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Space Science

Will We Ever Understand Black Holes? (theguardian.com) 92

Posted by msmash from the closer-look dept.
So, what would it feel like to fall into a black hole? From a report: "Well, at the moment you crossed the horizon, you wouldn't feel anything -- there would be nothing dramatic," Peter Galison, co-founder of the Black Hole Initiative at Harvard University, says over the phone. Huh. Doesn't sound too bad. "But inevitably, you would be pulled towards the centre," he continues. "There's no going back; everything that falls into a black hole just keeps falling; there's no resisting that pull and things don't end well."

Ah. Go on. "Physicists have an expression called 'spaghettification' because if you were falling in feet first, your feet would be more attracted towards the centre than your head, and your sides would be pushed towards your middle and this process would extend and compress you." Right. So, terrifying, then. Especially when Galison adds with cosmic understatement: "In the long term that's not a good survival event." We are talking about his documentary film, Black Holes: The Edge of All We Know, four years in the making and available on Netflix from 1 June, which follows two scientific collaborations to understand the most mysterious objects in the universe. Among the highlights is being a fly on the wall as the late Stephen Hawking tries to figure them out.

It is Hawking's voice, that instantly recognisable computer speech synthesiser, that opens the film: "A black hole is stranger than anything dreamed up by science fiction writers. It's a region of space where gravity is so strong that nothing can escape. Once you are over the edge, there's no way back." City-sized black holes form when certain stars run out of fuel to burn and collapse under the force of their own gravity. Supermassive black holes -- millions or billions of times bigger than our sun -- are found at the centre of almost every galaxy including our own, the Milky Way.
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Will We Ever Understand Black Holes? More

Will We Ever Understand Black Holes?

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  • Ok so nothing comes out of the event horizon, sure. But while you're free falling and accellerating you don't feel the effect of gravity. And humans are so small maybe the gravitational force between our feet and heads isn't significant enough to cause any stretching? Maybe you wouldn't die until you collided with other matter or the acceleration became so great it causes your body to rupture?

    • Re: (Score:2)

      by JCW01 ( 5760250 )
      If you fall into a supermassive black hole the gravity delta is not that huge, you would not be spaghettified, but on smaller stellar size black holes you most definitely would be. At least that is how I understand it.

      • If you fall into a supermassive black hole the gravity delta is not that huge, you would not be spaghettified,

        You will. It will just take a while. The outer regions of the interior of the event horizon are quite safe and reasonable areas, minus the fact that you'll never manage to escape from them, and you need to be going significant fractions of the speed of light to maintain your orbital distance from the singularity.

        You're right that the average density of a supermassive black hole is generally less than water (as measured from the edge of the event horizon to the singularity).
        But ultimately, there's still a

    • I remember reading a short story where an alien species naturally lived in space. In addition, members of the species could telepathically link with humans.

      On a joint scientific expedition to study a black hole, the alien got too close and started to cross the event horizon. As he died, the human could "hear" its screams. However, because of time dilation and from the perspective of the human, the alien never died and its mental screams continued to forever torture the linked human.

      • the alien never died and its mental screams continued to forever torture the linked human.

        I once had a bad case of tinnitus that was similar to that.

      • [Telepathic alien falls into black hole] because of time dilation and from the perspective of the human, the alien never died and its mental screams continued to forever torture the linked human.

        Yeah, I read it too and thought it has a massive logical hole: as time slows down for the alien, its screams would also slow down. Their wavelength (assuming the telepathic transmission happens via some variety of field) should get red-shifted very quickly, until reception becomes impossible. And even if you handwave the reception issue away, how would a feeling of fear, diluted over years even be perceived?

        • Also from the perspective of someone outside the black hole the person crossing the event horizone falls to the singularity in the center at the speed of light (give or take). It is only the person travelling close to c that experiences time dilation.

          For the largest black hols in the Milky Way, A* at the center, that is 40 seconds.

          • Re: (Score:2)

            by nasch ( 598556 )

            I thought from the outside perspective, the infalling person/object never appears to cross the event horizon at all.

        • So did I.

          Another flaw: time dilation doesn't "stop time" at the event horizon, but at the singularity. The entity would be dead long before time dilation slowed its subjective time anywhere near zero. Presumably it would be dead before it even reached the event horizon, and certainly upon crossing it.

