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Author Topic: 50 Billion Suns! -The Biggest Single Object in the Universe !  (Read 40611 times)

Offline dlorde

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« Reply #50 on: 19/03/2009 21:53:30 »
I think I've remembered what that Blue Curtain thing is all about. It's from the perspective of an observer falling into the EH. Does that sound better?

I don't see it myself ;)

If you're falling towards the EH, you're accelerating. You'll be accelerating away from everything further out, so if you look back, it will all be red-shifted. Everything in front of you (toward the EH) is accelerating away from you into the BH, so that should be red-shifted too. So where does the blue light come in? Maybe if you were stationary at the EH, you'd see stuff coming in toward you as blue-shifted...
 

Offline DoctorBeaver

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« Reply #51 on: 19/03/2009 23:52:58 »
dlorde - I don't remember the details of it. It was at least 4 years ago and I didn't know much about physics in those days (I still don't, but I knew even less then). It didn't really sink in.
 

Offline dlorde

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« Reply #52 on: 20/03/2009 01:33:24 »
dlorde - I don't remember the details of it. It was at least 4 years ago and I didn't know much about physics in those days (I still don't, but I knew even less then). It didn't really sink in.
OK, I was just curious.
 

Offline itisus

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« Reply #53 on: 20/03/2009 05:46:16 »
"That's a hundred thousand tredagrams, and you'll never get the chance to use that word in relation to anything else."

It's a lot less than a googleplex though, and there is one of those in Mountain View.  I've been in Mountain View a few times; it's no big deal.

It's late, but my scratch paper gives about 10^(-8) grams /m^3.  Air at sea level is on the order of kg/m^3.

I have wondered whether the universe should be considered a black hole.  Nothing escapes unless it is evaporating, its size is unknown but much larger than 50 billion suns, and the usual Schwarzschild interior solution is dubious.
 

Offline LeeE

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« Reply #54 on: 20/03/2009 12:04:23 »
I think I've remembered what that Blue Curtain thing is all about. It's from the perspective of an observer falling into the EH. Does that sound better?

I don't see it myself ;)

If you're falling towards the EH, you're accelerating. You'll be accelerating away from everything further out, so if you look back, it will all be red-shifted. Everything in front of you (toward the EH) is accelerating away from you into the BH, so that should be red-shifted too. So where does the blue light come in? Maybe if you were stationary at the EH, you'd see stuff coming in toward you as blue-shifted...

You've got to remember the time-dilation effects too.
 

Offline Vern

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« Reply #55 on: 20/03/2009 13:25:17 »
The Event Horizon is a region around a black hole that marks the boundary where light closer in can never exit the black hole. Why does that fact give the boundary special properties?  The natural laws should still apply. A steel rod part way past the EH should still allow its internal construct to follow a force on the outside part that pulled it out of the EH.

I think we globally describe the event horizon then in our minds, give it special properties that the General theory of Relativity does not give it.

We like to say that, once past the event horizon, nothing can escape. Maybe that should be modified to say that nothing operating under its own momentum can escape.
 

Offline dlorde

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« Reply #56 on: 20/03/2009 20:51:15 »
You've got to remember the time-dilation effects too.
OK, please explain, I don't see how time-dilation would cause an observer falling into the BH to see blue-shifted light.
 

Offline dlorde

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« Reply #57 on: 20/03/2009 20:54:50 »
The Event Horizon is a region around a black hole that marks the boundary where light closer in can never exit the black hole. Why does that fact give the boundary special properties?  The natural laws should still apply. A steel rod part way past the EH should still allow its internal construct to follow a force on the outside part that pulled it out of the EH.

I think we globally describe the event horizon then in our minds, give it special properties that the General theory of Relativity does not give it.

We like to say that, once past the event horizon, nothing can escape. Maybe that should be modified to say that nothing operating under its own momentum can escape.
Yes, there is no special property at the EH, it's simply the point at which the escape velocity is > c. An observer falling through the EH wouldn't notice anything - assuming the BH is large enough that tidal forces don't rip him apart  ;)
 

Offline Vern

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« Reply #58 on: 20/03/2009 22:05:40 »
That was my thinking exactly. However, we seem to be thinking of the EH as the singularity itself. It seems to me that the EH would be some distance from the singularity.


