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SchillMcGuffin t1_jas8ecu wrote

Reply to comment by paleopuzzler in What exactly does Spaghettification mean? by mark0136

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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