The silicides would meanwhile say that the gaseous nature of CO2 makes carbon useless as a basis for life, and even go so far as to offer a plausible reason.

Sorry, no. CO2 being gaseous is a major availability and energy advantage. For example you can’t get plankton equivalents as dissolved silicon is parts per trillion at the oceans surface.

Hybrids are a different story, but I doubt that’s what anyone is talking about by silicon based life forms.

CO2 (or SiO2) is only one of the possible outputs, or inputs, of metabolism. Even on Earth its primacy came very, very late.

You don't get to constrain alien life to your preconceptions. Even some Earth life secretes SiO2. Using carbon for some things would not invalidate its Si-basedness any more than diatoms have given up carbon-basedness.

Silicon's near absence from our favored solvent, like iron's, is a product of toxic levels of biogenic atmospheric oxygen. Our early oceans were saturated with iron products, and are now starved of them. There is no reason to assume oxygen would be their favored oxidant.

> There is no reason to assume oxygen would be their favored oxidant

That goes back to astronomy no other option is going to be nearly as plentiful.

> You don’t get to constrain alien life with your preconceptions.

It’s not a preconception it’s the solubility of silicon and silicon compounds. Yes, CO2 and SiO2 aren’t the only option for respiration, but CO2 was and is a near endless supply of carbon and oxygen. Without that growth simply slows down dramatically. Even with a photosynthetic equivalent you simply don’t get the great oxygen event from silicon lifeforms in even 10 billion years.

PS: To be clear I am not saying silicon life would be impossible just that it’s at a huge disadvantage head to head and would have a vastly slower progression to large scale multicellular life.

Relative plenitude did not dictate the elements we and our relatives use, and would not for other life, either. In any case, many other elements are plentiful.

Carbon is a good choice for our temperature range, but there are plenty of temperature ranges as well or better represented. Life will not sneer at the others. Most life on Earth cannot make any use of the carbon in CO2, no matter how much of it there is; in fact, the more of it there is, the worse off most living creatures are, only green plants excepted. You would die in a room with the nitrogen replaced by CO2, despite that you use none of the nitrogen at all. (Only bacteria can "fix" nitrogen, and only some of them.)

Iron is very plentiful in the cosmos and in Earth's constitution, and most life depends on it, but it is practically nonexistent in the open ocean; yet life thrives there. Even where iron is plentiful, life uses hardly any. Silicon is extremely plentiful, too, yet used by our life even less -- until just lately!

So, your argument from plenitude utterly collapses. Life as we know it is a product of history and circumstance. Given different circumstances and different history, you should expect a different product. There will certainly be similarities dictated by physics: e.g., cells are very, very likely, and equally so use of liquids. Aperiodic fibers to encode genes, cognate to our DNA, and others to fold up and carry out processes, cognate to our peptide chains a.k.a. proteins, are quite likely. But there is no reason to imagine that any of the details of the encoding would match.

> Even where Iron is plentiful, life hardly uses any

That’s a huge misconception about biology and evolution. Basic cellular biochemistry is extremely well conserved. There is almost no pressure to use more iron, but significant pressure to use less. Photosynthesis for example is Iron dependent and that acts as a major limiting factor on ocean ecosystems. Yet, we don’t have an iron free alternative adapted to utilize that giant ecological niche. Life is seemingly using as little as possible.

That should suggest why primarily silicon life is such a poor option.

> There are plenty of temperature ranges as well or better represented.

For complex life it’s not just about temperature but also pressure and access to sunlight. Anyway, the normal argument is that silicon based life could thrive in extremely high temperatures. That’s all well and good for silicon but not so much the other chemicals it’s interacting with. Life is dependent on being able to reliably create and break relatively weak chemical bonds. However, when you start to look at the actual chemical options at those temperatures you run into serious problems. The kind of large molecules you want for complex life are either unstable or too stable at those temperatures.

Iron is not the key element in chlorophyll; magnesium is.

We don't know of many interesting Si compounds specifically because we have none of the organisms that would produce them. Our ignorance does not constrain nature.

"Weak", for bonds, is exactly a function of temperature. At higher temperature, life will have chosen constituents that bond with the right strength, wholly ignoring your preconceptions of what it ought to be using.

Chlorophyll contains magnesium, photosynthesis requires Iron at about 1000 magnesium atoms per iron atom. However, Iron still ends up the limiting factor in ocean ecosystems. https://zenodo.org/record/1258477

Anyway, Life can only chose chemical bonds that are possible. It’s perfectly reasonable to pick a specific temperature range and say here this is ideal for silicon lifeforms, but that means every chemical interaction must occur in that temperature range which is a major physical constraint.

We don’t know of many interesting and relevant Si compounds in large part because they don’t exist. It’s not even just chemical compounds life needs a water equivalent for all that cellular machinery to float around in and bump into each other.

Again: Relevant Si compounds don't exist here, because there is no Si-based life here. Our favorite carbon compounds would identically not exist in a Si-based ecosystem. That the useful compounds would not be simple substitutions of Si for C is of no importance. As I noted, some liquid-analog is likely involved in any spontaneously evolved ecology, but it does not need to be water, or even, technically, liquid-as-such.

Nature is not constrained to the limits of your imagination. I am frankly surprised to find you continuing to insist it is.

The tangible existence is irrelevant, computational chemistry is more than capable of exploring this territory. The issue isn’t simply finding giant complex molecules that would make up such hypothetical lifeforms it’s looking for the basic building blocks.

In the end all of chemistry comes down to quantum mechanics it really is quite constrained. Put another way you can list our every possible 2-9 atom molecule containing Silicon plus common elements and look at how the behave.

And, you claim to have not just made such a list, but also explored all of their possible interactions with one another and with what could be common non-Si compounds in their vicinity? You and what ten thousand universities?

You are just making things up. Who do you imagine believes you? Why do you want them to believe you?

No, can is not did. My point was it’s a finite space.

People have explored some of this space and found nothing.

"Sorry, no. CO2 being gaseous is a major availability and energy advantage"

At room temperature. Silicon life, if it exists, would be basically using lava instead of water. Glassblowers work at 600 degrees, i imagine thats the sort of temperatures where we will see any silicpn chemistry take place at non glacial pace

Silicon at those temperatures is still not a gas. But more interestingly photosynthesis equivalents become drastically less efficient at those temperatures. Life could still extract energy from chemical processes or even light, but ecosystems would be energy starved which slows down evolution. On top of this the chemistry capable of supporting complex life is again less efficient than carbon based life at lower temperatures.

Net result you might get the equivalent of very simple multicellular life, but intelligence becomes extremely unlikely.

Yet, there are simple Si compounds that are gaseous at such temperatures.

Chlorophyll photosynthesis does not work well at high temperature, but that says less than nothing about whether photosynthesis of different products using a different molecule and a different process could evolve. What we can say is that if it could, it would, somewhere.

You have no basis beyond your own preconceptions for any statements about likelihood. Thus, your estimate is 100% confabulation.

> Yet, there are simple Si compounds that are gaseous at such temperatures.

Silicon tetrafluoride might be a gas, but it’s not going to be plentiful. If you feel there are real options here feel free to list em.

> that says less than nothing about whether photosynthesis of different products using a different molecule and a different process could evolve.

Carnot efficiency does. You can’t simply scale up temperature and expect equivalent efficiency.

It's an interesting point.