          Not to mention there's some serious weirdness that happens to space-time as you cross the horizon, swapping time with one spatial axis. Essentially "inwards" becomes the timelike axis (one direction only), w

          • No, the time stop is at the event horizon. That's the point that the redshift becomes infinite and someone on the outside would never actually see the object pass. As you mentioned, inside the event horizon is where all the spacelike directions point inward and time gets wonky.

      • Re: (Score:2)

        by VAXcat ( 674775 )
        The story was "Kyrie" by Poul Anderson. Written in 1968 One of his best.

    • Re:Is spaghettification real? (Score:5, Informative)

      by ShanghaiBill ( 739463 ) on Wednesday May 26, 2021 @03:15PM (#61425130)

      There is no plausible mechanism for the creation of a BH smaller than about 3 solar masses. The smallest that we have detected is 3.8 SMs. The bigger the size, the smaller the spaghettification effect.

      The Schwarzschild radius of a BH with the mass of the entire Universe is about equal to the radius of the Universe. So we may be living inside of a black hole.

      Black hole cosmology [wikipedia.org]

      • Oh that's cool. Thanks for bringing this up, ShanghaiBill!

      • Could all the phenomenon we currently observe exist within a black hole?

        • Could all the phenomenon we currently observe exist within a black hole?

          Sure. Why not?

          The event horizon is only important to an outside observer. To someone passing through it or living inside it, there is nothing special about it.

          But it is like the Hotel California: You can check out any time you like, but you can never leave.

      • There is no plausible mechanism for the creation of a BH smaller than about 3 solar masses

        Does that statement include primordial black holes?

        • Does that statement include primordial black holes?

          Primordial black holes are possible, but it is not plausible that many still exist. They would have formed in the first few yottaseconds of the Big Bang, and nearly all would have decayed and disappeared a few microseconds later.

          If they continued to exist beyond the first few seconds, they would have rapidly grown due to the density of the early Universe. It is possible that they formed the massive black holes at the core of most galaxies. But those are way bigger than 3 solar masses.

          TL;DR: They either

      • >There is no plausible mechanism for the creation of a BH smaller than about 3 solar masses.

        At least not any more. During the inflationary period it's quite plausible that countless far smaller black holes were formed, most of the smaller end which would have long since evaporated to nothing... unless some QM process stops the process. Without a viable quantum theory of gravity it's impossible to predict how extremely small black holes would behave. But assuming evaporation stops at the Plank scale, t

      • I also wonder about the time dilation due to gravity. If your feet are feeling more gravity than your head, then your feet will also have a slower movement of time. If your feet are being pulled away from your head, but time is moving slower for them, is it possible these two effects cancel out, even partially, to make it so you don't really have spaghettification?

      • The bigger the size, the smaller the spaghettification effect.

        Only in relation to the size of the event horizon, and your distance from the singularity. Ultimately, the "force" of the spaghettification is higher the bigger the black hole, there's simply more space after which you're damned for crossing, but still quite safe from spaghettification.

        The principle difference between big and small ones, is whether spaghettification occurs inside or outside of the event horizon.

    • Remember, gravity is proportional to the products of the masses divided by their separation squared. For something the mass of the earth the event horizon (actually called Schwarzchild radius) is about 0.35 inches. Jupiter's mass would be 9 feet, and the sun 2 miles.

      A 6 foot tall person would span an immense range of gravitational force when dealing with something as dense as a black hole.

    • Sending data? [Re:Is spaghettification real?] (Score:5, Funny)

      by Tablizer ( 95088 ) on Wednesday May 26, 2021 @03:38PM (#61425206) Journal

      nothing comes out of the event horizon

      What about Hawking Radiation? Assuming a probe(s) could survive the EH entry, couldn't it somehow send a signal using Hawking Radiation and/or quantum entanglement?

      Is spaghettification real?

      Yes, it's called DOM and CSS.

      • Re: (Score:2)

        by nagora ( 177841 )

        nothing comes out of the event horizon

        What about Hawking Radiation?

        Hawking radiation doesn't technically come out of the black hole - virtual particles are created in pairs on each side of the EH.

        Assuming a probe(s) could survive the EH entry, couldn't it somehow send a signal using Hawking Radiation and/or quantum entanglement?

        Neither can be used to send a signal of any sort, let alone across the EH. HR can't be produced at will and QE just doesn't allow transfer of new information.

        • Hawking radiation doesn't technically come out of the black hole - virtual particles are created in pairs on each side of the EH.

          That's not actually how it works.
          The virtual particles are created with wavelengths that are larger than the blackhole, which is why the size of the blackhole is the deciding factor for how energetic the thermal photons (hawking radiation) are.

