Alien city lights could be detected across interstellar space

Top boffins in the US say that it should be possible to detect alien civilisations on planets orbiting other stars by looking for the light of their cities standing out at night. Concept art depicting the lights of an ET civilisation on an exoplanet. Credit: David A Aguilar (CfA) Hiveplexes Three to Five sadly gave away the …

[Nigel 11]

AC?

Surely the AC modulation of the alien-made light emission would also help, if it existed. Physics is universal, so it's a reasonable expectation that the technological evolution of an AC power grid would be like ours, locking them into a frequency not very far removed from the 50-60Hz here. (It would also help that they are very unlikely to use exactly 50Hz or 60Hz, so manmade interference can be filtered out).

Only snag is that AC lighting may be a short-lived phenomenon. Anyone know if LED lights contain an AC-DC converter, or do they economize by not bothering with a capacitor?

And where's the light-bulb icon?

[Lee Dowling]

LIGHTING is a very short-lived phenomenon. A hundred years ago we didn't have "city lights" at all and what we did have were gas-powered and probably wouldn't have showed up in local orbit, let alone a distant galaxy. A hundred years from now, when we would have had to repair *everything* that currently exists on the roads several times over, not to mention thousands of building regulations, and you probably *won't* have lights visible from the sky.

If they think reflected shine from the Sun on the planet's face is hard to detect, how do you intend to detect a "light pollution aware" civilisation (we ourselves are still doing things like turning off streetlights in tests, fighting energy-saving bulbs, making sure that useful light is focused only on the lit subject and not random sky etc.) against all the others.

And two hundred years, hell, call it a thousand, is a tiny, insignificant portion of a race's evolution, especially on astronomical scales. Most of the places we look, the light takes longer than that to get to us, so if we spotted a civilisation just at the peak of its energy production, they've probably been long dead anyway and we'd never be able to see anything more interesting of them. Hell, if we invented a faster-than-light ship, zoomed over to them using all the money and production capacity we have, they'd be finished in it by the time we got there and outrank even us.

That's not to say that detecting a civilisation wouldn't be huge news - but detecting them off the tiny amount of light beamed from their planet into space for literally fractions of an astronomical second isn't really a sensible way to find them. Better to look for the civilisation that came a thousand years after them and who were able to modify planetary orbits etc. that are millions of times easier to detect.

[Nigel 11]

Not convinced

I'm not convinced that a technological civilisation which once felt the need for night-time illumination would ever give that up as long as it remained a technological civilisation. That, even if current human solar and LED technologies are as good as it gets.

Efficiency, yes, but that means using less electricity to generate much the same amount of light. Efficient street-lighting may illuminate only downwards, but a significant fraction of that light must reflect back into space. Eyes would be useless if it didn't.

We can calculate the available solar energy resource. It's greater than the entire current energy usage by humanity (even before you consider the possibility of powersats). Night-time illumination at human levels using efficient light sources does not need an impossibly large fraction of a solar-powered civilisation's energy harvest.

So looking for artificial light is a good idea, if it's detectable over many light-years. Any detection of any technology being deployed outside the solar system would perhaps be the single most significant observation of all time.

BTW, powersats. Would the microwave leakage be detectable lightyears away?

[Cryo]

I think looking for "artificial" light on other planets is a reasonable prospect, probably more so than radio transmissions, at least once the hardware exists for it.

I do think a civilization could go quite far toward optimizing its light usage as it advances though. While you're not going to make outdoor lighting completely efficient, it would certainly be possible to greatly reduce the amount of light pollution, even using technology we have on hand today. Most streetlights are left on all night, even those that are only being used 1% of the time. If they were designed to operate only when needed, they could be far more efficient. Imagine a city with a "smart" lighting system, where the lights would detect whether someone was in range by means of an RFID tag, or similar. Also, mass transit like trains, trams, subways and so on, could potentially replace the need for most lighted roads. Looking into the future, it's even possible that most city-dwellers might have light-amplification implants installed in their eyes centuries down the line.

I also think there's a potential for false-positives. Who is to say that there isn't some abundant plant-like organism on the planet naturally creating bioluminescence at night with a similar spectrum to what we would expect to be artificial light sources. Or perhaps a chemical reaction caused by some specific oddity of the planet's surface. It would definitely require lots of other observations to verify, though it could provide targets to focus those on.

