# Do black holes evaporate or go bang ?

jimw
Do black holes "evaporate" or go "bang"?

A TV program last night quoted Hawkins saying (in effect) that, because a small amount of matter/energy escapes from black holes, they will eventually just disappear (evaporate). This raised a few questions to my non-physicist mind: (1) Since black holes "gobble up" matter, wouldn't the rate of increase be greater than the "evaporation" rate? Perhaps eventually there would be nothing left to feed it? (2) If a black hole did evaporate, it seems it would become less dense during the process and should explode when the gravitational force could no longer hold it together densely enough (the opposite of when a star implodes to create a black hole). perhaps a mini big bang?

Does this make sense? Thanks for any feedback.
Jim

jimw said:
A TV program last night quoted Hawkins saying (in effect) that, because a small amount of matter/energy escapes from black holes, they will eventually just disappear (evaporate). This raised a few questions to my non-physicist mind: (1) Since black holes "gobble up" matter, wouldn't the rate of increase be greater than the "evaporation" rate?
Yeah, as long as there's a continual supply of material to feed it a black hole can take in matter/energy faster than it radiates it out. The smaller the black hole, though, the faster it will be radiating energy away.
jimw said:
(2) If a black hole did evaporate, it seems it would become less dense during the process and should explode when the gravitational force could no longer hold it together densely enough (the opposite of when a star implodes to create a black hole). perhaps a mini big bang?
Actually smaller black holes are more dense than larger ones--you can see from the Schwarzschild formula here that the mass is proportional to the radius, while of course the volume is proportional to the cube of the radius, so mass/volume will decrease as radius increases (the black hole may evaporate when it reaches the 'Planck density', as discussed in this thread). So, a large black hole could have an arbitrarily low density. As for what happens at the moment the black hole evaporates, http://www.einstein-online.info/en/elementary/quantum/evaporating_bh/ [Broken] says that "For black holes with lesser masses, however, significant fractions of mass and energy are radiated away - the smaller the mass, the greater the power. This leads to a runaway process and to a final, gigantic flash of energy in which the black hole evaporates." Not sure how much energy would be released in the final moments, but I know one theory about gamma-ray bursts that are sometimes detected is that they are the signatures of small "primordial black holes" created in the big bang which are just evaporating now. See this paper on the subject, which is discussed in this New Scientist article.

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jimw
Thanks, Jesse. As indicated, I'm not well-versed in physics and expected others had thought about this and reached conclusions. I didn't know there were different densities of black holes. So, would an evaporating hole become less dense and/or larger? (and I hope Hawking would forgive my typo)

jimw said:
Thanks, Jesse. As indicated, I'm not well-versed in physics and expected others had thought about this and reached conclusions. I didn't know there were different densities of black holes. So, would an evaporating hole become less dense and/or larger? (and I hope Hawking would forgive my typo)
No, as you might expect from our intuitive notion of evaporation, an evaporating black hole keeps shrinking down until it disappears. The density is increasing as this happens, but the total mass of the black hole is shrinking (again, because the Schwarzschild formula says that the mass is proportional to the radius).

Gold Member
According to the most popular current models, a black hole could keep evaporating until it reached a Planck mass size [~E-08 kg], then go 'poof'. That would still be a pretty decent 'poof', but not spectacular [an e=mc^2 thing]. Of course there are other candidate models that predict a more [or less] satisfying conclusion. I'm a little shaky on the density thing JesseM mentioned. I don't think it changes. The spacetime curvature as you near the Swarzchild radius makes it very difficult to quantify density. According to the usual Einstein solutions, the singularity itself is generally considered to be infinitely dense. Pretty hard to get more dense than that.

gawd.iz.life

A TV program last night quoted Hawkins saying (in effect) that, because a small amount of matter/energy escapes from black holes, they will eventually just disappear (evaporate). This raised a few questions to my non-physicist mind: (1) Since black holes "gobble up" matter, wouldn't the rate of increase be greater than the "evaporation" rate? Perhaps eventually there would be nothing left to feed it? (2) If a black hole did evaporate, it seems it would become less dense during the process and should explode when the gravitational force could no longer hold it together densely enough (the opposite of when a star implodes to create a black hole). perhaps a mini big bang?

