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Are there any astronomers willing to answer this: How likely is it that a neutron star "crashes" with a black hole? Why do not they just fly past each other or

Are there any astronomers willing to answer this: How likely is it that a neutron star "crashes" with a black hole? Why do not they just fly past each other or just rotate like Earth and the sun?

readyplayernull 3y ago
Not an astronomer, but I can imagine a crashing window as a thin straight pipe that is bended by spacetime's gravitational waves, magnetic waves and BH ejection material.
mastazi 3y ago
If you look at the animation linked in the article, you can see that they did orbit each other, not only that but if you look at the time scale in the lower part of the screen, you can see that they were doing that at a crazy frequency (about 8 full revolutions in the last 30 milliseconds) https://www.youtube.com/watch?v=3PKsBwH_bJE https://www.youtube.com/watch?v=3PKsBwH_bJE
gosub100 3y ago
IANAA, but black holes often have an accretion disc orbiting them. Anything that would cross this plane would get impacted by the orbiting particles, which would slowly cause the object to begin to orbit. Neutron stars also have enormous gravitational and magnetic fields which could steal mass from the accretion disc. If you (the neutron star) gain mass near the gravitational field, it pulls you in closer.
- raattgift 3y ago
  > If you (the neutron star) gain mass near the gravitational field, it pulls you in closer. This conflicts with the Strong Equivalence Principle (SEP) in its universality of free fall form, or more colloquially a rule that the trajectory of a self-gravitating object depends only on its initial position and velocity and not on its internal composition (including the evolution of its mass). There is a good test of the SEP in a system involving the millisecond pulsar https://en.wikipedia.org/wiki/PSR_J0337%2B1715 https://en.wikipedia.org/wiki/PSR_J0337%2B1715 which is in a hard binary with a white dwarf; a second white dwarf orbits the inner binary. Here is a trace of the orbits https://www.youtube.com/watch?v=oDgfqq_W_uM https://www.youtube.com/watch?v=oDgfqq_W_uM -- I recommend reading the video description. Reference 2 (Archibald et al.) at the wikipedia page is an excellent paper. There are a couple of good astrobites aimed at (advanced, or possibly final-year) astrophysics undergrads at https://astrobites.org/2014/01/14/timing-is-everything-a-pulsar-in-orbit/ https://astrobites.org/2014/01/14/timing-is-everything-a-pul... (2014) and https://astrobites.org/2019/03/25/testing-einsteins-equivalence-principle-by-timing-a-pulsar-in-a-stellar-triple-system/ https://astrobites.org/2019/03/25/testing-einsteins-equivale... (2019) You're right that the flow of mass in a complex system might be relevant to the merging binary, but in general a stellar black hole's circumstellar accretion structure isn't going to do much to a close-in neutron star except create a lot of (relatively small) nuclear explosions on the latter's surface. There just isn't going to enough material in the accretion structure in comparison to the bigger bodies, which will have masses greater than our sun. Accretion structures are more relevant for softer (that is more widely separated) binaries, which are a long time before final merger. Circumstellar accretion discs can be in association with a circumbinary disc (CBD; outside the binary), with a flow of matter between them (one would expect the CBD to be the origin of the circumstellar discs, but jet outflows make that expectation complicated). CBDs tend to circularize wide binary orbits, for example. They also provide mechanisms to harden binaries that might seem so soft that a merger could be avoided. You're right that some neutron stars may have very strong magnetic fields that may affect its accretion disc or the (randomly aligned) accretion disc of its black hole binary partner, but again the flows of mass near the binary must be much less than the masses of the relativistic stars (the NS and the BH), as the each of the stars are very very strongly bound self-gravitation. Your idea bout mass flow is interesting -- it takes my mind to how much of the accretion disc might be blown into ~polar jets, and how complicated the alignment of spin axes are to the orbital plane can be late in the merger. The accretion structures are I guess likely to be blown into a CBD (or out of the system entirely) well before final merger. Either way, the masses involved likely won't change the inspiral significantly. If the NS shreds (as opposed to being swallowed whole, which is the more likely outcome, and the apparent outcome in this detection) the additional matter in an accretion structure (including any accretion material blown out of the system via jetting) could be relevant in the r-process and s-process heavy element formation processes. Realistically though that's the sort of question more relevant to binary neutron star mergers. (As a sort of self-bookmarking and because it's probably interesting to other readers here, Maria Massi's 2020 "Accretion" teaching slides at https://www3.mpifr-bonn.mpg.de/staff/mmassi/lezione2WEdd.pdf https://www3.mpifr-bonn.mpg.de/staff/mmassi/lezione2WEdd.pdf are awesome, and takes one to some work on x-ray binary circumstellar discs and even circumbinary accretion. More recent review on the latter <https://arxiv.org/abs/2211.00028 https://arxiv.org/abs/2211.00028>, and figure 1 is probably helpful for a couple of my paragraphs above).
