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The Interstellar Internet


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An interstellar web? (Original image by B. Torrissen)

A speculative but intriguing discussion that sometimes crops up when talking to people engaged in exoplanetary science goes like this; let’s suppose that we find an unmistakably terrestrial style planet around a relatively nearby star (less than about 30 light years away), perhaps even around one of the Alpha Centauri members, a touch over four light-years distant. Let’s further suppose that – possibly with the James Webb Space Telescope, or a next-generation ground-based super ‘scope – we gather evidence for an atmosphere and even find big chemical clues that there could plausibly be a biosphere on this world. What do we do next?

There are somewhat mundane answers – build better instruments, get better statistics – that may be the most realistic, but there’s also that nagging thought that the next thing to do would be to find a way to study such a planet up close. If enough coffee has been consumed then it’s a matter of finding a handy Tony Stark, willing to sink hundreds of billions into a robotic interstellar probe, on a long-shot for glory (or perhaps call up his real life role model, Elon Musk of SpaceX). There’s a problem though, unless you intend a very long round trip, how do you get the information back? While we are now pretty good at picking up signals from distant spacecraft – even from Voyager 2 at over 100 AU from the Earth – getting data back from a few light years is going to be hugely difficult. The required transmitter power, as well as interstellar scintillation, is conceivably a major hurdle.

A solution, that has cropped up in various guises, even in the idea of von Neumann probes, and the interplanetary internet, is that you don’t just send one probe. Rather, you send a chain of probes – pearls on a string – capable of communicating between themselves even if not individually directly back to Earth. It would take a long time, but as the furthest end of the chain crept towards a target stellar system we’d have ongoing feedback, the continuous relay of data as we crept through interstellar space. It might be optimal to build the biggest receiver and transmitter at the outermost practical limits of our solar system – the equivalent of an internet ‘backbone’ – with a clear line back to Earth. So how many probes would you need to get to somewhere like Alpha Centauri?

This system is about 278,000 astronomical units (AU) away. If we optimistically think we could build probes capable of high-bandwidth to-and-fro communication over a few hundred AU then we’re talking about a thousand or more devices. This sounds awfully challenging, but remember that we (as some hypothetical sublimely patient species) might not expect Probe-1 to reach Alpha Centauri for a few tens of thousands of years. We only have to launch every ten years or so. Even if each probe cost 10 billion dollars (allowing for lowered cost after the first few models) that’s peanuts over this timescale. In the meantime we have an ever extending tendril out into interstellar space. Being an innovative species we would undoubtedly think of more and more wonderful things to add to the probes, increasing the scientific return.

Powering transmitters and receivers, as well as sizing their antennae or dishes, is still a problem. Given the timescale to reach the target star then even radioisotopes are going to peter out (fission reactors are a no-go, the fuel burns out too fast – but perhaps carrying along enriched uranium is an option, as one of the commentators on an earlier Life, Unbounded post discusses). I personally think that supplementing fissile material with chemical energy might actually be the best option; carry a store of naturally chilled redox components, mix them periodically and recharge the batteries when a power-boost is needed, the ultimate fuel-cell.

One way to increase the efficiency of communication is to use lasers instead of more conventional radio frequency transmitters. In 1994 Lesh, Ruggier, and Cessarone of the Jet Propulsion Laboratory wrote up an interesting study of this in which they concluded that conventional radio communication from the vicinity of Alpha Centauri 4 light years away, with mega-watt power requirements, could perhaps be replaced by modulated lasers with a 20 watt output power (see also this excellent piece by Paul Gilster at Centauri Dreams). The bandwidth is nothing exciting, about 10 bits per second (no Netflix streaming then), but hey folks, it’s from around another star. Clearly if one instead used the pearls-on-a-string spacecraft configuration, the average comm-link distance could be drastically reduced, and the bandwidth and ease of signal-lock could be greatly increased.

All over-caffeinated speculation? Perhaps, but if we ever get serious about stepping beyond, then making sure we don’t drop the signal is going to be a very real issue, and building the outermost limbs of our information-obsessed species’ internet may be the easiest way to bring the universe back to us.