Does this make sense? Thanks for any feedback.
Jim

Yes, makes good sense. There is a threshold of mass wherein a dying star becomes either a neutron star or a black hole. If an evaporating black hole loses enough mass to breach this threshold, the "Strong Atomic Force" would kick in and there would be some type of an explosion. Nothing like a super nova, but nevertheless an explosion. I do not believe black holes evaporate into nothing. This is based on my good understanding of the 4 basic forces of physics: electromagnatism, which is extinguished when a small star becomes a white dwarf, leaving the other 3 remaining; the weak atomic force, which is extinguished when a larger dying star becomes a neutron star, leaving the other 2 remaining; the strong atomic force, which is extinguished when a larger dying star becomes a black hole, leaving only gravity remaining. This determination of what becomes of a dying star is based on the mass involved. Therefore, when a black hole loses sufficient mass, the strong atomic force might come back into play.

:)

twofish-quant

Yes, makes good sense. There is a threshold of mass wherein a dying star becomes either a neutron star or a black hole. If an evaporating black hole loses enough mass to breach this threshold, the "Strong Atomic Force" would kick in and there would be some type of an explosion.

I think this is confusing a lot of things.

First of all, one of the things that Hawking figured out is that the larger the black hole the more time it takes to evaporate. In the case of star-sized black holes, the amount of energy that the evaporating black hole produces is very, very small, and you aren't expecting those black holes to do anything over the many many times lifespan of the universe. For you to see a black hole evaporate now, it has to be a tiny black hole. However, no one knows how to form tiny black holes or even if they exist at all.

The threshold at which a star whether becomes a neutron star or a black hole is very unclear since we don't know a lot about this process. In particular, it's not know under what conditions you form a black hole directly and under want conditions you end up with an explosion and then a black hole.

This is based on my good understanding of the 4 basic forces of physics: electromagnatism, which is extinguished when a small star becomes a white dwarf, leaving the other 3 remaining; the weak atomic force, which is extinguished when a larger dying star becomes a neutron star, leaving the other 2 remaining; the strong atomic force, which is extinguished when a larger dying star becomes a black hole, leaving only gravity remaining. This determination of what becomes of a dying star is based on the mass involved. Therefore, when a black hole loses sufficient mass, the strong atomic force might come back into play.

Ummmm.. This is very different from what I learned when I was doing my Ph.D. What keeps things from collapsing isn't a force so much as the Pauli exclusion principle.

gawd.iz.life

I think this is confusing a lot of things.

First of all, one of the things that Hawking figured out is that the larger the black hole the more time it takes to evaporate. In the case of star-sized black holes, the amount of energy that the evaporating black hole produces is very, very small, and you aren't expecting those black holes to do anything over the many many times lifespan of the universe. For you to see a black hole evaporate now, it has to be a tiny black hole. However, no one knows how to form tiny black holes or even if they exist at all.

Agreed.

The threshold at which a star whether becomes a neutron star or a black hole is very unclear since we don't know a lot about this process.

Cosmologists have defined the masses of stars in clear ranges that either become a white dwarf, a neutron star or a black hole.

In particular, it's not know under what conditions you form a black hole directly and under want conditions you end up with an explosion and then a black hole.

The formation and death of stars is more clearly understood than evaporating black holes. Not sure what you mean here.

Ummmm.. This is very different from what I learned when I was doing my Ph.D.

Well my equivalent to an Master's Degree can't outdo your Ph.D, but that doesn't invalidate my opinions :)

What keeps things from collapsing isn't a force so much as the Pauli exclusion principle.

The 4 forces of physics are Gravity, the strong atomic force responsible for keeping subatomic particles together, the weak atomic force related to half-life decay and keeping the atom's nucleus together, and the electromagnetic force, which maintains the structure of electrons surrounding the nucleus. These 4 forces are clearly defined in physics.

Yes, when talking about black holes, we are talking about times of eternities. Still it's fascinating to understand what we can and to seek to understand more. Happy New Year.

qraal

Yes, makes good sense. There is a threshold of mass wherein a dying star becomes either a neutron star or a black hole. If an evaporating black hole loses enough mass to breach this threshold, the "Strong Atomic Force" would kick in and there would be some type of an explosion.

There are no particles in a standard Black Hole so there is no strong-force working against the gravity.
Nothing like a super nova, but nevertheless an explosion.

