Exploring Geometric Shapes of Gravity

[Dale]

Are you saying that the only thing defining a timelike geodesic is it's restriction to velocities below the speed of light? That's not complicated at all. Is it really that simple?

Yes, that is what it means for a worldline to be timelike. It is not just limited to geodesics, but applies to all paths in spacetime which are slower than light at each point.

 

[yuiop]

I think there IS change in motion of a particle resting on a table. The change is that it now isn't moving. It's inertial velocity is being held in check. The force to do this must be constant, which means that a particle resting on a table is undergoing a constant "change in motion" just to be "resting".

Now, subconsciously you are thinking in GR terms because you acknowledge that when a particle is "resting" on a table it requires a force to hold its natural inertial velocity (geodesic motion) in check. The next step is to realize that if the table breaks, the particle can follow its natural geodesic path and there are no forces acting on it (as it falls).

 

[Frame Dragger]

I do believe Hoku, that you've had the "A-Ha!" moment, also known as an Epiphany. Congrats!

 

[Hoku]

Now, subconsciously you are thinking in GR terms because you acknowledge that when a particle is "resting" on a table it requires a force to hold its natural inertial velocity (geodesic motion) in check. The next step is to realize that if the table breaks, the particle can follow its natural geodesic path and there are no forces acting on it (as it falls).

Ok, I acknowldege that there are no additional forces in play, but, I don't think that means gravity itself is not the product of a force. I believe the battle of whether or not gravity is a force has been fought on the wrong battlefield and that we need to broaden our view on the matter. Let's look again at the definition given from http://www.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture4/461_lecture4.html that says,

"A "force" is an action that changes, or tends to change, the state of motion of the body upon which it acts."

I think the problem is that we've restricted the realm of things that can be affected by a force to "bodies". Since spacetime is not a "body", then a force can have nothing to do with it. However, if we change the definition of a force to say: "A "force" is an action that changes, or tends to change, the state of motion of something else", then force can be applied to the changes that happen in spacetime. In the presence of gravity, spacetime moves. The state and motion of spacetime are changed from flat and still to curved and dynamic. It is the force inherent in objects that affects spacetime and causes gravity. Then, those same objects inertially follow the subsequent geometry.

 

[Frame Dragger]

Ok, I acknowldege that there are no additional forces in play, but, I don't think that means gravity itself is not the product of a force. I believe the battle of whether or not gravity is a force has been fought on the wrong battlefield and that we need to broaden our view on the matter. Let's look again at the definition given from http://www.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture4/461_lecture4.html that says, I think the problem is that we've restricted the realm of things that can be affected by a force to "bodies". Since spacetime is not a "body", then a force can have nothing to do with it. However, if we change the definition of a force to say: "A "force" is an action that changes, or tends to change, the state of motion of something else", then force can be applied to the changes that happen in spacetime. In the presence of gravity, spacetime moves. The state and motion of spacetime are changed from flat and still to curved and dynamic. It is the force inherent in objects that affects spacetime and causes gravity. Then, those same objects inertially follow the subsequent geometry.

If I read you correctly, especially the last part, you're asking "What is mass? Why does this deform spacetime, and spacetime in turn determines the path that mass follows?" The way this is formulated now is through mutual interaction and "feedback", at every step concievable in that geometry.

It's not possible for human being to imagine nothing, or a universe without some kind of "backdrop", but that doesn't mean that it isn't... not... isnt... ok, without getting into double and quintuple negatives... there may be NOTHING. Our inability to concieve of that, makes it no less likely, and evidence would seem to indicate that at some point the tower of turtles has to end.

Right now it's spacetime, but someday it could be Brane Cosmology, or something entirely different. Remember, there is a reason for the LHC; gravity is still mysterious, albeit not mystical. Describing it is one thing, explaining that behaviour is another.

Again, this is the "why" vs. the "how" question, I think.

 

[Hoku]

If I read you correctly, especially the last part, you're asking "What is mass? Why does this deform spacetime, and spacetime in turn determines the path that mass follows?"

Actually, I'm not asking at all. I believe mass is a manifestation of energy. I also believe that it deforms space because it is kinetic - meaning it's taking action. The action that it takes creates a field around the mass, which causes the phenomena of curved spacetime/gravity, similar to how a ball spinning in water creates suction. No questions from me.

It's not possible for human being to imagine nothing, or a universe without some kind of "backdrop", but that doesn't mean that it isn't... not... isnt... ok, without getting into double and quintuple negatives... there may be NOTHING. Our inability to concieve of that, makes it no less likely, and evidence would seem to indicate that at some point the tower of turtles has to end.

