⚫ Do black holes really have an event horizon?

A black hole is generally described as an object surrounded by a boundary called an event horizon. Once this limit is crossed, neither matter nor light can return to the outside. Some theories nevertheless propose extremely compact objects that would resemble black holes, but without a genuine horizon.

The difference could leave an observable trace. Matter continuously falls toward the massive objects located at the centers of many active galaxies. If the central object has a surface, this matter must eventually hit it. The energy from the impact would then strongly heat the surface and produce recognizable thermal radiation.

What the passage through a black hole's event horizon would look like.

What the passage through a black hole's event horizon would look like.
NASA illustration

With an event horizon, the scenario changes. Matter that crosses it disappears from our view without hitting a solid surface. Researchers therefore specifically looked for the luminous signature that a surface heated by an impact of matter would produce.

The team relied on BASS DR2, a survey of nearby active galactic nuclei observed notably in X-rays. This dataset provides detailed spectral measurements. The authors selected the objects that made it possible to compare their observed emissions with those expected in the presence of a surface.

Result: none of the objects studied displays the sought-after thermal signature. For a significant portion of the sample, the observations already make it possible to rule out a radiating surface such as the one predicted by many horizonless models. For the others, the current data are simply not yet precise enough.

This test is particularly valuable because it depends relatively little on the exact nature of the alternative object. Whenever a surface continuously receives matter, it should heat up and radiate. The absence of this light therefore narrows the range of several families of models proposed as alternatives to classical black holes.

CE
Centralement

Thank you! But we are well underway!
Everything in the universe rotates, a black hole rotates very fast, and moreover, it sinks into space-time, the veil that deflects light. Light follows this curve and does not come out because it cannot go around the black ball at the bottom of the black hole. That is the problem: the infinite mass of the ball at the bottom of the black hole!

CE
Centralement

The back of a black hole has never been found... Perhaps thinner than a needle, passing through our universe toward other spaces, or perhaps nothing but a complete void. Ah! By the way, it also moves just like everything else in the universe moves. So we should see the other end of the black hole, where the black noodle stretches out, very thin; we don't detect it from this side yet...?

Anonymous

It is difficult to mentally visualize an object where one side is its entire exterior regardless of where you are around it, and whose "other side" is the center of that object, since it warps space-time. To try and keep it simple: on Earth, it's roughly as if your "other side" were visible from the mantle or the Earth's core. Except with a black hole and its gravity, it's different because we can't "go inside"; the intense gravity warps everything, cf Einstein's Relativity.

SP
Spot

You're right, anonymous, we only see a black hole when it absorbs matter. Otherwise, it's an invisible ball, big or small. However, this ball warps space-time and sits within it... but by spinning rapidly. In fact, a fixed representation is impossible, and the funnel shape with its flame-thrower radius makes me laugh... even if we imagine it.

GG
GGH

"light does not escape" is because the force of gravity becomes so strong that the escape velocity required is that of light. Consequently, light "does not go fast enough" to escape gravity. This point constitutes the event horizon.
As for "the black ball at the center," it is the singularity, the famous "point that devours everything."
It is located, in a sense, at the center of the "sphere" represented by the event horizon.

GG
GGH

By definition, nothing can "escape" from the event horizon.
Except theoretically, a possible Hawking radiation.

But in fact, all of this is irrelevant because the article is not actually about a black hole :)

AY
Ayè

Haha! Out of subject...
That is indeed the case with black holes!