Lets talk about black holes again

You can always pull your arm back unless there are strong tidal forces involved. Orbiting the Sun or Pluto, moving your arm around takes exactly the same amount of force.

Like putting your hand into water. If you extend your fingers into the water, it's not like the entire hand's going to get wet. The event horizon is the surface of the water

Okay, I didn't explain good enough.

It not being a special space means that you could do ANY experiment in orbit around earth and get the same result orbiting at the event horizon, even passing through it, some how(this is where I'm struggling to see how.)

Just like there exists no special velocity's either.

To an outside observer time is slowed to a complete stop at the event horizon. You can never witness his hand going in.

For the person close to the event horizon, he won't really be able to tell where the event horizon is. The black hole will just take up more and more of his field of vision as he gets closer.

He doesn't need to know where it is. They can be a future alien civ that knows all the dimensions of the BH exactly. And they put him exactly in orbit where he just needs to extend his arm for it to be inside.

I don't think he could see his arm time dilate as its crossing. I could be wrong on that, but I never heard this.

>I don't think he could see his arm time dilate as its crossing. I could be wrong on that, but I never heard this.
To clarify, most stuff I read and seen, seem to agree that whoever falls in could not tell if he was in or out. His feet freezing for a second would be noticeable.

We already know the dimensions because of the Schwarzschild radius. The one orbiting will not see any barrier. He will stick his arm out towards the blackness and not notice anything at all.

The outside observers will see time stop for the one orbiting, and he will freeze in space and stop all movement, never seeing his arm extend.

But the arm would redshift

>He will stick his arm out towards the blackness and not notice anything at all.
I agree this is what physicists are saying, but how could he not notice anything if nerve signals and blood from his arm suddenly stopped coming back to his body?

The event horizon is not a barrier between time happening normally and time stopping. Time dilation is relative. To an outside observer the time dilation is gradual as you approach, going to zero at the horizon. To the approacher, time dilation doesn't happen at all to their local time.

So there's an area where time dilation does start to occur. What happens if you stick just a finger in that oh I guess the rest would get sucked in if gravity's acting on the finger but not tearing it apart. Matter gets flooded into the event horizon, yeah? But OP said there were no tidal gravitational forces

I'm not talking about time dilation when I say his arm couldn't go back. Simply that nothing can exit after dipping below the horizon.
I'm not sure that was what you were implying either though.

My best guess at the problem so far is that his whole orbit would have to change as he touched the horizon in such a way that there was no chance for anything to notice nothing was returning(even light I guess.)
I'm still not totally convinced this is the answer though since it seems like it would be a very drastic change in orbit

But aren't gravitational frames of reference agreed upon by all observers? Everyone would see the arm freeze in place / disappear / smeared across horizon, including the unfortunate person whose arm it is.

Gravitational time dilation is agreed to be slower, but you can never notice your own time going at anything other than a normal speed because your own head clock would be ticking slower .

At the event horizon it should be normal too, unless the gravity well has an extreme shape(small black hole) you shouldn't notice any difference when it crosses since space is continuous across the horizon. Its a bit confusing too for similar reasons as the problem in this thread.

It starts to occur in any gravitational field, so anywhere in the universe.

The dude would just reach out his hand and bring it back in. His frame of reference has nothing funny going on at all. To the outside observer, he is already frozen in time.

You will never experience time dilation happening to yourself.

>You can always pull back your arm
How do you figure?

Gravity becomes stronger the closer to a massive object you get. For a sufficiently large object or weak thrust you could not be able to gain altitude, such as in the photosphere of the Sun there's a pull of 28G. Let's say my hand weighs 500 grams, that's 14 kilogram. Unless the rest of my arm is weightless, it's probably not happening.

Gravity is not a force pulling on you. In space you are weightless and in free fall unless you are accelerating, you wouldn't notice anything.

Let's think of it it as not happening to you. It's your hand, but you're just observing it, falling into the black hole from 3ft away.

One means of observation is the pull of connective tissue.

Free fall assumes constant gravity, but gravity changes with distance.

That wouldn't happen. The event horizon isn't a physical barrier of sudden change. The whole transition is very gradual.

You can't orbit just outside the event horizon. The closest you can stablely orbit is the photon sphere, 1.5 times farther than the event horizon. Things can still move straight out from within the photon sphere, but any attempted orbit will go into the black hole. So if you're just outside the photon sphere, you can stick your hand in and pull it out, no biggie. If you're just outside the event horizon you're falling in anyway, so your arm, which is also falling in, has no problems moving relative to you.

