I think they are more drawing an analogy between the atom one or more black holes with a cloud of particles around them. Black holes are quantum mechanical and so the resulting system could behave much like an atom, including having things that look like energy levels. The universe rhymes.

&gt; I thought it is pretty well accepted there isn&#x27;t a &quot;gravaton&quot; like there are photons.Different branch of physics that we can&#x27;t quite mesh with GR yet. A graviton is the quantized &quot;piece&quot; of gravity, like phonon is for sound&#x2F;mechanical waves, or photons for EM. It exists as much as a photon or any other &quot;messenger particles&quot; exists, in that it&#x27;s a useful mathematical model. It&#x27;s not something we have isolated&#x2F;observed with equipment though.

&gt; I still do understand what a &quot;gravitational atom&quot; might be.It&#x27;s a vague gesture in the direction of a speculative theory of quantum gravity. It&#x27;s not anything we have any actual evidence for.

In an atom, the electrons orbiting the nucleus are only allowed to be at discrete energy levels (the energy levels are &quot;quantized&quot;). In a classical model of an orbit, the energy level of a spaceship orbiting a planet can be thought of as the speed it has that allows it to orbit at a certain altitude, assuming no friction or other effects. Velocity is energy in that case (E=m*v^2 in classical mechanics). Unlike electrons in an atom, the spaceship can be in any one of a continuous range of energy levels (up to escape velocity).The proposal in the article is (I think) that bosons orbit certain black holes in quantized energy states, like electrons would orbit an atomic nucleus.

As far as I know the existence of the graviton is 100% up in the air, not well accepted either way.But yeah, particles in energy levels gravitationally bound to a black hole in a similar way electrons are electrically bound to a nucleus.

Apparently I am not watching enough PBS SpaceTime because I still do understand what a &quot;gravitational atom&quot; might be.They are not implying a particle that causes gravity right? Because I thought it is pretty well accepted there isn&#x27;t a &quot;gravaton&quot; like there are photons.They also don&#x27;t mean atom-sized black-holes, so I still don&#x27;t get it.Hoping Matt does an episode on this so I can grasp it.<a href="https:&#x2F;&#x2F;www.pbs.org&#x2F;show&#x2F;pbs-space-time&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.pbs.org&#x2F;show&#x2F;pbs-space-time&#x2F;</a>

&gt; gravitational forcesGravity is not a force in GR. The spacetime curvature in a spacetime with two black holes that merge is not describable in terms of a simple &quot;gravitational field&quot;. Spacetime curvature is a fourth rank tensor with twenty independent components. In vacuum ten of those vanish identically, leaving ten independent components.

Regardless of the gravity canceling out in this region (which is more complicated and probably will not happen). The current LIGO and Virgo wouldn&#x27;t have enough spatial resolution to pinpoint tiny regions between black holes.To explain what will happen is that gravitational field in region you are describing would have a steep spacetime curvature and a point where it will cancel gravitational forces would be more of a saddle (lagrange point classically) point rather than zero gravity region.Now you also have quantum fluctuations that now with this strong gravitational field you will have virtual particle - anti particle paris pop in and out of vaccum. This is not going to be only in thia region but all around. Also merging process will enhance this phenomenon but deciding where actually this middle point will be difficult.Now it would be impossible at least for our current observational tools to have resolution for the scale we are talking about. Event horizon telescope for example is designed to observe areas around singularities on much larger scales that what you are interested in here.But the interesting part would be If matter and antimatter pairs were produced between merging black holes, they would likely be short-lived. In this intense gravitational environment, any particles created would be rapidly torn apart or accelerated towards one of the black holes. The annihilation of such pairs might emit gamma rays, but this signal would be extraordinarily faint compared to the other high-energy processes occurring during a black hole merge.So the answer is probably No, at least with our current technology.

very good, but I would say (since &quot;light cone&quot; is of such common parlance) that the physical 3d analogous projection would be two slightly overlapping 3d-venn diagram funnels conjoining at an &quot;indefinitely&quot; (asymptotically smaller) small space-time minkowski manifold.naked singularities themselves, however, do not exist.

But there&#x27;s still two singularities until one falls into the other.

&gt; between two singularities mergingTwo black holes merging does not mean two singularities merging. The singularity (singular, not plural) is a moment of time that is to the future of all other moments inside the hole. If two black holes merge, there is just one singularity inside the merged hole.Remember that GR is a model of spacetime, not space. In spacetime, a single black hole looks, heuristically, like a cylinder, and the singularity inside is at the future end of the cylinder. Two black holes merging look, heuristically, like a pair of trousers, and the (one) singularity inside the merged hole is at the future end of the trousers (the &quot;waist&quot;).

