I've got a PhD in physics but am not a theorist. I do, however, maintain an interest in this material and I must confess that when you dig pass the gloss that Susskind typically puts on the "quantum mechanics == gravity" spiel the actual theoretical case seems pretty distant/vague (to say nothing of the fact that there is nothing like experimental support for anything at all having to do with this because the experiments are presently impossible).
What all this comes down to is a "mere" correspondence between some equations governing the way entanglement develops in time and the way some other gravitational systems evolve in time in a very specific sort of set up universe which is quite different from our own. Lots of physical phenomena have similar dynamical laws. Given the tenuousness of ADS/Cft and the differences between that imaginary universe and our own in number of dimensions and structure of spacetime, I think the assertion that these two phenomena supervene upon a shared ontological substance of some kind is provocative but hardly anything I'd write a New York Times article about. I mean for the lay reader this is basically bullshit which is more likely to confuse than illuminate.
That said, if this kind of reporting sparks the interest of a young physicist out there, I guess its mostly harmless.
> if this kind of reporting sparks the interest of a young physicist out there, I guess its mostly harmless
I'm not sure I agree. If reporting like this funnels more young physicists into doing this kind of research, which has been going on for decades without making any successful experimental predictions, then I don't think it's harmless.
Not only that, but this kind of reporting has a subtext: Science is the Authority. Even when Science tells you what seems like obvious nonsense. And that pernicious effect goes far beyond a few young physicists.
That is not to say that there aren't scientific claims that seem highly counterintuitive to the lay person, but which are nevertheless true. There are. But those claims are nailed down by massive quantities of experimental evidence that matches the predictions of the models to many decimal places. The speculative claims discussed in this article have no evidence to support them. Big difference. But you'll never find that out by reading these kinds of articles.
Now you've made me feel like I was too hard on ADS/Cft!
Empirical confirmation is not, in my opinion, a requirement for taking a scientific idea seriously. There is a lot of discussion of relativity in the threads on this comment, but most of it misses what I think is the salient point: _general_ relativity was formulated not because of any compelling physical observation which demanded a theory of curved spacetime or some other relativistically invariant theory of gravity. General Relativity was formulated because _special relativity_ more or less demanded that the effect of gravity propagate in some way coherent with the idea that causes and effects are ordered and move from place to place at a finite speed.
There is nothing wrong, in my opinion, with undertaking a research program on the basis of a compelling outstanding but purely theoretical problem. I don't think the existence of such research programs alone, nor their discussion in public per se, tends towards a kind of scientific authoritarianism. My problem with the article is entirely in how it communicates the ideas here, which is badly and, more importantly, without situating them in the greater epistemological context of progress in physics. I suppose the presenting the material without that context does kind of support "scientism" but such is so common in media that this doesn't seem like a particularly outstanding example. Really, the inability of the public or the press to understand science is a deep cultural and political problem of which this kind of reporting is just a symptom.
> Empirical confirmation is not, in my opinion, a requirement for taking a scientific idea seriously.
Taking it seriously as far as investigating it, I agree; you don't need empirical confirmation for that, indeed it would make no sense to insist on it, since at least one major point of investigating it is to see if you can find empirical confirmation (though you might have to expend a lot of theoretical effort to get to that point).
Taking it seriously as far as claiming it is true, no. For that you need empirical confirmation.
You can investigate whatever, but at a certain scale of investigation it starts to make sense to think about how wise it is to spend the resources, time and collective effort on any given investigation carried out in the way it was meant to be carried out.
E.g. we could do a really hardcore investigation into whether the moon is made of cheese despite all evidence to the contrary. Then we could fly there have one guy be like "Yep it is not cheese". Then have a psychological analysis of that guy and send another one just to be sure, ad infinitum leaving nothing for actual serious, on the edge of knowledge investigations.
That overdriven would dadaist investigation is of course just a carricature I made up now, but the underlying issue is real: science costs us all and we should spend it where it helps. Sometimes that must also mean not knowing to which ends we do investigate something. But it should not become dadaist in a way similar to the outlined story.
Reminds me of an old joke about the dean of a university complaining to the head of the experimental physics department about how much his department costs. "You have all that expensive equipment," he says. "Look at your counterparts in the theoretical physics department. They're as cheap as the math department: all they ask for is pencils, paper, and erasers." He stops and thinks for a moment. "And the philosophy department is cheaper still. They don't even ask for erasers."
If physicists are getting their research directions from pop-sci NYT articles than we are in bigger trouble than merely having interesting theories not pan out.
