Interstellar Travel
Offline I think the overpopulation topic is becoming a bit absurd, so let’s talk about the other issue touched upon in that thread, interstellar travel. The process of doing this would be quite tricky and tremendously expensive. If given enough resources, it would probably be feasible. The trouble comes with the following:
We will be referring to this diagram a lot. On the y-axis is the gamma factor, also called the Lorentz factor. It is defined in the following way:
γ =[1-v2c-2]-0.5
For a spaceship moving away from Earth in an arbitrary vertice of direction when we refer to gamma henceforth, we will be discussing gamma of the spaceship as recorded in the Earth’s frame. There are several key assumptions which we need to consider here. The first and most important is that the destination star system does not move relative to the Earth. This is roughly true.
The trick with travel to faraway systems is that we must get the spaceship gamma in the Earth frame as high as possible because that way the length contraction measured by the spaceship in comparison to the Earth observers will be as high as possible and consequently, as far as the spaceship is concerned, the time taken to travel to the destination will be much less than that recorded by the Earth. However, since we are considering two reference frames, we must be precise about this. If the spaceship travels to a distant planet/star system, then the spaceship is measuring the proper time between the events because as far as the spaceship is concerned, the events “leaving Earth” and “arriving at the new planet” occur at the same point in space. As per the first postulate of special relativity, no inertial frame is more valid than any other and consequently, if the Earth observer states that the spaceship moves away at velocity v, relative to Earth, the spaceship observer is just as correct to say that Earth hurtles away from them at velocity v and the destination travels toward them at velocity v and they themselves have not moved. This is the tricky part not just about Relativity, but mechanics in general. Absolute space is completely meaningless. When we talk about “velocity” all we are saying is that an observer is measuring a certain object in his local coordinate system as changing coordinates over time. In special relativity, it becomes just as meaningless to assert that if the time between events A and B takes time t in reference frame A then it will in reference frame B. The same must consequently hold true for the rod-like distance between points in space. When we say “Betelgeuse is 780 light years away” what we really mean is that if an Earth observer had a giant ruler which started at Earth and ended at Betelgeuse, the Earth observers would state that the distance between the ends was 780 light years. The Earth measures proper length. This cannot be the case for any observer which is moving relative to the Earth frame since that would violate the second principle of special relativity. Consequently when we talk about stellar distances we will be making frame distinguishing from now on.
The reason this is important is because if we could get a spaceship to travel at 0.999c (exactly 0.999c, this is very important since gamma tends to infinity as v tends to c), then the gamma factor will be 22.4. This could be a good thing. If we wanted to travel to a planet that was 500 light years away in the Earth frame, then at 0.999c in the Earth frame, it would take 500.5 years in the Earth frame. However, in the spaceship frame, the destination planet (which is travelling toward them at 0.999c) is not 500 light years away, because the Lorentz factor contracts the rod like distance in their frame (this is important, it is meaningless to talk about space-like separation without reference to frame), as far as the planet is only (500/22.4)=22.3 light years away. Consequently, they state that the journey takes just over 22.3 years. The other way to look at this is from the Earth frame. The spaceship measures proper time, so the Earth measures dilated time. Since we have just stated the proper time interval to be 22.3 years, it follows that the dilated interval is 500.5 years since the Lorentz factor is the transform for both quantities.
So, this would appear to be no problem. If we found a close terrestrial planet at 20 light years away then at 0.999c in the Earth frame, a spaceship could get there in about 10 months as far as they were concerned. This would be good because then we would have to stock fewer provisions on the spaceship and consequently it would be less massive.
This is sort of important because this is where the major limitation comes in. As a consequence of the mass energy equivalence, the gamma factor dilates mass of an object relative to the rest frame of THAT object. The total energy of an object in an arbitrary frame is therefore the sum of the rest energy (intrinsic quantity of the object under discussion) and the kinetic energy (depends on the frame):
Et=Ek+Erest
Where:
Erest=m0c2
Here, m0 is the rest mass. From above, we have mframe= γm0 where gamma is as recorded for that object in the frame under discussion
The total energy recorded in an arbitrary frame of reference in which the speed of the object is recorded to be v is therefore:
Et= γ m0c2
Thus:
Ek=( γ-1) m0c2
This is where it gets a little tricky. We have a trade off here. We must have a high gamma factor so that the time taken to travel to a distant star is short in the spaceship frame of reference. This in turn implies fewer provisions need to be stocked. But it also implies that the m in the Earth frame is larger and consequently more fuel is required. In non-relativistic analysis, we would start with the rocket equation which is given as follows:
Vrocket (Earth frame) = vexhaust (rocket frame) [ln(mtotal/mempty rocket]
The quantity mempty rocket is the total mass of the rocket when it is unfuelled. It is assumed that all the fuel is used to accelerate the rocket to the maximum velocity (we would obviously have to take into account the fact that the rocket has to decelerate when it reaches the destination). This works because once the rocket reaches a maximum velocity, it no longer needs fuel to continue travelling through frictionless space at this constant velocity, as per Newton’s first law. This still holds in special relativity (not in General, though since objects follow the geosidics of warped space-time). However, every instance of m must be dilated by a factor of gamma in this case. We need to accelerate the rocket to this maximum velocity and it is this acceleration that requires the fuel. This is the hard part. It is very, very difficult to get a rocket accelerated to that close to c because a high gamma factor implies a higher mass in the Earth frame and consequently more energy required to accelerate the rocket. There are several prerequisites which would have to be in place to achieve something like this. First (obviously) the rocket would have to be assembled far above Earth in orbit. No project would be feasible without this. It’s hard enough to accelerate a rocket to 0.999c let alone with escape velocity of a gravitational potential well to cope with.
