The Power and Potential of Stellar Intensity Interferometry

Stellar Intensity Interferometry (SII for short) is a field that has recently starting to gain a lot of attention. Recently a Nature Astronomy paper was published on how it was recently used to take sub-milliarc measurements of stars, showing that Stellar Intensity Interferometry (SII) is a useful and powerful technique.

Many of the details of this post come from the recently published Nature paper found here.

Full disclosure, I Jonathan Davis, am a contributing author on the Nature Astronomy paper. Nolan Matthews is the lead author along with David Kieda. They have helped teach me much of what I know about SII. Nolan Matthews has also posted on the Nature Research Astronomy Community which goes into some more of the story here.

What Is the Deal With Stellar Intensity Interferometry?

It was used in the past to take some of the first measurements of star angular diameters in the 1960's with what was known as the Narrabri Stellar Intensity Interferometer

A picture of the Narrabri Stellar Intensity Interferometry in Australia. They were set on tracks and would track a Stellar Targets throughout a given night. I retrieved the image from here

Stellar Intensity Interferometry (SII) is a technique that measures a unique property of starlight, helping measure values on what is called the "Fourier Plane Visibility". I won't go into too much detail of what this exactly means, but basically, if you can measure the visibility in the Fourier Plane of an object in space extremely well, one can perform something called 'model independent imaging' which can basically produce an image of the object. If you are interested in learning about the Fourier transform, there is an awesome YouTube Video which gives a great intro to some of the basics.

You might recognize this picture which uses techniques similar to SII

The viral picture of the black hole which you can learn more about here. In order to take this, scientists use telescopes all over the world to measure the "Fourier Plane visibility" of this black hole. They then can reconstruct the image of the black hole using complex algorithms, something that is also theoretically possible to do with the measurements made using SII. The biggest difference is that this picture was taken using radio waves. SII uses optical light (this is what us humans see) which presents many different problems to overcome. SII can't as of yet produce a picture like this, but there is promise that it could do so in the future.
Credits: Event Horizon Telescope collaboration et al.

SII 'images' things in space using unique and complex techniques that involves a photo-multiplier tube (a really sensitive photon measuring device), incredibly fast cameras, and huge computing power to measure the coherence of starlight at different telescopes.

SII might one day be able to do what radio astronomers have done with the black hole image but SII is not yet at the level that it could produce an image like the black hole image. However, the recent star angular diameter measurements made using SII show that with upcoming, more powerful observatories, that there is the potential for SII to be used to image stars in a similar manner to how the black hole was imaged.

What Happened to the Narrabri Observatory?

Robert Hanbury Brown, John Davis (not to be confused with me, Jonathan Davis), and Richard Q Twiss, who have sadly passed away, were the leaders of the Stellar Intensity Interferometer field at the start. They helped create the underlying science, the creation of the Narrabri, and the creation of the first star diameter catalog using SII.

During the time of the Narrabri, SII was unparalleled in it's ability to measure star diameters. However, with emerging techniques with OAI observatories it was eventually found that with the technological limits of the 1960's and 1970's, that OAI was able to surpass SII. Because of this, even though SII was just beginning, the field began to meet an early demise as OAI observatories began taking over due to their better capabilities. The Narrabri observatory eventually shut down with plans for a second generation SII observatory being cancelled. 

Eventually, scientists at the University of Utah like Stephan Le Bohec and Dainis Dravins at Lund university started to theorize that current technology might make this field more viable than it was in the 60's and 70's. Others at the University of Utah like David Kieda, saw potential and helped get funding. This eventually led Nolan Matthews in assisting and leading the creation of a modern Stellar Intensity Interferometer with those previously mentioned. I of course then joined in on all the fun when I was tasked to help plan and eventually take some of the first modern Stellar Intensity Interferometry measurements along with Nolan Matthews and David Kieda.

What was Happening at 3am in 2019 During a December Night?

Nolan Matthews and I were actually making similar measurements that were made with the Narrabri observatory. However, instead of using the Narrabri telescope (which is impossible given the Narrabri no longer exists), we were using The Very Energetic Radiation Imaging Telescope Array System or VERITAS (I will be using the acronym as the full name is a mouthful) telescopes which are known as imaging atmospheric-Cherenkov telescopes, or IACT for short, to make angular diameter measurements of stars.

