Are There Dead Spots ?

[modeller]

We always think of the movement in space of objects , usually as being in orbit around something but there must be points where all forces cancel each other out and an object would/could remain motionless, as long as those conditions prevailed. However in a dynamic universe maybe those doldrum conditions would never last for long or do they.

[jim360]

Even if such places existed, how would the object get there in the first place? And how would it stay there?

Even so it's a good question. Science, often, is concerned with finding some fixed solution and seeing what happens when you change it slightly. Given that otherwise we wouldn't have the first clue what's going on now, let alone what might happen in future, this is the best way to try and understand a system. It's called, or certainly should be called, "instability analysis" (or in other contexts, "perturbation theory"), and is basically what Scientists spend their careers thinking of.

Why, by the way, "must" there be such points? In any simplified system, e.g. of two or three planets and a central star, such points to tend to crop up in special cases, but otherwise the only point of zero force is "at infinity" -- which in practice means nowhere.

[modeller]

I hesitated to use the word 'must' but on second thoughts if all the forces
acting on a point cancelled each other out, if only momentarally that would be a dead spot. I also thought if I flew towards say the moon and 'stopped'
in the neutral zone. Would I 'fall' towards the earth or the moon or neither ?
The same thing would apply if I flew towards any planet would I enter zero gravitational zones ?

[Teddio]

There is no absolute frame of reference. Yes there is a gravitational "dead spot" between the Moon and Earth where an object could remain at rest with respect to the two bodies. However there are always gravitational perturbations caused by the Sun, planets, nearby stars and even nearby galaxies.
The Moon orbits Earth, Earth orbits the Sun, the Sun orbits the centre of the Milky Way galaxy and the galaxies are gravitationally linked.

[LazyGun]

Are you thinking of Legrange points perhaps, modeller?

http://en.wikipedia.org/wiki/Lagrangian_point

[modeller]

//And how would it stay there?//
I assume it wouldn't have to do anything . If there are no forces acting upon it , it wont move.

//how would the object get there in the first place?//
I can imagine if say two slow moving objects were to collide with minimal force they could bring each other to a standstill.
It happens on earth when two vehicles gently slide into each other on ice,
there is no damage to either , they just stay there.

I remember reading about the rings of Saturn and the thousands rocks are grouping and regrouping and many break away .
I should imagine with the thousands of stray rocks in space some would find themselves at a standstill in a dead spot , if they exist.

[jim360]

Well my point was mainly that if such a point existed some object would have to travel to get there, and in so doing would be moving. At the point where the forces were zero it seems unlikely that the object in question would also have a speed of zero, at least in empty space unless the forces were such that they acted against the motion just enough so that when it finally reached the "dead spot" it would be stationary. Such a point would definitely be highly unstable - just kick it back the way it came and it would speed up away from the point.

In some way such spots would actually be quite common -- think of, say, a ball kicked up a hill and coming to a stop just at the very top of the hill. But almost all such spots, in empty space, would be highly unstable. And, as you say, the Universe is constantly changing so that any spots that do exist woudl undoubtedly be in a different place the next day.

[modeller]

LG // Are you thinking of Legrange points perhaps, modeller?//

Yes! in part, L2 is what I had in mind except it still orbits the sun. Whereas I was thinking of ' static ' neutral points possibly between several stars where objects could remain motionless in relation to those stars.
In other words dead spots between solar systems. and by extension to galaxies.

[modeller]

jim I like your hill top analogy but space is so vast that tops of hills might be bigger than we think . A few hundreds of miles could house many asteroids.

[jomifl]

You would need some kind of feedback to keep an object at one point, even in a 'dead spot'. Eventually it would succumb to the variations in the gravity field wherin it rests. No different to trying to balance a needle on it point.

[jim360]

Not really "my" hill analogy but thanks anyway. Yes, the hill tops could be large, but... they would certainly not be flat, over any significant range of space. Significant has a loose meaning here, depending greatly on the size of the object you are talking about, but in the end jomifl is right. The perturbations would be too great for an object to remain in a fixed position indefinitely.