Why does the Earth spin?

[The Black Flash]

Re: the origin of spin.
If you consider two rocks, both travelling in straight lines through space.
Their paths brings them close to each other.

0-------------------->

                        <---------------------0

As they get close, gravity starts to effect them and pull them together.
If you imagine that, when they get to the position below, gravity "locks" them together...

                   0--------------->

     <--------------0

You have this sort of situation:


                    0-->
                    |
                 <--0 

So their existing momentum causes the pair of them to rotate about each other.  

Multiply several billion times and you end up with a planet which is rotating.

[The Black Flash]

TheHeretic said:

Look up 'Hubble Deep field'. A postage stamp sized are of the sky that to us, had nothing on it. It was black. They pointed the telescope at this area, and kept it there for as long as they could, and this is the image that it produced.

Just updated with the eXtreme Deep Field (XDF) last week:
http://www.nasa.gov/mission_pages/hubble/science/x...

5,500 galaxies in an area of sky half the size of your fingernail held at arms length...

[mrmr96]

The Black Flash said:

Re: the origin of spin.
If you consider two rocks, both travelling in straight lines through space.
Their paths brings them close to each other.

0-------------------->

                        <---------------------0

As they get close, gravity starts to effect them and pull them together.
If you imagine that, when they get to the position below, gravity "locks" them together...

                   0--------------->

     <--------------0

You have this sort of situation:


                    0-->
                    |
                 <--0 

So their existing momentum causes the pair of them to rotate about each other.  

Multiply several billion times and you end up with a planet which is rotating.

^ This, IMO.

[TheHeretic]

The Black Flash said:

Just updated with the eXtreme Deep Field (XDF) last week:
http://www.nasa.gov/mission_pages/hubble/science/x...

5,500 galaxies in an area of sky half the size of your fingernail held at arms length...

Stunning, isn't it. Way more impressive than a burning bush.

[SpeedyDave]

The Black Flash said:

Re: the origin of spin.
If you consider two rocks, both travelling in straight lines through space.
Their paths brings them close to each other.

0-------------------->

                        <---------------------0

As they get close, gravity starts to effect them and pull them together.
If you imagine that, when they get to the position below, gravity "locks" them together...

                   0--------------->

     <--------------0

You have this sort of situation:


                    0-->
                    |
                 <--0 

So their existing momentum causes the pair of them to rotate about each other.  

Multiply several billion times and you end up with a planet which is rotating.

I think this example works for objects which start with some initial velocity and are not travelling directly towards each others center of mass. They make a close pass and each captures the other. A different explaination required for the origin of spin from a "cold start" since if we drop two objects in space far from the gravitational influence of other bodies they will fall directly at each other, ie


0--------------------> <---------------------0


rather than

0-------------------->

<---------------------0


In that case I think the explaination comes from the asymmetry of the actual objects. If they were perfect indestuctable spheres they might bounce back along the approach path and just occilate in and out like that, but since all the stuff out there is irregular in shape/structure the collision will exert a torque & get things spinning.

[SpeckledJim]

SpeedyDave said:

The Black Flash said:

Re: the origin of spin.
If you consider two rocks, both travelling in straight lines through space.
Their paths brings them close to each other.

0-------------------->

                        <---------------------0

As they get close, gravity starts to effect them and pull them together.
If you imagine that, when they get to the position below, gravity "locks" them together...

                   0--------------->

     <--------------0

You have this sort of situation:


                    0-->
                    |
                 <--0 

So their existing momentum causes the pair of them to rotate about each other.  

Multiply several billion times and you end up with a planet which is rotating.

I think this example works for objects which start with some initial velocity and are not travelling directly towards each others center of mass. They make a close pass and each captures the other. A different explaination required for the origin of spin from a "cold start" since if we drop two objects in space far from the gravitational influence of other bodies they will fall directly at each other, ie


0--------------------> <---------------------0


rather than

0-------------------->

<---------------------0


In that case I think the explaination comes from the asymmetry of the actual objects. If they were perfect indestuctable spheres they might bounce back along the approach path and just occilate in and out like that, but since all the stuff out there is irregular in shape/structure the collision will exert a torque & get things spinning.

But your clean and tidy example has a starting point at rest, and only two relevant actors.

That wasn't the case. It was, literally, chaos. Everything influencing everything.

[SpeedyDave]

SpeckledJim said:

But your clean and tidy example has a starting point at rest, and only two relevant actors.

That wasn't the case. It was, literally, chaos. Everything influencing everything.

Sure, I'm just explaining how you will still get spin even from seemingly 'nowhere' when things collapse under gravity.

[The Black Flash]

SpeckledJim said:

SpeedyDave said:

The Black Flash said:

Re: the origin of spin.
If you consider two rocks, both travelling in straight lines through space.
Their paths brings them close to each other.

0-------------------->

                        <---------------------0

As they get close, gravity starts to effect them and pull them together.
If you imagine that, when they get to the position below, gravity "locks" them together...

                   0--------------->

     <--------------0

You have this sort of situation:


                    0-->
                    |
                 <--0 

So their existing momentum causes the pair of them to rotate about each other.  

Multiply several billion times and you end up with a planet which is rotating.

