Viewing a single comment thread. View all comments

graveybrains t1_j2a5mwn wrote

Reply to comment by Most-Hawk-4175 in What is our current "best guess" about how to observers that entered a black hole on opposite sides would look to each other once they crossed the event horizon? by WittyUnwittingly

Except the subjective experience of time never stops, no matter how dilated it gets, so the first thing you’d likely experience crossing the event horizon is the evaporation of the black hole, or the end of the universe, whatever those might look like.

If you could hang out for a while, you’d see the black hole growing around you as it’s gravity leaves space more and more distorted. The event horizon would be the point where the black hole wraps all the way around and closes behind you.

At that point there is no physical way out, and any direction you can travel is going to be inward.

…I may have seen too much PBS as a kid.

11

WorstMedivhKR t1_j2amk4i wrote

You'd be spaghettified and die long before the evaporation of the black hole or hitting the singularity or the end of the universe.

4

graveybrains t1_j2at6ky wrote

Depends. A black hole the size of Sagittarius A* probably wouldn’t spaghettify you, but I don’t remember why

3

jaydfox t1_j2b665c wrote

If you double the mass of a black hole, its gravity doubles. But its radius also doubles. Gravity decreases with the square of the radius, so at double the radius, you have 1/4th the gravity. So double mass of the black hole, and you get 1/4th of double the gravity, or 1/2 the gravity at the event horizon. But the tidal forces that cause spaghettification decrease with the cube of the distance, so you get 1/2 the gravity and 1/4th the spaghettification. Make a black hole 10 times bigger, and you'll get 1/10th the gravity and 1/100th the spaghettification at the event horizon.

The black hole Sagittarius A* is about a million times more massive than a typical (stellar) black hole, so it's gravity at the event horizon is a million times smaller, and the spaghettification will be a trillion times smaller. Not sure if it's enough to survive being spaghettified near the event horizon, but a trillion times less stretching can't hurt. Black holes a thousand times more massive than Sagittarius A* exist in other galaxies, so they'd be even easier (a million times easier) to survive falling into. (Actually, there are a few known black holes about 10,000 times bigger or more than Sagittarius A*.)

7

graveybrains t1_j2bkolq wrote

This guy is PBS!

Thank you 😁

3

jaydfox t1_j2bqat6 wrote

Haha, no, but I do watch a fair amount or space channels (PBS Space Time, Dr. Becky, Anton Petrov, etc.).

One thing worth mentioning is that you'll eventually be spaghettified, no matter how big the black hole is. If a stellar mass black hole has an event horizon of about 3 km (I think, not positive), then a 1 million stellar mass black hole would have an event horizon of about 3 million km. But only when you get within 300 km of the singularity, would the tidal forces be as strong as at the event horizon of the stellar mass black hole. For the largest known black hole (66 billion solar masses, off the top of my head, but maybe there's a bigger one), the event horizon would be about 200 billion km in radius, about 2% of a light-year (so 0.04 light-years in diameter). But the tidal forces wouldn't be as strong as at the event horizon of a stellar mass black hole, until you were about 12,000 km from the singularity. This is due to the rapid decay of tidal forces with distance.

So in that sense, the person you were replying to was right. You will be spaghettified before you get anywhere near the hypothetical singularity, no matter how big. But with a big enough black hole, you'd get pretty deep into the black hole before you get ripped apart. Long enough to make interesting observations. Which is what the OP was asking about. What would you see if you fell into a large enough black hole? Long after you've crossed the event horizon, and long before you get close enough to the singularity to be ripped apart by tidal forces. I assume you would see stuff that fell in right after you, and stuff that fell in right before you. (If not, that implies you wouldn't even see your feet after they went in, if you went in feet first. And I seem to recall several different authors saying you would hardly notice yourself pass through the event horizon.) How much longer before and after? I assume the cosmic background radiation behind / above you would be blue shifted from microwave into infrared, visible, eventually ultraviolet, xrays, etc. Whether you get ripped apart before witnessing that, I don't know. I'd like to know.

5

WorstMedivhKR t1_j2dsjjw wrote

You will eventually be spaghettified regardless. In a large enough black hole it's after you cross the event horizon but before hitting the singularity.

1