# Magnetism, Gravity and Critical Momentum

1. Jun 11, 2009

### PRD

Please consider the following…

Suppose you were on a space-walk at the International Space Station and you had two refrigerator magnets. And suppose you gently released one magnet so that it remained “stationary” in space. And suppose you threw the second magnet as hard as you could in the direction beyond the solar system. Which of the following two scenarios would happen?…

1. The momentum of the second magnet would be so great that it would overcome the magnetic attraction between the two magnets and the two magnets would move away from each other forever.

Or,

2. The magnetic force between the two magnets (even though may be infinitesimally small at great distances) would be strong enough to slow the second magnet down and one day the two magnets would come back together again.

For what it’s worth my vote is for option one. If two magnets are moving away from each other there is a “critical momentum” (or escape velocity) which if exceeded will overcome the magnetic attraction between the two magnets and they will move away from each other forever. In effect, there is a certain distance beyond which the magnetic attraction between the two magnets becomes “zero“.

I postulate that exactly the same thing holds true for gravity. If two galaxies are moving away from each other there is a “critical momentum” (or escape velocity) which if exceeded will overcome the gravitational attraction between the two galaxies and they will move away from each other forever. In effect, there is a certain distance beyond which the gravitational attraction between the two galaxies becomes “zero“.

So what do you think? If in fact there is such a thing as a “critical momentum” it could explain the ever-expanding nature of the universe.

2. Jun 11, 2009

### ZapperZ

Staff Emeritus
I don't get it.

If you can derive the escape velocity of an object from the surface of a planet (and this a common intro physics exercise), then couldn't you just work backwards and figure out the minimum velocity from whatever the starting point that you want to use? So what am I missing here?

Zz.