[Harvardyard]

"...the civilisation that came a thousand years after them... " Everything you write is true except no one comes after them. No one will come after us. The probability of advanced civilisation is too infinitely small. Even the lifespan of humanoid activity on Earth is but a blink in cosmic time and unlikely to progress much further due to wars, the lemming urge or an "unknown unknown" to coin a phrase.

[OrsonX]

@Lee Dowling

your post makes for very depressing reading, I hope in some way that you are wrong in what you say, otherwise there is almost no point in even looking!

[Anonymous Coward]

"your post makes for very depressing reading, I hope in some way that you are wrong in what you say, otherwise there is almost no point in even looking!"

Of course there is ... There was a time in our past when we could only look across the oceans.

This is no different, we just don't have the technology yet - better to have some destinations when it does arrive (and it will (yes - I have blind faith in Humanity's better side)).

[Nuke]

@ Lee Dowling

Lee Dowling wrote : -

"if we spotted a civilisation just at the peak of its energy production, they've probably been long dead anyway and we'd never be able to see anything more interesting of them"

You are taking it lightly. Discovering any other life outside Earth. especially technologically capable life, long dead or not, reachable or not, would be the most profound event in the history of the human race. The imapact on the human psyche would be massive.

Spectral analysis of the planet's composition and atmosphere would likely establish that they could not be human, which would promptly scupper the basis of most religions for a start.

[Ken Hagan]

"Better to look for the civilisation that came a thousand years after them and who were able to modify planetary orbits etc. that are millions of times easier to detect."

No need. They will detect us. Having done so, if they want us to know about them, they'll make contact. If not, they will surely be able to arrange that we won't be able to find them.

But the essentials of your post are perfectly valid and much under-appreciated by the SETI fans. For any search that we conduct, the basic assumptions are EITHER that we've pretty much got all the technology that it is possible to have (and so looking for *our* technology *out there* is a reasonable strategy) OR that there's a civilisation not too far from us that just happens to be passing through the exact same hundred years or so of historical development whilst we are looking for them. Both assumptions are laughable. Therefore, so is SETI.

[Captain TickTock]

DC

If Edison had had his way, we would have had a DC distribution network.

(HVDC distribution is becoming a reality in some applications)

[Anonymous Coward]

Too long an integration interval

You would need to be able to sample the light at roughly the AC frequency. Astronomical telescopes sample light over VERY long times - minutes to hours. AC may as well be DC for that.

[Daniel Evans]

@Too Long An Interval

Actually, you can get some pretty speedy astronomy these days - pulsars are measured down to periods of 1.4ms, and sampling theory states you'll need to get a snap less than every 0.7ms for that.

You are right to say that you'd get nowhere near 60Hz for something as faint as city lights, though.

[sisk]

Alien civilizations may not have a Nikolai Tesla to promote AC before a DC based infrastructure could be build. Once the DC infrastructure is in place it would be highly unlikely that they would switch to an AC based one. Kinda like how one of the biggest obstacles in moving past gasoline powered cars is the lack of high speed charging/hydrogen/natural gas/whatever-else-you-want-to-burn stations to replace our gasoline stations, only on a much more costly scale.

[Anonymous Coward]

Assuming

That alien life forms even need lighting like we do.

[Ryan 7]

Assuming

That aliens even require light, or even 'see'.

Any chance of a telescope that can find large patches of echolocation?

[Tim #3]

Also assuming

That they are even alien species. A distant colony of badgers for instance...

Now where's my Sunday Sport?

[Ian Ferguson]

Dubious

This makes the major assumption that aliens see, and that they use a similar spectrum to us to do so.

Also, it's just as bad as looking for analogue radio signals; there's probably only a very small period in a species' history where their light would be visible in this way. Our cities' lighting would only have been visible for the past hundred years or so, and we're already aware of how much energy we're wasting by letting the light bleed off up in the air as well as down on the ground.

[Chris Miller]

Not so dubious

In an illuminated environment, being able to detect things at a distance (without announcing your presence, e.g. by echo-location) is a useful adaptation. Eyes have evolved independently at least 5 times on Earth, so it seems quite plausible they would evolve elsewhere too. They'd naturally be optimised for the local illumination, but most stars have peak luminance in the visible (at least, those where we think life would be most likely).

