https://johncarlosbaez.wordpress.com/2025/05/17/dead-stars-dont-radiate-and-shrink/ Azimuth * Home * About Dead Stars Don't Radiate [end_of_universe_coming_sooner_2] 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 will make the universe fizzle out sooner than 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 happen. 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 Jose 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 Carrion Alvarez, 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. Related 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. Post navigation << Previous Post 14 Responses to Dead Stars Don't Radiate 1. Clive Scott's avatar Clive Scott says: 17 May, 2025 at 3:23 pm Journalists measure their success by their ability to sell newspapers and get clicks. It follows that they have a perverse incentive to sensationalize any story. Reply 2. duetosymmetry's avatar duetosymmetry says: 17 May, 2025 at 3:41 pm In their defense, some science journalists do check with experts! I know of at least one journo who asked my colleague about this very paper, and correctly took the advise that the paper was wrong. PRL clearly asked the wrong referees about this manuscript... Reply + John Baez's avatar John Baez says: 17 May, 2025 at 4:27 pm I'm glad some journalists did check with experts. Unfortunately the journalists who did, did not publish articles with attention-grabbing headlines like PHYSICISTS PUBLISH FALSE CLAIM THAT THE UNIVERSE WILL END SOONER THAN EXPECTED So, it's like how experiments that report unsurprising "negative results" tend not to get published, while those that report surprising "positive results" do... leading to a a bias toward surprising but false papers. We do now have the Journal of Negative Results. But we need the Pop Science Magazine of Incorrect Discoveries. Reply 3. Toby Bartels's avatar Toby Bartels says: 17 May, 2025 at 3:45 pm So we have a rigorous proof that 2 + 2 = 4, but some people noticed that 2.3 + 2.4 = 4.7, did a rough approximation, and concluded that sometimes 2 + 2 = 5? Reply + John Baez's avatar John Baez says: 17 May, 2025 at 4:47 pm ... and then conclude that THE UNIVERSE WILL END AN HOUR LATER THAN EXPECTED!!! Reply o Simon Crase's avatar Simon Crase says: 17 May, 2025 at 11:16 pm Corrected for Daylight Saving Time? Reply 4. Wyrd Smythe's avatar Wyrd Smythe says: 17 May, 2025 at 4:18 pm Nice of you to lay it out like this, thanks. The way I had it explained is that an event horizon is necessary for Hawking radiation because it presents a "wall" that affects the QFT vacuum. It makes perfect sense that mere gravity wouldn't do it. Perhaps they took too seriously the (as I understand it, utterly incorrect) metaphor about one of a particle-antiparticle pair generated near the event horizon getting lost behind the horizon? Reply + John Baez's avatar John Baez says: 17 May, 2025 at 4:39 pm Sort of. In their mild defense, a static electric field can create positron-electron pairs: * Wikipedia, Schwinger effect. This is true even for a weak electric field, though the effect is exponentially suppressed and it's never yet been seen (see the article). So, you might think a static gravitational field could do something similar. The original paper, the rebuttals, and the attempted rebuttal to the rebuttals are all about this. Unfortunately, no amount of fiddling around with approximate calculations will take you to the truth. Or maybe it eventually could, but it's very inefficient. It's better to start with the correct answer as found by Ashtekar, Magnon and others, and then use that to study the approximations and see how well or poorly they do. That could be somewhat interesting, though I don't have the patience to do it myself. The key fact is that the geometry of spacetime affects our definition of what counts as a particle. If you don't handle this correctly you'll get nonsense. Luckily you can learn how to do it by reading Wald's textbook. Reply o Wyrd Smythe's avatar Wyrd Smythe says: 17 May, 2025 at 6:15 pm I'd never heard of the Schwinger effect. Very interesting! And I can see how an electric field would accelerate electrons and positrons in opposite directions. But for gravity to have that effect, it would seem the particles and antiparticles would need to be oppositely affected by gravity. And wasn't there just a recent experiment showing anti-hydrogen falls downwards just like hydrogen? (Thanks for inserting my missing word.) Reply 5. John Baez's avatar John Baez says: 17 May, 2025 at 5:21 pm I had a nice conversation on this topic with Tobias Fritz here. We got into some of the technical issues. Reply 6. Gregor's avatar Gregor says: 17 May, 2025 at 5:32 pm This might be a stupid question, but couldn't I read the claim that massive things dissipate as saying that there is not a timelike Killing vector field in the first place? After all, such a thing would generate an isometry that takes points prior to dissipation to points afterwards, which clearly can't exist. In other words, when you say 'static spacetime', aren't you assuming what you want to prove? Reply + John Baez's avatar John Baez says: 17 May, 2025 at 7:16 pm It's not a stupid question, it's a very clever one. In computations of the Hawking radiation for black holes, the issue you're worrying about actually comes up. There, we typically start by assuming a timelike Killing vector field. Starting from this assumption we compute the radiation of the black hole. This says the black hole is shrinking, which tells us our original assumption was not correct. It's only approximately correct. But the approximation should be good for a large black hole, because it mass changes very slowly. This sort of approximation, where we treat a slowly changing quantity as constant, is common in physics. However, people have gone further and computed what happens when you consider the 'back-reaction': the effect of the changing mass of the black hole on the Hawking radiation it emits. This back-reaction effect turns out to be tiny until near the end of the black hole's