[HN Gopher] Math proof draws new boundaries around black hole fo...
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Math proof draws new boundaries around black hole formation
Author : EA-3167
Score : 91 points
Date : 2023-08-16 14:54 UTC (8 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| raydiatian wrote:
| Here's a fascinating question:
|
| If an LHC collision were to form a blackhole,
|
| 1. How long would it last
|
| 2. Could we detect it
|
| 3. How much mass would need to be collided to suck in Earth?
| dumbfoundded wrote:
| It's not a practical possibility. The black hole wouldn't last
| long and would be too small to actually absorb anything. It's
| the equivalent of asking if a nuke would set the atmosphere on
| fire.
|
| Even a "large"ish primordial black hole would probably just
| pass straight through the Earth without anyone noticing.
|
| Strange matter on the other hand...
| EA-3167 wrote:
| Remember that Strange matter is only dangerous assuming a
| specific range of values for its surface tension, otherwise
| it's harmless and doesn't catalyze the conversion of normal
| matter into Strange matter.
| consilient wrote:
| > How much mass would need to be collided to suck in Earth?
|
| A significant fraction of the mass of the Earth. Black holes
| don't "suck" any harder than other objects of the same mass.
| YetAnotherNick wrote:
| 1 kg black hole will last 8*10^-17 seconds[1]
|
| [1]:
| https://en.wikipedia.org/wiki/Hawking_radiation#:~:text=The%...
| marius_k wrote:
| Could it find some equilibrium environment to be able to suck
| as much matter as it radiates. So it could last longer (?)
| thfuran wrote:
| It only has the gravity of a 1 kg object and is extremely
| small, so it's not likely you'd be able to find an
| environment where mass is going to be getting jammed into
| it at quadrillions of tons per second even ignoring the
| fact that it's sort of a continuous nuclear explosion --
| it's radiating all its mass energy at a rate a few million
| times higher than the output of the sun in the ballpark of
| a star actively going nova.
| kitanata wrote:
| [flagged]
| blovescoffee wrote:
| The article says 4+ dimension black holes can exist. Just curious
| about an oddball idea from complete laymen perspective - could a
| blackhole with a time dimension/component exist then? I'm sure
| someone's thought about that before me :)
| Paul-Craft wrote:
| I haven't read the paper, but I suspect the whole concept of a
| 4, 5, 6, or 7-dimensional black hole is nonphysical, but the
| math works out if you just close your eyes and assume such a
| thing can exist. The reason I suspect n=7 is as high as it goes
| is because the volume to surface area ratio of an n-dimensional
| sphere is r/n, which means that the higher n you have, an
| n-sphere of radius r has a much larger boundary for the same
| volume. Conversely, that means that for a given surface area,
| you have less volume, so, for a constant mass density, you're
| enclosing less "stuff" inside.
| akomtu wrote:
| What prevents the formation of many sub-universes with
| different space-time configurations? We happen to live a
| large 3+1 one, but others may exist. Speculating further,
| some black holes could be bridges between e.g. a 3+1 world
| and a 4+3 world. The observation about 7-dimensional spaces
| would mean that above that level there are no such boundaries
| between world, they all become one universe.
| seeknotfind wrote:
| Came to comment about the same thing. It's funny - higher
| dimension black holes can confidently exist in a model, but
| because the model isn't tied to reality, it's not a very
| meaningful statement.
|
| Time ends at black holes. They stretch out time infinitely,
| from an outside perspective.
| SanderNL wrote:
| Time ends at black holes. Is it even possible to empirically
| validate such a thing?
| make3 wrote:
| only for outside observers... you can validate it if you go
| in :)
| BSEdlMMldESB wrote:
| > Is it even possible to empirically validate such a thing?
|
| I wouldn't think so because empiricism implies experience
| which implies time
| bmitc wrote:
| I think what they meant by end is that time _appears_ to
| stop at the event horizon for outside observers. As
| something approaches a black hole event horizon, time
| appears to infinitely slow, for an outside observer, such
| that a frozen image of the object gets stuck on the event
| horizon. But this image can 't be seen because it gets
| infinitely red shifted such that no detection device can
| keep up with the longer and longer wavelengths.
| antognini wrote:
| Black holes already have a time component since they exist in a
| universe with 3-dimensional space and 1-dimensional time. The
| equations include both the spatial components and the temporal
| components.
|
| One of the curious features of black holes is that they reverse
| the roles of time and space in a sense. In ordinary space we
| have freedom to move around in space, but are constrained to
| only move forward in time. Different reference frames may move
| forward at different rates relative to us, but they always move
| forward.
|
| By contrast, once you pass the event horizon of a black hole,
| these properties swap. It's possible to "move freely" in time
| in the sense that you can find reference frames that appear to
| move backwards in time. But all these reference frames are
| constrained to move forward in space.
| cyberax wrote:
| > By contrast, once you pass the event horizon of a black
| hole, these properties swap.
|
| Sigh. No. They do not. Moreover, nothing special at all
| happens to you when you pass the event horizon.
