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Here's the one that has bugged me since I first learned of Hawking radiation as an undergrad: if it is true (it's still unobserved but let's go with it), that m

Here's the one that has bugged me since I first learned of Hawking radiation as an undergrad: if it is true (it's still unobserved but let's go with it), that means that merely passing through the event horizon allows for violation of the conservation of baryon number. I generally regard most of the "Star Trek physics" ideas like FTL with the hairy eyeball, but this one really gets me. Being able to violate the conservation of baryon number (and lepton number, for that matter) would point to -- in some scenarios -- mass-energy conversion being a lot more available than the laborious processes of fission, fusion, and particle/antiparticle mutual annihilation.

simonh 6y ago
Surely the conservation of baryon numbers is just a rule for reactions. I don’t see how crossing the event horizon changes anything. Remember from the POV of the baryon, it never crosses the event horizon. It’s not really ‘destroyed’, it just becomes unobservable to the external universe.
- qubex 6y ago
  True, but from the point of view of an observer at infinity, the baryon falls and comes to a halt, while the event horizon of the black hole expands and encompasses it (or would, if you could measure the diameter of the black hole with sufficient precision). This is just the essence of the information paradox, expressed in terms of baryon and lepton numbers. Either the Hawking radiation that comes out is ‘correlated’ with the matter that fell in (meaning that black holes have hair) and information is preserved, or they’re not, and information is destroyed. That’s literally the crux of the matter.
  - simonh 6y ago
    That’s a good point, I wasn’t thinking about it in terms of the information paradox, but then the recently proposed solutions look like they have a good chance of panning out. https://en.wikipedia.org/wiki/Black_hole_information_paradox#Recent_developments https://en.wikipedia.org/wiki/Black_hole_information_paradox... But anyway I still don’t think this is really an issue regardless. The same could be said of the cosmic event horizon. Vast swathes of the universe are unobservable and causally disconnected from us due to the expansion of the universe separating us at faster than the speed of light. Again, no Baryons are being destroyed in this process, they’re just becoming unobservable _to_us_. https://en.wikipedia.org/wiki/Event_horizon#Cosmic_event_horizon https://en.wikipedia.org/wiki/Event_horizon#Cosmic_event_hor...
    - qubex 6y ago
      As I see it, there’s a vast cognitive chasm between the cosmic event horizon and the event horizon of a black hole, as the former preserves “continuity of locality” (my term) whereas the latter does not. Basically the the former is only an event horizon for us who are very distant from it, whereas the black hole’s horizon is an abrupt threshold in space. I don’t think it’s healthy to confuse the two.
      - wnoise 6y ago
        The locality is all equally continuous in all these cases. Someone near the black-hole horizon will see nothing unusual, and will not experience a horizon unless they are accelerating like mad to stay there. But someone doing that in flat space will see a horizon too -- see the Rindler coördinatization[1] of Minkowski space. It even has an analogous Rindler horizon, which seems to emit particles from the Unruh Effect[2]. I say seems to because particle number isn't invariant under acceleration. An outside observer will see a particle detector go off, but will interpret the accelerated detector as causing the particles, rather than the horizon. [1] https://en.wikipedia.org/wiki/Rindler_coordinates https://en.wikipedia.org/wiki/Rindler_coordinates [2] https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect
- danbruc 6y ago
  Remember from the POV of the baryon, it never crosses the event horizon. It’s not really ‘destroyed’, it just becomes unobservable to the external universe. I think this is backwards - for an outside observer nothing ever crosses the event horizon, from the point of view of anything falling towards the event horizon it takes a finite amount of time to cross the event horizon.
  - heavenlyblue 6y ago