What you are talking about at this point is gravitational tidal forces, and they are only noticeable in extreme gravitational wells, which only occur around smaller black holes to my knowlage.

>You can't orbit just outside the event horizon. The closest you can stablely orbit is the photon sphere, 1.5 times farther than the event horizon.
Anywhere I can read about this? Or a video explaining it? Just curious, sounds very reasonable.

But the problem can be applied to a dude orbiting in the photon sphere with a jet pack then. Or just falling with a jet pack to simplify it.

en.wikipedia.org/wiki/Photon_sphere ?

I'm not sure what you mean with a jet pack. How are you imagining it would change the scenario?

You use the jet pack to maintain your "orbit"

>not sticking your peepee in a BH for science

Well the photon sphere is so named because theres a bunch of photons there, which are as close as they can be because they can go at the speed of light. If you want to maintain a circular orbit within the photon sphere you would need to orbit faster than the speed of light, which obviously, jetpack or no, is impossible. Same thing with nearing the event horizon. Your 'exit vectors' (that is, the best light can do, or anything easier) get narrower and narrower toward directly away from the event horizon.

I actually didn't think of it as tidal forces, but the thought experiment is set up to gauge tidal forces on the event horizon. It seems you should be able to tell, if only in principle.

I think that's the answer. What does the gradient look like though? Does it asymptote towards the event horizon?

Makes sense. Does hovering over the horizon have similar problems? (I can see why escaping is impossible for anything except photons)

Even descending slowly seems to me to have similar issues as I have been talking about in the thread.

If you are trying to argue that tidal forces exists then we have no argument here. And yes they can be theoretically measured where ever there is a gravity well, even if its a pretty normal one like earth's.

If you're using your jetpack to constantly blast away from the BH in order to hover just above it, you're no longer in free fall. You're now accelerating at a tremendous pace to attempt to outdo the BH's gravity. Needless to say this would be an ungodly amount of Gs, and the closer you get to the event horizon the more acceleration you need, until you need an infinite amount once you pass the event horizon. So imagine being strapped into a chair under a ten thousand Gs, and you can start to realize the sort of things that would happen. Assuming you remain conscious and don't liquefy for magical reasons, eventually you get close enough that a dangling arm's blood can't get pumped back up quickly enough, further still and neural signals can't make it, further still and your arm rips off, you get the idea. In general, unless you're standing on some sort of surface, it's very difficult to stay near a gravity well without being in a free fall orbit, and quite a bit more so if we're talking very near to a BH.

Didn't think Veeky Forums could come up with satisfying answers at this point. You're restoring my faith somewhat.

It totally makes sense what you are saying, but is there still theoretically a problem? If you imagine an instrument that uses photons for signal which is being hovered right at the limit of what is possible. Could you theoretically not detect the point where one side is inside and the other outside?

I guess at that point the rest instrument would be dragged down into the horizon with the other side in such a way that there wouldn't be any difference in time from when it was only outside the horizon when I think about it.

It doesn't even need to be hovered, the situation is basically the same isf you drop a device which is constantly emitting a signal to place well outside the BH, into the BH. The signal will slow down, redshift, and weaken as it gets closer due to time dilation. Eventually the signal weakens to such an extent that it can barely be said to exist at all. Whether the station it's transmitting to is far away or right outside the event horizon, the difference between the two approaches infinity either way.

I guess you could conclude something being inside. The thing is though that the event horizon is an information barrier. Whatever happens inside the rest of the universe will never know.

So maybe in theory you could have a sensor that sends out photons and track if they return or not, and from there deduce where the limit is

I don't think the light would redshift like that. The shift due to speed is countered by the blueshift as it gets absorbed by the falling instrument.

And the redshift due to gravity would only be as strong as the change in gravity potential from it being emitted to absorbed which doesn't have to be that different on the horizon of a supermassive black hole.

Doesn't this go against space not being special at the horizon(no experiment etc.(been over this.))?

The gravity is so strong that it will pull your whole body in

I just joined this conversation so I might have missed something but I'm not sure what you mean with space being "special" at the horizon. It is extremely warped, both spacially (as the event horizon will cover 50% of your field of view just outside of it) and in time (time dilation).

How is the speed of light constant for all observers? Imagine a space ship going 99% of the speed of light racing away from earth. Someone on earth shoots a laser at them. Both earth and the spaceship start a timer at the same time and end it when the pulse lands. Wont their clocks disagree on how long it took to hit?

Good question actually. The space ship would register being hit when the pulse reaches it while earth has to wait for both the pulse to hit and some time for the information to return.

