Was there enough time for a planet like earth to exist long before earth?

[Sisyphus]

But special relativity isn't a technology restriction.

 

Indeed. The fact that we're not constantly being visited by alien probes would seem to prove that there is a speed limit of some kind.

[Sayonara]

Highly improbable.

Not actually that improbable, "unlikely" is closer to the mark. Given particular building materials (and there aren't many you can use as the primary components of systems we would call "living") there are only so many efficient ways to solve adaptive problems.

 

That's one reason why evolutionary convergence occurs.

 

 

I absolutely agree. I have a problem with those who claim (Star Trek et al) that intelligent life is common. If so, where are the traces?

If there is intelligent life, there may be traces. However actually finding those traces is a whole different matter to "knowing they are there".

 

We have been around (as an intelligent race) for some 30,000 years (give or take), yet consider how far the radio signals our entire civilisation has produced must have travelled. Now imagine yourself on another planet at the leading edge of that EM expansion, trying to work out if you are listening to the signals of an alien world, or that pulsar just over to the right a bit.

 

 

NASA scientists estimate that humans will have begun our journey to visit and colonise other star systems within 1000 years.

That's a nice safe bet. I'd put it at 300.

 

And that's for colonising, not "visiting". Interstellar travel for any reason other than spreading the species is pretty much idiotic. Ask any space-faring race and they will confirm that.

 

 

While some other intelligent species might be 'stay at homes' or else kill themselves off early, if there are large numbers of intelligent species, then it appears probable that at least some would set out to explore the galaxy.

Yes, but you asked "where are the traces?".

 

If they exist, we will probably eventually come across signs that they are (or were) there. But we are hardly likely to stumble across anything before we get out there, are we?

 

 

At an estimated speed of 0.1c (we assume Einstein was right), and reasonable estimates for speed of population growth etc., we can assume they will fully explore and colonise the entire galaxy withing 2 to 10 million years (depending on which assumptions you use). Do the math yourself.

I think 10 million years is a tad optimistic (think volumetrically, not just radial planes) but I am willing to accept your figures for the sake of argument.

 

However, in the context of the thread title, there has been plenty of time for hundreds of civilisations spanning millions of years to have existed, with a good couple of million years between each one for all signs of their existences to fade into the darkness.

 

In this thread in particular it is important to remember the time factor - especially the time where nothing is happening, as opposed to "something".

 

 

Swansont, at 0.1 c, it would take about 700,000 years for humans to reach the far side of our galaxy (we are two thirds towards the outer edge of a spiral 100,000 light years across.). This would be most definitely a one way trip, so we need not talk about time to return. Information, on the other hand, would take only 70,000 years to arrive.

What you have failed to consider (apart from the fact that we would need to take a major detour around the galactic core) is that travelling from here to the far edge of the galaxy will expose us to some infinitesimally small percentage of star systems.

 

 

Thus, if lots of intelligences evolve, then plenty must have been around 2 billion years ago. If even one was expansionist, we can expect the early Earth to have been visited, and even colonised.

That is making major assumptions about the requirements of such a species matching conditions on Earth.

 

Also, we have to consider the fact that an expansionist empire that spans a galaxy will be reaching the point where it needs to start building Dyson Spheres (or similar), rather than colonising planets. Which incidentally makes them reallllllly hard to detect.

 

 

Once you have the intelligence and language ability to build a technological civilization, there really isn't a lot of selection pressure to be more intelligent than that.

That's not true at all. Selection operates due to intraspecific pressures as well as interspecific pressures, and in a population as numerous and widespread as ours competition is rife.

 

Where we see a change is that the responses to this pressure move away from biological adaptation, and towards social adaptation.

 

Our species may have conquered the Earth, but in order to survive and thrive it does need to make decisions about where the next hundred billion babies are going to grow up (because it sure as hell won't be here). Those decisions are basically going to be adaptations to pressure.

 

Indeed. The fact that we're not constantly being visited by alien probes would seem to prove that there is a speed limit of some kind.

"Well there are so many planets, and we only built fifty probes".

[john5746]

Unless most intelligent species kill themselves off. Or are, for some reason, inherently incapable of space travel. Or they never saw any reason to bother with our planet. Those are three good reasons they might not have visited, and if intelligent life is exceptionally rare, that might explain it nicely.

 

Nature kills also. If we have been around during the time of the dinosaurs, we would have been wiped out. We may end up being wiped out by an asteroid or some other cosmic event anyways. It takes time for evolution and intelligence to evolve to a point where galactic travel can even be attempted. Avoiding annihilation within that time period might be rare.

 

Look how long it took humans to use technology in a meaningful way. We shouldn't assume that intelligent life is rare because it hasn't visited or travel is impossible. The Europeans probably thought they were the only civilized people at one point, but there were many intelligent peoples scattered around the globe.