Also, if we were travelling to very distant stars then once the initial colonizers had travelled sufficiently far they would be more or less isolated entirely. A beginning student of Relativity would say something like “by the time the rocket reaches the new planet, everyone on board has aged very little even if everyone on Earth is dead”. This is in fact completely meaningless. We cannot compare the time coordinate of two events in two frames of reference in Relativity. The start of the sentence above, "by the time" is meaningful only in the reference frame of the spaceship. If the people on Earth sent a light signal back to Earth to say that they had arrived, then even though as far as the observers on the ship were concerned, this journey only took 20 years for a 500 light year (in the Earth frame) journey, the Earth observers will receive the signal 1000 years (in their frame) after they recorded the spaceship leave.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
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All of this assumes that you need to transport living bodies. There is a much cheaper and more likely scenario. Instead of investing in engine technology, you invest in two things: a) Self-sustaining and self-replicating biospheres, and b) artificial wombs and care-taking robots.
You send frozen cells instead of living humans, and you grow the humans once they arrive.
This allows you to send the payload at a much lower velocity and without the need for a continually operational star ship. It could simply boot up on arrival, dethaw the embryos and start the mission at the destination star. It's essentially minimal-tech panspermia.
By the time we are heavily into colonizing our current Sol system, most of this technology will be developed as a side-effect of the colonization effort. The cost, compared to the total energy economy of the solar system colonies will be proportionately miniscule.
It seems completely reasonable (extreme conservative estimate) that a velocity of 0.01c could be acheived. At that velocity, the gamma factor is negligible, and travelling 5 light years would likely take less than 1000 years, given acceleration and deceleration. If we are more liberal with our estimates, maybe 0.1c could be reached, and the travel time would be less than 100 years, with a gamma factor of only 1.005.
Since we have self-sustaining and self-replicating biospheres, we don't need to land on any planet to begin colonization. We can simply colonize the new star system itself, probably starting with asteroid belts or small moons for raw material. Therefore, there's no requirement to find an Earth-like planet first.
I think the only tricky part is to get a truly self-sustaining and self-replicating biosphere. The initial start of the mission will require robots to build the initial biosphere for the grown humans to inhabit. They will need sufficient software intelligence and starting materials to either build it from existing materials or start mining materials to build it from scratch. It will also need didactic computers to enculture and educate the humans. This does not require true AI, because much of the human culture can be passed on via recorded video and text. Fortunately, the humans will not be completely isolated, because they can keep in touch with Sol culture at a 5 year time lag.
Once the humans are mature enough to take over, they can begin colonization by self-replicating their initial biosphere.
So, the whole near-light-speed problem can be completely avoided and still have a viable interstellar mission without requiring 'cryosleep' or a generation ship. The payload can be minimal, the velocity moderate.
The interesting thing about interstellar travel will not be the travel itself. It will be the destination and the discovery of new worlds. All you have to do is get there in some practical fashion, and there's lots of ways to do it. You're really just sending a space spore -- the minimum tech to get a foothold in the new system.
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I suspect that for a venture of such cost, we would want people or at least AI (as opposed to mere computers) on board, because if something goes wrong on arrival, Earth won't know about it for a long time. Unless we find some method of superluminal communication, any colonies we have will be more or less isolated from each other .If we start by colonizing close stars in the 5-20 light year range,the problem is somewhat analogous to that faced by the Spanish in maintaining their empire. A letter from Hispaniola to Madrid took over a year, as did the response. Consequently orders were meaningless by the time they arrived again. So you'd probably want something conscious on location, biological or not.If on the other hand, superluminal communication was possible (I don't think it is, but if it turns out to be I suspect it will involve the EPR paradox) there would be less of a problem.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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D.G... stop fucking with my Sci-Fi fantasy's damn it!
Your such a downer all the time >.> (and no, the irony is not lost on me )
What Would Kharn Do?
Signature ? How ?
How does this contribute to the discussion at hand?
I'm not trying to be flippant but this doesn't really have anything to do with the OP and consequently I can only draw the conclusion that you haven't read it.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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Signature ? How ?
Care to justify that opinion?
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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I can't see humans in any real numbers reaching the stars without some new physics (as opposed to just new technology) that allows breaking or at least bypassing the speed of light. Until then we are probably stuck on this rock.
Bare in mind we can't even safely go into orbit, the fatality rate for a rocket launch is around 1% , which is just about acceptable if you are professional astronaunt but if 1 in every 100 passenger rockets explodes its not going to viable as a civilian project.
The OP which addresses that very point. You should probably read it. Not that it matters because the whole point of colonization is that we don't send people en masse. You are attacking a strawman. Is anyone apart from natural actually going to read the OP first and then respond?
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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Wasn't disagreeing with you (forums are only 99.9% of the time about conflict), was just adding a comment regarding reaching orbit.
Will be very interesting to see how these new civilian sub-orbital flights go I not sure how they can get anywhere near the current safety records of jet airliners. Will they be regulated in the same way? . I suspect as there will be relatively few of these flights it will be a considerably time until someone is killled but when one does happen it may well finish the project off.
If you blow an astronaunt up he/she is a hero, if you blow a civilian you have a serious legal problem (and no signing a disclaimer saying I understand this is very dangerous and if I die will not sue not protect Virgin)
Let me first start out by saying I read both posts by DG and Natural respectively. In my humble opinion, the best option would a self-sustaining biosphere with living humans on-board. We would have to throw away any notions of communications in their current state. A lag time of even a year will make communications completely useless, as mention by DG already. I would prefer living human beings as opposed to grown humans. Living humans can make corrections on the fly. When we consider such vast distances, control from the ground (Earth) is impossible for current technology and even most technology that can be conceived of.
There is still a problem of how to support life in space. It may be simple enough when you're near a star, but when you get into the gaping void that is space you'll have a serious problem. Perhaps studying deep sea life that lives with no sunlight will yield some answers. That or pack it with a larger payload of supplies and fuel.
After eating an entire bull, a mountain lion felt so good he started roaring. He kept it up until a hunter came along and shot him.
The moral: When you're full of bull, keep your mouth shut.
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My argument rests on the assumption of a fairly long period of solar-system colonization and harvesting of the sun's energy. Enough time to work out the kinks of self-sustaining and self-replicating ecosystems. By the time an interstellar colonization mission is considered, the solar system itself will be widely populated, and gathering large amounts of energy from the sun.
