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mixed nuts
Guest

Tue Jan 10, 2017 9:24 pm   



On 1/10/2017 8:04 AM, pcdhobbs_at_gmail.com wrote:
Quote:
Sure, having a black hole eat the planet would be bad. But how
would that relate to creating black hole in the lab? The idea that
even a tiny black hole would be catastrophic is great for science
fiction, but /very/ far from reality.

Okay, you sound very sure about that. Which of course you ought to
be, considering that the wager you propose is the lives of billions
of people plus all of human civilization forever (plus a few dogs and
cats and wild animals) versus some asshole's curiosity.

I wonder what odds Jimmy the Greek would offer on that one.


http://www.gocomics.com/calvinandhobbes/2016/12/31

--
Grizzly H.

Phil Hobbs
Guest

Tue Jan 10, 2017 9:33 pm   



On 01/10/2017 08:27 AM, David Brown wrote:
> /All/ black holes are very dense.

Nope. The bigger the mass, the lower the density. In fact not so long
ago there was significant doubt whether or not the whole universe was a
black hole (i.e. closed).

(Of course you can't really talk about volume in the neighbourhood of a
singularity, so it's usually computed as 4/3 pi r**3, where r is the
Swarzschild radius.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

Jeroen Belleman
Guest

Tue Jan 10, 2017 10:39 pm   



On 2017-01-10 14:27, David Brown wrote:
Quote:
On 10/01/17 13:13, bill.sloman_at_ieee.org wrote:
On Tuesday, January 10, 2017 at 8:26:19 PM UTC+11, David Brown
wrote:
On 10/01/17 02:55, bill.sloman_at_ieee.org wrote:
On Tuesday, January 10, 2017 at 12:11:14 PM UTC+11, rickman
wrote:
On 1/9/2017 11:07 AM, Phil Hobbs wrote:
On 01/08/2017 11:06 PM, rickman wrote:

snip

This newsgroup would be a lot nicer if you folks would learn how to
snip posts!

Someday we will create black holes in the lab and may
observe Hawking radiation. My understanding is that the
strength of the radiation would be proportional to the size
of the black hole, so if we created a very small one it
would in essence explode immediately. I think we could
detect that.


Man, I hope nobody is stupid enough to do that. A
miscalculation could literally destroy the planet.

How is that?

BTW, I think you overvalue the planet.

Having a black hole eat the entire planet would be a bad
outcome,


Sure, having a black hole eat the planet would be bad. But how
would that relate to creating black hole in the lab? The idea that
even a tiny black hole would be catastrophic is great for science
fiction, but /very/ far from reality.

http://www.livescience.com/4210-rumors-black-hole-factory-destroy-earth.html



https://www.quora.com/Is-it-possible-to-create-a-miniature-black-hole-on-earth-inside-a-lab-without-creating-a-massive-catastrophy

http://www.askamathematician.com/2012/07/q-is-it-possible-for-an-artificial-black-hole-to-be-created-or-something-that-has-the-same-effects-if-so-how-small-could-it-be-made/


That's interesting. I'd never actually noticed the point that small -
low mass - black holes would have to be very dense. The Chandrasekhar
limit is 1.3 solar masses, and the Tolman–Oppenheimer–Volkoff limit
is between 1.5 and 3 solar masses.

/All/ black holes are very dense. Even ignoring considerations of
different sizes of black holes having different densities, a small mass
black hole is going to be extremely small. It is also going to have
extremely low mass. This means its gravitational pull is going to be
extremely small, and it is extremely unlikely to capture any other
matter within its pull.

So if we assume there is no Hawking radiation and black holes can grow
but not "evaporate", if we made a black hole in the lab it would move
under the influence of gravity just like a point mass. It would not
interact with other matter in any significant way, as it would simply
pass through any atoms (just like neutrinos). Depending on its mass and
initial speed, it is likely to end up in orbit around the earth's centre
of gravity. Based on the values given in the links above (which I have
no reason to doubt), it would perhaps swallow a proton every hundred
years or so. It is not exactly a disaster.

