An intelligent species that lacked the tolerance for high G forces to survive a rocket ride into space might be able to get into space using space elevators. They'd have to send components of the elevator into orbit first, which would be an issue, but they might be able to handle that part by using robots.
*EDIT* Just as a hypothetical, let's say you parked one of our current deep space telescope satellites in, I dunno, Alpha Centauri (or another nearby star system) and pointed it at Earth - would you detect anything that would tell you an advanced, tool-using civilization was there?
Maybe? Our civilization does emit
a lot of radio signals, though they'd mostly be pretty weak at light-years distances. It might depend on whether they were lucky enough to have a powerful directional signal like an airport radar or military radar pointed in their general direction at the right time.
Worth noting though that a civilization that was basically like ours but some things a little differently might be a lot less detectable. The most powerful regular Earth signals were the Cold War warning radars; a more peaceful civilization wouldn't have anything like that (unless maybe it was an incoming meteorite detection system or something like that). A civilization that used air travel a lot less and something like high speed trains a lot more might have a lot fewer airport radars. A civilization that relied more on landlines, fiber optic cables, etc. and less on radio transmission might emit very little radio leakage compared to ours (notably, radio and TV signals are the ones that come closest to being omnidirectional). It's not hard to imagine a technological civilization around Alpha Centauri that we'd probably miss, even if
they could probably detect
us with something roughly equivalent to our present technology and radio astronomy/SETI spending. That might be an interesting asymmetry for science fiction: they might notice our radio broadcasts and recognize them as artificial, but they probably wouldn't be able to understand them, just know that somebody was sending out artificial-looking signals; they'd know very little about us and the ball would be firmly in their court whether they want to reveal their existence to us or not. It'd be funny if one side eventually sends a ship over to the other and finally meets them in 2100 or something and then we find out we could have had confirmation of intelligent aliens in 1970 and the only reason that didn't happen was the "we shouldn't call attention to ourselves until we know more about these aliens, they might be hostile and technologically advanced enough to reach our star system" side won the debate over there.
Carl Sagan actually did the math on this, turns out with the available space and resources in a star system and its close neighbors there is very little or no need to expand at the rate people have been suggesting, even mega projects such as dyson swarms or von Neumann machines are often completely unnecessary.
A solar system is vast but exponential growth/compound interest is powerful. A species that reproduced no faster than humans could easily fill up the galaxy in an eyeblink in cosmic time if they were inclined to do so and rate of reproduction was the only limiting factor (and aliens might easily reproduce faster than humans, though I suspect probably not
much faster; any intelligent species with sophisticated tool use and culture is probably a K strategist).
The whole Fermi Paradox is in effect driven by the assumption that intelligent life will behave as some humans want to.
I'd say it's driven by the assumption that
some intelligent life will behave as
some humans want to, which seems more plausible.
We might imagine some highly advanced species drawing up a list of sapient species in our galaxy noting how likely they are to colonize the galaxy some day. Entries from it might read something like this:
Species 1147 ("[complex ultrasonic signal]"):
Inhabit a hot and humid water-oxygen planet with 5.6 bars air pressure at the surface (atmosphere mostly nitrogen) orbiting a F8 star. Dwell outside water. Arboreal omnivore, moderate size (adult females 15-30 kg, adult males 25-50 kg), natural average lifespan 57 years (to old age), two sexes, high sexual dimorphism (males exhibit colorful plumage and have complex display structures, females are drab and lack these structures), oviparous typically laying one egg at a time with normal reproductive rate of 1 egg per every 2-4 years, communicate by ultrasonic signals. Six limbs: in females two wings and four hands, in males two wings, two hands, and two display structures. Females have moderate flight capability but primary method of locomotion is brachiation; males are unable to fly because their wings have evolved to serve as large display structures used in mating dances and are not functional for flight; males rely entirely on brachiation. Sapience evolved as a consequence of runaway sexual and social selection (with complex feeding strategies and predation pressure a contributing factor). Typically live in groups of between 5 and 200 individuals with complex social structures. Food is obtained by hunting and gathering. Brains are heavily specialized for social intelligence. Both sexes of species 1147 are highly intelligent, but mostly uninterested in physical problems, their intelligence almost entirely devoted to socialization and mating strategies. Tool use is limited to a complex array of stone, wood, and bone hand tools and woven nets and baskets. Species 1147, especially males, devotes a great deal of energy to art, chiefly wood carving, bone carving, jewelry, painting, body painting (body paint and jewelry are worn mostly by the males), textile arts, and ultrasonic song/poetry. History is somewhat difficult to reconstruct due to poor preservation due to the "jungle world" nature of this planet, but limited available fossils and genetic analysis suggests that Species 1147 achieved approximately its present physical and cultural form approximately 137 million years ago and has changed little since.
