It was Gandalf who said that of course. And before you try to contradict me, let me point out that Gandalf is a wizard that has no need to bother with silly things like spacetime continuity.
P.S.: https://quoteinvestigator.com/2014/07/13/truth/
> In conclusion, there exists a family of expressions contrasting the dissemination of lies and truths, and these adages have been evolving for more than 300 years. Jonathan Swift can properly be credited with the statement he wrote in 1710 [(that does not mention footwear yet)].
Both virtual particles-antiparticles survive (and promptly disappear because one didn't just cross an event horizon).
The man himself (Hawking) said: "One might picture this negative energy flux in the following way. Just outside the event horizon there will be virtual pairs of particles, one with negative energy and one with positive energy. It should be emphasized that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally."
Note that this is proper acceleration which is measurable locally with e.g. a weighted spring, contact with a piezoelectric scale, or any other sort of accelerometer apparatus.
> a black hole reuires an accelerated observer to not be pulled in
There are an infinite number of free-fall trajectories outside a black hole, most of which never go near the black hole in the first place. There are also infinite numbers of hyperbolic orbits which "graze" a black hole and an infinity of various elliptical and (quasi-)circular orbits.
An accelerometer on one of these trajectories will report zero proper acceleration. Yet none of these trajectories cross the horizon from the outside.
There are additionally free-fall trajectories which cross the black hole horizon from the outside. Who knows what happens not long after that: flat-space physics like the Standard Model of Particle Physics curved spacetime physics like General Relativity give conflicting answers.
Finally there are also an infinity of trajectories which are somewhere properly accelerated. Most of these won't cross the horizon, but some can: one can turn on one's rocket engines and carefully steer a course that crosses the horizon.
(There are multiple types of horizon; the interesting one is the apparent horizon which can be measured locally with various types of apparatus. An event horizon -- if there is one -- can only be determined with reference to the entire global spacetime and all its contents from infinite past to infinite future. There's a variety of other horizons too. Visser catalogued some of them in <https://journals.aps.org/prd/abstract/10.1103/PhysRevD.90.12...>).
Note that trajectories in my paragraphs above correspond to everywhere non-spacelike curves of all sorts. Free-falling trajectories are geodesics (timelike or lightlike) and one can grind out the geodesic equation for the central mass in a Schwarzschild spacetime, for example.
Finally, proper acceleration is difficult to maintain for long, let along perpetually. (Although one can stand on the surface of a rocky planet and with an accelerometer measure over a very very very long term practically constant acceleration, and might for practical reasons want to assign that approximately constant acceleration to e.g. little "g"). So most curves where there is some proper acceleration also have some free-falling segments eventually.
One consequence is that a traveller who undergoes different accelerations (including none -- free-fall) along its path through spacetime will count different numbers of particles at different points along the traveller's sometimes-geodesic/sometimes-accelerated timelike curve.
A black hole doesn't really change this other than that the formation of some types of horizon induces an acceleration between freely-falling observers before the horizon forms and observers after. The later freely-falling observers see more particles than the earlier ones, and they bunch up not very far (but not preciesly on) the appropriate horizon.
The Unruh effect introduces an Unruh horizon attached to an observer undergoing proper acceleration. That observer sees more particles when it is accelerating than when it is/was freely-falling.
Some of the radiation is massless or has so little mass that it flies out to infinity. Around some types of horizon near a central mass, we'd call that Hawking radiation (as opposed to e.g. Unruh radiation, or radiation associated with e.g. a cosmic horizon in an expanding universe). And as Baez notes, over lonnnnnnnnnng times one would want to take into consideration how the central mass evolves as these particles fly away. Over much shorter durations (fuel is limited!) one would want to take into consideration how Unruh radiation and proper acceleration co-evolve.
The idea in the paper (which Baez dismantles in the linked blog entry at the top) is that no type of horizon is necessary for an apparently particle-free vacuum to look like a particle-rich patch of spacetime. That's a pretty wild claim. Why don't electrons evaporate on their own, in that case?
"Hawking radiation doesn't just come from black holes but from any collapsed star"
This was followed in detail by Hawking 1975 (open access via Project Euclid) <https://projecteuclid.org/journals/communications-in-mathema...> where the most relevant bit surrounds eqns (2.27-28) and there is nothing wrong with talking about signed "contributions to the probability flux into the collapsing body" (as opposed to identifying those contributions as particles).
