Why does mass cause gravity

  • Thread starter gsingh2011
  • Start date
  • #1
gsingh2011
115
1
I've seen the images of a large mass creating a dent in the space-time fabric, and I've read the analogies as how the objects "fall in" towards the dent, causing gravity. But this is just an analogy, not an explanation. The objects would only fall in if there was gravity, so this cannot be an explanation; you can't use the term you're defining in the definition. So what actually causes the gravity?
 

Answers and Replies

  • #2
Nabeshin
Science Advisor
2,207
16
Find out and you're in for a nobel prize.

We can describe it very well mathematically, but as far as why is concerned... well that's a much more difficult question. We posit that mass causes gravity in a predictable way, and this appears to be very consistent with the universe we live in. For most physicists, I think, this is sufficient.
 
  • #3
DrGreg
Science Advisor
Gold Member
2,394
1,426
I've seen the images of a large mass creating a dent in the space-time fabric, and I've read the analogies as how the objects "fall in" towards the dent, causing gravity. But this is just an analogy, not an explanation. The objects would only fall in if there was gravity, so this cannot be an explanation; you can't use the term you're defining in the definition. So what actually causes the gravity?
You are quite right -- using gravity to explain gravity makes no sense, so the "ball on a rubber sheet" picture fails to explain gravity. The problem is that that picture depicts space, not spacetime.

For objects moving in one dimension of space, spacetime is just a graph of distance against time. An object in space becomes a line in spacetime. In the absence of gravity, a freely moving object is represented by a straight line drawn on a flat piece of paper. When there is gravity, we have to draw the line on curved paper. Two "straight" lines that start off as parallel will converge later on.

For further information, try http://www.relativitet.se/spacetime1.html
 
  • #4
Altabeh
660
0
I've seen the images of a large mass creating a dent in the space-time fabric, and I've read the analogies as how the objects "fall in" towards the dent, causing gravity. But this is just an analogy, not an explanation. The objects would only fall in if there was gravity, so this cannot be an explanation; you can't use the term you're defining in the definition. So what actually causes the gravity?

This is one of the questions that needs to change in such a way that one be really able to answer it! I mean that only given gravity as a fundamental force in nature (getting for one moment out of the explanation credited by GR about gravity) we can say why the objects fall in towards the dent. This has something to do with the fact that any question about the nature of gravitational force is an open problem and been largely dragged into consideration in the 1930s via introducing particles that mediate this force, the so-called gravitons. I.e. as an interacting force, gravity does welcome gravitons as the 'photons' of the gravitational interactions because they are postulated to be massless due to gravity having unlimited range. For a brief introduction to this subject, see http://en.wikipedia.org/wiki/Graviton

AB
 
Last edited:
  • #5
Temporal
11
0
I've seen the images of a large mass creating a dent in the space-time fabric, and I've read the analogies as how the objects "fall in" towards the dent, causing gravity. But this is just an analogy, not an explanation. The objects would only fall in if there was gravity, so this cannot be an explanation; you can't use the term you're defining in the definition. So what actually causes the gravity?

Objects don't cause gravity as they "fall in" to a gravity well. Gravity is the distortion of spacetime which is caused by matter embedded in spacetime. This distortion creates a "gravity well" with the bottom of the well being the center of mass of the object creating the distortion.

Here's a very simplified scenario. Say you have two objects, one of which is large and the other small. Now imagine the smaller object moving through flat spacetime and then encountering the curved spacetime created by the larger object's gravity well. The smaller object will merely travel along the line of least resistance...which would be the curved spacetime. The behaviour of the smaller object as it travels through this curved spacetime would be dependent on things such as its speed, its mass, the angle at which it enters the curved space, etc. The smaller object may fall down the gravity well until it strikes the larger mass...or it might be caught in orbit around the larger mass...or it may be moving fast enough to travel through the curved spacetime and leave the gravity well with a different trajectory.

