Astronomers are developing a new technology that could allow us to see the first-ever color image of a supermassive black hole!
This breakthrough comes from the Event Horizon Telescope (EHT) team. They have found a way to observe the radio sky at multiple frequencies simultaneously, which is like taking a “color photograph” of a black hole.
Color is an interesting phenomenon. In physics, the color of light is defined by its frequency or wavelength. The longer the wavelength or the lower the frequency, the closer the light tends toward the red end of the spectrum. Moving towards the blue end, the wavelength shortens and the frequency increases. Each frequency or wavelength has its unique color.
Why is a color image so special?
We cannot see radio waves with the naked eye, but radio telescopes can capture these waves and interpret them as different “colors” or frequency bands. In the past, most radio telescopes could only observe one frequency band at a time. This meant that to create a color image, astronomers had to observe the same object multiple times, imaging it at different frequency bands, and then overlay them.
For fast-changing objects, such as the dynamic environment around a black hole, this method doesn’t work. Imagine taking a motion picture with your phone, but each color needs to be exposed separately for a tenth of a second. The resulting image would undoubtedly be blurry and misaligned. The material around black holes moves at extremely high speeds, so it is necessary to synchronize the capture of data from different frequency bands.
Of course, this is not how we perceive color. Our eyes detect color through three different types of cone cells in the retina, which are sensitive to red, green, and blue light frequencies. Our brain then uses this data to create a color image. Digital cameras work in a similar way. They have sensors that capture red, green, and blue light. Then, your computer screen uses red, green, and blue pixels, tricking our brains into seeing a color image.
Although we cannot see radio light, radio telescopes can detect colors, which are called “bands.” Detectors can capture a narrow frequency range, known as a frequency band, similar to how optical detectors capture colors. By observing the radio sky in different frequency bands, astronomers can create a “color” image.
But this is not without problems. Most radio telescopes can only observe one frequency band at a time. Therefore, astronomers need to observe a target multiple times at different frequency bands to create a color image. This is no problem for many targets, but for fast-changing or small targets, this method fails. The image changes too quickly for the images to be aligned. Imagine if your phone camera needed a tenth of a second to capture each color. This would be fine for landscape or selfie photos, but for action shots, different images wouldn’t align.
This is where the new method comes into play. The research team used a technique called Frequency Phase Transfer (FPT) to overcome atmospheric distortion of radio light. By observing the radio sky at a 3-millimeter wavelength, the team can track how the atmosphere distorts the light. This is similar to how optical telescopes use lasers to track atmospheric changes. The team demonstrated how they could simultaneously observe the sky at 3-millimeter and 1-millimeter wavelengths, using this data to correct and sharpen images collected at the 1-millimeter wavelength. By correcting the atmospheric distortion in this way, radio astronomers can continuously capture images at different radio frequencies, all of which can be corrected, thus creating high-resolution color images.
This method is still in its early stages, and the latest research is only a demonstration of the technology. However, it proves that the method is feasible. Future projects, such as the next-generation EHT (ngEHT) and Black Hole Explorer (BHEX), will be able to build on this method. This means we will soon be able to see black holes in real-time, in full color.