The time span of evaporation, t, is the mass divided by the present mass-loss rate, then divided by 3. i.e. t = M/3.(dM/dt)

The mass loss rate is proportional to the inverse square of the mass. For example a 200,000 ton black hole is evaporating at 0.1 kg/s. Combining these facts tells us that the remaining 229 tons evaporates in 1 second. That's a 4.92 million megaton TNT equivalent explosion.

I do not believe black holes evaporate into nothing. This is based on my good understanding of the 4 basic forces of physics: electromagnatism, which is extinguished when a small star becomes a white dwarf, leaving the other 3 remaining; the weak atomic force, which is extinguished when a larger dying star becomes a neutron star, leaving the other 2 remaining; the strong atomic force, which is extinguished when a larger dying star becomes a black hole, leaving only gravity remaining. This determination of what becomes of a dying star is based on the mass involved. Therefore, when a black hole loses sufficient mass, the strong atomic force might come back into play.

:)

All the particle identities are merged in the Singularity in standard Black Hole theory so there is no strong force, no colour charge, left to cause such a re-expansion. You can make a theory in which such things are possible, but you'd have to explain how.

And the forces aren't "extinguished", they're overwhelmed. Each stage of collapse produces some quantum-powered opposition to the collapse, as you noted - but that's unstable for large masses. Because pressure is "energy divided by volume" it contributes to a collapsing mass's overall gravitation, thus increasing the amount of squeezing/pressure driving up the gravitation even more... in an 'endless' positive feedback loop that crushes the mass into a point. But that's in classical general relativity which has infinitely small sizes - we don't know if quantum space-time only allows finite volumes or not. Some theories say 'yes', others are less clear.

gawd.iz.life

There are no particles in a standard Black Hole so there is no strong-force working against the gravity.

The time span of evaporation, t, is the mass divided by the present mass-loss rate, then divided by 3. i.e. t = M/3.(dM/dt)

The mass loss rate is proportional to the inverse square of the mass. For example a 200,000 ton black hole is evaporating at 0.1 kg/s. Combining these facts tells us that the remaining 229 tons evaporates in 1 second. That's a 4.92 million megaton TNT equivalent explosion.

All the particle identities are merged in the Singularity in standard Black Hole theory so there is no strong force, no colour charge, left to cause such a re-expansion. You can make a theory in which such things are possible, but you'd have to explain how.

And the forces aren't "extinguished", they're overwhelmed. Each stage of collapse produces some quantum-powered opposition to the collapse, as you noted - but that's unstable for large masses. Because pressure is "energy divided by volume" it contributes to a collapsing mass's overall gravitation, thus increasing the amount of squeezing/pressure driving up the gravitation even more... in an 'endless' positive feedback loop that crushes the mass into a point. But that's in classical general relativity which has infinitely small sizes - we don't know if quantum space-time only allows finite volumes or not. Some theories say 'yes', others are less clear.

Isn't the evaporation time of a black hole is proportional to the cube of its mass?

There's what we KNOW about black holes. There's mathematics, the perfect language. There are theories utilizing mathematics to try to explain the universe, but always with missing links.

Black holes spin rapidly within their accretion disk. As they pull in more matter, they spin faster. MAYBE there's some limit in the universe, just as there are with collapsing stars.. Maybe there's some limit wherein gravity, the sole remaining force of the 4, ceases to exist for a 'split second'. What would you get? If we're talking about quasars, maybe all the matter would spin out into a brand new galaxy.

Do you think black holes are an irreversible state? All matters in the universe tend toward balance, from chaos. It's like the universe playing dice within certain laws, constants, ranges, etc.

I don't believe galaxies are formed like stars or planets by coagulating matter. But by their shapes, especially the spirals, and the fact the the outer arms go faster then the inner arms, so that the entire disk of the galaxy moves together.. I believe they are a reversal of the super massive black hole state.

Black holes contain immense potential energy.. pure matter, no particles, no strong force ONLY mass, gravity and MOVEMENT. They are super hot, super dense, and spinning very rapidly, as they travel thru space, yet containing no space within it.. solid matter.

What becomes of the universe? Does it fail to entropy, or is there some impact, some event, some limit.. wherein gravity ceases to exist..... at which point matter would be released, and all the forces would come back into play in a super CHAOS within the structure of the physics of our universe?