          The "either side of the EH" thing is a model used to teach laymen.

          • Re: (Score:2)

            by nagora ( 177841 )

            OK. Thanks.

            • I could say it a different way to demonstrate why the "virtual particle on either side of the EH" isn't a great model if you really want to understand what happens:

              The uncertainty in the position of any hawking radiation particle emitted from a blackhole will be the size of the entire black hole, plus an additional radius.
              This because "virtual particles" and classical "event horizons" are really just simplified analogues of the quantum field theory concept of vacuum polarization, and that the horizon-uns

      • >What about Hawking Radiation?
        That forms just outside the event horizon, when one half of a pair of virtual particles crosses the event horizon, freeing the other to depart and continue existing indefinitely. The resulting energy-debt then gets paid from the mass of the black hole (I'm not clear on the exact mechanism)

        Quantum entanglement shouldn't be affected by an event horizon - but there's no theoretical way to use entanglement to transmit information without also having a parallel classical informa

    • Re:Is spaghettification real? (Score:5, Informative)

      by nealric ( 3647765 ) on Wednesday May 26, 2021 @03:43PM (#61425210)

      Spaghettification is an observable phenomenon because for smaller black holes it begins to occur prior to crossing the event horizon. You can actually observe it in a very mundane way with any set of objects falling towards a massive object. For example, if you threw a bunch of beach balls at the earth from the moon in a line, the line would elongate as it fell towards the earth because the gravitational attraction of the first ball would be greater than the last by a very small amount.

      In the case of a black hole, as you approached the singularity, the delta between the attraction of your feet and your head would be nearly infinite. No massive object (including a human) could resist the stretching. By the time you reached the singularity, you'd be nothing but a string of subatomic particles. Eventually, energy (and mass) that used to make up part of you would be ejected from the black hole as Hawking radiation.

    • Not until you neared the mysterious singularity at the middle. Then the gravitational delta between your feet and head would grow massive, and your feet would fall faster, being tugged by weight of tons different, ripping you in half. All body parts would break smaller and smaller, finally molecules, then atoms, then atomic nuclei. Eventually (very quickly at this point actually) protons and neutrons would be ripped apart by the same process, and presumably quarks and electrons would have a party sharing

      • Eventually (very quickly at this point actually) protons and neutrons would be ripped apart by the same process,

        Time is relative and proportional to gravity. Sure it would be very quick to the person being ripped apart, but to the outside universe, it would take billions of years. Sorry for being pedantic, but how often do I get a chance to cite relativity :)

    • But while you're free falling and accellerating you don't feel the effect of gravity

      Unfortunately, the gravity gradient of a black hole is steep enough that the tidal force between your head and your feet is more than enough to pull you apart long before you impact with anything.

    • You don't feel gravity in a vacuum because there is a mostly uniform acceleration on your entire body. But if you are in free fall, there is a slight difference in the effective force on the parts of your body closer to the Earth than farther away which is proportionate to: (G*Earth's Mass) / (Earth's Radius^2) - (G*Earth's Mass) / ((Earth's Radius + Your height)^2). The Roche limit is an example of spaghettification which occurs in more conventional scales and it's the primary factor that creates planetary

  • Even worse black holes could be everywhere. [youtu.be]

  • How do we know it's not a Hawking impersonator?

    • How do we know it's not a Hawking impersonator?

      If it is, I'm impressed. I don't think I could hold that pose even for a few minutes.

  • Size Matters (Score:5, Interesting)

    by nealric ( 3647765 ) on Wednesday May 26, 2021 @03:31PM (#61425178)

    What happens as you approach the event horizon will depend on the size of a black hole.

    In a smaller black hole (one formed by the collapse of a single star, for example), an astronaut would be pulled apart by gravitational forces before reaching the event horizon. A space ship with sufficient thrust could theoretically escape at that point, but the gravitational force between the astronauts body parts would be sufficiently different as to pull it apart.

    In a supermassive black hole (such as the one at the center of the Milky Way), the description from the article would be true. Crossing the event horizon would appear to be a non event. In fact we already have "crossed" an cosmic event horizon of sorts. There are parts of the universe that are causally disconnected from earth, such that even light could never reach it. Crossing a black hole event horizon would just result in a much smaller portion of the universe you'd be "stuck" in. Theoretically, you could live your entire life on a space ship that has crossed the event horizon without anything noteworthy happening so long as you remained far enough away from the singularity and were not destroyed by the energy (kinetic, thermal, or otherwise) from all of the other objects that passed the event horizon. The event horizon of Messier 87 (the first photographed black hole) is about 120AU across (120 times the distance from the earth to sun), so you'd have nearly a whole solar system worth of "space" you'd still be able to interact with.. You'd be able to observe other objects that had crossed the event horizon with you and time would appear to pass normally to you.

