Comment by somat | Hacker News Reader

Comment by somat | Hacker News Reader

Parent Full thread

My rather pop-sci understanding is that when you start playing around with relativity math, trying various masses and densities you hit something rather worrisome. As you approach some great, but still possible, value the plot goes infinite, the singularity. This is bad for the theory because going asymptotic like that usually indicates a fundamental problem in the math. But relativity does so well everywhere else... What if it could? And thus the black hole was born.

The easy thing to miss, and blew my mind when I read it. is that general relativity is the concept of space-time, emphasis on the time, and this is also compressed by the mass, so if this singularity can actually occur it would also take an infinite amount of time to fall into it. So nothing can actually enter it. From the point of view of an astronaut(deliberately ignoring all the other relativistic implications) flying directly toward the event horizon. As you approach you will quickly see the rest of the universe age and die. and if hawking radiation is real the black hole will evaporate in front of you before you can reach it.

I believe this is a misunderstanding based on inadequate coordinate systems, and that an astronaut would fall through the event horizon, die, and reach the singularity in finite time.

See https://physics.stackexchange.com/questions/82678/does-someo...

Does time accelerate just for the astronaut approaching the black hole (meaning they would be witnessing the future) or does it accelerate for everyone (meaning approaching a black hole is going to make everything end quicker)?

My rather limited understanding is that. first there is a lot going on and due to the forces and gravitational gradients involved, matter itself probably would not survive the journey that close to the horizon. But ignoring that, When in compressed time, from the point of view of someone freefalling, timewise everything is normal, the rest of the universe has accelerated and in the case of a black hole has just aged out and died but you are fine. from the external point of view they are taking forever to get there.

Where my imagination fails(above my pay grade) is in the face of infinity, what are the implications of infinite time compression?(everything happens at once?)

> external point of view

So time is localised? I’m not sure what localised time means but I’m hoping the question makes sense.

Infinite time dilation means infinite Hawking radiation that will burn everything.

I can only guess as well, infinite time compression might result in a big bang when it applies to everything. All black holes could lead to the same moment causing the bang.

There are a lot of curious coincidences between properties of black holes and properties of the whole universe that lead some physicists to the idea that they could be connected. There's also that Penrose diagram showing another universe beyond a black hole. (Seems like a copout to me though. Penrose diagrams are drawn in 1D space, so the black hole cuts the universe in half. If you had 2D or better space couldn't you just travel around behind the black hole to the other side of it?)

Time acceleration is dependent on the gravitational flux.

All objects within a given radius of the black hole (possibly modulo spin) would experience the same time dilation. Remote objects in the universe would not experience the dilation.

From the perspective of the astronaut falling toward the singularity, the rest of the Universe would age at an ever-increasing rate.

From the perspective of a remote observer, the astronaut falling toward the singularity would be experiencing time at an ever-decreasing rate.

The notion of relativity is that time-perception is relative, and dependent on acceleration, whether from motion (as on a spaceship) or from gravitational acceleration (as near a black hole). Objects in orbit around Earth, further from Earth's centre, and hence subject to reduced gravitational acceleration, age more quickly than objects on Earth's surface. This is actually measurable using atomic clocks, though the effect is quite small. It is sufficient that GPS satellites require time correction.

If outside observers could watch the astronaut clearly, the astronaut would look like they're slowing down to a stop at the event horizon. The astronaut would not see themselves freezing in time, but they would see everything outside of the black hole speed up.

You could travel arbitrarily far into the future by getting close to a black hole's event horizon for a while without crossing it and then leaving, assuming you had the energy for it and you didn't get obliterated by all the mass and energy falling into the black hole in that timeframe.

AFAIK from the outside point of view it takes infinite time. We see the falling object redshift to infinity (blackshift, really) and merge into the black sphere we observe.

But in the object's own time coordinates the math says it does hit the singularity. If you fell in you wouldn't die of old age before you hit it.

Reply on news.ycombinator.com