[RayG]

See, and similar spectrum - this is not a dubious assumption.

Sight has evolved several independent times on Earth because it's too useful not to. Plenty of species on earth have different sensory ranges than humans but they're all in the same basic region - not just for evolutionary reasons but for physics ones. If there is life on other planets the biology and evolution will be different but physics should be the same.

Frequencies of the electromagnetic spectrum that are significantly higher (mid UV and above) are energetic enough to affect a much larger proportion of molecules a lot more rapidly than visible light (ie cause damage). Too much of them and it's not likely there will be anything around to have eyes at all. Too little and it's not worth evolving the ability to sense that portion of the spectrum.

Frequencies that are significantly lower (mid infrared and below) are sufficiently un-energetic that they are hard to sense, let alone image accurately.

We also know that there are a lot of stars out there putting out light in the visible spectrum. Plenty in the non-visible spectra too, but the above reasons make that less desirable.

However, I agree that wasted light is likely to be a brief phase for technology reasons.

[Neil B]

It doesn't assume *all* aliens need light to see, but it's a fair bet that some of them do.

[JeffyPooh]

Assumptions...

Assumed wavelength? Please don't answer "visible light".

Also, an intelligent species would ensure that their exterior lighting is aimed down, instead of lighting up the sky. Counter-examples (i.e. idiots) are readily spotted on *this* planet. Thankfully the local government here (NS, Canada) is switching street lighting to LED - lighting that is efficiently kept out of the sky. It's fantastic! Great lighting and dark skies! :-)!!!!!!!!!!!

[Nigel 11]

Visible / near IR band is universal

See my post below.

There's a transparency window between where UV starts getting absorbed by ionising molecules, and where IR starts being strongly absorbed by exciting vibrations of molecules. The next transparency window is right down at high microwave-radio frequencies and there's no strong natural source of those for passive vision.

Our visible is tuned to the emissions from our star. I'd expect for life around a redder star, their visible would exclude everything above yellow, and include some or all near IR (which we use for line-of-sight IR network links and suchlike).

[Nuke]

@ JeffyPooh

I'm not clear why lighting has to be by LED for it to directed downwards instead of half of it blasting into the sky. All that is needed is a reflector.

[Kyoraki]

"Sure it's impractical, the tech doesn't exist yet, and it most likely won't work, but on the 0.00000001% chance it does work, it would look bloody good on the evening news."

[Barry Tabrah]

We know aliens use light

All abduction stories seem to feature bright lights, and based on that scientific observation all aliens must be using bright lights at all times. I know that sounds technical, but that's what makes good science!

[Nigel 11]

Physics is universal, evolution is probably likewise

I'd expect a large fraction of alien animal life to "see" using light (visible or near IR), assuming they originate on rocky planets with somewhat transparent atmospheres.

Reason: solar radiation exists. As life evolves predators and prey, it becomes an evolutionary advantage for both to be able to see by reflection of such light. An evolutionary arms race leads to eyes as good as physics and biochemistry can allow. Physics dictates that other wavelengths are less suitable for sight by passive sensing. There's a window of transparency there in any sensible atmosphere. Ditto in any sensible liquid ocean. There's also a good fraction of the solar output in the visible/near-IR band.

Here on earth eyes have evolved several times independantly. Fish and higher life evolved from them, including humans. Molluscs. Spiders. Insects (compound eyes, not at all like spiders). Trilobites. (or are insects descended therefrom)?

Yes, there may be exceptions. An intelligent species descended from bats might have eyes atrophied to the point of uselessness, and see by sonar. From cetaceans, ditto, though it's hard to see how a marine-adapted species could acquire technology first-hand.

Sight is an essential for survival, and the most obvious route is evolution of sensory cells sensitive to solar illumination, leading to eyes as we know them.

[Annihilator]

Independent evolution

"Here on earth eyes have evolved several times independantly"

Correct, but the common factor isn't "physics", it's speculated that it's due to our star's spectral emission peaking in the "visible" light spectrum. In other words, although the Sun emits pretty much the entire spectrum, it's brightest at ~550nm - the centre of what we call visible light. Colours are effectively a myth, or at the very least just a handy guide to gradation that we came up with.

