As countless scientific and general news outlets reported today, the image of Sagittarius A*, the supermassive black hole at the center of our galaxy, is a marvelous scientific achievement. One aspect that hasn’t received quite as much attention, however, is the central role that simulations and synthetic data play in the discovery.
If you haven’t read this great science news yet, the Own contribution from Event Horizon Telescope is a great place to get the gist. Based on years of observations from around the world, a massive team at over a hundred institutions managed to piece together an image of the black hole our galaxy revolves around, despite its relative proximity and the interference of dust, nebulae, light-years, and other whims of the Empty.
But it wasn’t just about pointing the telescope in the right direction at the right time. Black holes cannot be directly observed with something like the Hubble or even the still warming Webb. Instead, all sorts of other direct and indirect measurements of the object have to be made – how radiation and gravity bend around it and so on.
That means data from dozens of sources must be collated and cross-checked, which is itself an enormous task and a big part of why observations from 2017 are only now being released as the final picture you can see below. But because this project really has no precedent (even the famous M87* image, although superficially similar, used different processes), it was necessary to essentially test several ways the same observations could have been made.
For example, if it’s “dark” in the center, is it because something’s in the way (and there’s about half the galaxy there) or because the hole itself has a hole (and it shines)? The lack of direct observational data makes it difficult to say. (Note that the images here do not simply show an image based on visible light, but the derived form based on myriad radiance measurements and other measurements.)
Remember to look at an ordinary object from afar. When viewed from the front, it looks like a circle – but is that a sphere? A plate? A cylinder seen from the front? Here on Earth, you might move your head or take a few steps to the side to get a little more information – but try it on a cosmic scale! To get effective parallax on a black hole 27,000 light-years away, you would have to travel quite a distance, probably breaking the laws of physics. So researchers had to use other methods to determine which shapes and phenomena best explain what little is could watch.
In order to systematically examine and evaluate the design decisions of the imaging algorithms and their impact on the resulting image reconstructions, we generated a series of synthetic datasets. The synthetic data has been carefully prepared to match the characteristics of Sgr A* EHT measurements. The use of synthetic data allows for a quantitative assessment of the image reconstruction by comparison with known ground truth. This, in turn, allows for the evaluation of design choices and the performance of the imaging algorithms.
In other words, they generated oceans of data related to various possible explanations for their observations and examined how predictable these simulated black hole environments were.
Lisa Medeiros from the Institute for Advanced Study explained a little bit about how and why the study looked at the spin of the black hole and how that, in a very interesting Q&A that is worth watching in its entirety if you have the time related to the rotation of the materials around them and to the galaxy in general.
“What was really exciting about this new result, compared to what we did for M87 in 2019, is that in Paper 5 we actually include multiple simulations where we examine that [i.e. the spin relationships],” she said. “So there are simulations where the spin axis of the black hole isn’t aligned with the spin axis of the matter spinning around the black hole, and this is a really new and exciting simulation that isn’t in the publications of 2019 was included.”
Of course, these simulations are incredibly complicated things that require supercomputers to process, and figuring out how many make sense and how close they should be together is an art and a science. In this case, the alignment question considered is of inherent scientific value, but could also help interpret, for example, the interference caused by gases and dust swirling around the black hole. When the spin pleases thisits gravity would affect the dust like thiswhich means the readings should be read as this.
“Our simulations, when we compare the simulations to the data, tend to favor models that are almost pointed at us — not pointed directly at us, but offset by about 30 degrees,” Medeiros continued. “And that would suggest that the black hole’s spin axis is not aligned with the spin axis of the galaxy as a whole, and if you believe what I said before, the disk prefers to be aligned with the spin axis of the black hole.” . It appears as if the disk and the black hole are aligned, but that neither is aligned with the galaxy.”
In addition to pursuing specific aspects like this, there was the more general question of what shape (or “underlying source morphology”) would produce the readings they get: essentially the “sphere vs. plate” question, but much, much more complicated.
In one of the papers published today, the team describes the makeup of seven different possible morphologies for the black hole, reflecting different arrangements of its matter, from ring to disk and even a kind of binary black hole – why not, right? They simulated how these different shapes would produce different results in their instruments and compared this to a more computationally (and linguistically) more challenging “general relativistic magnetohydrodynamic” or GRMHD simulation.
You can see these in a combination of two images from the paper here:
The idea was to find out which of the simulations gave results most similar to what they had actually seen, and while there was no runaway winner, the Ring and GRMHD simulations (which, it must be said, rather were ring-like) the most consistent results. This informed the way the data was interpreted for the final interpretation of the data and the resulting image (note that I’m roughly summarizing an extremely complex process here).
Considering that these observations were made about five years ago and a lot has happened since then, there is still a lot to investigate and run more simulations. But they had to hit print at some point, and the image above is their best-informed interpretation of the data produced. As observations and simulations accumulate, we can undoubtedly expect even better ones.
In fact, as Richard Anantua of the University of Texas, San Antonio put it in the Q&A session, you could even try it yourself.
“If you’re in sixth grade and you have access to some of your school’s computers, I think EHT imaging exists and we have all sorts of pipelines and tools that you can teach your class,” he said, seemingly only half to kid. “The data for some of these is public — so you can start working on it now, and by the time you’re in college you’ll pretty much have a picture.”
Simulation meets observation in first image of the supermassive black hole at our galaxy’s center – TechCrunch Source link Simulation meets observation in first image of the supermassive black hole at our galaxy’s center – TechCrunch