# View of Gravity?

#### tino lin

Einstein's view of gravity is a warping in space where smaller mass follow the curverture of the space created by the larger mass. I want to know if this is observed in all solar system where the smaller mass is rotating around the bigger mass above the X axis, assumming the bigger/center mass is at the center of the this X/Y axis.

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#### Integral

Staff Emeritus
Gold Member
All planets and stars, everywhere, orbit about the center of mass of the pair. So the sun and earth orbit about their center of mass, this is a point which is inside of, but not at the center of, the sun. There exist binary stars which orbit a center of mass outside of either star. Mizar in the Ursa Major and Algol in Perseus are 2 famous examples.

#### tino lin

Planet orbit at the center of mass, make sense, and I learn something new.
But what I am really interested in knowing is the Y cordinate position of the two planet. If this warping of space is true then the smaller mass must alway orbit above the bigger mass which ever way we want to define up as. In other word, the planets center of mass cannot line up 180 degree. Has this found to be true?

#### Integral

Staff Emeritus
Gold Member
I am sorry I do not understand your question. Can you express it mathematically.

Meanwhile I am going to move this to the cosmology forums. Perhaps someone more knowledgeable then I can help you.

#### RandallB

tino lin said:
Einstein's view of gravity is a warping in space where smaller mass follow the curverture of the space created by the larger mass.
It would be incorrect to consider Einstein’s view as LARGER Mass warping space for SMALLER Mass to follow the curve in. His view was that ALL mass distorts space even the small. And as such, even a large mass will be affected by the small "warp in space" caused by a small mass, just not by much.

To me it seems the warping idea gives a nice description but unnecessary. As an example you could describe electrostatic attraction and repulsion in a similar warp and reverse warped space maybe using an extra 3 of those eleven dimensions identified so far. But we do quite well working that all out without “warping”. I think the idea is overused on gravity.

RB

#### Janus

Staff Emeritus
Gold Member
tino lin said:
Planet orbit at the center of mass, make sense, and I learn something new.
But what I am really interested in knowing is the Y cordinate position of the two planet. If this warping of space is true then the smaller mass must alway orbit above the bigger mass which ever way we want to define up as. In other word, the planets center of mass cannot line up 180 degree. Has this found to be true?
I am also having trouble understanding what it is that you are asking, though I have a nagging suspicion. Could you express it with a diagram?

#### tino lin

It is very hard to explain in words the visual we get in our minds, so not to get everyone frustrated I will try to state it more mathematically.
Mathematically(describe), the closest I can state is when the smaller mass is orbiting the denser mass at the center, the circular or elliptical plane that the smaller mass draw is or is not on the same plane as the center mass? This might or might not be enough for people to visualize to the answer my question.
But basically what I am trying to get at is the orbit of the outer mass should not be on the same plane as the center mass if Einstein view of gravity was to make sense.

#### Janus

Staff Emeritus
Gold Member
tino lin said:
It is very hard to explain in words the visual we get in our minds, so not to get everyone frustrated I will try to state it more mathematically.
Mathematically(describe), the closest I can state is when the smaller mass is orbiting the denser mass at the center, the circular or elliptical plane that the smaller mass draw is or is not on the same plane as the center mass? This might or might not be enough for people to visualize to the answer my question.
But basically what I am trying to get at is the orbit of the outer mass should not be on the same plane as the center mass if Einstein view of gravity was to make sense.
Ah, I think the problem is that you are taking the "weight on a rubber sheet" analogy for gravity a little too literally. This is only an analogy. The plane of the planet's orbit will pass through the center mass in the Relativistic view of gravity also.

#### tino lin

Thanks Janus for clearing this up for me. But if that is the case then the question to "what gravity really is?" is not clearly answer, cause Einstein only give us a view but not what it really is.
Could it be that gravity don't really exist, it's only an effect of acceleration?
Everywhere that has the effect of gravity seem to have rotation where centripetal force is involved. Like planet rotation, solar system rotation, and black hole rotation at the center.

