadays physicists take that as the definition of force and as such its independent of any particular theory.
Will read the rest of your post when I have time, thanks for your link...even distribution of matter and cancelling out in a cavern in a sphere.
Just make sure you don’t confuse the difference between the absence of tidal forces (i.e. spacetime curvature) with the absence of gravitational forces (i.e. mass*gravitational acceleration). They aren’t the same thing.
Question: I assumed that you knew what tidal forces were when I wrote all of that. Perhaps I shouldn’t have made that assumption. Do you know what the definition of “tidal forces/tidal gradients” is?
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I’d like to explain something that might help clear things up.
A uniform gravitational field is defined by the relationship G_z = -mg where G is the gravitational force on the body. The acceleration of the body due to the gravitational field is in the –z direction and has the magnitude g. Note that the acceleration due to gravity is the same no matter where in the field the body is located.
The potential energy of a body in such a field is given by Phi = mgz. The tidal gradients in such a field is zero even though there is a gravitational acceleration. Since the tidal gradients are zero the spacetime curvature is zero. This is an example of a gravitational field with no spacetime curvature.
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Pmb
Question: I assumed that you knew what tidal forces were when I wrote all of that. Perhaps I shouldn’t have made that assumption. Do you know what the definition of “tidal forces/tidal gradients” is?
I think I do now after your post here…
Pmb The potential energy of a body in such a field is given by Phi = mgz. The tidal gradients in such a field is zero even though there is a gravitational acceleration. Since the tidal gradients are zero the spacetime curvature is zero. This is an example of a gravitational field with no spacetime curvature.
So, I would say tidal forces are the strength/ potential at different points of a gravitational field. If the field is uniform the accelerations are the same anywhere in the field. If the field has tidal gradients, then you would have different accelerations at different points of the field? Have I mis-understood?
I would describe the gravitational field around the sun has symmetrical in shape and
so symmetrically tidal as you rise from its surface. Same for the earth.
What do I mean by a Newtonian force? All I can say really is that he never involved altering space/spacetime in his ideas. Didn’t he say it would be an insane person that could describe the mechanism?
PmbWhat do you mean by “here”? In the case of the sun, no. In the case of the cavity that I explained, yes.
By here, I meant the case of an even distribution of matter and not the sun, I can understand your answer for both cases.
Thanks for answering. I feel I may have more questions, but I will digest your posts and link more now, the answers may be there
Thanks again for time.
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I think I do now after your post here…
Excellent!! It’s wonderful to know that all my work hasn’t been for not.
So, I would say tidal forces are the strength/ potential at different points of a gravitational field. If the field is uniform the accelerations are the same anywhere in the field. If the field has tidal gradients, then you would have different accelerations at different points of the field?
Awesome! Now you’ve got it down cold.
What do I mean by a Newtonian force? All I can say really is that he never involved altering space/spacetime in his ideas. Didn’t he say it would be an insane person that could describe the mechanism?
Force doesn’t always curve spacetime. In fact force can be defined outside of any use of spacetime. E.g. when one is doing relativistic electrodynamics they don’t have to use the spacetime approach and they don’t have to use 4-force. You can use the definition F = dp/dt. In fact many undergrad relativity texts still do that. But the existence of gravitational force does not mean that spacetime is curved.
Thanks for answering. I feel I may have more questions, but I will digest your posts and link more now, the answers may be there
Thanks again for time.
It’s my pleasure! I have to be confined to my house because walking too far is dangerous to me. If I’m going to be stuck at home for most of the rest of my life at least I can feel useful by doing things like this. So I get a lot out of helping you. So thanks for asking! :)
FYI - If you ever want to pick up an excellant text on this stuff but only know algebra then the best text that I know of is Gravity from the Ground Up. by Bernard Schuitz. It has an entire chapter on tidal acceleration which is wonderful because such descriptions are nearly impossible to find and most textbooks. In fact out of all the textbook that I have, which are listed at http://home.comcast.net/~peter.m.brown/ref/physics_textbooks.htm, this text is the only one which describes tidal forces and acceleations other than Ohanian's GR text.
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Thanks for the book recommendations.
I think I can understand the non-vanishing of tidal gradients leading to relativists calling it ‘curved’ space.
I understand a person in a rocket accelerating at one g could also believe he was standing on earth’s surface.
I don’t understand on a practical level why a uniform gravitational field can be made to vanish by changing to another co-ordinate frame. For instance… Only in regions throughout the universe do you find areas of tidal gradients and in other areas uniform gravitational fields.
So, this means at some point or location you have to enter or exit a uniform gravitational field. A rock would undergo an acceleration or deceleration at that point of entering/exiting such a uniform field. You can’t make that particular point vanish, it comes with the territory so to speak. Is that cross over point a tidal point?
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It's about experimentally proving that a gravitational field exist. If you are free falling in a black box without tidal influences on it you have no way of defining if there is a gravity or not. And what the experiments tell you is actually your reality inside that box. That someone else (on some planet maybe) seeing that box fall will define it as it is falling inside a gravitational field, following a geodesic is true for that observer, but as far as you is concerned the gravity he finds doesn't exist. I think you can call that different coordinate systems being used, yours and his.
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yor_on
Yes, I can understand and agree with your description, I think I was being trivial and perhaps over pernickety asking about entering and leaving points of a uniform field.
I can see why you answered, my ''I don’t understand on a practical level why a uniform gravitational field can be made to vanish by changing to another co-ordinate frame.'' Now I'm embarrassed, where's the smelling salts...
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No worries, know you're a smart guy. And furthermore I hope we all are allowed to question life, the universe and all, no matter where it takes us? Sometimes when I get into a certain set of mind I also ask questions that I already know the answer too, realizing it a day later :)
But sometimes it helps me to see the answer better too.
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And it's a question about what defines reality I think, and so in no way trivial. We have a universe in where we use repeatable experiments to decide that, and the results they present gives me no little headache.