henearkr3 days ago
About 9000 times the mass of the supermassive black hole at the center of our galaxy (Sagittarius A*).
(Sorry, I had to, with all the AI flood, I really was about to skip this info after the first 3 characters)
Hype is strong.
A cool achievement would be, observe the moon/earth separation event(s)
The bigger problem is all the dust and other stars in the way. I’m not aware of any black holes close enough that would have a direct path for the light to cross without being absorbed and scattered.
Also, the foreground galaxy/supermassive black hole in the Cosmic Horseshoe is 5.6 billion light years away, so any light that could come from our solar system, go around the black hole, and come back to our hypothetical hypertelescope would be over 11 billion years old - almost triple the age of our sun.
Saggitarius A* in our own galaxy is, of course, directly in the elliptic and therefore badly occluded by dust, but it would be interesting to look at as it's only 27k light years away. In the absence of that pesky dust, it would give us a picture of the solar system as of the Paleolithic. Andromeda, at 2.5 million light years away, would give us 5-million-year-old light. There are other black holes in the Milky Way on the order of a thousand light years away which are not at the center of the galaxy but have masses comparable to or slightly larger than our sun, these are far closer (within a few thousand years) but have much smaller gravitational fields. Luminous intensity drops off with the square of the distance, but I'm not sure how the gravitational field strength affects the ability of a particular galaxy to bend light.
It is possible to get a deflection angle of 180 but under a few million solar masses, hitting the “sweet spot” in between the photon sphere and the boundary of the shadow would basically be a once in the lifetime of the universe type probability, if it were possible at all. At billions of solar masses that sweet spot become much bigger, but then those are much further away.
In this insanely hypothetical scenario, would it be possible to see a sun before our sun? (In the same galactic vicinity)
https://en.wikipedia.org/wiki/TON_618
Event horizon radius would be about roughly 1000 times the distance between Earth/Sun.
Your “sightseeing tour” would be a kaleidoscope of light as it brushes past you on its way to the singularity.
Eventually your atoms will make their way to the center singularity.
in a black hole time and space get switched in a sense.
https://modern-physics.org/time-dilation-near-massive-bodies/
This is the origin of my favorite science fiction theory. (little to no actual science but you could write a fun space romp around it) If you get a large enough black hole where the tidal forces will not rip you to shreds instantly, you could just scoot across the event horizon right, now what happens? you can still move around, everything feels normal, but really you have lost half a dimension, everything "out" from the center is completely gone from the universe. Now the theory, back to our universe, What happened to time? why does time only go one way? we can accelerate and decelerate along the time axis, but can't reverse it. Where has our missing half of a time dimension gone?
Wouldn't that just mean that the singularity is located infinitely far into the future?
My understanding is that for extremely large black holes the tidal forces are negligible near the event horizon. So things should function pretty much the same other than you can't move in reverse and get out.
If two rockets fall past the horizon at the same time, one accelerating forward towards the singularity, and the other accelerating backwards away from the singularity, then shouldn't the distance between the rockets increase, even though they are both moving inexorably forward?
If the tidal forces are low, I'd assume that my muscles are still strong enough to "slow down my hand enough" to move it above my head.
Two rockets can diverge in distance, because one is slowing itself along the timeline space dimension toward singularity. If you are moving 1 m/s toward singularity, the fastest your hand can raise above your head is 1 m/s with infinite energy expenditure. The same goes for blood pumping to your head, electrical impulses to your brain, etc.
After you pass the event horizon, all your possible paths become elliptical. That doesn’t mean all possible paths instantly point directly at the center.
No. It's a fanciful analogy on a particular family of coordinate charts, particuarly systems of coordinates which do not smoothly/regularly cross the horizon. The black hole interior is still part of a Lorentzian manifold, there is no change of the SO+(1,3) proper orthochronous Lorentz group symmetry at every point (other than spacetime points on the singularity). One can certainly draw worldlines on a variety of coordinate charts and add light-cones to them, and observe that the cones interior to the horizon all have their null surfaces intercept the singularity. However, there's lots of volume inside the interior light cones (and on the null surfaces) and nothing really constrains an arbitrary infaller's worldline, especially a timelike infaller, to a Schwarzschild-chart radial line (just as nothing requires arbitrary infallers to be confined to geodesic motion).
The interior segment of a Schwarzschild worldline in general can't backtrack in the r direction, but there are of course an infinity of elliptical trajectories which don't. (That is to say that all orbits across the horizon are plunging orbits; but one can also say that of large families of orbits that cross ISCO, which is outside the horizon).