    How does this interact with the Hawking radiation? ie I throw something into the black hole, how much time do I wait before the Hawking radiation from that matter to manifest itself?
    - danbruc 6y ago
      From your point of view - assuming you do not fall into the black hole yourself - nothing you throw into the black hole ever crosses the event horizon so you will have wait forever. You can only see something cross the event horizon if you cross the event horizon yourself and you will see everything cross the event horizon at the moment you cross the event horizon.
      - coffeeling 6y ago
        My friend is kind of confused. Why wouldn't an observer outside of the event horizon see things falling into it? He says you can glean information from a thing that's outside of the horizon, but a thing falling into it should cease to be observable, at least by his understanding of black holes. My galaxy-sized brain understands this, of course, but my friend's pea-brain doesn't. So asking for a friend, ELI-aborted-CS-degree?
        danbruc 6y ago
        An observer outside the event horizon has to accelerate to avoid falling into the black hole itself and in the resulting frame of reference of this observer the horizon of the black hole coincides with future infinity and therefore everything falling into the black hole only reaches the event horizon infinitely far into the future. You can watch Leonard Suskind's lecture on that [1], falling into a black hole starts at about 53 minutes, but depending on your background you may want to start earlier, maybe even several lectures earlier. Looking at those Penrose diagrams is probably the easiest way to see what happens even if it will probably still not make intuitive sense. [1] https://www.youtube.com/watch?v=BdYtfYkdGDk&list=PLpGHT1n4-mAvcXwzOIz3dHnGZaQP1LEib&index=4 https://www.youtube.com/watch?v=BdYtfYkdGDk&list=PLpGHT1n4-m...
- at_a_remove 6y ago
  "Just a rule for reactions" drives quite a lot of physics. Conservation rules are how to describe all of the things that do not happen: why a hammer doesn't vanish, why all of your electrons do not turn into positrons, and so forth. That's what is so fascinating ... you have this odd topological boundary and it creates a violation in known rules.
willis936 6y ago
I don’t see why it’s unbelievable. High mass energy conversion is one of the least crazy things about black holes imo. For readers who want some quick info: https://youtu.be/t-O-Qdh7VvQ https://youtu.be/t-O-Qdh7VvQ This PBS SpaceTime video doesn’t talk about the practical uses of Kugelblitzes, but it isn’t a big leap from understanding what they are to how to get energy out of them. Hawking radiation rate increases as schwarzschild radius decreases. If you could make a properly sized black hole you could convert a little bit of mass into a lot of energy. Maybe this could be harnessed to solve all electricity demand issues. Maybe it could be used to make a room sized bomb that blows up Earth. Vote now on your phones. https://youtu.be/v3hd3AI2CAA https://youtu.be/v3hd3AI2CAA
- at_a_remove 6y ago
  I didn't say it was unbelievable, it's just ... very interesting, compared to all of the other space opera business. It implies that passing through this empty surface, this topological construct which itself need not have particles nearby (depending on how close you feel the singularity is to "nearby") makes this (and other typically conserved numbers) no longer all that important ... but only from the vantage point of an outside observer. In other words, relativity is intertwined with this conservation, as it is observer dependent.
  - willis936 6y ago
    I agree. It’s definitely very cool. It really highlights the mass energy equivalence. I’m not at a level of full intuition of relativity, but it’s one of those things that made a lot more sense once I understood mass energy equivalence. Like where the energy in exothermic fusion comes from: When the strong nuclear force overcomes the electrostatic force, the strong nuclear force increases the kinetic energy (momentum/speed) of the particles. At this point the mass of all particles from their own frames of reference is the same. Once the kinetic energy is lowered (particles are slowed back down to the same speed/temperature of their surroundings) does the mass lower.
scatters 6y ago
You don't need Hawking radiation for mass-energy conversion; simply dropping matter into a Kerr black hole gives up to 42% efficiency, and is far more controllable.