I'm not sure what "nothing" you're referring to but I can try to guess. Are you referring to when I said that the force of matter "acts" on spacetime, as if spacetime were something that already existed to act on? Are you approaching this from the "emergent" point of view in which spacetime did not exist before the big bang, so there could be no force to "change" it since it "emerged" in a curved fashion? I know this "emergent" view of space is popular, but it isn't the only accepted view, and it isn't the one that I choose to believe. (You should like the following quote, FD, because it talks about you! )

http://einstein.stanford.edu/SPACETIME/spacetime2.html

In 1918, Einstein described Mach's principle as a philosophical pillar of general relativity, along with the physical principle of equivalence and the mathematical pillar of general covariance. This characterization is now widely regarded as wishful thinking. Einstein was undoubtedly inspired by Mach's relational views, and he hoped that his new theory of gravitation would "secure the relativization of inertia" by binding spacetime so tightly to matter that one could not exist without the other. In fact, however, the equations of general relativity are perfectly consistent with spacetimes that contain no matter at all. [...] The bare existence of such solutions in Einstein's theory shows that it cannot be Machian in the strict sense; matter and spacetime remain logically independent. [...]

[...]Space and time do act on matter, by guiding the way it moves. And matter does act back on spacetime, by producing the curvature that we feel as gravity. Beyond that, matter can act on spacetime in a manner that is very much in the spirit of Mach's principle. Calculations by Hans Thirring (1888-1979), Josef Lense (1890-1985) and others have shown that a large rotating mass will "drag" an observer's inertial reference frame around with it. This is the phenomenon of frame-dragging, whose existence Gravity Probe B is designed to detect. The same calculations suggest that, if the entire contents of the universe were to rotate, our local inertial frame would undergo "perfect dragging" — that is, we would not notice it, because we would be rotating too! In that sense, general relativity is indeed nearly as relational as Mach might have wished. [...] Within the context of Einstein's universe, however, the majority view is perhaps best summed up as follows: Spacetime behaves relationally but exists absolutely.

Remember, there is a reason for the LHC;[...]

What is "LHC"?

 

[atyy]

Are you saying that the only thing defining a timelike geodesic is it's restriction to velocities below the speed of light? That's not complicated at all. Is it really that simple?

Yes. One way of visualizing this is that at each point in spacetime there is a light cone. A timelike path is one that always stays within the future lightcone. The difference between flat and curved spacetime is how the lightcones at different points are lined up.

http://physics.syr.edu/courses/modules/LIGHTCONE/introduction.html

I think there IS change in motion of a particle resting on a table. The change is that it now isn't moving. It's inertial velocity is being held in check. The force to do this must be constant, which means that a particle resting on a table is undergoing a constant "change in motion" just to be "resting". This seems like another classic "measurement problem" that we encounter in science. Here is that measurement problem, again. I wonder if the problem is that the an inertial particle, whether in flat space or curved, always has curved space around it. All particles curve space, right? They all produce "gravity". If all inertial movement is the result of different "gravities" interacting, then the particle is really just along for the ride. All the "work" is done in the spacetime surrounding the particle. As long as the gravities are free to be natural, the particle is "protected" by it's own "gravitational shield". Does this make sense?

The geodesic movement is a concept that applies only to test particles. A test particle is one whose mass-energy does not contribute to spacetime curvature. There is no such thing as a test particle, since all mass-energy contributes to spacetime curvature. However, in real life, there are particles whose mass-energy is small enough that their spacetime trajectories are very well approximated as test particles.

The full theory of general relativity does not contain test particles. It contains the gravitational field ("spacetime") and other fields like the electromagnetic field. Test particles are an approximation to the full theory, just like light rays are an approximation to light waves.

 

[AnthonySB]

My understanding or belief of Gravity has changed, I now no longer believe gravity is the curvature of space, indeed, "Gravity is Space".

 

[Frame Dragger]

Actually, I'm not asking at all. I believe mass is a manifestation of energy. I also believe that it deforms space because it is kinetic - meaning it's taking action. The action that it takes creates a field around the mass, which causes the phenomena of curved spacetime/gravity, similar to how a ball spinning in water creates suction. No questions from me.

I'm not sure what "nothing" you're referring to but I can try to guess. Are you referring to when I said that the force of matter "acts" on spacetime, as if spacetime were something that already existed to act on? Are you approaching this from the "emergent" point of view in which spacetime did not exist before the big bang, so there could be no force to "change" it since it "emerged" in a curved fashion? I know this "emergent" view of space is popular, but it isn't the only accepted view, and it isn't the one that I choose to believe. (You should like the following quote, FD, because it talks about you! )

http://einstein.stanford.edu/SPACETIME/spacetime2.html
What is "LHC"?

Well, it seems I have only one question left to answer "LHC" "Large Hadron Collider".
I'm guessing it was just the acronym, but just in case: http://public.web.cern.ch/public/en/LHC/LHC-en.html

Please, don't think I'm condescending however, it's just for anyone who reads this online and asks the same very reasonable question.