[Sisyphus]

I agree. I suspect the ratio of intelligent races in the universe to spacefaring intelligent races is very low. All kinds of things have to go right and all kinds of things have to not go wrong (or be doomed from the start). Maybe most of them are destroyed early or destroy themselves. Maybe there are intelligences that are hive minds, and can't travel anywhere without taking the whole hive. Maybe there are intelligences that never develop tool use because they can gain no advantage from it or have nothing to use for materials. Maybe they're just far too physically large, or can't survive out of liquid seas, or spend their whole lives attached to one rock, or just lack our monkey curiosity and never developed the instinct to explore. Maybe they're capable but just can't see any reason to leave their own worlds.

 

Still, despite all that, there's a LONG time frame within which it can happen. If it's remotely common, and interstellar travel is practical beyond the immediate "neighborhood," then it seems like there would have to be some visitation. And yeah, I guess just because there aren't any signs of it doesn't mean it didn't happen.

 

Incidentally, another reason I think intelligent life is very rare is because it's rare here on Earth. There are several species on the verge of complex communication and technology, but there is really only one, out of all the life on Earth, that is actually capable of them. Us. There's also no evidence that there was ever one before us. Although that doesn't prove anything, it should be noted that WE will certainly leave lots of traces for a LONG time. For one thing, all the oil is gone....

[scientistsahai]

.... there is the problem of heavy elements and planets. At the Big Bang there was only hydrogen, helium, and some lithium. Other elements up to iron are formed by fusion in stars. Some stars then explode in novae or supernovae ...

 

Where did the H and He come from originally?

[insane_alien]

The H came about through mattercoalecence as the universe cooled. the high energy gamma rays that existed at the time turned into matter-antimatter pairs and eventually the matter won out(at least in this part of the universe) helium and lithium formed from fusion reactions and beta decay of heavy hydrogen atoms H-3 and up.

[swansont]

I agree. I suspect the ratio of intelligent races in the universe to spacefaring intelligent races is very low. All kinds of things have to go right and all kinds of things have to not go wrong (or be doomed from the start). Maybe most of them are destroyed early or destroy themselves. Maybe there are intelligences that are hive minds, and can't travel anywhere without taking the whole hive. Maybe there are intelligences that never develop tool use because they can gain no advantage from it or have nothing to use for materials. Maybe they're just far too physically large, or can't survive out of liquid seas, or spend their whole lives attached to one rock, or just lack our monkey curiosity and never developed the instinct to explore. Maybe they're capable but just can't see any reason to leave their own worlds.

 

There's also the possibility that they are intelligent but lack the physical capability for tool use.

 

Then, has to consider that colonization is an enormous undertaking, since it it's years of travel outside one's solar system. Once you get to the next habitable planet, why would you want to pick up and move further anytime soon?

 

 

Incidentally' date=' another reason I think intelligent life is very rare is because it's rare here on Earth. There are several species on the [i']verge[/i] of complex communication and technology, but there is really only one, out of all the life on Earth, that is actually capable of them. Us. There's also no evidence that there was ever one before us. Although that doesn't prove anything, it should be noted that WE will certainly leave lots of traces for a LONG time. For one thing, all the oil is gone....

 

No evidence of a tool-using, massive-resources-depleting intelligence. But we have filled that niche, so it would be tough for another species to come along and encroach on that, unless they could do it better, or we died out for some other reason.

[swansont]

Look how long it took humans to use technology in a meaningful way. We shouldn't assume that intelligent life is rare because it hasn't visited or travel is impossible. The Europeans probably thought they were the only civilized people at one point, but there were many intelligent peoples scattered around the globe.

 

And there are reasons that the Europeans dominated the exploration and colonization; Jared Diamond discusses this in Guns, Germs and Steel. There are a lot of things that fell into place for the Europeans that allowed them to gain an advantage in development and exploration. One could imagine that the spread of northern hemisphere civilization would be slowed tremedously by something as simple as not having a north star for navigation purposes, and that having a domino effect on the advance of technology.

[SkepticLance]

A lot of reasons have been put up on this thread as to why no intelligent species ever got to the Earth. (They were stay at homes, died off early etc.)

 

All these are possible, but ONLY if the number of intelligent species that have evolved over the past 6 billion years is small. When Drake and Sagan first published the famous Drake Equation, they put in estimated values and calculated a total of one million intelligent species in our galaxy. That is patently wrong.

 

If we assume they exaggerated by a factor of 1,000, and 10% only came into being during the first 2 billion years our galaxy existed, then we still have 100 intelligent species living 2 billion years before the first mammalian life came into being on Earth. It is somewhat unlikely at every one of these species either died off or were pacifist, stay at home philosophers. Adaptations to multiply in numbers and expand geographically is universal for life on Earth, due to very basic principles of evolution. It is seriously probably that this would apply to extraterrestrial life also. Such adaptations in intelligent species are likely to be behavioural. ie. at least some of their population feel the need to explore and colonise.

 

If only 10% ended up as expansionist survivors, we still have ten intelligent species expanding throughout our galaxy more than 2 billion years ago. If we assume that their population growth is only one doubling per century (much slower than human population growth), they can still fully populate the entire galaxy in 2 million years. Spaceship speeds of only 0.1c is enough to permit this.