I'm talking along the order of galaxies... rather than The Big Bang. That's another discussion ;)

Isn't the evaporation time of a black hole is proportional to the cube of its mass?

Yes... given some assumptions such as no mass being absorbed, not even the cosmic background radiation; and the usual assumptions about Hawking radiation. Wikipedia has some useful equations at Hawking radiation.

The relation is
$$t_{ev} = \frac{5120 \pi G^2}{\hbar c^4} M^3 \approx 8.4\times 10^{-17} M^3$$​

Near the end of its life, a small black hole goes bang; the evaporation will release an enormous amount of energy in a very short period of time. For any black hole, we can calculate its temperature, its lifetime, its total energy, and its power output. For a black hole in empty space, soaking up the cosmic background radiation, it will be losing mass only when the temperature is higher than 2.726 K, which is the temperature of the background.

For example, a black hole weighing about a kilogram has a lifetime of about 84 attoseconds (0.000084 picoseconds) and releases as much energy as a 21.5 Megaton thermonuclear bomb. That qualifies as a "bang".

Here is a list of some different masses for black holes; with their total energy content, lifetime, instantaneous power output and characteristic temperature.

$$\begin{array}{l|llll|l} \text{Mass (kg)} & \text{Lifetime (s)} & \text{Power (W)} & \text{Energy (J)} & \text{Temperature (K)} \\ M & \frac{5120 \pi G^2}{\hbar c^4} M^3 & \frac{\hbar c^6}{15360 \pi G^2} M^{-2} & M c^2 & \frac{\hbar c^3}{8 \pi G k_b} M^{-1} \\ & 8.4 \times 10^{-17} M^3 & 3.56\times 10^{32} M^{-2} & 8.99 \times 10^{16} M & 1.23 \times 10^{23} M^{-1} \\ \hline 6\times 10^{-4} & 2\times 10^{-26} & 9.7\times 10^{38} & 5.44 \times 10^{13} & 2 \times 10^{26} & \text{= Hiroshima bomb} \\ 1 & 8.4 \times 10^{-17} & 3.56\times 10^{32} & 9 \times 10^{16} & 1.23 \times 10^{23} & \text{= 21.5 Megaton thermonuclear bomb} \\ 944 & 7.1 \times 10^{-8} & 4 \times 10^{26} & 8.5 \times 10^{19} & 1.3 \times 10^{20} & \text{= same power as Sun} \\ 2.3 \times 10^{5} & 1 & 6.84 \times 10^{21} & 2 \times 10^{22} & 5.4 \times 10^{17} & \text{= 1 second lifetime} \\ 7.2 \times 10^{7} & 3.16 \times 10^{7} & 6.85 \times 10^{16} & 6.5 \times 10^{24} & 1.7 \times 10^{15} & \text{= 1 year lifetime} \\ 1.7 \times 10^{11} & 4.1 \times 10^{17} & 1.24 \times 10^{10} & 1.5 \times 10^{28} & 7.24 \times 10^{11} & \text{= lifetime is age of universe} \\ 4.5 \times 10^{22} & 7.67 \times 10^{51} & 1.76 \times 10^{-13} & 4 \times 10^{39} & 2.7255 & \text{= temperature of cosmic background} \\ 6 \times 10^{24} & 1.8 \times 10^{58} & 1 \times 10^{-17} & 5.37 \times 10^{41} & 2\times 10^{-2} & \text{= mass of the Earth} \\ 2 \times 10^{30} & 6.73 \times 10^{74} & 8.9 \times 10^{-29} & 1.8 \times 10^{47} & 6 \times 10^{-8} & \text{= mass of the Sun} \\ 8 \times 10^{36} & 4.3 \times 10^{94} & 5.6 \times 10^{-42} & 7.2 \times 10^{53} & 1.5 \times 10^{-14} & \text{= Milky Way's supermassive BH} \end{array}$$

I can't believe I just did that. -- Sylas

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gawd.iz.life

And the forces aren't "extinguished", they're overwhelmed. Each stage of collapse produces some quantum-powered opposition to the collapse, as you noted - but that's unstable for large masses. Because pressure is "energy divided by volume" it contributes to a collapsing mass's overall gravitation, thus increasing the amount of squeezing/pressure driving up the gravitation even more... in an 'endless' positive feedback loop that crushes the mass into a point. But that's in classical general relativity which has infinitely small sizes - we don't know if quantum space-time only allows finite volumes or not. Some theories say 'yes', others are less clear.