As such, the relative cost of a space-spore mission would be very low. The main cost of it will be the fuel/energy to accelerate to 0.01c and then decelerate prior to arrival. Since the gamma factor is so low, I'm going to approximate it with Newtonian calculations, and then double it for a conservative estimate:
0.01c = 3 000 000 m/s
Mass of ship = 1 000 tonnes = 1 000 000 kg (random estimate; adjust results appropriately if necessary)
Energy of acceleration = 4.5 E18 J
Double for deceleration = 9 E18 J
10% Efficiency of conversion = 9 E19 J
Double for conservative est. = 1.8 E20 J
Total output of Sun's radiation = approx 4 E26 W
If we grant one year's time to harvest energy for the mission, then this represents:
Energy est. / (Sun x 1 Year) = approx 0.000 000 000 001 43 % of Sun's output
The Earth itself receives about 3.8 E24 J per year, and so it is:
Energy est. / Earth = approx 0.0048 % of the Earth's total solar energy budget
As you can see, the amount of energy required is actually quite small. Even if we needed to send a mass 1000 times greater than my estimate above, that would only require 5% of the surface area of the Earth (at Earth's distance; more if energy collection is at the asteroid belts; less if the collection occurs at a closer orbit than Earth).
Assuming we give sufficient time for solar-system colonization, then sending a space spore could be made arbitrarily cheap compared to total energy economy of the solar system.
If we are worried about something going wrong, we just send redundant systems. Whether they be redundancies in a single mission, or multiple independent missions. Since the cost of missions can be made arbitrarily low, relatively speaking, then sending multiple missions can also be quite cheap.
You could even have it so that the robots automatically establish orbit, and build the biosphere, but they wait until they receive a 'go ahead' from Sol before they unfreeze the embryos and start growing the humans.
In this way, only completely successful missions would begin colonization. The signal from the spore to Sol of 'all systems okay' would take 5 years, and the return signal of 'okay, begin colonization' would take 5 years. So just tack on 10 years to the mission and you eliminate all danger of interstellar travel.
The only remaining danger would be that of the artificial wombs and care-taking robots bringing up the first generation of human children. And all of that can be tested thoroughly here in the Sol system before sending the tech along on the spore mission.
Only the first generation would be 'isolated', and even they will not really be isolated. They can be receiving a constant stream of culture from Sol. Movies, literature, politics, news, etc.
The didactic computers only need to get the first generation to the point where they can begin their own government. The children would learn from these computers how to perform their duties, how to form a government, etc. It would be like more-advanced forms of computer learning we have today.
It's a bootstrapping process. All you really need to do is get a core society started. It can be as few as 150 people, maybe even less. Once they get started, they begin colonization, and they form their own culture. A constant stream of Sol culture would act as the 'mother' culture.
I don't see the need for specific orders to come from Sol. Sol wouldn't be running the show, it would be creating a daughter-culture which runs on its own.
We will already have experience with this during the colonization of our own solar system. There will be Moon colonies, Mars colonies, asteroid colonies, solar collector colonies, Kuiper belt colonies, etc. To some extent, they will have to operate independently. The new colony in another star system will merely be one that's further away. Instead of there being a 6 hour delay of messages (e.g. to Pluto), there will be a 5 year delay.
But again, once the colony gets started on its own, there will be plenty of people to interact with on a daily basis, and they can make their own day-to-day decisions. It won't be analogous to an Empire. In fact, the whole idea of a Galactic Empire seems silly to me. What? We learned all about democracy and the dangers of empire here on Earth, only to revert back to empire out in space? Galactic Civilization is more apt, in my opinion.
The colony will still be dependent on Sol for quite some time. Sol will be the source of much information and culture and technology. A small group of 150 people will have their time consumed with maintaining their colony, gathering energy/food, and replicating their first biosphere. It will take quite a while before their population gets into the millions and they can start innovating on their own. In the mean time, they will be dependent on Sol for new innovations. After all, their ship and initial biosphere are already 100-1000 years behind the times.
Assuming something goes wrong with Sol during their travel, they will at least have a copy of Sol's culture from the time they left. The information requirement to store this information will be quite small, compared to our information technology today. So, they'll at least be able to reproduce Sol's culture even if they are no longer receiving innovations from Sol.
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As soon as you add live people to the travel, it increases the complexity of the mission by a huge amount. You need more energy, you need faster speed, you need to ensure that any mechanical failures can be fixed (so the people don't die), you need to ensure there are enough people to avoid psychological problems from developing, etc. etc. etc. There are dozens more problems.
I am not saying it's not possible. I'm saying that a cheaper and earlier mission will be one that doesn't have live people. I'm saying, why go through all that trouble when there's a much easier way?
The technologies I propose are not difficult. We have already made leaps of progress on robotics, manufacturing, artificial wombs, software intelligence (not full AI, just adaptive systems), computer aided learning, etc. All I'm proposing are extensions to these trends. Given our capabilities now compared to 100 years ago, I think another 100 years should be plenty to begin colonizing the solar system. Once that begins, we will refine all the necessary technologies very quickly.
The first viable mission will be the cheapest and least complicated one. In order to send live humans on the flight mission, you will have to solve most of the problems a non-live mission would have to solve, plus a whole bunch of other problems. Plus it will take more energy to boot.
I just don't see any advantages to sending live humans. Are they going to pilot the craft by hand? I don't think so. It will all be automated anyway. Are they going to construct the first biosphere by hand? No. They'll use robots anyway. Just make the robots smart enough to do the initial work by themselves and you no longer even need a single human. It's just assembling parts. We have automated manufacturing already in modern factories. We already have a Mars rover that can operate without direct human intervention. Just make these a little smarter and you have the initial biosphere automatically constructed.
Heck, you could even send the initial biosphere fully constructed on the initial voyage, if you can be fairly sure that it won't cause problems on the journey. But sending live humans just doesn't get you anything. They would be sitting around playing cards for the whole journey. What do they add, besides risk and cost?
