If you look back outward then the outside universe still always takes up more than half of your sky (more than 180°). Think of it this way: when inside the black hole, light is still falling in along with you. Even though the black hole is "pulling it inward", the photons can still have some sideways motion compared to you, so you can see light from the outside universe coming from around you, not just directly overhead.I meant let's say you were looking from inside the event horizon back towards the event horizon while falling towards the singularity (in other words looking backwards relative to your direction of motion), would the universe not appear to get smaller and smaller as you got further and further away from the event horizon if you were looking in that direction as you get closer and closer to the singularity?
Yes, exactly. The tidal forces are more gentle close to the horizon of a larger black hole. The "lethal" (whether to you or to a planet) range of the tidal force grows with the cube root of the mass of the black hole, but the radius of the event horizon grows much faster -- linearly with the mass.But why can the distance a planet can be from a supermassive black hole be closer to that black hole's event horizon than a smaller black hole? Is it because a much larger black hole warps space-time in a much more gentle slope compared to a smaller black hole where you see much more extreme changes to the sloping of space-time?
In reality this is unlikely because black holes -- especially supermassive ones at the centers of galaxies -- are surrounded by extremely bright accretion disks, and material is constantly dumping onto them as well as the occasional star passing too close and getting tidally disrupted. So when imagining a habitable planet orbiting near one, we must imagine it to somehow be a black hole "in isolation".
They are thought to exist in the centers of some (not necessarily all) globular clusters, but the evidence for them is more sparse and indirect than what we have for supermassive black holes. For habitability though this probably isn't much better, as there are many stars that come very close to them from time to time. Both galactic centers and globular cluster centers are, on long timescales, pretty chaotic places.
Yes, but a rocky asteroid could also explain the explosion, and for being spread over such a large area in that kind of terrain, fragments could be very difficult to find.
All things considered, it is disturbingly easy for an advanced civilization (about K2.5 or K3 - 'easy' being a relative term) to make a blackhole out of a star. All they would need to do is concentrate enough mass in the star to essentially implode it. Vast amounts of heavy elements (iron is typically proposed) being pumped into the star's convection zone by magnetic fields would be one way of doing it. Eventually, the mass of the star is too great for fusion-driven gravitational equilibrium to overcome, and the star collapses onto itself. It would a massive stellar engineering project, with many nearby solar-systems and nearby stars themselves being harvested for their heavy elements. This is not considering the logistical nightmare of porting those harvested elements to the star you want to convert.
I've never really understood the microscopic black hole proposal for the Tunguska event. Not only why did it not get noticed exiting the other side of Earth (depositing that much energy through a whole column of ocean would definitely be noticed), but why did it not leave a channel of melted or vaporized rock where it entered the ground on the Sibera side?
It is true that within the horizon all allowed paths lead to the singularity, but that's not the reason the journey looks that way. To understand it, we must compute from which directions light rays intersect the viewer, at the same time that they are also falling inward. There are two different motions to consider together.
If the effects at exit point were similar and the exit was at a really remote place (like Antarctica), perhaps it could go undetected.I've never really understood the microscopic black hole proposal for the Tunguska event. Not only why did it not get noticed exiting the other side of Earth (depositing that much energy through a whole column of ocean would definitely be noticed), but why did it not leave a channel of melted or vaporized rock where it entered the ground on the Sibera side?
This would be reasonable if the object came in exactly vertically, but a vertical entry is pretty unlikely. In general, meteorite strewn fields are ellipses along the meteor's path, which in Tunguska's case is unknown. Larger fragments tend to travel farther, and the densest population of fragments may be found very far away from the center under the explosion. Wind can also deflect the fragments -- potentially displacing them from the meteor's ground track by many kilometers.
Assuming it was detonated in the same place?
Yes not at all exotic but can you imagine what the impact would be if a Tunguska event occurred today over a major metro area? Let's say it happened over NY or London or LA or Seattle or something- wow! I heard that it was felt on seismographs as far away as London? Wat how much stronger was the Tunguska event compared to the airburst event that occurred a few years ago in Siberia (ironically we were tracking a different near earth asteroid that day and this other one came out of the blue- literally lol!) Also about the Tunguska event, weren't there many thousands of trees burnt and blown down? I've seen pictures of that- can we discern anything about the blast radius from the shape and dimensions of the area of tree damage that occurred? We do that with tornadoes too!I've never really understood the microscopic black hole proposal for the Tunguska event. Not only why did it not get noticed exiting the other side of Earth (depositing that much energy through a whole column of ocean would definitely be noticed), but why did it not leave a channel of melted or vaporized rock where it entered the ground on the Sibera side?
Having the black hole evaporate fully before reaching the ground doesn't work either, because in that case the energy released would have been unfathomably greater (several teratons TNT equivalent!). Not to mention how absurdly improbable having a black hole both intersect Earth and evaporate just above the surface would be.
Given the character and energy of the event, I think that an airbursting asteroid or comet is the most sensible explanation. These are not that uncommon over human timescales, and no more exotic explanation is necessary.
It is true that within the horizon all allowed paths lead to the singularity, but that's not the reason the journey looks that way. To understand it, we must compute from which directions light rays intersect the viewer, at the same time that they are also falling inward. There are two different motions to consider together.
I think the common misconception is that the black disk that you see is the event horizon of the black hole. It isn't. The black disk corresponds to the directions from which light rays cannot have reached you.
If you hover just above the event horizon, then only photons coming in from directly overhead can reach you, because you're rushing upward against "a waterfall of space", with the photons rushing past you almost vertically. So your view of the outside universe is compressed to a tiny disk directly overhead. For analogy imagine driving down the highway in a heavy rain. The faster you drive, the more head-on the raindrops hit your windshield.
If you instead freefall into the black hole, then photons can reach you from many directions, including somewhat beneath you. Photons that you see coming from somewhat below are not moving outwards. They were always moving inward, but also sideways, so you may be moving inward faster. Thus the net effect is that you see them coming from the side or even slightly below, and the black disk takes up less than half your sky.