Office of Public Affairs While most science fiction books and movies portray a universe teeming with a plethora of space-faring alien species, the reality may be quite different. Few of us could look up into the night sky and not feel the enormity of the cosmos and the seemingly infinite number of stars that should, at least in theory, harbor a multitude of intelligent civilizations.
Yet so far, our attempts to identify signs of extraterrestrial life have all amounted to nothing. The apparent lack of observed intelligent life in the universe beyond Earth in the face of what would seem to be an almost limitless potential for such life to arise is known as the Fermi Paradox.
While borne of a lighthearted discussion, Fermi and others quickly realized the serious nature of the question and began to study the problem on several levels. Given the age of the universe, even a species that traveled among the stars at relatively low velocities should have had ample time to disseminate widely and make its presence felt.
Possible explanations to the Fermi Paradox can loosely be divided into two groups, focusing on issues either related to hypothetical alien intelligent civilizations or to the local situation of humans on the Earth, which I will term Them and Us, respectively. In this vein, perhaps alien civilizations have no interest in talking to other species in the first place.
As such, a species that is not actively attempting to communicate might be virtually impossible to detect. In a similar vein, while humanity has always been an expansive species, an intelligent alien race may simply have no interest in interstellar travel, or interstellar travel itself either via vessels carrying living beings or robotic probes may simply be impossible given the technology it would require and the vast distances between stars.
While humans have predominately searched for signs of alien life using radio or optical signals, our technologies may be hopelessly primitive or essentially useless for communication over interstellar distances.
By means of analogy, nobody I know uses Morse code when they have a cell phone on their hip. Anybody still transmitting in Morse code is going to be waiting a long time for a response to their SOS. Furthermore, while intelligent alien life could have theoretically evolved dozens, hundreds, thousands, or millions of times, we may simply be separated by too much time and distance to ever make contact.
A species that came to prominence , light-years from Earth would, for all intents and purposes, be too far away for us to meaningfully communicate with. The first suggestion came from Chandra Wickramasinghe , who proposed a polymeric composition based on the molecule formaldehyde CH2O. Usually this occurs when a molecule becomes ionized , often as the result of an interaction with cosmic rays.
This positively charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower.
Biologically relevant molecules identified so far include uracil , an RNA nucleobase , and xanthine. The molecule was found around the protostellar binary IRAS , which is located light years from Earth. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.
The chemicals, found in a giant cloud of gas about 25, light-years from Earth in ISM, may be a precursor to a key component of DNA and the other may have a role in the formation of an important amino acid. Researchers found a molecule called cyanomethanimine, which produces adenine , one of the four nucleobases that form the "rungs" in the ladder-like structure of DNA. Previously, scientists thought such processes took place in the very tenuous gas between the stars.
The new discoveries, however, suggest that the chemical formation sequences for these molecules occurred not in gas, but on the surfaces of ice grains in interstellar space. PAHs seem to have been formed shortly after the Big Bang , are widespread throughout the universe, and are associated with new stars and exoplanets.
Pyrimidine, like polycyclic aromatic hydrocarbons PAHs , the most carbon-rich chemical found in the Universe , may have been formed in red giants or in interstellar dust and gas clouds, according to the scientists.
Main article: Extraterrestrial life The chemistry of life may have begun shortly after the Big Bang , It is generally agreed that the conditions required for the evolution of intelligent life as we know it are probably exceedingly rare in the universe, while simultaneously noting that simple single-celled microorganisms may be more likely. However, some scientists hypothesize that faster-than-light interstellar space travel might be feasible.
This has been explored by NASA scientists since at least For the flu pandemic they hypothesized that cometary dust brought the virus to Earth simultaneously at multiple locations—a view almost universally dismissed by experts on this pandemic.
Hoyle also speculated that HIV came from outer space. The Lancet subsequently published three responses to this letter, showing that the hypothesis was not evidence-based, and casting doubts on the quality of the experiments referenced by Wickramasinghe in his letter. When the discovery was announced, many immediately conjectured that these were fossils and were the first evidence of extraterrestrial life — making headlines around the world.