Given this, I think the analogy you speak of is a correct, though overly simplified, explanation of what is occurring.

I think the big question is the mechanism behind the distortion. What is it about energy (matter being just another manifestation of energy) that distorts spacetime?
 
  • #6
Hoku
160
0
Using a term to define itself, as you said, is circular reasoning. This is the very thing I've been struggling with in the forums over the past few weeks, although from a different angle. The conclusion I've come to is that gravity works because it is a FORCE. However, since gravity doesn't have to be viewed as a force in relativity theory, lots of people have decided to abandon the connection between them.

I now believe that relativity theory is a symbolic language, in just the same way that math is. We use it as a tool to make predictions of objects in gravity. But it seems that people have taken that tool too seriously. We would be taking math too seriously to say that "math is geometry". Math is only symbolic of geometry. Likewise, I think we need to acknowledge that relativity theory is not gravity, it is symbolic of gravity. Gravity is best described as a force but relativity theory is the tool that we use to make predictions of that force. "Falling into the dent of spacetime fabric" is a symbolic representation that we should not take too literally.

This is something that I think is very important and I take it very seriously. So, as long as gsingh2011 feels this is in line with his/her question and permisses me, then I ask anyone who disagrees with what I've said to speak up. You may regret it later if you don't because, unless I have reason not to, I'm moving forward with this.
 
  • #7
gsingh2011
115
1
I understand Temporal's explanation about the line or least resistance (some of you may want to look up geodesics). Hoku, I believe what I said above is most likely the correct answer. Therefore, we don't just use relativity to make predictions about gravity, a separate force, the dent in spacetime actually causes the gravity. I hope I understood you correctly though...
 
  • #8
Temporal
11
0
"Falling into the dent of spacetime fabric" is a symbolic representation that we should not take too literally.

This is something that I think is very important and I take it very seriously. So, as long as gsingh2011 feels this is in line with his/her question and permisses me, then I ask anyone who disagrees with what I've said to speak up. You may regret it later if you don't because, unless I have reason not to, I'm moving forward with this.

What is it exactly that causes you to discount the idea that gravity is simply the curvature of spacetime due to the presence of matter? I'm not speaking of what actually caused the curvature (matter's interaction with the fabric of spacetime), but rather the behaviour we can directly observe due to the curvature itself. Are you saying that spacetime is not curved by the existence of matter or are you saying that the curvature really plays no role in how matter moves through spacetime?
 
  • #9
Hoku
160
0
If we take the image of "mass denting spacetime" literally, then we are making the statement that spacetime is a physical object that can be "dented". I am 90% certain the popular view is that spacetime is NOT a "thing", it is a "phenomena". A phenomena is useful and real, but that does not make it an "object". Does anyone disagree with this?
 
  • #10
Nickelodeon
181
1
...... So what actually causes the gravity?

The quantum gravity school have one idea and the general relativity school have another and I'm not sure how close they are to finding some common ground. I think the quantum gravity school get the most funding with their search for the Higgs boson.

Have you Googled 'What causes gravity'? Other theories tend to fall into three or four camps -
1) the expanding universe, where all matter including atoms are experiencing an accelerated expansion which provides our 1g.
2) Wave theories, which I guess would include the graviton idea.
3) Shadow effects, which can be similar to the previous idea but more particle based - Le Sage is one of the original proponents.
4) Matter decay is another.

take your pick ;-)
 
  • #11
Nabeshin
Science Advisor
2,207
16
Using a term to define itself, as you said, is circular reasoning. This is the very thing I've been struggling with in the forums over the past few weeks, although from a different angle. The conclusion I've come to is that gravity works because it is a FORCE. However, since gravity doesn't have to be viewed as a force in relativity theory, lots of people have decided to abandon the connection between them.

