[HN Gopher] The Unraveling of Space-Time
___________________________________________________________________
The Unraveling of Space-Time
Author : nsoonhui
Score : 87 points
Date : 2024-09-30 12:41 UTC (4 days ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| ackbar03 wrote:
| 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.
| danielmarkbruce wrote:
| Take the content, paste it into chatgpt and ask it to explain
| it with simpler language, more details and examples.
| Tagbert wrote:
| You might check out the PBS Spacetime series on YouTube.
| https://www.youtube.com/@pbsspacetime
| naught0 wrote:
| I love watching this channel even though it sounds like word
| salad to me
| groestl wrote:
| Judging by the comments, that seems to be a common
| sentiment among viewers :)
| Cthulhu_ wrote:
| 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.
| MaxikCZ wrote:
| 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.
| groestl wrote:
| 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).
| SoftTalker wrote:
| It seems to be well produced but the presenter rubs me the
| wrong way for some reason.
| benreesman wrote:
| 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.
| strogonoff wrote:
| 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.
| benreesman wrote:
| 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.
| strogonoff wrote:
| > 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.
| benreesman wrote:
| 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.
| rini17 wrote:
| Humans also desperately want to find something that
| transcends them. But so far failed to find matching QM
| interpretation with new explanatory power.
| benreesman wrote:
| 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.
| rini17 wrote:
| "We have people for that role" is exactly the copernical
| boundary, that he transgressed :)
| jgoldfar0nil wrote:
| 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.)
| bbor wrote:
| 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.
| verisimi wrote:
| 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.
| netfortius wrote:
| I recommend Carlo Rovelli's books.
| quantadev wrote:
| 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. :)
| NateEag wrote:
| And the explanation may or may not be total nonsense.
|
| Good luck figuring that out on subjects you don't understand.
| quantadev wrote:
| 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.
| emrah wrote:
| There are great video series on YouTube, for example:
|
| https://m.youtube.com/@pbsspacetime
|
| https://m.youtube.com/@HistoryoftheUniverse
|
| 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
| mazsa wrote:
| https://www.amazon.com/Physics-Symmetry-Undergraduate-Lectur...
| sandgiant wrote:
| 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]
|
| [1] https://www.preposterousuniverse.com/somethingdeeplyhidden/
| [2] https://www.preposterousuniverse.com/podcast/
| codethief wrote:
| > 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.
| bbor wrote:
| 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.
| jiggawatts wrote:
| _> 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?_
|
| You're not crazy, that's my pet theory too!
| mr_mitm wrote:
| > 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.
| raattgift wrote:
| 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)])
| raattgift wrote:
| > 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 x 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.
| raattgift wrote:
| > 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?
|
| I can't guess what you mean by that.
| dang wrote:
| [stub for offtopicness]
| leetrout wrote:
| I really am over the stupid scroll hijacking sites.
| king_magic wrote:
| Seriously. This page looks and interacts like shit on my
| iPhone. Gave up and went to reader mode. Still sucks.
| bmitc wrote:
| 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.
| jazzypants wrote:
| It's just gonna get worse now that it's easy to implement
| with CSS.
|
| https://developer.mozilla.org/en-
| US/docs/Web/CSS/CSS_scroll-...
| leptons wrote:
| Firefox and Safari don't support it.
|
| https://caniuse.com/mdn-css_properties_animation-
| timeline_sc...
| jazzypants wrote:
| That link mentions that Firefox already supports it if
| you change a setting, and Apple is literally currently
| implementing it.
|
| https://lists.webkit.org/pipermail/webkit-
| dev/2020-June/0312...
|
| https://github.com/WebKit/WebKit/pulls?q=Scroll+animation
| s
| MathMonkeyMan wrote:
| Fucked up my phone jeez.
| julianeon wrote:
| This "stupid site" is a national treasure.
|
| 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.
| chamomeal wrote:
| 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!
