New approach to SETI: Further confirmation of the utility of the Explanatory Filter

_{William Dembski

June 12, 2006

Intelligent Design}

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Here is an article about a new SETI telescope that searches light waves rather than radio waves. What kind of light are they looking for?

Today’s laser technology makes it possible for petawatt (10^15W) lasers to emit highly collimated nanosecond optical pulses that briefly outshine the Sun by a factor of 10000. Because no known astrophysical source could put out a bright nanosecond optical pulse, some SETI searchers have concluded that looking for signals from technologically advanced aliens is more promising with optical telescopes than with radio telescopes.

If we find nanosecond pulses, we can’t lose,” says Horowitz. “If it’s not from an alien civilization, at least we will have discovered an astrophysical phenomenon that no one anticipated. Not a bad consolation prize.

Physics Today, June 2006, http://www.physicstoday.org/vol-59/iss-6/p24.shtml

These SETI researchers are therefore using optical telescopes as an explanatory filter.

Comments

Mung: "Why did you drop the human-like civilisation in favor of mere civilisation? Are you aware of some non-human civilisations somewhere? Reinsert human-like and you may well have the answer to your query. Because this is a very human-like phenomenon. We have no reason to think it is a natural phenomenon. Reasoning from known causes and their known effects would lead one to infer the unknown cause of a known effect." I don't understand the requirement of the distinction between human or non-human civilisation. Of course we are the only known civilisation, or we wouldn't be having this discussion about SETI, but what human attribute, beyond intelligence, is it necessary to attribute to the "aliens" in order for them to come up with a big laser? And if none is required, then we are concerned only with the difference between intelligent and non-intelligent causes, which is what ID is about. This requirement to inject "human thinking" into the debate seems doubly strange to me for a generation (or two) that have grown up on science fiction involving every imaginable type of intelligent life, human-like and utterly alien, corporeal and non-corporeal, which have developed technologies parallel to our own. In the end, it is always the intellect that forms the common link. Still, it is not sufficient to show a phenomenon is "human-like" to infer intelligence; there are plenty of human-produced phenomonenon that are barely distinguishable from natural phenomenon (e.g. global warming is a good example where there's lots of debate). There is bound to be a gray area where no definitive conclusion can be made. It should be possible to establish criteria that definitively distinguish the two, when we stumble upon new phenomena, perhaps a phenomenological "universal probability bound".SCheesman_{June 14, 2006
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Re #11 and #12. I am respecting Dave's wish that I no longer participate in this blog. If you are interested in my dull and thoughtless comments you will find them on http://mark_frank.blogspot.com/.Mark Frank_{June 14, 2006
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So why do the SETI people think this particular phenomenon might be related to a civilisation?
Why did you drop the human-like civilisation in favor of mere civilisation? Are you aware of some non-human civilisations somewhere? Reinsert human-like and you may well have the answer to your query. Because this is a very human-like phenomenon. We have no reason to think it is a natural phenomenon. Reasoning from known causes and their known effects would lead one to infer the unknown cause of a known effect.Mung_{June 14, 2006
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I'd like to make one pitch for Mark's continual involvment in this forum; I have to agree with what he said here: "I come back to the main point. Are we to conclude an alien civilisation every time we come across a new phenomenon for which we do not know the source? I assume not. So why do the SETI people think this particular phenomenon might be related to a civilisation? There can be only one reason - because they have observed our civilisation make such a thing - as they note in the first sentence of the excerpt." I see nowhere to disagree, and would not dispute his presentation of the researcher's thoughts, especially in light of their closing paragraph in the original Physics Today article. I also have to agree with Dave Scot that it's highly unlikely we could find, or even invent a "natural" nano-second petawatt laser. But then I would have had a hard time explaining millisecond radio pulses with hyper-stable repetition rates if I knew nothing about neutron stars. How about a nuclear explosion near the event horizon of a black hole, with the resonant cavity being provided by collapsing magnetic field lines? Okay, it's crazy, and not likely to focus worth beans, and likely to take micro-seconds (which is an eternity in comparison to a nano-second) but I'm a geopysicist, not an astro, solid-state or nuclear physicist, so cut me some slack. There seems to have been a fair amount of discussion of what constitutes a design inference in biology (coding, multiple redundancy, complex interacting control systems, irreducibly complex structures etc.) as well as in astrophysics/cosmology (fine-tuning, "privileged planet" arguments etc), but not so much on how to distinguish between "designed" and "natural" e physical phenomenon, and it's worth investigating. I think the design filter is up to the challenge.SCheesman_{June 14, 2006
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Because if you know what laser light is (monochromatic, coherent) and how it is made (pumped optical cavity resonator) you know why itÃ¢â‚¬â„¢s exceedingly rare for any natural formation to generate it.