I now believe that relativity theory is a symbolic language, in just the same way that math is. We use it as a tool to make predictions of objects in gravity. But it seems that people have taken that tool too seriously. We would be taking math too seriously to say that "math is geometry". Math is only symbolic of geometry. Likewise, I think we need to acknowledge that relativity theory is not gravity, it is symbolic of gravity. Gravity is best described as a force but relativity theory is the tool that we use to make predictions of that force. "Falling into the dent of spacetime fabric" is a symbolic representation that we should not take too literally.

This is something that I think is very important and I take it very seriously. So, as long as gsingh2011 feels this is in line with his/her question and permisses me, then I ask anyone who disagrees with what I've said to speak up. You may regret it later if you don't because, unless I have reason not to, I'm moving forward with this.

I disagree completely. A force is no more real or correct than a geometrical explanation of gravity is. After all, what is a force? I can't measure it directly. I have to use other notions in order to describe this mystical "action at a distance".

Of course, perhaps you're referring to the fact that forces are traditionally represented by carrier particles, so we should view gravity as arising from gravitons. But this is still quite an unknown thing, and I don't think attributing gravity to tiny, mass-less particles is any more rewarding or true a description than the geometrical definition. In fact, I prefer the geometric one!

It's similar to the discussion which arises in classical mechanics when only potentials, not forces, enter into the fundamental equations. In this sense, the potential really is what's fundamental, not the force. In my mind, this situation is completely analogous.

Await your response!
 
  • #12
Temporal
11
0
If we take the image of "mass denting spacetime" literally, then we are making the statement that spacetime is a physical object that can be "dented". I am 90% certain the popular view is that spacetime is NOT a "thing", it is a "phenomena". A phenomena is useful and real, but that does not make it an "object". Does anyone disagree with this?

Isn't the theory of "Gravitational Lensing" based on curved spacetime? The more massive an object is, the greater the lensing effect? This would seem to directly corroborate "mass denting spacetime".
 
  • #13
Frame Dragger
1,477
1
The issue here is that "Why" is a philosophical or metaphysical question, not a question of Physics. Physics is all about "How", and "Why" is only asked of PHYSICAL PHENOMENA. "Why does the apple fall", not "Why is there gravity to make the apple fall (with continuing reducto...)". The former is a question for physics, the latter is metaphysics or philosphy.

As for Relativity, most of it is useful in describing HOW things occur, such as the motion of planets and stars, and far more complex phenomena. It is recent that access to some modicum of this knowledge was made so publicly available; without it seems, a similar focus on what is science, and what is not.

I'm not saying that "Why?" is not a good question to ask... just that if you're looking for something more than physical theories, you're barking up the wrong tree!
 
  • #14
Hoku
160
0
A force is no more real or correct than a geometrical explanation of gravity is.
I think what you're saying here is that they are essentially interchangable ideas. If so, I'm inclined to disagree. Gravity as a force is very different from gravity as geometry. This can be understood by looking at a definition of a force found here, http://www.uoregon.edu/~struct/cours..._lecture4.html: [Broken]

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

What this is saying, is that a "force" has energy that does work to an object. So, when gravity is a force, it acts upon objects. Contrairily, when we look at gravity as geometry, it's no longer something that has energy or does work. Instead, it simply becomes a "path" that an object with it's own energy follows. In one scenario, gravity is active. In the other it is passive. There is a very big difference between these.


I have to use other notions in order to describe this mystical "action at a distance".
The geometrical explanation of gravity still leaves "mystical" loopholes. Here's a link (courtesy of another user from another thread) that describes, very simply, how gravity works. http://www.adamtoons.de/physics/gravitation.swf . If you click on the "help" button at the bottom of the page and then the "examples" header at the top of the next page, you find this quote:

"When you push play, you see in the 3-D view, how the object is moving along its world line. The world line changes it's direction in regard to the dimensions because it is taking the straightest possible way."

This is the geometric view of gravity. It states that objects return to Earth because "it is the straightest possible way". However, there are countless geodesics or "straightest ways" that an object can follow depending on where it is going. So the "mystical" loophole that we can't avoid is, where is the object trying to go?

