6 ms·

As surrounding matter falls into the black hole, it heats up and emits radiation. There's also Hawking radiation, but that's not detectable.

alphydan 11y ago
To be slightly more specific: Astronomers think there is a black hole when you see gas moving very very fast towards an invisible and massive object. That gas heats up from the friction of its particles moving rapidly towards the black hole. This heated gas emits X or Gamma radiation when it travels that fast. However, once it goes past the event horizon nothing else is seen. So you have something very heavy which is attracting objects and accelerating the surrounding gas. At the very "edge", sometimes pairs of particles are formed out of the vacuum (for example an electron and an anti-electron (positron)). Hawking asked: What if one particle leaves away from the horizon, but the other one leaves towards the center of the black hole? We would see an object that is sending particles from its event horizon. So in some sense particles would escape from the edge of the black hole. That's Hawking radiation (in simplified terms).
- pflanze 11y ago
  Hmm, so there is this thing where physical theories seem to predict that each piece of matter should be complemented with an according piece of anti-matter (because those then make themselves "whole" by turning into pure energy), but we don't know where the anti-matter is. Your description made me think, could it all be in black holes?
  - Natanael_L 11y ago
    Antimatter is still regular matter, but with the exact opposite charge to the common particles we keep seeing. So technically yes, it could all have disappeared into black holes, but it wouldn't really be antimatter anymore after entering the singularity due to being effectively broken down into pure energy/matter.
    - danbruc 11y ago
      I don't think many - if any - physicist believe that there is a real singularity within a black hole. The existence of singularities in the solutions of our equations more likely just tells us that the equations are just good approximations that break down at the center of black holes.
    - lisivka 11y ago
      What will happen when black hole will merge with antimatter black hole of same mass?
      - Natanael_L 11y ago
        It is no longer antimatter after entering the black hole (edit: once reaching the presumed singularity, but either way you still won't see any difference from the outside). The mass is the same, only the charge is different. They'd merge normally. You might be thinking of negative matter, in which case they'd effectively cancel out, erase each other.
        danbruc 11y ago
        Nothing strange happens when something falls into a black hole. We could right now be beyond the event horizon of a newly forming black hole. Unless a black hole is particularly small, in which case there will be notable tidal forces near the event horizon, the event horizon of a black hole is a pretty ordinary place hardly distinguishable from flat space. So nothing happens to matter or antimatter when it falls into a black hole.
        raattgift 11y ago
        You're essentially right on the "no drama" conjecture. However, once through the horizon you switch from the exterior to the interior solution to the EFEs, and find yourself inevitably and very quickly (in proper time) colliding with the singularity or whatever is at the centre of a black hole. You will notice very quickly (seconds to minutes by your wristwatch) that you are inside the event horizon of even a 10^10 M_sun SMBH. That's one reason we can confidently predict we are not inside a black hole with a radius of approximately the Hubble length; we can't use an interior black hole metric to describe our patch of spacetime. If anything, because of the early hot dense universe, we look a bit more like a time-reversal of a interior black hole solution, but even that falls to pieces on inspection. Additionally, unless you believe that the universe is really really different a light year outside the Hubble volume, we also can't use an exterior black hole solution to describe that. (And if you decided for the sake of argument that physics just outside the observable universe is very different, the "outside" metric would still see an expanding surface, which is very not-blackhole-like. The "outside" observers should also be able to infer the spatial flatness of our observable universe; space is decidedly not flat inside a black hole, otherwise we wouldn't have the problem of the infinite density at the singularity.) Unfortunately GR and QFT make different and conflicting predictions about what happens to a positron and electron tossed into a black hole once they reach the region where curvature is on the scale of their Compton length.
