B Is there an inside to a Black Hole?

[PAllen]

I accept this.

Even from the infalling observer the event horizon would have no light come at them from the inside. But the EH isn't a physical barrier, the person falling in doesn't realize that time slows down, their own clock appears to them to tick normally.
If they were looking outward though, they would see the universe speed up and hot into heat death before they are able to cross the event horizon. (if they could see clearly outwards, i understand people have said there are red shift issues with being able to see outward).
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This is completely false. They would see the universe outside proceeding at a fairly normal rate, with moderate redshift. They would not see the end of the universe. In fact, if they were viewing a distant clock there is a specific time it would show as they approached the singularity. This time on the distant clock is the time when the event of infaller reaching the singularity is no longer in the (causal) future of the distant clock, and thus the distant observer can legitimately consider that the infaller reaching the singularity is “now” true.

 

[PeterDonis]

I'd actually be interested to know, how fast they would see the black hole evaporate. Would it evaporate before they are able to cross the EH?

No. For a black hole that evaporates, the spacetime geometry is not quite the same as in the non-evaporating case. We have had previous threads on this, though it has been some time since the last one.

Heuristically, you can get a reasonable first approximation to what happens in the evaporating case by thinking of the coordinate time the black hole finally evaporates as playing the role that $t = \infty$ plays in the non-evaporating case--i.e., the coordinate time the black hole finally evaporates is when a distant observer would see an infalling object cross the horizon (in fact the distant observer would see everything that happened on the horizon in the same outgoing light signal that shows the final evaporation of the hole). Here we are assuming Schwarzschild-type coordinates in which coordinate time is the same as proper time for the distant observer. (However, the coordinates aren't quite the same as standard Schwarzschild coordinates; those cannot be used to describe black hole evaporation. I am leaving out a number of technical points that are beyond the scope of a "B" level thread.)

An infalling observer would never see the black hole evaporate; they would fall into the hole, hit the singularity, and be destroyed, and their past light cone would never include any events anywhere close to the final evaporation of the hole.

 

[PeterDonis]

in some models of quantum gravity Black holes have no interior. This is related to something called black hole complementarity

I don't think it's quite correct to say black holes have no interior in the models you refer to. It would be more correct to say that the classical GR concept of a "black hole", or even the semi-classical concept of an "evaporating black hole" that Hawking and others originally investigated, does not even exist in these models.

 

[PeterDonis]

If they were looking outward though, they would see the universe speed up and hot into heat death before they are able to cross the event horizon.

As @PAllen has already said, this is wrong. The confusion you appear to be having is that you are thinking that "time dilation" for an infalling observer works the same as it does for a hovering observer--an observer who has nonzero proper acceleration and is keeping a constant altitude above the horizon. For the latter observer, yes, he sees the outside universe "speeded up" (blueshifted) compared to him. But the infalling observer does not; he sees the outside universe "slowed down" (redshifted) compared to him; his "time dilation" is different from that of the hovering observer.

 

[f todd baker]

I have a question: when a distant black hole eats a nearby star it has to gain mass. But if, from our perspective, we see the captured star stop at the event horizon, how do we see the mass increase? And clearly from gravity waves we see a merger of two black holes but we shouldn't see a merger from our reference frame. Obviously, I am no expert at cosmology!

 

[PeterDonis]

how do we see the mass increase?

We observe the gravitational effects of the increased mass--for example, if we are orbiting the hole, we observe our orbital parameters change. Those effects don't come from inside the hole's horizon; they come from the spacetime outside the horizon, in the past, whose geometry gets changed as the star falls into the hole.

 

[stevil]

The meaning of the term "black hole" is well established (a region of spacetime that is not in the causal past of future null infinity), so using it to refer to a model that obviously does not satisfy that meaning is, to say the least, not a very good idea. The fact that "many papers and talks" do it anyway just means scientists, like the rest of us, don't always do a good job of picking terminology.

I'm probably saying something stupid here.

But I find that definition to be assuming that SpaceTime is everywhere, at the very least it assumes that SpaceTime exists inside the EH therefore if it were found that there was no SpaceTime inside the EH then by that definition, that which we currently call a Blackhole could no longer be called a "blackhole". The EH would still exist, the immense gravity emanating from that region of space would still exist, but we would have to either modify the definition of BH or come up with a new name for it.

