I am writing this short story. It is part of the overall book that I am writing, but it is also a story that can be enjoyed completely on its own. In that story, a planet-sized ship approaches our Solar System, and, ultimately maybe, Earth. For dimensions, let's say it is equal to 1.5 of Earth's diameter. According to Google, that's 25,512 19,134 kilometers. The Planet-Ship is probably less dense than Earth, as it is largely occupied by biomass and weird alien electronics. You may think of it as a round Borg cube, from Star Trek.

My "Round Borg Cube" is completely black and spherical, with a smooth surface without any visible features.

What I wanna know is...

• Assuming that the object is on a very slow path towards Earth, at what proximity will its effects be known?
• At which point will scientists observe its effects, view it, or detect it with instruments?
• If that is at all possible, what would be a threshold in which the "Round Borgs" would have to interrupt their movement in order to remain undetected?
• At which distance will it be visible to the naked eye (if at all)?
• And if they chose to get dangerously close to Earth, what would be the impact on our environment?

I understand that is a lot to ask, but I just can't trust GPT for that kind of stuff, even if their answers sound plausible. Perhaps someone with astronomical knowledge as well as an interest in science fiction will find my questions enticing. I don't expect precise answers because I am not providing precise information. So feel free to speculate on that scenario. In any case, I am grateful for any answer I can get.

Thanks!

Hi all,

I am a PhD student focusing in cosmology. I wanted to up the science content here on Tildes, and I thought that one way to do so is to have an informal little Q&A session. As such, feel free to use this post to ask any questions you might have about cosmology specifically, and physics in general.

This may not be as exciting as some other science AMAs given that I am a rather early graduate student, so there may be a lot of questions I don't know the answer to. However, I'm willing to try my best and answer over the next few days, and to let you know I don't know if I don't!

A bit about myself: I did my undergraduate degree at the University of Chicago where I studied physics and mathematics, and then I was a student researcher in a computational cosmology group at a national lab. I subsequently enrolled at UC Davis to continue studying cosmology. Ask me anything about physics, cosmology, or high performance computing!

I also invite anyone else with expertise to chime in as well!

It is my understanding that if you are looking at an object falling into a black hole from a remote viewpoint, then the object will appear to take “forever” to complete the fall into the black hole. The object is effectively frozen in time at the black hole’s event horizon, from the remote viewer’s POV.

Is this the correct interpretation so far? If so, let’s remember that.

It is also my understanding that a black hole can increase in mass as it captures new objects. The mass does increase from an external viewpoint. Is this accurate?

If I understand known science on the above points, then the paradox I see here is that while the visual information is frozen in time from the external POV, the mass of the black hole does increase from the external POV. So is this where the Holographic Principle comes in? Or is there another explanation here, or am I off-base entirely?

Or is it just that the accretion disk gains mass and black holes never increase in mass from an external POV, after they are initially formed?

Is this known?

Please either attempt to answer my tortured question, or point me to material that might lead me ask a better question.

Thanks!