#### pervect

Staff Emeritus
tino lin said:
Thanks Janus for clearing this up for me. But if that is the case then the question to "what gravity really is?" is not clearly answer, cause Einstein only give us a view but not what it really is.

Do you really exist? How do you know that you really exist? Do I really exist? HOw do you know that I really exist? What is "real", anyway?

Sorry, but these philosophical questions (of which your gravity question is a very tiny subset) can be and are debated endlessly, and are notorious time-sinks.

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#### pmb_phy

tino lin said:
Einstein's view of gravity is a warping in space where smaller mass follow the curverture of the space created by the larger mass. I want to know if this is observed in all solar system where the smaller mass is rotating around the bigger mass above the X axis, assumming the bigger/center mass is at the center of the this X/Y axis.
Actually that was, more or less, Max Von Laue's view of gravity. To Laue the presence of a gravitational field is absolute in that the Riemann tensor (curvature) did not vanish when the field was present and not zero when there was more than one component of the metric tensor. For Einstein the existance of the non-vanishing of the Christofell symbols denoted by the non-vanishing of all of the components of the Christoffel symbols.

Pete

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#### Crosson

To me it seems the warping idea gives a nice description but unnecessary. As an example you could describe electrostatic attraction and repulsion in a similar warp and reverse warped space maybe using an extra 3 of those eleven dimensions identified so far. But we do quite well working that all out without “warping”. I think the idea is overused on gravity.
Yes, saying the words "gravity is curvature in spacetime" is meaningless for someone who hasn't studied general relativity. GR is about much more than curvature, here is a brief summary.

All things move through spacetime along geodesics (quickest paths). You may have heard that light does this; they always takes the shortest path through space, because photons don't move through time.

The equivalence principle says that free falling (gravity is the only force) reference frames are flat spacetime. Equivalent to the spacetime of Special Relativity.

From other points of view, free falling reference frames do not appear flat. This is because all objects move through spacetime on geodesics, and it must be curved spacetime making them move in that funny way.

#### two hands

As I understood Einstein gravity and electricity are one in the same, therefore if you insulated yourself from gravity as we do electricity then one could step off a high rise building and not fall to the ground. Then how do we insulate ourself from gravity without just using another platform to step on?

#### timestill

tino lin said:
Thanks Janus for clearing this up for me. But if that is the case then the question to "what gravity really is?" is not clearly answer, cause Einstein only give us a view but not what it really is.
Could it be that gravity don't really exist, it's only an effect of acceleration?
Everywhere that has the effect of gravity seem to have rotation where centripetal force is involved. Like planet rotation, solar system rotation, and black hole rotation at the center.
Speed of an object (moon, planet, star, starship) accounts for an increase in mass... so in a sense gravity is greater for the same object being at rest... but... The curvature of space-time (gravity) is the effect of mass .... You can have a 'static' object still exerting Gravitic forces....

#### Enos

Make something that repels the air with electromagnetism. Like a parachute!!! hehe.

#### m4sterm1nd

I understand the view where spacetime is warped and mass follows geodesics as an explantion for the attraction of bodies when there are in motion, but what is it that actually accounts for the initial movement of a mass from a position of rest when it is in a gravitational field? Is there a force-carrying particle as with the other forces? Or, is there some other aspect of relativity that I am missing which accounts for this?

#### pervect

Staff Emeritus
The intial movement of a mass from a position of rest when it is in a gravitational field can be understood in terms of geodesic motion.

An object follows a geodesic t(lambda), x(lambda), y(lambda), z(lambda). To simplify the concept, you can think of lambda as being the "proper time" for a timelike geodesic, i.e. the time measured by a clock following the geodesic. Technically, lambda can be any linear transformation of proper time, it's usually simplest to take it as being proper time, though.

An object may have a zero velocity dx/dlambda, dy/dlambda, dz/dlambda, but dt/dlambda will be non-zero. The fact that dt/dlambda is nonzero is what gives the intial accelration when the object is at rest.

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