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Is anyone else having trouble reading this? I am looking at a screen sliced in half and no manipulation will reveal the other half.
DG, a few questions (genuine questions, not rhetorical ones; I have no idea what he answers are):
- Once you reach relativistic velocity (even if you only accelerate at, say, 1G), don't you get... err... 'stretched'/elongated? If that's correct... wouldn't that be rather fatal?
- Wouldn't space debris be a rather large problem for a ship travelling at relativistic velocity? I mean, I presume there's no chance of steering to avoid an unexpected obstacle when one is going that fast?
Honestly, I think 'humanity' would mean something far different by this stage anyway. I imagine most of the 'population' would be stored digital information.
No, I don't mean 'downloaded brains'; I mean that there would be a small population of biological, breeding humans, some of who manage an interstellar 'database' that is the majority of humanity. Every person who ever lived essentially has their own 'Wiki', where they and their peers/family members store that person's history, personality, achievements, personal thoughts, etc... y'know, Wiki stuff. This information is processed in real-time and used to create a digital avatar of the person.
This being my hypothesis, I doubt that we'd have much use for growing too many biospheres or achieving relativistic space flight.
- Leon Trotsky, Last Will & Testament
February 27, 1940
No, you're thinking of travel into a black hole. At a Relativistic velocity, you get "contracted". The scare quotes are because you don't actually get contracted. It doesn't really mean anything to say that something happens to you. What actually happens is that an observer moving relative to you at relativistic velocity will say that you have been contracted. If you were to stretch your arms out, then in your frame of reference, your armspan would be your armspan, same as always. You would see everything else (moving relative to you in your frame of reference) to be contracted. But in someone whose frame of reference is moving relative to yours are relativistic velocities, your armspan and your body would appear contracted along the direction of travel. It's just about getting your head around the principle that absolute spatial distance is a meaningless concept outside of reference frames.
Yes. Fortunately, interstellar travel is mostly a distance problem, and one can safely assume that until they get into close proximity to their destination, space is empty.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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The speed of light is about 300,000 km/s.
The speed of Pioneer 10 (a space probe) was at max 36.7 km/s
Mankind's space probes are by a factor of about 5,000 away from getting into any trouble with relativistic effects.
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Who asks me inappropiate questions also has to live with the answers I may give.
I get different numbers when it comes to the costs:
I calculate with your 1.8E20 Joule of energy.
1.8E20 J are 50 trillion kWh.
my electricity company charges 21.9 euro-cent per kWh. So the costs of the needed amount of energy would be:
50 trillion kWh + 0.219 €/kWh = 10,950,000,000,000€
at the current exchange rate of 1 EUR = 1.3552 USD that would amount to:
$14,840,000,000,000
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Who asks me inappropiate questions also has to live with the answers I may give.
Hardly an insurmountable cost, considering that this will be a small fraction of the energy economy of a moderately colonized solar system.
$15 trillion dollars? The US already spends about $1 trillion annually on defense related programs. We're talking an economy many times the size of the US. An entire solar system filled with solar collectors. Also, by that time, solar will be much cheaper than 0.219 Euro/kWh.
But that does give a rough ballpark, so I appreciate it. It shows that such a mission is not unrealistic. More expensive than we could possibly afford right now, but not outside of the realm of probability in the future.
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Well I read your OP. However, I had to go to work so I was not able to post earlier. I have been watching this thread all day and I can't help but notice that nobody seems to be paying much mind to your OP. Everyone has their own ideas on how to go to the stars but they are not really taking special relativity into account.
Let's look at the graph that you posted above. The most obvious feature to me is the fact that the entire left side of the curve is nearly flat and horizontal. So it should be fairly obvious that below about 0.5c, the effects inherent to special relativity don't ever show up in any meaningful way. Past that point, they do begin to play a real role but only gradually at first. The real deal for this type of travel don't really get cooking until you get past about 0.9c when the curve mostly goes flat and vertical.
Here I would like to put some perspective into this. The reason why the space probes that we are sending out now take years to get anywhere is because there really is no need to get them there any faster. So we use “lowest possible energy” trajectories to save a huge bundle of cash.
Manned flight around the solar system is potentially in a different class as we might want to shorten the travel times by large amounts. For that type of mission, constant acceleration of the earth normal value of 9.8m/sec/sec are where matters get interesting. Here I do need to appeal to nuclear rocketry designs that are at least being considered as being on the drawing board.
At closest approach, the trip from earth to mars is under a week. Jupiter is a bit over a month away and pluto can be reached in under half a year. That much is reasonable with designs that are at least potentially reasonable. For some potential emergency situation, higher acceleration may be warranted. If there was some disease outbreak on mars and we needed to get a few tons of penicillin there quickly, then an unmanned vessels could make such a trip in a matter of hours at some arbitrarily higher acceleration (say 20g).
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Interstellar travel is a different game altogether. The first question that comes to mind is “Where do we want to go?” Hopefully, the Kepler space telescope will give us some real data. Until we have it, all that we have is guess work. That being said, I have no interest in even attempting to run the Drake equation.
That being said, the closest 100 stars are all within 22 light years. Just for shits and giggles, let's say that there are a dozen “Goldilocks planets”. Actually, I have a more stringent requirement than the popular press on that matter. The only planets that we are interested in must have a decent amount of atmospheric oxygen.
Here is where the Lorentz transformations come into play. At constant boost of 1g, a trip to a system that is less than 22 light years will simply not take us into the extreme end of the graph in the OP. It will get us up to such velocity that some interesting math becomes relevant but it will not get us to the far end of the curve. So ship time will be a bit shorter but not enough to make a difference in real terms. Especially considering that sending messages back to earth will take so many years that it is simply unreasonable for earth to be in command of such a ship.
=
@natural:
I detect a subtle flaw in your plan for slow colonization. Sure, it would work over the long haul. However, it will take your ships many centuries to get anywhere at all.
