Does anyone have a good recommendation for an introductory book on these concepts?
I read a bit of The Fabric of Reality but had trouble progressing too far with that one. The author talks about the quantum slit experiment, somehow arrives at the explanation of parallel universes, and then claims anyone who disagrees with this conclusion must have faulty logic. A huge chunk of reasoning behind parallel universes seems to be skipped and I have trouble taking it seriously every time it is brought up.
Sean Carroll (professor of physics, quoted in the article) has a highly-rated book titled "Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime" [1] which discusses exactly these topics. I haven't read it, but it's on my list.
I also highly recommend his podcast "Mindscape" where he discusses this and a range of other topics in science and philosophy. [2]
For German speakers, I also recommend Josef Gaßner's YouTube series "Von Aristoteles zur Stringtheorie" (I find it a bit more structured than PBS Space Time, albeit maybe less amusing).
I came here to reccomend this channel aswell. The "basics" were all covered years ago (6+), and now Matt is dipping into more complicated topics. If you find his recent videos overwhelming, I suggest to give his older ones a try.
The problem with the channel at the moment is that it builds on top of previous episodes, I got into it when it was still understandable to me; check out the older videos and go from there.
Just for context as a fellow interested noob: That book in particular is more like a manifesto than a survey, so it's not really intended to be 100% convincing, IMO. And AFAICT most of that book is about using the philosophy of the multiverse, not about justifying the physics -- so you definitely shouldn't feel bad coming away with that conclusion. An infinite multiverse is, to say the least, controversial!
For recommendations, assuming the linked articles themselves aren't up for grabs, I really liked The Rigor of Angels for a more historical-philosophical view on quantum physics, and how it compares to its predecessors. I also constantly reccommend this other Quanta article from a few years back, which is shorter and more cohesive than these: https://www.quantamagazine.org/what-is-a-particle-20201112/
Sadly, I think this whole field has an inherent level where it breaks down for non-experts, epistemologically speaking (how appropriate!). Watching Hossenfelder's engaging YouTube videos has taught me one thing above all else: I have no hope of critically engaging with the fine details of modern theories, only the metaphors and stories that surround the math.
What about if these people, despite their special terminology and maths, don't know either, and they're all just acting like they do? Stranger things have happened.
Great question and I’m hoping to see some other recommendations here.
In my opinion Coveney & Highfield’s 1990 survey “The Arrow of Time: A voyage through science to solve time's greatest mystery” is better than most for clearly connecting where theoretical approaches like many-worlds to the context they arise at the intersection of relativity, statistical thermodynamics, and quantum theory
https://en.m.wikipedia.org/wiki/Special:BookSources/978-1-85...
As the book is 30+ years old and predates what I have to assume to be significant progress in the respective fields I would be keen to know if the experts here are aware of updated sources at any level (peer reviewed papers, monographs, etc.)
> A and then claims anyone who disagrees with this conclusion must have faulty logic. A huge chunk of reasoning behind parallel universes seems to be skipped and I have trouble taking it seriously every time it is brought up.
I think your scepticism is generally warranted. There is a reason the many-worlds interpretation is called an interpretation, just like the Copenhagen interpretation and others. The interpretations cannot be logically deduced from the experimental evidence we have. They are attempts to try to make sense of the results.
As for book recommendations, I have really liked Tim Maylin's "Philosophy of Physics: Quantum Theory" because of the precision and clarity he brings to the discussion. I should mention, though, that I've got a physics degree.
I often find videos easier to follow due to the great visuals.
You can also listen to interviews with the authors on podcasts. They talk about their books and the concepts in them.
And as always, your favorite LLM is a great companion to help you understand. I've consulted it several times and the answers and links it provided were very helpful
It’s very much a Copernican boundary: human beings desperately want to be at the center of things.
At one time this meant being at the center of the universe, today it’s more like wanting to be at the center of a unique identity with a deterministic past but an undetermined future.
This has led to countless interpretations of e.g. the Born Rule where humans have a unique ability to force wavefunction collapse with their senses.
Hugh Everett and those who subscribe to his post-Copernican view assert that we’re better off discarding the parochial attachment to a unique identify of experience than we are discarding theory validated by experiment beyond credible doubt.
One cannot truly avoid being human-centric, if the territory of reality is never available to one other than a map constructed by one’s consciousness. The core conceptual constructs used to reason about reality, the ways of attending to reality, the approaches to measuring it, this is all inherently coloured by humanness and further distinct cultural predispositions (a Western natural scientist is biased in one way, a !Kung shaman is biased in another). To assume the possibility of an unbiased take is unreasonable.
(There is never a complete map of the territory that covers the entirety of the territory in a way equally suitable for every possible use. Such a map would be the territory itself, which is not made directly available. So it’s good to have different maps, and to acknowledge that none of them can be assumed to be correct/complete.)
That said, assuming the hypothetical ability to cause what we refer to as “wavefunction collapse” via observation does not seem to strictly imply a human-centric view. Humans may be treated as a particular kind of conscious observer, but in a monistic idealist take assuming they are alone in that seems insufficient to describe reality.
Your argument isn’t with me, it’s with iconic physicists spanning the range from Sean Carrol to David Deutsch.
I’m well aware that any argument against vibes around an asymmetrically deterministic universe with a clear arrow of time and a unique dualist window of self is going to be unpopular.
There isn’t any physics there: how one chooses to interpret and integrate the vanishing impossibility of free will or unique identity is a very personal matter.