If life were to evolve on a planet around HAT-P-9 for example (an exo-planet was discovered there, but a gas giant), the peak would be between "blue" and UV. Optical senses would likely be centred around there, making their "yellow" our "blue".

[Anonymous Coward]

We should be shining frikkin' pulsed lasers into space encoded with the message "Hello Alien Overlords!"

[Captain TickTock]

Then they'll think...

we're a civilisation of sharks..

Wait til I build the Alan Parsons Project!

[Stevie]

Fools!

If we can see them, it means that they can see us!

Aiee!

[Tchou]

What if they live underground and/or are super intelligent blind worms?

[Alister]

How typically arrogant and anthropocentric to assume that any "alien" species would use a sense organ that responds to the same part of the electromagnetic spectrum as our eyes.

Why, even on this planet, there are many species, even some mammals who use completely different methods to be aware of their surroundings, and many more species that see different bits of the spectrum compared to the frequencies we call visual.

[FIA]

Confused

Why is it typically arrogant? Typical of who?

Given that a statistically significant proportion of earth bound species use some form of EM detection in the visible or near visible part of the spectrum, and you're essentially trying to solve the needle in a haystack problem so should be choosing search parameters that you expect for whatever reason to return the most viable candidates, (and that are detectable, ruling out things like echo location), what criteria would you choose?

[slaterm61]

I wouldn't call it arrogant, i would rather human for us to assume that other beings are like us.

[Annihilator]

Seeing is believing

I thought the theory for why we see in "visible" light is due to that wavelength range being the peak of our sun's radiation spectrum.

Presumably, if eyes (and light) were to evolve around a different star, their "visible light" would be different to ours and their light emissions offset too. Not to mention when you consider red-shifts for each world, we'd be looking for any light, pretty much discernible from the system's light emissions.

[David Pollard]

Likely to draw visitors?

"... our own habit of broadcasting radio, TV etc was thought likely to draw visitors to us..."

A scan across some of the available channels seems to indicate that it already has.

[FrankAlphaXII]

This might be sarcasm

Yeah exactly, half of those people on TV really carnivorous reptilian shape shifting Aliens who all belong to the Freemason conspiracy run by the Illuminati and the Zeta Reticulans.

If you wear turquoise they can't see you though. David Icke said so, so it has to be true.

Roswell! Roswell!!

[EL Vark]

Local Matters

I'm looking forward to the pants-crapping number of false positives from the initial Kuiper Belt tests. "Incandescent Aliens at our Back Door!" exclaims Enquirer!

[Joe User]

Assuming

That the aliens don't live underground, or that their planet isn't swathed in clouds.

[sisk]

Lots of problems

1) It's possible that inventing artificial light is a rare phenominon. After all the natural human sleep cycle would have us going to bed at dusk and getting up at dawn, leaving little or no need for widespread artificial lighting.

2) Who says aliens can't be nocturnal, therefore having no need for artificial lighting.

3) There are plenty of critters here on Earth, even discounting the nocturnal ones, that see perfectly fine in the dark, such as housecats.

4) Perhaps they don't 'see' as we think of it. I can think of lots of sensory devices that could replace sight. Echolocation is just the start. Magnetic fields, other forms of radiation, heck, even a gravity sense (yeah, yeah, very science fictiony, but then we are talking about aliens here). Or maybe they're even blind, relying on olfactory, tactical, and auditory senses. And that doesn't even count things that they could sense that would be beyond my imagination (of which I'm sure there are plenty).

5) If you start with the assumption that an alien civilization developed as we did, you stand a very good chance of being wrong. For instance, our own path would have been far different had the oxygen crisis never happened.

[~mico]

Artificial light...

It seems so useful, that it was actually invented eons ago, long before first critters emerged from the sea. It's called bio-luminescence, and it is used where no light from our star can reach, like in deep ocean - and that's why creatures there have useful, non-degenerated eyes. As to nocturnal creatures, their eyesight suffers from low resolution and other optical problems, due to extreme adaptations to night vision. And arguably, developing advanced civilization requires visual acuity. Unless, of course, they developed huge eyes.

