# Is Optical VLBI theoretically feasible? If not why not?

There are plenty of optical interferometers in use with baselines of up to maybe 1km. As far as I can find out, they all work by directly collecting the light at all the telescopes, using mirrors to bring them together and then actually interfering them and observing the fringes with photodetectors.

Meanwhile, radio astronomers do interferometry over distances of thousands of km, but they do it quite differently. Essentially, they record the baseband electromagnetic waveform at each telescope, with an accurate timestamp, transport them all to a central location and combine them computationally.

So my question is whether in principle you could use the radio astronomers methods with an optical (or near IR) signal. Obviously the engineering and computational challenges would be formidable -- you would need to record data at speeds close to 1 Pb/s, timestamp it with femtosecond accuracy and do a ludicrously huge computation to recover an image, but that's not my question here.

On the one hand, it seems like the answer should be "yes" -- electromagnetic waves are electromagnetic waves. On the other hand, why should the phase of a photon observed in one telescope have anything to do with the phase of one observed in another? If you are digitising the data at each telescope, you are ruling out interference fringes on a quantum level, just as happens if you find a way to measure which slit a photon went through in the double-slit interferometer. This suggests that the answer should be "no".

If the answer is "no" then what wavelength does the cutoff happen at? We have radio-style VLBI at 1mm now and optical-style interferometry at 1 $$\small\mathsf{\mu m}$$, what is possible in between?

• For inferometry, don't the time measurement requirements (relative to wavelength) get tighter as the baseline increases? (i.e. higher frequency = more absolute precision needed) – Alex Hajnal Jan 8 '19 at 11:54
• @AlexHajnal Yes entirely, That's one of the formidable engineering challenges. – Steve Linton Jan 8 '19 at 14:17
• @SteveLinton You've answered it yourself. The problem is the huge time sampling frequency. – Florin Andrei Jan 9 '19 at 0:20
• I thought this has been done already (at least in IR), I'll try to look into it. If you observe a narrow emission line, you can mix it with a laser of slightly different frequency in a nonlinear junction and produce an RF signal at the frequency $f_2-f_1$. Optical down conversion is a well understood phenomenon and I have a hunch this is in fact "a thing". It's an exact analogy to the higher frequency bands (about 1 THz) at ALMA where they down-convert to a few GHz before digital sampling – uhoh Jan 9 '19 at 0:22
• You might want to look into the Van Cittert–Zernike theorem for the coherence of sources and thus photons in different pupils. It should apply both to radio and visible wavelengths. – Hannes Jan 18 at 22:12