IACT telescopes, even though these telescopes were designed to detect gamma ray showers which is completely different field entirely, IACT telescopes fill many of the necessary requirements for Stellar Intensity Interferometry.  As VERITAS happens to be an IACT it therefore fits the needs of a Stellar Intensity Interferometer.  David Kieda has been part of VERITAS since its creation and partly because of this, the VERITAS collaboration became an important part of building a modern Stellar Intensity Interferometer.
A picture of the VERITAS instrument I got from the VERITAS website. These telescopes are monstrously big, being close to 3 storied buildings in height. It is located close to the border in Arizona, USA. It wasn't designed for SII, but was designed to study cone shaped, photon showers, which come from high energy particles hitting the atmosphere. Regardless, we were able to use it to measure many different sizes of stars.

Using VERITAS, not only did we measure some of the same targets that were measured all those years ago with the Narrabri, we were able to do it orders of magnitude faster with lower error to boot. More specifically, we have shown that we were able to measure the angular diameter of Alnilam and Mirzam.
A beautiful picture of Orions belt and more specifically Alnilam, which is in the center and happens to be one of the stars we measured with VERITAS. We measured Alnilam's angular diameter or how large it appears in the sky. If one knows the angular diameter of a star and how far away the star is (which is often found using parallax), you can calculate how big the star actually is. You can download this picture here.
Davide De Martin & the ESA/ESO/NASA Photoshop FITS Liberator

If you know the angular diameter of a star and it's distance from earth, you can calculate how large the star is physically, something which is important for testing stellar physics models.

These measurements not only show that SII is useful, it shows that even with an observatory that wasn't designed for SII, the measurements we made can add to astronomy fields in unique and powerful ways. 

What this means

This isn't to say that SII will replace OAI or anything along those lines. It's to say that SII now has scientific evidence of its potential and utility for specific cases.

This leads to one of the biggest and coolest upcoming use cases of Stellar Intensity Interferometry, by using it with the Cherenkov Telescope Array (CTA)

An illustration of what some of the Cherenkov Telescopes will look like. Due to the massive number of telescopes planned to be built, it would be practically impossible to try to physically connect them like you would with a physical interferometer. Stellar Intensity Interferometry doesn't need physical connections and can digitally interfere light from all of the telescopes in unison. By using Stellar Intensity Interferometry, CTA might even be able to directly image stars, just like what was done with the black hole. You can learn more about CTA here, which is where I also got the image from.

Researchers have proposed that using a CTA like setup with Stellar Intensity Interferometry might even be able to produce images of stellar objects that can reveal surface features. Since Stellar Intensity Interferometry holds such powerful imaging potential, it could not only image stars, but might even be able to study things that are around stars as well, like gas flows. This type of super high resolution is something that is off in the future, but it is absolutely possible given the right instruments and sensitivity.

Honestly, I think it would be super cool if the original leaders of SII were still alive today. Not only would it be pretty funny if me and John Davis were both on a published paper together, possibly confusing editors and scientists around the world, they all would have been able to see what could likely be the revival of Stellar Intensity Interferometry.

Well so What?

I often get asked this, or something along the lines of "why does any of this matter anyway?". For a non-astronomer it can be hard to see why astronomy science is important in life beyond the impact it will have, or even the cutting-edge technology that will be created along the way.

However, if you think about it, you'll realize that it's only due to humanities understanding of the universe that you are able to read this post in the first place! The phones you talk on, the computers you use every day, the math used to model pandemics, high tech hospital equipment which saves lives, all stems from an initial understanding of how the world and universe works. Even around half of the food you find on your plate stems from when a chemist pulled nitrogen out of the air using a complicated machine that was designed using engineering and physics principles. 

Stellar Intensity Interferometry can seriously help improve our understanding of stars. By being able to better understand the physical structure of stars, the structure around stars, how they deform, etc. scientists can build and test models which predict how they came be that way, which leads to better understandings of all sorts of fundamental physics.

By studying the universe and trying to understand it, we add more pieces to our overall understanding, which leads us to be able to change the world around us. Stellar Intensity Interferometry has the potential to find pieces that we might currently be missing. Filling in the missing pieces of physics can, and has, changed the world. However, we can only understand the universe if we actually go out and try to understand it. This takes resources, time, and of course money, but in my opinion, it is absolutely worth the investment.