A black hole with horizon angular momentum and general charges offer up different possibilities, as does the presence of any matter near (including interior to) the horizon (all of these also split the ISCO radius, move the apparent horizon, and may split the apparent and event horizons). The Schwarzschild solution of course is a non-spinning, chargeless, vacuum solution everywhere, and is maximally symmetrical, and is usually probed with a test particle. An astrophysical system like a magnetic black hole formed that passes through a jet from a companion pulsar, for example, does not neatly admit the Schwarzschild chart (and has no known exact analytical solution to the field equations). At least one such astrophysical binary is known (in NGC 1851 from TRAPUM/MeerKAT) (and if you don't immediately run away from A. Loeb papers like you should, he added his name to one that argues there are thousands of such systems in the galaxy centre near Sgr A*, which itself is now known to have strong magnetic fields (thanks to EHT's study of the polarized ring)).
For a large-M black hole, there is "no drama" for a free-faller crossing the event horizon, as the KS gradient is tiny.
Since the crosser is in "no drama" free-fall he can raise his hands, toss a ball between his hands, throw things upwards above his head, and so forth. The important thing though is that all these motions are most easily thought of in his own local self-centred freely-falling frame of reference, and not against the global Schwarzschild coordinates. His local frame of coordinates is inexorably falling inwards. Objects moving outwards in his local frame are still moving inwards against the Schwarzschild coordinates.
You might compare with a non-freely-falling frame of reference. Your local East-North-Up (ENU) coordinates let you throw things upwards or eastwards, but in less-local coordinates your ENU frame of reference is on a spinning planet in free-fall through the solar system (and the solar system is in free-fall through the Milky Way, and the galaxy is in free-fall through the local group). That your local ENU is not a freely-falling set of coordinates does not change that the planet is in free-fall, and your local patch of coordinates is along for the ride.
A comparison here would be a long-running rocket engine imparting a ~ 10 m s^-1 acceleration to a plate you stand on. In space far from the black hole, you and the rocket engine would tend to move away from the black hole, but you'd be able to do things like juggle or jump up and down, and it'd feel like doing it on Earth's surface. This is a manifestation of the equivalence principle. Inside the horizon the rocket would still be accelerating the plate and you at ~ 10 m s^-1, but you, the plate, and the rocket would all be falling inwards.
If you're "travelling at 1m/s so you can only raise your hand above your head at 1m/s by expending infinite energy" then you're already travelling at c-1m/s away from the black hole through local space just to 'stay still' at 1m/s 'velocity'. No wonder you need infinite energy to accelerate your arm 1m/s further and things get weird - you're travelling at relativistic velocities.
It takes infinite time to reach event horizon, not the center.
But in this specific case, you get one odd conclusion. if it takes forever to enter a black hole. is it impossible for anything to pass the event horizon? It sounds like this is observation dependent. but from an external point of view you are unable to observe anything entering the black hole. and from an internal point of view, the universe will instantly age and die when you try and enter the hole.(and if hawking radiation actually exists you will see the black hole shrink and pop the instant you try and enter it) either way nothing is getting in.
Is most of the mass of the star that formed the black hole actually stuck in a time dilated shell just outside the event horizon? Or perhaps all the mass is eternally stuck collapsing. and never actually reaches the density required to pass the event horizon. is that another way to define the event horizon? the point where time stops.
Time dilation makes my head hurt.
Even that is only true to a distant observer, not the one crossing the horizon.
The outside observer’s view doesn’t stop physics inside. For a massive black hole, you absolutely do reach the singularity in finite time by your own clock.. likely minutes to hours for the largest ones we've known about so far.
The near-vacuum atmosphere of Mars seems very light...? What fundamental concept am I misunderstanding?
That sounds more like a description of the stuff neutron stars are made of. I don't think that description really works for black holes - how exactly do you take a teaspoon out of a black hole?
> The near-vacuum atmosphere of Mars seems very light...? What fundamental concept am I misunderstanding?
The linked Physics.SE answer does a decent job at explaining it, but the short of it is that for Schwarzchild black holes mass ~ event horizon radius, so if you define density as mass / (Schwarzchild volume) you get density ~ 1/(mass^2) - in other words, the more massive a black hole the less dense it is by that measure.
You actually can have a black hole with the volume of a teaspoon, and it's stable. It will eventually decay by Hawking radiation, but not for umpteen gazillion years until the CMB gets cold enough.