> Your argument isn’t with me, it’s with iconic physicists spanning the range from Sean Carrol to David Deutsch.
Of course, and (maybe more importantly) a number of philosophers. However, other philosophers (and, indeed, some iconic physicists) might not disagree.
> any argument against vibes around an asymmetrically deterministic universe with a clear arrow of time and a unique dualist window of self is going to be unpopular
Dualism is popular in general population, but monistic materialism is probably more popular among the tech crowd. Both approaches don’t strike me as elegant, naturally.
> how one chooses to interpret and integrate the vanishing impossibility of free will
Well, how one chooses to integrate the consciousness being the only thing that we can assume objectively exists (as the only thing we have direct access to, empirically) is also personal matter. The ways of waving it off (pretending it’s an illusion, etc.) are many… Once you stop doing that, though, suddenly free will is no longer such a crazy notion.
I’m not sure that a dichotomy between say illusory and real is the right category to examine consciousness with.
Consciousness is pretty difficult to define in any satisfactory way because we don’t have a way to know what another means by the word. I know how it feels to me, but not to you and vice versa. I’m aware that exceedingly clever people work on the problem and have for a very long time, and I hope they find some success, but I think it remains an open problem in most ways.
For my two cents I tend to think that pursuits like defining consciousness, or even higher level things like free will and even spirituality are very worthwhile, but likewise very difficult if not impossible to collaborate on with rigor.
I think I was a bit too flip in painting it as though consciousness and free will and spirituality are somehow inferior or less important topics than physics and cosmology and what not: put better I would say that they are very different pursuits and I think it perilous to try to integrate them under the guise of science. Physics can be done in collaboration with others, we have mediums for reaching some level of evolving consensus on that. Consciousness and free will are much less obviously amenable to any rigorous consensus.
Understood. I agree on difficulty with rigour, but then when you and I use “rigour” we mean it in a particular sense informed by the current philosophy of natural sciences. I suspect a possibility for rigour in other approaches, even if it looks not very rigorous to us in the framework of that philosophy.
> I think it perilous to try to integrate them under the guise of science
I may disagree on this somewhat, since physics can well be interpreted under monistic idealism where consciousness is a first-class citizen. The resistance may stem from it causing natural sciences (physics, etc.) to become subordinate to philosophy, which can be explained as an artefact of how science is done in our society (note how this anchors us again to humans and therefore consciousness): natural science allowed a lot of progress that benefitted our well-being (along with some consequences that potentially do not, cf. advanced weaponry), it gained certain status and many people don’t want it to lose that status for various reasons (maybe on occasion selfish ones). The resistance strikes me as misguided, natural sciences were in fact informed by philosophy, and philosophy may be evolving.
It’s like there was a healthy competition between different maps, natural sciences had won, but other maps are still useful (e.g., cases where modern medicine fails yet meditation/mindfulness do the trick are frequently mentioned even on HN), and competition would still be healthy. Perhaps I don’t need to be concerned and the conflict I want to remove via some universal philosophy is in fact evidence of competition keeping on and the plurality of maps. Maybe I in fact lament the lack of rigour that could benefit those other maps if modern science treated them seriously.
Physics and mathematics have yet to answer age-old questions about the meaning of life, and it’s understandable that this would be disappointing! Our most intelligent people, adequately if not always amply funded to probe the deepest mysteries of the universe have come back with: “there is no evidence that it means anything, or really that it even happened in the way you mean”. Who wouldn’t be disappointed?
But this is ultimately a failure of our priests and politicians and parents: it’s not the job of scientists to be our nursemaids in an indifferent cosmos. We have people for that role, at least on paper.
I only read "The Order of Time" by him but it was very mind-bending to me. Paraphrased below.
He argues that time arrow (mostly) does not exist in classical or quantum physics, all equations run as well forward in time as backward.
The only area different is thermodynamics, and there he argues it is all due to our feeble mental capabilities. If we could observe and track each molecule's movement, time arrow would disappear again, we could easily run the system backwards.
He concludes that time is just a consciousness' invention to help make sense of the observed chaos. A more powerful mind would have no use for time.
I think we're already at a point with LLMs (namely ChatGPT and Claude) where everyone has their own personalized Physics professor that you can ask any question you want. You can even say "explain it to me like I'm 5" and it will. You probably already knew this tho. :)
Yeah, that's a good point. I just mean if someone reads a book, and fails to fully understand an explanation of some topic, it's more likely than not that an LLM can help them understand it, and do as good a job as a human expert could.
My understanding of black holes is rather straightforward, gravity is so strong it prevents light and matter from escaping it's orbit. That does not mean that nothing is there or that all physics as we know it breaks down. Simply that our instruments can't observe what's there directly.
Not so simple. If there is no collapse into a singularity, what happens to the matter? Theoretically speaking, we have no clue to what happen to matter beyond neutron stars levels of degenerate pressure.
Specifically, what happens to fermions once all the quantum states are filled and they are still constrained by all the other fermions around them? We have no idea (and everything seems to point to a collapse into a singularity).
If, instead, there is an actual singularity (which has infinite density) it means that the curvature of space time is infinite, which our current theories can't cope with. Additionally, if singularities have infinite density, how is it that black holes can have different masses? A singularity can only be characterized by its position in spacetime since it has no size and so there is no space/surface for it to have any other property and yet we see that there are black holes with different masses. Another thing we can't explain with our theories.