  - danbruc 11y ago
    It sounds a bit like you think there should be one antimatter particle for each matter particle in the universe and we don't know where all the antimatter is but that is not the case. There is just a lot more matter than antimatter in the (observable) universe and we have no good explanation how this came to be because there is no obvious asymmetry in the equations and one would then naturally expect equal amounts of matter and antimatter in the universe. So we don't know why we see so little antimatter, but we don't really think that it exists and is hiding somewhere, we think it never came into existence. And that is a good thing, otherwise all matter could just have turned into radiation and we couldn't exist.
- ars 11y ago
  Can you explain the friction? To have friction you have to have two things moving at different speeds rubbing on each other. But infalling gas all moves in the same direction.
  - maaku 11y ago
    (1) Gas is composed of particles that are not moving in the same direction. (2) Electromagnetic waves come from the movement of an electric charge. Even a single molecule falling into an event horizon is a moving charge (plasma) or dipole (gas).
    - deleted 11y ago
      [deleted]
  - semi-extrinsic 11y ago
    The infalling gas will, in general, be rotating like water around a drain. The rotation leads to a shear (technical word for fluid rubbing against itself), and since the gas has non-zero viscosity (technical word for fluid friction coefficient), shear leads to heat production. In the purely non-rotating case, there is another source of heat as well: compression. As gas from far away moves closer to the black hole, the distance between gas molecules decreases, and the pressure increases. This causes the temperature to increase. Just feel the outside of a bicycle pump after quickly inflating two full tires.
  - raattgift 11y ago
    It's not really so much friction as collision, which in post-classical terms is objects coming close enough together for (mainly electromagnetic) quantum interactions to dominate. The objects around a black hole are on different intersecting geodesics; "collisions" result in energy exchanges that boost the particles onto different geodesics, typically that wind closer to the horizon. Near the horizon inverse Compton scattering becomes important, too: high energy photons emitted by matter-matter collisions end up colliding with charged massive particles with a transfer of momentum from the latter to the photon. The latter's change of momentum leaves it on a geodesic still closer to the horizon, while the photon is boosted to high energy (perhaps escaping to infinity as an X-Ray).
- sullyj3 11y ago
  I don't understand hawking radiation. People say it would eventually evaporate the black hole, but wouldn't the particle falling in increase its mass, whilst the emitted anti-particle would have no effect?
  - Retric 11y ago
    First it only applies to rotating black holes. Second it's particle and virtual particle not anti particle.
    - danbruc 11y ago
      I don't think that this is true, non-rotating black holes can also emit Hawking radiation. Besides that, there are probably no non-rotating black holes, having an angular momentum of (exactly) zero is a pretty improbable situation and a single particle falling not exactly radially into a non-rotating black hole would turn it into a rotating black hole albeit with a really tiny angular momentum.
      - Retric 11y ago
        I don't see why it would not work on stationary black holes, but I keep seeing it in reference to rotating black holes. Though, as you say all black holes rotate.
        danbruc 11y ago
        According to Wikipedia Hawking started his work after another physicist pointed out that rotating black holes should emit radiation. Maybe that stuck somehow. Hawking's work followed his visit to Moscow in 1973 where the Soviet scientists Yakov Zeldovich and Alexei Starobinsky showed him that, according to the quantum mechanical uncertainty principle, rotating black holes should create and emit particles.
        Retric 11y ago
        Ahh, ok thanks that's the connection I was missing.

