Hacker News Reader

Comment by nabla9 | Hacker News Reader

> ~17 planets in every cubic light year.

The change of hitting one while traveling one light year is in the order of 10e-16.

A few things make me thing it's a much larger risk than 10e-16.

1. My numbers are on uniform distribution of planets. The milky way is not uniformly distributed, and I suspect this isn't either. I hypothesis most of the planets will be on the spokes, just like most of the stars are. In addition, while it is 2kly tall, there is not a uniform distribution across the height.

2. Interstellar travel doesn't involves traveling a minimum of 4 lys, and a maximum distance without being intergalactic would be about 100,000lys.

3. Your assuming instantaneous travel across the light year. In reality we would consume at least 1 year in travel time, and likely much more.

4. The amount of resources it takes to build an intersellar ship is very large. The cost of a collision is high. This doesn't change the risk of a collision, but it does change what we consider acceptable risk.

About the same as getting shot while drowning in bathtub stricken by lightning?

That's how my uncle died.

Exactly, and I saw that when the Andromeda Galaxy collides with ours, it's unlikely many planets/systems will be colliding with one another.

Never really understood how galaxies can collide if space has been expanding since the big bang and everything is moving apart

It goes both ways depending on distance. Expansion is exponential (ish), gravity is inverse-square [1]. Only the few that happened to be close enough that gravity is more significant than expansion can collide.

[1] at least in Newtownian physics. The argument is geometry, so I suspect but am not qualified to say it isn’t exactly inverse square in GR.

Same reason I can collide with someone on the sidewalk while the universe is expanding at tremendous speed. Over short distances, the expansion can easily be overcome by others forces like gravity or my muscles excerting force through friction against the ground. However, given enough time, there will be less and less objects that are close enough to be able to collide at all. Lots of time though, and there are already now lots of things over galaxy could never ever collide with.

On very large scales gravity collected mater into what look like spider webs, with vast amounts of empty space and threads of interstellar gass which can form galaxies. Those galaxies form little clumps within those threads with gravity causing them to orbit each other. However, as it's a many body problem things are not stable over very long time frames and galaxies often hit each other. This does slow down over time as there are fewer remaining galaxies in a cluster.

Metric expansion of space has effect only on very large scale. Galaxy clusters have enough gravitational force to keep them together.

Reply on news.ycombinator.com