Tuesday, September 6, 2016

Almost certainly wrong: an alien megastructure speculation about KIC 8462852

Update: 20 September 2016 - with the Gaia DR1, we didn't really know which way the 300 micro arcsecond systematics would push, us, but now there is some evidence that the parallax measurements are systematically underestimated. Another nail in the coffin.

Update: 14 September 2016 - it would seem that today's Gaia data release invalidates this, as the the star is no further from us than what Boyajian, et. al., estimated from its brightness, and possibly a fair bit closer.  So what we are seeing is a real dimming.

OK, what follows is highly speculative, but as far as I can tell, is at least internally consistent and doesn't require any exotic new physics. I've got some facts in here, but if all you care about is the facts, this isn't for you.

As I pointed out recently, to find ET technological civilizations, we're going to have to be wrong a lot - unless they are trying to make it easy for us, which they very well may not be. So, I am a long term optimist but short term pessimist. Unfortunately, being persistently wrong is very painful for some people, many of which might be the most qualified to try and set out the theoretical parameters for ET technology.

So, let me have a crack at it for the case of the star KIC 8462852, commonly referred to on this blog as "Tabby's Star," and I could well be proven wrong in a few days with the first Gaia data release. I will stick to known physics exploited with unknown technology, and perhaps it may take a bit longer to prove me wrong.

The conjectured megastructure is actually a swarm (conceivably millions) of light sails flying close to the star, using light pressure in clever ways to maintain their positions (I won't detail this yet, because my model of "near field" stellar sailing isn't very good). The megastructure is a shell of reflectors, perhaps within one or two stellar radii (a few million kilometers) of the star's atmosphere. These sails are steered in a coordinated way such that they concentrate the star's light in a particular direction by a high magnification, for the purpose of accelerating (or possibly deaccelerating) a very large light sail and its payload up to interstellar speeds - perhaps a few percent of the speed of light. It would concentrate the star's light by several orders of magnitude.

Think about it - this star puts out about 10^27 Watts of power, most of it in visible light. If we could
Illustration from Robert Forward's 1984 Paper on Laser Sails
concentrate just one billionth of that power onto a reflective sail, that's 10^18 Watts, or a thrust of almost 10 billion Newtons - a ridiculous amount of thrust by terrestrial standards (probably more than you could use). In 1984, Robert Forward published a conceptual design for such a sail powered by an array of lasers.

If you build a really huge light sail out of aerospace grade unobtanium - say 1 million square kilometers (a bit bigger than Texas), it would receive much, much less than a billionth of the star's light at a distance of only a billion kilometers - not far away - so you'd want to concentrate it by a lot, and you'd want to control the amount of concentration - I'm guessing you'd want it to get as high as a million (6 orders of magnitude), but that of course is just a guess. Most of the acceleration would be in close to the star, since just 1 or 2 light years away the thrust would drop off to very low. This predicts that the concentrated beam of light would only be on for the order or decades, or centuries at most, but it would eventually get turned off or steered to another probe.

This predicts that we are on the edge of this beam (the sail is not being pushed straight at us), or are perhaps looking at a sidelobe of the beam formed from secondary reflections, just as a transmitting radio dish has sidelobes. So, the sharp dips in the star's brightness are glitches, or adjustments in the steering of the beam, and the slow dimming observed is the beam slowly steering off from our direction in a controlled manner.

This predicts six observable things and a possible seventh:

  1. The slow dimming will continue, and then stop when we are out of the beam altogether.
  2. The big dips will continue, but will not exhibit periodicity.
  3. The star is actually quite a bit further away than its brightness would indicate, but its proper motion indicates it can't be extremely further than the current estimate of roughly 1500 light years.
  4. We wouldn't see much IR excess, since the backs of the mirrors are away from us, but we might see a little.
  5. At some point, the probe would be too far away from the star to achieve much acceleration, so the beam might be "turned off" at that point and so there could be a sudden and permanent dimming. Alternatively, a deceleration phase could start, and the sail might become a detectable optical source.
  6. There should be some plausible destination for the interstellar probe, with a line of sight within a degree or so of our line of sight to Tabby's Star (if we want a concentration of 6 orders of magnitude). It should be quite a bit closer to Tabby's Star than we are. The star TYC 3162-879-1 might be a candidate. We don't know its distance directly, but it has a bit higher proper motion than Tabby's Star, so might be closer to us, and its line of sight from Earth is only 3 arc minutes (0.05 degrees) from our line of sight to Tabby's Star. There are quite a few other stars that might qualify.
  7. At the destination, there might be another mirror for deacceleration purposes, which would not be "turned on" right now, but would exhibit a significant IR excess when it is.
One candidate star only 3 arc-minutes from Tabby's Star in our sky (TYC 3162-879-1)
So, as I freely admit, probably wrong, but is it at least wrong? What do you think?

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