So yeah, black holes mess with our theories in a fundamental way because as soon as you start pondering what happens inside them we discover that general relativity or quantum mechanics or both break down and so they must be incomplete. Spacetime, being a construct of general relativity is therefore also an incomplete description of the real fabric of space time.
Ok, but in my mind it's a lot like a mystery box, there's a lot of speculation about what's in the box, but without new instruments or new observation techniques, if the light/information from inside isn't reaching us we'll probably not be able to prove what's going on in the box one way or the other. One can claim "space-time breaks inside the mystery box" all you want, but I haven't heard of any testable theories.
Perhaps, but in terms of getting things done and making progress, it isn't very useful to suppose that it is simply a mystery box and that's that.
Because if we just accept that, what do we do then? We just sit and wait for some new astronomical observation to give us a clue? that could take forever, and we'd be banking that we have the tech to observe this magic hint.
Better to suppose that the theories we have comprise an accurate approximation or partial model of reality, and from there strive to find a better model.
The process of doing so will either refine/entrench our current model and our conviction, or it will result in actually finding a better model. Win-win, and all the while, we can still have our telescopes and detectors on for the magic hint we'd be sat waiting for anyway.
When things break down into singularities, it can be a pretty good indicator that we've got something wrong. Not necessarily, but in this case, I think we missed something.
Sure, I just think it might have a more "boring" answer than people are hoping for. E.g. behind the veil is something extremely high mass and energy, but it's more akin to a new class of Star rather than something where space, time, mass and energy lose all meaning.
I mean based on the well tested theories we have now regarding general relativity… it isn’t just gonna be a new class of star. Unless general relativity is flat wrong, which it isn’t. That isn’t to say general relativity is the end all of our understanding of the universe—black holes are a perfect demonstration of where our understanding breaks down completely. Clearly there is a lot more going on than we currently can explain.
For it to just be a super dense “new class of star” would first require you to explain why general relativity is completely wrong.
And that is the problem. Black holes are weird because they break the well tested equations we currently use to describe what we observe in the universe.
If you want my opinion, figuring this shit out (including what we are calling “dark matter”) is gonna unlock a whole new realm of cool stuff for humanity. I suspect there is a reason why we haven’t solved the Fermi paradox and it is because most “intelligent life”, as we imagine it, is living outside our current understanding of the universe. To get into the “cool aliens club”, our understanding of the universe will need to change.
Not true. In classical general relativity singularity have 0 size.
Any other theory to avoid 0 sized singularities (LQG, string theory) has not been accepted and that's actually my point: true hard singularities are probability due to our theories not being complete and likely something else happens. However, you can't claim you know singularities have plank volume because no one managed to quantize spacetime in a coherent theory that explains everything we observe.
Doesn’t spacetime emerge from dimension? In other words, first there was a dimensionless point. Something interacted with this and from that point arose dimension. From dimension, spacetime emerged.
Spacetime in physics has always been an imaginary mathematical construct. Spacetime is not something that exists in reality. Take the deflection of light equation used to compute the deflection of light in General Relativity:
alpha = (4GM)/c^2*r
This equation contains no terms for something called “spacetime”. This equation does not say that the light is bent by spacetime or that this observation is done in spacetime. But physicists write a fictional story over this equation and pretend that spacetime bends the light. The equation doesn’t say that, physicists do. And now physicists decided that spacetime does not really exist, it is emergent etc. Just a few years back people who said these things in forums would be dubbed crackpots by physicists.
That simple equation you quoted has been derived from the geodesic equations. Geodesics are the equivalent of straight lines on curved manifolds, which is a mathematical object. The shape and geometry of the manifold in question, which in turn influences the shape of the geodesics, is given by the matter distribution, or more specifically the energy stress tensor. The relationship between the manifold and the energy stress tensor is described by the Einstein field equations.
Couple minor additions: geodesics are physical in that (i) they are the paths taken by objects in free-fall and (ii) light (massless waves, etc etc) in free space is always in free-fall: all allowable null curves are null geodesics. Massive objects and light may be accelerated (the direction part of the velocity vector changes with scattering even if the magnitude part does not) and during acceleration will not be bound to a geodesic because they aren't in free-fall.
Even though I agree with the way you put it, I'll say it's probably a bit too strong to write that stress-energy is the (sole) source of curvature given the extremely curved spacetime of our expanding universe. However, Einstein was OK with Schrödinger taking that view <https://arxiv.org/abs/1211.6338> (a super-interesting bit of science history), and we don't know enough about the cosmological constant vs dark energy, nor cosmic inflation.
This couldn't be further from the truth. Even just in special relativity, if you want to compute the age of an object on a very fast ship, you need to use the Lorrentz transforms, which categorically use a metric for the space-time. It's at the base of everything else in relativity.
A geometrical space in which closeness is defined and measurable by a numerical distance -- in this case r -- is a Riemannian manifold. "c" is a dimensional physical constant with units LT^-1 or length per time <https://en.wikipedia.org/wiki/Dimensional_analysis#Simple_ca...>. "Time" enters into the manifold making it semi-Riemannian or pseudo-Riemannian, and however you solve your equation, it can be represented in a pseudo-Riemannian manifold. If there are 3 spatial and 1 time dimension, the manifold is Lorentzian <https://en.wikipedia.org/wiki/Pseudo-Riemannian_manifold#Lor...>. Lorentzian manifolds are the spacetimes of relativistic physical theories.
