Relativity, Gravity, Attraction , Curved Space

  • #1

Main Question or Discussion Point

Relativity, Gravity, "Attraction", Curved Space

Okay, I am by no means knowledgable in this field, probably more dangerous than anything. But I have a question about gravity. If two masses are identical (in an idea situation) and have no other forces acting on them, and start from a stationary position (at rest and stopped - zero velocity) and some arbitrary distance between them. What makes them "attract" each other by curved space since they both curve space equally.

Does the space curve "force" them to move toward each other (in my mind that would mean there was a "normal line" in space and if curved away from the normal there is some force that causes the mass to move to the apex of the curve)? It may be over my skill level at this point, I am having a hard time comtemplating the attraction being caused by curve space when neither were in motion, what causes the force to act upon each other.

Hope this is clear enough to get my question across.

Thanks!
 

Answers and Replies

  • #2
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General and Special relativity are based on Maxwell's equations. The transformation, using Lorentz's transformation, is transforming the coordinate system. What is the transformed coordinate system representing? I believe it is Maxwell's equations but Maxwell's equations are induction equations yet light is not formed by induction. Can someone help me with this obvious missunderstanding of Einstein's general and special relativity. Is that relativity or relativities. Is special and general relatiavities the same or are they different?
 
  • #3
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Newton's classical approach defined a force between any two masses. He did not know it's origin, but thought space and time were absolute (fixed). Einstein extended the idea of such attraction to mass, energy and even pressure via curved spacetime...he found that space and time interchange..via Lorentz transforms...and that light speed is the universal constant.

Gravity is NOT described by Maxwell's equations...they cover electromagnetism....The Einstein field equations describe gravity in tensor formulation...like complicated vectors...

The usual way of thinking about visualizing gravitational curvature is a rubber membrane with some round masses placed on it....the combined masses depress the membrane between them relative to the distant edges and the round objects "roll" towards each other....

But fundamentally nobody knows how gravity originates, nor the other three forces either...what we do have are good descriptions about how they behave, but little understanding of how they arose.

The strong weak and electromagnetic forces have been unfiied (combined) mathematically and they seem to have a common origin...but gravity remains outside that combination.....so we really don't know what the forces are, nor mass, nor time, nor space either...

"What is the transformed coordinate system representing?" each viewers perspective....each observer in a different coordinate system (frame of reference) typically sees something different than other observers in other frames....the only thing everybody sees the same is the speed of light, c. It's called "relativity of simultaneity" meaning we will agree on what happens first, then second; but not on exactly when they happened....
 
  • #5


Awesome, I see it with better understanding. I know Maxwell's equations and I understand the Lorenz transformation. I was trying to figure out where the attraction comes from in Relativity. We really don't know at present and that is cool, gives a good study in one's spare time.

Do you know where to get more information on the pressure you mentioned in connection with Relativity? I have some math I have been doing with this concept and would love to dig deeper.

Thanks!
 
  • #6
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Awesome, I see it with better understanding. I know Maxwell's equations and I understand the Lorenz transformation. I was trying to figure out where the attraction comes from in Relativity. We really don't know at present and that is cool, gives a good study in one's spare time.

Do you know where to get more information on the pressure you mentioned in connection with Relativity? I have some math I have been doing with this concept and would love to dig deeper.

Thanks!
Have you studied the book "A first course in GR" by Schutz? In chapter 4, he discusses the perfect fluids in SR which is catchy if you are interested in learning beyond pressure, something like the components of the energy-stress-momentum tensor (e.g. pressure, density and non-gravitational force fields) can be well pinned down via his book at first glance if you have a good knowledge of calculus/SR.

The strong weak and electromagnetic forces have been unfiied (combined) mathematically and they seem to have a common origin...but gravity remains outside that combination.....so we really don't know what the forces are, nor mass, nor time, nor space either...
Well, I don't think the strong force has been combined with the other two. There is just an electroweak theory which covers electromagnetism and weak force together!