But not unheard of.
http://www.findarticles.com/p/articles/mi_pasa/is_199508/ai_1671653129

ItÃ¢â‚¬â„¢s almost inconceivable that any natural shutter or light source could be gated at one nanosecond intervals.
But not unheard of for pulses to be emitted at that frequency:
http://www.nature.com/nature/journal/v422/n6928/abs/nature01477.html
(sub-pulses; not "gated" per se)

Finding the exceedingly rare with the almost inconceivable in the same place at the same time moves you from almost inconceivable to fully inconceivable.
OK. I don't know. I'm not a physicist.

I tolerate bright, thoughtful contrarians and you just donÃ¢â‚¬â„¢t fit that category. You had no knowledge whatsoever from which to base your statements but you made them anyway. ThatÃ¢â‚¬â„¢s not thoughtful. Move along now. -ds
Radio sources are not at all rare but even so the Nature article said there's no satisfactory explanation for the 2 nanosecond pulse width. You also mistake frequency for pulse width in your response above. Frequency is the peak-to-peak time of a single electromagnetic wave. Pulse width is how long the transmitter is turned on. The Nature radio pulse lacks satisfactory explanation because of the power in the 2 nanosecond pulse. The generator must be less than a meter (the distance light can travel in 2 nanoseconds) and no one understands how that much power can get packed into a radio frequency generator that small. Naturally occuring laser sources, unlike radio sources, are exceedingly rare - to date only one has been discovered and the way it works is satisfactory explained. -ds franky17_{June 14, 2006
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Re #9. Dave - it would be shame to move to another blog just when the argument is getting somewhere - but if that is what you want I am happy to oblige. If you are interested in following the argument through: Why is it a safe bet for petawatt nanosecond laser pulses?

Because if you know what laser light is (monochromatic, coherent) and how it is made (pumped optical cavity resonator) you know why it's exceedingly rare for any natural formation to generate it. It's almost inconceivable that any natural shutter or light source could be gated at one nanosecond intervals. Finding the exceedingly rare with the almost inconceivable in the same place at the same time moves you from almost inconceivable to fully inconceivable. I tolerate bright, thoughtful contrarians and you just don't fit that category. You had no knowledge whatsoever from which to base your statements but you made them anyway. That's not thoughtful. Move along now. -ds
Mark Frank_{June 14, 2006
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Dave

I didn't say I could dream up a natural hypothesis. I am not a physicist. I am nevertheless willing to bet that a qualified physicist could, because you can think up a natural cause for almost anything. It doesn't have to be particuarly plausible. The alien civilisation is not very plausible either.

Rgds

Again I repeat the key point: Are we to conclude an alien civilisation every time we come across a new phenomenon for which we do not know the source?
Are we to conclude an alien civilisation every time we come across a new phenomenon for which we do not know the source? No, not every time, but in the case of petawatt nanosecond laser pulses it's a pretty safe bet. What isn't plausible about an alien civilization? We know technological civilizations that use lasers exist in at least one instance. It's therefore entirely plausible that a second such civilization exists. I think it might be time for you to move along to another blog. -ds Mark Frank_{June 13, 2006
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Re #7. That's another hypothesis. You could dream up any number of alternative hypotheses - some of which involved design and some of which did not.

