Mickymoody said:
It's proved to be the weakest of the forces, but unexplainable as to why.
That is correct. On a small scale it's so weak that meaningful experiments are difficult to do.
Example 1:
If you allow a hollow sphere to collapse under its own gravity, energy is released. Current thinking says that this energy comes from the extra gravitational field which has just appeared and so that field must consist of negative energy.
The alternative explanation is that the mass goes down. Which is it?
Example 2:
It's been accepted for some time that gravity deflects light. It would be nice to set this up in the lab and then check that the light also pulls back on the mass that deflects it.
Example 3: Move some mass and see how long it takes for the effect to be detected at a distance. That is to say, let's just measure the speed of gravity.
Unfortunately gravity is so weak that we can't make the measurements, though the third one might just be possible.
RedHerring2 said:
It's stronger than the force I can exert on 200kgs of ballast.
Although it's weak
on a small scale, gravity differs from the other known forces in having only one polarity. Moreover, it's the only known force for which like attracts like. While the electromagnetic and nuclear forces cancel out, gravity just keeps on adding up. Incidentally, if you apply Newton's law of gravitation to (hypothetical) negative masses you'll get some interesting results:
1) Negative masses repel each other. They can't coalesce into larger masses.
2) A negative mass is attracted towards a positive one but the positive mass is repelled.
3) If they are equal in magnitude, the negative one will chase the positive but never catch up.
4) If the positive mass is bigger, the negative one will catch up and annihilate, leaving a smaller positive mass.
5) If the negative mass is bigger, the positive one will outrun it.
The upshot of this is that, if negative mass particles had existed in the early universe, you wouldn't expect to find many of them now.
