Archive for the SETI Category

Failure Points

Posted in Astronomy, SETI, Speculation with tags , , , , on February 15, 2011 by davidnm2009

I had an odd thought earlier, whilst typing an e-mail.

Just what is the average metallicity of the galactic disk?

I’m not sure, but I have a suspicion that it might be at least a little bit subsolar. Why is this bothering me? Well, I have a suspicion this could impact on the Fermi Paradox. My thought goes something like this…

Consider a somewhat metal-poor star, say [Fe/H] = -0.3 – that is, about 50% the heavy-element abundance of the Sun [1]. Let’s assume its protostellar disk shares the same composition – not wholly unreasonable, since they formed in the same place! Presumably, the star’s relative lack of heavy elements will be reflected in the planets that it forms.

Now, first off, you won’t get as many planets around metal-poor stars; planet-formation seems to correlate with stellar metallicity. That said, you do get at least some planets around low-Z stars. But, until relatively recently in galactic history, most stars will have been ‘low’ metallicity. The interstellar medium didn’t originally contain metals; heavy atoms got there as a result of pollution by supernovae and red giants, and that process takes time. And the galactic disk still contains plenty of metal-poor regions even now. (A star having a subsolar metallicity is not a guarantee that it’s older than our Sun – something that’s irritatingly inconvenient for my own research!)

Anyway, I can imagine terrestrial planets existing around our [Fe/H] = -0.3 star. I can even imagine them having a reasonable amount of carbon, oxygen, nitrogen and hydrogen, so the absolute-minimum basis for CHON life is there. Probably there’s at least a dash of other stuff too. But what I imagine it particularly lacking is big, heavy atoms – like rare-earth elements and so on.

Supposing life gets started on such a planet. Suppose even some sort of civilisation gets going. But presumably you’re not going to be able to have much of an electronics industry without metals or rare-earth elements? And if building even just a radio is difficult – let alone TVs, radar etc. – then you’re not going to be leaking much EM radiation into space. And without much leakage, you’re certainly not going to be detectable at interstellar distances.

And if the galaxy as a whole is, on average, metal-poor relative to our Sun, does this also mean that advanced electronics industries are also rare? Food for thought…

[1] In astronomy, a ‘metal’ is anything heavier than helium – i.e., something that couldn’t have been formed during the Big Bang. The [Fe/H] scale is a logarithmic scale, based on the relative levels of iron[2] as compared to hydrogen; [Fe/H] = 0.0 is 100% solar, i.e. 10^0 = 1. [Fe/H] = -1.0 is 10% of solar and [Fe/H] = +1.0 would be 10x the solar content of metals. In practise, the highest values of [Fe/H] seem to be about +0.5ish. The record object for lowest metallicity, however, is HE 0107-5240, with a breathtakingly-low metallicity of [Fe/H] = -5.2.
[2] In stars with so-called ‘solar’ abundances, you can use the amount of iron as a sort of standard calibrator for other elements. In less well-behaved stars, those with non-solar abundances, this breaks down, and you have to be a bit careful. In these cases it can be more useful to use [M/H], which is exactly the same logic as Fe, except that M is the total amount of metals against the total amount of hydrogen.


Fermi, With or Without Exclusion

Posted in Astronomy, SETI, Space with tags , , on July 29, 2010 by davidnm2009

I’m not a big commenter, generally, but I felt motivated to leave one on this post on Charlie’s Diary. Basically it concerns the Fermi Paradox.

The Fermi Paradox is a difficult one for me. Obviously I’m interested – the fact I can’t quite shut up about it is the giveaway. But, and here is the problem, as I’ve said before, my considered opinion is that we can’t usefully speculate on it at the moment.

I was trying to wrack my brains again for anything useful that I could say. So, here is what I think we can say:

  • 1: We exist, so we can reasonably conclude that civilisations are at least possible, i.e. there is nothing in the basic physics that effectively rules them out.
  • 2: Our planet doesn’t appear to have been visited at any point in our fossil record. [1]
  • 3: If higher-rated societies on the Kardashev scale exist, then they can’t be anywhere nearby. Although our galaxy has lots of elegant little structural quirks, I don’t think anyone has ever suggested that they’re down to super-civilisation infrastructure projects! [2]
  • 4: Radio signals at interstellar distances – there is absolutely no agreement on this. I’ve seen it argued both way, with no clear sign of a consensus. So I suspect this means we can’t yet conclude anything useful here, either.
  • 5: This is a bit of a cheat as it hasn’t technically happened yet, but the next round of Kepler mission data (due in February 2011) should begin to tell us something about the mass function for terrestrial-type planets. At the moment, all we can really say is that they definitely exist, and some other stars have them.

Not looking very definitive, is it? It’s looking like the paradox might stay paradoxical for some time yet.

[1] But is there necessarily any reason why aliens would come here? Consider – if the typical life-bearing planet is a tide-locked world around an M-dwarf, why would you send a mission to a G-class star? They’re too hot, too short-lived, emit far, far too much ionising UV and any putative planet would have to orbit so far out that it would actually still have a day-night cycle! I mean, yuck, right?