In the end the pulse would hit for both parties at the same time, but the earth observer would have to wait a little longer for confirmation. The hit is still the same for both observers.

Let's say that I have a clock and I want it to get the maximum ticks possible before the end of the universe. Shouldn't I send it on a probe out to interstellar space, and then bring it to a complete stop? With basically zero speed and gravity, wouldn't everything else in the universe be aging much slower than it? So if you looked at all the clocks some point far in the future, mine should have ticked the most times.

But the people on earth know how far the ship is, so they can just correct for that in their pulse time calculation.

Then the clocks of the ship and on earth would align I guess...
Then there is time dilation from the near lightspeed travel..

Don't think I know the answer to this actually, relativity getting in the way. I'll leave it to someone more clever than me.

If the spaceship is moving at a constant speed, then both earth and the space ship would consider each other to be moving at 99% speed of light. So there is no time dilation for one that doesn't happen for the other.

If the hole is big enough that you weren't atomized on the way to the horizon, then you would not notice anything special when he stick your arm "over" the horizon, and you would be able to pull it back without issue. You are doomed only when your center of mass crosses the horizon, before that you can still escape (in theory, in practice you'd need an implausibly powerful engine to escape)

non physicist here.

just because the difference in gravity between one end of his body and the other is not strong enough to rip him apart doesn't mean that human biology will continue to work as normal right up until the event horizon, right?

But how? Information can't escape event horizon. Whatever body part of yours that crosses it, should follow the only possible path - the center of the black hole.

so long before the event horizon, you're already dead.

Why wouldn't it? You're just in free fall, same as anywhere. You'd be fine.

if you're orbiting you're not in freefall.

This is totally wrong but whatever.

>You can also have an object in a stable orbit around the black hole that gets close to the event horizon. (The orbit gets very funky, but that doesn't matter.)
source on this?

id like to know just how close you can get.

again, not a physicist.

You don't need to be a physicist to know an elementary school physics lmao.

Well, quick physics lesson here. If you're not actively accelerating yourself (with propulsion) and not standing on the ground, you're in free fall. Orbit is free fall.

Not a physicist, but I got to read a bit about black holes, and I believe an observer that close to the event horizon would either be on freefall or be ripped appart by the tidal forces any medium capable of avoiding freefall would provide.

But that doesn't answer the fundamental question, does it? In truth, I believe an observer that close to the event horizon would perceive the horizon itself to behave weirdly, in such a way that they never perceive to cross it while being inside.

Maybe by virtue of being in freefall, what to them is moving the hand back and forth (without ever touching a horizon) to us is an infinitely slow, infinitely redshifted person moving a hand forwards and backwards in time.

And I think your idea of the boundary not being special is a bit extreme. You would never touch a barrier, but time dilation is still in effect, even here on earth.

The event horizon isn't a visible limit you can cross. To the observer, the closer you get to it the more of your field of view is filled by it and when the event horizon encompasses more then 50% of your vision you are officially past it.

It's a Force to be Rekoned with

People are forgetting that by the time you get to the event horizon you are already dead
People have mass

In the case of an extremely large supermassive black hole, with no other significant source of matter nearby, assuming cosmic censorship isn't a thing (as the "current consensus among physicist now" is just the opposite of what you describe), the arm would be gone. He could pass, relatively unharmed, along with the arm (and should, everything sharing momentum), but nothing that goes beyond the event horizon is going to see the outside of it again.

Assuming cosmic censorship is a thing, there wouldn't be anything left of him to pass inside of the event horizon.

If it was a mid-sized black hole, or a black hole taking in any degree of matter, he'd be both spaghettified and obliterated before he got that close, and, if it was a very small black hole, disintegrated by hawking radiation.

But assuming the first scenario, if the, now one armed, man somehow managed to extract himself from that position, by the time he returned to Earth, it wouldn't be there anymore. At the event horizon of a large black hole, time is going to move so slowly that the whole life of the universe will pass you by in moments.

It's a bit odd how both the astronauts in and the writers of Interstellar only remember things like this when it's convenient.

>People are forgetting that by the time you get to the event horizon you are already dead

Not true, while small black holes have very high tidal forces, supermassive black holes have very small tidal forces. You could cross an event horizon of such a hole unharmed.

Unless cosmic censorship is a thing...

...In which case you could get to the edge, but be fucked after that.

Granted, either way, no one on the other side is ever gonna know, as either way, you aren't coming back, and if they are far enough away to be in some sane gravity well, they'll be long dead before you even get close anyways.