AB
 
  • #7
A.T.
Science Advisor
10,210
1,894


I am having a hard time comtemplating the attraction being caused by curve space when neither were in motion,
It is not curved space, but curved space-time. And everything is "in motion" trough space-time. The reason for the attraction is easy to understand, without any math:
http://www.relativitet.se/spacetime1.html
http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html
http://www.adamtoons.de/physics/gravitation.swf

Naty1 said:
The usual way of thinking about visualizing gravitational curvature is a rubber membrane with some round masses placed on it....the combined masses depress the membrane between them relative to the distant edges and the round objects "roll" towards each other...
That has nothing to do with the gravity model of GR, which is "free falling objects move straight ahead in curved space-time". It is also circular, as it uses gravity to explain gravity.
 
  • #8
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I should point out that this is not strictly accurate,
Altabeh said:
Well, I don't think the strong force has been combined with the other two. There is just an electroweak theory which covers electromagnetism and weak force together!
because

The standard model is a gauge theory of the strong (SU(3)) and electroweak (SU(2)×U(1)) interactions with the gauge group (sometimes called the Standard Model symmetry group) is SU(3)×SU(2)×U(1). It does not take into account gravitation, which is SO(3,1).
 
  • #9
George Jones
Staff Emeritus
Science Advisor
Gold Member
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Very roughly: SU(3)XSU(2)XU(1) splits cleanly, as a Cartesian product, into strong SU(3) and electoweak SU(2)XU(1). The electroweak SU(2)XU(1) does not split cleanly into a weak SU(2) part and U(1) electromagnetism part. Weak SU(2) is a "mixture" of stuff in SU(2)XU(1); electromagnetism U(1) is a "mixture" of stuff in SU(2)XU(1).

So, I agree with
Well, I don't think the strong force has been combined with the other two. There is just an electroweak theory which covers electromagnetism and weak force together!
 
  • #10


Have you studied the book "A first course in GR" by Schutz?
I actually purchased this book about three months ago, my plan was to go through it this summer. I am now finishing Calculus III and I didn't want to jump into the book before I was ready for it and become discouraged (I also have Wheeler's book, got it a great price at the same time, but it is definitely beyond my scope at this point, but I will tackle it soon). I am one who after being out of college for many years (several decades ago with a business degree) decided to go back and get my science degree and then pursue a masters and if I am lucky enough a doctrate, then I could retire and volunteer my services with some research group (what a cool way to spend the rest of one's life).

My areas of interest are gravity, time and light. I have only completed physics I & II at this point and no ODE in math formally, though I do work with them, that will be in the fall. Working 45-50 hours a week as an officer of a company and going to college 10-15 hours per semester has proven challenging, but when you enjoy this science, what else are you going to do??
 
  • #11
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George,

isn't that hair ( or group) splitting ? :biggrin:

Point taken. Insincere grovelling etc.
 
  • #12
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My areas of interest are gravity, time and light. I have only completed physics I & II at this point and no ODE in math formally, though I do work with them, that will be in the fall. Working 45-50 hours a week as an officer of a company and going to college 10-15 hours per semester has proven challenging, but when you enjoy this science, what else are you going to do??
You can't be in my situation ever and I assume you know if you're in a country that its government not only does not care about something called "Science", but also deprives people of the most fundamental rights through which dreams can come true, ah, you are in such a maze and better go pray that it is just job and college making you busy not "mental and physical torture"! I'm a physics maniac and I really do love it and when I see my future with this science, it sort of makes me calm down and heals my pains! Hope you'll enjoy physics more than now!

I should point out that this is not strictly accurate,
It is. The best ever known Grand theory to cover both three forces is Georgi-Glashow model which suffers many problems, most importantly, http://en.wikipedia.org/wiki/Doublet-triplet_splitting_problem" [Broken] which is not yet experimentally proven to be true.

AB
 
Last edited by a moderator:
  • #13
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Altabeh said:
It is. The best ever known Grand theory to cover both three forces is Georgi-Glashow model which suffers many problems, most importantly, doublet-triplet splitting problem and proton decay which is not yet experimentally proven to be true.
Enough already - I've been beaten up by George for this.
 

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