from another blog

I was browsing for DHTML countdowns, when just for the heck of it, I clicked on the blog link. This is the first entry and it was pretty fucking damn amazing.

http://andrewu.co.uk/blog/

thats the adress, or just click here if you are really interested.


I sometimes wonder where these thoughts come from. This one popped up between pondering whether there was anything worth watching on TV (no there wasn't) and whether or not I was hungry enough to warrant a snack (yes I was). Here's the thought:

It's popular knowledge that if the Sun disappeared in an instant, for argument's sake, that we wouldn't know about it for a little over 8 minutes due to the time it takes for photons to leave the solar surface and travel across the void to reach our eyes. After 8 and a bit minutes we'd suddenly see that the Sun had disappeared. The so far undisputed theory being that information cannot travel faster than c. So, it must also be true that the Earth would continue orbiting along its almost circular path for those 8 and a bit minutes too. It would continue to experience a centripetal acceleration towards the former centre of gravity.

The more interesting point though is that in large systems such as galaxies and galaxy clusters I wonder how this latency effects the dynamics of those systems? Consider that our Sun is in the region of 28-29 thousand light years from the centre of our Milky Way galaxy, that information about the centripetal acceleration due to the force of attraction towards the galactic centre and the direction of that force takes around 28-29 thousand years to arrive at the Sun. What effects arise due to this huge latency?

Mind you, when it comes to gravitational influences upon galactic disk members it's too simplistic to talk of the Sun orbiting the galactic centre in the same way that the Earth orbits the Sun as the inner parts of galactic disks tend to orbit with the characteristics of a solid — i.e. no distinct separate orbits but rather everything orbits together in a single amorphous lump. The comparison is still fair though for bodies at the far edge of the galactic disk, or perhaps not in the disk at all (not sure about that last bit).

I can't quite get my head around the idea of billions of gravitional interactions going on based upon the direction and magnitude of forces as they were several tens of thousands of years ago; stars and gas clouds reacting to influences as they were tens of thousands of years in the past, rather than as they are now. Still, it all seems to work out happily in the end since galactic disks appear to be quite stable and long-lived structures IIRC.

I don't recall anyone lecturing me about this latency effect at University at any point. It's one of those questions you'd really like to have asked at the time and not six years after the event. Seems like a bad case of treppenwitz!

now I know you are interested, so click here.