 

There are no alien artifacts on Earth. Yet current human technology is such that we are leaving non-biodegradable trash by the millions of tonnes that will still be around in 2 billion years as fossils in rocks. The only viable reason why there are no alien fossils or fossils of alien garbage is that aliens never got here in the first place. And the most likely reason for no early alien visits is that aliens are very, very rare.

[Sayonara]

If only 10% ended up as expansionist survivors, we still have ten intelligent species expanding throughout our galaxy more than 2 billion years ago. If we assume that their population growth is only one doubling per century (much slower than human population growth), they can still fully populate the entire galaxy in 2 million years. Spaceship speeds of only 0.1c is enough to permit this.

However:

 

a) For any number of reasons, not every planet in the galaxy will be visited,

 

b) Expansionism does not guarantee survival for long enough to make (a) false.

 

 

There are no alien artifacts on Earth.

That we have found, or recognised as such.

 

 

Yet current human technology is such that we are leaving non-biodegradable trash by the millions of tonnes that will still be around in 2 billion years as fossils in rocks. The only viable reason why there are no alien fossils or fossils of alien garbage is that aliens never got here in the first place. And the most likely reason for no early alien visits is that aliens are very, very rare.

Aliens "not getting here in the first place" is not the same as "no other intelligent civilisation existed before ours".

 

I would have thought that a much more likely explanation is that no matter how advanced the civilisation, no matter how much intelligence evolutionary progress will allow, interstellar travel is simply not practicable, even at 0.1c.

 

And then there is always the fact that Earth is a pretty boring planet, in a pretty boring part of the galaxy. Seriously, we are right in the Local Fluff of the Orion Arm... not the most enticing place for explorers or colonists. It's an "end of the list" neighbourhood.

[SkepticLance]

Sayonara

I have a personal theory which makes my logic fairly inevitable, if this theory is true.

 

That is : when humans ( and lots of possible hypothetical aliens) have developed sufficiently in terms of space technology, we will also develop large numbers of very substantial and highly advanced space habitats. I am talking about rotating (for gravity) cities in space of a million or more people.

 

After all, once we have made the effort to get into space, what would be more silly than to descend another gravity well (gravity trap) and imprison ourselves on another planet.

 

Human history can be seen as the history of separation from our natural environment, to the point where we now live in air conditioned buildings. The next step is total separation from the surrounding environment, as would happen in a space habitat.

 

Habitats can move freely from place to place simply by strapping on ion drive engines. They can harvest water from asteroids, moons, Saturn's rings etc. They can mine rubble in space for other raw materials. I know this is possible, since I have seen iron extracted from basalt rock which was melted in an electric arc furnace. With sufficient technological development, which is only a matter of time, they can become quite independent. They can use fusion power and lighting to grow their own food, and replenish their own oxygen. Independent cities in space.

 

Once a species reaches that stage in its development, what more simple than for one of these independent cities to simply head for the next star system? If it takes 100 years to get there, what the heck. When there, they can harvest water, rocks etc as before. When the population grows enough, make another habitat.

 

I am sure that humans will terraform and colonise planets. However, I suspect that, in say 20,000 years, there will be more people living in habitats than on planets, and these habitats will freely move from star system to star system. Over a long enough period, they will colonise the entire galaxy.

 

If a large number of intelligent alien species existed 2 billion years ago, they should have gone through this process way back then.

 

A comment about intergalactic travel.

This is many orders of magnitude more difficult that interstellar travel. Outside our own little cluster of galaxies (Milky Way, Magellanic Clouds etc), the next galaxy is Andromeda, 2 million light years away. If we assume 0.1c speed of travel, it would take 20 million years to get there. How do you keep a colony of people going 20 million years?

 

This is not totally impossible in theory. We could, for example, with sufficiently advanced computers, get them to travel, carrying frozen embryos. The computers are programmed to thaw out and incubate the embyos, and then raise and teach the new humans. Somewhat risky. To assume nothing going wrong over 20 million years seems a stretch.

[Edtharan]

Here is another reason that aliens might give our planet a wide bert (or at least not visit the surface and leave junk there):

 

Our planet is covered witha massive blanket of toxic gas. This gas is Oxygen.

 

Oxygen was not needed when life firs developen on earth, when it did, it produced a crisis and organisms had to adapt to it (that is why it is not extremely toxic to us).

 

We (life on earth) developed a resistance to it and eventually found a use for it. So this means that life on another planet might not have had to addapt to oxygen and so they could find our atmosphere hightly toxic.

[SkepticLance]

Edtharan

I seriously doubt that humans will find many planets to our liking once we are travelling interstellar. The vast majority will be too large, have toxic atmospheres, be too cold, too hot, too small etc. This situation would have greeted any alien civilisation that reached Earth.

 

There are two possible solutions.

1. Turn the inhospitable planet into a base for a habitat. That is, build a place to live screened off from the outside environment. This is what we will do if we set up bases on the moon.