Hawking himself wrote simpler pop-sci explanations in a number of places, including his book, where a reader would not be expected to understand how canonicalizations of wave functions in other than position space and directly-measured particles differ. However none of those works is the equivalent of the brief "explosions?" letter and its follow-on.
Is it really shocking (today)? I mean, isn't this a logical consequence of Hawking radiation for black holes? I thought we were shocked by this a long time ago, but now we're ok with it. The authors of the paper in question may very well be wrong in their calculations (I can't say), but this blog post doesn't smell good to me because of doubtful statements like these, passed off as so obviously true that you must be an idiot not to agree. That kind of emotional writing does not become someone whose profession should focus on scientific persuasion.
From Wikipedia [0], itself citing Daniel Harlow, a quantum gravity physicist at MIT:
> The conservation of baryon number is not consistent with the physics of black hole evaporation via Hawking radiation.
It's bloody John Baez, the man knows his stuff.
On you actual point, it is shocking because its claimed that baryon number is not conserved without black holes getting involved
Isn't it also speculated that there's hawking radiation caused by the event horizon at the edge of the visible universe in an accelerating frame?
So for example if you take a dead star in a vacuum with nothing else in the universe (and make certain technical assumptions) then you can prove that the star does not emit Hawking radiation. That's quite a strong result, and certainly does make the result seem shocking.
What you'd probably prefer reading is one of the sources John Carlos Baez cites [0]:
Comment on “Gravitational Pair Production and Black Hole Evaporation” Antonio Ferreiro1, José Navarro-Salas, and Silvia Pla
Where they take the equation used in the paper, and outline how there is a better way than using that equation
"... is obtained to the lowest order in a perturbative expansion, while the standard way to obtain the non-perturbative Schwinger effect using the weak field approximation is to perform a resummation of all terms"
and how the one in the paper being critiqued can't handle situations arising from electromagnetic cases, much less the gravitational one properly. These are the statements Baez makes but the cited paper gives in a much more professional tone and method.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13...
> Is it really shocking (today)?
Moreover, there are a few experiments that try to measure the proton decay (that would break the baryon number conservation.) They are run on Earth, far away form any black hole. For now, all of them failed to find a decay, and the conclusion is that the half life of protons is at least 2.4E34 years. https://en.wikipedia.org/wiki/Proton_decay#Experimental_evid...
I found an old article by quantamagazine explaining one of the experiment. It's a huge pool of very pure water and a lot of detectors. No black hole required. https://www.quantamagazine.org/no-proton-decay-means-grand-u... (HN discussion https://news.ycombinator.com/item?id=13201065 )
There are other black hole models that can conserve these quantum numbers!
Speaking of things that are so obviously true that you must be an idiot not to agree, there are statements so obviously false that you have to be an idiot to agree: People keep repeating the nonsense put out by Penrose, which require non-physical timelike infinities to work.
The current "pop science" (nearly science fiction) statement is that it is possible to fall into a black hole and there is "nothing special" about the event horizon.
Quite often, just one paragraph over, the statement is then made that an external observer will never observe the victim falling in.
The two observers can't disagree on such matters!
To say otherwise means that you'd have to believe that the Universe splits (when!?) such that there are two observers so that they can disagree. Or stop believing in logic, consistency, observers, and everything we hold dear as physicists.
This is all patent nonsense by the same person that keeps insisting that brains are "quantum" despite being 309K and organic.
If the external observer doesn't observe the victim falling in, then the victim never falls in, full stop. That's the objective reality.
Penrose diagrams say otherwise because they include the time at infinity, which is non-physical.
Even if the time at infinity was "reachable", which isn't even mathematically sound, let alone physically, Hawking radiation is a thing, so it doesn't matter anyway: Black holes have finite lifetimes!
There is only one logically consistent and physically sound interpretation of black holes: nothing can ever fall in. Inbound victims slow down relative to the outside, which means that from their perspective as they approach the black hole they see its flow of time "speed up". Hence, they also see its Hawking evaporation speed up. To maintain consistency with outside observers, this evaporation must occur fast enough that the victim can never reach any surface. Instead, the black hole recedes from them, evaporating faster and faster.
This model (and similar ones), can preserve all quantum numbers, because there is no firewall, no boundary, nothing to "reset" quantum fields. Everything is continuous, consistent, and quantum numbers are preserved. Outside observers see exactly what we currently expect, black holes look and work the same, they evaporate, etc...