&gt; I&#x27;m guessing it would create a bunch of angular momentum and throw out almost all their masses as gravitational waves. Will be a fun experiment for someone in a few hundred years.I&#x27;m not a physicist but it sounds to me like a gravitational bomb. I wonder how it will affect its surroundings? Stars being torn apart? I wouldn&#x27;t like to watch such an experiment in person. Though if someone tried it a couple billions light years away from me, and then sent a video record, I&#x27;m all for it. I&#x27;d prefer to wait a couple billions years for the record to be downloaded, than to watch an online stream.

One odd thing that never occurred to me until now is that two black holes that pass each other at near light speed, if they intersect by even a nanometer, will be guaranteed to merge. That nanometer has to fall into both singularities due to the properties of black holes, so transitively they have to merge. Kind of weird. I&#x27;m guessing it would create a bunch of angular momentum and throw out almost all their masses as gravitational waves. Will be a fun experiment for someone in a few hundred years.

What you&#x27;re asking for is a naked singularity. Nothing can produce that, not even another black hole.

In some Newtonian or pseudo-Newtonian models, possibly? Not in GR&#x2F;General Relativity.

Each black hole has tiny spot with zero gravitation in the centre.

Stupid question, but between two singularities merging, there is tiny space, with gravitation zeroing out and appearing plank matter being ripped towards one and the other. Can one spot that location in the middle where anti-matter and matter bleed from the nothing on modern telescopes?

yes, just heard about it on Physics Frontiers. Monsalve &amp; Kaiser are talking about primordial black holes, and are offering a theory that if they were imbalanced in color charge, they could be surrounded by a particle cloud, in a big quantum state.<a href="https:&#x2F;&#x2F;pca.st&#x2F;episode&#x2F;f0db6ab1-18ff-4201-a6b2-a30b780a266a" rel="nofollow">https:&#x2F;&#x2F;pca.st&#x2F;episode&#x2F;f0db6ab1-18ff-4201-a6b2-a30b780a266a</a>

Is the reason these are coherent quantum states that the postulated ultralight axions don&#x27;t strongly interact with anything except gravity, so they see very little environmental noise to decohere them? Would they also predict there&#x27;s (much smaller) dark matter halos around ordinary planets and stars, and these also have quantized atom-like states?

&gt; we&#x27;re all almost certainly inside a black holeNo, we&#x27;re not. The universe is rapidly expanding. Equating the Schwarzschild radius for a given blob of matter with the event horizon of a black hole requires that the matter be static or collapsing.The &quot;black hole cosmology&quot; models referred to in the Wikipedia article are misnamed. It is theoretically possible that our observable universe is a patch of a Schwarzschild spacetime, which is what the models referred to are asserting, but if it is, then, since the universe is expanding, it would be a patch of the white hole portion of the spacetime, not the black hole portion. And the &quot;horizon&quot; would be a white hole horizon, i.e., one from which the universe&#x27;s expansion would eventually cause us to pass out of.However, such a model is extremely unlikely because it has no way of explaining where the white hole horizon came from. A black hole horizon can come into being from gravitational collapse, but a white hole horizon would have to have been &quot;built in&quot; to the overall universe from the very beginning. Nobody has any reason to think that is actually possible, even if we have a theoretical mathematical model that includes it.

I guess that&#x27;d explain the big bang nicely. It does make it the &quot;outer universe&quot; disappointingly unobservable though.

Let&#x27;s not forget that the radius of the observable universe is ~= the Schwarzschild radius i.e. we&#x27;re all almost certainly inside a black hole ;)<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Black_hole_cosmology" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Black_hole_cosmology</a>

In quantum field theory electrons are excitations in the electron field.If they also were tiny black holes, what would it mean to be an excitation in the electron field which when the wave function collapses it would behave like a black hole. Does it mean that it&#x27;s not a black hole anymore when the wave function spreads out?

&gt; Then there’s spooky shit like:
&gt; <a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Black_hole_electron" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Black_hole_electron</a>From the article:&gt; However, Carter&#x27;s calculations also show that a would-be black hole with these parameters would be &quot;super-extremal&quot;. Thus, unlike a true black hole, this object would display a naked singularity, meaning a singularity in spacetime not hidden behind an event horizon. It would also give rise to closed timelike curves.I wonder if there&#x27;s a fun sci-fi story in the discovery that all electrons are in fact naked singularities.

If we did have a tennis-ball-sized black hole out beyond Neptune, it would be far beyond our capabilities to locate and track - we can barely track debris that small in low Earth orbit, and black holes aren&#x27;t even courteous enough to provide a radar return. We would not be able to send probes near it any time soon.

Some would, but the vast majority of matter is not found in planets and stars. Most stuff just floats around on it&#x27;s own out there.