  - gosub100 3y ago
    I appreciate the correction. It's too late to edit my post. I have been watching 8 years of astrophysics on YouTube and thought I might be able to pitch in on layman's questions but it seems as though I still have much to learn. Thanks
gilbetron 3y ago
Well, according to this: https://www.space.com/dancing-black-holes-merge https://www.space.com/dancing-black-holes-merge "Based on all the observations of gravitational waves, astronomers estimate that there are somewhere between 15 and 38 black hole mergers every year within every cubic gigaparsec of volume in the universe (about 1/12000th of the total volume of the observable universe)" According to this: https://bigthink.com/starts-with-a-bang/40-quintillion-black-holes-in-universe https://bigthink.com/starts-with-a-bang/40-quintillion-black... "By combining information about stars, black holes, and stellar and cosmic evolution all together, astronomers have the first robust estimate for black holes in the Universe: 40 quintillion" So, just for black hole mergers, it would be really, really rare. There are probably orders of magnitude more black holes that are just orbiting each other. Neutron star and black hole statistics I leave as an exercise for the reader ;)
raattgift 3y ago
Good question! I'll try an answer and hope it's a good balance between accuracy and what I guess is your (and other readers') knowledge of astrophysics. [tl;dr: a neutron star (NS) and black hole (BH) in a close binary are practically guaranteed to "crash" into each other eventually. There seem to be lots of those binaries. Neutron star - black hole mergers (NSBH) from other possible arrangements (wider binaries, isolated individual NSes and BHs) will be vanishingly rare in "the local universe".] There are two principal branches in an answer to your question. First branch: there are probably lots of individual isolated neutron stars and black holes within galaxies, and those are unlikely to form binaries with each other, or get anywhere close to each other (except maybe possibly in the very centre of galaxies, or in the very far future). I won't spend time on this branch in this comment, even though this is the branch which interests me personally (at a theoretical rather than astrophysical level). In reality, isolated stars and star systems within galaxies just do not seem to collide with one another and isolated neutron stars and (stellar mass) black holes are no different. Independent isolated stars may form binaries under rare circumstances such as galaxy-galaxy mergers or in radiating globular clusters within galaxies. The same goes for isolated neutron stars or stellar mass black holes. In those cases they can fall into the next paragraph. Second branch: there are probably lots of neutron stars and black holes already in close orbit with one another, and those orbits will over long times decay (because such binaries radiate nontrivial gravitational waves) leading to a "crash". Tight orbits involving two relativistic bodies (white dwarf (WD), neutron stars, black holes) are unstable and practically always lead to a merger; there are a number of ways a looser/softer binary of such bodies can become a tighter/harder one, including during the transition of a dying star into a WD, NS, or BH. Multi-star systems are commonplace[1] and likely mostly born as multi-star systems (e.g. within a nebula originating in the explosive demise of earlier more massive stars), rather than starting as individual stars that just get close enough to start orbiting each other. One could compare this to how Jupiter and the sun most likely formed from the same part of a giant molecular cloud (GMC), rather than a fully-formed Jupiter being captured later by the sun. Likewise, the Alpha Centauri system probably formed as triple-star system within a different part of the same GMC, rather than the inner AB binary capturing Proxima Centauri (C) or each other. (Triple star systems with an outer member orbiting an inner binary are pretty common, even very close to us in the galaxy). We'd expect therefore for a proportion of aging multi-star systems to evolve into binaries of "dead stars" including inner neutron star - black hole binaries. Those inner binaries will practically inevitably decay resulting in a neutron star - black hole merger (NSBH) producing a signal in-principle detectable by gravitational wave detectors like KAGRA, Virgo, and LIGO. (The outer