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Oh, How Our Cities Will Shimmer
The colourful weirdness of our urban future - or maybe not so much?
Before we begin - I’ve been wanting to link to a particular edition of’s The New Fatherhood newsletter for months now, and I’ve never quite got round to it, but I should have because it’s lovely and quietly profound, so today is that day.
It’s about what a rare and precious thing it is for us to all be here right now, and why it’s worth taking a moment to properly appreciate that fact:
No, it’s fine, there’s just something in my eye. I’m fine. Let’s move on.
Today we return for the last time to this season’s main topic: the practical science of colours. And if you’re still flinching from the horrors of the last one on Scheele’s Green, I promise this one will be a lot less traumatic. Trust me. You’re in safe hands.
Anyway, let’s begin by setting fire to everything.
In late 2021, a lot of people on social media became convinced that the best protection against forest fires was to coat their homes in tin-foil.
This may seem ludicrous, but there seemed to be proof: a photo from the small community of Phillips, near California’s South Lake Tahoe, taken in the aftermath of the colossal Caldor Fire that burned for over two months and consumed over a thousand buildings - but not the one in the picture being shared everywhere, which seemed to be clad in shiny foil, and stood intact, near the burnt-out shell of its neighbour. (The internet quickly nicknamed it the ‘Baked Potato Cabin’, because of course it did.)
So - could fighting off a fire be that simple?
Answer: um, sort of, ish? It’s true that a silvered surface can reflect a goodly amount of convective and radiative heat, the kinds that make short work of most structures in a big fire. But this wasn’t the aluminium baking foil we Brits use in our kitchens - this was double-layered aluminum-lined fire blankets, enough to protect your home for the five or ten minutes it takes for a wildfire front to roar over, but not much longer than that.
(And this isn’t really news: the US Forestry Service uses similar materials to protect historic cabins and other buildings in hard-to-access places.)
The owner of this surviving cabin, Eric Raymond, had a big advantage over other homeowners - he could silver-clad his home for free, getting permission to use a few unneeded rolls of thick aluminum foil from the Elk Grove vacuum valve manufacturer where he works as an engineering manager.
Over seven frantic hours, he banged around twelve hundred staples into the three most vulnerable sides of his mountain cabin, took some pictures in case that was the last time he’d see it, and left - and was as surprised as anyone when he discovered the firesheet-wrapped cabin had survived.
This isn’t really a practical solution to wildfires. It’ll give your building more of a fighting chance, but if a fire is slower to pass through, it’ll eat up pretty much anything. But it’s a nice reminder of the power of reflecting the infrared parts of the spectrum, which we know as radiated heat.
I started this season’s journey into the science of colour by looking at the blazingly white limewashed walls of the buildings in the Greek islands that make so many hillside villages look absolutely gorgeous:
So it feels fitting to circle back there to finish the season, by asking: hey, putting personal tastes aside, what’s the most useful colour to put on a building?
As I wrote about in October, the ancient Greeks and Romans and Egyptians used to splash colour around in a way we’d find, well, a bit tasteless - and they weren’t the only ancient peoples to do it. Colour for decorative reasons is never going out of fashion - and now we have branding, which is why, for example, KFC stores are that particularly aggressive, or if you like welcoming, shade of red, and so on.
But there’s an obvious reason that many buildings in the Mediterranean are blazingly white: it helps keep the heat off in a way that’s affordable to everyone.
(Yes, there are more sophisticated methods at a higher price, but if a technique’s only available to the wealthy, how fair is that, and since those wealthy own a small number of all the buildings in the world, how effective is that? Isn’t the aim to reach as many people as possible, to scale up the overall effect as widely as possible? Isn’t this supposed to be about everyone?)
Generally speaking, the darker the material, the more it absorbs heat. This is already a problem in many cities, and with the world warming up as it’s doing right now, it’s only going to get worse.
For example, roads. Fresh asphalt only reflects around 5% of the sunlight hitting it, so it can get up a shoe-melting 50 degrees hotter than the surrounding air temperature.
