Hi Bessy,

The extreme I was referring to is relating to the concept that lighter objects fall faster (i.e. with greater acceleration), for instance on the moon (it’s best not to think about black holes, they’re unnecessarily complicated).

If you take this to a logical conclusion, then very very light (low mass) things will experience very very high acceleration; atoms are pretty light (low mass), and so they would experience greater acceleration towards the ground in this model. So atmospheres (made of atoms and/or very light molecules) would experience more gravitational acceleration. That’s fly in the face of escape velocity.

Photons (sorry typo in my last post) have an even smaller mass (a result of them having energy and travelling at the speed of light), they’d be attracted to the moon even faster.

The moon isn’t a black hole, so I don’t think that makes sense.

When you’re talking about being next to a black hole, yes you will experience a different amount of force if you have parts of the object closer to (or even in) the event horizon of the black hole; this leads to Spaghettification (where you’d be stretched if you fell into a black hole, check Wikipedia); but my bowling ball example was considered from the more mild gravitational field of the earth, in this case, the distance of the centres of mass would be important, not the distance of the leading edge. If you think about it, if one half of the bowling ball is closer to the earth, then the other half is equally further from the earth, it evens out to make a centre of mass.

For planets not touching each other, we can generally use the concept of a centre of mass, check here:

http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation#Bodies_with_spatial_extentRead the section titled “Bodies with spatial extent”.

Also, I think you’re confused about the reason for refraction of photons; refraction is a process which results from the interaction between the photon (which is an electro-magnetic -wave) and the electrons in “orbit” (OK, a simplification) around the atoms, which are charged particles (charged particles can affect electromagnetic radiation); as the photon passes by the electron then it may be absorbed and re-emitted later, and this process will reduce the effective speed with-which the photon travels through the object and can (in some cases) allow the photon to be re-emitted back in the direction it came from.

This is a consequence of the electromagnetic force, as described by Quantum Mechanics, not relativity.

However, I think I see what you’re saying, that theoretically, if an atom was infinitely small, then if you get infinitely close you’d experience infinite gravity (if you plug in the maths); but remember that an atom is not infinitely small, it’s not a point in space, it is a collection of particles (protons, neutrons and electrons). Even the Protons and Neutrons are made up of smaller things (Quarks).

However, if you look at the basic building blocks (electrons and quarks) then these are point particles with no volume; these fundamental particles may (according to the maths of relativity) exibit near infinite gravity when you get near infinitely close; they may even have an extremely small event horizon (so be black holes, if you get close enough), but we believe this is more a sign that relativity breaks down as you get into subatomic scales. (Newtonian gravity shows this same weakness)

In order to actually understand what’s going on gravitationally at these scales, we need a theory of quantum gravity; which hopefully the LHC (the new particle accelerator) will help us develop.

Just to be honest and up-front, I’m a part-time student (not a graduate), so I’m still trying to learn all this stuff; hence I’m open to any mistakes or flaws that people find in my answers, I just think you’ve got things a little wrong here.

The problem is that you’re thinking of relativity from a non-mathematics stand-point; considering that most universities teach relativity in their maths department (not physics departments), that just shows how maths-heavy a subject general relativity is (and I’m nowhere near capable to answer that sort of maths).

If anyone wants to learn more about this stuff without the maths, then I highly recommend

http://www.astronomycast.com/, they have episodes on almost anything to do with this.