[2] Although you do have to wonder if the Type III equivalent of Capita was behind the Antenna Galaxies – definitely a PFI bodge if there ever was one!

[3] Yes, the post title is a physics pun. And not a particularly good one, no.

The First – And the Only?

Posted in SETI, Speculation with tags , on May 9, 2010 by davidnm2009

When I was in Tesco’s today, for no apparent reason, half the lights suddenly went out. It was a bit startling, albeit otherwise inconsequential. But it did get me to thinking about energy crises.

Energy crises have been a major factor in technological development. The Industrial Revolution was arguably first foreshadowed when late-Elizabethan England began to run out of wood. Trees burn far faster then they grow – by the end of the 17th Century, London had eaten all the woodland for miles around. People had to move to burning coal for heating and industry. Only there was a problem – the easy-to-get-at coal quickly ran out. Mines had to start digging deeper – except they had this awkward habit of flooding. Obviously it’s a bit hard to run a profit when all your miners keep drowning!

Enter the pump. Only, the problem was, horse-drawn pumps didn’t really run well enough. They were inefficient and required a lot of horses – and horses are expensive. They need feed, they need stables, they need rest. So when the first steam boilers arrived, in the early 18th Century, it was a godsend for the mines. Steam pumps don’t need hay or stables. So the mines could go deeper and deeper.

But this has led me to an odd thought. Suppose we haven’t quite got it right. Suppose there’s just something basically wrong in the way we’ve organised our economy or our society. It wouldn’t be the first time – a professional civil service is obvious in hindsight, but Rome’s republic never thought of it. Tax farming is a very bad idea, but the French monarchy carried on with it until the bitter end. Organising all your neighbours into a vast series of heavily-armed alliances is grist to the mill for security dilemmas – but that didn’t stop the continental alliance system breaking horrifically in World War One.

Suppose, then, that we go down, for whatever reason. Maybe the seas rise and flood all our ports, disrupting trade. Maybe a bacterium figures out how to eat plastic. Maybe the markets set the world up the bomb so badly that no amount of bail-outs can reverse a catastrophic decline in economic activity, leading to truly mass unemployment, hunger, riots, instability, civil wars and worldwide collapse. (In which case, in respect of moron bankers, there will be at least a shred of poetic justice. To quote what Jared Diamond had to say about the Norse Greenlandic elite, ‘..the only privilege they earned themselves was to be the last to starve.’)

Anyway, in this cheery scenario, what happens in 500-5,000 years’ time? Will we be the Atlantis or the fallen Rome to some successor-civilisation? Will English be preserved for centuries as the language of scholarship and the arts, much as Latin was in the West? Will distant-future tourists gape at the ruins of half-sunken skyscrapers and artists make charcoal sketches from the shade afforded by fallen concrete slabs?

Simple answer: no.

Why do I say no? Because I don’t think anyone will follow us. It comes back to the energy crises I mentioned above. You see, we’ve chopped down all the forests and dug up all the easily-accessible coal. Yes, trees will grow back eventually, climate allowing, but once coal or oil are burnt, they’re gone. There is very little left near the Earth’s surface that any putative successor-civilisation could use to advance itself. And it’s not just coal or oil – copper, iron, lots of other valuable metals all fall under the same category. (Smelting down all the Roadmakers’ abandoned cars is a non-starter, too – you need coal to get the kind of heat required to do that properly.)

Any successors to us are probably limited to windmills and water-driven wood-frame devices. It’s hard to see an industrial technological path that doesn’t involve petrochemicals. (And there’s a SETI thought – oil beds don’t form that easily, and accessible ones are even rarer. And how many other planets have had a Carboniferous-analogue?) Our successors will have wood, wind and water. And that is all they will ever have.

If I have a point with all of this, it’s this. We have an advanced, industrial civilisation. Yes, it’s a mess and yes, it has problems – but it’s a lot better then any of the alternatives. Back to nature *sounds* great, but I have hayfever, so I prefer my outdoors in moderate doses only. And anyway, to make it work, you’d somehow have to make more than 6,500 million people vanish. I can’t imagine they’d like that much.

You see, as we developed, we lifted the ladder up with us. We can’t climb down to the lower floors, but we can fall – and if we do, we’re staying down there. For a while as we lie injured on the floor, we’ll still be able to see the ladder, up above us, but it will remain forever out of reach. Basically, folks, our choice is simple.

We have to find some way to make this society work. Or it won’t.

Dyson Spheres, SETI and Old Stars

Posted in Astronomy, SETI, Speculation with tags , , , on April 21, 2010 by davidnm2009

…are a fun, but widely misunderstood, concept.

Centauri Dreams has an article on them that caught my eye earlier.

