# Can you build an interferometer between the stars?

We're getting some very good results here on Earth using interferometers, 'virtual' telescopes with dishes hundreds or thousands of kilometers across that are 'built' by combining the data from multiple sites in a computer. This got me to wondering. I know the theory behind such devices, but I'm a little fuzzy on the details of exactly how it is done - specifically, whether the data needs to be combined in real time, or whether recorded observations can be compiled and combined after the fact.

The Colonisation Program - an autonomous, non-governmental organisation that exists to train people as colonists and send them to new planets - has access to six of the twelve Faster-Than-Light starships in the human galaxy. One of their primary missions is to search for potentially habitable planets, gather as much information as they can about them, and then send missions to colonise them. To this end, they want to build a really big interferometer and I, as the God of their universe, have to decide whether they can or not.

So, here's the question: Given access to an FTL starship to courier data back and forth, is it theoretically possible to build an interferometer array from telescopes in multiple different star systems, with a baseline measured in light years? If it's possible, how much detail would such a telescope be able to see in a star system hundreds of light years away? If it's not possible, what would be the maximum practical size of an interferometer?

• To the question of how it is done: the data must be coherent, meaning the exact time delays between the signals can be measured and analyzed (to within a fraction of a wavelength). Often this is implemented by combining the data in real time, but it can be done by other means that guarantee the data remains coherent and then post processed later. – Cort Ammon Jan 2 '17 at 1:28
• I think you are mixing the ideas of interferometer and synthetic aperture. See How big can we make a telescope? for hard-science numbers. Also, I've seen other posts (here and on Physics) calculate the resolution and seeing ability — the same issues apply here, just scaled up. – JDługosz May 31 '17 at 21:00
• Aperture synthesis is a type of interferometry. They're the same thing. See the accepted answer's definition of Very Long Baseline Interferometry. – Werrf Jun 1 '17 at 17:07

What is the maximum practical size of an interferometer? Apart from the obvious fact that this depends on the maximum practical technological capacity any given technological society has at any given time, then the correct answer is it depends.

However, it worth considering what constitutes Very Long Baseline Interferometry (VLBI) and try to dtermine whether it is workable in the OP's scenario.

VLBI is an acronym for Very Long Baseline Interferometry and associated with radio astronomy and geodesy. Typically VLBI refers to experiments that do not process their data in real time, but record it for later correlation. In the world of increasing network connectivity, we are entering the realm of eVLBI (electronic VLBI), in which data are cross correlated in virtually real time. VLBI experiments have baselines of usually 100s or 1000s of km.

VLBI falls into several categories:

Continental – baselines of 100s to 1000s of km,
Global – Baselines of 1000s of km,
Space VLBI – involving the use of satellites, like VSOP.


VLBI is used in measuring pulsar parallaxes and proper motions, resolving the cores of radio galaxies and jets from supermassive black holes, among others.

Source: VLBI

The real question is whether "data are cross correlated in virtually real time" means that if data can be cross correlated using FTL spacecraft to transport data sets to combined and analyzed at a given node in the system, then this interstellar baseline interferometer is a practical working system.

Having multiple telescopes located in multiple planetary systems, while this might be convenient, doesn't seem like a good idea. The various interferometer component dishes would be better located in space, outside planetary systems to minimize gravitational perturbations, and with this elements are relative rest to each other. This is basically how space-based VBLI systems currently work. The same principles should apply not over interstellar distances.

Essentially the cross correlation centre should be located at the midpoint of the various interferometer telescopes, so that the data are analyzed as if it was part of the one single array. This won't be analyzing the data in "true" real time, but in a "time-shifted" virtual real-time.

Referring back to the basic principles of radio astronomy interferometry and this give a reasonable idea of the practically of an interferometer VLBI and its effective resolution of stars hundreds of light years distant.

A radio interferometer is an array of radio antennas or ‘elements’ that are used in astronomical observations simultaneously to simulate a discretely-sampled single telescope of very large aperture. To put it another way, a radio interferometer can be thought of as a single telescope with a very large and incompletely-filled aperture, of maximum size equivalent to the maximum spacing, or baseline, between any two of its component elements. This large ‘synthesized’ aperture is only sampled at the locations at which an element exists, and this is aided by the rotation of the Earth which effectively moves the elements within it, hence increasing the sampling. This is known as ‘Earth rotation aperture synthesis’. The size of the synthesized aperture dictates the resolution or ‘beam size’ of the array; the larger the aperture, the smaller the resolution.

If the aperture size is light years, then the resolution of stars hundreds of light years away should be extremely good. Certainly, enough to map the surfaces of any planets.

• You also need time synchronyzation. For correlation you have to measure time delay in 0.001 nanoseconds. It's hard since even for close space you need not only precise clocks but take into account relativistic effects and solve math task. Interstellar equations could be too complex because of gravity from any planet between them. 0.001 nanosecond is reality for today. On Earth orbit it's enough but for interstellar - maybe no. It could be solved in far future but possibly could NOT. – ADS Jun 1 '17 at 21:18
• @ADS Quite so. "It could be solved in far future but possibly could NOT." I assumed that in future it might be, but I didn't think that would be trivial. I remain amazed by the technological sophistication in the engineering of gravitational wave detectors. My proposal is an extrapolation of similar technological developments for interferometers. – a4android Jun 2 '17 at 4:41
• I agree it could be solved. Especially in its own world :) Just because of reality-check tag I added comment to note that something is hard enough (even with FTL) and something is just impossible (like Heisenberg's uncertainty principle) – ADS Jun 2 '17 at 6:17