Possibility of galaxy colonization: very low.
Species 42188 ("[complex ultrasonic signal]"):
Inhabit a hot water-oxygen planet with .8 bars air pressure at the surface orbiting a F7 star. Dwell outside water. Conditions on home planet land mass trend toward aridity due to continent configuration, but extensive humid regions exist near some of the coasts. Species 42188 is adaptable and has colonized most of the planet's land. Species 42188 is a large terrestrial omnivore: average adult weight 900 kg, six limbs (two arms and four legs). Hermaphroditic and oviparous, typically laying one egg at a time. Natural average lifespan 280 years (to old age). Communicate by ultrasound. Sapience evolved as a consequence of social selection, environmental stresses, and competition with other large omnivores and carnivores (those competing large omnivore and carnivore species are now extinct). Typically live in groups of between 5 and 100 closely related individuals. Kin are recognized by scent. Social cooperation is instinctively reserved for kin. Species 42188 are highly cooperative and form strong emotional bonds within their kin-group, but non-kin mostly meet only to mate. There is some limited capacity for cooperation with non-kin in service of some near-term goal, but the presence of non-kin make a member of Species 42188 highly fearful and aggressive; non-kin are regarded as competitors, threats, and/or opportunities for theft and/or cannibalism (mating among species 42188 is a rather fearful affair; it is done on "neutral ground" well away from the kin-groups of both partners, and engaging in mating means accepting a risk that one may be opportunistically killed and fed upon after or in lieu of the mating). This socialization pattern means Species 42188 is unable to form communities larger than villages. Species 42188 kin-groups in more humid and richer regions are larger (between 20 and 100 individuals) and exist as sedentary villages, usually dependent on agriculture and/or fishing. In areas unsuitable for agriculture or fishing kin-groups are usually smaller (between 2 and 15 individuals), nomadic, and exist as hunter-gatherers. Technology is relatively sophisticated (e.g. iron-working in village forges is common), but remains mostly limited to hand tools and relatively primitive structures of wood, mud-brick, etc.. Paleontological evidence indicates that Species 42188 achieved approximately its present physical form 32 million years ago and has made only slow and incremental progress since. Given their socialization pattern, it seems unlikely that they could ever develop a high-tech industrial civilization of the sort that might be able to build spacecraft.
Possibility of galaxy colonization: very low.
Species 26991 ("[complex infrasonic rumble"]):
Inhabit a cold water-oxygen planet orbiting a K1 star, with limited surface water and a .35 bar atmosphere (48% oxygen). Omnivore, average adult weight 2100 kg. Quadrupedal, with short thick legs, body is mostly covered by a bony shell, there is a tail protected by bony scutes that ends in a defensive heavy bone club. Manipulatory organs are two arm-like structures near the mouth which end in hand-like structures; they are distantly derived from mandibles. Communicate by infrasound generated by organs in the torso. Natural average lifespan: 420 years (to old age). Hermaphroditic, oviparous, typically lay one egg at a time, slow breeding (average one egg laid per 50 years). Curious, inventive, highly intelligent. Sapience evolved as a consequence of social selection, predation pressure, and environmental stress. Agricultural society with states and cities in the most fertile areas of the planet. Relatively sophisticated technology (primitive radios, aircraft, etc.), but development is constrained by limited resources, especially limited access to fossil fuels; economy still basically agricultural and preindustrial. Population constrained by limited ecological space: areas more than approx. 30 degrees from the equator are uninhabitably cold and majority of habitable temperature region is desert. Population constrained by large size and hence large food requirements of Species 26991. Planetary population: approximately 11 million. Technological progress and economic development constrained by small population size and limited resources. Paleontological and archaeological evidence and historical records indicate: Species 26991 achieved approximate present physical form 3 million years ago, achieved an agricultural society 100,000 years ago, achieved approximately its present level of technology 3000 years ago.
Possibility of galaxy colonization: moderate.