They tend to be imagined as big solid balls, rather like a hollow ball bearing which just happens to have a sun in the middle. Unfortunately, there are lots of engineering reasons why this probably wouldn’t work. The internal stresses of such an object would probably break it apart. Also, if it were even slightly off-centre from the star, tidal forces could cause problems – and it would have to be off-centre. If you wanted to simulate any reasonable ‘gravity’ on the inner surface, you’d have to spin the thing. Only the problem then is that it’d bulge at the equator, just like the Earth does. It’d also be squeezed along the polar axis. The overall shape would be basically be a squashed sphere – so different bits of it would end up different distances from the star by definition.

Dyson’s actual idea was a sort of cloud of asteroid-sized habitats surrounding the star, arranged so that all of the star’s light can be collected and used for industrial purposes. This is more workable then the giant ballbearing idea – but it still has some issues. For instance, an ablation cascade seems inevitable for a configuration this dense. Also, maintaining the orbits would be an absolute nightmare – this is well beyond the territory of the three-body problem. No analytical solution to the system will be possible. All you could hope to do would be try and keep the millions of habitats stable for a finite time – but sooner or later, chaos will ensue.

Eventually, all your habitats have ground each other to dust and you’ll be left with nothing, which is obviously a bit of a waste. (And, interestingly, the one actual search for Dyson Spheres that I’m aware of didn’t find any.)

No, it just doesn’t seem like an economically-sensible project. As Centauri Dreams points out, a more sensible project for such an old and capable society would be to try and keep their existing sun/suns going for longer. And, just for once in SETI, this leads to something that’s actually observable(!). Consider this very badly-drawn colour-magnitude diagram:

(Note, incidentally, that I haven’t tried to calibrate this – this image is here only for illustrative purposes, not quantitative ones.)

Briefly a ‘colour’ represents the ratio between light received in two different regions of an objects spectrum. So here B-V is the ratio between the B and V magnitudes. B and V mean ‘blue’ and ‘visual’, essentially. So, an object that’s hotter (emits more light in the B band) will have a ‘bluer’ B-V than an object that’s cooler (emits less light in the B band). I’ve noted the relative directions on the image.

Mv represents the absolute magnitude in the V band. This is useful as absolute magnitude is directly related to an object’s actual luminosity, while visual magnitude is sort of not. Also, the magnitude scale is a bit confusing – it’s a logarithmic scale, and brighter objects return smaller numbers. (Sirius – very bright – has a V magnitude of -1.4. Proxima Centauri – very faint – is something like +15.) So the faintest things on this plot are at the top.

Now, if you looked at a hypothetical sample of presumed ‘old’ stars (perhaps isolated by their kinematics), you’d expect to see two particular blobs. One for red giants (the bottom blob) and one for red dwarfs (the top blob). (I’ve not tried to show it here, but the red dwarf blob will be much bigger/denser as there will be far more objects in it.) But, suppose you did this and found a few bluish things with relatively-bright absolute magnitudes – say G-dwarf bright – you’d see them lying between the red dwarfs and red giants (greenish dots). Thus they’d really stand out on a diagram like this. And, I suspect, there’d be spectroscopic evidence as well. If someone was keeping a star alive long beyond its time, then it’s going to have a higher than normal helium-to-hydrogen ratio, simply because it’s been burning longer.

So … we’d be looking for a population of kinematically-old stars with unusually blue colours and odd spectra. It sounds like the kind of thing that the Sloan Digital Sky Survey would be suited to. And, unlike many things in SETI, it actually sounds like a practicable research project.

And, oddly enough, some globular clusters are known to contain blue stragglers,. Not that I’m suggesting anything, of course.

Absent Aliens?

Posted in Astronomy, SETI, Speculation with tags , , on January 31, 2010 by davidnm2009

Just a quick thought…

The Anthropic Principle – from an intellectual point of view, it’s largely useless. It makes few genuine testable predictions – Fred Hoyle is about the only person I’ve heard of who managed to get something usable out of it. And not only is the Anthropic Principle a useless bit of psychological fluff, at its worst extreme it can actually lead to some very, very silly intellectual places. So, on the face of it, the Anthropic Principle seems really a bit pointless.


It occurs to me that it might, just might, conceivably tell us something about the possibility of intelligent extraterrestrial life. The argument is this – we as human beings are an intelligent species, and we exist. Therefore, the logical inference is that intelligent life is definitely possible, and a basic scientific principle is that if it can happen once, it can happen again, given the right conditions.

So, this means that the debate regarding ‘aliens’ shouldn’t be ‘can they exist?’. Rather, we should be asking questions about the likelihood – and thus the spatial and temporal distribution. (10,000 civilisations at any given time in a galaxy the size of ours? Or one every billion or so years on average?)

And the nice thing about questions like this is that it should be possible to put at least some upper bounds on these numbers, even with today’s technology. The argument would be, if we can reliably detect (say) radio wave signals out to X parsecs, and we don’t, then we can argue an immediate upper bound as ‘no more than one civilization, i.e. us’ per X^3 cubic parsecs.

Divide the volume of the galactic disk by that number, and you have your estimate.