Black holes may not exist, but fuzzballs might, wild theory suggests
Universe

Black holes may not exist, but fuzzballs might, wild theory suggests

By far the most mysterious objects in the universe are black holes. They are objects in the cosmos where all of our knowledge of physics completely breaks down.

Nonetheless, they exist, despite their apparent impossibility. But what if these gravitational monsters aren’t black holes at all, but rather fuzzy, vibrating balls of string? New research implies that this may be the case, and that future observations may allow us to see them.

The problem of black holes

Black holes appear in Einstein’s general theory of relativity, and by all rights, they should not exist. According to the theory, gravity can become extremely powerful if a clump of matter crunches down into a small enough volume. This incredible gravitational compression can outperform any of nature’s other four fundamental forces, including the strong nuclear force that holds that clump of matter together. Once a certain critical threshold is reached, the clump of matter just squeezes and squeezes, compressing down into an infinitely tiny point.

That infinitely small point is known as the singularity, and it is surrounded by a surface known as the event horizon — the point at which gravity’s inward pull exceeds the speed of light.

Of course, there is no such thing as an infinitely small point, thus this image appears to be wrong. But in the mid-20th century astronomers began to find objects that looked like black holes, acted like black holes and probably smelled like black holes too. Regardless of their impossibility, they were floating around the universe.

That’s not the only problem. Stephen Hawking, a physicist, discovered that black holes aren’t absolutely black in 1976. Black holes slowly evaporate due to the weirdness of quantum mechanics. As a result, any information that falls into a black hole is trapped inside. However, Hawking’s radiation does not transport that information away (at least, as far as we understand). So when the black hole eventually evaporates, what happens to all that information?

A stringy solution

Theoretical physicists have been working for decades to find something — anything — to explain black holes. Something that explains the information paradox and something that uses math to replace the singularity.

Among those theorists are the ones working on string theory, which is a model of the universe that replaces all the particles and forces that you love with subatomic, vibrating strings. These strings are the fundamental constituents of matter in the universe, according to string theory, but we can’t see them as strings because they’re so small. Oh, and for string theory’s math to work, there must be extra dimensions – all small and curled up on themselves to subatomic scales, so we can’t see them.

String theory claims to be a theory of everything, capable of explaining every kind of particle, every kind of force, and basically everything in the universe (and, for completeness, the whole entire universe itself).

As a result, string theory should be able to explain the unexplainable: it should be able to substitute black holes with something less frightening. And, indeed, string theorists have proposed a less-scary replacement for black holes. They’re called fuzzballs.

String theory says that black holes are neither black nor holes. Instead, the greatest comparison for a fuzzball is to consider another compact and unusual object in the universe: neutron stars.

When an object does not have enough gravity to compress into a black hole, it becomes a neutron star. Matter is compressed to the maximum density possible inside a neutron star. Neutrons are fundamental atom constituents, although they usually interact with other particles such as protons and electrons. That kind of atomic camaraderie, however, breaks down and evaporates in a neutron star, leaving just neutrons crammed together as tightly as possible.

Fuzzballs occur when the fundamental strings cease to operate together and just crowd together, resulting in a big, well, ball of strings. A fuzzball.

Unraveling the yarn

Fuzzballs aren’t fully filled out, even in theory, because, as fascinating as string theory sounds, no one has ever come up with a comprehensive mathematical solution for it — and so fuzzballs are fuzzy not only in physical reality, but also in mathematical possibility.

Still, as described in a review article published in the preprint journal arXiv, we might be able to identify fuzzballs with upcoming surveys (opens in new tab). We are only now beginning to move past proving the existence of black holes and into the specifics of how they behave, and our best way to do it is through gravitational waves.

When black holes collide and merge, they release a tsunami of gravitational waves, which wash across the cosmos, eventually reaching our detectors on Earth. The gravitational wave signature is exactly what general relativity predicts black holes to accomplish in all of the dozens of black hole mergers we’ve seen so far.

However, future equipment, like as the improved Laser Interferometer Gravitational-Wave Observatory (LIGO) and Laser Interferometer Space Antenna (a projected space-based gravitational wave detector), may be sensitive enough to distinguish between normal black holes and stringy fuzzballs. I say “might” because different fuzzball models predict different variations from standard black hole behavior.

Finding evidence for fuzzballs would not only answer the puzzle of what black holes are; it would also expose some of nature’s deepest underpinnings.

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Rakesh Krishnan
6 months ago

Interesting!

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