# Black Hole: Why do we believe that matter could be such dense?

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Is that because we analysed the mass of rotating matter around them, and extracted that they shall weight enough to attracted it, at a certain rotation speed? And after deduced from their looking size, by our observation, what density they should have? Where is the Science on that, and the part of relativity to it ?

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There are various lines of evidence for the existence of black holes. Some of these are things like X-ray sources that correspond to what is predicted for accretion disks around black holes. That is quite indirect and requires a good understanding of the relevant physics.

A much more obvious example is the orbits of the stars around Sagitarius A*. These are orbiting an object that is very massive (determined by their orbits) and very small. Something that small and that massive could only be a black hole.

And, of course, we know have direct observation of the event horizon (or, more accurately, the "shadow" of the event horizon - not really shadow at all!)

20 minutes ago, MaximT said:

And after deduced from their looking size, by our observation, what density they should have?

The density decreases with the size of the black hole. This is because the mass is proportional to the radius, but the volume is proportional to the cube of the radius. Large black holes can have a density less than that of water.

21 minutes ago, MaximT said:

Where is the Science on that, and the part of relativity to it ?

This all comes from general relativity.

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2 hours ago, Strange said:

The density decreases with the size of the black hole. This is because the mass is proportional to the radius, but the volume is proportional to the cube of the radius. Large black holes can have a density less than that of water.

mind blown.

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The corollary of that is that the tidal forces near the event horizon are much smaller for a large black hole than a small one.

So, popular science articles always seem to say that you would be torn apart as you fell into a black hole. But in the case of a supermassive black hole you wouldn't notice anything at all as you fell through the event horizon. (Other than a slight sense of regret, maybe.)

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20 minutes ago, Strange said:

The corollary of that is that the tidal forces near the event horizon are much smaller for a large black hole than a small one.

1

is that due to Hawking radiation?

Edited by dimreepr
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People are often skeptical of Black Holes because they cannot understand how matter can be compressed to such a high degree in the central singularity ( near zero size ) to reach ( almost ? ) infinite density.

One has to remember that elementary particles, like leptons, are treated as if they have zero size, and only other properties, like the exclusion principle and the statistics they obey, keep an infinity of them from being stacked on top of each other.
As to mass, it is a property of these leptons, not a thing onto itself, and in a Black Hole, this property is conveniently left with the Event Horizon ( along with charge, angular momentum and in a non-classical treatment, entropy ). The EH is the mathematical construct where pre-collapse properties are stored, not the central singularity.

So we have a dimensionless singularity where a large number of dimensionless particles, that no longer have the property of mass, reside.
Is there actually a problem there ?

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6 minutes ago, MigL said:

People are often skeptical of Black Holes because they cannot understand how matter can be compressed to such a high degree in the central singularity ( near zero size ) to reach ( almost ? ) infinite density.

Agree. It really isn't a requirement anyway, is it?

I mean if there was some force that prevented compression beyond what we already know, how would that affect any external effects we might be able to observe from black holes?

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Do you know what small means?

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8 minutes ago, dimreepr said:

Do you know what small means?

In the context of "compressed to such a high degree in the central singularity"?

I can only imagine.

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1 minute ago, J.C.MacSwell said:

In the context of "compressed to such a high degree in the central singularity"?

I can only imagine.

me neither.

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Quote

Black Hole: Why do we believe that matter could be such dense?

The first predictions of Black Holes were in XIX century when early scientists figured out escape velocity formula for the Earth or other cosmic objects. It takes mass and radius as parameters.

If you will extrapolate mass further than any known cosmic objects and/or reduce radius, in escape velocity equation, at some point escape velocity is reaching speed of light value..

Therefor Black Holes used to be called Cold Stars. Cold because they were hypothesized to not emit any light (in thermodynamics any object is emitting light (photons) which correspond to temperature of the object).

Edited by Sensei
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8 minutes ago, dimreepr said:

me neither.

...or as they say as they enter the distorted space near a black hole:

"mean aether!"

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4 minutes ago, J.C.MacSwell said:

...or as they say as they enter the distorted space near a black hole:

"mean aether!"

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2 hours ago, Strange said:

The density decreases with the size of the black hole. This is because the mass is proportional to the radius, but the volume is proportional to the cube of the radius. Large black holes can have a density less than that of water.

Aaaargh!  What does that even mean!

As much as we say that we know little about BHs, some sure spout a lot.  Perhaps if some tried less to sound sooo intellectual, we could have a conversation.

First off, we know little about what's past the event horizon, so why talk about the density within?  For all we know, the "inside" of the event horizon could hide a super neutron (or quark) star just heavy enough with actual matter to suck in light and other EMs.  Some say the singularity inside has "infinite density."  WTH!  "Infinite" density would mean the entire universe would have to be a black hole singularity.  And if each black hole has "infinite" density, or close to it, how can any other black hole be bigger (heavier) than the other if they were "equally infinitely dense?"  Can one infinity be bigger than its sister infinity?