GP probably meant young people deciding to go into physics after reading these articles. I'm a physicist and many of us were massively influenced by the pop-sci books we read as kids before we knew better. You don't get specific research directions from NYT but you do get educated in what kind of ideas will get media attention, and that is as bad as the hypothetical big troubles you allude to.
Meh, lots of scientists first got that tendency by watching Star Trek and the like. NYT articles are a huge step upward in rigor, compared to that. I wouldn't worry.
> The speculative claims discussed in this article have no evidence to support them.
Can you highlight which claims that you find to be speculative without evidence? There's a mix of existing theories that are being actively studied in theoretical physics (holographic cosmology), and some claims that the author seems to call out in rhetorical question. I get that either will whoosh over the lay person.
> There's a mix of existing theories that are being actively studied in theoretical physics (holographic cosmology), and some claims that the author seems to call out in rhetorical question.
All of those are speculative claims without evidence to support them.
I'd argue that in the case of AdS/CFT correspondence, they've been mathematically proven to be _possible_. Doesn't mean that it is true. We also don't have all of the answers to AdS/CFT questions to prove a holographic universe theory with utmost certainty. I would, however, agree that evidence could have been presented in the article, but I'm guessing the target audience for this piece may have never clicked into an array of academic papers.
The classical wormhole as an extreme fold in spacetime is mathematically _possible_, but given what we've observed in the universe, extremely unlikely to naturally occur.
"Mathematically possible" is a much lower bar than "has evidence to support it". Yes, AdS/CFT is mathematically consistent, as are classical GR wormhole models. But there is no evidence to support either.
> I would, however, agree that evidence could have been presented in the article, but I'm guessing the target audience for this piece may have never clicked into an array of academic papers.
If you know of any scientific papers that present evidence for AdS/CFT (or for wormholes, for that matter), please post links. I'm not aware of any such papers.
Ah I understand your argument now and can agree we are on the same page. Can't say I know of any empirical/observed evidence either. I don't believe AdS/CFT can be used to make predictions of precise accuracy beyond serving as a toy model to reshape other physical observation.
To get off the tangent, I've backed up the comment chain and am thinking about your original comments on the harmfulness of articles like this. At first I shared OP's sentiment that it is generally harmless, but the more I think about it, the more I take your stance. Curious what your thoughts are on exposing more people to theoretical physics sans popsci buzzword articles?
> Curious what your thoughts are on exposing more people to theoretical physics sans popsci buzzword articles?
It's a great thing to try to do; I try to do it myself as a contributor to Physics Forums [1], for example.
I'm not sure how much good books like The Trouble With Physics actually do as far as exposing more people to physics sans popsci, because, while they might point out issues with how speculative research in physics is done, they don't actually teach any physics.
I personally would recommend Feynman's books for the layman, such as QED: The Strange Theory of Light and Matter or The Character of Physical Law, or Six Easy Pieces (followed by Six Not So Easy Pieces), as ways for people to get at least some exposure to physics without popsci buzzwords. IMO even those books are limited, because you can't really understand physics without actually doing the math and solving some actual problems. But they're still way better than popsci articles (or, for that matter, popsci books like those of Brian Greene or Michio Kaku).
> Yes, AdS/CFT is mathematically consistent, as are classical GR wormhole models
Nitpick (one which I'm sure you are aware of but it might be relevant for other readers): Classical GR (including wormhole spacetimes and others) is mathematically rigorous. All objects are precisely defined and you can write down actual mathematical theorems.
In contrast, AdS/CFT is anything but rigorous. It's basically just lots of handwaving; it's a hunch that people have, based upon lots of mathematically rather non-sensical calculations.
So I wouldn't want to equate the two in terms of rigorosity / consistency.
No, your claim regarding the historical facts is completely wrong.
If by "relativity" you mean special relativity, nobody used it for anything when it was published. The first real use of it theoretically was to explain the Compton effect in 1921, based on experimental results. It took another decade or so for it to be routinely used, mainly due to its success in explaining the fine structure of atomic spectra and in developing quantum field theory.
If you mean general relativity, the first event that could be said to have gotten it "accepted world wide" was the 1919 eclipse expeditions organized by Eddington, which confirmed the GR prediction of bending of light by the Sun. And even then acceptance of it was still limited (see below).
Also, even before that, GR was known to correctly predict the extra precession of Mercury's perihelion, which could not be explained by Newtonian gravity. So that was a piece of evidence for GR that was known before it was published.
Note that even after the 1919 eclipse expedition and the 1921 explanation of the Compton Effect, relativity was still considered too "out there" to justify a Nobel Prize; when Einstein got the prize in 1922 the Nobel committee specifically excluded relativity from the scientific work by Einstein that was the basis for it.
