Hacker News Reader

Comment by oblio | Hacker News Reader

> particle accelerators, stars, and supernovae

I have no clue about this stuff, but don't black holes also change matter... somehow? I mean, with all that gravity and stuff, crazy things must happen in there, right?

View on HN

What happens inside a black hole is basically unknowable. We can only ponder the math which leads to ideas like space and time swapping roles once you cross the event horizon.[0] The only thing that comes out is hawking radiation, which is sort of like... half of nothing.

[0] https://youtube.com/watch?v=KePNhUJ2reI

> space and time swapping roles once you cross the event horizon

This is a common misunderstanding. Space and time don't swap roles. It's just that there's one rather popular coordinate system (Schwarzschild coordinates) whose coordinates t, x outside the horizon correspond to temporal (timelike) and spatial (spacelike) directions, respectively, and inside they correspond to spacelike and timelike directions. What we mean by "timelike" and "spacelike", however, does not change.

That's what the video link (PBS SpaceTime) explains. Quite well, IMO.

Kind of a one way path though - unless you count the gamma radiation and split pairs. I'm no expert either but it's pretty cool stuff.

As I understand contemporary physics, once matter crosses the event horizon it becomes part of the singularity. The singularity behaves as a single super-sized particle, so nothing happens inside. However I also have heard that many physicists don't believe that singularities actually exist, it's just the best mathematical model we have for physics that are too extreme for us to measure.

>> it's just the best mathematical model we have for physics that are too extreme for us to measure

It's not only a measurement problem. Rather, the laws of physics, as we currently understand them, lead to this singularity. Sure, many physicists may doubt the existence of the singularity. They will need new physics, not only better equipment, to challenge it.

It does not become part of the singularity once it crosses the event horizon. The event horizon is actually rather uneventful as far as any particular piece of matter crossing it goes - it only means that this matter can never leave the boundary defined by the horizon, but it doesn't change it otherwise. The singularity (if it even exists) is the thing at the center of the black hole, far below its event horizon.

Technically yes. But also, things that enter the event horizon are compelled to hit the singularity on a very tight timescale. I forget the exact fraction of a second, but even for a supermassive hole it's very small. So it's not crazy to think of stuff entering the event horizon as immediately becoming part of the singularity (if it exists, as you mentioned. My bet is that it doesn't, but as far as our current understanding goes...)

The precise formula, assuming that object was at rest at the event horizon, is:

τ = (2√2)·R/3c

So "fraction of a second" is only true if you're talking about relatively small black holes. For a supermassive one, it can take hours or even days for the largest ones.

But note again that this assumes no orbit, just falling straight towards it from rest. For orbiting objects it could take much longer depending on their velocity.

Also, this all is from the perspective of the observer who is undergoing the fall. From the outside, time dilation means that objects never actually cross the event horizon at all - no matter how long you wait, you'll see them as they were getting closer and closer to it, but never the actual crossing.

Particles with any (rest) mass can have lots of things that happen inside.

https://profmattstrassler.com/articles-and-posts/particle-ph...

Reply on news.ycombinator.com