 

[stevil]

As @PAllen has already said, this is wrong. The confusion you appear to be having is that you are thinking that "time dilation" for an infalling observer works the same as it does for a hovering observer--an observer who has nonzero proper acceleration and is keeping a constant altitude above the horizon. For the latter observer, yes, he sees the outside universe "speeded up" (blueshifted) compared to him. But the infalling observer does not; he sees the outside universe "slowed down" (redshifted) compared to him; his "time dilation" is different from that of the hovering observer.

Yes, I've finally come to realize this is my issue. I mistakenly thought a free falling observer experiences time dilation. Staticboson has set me straight on this, as well as this post of yours. Thanks.

 

[PeroK]

Yes, I've finally come to realize this is my issue. I mistakenly thought a free falling observer experiences time dilation. Staticboson has set me straight on this, as well as this post of yours. Thanks.

Nothing ever "experiences" time dilation. It's a purely coordinate effect. This is not semantics. What an object experiences is the passing of its own proper time. The object cannot experience how others may measure that time.

 

[PeterDonis]

I find that definition to be assuming that SpaceTime is everywhere

That's not an assumption, it's a definition of spacetime. There is no such thing as a classical spacetime model where spacetime is not everywhere. The idea doesn't even make sense.

 

[PeterDonis]

I think I'd be turning the thread into something else then and derail it.

Yes.

I could discuss it on another thread.

Yes, if you want to discuss these models, please open a separate thread in the Beyond the Standard Model forum.

 

[stevil]

Nothing ever "experiences" time dilation. It's a purely coordinate effect. This is not semantics. What an object experiences is the passing of its own proper time. The object cannot experience how others may measure that time.

Yes, I'm not phrasing my thoughts correctly. I'm not up with the terminology.

 

[stevil]

That's not an assumption, it's a definition of spacetime. There is no such thing as a classical spacetime model where spacetime is not everywhere. The idea doesn't even make sense.

Is SpaceTime infinite in all directions?

I thought in a closed universe it's kind of like a bubble. There are no hard boundaries, no edge to the universe. But light going in a straight line doesn't always go further and further away from its source but eventually starts coming back just due to the curved shape of the universe. So there is no outside of the universe but if we were to model it on a three dimensional euclidean coordinate system (people intuitively think this way) we might point to a coordinate point and say but at this point which is outside the universe. That point doesn't exist, you can't get there, light can't get there, intuitively we think that place should exist but we can't get there.
Kinda like if there were multiple universes and if these were closed, we would have each universe in its own bubble but with no path to get from one universe to another.

Perhaps the above is just nonsense? I don't know enough about physics to know what is nonsense vs what is possible.
But I was wondering if the inside of a black hole were like that too. A space that we intuitively map out in a euclidean model and assume something must be there, but instead it just doesn't exist as there is no path for light to get there, not at any valid speeds.

The alternative is singularities
Singularities seem like nonsense to me (but I don't know enough and I'm not the right person to ask if singularities make sense or not).

 

[PeterDonis]

Is SpaceTime infinite in all directions?

That depends on the particular spacetime. Some are, some aren't.

I thought in a closed universe it's kind of like a bubble.

A closed FRW universe is spatially finite (but without boundary, as you note--topologically it's a 3-sphere), and also finite in time--it has an initial singularity, and (at least in the case of zero cosmological constant) a final singularity as well.

there is no outside of the universe

Yes. You can try to visualize the universe as embedded in some higher dimensional Euclidean space (as you do), but that causes more problems than it solves.

I was wondering if the inside of a black hole were like that too.

Like what? If you mean, like a closed FRW universe, no, it isn't. There are spacelike surfaces inside a black hole that are spatially infinite. As for trying to visualize an embedding of a black hole interior in a higher dimensional space, that's even more problematic than doing it for a closed FRW universe.

The alternative is singularities

Singularities aren't an "alternative" to anything. They are present in some spacetimes (such as both of the examples given above, a closed FRW universe and a black hole interior) but not in others. It just depends on the particular spacetime.

Singularities seem like nonsense to me

Many physicists believe that the presence of singularities in particular classical spacetime models is a sign that those models break down in those regimes, and that some new theory, such as quantum gravity, will be needed to model what actually happens in regimes where our current models have singularities. However, that is still an open area of research.