Think about this:
"The world line changes it's direction...because it is taking the straightest possible way."
So if we ask, "The straightest possible way to where?" the answer is Earth. So if we ask, "Why is it's energy directed at the Earth?"
Then, the we are brought back to the initial statement, "because it's the straightest possible way."
This is more circular reasoning!

...I don't think attributing gravity to tiny, mass-less particles is any more rewarding or true a description than the geometrical definition. In fact, I prefer the geometric one!
I think that gravitons are an attempt to explain the force of gravity (I'm not partial to gravitons one way or the other). Unfortunately, there is still something that even gravitons can't explain - basic, pure energy.
 
Last edited by a moderator:
  • #15
Dale
Mentor
Insights Author
2021 Award
33,032
10,291
I am 90% certain the popular view is that spacetime is NOT a "thing", it is a "phenomena".
How do you scientifically define "thing" and "phenomena"? What experiment can you perform to distinguish "things" from "phenomena"?
PS I was hoping that you would come back to the question of why doesn't a planet fall straight into the sun along a straight line geodesic.
 
  • #16
atyy
Science Advisor
14,769
3,296
The geometrical explanation of gravity still leaves "mystical" loopholes.

The laws of physics divide into dynamical laws and initial conditions. It is the initial conditions which determine if a test particle takes a spacetime geodesic that is a closed periodic orbit, or whether it falls radially. You can call the initial conditions "mystical" if you like. But it is not any different in Newtonian mechanics.
 
  • #17
Nabeshin
Science Advisor
2,207
16
Echoing atty's post, what do you find mystifying? Geodesics are given by:
[tex]\frac{d^2 x^\alpha}{d \tau^2} = -\Gamma ^\alpha _{\beta \delta} u^\beta u^\delta[/tex]
So with initial conditions, namely an initial 4-position and an initial 4-velocity, the geodesic should be determined on a given Riemann manifold, no?
 
  • #18
Hoku
160
0
The laws of physics divide into dynamical laws and initial conditions. It is the initial conditions which determine if a test particle takes a spacetime geodesic that is a closed periodic orbit, or whether it falls radially. You can call the initial conditions "mystical" if you like. But it is not any different in Newtonian mechanics.
I don't think this statement conflicts with anything I've said. Nabeshin was giving one reason why the "geometric" view of gravity is better than the "force" view of it. The reason he gave was that geometry can be explained and understood whereas a "force" necessitates some "mystical" unknown. I think this is a fine idea, however, even the geometric view - and the graviton view - can't avoid any "initial conditions" that may forever remain mysterious. In all three cases, the "mystical" unknown that Nabeshin is trying to avoid are exactly the same thing - energy that moves something. Consequently, I think this reason for choosing geometry over force is a little weak.

How do you scientifically define "thing" and "phenomena"? What experiment can you perform to distinguish "things" from "phenomena"?
You're right! "Things" and "phenomena" are both nouns. However, I think we can differentiate "tangible" nouns from "intangible" ones. An "idea" is a noun but I wouldn't expect to float a planet on it or move an apple with it because it's an "intangible phenomena".

I'm not suggesting we try to answer the "unanswerable", only that we resolve whether gravity is an active force or a passive geometry. The "unanswerable" will follow us wherever we go, so it's just not a good reason to choose one over the other.
 
  • #19
Frame Dragger
1,477
1
This may be terribly naive of me, but couldn't the notion of spacetime curvature (which seems pretty clearly demonstrated) as gravity and the notion of gravity as a force (with a QFT to describe it) doesn't seem to be a contradiction. They could be two accurate ways of describing the same phenomena, but in different conditions (not entirely unlike Newtonian vs. Relativity's predictions for orbits).