 

[Frame Dragger]

My understanding or belief of Gravity has changed, I now no longer believe gravity is the curvature of space, indeed, "Gravity is Space".

First, welcome to PF!

Second (and yes, this would be the hook lol)... what do you mean by "Gravity is Space"?

 

[AnthonySB]

Hi and thanks for the warm welcome.

It’s very strange but I have been working on a problem that has bothered me for some years. I am not a physics guru but I am completely hooked on physics, it’s totally brilliant, (I just wish I was, maybe things would be easier for me to work out! lol)

Well here goes, I shall try and explain if I can (me being a laymen in such matters), I believe now that space itself is actually Gravity, It is my understanding that gravity was actually the “curvature of space” for example,
A hypothetical situation
Suppose we have a universe which contains just one large round mass which is rotating clockwise, a rod is protruding out from the surface of the mass a mile high. One would naturally assume that the top of the rod is moving faster than the bottom as it appears that the top of the rod is traveling a further distance in the same amount of time.
I no longer believe this is an accurate assumption. I believe the top of the rod is covering the same distance as the bottom of the rod. The reason being is that I there is less space (gravity) at the top for the rod to pass through. The top of the rod is covering the same distance as the bottom, the reason being is that there is more gravity (space) at the base of the rod, so proportionally the distances covered are identical. Space is less compact (for want of a better word) at the top. It may be that distance between objects can only be truly measured by gravity (space). The strength of gravity is directly proportional to the amount of space, the two are the same. I maybe barking (or barking mad lol) up the wrong tree but its fun to think about regardless.

Take care, got to get back to work now 

Regards

 

[Mentz114]

For the second time this thread is hijacked .

AnthonySB you're off beam. If you took a literalist approach to GR, you might get away with 'gravity is space-time(curvature)'

 

[Frame Dragger]

For the second time this thread is hijacked .

AnthonySB you're off beam. If you took a literalist approach to GR, you might get away with 'gravity is space-time(curvature)'

It happens... I think he's sincere; he doesn't have that "here's my secret knowledge" air of a crank, and admits his lack of expertise. This is an educational site after all, and "the shape of gravity" is a bit vague. Heck, I'm not sure I could find a single unifying theme in this thread beyond a dialogue with Hoku and others about the nature of spacetime, gravity, mass, and related interactions.

After all, everyone can't just take "Oh, that's all in the SET" as an answer.

EDIT: @Anthony:... That being said, I think you're in way over your head. I don't mean that as an insult, but you just described an utterly impossible scenario, and one that is confirmed if you imagine a wheel. I think you've conflated notion of inflation of spacetime, gravity as the geometry of that spacetime, and how mass in turn further distorts that geometry, and (ideally) follows it. There are views of gravity as "force at a distance" (i.e. Newtonian gravity), but to say that gravity is space, is simplistic. You can definitely learn here, but starting at Z and working backwards never ends well in Relativity.

I hope this doesn't come across as insulting... I sometimes can be a bit tone-deaf to such things. If you ask questions here, people will leap to answer them, but meeting them with a personal theory is useful only as an illustration of what you do and do not understand.

If you do a search on this site for "parable of the apple" you might find something good. The notion of a Geodesic, may help you understand how local geometry at all points along "Alice's" worldline determine the perception of gravity as a force.

EDIT 2: Hey Mentz, sorry I was editing (adding, not subrating). I didn't think you were being critical, but it could be taken that way concievably by a new poster. If you told me I thread hijacked, I'd probably shut up and leave, rather than try to join the conversation and learn. Anyway, I wasn't trying to insult you either, but making an observation about Antony's level of knowledge regarding this subject. Refering to "space" and not "spacetime" (among other things... eh... heh) means that first principles are probably needed here. Consider it more of a critique on approach, not content or attitude.

 

[Mentz114]

It happens... I think he's sincere; he doesn't have that "here's my secret knowledge" air of a crank, and admits his lack of expertise. This is an educational site after all, and "the shape of gravity" is a bit vague. Heck, I'm not sure I could find a single unifying theme in this thread beyond a dialogue with Hoku and others about the nature of spacetime, gravity, mass, and related interactions.

After all, everyone can't just take "Oh, that's all in the SET" as an answer.

Hi FD,
what's your point ? I wasn't being critical of the poster. He's obviously not a crank and he did ask for comment on his idea.

 

[DaveC426913]

If you ask questions here, people will leap to answer them, but meeting them with a personal theory is useful only as an illustration of what you do and do not understand.

It would also be good to point out to AnthonySB that personal theories are forbidden here.

This is explicit in the PF Rules, to which he agreed when he signed up.

