String theorists propose that a record KM3NeT neutrino came from a black hole hidden in a fifth dimension

Every explanation of this particle rests on a single event ^[1]. Some theorists have put it down to a blazar, a type of highly active galaxy ^[3]. Others have reached for a primordial black hole. Such an object would have formed in the universe's first moments. It would lose mass through Hawking radiation and end in a burst of radiation and neutrinos ^[4].

The timing works on paper. Dan Hooper, a cosmologist at the University of Wisconsin-Madison, and others have suggested that processes such as inflation could have left regions dense enough to collapse into black holes of many masses ^[14]. The smallest would have evaporated long ago. Those with roughly the mass of an asteroid would be evaporating now ^[14]. Klipfel and Kaiser's earlier proposal, a black hole expiring near the solar system, depends on that population ^[15]. Other ideas have failed to fully explain the event, and the concept "still looks really beautifully consistent," Kaiser said ^[16]. Scientific American's account does not estimate how often a primordial black hole should die that close to the Sun, and it does not link these objects to dark matter.

The new paper keeps the exploding black hole and moves it into a fifth dimension ^[5]. The move is aimed at the missing photons ^[6]. Lüst, of the Max Planck Institute for Physics in Munich, describes the extra dimension as "an extra circle" about one micron wide. In his description it is curled up at every point of ordinary space and may arise as a side effect of dark energy ^[8]. "The only force which can penetrate the fifth dimension is gravity," Lüst said ^[9]. In his account, a black hole inside it would let only neutrinos escape into our universe ^[7]. Cumrun Vafa, a Harvard string theorist, said that "when the size of the black holes are smaller than a micron, they can sit inside other points in the extra dimension" ^[11].

In my view the absence of photons checks that the model is consistent but does little to test it, because the fifth dimension was introduced to keep everything except neutrinos out of our universe ^[7]. Whether that dimension exists is still disputed. Vafa argues it is "not only plausible; it is derived, in some sense, with minimal assumptions from string theory ideas," a case he made in a preprint posted in 2022 and published in 2023 ^[10]. Hooper is not persuaded. "We don't know that there is a fifth dimension," he said ^[12]. Lüst, Vafa and their colleagues, he said, "seem to be making some very strong statements that I wouldn't be able to defend" ^[13].