Gravity is ALREADY anamolous in all sorts of ways (range, strength, ALWAYS 'attracting' vs other forces), in ways that are as yet explicable only in conjecture or theory. Perhaps gravity is the equivalent of Electroweak at our energy levels, or perhaps it's a force field, but unlike others it distorts spacetime, while following its geometry.

If this were something which could be answered with confidence, there would be no Large Hadron Collider.

@Hoku: Mysterious is different from Mystical. We may never understand initial conditions, but they might be (in a global sense) very bland. Hell, maybe the Brane cosmologists are right, and we'e just the product of a collision between membranes. The point is, if you mean "mystery, that's one thing, but Mystical = magic, religion, supernatural. A mystery is accepted in science... a mystic should be shot on sight, then asked questions. :wink:
 
  • #20
Nickelodeon
181
1
"A "force" is an action that changes, or tends to change, the state of motion of the body upon which it acts."

What this is saying, is that a "force" has energy that does work to an object. So, when gravity is a force, it acts upon objects..

The trouble is with this definition it doesn't necessarily follow if you change it around i.e

" if the state of motion of a body changes a force is acting upon it."

The reason I say this is that if this were true then you would actually feel an acceleration in free fall and you don't. So gravity can't really be considered as a force.
 
  • #21
Frame Dragger
1,477
1
The trouble is with this definition it doesn't necessarily follow if you change it around i.e

" if the state of motion of a body changes a force is acting upon it."

The reason I say this is that if this were true then you would actually feel an acceleration in free fall and you don't. So gravity can't really be considered as a force.

That has to be the worst possible reasoning I've heard in a long time. Are you seriously making the case for Gravity as a pseudo-force, because we don't FEEL constant acceleration in free-fall? :eek:
 
  • #22
Nickelodeon
181
1
That has to be the worst possible reasoning I've heard in a long time. Are you seriously making the case for Gravity as a pseudo-force, because we don't FEEL constant acceleration in free-fall? :eek:

It's not a pseudo-force, or any force at all. It depends on whether you follow the Newtonian doctrine or Einstein's.
 
  • #23
Frame Dragger
1,477
1
It's not a pseudo-force, or any force at all. It depends on whether you follow the Newtonian doctrine or Einstein's.

I understand USING Newtonian formulations for everyday calculations where Relativistic effects are negligable, but what the hell do you mean by "doctrine"? The issue is utility in a given context, and neither are doctrine, nor is there some ongoing debate, "Was Newton right after all?!" I don't believe you know what you're talking about.

Gravity may be mediated by gravitons, in which case it is a force with a field. That is not the Newtonian view, it's just pure Standard Model. If Gravity is only a matter of geometry, then it wouldn't be a force. That is an unanswered question as of yet, and in the meantime treating Gravity as a fundamental force is hardly mad, if you don't forget the underlying principles in Relativity.

Anyway, my point wasn't that Gravity is a force, but that your way of reasoning and getting to that conclusion was... at best... odd.
 
  • #24
Ich
Science Advisor
1,931
1
That has to be the worst possible reasoning I've heard in a long time.
Ok, but 103 years ago, it proved to be very successful. Ein glücklicher Gedanke.
Are you seriously making the case for Gravity as a pseudo-force, because we don't FEEL constant acceleration in free-fall?
Actually, that's quite precisely the definition of a pseudo force. And the starting point of GR.
 
  • #25
Nickelodeon
181
1
I understand USING Newtonian formulations for everyday calculations where Relativistic effects are negligable, but what the hell do you mean by "doctrine"? The issue is utility in a given context, and neither are doctrine, nor is there some ongoing debate, "Was Newton right after all?!" I don't believe you know what you're talking about.

Gravity may be mediated by gravitons, in which case it is a force with a field. That is not the Newtonian view, it's just pure Standard Model. If Gravity is only a matter of geometry, then it wouldn't be a force. That is an unanswered question as of yet, and in the meantime treating Gravity as a fundamental force is hardly mad, if you don't forget the underlying principles in Relativity.

