Three guys claim that any heavy chunk of matter emits Hawking radiation, even if it’s not a black hole:
• Michael F. Wondrak, Walter D. van Suijlekom and Heino Falcke, Gravitational pair production and black hole evaporation, Phys. Rev. Lett. 130 (2023), 221502.
Now they’re getting more publicity by claiming this means that the universe will fizzle out sooner than we expected. They’re claiming, for example, that a dead, cold star will emit Hawking radiation, and thus slowly lose mass and eventually disappear!
They admit that this would violate baryon conservation: after all, the protons and neutrons in the star would have to go away somehow! They admit they don’t know how this would work. They just say that the gravitational field of the star will create particle-antiparticle pairs that will slowly radiate away, forcing the dead star to lose mass somehow to conserve energy.
If experts thought this had even a chance of being true, it would be the biggest thing since sliced bread—at least in the field of quantum gravity. Everyone would be writing papers about it, because if true it would be revolutionary. It would overturn calculations by experts which say that a stationary chunk of matter doesn’t emit Hawking radiation. It would also mean that quantum field theory in curved spacetime can only be consistent if baryon number fails to be conserved! This would be utterly shocking.
But in fact, these new papers have had almost zero effect on physics. There’s a short rebuttal, here:
• Antonio Ferreiro José Navarro-Salas and Silvia Pla, Comment on “Gravitational pair production and black hole evaporation”, Phys. Rev. Lett. 133 (2024), 229001.
It explains that these guys used a crude approximation that gives wrong results even in a simpler problem. Similar points are made here:
• E. T. Akhmedov, D. V. Diakonov and C. Schubert, Complex effective actions and gravitational pair creation, Phys. Rev. D. 110, 105011.
Unfortunately, it seems the real experts on quantum field theory in curved spacetime have not come out and mentioned the correct way to think about this issue, which has been known at least since 1975. To them—or maybe I should dare to say “us”—it’s just well known that the gravitational field of a static mass does not cause the creation of particle-antiparticle pairs.
Of course, the referees should have rejected Wondrak, van Suijlekom and Falcke’s papers. But apparently none of those referees were experts on the subject at hand. So you can’t trust a paper just because it appears in a supposedly reputable physics journal. You have to actually understand the subject and assess the paper yourself, or talk to some experts you trust.
If I were a science journalist writing an article about a supposedly shocking development like this, I would email some experts and check to see if it’s for real. But plenty of science journalists don’t bother with that anymore: they just believe the press releases. So now we’re being bombarded with lazy articles like these:
• Universe will die “much sooner than expected,” new research says, CBS News, May 13, 2025.
• Sharmila Kuthunur, Scientists calculate when the universe will end—it’s sooner than expected, Space.com, 15 May 2025.
• Jamie Carter, The universe will end sooner than thought, scientists say, Forbes, 16 May 2025.
The list goes on; these are just three. There’s no way what I say can have much effect against such a flood of misinformation. As Mark Twain said, “A lie can travel around the world and back again while the truth is lacing up its boots.” Actually he probably didn’t say that—but everyone keeps saying he did, illustrating the point perfectly.
Still, there might be a few people who both care and don’t already know this stuff. Instead of trying to give a mini-course here, let me simply point to an explanation of how things really work:
• Abhay Ashtekar and Anne Magnon, Quantum fields in curved space-times, Proceedings of the Royal Society, 346 (1975), 375–394.
It’s technical, so it’s not easy reading if you haven’t studied quantum field theory and general relativity, but that’s unavoidable. It shows that in a static spacetime there is a well-defined concept of ‘vacuum’, and the vacuum is stable. Jorge Pullin pointed out the key sentence for present purposes:
Thus, if the underlying space-time admits a everywhere time-like Killing field, the vacuum state is indeed stable and phenomena such as the spontaneous creation of particles do not occur.
This condition of having an “everywhere time-like Killing field” says that a spacetime has time translation symmetry. Ashtekar and Magnon also assume that spacetime is globally hyperbolic and that the wave equation for a massive spin-zero particle has a smooth solution given smooth initial data. All this lets us define a concept of energy for solutions of this equation. It also lets us split solutions into positive-frequency solutions, which correspond to particles, and negative-frequency ones, which correspond to antiparticles. We can thus set up quantum field theory in way we’re used to on Minkowski spacetime, where there’s a well-defined vacuum which does not decay into particle-antiparticle pairs.
The Schwarzschild solution, which describes a static black hole, also has a Killing field. But this ceases to be timelike at the event horizon, so this result does not apply to that!
I could go into more detail if required, but you can find a more pedagogical treatment in this standard textbook:
• Robert Wald, Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics, University of Chicago Press, Chicago, 1994.
In particular, go to Section 4.3, which is on quantum field theory in stationary spacetimes.
I also can’t resist citing this thesis by a student of mine:
• Valeria Michelle Carrión Álvarez, Loop Quantization versus Fock Quantization of p-Form Electromagnetism on Static Spacetimes, Ph.D. thesis, U. C. Riverside, 2004.
This thesis covers the case of electromagnetism, while Ashtekar and Magnon, and also Wald, focus on a massive scalar field for simplicity.
So: it’s been rigorously shown that the gravitational field of a static object does not create particle-antiparticle pairs. This has been known for decades. Now some people have done a crude approximate calculation that seems to show otherwise. Some flaws in the approximation have been pointed out. Of course the authors of the calculation don’t believe their approximation is flawed. We could argue about that for a long time. But it’s scarcely worth thinking about, because no approximations were required to settle this issue. It was settled over 50 years ago, and the new work is not shedding new light on the issue: it’s much more hand-wavy than the old work.
This entry was posted on Saturday, May 17th, 2025 at 2:44 pm and is filed under physics. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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