Weird Quantum ‘Magic’ of Black Holes


Quantum Black Hole


“Black holes are an incredibly unique and fascinating feature of our universe,” said University of Queensland theoretical physicist, Joshua Foo about their mind-bending ability to have different masses simultaneously. “They’re created when gravity squeezes a vast amount of matter incredibly densely into a tiny space, creating so much gravitational pull that even light cannot escape. It’s a phenomenon that can be triggered by a dying star. But, until now, we haven’t deeply investigated whether black holes display some of the weird and wonderful behaviors of quantum physics.

A UQ-led team of theoretical physicists, headed by Foo, ran calculations that reveal surprising quantum phenomena of black holes.


“One such behavior is superposition, where particles on a quantum scale can exist in multiple states at the same time. This is most commonly illustrated by Schrödinger’s cat, which can be both dead and alive simultaneously. But, for black holes, we wanted to see whether they could have wildly different masses at the same time, and it turns out they do. Imagine you’re both broad and tall, as well as short and skinny at the same time—it’s a situation which is intuitively confusing since we’re anchored in the world of traditional physics. But this is reality for quantum black holes.”

To reveal this, the team developed a mathematical framework allowing us to “place” a particle outside a theoretical mass-superposed black hole. Mass was looked at specifically, as it is a defining feature of a black hole, and as it is plausible that quantum black holes would naturally have mass superposition.

Research co-supervisor, Dr. Magdalena Zych, said that the research in fact reinforces conjectures raised by pioneers of quantum physics.

The universe is revealing to us that it’s always more strange, mysterious and fascinating than most of us could have ever imagined.”

“Our work shows that the very early theories of Jacob Bekenstein—an American and Israeli theoretical physicist who made fundamental contributions to the foundation of black hole thermodynamics—were on the money,” she said. “He postulated that black holes can only have masses that are of certain values, that is, they must fall within certain bands or ratios—this is how energy levels of an atom works, for example.

Does Quantum Gravity Cast Doubt On Fate of Black Holes?

“Our modeling showed that these superposed masses were, in fact, in certain determined bands or ratios—as predicted by Bekenstein. “We didn’t assume any such pattern going in, so the fact we found this evidence was quite surprising. The universe is revealing to us that it’s always more strange, mysterious and fascinating than most of us could have ever imagined.”


It is often in such extreme environments where interesting and unexpected phenomena emerge.”

The Last Word

In an email to The Daily Galaxy, Joshua Foo  wrote: “Our research is interested in the fundamental interplay between gravity and quantum mechanics, and so the scenario we’ve considered in the paper — a black hole in a superposition of masses — is obviously quite extreme. However it is often in such extreme environments where interesting and unexpected phenomena emerge — Stephen Hawking’s seminal discovery that black holes radiate particles is a good example of this. 

What we show in our paper is an in-principle way of detecting the signatures of a black hole with a superposition of masses, in an analogous manner to how the gravitational wave detectors at LIGO measured “signatures” of the binary black hole merger in 2015. Observation of these signatures, while technically difficult, would nevertheless demonstrate the first example of Einstein’s gravity obeying the quantum superposition principle.

It is natural to ask whether such astronomical superpositions could occur in nature, and it turns out there are conceivable scenarios where this is indeed possible. The motion of several orbiting black holes (like the binary black hole merger detected by LIGO) will often become “chaotic”, such that the “quantum wave function” describing the black holes will spread rapidly, allowing for them to be characterized as being in a “superposition of locations”.

The example we give in the paper is perhaps simpler: Sending a “wavepacket” of light with different energies into a black hole will simultaneously impart different energies to the black hole, thus creating a superposition of black hole energies (and from Einstein’s theory of relativity we know that energy is directly related to mass). 

Quantum Birth of the Universe

As I mentioned, the signatures of quantum black holes that we show in our paper may not be observable with current technology. However, studying the in-principle effects that quantum black holes produce is important for understanding how the elusive theory that unifies quantum mechanics and gravity may be constructed.” 

More information: Joshua Foo et al, Quantum Signatures of Black Hole Mass Superpositions, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.181301

Joshua Foo and University of Queensland


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