member of a triple or surrounding gas, dust, asteroids, and comets can help tighten the inner binary.) Gravitational wave detectors like LIGO are range-limited, with the current range about 200 megaparsecs (about 600 million light years), which is on cosmological scales quite local. We've counted about a quarter of a million galaxies[2] in that range. By comparison there are hundreds of billions of galaxies in our sky. A fraction of neutron stars are pulsars. Because of the practicalities of radiotelescopes, at present we can only really find pulsars nearby within our galaxy (dust obscures a lot of the galaxy at the relevant radio frequencies, and there is only one extragalactic pulsar known, and it's in the neighbouring Large Magellanic Cloud). However, we've found several thousand pulsars (see final paragraph below). A sizable fraction are in multi-star systems, which we know from tracking radial acceleration in several ways, and because among these are a handful of eclipsing binaries. There are known pulsar-pulsar pairs (famously, <https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_pulsar https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_pulsar>) as well as more complicated systems which include white dwarfs and maybe black holes. From gravitational wave observations, we can spot a NSBH even if we can't see either individual body (in radio or other parts of the electromagnetic spectrum) because of obscuration by dust and other foreground matter, or because the NS is not a pulsar and thus both objects are very faint in radio. We don't have a good census of dim neutron stars even in our part of the galaxy. However, there will be more dim neutron stars than radio-bright pulsars, and it is reasonable to guess that there will be more dim neutron stars in binary- and multi star systems with a black hole than there are radio-bright pulsars in such systems. So looking at one galaxy (ours), we can guess that if we waited a few million years we would see a decaying hard neutron star/black hole binary eventually merge. So naively, we might think that if we waited a few years we'd see an NSBH merger from among about a quarter of a million galaxies. And at gravitational wave detectors we're seeing more NSBHs than that naive estimate. So perhaps pulsarNS-BH binaries are rare compared to nonpulsarNS-BH ones. Or perhaps there are lots of pulsarNS-BH binaries in the Milky Way hidden behind dust and sources of radio noise. Finally, this interactive map of known pulsars https://ishivvers.github.io/maps/pulsars.html https://ishivvers.github.io/maps/pulsars.html is so cool I've submitted it separately at https://news.ycombinator.com/item?id=39970373 https://news.ycombinator.com/item?id=39970373 [1] "Stellar Multiplicity" (arxiv version of published ARA&A paper), Duchene et al. 2013 https://arxiv.org/abs/1303.3028 https://arxiv.org/abs/1303.3028 "Multiple Star Systems" (multimedia outreach) https://science.nasa.gov/universe/stars/multiple-star-systems/ https://science.nasa.gov/universe/stars/multiple-star-system... (examples) https://en.wikipedia.org/wiki/Category:Triple_star_systems https://en.wikipedia.org/wiki/Category:Triple_star_systems (more examples, quadruples and beyond) https://en.wikipedia.org/wiki/Star_system#Quadruple https://en.wikipedia.org/wiki/Star_system#Quadruple [2] section 5.3 of "Census of the Local Universe [..]", Cook et al. 2019 <https://ui.adsabs.harvard.edu/abs/2019ApJ...880....7C/abstract https://ui.adsabs.harvard.edu/abs/2019ApJ...880....7C/abstra...>
lazide 3y ago
Not an astronomer, but… They usually do fly past each other or (at first) rotate like the earth and sun. For similar reasons to if you drop a marble into a bathtub or sink, it will almost never go straight into the drain immediately. For it to immediately drop into the drain requires a bunch of velocity vectors to have very specific values (in a just right kind of way) that are statistically improbable in such a chaotic environment. It’ll occasionally, very rarely, happen of course. But more likely is that if they don’t just fly past each other (marble flies out of the sink!), is that eventually tidal losses/gravitational wave losses/mass transfers will slow the bodies down relative to each other until they merge. A much slower version of the marble rolling around, bouncing off things, and losing speed until it drops into the drain. Depending on the starting states of the system, it could take billions of years, or mere months. If there are other bodies involved like multiple stars or black holes, one of them might even just get ejected out of the system. Like if you drop a bunch of marbles into the sink at once, occasionally one of them will get knocked out even if none of them individually would have had the energy to do so, and it will be that much harder for any one of them to fall in since just when it is getting close another marble will come in to knock it away. That’s essentially what is happening with accretion disks, and why they get so hot.

