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JackDole
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12 Jan 2019 13:52

Can the Tunguska event not simply be explained by a comet? A comet consists mainly of ice and may have been completely vaporized during the explosion.
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Watsisname
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12 Jan 2019 15:13

A-L-E-X wrote:
Source of the post 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?

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.

This is really non-intuitive.  If you fall into a black hole, the black disk always appears to be beneath you, even when you're deep inside the horizon.

A-L-E-X wrote:
Source of the post 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?

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.

A-L-E-X wrote:
Source of the post Also could a planet near the center of the galaxy, that close to a supermassive black hole actually be habitable?

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".  

Perhaps in the distant future when the cores of galaxies have settled down, an extremely advanced civilization could find a suitable supermassive black hole and put a planet in orbit around it, constructing a habitable system for themselves.  Especially if the black hole is spinning rapidly (as most do), it could make for an excellent energy source, with far greater lifespan and amount of energy to be harvested than any Sun.

A-L-E-X wrote:
Source of the post I think we've also discovered intermediate mass black holes, at the center of globular clusters, haven't we?

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.
 
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Watsisname
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12 Jan 2019 15:58

JackDole wrote:
Source of the post Can the Tunguska event not simply be explained by a comet? A comet consists mainly of ice and may have been completely vaporized during the explosion.

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.

Whether the Tunguska event was caused by an asteroid or a comet has long been a matter of debate.  The cometary explanation took off after 1930 when a British meteorologist proposed that a small comet could explain the lack of a crater and meteorites.  But this view was later challenged.  A 1983 paper argued that a cometary fragment is not consistent with Tunguska's energy and altitude of the explosion, since cometary material should have been decelerated and disintegrated in the atmosphere more quickly.  It concluded that a more massive comet (with more energy released in the explosion) would have been needed to survive to that altitude, or a denser and more robust object like a rocky asteroid.  However, the energy of the event is itself somewhat uncertain, as different methods of estimating it lead to conflicting values.

More recently there has been extensive work for modelling the effects of atmospheric entry of objects with different compositions, velocities, entry angles, and so forth.  These actually have not helped settle the debate too much, but rather show that either a 50-60m stony asteroid or a 80-100m comet could explain the event.

Simulations have also shown that an airbursting asteroid easily explains the lack of material found on the ground, since the atmospheric passage leaves a channel of low pressure, which then pulls most of the exploded material back upward.  We can also see this effect in these simulations of ocean impacts:



So when the Tunguska remnants (whatever they are) finally made it to ground, they may be spread over an even larger area than initially expected from the explosion altitude.
 
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13 Jan 2019 01:59

Interesting!  I had always thought it was a comet myself but there seems to be some evidence of an asteroid also.  I agree with Mid that if it had been a micro black hole, there should have been an exit point also (but if the exit point was an ocean who knows what the effect may have been- a megatsunami?)

I like the idea of an advanced civilization making a planet near a black hole habitable and harvesting it for energy.  Perhaps it would also be able to actually convert a star into a black hole for this purpose?

Wat, does the black disk always appear to be beneath you because once inside the event horizon, all lines lead to the singularity?  The gravity makes it so that there is no other direction any of them can go.
 
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13 Jan 2019 02:16

A-L-E-X wrote:
Source of the post Perhaps it would also be able to actually convert a star into a black hole for this purpose?

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.
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A-L-E-X
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13 Jan 2019 02:43

I already have two target stars, eta carinae and R136a1.  I wonder how good either of these would be for conversion and as future sources of energy?
 
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13 Jan 2019 04:14

A-L-E-X wrote:
Source of the post I agree with Mid that if it had been a micro black hole, there should have been an exit point also (but if the exit point was an ocean who knows what the effect may have been- a megatsunami?)

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. :)


A-L-E-X wrote:
Source of the post Wat, does the black disk always appear to be beneath you because once inside the event horizon, all lines lead to the singularity?  The gravity makes it so that there is no other direction any of them can go.

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.
 
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13 Jan 2019 13:31

Watsisname wrote:
Source of the post 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?

