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Comment by ck2 | Hacker News Reader

what's really going to blow your mind is

while you probably assumed or knew spinning black holes move space around them

spinning black holes also move TIME around them

* https://www.science.org/doi/10.1126/sciadv.ady9068

so in theory a spinning black hole that's been around for billions of years has a time drag around it in a path that is billions of years old

(no we can't navigate it because yes that would be time travel to the past and violates causality)

black holes are just so weird with every new detail even more weird

oddly more interesting to me to try to grasp neutron stars (densest objects before black holes and are still visible, our entire solar system in a neutron star would be only 10km 6.2miles across)

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Another thing that may blow the minds of some is that M87* is less dense than air at 0.44kg/m³ so if you could bring it to sea level (in a large enough theoretical test area) it would float like a helium balloon (sea level air is 1.2kg/m³).

Of course if you did do that, the air itself would collapse into a black hole larger than M87*...

Consider the magnetar.

https://en.wikipedia.org/wiki/Magnetar

"A magnetar's 10^10 tesla field, by contrast, has an energy density of 4.0×1025 J/m3, with an E/c2 mass density more than 10,000 times that of lead."

Yes, magnetars are considerably more rare than black holes and considerably more interesting to study in terms of raw horsepower. Imagine a type-2 civilization using them as engines or launchers for spacecraft to zip around the galaxy.

the radiation from a magnetar exceeds any other star, overcoming that would seem implausible

still trying to wrap my mind around kilonovas (colliding neutron stars)

ie. they can pop out earth-sized chunks of gold, in theory, and since they aren't black holes that would be VISIBLE, albeit also "in theory" lol

* https://www.nasa.gov/image-article/unfolding-story-of-kilono...

maybe Roman can spot one someday, that would be something

> they can pop out earth-sized chunks of gold

The can produce Earth masses of gold, but of course not in the form of "chunks". The gold starts as atoms in a highly ionized gas and only much later cools enough to form solids (dust).

It blows my mind that, in the frame of an outside observer, time appears to stop at the event horizon. An observer falling through the horizon (who survived the radiation and tidal forces) would not perceive this.

Assuming they do fall through it. Remember they're about to receive an entire universe lifetime's stellar radiation compressed into a microsecond.

(Which is another reason some people think we might be living inside a black hole. An entire universe of energy released in zero time is literally a Big Bang. It would form into stars and galaxies.)

You would turn into plasma cloud long before getting there. How your specific atoms or parts of them, highly separated across long distances perceive that moment is practically irrelevant

Time travel breaks causality if you assume Copenhagen interpretation of quantum mechanics. Multi-world interpretation allows it as you just end up in a self-consistent "branch" and you would be stuck there.

Like you can travel back in time and kill one of your ancestors before he/she had children. In that branch you wouldn't be born, but since you come from another branch the system remain consistent.

(If you are interested look at David Deutsch’s quantum model of Closed Timelike Curves).

>what's really going to blow your mind is while you probably assumed or knew spinning black holes move space around them spinning black holes also move TIME around them

Well, isn't called space-time for nothing. You can't have one without the other. Like in electromagnetism. I thought it was kinda obvious since Einstein and Minkowsky.

> densest objects before black holes

Not quite, I think a (theoretical) quark star would be higher density?

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