[Jimbee (OP)]

I was going to start a question here by first saying that there is a wormhole at the center of every black hole. But Google AI says not necessarily true. There is a "singularity" at the center of every black hole. "A point of infinite density." (How is infinity possible?)

But, black holes are possible. But likely are they? Has one been ever observed?

And black holes would take you to another time and place in spacetime. So time travel is theoretically possible? What about temporal paradoxes? The the grandfather temporal paradox talks about an extreme example. What about on a smaller scale? If you went to your house when no one was home, wouldn't that create a temporal paradox? You changed the past because no one was supposed to be home at the time? Or am I mistaken?

And what about other ways of going back in time? In Star Trek they used the slingshot effect of the gravity of a star to build momentum while avoiding the change in mass that comes with speed. Would that work? Or is that just fiction?

[Janus]

Black holes do exist, as we have actually been able to image one. As for the singularity, that is an open question.  While some might describe it as a point of infinite density, it would be more accurately described as the point where our math breaks down.  It is very possible that when things get that small, other effects come into play which would prevent this from occurring.  As for being a passage to other points and times, while this isn't entirely ruled out by the math, the likelihood his small. Any representation of time travel you come across in a novel, TV show, or movie isn't going to be scientifically based, as the rules for such time travel will molded to fit the story being told. 

[Halc]

I was going to start a question here by first saying that there is a wormhole at the center of every black hole.

This is false. A wormhole is a valid solution to Einstein's field equations, but the conditions require exotic matter (say with negative mass) for this kind of thing to work. It has nothing to do with black holes.

There is a "singularity" at the center of every black hole. "A point of infinite density." (How is infinity possible?)

This is also completely wrong. AI has no clue. A singularity means a region where the equations no longer apply. It doesn't mean a point of infinite density. Matter is pulled apart by tidal forces near a large mass, not compressed at all. There is never a time during the fall that compression occurs.

Has one been ever observed?

Yes and no. They have pictures, yes, but the picture shows the X-ray emissions from the plasma surrounding one, not the BH itself, which is, well, black.

And black holes would take you to another time and place in spacetime.

So does sitting at your desk.

So time travel is theoretically possible?

You existed in 2024. You exist in 2025. If that's time travel, then yes. If you don't consider that to be time travel, then no because falling into a black hole is no different.

What about temporal paradoxes? The the grandfather temporal paradox talks about an extreme example.

There is no paradox here since you won't find your young grandfather by going into a black hole any more than you would by sitting at your desk. There is no worldline leading to 'the past' in any black hole.

And what about other ways of going back in time?

There isn't a first way to go back in time. Star Trek is fiction, as is any TV show or movie, including those that supposedly try to keep things plausible like 'Gravity'.

[paul cotter]

One can travel forward in time but going back is never possible.

[varsigma]

For what it's worth, black holes were predicted by Einstein's theories.

The physical evidence is these days that yes, they seem to be real celestial objects.
But, the theory describes a classical black hole, and the solutions to the field equations say there is a classical singularity.

We would like to know what a quantum theory says a black hole is. We don't have a theory like that yet.

So, the physics. There are two theoretical pictures of black holes. And there are observations to explain.
The classical theory is far from exhausted, the set of solutions is fairly small still. The quantum theory is still on the drawing board. I would say the theorists are probably thinking these objects might just bridge the gap between them.

One day.

[Eternal Student]

Hi.

Some good answers already...  but who doesn't love a bit of stuff about black holes, wormholes and Sci-Fi TV shows?  I've got a spare 20 minutes and intend to chip in to the discussion.

Jimbee said:   I was going to start a question here by first saying that there is a wormhole at the center of every black hole.
Halc replied:    This is false....

   Contrary to @Halc 's comments, there's quite a natural link between some notions of wormholes and some notions of black holes.   The maximally extended solution of the Schwarzschild metric allows for four regions of spacetime to be identified and these are creatively named regions I,  II,  III and IV in a Kruskal diagram.   I know that @Halc is aware of this but has possibly overlooked the bridge at the centre of the diagram.

