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acey

Are black holes real?

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Laura Mersini-Houghton was interviewed on Newsnight on Monday about her new paper arguing that collapsing stars may "bounce" rather than form a black hole. Here's the programme (4 days left - item begins at 29:30 min): 

http://www.bbc.co.uk/programmes/b046nd2n

Here's the abstract of her paper:

Particle creation leading to Hawking radiation is produced by the changing gravitational field of the collapsing star. The two main initial conditions in the far past placed on the quantum field from which particles arise, are the Hartle Hawking vacuum and the Unruh vacuum. The former leads to a time symmetric thermal bath of radiation, while the latter to a flux of radiation coming out of the collapsing star. The energy of Hawking radiation in the interior of the collapsing star is negative and equal in magnitude to its value at future infinity. This work investigates the backreaction of Hawking radiation on the interior of a gravitationally collapsing star, in a Hartle-Hawking initial vacuum. It shows that due to the negative energy Hawking radiation in the interior, the collapse of the star stops at a finite radius, before the singularity and the event horizon of a black hole have a chance to form. That is, the star bounces instead of collapsing to a black hole. A trapped surface near the last stage of the star's collapse to its minimum size may still exist temporarily. Its formation depends on the details of collapse. Results for the case of Hawking flux of radiation with the Unruh initial state, will be given in a companion paper II.

Here's the full paper:

http://arxiv.org/abs/1406.1525

Edited by acey
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I watched a good one the other night about gravity saying a similar thing

Pat

Edited by todd8137
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Black holes might not be real, but something heavy and very compact has been observed in galaxies. If they are not supermassive black holes, what are they?

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The problem is that if they do not exist then what is the massive object that the centre of the Milky Way that the set of core stars are orbiting around at very high velocity. Using basic Newtonian mechanics whatever is there is some 4 million solar masses, and there is nothing else that could have that mass.

So arguing against them is one aspect but then comes the need to supply answers to known situations. If they are not real what is the huge gravity mass at the centre of the milky way.

Also a black hole could simply be looked on as a normal progression - White Dwarf, Neutron Star, Black Hole, and they were predicted by General Relativity which is holding pretty firm and solid after many years. Black Hole is I suspect a bad name.

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Are they just super dense, but not a singularity? Could such a body be massive enough to account for what we see at the centre of galaxies?

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Does the centre spin faster than the outer edge of a galaxy

Pat

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Thanks for posting. If I understand correctly then the contention is that 'stellar collapse does not lead to BH formation' rather than 'BH's do not exist'.

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Does the centre spin faster than the outer edge of a galaxy

Pat

from wot i understand everything rotates at the same pace, like when looking down on a record :D (showin me age)

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I have loads of vinyl from my youth and am not that old ,well that blows my theory on what's at the centre as if you spin water in a bowl with your hand,the centre spins faster than the outside I thought the same for the galactic centre and that would explain why the stars spin faster at the centre am sure missed that on Bluepeter :)

Pat

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haha i still have a lot of vinyl too glad my taste today has changed :D

i use to think that it was more whirlpooly but did read somewhere that its summit to do with dark matter not sure thou, ill have to have a read on it again

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...well that blows my theory on what's at the centre as if you spin water in a bowl with your hand,the centre spins faster than the outside I thought the same for the galactic centre and that would explain why the stars spin faster at the centre am sure missed that on Bluepeter :)Pat

I was thinking the same. And are they held tigether with some Blue Peter 'stickyback plastic', me thinks!

Edited by Philip R
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Of course they are real. Drive down any UK road and you will end up in one.  

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Of course they are real. Drive down any UK road and you will end up in one.

Yep! plenty on Eastern Perimeter Road, LHR.

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Can't explain some thing ,it must be a black hole ('Astros physics)

Pat

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STEPHEN CROTHERS: Black Holes & Relativity, Part One

nice talk given at a level of common sense..