Concrete, estimated to be the most widely-used substance on the planet apart from water, is a problem in all sorts of ways, but one is how dull it is, optically speaking. It dominates our cities and it absolutely guzzles heat, helping create what are now called urban heat islands - settlements that are noticeably hotter than the surrounding countryside.
(And to be clear, this isn’t blaming concrete as such - it’s transformed a lot of lives for the better, simply because it’s empowered billions of people to acquire cheap homes for themselves. Concrete is not innately “bad” in this regard - but it certainly causes a lot of environmental problems.)
The wider problem is rising global temperatures, and there’s plenty that should be done about that - you’d best go read Bill McKibben’s newsletter for the details - but in terms of alleviating the symptoms in cities, speaking very broadly, there’s two things in particular that seem to work really well: planting loads of trees, and painting all those concrete surfaces a different colour.
For example, in 2017 the western Indian city of Ahmedabad ran a pilot scheme where 3,000 rooftops were painted with a mixture of white lime and a reflective coating. This cooled the buildings underneath by between 2 and 5 degrees Celsius. Something similar is happening with New York’s tar roofs, under the CoolRoofs programme - and Los Angeles has coated some of its streets in a light grey paint. It helps, and it works.
But - how white can white get? Not a social question, but a scientific one: what’s the maximum possible reflectivity of white paint? A team at Purdue University in Indiana has been experimenting with different compounds that absorb less UV light than usual paint materials, and in 2021, they created a paint that reflects 98.1% of all the sunlight landing on it - which means it could cool the surfaces it’s painted upon below the ambient temperature, without aircon. Pretty spectacular!
But I’ll tell you one thing from experience: walk down a white-painted street in the Greek islands on a blazingly hot day, and your eyes are going to get a battering.
All that reflected light still has to go somewhere, and where it goes is into your face. Simply bouncing the sunlight off would condemn future inhabitants of our cities to the urban equivalent of snow blindness, unless everyone wears the kinds of cool sunglasses that science fiction’s been telling us everyone in the future should be wearing. Nevertheless: doesn’t feel like a complete solution.
So how about less reflective colours that deliberately absorb heat - then process it into an energy source? This seems a very fruitful avenue of research, and I’m sure solar panels are the uppermost tip of the iceberg here.
But it could be that the most useful colour for buildings of the future is - well, all of them, all at the same time.
You know that gorgeous wash of colour you get on the side of a soap bubble
when you’re washing the dishes and you really froth the water up, and then your girlfriend asks you to stop making a mess, but you’re having great fun so you reply defensively that you’re doing a science experiment that’s going in your newsletter…..
You know soap bubbles? That shimmering rainbow on their surface is called iridescence, and it’s a different way of creating colour. Basically, as light enters the side of that bubble, the light impacting it it head-on travels a shorter path through its surface than light entering at a wider angle which creates different wavelengths undergoing constructive and destructive interference, and - uh, some science happens, and so we see all those different colours. That’s how it happens. Yes.
This is called structural colour, because it’s created by the way light interacts with the structure of a surface.
There’s a butterfly in the Amazon called the blue morpho, and when I say blue I mean the most intense and luminous of blues you’ll ever see. Only thing is, blue is an incredibly rare pigment colour in nature, so this butterfly cheats: it actually has dully-pigmented, almost colourless wings that appear blue when the light hits them. This is done via diamond-shaped scales that create a wavelength-specific form of iridescence, which is that piercing blue.
The research team at Lexus recently tried a similar technique to coat a model of car, and they created a luminously blue finish that doesn’t contain any blue pigment, all for the low low low price of $100,000.
But this matters beyond the shiny toys for wealthy people, because in theory, structural colour could be changed on the fly. Remember when I talked about new technologies for changing the colour of things? I said this about its potential for buildings:
“This could be an incredibly useful thing for the outsides of buildings for heat regulation. Just imagine it - on a really hot and sunny day, the outside of your home brightens up to reflect more heat away and keep the indoors a bit cooler, like the white-painted walls of homes in the islands of Greece - and on a bitterly cold day, it darkens to absorb as much heat as possible. Maybe even instantly, in response to the sun coming out from behind a cloud. Imagine seeing a whole street doing that, or a city, in response to a sunbeam. Wouldn’t that be trippy?”