And thank you for discussing pressure. Actually, given the masses of the stars that fall into the 3 specific ranges, white dwarf, neutron star, or black hole.. the force of the collapse itself exerts a huge amount of pressure.

So if you have a neutron star taking in the material of a binary star (which is not uncommon), you are increasing the mass of the neutron star, but there is not the force of pressures involved with an implosion.

But it may be possible for a neutron star to gain a LOT of mass and then become unstable. This would be due to the pressures of gravity alone, opposing the Strong Atomic force, which keeps the neutrons intact.

Maybe this is what magnetars are.. They are rare, and are the most magnetic objects in the universe, and they are neutron stars. Maybe they are neutron stars.. going black.

As for the converse, a black hole evaporating into something unstable enough for the other forces to overcome the gravity.. I think this would be much less common. I was asserting an alternative possibility, as I don't believe black holes evaporate away into nothing.

If a black hole were to evaporate down to an unstable state, such that the reduced pressures of gravity could no longer overtake the other forces, there would be an explosion.. not exactly a neutron star as a result :) And there are random flashes throughout this galaxy that scientists cannot explain. ;)

Thanks for an interesting discussion!

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gawd.iz.life

I can't believe I just did that. -- Sylas
Thank You!

gawd.iz.life

Near the end of its life, a small black hole goes bang; the evaporation will release an enormous amount of energy in a very short period of time. For any black hole, we can calculate its temperature, its lifetime, its total energy, and its power output. For a black hole in empty space, soaking up the cosmic background radiation, it will be losing mass only when the temperature is higher than 2.726 K, which is the temperature of the background.

For example, a black hole weighing about a kilogram has a lifetime of about 84 attoseconds (0.000084 picoseconds) and releases as much energy as a 21.5 Megaton thermonuclear bomb. That qualifies as a "bang".

Have we defined the smallest possible black hole?

I don't believe black holes can be tiny singularities.. or that they can be created in a particle accelerator. They contain mass, they have a size & shape, they possess gravity. They are more than just a "point".

The smaller you get when observing matter, down to atoms and subatomic particles, the less significant the force of gravity becomes. It is actually the least of the forces in the realm of quantum particles.

Since black holes by definition possess only gravity in defining what they are...... we can never create a black hole "singularity" in an accelerator. It just makes sense to me.

I don't believe black holes can be tiny singularities.. or that they can be created in a particle accelerator. They contain mass, they have a size & shape, they possess gravity. They are more than just a "point".

The reference to "singularity" is simply an indication that classical physics (GR) breaks down. The "size" of a black hole is usually a reference to the event horizon. But we can still describe reasonably well with classical physics what goes on inside a black hole... up until you get to the center, which is the "singularity"; a point where density diverges to infinite.

To really describe what goes on inside a black hole, where classical physics diverges in a singularity, we'll need to have a theory that combines quantum physics and gravity.

Be that as it may, I don't see any reason in principle to say that a black hole could not be made by a particle accelerator. The problem is that it would take one heck of a lot of energy, and even then any hole would be extremely unstable, because it is so small. It would evaporate almost immediately.

Since black holes by definition possess only gravity in defining what they are...... we can never create a black hole "singularity" in an accelerator. It just makes sense to me.

I don't even see how you make that association or inference.

But in any case, a black hole has mass, charge, and spin (angular momentum); so there are three defining properties for a black hole.

Cheers -- sylas

PS. I don't know how reliable this is, but the wikipedia article suggests that Planck mass is a lower limit for a black hole; below that general relativity breaks down completely. This mass is around 2x10-8 kilograms, smaller than any of the holes in my table.

gawd.iz.life

I don't even see how you make that association or inference.

But in any case, a black hole has mass, charge, and spin (angular momentum); so there are three defining properties for a black hole.

Cheers -- sylas

PS. I don't know how reliable this is, but the wikipedia article suggests that Planck mass is a lower limit for a black hole; below that general relativity breaks down completely. This mass is around 2x10-8 kilograms, smaller than any of the holes in my table.