|
| In fact, if you are free-falling then you should not even be
| able to detect the crossing using only local experiments.
|
| > It's possible to "move freely" in time in the sense that
| you can find reference frames that appear to move backwards
| in time.
|
| Nope.
|
| What happens is that your spatial directions become more and
| more constrained, until they collapse into a single point
| (the singularity). And then you'll just exist in this single
| point forever, according to GR.
|
| From your viewpoint, it'll look like the singularity becomes
| an infinite plane that cuts off most of your field of vision.
| You'll be falling towards this plane, but until the last
| moment you'll be able to receive signals from outside of the
| black hole.
|
| If someone puts a stationary clock outside the black hole's
| event horizon, you won't see it going faster or slower. And
| for any realistic black hole, your trip to the singularity
| will consume only a short time according to that clock.
| dvt wrote:
| > Sigh. No. They do not.
|
| Technically, GP is right that they (mathematically) swap,
| but yeah, it has a meaningless physical effect (the
| geodesic is always smooth). It's akin to describing the
| rotation of a kicked ball with imaginary exponentiation and
| thinking something spooky is going on.
| antognini wrote:
| So I would agree with this for the Schwarzschild metric.
| The swapping of signs in the space and time components of
| the metric does not have any real physical consequences.
| But when you move to a Kerr metric it absolutely does
| because it permits the construction of closed timelike
| curves.
| Spivak wrote:
| And to add to this a singularity is _predicted_ by GR, we
| don 't actually know if such a thing exists and there are
| models where it doesn't like LQG where black holes are just
| a different kind of star whose collapse is opposed by a
| repulsive force that is predicted to exist by the
| uncertainty principle.
| Zamicol wrote:
| According to GR, time stops at the event horizon. It would
| take infinite time to cross the event horizon.
|
| Everything past the event horizon is speculative.
| magicalhippo wrote:
| Not really. It just appears to take an infinite amount of
| time for an outside observer.
|
| For someone falling into a black hole, it takes a finite
| amount of proper time[1] to reach the event horizon.
|
| For that infalling observer, the horizon is a boundry
| where, once beyond, the singularity is always in their
| finite future. No matter what you do inside, you _will_
| reach it at some point.
|
| [1]: https://en.wikipedia.org/wiki/Proper_time
| Zamicol wrote:
| How long does it take a infalling observer to cross the
| event horizon of a black hole?
| EA-3167 wrote:
| From the perspective of the person falling, it happens
| from one moment to the next. You as the person being
| accelerated don't notice any change in your own proper
| time, only distant observers will see that.
| magicalhippo wrote:
| If you were to fall radially inwards towards a non-
| rotating black hole, that is not spiraling around it but
| "straight in", it would be the same as with plain
| Newton's law[1].
|
| There's a nice graph in the midde of this[2] page that
| shows the difference between the proper time and the
| aparent time observed by the outside observer. At r = 2m
| you can see the aparent time goes to infinity and the
| quickly back again.
|
| [1]: https://physics.stackexchange.com/questions/718222/p
| roper-ti...
|
| [2]: https://www.mathpages.com/rr/s6-04/6-04.htm
| antognini wrote:
| Yes, this is correct. If you look at the Schwarzschild
| solution to the Einstein field equations you'll find that
| there are two apparent singularities: one at the center
| of the black hole, and another at the event horizon.
| However, the apparent singularity at the event horizon is
| not a true singularity because you can perform a
| coordinate transformation in which it disappears.
| Mathematically, this is doing what you describe: going
| from the reference frame of an external observer to one
| who is falling into the black hole. To the outside
| observer you appear to take an infinite amount of time to
| fall in, but from the perspective of the person falling
| in, it happens in a finite amount of time.
|
| The other singularity at r = 0 is different, though. It
| is a true singularity because there is no coordinate
| transformation you can make in which it disappears.
| qubex wrote:
| Aside: the fact that the radial spatial dimension towards the
| centre of the black hole where the singularity lies becomes
| time inside the event horizon is why all the drivel about
| "what you'd see if you crossed the event horizon" is drivel.
| You'd die instantly since circulating your blood or sending
| nerve impulses would essentially become impossible because it
| would involve reverse time travel -- i.e., blood moving
| toward the centre would never be able to move out again
| because that would involve it travelling back in time. Greg
| Egan mentioned this in a short story of his.
| antognini wrote:
| This is not true. It is possible for two observers inside
| the event horizon to send signals to each other. It's just
| that those signals cannot be sent to any radius larger than
| the radius they originated at.
|
| The only danger to a human would be the tidal forces. For a
| supermassive black hole, the tidal forces at the event
| horizon are quite modest, so you really would not notice it
| when you passed across the event horizon. It's only when
| you get close to the singularity that the tidal forces tear
| you apart.
| Sniffnoy wrote:
| The article explicitly says that it's talking about number of
| spatial dimensions, it's not including time. It says the new
| theorem shows that black holes can form into up to 7 spatial
| dimensions (plus 1 for time). (This isn't to say they can't
| form beyond that, just this new theorem doesn't show that.)
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