Is it true that black holes don't have an interior?

 Watch someone fall into a black hole, and you will see them freeze at the edge forever, never actually going inside. So does a black hole's interior even exist?

The answer depends entirely on who is doing the looking and which branch of physics is used to describe the phenomenon. Under classical physics, the interior is a very real, very terminal place, but modern quantum theories suggest it might not exist in the way traditional science fiction portrays it.

According to Einstein's theory of general relativity, black holes absolutely have an interior. In this framework, the event horizon is not a physical surface or a solid barrier. It is simply a mathematical boundary marking the point of no return. If an astronaut were to fall past the event horizon of a supermassive black hole, they would not hit a wall. They would cross an invisible threshold into an interior region where space and time are warped so severely that all possible directions point inevitably toward the center, known as the singularity.

However, the picture changes drastically depending on the observer. Because immense gravity warps time, an outside observer watching an object fall toward a black hole sees a very different reality. As the object gets closer to the event horizon, time for that object appears to slow down relative to the observer. The light emitted by the object stretches into longer, redder wavelengths until it eventually fades to nothing. To the outside universe, the object freezes at the edge and never actually crosses the horizon. Because no information can ever leave the interior, from the practical perspective of the rest of the universe, the inside is completely inaccessible and effectively severed from reality.

The debate becomes even more complex when introducing quantum mechanics. Theoretical physicists have spent decades trying to reconcile the smooth geometry of general relativity with the erratic nature of quantum physics, leading to several theories that eliminate the traditional black hole interior altogether:

• The Fuzzball Theory: Originating from string theory, this concept suggests that a black hole is not an empty void with an infinitely dense singularity at the center. Instead, it is a densely packed, fuzzy sphere of fundamental strings extending all the way out to the event horizon. In this model, there is no empty interior to fall into; the black hole is solid right up to its edge.
• The Holographic Principle: This theory proposes that all the three-dimensional information of the matter that formed the black hole is entirely encoded on the two-dimensional surface of the event horizon. If all the data of the black hole exists on its surface boundary, a traditional volume of space inside might be an illusion.

Until physicists successfully merge general relativity and quantum mechanics into a unified theory of quantum gravity, the exact nature of the space inside an event horizon remains one of the most profound mysteries in science.

What happens if two black holes collide?

 A massive outburst of energy,for a brief moment,it (collision of two black holes) outshine the entire observable universe.Scientists measured gravitational waves (which is caused by twisting space-time curvature)in LIGO and CERN where that waves can created by twisting space-time curvature by these two black holes.

For an example the collision of Milky way and Andromeda galaxy leads to a giant galaxy with a very giant black hole at the center.

The gravitational wave detectors include:

• LIGO (two facilities in the US)
• Virgo (in Italy)
• KAGRA (in Japan)…

• Image credit: The Guardian news

What would happen if two black holes with different spins collide?

 It would be the most extreme events in space. When spins are not aligned to each other,then the event horizons of the new black hole forms and so unstable that it could complex ripple in space and resulting gravitational waves are spreading.

A significant portion of the total mass (roughly 5% to 10%) is converted into gravitational wave energy.

If the spins are not perfectly aligned or if the masses are unequal, gravitational waves are emitted more strongly in one direction.

Image Source: Google

Why does the universe allow black holes to exist at all?

 There exist black holes in the universe because any star which is greater than 3 times solar masses can go into three types objects in their final phases.They may be white dwarf,neutron star or black hole. So forming a black hole is an inevitable phenomenon In our cosmos.

Now turn to the main question, why?

Because black hole regulates star formation in it's host galaxy. There is a huge correlation between black hole and galaxy co-evolurion. So without black hole, galaxy might not be fully structured.

In center of each galaxy ,there is a supermassive black hole. It preserves angular momentum of that system. When there exist too many masses in a small region,they don't let go anything outside nor even light. So these masses creates curve in space-time curvature and gravity is too high.This high density region is known as black hole.It is also a consequence of theory of general relativity by Einstein.

So existence of black hole in space proved by theoretically and experimentally both.

Could black holes be artificial structures built by advanced civilizations?

 The notion that black holes might not only be natural astrophysical objects but could also be artificial constructs engineered by advanced civilizations is a fascinating, though highly speculative, idea that straddles the frontier between theoretical astrophysics and the search for technosignatures. Below is a detailed, scientific exploration of this concept.