If the effects at exit point were similar and the exit was at a really remote place (like Antarctica), perhaps it could go undetected.
It could be an interesting exercise to study what the effects on Earth would be if a black hole passed through.  It would be such an exotic event and there could be surprises.  In any case, Tunguska is sufficiently explained by a comet coming in at the right speed and angle.
It's also hard to rule out that there aren't meteorites in Tunguska.  Most people would perhaps imagine a crater in the centre of the blast zone, but that's very unlikely in any case.  The entry angle could have been shallow and there could be thousands and thousands of small meteorites scattered over a large area quite far from the centre, possibly even outside the flattened area.  Without knowing anything about the track, the search area would be something like 10,000 km², and to find meteorites, most of which could just be a few grammes, is near impossible after that many years even if there are 100,000 of them.  And even if there are one or more 1 tonne pieces, even those would be unrealistic to find in such a large and remote area.
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JackDole
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13 Jan 2019 14:57

Of course, the majority of meteorite chunks would have to be distributed over a very large radius. But if one assumes that the explosion was approximately spherical, also chunks would have to be found in the center under the explosion. If the explosion was not quasi disk-shaped.
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13 Jan 2019 19:25

JackDole wrote:
Source of the post But if one assumes that the explosion was approximately spherical, also chunks would have to be found in the center under the explosion.

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.

Example: The strewn field from the Chelyabinsk meteor

With so many unknowns for the Tunguska event's size, composition, velocity, entry angle, etc, we really know very little about what the distribution of fragments on the ground might be.  The best way to figure out what it looks like would be to actually find and map it out, but as Midtskogen said, this is enormously impractical for such a wide area in that terrain and after so many years.

There was a paper from 2013 claiming that microscopic study of material found in peat near the Tunguska event show evidence of meteoric origin and that the parent body was actually an iron meteorite and not a comet.  I'm not sure if this evidence is entirely convincing, however.
 
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13 Jan 2019 22:18

A single airburst, which would appear pretty spherical, doesn't stop the object and meteorites drizzle down at terminal velocity at that point.  Meteorites, unless their weight is in the tonnes, will lose their cosmic velocity and basically freefall before reaching ground, but if the airburst is above the troposphere, the ground speed can still be very high (many times the speed of sound).  Only in the lower troposphere can we expect meteorites to fall more or less straight down, only carried by the wind.
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14 Jan 2019 00:04

Speaking of several teratons...

Whats the nitty gritty of what would have happend if Tsar Bomba had made a 7 teraton explosion?
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14 Jan 2019 01:28

PlutonianEmpire wrote:
Source of the post Whats the nitty gritty of what would have happend if Tsar Bomba had made a 7 teraton explosion?

Assuming it was detonated in the same place?

Neglecting the radiation (which I'm not sure how to handle the details of, aside from the fallout probably being a big problem wherever the wind takes it), the closest analogy to this situation would be the impact of an asteroid a few kilometers in diameter.  For example, an asteroid 4.5km in diameter coming in at 20km/s and having 3g/cm3 density would have the same kinetic energy.  Impacts of that magnitude happen on average about once every 20 million years or so.  So this implies it would be pretty devastating to a huge area, but not quite the same mass-extinction-causing magnitude as the KT impact.

To get an idea of the immediate local effects, we can use the Earth Impact Effects online calculator.  This suggests the thermal radiation from the fireball would be lethal to over 700km from the detonation.  Everything on Novaya Zemlya would be incinerated.  The airblast would shatter windows as far as Moskow, 2000km away, and the sound heard all the way around the world.  

I must caution however that there may be some important differences between how the energy of a nuclear weapon is released vs. a large asteroid impact, particularly in terms of what fraction of it goes to the atmosphere (fireball and pressure wave) versus the ground (seismic shaking).  I don't know how that would work out for a nuclear explosion this large.  But this is perhaps a good first estimate.
 
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14 Jan 2019 02:58

Watsisname wrote:
A-L-E-X wrote:
Source of the post I agree with Mid that if it had been a micro black hole, there should have been an exit point also (but if the exit point was an ocean who knows what the effect may have been- a megatsunami?)

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. :)


A-L-E-X wrote:
Source of the post Wat, does the black disk always appear to be beneath you because once inside the event horizon, all lines lead to the singularity?  The gravity makes it so that there is no other direction any of them can go.

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.

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!

Ah, I see how it would be different inside the event horizon now and how photons could appear to be coming upward if I was falling faster than they are- that's like when an outer planet is in retrograde- it's still moving in the same direction but ours is moving faster and has just passed it so it appears to be moving backwards!  If I were in free fall inside the black hole does that mean I could actually be moving faster than the photons?  Does that mean my mass would be converted into light?
 
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14 Jan 2019 19:52

Y'all have any thoughts on the recent FRBs detected this month?
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