HGST note: Space-time continuum. It might sound a bit like science fiction, but to Shep Doeleman, it’s an everyday reality. We are so excited to have Shep, principal research scientist at MIT, assistant director of MIT Haystack Observatory and astronomer at the Harvard Smithsonian Center for Astrophysics, share how one creates an image of a black hole boundary, and tests Einstein’s theories using radio dishes that span the globe. Did we mention that this has never been done before? Welcome to the Event Horizon Project.
Chasing Black Holes and a 100 Year Old Theory
I spent two weeks this March at the Large Millimeter Telescope (LMT) atop Sierra Negra, a 15,000ft peak near Puebla, Mexico with a group of astronomers trying to take a “picture” of a black hole.
It’s not easy. For one thing, black holes, even the supermassive variety that can weigh millions or billions of times what our Sun does, are the smallest objects thought to exist in the Universe. They are formed when matter becomes so dense and is confined to such a small volume that the force of gravity is irresistible, resulting in catastrophic collapse into a ‘singularity’: a single point of infinite density. Surrounding this singularity is the event horizon, a boundary where gravity is so strong even light cannot escape its pull. But contrary to what you might think, black holes are not dark. The gas and dust they attract becomes squeezed into ever smaller volumes, heating to billions of degrees before crossing the event horizon, making black holes some of the brightest objects in the sky.
One hundred years ago, Einstein developed his theory of general relativity, describing how matter warps space-time, and how space-time – in turn – affects the motion of matter and the paths of light beams. His theory has passed every test so far. We were on Sierra Negra as part of an investigation to see whether Einstein’s theory of gravity holds at the edge of a black hole – the most extreme environment and the ultimate proving ground.
General relativity predicts that a ‘shadow’ should form around the black hole as the light generated by heated in-falling gas is bent by gravity into a ring that encloses a dark center. Our goal is to make an image of this ring to see whether it has the predicted shape and size, as predicted by Einstein. The closest supermassive black hole – and the one whose event horizon and shadow would be the largest – is Sagittarius A* (SgrA*): a black hole at the center of the Milky Way, with a mass of 4 million Suns. Even SgrA*’s shadow is only 50 micro arcseconds across, so to snap its picture requires a telescope with a magnification capable of seeing a grapefruit on the moon!
Our project, called the Event Horizon Telescope (EHT) achieves this magnifying power – 2000 times better than the Hubble Space Telescope – by creating a virtual telescope as big as the Earth. We do this by synchronizing an array of telescopes across the globe to simultaneously stare at SgrA* over the course of the night. Together with the LMT in Mexico, and with other colleagues manning radio dishes in Hawaii, California, Arizona, Chile and Spain we have begun assembling a global array. At the high altitude and remote sites, radio waves from the event horizon of SgrA* are captured and stored in real time using HGST** high-capacity and high-speed storage solutions (learn why HGST helium-filled drives are critical to the EHT project*).
When we play the recordings back at a central facility (many weeks later) we can combine the signals in exactly the same way that an optical telescope uses a mirror to combine light at a focal point (where a camera or eyepiece sits to
relay an image back to a human eye). This process effectively produces a mirror whose size is equivalent to the distance between the EHT sites around the world. But this process is an agonizing case study in delayed gratification. We don’t know if everything worked well until all the data is brought together at a central computer cluster.
This summer, our team started processing the data we collected in March of 2015, and the news is good. We know that the EHT did work, and we will be analyzing the results in the coming months. It’s incredible that this technique works at all, but it does; and now we are on track to test Einstein’s theories using radio dishes that span the globe – listening for radio whispers from the event horizon.
– Shep Doeleman
Talk to Shep and the Event Horizon Team Live on Ask Me Anything – Reddit!
Shep Doeleman and scientists from the Event Horizon Telescope Team will be holding an Ask Me Anything (AMA) session on Reddit about black holes and what it takes to capture the world’s first image of a supermassive black hole on August 26, at 2pm EDT. AMAs are open to all Reddit users and use the site’s comment system for both questions and answers. Click here to learn more. Read HGST’s press release here.
Cover picture: Part of the Event Horizon Telescope project, antennas of the Atacama Large Millimeter/submillimeter Array (ALMA), on the Chajnantor Plateau in the Chilean Andes. Credit: ESO/C. Malin
Insert picture: Event Horizon Sites. Credit: Gopal Narayanan