[Physics] Thought Experiment – Poking a stick across a Black Hole’s Event Horizon

black-holesevent-horizongeneral-relativitytidal-effect

The classical explanation of a black hole says that if you get to close, you reach a point – the event horizon radius – from which you cannot escape even travelling at the speed of light. Then they normally talk about spaghetti.

But here's a thought experiment. What if I have a BH with event horizon radius R such that the gravitational gradient at the event horizon is far too weak for creating pasta. I build a Ring with radius R+x around the BH. Then I lower a pole of length x+d from my ring towards the BH, so that the tip passes beyond the event horizon.

Now what happens when I try to pull the pole back?

Best Answer

Funnily enough, you will never get to find out what happens when you try to pull it back, because you won't live to see the stick pass through the event horizon. That's not because you will suffer some kind of violent death (although you probably would), it's because you will die of old age before the stick reaches the event horizon. As you push the stick towards the black hole, the subjective time of the end of your stick moves more and more slowly relative to your subjective time.

The Schwartzschild metric can tell us $\frac{d\tau}{dt}$, the rate of time passage at a particular radius $r$ compared to the rate of time passage infinitely far from the black hole:

$$\frac{d\tau}{dt}(r) = \sqrt{1 - \frac{r_s}{r}}$$

where $r_s$ is the event horizon radius. Now, if you are at radius R, and the end of your stick is at radius $R_s$, then the relative rate of time passage of the end of your stick to you is

$$\frac{d\tau}{dt}(R_s) ~/~ \frac{d\tau}{dt}(R) = \frac{d\tau_{stick}}{d\tau_{you}} = \sqrt{\frac{1 - \frac{r_s}{R_s}}{1 - \frac{r_s}{R}}}$$

Notice, that as $R_s$ approaches $r_s$, that ratio approaches zero! By the time your stick is nearly at the event horizon, the end of your stick is experiencing almost no passage of time compared to you.

Now, it seems to me that an interesting question is, if you push on this stick, what do you feel? A force pushing back? What kind of force is it?

When you push on a rigid body like the stick, you are actually sending a wave of pressure through the atoms of the stick that travels at the speed of sound in that material; that's how you effect a force on the front end of your stick without actually touching it. Such a wave would also slow down as it propagates down the length of the stick towards the event horizon, so the front side of the stick would not respond to your force as it normally would. I think that you would experience a kind of "pseudo inertia" - a time-dilation-derived inertia, as though your stick had an enormous mass. But I'll have to think about that some more to be sure.