2. Change the planet to suit our needs. To change a planet into one similar to Earth is called terraforming. There has already been at least one international conference on terraforming Mars. Those scientists think it is feasible.

 

It is probable that any hypothetical alien civilisation touring the galaxy 2 billion years ago would have to do one of the above for pretty much each and every planet they colonised. An expansionist species would need living space, meaning the need to colonise. (Though I still think most would live in space habitats)

 

Again, I suggest that, if there were numerous intelligences, at least one would have been successfully expansionist, and Earth would have been colonised, with 'fossil' remains left behind. The only explanation I can see that makes sense for the lack of remains is that the number of alien intelligences is very low.

[Sayonara]

Sayonara

I have a personal theory which makes my logic fairly inevitable' date=' if this theory is true.[/quote']

You know much better than to bandy the word theory about for untestable speculations.

 

 

That is : when humans ( and lots of possible hypothetical aliens) have developed sufficiently in terms of space technology, we will also develop large numbers of very substantial and highly advanced space habitats. I am talking about rotating (for gravity) cities in space of a million or more people.

I am sure we will do this sooner or later (and the rest of it). However it doesn't mean that in a billion years time every planet in the galaxy will be covered with fossilised Coca Cola bottles and such.

 

 

After all, once we have made the effort to get into space, what would be more silly than to descend another gravity well (gravity trap) and imprison ourselves on another planet.

Errr... isn't this contradicting your earlier posts a bit?

 

 

Human history can be seen as the history of separation from our natural environment, to the point where we now live in air conditioned buildings. The next step is total separation from the surrounding environment, as would happen in a space habitat.

Not so sure about that. We separate ourselves from our environment because it makes us moderately more comfortable to do so, and it is not a true separation in that all we have to do is walk outside to end it. All we have done is created shelter that is more effective than a cave.

 

Given the choice, I think most humans would prefer to live separately from the rain, wind, biting things etc, but still have the ability to walk outside, rather than remain entombed in a metal pod that shields them from the endless gulf of hard vacuum and lethal radiation.

 

 

With sufficient technological development, which is only a matter of time, they can become quite independent. They can use fusion power and lighting to grow their own food, and replenish their own oxygen. Independent cities in space.

Probably, yes. Ironically the things that make this difficult to achieve and maintain are biological, and not technological.

 

 

I am sure that humans will terraform and colonise planets. However, I suspect that, in say 20,000 years, there will be more people living in habitats than on planets, and these habitats will freely move from star system to star system. Over a long enough period, they will colonise the entire galaxy.

I thought habitats were an alternative to planetary colonisation?

 

 

If a large number of intelligent alien species existed 2 billion years ago, they should have gone through this process way back then.

All I am trying to get through to you is that if they did, we might not necessarily know it.

It is foolish and arrogant to take the absence of recognisable evidence for alien visits to Earth in the distant past as evidence against intelligent life elsewhere in the galaxy.

 

And there is no getting around that.

 

Again, I suggest that, if there were numerous intelligences, at least one would have been successfully expansionist, and Earth would have been colonised, with 'fossil' remains left behind.

Earth COULD HAVE BEEN colonised.

 

As I said before, it is hardly in a prime location, and just because an expansionist race spreads throughout the galaxy it does not necessitate colonisation of every planet there is. Especially when building these "habitats" (I wish you would not give ecological terms your own meanings in a biology forum) would be a lot simpler and cheaper than terraforming an entire world.

 

 

The only explanation I can see that makes sense for the lack of remains is that the number of alien intelligences is very low.

That's not the only explanation you can see, it's the one you prefer.

[Edtharan]

I seriously doubt that humans will find many planets to our liking once we are travelling interstellar. The vast majority will be too large, have toxic atmospheres, be too cold, too hot, too small etc. This situation would have greeted any alien civilisation that reached Earth.

I also seriously doubt this too. I was saying this exact thing. Earth's atmosphere is greatly out of equilibrium. The oxygen in our atmosphere is not usual for planets. A visiting alien race might well have sen Earth and looked at another planet (or moon) in our solarsystem and though it would be easier to colonise that one (it would more likely suit their atmosphere. Also the preasence of life on earth that is producing this toxic atmosphere of ous would be very difficult to get rid of (you would essentially have to blast the planet to bedrock to get rid of it all or it would just start producing more toxins). It would be much easier to start from a location without much life (hence why Mars is actually feasable).

 

I think that if ailens have visited our solarsystem, we will find the evidence on other planets or moons (none of which have been explored even near enought to make any predictions about it).

[SkepticLance]

Sayonara said :

 

It is foolish and arrogant to take the absence of recognisable evidence for alien visits to Earth in the distant past as evidence against intelligent life elsewhere in the galaxy.

 

And there is no getting around that.

 

This is an assertion, not an argument. Present your logic or your evidence, or else it is just a statement to be taken on faith.

