4 ms·

10^17g is about 20,000 times heavier than the great pyramid of giza. Funny to imagine something as massive as a pyramid pass through your body and not noticing

10^17g is about 20,000 times heavier than the great pyramid of giza. Funny to imagine something as massive as a pyramid pass through your body and not noticing it.

kypro 2y ago
I'm trying to get some intuition for this. Is the difference that a massive objective hitting you would typically do damage via the electromagnetic force? But if you consider the gravitation force of the pyramids obviously that's extremely weak compared to the other forces on our body, so we shouldn't expect it to do damage in the same way?
- mystified5016 2y ago
  Remember that gravity falls off in strength with the square of distance. The gravitational field around such a black hole would be immensely strong and absolutely miniscule. The event horizon would be the size of a few atoms. Locally, the black hole would swallow any atoms it comes into contact with and would probably scatter nearby molecules, but that's it. It'd pass straight through you.
  - bookofjoe 2y ago
    Like a neutrino
    - isoprophlex 2y ago
      Only not weighing millions and millions of kilograms
    - adastra22 2y ago
      No a neutrino would literally leave no trace. The black hole would harm the cells in its path.
      - Tuna-Fish 2y ago
        A small enough black hole would be very unlikely to hit even one atom in one cell while going through you.
        adastra22 2y ago
        TFA is about the effects it would have moving through cells.
        Tuna-Fish 2y ago
        Because its gravity would cause a shock wave as it rapidly moved through you, not because it would absorb any material doing so.
        adastra22 2y ago
        I think you are drawing a distinction that is not what I said and not relevant.
  - perihelions 2y ago
    The gravitational field would be enormous: 1,000–500,000 times Earth gravity, at one meter's distance, for the range of PBH masses this paper mentions. That's balanced out by the relative speed being a brisk 200 km/s, so the interaction time would be short (microseconds). Still a pretty significant momentum impulse if one flew by.
    - H8crilA 2y ago
      This is analyzed in the paper. There are two sources of problems: gunshot-wound-like, and the effect of transiently highly increased gravity on nearby tissues. The first one seems worse, and is the limiting factor. PS. The paper is very easy to read :)
    - bloopernova 2y ago
      So gravity builds up over time? (Forgive the layman's attempt at understanding) If Bob is floating in space in a capsule, and a very heavy asteroid floats past, the amount that Bob's capsule is moved is dependent on the speed of the asteroid? Does that sort of thing have to be considered when planning the orbits of probes etc? Is Earth traveling fast enough, from the viewpoint of Alice floating near the sun, to have a gravity "tail" or "wake" trailing after it? (Alice near the sun thing is my attempt at a mostly static observer) And of course due to relativity, if Alice was orbiting retrograde to the big heavy object, she'd observe even more effect? I think I need about 6 more cups of tea before I can think about this!
      - sapiogram 2y ago
        > If Bob is floating in space in a capsule, and a very heavy asteroid floats past, the amount that Bob's capsule is moved is dependent on the speed of the asteroid? Yes > Does that sort of thing have to be considered when planning the orbits of probes etc? Yes, it's a primary concern when sending probes to other planets > Is Earth traveling fast enough, from the viewpoint of Alice floating near the sun, to have a gravity "tail" or "wake" trailing after it? If I understand you correctly, then classically (i.e. ignoring relativity), no. Your gravitational acceleration towards the earth depends only your distance to it. If you consider general relativity? It's... complicated.
  - Tuna-Fish 2y ago
