11 ms·

To state that point in another way, the key here is time dilation. (Please correct me if that's wrong.) Let's assume that your feet send a periodic signal to th

by codeflo 13y ago

To state that point in another way, the key here is time dilation. (Please correct me if that's wrong.) Let's assume that your feet send a periodic signal to the brain, a "tingling" that let's you know your feet are still there. As your feet approach the event horizon, the tingling will get slower and slower, because time passes much faster for your head than for your feet. As your feet pass the event horizon, the tingling has become infinitely slow (relative to your head). But there's no sharp line where suddenly something happens as your feet touch the event horizon. (All of this of course only in principle and under the premise that you're not smashed to bits at that point.)

millstone 13y ago
I would say this isn't quite right. It's true that time passes more slowly for your feet than your head, but this is due to tidal forces: your head and feet are different distances away from the black hole. For very large black holes, the tidal forces at the event horizon will be small, and the event horizon is the unexceptional, smooth spacetime that Steuard described. So in your example, for a large black hole, you will experience no difference in the rate of tingling as you cross the event horizon. This is what the principle of relativity tells us: you are in free fall, so the laws of physics must appear the same to you.
Steuard 13y ago
I'm not sure that this captures the physics accurately from the perspective that you're describing here. The extreme time dilation that occurs as you approach a black hole's event horizon is really a "global" effect rather than a "local" one: it is defined by the relative difference in how time progresses close to the black hole vs. very far away. When you're falling through the horizon (particularly for a black hole so large that the tidal forces at the horizon aren't deadly), your whole body is "close", so there isn't substantial time dilation between your head and your feet. This is a little bit weird, I know!
pdonis 13y ago
(Please correct me if that's wrong.) It's wrong. Time dilation is not something you observe yourself to experience; it's something another observer observes you to be experiencing. "Time" is relative, so you can feel your own time to be ticking away perfectly normally while it appears to be ticking much more slowly to someone else. As your feet approach the event horizon, the tingling will get slower and slower, because time passes much faster for your head than for your feet. No, this is not correct. As you fall toward and through the horizon, signals travel between your head and your feet just fine, and you don't notice any difference in how fast they travel. But, as I posted upthread, if your foot emits a signal towards your head when it's below the horizon and your head is above it, then by the time the signal reaches your head, your head will have fallen below the horizon.

by codeflo 13y ago

millstone 13y ago

I would say this isn't quite right. It's true that time passes more slowly for your feet than your head, but this is due to tidal forces: your head and feet are different distances away from the black hole. For very large black holes, the tidal forces at the event horizon will be small, and the event horizon is the unexceptional, smooth spacetime that Steuard described. So in your example, for a large black hole, you will experience no difference in the rate of tingling as you cross the event horizon. This is what the principle of relativity tells us: you are in free fall, so the laws of physics must appear the same to you.

Steuard 13y ago

I'm not sure that this captures the physics accurately from the perspective that you're describing here. The extreme time dilation that occurs as you approach a black hole's event horizon is really a "global" effect rather than a "local" one: it is defined by the relative difference in how time progresses close to the black hole vs. very far away. When you're falling through the horizon (particularly for a black hole so large that the tidal forces at the horizon aren't deadly), your whole body is "close", so there isn't substantial time dilation between your head and your feet. This is a little bit weird, I know!

pdonis 13y ago

(Please correct me if that's wrong.) It's wrong. Time dilation is not something you observe yourself to experience; it's something another observer observes you to be experiencing. "Time" is relative, so you can feel your own time to be ticking away perfectly normally while it appears to be ticking much more slowly to someone else. As your feet approach the event horizon, the tingling will get slower and slower, because time passes much faster for your head than for your feet. No, this is not correct. As you fall toward and through the horizon, signals travel between your head and your feet just fine, and you don't notice any difference in how fast they travel. But, as I posted upthread, if your foot emits a signal towards your head when it's below the horizon and your head is above it, then by the time the signal reaches your head, your head will have fallen below the horizon.