Why not?
If a spaceship fell toward a black hole and, as it approached the event horizon, one observer saw it turn into a horse and the other saw it turn into a cat, that would be very strange indeed, and one would suspect at least one of the observers of being wrong.
But if one observer sees it fall through the event horizon and the other observer waits… and waits… and gets bored and starts doing some math and determines that they could spend literally forever and never actually observe the spacecraft falling through the event horizon, then what’s the inconsistency? You might say “well, the first observer could fire up their communication laser and tell the second observer that ‘yes, the spaceship fell in at such-and-such time’, and the second observer would now have an inconsistent view of the state of the universe”, but this isn’t actually correct: the first observer’s message will never reach the second observer!
Because that's not how relativity works! Two observers can disagree only on the order and relative timing of events, not what the events are or the total number of events. There are far more restrictions than that, but those are sufficient for my point.
The whole quantum information loss problem is just this, but dressed up in fancy terminology. It's the problem with black holes that the "number of things" (particles, events, whatever) is "lost" when matter falls into them.
The modern -- accepted -- resolution to this problem is that this information is not lost, preserving quantum numbers, etc...
How exactly this occurs is still being debated, but my point is that if you believe any variant of QM information preservation, then the only logically consistent view is that nothing can fall past an event horizon from any perspective, including the perspective of the infalling observers.
If you disagree and believe the out-dated GR model that an astronaut can't even tell[1] that they've crossed the event horizon, ask yourself this simple question: When does the astronaut experience this "non-event"[1]? Don't start with the mathematics! Instead, start with this simple thought experiment: The non-victim partner far away from the black hole holds up a light that blinks on an off once a second. The victim is looking outward and is watching the blinking speed up. How many blinks do they count at the time they cross the horizon?
Now think through the scenario again, but this time assume the spaceship turns the light off when they observe that the black hole has finished evaporating. When does the in-falling astronaut observe the blinking stop? Keep in mind that every "toy model" makes the simplification here that the blinking rate goes to infinity as the astronaut falls in! (I.e.: "They see the entire history of the universe play out." is a common quote)
[1] Isn't that a strong enough hint for everybody that there is no horizon!?
No, and this has nothing to do with quantum mechanics or the no-hair theorem or anything particularly fancy.
As a toy example, suppose you have a frame with a (co-moving, but it doesn’t really matter) time coordinate t. A series of events happen at the origin (x=y=z=0 in this frame) at various times t.
There’s another observer in a frame with a time coordinate t'. The frames are related by t' = t - 1/t for t<0. The t=-10 event happens at t'=-9.9. The t=-4 event happens at t’=-3.25. The t=-1 event happens at t’=0. t=-1/100 happens at t'=99.99. t=0 gets closer and closer to happening but never actually happens. t=1 doesn’t even come close.
Critically, the t' observer does not observe t=0 or t=1 in some inconsistent manner. There is no disagreement between the observers as to what happens at t>=0. To the contrary, those events are simply not present in the t' observer’s coordinate system!
Note that the transformation above isn’t about when light from an event gets to the t' observer — it’s the actual relativistic transformation between two frames.
The Schwarzchild metric has a nastier transformation than this. If you toss a rock into an isolated black hole from far away, you will see the rock get progressively closer to the event horizon, and you will never see it fall in. But the rock is in trouble: its co-movimg coordinate system ends not long after it crosses the horizon. That latter phenomenon is called the “singularity”, it’s solidly inside the event horizon, and it’s not avoidable by coordinate system trickery. While general relativity does not explain what happens when one encounters the singularity, one might imagine that it’s fatal to the rock the reaches it.
edit: FWIW, you also say:
> The current "pop science" (nearly science fiction) statement is that it is possible to fall into a black hole and there is "nothing special" about the event horizon.
I’m not sure what you’re talking about. In a pure GR model of an isolated black hole, as you fall in, you will observe tidal forces. In a smaller black hole, the tidal forces will squash you long before you reach the event horizon. In a large enough black hole, they will not! Your view of the sky would certainly look very, very distorted and delightfully and possibly dangerously blue-shifted, but we’re talking about an isolated black hole. Nothing to see in the sky, and you may well survive your visit to the event horizon. Then, dramatically less than one second later for any credibly sized black hole, you will meet the singularity, and IIRC you should probably expect to be squashed by tidal forces before that. Source: I took the class and did the math. I assume this is what the “pop science” you’re talking about is saying, and it’s not wrong.
