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paleopuzzler t1_jariz1s wrote

It's a good short hand for describing what happens to matter at the event horizon without getting too graphic. I don't believe most objects would actually get stretched out — they'd probably be torn apart before then.

Essentially, due to the strong tidal forces, an object will either become stretched out or break out into a vertical line of debris. Depending on what the material is would change the exact nature (ex. a star may get stretched out as it is gaseous, but an astronaut would probably fall apart. Neither is very pleasant though)

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OlympusMons94 t1_jasfhg2 wrote

Sphaghettification is not linked to the event horizon, or necessarily black holes or general relativity at all. For a supermassive black hole, spaghettification would not occur until well within the event horizon. For a stellar black hole, spaghettification would occur outside the event horizon.

Even just strictly following Newtonian mechanics, the tidal forces from being sufficiently close to a sufficiently high and compact mass would stretch you out and rip you apart. Of course, Newtonian gravity is not a very good approximation in situations where that would apply to objects as small as the human body, let alone molecules. (But for large objects, see Roche limit and the "stretch marks" on Mars' moon Phobos caused by tides.)

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BoxOfNotGoodery t1_jauj9yn wrote

It's not necessarily at the event horizon. Large black holes would allow for objects to cross the horizon without being pulled apart.

But past the horizon it's inevitable

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annomandaris t1_jaryr0b wrote

Imagine your hands are tied to the ceiling, and then they attach 5 ton weights to each of your legs.

That’s what spaghetification like in real life.

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SchillMcGuffin t1_jas8ecu wrote

How large does something have to be, though, before tidal forces can be destructive? Sure, something the size of the Earth can experience, well... tides... But down at human size how much can the difference between the force on your head and your feet really be? Is it just a function of the immensity of the overall force? And if the underlying force is that immense, might it distort space itself enough to limit the actual effect experienced on the occupying matter?

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RWDYMUSIC t1_jasbqga wrote

This is a good point, a human could actually fall through an event horizon without knowing. Gravity doesn't increase linearly with distance. So at some point when you get close enough to the singularity gravity experienced by your head and feet could be immensely different, but I can't imagine anyone falling that deep could survive long enough to get to that point.

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purpleoctopuppy t1_jauoh8j wrote

How would you fall through the event horizon without noticing? Your feet would be causally disconnected from your head (e.g. a nerve signal from your feet wouldn't be able to cross the event horizon to reach your brain), surely that would be noticeable? I guess my question is how can you remain a coherent object when no information can be sent radially outwards to the rest of you?

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mutandis57 t1_jazj3ym wrote

Everything inside the event horizon must travel towards the singularity, but the speed at which they travel inwards can still be different. This allows things like "blood" or "nerve impulses" to subjectively travel "upwards", such that from your perspective you will feel physically normal, even as you cross the horizon.

Here's an example using fake units: you are falling through the horizon, at t=0 your head is 1m above it, your legs 1m below it. Your toe sends a nerve signal. At t=1, your head enters the horizon, your leg is 2m below, and the nerve signal is 1.75m below. The signal is "deeper" into the black hole than when it started, but it still travels up your leg. At t=8, your leg is 9m below, your head is 7m below, and the nerve signal finally reaches your head. You can feel you toe!

You could even have two spaceships fall in separated by 1km (a rope linking them together optional!) and you could see your buddy on the front spaceship fall in past the event horizon and wave at them and then see them wave back in response. Of course, by the time you see the response, you yourself will already be inside the event horizon and way past the point where they were when they waved back.

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RWDYMUSIC t1_jba6e1z wrote

I guess a decent analogy would be like if you were a salmon swimming in a river that is increasing its flow rate as you move down. Eventually the water starts flowing so fast that you can't out swim it and you can't make it back where you started. If you were swimming next to a salmon friend that friend wouldn't suddenly get launched to oblivion when they hit the no-swimming-back point, you could watch them appear to swim normally next to you but neither of you can make it back up river. Your only option at that point is to keep moving in the direction the water is flowing. In the black hole case, just replace the water with space-time and its the same scenario.

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DCSMU t1_jasp9ed wrote

What would this look like from the point of view of the poor sob who falls in? Which effect gets to him first, time dialation or tidal strain (assuming he falls straight in and doesnt begin to orbit)?

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pzerr t1_jatn8f8 wrote

The guy falling in does not experience any time dilation. From his perspective, it will happen at normal speeds. Ignoring that radiation would likely kill you before the tidal effect, it would be quite painful but possibly too fast to be noticed.

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Speed_Alarming t1_jatx12a wrote

Yeah, I always found this whole thing to be hyperbole and sensationalism by people looking for a cool sound-bite for a tv bit. For a human-sized human crossing the event horizon of a black hole from a gravitational viewpoint you’d not even notice unless the black hole was super tiny and you were insanely close to the singularity itself. From your own perspective you’d just continue accelerating. The fact that no known force could prevent your inevitable “swallowing” is largely irrelevant. Going from almost an infinite amount of energy required to an infinite amount of energy? What’s the difference in the real universe? I imagine that the radiation environment from things being almost caught but instead yeeted out into the void would be more of a pressing issue. There’s likely layers of that depending on the size, nature and velocity of things in orbit.

From an outside observer’s perspective all sorts of crazy things would appear to happen, depending on your relative distances and the size of the black hole and the radius of its event horizon etc. None of that would be experienced by you, the poor hapless chappy in peril, you’d be dead from something long before you got close enough to get actually super-stretched.

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