[Cryo]

I wouldn't really consider those "problems". We're not looking for one alien civilization in particular, but rather one of potentially many, that happens to use artificial lighting as we do. Sure, it's possible that the vast majority of civilizations don't use detectable light sources, but based on our knowledge, it stands to reason that at least some would, so that would be something worth looking for. It's the same reasoning behind looking for radio transmissions. Most advanced civilizations might not use detectable transmissions, but some may, at some point during their development. It's simply a matter of looking for those things that we know we can detect.

[Anonymous Coward]

But.....

What if the aliens don't see on the same wavelengths (and therefore use different lighting) than we do!!

For example, maybe we should be looking for the ultraviolet frequencies used in our tanning booths. This would have the added benefit of a real-time update on the location of the cast of "The Jersey Shore". I for one would be entirely in favor of being warned that Snooki was in my vicinity.

[Pascal Monett]

I have a question

Independantly of the lighting issue, which is a rather interesting thread, I have a question on a different matter.

All I have read about detecting exoplanets indicates that we can only "see" them when they are between us and their Sun.

To me, that says that we see their "night" side.

Now could someone explain to me just how we are going to make the difference between a lit night-side indicative of civilization, and a dark night-side indicating nothing at all ?

At however many light-years away the planet is ?

Overshadowed as it is by its Sun's glare ?

I somehow have trouble believing that we are going to be able to make a difference, from Earth, between a civilization with night-time lighting and a planet made of rather light-coloured regolith.

Now, the day we can start those evaluations from the Kuiper belt of that planet's star, I will be overjoyed and duly impressed - if I am on the ship that is doing the surveying, that is.

[Cryo]

As the article mentions, this would require more powerful telescopes than we have today, but that we will likely have decades down the line. Their method looks for frequencies of light that differ from the parent star and its light reflected off the planet's daylit side. If the equipment were powerful enough, one might also be able to compare how those light levels vary depending on the planet's rotation, as an uneven distribution of "cities" cross over to the night side. They're simply saying that in theory, given a powerful enough telescope and enough "artificial" light from a planet, that light could be detected.

[AndrewG]

Its not quite that bad

Actually we can't only "see" them when they are between us and the sun. The usual method for detecting exoplanetary systems is Doppler Specroscopy (the minute change in the frequency of the light given off by the sun that we see as the planet orbits it). We can detect a change of about 3 m/s last time I looked which means of course its way easier to find a superjovian than an earth like world. However the limits are currently that we can detect a heavy world as long as its orbital plane is within about 20 degrees of our line of site.

That brings us to problem 1, geometry is not our friends in this case so theres about a 60% chance we will just have the wrong viewing angle to see nightside or even be able to detect an exoplanet.

Problem 2 is as pointed out...the small earth like worlds have to be on a really close orbital plane to viewing angle and that means theres a ruddy sun in the way half the time

Problem 3 is that your detecting exoplanets by essentially looking at how the mass distribution changes over time so you've got no way of working out when the intelligent aliens will be pointing their nightside to us

Problem 4 is I think lighting will be like radio, we'll always have it, but waste light going up not the sky will be a passing fad.

[JeffyPooh]

BTW: It is possible to integrate AC

Trivial if you can guess the exact frequency in advance. Not that much more difficult even if you don't: sample and store, then FFT it.

It still won't work for other reasons. But signal processing is not an issue.

[General Pance]

Irrelevant

Aliens don't need lights. They can see in the dark.

[Anonymous Coward]

shurely

we just need to decrypt their pron channels?

[Andus McCoatover]

Given the size and age of the universe...

...and assuming there are other civilisations out there, they may be eons older, or eons more advanced than us. If they're intelligent, they must've asked themselves the same question - "Are we alone?".

Now, given the number of planets in the universe, why are we loooking for them? Surely the more advanced should've found _us_ by now.

[boothamshaw]

They're probably beaming - oh I dunno - modulated super-luminal neutrinos or whatever at us right now and wondering why we haven't picked up the 'phone. I wonder if looking for ET radio is a bit like an uncontacted stone-age tribe trying to call us by thumping really hard on some bits of wood.

[xlq]

Surely the more advanced should've found _us_ by now.

They're probably keeping quiet and waiting for an even more advanced civilization to find them.

[Anonymous Coward]

if they're super intelligent aliens...

they'll have a blackout like during the Blitz. I could just picture Earth as the scum in the interstellar tv show "my intergalactic neighbour from hell."