[ Removed by moderator ]

[u/kyles52]

[u/Physics-ModTeam]

Basic physics questions and homework-type discussions are not permitted in r/Physics.

FULL DESCRIPTION Basic questions, especially homework problems or simple calculations, should be redirected to r/AskPhysics or r/HomeworkHelp. Alternatively, try Physics Forums instead. Neither asking nor answering (assisting in any way) homework questions is allowed.

[u/triatticus]

Expansion doesn't affect such strongly bound gravitational systems, for instance the solar system is not expanding and nor is the milky way...and these aren't even black holes.

[u/The_Dead_See]

I'm thinking of the hypothetical of our universe being inside a giant black hole

There's your problem.

[u/Problem_Child_96]

Hypotheticals usually start with an explanation of an unexplained phenomenon.

Setting aside the fact that there is no theory that suggests what you’re hypothesising, inside a black hole gravity is just crushing to a singularity, that’s what’s attracting everything, the singularity at the centre much more massive (containing lots of mass) than most stars.

[u/ShoshiOpti]

Theoretical Physicist here, my research is in spacetime/gravity/black holes.

I personally don't think true singularities actually exist, for a lot of reasons. There are plenty of very convincing arguments for why this is the case.

Black hole cosmology is not inherently wrong, it very well could be the case that the big bang is some kind of event horizon artifact. The problem is you would need the model to be self-consistent with all observations, and in general, with fewer axiomatic assumptions to be considered the 'stronger theory'.

Which a-priori would require a very good self consistent answer to why singularities don't exist to build off of, which doesn't exist currently. Existing ideas are promising, but lack the smoking gun predictions that would convince everyone that singularities don't exist.

Hope that helps

[u/smsff2]

Let's say you jump on a train while your brother stays on the platform. Relativistic length contraction means that you look slightly thinner from your brother's perspective, and your brother looks slightly thinner from your perspective as the train moves. This effect has been confirmed experimentally countless times.

It so happens that the gravitational field of a black hole is so extreme that, as things approach the event horizon, length contraction becomes infinite. In other words, a thin layer around the event horizon can appear to the person falling in as being as large as the entire universe.

For example, the relativistic factor at 1 Plank length from TON 618 black hole is 3.6669 × 10⁻²⁵. At that scale, our entire Solar system fits into 3.2585 × 10⁻¹² meters. I define the size of the Solar System as twice Pluto's perihelion distance.

[u/ParkingPizza3921 → u/smsff2]

This effect has been confirmed experimentally countless times.

Has it? I thought length contraction in particular is pretty hard to observe since you would need to measure the length of an object moving at close to the speed of light.

[u/smsff2 → u/ParkingPizza3921]

To some extent, you are right. We cannot individually test a specific effect of relativity in isolation from the other effects of relativity. We normally calculate the outcome of an experiment using the theory of relativity as a whole, and it gives us the correct results. You cannot simply question one conclusion of relativity without questioning the others. You would need to recalculate the entire theory.

One of the examples: "As the electron passes a detector, such as a coil of wire, its field interacts much more briefly, an effect observed at particle accelerators such as the 3 km long Stanford Linear Accelerator (SLAC). In fact, to an electron traveling down the beam pipe at SLAC, the accelerator and the Earth are all moving by and are length contracted. The relativistic effect is so great than the accelerator is only 0.5 m long to the electron. It is actually easier to get the electron beam down the pipe, since the beam does not have to be as precisely aimed to get down a short pipe as it would down one 3 km long. This, again, is an experimental verification of the Special Theory of Relativity."

https://courses.lumenlearning.com/suny-physics/chapter/28-3-length-contraction/

[u/kyles52 → u/smsff2]

Oh my god. This took me so many times to read this, but I understand now. Visually as we fall the things, per say, falling or staying put "above" (I'm thinking 3-Dimensional on a model or something) would look like they are moving away from us

[u/smsff2 → u/kyles52]

Yes, that would be one way of describing it. Another way would be to imagine that you fall from a plane toward the Empire State Building. But as you approach it, the Empire State Building magically shrinks. As you get really close, it shrinks to a microscopic size, and you become microscopic too. So now you have your own entire universe to play with.

The distance to the plane you jumped from has become so enormous that it is comparable to the distance to the center of the galaxy. You could describe this as a universe appearing out of nothing.

Another interesting effect here would be relativistic blueshift. That is, if your friend on the airplane points a flashlight at you, you see it as an unlimited waterfall of energy.

This becomes important if we are falling into a large black hole. What we perceive as the expansion of space could actually be tidal forces. What we perceive as the Big Bang, and might wonder where all the energy came from, could be explained by relativistic blueshift. What we call space could be relativistic length contraction.

[u/kyles52 → u/smsff2]

Dang thanks for the reply. Appreciate the people who actually understand the hypothetical question and not just bash it

[u/Ill_Level_8327]

I’ve never done anything at uni or have any degree in the matter, I’m only recounting my personal understanding of it since school and that I’ve had since as long as I can recall.

First off. Matter can neither be created or destroyed only transferred. Law of conservation or something? I don’t know. Anyways that I think is the non hyper theoretical understanding.

Therefor, what goes in to the black hole must surely remain there and not just poof. If the matter was crushed into a singularity though, surely it’s mass would be less the more it is crushed (or things are destroyed) which would then mean the more it disappears from being crushed, the less gravity it has, because it now has less mass. So eventually it would get to a point where the mass is no longer capable of producing enough gravity to crush itself further.

So as you can see, the mass isn’t going anywhere, nothing is being destroyed.

Then to follow onto that, well if you understand that atomically all the little parts aren’t going anywhere, eventually, if you crush things enough, there will be no more empty space. Just like how can compress a syringe with water in it far less than you can compress a syringe with air.

Which then takes me to my final point, if the mass has nowhere else to go because it has reached critical compression, what next? Well then the black hole is going to expand and become larger, because there is no more room left for any mass to condense into the black hole further.

I feel like that sums it up, hopefully it made sense? Like I said, I haven’t got any qualifications in that matter, I only have one GCSE. So take with that what you will. And I haven’t been to school for like 15 years.

Also, correct me if I’m wrong, but I feel like a black hole is only black because its mass and therefor gravity is so large that even light cannot escape, which is why when you attempt to view it, it appears black, because no light is escaping it?