> There's a Sci-fi story - I forget the name - where space travel is actually quite simple if you know the math to make it happen, and when a peaceful alien race visits Earth, the humans - with their thousands of years of armed conflict - kill the aliens and take their spaceships.

I think you might be thinking of Harry Turtledove's "The Road Not Taken". They weren't exactly friendly, but they were quite surprised by Man's ingenuity.

I recall the story too: if I recall correctly, it was actually a branch of chemistry we had missed, not a math problem per se. The aliens stormed off the ships with muskets and we slaughtered 'em.

Not doing any kind of reconnaissance before a conflict would be crazy! But I guess maybe they didn't think of it!

It's also my working theory of why we won in Independence Day the movie :) Aliens were never warring among themselves and/or allowing any kind of a hacking culture, so didn't think to secure their systems!

I had come to the same conclusion with respect to ID which made the plot a lot easier to swallow.

They were essentially a hive mind working with a single purpose and were therefore incapable of conceiving the notion that they would need to secure their systems against an attack that essentially used their own technology against them.

It would be nice, and is certainly the result I hope for (with all the good wishes do), as I'm not sure we will work very well with slow and expensive growth. Reading 'Against a Dark Background' was certainly interesting as it considers a system which essentially has its growth capped by the distance between itself and the rest of the universe.

Something I noticed in that novel is that they still had incredibly advanced technology. The Lazy Guns alone hint at what they lost!

Maybe there's a reason Golter is where it is, and maybe it wasn't always there.

I read _Against A Dark Background_ as a teen an thought of it as a pretty fun sf, almost space-opera adventure story. Then I read it again as an adult and... wow. Bank's SF work is very good, but the quiet existential horror that lurks behind the scenes in that novel really merits the title. Makes good old Lovecraft seem like a piker by comparison.

You write a good comment, I don't know why you were down voted.

There's the EM drive, maybe. Some recent discussion: https://news.ycombinator.com/item?id=12995125

If you can create trust out of electricty (ion drives also come to mind), then if you have nuclear power or fusion, things get interesting again in terms of propulsion.

To reach orbit, there are a bunch of interesting proposals: skyhooks (i.e. space elevator light), launch loops, railguns.

The EM drive is reported as producing maybe 1 mN per kW. The largest power plant in the world produces about 22 GW of energy--at that ratio, an EM drive could produce maybe 22 kN. The first stage of the Saturn V rockets produced 35,000 kN of thrust.

As a practical engine, EM drives are worse than chemical rockets or even ion thrusters (which produce about 25-250 mN/kW, according to Wikipedia).

If your numbers are correct, and I did the math correctly in my head, that's about five thousand pounds of thrust. A nuclear submarine might weigh ten thousand times that. Obviously a nuclear sub operates at much lower power level and obviously the hull doesn't have to be as thick for a spacecraft so I think it not a ridiculous comparison. Also you're spacecraft need not carry dozens of ICBMs, torpedoes, hundreds of sailors, etc.

Obviously the first engine ever exhaustively tested is usually not the highest performance achievable. A factor of a thousand seems possible although a hundred is more likely.

So for a very large nuclear plant, if the EM drive were weightless (LOL) then maybe 1/10th G applied to that nuclear sub is some kind of theoretical limit not entirely ridiculous but possible.

Now consider that to one sig fig a G of acceleration for a year is the speed of light. And if you're careful maybe you could run the plant for three decades.

So a round trip to the speed of light and back again is more or less back of the envelope possible to visit some distant star.

Relativity rears its ugly head and ten years of ship time around the speed of light is a lot further than 10 lightyears and time passing on earth is a lot longer than ten years.

Of course at the speed of light you're going to need a rather substantial hull, an active radar that can avoid interstellar crud, etc. If you can detect "stuff" at a day out, at a tenth of a G accelerating for an entire day you can be quite far away indeed by the time you pass it.

I can't run the numbers in my head like above, but even 1/100th G slices a very substantial amount of time off a Mars mission. Some unverified google results imply 1/100th G means Mars in a month not a year and a half.

At least as a back of the envelope calculation its quite a useful vehicle.

This was a response to 'Problem is we don't really know where to go next'. Saying that there's basically nothing beyond chemical rockets.

My point is that there are plenty of possibilities to continue to develop propulsion technology.

Once we understand the physics, if it's a real phenomenon, we might be able to make it much more efficient. Compare a modern li-on battery to a lemon, coin and nail, for example.

If you have a fusion power plant, you also have plenty of spent fuel to use as reaction mass.