Gravity And Straight Lines

In summary: This is why everything, from planets to stars, follows an elliptical path in space. In summary, mass curves spacetime, which causes light to be bent. This is why the light from a star behind Sun is bent. Things that leave Earth follow a unique path in spacetime called a geodesic.
  • #1
Nim
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The explanation I've heard for gravity as depicted by relativity, is that it's the consequence of mass curving spacetime. This means that the reason why the light from a star behind Sun is bent is because evening though the light is traveling a straight line, since space itself is bent, traveling a straight line will cause the light to bend.

If this is the reason why we are stuck to the Earth, then what is happening when something leaves Earth? Is it no longer traveling a straight line through the curved space?
 
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  • #2
That is essentially true. When something leaves Earth, it must undergo some acceleration to get away. While accelerating, the object is no longer traveling in a strait line.
 
  • #3
Nim said:
The explanation I've heard for gravity as depicted by relativity, is that it's the consequence of mass curving spacetime. This means that the reason why the light from a star behind Sun is bent is because evening though the light is traveling a straight line, since space itself is bent, traveling a straight line will cause the light to bend.

If this is the reason why we are stuck to the Earth, then what is happening when something leaves Earth? Is it no longer traveling a straight line through the curved space?


First of all, it's spacetime that's curved not just space. That's important as we will soon see.

Second, not just mass but all momentum and energy curves spacetime. Mass is a part of that because it's a form of energy (Einstein's famous equation), but there are other forms too. Light in particular has no mass but it does have momentum (which in its case modifies its frequency).

Third, a truly straight line cannot exist in a curved geometry. What you have instead are curves that are as stright as possible (this can be defined mathematically). They are called geodesics (gee-oh-dessiks is the way I say it; YMMD), and these are the curves that go through the curved geometry of spacetime without being deviated by outside forces.

Now as an example of something that has left Earth, let's take a space station in orbit. Its motion in space is close to an ellipse, but we have to consider its track in spacetime. As it goes around one orbit, time passes and it comes back not to its original position but to that position at a later time. In spacetime it describes a kind of elliptical helix, and this can be shown to be the unique geodesic curve that goes from its original spoacetime position (arbitrariy, (0,0,0,0) ) to its new one (0,0,0,t).

And everything that is acted on only by gravity (i.e. moves through spacetime without any other constraint than the geometry itself) follows a geodesic.
 

What is gravity?

Gravity is a natural force that all objects with mass possess. It is the force that pulls objects towards each other. On Earth, gravity is what keeps us grounded and prevents us from floating off into space.

How does gravity work?

Gravity works by the principle of mass attracting mass. The larger the mass of an object, the stronger its gravitational pull. This is why planets have a strong gravitational pull and can hold their moons in orbit.

What is the relationship between gravity and straight lines?

The relationship between gravity and straight lines is that gravity causes objects to fall in a straight line towards the center of the Earth. This is known as free fall. However, other forces such as air resistance can affect the path of the falling object.

Can gravity be stopped or reversed?

No, gravity cannot be stopped or reversed. As long as an object has mass, it will have a gravitational pull. However, the strength of gravity can be reduced by increasing the distance between objects.

How does gravity affect light and electromagnetic waves?

Gravity does not affect light or electromagnetic waves directly. However, it can cause the bending of light and other electromagnetic waves when they pass through areas with strong gravitational fields, such as near massive objects like stars or black holes.

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