A Telescope the Size of Earth

On April 10, 2019, a team of astronomers made history: They published the first photo of a black hole.

The astronomers took the image using radio telescopes positioned around the world, including at the South Pole. The telescopes were linked using a technique known as very-long-baseline interferometry, or VLBI.

What didn’t make the headlines is that the photo, which was viewed by people around the world, would not have been possible without atomic clocks.

As their name suggests, radio telescopes take in electromagnetic radiation in the form of radio waves, similar to those that bring us music and news in our cars. Every radio emission captured from space carries two key pieces of information: wavelength (the distance between successive wave peaks) and phase, or where in its cycle the radio wave was detected. This information tells us about the distant objects that emitted these radio waves.

For VLBI, the radio waves' phase is key. By comparing phase information collected by telescopes around the globe, astronomers can effectively create a telescope the size of the Earth. This vastly increases the resolution of the images that can be captured.

But radio waves cycle fast — thousands, millions or even billions of times per second. So very accurate clocks are needed to record exactly when light arrived at a particular telescope. Only atomic clocks can make these pinpoint time stamps. Most radio telescopes use a type of clock called a hydrogen maser to time-stamp their observations.

Data from far-flung radio telescopes are then sent to a central location, where scientists use the phase and time information to construct an image of a distant object. To make the black hole photo, astronomers stored data on high-performance helium-filled hard drives, then flew these hard drives to specialized supercomputers where the data were combined.

VLBI isn’t used just to make pretty pictures of black holes. Astronomers have used the technique to image the surfaces of distant stars and planets, and to create a reference frame based on distant galaxies that can be used for navigation. VLBI is also used closer to home to measure the motion of Earth's crust and fluctuations in the length of the day. It can even be used to compare atomic clocks located on different continents.

Next: Future Directions: Atomic Clocks Meet Quantum Entanglement