Milky Way & Andromeda Merger

Barry Schwarz writes:

On Wed, 09 Sep 2020 20:34:07 GMT,
wrote:

Common notion of collision is 2 entities on intersecting
trajectories, and making some kind of PHYSICAL contact, thus
altering both trajectories. It just doesn't fit the upcomming merger
IMHO.

When two ordinary objects (e.g., billiard balls) collide, the atoms of
the objects do not come in contact. The actual interaction is between
the forces holding the molecules in place relative to each other. In
an elastic collision, the centers of mass will never be closer that 2
times the radius.

When two galaxies collide, the stars of the galaxies need not come
into contact. The actual interaction is between the (gravitational)
forces exerted by each star (and the other mass in each galaxy). As
the galaxies merge, the centers of mass can get as close as the
trajectories allow.

What is the real difference that makes "collide" an appropriate word
for one interaction and not the other?

It's really a stupid argument and makes absolutely ZERO difference on
the reality of the upcoming merger. Neither the milky way or andromeda
care if it's a collision or not.
--
Daniel

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