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PeterDonis submitted a new PF Insights post
Do Black Holes Really Exist?
Continue reading the Original PF Insights Post.
Do Black Holes Really Exist?
Continue reading the Original PF Insights Post.
jambaugh said:Yes. They really do... They are observed in galactic centers.
I find people get most confused by the characterization of event horizons, as if the proverbial event horizon of a black hole is some unique new physical entity. We pass through event horizons constantly. space-like hyper-surface is an event horizon,
Bernie G said:Do we need complex ideas like GR or curved space to predict black holes?
QuantumQuest said:In my opinion, this sounds like some people beginning with Albert Einstein, just wanted to make things look complex, which can in no way be true.
My suspicion is that you are using a definition of "direct observation" here that it far more limited than you would use in other situations (just seeing with your eyes?). Because there are several direct observations of properties of black holes. Gravitational field strength is measured by timing orbits. Size is measured by observing radiation from infalling matter._PJ_ said:I disagree. I am still unaware of any DIRECT OBSERVATION of Black Holes...
Light always travels in straight lines. In curved spacetime, this straight path is seen to be curved.Bernie G said:Is the concept of curved space required to predict black holes?
_PJ_ said:Light always travels in straight lines... in some ways, yes, curved space is necessarily part of the actual definition of what a Black Hole is ...
Because Light must travel in straight lines. If light was "bent" or curved, it would necessitate a change in velocity which necessarily entails a temporal metric which implies that light is not relativistic and violates both of Einstein's theories in one go.Bernie G said:Why can't we simply say that light bends around an object??
_PJ_ said:In your given equations, when dealing with relativistic speeds, one must factor in the Lorenz transformations, which you seem to be missing.
It will have a constantly changing velocity anyway, if it's "falling".Bernie G said:Sure, a particle falling straight down towards a black hole will have Lorenz transformations, but do the Lorenz transformations at any point affect the velocity it will have?
_PJ_ said:It will have a constantly changing velocity anyway, if it's "falling".
So do you accept that the gravitational field strength and size measurements are "direct" measurements? I can't tell from what you are saying. What is the difference between/definition of "direct/indirect" measurements"? Rather than direct/indirect measurements, you now seem to be talking about some sort of indirect properties, and I've never heard of such a thing either._PJ_ said:No. By Direct Obsevration of a Black Hole, I mean, any measurement that detects the actual properties of a Black Hole directly, rather than an indirect inference from a measurement of some other property...
That's a different issue than whether the measurements are "direct". In science, theories predict properties and if properties are detected that match the theory and no other viable theories exist, then the theory is validated. Your line of logic sounds more like wishing another explanation will be found than accepting the scientific process that already found a viable explanation....which (ALTHOUGH HIGHLY UNLIKELY) may still be yet shown to be due to some other process.
Gravitational acceleration is caused by mass. Mass is a property of objects. So that's an observation of the property of mass of the object it is falling into.Infalling Matter tells us the gravitational power accelerating objects, there is no observation of to-what this matter is falling into.*
That's just the vanilla "you can't prove anything absolutely" fundamental reality of science. It's true of anything in science and nobody would ever claim black holes or anything else were 100% proven.I maintain that it's simply not enough to warrant any claim of confirming the definite, undeniable such a phenomena as a Black Hole.
Actually you may read more about BH Firewall Paradox.rootone said:A black hole is a prediction of GR, but nobody has made any claims of knowing exactly what happens inside of the event horizon of a BH,
and obviously whatever does go on cannot be observed directly.
The fact that the simplest models end up with a mathematical singularity is a strong indication that some kind of presently unknown physics comes into play.
However which ever way one chooses to interpret it, 'black hole candidates' do exist, and in particular the evidence for the SMBH in our galaxy's centre is overwhelming.
There are beyond any doubt star systems which are rapidly orbiting an extremely massive yet small invisible object.
Whatever object exists there it fulfils the GR description of a black hole, so until such time as there is contrary evidence we may as well call it a black hole.
I tend to agree. Mass appears to be the determining factor. A star with ≤ 4.8 M☉ will eventually form a degenerate white dwarf capable of resisting gravity's effects. Whereas a star > 4.8 M☉ ≤ 10 M☉ will eventually form a neutron star capable of resisting gravity's effects. It certainly seems plausible that ultra-relativistic matter, such as quarks or dileptons, in stars with a certain mass range (> 10 M☉) could be capable of resisting gravity's effects. The apparent effect, to an outside observer, would be identical to a black hole with a Schwarzschild radius event horizon and an apparent horizon. While being incredibly dense (possibly an order of magnitude more dense than a neutron star) it still would not be "infinitely" dense, as in a "singularity."Bernie G said:"Do Black Holes Really Exist?"
Probably, but could what we think are black holes be compact stars (larger than their Schwarzschild radius) if they had the following characteristics?: (1) They were a mixture of normal matter and ultra-relativistic matter. (2) They had a crust that was mostly a light absorber.