> If by "relativity" you mean special relativity, nobody used it for anything when it was published. The first real use of it theoretically was to explain the Compton effect in 1921,
The Michelson-Morley experiment was in 1887 and a whole bunch of theories resembling relativity were developed to explain it. SR itself was one of those, and the cleanest.
Yes, good point. Also, classical electrodynamics based on Maxwell's Equations was already known to be Lorentz invariant, not Galilean invariant, so SR can be thought of as simply taking the invariance properties of electrodynamics and "porting" them to mechanics (whereas pretty much all other approaches went the other way, trying to "port" the invariance properties of Newtonian mechanics to electrodynamics).
Einstein's speech in the Nobel ceremony was entirely about relativity, which had been accepted as fact since 1908 within the physics community. General relativity was immediately accepted as fact on being published. The same is true for the discovery of gravitational waves.
Relativity was excluded from the prize for relatively obscure reasons, the then famous philosopher Henry Bergson objected to Einstein's concept of time at the time that the committee met to decide on the prize[1]. To top it off Alfred Nobel was not a fan of theoretical physics and made it clear that only experimentally verified theoretical results should be awarded[3]. This is at odds with the physics community. No prominent physicist has objected to relativity since 1910.
If you have actually studied physics, you will see that special relativity is "obviously" correct. There is no reasonable explanation of electromagnetism that doesn't logically imply special relativity. If you accept Maxwell's electromagnetic field equations, special relativity follows mathematically from it.
Theoretical Physics proceeds by exposing contradictions within theories and solving them.
1. Galileo challenged Aristotle's physics by assaulting the contradictory laws for heavenly and earthly bodies
2. Newton gave it a sound mathematical footing resolving contradictions between the laws of heavenly and earthly bodies
3. Maxwell resolved contradictions in Biot Savarts law by introducing displacement current and thus predicting that light is an EM wave
4. Special relativity eliminated the extra ordinary coincidence that prevented any physical experiment from distinguishing an electric field from a magnetic field, even in a theoretical setting, by proposing that the speed of light is constant wrt the observer [2]
5. General relativity solved the extra ordinary coincidence that we cannot distinguish gravity from acceleration while also resolving contradictions between gravity and special relativity.
These theoretical explanations were adequate by itself and did not need experimental verification to be celebrated. Because they solved long standing contradictions and coincidences in a simple compact explanation that reduced the axiom set in physics, not increase them.
That's what differentiates theoretical physics from experimental physics. Alfred Nobel had an aversion to theoretical physics, that's why Higgs had to wait for his prize for decades. But the physics community always knew that the Higgs boson existed. Otherwise, they wouldn't spend billion plus dollars on speculation.
Stanford would not hire Susskind as a professor in theoretical physics if the physics community thought his works are flights of fancy. The same goes for the Lucasian chair conferred on Stephen Hawking, who worked almost entirely in theory with very little experimental verification for his work. Einstein was offered professorships in theoretical physics at various institutions in the 1910s, including the prestigious ETH Zurich well before any sort of experimental verification of his theories.
> There is no reasonable explanation of electromagnetism that doesn't logically imply special relativity.
In other words, the experimental confirmations of Maxwell electrodynamics can also be considered as evidence for SR. That's fine, but it contradicts your claim that SR's acceptance was purely "theoretical" and abstract.
> These theoretical explanations were adequate by itself and did not need experimental verification to be celebrated.
Einstein took a similar position. As one story goes, when he was asked what he would think if experimental tests contradicted the predictions of relativity, he said something like "Then I would feel sorry for the Lord, because the theory is correct."
All of which does not at all change the fact that, unless and until a theory makes accurate predictions, it cannot claim to be "scientific truth". "Solving theoretical contradictions" is fine and a worthwhile endeavor, but it's still not the same as making accurate predictions. String theorists wax lyrical about the same wonderful things you mention, but that doesn't change the fact that, unless and until string theory makes accurate predictions, it cannot claim "scientific truth".
> the physics community always knew that the Higgs boson existed. Otherwise, they wouldn't spend billion plus dollars on speculation.
By the same argument, supersymmetry would have been discovered at the LHC, since the physics community "knew" that supersymmetry was "right". But of course this prediction by many physicists is now notorious for its wrongness. Beautiful theories sometimes get spoiled by ugly facts. Nature doesn't always do what theorists expect.
As for professorships, obviously a professorship in theoretical physics is going to be based on someone's theoretical work, which might be judged by different standards than the standard of "scientific truth". Many theorists have come up with theories that didn't turn out to be right. That doesn't make them bad theorists. It just shows that theories need to be tested to see whether they make accurate predictions. Sometimes they do, sometimes they don't. Nobody gets it right all the time.
