I've noticed she gets some more attention outside her blog and YT channel recently.
String theory in particular has had a testability problem from the very beginning. https://www.math.columbia.edu/~woit/wordpress/?p=533
All current observations are also consistent with quark stars. We don't know what the quark degeneracy pressure is, so there's no current way to differentiate.
LIGO has been able to confirm theoretical properties of the event horizon. The EHT has imaged a black hole, which effectively images the event horizon.
> All current observations are also consistent with quark stars.
Is there a reference for that?
What specific properties has LIGO confirmed about the event horizon? It is not quite clear to me how the gravitational waves of two merging bodies would be different between a merger of normal stellar objects and black holes.
> The EHT has imaged a black hole, which effectively images the event horizon.
The EHT image is mostly an accretion disk. I think accretion disks can also form around things that are not black holes. If something were of the sufficient density, I believe it could make the same image without having an event horizon. I am no astrophysicist though so maybe that image would force the density to be sufficiently high to make a black hole.
No "normal stellar object" can be more compact than 9/8 of the Schwarzschild radius for its mass. (This is a result known as Buchdahl's Theorem, first discovered in the 1930s.) Gravitational wave emission when objects of that size or larger merge is much less energetic compared to the total mass involved than the merger of black holes, which merge at the Schwarzschild radius; also the predicted waveforms are different, because there is actual matter (nonzero stress-energy) present when normal stellar objects merge, whereas black holes are vacuum (zero stress-energy).
Hawking's area theorem is one: https://news.cornell.edu/stories/2021/07/hawkings-black-hole...
> It is not quite clear to me how the gravitational waves of two merging bodies would be different between a merger of normal stellar objects and black holes.
I'm not sure one can be so loose here. LIGO uses detailed models of black hole and neutron star mergers to match the signals against. What are "normal stellar objects" that would yield the same magnitude of gravitational waves emitted during a merge as black holes and neutron stars?
> The EHT image is mostly an accretion disk. I think accretion disks can also form around things that are not black holes.
There's also a huge hole in the middle of the accretion disk. What other object, other than a black hole, has such a large accretion disk but yet no emitted radiation?
Gravitational waves are a prediction of GR but not of quark stars, so GR explains more of the available evidence than quark stars alone.
Quark stars have plenty of mass, why would you think they would not generate gravitational waves? I don't understand what you could be thinking of.
In fact a black hole would make things more complicated because of the infinite time dilation.
If anything gravitational waves are evidence for extraordinarily massive objects, but against black hole singularities with their problematic infinite time dilation.
This is way too simplistic. Gravitational waves are waves of propagating spacetime curvature. They can certainly be produced by "mass", i.e., by objects with nonzero stress-energy, as long as those objects (or mergers of such objects) have time-varying quadrupole moments. But they can also be produced by objects like black holes (or mergers of such objects), which have zero stress-energy and are made purely of spacetime curvature.
> a black hole would make things more complicated because of the infinite time dilation
There is no such thing. The effect you are talking about is an artifact of a particular choice of coordinates and has no physical meaning. It certainly does not prevent black hole mergers from emitting gravitational waves.
From the point of view of Earth black holes can never merge because it would take an infinite amount of time to do so.
I do not care about the point of view of the black hole itself, or the point of view of an object falling into the black hole.
And it most definitely has physical meaning, and while gravitational waves could be emitted, they can never actually get to the merger.
The gravitational waves would essentially slow down and fade out if it was a black hole.
And since that is not what we see, I do not find the data compelling for black holes, but rather for ultra massive objects.
This is not correct. What is correct is that from Earth you cannot see events on or inside the horizon of a black hole (or of a merger). But the fact that you cannot see those events does not mean they don't happen or that you cannot have indirect evidence for them, any more than the fact that you cannot see objects over your horizon on Earth itself means that objects can't travel there.
> I do not care about the point of view of the black hole itself, or the point of view of an object falling into the black hole.
Nothing I am saying has anything to do with any "point of view". I am stating invariant facts about the spacetime geometry and events in it.
> it most definitely has physical meaning
No "point of view" has physical meaning. "Points of view" are mathematical conveniences that we humans use for calculations. They are not physical things.
> while gravitational waves could be emitted, they can never actually get to the merger.
> The gravitational waves would essentially slow down and fade out if it was a black hole.
Neither of these claims correctly reflect what GR predicts for black holes and black hole mergers. What GR predicts matches observations.
In a different reality, i.e. in a different point of view, it may have happened. And this is not a contradiction, it is merely the way the universe works.
And not only what can we not see beyond the event horizon, as things approached the event horizon they slow down tremendously. Even at quite a distance they slow down so much that our lifetime is insufficient to see them change.
In our lifetime it is impossible to see black holes merge because the time dilation is sufficient to slow that down to imperceptibility.
Sorry, but this is simply wrong as a description of what the actual theory of relativity says. So is the rest of your post. I've already explained why.
Using the mathematics of a physics theory to deny a tautological(?) statement seems to get at the very issue that Sabine is raising in the original article. Alternatively, you're literally arguing that the mathematics of the theory of relativity bears no meaning in reality. (i.e. "The effect you are talking about is an artifact of a particular choice of coordinates and has no physical meaning.") I'm not sure whether your statement is true, but if it is, it seems to prove Sabine's point...