Something called "electroweak star" is proposed where neutrons/quarks would be burned to leptons in small core of otherwise "normal" neutron star.Bernie G said:Questioning conventional theory is a good thing but as was said above its pretty much a fact that there are small-size large-mass objects that are not visible. The GR description of a black hole is a logical explanation although we don’t have to accept it as gospel. An object that gravitationally contains light could be predicted without GR or curved space. A compact star with a light absorbing surface could also appear as a black hole. And there are other theories mentioned above.
There might be a clue from the probable fact that neutron star mass is limited by some process to 2 solar masses. Logically this process must occur within the star. Jets forming outside the star that limit the stars mass by preventing all material from falling to the star seem illogical. In older neutron stars large amounts of mass-energy could be shed by super extreme surface temperature, but this is not observed. Nuclear surface explosions are not an explanation. The alternative is the direct ejection of ultra-relativistic matter from the star. (Matter with a velocity of only 0.1c can’t escape the star). Ultra-relativistic matter is a logical result of the collapse of neutrons in the core, and this would explain the jets from neutron stars, not so interesting except this would probably have implications for the 5 – 10 solar mass “black holes” with jets. It seems that many people have a mental block to considering the possibility of neutron collapse at the core of a neutron star. It would be good to hear any suggestions for a process that limits the mass of neutron stars.
You're saying this in a way which suggests that you think some mechanism prevents an existing neutron star from becoming any larger. I'm not aware of any evidence for this. What is thought to happen is that there is a threshold mass at which a neutron star will collapse to become a black hole, and there may be intermediate phases at which a neutron star might be transformed to a more dense hypothetical object such as a "quark star".Bernie G said:There might be a clue from the probable fact that neutron star mass is limited by some process to 2 solar masses.
I note that this suggestion violates baryon and lepton number, which contradicts all experimental evidence to date. (But so do black holes).Martin0001 said:Something called "electroweak star" is proposed where neutrons/quarks would be burned to leptons in small core of otherwise "normal" neutron star.
http://arxiv.org/abs/0912.0520
Jonathan Scott said:You're saying this in a way which suggests that you think some mechanism prevents an existing neutron star from becoming any larger. I'm not aware of any evidence for this. What is thought to happen is that there is a threshold mass at which a neutron star will collapse to become a black hole, and there may be intermediate phases at which a neutron star might be transformed to a more dense hypothetical object such as a "quark star".
Also, neutron stars are currently primarily distinguished from any more compact form by X-ray bursts which are thought to be from fusion of accumulated helium (produced by hydrogen immediately fusing to helium when it falls to the surface). I've just started another thread to ask about whether a neutron star might be able to become sufficiently massive that falling hydrogen might have enough energy for much of it to fuse beyond helium immediately, in which case there will be no accumulation of helium to cause an X-ray flash, making it more difficult to distinguish it from a black hole. Here's the thread: https://www.physicsforums.com/threads/x-ray-bursts-might-not-happen-for-larger-neutron-stars.850627/
You seem to have missed the point. It is theoretically expected that there will be a maximum possible mass for a neutron star between 1.4 and 3 solar masses, after which the neutron star will collapse to some other state (starting from the core with what could indeed be described loosely as "neutron collapse"). If the initial state is not a black hole, it is expected that only a relatively small further increase in mass would be enough to create a black hole. Regardless of whether the new state is a something like a quark star or a black hole, it could appear very similar to the original neutron star, as the appearance is normally dominated by radiation from the accretion disk.Bernie G said:You're saying this in a way which suggests that you think some mechanism prevents an existing neutron star from becoming any larger.": YES!
"I'm not aware of any evidence for this.": The maximum observed mass of neutron stars is thought to be about 2 M☉.
Jonathan Scott said:It is theoretically expected that there will be a maximum possible mass for a neutron star between 1.4 and 3 solar masses, after which the neutron star will collapse to some other state (starting from the core with what could indeed be described loosely as "neutron collapse".
Please do not continue to promote this extremely speculative idea which has already been the subject of another thread. I have already pointed out that if you wish to discuss it further, you first need find appropriate references then start a new thread. (In that thread I already pointed out that no additional kinetic energy can be found without violating baryon number conservation, and your hand-waving assertion that such relativistic material could find its way from the core to the surface at such a speed as to escape the gravitational field seems totally fanciful).Bernie G said:Then when the neutrons at the core collapse, what happens to them? What would be the "other state"? Recent high energy collider experiments indicate that when a nucleus collapses the mass is converted roughly 10+% quark type matter and roughly 90% energy. I think the same would probably happen when some core neutrons collapse. If the resulting quark matter and energy quickly exit the star by some process, pressure is then relieved and core collapse should stop.
Jonathan Scott said:It is theoretically expected that there will be a maximum possible mass for a neutron star between 1.4 and 3 solar masses, after which the neutron star will collapse to some other state (starting from the core with what could indeed be described loosely as "neutron collapse".