Newton's big G incorporates length and duration; "4M" is the only term in your equation which includes neither.
So it seems less like you have a problem with spacetime (having shown you're OK with length and duration), than that gravitation is identified with spacetime curvature, and that this is captured by the metric tensor which encodes how lengths, durations, and angles change with tiny displacements from a point on the manifold.
> physicists decided that spacetime ... is emergent
Nobody's decided that, but some physicists are chasing that as a hypothesis. The idea is that there is some underlying theory that recovers equations like the one you quote in some low-energy limit of a theory that is not set in a smooth pseudo-Riemannian manifold.
See, "every smooth manifold admits a Riemannian metric" <https://en.wikipedia.org/wiki/Riemannian_manifold#Every_smoo...>. Smoothness is really a question of "no sharp corners", and tends to be expressed in terms like "everywhere differentiable". One can imagine a spacetime that is made up of spacetime-tetrahedra (you and three friends run together towards a point) and see that the triangular features can be non-smooth when you look close but become smooth as you put lots and lots of tetrahedra together and look at them in bulk. (Loop quantum gravity considers something like this).
However, practically all the differential equations we use in practice rely upon smoothness, and modern ones (since the 20th century introduced tensors and general covariance) would break if the universe around us were not very very very very similar to a Lorentzian manifold.
There are alternatives to curvature of the Lorentzian manifold (i.e., they keep special relativity and its flat spacetime) for encoding the gravitational interaction, with various trade-offs and exposure to narrowing limits obtained from astronomical observation. Gauge theory gravity (GTG) is an example of one that is in good shape.
> people who said these things in forums would be dubbed crackpots
I'd reserve crackpot for someone who pesters scientific journals and working scientists (yeesh, some will call you at home) with their non-standard and often quite verbose ideas even after they are shown conclusively to be wrong in some way. Especially if they think the non-standard idea will revolutionise the field. Pursuing a mathematically-expressable theory from which General Relativity (or something very close to it) emerges in some limit (and Newton/Poisson/Gauss gravity in the low-speed low-escape-velocity local limit of that) isn't cranky. Insisting that the new theory is world-changingly revolutionary probably is cranky, but at least they will have shown they understand existing theory and how theirs differs in detail.
What word should be used for someone who denounces an area of research while obviously not understanding the research denounced?
Ignorant? Ignorance is curable. Every working scientist was once an ignorant kid. They read a lot of textook material, got plenty of help from people with more experience, became used to being shown how they misunderstood something or other, and learned how they could rigorously demonstrate how someone else -- including authors and editors of textbooks -- got something wrong. The result: a firehose of papers on arxiv and elsewhere proposing and exploring in detail something that is not the standard model of gravitation (or particle physics, or fluid mechanics, or whatever).
Loaded in Chrome. Various effects were annoying. Right click - load in new tab didn't work for linked articles. And no changed link colors to show which articles I'd read before. Fancy graphics break web conventions.
A) I like cute scroll effects, but maybe we should consider making them additive rather than completely ephemeral (when screen size permits)? The intro is beautiful on mobile, I'm sure, but it's just kind of irritating on desktop.
B) Thanks so much for posting, I had skimmed a few of these articles but didn't realize it was part of a cohesive series. For others, since it's not really obvious at first: the whole series is built to setup the three articles at the bottom, where Charlie Wood examines three competing research groups. Quanta is on fire these days... This is an amazing evolution upon 2020's What is a Particle?, which is an article I've been bringing up in like every other conversation b/c it's so fascinating and accessible.
C) Other than the initial complaints about the intro, the article itself is downright groundbreaking webdev-wise. And I called it when I started this comment -- the rest is additive!
The way the text deforms around the sliding divs (but quick enough that it probably won't interrupt you since you haven't gotten to it yet) is just beautiful, I don't think I've seen that before. The dynamic, freeform cutouts fit perfectly with the theme, and the background is bold (how many sites are purple??) but indescribably perfect. Of course, the star of the show is the animated "thought experiment" page, even if the transitions are atomic (can't be reversed halfway through by scrolling back up). And the final summary + animation is worth scrolling to the bottom for, even if you don't read the rest of it.
Well done Quanta web team, you're seriously raising the bar with this article, IMHO. Inspiring stuff. Is there any magazine ~~article~~ "series" (issue?) that even approaches the beauty and cohesiveness of this one? I'd love to proven wrong by the experts on here ;)
[ETA: wow everyone hates the UX, I'm shocked. Why, in particular...? At the end of the day the linked page is just a list of links that appears right near the top, I don't understand the ubiquitous hate! What am I missing?]
D) I'm pretty far from critically evaluating the physics surveys themselves, but I certainly found them helpful. Some quotes that are just insanely insightful, the kind of simple, boring statements that edge on superstition through their pure profundity alone:
In periods when we are looking for new theories, physics has always become philosophical.
Our natural perspective as beings with locations separated by space sticks out as a mathematical oddity. “It’s a reminder that the laws of physics that we perceive in our world don’t seem to be random,” said Sean Carroll (opens a new tab), a physicist at Johns Hopkins University. “They seem to be specific.”
“AdS/CFT is an insane suggestion that should be stupid,” said Geoff Penington, a physicist at the University of California, Berkeley who studies holography. “But then you try all these things, and it all ends up being consistent.”