Technically speaking that sure sounds like scooping out a teaspoon of neutronium to me. Nothing said it had to be stable :P
But in any case, I suppose what doesn't work for me is that when the teaspoon illustration is being used it's in the context of picking out some sample/subset of a larger whole - take a whole neutron star and examine the properties of this supposed representative part of it, same way one might scoop out some ice cream out of a container. While that's technically not totally correct for neutron stars since they don't exactly have a uniform density, I feel that it's usefully-close-enough compared to black holes, since as far as we know all the mass of a black hole is concentrated in a point at its center so your "scoop" is either going to get nothing or everything.
> You actually can have a black hole with the volume of a teaspoon, and it's stable.
Sure, but at that point I wouldn't use the wording "a teaspoon of black hole"; something more like "teaspoon-sized black hole" would be more appropriate (though to be fair that's still technically somewhat ambiguous).
Radius is linearly proportional to the mass: r = 2GM/c²
(So volume grows faster than mass)
Passing the event horizon doesn’t mean you’ve reached the potentially ultra dense singularity, but it does mean you won’t escape.
So... how long before we see the shape change? How fast do galaxies move anyway?
Once we wanted to simulate the dynamics of galaxies. I don'it think it was an SA article, but we did it the slow way by calculating the force on every star individually from each other star. It was excruciatingly slow and boring.
Then some time later, I don't recall where I picked that up, I updated the simulation to just model the force on each star coming from the galaxy's centre of mass.
I could simulate many more stars, have galaxies collide and see them spin off with their stars scattering around.
What struck me was that they looked like real galaxies we see out there.
I wasn't aware of the postulations made in the 60s/70s about there being supermassive black holes at the centre of galaxies, but to me, this simplified simulation was kind of like a smoking gun for that... from an 80286 IBM PC AT.
This wouldn't work for something like the Solar system with a very sparse distribution of mass, but at the galaxy level it seems right even without the presence of a black hole.
It's a long time ago, but what I remember was being fascinated by the shapes of the galaxies emerging from a collision under this centre-of-mass approximation, and that it created shapes we see out there. It was as if the main effect were a central mass in each galaxy dominating the dynamics.
There are other larger ones out there, looming in the darkness.
Not to be that guy, but a physician is a doctor.
In the time that it took you to type that response, you could have learned 10 new words.
I do it because I appreciate it when people do it for me.
That was the purpose of my comment. What was the purpose of yours?
I try to remember that when I'm tempted to point out mistakes that are fine to overlook.
I'll try to dig it up when I'm not at work (or if I remember the exact episode through the day).
The episode is called “The NEW Ultimate Energy Limit of the Universe”. https://youtube.com/watch?v=0rzgYzbzq5Q
https://scitechdaily.com/cosmic-heavyweights-collide-ligo-detects-largest-fastest-spinning-black-holes-yet/
https://www.youtube.com/watch?v=doS85Mh78Vc
This is what they look like when they merge, its pretty darn cool
> [270B solar masses] is the maximum mass of a black hole that models predict, at least for luminous accreting SMBHs.
as well as:
> The limit is only 5×10^10 M [50B solar masses] for black holes with typical properties, but can reach 2.7×10^11 M [270B solar masses] at maximal prograde spin (a = 1).
However in the chapter before, it's stated:
> New discoveries suggest that many black holes, dubbed 'stupendously large', may exceed 100 billion or even 1 trillion M.
For everyone else reading the thread, let me summarize. The article agrees with me:
> the entire observable universe exists within a black hole—except, that is, for all the evidence to the contrary
>....
> It does not seem likely that we live inside a rotating universe, let alone a black hole.
(I'm not thinking this is too much to ask; saying it's wrong might require empirical support, but the claim that it's "nonsense" should be easier to justify.)
It really looks nothing like a black hole.
I mean, everything in our universe does move towards something. The future.
As another comment pointed out, in GR our "future" is a singularity which everything moves towards (so what we see as a time dimension).
"Expanding" and "contracting" depend on your coordinate system. If your rulers are shrinking, you can't tell this from space expanding.
The common factor here is you are wanting to use our reference frame (somewhere in this universe, not near a black hole) to describe things as they would be seen from other reference frames.
How would we know the size? Relative to what?
EDIT: I believe the above could be incorrect - if the universe has too much electrical charge or angular momentum. (And some other cosmological properties, so you couldn't get around the charge & spin issues.)
Might there be a black hole astrophysicist in the house, to comment on this?