Maybe it does to you, but it isn't a tautology. It's a false claim. See below.
> Is there a substantial difference between "from our point of view" and "as far as we are concerned ... in reality" under the theory of relativity?
Yes. A "point of view" in relativity (by which is meant something like a coordinate chart or a reference frame) might not be capable of describing all of reality. So you cannot infer, just from the fact that a "point of view" does not contain some particular event, that that event is not part of reality. Before you can draw that inference, you first have to show that the "point of view" in question is capable of describing all of reality (which in relativity means all of whatever spacetime manifold you are dealing with).
The "point of view" the GP is trying to use is not capable of describing all of the spacetime manifold we are dealing with. So it does not justify the claim the GP is making, that any event not described by that point of view cannot be part of the spacetime manifold.
> Using the mathematics of a physics theory to deny a tautological(?) statement
Is not what I am doing. I am using the mathematics of relativity to correct a wrong claim by someone who (apparently) doesn't understand that mathematics properly. Everything I am saying is basic general relativity and can be found in any GR textbook.
Doesn't have anything to do with the mistake the poster I have been responding to has been making. Sabine is talking about people who understand a given theoretical model but aren't willing to give it up when it either fails a test against experiment, or is shown to be in principle untestable by experiment. The poster I have been responding to does not understand the theoretical model being discussed, so is in a different position from the people Sabine is criticizing.
> Alternatively, you're literally arguing that the mathematics of the theory of relativity bears no meaning in reality.
I am arguing no such thing. I am simply making clear that, like any physical theory, GR has parts of its math that represent physically meaningful parts of reality, and parts of its math that represent human conventions that make calculations easier but have no physical meaning. Coordinate charts/reference frames, which is what the poster I have been responding to means by "point of view", are in the latter category. But invariants--quantities that are the same regardless of any choice of coordinates or reference frame--are also part of the math of GR, and those do have physical meaning.
I don't know where you're getting that from. Gravitational waves can be emitted by any system with a time-varying quadrupole moment. Mergers of quark stars would certainly qualify. Even a single quark star that was wobbly enough would.
This is not correct. There is a maximum mass limit for any object supported by quark degeneracy pressure, regardless of the equation of state. That is a fundamental fact about relativistic degeneracy that has the same general consequence for any kind of degeneracy pressure (it was originally discovered by Chandrasekhar when he discovered the maximum mass limit for white dwarfs that is now named after him). What the exact limit is depends on how stiff the equation of state is, but even the stiffest equation of state leads to a mass limit of about 3 solar masses. We know of plenty of compact objects that are far more massive than that.
No one knows the actual quark degeneracy pressure, so no one knows if stars collapse to black holes or if they get stopped.
Because we have these objects with huge masses but we can't see them, and such objects have to be compact. Objects with those masses that do not emit huge amounts of light that make them easily visible (i.e., that are not stars) can't not be compact. They either have to be held up by degeneracy pressure, which means they are either white dwarfs or neutron stars/quark stars, or they have undergone complete gravitational collapse and are black holes. All of those objects are compact, and there are no other possibilities.
> No one knows the actual quark degeneracy pressure, so no one knows if stars collapse to black holes or if they get stopped.
This is not correct. I have already explained why: the maximum mass limit. That limit applies regardless of what the actual quark degeneracy pressure turns out to be in an actual gravitational collapse. It is a fundamental fact about relativistic degeneracy.
You have not explained sufficiently about the maximum mass limit, there are plenty of suns that are enormously massive, but they're not black holes because heat is keeping the density from increasing.
The same thing would happen with a quark star or a strange star, the degeneracy pressure would keep it from ever becoming a black hole, at least until the pressure was overcome but we don't know what that pressure is.
What I'm saying is we cannot distinguish between a black hole and a quark star, not with our present observations.
Yes, I have: I said it applies to objects supported by degeneracy pressure. Ordinary stars are not; as you note, they are supported by thermal pressure. But thermal pressure never lasts forever; eventually any star will stop being able to produce heat and will collapse until it is either supported by degeneracy pressure, or forms a black hole. The maximum mass limit is a limit on the first alternative.
Also, as I said, an object supported by thermal pressure will be easily visible since it will emit lots of radiation. The objects that we believe are black holes are not doing that, so they can't be supported by thermal pressure anyway.
> The same thing would happen with a quark star or a strange star, the degeneracy pressure would keep it from ever becoming a black hole
Not if it is over the maximum mass limit. Sorry, but you are simply wrong as regards what the actual physics says. For the gory details, look up the Harrison-Wakano-Wheeler equation of state, or consult the classic textbook by Shapiro and Teukolsky.
> at least until the pressure was overcome but we don't know what that pressure is.
The whole point of the maximum mass limit is that any pressure, no matter how large, will be overcome if an object is over the limit. That's why we don't need to know what the actual pressure is.
> What I'm saying is we cannot distinguish between a black hole and a quark star, not with our present observations.
And what you are saying is wrong. See above.
I'm probably an order of magnitude less intelligent than her, but her arguments seem extremely reasonable to me. She seem to me to be a bridge from the PBS, pop-science mentality to more serious considerations.
Also, she's a fox. :-)