alphydan 11y ago

To be slightly more specific: Astronomers think there is a black hole when you see gas moving very very fast towards an invisible and massive object. That gas heats up from the friction of its particles moving rapidly towards the black hole. This heated gas emits X or Gamma radiation when it travels that fast. However, once it goes past the event horizon nothing else is seen. So you have something very heavy which is attracting objects and accelerating the surrounding gas. At the very "edge", sometimes pairs of particles are formed out of the vacuum (for example an electron and an anti-electron (positron)). Hawking asked: What if one particle leaves away from the horizon, but the other one leaves towards the center of the black hole? We would see an object that is sending particles from its event horizon. So in some sense particles would escape from the edge of the black hole. That's Hawking radiation (in simplified terms).

pflanze 11y ago
Hmm, so there is this thing where physical theories seem to predict that each piece of matter should be complemented with an according piece of anti-matter (because those then make themselves "whole" by turning into pure energy), but we don't know where the anti-matter is. Your description made me think, could it all be in black holes?
- Natanael_L 11y ago
  Antimatter is still regular matter, but with the exact opposite charge to the common particles we keep seeing. So technically yes, it could all have disappeared into black holes, but it wouldn't really be antimatter anymore after entering the singularity due to being effectively broken down into pure energy/matter.
  - danbruc 11y ago
    I don't think many - if any - physicist believe that there is a real singularity within a black hole. The existence of singularities in the solutions of our equations more likely just tells us that the equations are just good approximations that break down at the center of black holes.
  - lisivka 11y ago
    What will happen when black hole will merge with antimatter black hole of same mass?
    - Natanael_L 11y ago
      It is no longer antimatter after entering the black hole (edit: once reaching the presumed singularity, but either way you still won't see any difference from the outside). The mass is the same, only the charge is different. They'd merge normally. You might be thinking of negative matter, in which case they'd effectively cancel out, erase each other.
      - danbruc 11y ago
        Nothing strange happens when something falls into a black hole. We could right now be beyond the event horizon of a newly forming black hole. Unless a black hole is particularly small, in which case there will be notable tidal forces near the event horizon, the event horizon of a black hole is a pretty ordinary place hardly distinguishable from flat space. So nothing happens to matter or antimatter when it falls into a black hole.
        raattgift 11y ago
        You're essentially right on the "no drama" conjecture. However, once through the horizon you switch from the exterior to the interior solution to the EFEs, and find yourself inevitably and very quickly (in proper time) colliding with the singularity or whatever is at the centre of a black hole. You will notice very quickly (seconds to minutes by your wristwatch) that you are inside the event horizon of even a 10^10 M_sun SMBH. That's one reason we can confidently predict we are not inside a black hole with a radius of approximately the Hubble length; we can't use an interior black hole metric to describe our patch of spacetime. If anything, because of the early hot dense universe, we look a bit more like a time-reversal of a interior black hole solution, but even that falls to pieces on inspection. Additionally, unless you believe that the universe is really really different a light year outside the Hubble volume, we also can't use an exterior black hole solution to describe that. (And if you decided for the sake of argument that physics just outside the observable universe is very different, the "outside" metric would still see an expanding surface, which is very not-blackhole-like. The "outside" observers should also be able to infer the spatial flatness of our observable universe; space is decidedly not flat inside a black hole, otherwise we wouldn't have the problem of the infinite density at the singularity.) Unfortunately GR and QFT make different and conflicting predictions about what happens to a positron and electron tossed into a black hole once they reach the region where curvature is on the scale of their Compton length.
- danbruc 11y ago
  It sounds a bit like you think there should be one antimatter particle for each matter particle in the universe and we don't know where all the antimatter is but that is not the case. There is just a lot more matter than antimatter in the (observable) universe and we have no good explanation how this came to be because there is no obvious asymmetry in the equations and one would then naturally expect equal amounts of matter and antimatter in the universe. So we don't know why we see so little antimatter, but we don't really think that it exists and is hiding somewhere, we think it never came into existence. And that is a good thing, otherwise all matter could just have turned into radiation and we couldn't exist.
ars 11y ago
Can you explain the friction? To have friction you have to have two things moving at different speeds rubbing on each other. But infalling gas all moves in the same direction.
- maaku 11y ago
  (1) Gas is composed of particles that are not moving in the same direction. (2) Electromagnetic waves come from the movement of an electric charge. Even a single molecule falling into an event horizon is a moving charge (plasma) or dipole (gas).
  - deleted 11y ago
    [deleted]
- semi-extrinsic 11y ago
  The infalling gas will, in general, be rotating like water around a drain. The rotation leads to a shear (technical word for fluid rubbing against itself), and since the gas has non-zero viscosity (technical word for fluid friction coefficient), shear leads to heat production. In the purely non-rotating case, there is another source of heat as well: compression. As gas from far away moves closer to the black hole, the distance between gas molecules decreases, and the pressure increases. This causes the temperature to increase. Just feel the outside of a bicycle pump after quickly inflating two full tires.
- raattgift 11y ago
  It's not really so much friction as collision, which in post-classical terms is objects coming close enough together for (mainly electromagnetic) quantum interactions to dominate. The objects around a black hole are on different intersecting geodesics; "collisions" result in energy exchanges that boost the particles onto different geodesics, typically that wind closer to the horizon. Near the horizon inverse Compton scattering becomes important, too: high energy photons emitted by matter-matter collisions end up colliding with charged massive particles with a transfer of momentum from the latter to the photon. The latter's change of momentum leaves it on a geodesic still closer to the horizon, while the photon is boosted to high energy (perhaps escaping to infinity as an X-Ray).
sullyj3 11y ago
I don't understand hawking radiation. People say it would eventually evaporate the black hole, but wouldn't the particle falling in increase its mass, whilst the emitted anti-particle would have no effect?
- Retric 11y ago
  First it only applies to rotating black holes. Second it's particle and virtual particle not anti particle.
  - danbruc 11y ago
    I don't think that this is true, non-rotating black holes can also emit Hawking radiation. Besides that, there are probably no non-rotating black holes, having an angular momentum of (exactly) zero is a pretty improbable situation and a single particle falling not exactly radially into a non-rotating black hole would turn it into a rotating black hole albeit with a really tiny angular momentum.
    - Retric 11y ago
      I don't see why it would not work on stationary black holes, but I keep seeing it in reference to rotating black holes. Though, as you say all black holes rotate.
      - danbruc 11y ago
        According to Wikipedia Hawking started his work after another physicist pointed out that rotating black holes should emit radiation. Maybe that stuck somehow. Hawking's work followed his visit to Moscow in 1973 where the Soviet scientists Yakov Zeldovich and Alexei Starobinsky showed him that, according to the quantum mechanical uncertainty principle, rotating black holes should create and emit particles.
        Retric 11y ago
        Ahh, ok thanks that's the connection I was missing.