 

My statements on space habitats (which is more than just a term of ecology) do not contradict anything I said earlier - just expand on them. Any successful expansionist species moving through the galaxy would be expected to live primarily in space habitats for the reasons given. However, they would also move onto planets. My logic assumes some population pressure. One thing much bigger than the galaxy is the ability of living things to grow in numbers. Unless they had a brake on reproduction, sooner or later, they would be looking for each last bit of habitable acreage. And, if there were many intelligent species, the ones with brakes on reproduction would be overwhelmed by the ones that did not.

 

That is also the reason why the argument that Earth may not have been a hospitable environment is not a good one. A sufficiently advanced technology can produce liveable buildings in any environment. When population pressure is sufficient, even inhospitable Earth would be colonised.

 

The arguments Sayonara etc have put forward are valid, but only if the numbers of intelligent species are small.

 

Let me say one other thing, to avoid looking arrogant. This is all speculation. In science, we know that speculation, theorising, calculating, computer modelling, and other exercises that take place in the human mind or a computer rather than the real world, are ultra prone to error. All my 'logic' may be just so much hot air, and is very likely to be. However, I enjoy speculating, and that is the justification.

[swansont]

Sayonara said :

 

It is foolish and arrogant to take the absence of recognisable evidence for alien visits to Earth in the distant past as evidence against intelligent life elsewhere in the galaxy.

 

And there is no getting around that.

 

This is an assertion' date=' not an argument. Present your logic or your evidence, or else it is just a statement to be taken on faith.

 

[/quote']

 

 

How about you first? I asked you to give more of the assumptions/calculations to which you referred, and you have thus far declined.

 

One problem with the Fermi paradox is that it encompasses many assumptions, but the conclusion drawn from the contradiction is that there are no intelligent species out there, instead of one of the other assumptions being wrong.

 

And there's the Park hypothesis

[Sayonara]

This is an assertion, not an argument. Present your logic or your evidence, or else it is just a statement to be taken on faith.

I made no claim that it was an argument, and I have already stated my logic throughout the thread, to which (until now, in the below) you have made few specific responses.

 

 

My statements on space habitats (which is more than just a term of ecology) do not contradict anything I said earlier - just expand on them.

As to habitat being "more than a just a term of ecology", this is quite so. However your particular use of the word is introducing a slight ambiguity to the thread. Can we call them space hotels, or something?

 

These statements do contradict your assertion that an expansionist spacefaring civilisation would colonise planets. A race capable of creating a stable biome within an artificial structure that can withstand multi-generational voyages, and replenish its supplies by mining as needed, would have little use for dirty and immobile planets.

 

It would be far easier and more efficient for them to keep building space hotels, and when population becomes an issue, to build Dyson Spheres, which I mentioned some several posts ago.

 

A single Dyson Sphere is equivalent to thousands of Earth-like planets, and as such represents a massive exploratory saving in distance travelled alone. Also it does not incur the same risk of failure as exploration.

 

 

Any successful expansionist species moving through the galaxy would be expected to live primarily in space habitats for the reasons given. However, they would also move onto planets.

As you said to me, "present your logic or your evidence, or else it is just a statement to be taken on faith".

 

 

My logic assumes some population pressure. One thing much bigger than the galaxy is the ability of living things to grow in numbers. Unless they had a brake on reproduction, sooner or later, they would be looking for each last bit of habitable acreage. And, if there were many intelligent species, the ones with brakes on reproduction would be overwhelmed by the ones that did not.

I have absolutely no issue with this. What I do take issue with is the fact that you are disregarding all the possible brakes that could be applied to the reproductive rate of any given pan-galactic species.

 

It doesn't even need to be that complex. Assume the galaxy was over-run with these creatures: it might just be that the space hotel sent here blew up before it arrived, and central command doesn't know about it.

 

 

That is also the reason why the argument that Earth may not have been a hospitable environment is not a good one. A sufficiently advanced technology can produce liveable buildings in any environment. When population pressure is sufficient, even inhospitable Earth would be colonised.

Not when there are safer, more energy- and time-efficient options available to a species with the level of technology you are ascribing to them.

 

 

The arguments Sayonara etc have put forward are valid, but only if the numbers of intelligent species are small.

We don't have any information on galactic ecology, only spurious theoretics, so I am classing that as speculation. The ecological models we have don't necessarily scale up.

 

However I will say that it does not mean the numbers of intelligent species are small. Instead it suggests that the number of intelligent species that arose and avoided extinction several million years before us - with the will and capacity for galactic domination - was small. I thought that was what we were arguing about? You know - the time factor

 

 

Let me say one other thing, to avoid looking arrogant. This is all speculation. In science, we know that speculation, theorising, calculating, computer modelling, and other exercises that take place in the human mind or a computer rather than the real world, are ultra prone to error. All my 'logic' may be just so much hot air, and is very likely to be. However, I enjoy speculating, and that is the justification.

And please don't get me wrong - I am not trying to thrash your logic into the dust. I am just trying to point out areas you obviously haven't thought of, because you look like you consider this speculation to be more logically valid than you should.