"Have a density less than that of water?"  Yeah, I've read that before too but how can that be proven?  Does the area just inside a BH constitute matter or just a "space" where outside matter like rogue spaceships or bodies of scientists whiz by on their way to the "infinite" singularity?  If it's just a "space" how can it weigh like water; if it's matter (it's probably not) then "weigh like water," really?  And if it's not space or matter, then.....

OK, thanks for the venting opportunity.  Bottom line: Speculation about the inside of BHs should be just that -- speculation.  But it shouldn't be: pseudo-scientist 1, black holes have extreme density thus preventing even light from escaping, and pseudo-scientist 2, large BHs have a density like that of water.

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1 minute ago, EWyatt said:

First off, we know little about what's past the event horizon, so why talk about the density within?

Density is mass divided by volume. Volume of sphere is $\frac{4}{3} \pi r^3$ where r you can use the same radius as you use in escape velocity formula when vescape = c..

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2 hours ago, dimreepr said:

is that due to Hawking radiation?

No. Tidal forces are differences in the force due to distance between two points feeling the force. dF/dr. Since gravity drops off as 1/r^2 (nominally), the tidal forces drop off as 1/r^3, i.e. faster than the force.

1 hour ago, EWyatt said:

Bottom line: Speculation about the inside of BHs should be just that -- speculation.

Nothing in the discussion is about what goes on inside a BH. We can measure what the mass of the BH is because we can see how it gravitationally affects masses that are outside the event horizon.

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1 hour ago, EWyatt said:

First off, we know little about what's past the event horizon, so why talk about the density within?

That is a good point. I did intend to say in my previous reply that "density" is a pretty meaningless concept for a black hole.

However, we know the mass and the volume (although that isn't really well defined) so we can calculate an average density, even if it doesn't mean much.

1 hour ago, EWyatt said:

Some say the singularity inside has "infinite density."﻿  WTH!  "Infinite" density would mean the entire universe would have to be a black hole singularity.  And if each black hole has "infinite" density, or close to it, how can any other black hole be bigger (heavier) than the other if they were "equally infinitely dense?"  Can one infinity be bigger than its sister infinity?

The infinities come from a naive application of GR, which we are pretty certain does not apply any more at that point.

But you seem to be confusing density and mass. The density can (in principle) be infinite with a finite mass, and it is the mass that makes one black hole larger than another.

(But yes, one infinity can be larger than another. But that is a separate discussion!)

1 hour ago, EWyatt said:

OK, thanks for the venting opportunity.  Bottom line: Speculation about the inside of BHs should be just that -- speculation.

I wasn't talking about the inside of the black hole, but the black hole overall. (There is another thread where I have made exactly the same point: we know nothing about what happens inside a black hole.)

1 hour ago, EWyatt said:

But it shouldn't be: pseudo-scientist 1, black holes have extreme density thus preventing even light from escaping, and pseudo-scientist 2, large BHs have a density like that of water.﻿

It is the mass that stops light escaping, not density. The (probably non-existent) singularity has infinite density. The average density of the black hole (assuming a sphere of the same radius, with that mass) is relatively low.

Quote

### In the Milky Way

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic center[44]

Astronomers are very confident that the Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A*[45] 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 or 120 AU) from the center of the central object.[46]
• From the motion of star S2, the object's mass can be estimated as 4.1 million M,[47][48] or about 8.2×1036 kg.
• The radius of the central object must be less than 17 light-hours, because otherwise S2 would collide with it. Observations of the star S14[49] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
• No known astronomical object other than a black hole can contain 4.1 million M in this volume of space.

Is a sum of mass orbiting a Black Hole, could contribute to the gravity force maintaining the rotation in orbit around the center of the galaxy, to make that galaxy objects spin around the center?

Is that sum of mass (all orbiting objects) could reduced the theoretical mass of the Black Hole itself, creating a empty, cannot be seen, zone around it, at the place of dilating it's volume to achieved the same density?

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8 minutes ago, MaximT said:

Is a sum of mass﻿ orbiting﻿ a Black Hole﻿, could﻿ contribute to the gravity force maintaining the rotation in orbit around the center of the galaxy, to make that galaxy objects spin around the ﻿center﻿?

The mass of the black hole is a tiny fraction of the mass of the galaxy so it will only have a significant effect on the stars very close it to. It has no effect on the orbit of the Sun around the galaxy for example.

9 minutes ago, MaximT said:

Is that sum of mass (all orbiting objects) could reduced the theoretical mass of the Black Hole itself, creating a empty, cannot be seen, zone around it, at the place of dilating﻿ it's volume to achieved the same﻿ density?﻿﻿

I don't see how the orbiting objects would reduce the mass of the black hole. The Moon doesn't reduce the mass of the Earth.