Anyway, my point wasn't that Gravity is a force, but that your way of reasoning and getting to that conclusion was... at best... odd.

You seem to have opened up several independent issues here - doctrine and utility, whether I know what I'm talking about, what constitutes the standard model and back to the original point which is should we be thinking of gravity as a force or whether it is a geometric effect?
 
  • #26
Frame Dragger
1,477
1
Ok, but 103 years ago, it proved to be very successful. Ein glücklicher Gedanke.

Actually, that's quite precisely the definition of a pseudo force. And the starting point of GR.

103 years ago, yes, but thankfully we percieve the passage of time, and a century or later it's really just a 'happy thought'. As far as that being the definition of a pseudo-force, yes, but is that how you would make the case for gravity as a pseudo-force, NOW?! I didn't say that wasn't a fine definition of a fictitious force, I said it wasn't a good way to make the case for GRAVITY being a fictitious force. Einstein thought of a free-falling object in a box, but to be blunt, we're at a crossroads where the nature of gravity is uncertain!

As I said, if there is a graviton, it would be a force, and if it's geometry only, it wouldn't. I still find it odd using first principles that Einstein used to formulate GR as a means to define gravity now, when it defies explanation at the moment. Relativity and Newton are just 2 ways of partially describing something, and quantum-gravity (as hoped for) would imply a real force, with carriers.
 
  • #27
Ich
Science Advisor
1,931
1
103 years ago, yes, but thankfully we percieve the passage of time, and a century or later it's really just a 'happy thought'.
Now, a century later, it's still the only theory of gravity that we have.
As far as that being the definition of a pseudo-force, yes, but is that how you would make the case for gravity as a pseudo-force, NOW?!
What has changed?
I didn't say that wasn't a fine definition of a fictitious force, I said it wasn't a good way to make the case for GRAVITY being a fictitious force.
But it is, independent of theory. No matter what theory one invents, it must explain why gravity acts like a fictitious force.
Einstein thought of a free-falling object in a box, but to be blunt, we're at a crossroads where the nature of gravity is uncertain!
With the traffic lights red since 50 years. You know, we're camping there, at the crossroads.
As I said, if there is a graviton, it would be a force, and if it's geometry only, it wouldn't.
Why would it be a force then? Why couldn't the graviton quatize geometry?
And if it were a normal force, why don't physicists simply write it down instead of making such a fuzz?
I still find it odd using first principles that Einstein used to formulate GR as a means to define gravity now, when it defies explanation at the moment.
The status quo in gravity is the same since many decades.
Relativity and Newton are just 2 ways of partially describing something, and quantum-gravity (as hoped for) would imply a real force, with carriers.
On a static background? With all kinds of energy coupling exactly the same way to it?
I'd say, you're not only speculating, you're also oversimplifying.

My advice: Stay with GR in the relativity forum. It is not clear which part of it will survive, but gravity being geometry might well be one of those that do.
 
  • #28
utesfan100
105
0
If we take the image of "mass denting spacetime" literally, then we are making the statement that spacetime is a physical object that can be "dented". I am 90% certain the popular view is that spacetime is NOT a "thing", it is a "phenomena". A phenomena is useful and real, but that does not make it an "object". Does anyone disagree with this?

Disagree with objection to denting space time

I like the denting idea. Suppose mass is modeled as a positive curvature dent, like a sphere. For simplicity I picture a dent shaped like a normal bell curve that I can rotate around the z axis to get a 2D analogy in a radial plane.

First we see that the curvature in the direction tangent to r is down in all cases. For r the curvature is down unti we reach the point of inflection then is up. The mixed partials are 0, so the regions where the direction are the same are the areas of positive curvature and the other areas are negative curvature.

In the center of the dent is a positive curvature representing the positive mass-energy of the object, surrounded by a negative energy gravitational field.