    > Locally, the black hole would swallow any atoms it comes into contact with and would probably scatter nearby molecules More likely, it would go straight through most atoms without interacting with them. They are tiny and move very fast, if you do the math the likelihood of direct, non-gravitational interactions with matter are somewhere near neutrinos, of which trillions passed through you while reading this sentence. When one does end up eating something, the most significant effect is probably that it wouldn't eat the whole atom, but either just the nucleus, freeing all the electrons, or just an electron, leaving the atom positively charged.
    - cogman10 2y ago
      I wonder what happens to the electrons when/if they interact with the black hole. There's already weird stuff going on there with electrons being particles and waves. So, I wonder what happens when you introduce something like a black hole to the equation. Does it have a charge? I'm assuming it'd break through the electron shell; would it absorb the electron/s upon interacting with the shell?
  - mr_toad 2y ago
    > Locally, the black hole would swallow any atoms it comes into contact with Would it be capable of doing that? If the radius of the event horizon is significantly smaller than the radius of a proton, would it be capable of swallowing a proton (and how)?
- Ajedi32 2y ago
  Yes. And a 10^17g black hole would have a radius of 1.485×10^-13 meters[1], 50 million times smaller than a red blood cell[2]. [1]: https://www.wolframalpha.com/input?i=schwarzschild+radius+calculator&assumption=%7B%22F%22%2C+%22EventHorizonRadius%22%2C+%22M%22%7D+-%3E%222%C3%9710%5E17g%22 https://www.wolframalpha.com/input?i=schwarzschild+radius+ca... [2]: https://www.wolframalpha.com/input?i=%28red+blood+cell+size%29%2F%281.485%C3%9710%5E-13+meters%29 https://www.wolframalpha.com/input?i=%28red+blood+cell+size%...
- cwillu 2y ago
  That's more or less my understanding, yes.
- hnuser123456 2y ago
  >Is the difference that a massive objective hitting you would typically do damage via the electromagnetic force? Yes. PBHs, if they exist, have been constrained to asteroid mass ranges, and at the 10^17g the author concludes would be harmful, their radius would be 0.15 femtometers, or about 1/5 the radius of a proton. Combine that with galactic orbital speeds, and it's just too small and too fast to leave a mark, unless the mass is multiple mountains worth.
kamaal 2y ago
How can some thing this big pass through one's body without causing considerable damage, let alone one not notice it?
- gostsamo 2y ago
  high speed means that their is no momentum spread through your tissues. the damage will be the diameter of the thing and this is rather small diameter.
- neallindsay 2y ago
  It's extremely small, just very massive.
- thrance 2y ago
  Because a black hole of that mass would be tiny (smaller than a micrometer according to the article), and gravity is a very weak force. It would pass through you like through butter and bore a tiny tunnel, but it's so small it would probably leave you unharmed.
  - scarmig 2y ago
    > It would pass through you like through butter I know this is just an idiom, but it got me to thinking about just how much easier it would be for the PBH pass through you than a knife through butter. Some back of the envelope calculation gets me 40 orders of magnitude. If you compare the ease of a knife through diamond to a knife through butter, even that difference is many orders of magnitude less than a PBH through the human body.
- mytailorisrich 2y ago
  We're mostly empty space so anything small enough and not charged can pretty much just fly through us. If it does not hit anything then mass does not really matter as long as small enough not to have big gravitional effect.
- rachofsunshine 2y ago