For his part, he feels that holography isn’t radical enough. It shows how one dimension of space could emerge, but otherwise all the familiar ingredients of quantum theory are there from the start: some space, locality, and a clock to mark time. Arkani-Hamed feels that all of those elements should emerge together from something more primitive — as they do in surfaceology.
It might seem like I’m interested in lots of different things, but I’m not sure if they’re really all that disconnected. At least within math, it is kind of true that sufficiently beautiful things tend to be connected with one another.
E) Any friendly experts around to explain what a "lower dimension" representation of a black hole would look like? What's a periodically-repeating 2D manifold, given that circles (flattened spheres) don't pack? I guess it warps to pack nicely? This is a core example they use to explain the broader thesis--which I thought I understood--but I don't really understand in the topological shape of what they're describing, so it ended up confusing me more. In particular I'm commenting on this:
So the key to black holes’ underlying structure exists on their surface. “People began to think,” Law said, “that maybe whatever microscopic theory describes black holes lives in a space-time with one lower dimension.”
F) Even more fundamentally, can someone explain why we can't just assume that there isn't spacetime inside black holes? If everything's pointing towards their contents being entirely contained within their infinitely-thin surface, why can't we just embrace that? In other words, that whatever determinations make up the universe are undetermined in those places? That the diameter of every black hole is 0, when considered from the inside? I'm assuming the answer is "complicated math says no", but I'm failing to confirm that.
Sorry for the spam, won't mind if this is deleted -- it's just terribly helpful to write as I go. If you're scrolling by, I highly recommend bookmarking this series in full! And if you work at Quanta and are reading this: you're awesome.
> F) Even more fundamentally, can someone explain why we can't just assume that there isn't spacetime inside black holes? If everything's pointing towards their contents being entirely contained within their infinitely-thin surface, why can't we just embrace that?
It's not that simple. The surface at the event horizon is not a physical thing, it's a coordinate singularity which is only apparent for a very distant observer. If you imagine two spaceships flying towards the center of a black hole within close distance, they would notice nothing unusual when crossing the event horizon. They would be in constant radio contact. It's mind bending stuff, really.
The idea of making the black hole membrane paradigm generally covariant and everywhere locally Lorentz invariant gives me a migraine.
Dropping charges onto a superthin surface (and keeping it cold, we have to consider arbitrarily large SMBHs too) or having the superthin surface emit Hawking radiation is hard to make not-a-toy (step 0: spectral structure evolution; step 1: ultraboost something, Aichelburg-Sexl style pancake; step 2: astronomer near ISCO around a distant SMBH; step 3: Rindler particles from the membrane's pole; step 4: incoming ultra-high-energy cosmic rays, GZK-like cutoff induced above membrane; ...; step N how the hell do they evolve during the PN phase of a BHB merger, and how do you match that up with the numerical relativity?).
(This line of thought has taken me somewhere that seems crazy: following Giddings & Rychkov <https://arxiv.org/abs/hep-th/0409131v1> and Yoshino & Mann (RN metric and during the LHC-will-destroy-the-world silliness) <https://arxiv.org/abs/gr-qc/0605131v3> can UEHCRs on a hyperbolic orbit peel off small black holes from the electromagnetic field around a black hole membrane? If a standard quiescent BH is massive enough there won't be much Hawking anything to collide with, so we won't see evaporation signals; but isn't there a nontrivial electromagnetic field strength close to even a big horizon in the membrane paradigm or anything reasonably like it? [note to self, in case I care later: Parikh & Wilczek (1997) "An Action for Black Hole Membranes" <https://arxiv.org/abs/gr-qc/9712077> just after eqn. (3.8)])
> F) Even more fundamentally, can someone explain why we can't just assume that there isn't spacetime inside black holes? If everything's pointing towards their contents being entirely contained within their infinitely-thin surface, why can't we just embrace that?
> F) ... why we can't just assume that there isn't spacetime inside black holes
Spacetime isn't a substance.
I guess what you are asking is, can our universe be non-simply-connected 3+1-dimensional manifold (see 2-dimensional version at first diagram @ <https://en.wikipedia.org/wiki/Simply_connected_space>, imagining the additional 1+1 dimensions is an exercise for you) with what we think of as black hole horizons being a region very very very close to a "hole" in the manifold.
The answer is probably not, and depends on the Topological Censorship Hypothesis <https://en.wikipedia.org/wiki/Topological_censorship> being correct. If the universe works the way we think it does pretty much everywhere far enough away from black holes, then a hole-like defect can't last for the billions or trillions of years that an astrophysical black hole will (we have good evidence that some galaxy-centre black holes have been around at least hundreds of millions of years, and it is safe to assume we'll be able to expand that out to billions). Hole defects also can't grow like black holes do by ingesting partner ordinary gaseous stars, or by colliding with one another and merging, or rather the electromagnetic signals would be radically different.
This makes a couple of assumptions about the massless wave function for light. Things get tricky if we introduce an invariant mass to light (i.e., electromagnetic radiation has a sub-infinite range it can travel) or otherwise break the equality of c and the speed of light in free space. Maybe one can contrive a longer lifetime for regions that break simple-connectedness. However, you still have to pile things up around the hole, and there's no reason to expect gravitation of the pile of things will help you there.