 

For all we know, travelling at 0.1c might liquefy organic material regardless of technological countermeasures. Or there could be some non-duplicatable energetic quality inherent to all star systems without which native life cannot exist.

 

Either of those, or similar unpredictables, would render this whole conversation moot.

 

 

 

Anyway, having gone back and read post 1, I think we are a little off-topic. The question as asked is answered by "yes, an intelligent race as smart or smarter than us could have existed before we did, and could still exist."

[SkepticLance]

Sayonara.

I think it is important we all accept that this is a speculative discussion, with no proper scientific way of determining who is 'right' and who is 'wrong'. I enjoy this kind of discussion and am happy to engage purely for the pleasure of discourse. Either side may be correct, but let's explore.

 

I do not dispute your ideas on why a particular intelligent species never got to Earth. Perhaps their travelling space city did blow up. The thing is, though, that these possibilities can only apply to a small number of species. If Sagan and Drake were correct, and the galaxy supports a million intelligent species, then suggesting that no-one got to Earth because of this or that individual reason no longer makes sense, because it could not have happened to everyone. At least one alien species would have lived long enough, and been expansionist enough, to make it to Earth.

 

Incidentally, the reason I used the term 'habitat' was because I saw it used that way in a Scientific American article on our future in space. Space 'hotel' does not quite cover it. I suggest we call them space cities.

[lucaspa]

Isnt that egotistical

 

It may be possible that we are the most advanced civilization in our galaxy (or at least' date=' any other civilzations are a million or less years ahead).

 

How about inter-galactic travel?

statistically, there HAS to be intelligent life in the universe. If not in our galaxy, then perhaps in another. If they had a 1 billion yr head start then I'm sure they could make it intergalactically[/quote']

 

1. Instead of "intelligent", you need to say "technological" species. Chimps (and perhaps whales) are intelligent, but they are not technological.

 

2. Intergalactic travel below lightspeed is going to be technically and, more importantly, economically VERY, VERY, difficult. You need a ship that would last for a billion years, be generational, and can carry enough consumables for an intergalactic trip. No recycling is 100% efficient and material between galaxies is very sparse. And, even if you are using Bussard ramjets as propulsion, is there enough hydrogen for fuel? If we are restricted to lightspeed, then intergalactic travel is damn near impossible.

 

 

Sayonara said :

 

It is foolish and arrogant to take the absence of recognisable evidence for alien visits to Earth in the distant past as evidence against intelligent life elsewhere in the galaxy.

 

And there is no getting around that.

 

This is an assertion' date=' not an argument. Present your logic or your evidence, or else it is just a statement to be taken on faith. [/quote']

 

As stated, it is a sound scientific principle: absence of evidence is not evidence of absence.

 

There are many, many hypotheses to explain the lack of evidence today of visits in the past. Especially the geological past. Many of these are the same hypotheses that explain the absence of transitional series of fossils between most taxa: only a small area in a limited time was visited, that geological strata is either not exposed on the surface or has already eroded away, there are a limited number of paleontologists and they simply haven't found the site yet, the visitors did not leave any evidence that would have survived to today.

 

Any successful expansionist species moving through the galaxy would be expected to live primarily in space habitats for the reasons given. However, they would also move onto planets. My logic assumes some population pressure. Unless they had a brake on reproduction, sooner or later, they would be looking for each last bit of habitable acreage.

 

Bad assumption. Remember birth control? The distance between stars using lightspeed is such that birth control MUST be practiced most of the time. Such ships are going to be generational: several generations living and dying on the ship. So, in order to undertake such a journey in the first place, they must have a brake on reproduction. Otherwise they couldn't maintain a stable population on the ship.

 

Such a society might not want to live on a planet. Blish's city ships and Poul Anderson's "Kith" and spacefarers in Spaceship are just 3 examples where it can be imagined that humans establish a nomadic culture living only on spaceships and not on planets. One thing much bigger than the galaxy is the ability of living things to grow in numbers. And, if there were many intelligent species, the ones with brakes on reproduction would be overwhelmed by the ones that did not.

 

All my 'logic' may be just so much hot air, and is very likely to be. However, I enjoy speculating, and that is the justification.

 

I'm glad you enjoy the speculation and I hope you enjoy my testing of those speculations.

 

 

Occam's razor in its original form states simply that entities should not be multiplied unnecessarily. In other words, make as few assumptions as possible, and discard those aspects of an explanation which are not necessary to it. He also said that, given two explanations with equal predictive power, one should favor the simpler one. So how was bascule's usage incorrect?

 

Because that isn't Ockham's Razor. The Razor has nothing to do with assumptions. What Ockham said was "Refrain from adding unnecessary entities to describe the phenomenon." His example was the statement:

 

“A body moves because of an acquired impetus” vs “a body moves”. In Ockham's day a force called an "impetus" was supposed to make things move. But movement is a succession of points in space over time. So, the statement "a body moves" is all you need to describe the phenomenon.

 

Ockham argued against favoring the simpler explanation. That's because others of his time were arguing that the simplest explanation HAD to be the correct one. Ockham realized that this was not true (Ockham hated the idea because it limited God's power).