Note also that while CERN have managed to make collisions with
impressively high energies, we are regularly hit by cosmic rays with
/much/ higher energies. If we had the remotest chance of making a black
hole in the lab, and if that black hole posed the remotest danger, then
we would already have been gobbled up by a black hole from a cosmic ray
collision.


Regarding black hole density, the universe and all that:
You better start believing in black holes. You're in one!

If the universe is closed, that is. Yes, I know that view
is passé.

Jeroen (savvy?) Belleman

rickman
Guest

Wed Jan 11, 2017 3:51 am   



On 1/10/2017 8:04 AM, pcdhobbs_at_gmail.com wrote:
Quote:
Sure, having a black hole eat the planet would be bad. But how would
that relate to creating black hole in the lab? The idea that even a
tiny black hole would be catastrophic is great for science fiction, but
/very/ far from reality.

Okay, you sound very sure about that. Which of course you ought to be, considering that the wager you propose is the lives of billions of people plus all of human civilization forever (plus a few dogs and cats and wild animals) versus some asshole's curiosity.

I wonder what odds Jimmy the Greek would offer on that one.


We all die. What's the difference of when? Death by the sort of black
hole you are talking about, it it could happen, would be pretty quick.
Once the black hole reached some size I expect it would contain the
entire earth rather quickly.

We are only on this planet temporarily anyway. In a few billions years
the sun dies and takes us with it. Mankind is doomed anyway.

--

Rick C


Guest

Wed Jan 11, 2017 5:30 am   



On Wednesday, January 11, 2017 at 7:51:50 AM UTC+11, rickman wrote:
Quote:
On 1/10/2017 8:04 AM, pcdhobbs_at_gmail.com wrote:
Sure, having a black hole eat the planet would be bad. But how would
that relate to creating black hole in the lab? The idea that even a
tiny black hole would be catastrophic is great for science fiction, but
/very/ far from reality.

Okay, you sound very sure about that. Which of course you ought to be, considering that the wager you propose is the lives of billions of people plus all of human civilization forever (plus a few dogs and cats and wild animals) versus some asshole's curiosity.

I wonder what odds Jimmy the Greek would offer on that one.

We all die. What's the difference of when? Death by the sort of black
hole you are talking about, it it could happen, would be pretty quick.
Once the black hole reached some size I expect it would contain the
entire earth rather quickly.


If you'd paid any attention to the subsequent discussion, you'd have realised that anything we could have created would have been small enough to radiate itself away faster than it could collect new mass. Cosmic rays must be generating more very small black holes than CERN ever could - if CERN were capable of generating them, which does seem unlikely (though perhaps not entirely impossible if you accept the current estimate for the size of the smallest possible black hole)

Quote:
We are only on this planet temporarily anyway. In a few billions years
the sun dies and takes us with it. Mankind is doomed anyway.


The sun doesn't die - it blows up into a red giant whose surface would include the earth's orbit, before collapsing into a white dwarf (to skip over a lot of fascinating detail).

http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html

It would take some fancy geo-engineering to save the planet - though whatever is living here by then would probably have started moving it away from the sun rather earlier to cancel the increase in solar output.

If we were silly enough to be stuck to the planet, and dumb enough not to able to move it, we'd deserve to die.

The chance that the human species would still be around by then are pretty remote. As a successful species we can be expected to evolve different sub-species to exploit different environments, and generate a bunch of new species, each adapted to a different environment.

The average survival time of a mammalian species is apparently about 10 million years - after that something different shows up that that is better adapted to the particular ecological niche than the species that used to occupy it.