Squeezing matter up against the "edge" of a topological hole will also brighten the matter, and you really want it to dim and fade away for observers around here, since that's what we see from astronomy in the aftermath of tidal disruption events, binary merger ringdowns, and infalls of gas clouds. In standard theory black holes get this dimming by directing some near-horizon radiant energy inwards. If there's no inwards, how do you make the heat glow become invisible? As well as the dimming, visible spectral lines shift into the infrared, and the apparent angles and areas of an infalling object should change.
> In other words, that whatever determinations make up the universe are undetermined in those places?
> E) Any friendly experts around to explain what a "lower dimension" representation of a black hole would look like?
It would be a 2-spatial-dimensional oscillating (that's where the time dimension comes in) membrane around the black hole. Where? Super close to the horizon or super far from it? The answer varies, but the former is more popular. A better question, and I do not know a proper canonical answer in this particular case you've asked about, is: is the membrane or its oscillations tangible, or more precisely, how does one characterize (microscopically) the interaction with light and matter incident upon an oscillation?
Also not very clear in the article is the popularity of adding dimensions beyond the 3+1 spacetime and hiding them where gravitation isn't strong. For example, ADS/CFT comes up in the article but that's an equivalence between AdS_5 × S_5 and N = 4 supersymmetric Yang-Mills theory, or ADS5/CFT4. The value of N is essentially the number of (independent) operations that transform the spins of bosons and fermions; the subscripted 5 on AdS means an embedding of a higher-dimensional contracting, bounded space into 5 spacetime dimensions, and the subscripted 5 on S means there are five extra compact spacelike dimensions as well. There are other (less proven? I don't really pay attention to that area of string theory) correspondences with different numbers of extra dimensions and symmetries compared to our universe. In any of these a black hole (or something very like it) has some complete description on a membrane somewhere around it, and there's still some metaphysics about whether the CFT is outright sourced by the bulk or if ADS & CFT are just equivalent representations of the same bulk system.
"Holography" is a rough analogy - a 2-d film on a credit card looks like a 3-d image of e.g. a bird; a 4-dimensional conformal field's configuration can represent a 5-dimensional object (including a black hole), provided there are extra symmetries in the field theory and extra spatial dimensions available around the object into which one can shove some excess energy.
In these approaches the boundary of the collapsing spacetime and the membrane on which the CFT lives are very far away from everything (our universe doesn't appear to have a boundary or a membrane full of high-energy particles surrounding everything in the sky, after all), the anti-expansion is very slow (as the metric expansion of our universe is pretty obvious with all those redshifted supernovae, quasars, hot clouds of hydrogen, etc), the extra dimensions are made invisible by making them small and/or curled up, and so on.
Various flavours of the holographic principle have their adherents who believe their preferred form of holography say something important about gravitation in our universe, and might explain various features of collapsed stars and supermassive black holes.
A contrasting approach is to assume the Standard Model and 3+1 dimensional General Relativity and to use numerical simulations of gravitational collapses and black hole evaporations and so forth.
There is insufficient evidence from astronomy to really hint at which approach is better, although there are now limits from astronomy on the numbers and natures of possible extra spatial dimensions (they tend to leave imprints on the spectra of distant light sources, for example) and limits from particle physics on the minimum energy levels of extra symmetries beyond the Standard Model.
This looks to be some fashly extra content to go along with the recent and wonderful John Wheeler retrospective. [1] But truth be told I couldn’t get through more than a few links before I got frustrated with the design.
Wow, hot takes. All of this is because the intro quotes appear and dissapear? Otherwise it doesn't actually hijack your scrolls other than to slide in some incidental images, and this is just the index, anyway -- all the articles are plain print articles with no animations at all.
In my case there was a floating nav bar bouncing around at seemingly random vertical heights over top of the document as I scrolled, which I found very distracting. Additionally, the buttons that are meant to link to the subtopics did not work, the vertical line height and scroll-jacking necessitated excessive scrolling, and and no scroll bar was visible. I've since figured out that FF mobile's reader mode makes it workable.
"Please don't complain about tangential annoyances—e.g. article or website formats, name collisions, or back-button breakage. They're too common to be interesting."
This is Quanta Magazine, which is printing high quality science stories that no one else approaches. If they went under those stories, like this one, would simply be lost.
How can Quanta Magazine do that, when no other magazine can? It's because they're bankrolled by Jim Simons, the MIT-trained mathematician who co-founded Renaissance technologies and was called the "greatest hedge fund manager of all time" (he died this year). It's basically his gift to the world. We are lucky to have it.
I love quanta magazine!! Always exciting to see quanta articles pop up on hacker news. They’re usually technical enough to be fascinating, but also accessible (and well-explained) enough that I can send them to my dad lol.
I had no idea about Jim Simons. What a legend.
I recently was looking to see if they had a physical subscription, and couldn’t find one. I actually didn’t even see anywhere to donate on the site!
Agreed. Quanta has great content, but man alive, this particular "article" (?) is completely unbearable. I wish they'd just release these special issues as actual print magazines. I'd love to have them in the flesh, but Quanta very rarely publishes their stuff on paper.
I know it is not going to happen, but ideally the browser would be more configurable than they actually are so that you could configure yours for high visual stimulation whereas I could configure mine to make pages standardized and plain for rapid "navigation" (scrolling, searching in the page, copying): i.e., ideally the site would publish just the words and the browser would be responsible for almost all of the UI.
If there are experiments that can be tried to challenge my own current perspective on the matter, I’m interested.
Because to my mind, anything that is said to emerge implies that the dimension we usually refer to as time is already an implicit hypothesis of this anything being able to emerge.
