Does the time exist?

[geordief]

42 minutes ago, KJW said:

Considering the Schwarzschild metric, there is no upper limit to mass, and therefore no limit to the size and how close to zero the density can be. However, there may be limits at the cosmological scale. For example, I doubt that a black hole can be less dense than the universe as a whole.

If the region surrounding a black hole is denser than the black hole, then the total mass of the black hole and the surrounding region would be large enough for the surrounding region to be also inside the black hole

 

Is it at all possible that the observable universe is inside a black hole with the event horizon  between the observable and the  non observable regions?

[Bufofrog]

40 minutes ago, geordief said:

Is it at all possible that the observable universe is inside a black hole

No.

[iNow]

The logical question back is, where is the backhole located if the universe itself (which is everything including all black holes) could fall into it?

[MigL]

The geometry of space-time 'starts' at the Big Bang; geodesics extend forward in time, but not backwards.
Somewhat like lines of latitude and longitude at the North Pole; they only extend in one direction ( to use Markus' analogy ).

A Black Hole is different.
The Event Horizon of a Schwarzschild BH ( non-rotating and non-charged ) appears differently to different observers.
While an infalling observer may note nothing peculiar falling through the EH ( no time slow-down or tidal forces for large BHs ),  a distant observer will note all information transmission from the infalling object slowing on approaching the EH, and finally stopping at the EH.
To the outside observer it would appear as if time had stopped at the EH, along with geodesics leading into the EH.
The EH is known as a co-ordinate singularity, and this apparent freezing of time does not happen in the proper time of the infalling observer.

[geordief]

@iNow don't know .Everywhere?

That would seem to be an answer in the spirit of "where is the centre of the universe ?"

(answer : "everywhere")

Do we know that black holes have "back holes" ,anyway?

I thought we just knew as far as the event horizon and some have suggested all the matter could actually collect there -with there being nothing inside .

Would the "back hole" be on the event horizon in that case?

Obviously ,I know zilch about this

9 minutes ago, MigL said:

The geometry of space-time 'starts' at the Big Bang; geodesics extend forward in time, but not backwards.
Somewhat like lines of latitude and longitude at the North Pole; they only extend in one direction ( to use Markus' analogy ).

A Black Hole is different.
The Event Horizon of a Schwarzschild BH ( non-rotating and non-charged ) appears differently to different observers.
While an infalling observer may note nothing peculiar falling through the EH ( no time slow-down or tidal forces for large BHs ),  a distant observer will note all information transmission from the infalling object slowing on approaching the EH, and finally stopping at the EH.
To the outside observer it would appear as if time had stopped at the EH, along with geodesics leading into the EH.
The EH is known as a co-ordinate singularity, and this apparent freezing of time does not happen in the proper time of the infalling observer.

Are you saying that the idea of the universe having an event horizon corresponding to  what a black hole might is  more or less a non starter ?

Or maybe you were replying to someone else?

[MigL]

Not replying specifically to you; just some general observations.

24 minutes ago, geordief said:

Are you saying that the idea of the universe having an event horizon corresponding to  what a black hole might is  more or less a non starter ?

right.

And no, nothing stops at the EH; using different co-ordinate systems for a Schwarzschild BH will yield a non-singular EH.

[J.C.MacSwell]

On 1/10/2024 at 9:33 PM, StringJunky said:

Your explanations on time over the years has always been internally consistent, in my opinion.

 

Like clockwork...

On 1/13/2024 at 12:30 PM, iNow said:

No one has distance in a bottle, either. So what?

”Hey barkeep! Give me a pint of centimeters and a plate full of inches, please.”

Pint of centipedes and plate of inchworms coming right up!

[iNow]

1 hour ago, geordief said:

Would the "back hole" be on the event horizon in that case?

No. It’s a misspelled autocorrect version of blackhole. Sorry for the confusion and not catching that in previous post. 

[KJW]

6 hours ago, geordief said:

Is it at all possible that the observable universe is inside a black hole with the event horizon between the observable and the non observable regions?

I have read the suggestion that the universe, represented by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, can be regarded as the time-reversal of a collapsing matter black hole. I don't know if this is true, but I do see it as plausible. It should be noted that the radial coordinate of a Schwarzschild black hole is timelike to an observer who is inside. Thus, to an observer inside the black hole, the singularity is not located anywhere in space, but in the future. Also, the FLRW metric is conformally flat, which is exactly not the spacetime of a Schwarzschild black hole. But the interior of an infalling matter black hole is not a Schwarzschild metric and may be more like the FLRW metric.

 

 

3 hours ago, geordief said:

Do we know that black holes have "back holes", anyway?

A black hole can exist inside a larger black hole. Given that nothing special happens when an object crosses the event horizon of a large black hole, it is reasonable to conclude that a small black hole can cross the event horizon of a much larger black hole completely intact.

 

 

Edited January 15 by KJW

[MigL]

1 hour ago, KJW said:

it is reasonable to conclude that a small black hole can cross the event horizon of a much larger black hole completely intact.

Not 100% sure, but I would think as soon as the Event Horizons of the two BHs come into contact, they would merge and immediately assume a larger spherical configuration.
As to the interior configuration/composition, I don't think we can say much.

I'm basing this on Oppenheimer/Wheeler theory, however; I'm sure some computational modelling has been done for merging BHs in relation to gravitational waves.

[KJW]

28 minutes ago, MigL said:

Not 100% sure, but I would think as soon as the Event Horizons of the two BHs come into contact, they would merge and immediately assume a larger spherical configuration.

