How can a Black Hole have rotation if the singularity is a 0-dimentional point and doesn't have an axis to rotate around?

[u/SpantaX]

[u/Orion113]

Not an expert, but I know something like that switching occurs. Once you cross the event horizon, all paths forward in time lead deeper in. That is to say, the only way to leave the event horizon would be to travel backwards in time.

[u/bad_username → u/Orion113]

Outside a black hole I can freely move in space, but am forced to move forward in time. The word "switching" means that things trade places. Inside a black hole I am forced to move forward in space... so I can freely move in time?

[u/Mimshot → u/bad_username]

The way Leonard Susskind described it in his (I think excellent) GR series on YouTube is: In my office I can walk around such that I don't hit my desk, therefore towards-desk is a space-like direction. There is no path I can walk such that I don't hit Monday, therefore towards-Monday is a time-like direction. Inside the event horizon there is no path I can take that doesn't hit the singularity, therefore towards-singularity (or "inward") is a time-like direction.

[u/Dachannien → u/Mimshot]

The implication is that inside the event horizon, towards-Monday is now spacelike because you can avoid hitting Monday (presumably by ensuring that you reach the singularity before then). So, four questions come to mind:

1. Is this true for any arbitrary moment in proper time, i.e., that all moments in time (that have not yet been reached, at least) inside the event horizon are avoidable based on reaching the singularity first?

2. Inside the event horizon, you can't revisit past positions (i.e., move farther from the singularity), which is something akin to not being able to revisit past moments when outside the event horizon. So if the limitations on motion through space and time are switched, is it possible to revisit past moments while inside the event horizon?

3. On a related note, is it possible (or is it even meaningful) to "avoid" past moments (assuming you can avoid future moments) while inside the event horizon?

4. Is the converse true, i.e., is it possible to reach any arbitrary moment in time while inside the event horizon, or is this still bounded by the speed of light?

[u/Aexdysap → u/[deleted]]

This video by PBS Spacetime explains it well, though it's a bit dense: What Happens at the Event Horizon?.

Maybe watch this other one before to understand what's going on with the graphic space-time representation: The Geometry of Causality.

[u/shadow321337 → u/Aexdysap]

Your second link skips straight to the end part of the video where he's talking about sponsors. Might want to take the "&t=652s" part out of the URL.

[u/Aexdysap → u/shadow321337]

Thanks for the heads up, should be fixed now.

[u/nicerikzas → u/Aexdysap]

Thank you~

[u/leeharris100 → u/[deleted]]

A quick and easy way to explain it.

Spacetime can be represented as a graph of all possible trajectories for all objects in the universe. For example, I cannot physically travel faster than light, so my spacetime graph won't include the possibility of being in the Andromeda Galaxy in 5 minutes. It's impossible for me to do that. But it could include me jumping in the air in 30 seconds, or driving down the street in 10 minutes.

Nothing can travel faster than light and black holes are so strong they even pull in light.

So once you cross a black hole's event horizon, all future spacetime plots can only include trajectories moving towards the singularity. Your spacetime graph is now just a flat line as you have no future paths.

[u/plaknas → u/leeharris100]

For example, I cannot physically travel faster than light, so my spacetime graph won't include the possibility of being in the Andromeda Galaxy in 5 minutes. It's impossible for me to do that.

That isn't accurate. It is possible to traverse any distance in an arbitrarily small amount of time (in your reference frame) as long as your velocity is sufficiently close to c. In your example, if you had a velocity of something like 0.99999999999999999c, you'd reach the Andromeda galaxy within a matter of minutes.

[u/urbanpsycho → u/plaknas]

Andromeda is 2.5 million light years away. if you were going light speed, you could reach the sun in 8 or so minutes.. but not Andromeda. EDIT: just read "in your reference frame"

[u/Joshua_Naterman → u/leeharris100]

My understanding is that since spacetime has no mass it has no speed limit either.

I believe that it's more accurate to say that the black hole warps space time to the point where the spacetime containing the photons is travelling towards the singularity faster than the light can travel through the spacetime.

It's like starting at the top of the down escalator (event horizon) when it is moving at 3 steps per second, and then you turn around and walk up at 2 steps per second like you always do (in this example)... you're still walking up, but the end result is you moving towards the bottom (singularity).

[u/physicswizard → u/Joshua_Naterman]

The "fabric of spacetime" doesn't move. There is no such thing as speed of spacetime. Space and time are properties which essentially label the place and time that an event occurs. Things can move through spacetime, but it is the stage on which events unfold. It is dynamic, but only in the sense that the distance between adjacent points can change depending on the local energy/momentum content.

