How does curved space create gravity?

In summary, the "ball on a rubber sheet" analogy is not a good explanation of gravity because it only uses curvature of space. Curvature of space-time causes the person to start moving in space towards the planet.
  • #1
paulicator
1
0
We've all seen the "ball on a rubber sheet" analogy, showing how warped space near a planet can cause a light beam to alter its path. We are told that the light is actually following the shortest path in curved space.

When it comes to a *stationary* object near a planet, however, I have a harder time making sense of things.

If I assume a person (for example) is statically floating near a massive planet, *why* does curvature of space cause the person to experience a gravitational force?
 
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  • #2
paulicator said:
We've all seen the "ball on a rubber sheet" analogy, showing how warped space near a planet can cause a light beam to alter its path. We are told that the light is actually following the shortest path in curved space.

When it comes to a *stationary* object near a planet, however, I have a harder time making sense of things.

If I assume a person (for example) is statically floating near a massive planet, *why* does curvature of space cause the person to experience a gravitational force?
You can't "statically float" near a massive planet. If no force is applied to you, you will be in freefall and not feel a force, but you will not remain "stationary" with the planet. If a force is applied to you to keep you "stationary" with the planet, you will of course feel that force.
 
  • #3
paulicator said:
*why* does curvature of space cause the person to experience a gravitational force?
The only thing that could possibly give you some sort of answer to that is a new theory of gravity that has an even better agreement with experiments than GR, and also describes things in different terms. There is no such theory today, and even if there was, it would give you a new question to ask.
 
  • #4
It's curvature of spacetime, not curvature of space. That makes a big difference!

To make an analogy, consider two airplanes flying north, starting from nearby points along the Earth's equator. Each plane continues to fly straight north by its own reckoning, along its own meridian of longitude. As they proceed further north, the meridians converge towards the North Pole, so the planes approach each other even though they started out flying in parallel straight lines.

Of course, in the example above, the airplanes have to move in order to make this work. But now suppose the two dimensions on the sphere are not both spatial dimensions, but instead the "north-south" direction represents time and the "east-west" direction represents a one-dimensional space, that is, the two planes both move back or forth along the same line. As time passes, the planes automatically proceed "northward" in time, and the spatial "east-west" distance steadily decreases.
 
  • #5
jtbell said:
Of course, in the example above, the airplanes have to move in order to make this work. But now suppose the two dimensions on the sphere are not both spatial dimensions, but instead the "north-south" direction represents time and the "east-west" direction represents a one-dimensional space, that is, the two planes both move back or forth along the same line. As time passes, the planes automatically proceed "northward" in time, and the spatial "east-west" distance steadily decreases.

And note that there is no force required to bring the two planes together.
 
  • #6
paulicator said:
If I assume a person (for example) is statically floating near a massive planet, *why* does curvature of space cause the person to experience a gravitational force?
As jtbell said: Curvature of space alone doesn't cause this, therefore the rubber-sheet analogy is not good explanation of gravity. Curvature of space-time causes the person to start moving in space towards the planet.

Here a strongly simplified explanation of curvature space vs. curvature of space-time effects:
http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html
 
  • #7
I think by taking Einstein's view violation of geodesi creates the gravity and curved space time
 
  • #8
I think I found a pretty good description here http://www.allsubjects.org/viewtopic.php?f=14&t=4&sid=5c02ae9e2b2a7d9cff07cb87c89a0263" [Broken]
 
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1. How does curved space create gravity?

Curved space creates gravity by warping the fabric of space-time. This warping is caused by the presence of massive objects, which create a gravitational field that curves the space around them. The curvature of space-time is what causes objects to experience the force of gravity.

2. What is the relationship between curved space and gravity?

The relationship between curved space and gravity is that curved space causes gravity. The more massive an object is, the more it warps the space-time around it, and the stronger its gravitational pull becomes.

3. How does Einstein's theory of general relativity explain curved space and gravity?

Einstein's theory of general relativity explains curved space and gravity by proposing that massive objects cause a curvature in space-time, which is what we experience as the force of gravity. This theory has been proven through various experiments and observations.

4. Does the curvature of space-time change over time?

Yes, the curvature of space-time can change over time. This is because the distribution of mass and energy in the universe is not constant and can change, thus affecting the curvature of space-time and the strength of gravity.

5. Can we see the curvature of space-time?

No, we cannot directly see the curvature of space-time. However, we can observe its effects, such as the bending of light around massive objects like stars, which is known as gravitational lensing. We can also measure the curvature of space-time through precise experiments and observations.

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