It is against our Posting Guidelines to discuss, in most of the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion. Personal theories/Independent Research may be submitted to our Independent Research Forum, provided they meet our Independent Research Guidelines; Personal theories posted elsewhere will be deleted. Poorly formulated personal theories, unfounded challenges of mainstream science, and overt crackpottery will not be tolerated anywhere on the site.

https://www.physicsforums.com/showthread.php?t=5374

Further persual of this speculation will result in deletion of posts, possibly warnings, and ultimately, banning.

 

[Frame Dragger]

It would also be good to point out to AnthonySB that personal theories are forbidden here.

This is explicit in the PF Rules, to which he agreed when he signed up.


https://www.physicsforums.com/showthread.php?t=5374

Further persual of this speculation will result in deletion of posts, possibly warnings, and ultimately, banning.

You're right, but I didn't feel like leading with a strong right-cross to the jaw.
I think there is a difference between discussing or promiting a personal theory, and essentially saying: "This is what I think", while admitting one is not a professional, and that this is not realistic. To be even more fair, I asked what he meant, and he answered... so if someone is really at fault here, it's probably me.

Unless he's a crackpot trying to evade a previous ban or the like, but as I'm not staff, I don't have to make those hard assumptions. Beyond that, I leave the responsiblity for forum guidlines being read to the person signing up, and the enforcement to staff. (beyond reporting of course).

 

[DaveC426913]

You're right, but I didn't feel like leading with a strong right-cross to the jaw.

Well, a right-cross would have been of he had gotten an official warning or points. I figure it's better if he hears it peer-to-peer.

... as I'm not staff...

Neither am I, btw.


Andrew, the better approach is, instead of forming a theory, just state your assumptions that lead you to think this way and we'll guide you to better answers.

 

[Hoku]

Andrew, the better approach is, instead of forming a theory, just state your assumptions that lead you to think this way and we'll guide you to better answers.

I support this 100%. I also want to add, for those in a helping position, that it can sometimes be difficult for a layman to recongnize the difference between "forming their own theory" and simply going on incorrect assumptions. This quote from DaveC is a good way to help awaken the asker about it so that progress can be made. Andrew, however, has suggested that he doesn't really have the time or energy to work through the misunderstanding, so he's putting his progress on hold.

Frame Dragger, when I opened my e-mail and read Andrew's first post I thought, "Oh no!" because I expected it to provoke some tradmark cruelty (from you, FD, as well as others). I was happy and surprised to see that you were trying a newer, ultimately more productive approach. I think you're trying to find a balance between "kindness" and "forthrightness". It's not an easy balance to find and I respect your efforts.

At any rate, unless there are any objections, I think I'm ready to wrap this thread up. It's been a long haul and I am SOOO grateful to everyone that has been kind and offered valuable insights. PIZZA PARTY! You're all invited, let's whoop it up! I've put some important projects on hold that I must get back to, after the pizza party, or course, so I may not be around the Forums much but I will be around.

 

[Frame Dragger]

Well, a right-cross would have been of he had gotten an official warning or points. I figure it's better if he hears it peer-to-peer.


Neither am I, btw.


Andrew, the better approach is, instead of forming a theory, just state your assumptions that lead you to think this way and we'll guide you to better answers.

Hmmm, good point. I think in this case I hesitate to do it because, although it's true that we're both not staff, you're... well... look, I'm not always the most diplomatic person.

That, and to be blunt, I'm quite new to this forum. Where you have the gravitas (pun!) to make such a warning stick, and my nature is such that once I scent "crank" I tend to become fascinated, which is no good for this forum of course. I don't mean fascinated by the theory, but the person; after all, every person is unique, and yet so many common themes pervade. Anyway, my agenda in these situations can rapidly turn to one that is an artifact of old, bad habits as an admin on rougher seas, which is to toy with, and then k-line the "barbarians storming the keep."

Needless to say, this makes my judgement suspect, and my lack of expertise in physics in general contributes as well. Of course, that being the case the lesson here is that I should have refrained from engaging in the first place, but like the cat I am curious to the end. Then again, my internal rejoinder is, "Curiosity killed the cat, but satisfaction brought it back." *shrug*

Your last sentence, is one I agree with entirely, but it seems not be the way it is for most people. I don't know why.

@Hoku: Thanks Hoku, it was as you describe, an effort to be civil and and respectful in an educational forum. I'm also glad that you've found this thread useful, and I am sorry for insults or casual cruelty when I first encountered you.

@Andrew: For the record, when I asked that question, I didn't expect a full theory, nor was I trying to trouble you. I just tend to prefer something along the Socratic method when unsure of another person. From there, I recognized that I was no longer the person to carry on there.

 

[Dale]

I am SOOO grateful to everyone that has been kind and offered valuable insights.

You are very welcome, I enjoyed the discussion.