Personally, I like the idea of a graviton, and the fact that its momentum must be opposed to its direction of propogation supports the idea of a negative value for its energy.

agree with objection to bent space-time

If two satalites in geosyncronous orbit are connected by a tether that is moving along the the same geodesic as the satalites the teather would appear to be curved. A shorter tether could connect the same satalites through a euclidean path, but would be subjected to forces.

Observing that space is hyperbolic removes the restriction that a geodesic is the shortest path between two points, a distinctive characteristic of space-like curves.

The hyperbolic nature of space-time deviates from the intuitive nature of space.

The real issue between gravity as a force or as space-time curvature

Do we use the Einstein convention for setting clocks or do we select a frame of reference to use as a standard?

Saying that no frame of reference is the definative standard does not mitigate the fact that having one around is often helpful.
 
  • #29
Dale
Mentor
Insights Author
2021 Award
33,032
10,291
The geometrical explanation of gravity still leaves "mystical" loopholes. ...

This is the geometric view of gravity. It states that objects return to Earth because "it is the straightest possible way". However, there are countless geodesics or "straightest ways" that an object can follow depending on where it is going. So the "mystical" loophole that we can't avoid is, where is the object trying to go? ...

This is more circular reasoning!
Hoku, I find this portion of your post very distressing. You are at the very beginning of learning this subject and somehow you think that your current lack of understanding implies a failure in GR such that you can justifiably label it "mystical" or "circular reasoning". Neither is the case and it is upsetting for you to do so. Nothing anyone has stated on the subject here has given you the least reason to call it "mystical" and I personally directly addressed the "circular reasoning" misunderstanding in my previous posts.

https://www.physicsforums.com/showpost.php?p=2636486&postcount=70
https://www.physicsforums.com/showpost.php?p=2637761&postcount=79

However, this topic, IMO, is really the key issue for you at this time, much more important than the discussion about forces. So I would encourage you to pursue this line of questioning, but just without the inflammatory language.

Did you understand my description of the local and global definitions of a geodesic? In the global case you define the starting point and the end point and then you choose the path which minimizes the distance, so your input is two points. In the local case you define the starting point and a direction (tangent vector), and then you choose the path which goes in the same direction at each point, so your input is a point and a direction. Now, when we are talking about spacetime instead of just space, the "direction" includes the speed. So, a projectile launched straight up from Paris at a speed of 100 m/s is starting at the same point but along a different spacetime tangent than a projectile launched straight up at the same time from Paris at a speed of 1000 m/s. This is analogous to the fact in spherical geometry that each longitude line is a geodesic starting from the North pole in a different direction.
 
  • #30
Nabeshin
Science Advisor
2,207
16
I don't think this statement conflicts with anything I've said. Nabeshin was giving one reason why the "geometric" view of gravity is better than the "force" view of it. The reason he gave was that geometry can be explained and understood whereas a "force" necessitates some "mystical" unknown. I think this is a fine idea, however, even the geometric view - and the graviton view - can't avoid any "initial conditions" that may forever remain mysterious. In all three cases, the "mystical" unknown that Nabeshin is trying to avoid are exactly the same thing - energy that moves something. Consequently, I think this reason for choosing geometry over force is a little weak.

I think you misunderstood my intention. I was trying to illustrate that neither picture is more correct, better, or any more real. Your original post seemed to be implying the force description was indeed the "real" description of gravity, which is hogwash. I personally, when thinking about gravity, prefer to think geometrically, but I am not saying that this is any better than thinking in a newtonian force framework! But the discussion of the interpretations appears quite beyond the point for this discussion.. the answer to the OP's question is here: https://www.physicsforums.com/showpost.php?p=2645913&postcount=13
 
  • #31
Naty1
5,607
40
Nobody knows why matter causes gravity....nor why energy and pressure also "causes" gravity....nor forces,energy,mass,time nor space for that matter. The best can do (so0 far) is assign some mathematics that seems to capture most conditions we observe, some we don't, so we can predict SOME things....and test them experimentally.....If you ask what is gravity in the quantum mechanics forum, you'll get an almost entirely different set of reasoning, logic and mathematics....
 
