[olegkikin]

That's the thing that never made sense to me about the black holes. The closer you are to the event horizon, the faster the time passes for the universe around you. So reaching the event horizon should take infinite amount of the "outside time", so right before you reach the horizon, you should see the whole future of the universe, including its end, if there's any.

So how do black holes gain any mass then?

[grondilu]

> So how do black holes gain any mass then?

Well, from our point of view all incoming mass gets stuck very close to the event horizon. That's not too surprising considering the amount of information of a black hole is proportional to its surface area. So from a far away observer a black hole is more like a sticky sphere than an hollow ball.

[metaobject]

Doesn't the surface area increase as the BH takes on more mass?

[nightcracker]

I like the analogy of a snowball, nothing ever enters it yet it grows.

[booleandilemma]

This accurately describes how I gain weight as well.

[johnhenry]

This joke is in direct violation of the First Law of Thermodynamics.

[grondilu]

Yes, but the mass does not have to cross the event horizon for that, does it?

At that point we probably can't tell more without diving into the math, and it's beyond my abilities. Yet the gross idea does not seem absurd to me.

[joemag]

Yes, the radius of its event horizon is directly proportional to its mass. The relationship is described by the Swartzchild equation.

[IntronExon]

What is important to remember is thst our point of view is not special or “right” in any way. There is a difference between our observation of a process like matter falling into a black hole, and the reality of it. Black holes do “eat” and grow, and that occurs despite the intense time dilation near their surface. The light which returns to us will be redshifted to black, and we won’t see the final event, but it does happen in its own proper time, in finite time. When black holes merge we can detect their gravitational waves, evidence that black holes are not “frozen” in reality.

It is also important to distinguish between the implications of a model black hole formed from the uniform collapse of a perfectly spherical dust cloud, from infinity, in an otherwise empty universe, with no charge or angular momentum, from what happens in nature.

[mtgx]

Doesn't it mean that only the space around the black hole sees that "infinite amount of time" pass, and not the whole universe?

So in that case, you wouldn't be seeing the "future of the universe" just the "future" (I guess) of the space around the black hole.

I kind of see blackholes in space as vortexes in a lake. Anything that gets "trapped" in the vortex moves much faster, including the water (space-time) itself. It doesn't impact the rest of the lake, except for the things that are on a collision course with the vortex, and then get swallowed by it (and spit back out?).

[AgentME]

I think you've got it backwards. As on object approaches a black hole, an outside observer will see that object's clock slow down and eventually stopping as it hits the event horizon. Conversely, the object falling into the black hole will see the rest of the universe's clocks speed up. It could watch stars (far from the black hole) be born and die. If you take a trip close to a black hole's event horizon and then fly away, you could find yourself in the far future.

[shullbitt0r]

> As on object approaches a black hole, an outside observer will see that object's clock slow down and eventually stopping as it hits the event horizon.

So I had the idea that smaller black holes are at the center of the sun, the earth and so on, being the principle source of gravity and the "movement" that we see is just us falling into different black holes at the same time, which are also falling into each other. So micro black holes must be at the center of massive particles too. The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements. But you can have vastly more complicated geometries and thus assembles of entangled particles.

I don't know the "standard model" well enough to take the analogy any further, not to mention string theory and all that jazz.

[AgentME]

It's really reaching a lot to assume black holes are the sole source of gravity or time. (If you want to distill it really far, I think it's more right to say they counteract time than that they cause or provide it. You go near a black hole, and you age less and have less time to do things between the times of external events not near the black hole.)

>So micro black holes must be at the center of massive particles too.

If an object has a schwarzschild radius smaller than its own radius, then it isn't a black hole. That literally describes all non-black-hole objects with mass. That's just the standard manifestation of gravity.

>The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements.

Is there any connection here besides that an event horizon and the sum of all possible paths of a photon in a given amount of time are both spheres?

If there were a micro-black-hole inside of a particle, its event horizon would have to be within the particle, or else the particle would just be indistinguishable from a black hole. The particle wouldn't be like a normal particle with an invisible spherical event horizon surrounding it and affecting its interactions.

[shullbitt0r]

> including the water (space-time)

I think you meant to say ether.

