h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

We like to think that space is a vacuum and it is in all practical terms, but there are a few particles of, correct me if I'm wrong, ionized hydrogen every few cubic feet or so. So my question is this: if you had an object traveling in the inter-stellar medium, would it, independent of gravitational pull of anything else and assuming it would never run into any other object, ever come to a stop due to friction or are the particles so few that it would have no effect on the object. If so, has anyone ever calculated how long it would take?

Forever is a long time.

If there is hydrogen, no matter how thinly spread it is, it will slow down any moving thing and eventually it will be going so slow it will effectively have stopped. But such viscosity will never bring it to a complete stop because the slower it's going the less of an effect the viscosity will have.

Of course, in the "stopped" state, both the object and the interstellar hydrogen will still be orbiting around the centre of the galaxy.

>>,,, the slower it's going the less of an effect the viscosity will have.<< : Gnomon.

... and presumably an object of substantial size would start to attract such hydrogen and in a short time (universe-wise) become something else.

yes/no?

"...ever come to a stop due to friction or are the particles so few ..."

Coming to a stop assumes there is a privileged inertial reference frame, which there isn't. I suppose you mean stop as in relative to the background hydrogen atoms / ions (there's also Helium atoms etc but in lower proportion) - but I think their relative motion is not uniform and dependent on where they are located in the universe (??) I suppose the question needs to be framed within a model universe before answering. Using the model universe the answer would be it would never "stop" - it would slow down to an infinitesimal speed but not zero speed.

ps I am not sure what effect the "vacuum fluctuations" would have on the speed of the spacecraft.

pps Radiation pressure certainly will have an effect on the speed of the "object". So once again we would need to have a "model universe" to answer the question.

Thanks everyone, it's given me a lot to think about!

Okay a back of the envelope calculation:

A 10 tonne piece of rock (chondrite) travelling at 10 km per second through interstellar space (a million atoms per cubic metre average density) would take about 0.3 billion current universe "lifetimes" (current universe "lifetime" (current age) = 13.7 billion years) to slow down to about 0.01 metre per second speed (let's call that as good as stopping).

I am not sure what affect dark matter would have nor the affect of the electromagnetic radiation.