P.S. I’ve never tried to calculate how lethal the blue-shifted sky would be. Naively considering just the time transformation, it should be infinitely lethal at the event horizon. But trying to apply intuition based on only part of a relativistic transformation is a great way to reach incorrect conclusions.
Don't we know about a bunch of black holes best measured in AU? Won't you have a good chunk of time inside those? Does time dilation work severely against you?
Even worse: the way to maximize how long you have before you hit the singularity, you should do nothing. Firing your rocket in any direction gets you to the singularity in an even smaller amount of proper time: the singularity isn’t in front of you in space — it’s ahead of you in time.
Keep in mind that all of this is for the Schwarzchild metric, which is a nice solution to Einstein’s equations in the sense that you can derive it on a blackboard. It can’t describe what we think of as a real black hole for plenty of reasons, including the major one that a Schwartzchild black hole has existed forever and therefore could not have formed in a supernova. You need a different solution for a black hole that has only existed for a finite time.
You only get one microsecond as you cover over half a light minute? Huh.
There is no consensus, quite the opposite: it was very well known that neither classical GR nor quantum mechanics are able to model a black hole!
People like to argue this as if it is settled science, right after saying two contradictory things about it, both from simplified, incomplete models.
If this is radiating a star's mass worth Hawking radiation particles, is it like the Solar Wind, and if it's happening ever faster is there a point where it would start pushing the victim away from the black hole again? (the 'victim' can be a solar sail if that helps)
Arvin Ash just did a video on this
https://www.youtube.com/watch?v=UxVssUb0MsA
It appears to occur outside the event horizon in a large area.
Outside observers see the victim's own black body radiation become extremely redshifted, asymptotically matching the black hole's black body radiation.
If you mathematically "undo" this distortion for both, then what you are really observing from the outside is a star's worth of matter getting converted to pure energy and the infalling victims getting blasted in the face by that.
The victims can't make it back out "whole and intact" in the same sense that you're not going to keep your atomic integrity if you're up close and personal to a supernova.
Your quantum numbers however... those can be preserved nicely.
I don't think that's true. What kills you isn't radiation of the singularity, but cosmic microwave background (and other infalling radiation) turned to visible light, then x-rays, then gamma rays.
How is this not true? From the point of view of whoever is falling, and supposing the black hole is very large
For example there's no mathematics at all that mankind has ever known where an asymptotic approach towards some limit doesn't have a mirror version (usually inverted) on the other side of the asymptote. If we see time stop, at the EH it seems wrong to assume there's nothing "stopped" similarly from the other side too. So this means the surface has to be very special. You don't just pass by it and not notice as you fall in, imo.
That’s a strong statement. 1/sqrt(x), over the reals, doesn’t have an inverted world for x<0. Maybe you could argue that it does exist, weirdly rotated, outside the reals?
In any event, the Schwarzchild metric itself is an actual example of this. From the perspective of a doomed spaceship at the event horizon, the Schwarzchild metric is quite civilized.
The stuff after the horizon is a different story, but that’s not immediately after crossing the event horizon — it might be whole nanoseconds later :)
Go take a GR class. It’s fun and mind-bending.
As you probably know, horizontal asymptotes are never what we think of as the 'problematic' parts of Relativity, because when something approaches a constant that's never something that breaks the math.
The Schwarzchild metric, being a relationship of 6 different variables I think, has some relationships that go to infinity asymptotically at the EH radius and some things that approach a constant at that radius, so it's an example of the kind of asymptotic I was talking about _and_ one like your "horizontal" example.
Closer to the centre, including your feet, the escape velocity is higher.
Electrical impulses wouldn’t be able to travel from the bottom of your brain to the top, so you’d be unconscious anyway.
An event horizon is not like the surface of a planet - you will not be accelerated as you pass through it.
It is, once again, irrelevant that light cannot propagate outward once you're behind the horizon because, again, you are falling towards the center, and in particular you are falling through the future light cone of your feet. Please look at some spacetime diagrams if you do not believe me, preferably ones in Kruskal-Szekeres coordinates.