  Because it isn't big. It's massive. Those are not the same thing. Our everyday intuitions deal with solids with densities within a couple of orders of magnitude (from very light foamy solids at perhaps 0.1 g/cm^3 to very dense metals around 20 g/cm^3), so the two can never be that far apart in everyday experience. But small black holes are many, many, many, many orders of magnitude denser than that. When a big solid object (where "big" is "macroscopic") strikes your body at a low speed (where "low" is "a few km/s"), it interacts with the atoms in your body. It applies pressure to your body, primarily the degeneracy pressure[1] that pushes back when electron clouds push up against one another. The interaction tine is long (on the order of milliseconds to seconds), so not only is the applied force high, the force has time to do its work. The push overcomes the mechanical strength of the structures in your body, like the walls of your blood vessels or the membranes of your cells, shattering them and causing the secondary damage of bleeding, organ dysfunction, inflammation, vulnerability to infection, etc. When our black hole here passes through you, though, it is both extremely small (smaller than an atom, if by "size" we mean its event horizon) and moving extremely fast (about ten times Earth's orbital velocity, or about a hundred times faster than a bullet). It's too small to directly "eat" your tissues, and it isn't "pushing them out of the way" by much, either. It interacts with your body by tugging on it. That tugging would be enough to tear your body's structures apart if it were sustained (the tidal forces here are very extreme), but the hole is moving so quickly that while the force is very large, the impulse (force times time) is not. It does devastating damage along the very narrow corridor where it's munching up an atom or three as it goes and where it's applying ultra-extreme forces that matter even over these millisecond timescales, but the corridor of damage is so narrow that it doesn't disrupt the function of your body. (The paper establishes the mass cutoff where this would no longer be so, and where the gravitational shockwave would indeed be enough to start tearing at your body's structures.) It's kind of like how you can snuff out a candle with your fingers, even though a typical candle flame is not much cooler than the surface of the Sun. The heat flux from the flame to your skin is extreme, but it's applied for such a short period of time that the total energy delivery is tiny and does not deal meaningful damage to the skin. [1] Electrostatic repulsion plays a role, but degeneracy pressure is the primary thing that makes matter take up space.
IsTom 2y ago
Gravitationally maybe, but you'd certainly notice the 9 kW of hawking radiation.
- simiones 2y ago
  I believe black holes are not currently emitting any Hawking radiation, because the CMB is far too hot for now. They will start emitting Hawking radiation in the far future, after the Universe has cooled down significantly.
  - IsTom 2y ago
    Hawking radiation depends on mass of the BH, the smaller the hole the hotter it is.
  - sapiogram 2y ago
    No, the formula for the power of a black hole's Hawking Radiation is very straightforward, and it only depends on the black hole's mass.
    - ithkuil 2y ago
      I think the idea is that black holes can never shrink to a size small enough to emit a hawking radiation hotter than CMB as long as it receives photons from the CMB that that will balance the mass/energy of the black hole and keep it in equilibrium. This obviously is not a problem if you create a new very small black hole from scratch as it only applies to pre-existing black holes that are massive enough so that their hawking radiation temperature is lower than the CMB
  - eigenspace 2y ago
    That's nice that you believe that, but your belief is wrong (for sufficiently small black holes)
  - rachofsunshine 2y ago
    Black holes emit Hawking radiation regardless of ambient temperature. It's just that right now, the mass(-energy) of photons falling into the hole from the CMBR is outpacing the mass loss to Hawking radiation for large black holes, so large black holes aren't currently evaporating from their own Hawking radiation. It's like standing next to a campfire. Your body is still emitting thermal radiation in the IR, it's just that you're receiving more thermal radiation from the fire than you're putting out.

