Ask Ethan: How Does Very-Long-Baseline Interferometry Allow Us To Image A Black Hole?
“[The Event Horizon Telescope] uses VLBI. So what is interferometry and how was it employed by [the Event Horizon Telescope]? Seems like it was a key ingredient in producing the image of M87 but I have no idea how or why. Care to elucidate?”
If it were easy to network radio telescopes together across the world, we’d have produced an image of a black hole’s event horizon long ago. Well, it’s not easy at all, but it is at least possible! The technique that enabled it is known as VLBI: very-long-baseline interferometry. But there are some critical steps that aren’t very obvious that need to happen in order for this method to succeed. Remarkably, we learned how to do it and have successfully employed it, and the Event Horizon Telescope marks the first time we’ve ever been able to get an image with a telescope that’s effectively the size of planet Earth!
Come get the incredible science behind how the technique of VLBI enabled the Event Horizon Telescope to construct the first-ever image of a black hole’s event horizon!
6 Supermassive Questions On The Eve Of The Event Horizon Telescope’s Big Announcement
“3.) Is a black hole’s event horizon circular, as predicted, or does it take on a different shape? Although all physically realistic black holes are expected to spin to some degree, the event horizon’s shape is predicted to be indistinguishable from that of a perfect sphere.
But other shapes are possible. Some objects bulge along their equators when they rotate, creating a shape known as an oblate spheriod, such as planet Earth. Others creep up along their rotational axes, resulting in a football-like shape known as a prolate spheroid. If General Relativity is correct, a sphere is what we anticipate, but there’s no substitute for making the critical observations ourselves. When the images come out on April 10, we should have our answers.”
On April 10, one of the longest-awaited advances will finally be upon us, as the Event Horizon Telescope collaboration will reveal their very first image of a black hole’s event horizon. It should test General Relativity as never before, including measuring the black hole’s event horizon, its sphericity, the radiation coming from around it, and many other properties.
What’s the full suite of what our first direct image of a black hole might teach us? Come get the amazing science behind a spectacular discovery we’ve all been waiting for!
This Is How We Will Successfully Image A Black Hole’s Event Horizon
“Normally, the resolution of your telescope is determined by two factors: the diameter of your telescope and the wavelength of light you’re using to view it. The number of wavelengths of light that fit across your dish determines the optimal angular diameter you can resolve. Yet if this were truly our limits, we’d never see a black hole at all. You’d need a telescope the diameter of the Earth to view even the closest ones in the radio, where black holes emit the strongest and most reliably.
But the trick of very-long baseline interferometry is to view extremely bright sources, simultaneously, from identical telescopes separated by large distances. While they only have the light-gathering power of the surface area of the individual dishes, they can, if a source is bright enough, resolve objects with the resolution of the entire baseline. For the Event Horizon Telescope, that baseline is the diameter of the Earth.”
The Event Horizon Telescope is one of the best examples of international collaboration, and its necessity, in answering questions that are too big for any one nation to do alone. Part of the reason for that is geography: if you want to get the highest-resolution information possible about the Universe, you need the longest-baseline of simultaneous observations that it’s possible to make. That means, if you want to go as hi-res as possible, using the full diameter of the Earth. From the Americas to Eurasia to Africa, Australia and even Antarctica, radio astronomers are all working together to create the first image of a black hole’s event horizon.
What does it look like? Is General Relativity correct? As soon as the Event Horizon Telescope team releases their first images, we’ll know. Come watch a live-blog of a talk from their team today, and get the answers as soon as we know them!
2018 Will Be The Year Humanity Directly ‘Sees’ Our First Black Hole
“No longer will we need to rely on simulations or artist’s conceptions; we’ll have our very first actual, data-based picture to work with. If it’s successful, it paves the way for even longer baseline studies; with an array of radio telescopes in space, we could extend our reach from a single black hole to many hundreds of them. If 2016 was the year of the gravitational wave and 2017 was the year of the neutron star merger, then 2018 is set up to be the year of the event horizon. For any fan of astrophysics, black holes, and General Relativity, we’re living in the golden age.”
Black holes have been dreamed up by theorists for centuries. Even in the aftermath of Newtonian gravity, a spectacular realization came about that if you gathered enough mass together into a small enough volume, the gravitational effects would be so pronounced that nothing, not even light, could escape. These black holes show up with very specific properties in General Relativity, and today in modern astrophysics, we know of three independent ways to make them. But despite observing their effects in many different wavelengths of light, such as the radio and X-ray, we’ve never imaged an event horizon directly. Although a telescope the size of Earth would be able to, that technology is out of reach. Owing to collaboration and human ingenuity, however, we’ve developed an array known as the Event Horizon Telescope that should reveal its first image next year!
Will we see the event horizon of a black hole for the first time? My bet is on yes. Come get the science as to why!
Ask Ethan: What Should A Black Hole’s Event Horizon Look Like?
“Shouldn’t the event horizon completely surround the black hole like an egg shell? All the artist renderings of a black hole are like slicing a hard boiled egg in half and showing that image. How is it that the event horizon does not completely surround the black hole?”
Black holes were one of the first consequences of general relativity that were predicted to exist, and the more we’ve studied the Universe, the more interesting they’ve become. We’ve calculated their physical sizes, their effects on the curvature of spacetime, their apparent angular sizes, and the properties of matter that gets caught in an accretion disk around them. But we’re about to take another giant leap forward: we’ve about to directly observe one for the first time. Sure, it will be in radio frequencies rather than visible light, but we should be able to directly image the event horizon, and contrast those observations with our best predictions. What should that event horizon look like, though, and why – if it’s completely black – should we be able to see it at all? The answers are both fascinating and informative, and when the results are released later this year, you’ll want to know.
Come learn all about it on this week’s Ask Ethan!