1. The Natural Phenomenology of Black Holes

Black holes arise naturally from the gravitational collapse of massive stars or via dynamical processes in galactic centers. Their properties—characterized by intense gravitational fields, event horizons, accretion disks, and relativistic jets—are well described by general relativity and quantum field theory in curved spacetime. In particular, processes such as Hawking radiation (a quantum-mechanical emission of particles from the horizon) and the Penrose process (energy extraction from a rotating black hole) illustrate how black holes can both emit energy and interact in complex ways with their surroundings.

2. Advanced Civilizations and the Kardashev Scale

On the Kardashev scale—a framework that classifies civilizations based on their energy-harvesting capabilities—a sufficiently advanced civilization (Type II or III) might seek to harness astronomical energy sources beyond stellar fusion. The idea here is that a civilization with access to enormous energy resources could conceivably engineer structures that either mimic or modify black holes. For instance, theoretical models have considered Dyson spheres not only around stars but also around black holes to capture energy from accretion disks or even from Hawking radiation. Such proposals naturally arise when one contemplates that a highly advanced society might be capable of manipulating gravity, energy, and even spacetime at a fundamental level.

3. Theoretical Proposals for Artificial Black Holes

Several speculative frameworks suggest how black holes could be artificially created or modified:

Energy Extraction and Quantum Computing:

Researchers like Gia Dvali and Zaza N. Osmanov have proposed that black holes may be the most efficient capacitors of quantum information. In their 2023 study, they argue that advanced civilizations could intentionally create microscopic black holes to serve as components in ultra-powerful quantum computers. The accompanying Hawking radiation—emitted in a “democratic” manner across particle species—could serve as a detectable technosignature if these artificial black holes are engineered for information processing or energy extraction .

Megastructures Around Black Holes:

Other studies have explored the concept of constructing Dyson sphere–like structures around black holes. For example, research published in the Monthly Notices of the Royal Astronomical Society and reported by Universe Today has suggested that a civilization might build an “Inverse Dyson Sphere” around a black hole to capture the energy released by its accretion disk and jets . In this scenario, the structure would intercept high-energy radiation and possibly reprocess it into useful power.

Artificial Creation as Propulsion Sources:

In the realm of speculative astronautics, proposals exist (for example, by Crane and Westmoreland) to create microscopic black holes that could power starships by converting their Hawking radiation into thrust. The concept of a “black hole starship” relies on engineering a black hole with just the right balance of mass, lifespan, and radiative output to serve as an efficient energy source for interstellar travel. Although this idea is far beyond current human technology, it illustrates the potential utility of controlled black hole creation in advanced technological contexts .

4. Astrophysical Mechanisms and Engineering Challenges

While these ideas are captivating, they rest on numerous formidable challenges:

Energy and Material Requirements:

The creation or manipulation of a black hole, even a microscopic one, would require control over energy scales and material densities far exceeding current capabilities. For instance, the production of an artificial black hole for propulsion or computing purposes might require particle collisions at energies near the Planck scale or novel forms of matter that can withstand extreme conditions.

Stability and Control:

Any artificially created black hole would need to be stabilized and integrated into an engineering framework. This includes controlling its evaporation via Hawking radiation, preventing uncontrolled accretion, and managing the directional emission of energy for applications such as thrust or computation.

Observational Technosignatures:

If advanced civilizations were to build such structures, they might leave behind unusual signatures—such as excess infrared or high-energy radiation that does not match natural astrophysical processes. Telescopes like ALMA, the Hubble Space Telescope, or neutrino observatories like IceCube are among the instruments that could, in principle, detect these anomalies. Some recent studies have proposed observational methods to look for waste heat or spectral peculiarities consistent with engineered energy-harvesting structures around black holes.

5. The Current Scientific Consensus and Speculative Nature

Despite the intriguing theoretical possibilities, there is no observational evidence today that any known black hole is artificial. The astrophysical community broadly regards black holes as natural endpoints of gravitational collapse. However, these speculative proposals expand the range of technosignatures that researchers search for in SETI (the Search for Extraterrestrial Intelligence). The idea that an advanced civilization might harness black holes either as energy sources or as components in quantum computing systems encourages the development of novel observational strategies that could, in principle, detect anomalies in the high-energy sky.