Human being show a remarkable ability to exploit different ecological niches, so this isn't going to be quite the way it works for us. At present there are people who think that we could exploit the asteroid belt, and it doesn't take much extrapolation to see the next generation exploiting the Oort Cloud - the sky doesn't seem to be the limit. Olaf Staptleton was a piker.

https://en.wikipedia.org/wiki/Last_and_First_Men

--
Bill Sloman, Sydney


Guest

Wed Jan 11, 2017 3:16 pm   



Quote:
But then they say that nothing actually crosses the event horizon, or is
that only from the perspective of an outside observer?


You can't observe an object crossing the event horizon--the light coming from such an object fades out because it takes longer and longer to reach you the closer the object gets to the horizon.

From the object's point of view nothing very remarkable happens when it crosses the horizon. If the hole is massive enough, you wouldn't even notice. Of course then you intersect the singularity after a finite amount of time.

I haven't been through the math myself--I decided that I'd rather take quantum field theory than GR in grad school, and then my study partner flaked on me so I had to drop that too, about 2/3 of the way through. A pity.

Cheers

Phil Hobbs

David Brown
Guest

Wed Jan 11, 2017 3:40 pm   



On 10/01/17 15:33, Phil Hobbs wrote:
Quote:
On 01/10/2017 08:27 AM, David Brown wrote:
/All/ black holes are very dense.

Nope. The bigger the mass, the lower the density. In fact not so long
ago there was significant doubt whether or not the whole universe was a
black hole (i.e. closed).


I have just done a few sums, and you are right - for supermassive black
holes, the density can be surprisingly low. The biggest ones would
float on water.

Stellar black holes are very dense, and of course tiny ones (if there
are any) are ridiculously dense.

Quote:
(Of course you can't really talk about volume in the neighbourhood of a
singularity, so it's usually computed as 4/3 pi r**3, where r is the
Swarzschild radius.)


Yes, and since r is proportional to the black hole's mass, the density
is inversely proportional to the square of the mass.

Quote:

Cheers

Phil Hobbs


David Brown
Guest

Wed Jan 11, 2017 3:43 pm   



On 10/01/17 16:39, Jeroen Belleman wrote:

Quote:
Regarding black hole density, the universe and all that:
You better start believing in black holes. You're in one!


I have also heard the idea that we are not /in/ a black hole, but on the
/surface/ (event horizon) of a black hole of more dimensions.

I suspect that theory will be even harder to verify than Hawking radiation!

Quote:
If the universe is closed, that is. Yes, I know that view
is passé.

Jeroen (savvy?) Belleman


George Herold
Guest

Wed Jan 11, 2017 4:15 pm   



On Wednesday, January 11, 2017 at 8:16:27 AM UTC-5, pcdh...@gmail.com wrote:
Quote:
But then they say that nothing actually crosses the event horizon, or is
that only from the perspective of an outside observer?

You can't observe an object crossing the event horizon--the light coming from such an object fades out because it takes longer and longer to reach you the closer the object gets to the horizon.

From the object's point of view nothing very remarkable happens when it crosses the horizon. If the hole is massive enough, you wouldn't even notice. Of course then you intersect the singularity after a finite amount of time.

I haven't been through the math myself--I decided that I'd rather take quantum field theory than GR in grad school, and then my study partner flaked on me so I had to drop that too, about 2/3 of the way through. A pity.

Huh, you couldn't find some other grad student to help?
My memory of grad school is that I was always pestering older grad students to
help me with problem sets... Badut, genius-type guy in the lab next door, would
help me, but then give me some other problem to solve as sorta payment.
Wonderful time!

George H.
Quote:

Cheers

Phil Hobbs



Guest

Wed Jan 11, 2017 4:26 pm   



Quote:
uh, you couldn't find some other grad student to help?  
My memory of grad school is that I was always pestering older grad students to
help me with problem sets..


Yeah, but I was already working on my thesis, had a wife and child, and none of my other friends was in the class, so I bailed too. I was really only taking it for interest, since I'm not a theorist or high-energy type. I did do a bunch of Feynman diagrams and stuff though. ;)

Cheers

Phil Hobbs

rickman
Guest

Wed Jan 11, 2017 4:28 pm   



On 1/11/2017 3:40 AM, David Brown wrote:
Quote:
On 10/01/17 15:33, Phil Hobbs wrote:
On 01/10/2017 08:27 AM, David Brown wrote:
/All/ black holes are very dense.