Now, let me ask you about the possibility of some future civilization being prepared to use a relativistic travel system. Are you ready for your ships to arrive on planets that have already been colonized a few centuries in the past?
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Rockets do not constantly accelerate. That's the whole point. If we tried to get the rocket to accelerate constantly we would simply come up against the Lorentz factor again. What we really want is to use as much fuel as possible to accelerate the rocket the a final velocity and then, by Newton's first law (which is still valid) it will continue at that velocity in a straight line. A rocket travelling on such a long flight would spend the initial phase accelerating until it reachet its final velocity near to c.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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You're unlikely to find oxygen, as it's highly reactive and if it weren't for photosynthesis, it would rapidly oxidize out of the atmosphere. The only likely place you'll find oxygen is on a planet with life.
Fortunately, oxygen is plentiful in the universe, and we can generate it by hydrolyzing water (equivalent of photosynthesis, but doesn't necessarily require plants).
Actually, at 1g acceleration, you quite quickly reach relativistic speeds. Check out this page: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html
According to this guy's calculations, a trip to Vega 27 ly away would only take 6.6 subjective years.
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The scenario I propose is a proof of concept. I try to use conservative estimates and known, existing technologies, extrapolated into the future. For example, I actually think that AI will be widely available by that time, and even machine consciousness, such that you won't have to send living tissue at all. You can just send transhumans, robots with superhuman minds.
However, if I tried to lump in all the technology I thought were really possible, then I'd have to waste my time defending these speculations, when they are not really crucial to my point.
My point is simply that interstellar travel is possible in concept, and we will be able to send biological humans if they are necessary. A galactic civilization is possible in principle, given enough time, *and* assuming we don't blow ourselves up first.
So, if someone figures out a way to travel at relativistic speeds, then that doesn't hurt my argument, it strengthens it. I chose 0.01c because it seems like a conservative estimate, and to travel to the nearest star could be accomplished in less than 1000 years, which is within the realm of human cultural timelines (we have some concept of what the world was like 1000 years ago, 2000 years, 5000 years, even 10,000 years).
However, if someone gets there in 100 years, or 10 years, so much the better.
I tend to think travelling at relativistic speeds will be more trouble than it's worth, due to the excessively huge amount of energy needed. But if it turns out to be economical, why not?
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In fact, my scenario is part of a larger argument, which is an anti-Drake equation argument that we humans are the only space-faring species in our galaxy.
Drake's equation fails to take into account the Anthropic Principle. (Look at my signature below for a refresher on the Anthropic Principle.)
The reason we are likely the only space-faring species in our galaxy is because we are not surrounded by a galactic civilization preceding us. In fact, I would argue that the first space-faring species will spread so quickly throughout the galaxy that they will displace any less-intelligent species. As the Fermi Paradox asks, "Where are they?" The Anthropic Principle replies, "We are them."
It is similar to the reason there is only one common ancestor to all life on Earth. The first life that could self-replicate wiped out all the other proto-life. It spread so fast and so far that it swallowed every competitor up and created a monopoly. Now it's impossible for new life to form on its own.
If we ever manage to colonize another star system, then it will only be a matter of about a million years or so before the entire galaxy is filled with colonies. Just like bacteria, we would reproduce exponentially, filling up all available niches.
Like the first life, we would crowd out any potential competitors, proto-civilizations.
Since this is how we would do it, as a simple consequence of being self-replicating space-faring life, then if there had ever been self-replicating space-faring life in the past, it would have already colonized our entire galaxy, and we would either not have survived (they would likely colonize Earth) or we would find ourselves alive, but surrounded by an alien civilization.
Since we do not find this situation, it is logical to conclude that there was no self-replicating space-faring life in our galaxy more than about a million years ago.
Since the universe is 13.5 billion years old, the average age of stars in the Milky Way about 6.5 billion (oldest about 13 billion), and our own Sol about 4.5 billion years old, it seems a very safe bet to say that there's a greater than 90% chance that we are the only space-faring species in this galaxy. Probably closer to 99%, but again, I'm being conservative.
This argument requires a minimalistic case for the inevitability of going from self-sustaining and self-replicating biospheres in space all the way to complete galactic colonization/civilization. With my conservative estimates, it is inevitable within 1-10 million years, depending on your assumptions about velocity. If you're more liberal with your estimates, it could be as low as the diameter of the galaxy in light years, which is about 100,000 lightyears, so near-light-speed colonization could occur in 100,000 years.
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All of this is to say that if you're asking me about how relativistic travel affects my argument, then it increases the likelihood that we are alone in our galaxy to somewhere in the neighbourhood of 99.99% likely. So, it actually strengthens my argument a lot.
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Oh, very funny. You meant the time derivative of acceleration of the rocket did not change during its acceleration phase.
I had to assume you were talking about the rocket undergoing acceleration constantly because the rocket does not undergo "constant acceleration" (i.e da/dt=0) during the initial acceleration. The rocket experiences a constant force, but because mass is being lost at a constant rate (the propellant), the acceleration a(t)=F/m(t). However, m(t)=m0-kt where k is the flow rate and thus a(t)=F/[m0-kt].
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
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Yah, dude. I could have chosen to wax poetic about another way to get to the stars but that would only have added “and another way to do this...” to the thread. What I am really after is to get back to your OP.
Remember that Einstein started working on special relativity by assuming that “if I was traveling at the speed of light” and asking questions from there. Proceeding on those grounds, if I had a rocket that was capable of a constant boost for all of eternity, what conditions would special relativity impose?
From that POV, nothing really special happens at any velocity less that about 0.5c. From about 0.5c to about 0.9c, the Lorentz transformations provide for effects that are only mildly interesting but of no great consequence even in terms of “ship time”. Past 0.9c, the curve goes essentially vertical and then you really have to deal with relativistic effects.
As far as making a point goes, how far does one have to travel at a constant acceleration of 1g before the consequences of special relativity become worth dealing with? I am aware that you are capable of doing the math yourself but to save you the time, nothing remarkable happens until you have gone at least ten light years. Allowing for a turnaround and deceleration, one really doesn't even need to think in terms of special relativity until we are talking about trips over twenty or so light years.