 

So, we can have the statements:

1. Planets move in elliptical orbits.

2. Planets move in elliptical orbits because of gravity.

3. Planets move in elliptical orbits because alien spacehips push them.

 

Both #2 and #3 should not be used. In each case, anything beyond "orbits" is unnecessary to describe the motion of planets. In both cases, those are theories about WHY the planets move as they do, but have nothing to do with describing their motion.

 

 

Where did the H and He come from originally?

 

Insane has part of it, altho what he said is not what I have read. Remember E= mc^2. Matter and energy are two forms of the same thing. The early universe was much too hot for matter it exist, there was only energy. As the universe cooled there was a phase transition and the energy converted to matter. It did so in both matter/antimatter. Those tended to annhilate each other and form the gamma ray photons, BUT, there was a slight excess of matter. That became the hydrogen and helium.

 

The gamma ray photons became the cosmic microwave background radiation. With the expansion of the universe the wavelength was strected from the very short wavelength of gamma rays to the much longer wavelength of microwaves.

[SkepticLance]

Lucaspa said :

 

The distance between stars using lightspeed is such that birth control MUST be practiced most of the time. Such ships are going to be generational: several generations living and dying on the ship. So, in order to undertake such a journey in the first place, they must have a brake on reproduction. Otherwise they couldn't maintain a stable population on the ship.

 

Sorry. This is faulty logic. The distance from here to Alpha Centauri is 4.3 light years. At 0.1 c, that journey would take 43 years. If our hypothetical invading alien had a generation time of 25 years (human average), then the population increase would be manageable. It would be easy to design a vessel able to cope with such an increase in numbers. On the other hand, they might use short term population control - just for the ship's journey - and switch to uncontrolled growth after arrival.

 

Either way, we cannot assume that our alien species will not grow dramatically in numbers after arriving at their destination. In fact, assuming numerous species competing, the species that increases fastest has a substantial competitive advantage.

 

The average distance between stars within our galaxy is, in fact, less than 4.3 light years. Thus, a rapidly growing population could easily cross the void, stellar system to stellar system. In fact, such a species is the one most likely to be involved in rapid expansion. Population pressure will pressure the explorer types to get out there.

 

Humans are likely, in the next 50 years, to develop a 'space elevator' which can get us into space cheaply and in large numbers. Assuming our alien species has this technology, they will have no major brakes upon their ability to get into space cities, and from there to expand in all directions.

[lucaspa]

Lucaspa said :

 

The distance between stars using lightspeed is such that birth control MUST be practiced most of the time. Such ships are going to be generational: several generations living and dying on the ship. So' date=' in order to undertake such a journey in the first place, they must have a brake on reproduction. Otherwise they couldn't maintain a stable population on the ship.[/i']

 

Sorry. This is faulty logic. The distance from here to Alpha Centauri is 4.3 light years. At 0.1 c, that journey would take 43 years.

 

IF Alpha Centauri has habitable planets. We don't know that. If you begin to extend the distance beyond Alpha Centauri (and particularly intergalactic), then you have many more than 2 generations. If population increases only 10% per generation, then you quickly run into troubles.

 

Either way, we cannot assume that our alien species will not grow dramatically in numbers after arriving at their destination. In fact, assuming numerous species competing, the species that increases fastest has a substantial competitive advantage.

 

Your assumptions are:

1. The species would want to colonize. Maybe not.

2. That population would increase dramatically. I doubt that. In a technological civilization, everyone wants the wealthy lifestyle associated with technology. That can't be maintained until the technological base is constructed. How long does it take to grow the food to feed the new numbers, mine the metals, refine them, manufacture the tools to make the tool to make the tools to make the factories to produce the technology?

3. That the death rate will be minimal. Look at the history of colonization of N. America by Europeans. Roanoake colony failed and the colonists died. Plymouth suffered 60% deaths the first winter. Both Plymouth and Jamestown survived only because there were additional colonists arriving from Europe. And that was on the same planet on which humans evolved! Imagine the potential problems for colonists evolved on a different planet, cut off from reinforcements, and facing all the hazards of a new environment.

 

The average distance between stars within our galaxy is, in fact, less than 4.3 light years.

 

That's misleading. Average includes the stars in the core -- and organic life can't live there because of the radiation. Out in the toroid where habitable planets exist, the average distance is more like 10 light years.

 

In fact, such a species is the one most likely to be involved in rapid expansion. Population pressure will pressure the explorer types to get out there.[/quote

 

Bad assumption. We have population pressure on earth and we are expansionist. Look at the exploration and colonization conducted by Europeans. Yet where is the construction of generation ships? We have the technology.

 

Instead, the economic and political cost is too high. Instead of spending resources to build such ships, we use the resources to feed the people on the planet and to increase wealth. Also, as wealth increases, birth rate drops. Any species as technologically advanced as you propose is going to have a means of birth control. After all, it must be used on generation ships. I don't see why an alien species would be exempt from economic principles.

