how exactly does "gravity" work? what is the initial force to set planets in motion?

[xxplexx]

i have learned that gravity is caused by mass bending the space around it, how ever, this makes no sense at all to me and does not explain how the planets actually move. sure, it explains why planets move in orbits around a larger mass, but it does not explain how. there must be some type of force present that actually gives the planets their motion. it would be impossible for objects to just decide to move because the space around it is bent.

i mean, space can be bent all it wants, but this alone can not induce motion on other objects, there has to be some force that pulls or pushes the planet in the first place, other wise, all planets are using magical powers to move on their own.

 

not sure if anyone understands my point, so i will give a good example. hypothetically of course.

 

lets pretend we remove all mass from the universe except for the fabric of space-time. now pretend that we bend space-time to the same shape as a trampoline bends when you put a bowling ball in the center of it, now lets pretend we put a small ball at rest (or completely still) on the curved surface of space-time, the ball should not move or fall, it should just stay in that one spot because there is no force present that would actually make the ball roll down the bent space-time.

so what actually causes planets to move in the first place? i read that time is the cause of a planets motion, but that just sounds silly to me because time is not a physical force that pushes object forward, right?

 

 

 

[StringJunky]

i have learned that gravity is caused by mass bending the space around it, how ever, this makes no sense at all to me and does not explain how the planets actually move. sure, it explains why planets move in orbits around a larger mass, but it does not explain how. there must be some type of force present that actually gives the planets their motion. it would be impossible for objects to just decide to move because the space around it is bent.

i mean, space can be bent all it wants, but this alone can not induce motion on other objects, there has to be some force that pulls or pushes the planet in the first place, other wise, all planets are using magical powers to move on their own.

 

not sure if anyone understands my point, so i will give a good example. hypothetically of course.

 

lets pretend we remove all mass from the universe except for the fabric of space-time. now pretend that we bend space-time to the same shape as a trampoline bends when you put a bowling ball in the center of it, now lets pretend we put a small ball at rest (or completely still) on the curved surface of space-time, the ball should not move or fall, it should just stay in that one spot because there is no force present that would actually make the ball roll down the bent space-time.

so what actually causes planets to move in the first place? i read that time is the cause of a planets motion, but that just sounds silly to me because time is not a physical force that pushes object forward, right?

 

 

 

The energy was always there since the big bang and things are always moving. The idea of something being stationary is an observer-effect; if you are moving in the same direction and with the same speed alongside some object it will look stationary to you.

[Phi for All]

The idea of something being stationary is an observer-effect; if you are moving in the same direction and with the same speed alongside some object it will look stationary to you.

 

I recently re-read The Martian Way. Asimov describes a great bit where astronauts are on their way from Mars to Saturn, and to pass the time they go outside the ship, release their magnetic boots, slowly drift away from the ship while barely gripping the tether until they are stationary with respect to the ship. They can then hang there watching the stars (which don't seem to be moving either since they're so far away) while there is slack in their tether line, and nothing seems to be moving in the whole universe, when in reality they're moving at tens of thousands of miles an hour.

[Lord Antares]

First of all, to say that time is the cause of motion is wrong. Motion and time are closely connected, but saying that one causes the other is ill-conceived.

 

Secondly, it isn't know what gravity actually is. General relativity doesn't attempt to explain it because no explanation is known. It simply describes how and by which principles it operates, but it doesn't answer the question which is ''what is gravity?''.

 

I have a speculation on why it works, but don't we all? Without tested and statistical knowledge, saying anything is just a stab in the dark.

 

Answering that question just brings up more questions above our knowledge, such as ''why was the energy there from the start'', ''how did energy come to be'' and you faintly start to wander into philosophy.

So, short answer: we don't know.

 

It's easy to understand why objects follow the curved path of space caused by a mass. It's because that path is actually ''straight'' in that curved space, so in a way, it doesn't change direction per se, but why the objects are moving in the first place is a more difficult question.

[StringJunky]

 

I recently re-read The Martian Way. Asimov describes a great bit where astronauts are on their way from Mars to Saturn, and to pass the time they go outside the ship, release their magnetic boots, slowly drift away from the ship while barely gripping the tether until they are stationary with respect to the ship. They can then hang there watching the stars (which don't seem to be moving either since they're so far away) while there is slack in their tether line, and nothing seems to be moving in the whole universe, when in reality they're moving at tens of thousands of miles an hour.