In GR spacetime is locally flat and for an inertial observer special relativity applies, up to tidal corrections which can be made arbitrarily small at the horizon by considering a suitably large black hole. This is a deep and important fact about GR. The idea that falling through the horizon causes you to suddenly not be able to see your feet anymore appears to obviously violate this basic principle, so if you think your assertion is true you should be able to explain why either this principle of GR is actually not true, or why your assertion does not actually violate this principle.
If your head is further from the singularity than your feet then you can't see them.
Happy not to discuss further!
Say the trajectory that's drawn on the diagram is the trajectory of your feet. Now consider a second trajectory which begins slightly displaced "outwards" (that is, rightwards at t=0 on the diagram) from this first one - that's your head. Hopefully you agree that the head-trajectory would have to do something pretty strange to avoid crossing through the future lightcone of your feet, even behind the horizon. This doesn't require signals from your feet to travel "outward" - it's just that your head is travelling "inward".
K-S coordinates make it pretty clear that nothing drastic happens to the structure of spacetime at the event horizon - everything is perfectly regular. It's just that once you cross the horizon, the singularity (the thick hyperbola at the top of the diagram) is inevitably in your future: there is no trajectory within any future lightcone behind the horizon that doesn't run into the singularity. You're doomed to run into it in finite time, and all your future lightcones lie entirely behind the horizon.
[1]: https://en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coord...
[2]: A useful feature of K-S coordinates is that lightcones are always at +-45 degrees
I completely agree that spacetime can be "flatish" for a large block hole, but the event horizon does still represent a boundary right?
Consider the edge case of crossing the event horizon itself at some speed <<c (because you've got magic thrusters fighting the pull). At some point your feet will be through the event horizon and your head won't be. Do you agree that at that point you won't be able to see your feet?
I agree that your head will pass through the future light cone of your feet, and so could do somethign to affect your head (by emitting something falling slower than your head), but I'm not sure any light rays could follow that path.
I'm not quite sure from that discussion why an event horizon is equivalent to a body moving outwards at the speed of light but it does make some sense. GR is always fun!
I still don't have a good idea of the "slow moving crossing the event horizon" case" but I'll read around it some more
Maybe the difference is between "free fall through an event horizon" vs "hover" (as much as is possible) at an event horizon
Space being flat locally is important because if the gravitational gradient is too high (i.e. you get too close to the singularity) your feet will be accelerated much faster than your head.
If you want to take into account the evaporation of the black hole, then you should look at something like the vaidya metric. The mass function is a function of the ingoing Eddington coordinate v, which takes on a specific value when you cross the event horizon, and so you observe the black hole at a specific mass as you cross the event horizon. Contradicting your layman understanding of time dilation for the observer relative to the black hole.
Once you cross the horizon, the r coordinate becomes timelike, and so you are forced to move to decreasing r value just like a regular observer is forced to move to increasing t value. Your entire future, all your future light cone is within the black hole and it all terminates at the singularity. Minewhile, the t coordinate is space like which is what gives you space like separation from the mess that had happened in the original gravitational collapse. You wouldn't be blasted by a frozen supernova like you have said.
You can kind of say the universe splits at the event horizon, the time like coordinate changes from t to r and the future of the black hole branch of the universe is permanently cut off from the rest of the universe.
In rotating and charged black holes it is different, and you observe the evaporation of the black hole once you cross the Cauchy horizon. If the black hole is eternal (because someone kept feeding radiation to the black hole, maybe by reflecting the hawking radiation inwards), then you would in fact see timelike infinity as you reach the Cauchy horizon, so this time like infinity is quite physical. You would need to avoid being vaporized by blue shifted incoming radiation.
Those closer-and-closer line spacings are hiding a mathematical infinity, which isn't physical for finite-lifetime black holes.
Conversely, look at: https://en.wikipedia.org/wiki/Eddington%E2%80%93Finkelstein_...
The ordinary Schwarzschild metric diagram in that article makes it crystal clear that in-falling observers asymptotically approach the horizon, but never cross it.
Read the next section as well, which uses the "Tortoise coordinate"... which again uses the mathematical infinity to allow the horizon to be crossed.
I really don't understand why people keep arguing about this!
If you find yourself writing an infinity symbol, you've failed at physics. Stop, go back, rethink your mathematics.
You can choose stupid coordinates that introduce a singularity wherever you like, in GM or in classical mechanics just the same. The coordinates have no meaning.
