pac0master said:
This is something that got me thinking for a while.
Especially after learning the basic of physics and chemistry.
Basically, pretty much every movies, novel, etc that feature an advanced race of aliens(or any alien in general), they gives them some sort of technology or their organic composition that is made of materials unknown to us.
Since the Periodic Table is pretty much "complete" (no missing(space) between elements) It would means that they build pretty much everything from Very heavy/dense elements right?
"materials" are made of molecules, not just elements. So the "unknown material" could just be a collection of molecules that is not currently known to the people in the movies. ie an flask of fullerene, or anything made of carbon nanotubes, would have counted as an "unknown material" if found on the Roswel craft. Mind you - it would probably get misidentified as something else ... and only show up as odd under closer examination.
The main question I have at that point is, "how did they determine the material is unknown"?
Is it just unknown to them at the time or did they manage to eliminate every possible combination of atoms and molecules in the time spent checking?
To put this in perspective: get some sugar and some butter in a pan and heat them up till you get a brown sticky substance. That is caramel.
Caramel is a very complicated substance - iirc not all the constituents have been identified, though the main ones are well known.
http://sciencegeist.net/the-chemistry-of-caramel/
... does caramel contain materials unknown to science?
However, it is also common for the script to say that the elements themselves are unknown, and that speaks to your question:
Wouldn't that mean that they are basically very radioactive and unstable?
... we do not know if there is another stability valley somewhere off the end of the periodic table, or if there could be an extension of current physics that would allow very heavy elements to be stablised. It may also be that the unknown elements are made of exotic matter, or dark matter - or some currently unknown kind of stuff.
(There is a kinda stability valley when some other force like gravity can help hold the nuclei together - see "white dwarf" and "neutron star". Maybe an undiscovered 5th force is responsible?)
If it's right, it would mean that any actual aliens would have pretty much the exact same materials available as us.
...yes. The alien materials must be available to us or they would not appear in the movie. They would be materials that are, in principle, able to be manufactured by us... well... the usual movie ones would.
How about magnetic monopoles or quantum black holes ... these may be things that cannot be made by humans but may be discovered as remnants of the Big bang. Perhaps the aliens evolved near a large collection of them? In that case, the material would not be so readily available to us.
You can also consider the AK47 ammunition in
Guns of the South, the Confederate gunsmiths could recognise the constituents of the modern gunpowder but had a great deal of trouble manufacturing any with the tech they had available.
I'm alright giving them the benefit of the doubt of a superior technology so they can create different isotopes which could result by a better and more efficient material.
The question to ask is "how do the characters know the material is unknown?"
The best they can get is that the material does not register as anything whatever tests they ran on it could identify.
We see this in weird claims quite a lot: someone finds a bit of stuff under their skin and gets someone else to put it through a spectrograph ... the spectrograph does not know what the material is so they report it as "unknown".
It's a bit like how some DNS addresses get flagged at server level as "martian".
I always feel like the authors of such scenarios went the lazy path and decided to use the excuse that it's sci-fi so it doesn't matter.
Yep. It's like invoking magic.
There are some clear rubbish bits like when a spacecraft hull is made of degenerate matter (the author has heard it is the densest stuff known and, by extension, hard to penetrate with anything) not realizing that degenerate matter requires a really strong field to hold it together. The writer has been impressed by stories of how much a teaspoon of the stuff would weigh without thinking you can't get just one teaspoon of it. If the aliens have solved that problem, they probably don't need physical spacecraft .
Some authors get around this by having the supertech be a discovered artifact [see: Niven (1985)
Footfall]... a bit like apes are able to swing on nylon ropes but cannot make them.