I've been told that once you go through a black hole, you never wish to go back to a white hole.

I think this is more to do with birth trauma than transgressive sex.

I don't at all believe that black holes aren't simply mathematical constructs at this point.

Is a white hole a mathematical necessity? Does the math suggest its existence?

Something is there that's being described by the math, given the massive gravitational effects on nearby objects. And it's condensed to a small area for that much gravity. It also doesn't give off light beyond a certain point. There is real data about the objects we model as black holes.

Something is there that's being described by the math, given the massive gravitational effects on nearby objects. And it's condensed to a small area for that much gravity. It also doesn't give off light beyond a certain point. There is real data about the objects we model as black holes. — Marchesk

That's assuming general relativity provides us with an accurate model of things at this scale. But we can consider that the concept of "event horizon" is evidence that general relativity doesn't provide us with an accurate model.

That's assuming general relativity provides us with an accurate model of things at this scale. But we can consider that the concept of "event horizon" is evidence that general relativity doesn't provide us with an accurate model. — Metaphysician Undercover

Whether GR is accurate or not doesn't change the astronomical data. There is something there. Our understanding of it might be inaccurate, but that doesn't change the data.

Yeah, sure, I agree. It's just that the concept "black hole" itself might be misleading.

Whether GR is accurate or not doesn't change the astronomical data. There is something there. Our understanding of it might be inaccurate, but that doesn't change the data. — Marchesk

It does change whether what we are seeing is a black hole, because a "black hole" is not a theory-free observation, it is a theoretical entity that happens to fit observations in the context of modern physics and astronomy. That is not to say that there is something particularly suspect about black holes: we could say the same about just about anything: atoms, eclipses, electrical currents, etc.

The only really suspect thing about black holes is the theoretically-predicted singularity at their center - many consider this to be problematic as such, and especially so if we assume that quantum mechanics is valid at the same time.

I am 24 hours fresh from a talk about mathematical structuralism which I only one-tenth understood. I believe I have grasped though that we could safely assert black holes as abstract but instantiated mathematical structures, and white holes as abstract, uninstantiated mathematical structures. As ever, though, there are then several possible forks in the path to ontology, and my grip on the complexity weakens towards zero.

Something is there that's being described by the math, given the massive gravitational effects on nearby objects. And it's condensed to a small area for that much gravity. It also doesn't give off light beyond a certain point. There is real data about the objects we model as black holes. — Marchesk

The only thing that's definitely there is numbers from our instruments that don't match what we're expecting given our current gravitational models. So we make up something to explain the unexpected numbers.

The existence of white wholes is close to being a logical consequence of the existence of black holes (plus some plausible symmetry assumptions). Most black holes are rotating. That is, they have non-zero angular momenta inherited from the conserved angular momenta of the collapsing stuff that made them up and that they later accreted. Rotating black holes are called Kerr black holes (strictly speaking only when they have zero electric charge, but that is inessential). Rather than having a point singularity (as have non rotating black holes) they have an annular singularity. If one falls into a Kerr black hole from a direction outside of its equatorial plane, then one ought to be able to go trough the annular singularity while not getting crushed, nor stretched by huge tidal forces, provided only the Kerr black hole is large enough, and it has sufficient angular momentum, as might be the case with many massive black holes that are located in galactic cores. If the metric of space time past those inbound trajectories into the annular singularity is symmetrical with the metric in the "inbound" region, then a falling traveler (or any inbound object) ought to emerge from the black hole somewhere, from a white whole, presumably. Where would one emerge, and whether this would occur somewhere in this universe at all, is anyone's guess.

The only thing that's definitely there is numbers from our instruments that don't match what we're expecting given our current gravitational models. — Terrapin Station

They match what we're expecting given our current gravitational models to a high degree of accuracy.

we could safely assert black holes as abstract — mcdoodle

I'm not sure I understand, are you saying that black holes are merely theoretical? Perhaps elements of the subject do not completely ameliorate its external properties, but the emission of electromagnetic wavelengths from quasar radiation have been observed where redshift surveys that use parametrical time delays have been compared. That is, the interaction between matter and electromagnetic radiation - visible through the wavelengths of spectral lines - have been observed.

https://academic.oup.com/mnras/article/337/1/109/1034129/Measuring-the-black-hole-masses-of-high-redshift

White holes are hypothetical though.

I'm not sure I understand, are you saying that black holes are merely theoretical? — TimeLine

No, my mind was addled by mathematical structuralism, point taken.

They match what we're expecting given our current gravitational models to a high degree of accuracy. — SophistiCat

The structure and apparent motion of stars doesn't match what we're expecting given our gravitational model. Hence the need to invent black holes.

The structure and apparent motion of stars doesn't match what we're expecting given our gravitational model. Hence the need to invent black holes. — Terrapin Station

No. Black holes are a generic prediction of General Relativity. If GR is our gravitational model, then black holes are part of the package.

No. Black holes are a generic prediction of General Relativity. I — SophistiCat

That they're consistent with GR doesn't make them a prediction of GR. We invented them so that they'd be consistent with GR, otherwise we'd need to retool our gravitational theory.

