Crossing Event Horizon: Physics of Disintegration?

[arusse02]

As you fall through the event horizon, for a time one part of your body is inside the event horizon and the other part is outside. At that point the two parts of your body are casually disconnected. So shouldn't it be severing the chemical bonds holding your body together at the point of the event horizon? It doesn't seem like chemical bonds would be able to communicate electromagnetically between the event horizon and so you should disintegrate as you fall through the event horizon. The other way I was thinking about this is you have an extremely powerful rocket hovering over the event horizon. If you stick your arm into the event horizon then it's gone as soon as you cross. In fact it seems like nothing but point particles should be able to cross but please correct me if I'm wrong, thanks.

 

[Ibix]

It doesn't seem like chemical bonds would be able to communicate electromagnetically between the event horizon and so you should disintegrate as you fall through the event horizon.

The event horizon passes through you at light speed, so the bonds don't have a problem passing through it - they're through before they find out they're on opposite sides of the horizon. Unless, as you suggest, you are sitting on a powerful rocket. Then you leave an arm behind, yes.

 

[Fred Wright]

You may find this article of interest; https://www.sciencealert.com/what-would-happen-if-a-human-tried-to-enter-a-black-hole-to-study-it

 

[PeterDonis]

At that point the two parts of your body are casually disconnected.

No, they aren't. Say your feet are below your head, so your feet cross the horizon first. Your feet can still send light signals upwards to your head; by the time those signals reach your head, your head will be below the horizon, so no light signal ever has to cross the horizon going outward.

The error you are making here is the common one of trying to think of the horizon as a place in space. It's not. It's an outgoing lightlike surface--it is moving outward at the speed of light. Such a surface is fundamentally different from a place in space (which is described, in spacetime terms, as a timelike curve or surface), and cannot be thought of as one.

 

[PAllen]

As you fall through the event horizon, for a time one part of your body is inside the event horizon and the other part is outside. At that point the two parts of your body are casually disconnected.

Stop right here. This is false. As long as you are falling past the horizon, all parts of your body are causally connected at all times. Only if part of your body is equipped with rockets to keep it outside the horizon does that part of your body become causally disconnected from the part that crossed before. Further, it is only due to this rockets extreme acceleration pulling your body apart, that this happens.

A few clarifying points:

The event on your head and your feet, at this very moment (as defined by the simultaneity of a standard inertial frame) are causally disconnected - precisely because they are simultaneous in some frame. What matters is that each part of your body remains past and future causally connected to the history of every other part.

In the case of a body falling through a horizon, as along as all parts are in free fall , all parts will remain causally connected exactly as described above. A nerve signal from your feet after they are inside will reach your head just fine after it is inside

 

[PeterDonis]

You may find this article of interest; https://www.sciencealert.com/what-would-happen-if-a-human-tried-to-enter-a-black-hole-to-study-it

The basic issue this article is talking about is tidal gravity, which is a different issue from the one the OP is raising. It is true that a human falling into a black hole of stellar mass would be torn apart by tidal gravity, probably before they even reached the horizon, whereas a human falling into a supermassive black hole would not because tidal gravity at the horizon of such a hole is much, much smaller than it is at the horizon of a stellar mass hole. But this has nothing to do with any inherent "lack of causal connection" between the parts of objects that cross the horizon.

 

[Fred Wright]

The basic issue this article is talking about is tidal gravity, which is a different issue from the one the OP is raising. It is true that a human falling into a black hole of stellar mass would be torn apart by tidal gravity, probably before they even reached the horizon, whereas a human falling into a supermassive black hole would not because tidal gravity at the horizon of such a hole is much, much smaller than it is at the horizon of a stellar mass hole. But this has nothing to do with any inherent "lack of causal connection" between the parts of objects that cross the horizon.

Please excuse my ignorance and I'm not being facetious (I haven't made a concerted effort to study GR) but what is the event horizon? Can it be thought of as an infinitesimally thin membrane which surrounds the BH? Is it that if I'm falling into a supermassive BH and my feet precede the rest of my body I won't see my feet any more and the nerves in my feet can't communicate with the rest of my body?

