The plasma jet from the center of a distant galaxy is like a river that flows from its source and gradually becomes clear. Near the black hole, the flow is dense and opaque to radio waves, like muddy water. Only at some distance does the jet 'clear up,' and it is there that radio telescopes see the bright core. But this core is not the black hole itself; it's the point where the jet first becomes visible.
Astronomers observed BL Lacertae — a galaxy whose jet is aimed almost directly at Earth. They used a network of radio telescopes spread over thousands of kilometers to get an ultra-sharp image. They found that at a frequency of 8.4 gigahertz, the core appears 250 microarcseconds farther from the black hole than at 43.2 gigahertz. This shift is a thousand times smaller than the thickness of a human hair seen from a kilometer away.
From the magnitude of the shift, they estimated how quickly the jet becomes transparent. This allowed them to calculate the magnetic field strength at the jet's base. It turned out that the energy of particles and the energy of the magnetic field there are nearly equal, like a river where the energy of the current and the energy of the eddies are balanced. Such an equilibrium has long been predicted by theory, but this is the first time it has been tested so close to a black hole.
These measurements were made possible by a neighboring radio galaxy that served as a fixed reference point. Astronomers compared the position of BL Lacertae's core with that of this neighbor, thus eliminating atmospheric jitter and other disturbances. The precision was fantastic: 5–30 microarcseconds — the angle under which a coin is seen from a distance of 4000 kilometers.
250 microarcseconds is the angle under which a penny is seen from 4000 kilometers away (roughly from Moscow to Krasnoyarsk).
In the series 'Star Trek,' Federation starships use plasma jet deflection for navigation, but real astronomers measure their shift to peer into the very heart of a galaxy.