Wiki article explaning the Schwarzschild radius

Quote from: the article.
In 1916, Karl Schwarzschild obtained an exact solution[1][2] to Einstein's field equations for the gravitational field outside a non-rotating, spherically symmetric body (see Schwarzschild metric). The solution contained a term of the form 1 / (2M − r); the value of r making this term singular has come to be known as the Schwarzschild radius. The physical significance of this singularity, and whether this singularity could ever occur in nature, was debated for many decades; a general acceptance of the possibility of a black hole did not occur until the second half of the 20th century.
 

Offline DoctorBeaver

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« Reply #59 on: 21/03/2009 08:48:47 »
Apparently you don't actually need a singularity for an event horizon to form. If the mass is spread over a large enough area you could still get an EH. I've seen this theory proposed for the universe. It was referred to as a brown hole (sounds a bit dodgy, that).
 

Offline LeeE

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« Reply #60 on: 21/03/2009 19:28:06 »
The Event Horizon is a region around a black hole that marks the boundary where light closer in can never exit the black hole. Why does that fact give the boundary special properties?  The natural laws should still apply. A steel rod part way past the EH should still allow its internal construct to follow a force on the outside part that pulled it out of the EH.

I think we globally describe the event horizon then in our minds, give it special properties that the General theory of Relativity does not give it.

We like to say that, once past the event horizon, nothing can escape. Maybe that should be modified to say that nothing operating under its own momentum can escape.

The event horizon isn't a region but a boundary.  It doesn't occupy a volume of space but separates two regions of space that have different characteristics.  In the region of space outside the event horizon the rate of time is greater than zero but reduces as one gets closer to the event horizon.  Exactly at the event horizon, the rate of time reduces to zero.  What happens on the other side of the event horizon is anyone's guess, and a guess is all anyone can give you, but one thing for sure is that you couldn't poke a steel rod through it.

It's debatable that the steel rod could even actually reach the event horizon, for if space is distorted just as time is, there may be an infinite amount of space compressed around the event horizon, in which case you could fall forever and never reach the event horizon, not only from your point of view, in a slow time-frame, but also from the point of view of an observer, who would seem to see you perpetually receding from them, both shrinking and fading from sight.  Like I said though, this interpretation of the distortion of space-time around a black hole is open to debate.

However, one thing is for sure, if the rate of time reduces to zero at the event horizon we cannot talk of anything happening inside it for there appears to be no time, from our point of view, on the other side of the event horizon for anything to happen within.
 

Offline LeeE

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« Reply #61 on: 21/03/2009 19:44:22 »
You've got to remember the time-dilation effects too.
OK, please explain, I don't see how time-dilation would cause an observer falling into the BH to see blue-shifted light.

Although light produced closer to the event horizon than the observer is red-shifted, effectively reducing it's frequency when viewed by the observer, the time-dilation experienced by the observer means that less time has passed for the observer, which has the effect of raising the frequency of the light.

Also, iirc, I think the main factor for the Blue curtain effect is that matter falling into the BH produces gamma frequency light, which after the red-shift and time-dilation factors are taken in to consideration, ends up as blue in the visible spectrum.   Or something like that.
 

Offline Vern

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« Reply #62 on: 21/03/2009 20:33:11 »
Quote from: LeeE
However, one thing is for sure, if the rate of time reduces to zero at the event horizon we cannot talk of anything happening inside it for there appears to be no time, from our point of view, on the other side of the event horizon for anything to happen within.
Are you sure about this? It seems that time should be zero at the singularity. How does time get to be zero at the Event Horizon?
 

Offline LeeE

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« Reply #63 on: 21/03/2009 20:55:38 »
Quote from: LeeE
However, one thing is for sure, if the rate of time reduces to zero at the event horizon we cannot talk of anything happening inside it for there appears to be no time, from our point of view, on the other side of the event horizon for anything to happen within.
Are you sure about this? It seems that time should be zero at the singularity. How does time get to be zero at the Event Horizon?

Have a look at:

http://en.wikipedia.org/wiki/Gravitational_time_dilation#Outside_a_non-rotating_sphere

for the simplest solution (outside a non-rotating sphere).  The formula is pretty simple and it shows that at the Schwarzchild radius you end up with 0.

Interestingly, there's another solution just below, for inside the event horizon, but it doesn't deal with how anything can actually pass that zero-time boundary, and while the outside solution might be provable, the inside solution isn't; if someone were to be able to get inside to prove it, they couldn't convey that proof back to us.
 