Conclusion

In summary, while black holes are naturally occurring and are explained well by established physical theories, some theoretical models suggest that advanced civilizations—especially those on the upper rungs of the Kardashev scale—might have the capability to engineer or repurpose black holes for energy extraction, quantum computation, or even propulsion. These ideas, while scientifically imaginative and grounded in aspects of general relativity and quantum mechanics, remain speculative. Their exploration, however, is valuable because it broadens our search for technosignatures and deepens our understanding of what might be possible in a universe governed by the known laws of physics.

Is it scientifically possible to use dark energy or black holes as shortcuts for Interstellar travel through time and space?

 Difficult one to answer it.Christopher Nolan presented a theory of time travel in his movie Interstellar. Now there he took massive black hole so that it was not so much dangerous as it's eating rate of material is slow than stellar one. When I researched about that I find it's possible to do travel in time by a wormhole.when I wrote bout it, I took help of AI. there I saw if time rate of flow is different in different spaces(I imagine they are far away so) then synchronizing frequency we can get time travel. Now travel through space? Nolan showed us inside black hole through singularity we may go to another space of this universe. Now scientifically it is true? Possibly a wormhole connects two distant region of space as shown in figure below. While doing chat with AI, it shows me that to create such a wormhole we need black hole. More theoretically I don't talk here. Here the role of dark energy is not known to me. I know exotic or negative matter play a role in creating such a wormhole.

What came first, black holes or stars?

 According to our current cosmological models, the very early universe was featureless and dark, with the first gas clouds capable of forming stars only appearing between 100 million to 250 million years after the Big Bang. Thus, we can reasonably infer that the first stars only appeared about 100 million years after the Big Bang, nearly a billion years before galaxies proliferated across the cosmos.

Now, the early universe was also characterized by very high densities and non-uniform conditions that could have led sufficiently dense regions to undergo gravitational collapse, thus forming black holes. These hypothetical structures, called primordial black holes (PBHs), are thought to have formed soon after the Big Bang. Interestingly, since PBHs did not form from the gravitational collapse of stars, their masses can be much lower than typical stellar masses.

Thus, in conclusion, black holes - most likely - have formed before stars!

Have some black holes already swallowed entire galaxies?

 Stars in a galaxy? Say, 100 billion, typical. But some are very small, down to 100 million.

Our galaxy supermassive blackhole, Sag A*, is only 4 million solar masses. The Milky Way has over 100 billion stars with a lot of them, many more times massive than our Sun. in fact the Milky Way is estimated at over 1.5 trillion solar masses. Did Sag A* eat a galaxy? No.

So, how about the most massive black hole, TON 618? That is 66 billion solar masses. That does put it in range of the smaller galactic masses. However, there is the problem of velocity. Things are not just floating around in space. Actually, everything has massive velocity. in one sort of orbit or another. The sort of orbit for galaxies, is a complex spiral, around all the galaxies around it. So, just imagine what this means. We can measure the proper velocity of the Milky Way, against the Cosmic Microwave Background. We are moving at 1.3 million miles per hour across the CMB. At these velocities black holes cannot eat everything, but they will gulp the part they are exposed to.. This means, the answer is no, black holes don’t swallow galaxies. Then how do they get so big?

They are part of the formation of some types of galaxies, as they undergo collisions with each other. They represent the core stars and dust collapsing, and merging, as another galaxy passes through it. Then, those galaxies collide. That can result in much more consumption of stars, and even the merger of the core black holes, over millions of years, of course.

Does every Galaxy have a black hole? Is that what keeps galaxies a Galaxy?

 There are many Galaxies with no detected black holes (BH). However, as the saying goes, absence of evidence is not evidence of absence. The fact is that it’s very hard to detect any single object at galactic distances unless it’s extremely luminous. In fact, we can only detect those BH’s that produce powerful astronomical jets. These jets are made up of a collimated beam of high-speed particles (see illustration below). To produce these jets a BH must be actively “swallowing up” large amounts of matter. But in older galaxies the BHs already swallowed everything in its vicinity (i.e. cleared its neighborhood). So there is no jet and they become nearly undetectable.

However, this is not a permanent state. If we wait long enough (perhaps thousands or millions of years), an unlucky star or nebula will eventually stray too close to the BH, fall into it, and produce a new jet.

Q. Is that what keeps galaxies a Galaxy?

The OP’s second question is much easier to answer: A typical galaxy has many millions of BHs of various sizes, among them is often a supermassive BH at the galactic center. As far as we know, we can remove all of them, and almost nothing will change in the structure or dynamics of the galaxy. They are just there, adding some small fraction of gravitational pull, but not much else. They are not needed to keep the galaxy a “Galaxy”.