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3 minutes ago, Strange said:

The Moon doesn't reduce the mass of the Earth.

In fact, yes, when it's at the opposite of the Sun, our star. I'm asking to myself, if that phenomena's happening at galaxy level?

After some thinking, I'm expecting that the galaxy, the milky way, is not orbiting around it's Black Hole, but around it's own mass. A mass that is One Million time bigger than it's suppose Black Hole, at Sagittarius. The density of the BH, is extract from the star S2 motion, and has nothing to do with the whole rotation of the Galaxy...

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1 hour ago, MaximT said:

In﻿ fact﻿, yes, when it's ﻿at the opposite of the Sun, our star.﻿ ﻿

I don't know where you get that idea from. It is wrong.

1 hour ago, MaximT said:

After some thinking, I'm expecting that the galaxy, the milky way, is not orbiting around it's Black Hole, but around it's own mass. A mass that is One Million time bigger than it's suppose Black Hole, at Sagittarius. The density of the BH, is extract from the star S2 motion, and has nothing to do with the whole rotation of the Galaxy...

Exactly.

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3 minutes ago, Strange said:

I don't know where you get that idea from. It is wrong.

Misunderstanding there, when the moon is at the opposite of the sun, the moon attraction retrieved from the force of the Sun on the Earth. I mean both attract the earth, so the sum of force will be less on the Sun side, by that fact. But I understand that it's not modifying the gravity...

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5 hours ago, EWyatt said:

Aaaargh!  What does that even mean!

As much as we say that we know little about BHs, some sure spout a lot.  Perhaps if some tried less to sound sooo intellectual, we could have a conversation.

First off, we know little about what's past the event horizon, so why talk about the density within?  For all we know, the "inside" of the event horizon could hide a super neutron (or quark) star just heavy enough with actual matter to suck in light and other EMs.  Some say the singularity inside has "infinite density."  WTH!  "Infinite" density would mean the entire universe would have to be a black hole singularity.  And if each black hole has "infinite" density, or close to it, how can any other black hole be bigger (heavier) than the other if they were "equally infinitely dense?"  Can one infinity be bigger than its sister infinity?

"Have a density less than that of water?"  Yeah, I've read that before too but how can that be proven?  Does the area just inside a BH constitute matter or just a "space" where outside matter like rogue spaceships or bodies of scientists whiz by on their way to the "infinite" singularity?  If it's just a "space" how can it weigh like water; if it's matter (it's probably not) then "weigh like water," really?  And if it's not space or matter, then.....

OK, thanks for the venting opportunity.  Bottom line: Speculation about the inside of BHs should be just that -- speculation.  But it shouldn't be: pseudo-scientist 1, black holes have extreme density thus preventing even light from escaping, and pseudo-scientist 2, large BHs have a density like that of water.

First up, let me say poor attitude. Secondly, the reason why density when speaking of a BH is a meaningless concept, is that all the mass according to GR resides at the singularity, The rest according to GR is nothing but critically curved spacetime, ignoring any in-falling matter/energy.

GR tells us that when the Schwarzchild radius of any mass is reached, further collapse is compulsory. So while we will never have any real observational evidence about what is, or what isn't beyond the EH, we do have the overwhelmingly supported theory of GR to guide us to the best explanation.

While GR tells us that further collapse is compulsory, it also fails to be of any use or application at the quantum/Planck level. This is why most reputable astronomers and cosmologists, do not accept the singularity of infinite density and infinite spacetime curvature, rather just a singularity defined by the failure of our current laws of physics and GR to describe. Absorbing all that, its a reasonable bet to assume a surface of sorts exist at or below the quantum/Planck level.

I see the  picture of what is inside a BH, as dictated by GR, as reasonable, despite the fact we will never be able to observe beyond the EH. Much the same way that we have a reasonable assumption of what powers the Sun and other stellar objects with out having to visit them.

In answer to the original question as per the thread heading, the effects we observe of matter/energy, and spacetime  in the vicinity of what appears to be nothing, or invisible, leads us to theorise BH's. The discovery of gravitational waves as per colliding BH templates and the EHT image has now made their existence a near certainty.

Edited by beecee
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I believe the term 'black hole' was coined by J A Wheeler, Sensei.
However the term has rude connotations in Russian, so Y B Zeldovich called them 'frozen' stars instead. No-one has called them cold stars that I know of. This on account theory predicts that, from an external FoR, a collapsing star will only collapse to its Swartzschild radius, and temporally 'freeze' there.

These days, even in Russia, 'collapsar', or black hole, is used.

Edited by MigL
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1 hour ago, MigL said:

I believe the term 'black hole' was coined by J A Wheeler, Sensei.

He thought the terminology of a "gravitationally completely collapsed object"as too much of a mouth full.

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