Gravity (over extremely long distances)

[.physics]

Existing "correct" theories tend to break when pushed far beyond observed ranges of operation, with the old theory being a limiting approximation in that regime. E.g. SR replaces Newton's laws, when speeds get high.

Nobody has studied very small accelerations or weak gravity. If current theory were to be wrong, this is an area where it might show up. Meanwhile, QM + Gravity is a major unsolved problem. So, the behavior of gravity outside of its known regime is a point of, at the very least, humility in our confidence.

You are right. That's what I was trying to say. We would better find out some new formula for extreme condition.

 

[diazona]

But sometimes theories continue to apply without modification well outside their original observed ranges of operation. So you can't automatically say that you need a new formula for extreme conditions. At least, not until you find some reason to believe that the existing theory is inadequate.

 

[DaveC426913]

You are right. That's what I was trying to say. We would better find out some new formula for extreme condition.

Why do you think a new formula is needed?

Do you mean to say that, as a rule, all models breakdown at their extremities, therefore this one must too?

Shouldn't we wait to see if our existing model actually breaks down first? Hm?

 

[Thecla]

Hi
The Physics Teacher(vol32 Nov 1994 p.493) addresses a similar problem in a paper "Surprising Facts About Gravitational Forces" by Mallmann, Hock, and Ogden.

Two hydrogen atoms initially at rest and 1.0 millimeter apart would require almost two million years to fall toward one another to be 0.5 millimeter apart.
In the first 1000 years the separation of the atoms would be reduced from 1.0 mm to
0.99999989 mm.(a change in distance of about one atomic diameter)

The equation of time for a free fall of two objects is derived in this paper ,the final result is:

t={r(i)^1.5/sqrt(2G(m(1)+m(2)))} X {sqrt(R) x sqrt(1-R) +arccos(sqrt(R))}

Where t= time for the two masses,initially at rest, to travel the distance from r(i) to r(f)
r(i) is the initial separation of the center of masses of the two objects
R is the Ratio of r(f)/r(i) where r(f) is the final separation
G is the gravitational constant
m(1) and m(2) are the masses of the two objects

 

[diazona]

Why do you think a new formula is needed?

Do you mean to say that, as a rule, all models breakdown at their extremities, therefore this one must too?

Shouldn't we wait to see if our existing model actually breaks down first? Hm?

And using that logic, the idea that all models break down at their extremities is itself a model, and therefore must fail when applied to such an extreme theory as gravity

(well said DaveC)

 

[DaveC426913]

And using that logic, the idea that all models break down at their extremities is itself a model, and therefore must fail when applied to such an extreme theory as gravity

Good one!

 

[13habelbrea]

That's one hell of a force!

Assuming the force stays constant on both neutrons attracting each other, then it would take approx 10⁵³ years for them to collide. At the time of their collision, they'll be slamming into each other at a whopping 200 Planck lengths per second.

But what bothers me is that I've used a simplified version of events with my assumption. Of course the attractive force will increase as they get closer to each other. Anyone know how this could be calculated?

On that that makes me have a question, Would any droppler effect/ expansiton of space be happening on the two neurons?

 

[ABW]

I thing, this Question actually is connected with "model" of the Universe in Future.
In future many stars become Neutron stars and Black holes.
And question also is: "Will gravity eventually pull them together"