I’m not certain which designs you are referring to, but I suspect you are either referencing Project Daedalus concept or the ORION nuclear pulse propulsion. Both concepts and derivatives thereof have specific impulses on close order of I[sup]sp[/sup] ~10,000 s, which is certainly adequate for interplanetary travel, but too little by at least an order of magnitude to achieve velocities that are an appreciable fraction of c and carry enough propellant to decelerate at the destination. (The Project Daedelus concept was never planned to deceleate; it would fly right through a system at >0.10c, for whatever use that would be.) Neither would be suitable for sending people to another system.
The problem with antimatter–aside from issue that reaction products are either highly energetic gamma rays that are difficult to thermalize, or heavy mesons that carry much of the energy reaction bound up inefficiently as mass–is that it takes enormous amounts of energy to produce the environment in which pair production can occur, far more than you’ll ever recover from the annihilation reaction. By any conceptual antimatter production method it would literally take a substantial fraction of the energy output of a medium sized star to produce enough antimatter to propel a sizable craft across interstellar distances.
I don’t know of any workable propulsion concept that could plausibly reach a speed that would permit transporting a useful colony-sized payload across interstellar distances save for the Bussard fusion ramjet, which itself has some significant limitations (minimum operating speed 1-2% of c, thrust/drag relationship that limits its maximum speed to 8-12% of c, and the whole difficulty of fine control of a magnetic field hundreds or thousands of kilometers wide). There just isn’t any current concept that is viable within known physics that makes it plausible to transit interstellar distances in something like a human lifetime.
Stranger
No No No when he said the 8th month of the year 1999…no er um…when he said October 13, 2010…he didn’t mean OCTOBER 13, 2010. He meant some other time…in the future. October in Old English means March…or possibly September. Also some configuration of time/year that somehow can be manipulated to be 2010.
No 10/13/2010 is really meant to be 3/10/2013.
Oh yeah? Well you’ll be blinking out the other side of your beak (no, I’ve no idea what that means either…) after the aliens vaporize your favorite city later today! :eek:
space travel is only expensive to us. For all we know, there are aliens out there who can travel light years on one Butterfinger bar.
[QUOTE=Stranger On A Train]
I don’t know of any workable propulsion concept that could plausibly reach a speed that would permit transporting a useful colony-sized payload across interstellar distances save for the Bussard fusion ramjet, which itself has some significant limitations (minimum operating speed 1-2% of c, thrust/drag relationship that limits its maximum speed to 8-12% of c, and the whole difficulty of fine control of a magnetic field hundreds or thousands of kilometers wide). There just isn’t any current concept that is viable within known physics that makes it plausible to transit interstellar distances in something like a human lifetime.
[/QUOTE]
What about the concept of contracting space in front of the ship while expanding it behind? Granted, we don’t know how to do this (no more than we know how to make anti-matter in sufficient quantities to be useful even if you could get around the radiation effects), but it’s theoretically possible from what I understand, and could propel a ship at over light speed (well…that would be the effect to an outside observer, again from what I gather). You just need exotic stuff like negative matter and negative energy to do it…
The point, of course, is that our understanding of the universe, how it works and what technologies are or aren’t available might be different than another species, especially if said species is more advanced. But even given our perhaps limited understanding, I’ve certainly seen ‘current concepts’ that are ‘viable within known physics’ that COULD make interstellar distances something that could be traveled in a humans lifespan.
I don’t see why another species that had such technology would travel interstellar distances to conquer another planet…given such technology the entire galaxy would be open to such a species, after all. Pretty much unlimited resources by any measure would be available…hell, it would be available to mankind if we could simply travel freely and inexpensively in our own solar system.
-XT
The really interesting question posed by the Fermi paradox is “Why do we exist in the first place?” instead of “If there are aliens, why haven’t we heard from them/met them?” If alien races that expand everywhere were common, it’s far more likely that one of them would have colonized Earth in the last 1 billion years already. If Earth had become an alien colony, then we would have never evolved.
Even if they had later died out on Earth, it’s very unlikely they wouldn’t leave any evidence behind. There’s no evidence of any long term alien visitation that we’ve found anywhere in our solar system. As far as we can tell, there have been no invasion fleets interested in Earth as a planet in the solar system’s long history. How unlikely is it then that one would be coming now in the brief period of human existence?
Whether or not there are aliens somewhere in the galaxy, it’s not at all surprising that we haven’t found them yet. We have only searched for an extremely tiny part of space, for an extremely tiny amount of time, at an extremely limited wavelength. Our existence and there being no sign of aliens in the history our planet is a far more compelling argument for the rarity of life, than SETI not finding anything.
This mostly concerns finding life in the same galaxy. Even if it’s unlikely for two intelligent species to exist in the same galaxy at the same time, there could still be a hundred million intelligent species in other galaxies. The space between stars is already hard enough to cross even with imagined future technology. The space between galaxies may be impractical to cross with any technology.
That assumes that interstellar colonization is at all plausible or even possible.
You cannot do a lot of expanding if interstellar colonization isn’t possible, so that was already in the assumption.
The point your quote raised was, if there are aliens, why haven’t they colonized Earth? I provided the answer. Like you said, it seems difficult to imagine spreading across the stars. I was more or less agreeing with your ending sentiment.
Don’t give 'em jobs, take away their welfare, damn aliens will leave.
True. 510 million square kilometers is the surface of the Earth. Not an unreasonable barrier to keep out aliens., Let’s start on a fence…
The problem is that human beings have not yet discovered faster than light travel, and until we do, no alien race will come talk to us and will leave us alone as not to disturb the timeline. We humans are too primitive and scary and will kill and possibly cook any green man from the heavens.