  • #32
Altabeh
660
0
It's similar to the discussion which arises in classical mechanics when only potentials, not forces, enter into the fundamental equations. In this sense, the potential really is what's fundamental, not the force. In my mind, this situation is completely analogous.

In fact, say, the gravitational potential and the gravitational force entering in Newton's gravitation law are not two separate things and indeed one implies the existence of another in the form required. Talking about which one of them is more fundamental than the other is a matter of realization and does not necessarily pertain to one and all though I think forces are more fundamental not only because they were the first "objects" to be found having a role in the conduction of all efforts made to describe the most simple kind of motion, but because the potential energy only exists when there is a force that tends to pull an object back towards some lower energy position so force foregoes potential.

AB
 
  • #33
Frame Dragger
1,477
1
I SAID that "why" questions never end well when people expect a science to produce "TRUTH". Physics produces amazing lenses and tools to look at and deal with the world. The ultimate "why" questions MIGHT be answerable by physics, but no sane physicist claims to HAVE that knowledge or be anywhere near it.

What is spacetime made of? Why does it bend?

No, for physics the question can begin with "why", but the GOAL is "HOW", as I said earlier in this thread. Really, sometimes these threads are positively deterministic except for specific kinks (say, Ich handing my *** to me for instance :wink: )
 
  • #34
Hoku
160
0
Nothing anyone has stated on the subject here has given you the least reason to call it "mystical"...So I would encourage you to pursue this line of questioning, but just without the inflammatory language.
If I may direct your attention to post #11 on this thread by Nabeshin. His first paragraph uses the word "mystical", which is what I was addressing. I'm just as happy to call it, "mysterious", "unknown", "initial condition" or whatever. If Nabeshin wants to use the word "mystical" I don't have a problem using his semantics. That's what I did when I initially addressed this in post #14.

@ Nabeshin: You said, "I was trying to illustrate that neither picture is more correct, better, or any more real. Your original post seemed to be implying the force description was indeed the "real" description of gravity, which is hogwash."

You said it yourself, "neither picture is more correct", which is just as much saying, "both pictures are equally correct". Therefore, it is NOT hogwash for me to suggest that the "force" description is "real". Let's not forget that we are dealing with a "law" of gravity, which came from Newton, and a "theory" of relativity.

My point in this thread was to demonstrate that there is a lot of conflicting information about gravity. Yet, many people are not hearing both sides of the story, so they don't realize there IS another side! I think this is a problem.

What I would like to see is a reconciliation of these two views such that both views are understood, integrated and accepted across the board. I think this is a reasonable and worthy desire.

However, this topic, IMO, is really the key issue for you at this time, much more important than the discussion about forces.
This is important. I will begin a new thread soon where we can continue exploring it.
 
  • #35
Dale
Mentor
Insights Author
2021 Award
33,032
10,291
If I may direct your attention to post #11 on this thread by Nabeshin. His first paragraph uses the word "mystical", which is what I was addressing. I'm just as happy to call it, "mysterious", "unknown", "initial condition" or whatever. If Nabeshin wants to use the word "mystical" I don't have a problem using his semantics. That's what I did when I initially addressed this in post #14.
Ahh, my apologies, I completely missed that.

This is important. I will begin a new thread soon where we can continue exploring it.
Excellent. I look forward to it.

Btw, once you understand the geometric view I think that it will not be very difficult for you to understand the connection between it and the forces view.
 

Suggested for: Why does mass cause gravity

  • Last Post
Replies
24
Views
17K
  • Last Post
Replies
26
Views
701
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
5
Views
927
Replies
4
Views
990
Replies
27
Views
2K
Replies
3
Views
1K
Replies
3
Views
3K
Replies
21
Views
20K
  • Last Post
Replies
32
Views
5K
Top