Offline Vern

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« Reply #64 on: 21/03/2009 23:57:17 »
Thanks for the link LeeE. I notice that the Schwarzchild radius is part of the equation. It is not immediately obvious to me that t = 0 at that radius. I don't doubt that it might, I just notice that many folks think that t = 0 closer in toward the singularity. I'll have to do some arithmetic.  :)
 

Offline dlorde

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« Reply #65 on: 22/03/2009 01:11:29 »
The event horizon isn't a region but a boundary.  It doesn't occupy a volume of space but separates two regions of space that have different characteristics.
In the region of space outside the event horizon the rate of time is greater than zero but reduces as one gets closer to the event horizon.  Exactly at the event horizon, the rate of time reduces to zero.  What happens on the other side of the event horizon is anyone's guess, and a guess is all anyone can give you, but one thing for sure is that you couldn't poke a steel rod through it.
Not really - the time dilation experienced by objects at the EH is relative to the outside observer. Because of the extreme curvature of spacetime, external observers can't see beyond the event horizon in space or time, but the only thing special about spacetime at that point is the amount of curvature it has. From outside, we can't see inside because at that point the curvature is too great, but the curvature is smooth. If the black hole is sufficiently large (e.g. 10 million x solar mass), the curve will be shallow enough at the EH that an observer falling through the EH would not be ripped apart by the tidal forces, and would, in principle (ignoring radiation, etc) be able survive for some time on the other side before being pulled apart.

Quote
It's debatable that the steel rod could even actually reach the event horizon, for if space is distorted just as time is, there may be an infinite amount of space compressed around the event horizon, in which case you could fall forever and never reach the event horizon, not only from your point of view, in a slow time-frame, but also from the point of view of an observer, who would seem to see you perpetually receding from them, both shrinking and fading from sight.  Like I said though, this interpretation of the distortion of space-time around a black hole is open to debate.
Again, that's not accurate, there is no special discontinuity at the EH - the singluarity sits at the bottom of almost infinitely deep gravity well - we don't know what's near the bottom or if there is one, but spacetime is being effectively stretched down into this well. Space-time may be compressed at the singularity, but no-one knows. But outside the singularity tidal forces will pull objects apart in the direction of travel and compress them across it (spaghettisation) as the gravity differentials across them strengthen as space-time stretches. Other than that (ouch), and some odd visual distortions, not a lot is different.

Quote
However, one thing is for sure, if the rate of time reduces to zero at the event horizon we cannot talk of anything happening inside it for there appears to be no time, from our point of view, on the other side of the event horizon for anything to happen within.
That doesn't mean that an observer falling through the EH experiences that time distortion. It is possible to predict what they might experience right up until they approach the region of singularity. In 'The Emperor's New Mind' (pp 433,434), Roger Penrose describes what an observer B, falling into a BH, away from an observer A outside the EH, would experience:

".. It should first be pointed out that there will be nothing whatever noticeable by B at the moment of his crossing the horizon. He glances at his watch ... and he sees the minutes pass regularly by. He can look back at A, and will find that A remains continuously visible the whole time. He can look at A's own watch, which appears to be proceeding in an orderly and regular fashion. Unless B has calculated that he must have crossed the horizon, he will have no way of knowing it. ... The second law [of thermodynamics] will hold sway just as much inside a BH as it does elsewhere. The entropy in B's vicinity is still increasing, right up to the time of his final crunch." Penrose should know - he helped Hawking develop the physics of BHs.

ISTM that in principle, it would be possible to drop down a cable or tether across the EH from orbit around a BH - but of course, you couldn't pull it back. In practice, the tether would have to be incredibly strong, and if it didn't break, it would start pulling you in towards the EH well before it got there. Once it reached the EH, you'd have to let go or be pulled in with it.
 

Offline om

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« Reply #66 on: 22/03/2009 05:16:26 »
Detailed studies of the properties of ordinary nuclei reveal strongly repulsive interactions between neutrons.

[See: "Neutron repulsion confirmed as energy source", Journal of Fusion Energy 20, 197-201 (2003)].

http://www.omatumr.com/abstracts2003/jfe-neutronrep.pdf

These studies show that neutron-emission from a neutron star "may release up to 1.1%-2.4% of the nuclear rest mass as energy".  By comparison, only about 0.8% of the rest mass is converted to energy in Hydrogen fusion and only about 0.1% of the nuclear rest mass is converted to energy in fission.

Therefore massive, energetic celestial objects are probably not black holes at all, but neutron stars that are highly energized by repulsive interactions between neutrons.

