One thing people tend to forget about "alternate biochemistries" is that while they might be possible, they tend to be quite chemically limiting. For example, water surpasses methane as a solvent, and its liquid range is at a far higher temperature, allowing faster/higher chemical interaction. Methane is also liquid over a short temperature range, which makes an ecological reliance on it pretty tricky.
Silicon also has problems. For example, it is not as chemically active as carbon, is less abundant, and its oxide (silicon dioxide, or
sand) is not soluble in water.
It is for reasons like these, among others, that make it improbable for 'alternate' life to evolve into complex, advanced organisms that might build spacecraft, for example. It is not narrow-mindedness, rather, they are at a simple and clear chemical disadvantage to life similar to us. I'm not excluding anything that is not carbon-and-water-based, but rather excluding most, if not all, of the currently proposed biochemistry concepts.
Life existing on any of the noble gases is, dare I say it,
impossible. These are gases that are inert, they will not even interact with the environment or form any sort of biologically useful chemicals, let alone be metabolically advantageous.
It is also not egotistical to suggest that life chemically similar to our own will predominate. Hydrogen is the most common element in the universe. In our own galaxy, it is followed- after inert Helium, by oxygen (third most common element). Carbon is the fourth most common element. Nitrogen is the seventh.
All the components of CHON are in the top ten most abundant elements, and all but one of the components of CHONPS are in the top ten- sulfur makes an apperance as #10, but phosphorous is nowhere to be seen (though presumably is not that far down the line). This is not critical as phosphorous is not required that much by living things, and even then, alternatives like arsenic are probably far less common (phosphorous makes up some 1000 ppm of the Earth's crust, arsenic is only 1.5-2.5 ppm).
That doesn't mean that extraterrestrial life could not be chemically novel- they could be made out of the same (or similar) chemical "building blocks", but arrange them in totally different ways. The side effect is that alien sirloin steaks could be fulling without having any calories (alternate chirality of the molecules), or could even kill you, act as an illicit drug, or be medicinally important (have natively common but otherwise unknown biochemicals with unforseen effects).
Stephen Hawking said that aliens might kill us all.
Stephen Hawking is a theoretical physicist and a cosmologist. He is not a historian, an anthropologist, a sociologist, or even a biologist...
His statement that they
might try to kill us all is valid, but whether they would or not is up to debate, can't really be answered by anyone with current data, and should not elicit scaremongering...
It's also egotistical to think that "sentient" has any real meaning in this discussion - there are likely many kinds of sentience that we simply wouldn't recognize as being sentient - we define sentience according to our own human perception of sentience.
The proper description for "intelligent" life is
sapient, not sentient. The word
sapience is derived from the Latin
sapientia, meaning wisdom. Related is the verb
sapere, which means- among other things- "to be wise and to know".
Sentience is ability to feel or percieve. Most metazoans have that ability to a degree, so it is nothing special. In my own (shameless plug here...) words, "sentience is awareness. Sapience is awareness of being aware."
I would not go as far as to regard a bacterium, a protist, or even a tree as sentient, even though they react and adapt to stimuli. I would not regard an ostrich or an octopus, or an antelope, as sapient, even though they display varying degrees of intelligence.
The appearance of "behavioural modernity" in
H. Sapiens some 50 000 years ago shows that there is something special that we have, that no other known organism has. Sapience is not a fuzzy line, but a trait that can be clearly defined.
It's not necessarily a trait that is unique to us, and it does not make us gods of the cosmos. But we are an example of the trait- and so far, the only known example.
Water is hardly a requirement. Consider that the Internet is a life-form already. It is silicon and petroleum based, more or less depending on what parts you look at.
The internet is no organism, it is a computer network. A vast, dynamic and in some respects life-like network, but still a nonliving information network. Individual pieces of software- such as some viruses- better fit the definition of life, but are not organisms themselves.
Nevertheless, a computer network or a malicious software program is something that is extremely unlikely to form naturally, with our current scientific understanding...
There is actually a scientific definition of life... it basically boils down to having heritable reproduction, meaningful metabolism, and a defined structure... Of all the three I can't even see one the internet fits partially.
Even if there are hundreds of advanced alien races, the odds of them noticing us are very slight.
I am doubtful. If they are interested in carefully catalouging star systems, then it's entirely plausible that they would have detected Earth, but they might not know explicitly that humans exist on the surface.
If Kepler (hypothetically) found a habitable planet some 100 light-years away, the scientific community wouldn't have a clue if the most advanced organisms on the surface were more like
Haikouicthys or
Homo erectus...
Of course, an advanced alien civilisation would have telescopes and such equipment that would be far more advanced than our own, but as you said- it depends on how interested they are. I think they will be interested,
to a degree- and some more interested than others.
...and even if we WERE being studied by some intelligent species, they'd probably be so far beyond us in technological abilities that chances are, we wouldn't even know we were being studied. Does an ant know when a scientist is examining it ? Even when handled, it has no concept of what's going on.
That argument is broken down by the mere fact that we can speculate about being studied by an advanced alien species, and that we can compare ourselves as subjects for scientific study, to ants.
Humans cannot be directly equated to ants, because of very fundamental traits... just like how an ant being studied is already vastly different to a bacterium being studied.
I recommend having a read of the Centauri Dreams blog. It has several articles about why SETI might be a failure, how our radio leakage, even back in the 1960's couldn't be detected without some very serious infrastructure and how SETI might be giving way to SETA of which there is some interesting circumstantial evidence.
The circumstantial evidence is what? 1991 VG?
I find the search for artifacts within our solar system alone to be quite limiting- the probability of an alien civilisation sending some sort of artifact to our solar system is probably very low- maybe speculation in this regard should offer the possibility of a SETA equation, similar to the Drake Equation.
If an interstellar probe is considerably more costly than blaring a radio/laser signal at someone (which it probably is), and very few civilisations want to blare radio signals at potentially inhabited planets (or so it seems, from our observations), then the number of civilisations that are sending out interstellar artifacts/have sent out interstellar artifacts is slim at best.
Of course that does not mean we cannot try to detect extrasolar advanced civilisations... an advanced civilisation should be relatively easy to detect from their energy emissions and energy collection structures.