This post is a reworking of an old Life, Unbounded piece from back in 2010. It seems like an appropriate followup to the previous post on Mass Effect and the Fermi Paradox, perhaps a glimmer of where we go next…

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His latest book is 'Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos', and he is working on 'The Copernicus Complex' (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jduringer 11:54 am 03/26/2012

    I’d bet a dollar that economic capital formation for such a venture would converge with entangled particle communication breakthroughs. What say you, holy seers, any dogmatic pedantic enlightenment on such speculation from such a sycophant?

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  2. 2. EyesWideOpen 4:26 pm 03/26/2012

    As I understand, you propose to build a cosmic Internet with relaying probes arrayed in a string — at $10 billion per pearl and just 1,000 so so pearls between here an Alpha Centauri — over the next million years (or more precisely a few tens of thousands of years). That’s an estimated $10 trillion for that one strand of probes, but it’s spread out over a few tens of thousands of years. Considering the Egyptian empire lasted thousands of years, I suppose it’s optimistically possible our civilization could last 10x that many years. (I didn’t project the rate of inflation over 50,000 years, incidentally, or whether currency of a value we understand will exist when the first pearl is strung.)

    If we avoid an extinction-level event (“ELE”) before the first 10,000 years from now, and we even beat the odds and get to 50,000 years when transmission reaches Earth, will we be here to receive it? And if so, will some smart-alec quip at NASA as it may exist then, something like, “Great, we got the first transmission and there are hundreds of habitable worlds like Earth in Alpha Centauri. Now if only we had spent that $10 trillion in 2012 dollars, starting fifty thousand years ago, to find a friggin’ way off this rock, we could evacuate Earth before that rogue planet the size of Jupiter sucks us into is atmosphere like a whale sucking krill into its lungs, as it passes by us five years from now. Yet we spent our budget on the probes, and discovered we could have spent the money to build inter-stellar maybe even FTL travel from here to there.”

    An intern from MIT, wanting to break the somber mood, says “But look at the bright side…” (fill in the blank).

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  3. 3. Dean8 6:14 pm 03/26/2012

    Once we settle on a direction for the string, keeping them inline while still orbiting Sol until totally free of its gravity well may require several probes launched per year.

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  4. 4. GreenMind 6:22 pm 03/26/2012

    No need for all this fuss. Just ask the Tibetan Buddhists to go visit the planet using the Bardo.

    More realistically, this plan leaves out a critical factor, which is that each pearl would have an unknown probability of failure over thousands of years. To prevent the entire chain from failing when a single pearl fails, you would have to launch them closer together so that the transmissions could skip one or more pearls. I’d suggest five thousand pearls, launched every two years or so, so that multiple adjacent pearls could fail without disrupting the chain.

    I don’t see why you couldn’t provide each pearl with very large concentrating solar collectors to collect starlight for electricity. The bigger the collector, the more electricity.

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  5. 5. poihths 6:31 pm 03/26/2012

    To me, this is all nothing but intellectual thumb-twiddling. Here on Earth, where I live, we have huge problems trying to maintain directed purposeful activity on a scale of months or years. The idea that we would be able to maintain such an activity for millenia isn’t even worth laughing at.

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  6. 6. Caleb A. Scharf in reply to Caleb A. Scharf 10:21 pm 03/26/2012

    Thanks for comments. I agree that the fail rate is an issue that I skirted here, but whether one would need entire probes or just more redundancy in each is I think an open question. I also agree that given the way we humans seem to behave then the notion of this kind of extremely long-term effort sounds a bit ludicrous (it would be like sticking to the same plan the cave-painters of Lascaux had 20,000 or so years ago). A potential solution would be through automation (admittedly this may also sound like pie in the sky), a strategy considered by many (e.g. von Neumann probes etc) where the entire system is out of human hands once initiated – from probe manufacture to launch.

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  7. 7. DaniEder 11:04 pm 03/26/2012

    Caleb, I think you entirely missed the idea of using stars as gravitational lenses (despite SA covering the topic several times). Then you only need two relays, one at each star’s focal point. For the Sun that is 544 AU or more in the direction *opposite* alpha Centauri. From the relay points, then you do local communications to any point in that system.

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  8. 8. nathanielv 4:35 am 03/27/2012

    Interesting stuff. If it’s really that hard to send a signal from one planet to another then the failure of SETI to receive anything by now seems not surprising at all.