The outer event horizon would expand in accordance with the extra mass of the infalling black hole, but the inner event horizon would remain intact as the boundary from which light can't even escape to inside the outer black hole.

 

 

[MigL]

I had considered only an outside observer.
An infalling observer will see things differently I suppose.
But if you fell through the EH of a huge BH, along with a much smaller BH, would you be able to see it ?
It it was ahead of you, I don't think you would, and I have my doubts even if the small BH was alongside you.
You may be able to see it if it followed you through.

And even if we don't consider different observers, how long can the two EHs remain distinct?
They are essentially mathematical constructs denoting a region of extremely curved space-time

[geordief]

1 hour ago, MigL said:

I had considered only an outside observer.
An infalling observer will see things differently I suppose.
But if you fell through the EH of a huge BH, along with a much smaller BH, would you be able to see it ?
It it was ahead of you, I don't think you would, and I have my doubts even if the small BH was alongside you.
You may be able to see it if it followed you through.

And even if we don't consider different observers, how long can the two EHs remain distinct?
They are essentially mathematical constructs denoting a region of extremely curved space-time

This seems a silly question ,but if two objects follow each other through the event horizon of a  very large BH ,does the second  object see the first  object as it passes the EH? 

Even if the two  are very close together...

 

Alternatively would a  Pinocchio  lose the sight of the end of his nose first   as he passes the threshold?

[Genady]

1 hour ago, geordief said:

if two objects follow each other through the event horizon of a  very large BH ,does the second  object see the first  object as it passes the EH?

Yes.

 

1 hour ago, geordief said:

would a  Pinocchio  lose the sight of the end of his nose first   as he passes the threshold?

No.

Edited January 15 by Genady

[geordief]

1 hour ago, Genady said:

Yes.

 

No.

What about a small BH?

[Genady]

Just now, geordief said:

What about a small BH?

The same. As long as the falling objects are much smaller than the BH.

[MigL]

18 hours ago, Genady said:

Yes
.
.
No

You're going to have to elaborate, Gemady.
I don't see how an observer would be able to see anything ahead of himself upon passing through the Event Horizon, as there are no geodesics for light to follow in the outwards direction.
The only available geodesics are forward in time to the center.

[Genady]

1 hour ago, MigL said:

You're going to have to elaborate, Gemady.
I don't see how an observer would be able to see anything ahead of himself upon passing through the Event Horizon, as there are no geodesics for light to follow in the outwards direction.
The only available geodesics are forward in time to the center.

The primed observer, falling through the horizon at event C', sees the unprimed observer, which is falling through the horizon at event C.

[sethoflagos]

1 hour ago, MigL said:

I don't see how an observer would be able to see anything ahead of himself upon passing through the Event Horizon, as there are no geodesics for light to follow in the outwards direction.
The only available geodesics are forward in time to the center.

Does the light need to be travelling in the outward direction?

If the infalling observer is falling faster than the wavefronts are receding, he catches up with them doesn't he?

[Bufofrog]

19 minutes ago, sethoflagos said:

If the infalling observer is falling faster than the wavefronts are receding, he catches up with them doesn't he?

Why would he fall faster than light?

[sethoflagos]

42 minutes ago, Bufofrog said:

Why would he fall faster than light?

He wouldn't be.

Just faster than the light heading 'in his direction' was receding from the event horizon.

[Luc Turpin]

5 hours ago, Genady said:

The primed observer, falling through the horizon at event C', sees the unprimed observer, which is falling through the horizon at event C.

The unprimed observer would also see the primed observer, but motionless as he has already fallen through the horizon! Correct?

 

Just now, Luc Turpin said:

The unprimed observer would also see the primed observer, but motionless as he has already fallen through the horizon! Correct?

 

 

5 hours ago, Genady said:

The primed observer, falling through the horizon at event C', sees the unprimed observer, which is falling through the horizon at event C.

The opposite, I should have said, its the primed observer that sees the unprimed observer motionless as he has fallen through the horizon! correct?

And the unprimed observer would also see the primed observer, but moving as he has not fallen through the horizon! Correct?

Disregard the second post, I had it correct the first time.

[joigus]

On 1/15/2024 at 11:15 PM, Genady said:

On 1/15/2024 at 10:13 PM, geordief said:

if two objects follow each other through the event horizon of a  very large BH ,does the second  object see the first  object as it passes the EH?

Yes.

IMO, this would be a "no", and I think @MigL's objection,

7 hours ago, MigL said:

I don't see how an observer would be able to see anything ahead of himself upon passing through the Event Horizon, as there are no geodesics for light to follow in the outwards direction.

still stands. Never mind your diagram. Frequency of backward-sent photon being zero. How do you detect a zero-frequency photon?

Mind you, I might be obfuscated by ungodly-late hour at place of present statement. 😆

 

 

[Genady]

23 minutes ago, joigus said:

backward-sent photon

All photons behind the horizon, in this diagram, go toward the singularity. There are no "backward-sent photons" there.

52 minutes ago, Luc Turpin said:

The unprimed observer would also see the primed observer, but motionless as he has already fallen through the horizon! Correct?

Incorrect.

Use the Kruskal diagram to find who sees whom and how.

[Luc Turpin]

59 minutes ago, Genady said:

All photons behind the horizon, in this diagram, go toward the singularity. There are no "backward-sent photons" there.

Incorrect.

Use the Kruskal diagram to find who sees whom and how.

Will do!