life, when it's losing mass quickly. For the case of a static dead star, I believe the assumption of a timelike Killing vector field is fine and exactly correct, since it leads to a perfectly self-consistent result: no radiation and no change (unlike the case for black holes, where this assumption leads to radiation and a slowly changing mass of the black hole). The skeptic (like you) could argue that while we get a self-consistent solution, the reasoning is circular. Such skeptics should do a calculation where they allow the dead star's mass to slowly change, and see if there's also a self-consistent solution where there's a little radiation and the dead star slowly shrinks. I don't think there will be. But if there were, there would then be two self-consistent solutions to the same physics problem, an unusual situation. Then one could try to study whether one solution was 'better' than the other. Of course we already have one reason the (known) static solution is vastly better than any (purely hypothetical) alternative solution: in the static solution, protons and neutrons don't disappear into thin air! Any 'slowly shrinking star' solution would violate conservation of baryon number--and in a completely mysterious way, since nothing about the process of particle-antiparticle creation that supposedly drives this hypothetical solution violates baryon conservation. Reply 7. culeigh's avatar culeigh says: 18 May, 2025 at 3:08 am Thank you very much for the nice summary of the situation! It did seem awfully strange that such an effect could have been missed for so long, when similar issues have been studied to death on the context of Hawking radiation. I had two basic follow-up questions, if you don't mind: In the Schwinger effect, electric fields pull electrons one way and positrons the other. However gravity attracts both particles and antiparticles. So how could gravity create anything analogous to the Schwinger effect? Also I've never heard of an antiphoton, and Hawking radiation is mostly photons... I had always written off this explanation of Hawking radiation via analogy to the Schwinger effect as unhelpful because of these reasons, but it seems like you take it fairly seriously. Can it be made precise? Don't black holes already force violation of baryon number? Is your argument that this hypothetical process would be even stranger, because now the baryon number violation isn't hidden behind an event horizon, where quantum gravitational effects could more plausibly change the story? Reply + John Baez's avatar John Baez says: 18 May, 2025 at 9:55 am In the Schwinger effect, electric fields pull electrons one way and positrons the other. However gravity attracts both particles and antiparticles. So how could gravity create anything analogous to the Schwinger effect? Please ask Falcke et al, since that is their theory not mine. Or read their paper (which is incorrect). Also I've never heard of an antiphoton, and Hawking radiation is mostly photons... The photon is its own antiparticle. I had always written off this explanation of Hawking radiation via analogy to the Schwinger effect as unhelpful because of these reasons, but it seems like you take it fairly seriously. Can it be made precise? I didn't say Hawking radiation is like the Schwinger effect. I was explaining to Wyrd Smythe why Falcke et al erroneously believe that a strong static gravitational field can create particle-antiparticle pairs. They believe it's analogous to the Schwinger effect. So I had to tell him about the Schwinger effect, and I said "in their mild defense" that in this effect, a strong electromagnetic field indeed creates electron-positron pairs. I'm glad I told him this, because he hadn't known about the Schwinger effect. But I wasn't saying anything in support of Falcke's claim that a strong gravitational field creates particle-antiparticle pairs. Don't black holes already force violation of baryon number? Nobody knows, because we don't have an accepted theory of what happens when black holes completely decay, and we don't have any experimental evidence either. Of course people have discussed this issue a lot, and suggested that under the extreme and mysterious conditions inside a black hole baryon number conservation is strongly violated. None of that is relevant to Falcke et al, since they're considering something like a rock or dead star, governed by known physics. Is your argument that this hypothetical process would be even stranger, because now the baryon number violation isn't hidden behind an event horizon, where quantum gravitational effects could more plausibly change the story? Right, exactly. Falcke et al are studying something like a dead star floating in empty space. They claim that it emits Hawking radiation. Because this would violate conservation of energy, they conclude that the dead star must slowly shrink and disappear, without giving any mechanism for how this would work. (Their Hawking radiation calculation does not involve any conversion of matter into radiation.) Then they notice that the "mysteriously disappearing dead star" scenario would violate baryon conservation, because protons and neutrons are somehow disappearing. They mention that perhaps baryon number is not conserved. But they don't connect any candidate processes that might violate conservation of baryon number to their calculation of how the gravitational field supposedly creates radiation. I hope you see why I find this unsatisfying. After a physicist makes one mistake, they may be led into other dubious hypotheses to avoid breaking fundamental principles like conservation laws, but usually they at least propose some sort of details. Here there's no explanation at all of why the dead star shrinks and eventually disappears. Reply You can use Markdown or HTML in your comments. You can also use LaTeX, like this: $latex E = m c^2 $. The word 'latex' comes right after the first dollar sign, with a space after it. Cancel reply Your email address will not be published. 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