simonh 6y ago

Surely the conservation of baryon numbers is just a rule for reactions. I don’t see how crossing the event horizon changes anything. Remember from the POV of the baryon, it never crosses the event horizon. It’s not really ‘destroyed’, it just becomes unobservable to the external universe.

qubex 6y ago
True, but from the point of view of an observer at infinity, the baryon falls and comes to a halt, while the event horizon of the black hole expands and encompasses it (or would, if you could measure the diameter of the black hole with sufficient precision). This is just the essence of the information paradox, expressed in terms of baryon and lepton numbers. Either the Hawking radiation that comes out is ‘correlated’ with the matter that fell in (meaning that black holes have hair) and information is preserved, or they’re not, and information is destroyed. That’s literally the crux of the matter.
- simonh 6y ago
  That’s a good point, I wasn’t thinking about it in terms of the information paradox, but then the recently proposed solutions look like they have a good chance of panning out. https://en.wikipedia.org/wiki/Black_hole_information_paradox#Recent_developments https://en.wikipedia.org/wiki/Black_hole_information_paradox... But anyway I still don’t think this is really an issue regardless. The same could be said of the cosmic event horizon. Vast swathes of the universe are unobservable and causally disconnected from us due to the expansion of the universe separating us at faster than the speed of light. Again, no Baryons are being destroyed in this process, they’re just becoming unobservable _to_us_. https://en.wikipedia.org/wiki/Event_horizon#Cosmic_event_horizon https://en.wikipedia.org/wiki/Event_horizon#Cosmic_event_hor...
  - qubex 6y ago
    As I see it, there’s a vast cognitive chasm between the cosmic event horizon and the event horizon of a black hole, as the former preserves “continuity of locality” (my term) whereas the latter does not. Basically the the former is only an event horizon for us who are very distant from it, whereas the black hole’s horizon is an abrupt threshold in space. I don’t think it’s healthy to confuse the two.
    - wnoise 6y ago
      The locality is all equally continuous in all these cases. Someone near the black-hole horizon will see nothing unusual, and will not experience a horizon unless they are accelerating like mad to stay there. But someone doing that in flat space will see a horizon too -- see the Rindler coördinatization[1] of Minkowski space. It even has an analogous Rindler horizon, which seems to emit particles from the Unruh Effect[2]. I say seems to because particle number isn't invariant under acceleration. An outside observer will see a particle detector go off, but will interpret the accelerated detector as causing the particles, rather than the horizon. [1] https://en.wikipedia.org/wiki/Rindler_coordinates https://en.wikipedia.org/wiki/Rindler_coordinates [2] https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect
danbruc 6y ago
Remember from the POV of the baryon, it never crosses the event horizon. It’s not really ‘destroyed’, it just becomes unobservable to the external universe. I think this is backwards - for an outside observer nothing ever crosses the event horizon, from the point of view of anything falling towards the event horizon it takes a finite amount of time to cross the event horizon.
- heavenlyblue 6y ago
  How does this interact with the Hawking radiation? ie I throw something into the black hole, how much time do I wait before the Hawking radiation from that matter to manifest itself?
  - danbruc 6y ago
    From your point of view - assuming you do not fall into the black hole yourself - nothing you throw into the black hole ever crosses the event horizon so you will have wait forever. You can only see something cross the event horizon if you cross the event horizon yourself and you will see everything cross the event horizon at the moment you cross the event horizon.
    - coffeeling 6y ago
      My friend is kind of confused. Why wouldn't an observer outside of the event horizon see things falling into it? He says you can glean information from a thing that's outside of the horizon, but a thing falling into it should cease to be observable, at least by his understanding of black holes. My galaxy-sized brain understands this, of course, but my friend's pea-brain doesn't. So asking for a friend, ELI-aborted-CS-degree?
      - danbruc 6y ago
        An observer outside the event horizon has to accelerate to avoid falling into the black hole itself and in the resulting frame of reference of this observer the horizon of the black hole coincides with future infinity and therefore everything falling into the black hole only reaches the event horizon infinitely far into the future. You can watch Leonard Suskind's lecture on that [1], falling into a black hole starts at about 53 minutes, but depending on your background you may want to start earlier, maybe even several lectures earlier. Looking at those Penrose diagrams is probably the easiest way to see what happens even if it will probably still not make intuitive sense. [1] https://www.youtube.com/watch?v=BdYtfYkdGDk&list=PLpGHT1n4-mAvcXwzOIz3dHnGZaQP1LEib&index=4 https://www.youtube.com/watch?v=BdYtfYkdGDk&list=PLpGHT1n4-m...
at_a_remove 6y ago
"Just a rule for reactions" drives quite a lot of physics. Conservation rules are how to describe all of the things that do not happen: why a hammer doesn't vanish, why all of your electrons do not turn into positrons, and so forth. That's what is so fascinating ... you have this odd topological boundary and it creates a violation in known rules.

willis936 6y ago

I don’t see why it’s unbelievable. High mass energy conversion is one of the least crazy things about black holes imo. For readers who want some quick info: https://youtu.be/t-O-Qdh7VvQ https://youtu.be/t-O-Qdh7VvQ This PBS SpaceTime video doesn’t talk about the practical uses of Kugelblitzes, but it isn’t a big leap from understanding what they are to how to get energy out of them. Hawking radiation rate increases as schwarzschild radius decreases. If you could make a properly sized black hole you could convert a little bit of mass into a lot of energy. Maybe this could be harnessed to solve all electricity demand issues. Maybe it could be used to make a room sized bomb that blows up Earth. Vote now on your phones. https://youtu.be/v3hd3AI2CAA https://youtu.be/v3hd3AI2CAA

at_a_remove 6y ago
I didn't say it was unbelievable, it's just ... very interesting, compared to all of the other space opera business. It implies that passing through this empty surface, this topological construct which itself need not have particles nearby (depending on how close you feel the singularity is to "nearby") makes this (and other typically conserved numbers) no longer all that important ... but only from the vantage point of an outside observer. In other words, relativity is intertwined with this conservation, as it is observer dependent.
- willis936 6y ago
  I agree. It’s definitely very cool. It really highlights the mass energy equivalence. I’m not at a level of full intuition of relativity, but it’s one of those things that made a lot more sense once I understood mass energy equivalence. Like where the energy in exothermic fusion comes from: When the strong nuclear force overcomes the electrostatic force, the strong nuclear force increases the kinetic energy (momentum/speed) of the particles. At this point the mass of all particles from their own frames of reference is the same. Once the kinetic energy is lowered (particles are slowed back down to the same speed/temperature of their surroundings) does the mass lower.

scatters 6y ago

You don't need Hawking radiation for mass-energy conversion; simply dropping matter into a Kerr black hole gives up to 42% efficiency, and is far more controllable.