[phamilton]

Wouldn't it just depend on your definition of the area of the black hole? By your logic, I certainly agree that black holes having mass beyond their event horizon is an impossibility from an observation perspective, but how far outside the event horizon do we include in our definition of a black hole? Far enough out, we could certainly observe them gaining mass.

[ajnin]

I think one way for matter to reach the black hole might be that when more matter comes close to the black hole the event horizon expands, so that matter that is already close to the event horizon gets incorporated even without moving. I don't know what happens to the matter that gets inside that way, does it remain stuck, giving an onion-like structure to the black hole, or something else ?

[raattgift]

Observers outside a black hole (BH) are free to disagree about the location and shape of the horizon. However, there is no observer which is free to say that there is no horizon [1]. This latter point is why the horizon is a physical feature of the universe containing the BH. The former point is why infallers can cross the horizon according to them, while some outside observers will never actually see the crossing [2].

This is not an analogy as much as an example of an external-vs-internal observer problem. When you close the (opaque, insulated) door of your fridge, observers inside will see the (filament of the incandescent) light significantly dim, and if the door stays closed long enough, will see the light thermalize with rest of the internal volume. Someone standing outside the fridge might not even see the initial dimming; indeed, that observer may only ever see the light as "on" (rather than "heating from cold" or "cooling from hot").

[1] We could talk about naked singularities a bit: this usually means that there is at least one outside-the-black-hole observer for which the shape of the horizon is such that the centre of mass-energy of the BH is outside the horizon, rather than an observer for which there is no horizon at all. However, even these scantily clad BHs don't arise in realistic universes described by General Relativity. Fully naked singularites (where at least one observer exists which does not see any horizon at all) require an alternative theory of gravitation, or conditions extremely unlike those anywhere in our universe.

[2] Consider the observation of a supermassive black hole at the edge of the observable universe. From our view here around Earth, we see a race between a very bright star about to cross the black hole's event horizon and the black hole about to cross our Hubble horizon. Observatory A sees the star vanishing behind the horizon just in time; Observatory B sees the BH cross out of observability before the star vanishes behind the BH horizon. A and B have (very slightly) different Hubble horizons focused on them [3], and also with a (n also slightly) different radial distance to the BH horizon. "B" can never directly see the same coincidence of events that "A" sees; should "B" deny the infalling?

[3] Maybe this is illustrative of observer-centred observables? Glories (an optical phenomenon similar to rainbows) are so observer-specific that you and your handheld camera will have different ones (and each of your eyes will have different ones). As noted in the "From the air" subsection, we can tell what seat a photographer of a glory from a plane must have been sitting in. https://www.atoptics.co.uk/droplets/gloim1.htm Likewise, we can determine the location in spacetime of an observer of a star-into-black-hole event from that observer's detailed description.

[make3]

[Deleted]

[v_lisivka]

so, because black hole rotates, at some point in far future, speed of the shell will cross speed of light just because of geometry.

[techdragon]

That’s a really interesting point. I wonder if there’s any proper theoretical work done on this possibility.

My immediate, naive instinct is that by crossing this limit, the frame dragging effect, would be extremely powerful, to the point where it might increase the radius where Hawking radiation is formed/emitted and increase the rate of Hawking radiation to avoid passing the limit. A sort of self limiting process to prevent breaking the speed limit of c.

But in the time it took to write this out, I remembered angular momentum and realised that notwithstanding the additional angular momentum of the infalling mass, the conservation of angular momentum would just make the event horizon slower as it expands. Which makes the superluminal event horizon unlikely in my mind.

[v_lisivka]

It was response to deleted comment about "sticking shell" inside of a black hole, to illustrate that such shell will have superluminal speed, thus it cannot exist, because any matter will decay.

[IntronExon]

There are two frames of reference here, and only from the frame of the external observer does what you say hold true. From the point of view of infalling mass, there is no “freeze-frame” and time proceeds normally. The classic example to illustrate this is a black hole with so much mass that you can fall past the event horizon without experiencing significant tidal forces. You could survive the fall, Andy live for hours inside the hole before tidal forces turned you into subatomic sphagetti.

