When does a wormhole cease to exist?

The limits of a black hole

Where does a black hole end? At the event horizon, one possible answer would be. That border, once crossed, can never be left again by either light or matter. If astronomers were able to observe the shadow of an event horizon, that would be the first direct evidence that black holes exist.

The idea of ​​celestial bodies with such an immense force of attraction that not even light can escape is already several hundred years old. The British natural philosopher John Michell and the French mathematician Pierre-Simon Laplace wrote independently of one another about the existence of such "dark stars" as early as the end of the 18th century. At that time, however, your statements were ignored. It was not until 1915 that Albert Einstein gave this idea a solid theoretical foundation with his general theory of relativity.

Illustration of a galaxy with a black hole in the center

This theory describes the cosmos as a four-dimensional space-time that is curved by objects with a mass. If, for example, light propagates in spacetime, it does not follow a straight line, but the shortest line in curved spacetime: the greater the mass of an object, the greater the curvature and therefore the deflection effect on light or matter. A solution to the equations of general relativity describes a black hole as a singularity - as a point at which space-time is no longer defined: the entire mass of the object is united in a point with an infinitely high mass and an infinitely strong gravitational field. The closer you get to the black hole, the more space-time is curved.

"Black holes are defined by the existence of an event horizon," says Norman Gürlebeck from the Center for Applied Space Technology and Microgravity at the University of Bremen, "without the area of ​​the event horizon, there would be no black hole." The event horizon was first described by the German astrophysicist Karl Schwarzschild in Year 1915 - even if neither he nor other scientists, including Albert Einstein, understood the relevance of these calculations at the time. Schwarzschild solved Einstein's equations of spacetime because he wanted to calculate the gravitational field of a single point of mass. In this way he received solutions for a symmetrical spacetime, which among other things also contained the Schwarzschild radius named after him.

theory and practice

This describes the volume for the simplest type of black hole, which is spherical and does not rotate. If matter is compressed so that it falls below the Schwarzschild radius, it inevitably collapses into a black hole. This applies, for example, to very massive stars that collapse at the end of their evolution. The extent of the event horizon can be calculated using the Schwarzschild radius - but only for a special case of black hole. “The Schwarzschild radius describes a spherical event horizon,” says Gürlebeck. “Such an event horizon would be expected if the black hole is static, ie not rotating. But when the black hole rotates, the sphere is flattened and it becomes an ellipsoid, much like the earth. ”Scientists currently assume that there are only rotating black holes, since their predecessor objects also rotated and the angular momentum also when collapsed to a black hole is preserved. The spherical event horizon is therefore more the exception than the rule.

The shadow of a black hole

“In theory, the event horizon is clearly defined, there is a sharp limit,” says Michael Kramer from the Max Planck Institute for Radio Astronomy in Bonn. “For an astronomer, however, this limit is less clear.” The event horizon gives the black hole its size - behind it hides the singularity. And the more massive the black hole, the more extensive it is. But the event horizon is also invisible: when matter or light passes through it, there is no going back.

“You can imagine a particle that falls into the black hole and sends out light pulses on this path, that is, radiation that we could pick up with telescopes,” says Gürlebeck. Since the black hole bends spacetime due to its gravitational pull outside of the event horizon, time also passes more slowly in its vicinity compared to regions that are less curved. Therefore, these light pulses are shifted more strongly into the red area of ​​the electromagnetic spectrum, the closer to the event horizon they are emitted by the particle: their frequency is smaller, so their wavelength is larger. “This process is actually fluid,” says Michael Kramer. "The signals are increasingly redshifted and at some point they are so long-wave that you simply can no longer receive them." So you will never be able to observe a particle that actually falls into the black hole - and that's why there is none for astronomers pinned event horizon.

The shadow of the event horizon

So far, astronomers have mainly obtained the best indirect evidence of black holes from their accretion discs. These are debris, dust and gas orbiting the black hole. This material is heated so much that it glows brightly. The first stellar black hole observed, Cygnus X-1 in the constellation Swan, which astronomers tracked around fifty years ago, is one of the most powerful X-ray sources in the sky.

Simulation of the event horizon of a black hole

In distant galaxy nuclei there are black holes with billions of solar masses, which accelerate and heat the material in their accretion disk so strongly that their radiation can be captured with earthly telescopes after a journey of billions of light years. “The accretion disk doesn't stop exactly at the event horizon,” says Gürlebeck, “where exactly it stops says something about how fast the black hole rotates. That is why there is an area between the event horizon and the accretion disk that is as good as material empty - except for the particles that are just falling into it. "

In the meantime, however, scientists are no longer satisfied with indirect observations. They want to take a picture of the event horizon of the extremely massive black hole in the center of our galaxy, for example as part of the “Event Horizon Telescope” or the “Black Hole Cam” project. The principle is the same for both projects: By merging several powerful radio telescopes around the world, they want to improve the spatial resolution so that they can look right over the inner edge of the accretion disk. Because there they expect to see, as it were, the shadow of the black hole, which is only slightly larger than the black hole itself.

“The resolution has to be smaller than the shadow,” says Kramer. “Such a picture would be the final proof that there are black holes.” However, the scientists have no real doubts about the existence of black holes: “We cannot see the black hole ourselves,” says Gürlebeck, “but all particles are in near these celestial bodies behave as they should behave near a black hole. Everything is in line with our predictions. "