My immediate reaction to the headline was "oh, so NYT is now adding pop-sci clickbait to their repertoire?"
Given all the other ways in which they've degenerated and embarrassed themselves in the last decade, I supposed they were bound to start doing this one too.
Hah... Maybe they've figured out that most of their readers are the same pseudo-intellectuals that will misunderstand things like Schrodinger's Cat and refer to it like magic, quietly hoping that other people will look up to them as all the wiser for having referenced it. Like a bartender I met a while ago that confessed to me that he and his wife interpreted quantum mechanics in a way that borders on religious, using it to discredit the concept of free will altogether.
> Take gravity, add quantum mechanics, stir. What do you get? Just maybe, a holographic cosmos.
Something tells me I'll be hearing a lot of confused misinterpretations of this over the next few days... A lot like the confusion over entanglement after this year's Nobel prize.
Have you considered why the best explanations that the field of physics can offer are all interpretations? Maybe it's because there is a lot we don't know, and the patchwork of scientific theory we've built up iteratively over centuries is leaking in places for reasons we don't understand.
If you accept this, then it should be hard to mock someone who offers their own interpretation, since none of us know the answer to so many fundamental questions underpinning the physical universe in our cosmological plane of existence. At best, you knew more than he did about what we don't know.
The best explanations that the field of physics can offer are systems of equations. Which make a lot more sense than the eli5 woo that popular science turns it into.
> Have you considered why the best explanations that the field of physics can offer are all interpretations?
Or you can look at the maths but I doubt you would want to do that... Anytime you dumb something down, something is lost. If you try to build back from something that's dumbed down then you get a mess.
confessed is an interesting word choice? the bartender was being somehow deceptive about how they present their interpretation of quantum mechanics normally?
IMO, GPT-3-like articles like this are no different from blog spam recipe websites.
What even is the point of this article? What is the mind bending secret? The holographic principle? Wormholes? Entanglement? It's unclear what the title was referring to, but none of these things are news. There's not even a well defined recipe at the end; we're just left with some kind of pop-physics stew.
That's the thing that sucks about titles. Editors choose them, the journalist has zero control over them, and they test different variations on the site and then settle on the one with the most clickthroughs.
Definitely lots of cases of Op-Ed contributors, especially, unhappy with seemingly borderline misleading titles given to their pieces.
I don't mind it too much as a reader, but it can suck as a contributor because everyone thinks they're your words.
“… hardly anything I'd write a New York Times article about.”
Given that NYT devotes an entire section to the goings-on of old money New Yorkers, I’m not sure the bar is all that high.
And hey sometimes coincidences are just that, but other times they can lead to profound theories. I’m reminded of Monstrous Moonshine for which Borcherds earned the Fields Medal in 1998: https://en.m.wikipedia.org/wiki/Monstrous_moonshine
The term "monstrous moonshine" was coined by Conway, who, when told by John McKay in the late 1970s that the coefficient of
q (namely 196884) was precisely one more than the degree of the smallest faithful complex representation of the monster group (namely 196883), replied that this was "moonshine" (in the sense of being a crazy or foolish idea). Thus, the term not only refers to the monster group M; it also refers to the perceived craziness of the intricate relationship between M and the theory of modular functions.
As someone with a physics background does this sound right?
From the article:
According to Einstein’s general relativity, the information content of a black hole or any three-dimensional space — your living room, say, or the whole universe — was limited to the number of bits that could be encoded on an imaginary surface surrounding it.
I thought this was more according to the Bekenstein bound than GR.
The piece feels a bit fluffy, but I agree that if it excites a new generation, a new bunch of physicists with a new way of looking at things, it's completely worth reading.
On the other hand, I feel like it really missed the boat when it comes to the relation of entanglement to other properties of the macro universe. The notion that "geometry of spatial dimensions as emergent from networks of entanglement" is such an evocative idea that I don't understand how people aren't thinking about it all the time.
What if certain systems are in fact in the same "place", even if they appear to be light years apart? What does "appear to be apart" even mean?
To overuse an adjective, it's evocative. I do realize my understanding is barely that of a sparse layman, but it's still a fun mental toy to play with.
A talk came across my screen and a third of the way through a misleading and glossy description of QKD prompted me to stop watching and do something else with my time.
I agree though that kind of optimism can be very motivational, which is great in an education system that is basically a water wheel of slapstick white gloves for you to stick your face in.
Vague, sure, but does physics have anything better to offer when it comes to quantum gravity? Granted I'm biased as a (lay) fan of ER=EPR. And I don't entirely disagree with you; the NYT is probably not helping here and maybe shouldn't have waded in.