Then the fun kicks in. In terms of “ship time” it really does not matter how far you are traveling. Whether you are going 50 light years of 500 light years, the ship time is going to be just a bit over 20 years. In fact, a trip of 100,000 light years would take about 30 years of ship time.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Let me make a conclusion here:
Special relativity only comes to be something we have to consider if we are using a constant acceleration of 1g for at least 20 years.
There are 100 stars that are closer than that.
We do not know today just how common Goldilocks planets are but with a hundred stars to work with, there are probably a few in that range.
From that, it seems reasonable that we can set up colony worlds without having to worry about special relativity to any great extent.
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Spending untold hours with really smart people debating the mechanics, pros and cons of human interstellar exploration. The sad reality, I have found, is this:
Reality, not potentialities, not capabilities, not ifs, ands, buts and whens are the clincher to this one. The realities are that we have not stepped foot on our natural satellite in well over 4 decades. Mind, the self-same natural satellite that we still have a multitude of questions about. Luna, at only 2+ days travel time, is our backyard training ground to a lot of what is debated about interstellar exploration. But we sure don't act like it.
Why not? Why did we abandon our first, best science-gathering platform off-world, after multiple return trips with computers and technology that by todays' standards do not deserve the name? Keep in mind that I am making a point here, not trying to denigrate heroic efforts and accomplishments of the past. Why do we not have experimental self-contained biospheres on the moon? Experimenting with biospheres on Earth becomes nothing more than a monumental squandering of time, resources and effort after a while. Acknowledging that I don't have all the answers, here's my take-
Let's put aside what we think we could do, apply Occam's Razor, and attempt to discern what most likely will occur. O.R. tells me that real world space exploration is entirely too expensive. And, sadly, that's the bottom line. Of secondary importance to the expense is the safety factor, and yes I am keeping in mind that nearly all effective space exploration will be conducted robotically. Interstellar travel represents far more of a challenge than that of just getting from point A to point B. And at our current pace, it will be literally many, many decades before we can begin to make a good effort at it. If then.
Don't get me wrong, I agree with you that far. However, as far as sending out colonies, nobody is going anywhere for quite a while. How long it will be, I can't say with any degree of confidence. On that, I am given to say “In about a thousand years” which you can take as shorthand for so fart in the future that nobody can really say what conditions will be present (apart from known physics, which I don't expect to be revolutionized to the point of FTL drives).
If I am reading you correctly, what you are proposing is something along the lines of Noah's Ark but with a cargo of several hundred thousand test tubes packed in liquid nitrogen. That much is fine and in a world where we already have dozens of office buildings that are 100+ floors tall, the engineering for such a ship are clearly in reach.
Let me take this a step further. The Kepler space telescope is going to find us some interesting places to look at but before we start sending out space arks, we probably want to send out some robotic probes. Have a real look at local conditions to see which of the candidate planets are really worth looking at.
Say for example that we find a world that looks just about right in the telescope. But without a robotic probe, we might get there only to find that there just is no dry land. Or the biochemistry has gone far enough different that despite the oxygen atmosphere, the local biosphere is instantly lethal. Any number of such scenarios are possible.
Even so, your scenario of slow ships, while certainly reasonable, leaves such ships taking a few thousand years to get anywhere. But some future civilization could launch faster ships hundreds of years later and get there first.
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Two things: First, special relativity is insufficient to deal with acceleration. You need to use general relativity.
Second, it depends on whether you are assuming constant g acceleration from the frame of the ship, or from the frame of someone on Earth. If you are talking about from the frame of the ship (as would be assumed, since otherwise why pick 1g?), then you reach relativistic velocities much faster than 10 years. Did you read the linked article I posted?
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Seeing as how we apparently aren't going to finish in the other topic, I'm relcutant to say anything in this one for fear of assault on all sides without explanation.
But I vaguely recall an ion engine that NASA equipped to a probe recently, and am wondering what, if any, impact such propulsion has on this conversation, as I haven't seen it mentioned anywhere.
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Interested to hear other's opinion's about solar sails?
http://en.wikipedia.org/wiki/Solar_sail
One could use lasers/microwaves to accelerate a ship to near c. The people inside in suspended animation. When you reach the new solar system, wake the people up if it looks like an inhabitable planet. Otherwise use the star's energy and matter in the solar system to propel the ship to a new star.
Taxation is the price we pay for failing to build a civilized society. The higher the tax level, the greater the failure. A centrally planned totalitarian state represents a complete defeat for the civilized world, while a totally voluntary society represents its ultimate success. --Mark Skousen
The reality is that the only reason space exploration is expensive is because we don't yet have good enough solar technology to harvest enough energy to make it worth it. However, the reality is that will eventually change, especially now since we're running out of oil and will be forced to go solar.
Google 'solar satellite'. It's probably too expensive to build one now, but that will change, and eventually we'll have people in space servicing solar satellites. It will be cheaper to keep them in orbit for a while, rather than doing short missions. Therefore, we'll need to work on space station technology to make it cheaper and safer to stay in space for longer periods.
The reality is that we've already got a plan for a manned Mars mission. It is possible. It only requires political will.
And, the reality is that someone will eventually find the political will, if only for the reason, "We did it first." People put on big shows just to show how big they are. That's human nature. What bigger show than to be the first to successfully land on Mars and plant your flag there?
So, there are short term motivations to improve orbital space stations, and there are long term motivations to explore and eventually colonize.
Aside from not making Moon missions, there is far more space travel now than there was in the 60s, 70s, and 80s, if ony to launch satellites. That's not going to stop, and it's going to get cheaper.
Getting all the way to interstellar travel will be done in tiny steps, just like evolution. First we build more sophisticated satellites, and start harvesting more solar energy, then we send out a few manned missions here and there (likely Mars and the Moon), then we start to become more dependent on space energy (did I mention there's a shitload of energy in space, from the Sun?), so we need more maintenance crews, then we start to build services for these crews (such as medical treatment, agriculture, etc.), and eventually more sophisticated space stations, etc. etc. until finally you have a fully self-sustained biosphere in space. Once you have that, then colonizing the rest of space (to gather more solar power) becomes a no-brainer.
