# Empirically testing black holes.

Gold Member
How much is it feasible for us to send some probe/s to the event horizon of a black hole?

Perhaps Voyager will arrive at the centre of our galaxy, where if I remember correctly there's a massive black hole. How much time will that take? Probably Voyager will collapse way before arriving at the centre of a black hole.

I mean for me, I would like to know what goes inside a black hole, cause you can do a lot of theorizing about what goes inside a black hole, but without any data it's really like believing in fairy tales.

## Answers and Replies

It's completely unfeasible with current technology. The time scale for interstellar travel is too long, even at speeds approaching c.

Take the Voyager probes and the central black hole as an example.
1.The plutonium pellets providing electricity will be unable to power the instruments within about a decade or two.
2. The probes travel at ~17km/s with respect to the sun in the direction roughly towards the direction of the orbital motion of the solar system. This means they still have ~250 km/s tangential velocity associated with the revolution of the solar system around the galactic centre. With no plausible means to lose that speed, they will never move appreciably closer to the galactic core.
3. Even if they could somehow lose all that speed and move in a straight line towards the central black hole, and even if they could be accelerated to 99% speed of light, they would take ~30000 years to get there. At their current asymptotic velocity of ~16km/s, it'd take ~4000 years just to cover one light-year of the distance.
4. There's no way we'd be able to send or receive any signals at such a distance, so the probe would have to swing around the black hole and physically get back to us with the data. This means another 30000 years at best. Also no means to correct the course as it travels.
5. Finally, and perhaps most importantly, once it gets there, it can not probe what's going on iside the black hole, as it's in principle impossible to receive any information from beyond the event horizon. It could only look at the effects the black hole has outside the horizon, like the accretion disc, Hawking radiation, maybe frame dragging.

Also, I don't think it's fair to compare the theories concerning black holes with fairy tales. There is a strong theoretical background behind the former(General Relativity).

Genocide
Absolutely agreed with Bandersnatch. Further I guess we would not even know if Voyager actually enters into a black-hole because Kruskal diagram reveals that we see infinite time from our frame. So it would be like getting stuck in event horizon from our point of view although it may go deeper inside towards singularity, from "voyager's" point of view. And theres no way to communicate after that. And yeah the Photonsphere (3GM/c^2 radius) would make matter even worse for communication .

For inside black-hole question .. since the geodesic and Reimann curvature tensor are smooth except at singularity I think nothing so special occurs except for maybe some exotic pair-productions and stuffs like that :)

Peace,
Genocide!

I mean for me, I would like to know what goes inside a black hole, cause you can do a lot of theorizing about what goes inside a black hole, but without any data it's really like believing in fairy tales.

Unfortunately the only way for you to be satisfied then is to jump in.

We can, and have, studied the environment around black holes (via close stellar orbits and accretion disk phenomena). The only thing 'left' really, is to see gravitational wave signatures from merging black holes, which is the ultimate probe of the strong field.

David Mentor
That’s not quite correct. We have studied the environment around black hold candidates at vast, sometimes intergalactic distances. No one knows for sure what they really are other than blobs, anomalies, sources of radiation.

Mentor
That’s not quite correct. We have studied the environment around black hold candidates at vast, sometimes intergalactic distances. No one knows for sure what they really are other than blobs, anomalies, sources of radiation.
With a tiny set of exceptions, all stars appear as point-like sources of radiation to our telescopes. Think of all the things we know about those stars!

It is possible to measure the size of accretion disks indirectly via doppler shift measurements. You can "see" the disk with that method.

Genocide said:
So it would be like getting stuck in event horizon from our point of view
It would not. You can calculate that the observed intensity of emitted radiation drops quickly - you won't see it there for a long time until no further photons will hit you.