The path to detecting extraterrestrial life with astrophotonics

The path to detecting extraterrestrial life with astrophotonics




> This will require measuring a velocity change in the speed of light of <50 cm/s and more likely 10-20 cm/s (1 part in 3 × 10^9).

Can someone explain to me why the speed of light changes?


It doesn't; that language is garbled. It's the velocity of the star they're observing that changes by several cm/s. This translates to a Doppler shift in the starlight being detected: a change in its frequency and wavelength.

see also


Thank you! I was massively confused there for a second.


If and when we detect extrasolar technological life it's highly unlikely we'll do it by detecting planetary emissions. It'll also be a whole lot more obvious. Let me explain.

The future of humanity is likely to be in orbitals. With existing materials these are likely to be 2-3 miles in diameter and 10-20 miles in length, producing EArth-like gravity by simply spinning. You then cover the surface with solar panels. If graphene ever becomes viable as a mega-engineering material, you can scale these up 100x+ larger. Why? Lots of advantages:

1. You can produce this with existing materials. No magic theorized materials are required;

2. No magical new technology is required. Even something like nuclear fusion power generation, which I'm not yet convinced will ever be commercially viable, isn't required;

3. Spin gravity is identical to normal gravity. Earthlike gravity means we don't have to worry about our ability to live in microgravity or even low-G (eg Moon, Mars);

4. You can largely be shielded from cosmic radiation with water tanks beneath your feet;

5. You get to harness the full output of the Sun for energy. For reference, only about one billionth of the energy of the Sun hits Earth. This is a Dyson Swarm;

6. This is incredibly efficient in creating living space per unit mass. Roughly 1% of Mercury's mass is enough to create a Dyson Swarm with billions of times of the living area of Earth.

So, by looking for technological signatures from exoplanet atmospheres we're essentially hoping to find the ~1000 year window where there are detectable signals and we're not looking for a Dyson Swarm instead. SETI has the same problem with a much smaller window.

So why is this much easier to detect? Because of thermodynamics. You absorb a star's light output and you heat up in anything less than perfect energy efficiency (which is impossible). The only way to dispose of heat in space is to radiate it away. Technically you could eject mass but then you have to replace that mass. And the frequency of the radiated heat is determined by the temperature of the radiating object. For any reasonable temperature, that's in the IR range.

The Sun is less than a million miles in diameter. If we were looking for a Dyson Swarm around a star like our Sun, we'd see an object that was more than 200 million miles across (ie the cloud of orbitals) that is producing much of that energy as infrared. It would stick out like a sore thumb.


Yet we seem to not be finding any sore thumbs.


You know how it is, when all you have is a hammer...


This article isn't about a search for technological civilizations. Its about spectographic analysis of exoplanets, which can be used to detect organic molecules in the atmosphere.


It should support the work of lee cronin and sarah walker (Assembly Theory), a novel way to identify compounds that are indicators of life by virtue of their complexity (so we don't need to make any dogmatic assumptions about what life can be made of). We still need to detect these things at a distance, so this paper is relevant, I think, but I also love Assembly Theory.

They have a new preprint that argues that Assembly theory can explain and quantify selection and evolution in complex systems: (I think that it is one of the more under-appreciated papers out there).


> to detect organic molecules in the atmosphere

You don't really need to detect organic molecules. If anyone detected 21% oxygen (O2, inorganic) in Earth's atmosphere, they sure knew something was going on. Lifeless planets like Venus, Mars, have atmospheres that are close to chemical equilibrium. And Earth's atmosphere is far from equilibrium, free oxygen it much too reactive and wouldn't exist without photosynthesis continuously producing it.


According to NASA, abiotic production of O2 at Earth like levels is plausible.

Our understanding of astrochemestry is still limited enough, that we cannot say with confidence that a given signature requires a biotic process to produce.


> we'd see an object that was more than 200 million miles across (ie the cloud of orbitals) that is producing much of that energy as infrared. It would stick out like a sore thumb.

Unfortunately large objects emitting a lot of total energy in infrared don't stick out like a sore thumb. It's very difficult to distinguish a Dyson swarm from a Dyson swarm sized ball of Dyson swarm temperature gas and dust, which is a reasonably commonplace phenomenon. Indeed for one of the simpler construction methods, a dyson swarm may essentially be a big ball of dust where each mote is a solar energy collector. Further, over long time scales without active maintenance you'd expect dyson swarms to break down into literal clouds of gas and dust. While dyson swarm candidates might be good places to focus other SETI experiments like radio telescopes, it's unlikely that the thermal signature of the swarm alone would ever be unambiguously artificial.