I come back to the main point. Are we to conclude an alien civilisation every time we come across a new phenomenon for which we do not know the source? I assume not. So why do the SETI people think this particular phenomenon might be related to a civilisation? There can be only one reason - because they have observed our civilisation make such a thing - as they note in the first sentence of the excerpt.
You could dream up any number of alternative hypotheses - some of which involved design and some of which did not. I'm going to call your bluff. I don't think you can dream up a single plausible hypothesis for a natural source of nanosecond petawatt laser pulses. Making things up out of thin air is going to get you blacklisted here. I don't relish spending my time fisking empty rhetoric. -ds Mark Frank_{June 13, 2006
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Human-like civilization? Why couldn't it be an expanding hive-mind that uses tight-beam lasers for communicating (syncing itself) with remote copies on planets in its solar system? :PPatrick_{June 13, 2006
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So now if we were to observe this phenomenon we have some reason for adopting a human-like civilisation as an explanation.
If we were to rewind the tape of life and replay it on a different planet, it would produce a human-like civilisation. But if we were to replay it here on earth, it would not.Mung_{June 13, 2006
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DS: "I donÃ¢â‚¬â„¢t have access to the article but IÃ¢â‚¬â„¢d bet SETI is looking for a modulated nanosecond pulse stream. The nanosecond pulse is just an attention getter. Any technological entity capable of generating high power nanosecond laser pulses is going to encode a message of some sort in a stream of pulses." I just received my copy of Physics Today, and it seems this is not the case: "And one is not looking for spectroscopic preceision or for the kind of phase-encoded messageing one seeks with radio-telescope heterodyning." Instead, it is the aiming ability of the laser pulse which is relied upon, since its estimated 10^-7 radian spread at the top of the atmosphere concentrates the beam to just 1 part in 10^15 of celestial sphere. The alien's strategy, presumably, would be to aim the beam at one start after another. It seems to me that it is partly the remote chance of receiving such a signal, in addition to the nature of signal, that contributes to the design inference. Note also that the Physics Today article makes reference to a previous, related study, with a less optimized and dedicated system: (Howard et al., Astophysis. J., 613, 1270, 2004.)SCheesman_{June 13, 2006
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The paper is subscription only and rather expensive so I have to base this on the quote only. Consider for a moment why they are looking nanosecond impulses. There must be an infinite amount of potential phenomena that have "no known astrophysical source". Are we to conclude an alien civilisation every time we come across a new phenomenon for which we do not know the source? Of course not. The reason is that man has developed laser technology that makes this particular phenomenon possible. So now if we were to observe this phenomenon we have some reason for adopting a human-like civilisation as an explanation. From the quote it appears that despite this they would not dismiss the alternative type of explanation "an astrophysical phenomenon that no one anticipated". So if the phenomenon occurred there would then be some further assessment of possible causes. What role does the filter play in all this reasoning?Mark Frank_{June 12, 2006
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Oops - looks like the posting software lost my HTML superscript tag (around the number 15) also.sagebrush gardener_{June 12, 2006
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Note to moderator:

"petawatt (1015W) lasers" should be "petawatt (1015W) lasers", or if the posting software can't handle the HTML superscript tag, "petawatt (10^15W) lasers".
Done. Thanks. -ds sagebrush gardener_{June 12, 2006
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Wouldn't the explanatory filter, upon seeing this pulse, conclude intelligence, whereas these guys are concluding either intelligence or law?

The filter would have us note that we know of no law that could produce the pulse (as the SETI people do) and then make a design inference. The SETI people aren't doing that - they leave open the law possibility and don't make the inference.
I don't have access to the article but I'd bet SETI is looking for a modulated nanosecond pulse stream. The nanosecond pulse is just an attention getter. Any technological entity capable of generating high power nanosecond laser pulses is going to encode a message of some sort in a stream of pulses. -ds Tiax_{June 12, 2006
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