D-1: Just Over The Horizon

The following is Murmeldjuret’s final guest column in this series. It was originally published on the Stellaris forums.


Now that we are getting close to release, you know what would really suck? Time slowing down or even stopping.

Gravity slows time down, very marginably. The GPS satellites experience time slightly faster than we do at the surface. It is only a few microseconds each day. The closer you are to a densegravitational source, the more spacetime is curved, and the slower time passes. If you follow this to its extreme, you can with a gravitational singularity stop time as we know it.

(The black hole from Interstellar)

The most common singularity is what we call a black hole. We call it black because the most used description is that light can’t escape it. This is a simplification. Nothing escapes a black hole. The direction of out no longer exists. Just at the event horizon or surface of the black hole, the spacetime paths that light can take all are bent into leading further into the hole.

For an outside observer, anything falling in will slow down and be redshifted. The moment of it arriving at the event horizon, from the view of an outside observer, it will have stopped entirely. The redshift is now infinite as the time it will take the light to leave is infinite. That last light that never can leave is the event horizon. The event horizon is simply put a stitched together canvas of all the things that have fallen in. The surface is the last event of anything that has fallen in spread out for all future to witness. If a million years passed, that same event would be on the surface, still trying to send out the same light.

Now here is the weird bit. Black holes gain mass when things fall in. Yet nothing ever falls in, as time has stopped on the surface for everything outside the black hole. The weight is from all future events. So in a million years, its mass is felt outside, but its past is still stuck on the horizon.

So how does it look from the perspective of anyone falling in? For them time passes normally. It is like any gravitational free fall. Eventually the gravitational difference will be so great matter gets torn apart. Probably. For us on the outside, nothing happens in a black hole. It just happens to be a heavy collection of futures with a surface of pasts.

So what happens when to black holes merge?
This is a simulation of how the gravitational lensing would look like.
Oj287 is probably a supermassive binary black hole and it will merge sometime within 10000 years. PG 1302-102 is a very likely candidate for a supermassive binary black hole and if so it has already merged and the event will reach us in just shy of a million years. It will shine brighter than a supernova even though it is billions of lightyears away. We know that black holes merge, we have even detected the gravitational waves now, so even if black holes are still weird, logic defying things, we have found them.

We know what happens when two black holes collide from the outside. What happens on the inside? What happens when two events that do not occur collide? Does it even happen if nothing can observe it? What is even the inside of a black hole?

Quantum effects on the black hole surface create Hawking radiation, often called black hole evaporation. Entropy suggests black holes shrink, and it does indeed seem they can do so. Freeing up the events that have already happened as radiation. The universe truly is weird.


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