    • "Well, at the moment you crossed the horizon, you wouldn't feel anything

      Well, they're right about that at least. No matter how gentle the gravitational gradient at the horizon of a large black hole, your brain still requires electro-chemical signals bouncing around in it to function, and as you cross the event horizon, those signals can no longer propagate outward. Effectively, your brain would cease to function. So would every single cell in your body.

      Or would it? The signals may not be able to propagate outward, but if they just fall inward more slowly than the far side o

      • Re:Would feel nothing... (Score:5, Interesting)

        by nealric ( 3647765 ) on Wednesday May 26, 2021 @04:23PM (#61425332)

        It's true there would be a moment where part of your body would be inside the event horizon and parts would be outside. A nerve signal from your toe inside the event horizon could never reach your brain so long as your brain is still outside. It's mostly a matter of speculation how that might impact you physiologically, but if you were on a space ship capable of traveling across the relativistic distances necessary to reach the black hole in the first place (at least a pretty significant fraction of the speed of light), your body would probably cross the event horizon quickly enough that it wouldn't have any noticeable effect.

        Also, to clarify my post above about being able to spend long time inside the event horizon as if nothing happened: you'd still eventually end up spaghettified. Once you cross the event horizon, going to the singularity is as inevitable as the passage of time. But depending on the size of the black hole, that could be a very long time (perhaps longer than a natural lifetime).

        • Re:Would feel nothing... (Score:4, Interesting)

          by Immerman ( 2627577 ) on Wednesday May 26, 2021 @04:57PM (#61425414)

          Note that the event horizon is nothing terribly special - it's only the outermost edge of a region in which information cannot flow outward - the deeper you fall into the black hole, the stronger the gravity and the more intense that effect will become. Much like the surface of the ocean is just the first point where your toe gets wet - it's not going to get any drier as you go deeper.

          There is actually some serious weirdness at the the horizon though - essentially space and time swap axes , with "inward" becoming the one-directional timelike axis, while physical acceleration would move you through "time" in either direction across the lifetime of the black hole, at least as defined by the order in which you observe outside events ("Ooh, that supernova was a good one, let's go past-ward and watch it in reverse!") It's possible that non-infinitesimal objects might continue to exist in the newly defined three spatial dimensions, but it seems more likely that the transformation would utterly destroy them.

      • Adding to my comment below: forces would still be able to propagate on the other side of the event horizon. An object that has crossed the event horizon must inevitably fall to the singularity, but it doesn't have to do so in a linear fashion. An explosion would still propagate outward same as outside the event horizon. The only difference is that an explosion occurring inside the event horizon could not radiate matter or energy outside.

        An astronaut inside the event horizon would be subject to enormous grav

        • I think I already addressed that with my previous reply. No - nothing can travel outward from any point within a black hole - the event horizon is infinitely deep. The leading edge of an "outward" explosion will still be falling inward, just not as fast as the original explosive. You can travel inward faster or slower, but you cannot stop or reverse direction. "Inward" becomes your timelike (one-directional) axis.

          • The event horizon is not infinitely deep. It has a measurable radius (at least from the perspective of someone outside of it).

            But yes, the leading edge the outward explosion will still be falling, but not necessarily relative to any object inside the event horizon. That's why human physiology should theoretically still operate normally on the other side of an event horizon. You and your nerve signals may all be traveling towards the singularity, but they can still travel from your toe to your brain. You cou

            • Except that black holes are much larger on the inside. Infinitely might be overstating it (I'm not actually sure - I think it would be infinite for a true singularity), but the distance from the event horizon to the singularity (the "depth" of the black hole) is much, much larger than the diameter of the event horizon.

              Of course, thanks to the one-way nature of the space within the black hole, the full distance can only be observed from the singularity itself.

              >You and your nerve signals may all be travel

              • I guess itâ(TM)s hard to describe what happens inside a black hole without specifying from whose perspective you are talking. To an outside observer, a spaceship crossing the event horizon is obliterated.

    • In a way this is like dying, maybe some aspect of you will continue to live in another realm but you can't ever tell anyone.