That they're consistent with GR doesn't make them a prediction of GR. We invented them so that they'd be consistent with GR, otherwise we'd need to retool our gravitational theory. — Terrapin Station

I agree with this. There are areas where GR has difficulties, one being the phenomenon which is called a black hole. It's not that GR predicted these things, more the opposite, it's a place where GR cannot predict. This indicates that GR is an incomplete, or deficient theory for representing gravitation.

That they're consistent with GR doesn't make them a prediction of GR. We invented them so that they'd be consistent with GR, otherwise we'd need to retool our gravitational theory. — Terrapin Station

We "invented" them only in the same sense that we "invent" solutions to equations. Black holes are what we can expect to see, given GR. And what we do see is in close agreement with what we expect to see. So whatever semantic point you are trying to score, it is irrelevant. Black hole physics is not an ad hoc addition to our gravitational models, as you implied.

Black holes are what we can expect to see, given GR. — SophistiCat

I don't see how this could be true. What is it inherent within GR which would make you expect to see a black hole?

I don't see how this could be true. What is it inherent within GR which would make you expect to see a black hole? — Metaphysician Undercover

General Relativity is a theory of gravitation that lawfully relates the distribution of energy and momentum in space-time with the metric of space-time (thus specifying its "curvature"). It follows from this lawful relation (i.e. Einstein's field equations) that whenever a spherical distribution of mass achieves a density such that it is contained within its Schwarzschild radius, then the escape velocity at the surface attains the speed of light. Past this point, the mass can't possibly not collapse into a singularity (and there can't possibly not occur the formation of an event horizon at the Schwarzschild radius) consistently with Einstein's field equations.

It also is the case that for a stellar mass to achieve a density such that it is contained within its Schwartzschild radius is a very common occurrence with dying stars only slightly more massive than our Sun, or with dense galactic cores.

That doesn't indicate that there is anything inherent within GR which would make you expect to find a black hole, it indicates that certain types of stars when understood under GR make you expect to find a black hole.

That doesn't indicate that there is anything inherent within GR which would make you expect to find a black hole, it indicates that certain types of stars when understood under GR make you expect to find a black hole. — Metaphysician Undercover

Of course. General relativity is a deterministic theory. It only tells you what to expect given some empirically realized initial conditions. Likewise, there isn't anything inherent to Newtonian mechanics which would make you expect to find a billiard ball in a pocket. But, given that some billiard balls are subjected to impacts thus and so and then let roll freely, then they are expected to end up in a pocket. Likewise, given that some stars naturally evolve in such a way that they contract within their Schwartzschild radii, then they are expected to become black holes.

So the issue here, appears to be that since there are such objects, black holes, whose mass is contained within the Schwartzchild radius, doesn't this indicate that GR is inadequate for understanding some aspects of the universe?

The structure and apparent motion of stars doesn't match what we're expecting given our gravitational model. Hence the need to invent black holes. — Terrapin Station

Ha. A paid up member of the flat earthers. It's only a rumour things disappear over the horizon because the world is curved.

That doesn't indicate that there is anything inherent within GR which would make you expect to find a black hole, it indicates that certain types of stars when understood under GR make you expect to find a black hole. — Metaphysician Undercover

Another flat earther. In 1915, Schwarzchild had already extracted the basic cosmological implication of general relativity being true. A dense enough lump of matter would have to produce the local curvature that would become a complete gravitational collapse. That comes directly out of the equations.

And it is one of the ironies of intellectual history that Roy Kerr announced his simplification of Einstein's equations that could account for a realistic solution for a spinning black hole at the very same conference called to discuss the discovery of quasars. Unlucky for Kerr, it took quite a few more years for it to be realised that quasars were the product of black holes. So at the conference many wandered off or catnapped as he gave what seemed like an obscure mathematical technical paper at the time.

Now of course astronomers are drumming up public money so they can take a "photograph" of Sagittarius A*, the super massive black hole at the centre of our own galaxy.

So the issue here, appears to be that since there are such objects, black holes, whose mass is contained within the Schwartzchild radius, doesn't this indicate that GR is inadequate for understanding some aspects of the universe? — Metaphysician Undercover

It's merely a theory of gravitation. It is adequate for the very restricted purpose of understanding the phenomenon of gravitation as a manifestation of the metric of space-time (and what it is that determines this metric: spatial distributions of energy and momentum). But it is also limited in scope since it is a "classical" theory that becomes inadequate for energy and density scales where quantum fluctuations become relevant. Hence, we can't really know, within the general relativity framework, what happens to the metric of space-time near singularities or in the very early universe.

The structure and apparent motion of stars doesn't match what we're expecting given our gravitational model. Hence the need to invent black holes. — Terrapin Station

What a load of rubbish. If something is inconceivably dense like a black hole, even without the emission of radiation including light, it doesn't mean it is without energy and they have been detected through the x-rays emitted by expelled gas falling toward a black hole. It's not an invention.

We "invented" them only in the same sense that we "invent" solutions to equations — SophistiCat

Solving equations has nothing to do with positing real ontological entities.

And yet the application of GR equations to cosmology immediately had the effect of predicting spacetime curvature so extreme that an event horizon must result. So the equations did exactly that - obliged us to posit ontologically real outcomes (of which black holes are one of many now empirically supported examples).