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The black hole in my wallet is very real LOL

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There's an object at the centre of our own galaxy that, using the orbits of stars swirling around it, has been proven to be 4 million times the mass of the Sun. Such an object would have gravity strong enough to prevent light leaving...and would thus be a black hole, and is indeed invisible to telescopes. So if black holes "can't" exist....nobody has told this object about it.

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The question is whether the mass at the centre of galaxies is smaller than the Schwarzschild radius. It is inferred from observations of our own galaxy that the radius is indeed smaller. However Laura Mersini-Houghton's work implies that Hawking radiation would prevent matter from collapsing to a size smaller than the Schwarzschild radius, hence a black hole would not form. There is a contradiction between inference from observation and inference from theory.

Here's a recent article which references the same paper I mentioned in my original post, and also a more recent paper written with H.P. Pfeiffer:

http://phys.org/news/2014-09-black-holes.html

Here's the new paper:

http://arxiv.org/abs/arXiv:1409.1837

Here's the abstract:

(quote)

A star collapsing gravitationally into a black hole emits a flux of radiation, knowns as Hawking radiation. When the initial state of a quantum field on the background of the star, is placed in the Unruh vacuum in the far past, then Hawking radiation corresponds to a flux of positive energy radiation travelling outwards to future infinity. The evaporation of the collapsing star can be equivalently described as a negative energy flux of radiation travelling radially inwards towards the center of the star. Here, we are interested in the evolution of the star during its collapse. Thus we include the backreaction of the negative energy Hawking flux in the interior geometry of the collapsing star and solve the full 4-dimensional Einstein and hydrodynamical equations numerically. We find that Hawking radiation emitted just before the star passes through its Schwarzschild radius slows down the collapse of the star and substantially reduces its mass thus the star bounces before reaching the horizon. The area radius starts increasing after the bounce. Beyond this point our program breaks down due to shell crossing. We find that the star stops collapsing at a finite radius larger than its horizon, turns around and its core explodes. This study provides a more realistic investigation of the backreaction of Hawking radiation on the collapsing star, that was first presented in [1].

(endquote)

Strong empirical evidence that the mass at the centre of our galaxy is indeed smaller than its Schwarzschild radius comes from observations of Sgr A* (a compact radio source) and objects orbiting it. From Wikipedia:

(quote)

Astronomers are confident that these observations of Sagittarius A* provide good empirical evidence that our own Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System[6] because:

  • The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours (1.8×1013 m) from the center of the central object.[17]
  • From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses.[3] (The corresponding Schwarzschild radius is 0.08 AU/12 million km/7.4 million miles; 17 times bigger than radius of Sun.)
  • The radius of the central object must be significantly less than 17 light hours, because otherwise, S0-16 would collide with it. In fact, recent observations[18] indicate that the radius is no more than 6.25 light-hours (6,75 billion km/4.2 billion miles/45 AU), about the diameter of Uranus' orbit, leading to density limit 8.55×1036 kg / 1.288×1039 m3 = 0.0066 kg/m3.
  • The only widely hypothesized type of object which can contain 4.1 million solar masses in a volume that small is a black hole.

While, strictly speaking, there are other mass configurations that would explain the measured mass and size, such an arrangement would collapse into a single supermassive black hole on a timescale much shorter than the life of the Milky Way.[8]

(end quote)

So this is a case of one theoretical line of reasoning against a lot of observational reasoning. But the challenge would be to show how Mersini-Houghton's reasoning is wrong, not just to say it must be wrong.

Edited by acey

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The paper hasn't been peer-reviewed yet, has it?

I'd wait until it has been reviewed and verified before ripping up black-hole theory....

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First paper has been peer reviewed and published in Phys Lett B, second is presumably submitted and under review.

I'm not qualified to express an informed opinion on it, but just think it looks pretty interesting.

  10.1016/j.physletb.2014.09.018 Edited by acey

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to go all resident evil on you, "But what is it?"

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Singularities are problematic even with Relativity also I understand that Stephen Hawking has now referred to grey holes! Tony

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