10^17g is about 20,000 times heavier than the great pyramid of giza. Funny to imagine something as massive as a pyramid pass through your body and not noticing it.

kypro 2y ago

I'm trying to get some intuition for this. Is the difference that a massive objective hitting you would typically do damage via the electromagnetic force? But if you consider the gravitation force of the pyramids obviously that's extremely weak compared to the other forces on our body, so we shouldn't expect it to do damage in the same way?

mystified5016 2y ago
Remember that gravity falls off in strength with the square of distance. The gravitational field around such a black hole would be immensely strong and absolutely miniscule. The event horizon would be the size of a few atoms. Locally, the black hole would swallow any atoms it comes into contact with and would probably scatter nearby molecules, but that's it. It'd pass straight through you.
- bookofjoe 2y ago
  Like a neutrino
  - isoprophlex 2y ago
    Only not weighing millions and millions of kilograms
  - adastra22 2y ago
    No a neutrino would literally leave no trace. The black hole would harm the cells in its path.
    - Tuna-Fish 2y ago
      A small enough black hole would be very unlikely to hit even one atom in one cell while going through you.
      - adastra22 2y ago
        TFA is about the effects it would have moving through cells.
        Tuna-Fish 2y ago
        Because its gravity would cause a shock wave as it rapidly moved through you, not because it would absorb any material doing so.
        adastra22 2y ago
        I think you are drawing a distinction that is not what I said and not relevant.
- perihelions 2y ago
  The gravitational field would be enormous: 1,000–500,000 times Earth gravity, at one meter's distance, for the range of PBH masses this paper mentions. That's balanced out by the relative speed being a brisk 200 km/s, so the interaction time would be short (microseconds). Still a pretty significant momentum impulse if one flew by.
  - H8crilA 2y ago
    This is analyzed in the paper. There are two sources of problems: gunshot-wound-like, and the effect of transiently highly increased gravity on nearby tissues. The first one seems worse, and is the limiting factor. PS. The paper is very easy to read :)
  - bloopernova 2y ago
    So gravity builds up over time? (Forgive the layman's attempt at understanding) If Bob is floating in space in a capsule, and a very heavy asteroid floats past, the amount that Bob's capsule is moved is dependent on the speed of the asteroid? Does that sort of thing have to be considered when planning the orbits of probes etc? Is Earth traveling fast enough, from the viewpoint of Alice floating near the sun, to have a gravity "tail" or "wake" trailing after it? (Alice near the sun thing is my attempt at a mostly static observer) And of course due to relativity, if Alice was orbiting retrograde to the big heavy object, she'd observe even more effect? I think I need about 6 more cups of tea before I can think about this!
    - sapiogram 2y ago
      > If Bob is floating in space in a capsule, and a very heavy asteroid floats past, the amount that Bob's capsule is moved is dependent on the speed of the asteroid? Yes > Does that sort of thing have to be considered when planning the orbits of probes etc? Yes, it's a primary concern when sending probes to other planets > Is Earth traveling fast enough, from the viewpoint of Alice floating near the sun, to have a gravity "tail" or "wake" trailing after it? If I understand you correctly, then classically (i.e. ignoring relativity), no. Your gravitational acceleration towards the earth depends only your distance to it. If you consider general relativity? It's... complicated.
- Tuna-Fish 2y ago
  > Locally, the black hole would swallow any atoms it comes into contact with and would probably scatter nearby molecules More likely, it would go straight through most atoms without interacting with them. They are tiny and move very fast, if you do the math the likelihood of direct, non-gravitational interactions with matter are somewhere near neutrinos, of which trillions passed through you while reading this sentence. When one does end up eating something, the most significant effect is probably that it wouldn't eat the whole atom, but either just the nucleus, freeing all the electrons, or just an electron, leaving the atom positively charged.
  - cogman10 2y ago
    I wonder what happens to the electrons when/if they interact with the black hole. There's already weird stuff going on there with electrons being particles and waves. So, I wonder what happens when you introduce something like a black hole to the equation. Does it have a charge? I'm assuming it'd break through the electron shell; would it absorb the electron/s upon interacting with the shell?
- mr_toad 2y ago
  > Locally, the black hole would swallow any atoms it comes into contact with Would it be capable of doing that? If the radius of the event horizon is significantly smaller than the radius of a proton, would it be capable of swallowing a proton (and how)?
Ajedi32 2y ago
Yes. And a 10^17g black hole would have a radius of 1.485×10^-13 meters[1], 50 million times smaller than a red blood cell[2]. [1]: https://www.wolframalpha.com/input?i=schwarzschild+radius+calculator&assumption=%7B%22F%22%2C+%22EventHorizonRadius%22%2C+%22M%22%7D+-%3E%222%C3%9710%5E17g%22 https://www.wolframalpha.com/input?i=schwarzschild+radius+ca... [2]: https://www.wolframalpha.com/input?i=%28red+blood+cell+size%29%2F%281.485%C3%9710%5E-13+meters%29 https://www.wolframalpha.com/input?i=%28red+blood+cell+size%...
cwillu 2y ago
That's more or less my understanding, yes.
hnuser123456 2y ago
>Is the difference that a massive objective hitting you would typically do damage via the electromagnetic force? Yes. PBHs, if they exist, have been constrained to asteroid mass ranges, and at the 10^17g the author concludes would be harmful, their radius would be 0.15 femtometers, or about 1/5 the radius of a proton. Combine that with galactic orbital speeds, and it's just too small and too fast to leave a mark, unless the mass is multiple mountains worth.

kamaal 2y ago

How can some thing this big pass through one's body without causing considerable damage, let alone one not notice it?