I was suggesting that a significant amount of mass would be required to initiate a black hole. However, since e = m c squared, then with a sufficient amount of energy, I suppose it might be possible for humans to create a small black hole. But, it would be a LOT of energy, maybe an infinite amount.. and does the charge of a black hole enable magnetic suspension within a partical accelerator? Maybe my assertion about a minimim amount of mass required, is wrong, but I still don't think we can ever do it.

Another thing I find very interesting in the universe, is if you have a photon from a star traveling, say.. East. And in the opposite direction a photon is traveling due west. then the space traveled between the 2 photons is equivalent to twice the speed of light. Does this create a straight-line red shift from the perspective of either photon?

And, would it be true that if 2 galaxies were traveling away from each other such that the net distance growing between them was say exactly the speed of light, then would the light seen from either galaxy of the other, never change? In other words, as they became say 10 billion light years away from each other, they wouldn't necessarily be seeing each other as they were 10 billion light years ago, would they? :)

I was suggesting that a significant amount of mass would be required to initiate a black hole. However, since e = m c squared, then with a sufficient amount of energy, I suppose it might be possible for humans to create a small black hole. But, it would be a LOT of energy, maybe an infinite amount.. and does the charge of a black hole enable magnetic suspension within a partical accelerator? Maybe my assertion about a minimim amount of mass required, is wrong, but I still don't think we can ever do it.

Another thing I find very interesting in the universe, is if you have a photon from a star traveling, say.. East. And in the opposite direction a photon is traveling due west. then the space traveled between the 2 photons is equivalent to twice the speed of light. Does this create a straight-line red shift from the perspective of either photon?

And, would it be true that if 2 galaxies were traveling away from each other such that the net distance growing between them was say exactly the speed of light, then would the light seen from either galaxy of the other, never change? In other words, as they became say 10 billion light years away from each other, they wouldn't necessarily be seeing each other as they were 10 billion light years ago, would they? :)

We are diverging from topic somewhat. It takes a lot of energy to make a black hole; even a small one... but not infinite. There's no such thing as infinite energy; this is just saying you can't make small black holes at all, and there's no reason to claim that.

The exact amount of energy required for a very small black hole is unclear; it would depend on the physics of both relativity and quantum physics and we don't have a complete theory for that. However, any such black hole would be unstable and vanish almost as soon as it was made. The Planck mass (21.8 μg, or 1.22*1016 TeV) is very large by comparison with particles or available energies in an accelerator (The LHC has got up to about 1 TeV, and hopes to get up to as much as 7 TeV). Any black hole made in an accelerator collision would need to be much smaller than planck mass, and we really don't have a good handle on the physics of that scale. In some theories, involving additional dimensions at small scales, it might be possible to make black holes at the TeV scale, which means that it could be possible with the LHC. See Theoretical survey of tidal-charged black holes at the LHC (arXiv:0911.1884); but they would be far too small to be any danger.

As for the points on speed of light; when two photons approach each other, the distance between them decreases at twice lightspeed, for any observer, but nothing is traveling at that speed. For galaxies at large separations, the notion of distance itself becomes ambiguous. This really should be an FAQ... it has been discussed in various threads but I can't pick a good one quickly. But basically, we can see galaxies that have a "proper" recession velocity greater than light speed and the details of why belongs in another thread.

Cheers -- sylas

gawd.iz.life

We are diverging from topic somewhat.

I have thought a lot about these things. I have a passion for physics and cosmology in particular. I will look for other threads as things come up. For instance, I've heard of neutron star quakes and neutron stars having a crust.. and there is some space within a neutron star so that is possible. But black holes are solid all the way thru. So, when they rotate, the outer equator would go faster than the inner core, and of course the center would be gravity-less. For blazars to shoot out jets, from incoming matter at the accretion disk, at near the speed of light from its poles, that thing has to be rotating VERY fast. What would happen if the rotation of the black hole DID reach the speed of light? Maybe THAT's what cancels out gravity for a "split second", with the solid black hole would "unwind" all of its matter outward, in a massive Big Bang, forming a beautiful galaxy, where the outer arm is spinning faster then the inner, just as its solid source, the super massive black hole. I believe this is how galaxies are formed. But I've been away from the math for a long time (I have 152 credits as a math major)....... so I cannot discuss this in terms of mathematics at this time, but I would love to. As for The Big Bang....... I have no idea. I can't comprehend all the mass in the universe altogether at one point. I'm still trying to understand galaxies :)

DiamondGeezer

Yeah, as long as there's a continual supply of material to feed it a black hole can take in matter/energy faster than it radiates it out. The smaller the black hole, though, the faster it will be radiating energy away. Actually smaller black holes are more dense than larger ones--you can see from the Schwarzschild formula here that the mass is proportional to the radius, while of course the volume is proportional to the cube of the radius, so mass/volume will decrease as radius increases (the black hole may evaporate when it reaches the 'Planck density', as discussed in this thread).