The rest follows from that naturally.
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Why do you need to colonize a planet? If we already have fully self-sustained and self-replicating biospheres that can survive in space, then all you need are raw materials (asteroids are a good place to start) and energy, which is what the star at the centre is for.
You just build more biospheres. Eventually, you might want to risk landing on a planet, but that is not your main goal. Planets are giant gravity wells. They take energy to land on and then later to escape. They present a lot of hazards and risks that can be avoided by just staying in space. Eventually, of course, people will want to live on them. However, the initial colonization effort will occur in space, probably near an asteroid belts or possibly on small moons or plantary rings.
Once the initial colonization effort has got a strong foot hold, they can then try out landing on planets if they think it necessary.
Answered in a previous post. So what? So someone got there earlier. It still shows that someone will get there. I'm sure some space probe will eventually overtake Voyager, too. No biggie.
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It's the logical next step for interplanetary travel. It's efficient and likely to scale-up. It's a slow, but constant acceleration. The constancy is what matters, and what makes it efficient.
It will make a Mars mission realistic and less expensive.
In my scenario, it will allow people to travel pretty much wherever they want, rather than having to remain in orbit around a planet or moon. It will allow shuttles to transport raw materials from asteroids and the Kuiper belt into closer solar orbits where there is more concentration of sunlight to harvest. Thus, we may be able to build large structures in Earth orbit, and we may be able to build solar harvesting stations closer to Venus' orbit where they can harvest large amounts of sunlight. They could beam this harvested energy pretty much anywhere in the solar system, making an energy economy viable: Harvest raw materials from far out, harvest energy from close in, and trade them. The trade would be done with ion shuttles and microwave energy beaming.
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artificial gravity, as we humans seem to have an issue staying long times in space with minimal gravity. As I suspect so would any other lifeforms we bring along for the ride.
"The reality is that we've already got a plan for a manned Mars mission. It is possible. It only requires political will."
Perhaps my command of the English language is not up to snuff? Perhaps because I did not go to a public school, I do not know the requisite catch phrases that tell others that this is what needs to be paid attention to? Perhaps these things and more are why I seem unable to get my point across to some.
It matters not one whit what plans exist. We collectively have done enough by this point , though it be miniscule in the extreme, to understand what and where the next set of signiically important questions need to be formulated, let alone asked. We (humanity) are scalp-deep in "plans", and approximately toenail-deep in practical, applicable positive results. I care not one little bit about plans. Especially when those plans have been on the shelf for a very, very long time. What I care about are results. And, again, there exists not the slightest, insignificant shred of evidence that there is ANY political will to act on the plans that very intelligent people promulgated, a very long time ago, to do what we are talking about here.
This is not a rant on hopelessness. It is intended to be a wake-up call to those who may yet be able to turn the tide.
It's the logical next step for interplanetary travel. It's efficient and likely to scale-up. It's a slow, but constant acceleration. The constancy is what matters, and what makes it efficient.
It will make a Mars mission realistic and less expensive.
In my scenario, it will allow people to travel pretty much wherever they want, rather than having to remain in orbit around a planet or moon. It will allow shuttles to transport raw materials from asteroids and the Kuiper belt into closer solar orbits where there is more concentration of sunlight to harvest. Thus, we may be able to build large structures in Earth orbit, and we may be able to build solar harvesting stations closer to Venus' orbit where they can harvest large amounts of sunlight. They could beam this harvested energy pretty much anywhere in the solar system, making an energy economy viable: Harvest raw materials from far out, harvest energy from close in, and trade them. The trade would be done with ion shuttles and microwave energy beaming.
Seems reasonable to me. It could also be a competitor with ion drives for interplanetary travel. I wonder which is more efficient. Solar sails don't need any fuel, but they require large sails, which could otherwise be solar collectors.
For interstellar travel, it seems the way to go, since it would drastically reduce fuel requirements. However, you still need a way to slow down, so I'm guessing you'll still need something like an ion drive. You can't just use the destination star, since you'll be going too fast, boosted by the laser.
Again, I don't see why they would need to pass on by. Why not stop and build a colony of biospheres? Why do you need a planet?
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Has anybody here ever read any science fiction? More to the point, does anybody here understand that the very best science fiction writers are themselves scientists, or former scientists? Even more succinctly, much - if not all- of science fiction from back in the day ( < 1920s- approximately mid-1950s) is now "science fact". But since then, what have we got? Nothing, really. All we are doing is pretty much attempting to conirm or deny theories that were first put forward years and years ago.
We cannot duplicate conditions that lead to phenomena that we observe remotley from planetary or solar orbit. Only can we go so far off the voluminous data we so receive. Soon enough, questions of analysis and interpretation begin to seep in otherwise.
Pretty much a complete and total lack of any need to be on the moon at the time. It really ought to be common knowledge that the trips that we made to the moon were motivated politically far more than scientifically but let me cover that ground in a bit more detail than most people ever consider.
OK, so the Russians beat us into orbit. That makes for a huge splash politically but the basic reality is that they only beat us by a couple of weeks. From a purely technological POV, it was not a matter of the Russians being vastly far ahead of us but really only very slightly ahead. Still, it was a big deal at the time.
So president Kennedy called Werner Von Braun to the oval office and basically asked him what he could do that would be a much bigger deal. Well, since we were so close behind them, what Werner Von Braun offered was that even given massive funding, we would be playing a game of technical leapfrog for a few years. However, if that was allowed to happen, it would be possible to be the first nation on the moon.
Actually, the Russians beat us to several intermediate steps such as the first man in space and the first space walk. They came within weeks of beating us to the moon as well but we did finally get there first. Once we had that under our belt, the Russians stopped trying to go to the moon and put all of their effort into much more practical areas of space technology. Sure, we went back to the moon a few times but that was more because we could than because there was any relevant scientific goals to meet.