[diagram taken from Wikipedia.    https://en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coordinates ]

Regions I and III are the exterior of the black hole.   By convention we consider ourselves to be in region I.  Meanwhile Region III is just another patch of spacetime, it's not necessarily the same bit of spacetime that we have been in or will ever be in but it is otherwise physically similar to the exterior region I.  For example region III could be associated with another universe  (or "parallel universe").     The bit right in the centre of the diagram (which actually marks a place in space and a whole range of times) can be called a wormhole but a more commonly used name is an "Einstein-Rosen Bridge".     Warning:   These two terms can be used interchangeably - but most commonly the term "wormhole" is kept for something that would require a negative energy density and the term "Einstein-Rosen bridge" for the thing at a black hole.

    Interestingly, the E-R bridge associated with a black hole is NOT at Schwarzschild radial co-ordinate r=0 or what we might naively treat as the centre of the black hole.   (Why did I say "naively" ?   .... because where r<1, it turns out that this radial co-ordinate is no longer telling us about spatial location, it's behaving like time instead).   It's actually at r=1 which would be identified as the event horizon around the black hole where there would be a path (a trajectory you can take) to stay on the border between region II (the interior of the black hole) and region III.   We are about to describe two different co-ordinate systems, so it's important to establish the symbols and terminology I will be using:   The Schwarzschild co-ordinates will always be lower case letters,  we have r (the radial co-ordinate) and t (the time co-ordinate).   The Kruskal co-ordinates will have upper case letters, T and X and it is these Kruskal co-ordinates that are on the axis of the diagram above.   Some lines of constant Schwarzschild co-ordinates (constant r or constant t) are shown on that diagram.   

This bridge (at spatial location r=1) exists at all times, t, in the Schwarzschild co-ordinates.  In the more convenient Kruskal co-ordinate system, we would tend to think of it as just a single point at T =0 and X =0 which is right at the centre of this diagram.   Indeed, that is the only location on the diagram where a sensible Schwarzschild time co-ordinate (one that isn't + or - infinity) can be associated with this location.   However the situation is a bit more complicated, if you wish to make it so....   the spatial location with radial co-ordinate r=1 exists all along both dotted diagonal lines in this diagram (if we allow the Schwarschild t co-ordinate to be +/- ininfity  BUT  we'll leave this alone for a paragraph or two.  LATE EDITING: The post is already too long, we'll just leave it well alone for the whole of the post).   There is no sensible way to know what Schwarzschild t (time) co-ordinate should be associated with the spatial location r=1.  The degeneracy of the Schwarzschild co-ordinates, in particular that multiple finite times at r=1 specify precisely the same event,  i.e. that (r=1, t=1)  is no different to  (r=1, t=2) or (r=1, t= -100)  etc.  is essentially why Kruskal co-ordinates  (large T and large X)  are used and are better for describing the spacteime manifold around black holes.   The Schwarzschild co-ordinates fail to uniquely identify an event (a 4-dimensional space and time location) in this spacetime manifold but the Kruskal coordinates will uniquely identify an event in spacetime.
    So the simplest notion of an E-R bridge is obtained just by noting that on a Kruskal spacetime diagram, the paths you can take (usually called worldlines for the object) are at most 45 degree diagonal lines.   That's the path you would take if you're travelling at light speed radially inward or outward from the black hole.  Any slower speed produces a worldline that is more upright  (going more up the Kruskal T axis rather than along the Kruskal X axis).   So, if you start from the centre of the diagram (say having just come in from the region I almost directly along the line of constant time we can set as t=0),  that is to say you are spatially located right at the event horizon of the black hole when we set our clock to t=0,  then there is a path (a trajectory at light speed) you can take that will keep you on the border between region II (the interior of the black hole) and region III (some other universe possibly).   Travelling at less than the speed of light can only take you into region II (the interior of the black hole) or having an incorrect trajectory (direction) of travel will also usually take you into the interior of the black hole.   The two interesting directions of travel are directly radially outward from the black hole (that trajectory follows the dotted diagonal line on the top right of the diagram and keeps you on the border between region II and region I)  and  directly radially inward to the black hole (that trajectory puts you on the border between region II and region III).   That's also a little bit interesting when you think about it, travelling inward to the black hole at some sub-light speed can only get you into the interior of the black hole (region II) as you might expect and seems like the last thing you'd want to do.   However, if you want to find the border or path to region III then you really need to get brave, put your foot down and race directly into the black hole at light speed.
     Anyway, that's a simple explanation of an E-R bridge that exists at the event horizon of a black hole.   There is a realistic path or trajectory that would have you on the border of region III.   However, you cannot properly enter deeper into region III unless you were somehow able to travel just a bit faster than light (which is not mainstream physics) - then you can take a path at a lesser angle than 45 degrees and emerge somewhere properly inside region III.    This is sometimes paraphrased and pictured as if there is a bridge, a path or viable trajectory to reach region III for an instant when you at the event horizon but it shuts just as instantly, you would need to travel a bit faster than light to properly cross that bridge before it closes.
    Time is of the greatest importance to all of us (of course) but there's a big problem for any object that is travelling at light speed.  The path taken at light speed is refered to as a null path, which means that no proper time elapses for the object between any two points along its worldline.   So, you may have put yourself on the border of region III but you'll have no time to enjoy the view or scenery from over there.   No proper time will elapse for you between being at T=0 and X=0 and having raced along right to the end of the dotted diagonal line on the top left hand side of that diagram (the border of region II and region III) where you hit the singularity at r=0.  At Schwarzschild co-ordinate r=0, all worldlines terminate, so there will be no more experiences you would have.