Species 72244 ("The Last Generation, Immortal, Who Dwell In Paradise"):
This species once had more conventional bodies, but now exist as disembodied brains of approximately 70 kg sustained by artificial machinery. Sophisticated medical technology has given these beings an indefinite lifespan, and they have been alive since their species made its transition into this form 14 million years ago. They do not reproduce, though they could easily create more of themselves if they desired to. These brains are connected to an extremely complex shared dreamworld that they collectively control. They generally feel that this dreamworld is vastly superior to the real physical world. They consider themselves a very successful culture and the successful culmination of the hopes and dreams of their ancestors. They consider the real physical world boring and mostly want as little as possible to do with it. The surface of their planet has largely been allowed to return to a wilderness state, the ancient cities abandoned to crumble into dust; all that remains is a minimalistic support infrastructure for the brains and the dreamworld, mostly maintained by automated systems, although the infrastructure does occasionally require some attention from a few of the brains via teleoperation (this duty is mostly performed by a few eccentrics who still feel some lingering affection for interacting with the real physical world). The brains are friendly to aliens who visit their world, but mostly have little desire to interact with the universe beyond their dreamworld. In material terms, their culture has changed very little in 14 million years (they would say this is because you can't improve on perfection). Their society is limited to one solar system, and mostly to their original homeworld (the only space presence is a few asteroid mines, an anti-meteorite defense system, etc.). Occasionally some eccentric among them will commission the construction of a highly automated starship-body which they then use to leave and explore the galaxy, but this is usually an enterprise of a only a single individual or a small group and motivated only by curiosity.
Possibility of galaxy colonization: low.
Species 67968 ("Humans"):
Inhabit a cool-temperate water-oxygen planet with a 1 bar atmosphere orbiting a G2 star. Dwell outside water. Omnivore, adult weight 50-200 kg, four limbs (two arms and two legs), two sexes, moderate sex dimorphism, natural average lifespan 80 years (to old age). Communicate by sound. Curious, inventive, relatively rapid technological progress; achieved approximately present physical form 200,000 years ago, developed an agricultural society 10,000 years ago, invented coal-fired steam engines approx. 200 years ago, presently a global-industrial society with approx. 8 billion individuals, nuclear power, limited space exploration, experiencing rapid economic growth and technological progress (this description likely to be outdated within 10-100 years). Viviparous, reproduction painful, difficult, and dangerous (for females), under high-tech conditions reproduction trends toward replacement rate or below replacement rate. Relatively short lifespans lead to difficulty with long-term planning and long-term coordination; civilization trends toward instability.
Possibility of galaxy colonization: moderate.
Species 97003 ("[a series of notes like a flute might make]"):
Omnivore, six limbs (two arms and four legs), average adult weight 50 kg. Originally native to a warm-temperate water-oxygen planet with a 1 bar atmosphere orbiting a G8 star. Communicates by sound, generated by a vocal apparatus similar to a complex wind instrument. Natural average lifespan 55 years (to old age). Hermaphroditic, viviparous, infants are born small (less than 1 kg) and nearly mindless but relatively robust and capable of simple physical activities like walking within a few minutes of birth, and then grow to physical, mental, and sexual maturity within 15 years. Sapience evolved as a consequence of social selection, predation pressure, and complex feeding strategies. Curious, inventive, communitarian, long-term thinkers, strong group solidarity, tendency toward xenophobia (Species 97003 has very much been shaped by an evolutionary history as a prey species - paleontological evidence suggests exterminating their predators to extinction was one of the first things they did upon developing weapons better than stone hand axes). Individuals of Species 97003 seem to instinctively desire to reproduce as an end in itself; producing and raising offspring is one of the greatest satisfactions of a typical Species 97003 individual's life, and they take so much joy in it that they almost universally desire large family sizes. A Species 97003 mated pair will typically produce something between 3 and 10 offspring in their lifetimes. Replacement rate reproduction in Species 97003 can be maintained only by severe coercion, and the brief historical period when this was necessary because of the resource limitations of their homeworld is remembered by them with horror. This might be a legacy of Species 97003's evolutionary history as a prey species. The history of Species 97003 is one of rapid progress (though marred by extremely violent wars caused by population pressure, including four nuclear holocausts), they reached a space age level of technology within 3000 years of the emergence of their first cities, and they responded to space travel and then star drives by expanding rapidly. Species 97003 has now been starfaring for less than 10,000 years and in that time they have colonized several million solar systems. The vast majority of Species 97003's population lives in space habitats, which allows them to colonize almost any star system and to colonize those solar systems extensively; a typical solar system inhabited by Species 97003 has trillions or quadrillions of inhabitants. The total Species 97003 population is estimated to be at least 10^21 individuals, possibly much higher, and growing fast. The Species 97003 colonization volume is advancing across the galaxy at close to the speed of light.
Possibility of galaxy colonization: very high.
Notably, Species 97003 is the only one where you can take a look at them and say "yeah, if a species like that started spreading out 10 million years ago, we'd probably either know they existed pretty quickly or wouldn't be here"... but that's all it takes.