Are there any astronomers willing to answer this: How likely is it that a neutron star "crashes" with a black hole? Why do not they just fly past each other or just rotate like Earth and the sun?

readyplayernull 3y ago

Not an astronomer, but I can imagine a crashing window as a thin straight pipe that is bended by spacetime's gravitational waves, magnetic waves and BH ejection material.

mastazi 3y ago

If you look at the animation linked in the article, you can see that they did orbit each other, not only that but if you look at the time scale in the lower part of the screen, you can see that they were doing that at a crazy frequency (about 8 full revolutions in the last 30 milliseconds) https://www.youtube.com/watch?v=3PKsBwH_bJE https://www.youtube.com/watch?v=3PKsBwH_bJE

gosub100 3y ago

IANAA, but black holes often have an accretion disc orbiting them. Anything that would cross this plane would get impacted by the orbiting particles, which would slowly cause the object to begin to orbit. Neutron stars also have enormous gravitational and magnetic fields which could steal mass from the accretion disc. If you (the neutron star) gain mass near the gravitational field, it pulls you in closer.

raattgift 3y ago
> If you (the neutron star) gain mass near the gravitational field, it pulls you in closer. This conflicts with the Strong Equivalence Principle (SEP) in its universality of free fall form, or more colloquially a rule that the trajectory of a self-gravitating object depends only on its initial position and velocity and not on its internal composition (including the evolution of its mass). There is a good test of the SEP in a system involving the millisecond pulsar https://en.wikipedia.org/wiki/PSR_J0337%2B1715 https://en.wikipedia.org/wiki/PSR_J0337%2B1715 which is in a hard binary with a white dwarf; a second white dwarf orbits the inner binary. Here is a trace of the orbits https://www.youtube.com/watch?v=oDgfqq_W_uM https://www.youtube.com/watch?v=oDgfqq_W_uM -- I recommend reading the video description. Reference 2 (Archibald et al.) at the wikipedia page is an excellent paper. There are a couple of good astrobites aimed at (advanced, or possibly final-year) astrophysics undergrads at https://astrobites.org/2014/01/14/timing-is-everything-a-pulsar-in-orbit/ https://astrobites.org/2014/01/14/timing-is-everything-a-pul... (2014) and https://astrobites.org/2019/03/25/testing-einsteins-equivalence-principle-by-timing-a-pulsar-in-a-stellar-triple-system/ https://astrobites.org/2019/03/25/testing-einsteins-equivale... (2019) You're right that the flow of mass in a complex system might be relevant to the merging binary, but in general a stellar black hole's circumstellar accretion structure isn't going to do much to a close-in neutron star except create a lot of (relatively small) nuclear explosions on the latter's surface. There just isn't going to enough material in the accretion structure in comparison to the bigger bodies, which will have masses greater than our sun. Accretion structures are more relevant for softer (that is more widely separated) binaries, which are a long time before final merger. Circumstellar accretion discs can be in association with a circumbinary disc (CBD; outside the binary), with a flow of matter between them (one would expect the CBD to be the origin of the circumstellar discs, but jet outflows make that expectation complicated). CBDs tend to circularize wide binary orbits, for example. They also provide mechanisms to harden binaries that might seem so soft that a merger could be avoided. You're right that some neutron stars may have very strong magnetic fields that may affect its accretion disc or the (randomly aligned) accretion disc of its black hole binary partner, but again the flows of mass near the binary must be much less than the masses of the relativistic stars (the NS and the BH), as the each of the stars are very very strongly bound self-gravitation. Your idea bout mass flow is interesting -- it takes my mind to how much of the accretion disc might be blown into ~polar jets, and how complicated the alignment of spin axes are to the orbital plane can be late in the merger. The accretion structures are I guess likely to be blown into a CBD (or out of the system entirely) well before final merger. Either way, the masses involved likely won't change the inspiral significantly. If the NS shreds (as opposed to being swallowed whole, which is the more likely outcome, and the apparent outcome in this detection) the additional matter in an accretion structure (including any accretion material blown out of the system via jetting) could be relevant in the r-process and s-process heavy element formation processes. Realistically though that's the sort of question more relevant to binary neutron star mergers. (As a sort of self-bookmarking and because it's probably interesting to other readers here, Maria Massi's 2020 "Accretion" teaching slides at https://www3.mpifr-bonn.mpg.de/staff/mmassi/lezione2WEdd.pdf https://www3.mpifr-bonn.mpg.de/staff/mmassi/lezione2WEdd.pdf are awesome, and takes one to some work on x-ray binary circumstellar discs and even circumbinary accretion. More recent review on the latter <https://arxiv.org/abs/2211.00028 https://arxiv.org/abs/2211.00028>, and figure 1 is probably helpful for a couple of my paragraphs above).
- gosub100 3y ago
  I appreciate the correction. It's too late to edit my post. I have been watching 8 years of astrophysics on YouTube and thought I might be able to pitch in on layman's questions but it seems as though I still have much to learn. Thanks