Is that so? Isn't that a continuous effect? Things falling into the black hole appear to be frozen at the event horizon only for an observer at infinity.
Universe expected to decay in 10⁷⁸ years, much sooner than previously thought (phys.org) https://news.ycombinator.com/item?id=43961226 223 points, 5 days ago, 323 comments
https://news.ycombinator.com/item?id=43964524
It's true, that paper is nonsense. There's not really much else to say. Preprint servers sometimes publish the sort of stuff that wouldn't pass peer review. (Remember that S.Korean "superconductor" from about two years ago!?) The press should be cautious when writing about it.
> 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.
An attitude like that would have us all believing the earth is flat or that the sun revolves around the earth. After all, experts of the time believed both wrongly.
We shouldn't take the experts on blind faith, but we definitely shouldn't take the challenges on blind faith either.
Without being up to date in the minutia of every field one will be ill disposed to judge which of 100 piles of nonsense extant at any given time are real discoveries and which bunkum and ones work as a journalist would be so compromised by the 1000 lies one spread that none should believe the one truth they uncovered.
I wouldn't credit anyone so stupid as not to consult ones peers.
As an aside, nobody really believed the earth was flat: https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth.
Your link only debates that during the Middle Ages people thought the earth was flat.
Those living in ancient Mesopotamia and Egypt believed we all lived on a flat disc or plane floating in the ocean.
That's not a good thing if your goal is to advance everyone's knowledge. Whatever is going on in academia is failing relatively closely related fields which is not good.
I don't think they were stupid per se, nor malicious, but perhaps cavalier in pushing a result with such unexpected consequences without getting a consult.
1: https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved...
But yes: the world is complicated and it's easy to make mistakes outside your core field. The point of the scientific process is to get things in front of eyeballs who can spot the mistakes, c.f. the linked blog post. Then everyone fights about it or points and laughs or whatever, and the world moves on. The system worked.
What the process is not good at is filtering new ideas before people turn them into news headlines. And sure, that sucks. But it's not a problem with "academia failing", at all. The eyeballs worked!
Kinda classic. Kinda boring.
I don't think it's quite that, since the eventual goal is to publish, not only publicly, but as publicly as possible. More like it seems like everyone tends to hold their cards quite close to their chest until the moment of pre-print publication. Which means you can be working on something that someone could have told you months or years ago you have a problem.
The scientific equivalent of polishing a branch before making a pull request, only to be told "this has a huge memory leak and moreover what you want already works if you use this other API".
I'm not really sure there's a human-scale solution: the research landscape is so vast that you can't connect everyone to everyone else and have everyone in need of valuable input get it, and have everyone able to give it not be inundated with half-baked rubbish. Even if you assume everyone from the top to bottom has pure motivations and incentives for doing the research in the first place (in the pull request analogy CVE spammers, for example).
Perhaps not having the universities themselves so keen for PR that they'll slap a press release together about anything that looks clickable without due diligence would at least prevent making a public spectacle outside of the academic circle now and then, but it wouldn't solve the fundamental issues.
I think the real issue this highlights -- which is something everyone knows and still everyone does -- is that people love to spread and discuss sensational stories, and no one likes to hear naysayers ruining the fun.
Look the discussion of the original story here in HN[1]. There's a comment by A_D_E_P_T way down in the discussion explaining why the paper is nonsense and pointing to one of the replies objecting to it mentioned in the article from this post. That comment was downvoted by HN readers. I know because it was greyed out when I upvoted it days ago.
So there's no knowledge silo -- us simple folk just want to discuss the newest breakthrough without looking too hard, because that spoils the fun.
I skip that whole thread because I was expecting an overhyped result and I have to sleep from time to time https://xkcd.com/386/ . I'd have upvoted that comment, especially if it was gray.
The comment is like ELI35[1], but for HN it's better to write a ELI25[2] version. Or perhaps a ELI25 introduction and a second ELI35 part with even more technical details. (I never liked ELI5[3].)
[1] I just finished my postdoc in General Relativity.
[2] I just finished my major in Geology. I know atoms and calculus, but I have no idea what covariant is. Moreover, whatever gauge means is not the type of gauges I know.
[3] I just want a lollipop.
Also, the comment you reference was probably downvoted because of the tone, not because of some HN bias against naysayers. Starting out your comment with "It's nonsense." is about as conducive to a productive conversation as starting it out with "You're wrong."