There are lots of quantum gravity theories. Take your pick. The problem isn’t so much that we don’t know how to reconcile QM with GR, but that there are many ways to do it and we don’t know which is physical.
Adding gloss to a topic without leaving the realm of truth can be helpful in inciting interest, as you mention. Similarly, Dr. Michael I. Jordan of UC Berkeley describes in no uncertain terms that the phrase "Artificial Intelligence" is misleading in the field of the same name [1]. He basically says our AI thus far is recommendation engines and a self driving car.
Is the glossy, somewhat simplified discussion of quantum mechanics == gravity == anti-god particle also misleading? I'm still trying to wrap my mind around the 10th grade Algebra's imaginary number √-1 .
regarding √-1, the mystery is a bit simpler than with physics, because with math it is what it is, and there's no arguing about how real the math is :)
that said, just as in physics we went from the concept of an atom (meaning an undividable whole) to a big ball of nuclear soup with protons and neutron and bound electrons raving around, then even further to noticing an even crazier confluence of chaos of quarks and gluons constantly churning in the nucleus, like that in math we went from integers to reals to complex numbers to saying hold my beer and "dividing number fields with irreducible polynomials" (see algebraic extension) and discovering all kinds of madness.
in physics we used mightier and meaner matter smashers, in maths we used ... well, a similar amount of brute force of a certain kind, the kind that solved problems, rules of square roots be damned. and then it turns out that the rule was different all along, and congratulations now you have even more complicated numbers, the complex ones.
just as in physics there's the always gnawing question of "okay, but what explains that small blip in the data?" there's the one in math about "okay, sure, that question is so simple-looking it's outright ridiculous/dumb, but how come nobody was able to answer it in decades/centuries? what if ...?"
Nice job describing why ER=EPR is so exciting, and conveying conflicts within physics. The idea that Einstein held conflicting views is important to keep in mind because we see him being led by the data rather than dogma. Susskind has a lot of interesting ideas and excels at sharing his excitement. These are interesting problems and quantum physics has great difficulty communicating well, and still has plenty of nonsense sadly. Yes, Einstein was conflicted, sans acknowledged these things. Susskind used to be a string theorist. People grow and learn.
One theory I have is that advanced intelligences are out there harvesting energy, so they deliberately start black holes to power their spacecraft. Imagine some sort of collection depot on the other side of a black hole that is harvesting exotic matter so they can travel vast distances with ease. </tinfoil>
Perhaps space itself is the spacecraft, with extra-dimensional links between matter hiding in dark energy, creating an almost infinite capability for sensing and affecting matter? Perhaps black holes are the homes from which these aliens reflect their powers across the event horizon, providing powerful reach and safety? Or maybe they just live in the ocean and they don't like black holes either. I mean if we're going to go tin foil, might as well go all the way.
What has always bothered me is the nomenclature; "black hole," is an astounding misnomer. A hole is not something, it is a cavity in something else. Black holes are definitely something and a lot of it. Space is the black hole of the Universe, not black holes. Those are the mountains.
Well, it's not a description, it's a name. Names are allowed to not be accurate to the thing (a girl named Sandy is not necessarily made out of sand, and light is not light, it's weightless).
And itself is based on two things: that they're absolutely black as in they don't reflect anything (that's what they believed when then the name came about), and that things can fall into them.
>A hole is not something, it is a cavity in something else
Well, and black holes can behave like cavities in the universe. They bend the surrounding area with their gravity so much that they end up forming a kind of cavity.
"Black holes" absorbs light, so they can be reasonably thought of as "black" (despite lighting effects as it interacts with surrounding matter), and anything that gets too close falls in, like a "hole" (or a cavity).
Compound names mostly come from associations, not careful logic.
Names in general are arbitrary symbols, not definitions.
My issue is not with the descriptor, "black," but in calling it a hole. You get too close to a star or planet, you will also fall in, but we don't consider stars or planets as holes. "Gravity well" is a better descriptor, but not when considered a hole, rather only when considered full of water, but here the water is instead gravity, and gravity always implies something massive, not the absence of something else.
Black holes, as we think of them as something, are really shadows. It'd be more accurate and less confusing to call them singularity shadows.
It's a hole in spacetime in the sense that the inside of the Black Hole is completely cutoff from the outside, with the exception of Hawking radiation. A black hole's interior is causally separate from the rest of the universe.
This doesn't solve the problem. Spacetime itself is a hole, so you're saying a black hole is a hole within a hole, and we can designate an infinite number of holes within a hole.