And if there is something out of time, then by its very definition it can’t interact with time:
- either there’s a connection with time and then at some point there is a change to that thing that can no longer be considered an absolute static thing as there is a before and after event that this thing is attached to,
- or this thing is without any relation to time and thus can not be something that engender time itself.
To be clear, I think there is still large room for interpretation anywhere formal mathematical means can give us great insights that would be unreachable without them. Agreeing with equations is not agreeing on the interpretation to give to the equations at a larger epistemological picture, and even less on ontological genesis.
The bits of a movie on a DVD depict regions of space evolving over time. The DVD and its bits exist in space and time but they are static and the spatial arrangement of the bits is pretty much unrelated to the space depicted by the movie. If you had sufficiently good eyes and mental DVD decoding capabilities, you could look at the DVD and get a God's eye view of the world, see all of space and time at once, at least the parts captured in the movie.
Similarly in a computer game, to run into a wall does not require the player and the wall to be next to each other in any real sense, it is sufficient that some code knows which memory locations to inspect to detect a collision and constrain the motion accordingly. Similarly running into a wall in the real world does not require that you are next to the wall in some fundamental sense, the universe could at a very deep level just be a list of object coordinates and the laws of nature would cause a force whenever two coordinates become very close.
I will agree with you that it is much harder to think of time in a similar way but I am willing to consider this potentially a limitation of human brains. From Einstein we know about the close relationship between space and time and I can relatively easily make sense of the idea that spatial relationships are not fundamental, so maybe that just carries over to time, even though my brain can not make sense of it.
This is one take on what Bohm called the implicate order, and which is implied in non-locality and superdeterminism.
Reality is a projection of a hidden system which operates under completely different rules.
And when we get down to the quantum level, we start seeing that essential information is somewhere else.
You can't look at a single electron and see that it's entangled. So where does the entanglement information live? It's clearly not inside the electron. Is a measurement really an interaction with the projection mechanism?
This sounds like simulation theory, but it doesn't have to be. There might be a super-causal system of relationships and meta-objects without needing a "computer" made by a conscious entity to keep track of their relationships.
You seem to be talking about the "hidden variables" interpretation. As far as I understand, that theory has been disproven experimentally.
The core idea is that entangled particles 'know' their final state when they are created. That information is hidden from observers until measured, but the result is not truly random in the way that QM implies. Experiments have proven that this cannot be true, but I don't understand it well enough to descibe here.
> Because to my mind, anything that is said to emerge implies that the dimension we usually refer to as time is already an implicit hypothesis of this anything being able to emerge
To the mind of a physics theorist, emergent means that one theory can be mathematically derived from another theory.
Often this is a one-way street where one of two theories being compared is more fundamental in the sense that it cannot be derived from the other, but the other can be derived from it. The other theory is the emergent theory.
There are a variety of ways of communicating this idea, none of which are especially satisfactory.
For example, the Charlie Wood article in the link at the top says, "Sean Carroll proposes the following working definition: A system is emergent when you describe it with two theories, one of which is more complete than the other."
There are a variety of attempts to make this more rigorous in philosophy of science literature - if you're interested in how to nail down "emergent theory" and can't find anything on your own, I'll dig up some references.
None of this has anything to do with a definition of time. There are a variety of pairs of time-independent physical theories where one can be derived from the other, satisfying my definition above (and usually Carroll's too). In general it is perfectly reasonable to use different formalisms to describe the same unchanging system, and sometimes one or more of the formalisms can be derived (i.e., can emerge) from another, or at least (using Carroll's definition) one of the descriptions is less general than the others (in the sense that a small change of the system can be captured by the more complete theory, but not by the other less-complete one).
This is one thing to say that a system can encompasse/handle all the analysis that an other system can represent, and not the over way. It feels like here there is something very different with a claim like "time is better understood as an emergent property and if you disagree your logic is broken" as can be found in this article.
Maybe there it's about some isomorphism where time is in a bijective relationship with something else?
I don't really understand what you're trying to say.
I think there's two things going on in your comment: you want more information about emergent theories, and you want more information about how time emerges in a theory that is symmetrical under time reversal.
For the second: your paraphrasing, particularly "if you disagree your logic is broken" is in my view unfair to the quoted scientists and the author & editor. However here's a handful of paragraphs on how time can be understood as an emergent property.
tl;dr of below: the behaviour of matter in the bulk breaks the time-reversal symmetry of fundamental physical equations and provides a "clock" that solutions of these equations do not. Since a direction of time can't be obtained by these fundamental equations, but can be obtained by non-fundamental bulk behaviour theories (like statistical mechanics, which captures the collective behaviour of many items described by fundamental theories, with that collective behaviour being qualitatively different than individual behaviour) time can be said to be emergent.
I'm hoping that your "logic is broken" paraphrase is just that you don't understand that any monotonic function on a path through spacetime can serve as a clock, and while there is no a priori correct path, there is always at least one extreme path through spacetime from which one can take a global notion of time. For example, in the FLRW universe, the comoving coordinate time is suitable. However, this time is not the wristwatch time of an observer who sometimes accelerates to a significant fraction of c, and sometimes decelerates to a tiny fraction of c, and the function transforming the two times (comoving time and this particular proper time) is complicated.