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  9. 9. Caleb A. Scharf in reply to Caleb A. Scharf 10:18 am 03/27/2012

    DaniEder – I know about the idea of using stellar masses as lenses for communication, this is certainly a clever notion. However this too has some very significant technical hurdles and the reason I didn’t get in to it here is that it obviously only works once a probe has arrived in another star system and positioned itself or a relay at the focal point of that star relative to the Sun, so you’d be flying silent until then – my pearls on a string idea is a way to maintain a constant downstream/upstream channel during the thousands of years trip, returning science and allowing for early decisions to be made about destination targets.

    On a slightly different note. The issue of how expensive this all gets to be is important, because it’s really not. Right now the world spends close to $1.5 trillion per annum on military activities (maintenance of armies, campaigns, infrastructure etc etc) – that’s about $4 billion per day. So a few days a year of global military spending would probably be enough to set us in motion towards another star. The cost is minimal, the question is indeed whether we can get our act together to survive long enough to do it.

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  10. 10. jtdwyer 10:35 am 03/27/2012

    Regardless of how any interstellar communication might be implemented, the fundamental propagation delays would render any dialog meaningless. Unless ‘instantaneous’ response times can be provided for queries to some information provider even 30 light years away, any meaningful conversation would exceed the historical duration of our current technological society. A meaningful dialog would require some fundamental technological breakthrough.

    If it takes 30 years to receive an answer to the question: “Are you friends or foes?” our subsequent destruction by some high energy beam might seem to be an unexpected response.

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  11. 11. styman 10:47 pm 03/27/2012

    forget all of that. we should be thinking of how we can get
    off of this planet.

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  12. 12. Wayne Williamson 5:17 pm 03/30/2012

    This article is a piece of crap. If this is what forward thinking comes up with, then we need some different forward thinkers.

    An inter stellar probe should be able to get up to at least 1/2c, dropping relays along the way. I’m guessing you would need at least two to try boost the success. The second one would monitor the first (via its relays) and if contact was lost then change course. Also, I’m talking about million to billion kilo ships. Trying to do this from Earth is stupid.

    Within the solar system is a different issue. The initial ships would be very expensive but their main purpose would be to go and get additional material and fuel to build additional ships and fuel for them(read iron and water(or ice if you prefer;-)). No reason why they couldn’t be manned, with only 1 or 2 month turn arounds.

    While I’m on it, the propulsion would be electric, splitting water to create the ions. Nuclear reactors in the 100MW range would also be needed. I think the Russians had designs up to 20MW.

    Just some thoughts…..

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  13. 13. Postman1 5:15 pm 03/31/2012

    Caleb – You should check out
    http://www.centauri-dreams.org/ Paul Gilster has had several recent posts on subjects like this. Check out the archives for the last couple of months. A good one is
    http://www.centauri-dreams.org/?p=21979
    There are some very informed comments too.
    Slow migration outward through the Kuiper Belt and into the Oort cloud, jumping planetoid to planetoid. It will take longer, but some estimates place trillions of objects out there. Many believe they could meet up with the Oort cloud objects from other nearby stars, making it possible to slowly migrate without a large jump.

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  14. 14. Postman1 5:23 pm 03/31/2012

    Caleb- I meant to add ‘colonizing the objects’ as we go. Makes me wonder if we are looking in the wrong place for ETI. Perhaps they are between the stars and only venture nearer a star for science and exploration. A train of thought for another day.
    Good article!

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  15. 15. Caleb A. Scharf in reply to Caleb A. Scharf 9:35 am 04/2/2012

    Postman1, thanks for the comments and links, yes great stuff, Centauri Dreams is definitely the go-to source for careful and thoughtful coverage of these ambitious ideas!

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  16. 16. cubeboy 2:38 pm 04/4/2012

    Interesting … makes SETI seem even more futile. And the old worry about how aliens would underestimate our culture by picking up I Love Lucy reruns is equally ridiculous — a crummy 50KW omnidirectional signal ain’t gonna make it to any stars.

    Given how our technology has changed so radically in say, the last 100 years, it also seems that by the time the second pearl was launched, the first would be incomprehensibly obsolete. We can’t even come up with a digital data storage medium that lasts 50 years.

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