   Black holes, they are interesting things.....

    Final comments:   The above description of a black hole is called the maximally extended Schwarzschild solution and it is associated with what we would call an "eternal black hole".   A black hole that started out as just some dense material and then underwent a dramatic collapse at some later time is NOT meeting the criteria of an "eternal" black hole, indeed there was a time when the black hole was not there.   These sorts of black holes will be associated with a different metric (not quite the Schwarzschild metric) so they will not have the same spacetime diagrams.   Specifically, they may not have an Einstein-Rosen bridge to a region III of spacetime and this explains why a few sources of information may state that real world black holes do not feature any sort of wormhole or E-R bridge.  So are there any genuine eternal black holes in existence or were they all originally just some matter that gradually became over-dense?  I don't know... but it seems that black holes are both too numerous and some also have a mass parameter that is just too large to explain away as if they were created at some non-zero cosmological time by matter coming together and forming over dense regions.   There hasn't been enough time since the big bang to get all the matter together to make these massive black holes, so it is possible that there are some black holes that have always been there, genuine eternal black holes that are well described by the Schwarzschild metric.

   

But, black holes are possible.....? Has one been ever observed?

     We have some images as stated by @Halc  but most of the evidence for their existence is a bit indirect.   For example, we have studied the motion of stars in our own galaxy and found that they move as if they are being acted on by the gravitational force from a massive but extremely small object at the centre of the galaxy.   Only a black hole would fit the characteristics required.    We also have a lot of recent evidence from gravitational wave analysis, such as from the LIGO site.   Here we have results indicating that some massive things have been merging together in space and for some of these events black hole mergers are the only thing that would seem to fit the data.

Best Wishes.

[paul cotter]

Very interesting, ES, but I have to admit that I understood only some of it. I found the wiki article that you linked to be very informative- I have always been troubled as to how anything could actually fall into a black hole rather than getting stuck at the event horizon. I did find the reference to a "white hole" disturbing to my matter of fact nature as such an entity could only have a very transitory existence as opposed to a black hole which can last for multiple billions of years.  More studying needed!