gostsamo 2y ago
high speed means that their is no momentum spread through your tissues. the damage will be the diameter of the thing and this is rather small diameter.
neallindsay 2y ago
It's extremely small, just very massive.
thrance 2y ago
Because a black hole of that mass would be tiny (smaller than a micrometer according to the article), and gravity is a very weak force. It would pass through you like through butter and bore a tiny tunnel, but it's so small it would probably leave you unharmed.
- scarmig 2y ago
  > It would pass through you like through butter I know this is just an idiom, but it got me to thinking about just how much easier it would be for the PBH pass through you than a knife through butter. Some back of the envelope calculation gets me 40 orders of magnitude. If you compare the ease of a knife through diamond to a knife through butter, even that difference is many orders of magnitude less than a PBH through the human body.
mytailorisrich 2y ago
We're mostly empty space so anything small enough and not charged can pretty much just fly through us. If it does not hit anything then mass does not really matter as long as small enough not to have big gravitional effect.
rachofsunshine 2y ago
Because it isn't big. It's massive. Those are not the same thing. Our everyday intuitions deal with solids with densities within a couple of orders of magnitude (from very light foamy solids at perhaps 0.1 g/cm^3 to very dense metals around 20 g/cm^3), so the two can never be that far apart in everyday experience. But small black holes are many, many, many, many orders of magnitude denser than that. When a big solid object (where "big" is "macroscopic") strikes your body at a low speed (where "low" is "a few km/s"), it interacts with the atoms in your body. It applies pressure to your body, primarily the degeneracy pressure[1] that pushes back when electron clouds push up against one another. The interaction tine is long (on the order of milliseconds to seconds), so not only is the applied force high, the force has time to do its work. The push overcomes the mechanical strength of the structures in your body, like the walls of your blood vessels or the membranes of your cells, shattering them and causing the secondary damage of bleeding, organ dysfunction, inflammation, vulnerability to infection, etc. When our black hole here passes through you, though, it is both extremely small (smaller than an atom, if by "size" we mean its event horizon) and moving extremely fast (about ten times Earth's orbital velocity, or about a hundred times faster than a bullet). It's too small to directly "eat" your tissues, and it isn't "pushing them out of the way" by much, either. It interacts with your body by tugging on it. That tugging would be enough to tear your body's structures apart if it were sustained (the tidal forces here are very extreme), but the hole is moving so quickly that while the force is very large, the impulse (force times time) is not. It does devastating damage along the very narrow corridor where it's munching up an atom or three as it goes and where it's applying ultra-extreme forces that matter even over these millisecond timescales, but the corridor of damage is so narrow that it doesn't disrupt the function of your body. (The paper establishes the mass cutoff where this would no longer be so, and where the gravitational shockwave would indeed be enough to start tearing at your body's structures.) It's kind of like how you can snuff out a candle with your fingers, even though a typical candle flame is not much cooler than the surface of the Sun. The heat flux from the flame to your skin is extreme, but it's applied for such a short period of time that the total energy delivery is tiny and does not deal meaningful damage to the skin. [1] Electrostatic repulsion plays a role, but degeneracy pressure is the primary thing that makes matter take up space.

IsTom 2y ago

Gravitationally maybe, but you'd certainly notice the 9 kW of hawking radiation.

simiones 2y ago
I believe black holes are not currently emitting any Hawking radiation, because the CMB is far too hot for now. They will start emitting Hawking radiation in the far future, after the Universe has cooled down significantly.
- IsTom 2y ago
  Hawking radiation depends on mass of the BH, the smaller the hole the hotter it is.
- sapiogram 2y ago
  No, the formula for the power of a black hole's Hawking Radiation is very straightforward, and it only depends on the black hole's mass.
  - ithkuil 2y ago
    I think the idea is that black holes can never shrink to a size small enough to emit a hawking radiation hotter than CMB as long as it receives photons from the CMB that that will balance the mass/energy of the black hole and keep it in equilibrium. This obviously is not a problem if you create a new very small black hole from scratch as it only applies to pre-existing black holes that are massive enough so that their hawking radiation temperature is lower than the CMB
- eigenspace 2y ago
  That's nice that you believe that, but your belief is wrong (for sufficiently small black holes)
- rachofsunshine 2y ago
  Black holes emit Hawking radiation regardless of ambient temperature. It's just that right now, the mass(-energy) of photons falling into the hole from the CMBR is outpacing the mass loss to Hawking radiation for large black holes, so large black holes aren't currently evaporating from their own Hawking radiation. It's like standing next to a campfire. Your body is still emitting thermal radiation in the IR, it's just that you're receiving more thermal radiation from the fire than you're putting out.