But maybe nobody will want that. And we certainly won’t want urban snow-blindness either. As fun as strange-looking building are, if everyday office blocks started shimmering with iridescence…well, just go and watch the film version of Jeff VanderMeer’s Annihilation and see how creeped out you feel. (But also, please read the books too, they’re even eerier.)
I reckon there’s a good chance we’ll just want most of our urban infrastructure to remain that reassuringly boring everyday range of colours we’re all used to (the nice ones, anyway). Is it possible to do that and help reduce temperatures inside, or absorb heat for energy-generating purposes? Is it possible for everyone to be happy here?
The answer might be yes, if artificial iridescence fulfils its promise. This new wave of artificial structural colours that can keep surfaces cool - they don’t actually have to be coloured white, or be light colours at all.
For example, there are the coatings that Yuan Yang and his team have developed at New York’s Columbia University, using an iridescent layer to reflect 90% of the infrared light, and another layer over the top of it that’s regular old pigmented paint - so onlookers get the visual benefits of a fashionable colour within our visible spectrum, but outside of those wavelengths, it’s also blocking incoming heat. A black version of this was seen to keep surfaces almost 16 degrees cooler than normal black paint.
So maybe what’s going to happen is - well, not much, at least to our eyes? If so, the colours of the outside of new buildings aren’t going to get a lot weirder, but under the surface (quite literally), structural colour could be keeping things a lot cooler inside our homes. If that can be produced at a big scale, that would be great, considering the energy spent on aircon around the world is already about 10% of global electricity consumption. Putting a meaningful dent in that would have a big effect.
But if you’re a bit disappointed that the future might not look like one of Salvador Dali’s nightmares, here’s a throwaway paragraph from theoretical physicist Michio Kaku’s book The Future Of Humanity, a speculative hard-science look at the next century or so.
In this section he’s talking about graphene, a form of carbon that is incredibly strong - if it’s laid down in a layer one atom thick, it’s over a hundred times stronger than the best steel at that same thickness (making it stronger than diamond). Graphene also conducts electricity, weighs practically nothing, and has some…really weird optical properties, to say the least.
Here’s Michio Kaku:
“Carbon nanotubes are sheets of graphene rolled into long tubes. They are practically unbreakable and nearly invisible. If you built the suspension for the Brooklyn Bridge out of carbon nanotubes, the bridge would look like it was floating in midair.”
If you want to see what this might look like in action, here’s an underwater carbon nanotube “invisibility cloak” being demonstrated in 2011:
What’s happening here isn’t ‘invisibility’ in the Harry Potter sense - it’s a nanotube-induced equivalent of the light-bending heat shimmer you can see above roads on a hot day, or the Fata Morgana mirage I wrote about in season 3.
But however it’d work, there’s certainly a weird thought: practically invisible buildings. Or parts of them, invisibly holding up other parts in a way that makes the whole structure look like it’s about to collapse.
How would we feel about that? I bet influencer thrillseekers would take to them in the same way they’ve eagerly leapt into those glass-bottomed swimming pools hanging off the tops of skyscrapers - but the rest of us? Not so much?
(Maybe they’d end up painted to make them visible, in the same way that digital SLRs or electric vehicles have artificial sounds to add that reassuring shuttery *click* or prevent us from being run over.)
However our cities of the future will look, it seems they’ll be using colour in ways it’s hard to imagine right now, keeping us cool, lowering heat island temperatures and even turning currently-wasted heat into the juice to power our homes - even if the overall effect might not look too different to what we’re used to right now…
Or hey, maybe it will, and we’re in for a lot of eyepopping weirdness!
(I’d like to cast my vote for buildings swiss-cheesed with holes for nesting birds. That feels like a future worth having.)
Next time: Season 5 is a wrap! What’s next?
“How the shimmering secrets of iridescence could keep us cool” - Michael Allen, New Scientist
“How one foil-wrapped home survived the Caldor Fire as everything around it burned” - Michael Cabanatuan, San Francisco Chronicle
“‘Mirage-effect’ helps researchers hide objects” - EurekAlert!
“The white roofs cooling women’s homes in Indian slums” - Vandana K, BBC Future