With kind regards,
Oliver K. Manuel
http://www.omatumr.com/
 

Offline dlorde

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« Reply #67 on: 22/03/2009 10:33:24 »
Another good link on falling into a Black Hole (it's not habit forming  ;)) : Fall Into A Black Hole
« Last Edit: 22/03/2009 18:01:38 by dlorde »
 

Offline DoctorBeaver

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« Reply #68 on: 22/03/2009 10:57:54 »
There's a problem with your link. You got http:// twice. Try editing it.

It's an interesting article but I'll have to read it a few times to fully absorb it.
« Last Edit: 22/03/2009 11:11:57 by DoctorBeaver »
 

Offline om

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« Reply #69 on: 22/03/2009 13:50:08 »
DoctorBeaver,

Thanks for your comment.  Try this link to: "Neutron repulsion confirmed as energy source", J. Fusion Energy 20 (2003) 197-201,

www.omatumr.com/abstracts2003/jfe-neutronrep.pdf


If it doesn't work, see: "Nuclear systematics: III. The source of solar luminosity", Journal of Radioanalytical & Nuclear Chemistry, Vol. 252, No. 1 (2002) 3-7

www.omatumr.com/abstracts2001/nuc_sym3.pdf

With kind regards,
Oliver
www.omatumr.com/index.html

 

Offline Vern

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« Reply #70 on: 22/03/2009 13:53:39 »
Another good link on falling into a Black Hole (it's not habit forming  ;)) : Fall Into A Black Hole
It seems to still not work when edited.
 

Offline yor_on

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« Reply #71 on: 22/03/2009 15:12:03 »
Ah, Vern, wrong dude :)
 

Offline Vern

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« Reply #72 on: 22/03/2009 15:38:27 »
Well; I don't know; the link did have, http://http://, doubled like that. But I fixed it in the quote and the link still didn't work. However the home site is really nice :)
 

Offline dlorde

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« Reply #73 on: 22/03/2009 18:05:06 »
Sorry guys, it's fixed now. Looks like this forum software adds the http prefix automatically.
 

Offline LeeE

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« Reply #74 on: 22/03/2009 21:07:37 »
The event horizon isn't a region but a boundary.  It doesn't occupy a volume of space but separates two regions of space that have different characteristics.
In the region of space outside the event horizon the rate of time is greater than zero but reduces as one gets closer to the event horizon.  Exactly at the event horizon, the rate of time reduces to zero.  What happens on the other side of the event horizon is anyone's guess, and a guess is all anyone can give you, but one thing for sure is that you couldn't poke a steel rod through it.
Not really - the time dilation experienced by objects at the EH is relative to the outside observer. Because of the extreme curvature of spacetime, external observers can't see beyond the event horizon in space or time, but the only thing special about spacetime at that point is the amount of curvature it has. From outside, we can't see inside because at that point the curvature is too great, but the curvature is smooth. If the black hole is sufficiently large (e.g. 10 million x solar mass), the curve will be shallow enough at the EH that an observer falling through the EH would not be ripped apart by the tidal forces, and would, in principle (ignoring radiation, etc) be able survive for some time on the other side before being pulled apart.

While the degree of time-dilation is relative to the observer, as it must be unless when compared with a hypothetical space-time frame outside of our universe, it doesn't mean that the effect is not real.  Experiments, both with moving clocks and with clocks in different gravitational potential, show that different amounts of time have passed for the two separated clocks when they are subsequently brought back together.  The difference in the duration of time that has passed for the two separated points of view is not illusory.  Thus, as something approaches an event horizon, the absolute amount of time that passes for the approaching object is less than the absolute amount of time that has passed for a distant observer, and when the approaching observer reaches the event horizon zero time will pass for it.  At this point, the ratio between the rate and duration of time for the approaching observer and that for the distant observer becomes infinite, and from that point onwards, physics breaks down.  Yes, the approaching observer will not be aware that time is running slow, at least from their point of view, in their own space-time frame; they will not 'feel' that they are running slow, and neither, when/if they reach the event horizon, will they realise that no time is passing for them, because everything will have stopped.  Even though the difference can only be expressed in relative terms, if one set of values equals zero the difference is absolute.

If the approaching observer only closely approaches the event horizon, and then returns to the distant observer, their two space-time frame can be reconciled because the difference between the rates and durations of time that have passed for both of them will be finite.  If the approaching observer were to be able to actually reach the event horizon, however, the difference between the two rates and durations cannot be reconciled because for the approaching observer, they will both be zero.

I think that the other two points you raise are dependent upon your first point.
 

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