Isn't the idea that the universe (or the superset of universes) a hole within a hole (or an infinite number of holes within a hole) something physicists already entertain?
E.g. there are several theories about how our universe itself is the inside of a black hole, and so on.
A water well is a special type of hole known as an excavation, meaning, of course, that it was excavated. Wells are not made by accumulating so many things that they just sink into the ground, and nothing is taken out of a black hole to create it.
> You get too close to a star or planet, you will also fall in
An object with a relatively much smaller mass can take a hyperbolic orbit arbitrarily close to a star or planet without "falling in". Practical examples include <https://en.wikipedia.org/wiki/List_of_hyperbolic_comets> and many small near-earth objects. Theoretical details in Newtonian gravity at <https://en.wikipedia.org/wiki/Hyperbolic_trajectory> and there is a literature exploring post-Newtonian corrections for such "orbits" (in e.g. General (or Numerical) Relativity, or formalisms like gravitoelectromagnetism or effective one body, for cases where one or both bodies are "compact" (like white dwarfs or neutron stars), the mass-ratio of the bodies is close to 1, or one or both bodies are moving at large fractions of c, or there is some combination of these features).
An object can take a hyperbolic orbit arbitrarily close to a black hole without falling in.
Black holes were first formalized in the context of General Relativity; commonly one would use a different term to describe something phenomenologically similar but set in a different theory ("fuzzballs", gravastars, and so on). Fevers and spots appear in diseases with very different causal agents (any number of quite different bacteria, viruses, and other things may cause grossly similar symptoms in the victim). Likewise, apparent trapping surfaces can appear in many ways in different theories of gravitation or in different configurations of variables within a single theory of gravitation.
The important word in the previous sentence is "trapping": anything crossing that surface from the outside to the inside cannot cross back to the outside for arbitrarily long times. Differences in configurations on the inside of a trapping surface do not materially affect the outside at all. The "apparent" qualifier captures the possibility that the trapping surface does not go to the eternal future because of (for example) instabilities from quantum effects (Hawking-style), so we can take at all as meaning "for a very very very very verrrrrrrrry long time".
The observables of a black hole in General Relativity (in its form as a physical theory that adequately represents many physical features of our universe) are all outside the event horizon. Anything inside the horizon stays permanently inside. From the outside one cannot test the internal configuration. While the internal configuration is described in several black hole solutions to the Einstein Field Equations, nobody expects that just because the external configuration (outside the black hole) is a good physical model, that therefore the internal configuration must be a good physical model too. Roy Kerr makes this point almost every time he lectures about his solution for black holes with nonzero angular momentum (example: 48m04s mark <https://youtu.be/nypav68tq8Q?t=2884>, where he points out that the Kerr solution is a vacuum solution, and that adding matter inside the horizons is likely to dramatically change the black hole internal configuration. Note however that adding matter to the outside part of the Kerr solution is highly likely to be undramatic, and that is one reason why the Kerr solution is astrophysically useful).
Stars and planets differ from black holes in that there is no apparent trapping surface. You can shoot an electron neutrino right through the Earth or the Sun. You can't shoot an electron neutrino through a black hole: if it goes in, it stays trapped inside.
Your term "singularity shadows" presupposes that as we develop better solutions (numerical or analytical) of the Einstein Field Equations for (apparent) astrophysical black holes, the singularities that appear in e.g. the Schwarzschild or Kerr solutions will remain. That may not be true. I don't think the "shadow" part adds any accuracy.
It is not true that "gravity always implies matter". In General Relativity there exist several exact solutions to the Einstein Field Equations where there is significant spacetime curvature but no mass. Some of these usefully approximate features of the universe we observe, even though as far as we can tell there is no part of our universe that is completely free of matter (in the most general sense, including electromagnetic radiation), although large and growing regions are so sparse that the matter in them does not collapse gravitationally into clumps. This trend is just as much an effect of spacetime curvature as is the gravitational collapse of dust clouds into stars.
Finally, you can use whatever nomenclature makes you happy. It's just a fanciful term that covers a wide range of theoretical descriptions and astrophysical phenomena. Astrophysicists and theorists use "black hole" knowing that they may be talking about quantitatively and qualitatively different objects with fairly similar symptoms being presented. But they also know how to find, read, and understand a precise mathematical description that removes the ambiguities of English (and other languages) and any inaccuracies (in some settings a black hole may be a very weak greybody radiator; and in some settings "hole" may be less poetic and more descriptive, e.g. in Wheeler's bag-of-gold solution). Substituting some other pithy name for "black hole" doesn't help these physicists, and is unlikely to help anyone who doesn't know how to deal with the formal, unambiguous descriptions of them.
