Speaker
Description
THE PAST: Concepts of astronomical interferometry were proposed by Fizeau in the 1860’s and tried out by Stéphan in Marseille some decade later. After achieving the first successful stellar observations on Mt.Wilson around 1920, the field saw little development until Hanbury Brown and Twiss measured intensity correlations in the 1950’s. The Narrabri intensity interferometer followed, but its further development was stalled by the concurrent success of revived phase interferometry. Intensity interferometry instead saw a vigorous development in particle physics, where the boson statistics of particles with integer quantum spin are equivalent to those of photons. The Nobel prize to Glauber followed, while instrumentation was developed for quantum optics, in particular for laser light scattering against microscopic media. Discoveries of transient phenomena stimulated work in high time-resolution astrophysics, with instrumentation reaching nanoseconds, enabling also intensity interferometry. Design studies for extremely large telescopes were paired with laboratory experiments, together with a realization that emerging air Cherenkov telescopes could be used for the task. In 2009, various groups met in Salt Lake City for the first conference on intensity interferometry in the modern era, with the following years marked by a strongly increased activity worldwide.
THE FUTURE: Strengths lie in very long and very many baselines, a very good Fourier (u,v)-plane coverage, and 2-D image reconstruction, possibly enhanced with correlations between triplets of telescopes. Image reconstructions are common in laboratory X-ray intensity interferometry, from where software experience might be brought. To improve signal-to-noise, time resolution could be pushed to perhaps 10 ps, although only feasible in ‘ordinary’ telescopes (intrinsic time spread in Cherenkov instruments may be a limiting parameter). Great improvements will result from multiple spectral channels. However, ordinary spectrometers need small focal-plane images, requiring ‘ordinary’ telescopes, of which groups of sufficient size might be awkward to secure. A possible path could be telescopes with optical fidelity between Cherenkov and ‘ordinary’ ones; still inexpensive and not requiring dome enclosures, yet adequate for spectroscopy. If large-format energy-resolving detectors become available, also Cherenkov telescopes would become multi-wavelength instruments. While the cost of a dedicated interferometer array could be modest compared to other large facilities, its realization would likely be limited by the current size of the community, prompting some collaboration. One worldwide community studies astrophysical transients with multi-messenger techniques. A priority and bottle-neck is to get source spectra soon after their discovery. For this, concepts like the “Time-Domain Telescope” are proposed, consisting of about 100 telescopes, feeding multiple spectrometers with high-speed detectors. If such a group of telescopes would be suitably spread out and equipped with nanosecond-resolution detectors, it could serve multi-messenger astronomy and intensity interferometry alike.