 

Even IF there is colonization, how long before the colonists have populated the new world and are able to build their own generation ships to continue the process?

 

Humans are likely, in the next 50 years, to develop a 'space elevator' which can get us into space cheaply and in large numbers. Assuming our alien species has this technology, they will have no major brakes upon their ability to get into space cities, and from there to expand in all directions.

 

Even if we get into "space cities", (and I disagree with your "likely") what is the incentive to leave the solar system? If we can build habitats in space, then we can build them all over the solar system. Think of the massive amount of resources still within the solar system! Your population pressure disappears and we have no incentive to build generation ships.

 

And maybe that is the answer to "Where are they?" Once the level of technology is such that the species can build habitats on the other real estate within their solar system, there is no economic or population pressure to go to the stars if we are restricted to generation ships. Only if there is an FTL drive would colonization become attractive.

[lucaspa]

Incidentally, another reason I think intelligent life is very rare is because it's rare here on Earth. There are several species on the verge[/i'] of complex communication and technology, but there is really only one, out of all the life on Earth, that is actually capable of them. Us. There's also no evidence that there was ever one before us. Although that doesn't prove anything, it should be noted that WE will certainly leave lots of traces for a LONG time. For one thing, all the oil is gone....

 

See that you are equating technology with intelligence? Not the same thing. A species can be very intelligent (perhaps as whales are) but lack the necessary traits to make technology. In this case, whales lack hands and living in water handicaps them in building technology. Difficult to refine metals underwater.

 

Also, 30,000 years ago there were three (at least) species of Homo on the planet: H. neandertal, H. erectus, and H. sapiens. There may have been more but not have the skeletal features necessary to distinguish them from H. sapiens. For instance, Mungo man in Australia is skeletally modern, but has DNA as different from us as neandertals.

 

So, having only one species of Homo on the planet is unusual and recent (in evolutionary terms). It may be that one technological species will always supplant all the others. But then again ...

[Sayonara]

I think it is important we all accept that this is a speculative discussion, with no proper scientific way of determining who is 'right' and who is 'wrong'.

Well yes, but there is still plenty of ground for scientifically determining which evolutionary pathways are more likely, and for identifying stacks of bad assumptions.

 

I enjoy this kind of discussion and am happy to engage purely for the pleasure of discourse. Either side may be correct, but let's explore.

You have the bridge!

 

I do not dispute your ideas on why a particular intelligent species never got to Earth. Perhaps their travelling space city did blow up. The thing is, though, that these possibilities can only apply to a small number of species. If Sagan and Drake were correct, and the galaxy supports a million intelligent species, then suggesting that no-one got to Earth because of this or that individual reason no longer makes sense, because it could not have happened to everyone. At least one alien species would have lived long enough, and been expansionist enough, to make it to Earth.

Not so. This is a bit like the "national lottery" problem of probability - although the odds are millions to one, someone always wins. Problem is it's never you!

 

In our case there may have been thousands of intelligent races that arose, lived and died before ours, all with the capability and will to spread throughout the galaxy, who for whatever reason never made it to 100% dominance. The successful race could have started its mission of conquest fifty million years ago, before we were even here, and just not have made it to our neck of the woods yet.

 

Alternatively Earth may have been colonised or used as an outpost a dozen times already, but for any number of reasons (and some good ones have been given) we simply haven't detected the evidence.

 

It's possible Earth is so toxic to non-Terran life we have been consistently ignored by colonists for the past billion years.

 

There is just no way of knowing (at the moment), and the ideas that it 'only takes one species to dominate the galaxy' and 'enough time should have passed for that to occur already' still do not mean that every planet in the galaxy would or could be jammed full of easily-spotted artifacts.

 

 

Incidentally, the reason I used the term 'habitat' was because I saw it used that way in a Scientific American article on our future in space. Space 'hotel' does not quite cover it. I suggest we call them space cities.

I used hotel because you were implying that they were not a permanent dwelling (at least for some inhabitants). City pretty much does the job though.

 

 

That's misleading. Average includes the stars in the core -- and organic life can't live there because of the radiation. Out in the toroid where habitable planets exist, the average distance is more like 10 light years.

And let's not forget that just because Alpha C is 4.3 light years away, it does not mean we can get there in 43 years at 0.1c. Safe acceleration and deceleration, and avoiding intervening matter and energy - both of those will likely double the time.

 

Ships travelling interstellar distances are also likely to encounter lethal phenomena that cannot be detected by the onboard equipment.

 

 

And maybe that is the answer to "Where are they?" Once the level of technology is such that the species can build habitats on the other real estate within their solar system, there is no economic or population pressure to go to the stars if we are restricted to generation ships. Only if there is an FTL drive would colonization become attractive.

I have already probed this highly likely possibility several times, but it keeps being ignored in lieu of interstellar colonialism.

[darkangel199]

well, i think maybe they just don't want to be bothered? I don't see why a alien species should act like a human when it comes to exploring things. And if they can travel interstellar distances their technology is so far advanced past our own would we even know they were here to begin with?