Some of them would, some wouldn&#x27;t, depends on where they were and how they were moving.This is a bit like asking why Jupiter isn&#x27;t part of the Sun but other stuff is.

Why wouldn&#x27;t the primordial black holes condense into bigger ones like the matter that makes up planets and stars did?

If the Hawking temperature is below the CMB no net evaporation happens. This means there is a mass cutoff and it’s below asteroid mass. Any smaller PBHs would have evaporated by now assuming we are right about Hawking radiation. The math says it should exist but we have AFAIK not proven it.The big black holes will last insanely long amounts of time.

&gt; The universe could be full of black holes in the asteroid mass range the size of hydrogen atoms.? Not if there&#x27;s Hawking radiations.

That&#x27;s another leading hypothesis. Some classes of WIMPs have been ruled out but the whole idea hasn&#x27;t been.BTW primordial black holes could still exist even if they aren&#x27;t a significant dark matter component.

Black holes can&#x27;t be dark matter.Dark matter is a WIMPThe universe was built on a scaffold of dark matter like an old spiderweb that slowly collects dust.

My most recent physics rabbit hole was the black hole hole. They are fascinating.My favorite is the idea of primordial black holes which formed in the instants after the Big Bang. Many models and theories predict them and they could be an excellent dark matter candidate. The universe could be full of black holes in the asteroid mass range the size of hydrogen atoms.There is also a hypothesis that the predicted (by many solar system simulations and models) planet nine far beyond Neptune could be a captured primordial black hole in the 1-5 Earth mass range and about the size of a golf ball to a tennis ball.I really really hope that exists because if it did it would be within probe range. Going and checking out a black hole could allow us to solve physics and develop a complete tested unified theory.Then there’s spooky shit like:<a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Black_hole_electron" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Black_hole_electron</a>

&gt; Gravitational waves carry momentum and energy (and therefore mass), just like electromagnetic waves.No, EM waves do not have mass. They are massless. They carry momentum and energy, yes, but not mass.In GR, the source of gravity is not &quot;mass&quot;, it&#x27;s stress-energy. EM waves carry stress-energy even though they are massless.Gravitational waves do have some aspects that are analogous to EM waves, but there is a key difference: gravitational waves do not have any stress-energy. They are pure spacetime curvature in vacuum. So while there is a sense in which they carry momentum and energy, since properly constructed detectors can extract momentum and energy from them, they do not carry any stress-energy and the momentum and energy they carry cannot be localized the way momentum and energy in EM waves can.

&gt; But gravity is the curvature of space caused by mass, it is not described as a form of mass.Gravitational waves carry momentum and energy (and therefore mass), just like electromagnetic waves. Theoretically, you can extract that energy from gravitational waves, by using oscillating masses tuned to the wave&#x27;s frequency.

&gt; Nor do they &quot;escape&quot; the black hole; they are emitted from outside the horizon.That&#x27;s interesting, I&#x27;d never really thought about that before. Does GR predict that there would be any waves confined to the inside of the merged black hole?

&gt; matter, they are made of spacetime curvatureWhich is caused by matter achieving a particular density. What we observe is not the matter but the _effects_ of the curvature created by that matter but to say they&#x27;re not &quot;made of matter&quot; seems overly reductive here.&gt; some of the spacetime curvature doesn&#x27;t get included in the merged holeThe observable mechanics of a black hole are (probably) controlled by it&#x27;s surface area and not it&#x27;s volume. When two spherical objects merge the surface area is less than the sum of the two original objects.

&gt; How do you understand this process of matter escaping a black hole?No matter escapes. Gravitational waves are not matter. They are spacetime curvature. Nor do they &quot;escape&quot; the black hole; they are emitted from outside the horizon. The reason the mass of the merged hole can be smaller than the combined masses of the original holes, with the difference being emitted as gravitational waves, is that black holes are not made of matter, they are made of spacetime curvature, and when they merge, some of the spacetime curvature doesn&#x27;t get included in the merged hole. That&#x27;s just how spacetime curvature works.

&gt; We were told that nothing can escape a black hole.Stephen Hawking told otherwise, actually: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hawking_radiation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Hawking_radiation</a>In 1974, and that&#x27;s now 50 years ago.

We were told that nothing can escape a black hole.But when black holes merge, the combined mass is smaller.It is described that the lost mass is converted to gravitational waves.But gravity is the curvature of space caused by mass, it is not described as a form of mass.How do you understand this process of matter escaping a black hole?Is it that the gravitational waves are caused by a &quot;quantum gravity&quot; particle that can&#x27;t be converted back to any of the other quantum particles?

Signatures of gravitational atoms from black hole mergers