# How would a clock with hands behave if half of it is outside a black hole and half inside? [closed]

I am thinking of a clock moving close to the speed of light at a tangent to the event horizon so it doesn't fall in instantly Would the time like spacetime inside the black hole be reflected in the way time passes outside the hole in space like spacetime?

• Firstly, for an external observer, nothing can enter a black hole. Secondly, for the very clock, if assimilated to a point, there is no problem of any sort when crossing the horizon. A more complete picture is found by considering the clock as being made of particles. Commented Jan 19, 2023 at 16:29
• I don't think any physical object could exist half inside and half outside, because the intermolecular forces holding it together cannot propagate from inside to outside. The idea of an event horizon is that the inside is not a region of concurrent happenings with the outside. They are completely separated causally. Commented Jan 19, 2023 at 16:30
• Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
– Community Bot
Commented Jan 19, 2023 at 16:58
• @RC_23 If force can't propogate from inside to outside how does an electric charge at the centre of a black hole give it an external electric field that can exert force on charges outside the black hole?
– user355436
Commented Jan 19, 2023 at 17:47
• It would make sense if the charge isn't in the centre but spread out outside the event horizon .But this would mean even one electron would have its charge divided many times into smaller charges.Perhaps that has happened to the universe's missing antimatter.
– user355436
Commented Jan 19, 2023 at 23:00

A clock on a path aimed to be tangent would not hit at a tangent. Everything, even light, is deflected inward. Gravity around a black hole is so strong that light fired tangentially at 2.6 times the radius of the event horizon orbits the black hole. Matter cannot have a stable orbit below 3 times the event horizon radius. See Veritasium's How to Understand the Black Hole Image for more.

It is theoretically possible to escape at any height above the event horizon. You would need an initial velocity away from the black hole, or a rocket engine.

But even if you arrange for the top of the clock to escape, or if you stand back and take pictures of the top half, you don't learn anything about the bottom half. The part inside the event horizon is on a path inward. It cannot affect the part outside.

One possible scenario is that the clock might be ripped apart. Gravity gets stronger as you get closer. The inner part might be attracted so much more than the outer part that it gets pulled in, leaving the outer part behind. This is called a tidal force. It is a more extreme version of how the moon makes the ocean level go up and down.

But this need not happen. For a larger, galaxy core sized black hole, the tidal forces might be gentle enough to survive. In this case, what you see depends on your frame of reference.

If you fall in alongside the clock, you see nothing special happen to the clock. It keeps ticking, even inside. Very soon you get closer to the center, and tides get strong and rip you apart.

From the point of view a long distance away, the curvature of spacetime gets very strong. Times and distances get distorted. Through your telescope you see light reflected from the clock. As the clock approaches the black hole, time runs slower. You see the hands slow down. You see the speed of approach to the black hole slow down. Furthermore, light from the clock gets red shifted.

As it approaches the black hole, this gets extreme. Time slows so much that you don't see it actually cross the event horizon until an infinite amount of your time passes. From your point of view, it hovers over the horizon.

Of course you don't see it hovering. From the clocks point of view, nothing is unusual. The clock reflects a certain number of photons each second. From your point of view, the clock's time slows. These photons arrive at a slower and slower rate in addition to being redder and redder. The clock quickly fades from view.

So you can't learn anything about what goes on inside because you can't see the clock ever get there.