>> An object can take a hyperbolic orbit arbitrarily close to a black hole without falling in.
It occurs to me that in your various comments here you are thinking of the point mass (or divergence of the Kretschmann scalar or whatever) as the black hole. Conventionally, and for good practical reasons, practically everyone working with astrophysical and theoretical black holes define the horizon as the black hole.
It's frequently tempting to think of the point-mass in Schwarzschild as the generator of the event horizon. After all, it's usually described as being a surface at r = 2GM/c^2, with "M" doing the heavy lifting, if you'll pardon the expression. However, Schwarzschild is an eternal black hole, rather than one that forms by gravitational collapse. For the case where there is some matter and no black hole -> some matter + a black hole, it is the early configuration of the "some matter" that generates the event horizon.
If one, following Lemaître-Tolman <https://en.wikipedia.org/wiki/Lema%C3%AEtre%E2%80%93Tolman_m...>, takes a spherical shell of radiation with a total momentum-energy comparable to a galaxy all moving radially inwards, and starts the spherical shell at billions of light-years from the shell's centre, then anything already at the centre (even a small interplanetary civilization) is already inside the event horizon before the civilization's home planet formed. Barring faster-than-light travel, nothing within (or produced by) the "victim" solar system will be able to cross outside a surface near the trailling edge of the inrushing radiation: at early times all possible low-speed trajectories from the victim solar system ultimately recurve back to a (set of) point(s) within it and at late times all possible high-speed trajectories recurve inwards.
Near the latest time in the collapse, everyone outside the victim solar system can conclude the victims are inside a black hole, even though for hours to days (and much longer, if we make the total mass of the shell extremely large) the victims inside will still be going about their business wholly unaware (because "c") of their fate. Horizon = yes. Singularity = no ("not yet", perhaps).
Finally,
> 90% of the matter orbiting a black hole will never fall into it.
is probably wrong, especially if one takes "never" literally. The dynamical spacetime in the near horizon region is on its own probably enough for orbital decay of anything in close orbit (a few tens of R_{crit} ~ R_{Schwarz.}) and there are plenty of forcing functions on bodies in elliptical orbits at greater remove, particularly in galaxy cenrtres and globular clusters.
You're thinking that a snarky response is a good fit for the parent, who in good will provided a detailed answer trying to charitably deduce what you were asking and how you conceive the topic in discussion, using the minimal information your comments provided (this is what the phrase you quoted kindly implies)...
I wasn't willing to argue this before because my issue was not with the adjective. But actually black holes are not really black, they emit radiation.
> - "Hole" — things fall in and don't come out (more or less).
But things fall into any massive object like planets or stars, so "hole" is ambiguous. Things also fall out of holes, such as holes in a ceiling or screw holes.
> Plus they look like holes on those 2d representations of space-time.
Seems reasonable to me.
It is a fair point to say that the hole of a black hole is a 4 dimensional hole, except that space-time is itself a 4 dimensional hole, leading to more ambiguity.
Definitely not true in math - if you take a circle (S^1 = { (x, y) : x^2 + y^2 == 1 }) it has a hole as defined by homotopy in the middle and there is no 'space' there. If you fill it with space you get a disk (D^2 = { (x, y) : x^2 + y^2 <= 1 }) there is no longer a hole as defined by homotopy/is contractible.
> Black holes are real, but your circle can only "exist" in mathematics.
You are aware that General Relativity is about the most mathematical (and mathematically rigorous and mathematically advanced) theory we have in physics? Several predictions were made purely based upon mathematical arguments, without much physical input. So your argument that math were a "different realm" with no connection to reality is full of, uhh, holes.
In fact, I would argue mathematics is mainly a language that provides us with the precision that everyday language lacks, it's a tool that allows us to make precise arguments while ruling out logical fallacies.
In the present case it allows us to say very precisely what we mean by a hole. This definition[0] has been employed in countless predictions in physics and its usefulness has been confirmed by experiment. Meanwhile, your definition is vague at best.
[0]: Look up "simple-connectedness" or, more generally, "homotopy groups".
> 1) If a hole is made of space, it doesn't follow that space is a hole.
Yes, it absolutely does, so long as there is no category error, and there is none here. The Universe is just an inconceivably large, curved hole with a tiny little bit of stuff in it.
> 2) Space is not spacetime.
Einstein and I disagree with you. Prior to Einstein's work, space and time were independent dimensions. But over 100 years ago Einstein showed that relativity of motion mathematically combines space and time into one, spacetime. For all intents and purposes, anyone talking about space today under any circumstances is also talking about spacetime whether time is specifically mentioned or not it is always there along with space.