The fact is, there are just too many people doing too many things. When any technical paper sounds like gobblygook to even people in the same field but in a different specialty, it's no surprise this happens, especially when coupled with the modern pressure to scientifically publish and modern "journalism" trends.
MY mother and father also have an untested theory that explains all this too it's called "God", most Sci-Fi authors have plenty, and I am sure AI's will soon add to this pile.
Kudos to those scientists that create testable papers or experimentally prove stuff.
This mistake of language plays into the weirdos who opine we should disregard inconvenient ideas because they are just a theory failing to understand the difference between their own usage and the scientific usage.
Layman here, but if you're standing, you're not actually accelerating, right? You'd only be accelerating if there was nothing under you holding you up, meaning if you were falling down.
What you say makes intuitive sense, but it was actually the opposite logic that lead Einstein to his general theory of relativity. Here's a slightly dorky but very good Veritasium video that explains this issue and general relativity https://youtu.be/XRr1kaXKBsU?si=1iudoAx5kWgWHHt-
My understanding from pop science videos is that they can indeed evaporate, but only through decay mediated by the weak force.
> [in their 1975 paper] Ashtekar and Magnon also assume that spacetime is globally hyperbolic
Isn’t the modern assumption that spacetime is globally flat?
It's not an assumption that space is flat. GR doesn't specify the global space curvature, so it's possible that it has a globally negative or positive curvature, but so far there's no evidence of any.
Much of where Relativity "breaks" spacetime (i.e. problems with infinities and divide-by-zero) can be solved by looking at things as a loss of a dimension. For example, length contraction is compressing out a dimension (at light speed), and also time dilation (at event horizons, or light speed) is a removal of a dimension as well. Yes, this is similar to Holographic Principle, if you're noticing that. In my view even Lorentz equation itself is an expression of how you can smoothly transform an N-Dimensional space down to an (N-1)-Dimensional space, which happens on an exponential-like curve where the asymptote is reached right when the dimension is "lost". I think "time" always seems like a special dimension, no matter what dimensionality you're in, because it's the 'next one up' or 'next one down' in this hierarchy of dimensionality in spaces. This is the exact reason 'time' in the Minkowski Space distance formula must be assigned the opposite sign (+/-) from the other dimensions, and holds true regardless of whether you assume time to be positive v.s. negative (i.e. called Metric Signature). This of course implies our entire 4D universe is itself a space embedded in a larger space, and technically it's also an "event horizon" from the perspective of higher dimensions.
I don't think this is a good way to think it. If black hole is big enough, there is nothing strange happening in the event horizon, no significant length contraction, nothing.
For example, when you see a clock fall into a BH you see it stop ticking at the EH, not at the center. It's a common misconception that everything about them is at the center, but everything interesting is at the surface.
Even if all information is encoded in two-dimensional surface that forms the Bekenstein bound, that does not mean that anything changes when you cross the area. It only means that radiation which black holes emit can't emit information of the matter it has absorbed.
Sounds tempting, but then what happens at the transition : when a sphere of matter gets just a little bit too dense ?
But you're raising a good point that maybe Lorentz is pointing to 'non-integer dimensionality' where even enough mass crammed into a small enough space causes the "new maths" to begin to noticeably take hold. Like I said I see Lorentz as a way to transform dimensionality from N-D to (N +/- 1)D, but in a continuous and 'differentiable' way.
In super simplistic terms Lorentz is a "compression" function where one dimension of space is compressed perfectly flat, which is the mathematical equivalent of removing that dimension from the 'degrees of freedom' of the system.
But what can be done? Science is not supposed to be the realm of disinformation, but it seems to have no real defenses. People are being paid to lie, no one is being paid to say they are liars, and from the outside scientific dispute looks a lot like politics, so scientists lose credibility by association.
That's a real problem.
Well played.
It’s a very old saying but we all learn it at some point
White dwarfs and neutron stars are generally considered "dead stars", since they no longer have active fusion processes. But they do radiate from energy left over from the star's "death". (Mostly thermal energy for a white dwarf, for neutron stars there is also a lot in angular momentum and the spinning magnetic field.) In theory, they will eventually radiate all of their energy away and become black dwarfs or cold neutron stars, but IIRC, that would take longer than the current lifetime of the universe.