Public interest soon started to dwindle as most experts started to agree that these structures were not indicative of life, but could instead be formed abiotically from organic molecules. However, in November , a team of scientists at Johnson Space Center , including David McKay, reasserted that there was "strong evidence that life may have existed on ancient Mars", after having reexamined the meteorite and finding magnetite crystals.
Geologist Bruno D'Argenio and molecular biologist Giuseppe Geraci claim the bacteria were wedged inside the crystal structure of minerals, but were resurrected when a sample of the rock was placed in a culture medium. The labs found 12 bacterial and 6 different fungal species in these samples. The fungi were Penicillium decumbens , Cladosporium cladosporioides , Alternaria sp. Out of the 12 bacterial samples, three were identified as new species and named Janibacter hoylei after Fred Hoyle , Bacillus isronensis named after ISRO and Bacillus aryabhattai named after the ancient Indian mathematician, Aryabhata.
Find the stellar luminosity that corresponds to the stellar mass that you labeled in question N. An earlier implicit mention was by Konstantin Tsiolkovsky in an unpublished manuscript from
With funding, within 50 years we will certainly be able to do that. The motivation may be ethical encouraging humanity's independent development or strategic aliens wish to avoid detection and possible destruction at the hands of other civilizations. This possibility has been examined in terms of percolation theory: colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the fact that colonies will inevitably develop a culture and civilization of their own.
As Charles Lineweaver has noted, planets began forming in our Galaxy as long as 9 billion years ago. The outer "fusion layer" was in fact glue. It is now known that extremophiles , microorganisms with extraordinary capability to thrive in the harshest environments on Earth, can specialize to thrive in the deep-sea,    ice, boiling water, acid, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life. How will that help us?
In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. This idea is most plausible if there is a single alien civilization within contact range, or there is a homogenous culture or law amongst alien civilizations which dictates that the Earth be shielded. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close range communication with the discovered civilization.
Therefore, averaged over all universes, universes with civilizations will almost always have just one, the first to develop. The intelligent species has managed to survive major environmental changes such as ice ages and has built a civilisation. Television, as developed in the mid-twentieth century, employs transmitters with strong narrow-band carrier signals that are perhaps the most detectable human signals at stellar range; however digital television is replacing this technology and uses wide-band spread spectrum modulation with much lower carrier power. Even life in the ocean depths, where sunlight cannot reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. Have your views changed? We also live in a highly fertile Galaxy that's friendly to extremophiles.
The idea that someone, or something, is destroying intelligent life in the universe is well explored in science fiction, for instance. This includes both efforts to find any indication of life,  and efforts specifically directed to finding intelligent life.
SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory , Earth's television and radio broadcasts would only be detectable at distances up to 0. One of the factors that scientists believe to be necessary is a long period of relatively stable climate resulting from a stable planetary orbit at just the right distance from an appropriate type of star. Imagine that Lmax1 is greater than Lmax2 for a star, which value sets the limit for how bright a star can be and still have complex life on a nearby planet? There is a possibility that archeological evidence of past civilizations may be detected through deep space observations — especially if they left behind large artifacts such as Dyson spheres — but this seems less likely than detecting the output of a thriving civilization.
It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Signals might be sent from non- main sequence star systems that we search with lower priority; current programs assume that most alien life will be orbiting Sun-like stars. In this view, the aliens have arrived on Earth, or in our solar system, and are observing the planet, while concealing their presence.
Dyson spheres might have different emission spectra depending on the desired internal environment; life based on high-temperature reactions may require a high temperature environment, with resulting "waste radiation" in the visible spectrum, not the infrared. Thus, even if intelligent life expands elsewhere, it remains statistically possible that terrestrial life will go undiscovered. Those species which did survive had their numbers drastically reduced. The giant planet Jupiter is more than times the mass of the Earth. To build granite you need a planetary subduction [or plate tectonic] process. Tipler in extending Hart's arguments.