gilbetron 3y ago

Well, according to this: https://www.space.com/dancing-black-holes-merge https://www.space.com/dancing-black-holes-merge "Based on all the observations of gravitational waves, astronomers estimate that there are somewhere between 15 and 38 black hole mergers every year within every cubic gigaparsec of volume in the universe (about 1/12000th of the total volume of the observable universe)" According to this: https://bigthink.com/starts-with-a-bang/40-quintillion-black-holes-in-universe https://bigthink.com/starts-with-a-bang/40-quintillion-black... "By combining information about stars, black holes, and stellar and cosmic evolution all together, astronomers have the first robust estimate for black holes in the Universe: 40 quintillion" So, just for black hole mergers, it would be really, really rare. There are probably orders of magnitude more black holes that are just orbiting each other. Neutron star and black hole statistics I leave as an exercise for the reader ;)

raattgift 3y ago

Good question! I'll try an answer and hope it's a good balance between accuracy and what I guess is your (and other readers') knowledge of astrophysics. [tl;dr: a neutron star (NS) and black hole (BH) in a close binary are practically guaranteed to "crash" into each other eventually. There seem to be lots of those binaries. Neutron star - black hole mergers (NSBH) from other possible arrangements (wider binaries, isolated individual NSes and BHs) will be vanishingly rare in "the local universe".] There are two principal branches in an answer to your question. First branch: there are probably lots of individual isolated neutron stars and black holes within galaxies, and those are unlikely to form binaries with each other, or get anywhere close to each other (except maybe possibly in the very centre of galaxies, or in the very far future). I won't spend time on this branch in this comment, even though this is the branch which interests me personally (at a theoretical rather than astrophysical level). In reality, isolated stars and star systems within galaxies just do not seem to collide with one another and isolated neutron stars and (stellar mass) black holes are no different. Independent isolated stars may form binaries under rare circumstances such as galaxy-galaxy mergers or in radiating globular clusters within galaxies. The same goes for isolated neutron stars or stellar mass black holes. In those cases they can fall into the next paragraph. Second branch: there are probably lots of neutron stars and black holes already in close orbit with one another, and those orbits will over long times decay (because such binaries radiate nontrivial gravitational waves) leading to a "crash". Tight orbits involving two relativistic bodies (white dwarf (WD), neutron stars, black holes) are unstable and practically always lead to a merger; there are a number of ways a looser/softer binary of such bodies can become a tighter/harder one, including during the transition of a dying star into a WD, NS, or BH. Multi-star systems are commonplace[1] and likely mostly born as multi-star systems (e.g. within a nebula originating in the explosive demise of earlier more massive stars), rather than starting as individual stars that just get close enough to start orbiting each other. One could compare this to how Jupiter and the sun most likely formed from the same part of a giant molecular cloud (GMC), rather than a fully-formed Jupiter being captured later by the sun. Likewise, the Alpha Centauri system probably formed as triple-star system within a different part of the same GMC, rather than the inner AB binary capturing Proxima Centauri (C) or each other. (Triple star systems with an outer member orbiting an inner binary are pretty common, even very close to us in the galaxy). We'd expect therefore for a proportion of aging multi-star systems to evolve into binaries of "dead stars" including inner neutron star - black hole binaries. Those inner binaries will practically inevitably decay resulting in a neutron star - black hole merger (NSBH) producing a signal in-principle detectable by gravitational wave detectors like KAGRA, Virgo, and LIGO. (The outer member of a triple or surrounding gas, dust, asteroids, and comets can help tighten the inner binary.) Gravitational wave detectors like LIGO are range-limited, with the current range about 200 megaparsecs (about 600 million light years), which is on cosmological scales quite local. We've counted about a quarter of a million galaxies[2] in that range. By comparison there are hundreds of billions of galaxies