Connes & Rovelli in the early 1990s originated the idea of using Boltzmann entropy as a "universal" clock comparable to comoving time: in both cases the behaviour of bulk matter allows for a set of coordinates that most observers can relate to their proper time. Entropy is more general since it's not restricted to an expanding and at large scales isotropic and homogeneous universe; it just requires thermodynamic matter. The details are in papers which are probably too technical for most -- a copy of their 1994 paper in Classical and Quantum Gravity can be found at <https://arxiv.org/abs/gr-qc/9406019>.
One could summarize: galaxy clusters spread apart with high-quality vacuum growing in between them; in galaxy clusters, "metal-poor" gas clouds collapse into stars, chemical enrichment occurs through stellar evolution, successor clouds collapse into stars with more "metal", and so on; black holes form, merge, and grow. All of this can be time-oriented such that the future having more entropy.
There are other "decays" of matter that can serve to break microscopic time-reversal symmetry which have been proposed and even to some extent demonstrated. Practically all of them rely on a sizable collection of in-principle reversible microscopic changes that in practice -- and in large numbers -- will not reverse. Consequently you can examine such a clock and tell if it is ticking forwards or backwards. Again, that's what's meant by time as an emergent property.
Consider an unchanging singly-linked list as a universe. Time (or perhaps "the passage of time") in the intuitive sense does not exist for this universe, but it does exist in the sense that there is a global ordering of values. But does the ordering of the values come from the edges or the nodes? The ordering requires both.
Or, more humanly(?), a film reel.
At least, this is how I would think of it if required to give a simple-but-possibly-off-base answer for a possible way time could be emergent (would love it if someone more educated could correct me). This is also known as the Einstein block universe model.
Otherwise, since my physics education is far below the actual bar needed to understand quantum gravity stuff, I think back to thermodynamic microstates and macrostates, and how temperature is emergent.
There are no such things as absolute static things in relativity theory. "Before" and "After" depend entirely on the observer. Energy/matter and spacetime are deeply connected through the Einstein equations, so much so that they may indeed be "the same underlying thing".
Approximate static time and space are convenient illusions/approximations, that happen to be very useful for us as a species when it comes to surviving and replicating, but the Universe has no obligation to cater to our feeble minds or senses when it comes to reality.
> "Before" and "After" depend entirely on the observer.
This is only true for space-like separated events. But the earth gets wet After the rain started, for any observer in any frame of reference whatsoever. The eggs have to break Before you can make the omelette.
Lorentzian manifolds come with a definition of causality. The spacetime of Special Relativity is a Lorentzian manifold. An additional condition of time-orientability obliterates the free choice of an observer on a "comes-before"/"comes-after" relation between observable events.
A further condition of global hyperbolicity also determines the "comes-before"/"comes-after" relation between unobservable events. This condition can be fixed by (i) a non-Minkowski metric, or (ii) by constraints on the pattern of events in the gravitation-free metric of Special Relativity (an example of such constraints is thermodynamics). Sloganizing this: "states of matter tells you what configuration came before/came after" in the second case, and also in the first case if the non-Minkowski metric's source is only matter; otherwise you need to do a causality analysis, e.g. by fixing causal cones on curves (paths, trajectories - they don't have to be geodesics) of interest or solving the relevant wave equations).
As a practical matter, the initial value formulation of General Relativity <https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...> (and numerical relativity built on that) is popular and of practical use because so far there is no reason to describe a natural system (where gravity isn't just ignored) in a way that breaks global hyperbolicity.
>There are no such things as absolute static things in relativity theory.
Well there is light-speed, which is an "absolute universal constant" - though of course light-speed is directly related to time. Also general relativity as a bit more to say on the topic.
>Approximate static time and space are convenient illusions/approximations, that happen to be very useful for us as a species when it comes to surviving and replicating, but the Universe has no obligation to cater to our feeble minds or senses when it comes to reality.
Sure. Even theories that don’t rely on time to be a fundamental dimension fall in the same category of "universe doesn’t need our small minds to be able to grab its actual complexity".
I feel like someone could describe foam emerging from the sea without implying that it is a process in time. It could be thought of as a process relating to the properties of seawater, wave agitation, sand etc. It is possible for a picture, not a video, to depict foam emerging from the sea. Also, there need not be a before or after, or even quantifiable extrinsic changes.
Emergence has nothing to so with time. Emergence means that a lower-level description logically entails a certain higher-level description in some nontrivial way. Both exist at the same time, there is no chronological sequence. It’s a process of logical implication, not a process in time.
"The act of" is literally the first words of the first definition. There nothing that evokes better the idea of time than an act(ion), doesn’t it?
>Emergence means that a lower-level description logically entails a certain higher-level description in some nontrivial way.
The description doesn’t occurs out of time, it’s conceptualization doesn’t happen out of time, the proposal that something could happen out of time is happening within time.
>Both exist at the same time, there is no chronological sequence.
Just because one abstract away chronological sequences doesn’t mean that time doesn’t actually exist as a fundamental dimension.
>It’s a process of logical implication, not a process in time.
That’s not what documenting on history of logic taught me. I warmly recommend everyone to document on the topic. It’s amazing how different the logical firm fundamental beliefs can diverge over time (and even space actually).
I read a bit of The Fabric of Reality but had trouble progressing too far with that one. The author talks about the quantum slit experiment, somehow arrives at the explanation of parallel universes, and then claims anyone who disagrees with this conclusion must have faulty logic. A huge chunk of reasoning behind parallel universes seems to be skipped and I have trouble taking it seriously every time it is brought up.