This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
On one small rocky planet orbiting a middle-aged main-sequence star, in an otherwise rather dull backwater of a rather ordinary spiral galaxy, a sentient species is about to abandon itself to fantasy.
Specifically, a lot of us humans are preparing to part with our hard-earned credits to gaze at, and listen to, the next chapter of one of the most popular fairy tales of the late 20th century. Yes folks, Star Wars is here (again).
At some point during my blogging career at Scientific American I'm pretty sure I swore to never succumb to the temptation of such tropes as 'The Science of (insert science fiction/super hero title here)'. And I mostly still stand by that. I don't care how dense Thor's hammer has to be, or how massive Darth Vader is, or how uncomfortable Ant Man's underpants must be, and I'm not convinced these are great ways to educate us about science.
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But when a fictional and hugely appealing tale constructs a whole universe around itself, it's hard to resist asking whether any of this could actually be going on around us, unnoticed thus far? So I'll abandon my qualms for the next few paragraphs to try to answer that question.
Here we enter the realm of efforts like the Search for Extraterrestrial Intelligence (SETI), as well as the emerging field of time-domain astronomy. In the latter case, observatories like the future Large Synoptic Survey Telescope (due for first-light in 2019), or Pan-STARRS, are set to transform our view of the cosmos by constantly monitoring a large fraction of the night sky. The discovery and study of so-called 'transient' phenomena is a major scientific output - everything from small faint objects orbiting in our solar system to distant supernova and other astrophysical one-offs. SETI too is going to expand its efforts in coming years, helped by Yuri Milner and the Breakthrough Listen projects.
But despite all of this enhanced sensitivity to the cosmos around us, would we actually be able to detect the goings-on of a galaxy awash with feuding species and interstellar empires?
I've written about a number of related issues more extensively elsewhere, but let's do a quick thought experiment on one particular facet of the question.
In the Star Wars saga, the most visible astrophysical signature of galactic shenanigans is arguably the destruction of planets and the destruction of the planet-destroying tool itself - the Death Star. So a simple question to ask is whether we'd even notice if planets started going pop around other stars (in a galaxy far, far away, or right here in our neighborhood)?
We can calculate the energy requirements quite easily. Let's take an Earth-sized planet. The total gravitational binding energy of a uniform sphere of mass M and radius R is (with gravitational constant G):
For the Earth this yields a total binding energy of about 2.3 x 10^32 Joules - in other words you'd need to put in this much total energy to disperse the Earth until it was no longer held together by its own gravity.
How does this stack up to known astrophysical phenomena?
Well, the Sun pumps out about 3.8 x 10^26 Watts of power. One watt times one second equals 1 joule, and therefore it would take the Sun roughly 605,300 seconds, or 7 days to produce the same energy as required to disrupt the Earth.
But, with your handy dandy Death Star the destruction of a planet seems to take mere seconds. So that would take a burst of power equivalent to cranking up the solar luminosity by roughly a factor of 605,300 - to get all the energy out in a second (for the sake of argument).
Would we notice an event like this taking place? If we saw a star appear to brighten by a factor of ~6 x 10^5 (which is a few times more than something like a fast nova that evolves over many days, but many, many orders of magnitude fainter than a supernova) we might indeed sit up and pay attention. However, if the event only lasts a second or two we might very well miss it altogether unless we happened to be monitoring that part of the sky very actively.
I've also assumed that all of the energy that's supposed to be destroying a rebel planet is instead flooding out as electromagnetic radiation - which wouldn't really be the case. So perhaps it's better to consider a Death Star destruction event. Estimating the luminosity of an explosion like that is a bit tricky (you may laugh), but we could assume that the event has the potential to release perhaps 10 times more energy than the weapon itself. Now we've got a few seconds of output roughly a million to ten million times the luminosity of the Sun. But once again, cosmic distances can make all things faint, and this outburst lasts only as long as it takes to reach the end credits and could require great serendipity to detect.
If a significant fraction of that energy came out as (for example) gamma-rays, we might be in better shape - if the event happened in our general cosmic neighborhood and was detected by something like the Swift observatory. Yet a phenomenon that doesn't last long, or ever repeat itself in the same place, is awfully tricky to interpret - as the history of gamma-ray bursts has shown us.
In the era of exoplanetary science we might notice if a well-studied planet suddenly vanished - no longer transiting its parent star, or tugging at a system's center of mass. But I'm not sure that the first thought that would pop into astronomers' minds would be 'Death Star!' More likely would be, 'lousy data!'
The bottom line is that, in the case of a Star Wars universe, I don't think we can yet rule out the possibility of planet-destroying galactic warfare going on all around us. It could be, and we've not noticed. Equally, we have absolutely no good reason to assume that anyone is out there and buzzing around. Until our ability to monitor the universe around us gets better we have very few constraints to place on the possibilities.
So when you buy a movie ticket in the coming weeks, consider also supporting the astronomical sciences!