The idea of the Dyson swarm is just taking humanity as we've seen it over the past 200 years and assuming the exact energy curve would continue for 2K more years. It's kind of silly. People don't need infinite energy to live. Many people are happy to live as much in harmony with nature as they can.

This whole line of thinking is short sighted. For example "No magic theorized materials are required;" but we literally WILL have 'magic' materials in the future. Of course we can't predict the things we don't have yet, but assuming we'd just move in some linear line from where we are is probably unlikely.


This stuff made a lot more sense when it wasn’t yet crystal-clear that we’re on a shortish path to zero or negative population change.

Why build and pay for a complex life support system in space when… I’m living on a really big and really good one for free? It doesn’t seem like we’re going to run out of space for people, so why expend resources making mediocre space environments for people to live on, except for certain narrow purposes?

[edit] I think the problem becomes obvious if one considers colonizing the oceans with big ships and floating platforms. We haven’t really done it, and it’s hard to imagine why we would—it’s just so much more expensive and less convenient than living on land. Space is like that, except all the bad parts are even worse. What do we have on the oceans? Resource-gathering operations, shipping, exploration ships. I don’t see why space would see more colonization than the oceans have (which is nearly zero, mostly temporary purpose-specific habitation and movement across it to land)

I’d include Mars and such as places much worse and more expensive to colonize than the oceans. I don’t see it happening any time soon—why would it?


I do wonder if humankind will have 3 phases: 1) not knowing if life is out there, 3) discovering extraterrestrial life, but mainly:

2) 1000s of years where we're pretty sure that oxygen rich planet has life, but can't get there and have to concede it might be just a flaw in our theory of planet formation.


That's a really interesting breakdown. I think we'll be able to build larger and larger telescopes and figure out more and more of what's in atmospheres of planets. I feel like we'll be increasingly seeing more and more life signals without it being 100% decided. I want to see articles that consider what you could see about the earth from 100 light years away with just a little bit more advanced technology than we have.


Optical engineer here. For anyone not in the field, “photonics” is a fancy academic term for optical systems that involve waveguide structures. You can do some neat things with waveguides that you can’t do with more traditional optics like lenses, prisms and mirrors.

I think the paper’s title is a bit sensationalist (a callout to extraterrestrial life in an optics paper? Really?) but it’s basically talking about how these devices can help improve spectroscopy and imaging, which are the only two tools astronomers have.


And polarimetry!


Lots of exciting possibilities in here and good work by the authors previously but this one is a bit of "white paper" paper describing what could be done in the future if the money is there


The other end of the spectrum from detecting bio-signatures of life on individual planets, would be mega structures.

Once a civilization began harvesting the energy of a few star systems, it would have a good chance of colonizing their whole galaxy in a blink of an eye.

(100 thousand years, plus or minus an order of magnitude, as compared to universe time in billions of years.)

So we would be much more likely to see totally colonized galaxies, rather than partially colonized.

I would be curious about which instruments would detect infra-red galaxies, I.e. galaxy sized objects where all starlight was put to use, with only infra-red light as waste.

And would that be an object that would stand out if a major telescope came across it, or require specific initiative to search and find?

And speaking of optics, is there any reason to suspect an efficient civilization would leak lower frequency light than infra-red?


Any advanced civilization will quickly lose interest in space exploration. Just a phase.


> would have a good chance of colonizing their whole galaxy in a blink of an eye

But what's the motivation to do that, so that the collapse is bigger? Beyond just because it could also many good reasons not to do that...


I strongly disagree, it's not beyond the dreams of humans to travel a few light years away with generational starships. How are you going to travel all the way across the galaxy without faster than light travel. We could send humans in the journey to the nearest stars within the next thousand years. At least we could start the ships. That's with only a little bit more technology than we have today. But colonizing the galaxy takes imagining things we have no idea if they will ever be possible, like faster than light travel.


Let's build a second James Webb and send it in the other direction for a massive parallax camera (parallax is the wrong word but my brain can't remember the right one)

I just wish I could live long enough to see the SGL


I think you could make a case for whatever replaces James Webb going to L4 and L5 instead of L2.

They put JW at the L2 point. I don't think L1 would make a good astronomical location (we put Sun monitoring satellites there, which is probably a good idea), and L3 wouldn't be able to transmit to earth because there's a big ball of fusion in the way.

L4 and L5 have the advantage of being stable. No reaction mass needed for orbital correction. I'm not sure how much reaction mass you need for the orbital corrections versus going to L2. Anyone know?