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I've noticed she gets some more attention outside her blog and YT channel recently.

by emsign 4y ago

cratermoon 4y ago
She's refreshingly keen on evidence and testability for physics advances. The entire niche of taking the equations and treating the results they give when pushed to the limits as reality is very dodgy. This is true even though black holes we predicted by plugging extreme values into general relativity. While black holes do exist, we don't know yet how well their real physical properties match those predicted by GR. String theory in particular has had a testability problem from the very beginning. https://www.math.columbia.edu/~woit/wordpress/?p=533 https://www.math.columbia.edu/~woit/wordpress/?p=533
- ars 4y ago
  Actually the event horizon of a black hole has never been observed. 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.
  - bmitc 4y ago
    What does it mean to observe the event horizon? 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?
    - thethirdone 4y ago
      > LIGO has been able to confirm theoretical properties of the event horizon. 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.
      - pdonis 4y ago
        > 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. 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).
      - bmitc 4y ago
        > What specific properties has LIGO confirmed about the event horizon? Hawking's area theorem is one: https://news.cornell.edu/stories/2021/07/hawkings-black-hole-theorem-observationally-confirmed 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?
  - naasking 4y ago
    > All current observations are also consistent with quark stars. Gravitational waves are a prediction of GR but not of quark stars, so GR explains more of the available evidence than quark stars alone.
    - ars 4y ago
      That is not true. General relativity concerns itself with mass, and gravitational waves are function of mass. 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.
      - pdonis 4y ago
        > General relativity concerns itself with mass, and gravitational waves are function of mass. 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.
        ars 4y ago
        The choice of coordinate is Earth. 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.
  - pdonis 4y ago
    > 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. 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.
    - ars 4y ago
      How do you know the object is compact? The idea is that quark degeneracy pressure is enough to keep the object from ever becoming dense enough to form a black hole. No one knows the actual quark degeneracy pressure, so no one knows if stars collapse to black holes or if they get stopped.
      - pdonis 4y ago
        > How do you know the object is compact? 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.
        ars 4y ago
oceanghost 4y ago
She's got someone doing good PR for her. I've been a subscriber for years and never seen her on HN except for the last few months. 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. :-)
deleted 4y ago
[deleted]

I've noticed she gets some more attention outside her blog and YT channel recently.

by emsign 4y ago

I've noticed she gets some more attention outside her blog and YT channel recently.

She's refreshingly keen on evidence and testability for physics advances. The entire niche of taking the equations and treating the results they give when pushed to the limits as reality is very dodgy. This is true even though black holes we predicted by plugging extreme values into general relativity. While black holes do exist, we don't know yet how well their real physical properties match those predicted by GR. String theory in particular has had a testability problem from the very beginning. https://www.math.columbia.edu/~woit/wordpress/?p=533 https://www.math.columbia.edu/~woit/wordpress/?p=533

ars 4y ago
Actually the event horizon of a black hole has never been observed. 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.
- bmitc 4y ago
  What does it mean to observe the event horizon? 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?
  - thethirdone 4y ago
    > LIGO has been able to confirm theoretical properties of the event horizon. 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.
    - pdonis 4y ago
      > 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. 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).
    - bmitc 4y ago
      > What specific properties has LIGO confirmed about the event horizon? Hawking's area theorem is one: https://news.cornell.edu/stories/2021/07/hawkings-black-hole-theorem-observationally-confirmed 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?
- naasking 4y ago
  > All current observations are also consistent with quark stars. Gravitational waves are a prediction of GR but not of quark stars, so GR explains more of the available evidence than quark stars alone.
  - ars 4y ago
    That is not true. General relativity concerns itself with mass, and gravitational waves are function of mass. 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.
    - pdonis 4y ago
      > General relativity concerns itself with mass, and gravitational waves are function of mass. 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.
      - ars 4y ago
        The choice of coordinate is Earth. 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.
- pdonis 4y ago
  > 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. 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.
  - ars 4y ago
    How do you know the object is compact? The idea is that quark degeneracy pressure is enough to keep the object from ever becoming dense enough to form a black hole. No one knows the actual quark degeneracy pressure, so no one knows if stars collapse to black holes or if they get stopped.
    - pdonis 4y ago
      > How do you know the object is compact? 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.
      - ars 4y ago

oceanghost 4y ago

She's got someone doing good PR for her. I've been a subscriber for years and never seen her on HN except for the last few months. 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. :-)

deleted 4y ago

[deleted]