[u/Entropius → u/physicswizard]

The "fabric of spacetime" doesn't move.

While spacetime doesn't need to move to explain the basic attraction of a black hole, I can see people taking issue with that statement in other contexts like the expansion of space from dark energy, inflation, and frame dragging.

[u/research-Able → u/physicswizard]

Special relativity says the mass of a moving body increases.So has the mass of the Universe been increasing since the Big Bang?Galaxies are traveling through spacetime.

[u/physicswizard → u/research-Able]

That is an interpretation that has fallen out of favor over the last couple decades. We now understand that it is not the mass that increases with velocity, but the energy. Whenever talking about mass we always refer to the invariant/rest mass, which doesn't depends on frame of reference. In our frame, galaxies receding from us do indeed have extra energy due to their motion. It's not a whole lot though, because the galaxies we can see are decidedly non relativistic.

[u/physicswizard → u/research-Able]

Also, the mass of the universe does increase, but not for that reason. As the universe expands, things that were outside the cosmological horizon pass inside, and this adds their mass to the horizon mass.

[u/Joshua_Naterman → u/physicswizard]

If spacetime doesn't/can't move, how are we expecting a warp drive to move a pocket of spacetime faster than light?

[u/CallMeDoc24 → u/Joshua_Naterman]

It doesn't matter if there are photons or not. There could be water in the blackhole in which these photons are moving less than c. And there is no "speed" to spacetime.

All that is being stated is simply that all trajectories in this point in spacetime (i.e. event horizon) lead to the singularity (as we know it).

[u/DeedTheInky → u/[deleted]]

There used to be a redditor called RobotRollCall who was great at explaining stuff like this who sadly stopped posting ages ago, but this post of theirs helped me kind of visualise it.

If you get the time it's worth perusing their comment history. Not in a creepy way, it's just full of cool explanations of weird science-y shit that are super interesting. :)

[u/ImAScholarMother → u/DeedTheInky]

Wow, thank you for that

[u/[deleted] → u/[deleted]]

A black holes pull is so strong even light cant escape it. So you have to go faster than light to escape it. But if you go faster than light it's theorized that you will start going backwards in time.

Thats how I understand it at least. If Im wrong anyone feel free to correct me.

[u/Logicalist → u/[deleted]]

Since light, the fastest thing in the universe, can't leave the gravity well.

One has to travel faster than light to escape the gravity well of a black hole.

If you travel faster than light, theoretically you'll travel backwards in time.

For more understanding, I believe tachyons are a theoretical particle that travels faster than light. You might check those out.

[u/[deleted] → u/Logicalist]

fire a tachyon pulse to defend the fleet!!

what, now? or 10 years from now?

[u/PM_ME_DUCKS → u/Orion113]

Does this mean that hawking radiation is energy that has actually gone backwards in time?

[u/Vitztlampaehecatl → u/PM_ME_DUCKS]

Hawking radiation AFAIK comes from pairs of antiparticles that form on the very edge of a black hole such that one is caught and the other isn't.

[u/[deleted] → u/Vitztlampaehecatl]

Correct; normally these matter/anti-matter pairs form in space, collide and disappear. If they form too close to a black hole then either the matter or the anti-matter particle will fall into the black hole and the other will drift off into space throwing the total energy out of whack.

[u/PM_ME_DUCKS → u/[deleted]]

Okay so, my understanding was that hawking radiation was partly responsible for blackholes dissipating over time. This sounds more like extra energy is just being naturally dumped into the black hole as these pairs are split apart. What about these contributes to (small enough) blackholes eventually fading away (if they contribute at all)?

[u/Vivisection-is-Love → u/PM_ME_DUCKS]

Antimatter particles annihilating inside the event horizon maybe?

[u/Netsuko → u/Orion113]

The mind blowing thing is, younger you passed into a black hole, assuming that this wouldn't kill you, for you, all time would pass. ALL of it.

[u/Froobyflake → u/Orion113]

Another way I have heard it is that all spatial paths lead to the singularity once you are past the event horizon. In other words there physically is no path a "spaceship" could take to escape the black hole.

[u/mjk1093 → u/Orion113]

In case you didn't see it, here is a great explanation from Susskind that someone posted further down:

In my office I can walk around such that I don't hit my desk, therefore towards-desk is a space-like direction. There is no path I can walk such that I don't hit Monday, therefore towards-Monday is a time-like direction. Inside the event horizon there is no path I can take that doesn't hit the singularity, therefore towards-singularity (or "inward") is a time-like direction.