# Confused with General Relativities explanation of Gravity

• mosfet_2005au
In summary, General Relativity explains gravity as the warping of spacetime by mass or energy. This warping of geometry results in objects appearing to move in curved lines, which we perceive as the force of gravity. This can be seen in the example of two stationary masses bending the spacetime around them and causing gravitational effects between them. This is due to objects following geodesics in spacetime, which are the shortest paths between two points while remaining on the curved surface.
mosfet_2005au
I am rather confused with General Relativities explanation of Gravity!
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Gravity warps space-fabric (eg. rubber sheet model)
How does this explain gravity?
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Can anyone help here?

Gravity doesn't warp space-time fabric. Energy(which includes mass) does. The spacetime near any massive object is bent, in the sense that it differs from the usual kind of geometry. This warping of geometry is what appears to us as gravity. In warped spacetime, the usual straight lines or "Geodesics" are not what they used to be. Objects moving in a straight line(in the 4-d spacetime) appear to be moving in a curved line to us in the 3 space dimentions.

(Ok, I meant to write “Gravity – warped space-fabric”)

So, I guess my question is
How does the geometry warping appear as gravity?
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Gravity = accelerated mass
So, how does the bending of space-time by mass appear as an intrinsic acceleration?
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For example:
Mass “m1” exists in space-time near another mass “m2”
Both masses are stationary relative to each other.
Mass “m1” bends space-time near mass “m2” and vice versa.

How does this explain the gravitational effects between the 2 masses “m1” and m2”?

As I have already said, a straight line in spacetime would not appear as a straight line in space to us. Objects always travel in straight lines in spacetime, but, this straight line in spacetime is not a straight line in space. Any deviation from a straight line in space indicates acceleration.

Particles follow "geodesics" in spacetime. On a curved 2D surface, a geodesic would be the shortest path between two points on the surface--for example, on a sphere the shortest path between points would be a section of the great circle that passes through both points. In curved spacetime, a geodesic is the worldline between two events with the greatest proper time (time as measured by a clock that follows that worldline). For example, in the "twin paradox" the twin who doesn't accelerate is the one who follows a geodesic, that's why he'll be older than his brother who did accelerate when they reunite. Planets in orbit are also following geodesics through the curved spacetime around the sun, although I don't know the details of how you'd show this.

Remember that one second of time is equivalent to 300,000 kms of space. In a space-time diagram drawn to scale the orbit of the Earth, spiralling through space-time, would be 1 A.U. across but I light year in 'pitch'. Therefore the amount of curvature is actually very slight.

Garth

Just some added notes. You can draw a space-time diagram by plotting position as a funciton of time. You usually do this on a flat sheet of paper.

What if you did this plot on a curved sheet of paper? Specifically, one curved like the surface of a sphere.

It's too hard to draw pictures on the internet, but if you manage to carry out this expeirment, you'll see that the worldlines of neighboring particles act a lot like they attracted each other - even though one is just drawing "straight" lines on a curved surface.

"Straight" lines in this context are the shortest paths that join two points while remianing entirely on the curved surface (it's not allowable to leave the surface). They are also called geodesics.

A picture or two might really help, but you'll need to go to a textbook to find one AFAIK - I haven't run across any on the internet, and ascii is certainly not up to the job.

## 1. What is General Relativity and how does it explain gravity?

General Relativity is a theory proposed by Albert Einstein in 1915 that describes gravity as the curvature of spacetime caused by the presence of mass and energy. It explains that objects with mass create a dent in the fabric of spacetime, and other objects with less mass are pulled towards this curvature, resulting in the force of gravity.

## 2. How does General Relativity differ from Newton's theory of gravity?

Newton's theory of gravity, proposed in the 17th century, described gravity as a force acting between two objects with mass. General Relativity, on the other hand, explains gravity as the curvature of spacetime. It also accounts for the effects of gravity on the motion of objects in the universe, such as the bending of light around massive objects.

## 3. What evidence supports General Relativity's explanation of gravity?

There is a considerable amount of evidence that supports General Relativity's explanation of gravity. For example, the phenomenon of gravitational lensing, where light from distant galaxies is bent by the gravitational pull of massive objects, can only be explained by the curvature of spacetime. Additionally, the precise predictions made by General Relativity, such as the precession of Mercury's orbit, have been confirmed through observations.

## 4. Why is it often difficult to understand General Relativity's explanation of gravity?

General Relativity is a highly complex theory that involves concepts such as spacetime, curvature, and relativity. These concepts are not easily visualized and can be challenging to grasp for those without a background in physics or mathematics. Additionally, the mathematical equations used to describe General Relativity can be daunting for some individuals.

## 5. Are there any limitations to General Relativity's explanation of gravity?

While General Relativity is a highly successful theory, there are still some limitations to its explanation of gravity. For example, it does not account for the behavior of particles at a quantum level, and it does not provide a complete understanding of the nature of dark matter and dark energy. Scientists are still working to reconcile General Relativity with quantum mechanics to create a more comprehensive theory of gravity.

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