| patcon wrote:
| Thanks for this context!
| tonfry wrote:
| After waiting for a few seconds I gave up and came here to
| determine if it was just me... nope. The site just sucks on
| mobile.
| dang wrote:
| Personally I agree; however:
|
| " _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._"
|
| https://news.ycombinator.com/newsguidelines.html
| smolder wrote:
| I am not sure what this article is about. I assumed it would be
| redundant but didn't get to find out because of the weird
| intolerable formatting.
| zzo38computer wrote:
| It does not display at all for me. If they have a plain text
| file available then I might try to read it.
| wizardforhire wrote:
| 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.
|
| [1] https://www.quantamagazine.org/john-wheeler-saw-the-tear-
| in-...
| RajT88 wrote:
| I come here for the plaintext.
|
| Scrolling through that site is like geek kryptonite. Terrible.
| I will monitor this comment thread for other articles which
| cover the same content.
| bbor wrote:
| 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.
| RajT88 wrote:
| I had just whitespace on mobile for pages and pages before
| text started appearing.
|
| This site deserves my wrath, and the wrath of others. You
| have a browser which works with it obviously.
| rezmason wrote:
| Sorry, folks.
|
| I think Quanta discovered people like me rarely get past the
| third paragraph, and they made this to keep my attention. And
| it's working.
|
| --By "working", I mean I can experience it in Chrome and I feel
| like I'm retaining information. In Safari 17.5, the images
| don't load.
| hollerith wrote:
| No need to apologize. You did nothing wrong.
|
| 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.
| SvenL wrote:
| Quantamagazine is pretty amazing. The content, the
| visualizations, no ads, it's loading pretty quick - I think this
| is how the web should be
| nyc111 wrote:
| 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.
| mr_mitm wrote:
| It's a bit more involved than that.
|
| 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.
|
| We gave that manifold the name "spacetime".
| raattgift wrote:
| 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 Schrodinger 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.
| tsimionescu wrote:
| 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.
| raattgift wrote:
| > alpha = (4GM)/c^2*r
|
| r has units of length.
|
| 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/w
| iki/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).
| ninacomputer wrote:
| Just in passing, Quanta magazine has been excellent lately!
| dist-epoch wrote:
| It was also excellent in the past!
|
| It's excellent in general
| nuancebydefault wrote:
| It was observed as excellent in your universe, in our
| universe...
| soup10 wrote:
| 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.
| marcyb5st wrote:
| 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.
| soup10 wrote:
| 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.
| wanda wrote:
| 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.
| soup10 wrote:
| 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.
| cruffle_duffle wrote:
| 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.
| tiborsaas wrote:
| If you deepen your understanding it suddenly stops being rather
| straightforward.
|
| Here's a good playlist to warp your head around the subject.
|
| https://youtube.com/playlist?list=PLsPUh22kYmNBl4h0i4mI5zDfl...
| psychoslave wrote:
| 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.
| winwang wrote:
| 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.
| guy234 wrote:
| 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.
| danbruc wrote:
| 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.
| TheOtherHobbes wrote:
| 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.
| mystified5016 wrote:
| 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.
| sandgiant wrote:
| 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.
| tsimionescu wrote:
| > "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.
| raattgift wrote:
| 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).
|
| https://en.wikipedia.org/wiki/Causal_structure
|
| https://en.wikipedia.org/wiki/Causality_conditions
|
| As a practical matter, the initial value formulation of
| General Relativity <https://en.wikipedia.org/wiki/Initial_val
| ue_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.
| psychoslave wrote:
| >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".
| dist-epoch wrote:
| You can have time without having time - block time
|
| https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time...
| raattgift wrote:
| > 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).
| layer8 wrote:
| 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.
| psychoslave wrote:
| Look up some definitions for emergence:
| https://en.wiktionary.org/wiki/emergence
|
| "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).
| IAmGraydon wrote:
| 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.
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