 

[Ibix]

Please excuse my ignorance and I'm not being facetious (I haven't made a concerted effort to study GR) but what is the event horizon? Can it be thought of as an infinitesimally thin membrane which surrounds the BH? Is it that if I'm falling into a supermassive BH and my feet precede the rest of my body I won't see my feet any more and the nerves in my feet can't communicate with the rest of my body?

It isn't anything. There's nothing there to mark it, any more than there's anything to tell you you're at the north pole. It's just the surface (it isn't really a place, since it's a null surface) beyond which light signals can no longer escape.

Whether or not you can see your feet depends on what you are doing. Light signals (or nerve signals) from your feet cannot escape the hole, but if you are falling into the hole, you'll soon be inside the hole and the light signals will reach you. So if you fall in, there's no time when you can't see your feet. If, however, you let your feet fall in and then turn on your super-powerful rocket backpack so that you escape, your feet will not escape and you will not see anything of them after they crossed the horizon.

 

[PeterDonis]

what is the event horizon?

It is the boundary of the black hole, which is a region of spacetime that cannot send light signals to infinity.

Can it be thought of as an infinitesimally thin membrane which surrounds the BH?

No. As I said, the horizon is not a place in space. That means it is not any kind of ordinary "object", whether a membrane or anything else.

Is it that if I'm falling into a supermassive BH and my feet precede the rest of my body I won't see my feet any more and the nerves in my feet can't communicate with the rest of my body?

No. The previous posts from me and others in this thread, in response to the OP (whose question was basically the same as this one), should already have made it clear that that's not the case.

 

[DaveC426913]

It's just the surface (it isn't really a place, since it's a null surface) beyond which light signals can no longer escape.

While correct, I think even this description as "a surface" is misleading to the layperson because it still seems suggest that there's a physical difference between what's above the surface and what's below it.

The difference between above the surface and below the surface of the EH is no more physical than the "surface" of the sphere made by all the points that a frog can jump above (an idealized) Earth. This would form a rough sphere about 3900 miles and 2 feet in radius.

But it's not physical - it's abstract. There's no way of knowing where it is without taking careful measurements and deducing by calculation the location and extent of the invisible boundary that we call the Frog Horizon.

Right?

I wish I could find a better analogy.

 

[Ibix]

Right?

Almost. There is a geometrical significance to the event horizon that's lacking with the frog's altitude ceiling. Also, frog ceiling is a spacelike surface and the event horizon is null.

But I think you are trying to get at the same thing I was doing by commenting that it's similar to the north pole. There is no marker, nothing to tell you you have reached it. This is not a perfect analogy either - I think it has the same defects yours does.

 

[DaveC426913]

...the event horizon is null.

OK, but surely that's still an abstract trait in the sense that it can only be deduced as opposed to tested. It's not like there's anything you can directly detect near the EH that says you are inside versus outside. Photons don't change direction or some such. (I'm a bit digressive here because I'm tying to describe in a way laypeople can grok.)

 

[PeterDonis]

I think even this description as "a surface" is misleading to the layperson

The key thing to remember is that it's a surface in spacetime, not space. The layperson's intuitive understanding of "surface" is a surface in space, and the horizon is not a surface in space.

Perhaps drawing that distinction would help.

 

[Ibix]

The key thing to remember is that it's a surface in spacetime, not space. The layperson's intuitive understanding of "surface" is a surface in space, and the horizon is not a surface in space.

Perhaps drawing that distinction would help.

Part of the problem is that "null surface" is one of those things that doesn't have a non-relativistic analogue. An acausal spacelike surface is what a layperson would call "a moment in time". A timelike worldline is what a layperson would call "a place in space" (or several more general concepts, like my personal path through the world). But relativity allows a lot more concepts, including null surfaces. Null surfaces have some of the properties of a moment in time (you can only pass through them once), and some of the properties of a place in space (events on it are causally connected so something that happens there earlier on can affect something later). They're kind of betwixt-and-between the everyday concepts.