Nope. The bigger the mass, the lower the density. In fact not so long
ago there was significant doubt whether or not the whole universe was a
black hole (i.e. closed).


I have just done a few sums, and you are right - for supermassive black
holes, the density can be surprisingly low. The biggest ones would
float on water.

Stellar black holes are very dense, and of course tiny ones (if there
are any) are ridiculously dense.


If I am not mistaken, what spaghettifies matter falling into a black
hole is the delta in gravitational force or the gradient in other words.
With a very large black hole wouldn't the gradient be a lot less and
so less spaghettification forces? Or is this more an issue of
approaching the event horizon where the actual gradient doesn't matter?

I know from the inside the event horizon doesn't appear noticeable in
any way. I believe crossing the event horizon would be noticeable from
the outside since that it the point where even light can't make headway
against gravity. Until then you can see things between you and the
event horizon. After crossing that you can only see other things
crossing behind you.

I think this pretty clearly shows it is impossible for the universe to
exist inside a black hole. We observe the universe expanding, but
inside a black hole everything falls inward. Even light can't make
headway against gravity.


Quote:
(Of course you can't really talk about volume in the neighbourhood of a
singularity, so it's usually computed as 4/3 pi r**3, where r is the
Swarzschild radius.)

Yes, and since r is proportional to the black hole's mass, the density
is inversely proportional to the square of the mass.


Cheers

Phil Hobbs




--

Rick C

David Brown
Guest

Wed Jan 11, 2017 4:51 pm   



On 11/01/17 10:28, rickman wrote:
Quote:
On 1/11/2017 3:40 AM, David Brown wrote:
On 10/01/17 15:33, Phil Hobbs wrote:
On 01/10/2017 08:27 AM, David Brown wrote:
/All/ black holes are very dense.

Nope. The bigger the mass, the lower the density. In fact not so long
ago there was significant doubt whether or not the whole universe was a
black hole (i.e. closed).


I have just done a few sums, and you are right - for supermassive black
holes, the density can be surprisingly low. The biggest ones would
float on water.

Stellar black holes are very dense, and of course tiny ones (if there
are any) are ridiculously dense.

If I am not mistaken, what spaghettifies matter falling into a black
hole is the delta in gravitational force or the gradient in other words.
With a very large black hole wouldn't the gradient be a lot less and so
less spaghettification forces? Or is this more an issue of approaching
the event horizon where the actual gradient doesn't matter?


My understanding is also that it is the gradient of the gravitational
force. The gravitational force is proportional to 1/r² distance from
the centre of mass, and the gradient is then going to be proportional to
1/r³ (as well as to the mass). That means the gradient is going to be
very big for something like a stellar black hole where you can get close
before reaching the event horizon. For tiny black holes, the low mass
means that there is very little gravitational force until you are
extremely close - it doesn't really count as "speghettification" if it
only affects a water molecule on the surface of your skin. And for
supermassive black holes, the gradient will be low because you can't get
close before reaching the event horizon.

Quote:

I know from the inside the event horizon doesn't appear noticeable in
any way. I believe crossing the event horizon would be noticeable from
the outside since that it the point where even light can't make headway
against gravity. Until then you can see things between you and the
event horizon. After crossing that you can only see other things
crossing behind you.

I think this pretty clearly shows it is impossible for the universe to
exist inside a black hole. We observe the universe expanding, but
inside a black hole everything falls inward. Even light can't make
headway against gravity.


Inside the black hole, light can't escape to get out. Within it, it may
be a different matter. "Falling inward" is not in itself a problem -
remember, the moon is continually falling inward towards the earth, and
the earth is continually falling inward towards the sun. But I don't
know enough about the insides of black holes to go on - I could
speculate and have Phil or others correct me, but it is perhaps best to
let those that know more answer this point without me getting it wrong
first Smile All I can say for sure is that since serious physicists have
done serious maths regarding these theories, it is /not/ "pretty clearly
impossible".