[olegkikin]

But from our perspective, the outside observers, why do black holes gain mass? It would take an infinite amount of time from our perspective for any mass to even touch the event horizon.

[IntronExon]

At this point a few issues arise. The first is that what you’re describing is a feature of the Schwarzschild metric, which applies to a model black hole, which is time-independent and eternal. There is no particular reason to believe that this accurately describes black holes in nature. For example this metric can not describe the merger of two black holes, but we now have observational evidence that this does indeed take place.

The biggest issue, aside from the model, is that time dilation is something which only matters when two observers “compare clocks.” Neither observer alone ever experiences a difference. The crew of a 99.9% lightspeed ship doesn’t experience time dilation... until they return home. It makes no sense to talk about the effects of time dilation from the point of view of a one-way trip to the event horizon.

[olegkikin]

> The crew of a 99.9% lightspeed ship doesn’t experience time dilation until they return home.

That's not true. They see the universe around them moving much faster.

Time dilation has nothing to do with "returning home".

[wallace_f]

I've never been able to feel comfortable understanding reference frames. Even considering simpler examples: So what if a probe is launched to catch our solar system's recent cigar-shaped visitor. Assume we catch it and want to bring back a sample of equal mass to the probe. So what determines the kinetic energy required to return this sample to earth? Is the delta relative to that of the probe, to the solar system, or to its origin? What if it is was accelerated to 0.1c relative to its launch site in another galaxy, but is only travelling at 0.001c relative to us?

Common sense dictates that the probe and the sample would require equal fuel to return to earth; but to an observer riding this cigar-rock, why would the universe cut our probe some slack if we changed its kinetic energy rather than the observer?

[IntronExon]

It’s from the frame of reference of the probe you make fuel calculations. The various observers don’t necessarily need to agree on the ordering of events (Relativity of Simultaneity), but they will always agree on the laws of physics, which are the same everywhere. If it takes a given amount of fuel to accelerate a mass to a given degree, everyone will agree on that point. It might take som calculation to make that clear to all of the observers, but they will agree regardless if some are in accelerating reference frames, and others inertial.

[IntronExon]

Seeing the universe around them is comparing clocks with another frame of reference. On the ship, life at 99.9% and life at 1% of c is identical. By the same token, if you fell into an (inactive) supermassive black hole, your watch would trick the same way into and past the event horizon. You would in fact live inside the hole for hours until you were torn asunder.

[simonh]

The point is, the crew onboard the ship can compare clocks with those in different frames of reference at any time. All they have to do is observe the period of a pulsar for example, or measure the orbits of binary star systems, etc. It would be quite apparent to them that there is a time dilation effect for them with respect to most of the rest of the universe. See Tau Zero, by Poul Anderson. It has a few mistakes, but it's still a great read.

[shullbitt0r]

> Seeing the universe around them is comparing clocks with another frame of reference

So, if you don't sense anything, you don't sense time dilation either?

This is slightly more complicated. First of all, you haven't given a frame of reference. If you claim someone were moving at 0.99c then you have already set the frame of reference. And they would have to gain near infinite mass and would die. You seem to assume a restricted frame of reference though, inside the spaceship. So, a point of reference inside the spaceship would see light moving with c inside the spaceship. And would assume his own point of reference as the origin of the inertial frame of reference. So baring any outside measurement, how do you know the spaceship is moving with 0.99c and in which frame of reference?

[xelxebar]

I just wanted to mention that the Schwartzchild metric doesn't only apply to black holes. It's the general metric for a spherically symmetric, static space-time, so it's good for roughly spherical bodies like stars and planets over small timescales.

Recently, I worked out the ISS orbit using the Schwartzchild metric as an approximation of Earth. It's cool to see the orbital period pop out and agree with real life! It's then just a small step to calculate the time dilation experienced by ISS astronauts.

[shullbitt0r]

First you have to define mass. It's currently "defined" extrinsically, by a piece of metal machined by the SI. That doesn't allow an intrinsic answer definition of a black holes mass.

The answer is kinda easy if I can make up my own intrinsic definition. The mass is the mass of the stuff around the black hole. A black hole is a singular point, it can't have mass, don't be silly.