I think this example works for objects which start with some initial velocity and are not travelling directly towards each others center of mass. They make a close pass and each captures the other. A different explaination required for the origin of spin from a "cold start" since if we drop two objects in space far from the gravitational influence of other bodies they will fall directly at each other, ie


0--------------------> <---------------------0


rather than

0-------------------->

<---------------------0


In that case I think the explaination comes from the asymmetry of the actual objects. If they were perfect indestuctable spheres they might bounce back along the approach path and just occilate in and out like that, but since all the stuff out there is irregular in shape/structure the collision will exert a torque & get things spinning.

But your clean and tidy example has a starting point at rest, and only two relevant actors.

That wasn't the case. It was, literally, chaos. Everything influencing everything.

I think he's right though, for direct impacts, albeit that those might well be very rare. I was just trying to keep things simple

[SpeckledJim]

The Black Flash said:

SpeckledJim said:

SpeedyDave said:

The Black Flash said:

Re: the origin of spin.
If you consider two rocks, both travelling in straight lines through space.
Their paths brings them close to each other.

0-------------------->

                        <---------------------0

As they get close, gravity starts to effect them and pull them together.
If you imagine that, when they get to the position below, gravity "locks" them together...

                   0--------------->

     <--------------0

You have this sort of situation:


                    0-->
                    |
                 <--0 

So their existing momentum causes the pair of them to rotate about each other.  

Multiply several billion times and you end up with a planet which is rotating.

I think this example works for objects which start with some initial velocity and are not travelling directly towards each others center of mass. They make a close pass and each captures the other. A different explaination required for the origin of spin from a "cold start" since if we drop two objects in space far from the gravitational influence of other bodies they will fall directly at each other, ie


0--------------------> <---------------------0


rather than

0-------------------->

<---------------------0


In that case I think the explaination comes from the asymmetry of the actual objects. If they were perfect indestuctable spheres they might bounce back along the approach path and just occilate in and out like that, but since all the stuff out there is irregular in shape/structure the collision will exert a torque & get things spinning.

But your clean and tidy example has a starting point at rest, and only two relevant actors.

That wasn't the case. It was, literally, chaos. Everything influencing everything.

I think he's right though, for direct impacts, albeit that those might well be very rare. I was just trying to keep things simple

I think if it looks like a direct impact, you aren't measuring it to a sufficient degree of accuracy.

Only on the blackboard could two objects be solely attracted to each other, to the exclusion of everything else, and so have a genuinely 'head-on' collision.

[SpeedyDave]

I agree, again the point was to explain that spin will even arise when the tragectories don't appear to promote it. Spin is inevitable, its the way of things

[Gene Vincent]

The really clever thing to explain is why any objects move at all if there is insufficient gravity around to start them moving...

[TheHeretic]

Is it not because god is a DJ?

[MiseryStreak]

Gene Vincent said:

The really clever thing to explain is why any objects move at all if there is insufficient gravity around to start them moving...

I don't understand what you mean, objects are caused to move without gravity all the time. In the grander scheme doesn't gravity actually reduce movement (by countering the inflaton field)?

[SpeckledJim]

Gene Vincent said:

The really clever thing to explain is why any objects move at all if there is insufficient gravity around to start them moving...

Doesn't answering that question force an acceptance that only gravity can start things moving?

Erroneously?

[Asterix]

TheHeretic said:

I'd move to the sunny side, and then opt to work nights.

You'd also find it an Islam free place - Ramadan would be a hoot!

[Gene Vincent]

Go to the very start of the accretion process.

In free-space before any accretion existed, the ratio of matter is roughly equivalent to having 3 marbles on a Table about the size of Europe, that is perfectly flat and frictionless.

One marble is in London, another in Munich and the third in Gibraltar.

How do they move at all?

[SpeckledJim]

Gene Vincent said:

Go to the very start of the accretion process.

In free-space before any accretion existed, the ratio of matter is roughly equivalent to having 3 marbles on a Table about the size of Europe, that is perfectly flat and frictionless.

One marble is in London, another in Munich and the third in Gibraltar.

How do they move at all?

Is that the very start though?

How did those marbles get to those places and is there any kinetic energy remaining from made them and put them there to provoke any movement?

[MiseryStreak]

I have too many questions about your questions to begin to attempt an answer! Do you mean because the strong nuclear force has no effect at that range? Before gravity existed as a standalone force, were there any 'objects' to move?

[Gene Vincent]

SpeckledJim said:

Gene Vincent said:

Go to the very start of the accretion process.

In free-space before any accretion existed, the ratio of matter is roughly equivalent to having 3 marbles on a Table about the size of Europe, that is perfectly flat and frictionless.

One marble is in London, another in Munich and the third in Gibraltar.

How do they move at all?

Is that the very start though?

How did those marbles get to those places and is there any kinetic energy remaining from made them and put them there to provoke any movement?

Pretty much, after inflation there cosmos was very thinly sprinkled with minute particles almost all with that sort of distances between them.

So, what started them moving?

What process caused such isolated particles to start moving at all, something must have been 'present' to get them whizzing across vast distances so as to eventually attract and eventually collide with one another.

What caused this to happen?

[MiseryStreak]

Gene Vincent said:

Pretty much, after inflation there cosmos was very thinly sprinkled with minute particles almost all with that sort of distances between them.

So, what started them moving?

What process caused such isolated particles to start moving at all, something must have been 'present' to get them whizzing across vast distances so as to eventually attract and eventually collide with one another.

What caused this to happen?

Irregularities in the quantum 'jitters'? So the matter was not evenly distributed and entirely homogenised, resulting in some particles being closer to some neighbours than others and therefore having a tendency to move towards those and eventually coalesce? The gravity would be extremely small but non-zero.