AFAIK everything above above absolute zero radiates, which effectively means that everything radiates. Black holes would be an exception if it wasn't for Hawking radiation.
In addition, (stellar) black holes are dead stars. Or at least, that's one way to see them.
What really matters is temperature relative to surroundings. Something at the same temperature as everything around it won't lose any net energy to radiation.
The temperature inside could be anything. You could well be inside a black hole right now.
Even if we were inside one we couldn't really talk about the temperature of the singularity. The singularity is a divide-by-zero error. It probably doesn't physically exist at all, and whatever does exist is beyond our ability to model.
Not if you know the reputation of John Baez: Anyone familiar with him or his writings would know without hesitation that he understands black-body and E&M radiation, so his choice of title is clearly meant to be provocative.
It says to the reader "I wonder what he means?" To this reader, I'll also say that he delivered a terrific blog post.
This has become affectionately known as “click bait”.
No disrespect to the pedigree of the clearly distinguished author.
All the power to them by the way. It's the crushing power of the algorithm. No hard feelings, just something I've been wondering.
So full disclosure: I've directly interacted with John Carlos Baez only in social media, with the topics as disparate as music and observational astronomy. My own QFT & GR background is grad course level but with little actual usage in my career. (I've done more solid-state + high-speed electronics work, with a bunch of programming as well.) With that background, and turning the pedantry dial up to 11:
To me, one distinguishing element of clickbait is that the post is ultimately disappointing. The usual M.O. for clickbait is that the website needs eyeballs for advertising, so they beef up a headline of an uninteresting article with the expectation of getting extra monetization compared to an honest headline.
I would venture a guess that he doesn't actually care about monetization, or really even extra clicks, with this post. The screenshot with the big red X through the popsci article sets the expectation pretty quickly, and the tone of the rest of the post is really a rant that mediocre science made it into PRL and then into the popular science literature. He explicitly calls out the popsci journalists for laziness, but in a clever (I'm pretty sure Mark Twain would approve of his name being taken in vain) and erudite (correct use of the subjunctive) way.
Would I have clicked on the title without seeing the authorship johncarlosbaez.wordpress.com? Maybe but I doubt it. There is so much bad popsci physics out there that I'm pretty trained to ignore obviously inadequate headlines. So on a scale of 1-10, I'd rate the click-baityness of the headline no more than a 3. He got me to click, but only because I knew it was his post.
As for others, the set of people who understand that Hawking radiation exists has nearly 100% overlap with those who know that black bodies and spinning magnets radiate, so for those folks who are in the set who are also unfamiliar with the author, perhaps it's more clickbaity.
[edit: And I can't believe you got me to write that many words on the clickbait philosophy. Have I been baited? :) ]
I have a degree in Physics with an emphasis in Astronomy, and my thought on reading the title was "that's absurd". Even if you somehow infer that "radiate" specifically means "emit hawking radiation" which I don't know how you would without more context, "dead stars" generally is considered to include black holes, which do emit hawking radiation.
> As for others, the set of people who understand that Hawking radiation exists has nearly 100% overlap with those who know that black bodies and spinning magnets radiate, so for those folks who are in the set who are also unfamiliar with the author, perhaps it's more clickbaity.
So according to my theory, you must in the set that understands Hawking radiation + black bodies + E&M, but not in the set familiar with Baez.
I worked hard on my theory, please don't let me down and be a counterexample. :)
Dead Stars Don't Hawking Radiate
The whole point of respectable journals is that they filter out bad quality papers.
Bluntly, science journalism is not that well paid and most people won't have given the articles he calls out more than a moments thought. Of course journalists are going to be lazy if they aren't paid not to be. (Which is a problem itself, but I don't see many people advocating for better paid science journalists).
But journals should be a kind of guard rail against that. If their peer reviewers were too lazy to find experts or too arrogant to admit they didn't understand what they were reviewing, then that is a real problem.
Besides some high level ideas, which even us normal people can understand, there are so many details linked in the original post that you need an MSc/PhD to fully understand them.
For the time being, let’s just keep that the universe has a few extra trillion years, and isn’t expected to decay in 10⁷⁸ years ;-)
I feel like the only way not to emit is to absorb.
The original paper is 2023 (Phys Review Letters). There was a rebuttal in PRL in 2024. I don't know why this is still a big deal now in 2025 other than Science Alert decided to write (another?) hyperbolic article based on crap. Still boring.
