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)

In short, it’s basically a hybrid.

From the point of view of a body experiencing it, it’s just an extreme tidal force (which is not actually a force, but the rate of change of acceleration induced by some force with respect to position). It is space time being stretched, and that would produce tension on an object falling into a black hole (and I think there’s also compression in directions perpendicular to that tension, as depicted in the image you shared, but I don’t recall the details off the top of my head). Early on, that tension is not sufficient to break apart molecular bonds, but eventually it is, and the object will indeed get ripped apart. Eventually that tidal force might be strong enough that molecules get ripped apart, and at some point the curvature could be enough that you have to modify the very description of fundamental particles, at which point you’re getting into questions of quantum gravity, which are beyond the scope of experiment at present, and for which there are no generally accepted theories.

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.

I feel like the ripping of molecules would happen due to the magnetic field way before gravity has a chance to do so. And also gravity would first cause the ripping of the physical structure across the weak points ( joints and such? idk too much about anatomy).

> would happen due to the magnetic field way before gravity

There isn’t necessarily any magnetic field. Spaghettification happens even in an idealized non-rotating, charge-neutral black hole.

> ripping of the physical structure across the weak points

Those points are bonded together by some kind of chemical bond, like hydrogen bonds between different atoms.

so the human body cannot snap in half if pulled apart with huge amounts of force?

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> But is it a separation of molecules? That is, would an object just rip apart into pieces?

It's this one.

Tidal forces stretch objects along the radial direction (toward and away from the gravitating body) and compress them along the other directions. Spaghettification is the result of tidal forces taken to the extreme.

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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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.

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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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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Wouldn't it be like a Roche limit for non-gravitational structures?

With a biological structure orbiting a black hole, eventually the part closest will be moving so fast that it'll be ripped away/apart at the macro level, then cellular, then molecular, etc etc until total annihilation.

Like pressing a finger against the ultimate rotary sander.

(Uneducated but curious, so my terminology is probably inaccurate, please be gentle.)

imagine the front of ur car accelerates stronger than the back. eg. at some point the front is already moving whit 100mph ur back is still only moving 5mph. 1sec later ur front has accelerated to 200 while the back only got to 10mph.

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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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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.

A car is a pretty rigid, contiguous unit tho. Pull hard on the bumper(fender, whatever) and it’ll rip right off, but pull hard on the chassis you take the whole car with you. Humans aren’t as rigid or strong, but we’re also much smaller. You’d have to be super close to the centre to feel a difference from one part of you to the next and by then you’ve got plenty of problems to deal with.

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Spaghettification is due to strong gradient in gravitational pulls between different parts of an object while approaching a black hole

Getting close to a black hole, gravitational pulls can vary significantly also in a matter of microscopic distances, and because of that the closest parts of the object get accelerated before the others so strongly, that the object breaks and gets "spaghettified", meaning "reduced to strings of atoms orbiting or falling into the black hole", instead of falling into it or orbiting it while maintaining its original shape

You can experience a similar event if you drop some ink close to a water vortex, you will notice it creating strings following the path described by the vortex itself, because part of the ink gets captured by the circulating waves simulating the gravitational pulls of the black hole (you can look for something about using fluid dynamics to simulate conditions close to black holes)

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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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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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Yes the molecules are being pulled apart due to the stronger gravity closer to the black hole. Space-time stretching may have some effect in the last millionth of a second before the object is pulled in, but the object will already have been spaghettified before that happens.

Here’s a bit of math to back that up:

a=G*M/r2

Acceleration = gravitational constant * mass / radius(distance to mass center) squared

G=6.7e-11

Earth check:

M= 6e24

r=6378000m=6.4e6m

a= 6.7e-11 * 6e24 / 6.4e6^2 = 9.8m/s^2

3 solar mass black hole:

M= 6e30

r_feet = 1000000m (your feet at 1000km away)

r_head= 1000002m (your head)

a_feet = 402,000,000 m/s^2

a_head = 401,998,392 m/s^2

Looks almost the same but your feet are being pulled away from your head at 1608m/s^2 , or 164x Earth gravity! You’re also going close to speed of light and have only a few milliseconds left to live.

Above is Newtonian math and is good enough to answer your question. Even if you’re going 99% the speed of light, spacetime dilation is only 14%:

γ = √(1 - v²/c²) — Lorentz factor

= √(1 - 0.99²) = 14%

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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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.