The Universe is a big place; we have that idea down pretty tight.  Even with all that room, sometimes moving bodies run into each other.  In about 4.5 billion (with a “b”) years, that may happen to the Milky Way and M31; the Andromeda Galaxy.

This is a CGI visual of the Andromeda/Milky Way collision, by HubbleSite:

Andromeda and Milky Way collide

We know that Andromeda is getting closer to the Milky Way by about 120 km/s (by Doppler redshift, which you know about), but we don’t have a way yet to measure its transverse velocity.  That may change in 2012 with the ESA’s Gaia Mission (read about that here).

From a distance (and over time), the force of the two galaxies colliding will be impressive.  Close up, it probably won’t be much to shout about because galaxies are diffuse things.  For perspective, if our sun where the size of a quarter (small American coin), the next closest “quarter” would be about 475 miles away.  Of course, when our galaxy cores merge, with our supermassive black holes, that will likely cause some shock waves.

Scientists are saying there is a 12% probability that our solar system will be ejected from the galaxy in the collision.  That might be fairly impressive to witness – but we won’t.  We will be long gone from the Earth by then, either through relocating our species or through extinction.  Remember, in about a billion years the sun will be too hot for there to be liquid water on the Earth.

4.5 billion years is a long time for any species, but maybe there will still be humanity somewhere to witness the collision.