Quote:


(Of course you can't really talk about volume in the neighbourhood of a
singularity, so it's usually computed as 4/3 pi r**3, where r is the
Swarzschild radius.)

Yes, and since r is proportional to the black hole's mass, the density
is inversely proportional to the square of the mass.


Cheers

Phil Hobbs





George Herold
Guest

Wed Jan 11, 2017 5:26 pm   



On Wednesday, January 11, 2017 at 9:26:48 AM UTC-5, pcdh...@gmail.com wrote:
Quote:
uh, you couldn't find some other grad student to help?  
My memory of grad school is that I was always pestering older grad students to
help me with problem sets..

Yeah, but I was already working on my thesis, had a wife and child, and none of my other friends was in the class, so I bailed too. I was really only taking it for interest, since I'm not a theorist or high-energy type. I did do a bunch of Feynman diagrams and stuff though. ;)

Cheers

Phil Hobbs


Fair enough. I audited a Solid State theory class, (full of these
renormalized electrons and such.).. by the end of it I wasn't even
treading water.. but slowly sinking into the deep end.

George H.

rickman
Guest

Wed Jan 11, 2017 5:59 pm   



On 1/11/2017 4:51 AM, David Brown wrote:
Quote:
On 11/01/17 10:28, rickman wrote:
On 1/11/2017 3:40 AM, David Brown wrote:
On 10/01/17 15:33, Phil Hobbs wrote:
On 01/10/2017 08:27 AM, David Brown wrote:
/All/ black holes are very dense.

Nope. The bigger the mass, the lower the density. In fact not so long
ago there was significant doubt whether or not the whole universe was a
black hole (i.e. closed).


I have just done a few sums, and you are right - for supermassive black
holes, the density can be surprisingly low. The biggest ones would
float on water.

Stellar black holes are very dense, and of course tiny ones (if there
are any) are ridiculously dense.

If I am not mistaken, what spaghettifies matter falling into a black
hole is the delta in gravitational force or the gradient in other words.
With a very large black hole wouldn't the gradient be a lot less and so
less spaghettification forces? Or is this more an issue of approaching
the event horizon where the actual gradient doesn't matter?

My understanding is also that it is the gradient of the gravitational
force. The gravitational force is proportional to 1/r² distance from
the centre of mass, and the gradient is then going to be proportional to
1/r³ (as well as to the mass). That means the gradient is going to be
very big for something like a stellar black hole where you can get close
before reaching the event horizon. For tiny black holes, the low mass
means that there is very little gravitational force until you are
extremely close - it doesn't really count as "speghettification" if it
only affects a water molecule on the surface of your skin. And for
supermassive black holes, the gradient will be low because you can't get
close before reaching the event horizon.


I know from the inside the event horizon doesn't appear noticeable in
any way. I believe crossing the event horizon would be noticeable from
the outside since that it the point where even light can't make headway
against gravity. Until then you can see things between you and the
event horizon. After crossing that you can only see other things
crossing behind you.

I think this pretty clearly shows it is impossible for the universe to
exist inside a black hole. We observe the universe expanding, but
inside a black hole everything falls inward. Even light can't make
headway against gravity.

Inside the black hole, light can't escape to get out. Within it, it may
be a different matter. "Falling inward" is not in itself a problem -
remember, the moon is continually falling inward towards the earth, and
the earth is continually falling inward towards the sun. But I don't
know enough about the insides of black holes to go on - I could
speculate and have Phil or others correct me, but it is perhaps best to
let those that know more answer this point without me getting it wrong
first Smile All I can say for sure is that since serious physicists have
done serious maths regarding these theories, it is /not/ "pretty clearly
impossible".