      Or can you? Maybe information escapes from the afterlife to another's dream as it apparently can escape the black hole.

  • We can't yet directly observe black holes in any even loose approximation of real time.

    All we can do is treat what we "can't see" along with gravitational lensing as showing us where they are, confirm what we already guessed using equations, and think up experiments when we do get close enough to play with one without taking lifetimes to see any results.

    The fascinating thing to me is that in theory all that matter is stuck together in basically the space of one atom, like a condensate state of matter.

    That m

  • We don't know what we don't know so we work to find out.

    More passive-aggressive space filler SEO posts do not add quality to Slashdot.

    Does msmash have a boss or own stake in Slashdot? That's a serious question because the editors are solely responsible for the decline of Slashdot as a tech site and its transformation into a (mostly) generic blog padded with crap that adds nothing.

    I'll keep questioning but the lack of answers is telling and I'm far from the only reader to notice. There is no logical revenue

    • Well I for one found it a refreshing change from Bitcoin, Global Warming, Covid, and FANG articles

  • Certainly, Hawking Radiation -- which has been theorized -- is created from the disintegration of matter. The weird part is trying to think up a particle that is massless and can escape; an electron has mass, as does any other fundamental particle so we get into weird matter -- muons, gluons, quarks, etc.

    I personally think that dark matter is just the weird matter exhausted by the "furnaces" within stars and especially black holes. It, having no mass or doesn't interact with the electromagnetic spectrum,

    • Photons are massless and still can't escape.

    • No, we don't know whether black holes have Hawking radiation, never been detected (nor can it be with stellar mass black holes, too little of it)

      We don't have a complete model of black holes that takes into account both our best models. Contrary to popular misconception, we don't even have model of what interior of real black hole is like, only four oversimplified cases of eternal black holes that don't and can't exist in reality. A real black hole that has a formation time is too complex to model in GR.

  • You might be inexorably drawn to the center, but owing to the extreme curvature of space, the path that you actually take to get there will take you something on the order of a google years, by which point the black hole will have long since evaporated.

  • They like to look at a table of billiard balls move in motion: you tell us how fast they’re moving and where they are, we’ll tell you where they were and where they’re going to be.

    In motion is the important part here. If you look at a table of billiard balls sitting still, you can tell me nothing about how they came to be there. What direction did they come from and how fast were they moving before coming to a stop. Why should it matter if it stops in a black hole or it simply stops on the pool table somewhere? Either way you can tell nothing about it's past motion.

  • You know the theory that a previous universe ... (Score:3)

    by lsllll ( 830002 ) on Wednesday May 26, 2021 @05:25PM (#61425484)

    You know the theory that a previous universe shrank and then we had a big bang? Maybe after the black holes grow to be so big, they swallow each other (kinda like that bubble game) and it goes on and on until you're left with just one black hole and then -- fill in the blank here -- which leads to another big bang.

    For the "fill in the blank here" part, it could be that the one black hole becomes so dense that it starts to crush nucleons to the point that they end up converting to energy en-masse and create a big bang.

    Man, this weed is really good shit!

    • That is exactly what I used to think.
      Now I just don't think anymore....
      Stupid 9-5, a man's gotta eat.

    • Re: (Score:2)

      by gawbl ( 941021 )

      You know the theory that a previous universe shrank and then we had a big bang? Maybe after the black holes grow to be so big, they swallow each other (kinda like that bubble game) and it goes on and on until you're left with just one black hole and then -- Profit! -- which leads to another big bang.

      FTFY

    • Aren't black holes like cosmic roombas? Cruising around the 'verse, slurping down matter and each other till There Can Be Only One?

  • Feeding a black hole a steady stream of congressmen. If you told the there were votes to be had theyâ(TM)d work out how to communicate back in ways undreamed of.

  • Oh yes, the underdogs of the universe. Black Holes are subject to ridicule and abuse around the universe. How can we, mere humans, understand the angst that Black Holes feel? On the grand scale of things, our individual understandings are of little comfort. Join now in the nascent organization Black Holes Matter and help support the spiritual essence of these majestic celestial magnatrodes!

  • Maybe, maybe not.

    But we (for values of "we" at the species or society level) are very unlikely to ever stop trying to understand (etc etc)

  • Yes, eventually if we live long enough as a species and don't atrophy and all die from boredom we will understand life the universe and everything.
    That includes black holes, women, and the American voting system.
    Thank you, I am here all week.
    NEXT!

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