10 December 2015

Sci-fi tropes: Nanobots can't exist...

I've not had a lot of time to write a review of any of the games I've been playing but I did want to publish at least one article a month and so we're stuck with a simple post from the meanderings of my mind. (And now I'm late because I didn't manage to get this post done in time for the end of November! :) )

Take a look up there at the title. It's a little inflammatory and simplistic but it's also true. Let me summarise the point of this post neatly:

Nearly every depiction of nanobots in sci-fi and popular culture is incorrect. 

It's a terrible problem - mostly because the general public don't realise the gross over-exaggeration of nanobot capabilities and so we get ridiculous 'scenarios' like the 'Gray Goo' story mentioned in the last Sci-fi tropes post.

If you're a chemist (and perhaps biologist) you'll have a good understanding as to just why these sorts of scenarios are very unlikely, if not technically impossible, and the beginnings of the evidence is listed right there in the Waitbutwhy article:

"this 1-100 range encompasses viruses (100 nm across), DNA (10 nm wide), and things as small as large molecules like hemoglobin (5 nm) and medium molecules like glucose (1 nm)." 

That's a pretty good indication of the size of certain types of chemical/biological entities. However, you've got to look at those molecules' capabilities to put this scare and the sci-fi into perspective. On their own, glucose and haemoglobin don't do much of anything at all... and haemoglobin only has one function - the interaction with oxygen.

Let's take that up a notch and switch to an enzyme - something that we know does something rather than just existing as a simple chemical block waiting to interact with something else in a kinetically or thermodynamically favourable reaction. Something like glucose oxidase might be of the order of 6 x 5 x 7 nm. That's 210 cubic nanometres! Relatively huge compared to haemoglobin that has a volume of around 90 nm3 and yet it is still only able to do one thing...

"But wait!" You might say, "DNAs are the building blocks of most species on the planet! (if you exclude certain primitive RNA-based forms). If we can construct something of that size we can do pretty much anything!"

Well, not really. DNA doesn't exist in a vacuum - it only functions as part of a nucleus in a cell... The diameter of a nucleus for a complex species is around 2 - 10 um. That's 2000 - 10,000 nm. That's a volume of 33 US billion - 4,200 US billion nm3. Even if you ignore the volume and just go on diameter you're way beyond the term nanobot now... You can't even really take DNA out of the nucleus because it gets oxidised and broken up really easily - hence the reason for the nucleus in the first place from an evolutionary perspective!

PLUS, expression of genes is highly dependent on various factors like hormones and steroids which means that if we want to make a nanobot that can do more than one thing it will need a whole load of supporting systems in order to work at which point it starts to become a genetically engineered organism and no longer really of nano-sized proportions.

But let's say that you only want your nanobot to do one thing: Say it has to break down matter like the grey goo example so that it's self-replicating (ignoring the fact that function in isolation is pretty worthless); seems simple enough, right?

A superlative plan but there are just two minor drawbacks to that:

1) Thermodynamics


2) Bohr's model of the atomic structure and electronic shell structure

You see, it turns out that the laws of nature are pretty well defined and chemical reactions require a certain amount of energy (or release a certain amount of energy if the end product is more thermodynamically stable) to work.... even then, a large portion of chemical reactions require a kick-start to reach a starting point whereby the rest of the reaction becomes favourable. This energy isn't free and it doesn't normally happen to be lying around either. Luckily for humans (and most other life on Earth) chemical entities like enzymes and other catalytic-type complexes were formed and evolved to reduce the energy required to perform a certain chemical transformation operation (e.g. reducing a nitro group or breaking a particular bond). Without those adaptations, life would be much more simple (if existing at all) and less able.

Of course, you can just brute-force the destruction of a chemical entity but that requires an even larger, huge in fact, amount of direct energy. No nanobot could even harness or harvest these energies to break down all compounds or even just biological carbon compounds.... nor could it have all the necessary bits and bobs needed to reduce the activation energy for every particular reaction pathway to remake the myriad biological carbon compounds in a human body into whatever form the nanobot took.

If it were that simple to make a molecule (even a complex one) then chemistry and biology would be more than easy. They almost wouldn't even constitute having to be learned and we'd have wonders unbounded in the world we live in.

Speaking of energies. A nanobot would need to have an energy source... and be able to store it for periods where it was no longer receiving that energy. Guess what? Molecules themselves do not have this ability. Yes, molecules can be used to store energy by changing electronic and/or chemical state but this ability needs to be linked into other molecules that make use of these changes. If this is sounds like the sort of thing that goes on in animal and plant cells that's because it is!

Guess what? If a cell nucleus is too large to be a nanobot then a whole cell is magnitudes larger than it should ever be to be called a nanobot. To give you a size example: a human muscle cell is around 1,000-40,000 um in length and 10-50 um in width... That's just one type of cell and that doesn't exist in a vacuum - it needs transport cells to get the energy units to it and it's the same for all viruses and bacteria - (although bacteria might need chemical constituents instead of energy if they photosynthesise but it's the same premise). Again, we're brought up short in our ability to operate anything remotely complex at even the micro-scale, let alone the nano-scale!

Even then these cells, bacteria and viruses are hugely limited in their effective range. The world hasn't been wiped out by any of these things, even though they all each have the individual ability to self-replicate and attack other types of cells.

One final thought on all this before I come to the inevitable conclusion: viruses can be of the order of 400 nm in diameter. Yes, you could say they are nanobots. However, there is this thing called the immune system in most animals. If a virus can be identified and attacked and neutralised by the immune system - why wouldn't a nanobot?

Sure, some whacky-advanced bioterrorist might release a nanobot plague into the populace in order to reduce them to goo... but it would be stopped by our immune systems ganging up on it and destroying it before it would even get anywhere.

As a science fiction device, nanobots are interesting and appealing. However, there's just no way they can exist as they are portrayed. Even the more realistic healthcare evangelists are exaggerating the capabilities of nano-devices. Drug delivery entities? Sure! That's a possibility. Beyond that? Not much - and certainly not alterations on a massive biological scale...

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