I think inside the event horizon there is no place where light will
behave remotely normally. Regardless of how fast you are falling, the
light leaving you can't make any headway directly toward the event
horizon. I'm pretty sure it makes no sense to try to say you are
falling faster than the light.

But then they say that nothing actually crosses the event horizon, or is
that only from the perspective of an outside observer?

--

Rick C

Jeroen Belleman
Guest

Wed Jan 11, 2017 7:43 pm   



On 2017-01-11 11:59, rickman wrote:
Quote:
On 1/11/2017 4:51 AM, David Brown wrote:
On 11/01/17 10:28, rickman wrote:
On 1/11/2017 3:40 AM, David Brown wrote:
On 10/01/17 15:33, Phil Hobbs wrote:
On 01/10/2017 08:27 AM, David Brown wrote:
/All/ black holes are very dense.

Nope. The bigger the mass, the lower the density. In fact not so
long
ago there was significant doubt whether or not the whole universe
was a
black hole (i.e. closed).


I have just done a few sums, and you are right - for supermassive black
holes, the density can be surprisingly low. The biggest ones would
float on water.

Stellar black holes are very dense, and of course tiny ones (if there
are any) are ridiculously dense.

If I am not mistaken, what spaghettifies matter falling into a black
hole is the delta in gravitational force or the gradient in other words.
With a very large black hole wouldn't the gradient be a lot less and so
less spaghettification forces? Or is this more an issue of approaching
the event horizon where the actual gradient doesn't matter?

My understanding is also that it is the gradient of the gravitational
force. The gravitational force is proportional to 1/r² distance from
the centre of mass, and the gradient is then going to be proportional to
1/r³ (as well as to the mass). That means the gradient is going to be
very big for something like a stellar black hole where you can get close
before reaching the event horizon. For tiny black holes, the low mass
means that there is very little gravitational force until you are
extremely close - it doesn't really count as "speghettification" if it
only affects a water molecule on the surface of your skin. And for
supermassive black holes, the gradient will be low because you can't get
close before reaching the event horizon.


I know from the inside the event horizon doesn't appear noticeable in
any way. I believe crossing the event horizon would be noticeable from
the outside since that it the point where even light can't make headway
against gravity. Until then you can see things between you and the
event horizon. After crossing that you can only see other things
crossing behind you.

I think this pretty clearly shows it is impossible for the universe to
exist inside a black hole. We observe the universe expanding, but
inside a black hole everything falls inward. Even light can't make
headway against gravity.

Inside the black hole, light can't escape to get out. Within it, it may
be a different matter. "Falling inward" is not in itself a problem -
remember, the moon is continually falling inward towards the earth, and
the earth is continually falling inward towards the sun. But I don't
know enough about the insides of black holes to go on - I could
speculate and have Phil or others correct me, but it is perhaps best to
let those that know more answer this point without me getting it wrong
first Smile All I can say for sure is that since serious physicists have
done serious maths regarding these theories, it is /not/ "pretty clearly
impossible".

I think inside the event horizon there is no place where light will
behave remotely normally. Regardless of how fast you are falling, the
light leaving you can't make any headway directly toward the event
horizon. I'm pretty sure it makes no sense to try to say you are
falling faster than the light.

But then they say that nothing actually crosses the event horizon, or is
that only from the perspective of an outside observer?


The whole concept of an event horizon is fallacious, anyway.
There are no singularities in nature. It would imply that an
object falling into a black hole would reach c, and therefore
have infinite kinetic energy. However, the mass of the BH is
and remains finite, so we have a contradiction.

A bit of renormalization is needed. There are lots of places
in physics where that need arises. As a simple example, the
electrostatic energy of an electron --considering it as a point
charge-- is infinite too, but we know it's really finite, at
about 511keV. That gives us a size estimate for the electron,
too. It should be possible to do something similar for a BH.
I'd be surprised if it was the same as the basic Newtonian
r=2GM/c^2 which we see everywhere.

I didn't attempt to do it.

Jeroen Belleman

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