If you dig a hole and toss something in you don't really consider the thing part of the hole (it's in the hole, but it is not the hole) but with the "black hole" the meaning of "hole" is "the volume enclosed by the event horizon plus all the stuff inside it".
You seem to be working from an arbitrarily tight definition of hole that's excluding black holes.
There are lots of different definitions of "hole", and I don't think black hole is misleading to the layfolk. They're typically dark, like some holes. And like some holes, stuff falls in and generally doesn't survive the journey. To the physicist, they know that the words "black hole" just reference a physical object with lots of mathy description.
> You can say black holes are holes in spacetime with a big rock at the bottom.
The more I've thought about this, the more it seems wrong. Black holes are not a hole in spacetime like a puncture hole in fabric. There is no missing spacetime like there would be missing fabric. Black holes are more accurately a round 4D valley in spacetime rather than a hole in spacetime.
There kinda is missing space time. Everything inside the event horizon is fundamentally separate from everything outside. It takes an infinite amount of time (from the perspective of an exterior observer) for something to fall into the event horizon, and it is fundamentally impossible for anything inside to get out.
Not really, unless you can find a contrasting definition to one found on WorldReference. Puncture is defined as making a hole in surface (the boundary) rather a volume (the solid). In any case this goes off topic. You've apparently misunderstood what people usually mean with hole. (edit: Or maybe I've. Now I'm confused. Here goes the rest of my day thinking about the meaning of holes.)
> A hole is not something, it is a cavity in something else
this reasoning also applies to "infinity". it literally means NOT-finite. it refers to the lack of a thing: namely a biggest number.
then again, freaking language and maths sure make us able to think about these 'lack of [blank]' as if they were actual things (to the point that abstractly they are as real as it gets)
"Infinity" applies to the reasoning of what a hole is, but not to what a black hole is. A black hole is not not something. It is something, yet a hole is not something. Unlike a hole, a black hole is defined by its inherent characteristics, mass, spin and electric charge. A hole can only be defined by characteristics of what it is not, the empty volume of missing substrate.
You are right about language, with contronyms and oxymorons. Especially in nomenclature, many things are named incorrectly or have contradictory names, such as Koala Bear, Whale Shark, Killer Whale, Starfish, Prairie Dog, King Cobra, Red Panda, Guinea Pig, Bearcat, Flying Lemur, Flying Squirrels, etc. Also, asteroid, which are not at all particularly "star like."
The article spends a lot of time talking about black holes, I'd rather hear more about wormholes and the quantum entanglement problem. That is something that you can easily demonstrate on a desktop and is going on all around us (whereas evaporating black holes are not). An entangled quantum state being composed of a wormhole is more or less what I've thought should be the way to resolve the non-locality of the measurement problem. You need a physical theory of quantum wormholes though which doesn't violate causality and cannot be used for superluminal communication. Then some sort of conditions which causes the entanglement to "snap" and become observed (of course that might not help with the black hole information paradox).
Dumb question: are wormholes real, or are they just a theoretical speculation that science fiction latches on to in order to explain/justify long distance space travel?
As far as I understand, wormholes are a valid solution to the equations governing relativity. Our best theories suggest that if a wormhole magically popped into existence, it would simply continue to exist without violating any known laws. It's possible wormholes have existed since the beginning of the universe, and this wouldn't require any revision in our understanding of cosmology.
The more thorny question is whether/how wormholes could be created in the "modern universe" where they didn't exist before. My interpretation is that opinions vary widely on this, from "it's impossible because it would take an infinite amount of energy" to "we're pretty sure humans will be making these some day and we have a rough idea how they'll be doing it".
Of course, wormholes simply not existing anywhere is also compatible with relativity. None have ever been observed.
None of the HN commentors have a background in theoretical physics. These are low quality bike shedding comments.
The ny times article itself is of a much higher quality. The holographic principle has been established for decades now. HN is not the place where you will find useful discussions on physics.
Ask physics reddit might be better. But I have never engaged there.
I am a physics novice too, but I am familiar with Electro magnetic field theory and special relativity. Any physics discussion on HN is bike shedding, if not outright wrong. Veritasium on YouTube is probably the best educational physics resource out there.
This is pop-sci over speculative theoretical physics. Whether it makes sense to "popularize" this stuff or not, I think it makes for a great mind bending and fun read --also generates incoming traffic. So why not?
it was an interesting read indeed. "What if a black hole was actually a hologram, with the event horizon serving as the “film,” encoding what was inside?" got me good
Who knows. I've replaced the baity title* with representative language from the article body. No doubt someone else can do better, and we'd be happy to change it again.