I imagine it would be a feeling that you were at the centre of universe; a point of perfect stillness.

[Strange]

so what actually causes planets to move in the first place? i read that time is the cause of a planets motion, but that just sounds silly to me because time is not a physical force that pushes object forward, right?

 

 

Everything moves forward in time, kinda by definition. In non-curved space-time two objects will move forward on parallel paths.

 

A useful analogy might be to think of lines of longitude on the Earth. These converge on the North Pole. That means that as two objects move along their lines of longitude, they get closer together.

 

Now, imagine that the North Pole is "the future", so moving north is like moving forward in time.

 

So it is in curved space-time: as two objects move forward in time, their paths are no longer parallel and they get closer together as they move through time. Just as if a force were operating on them. (But there is no force.)

Edited February 27, 2017 by Strange

[swansont]

 

I recently re-read The Martian Way. Asimov describes a great bit where astronauts are on their way from Mars to Saturn, and to pass the time they go outside the ship, release their magnetic boots, slowly drift away from the ship while barely gripping the tether until they are stationary with respect to the ship. They can then hang there watching the stars (which don't seem to be moving either since they're so far away) while there is slack in their tether line, and nothing seems to be moving in the whole universe, when in reality they're moving at tens of thousands of miles an hour.

 

 

Not sure why would you need to leave the ship to do this. That's what it would look like just looking out the window. It's not like e.g. the display on Star Trek (TOS), with stars whizzing by.

[Janus]

i have learned that gravity is caused by mass bending the space around it, how ever, this makes no sense at all to me and does not explain how the planets actually move. sure, it explains why planets move in orbits around a larger mass, but it does not explain how. there must be some type of force present that actually gives the planets their motion. it would be impossible for objects to just decide to move because the space around it is bent.

i mean, space can be bent all it wants, but this alone can not induce motion on other objects, there has to be some force that pulls or pushes the planet in the first place, other wise, all planets are using magical powers to move on their own.

 

not sure if anyone understands my point, so i will give a good example. hypothetically of course.

 

lets pretend we remove all mass from the universe except for the fabric of space-time. now pretend that we bend space-time to the same shape as a trampoline bends when you put a bowling ball in the center of it, now lets pretend we put a small ball at rest (or completely still) on the curved surface of space-time, the ball should not move or fall, it should just stay in that one spot because there is no force present that would actually make the ball roll down the bent space-time.

so what actually causes planets to move in the first place? i read that time is the cause of a planets motion, but that just sounds silly to me because time is not a physical force that pushes object forward, right?

There are two different things to deal with here. What keeps the planet's in motion, and how do we treat gravity as a space-time curvature.

 

The first can be explained without delving into Relativity and sticking simply to Newton. The planets move because something gave them a motion in the past and they have retained it. Once you start something moving, you don't need to keep applying a push to keep it doing so. As long as no other forces act on it it will continue with that same motion forever. Now this might seem contrary to common sense, because in everyday life, if you give something a push, it eventually comes to a stop unless you keep pushing it. But that is because there are other forces involved, generally caused by friction. It is these forces acting against the motion of the object that slow it down and eventually bring it to a halt. The reason you need to keep pushing, is to counter these forces so that there is a zero net force acting on the object.

The planets traveling around the Sun travel through an environment that is about as friction-free as its comes. The material that makes up the planets was formed in the hearts of stars that later exploded, spewing this material out into the galaxy. So when it came together it already had quite a bit of motion relative to the center of the solar system and what eventually became the Sun. They continue to move around the Sun because there isn't enough friction forces to slow them down significantly.

 

The other issue, How we deal with space-time curvature is a bit more abstract. The clue is in the fact that gravity is a curvature of space-time. The reason that this is important is that all things move through time, even if they are "at rest" in space. Everything flows from past to future. So what happens when you get a space-time curvature, is that space is curved a bit into time and vice-versa.

For that object sitting on the"trampoline", that bending of the surface represents a curving of space-time. One way to put it is that the object's "future" now point towards the bowling ball. Some of that "motion through time" is being expressed as motion through space.

I know that this sounds a bit odd, but that's simply because we can't readily form a mental picture of four dimensional space-time. Our brains just aren't wired for it (Our primate ancestors never had to have an understanding of space-time geometry in order to work out how to swing to that next branch).