in our sky. A fraction of neutron stars are pulsars. Because of the practicalities of radiotelescopes, at present we can only really find pulsars nearby within our galaxy (dust obscures a lot of the galaxy at the relevant radio frequencies, and there is only one extragalactic pulsar known, and it's in the neighbouring Large Magellanic Cloud). However, we've found several thousand pulsars (see final paragraph below). A sizable fraction are in multi-star systems, which we know from tracking radial acceleration in several ways, and because among these are a handful of eclipsing binaries. There are known pulsar-pulsar pairs (famously, <https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_pulsar https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_pulsar>) as well as more complicated systems which include white dwarfs and maybe black holes. From gravitational wave observations, we can spot a NSBH even if we can't see either individual body (in radio or other parts of the electromagnetic spectrum) because of obscuration by dust and other foreground matter, or because the NS is not a pulsar and thus both objects are very faint in radio. We don't have a good census of dim neutron stars even in our part of the galaxy. However, there will be more dim neutron stars than radio-bright pulsars, and it is reasonable to guess that there will be more dim neutron stars in binary- and multi star systems with a black hole than there are radio-bright pulsars in such systems. So looking at one galaxy (ours), we can guess that if we waited a few million years we would see a decaying hard neutron star/black hole binary eventually merge. So naively, we might think that if we waited a few years we'd see an NSBH merger from among about a quarter of a million galaxies. And at gravitational wave detectors we're seeing more NSBHs than that naive estimate. So perhaps pulsarNS-BH binaries are rare compared to nonpulsarNS-BH ones. Or perhaps there are lots of pulsarNS-BH binaries in the Milky Way hidden behind dust and sources of radio noise. Finally, this interactive map of known pulsars https://ishivvers.github.io/maps/pulsars.html https://ishivvers.github.io/maps/pulsars.html is so cool I've submitted it separately at https://news.ycombinator.com/item?id=39970373 https://news.ycombinator.com/item?id=39970373 [1] "Stellar Multiplicity" (arxiv version of published ARA&A paper), Duchene et al. 2013 https://arxiv.org/abs/1303.3028 https://arxiv.org/abs/1303.3028 "Multiple Star Systems" (multimedia outreach) https://science.nasa.gov/universe/stars/multiple-star-systems/ https://science.nasa.gov/universe/stars/multiple-star-system... (examples) https://en.wikipedia.org/wiki/Category:Triple_star_systems https://en.wikipedia.org/wiki/Category:Triple_star_systems (more examples, quadruples and beyond) https://en.wikipedia.org/wiki/Star_system#Quadruple https://en.wikipedia.org/wiki/Star_system#Quadruple [2] section 5.3 of "Census of the Local Universe [..]", Cook et al. 2019 <https://ui.adsabs.harvard.edu/abs/2019ApJ...880....7C/abstract https://ui.adsabs.harvard.edu/abs/2019ApJ...880....7C/abstra...>

lazide 3y ago

Not an astronomer, but… They usually do fly past each other or (at first) rotate like the earth and sun. For similar reasons to if you drop a marble into a bathtub or sink, it will almost never go straight into the drain immediately. For it to immediately drop into the drain requires a bunch of velocity vectors to have very specific values (in a just right kind of way) that are statistically improbable in such a chaotic environment. It’ll occasionally, very rarely, happen of course. But more likely is that if they don’t just fly past each other (marble flies out of the sink!), is that eventually tidal losses/gravitational wave losses/mass transfers will slow the bodies down relative to each other until they merge. A much slower version of the marble rolling around, bouncing off things, and losing speed until it drops into the drain. Depending on the starting states of the system, it could take billions of years, or mere months. If there are other bodies involved like multiple stars or black holes, one of them might even just get ejected out of the system. Like if you drop a bunch of marbles into the sink at once, occasionally one of them will get knocked out even if none of them individually would have had the energy to do so, and it will be that much harder for any one of them to fall in since just when it is getting close another marble will come in to knock it away. That’s essentially what is happening with accretion disks, and why they get so hot.