So why are we talking about going back to the moon with any seriousness today? Well, consider that China, India and Japan are all making noise about going to the moon. Iran just launched it's first satellite a few weeks ago. There really is not a huge commercial incentive to do anything on the moon but once again, national pride plays a role here. We want to be back there first. Because we can/because it is cool.
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I don't think that the Original post was talking about what we have the capability to do now. I think everyone here is working with the premise that this is only possible once we move higher (specifically TI) on the Kardashev scale, however long that may take.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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Yes, of course.
Depends what you mean by 'best' SF. There's some really boring hard SF by scientists. There's some great classics that are mostly speculative and 'soft' SF by non-scientists. Even hard SF often gets the speculations wrong.
That's a bullshit claim if ever I heard one. Much, if not all, of SF from the 20s to the 50s was trash (90% of everything is garbage). Much of it never came true. Much of it never predicted things that *actually* came true, such as the computer revolution.
We don't judge SF solely by the accuracy of their speculations, nor solely by the accuracy of their explications of existing theory.
This is vague and meaningless gobbledygook. What are you trying to say? Be specific. I can't make heads or tails of what you're saying here. Are you talking about SF, actual science, or this particular forum thread?
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I think that we're going to have to make the acceleration a lot greater than 1g.
Actually, this may be a good argument against sending people, unless we come up with new shock absorbing technology that allows the survival of tremendous G-Force of the initial acceleration (and to make this worthwhile, the initial acceleration has to be huge). But that is a problem of engineering, not phyiscs, and since I'm not an engineer I can't really talk about that.
So are some of us here.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
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"Depends what you mean by 'best' SF. There's some really boring hard SF by scientists. There's some great classics that are mostly speculative and 'soft' SF by non-scientists. Even hard SF often gets the speculations wrong."
"Boring" usually means the concepts being imparted are beyond the capacity of the recipient to grasp at the moment. Aside from that, no idea what you are talking about.
"That's a bullshit claim if ever I heard one. Much, if not all, of SF from the 20s to the 50s was trash (90% of everything is garbage). Much of it never came true. Much of it never predicted things that *actually* came true, such as the computer revolution.
We don't judge SF solely by the accuracy of their speculations, nor solely by the accuracy of their explications of existing theory."
Which wins a prize for bullshit claims if I ever saw one. Got news for you, cowboy. way more than 90% of everything that you will ever read, anywhere, is utter and complete BS. And your point is what exactly? Mine is that my presumption is thatvthose I interact with are better at filtering out BS from non-BS.
"This is vague and meaningless gobbledygook. What are you trying to say? Be specific. I can't make heads or tails of what you're saying here. Are you talking about SF, actual science, or this particular forum thread?"
If you don't like it, change the channel. I'll help you- and by implication, everybody else that has real difficulty with reading comprehension- what that means is, if you have dificulty with what someone else is trying to communicate to you, you ask politely for clarification. If your intent is simply to belittle someone else's ideas, proceed as you have.
That aside, I am always ready for civil discourse. Your choice.
Why would we want to go to the moon?
I think you're arguing with a false analogy.
People who think there is something they refer to as god don't ask enough questions.
I don't know if it's true, but I've heard that every kilogram of matter put on orbit costed us as it's double weight in gold. Our cosmic visitors must laugh on the very idea of having a big spaceship propelled by engines on one end and limited by the ridiculous slowness of light. Having a hibernated humans in there or whole biospheres is also ridiculous. So many new technologies, developed for a lack of one important technology.
We should focus on saving the world from poverty, drug abuse, pollution of environment and wars. (as someone wr) It's technically impossible for extraterrestrials to contact any government, because in this divided world any advantage of one part of humankind would be used as a weapon against another part of humankind. Only a peaceful, united humanity would be ready to receive technologies like a real space travel. And also developing them would be more likely.
Beings who deserve worship don't demand it. Beings who demand worship don't deserve it.
Oh shit...this previously intelligent discourse has automatically been tanked
Yeah...uh, everything is limited by the "ridiculous slowness" of light. Didn't you read the OP where this was explained?
As for the part about extraterrestrials visiting us, I'm not even going to respond. This is supposed to be a discussion about interstellar travel not conspiracy bullshit.
"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.
-Me
Books about atheism
"Why would we want to go to the moon?
I think you're arguing with a false analogy."
I'm sorry, but are trying to imply that Luna is scientifically insignificant? Really!!!???!
I just do not have anything else to say to that. I thought this was a rationally-oriented site.
But then, I have been mistaken beore.
You made this claim:
And now you're admitting that "90% of everything that you will ever read, anywhere, is utter and complete BS". So, which is it? Is it the case that 'much - if not all - of science fiction [from 20s to 50s] is now "science fact"', or is it the case that '90% of everything that you will ever read, anywhere [presumably including SF], is utter and complete BS'?
You made both claims. Don't ask me to sort them out. They're your contradiction.
I didn't belittle you, I belittled what you wrote. You are not what you write. I'm sorry for you if your ego is so attached to what you write that you can't take a little criticism of it.
I, and pretty much everyone here at RRS, reserve the right to criticize ideas, and yes, even ridicule them and make fun of them. This is with the understanding that people deserve respect, but ideas do not automatically deserve respect.
Aside from me calling what you wrote gobbledygook, what are you angry about? I asked you fairly politely to elaborate what you were saying. There were no insults directed at you personally.
Part of civil discourse is the ability to separate yourself from your ideas and to take criticism of your ideas as merely that, criticism of ideas.
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I know this is not addressed by the OP, but it is related to the physical problems associated with interstellar colonization, and AIGS's requirements for the goldilocks zone. Should we not also restrict our search to essentially earth-sized planets as well? Humans are built for earth gravity, and if we try to put a permanent colony on a planet too far out of earth-gravity range -- either way -- the long term physiological problems would be very great.
Atheism isn't a lot like religion at all. Unless by "religion" you mean "not religion". --Ciarin
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