Unfortunately the volume of a Schwarzschild Black Hole is zero using Schwarzschild coordinates, infinite if using Kruskal Szekeres coordinates (but if you include the event horizon the volume is [zero times infinity] which could be anything).

This makes calculating the density of a black hole rather difficult.

But not to worry, because in 30 minutes George Jones will lock the thread saving the result for "pedagogical reasons", so no need to worry about replying. I certainly won't.

Ref: "Volume of a Black Hole" Brandon S. DiNunno, Richard A. Matzner Gen Relativ Gravit (2010) 42:63–76 DOI 10.1007/s10714-009-0814-x Link: http://www.springerlink.com/content/dl8mu550u7736567/

twofish-quant

Cosmologists have defined the masses of stars in clear ranges that either become a white dwarf, a neutron star or a black hole.

No they haven't. If you look at an intro astronomy text book, it makes it look like we know a lot more than we really do. The separation between a white dwarf and a neutron star is pretty well understood, but the separation mass between a neutron star and a black hole, and what happens at the separation mass is very, very poorly understood.

The formation and death of stars is more clearly understood than evaporating black holes. Not sure what you mean here.[/QUOTE

Curiously that's *NOT* true. Evaporating black holes are very, very simple objects to model theoretically. The formation and death of stars are extremely complex and there are about eight pieces of physics, each of which are badly understood, and together they form a total mess.

One mistake people make is to think "exotic" things are more mysterious than things that we see everyday. This is often not true, and it's certainly not true in the case of black holes versus star formation and death.

Well my equivalent to an Master's Degree can't outdo your Ph.D, but that doesn't invalidate my opinions :)

It may not, but my Ph.D. dissertation was on modeling supernova. One thing that tends to be the case is that the more you know, the more you know that the less you know. A undergraduate freshmen would think the question of what stars become neutron stars and what stars become black holes is "solved" whereas someone with a Ph.D. in the topic knows that it isn't anywhere close to being solved.

gawd.iz.life

One thing that tends to be the case is that the more you know, the more you know that the less you know.

A undergraduate freshmen would think the question of what stars become neutron stars and what stars become black holes is "solved" whereas someone with a Ph.D. in the topic knows that it isn't anywhere close to being solved.

The problems with ppl are always in the basic areas of the human condition. This is often why ideas ofteh cannot be discussed openly with enthusiasm. Human arrogance and desire to control are the biggest human flaws, and I see this even in episodes of The Universe that I watch. For everything we learn, we seek to control. We demand eternity. We see this in all of our religions as well. We still haven't evolved out of the "we are the center of the universe" mode. All throughout history, there have been ppl with new and creative ideas, who ended up contributing a lot to science, who were oppressed by their peers, and even jailed, or driven to suicide. I always enjoy discussing with enthusiasm matters of the universe, and I am one of many intelligent ppl capable of understanding such things. I'm not real sure why you need to oppose my opinions, or assert your authority as an "expert" over me, but I'm just enjoying discussing these things about which I am very passionate. With this, I exit this thread.

MotoH

If a very large black hole did evaporate? what would happen to the space time that it deformed? Would it spring back to normal like there was no black hole there distorting space-time, or would there be some sort of space-time wave?

twofish-quant

I'm not real sure why you need to oppose my opinions, or assert your authority as an "expert" over me, but I'm just enjoying discussing these things about which I am very passionate. With this, I exit this thread.

Because we have to make distinctions between facts and opinions, and we have to be clear about who is saying what. If you think that the received wisdom on star formation and destruction is totally wrong, that's a fine thing to say. What I'm arguing with you about is that I think you are mischaracterizng what is known and what is not, and if you don't accurately describe what people really believe, and what the observational data is, then it's hard to come up with useful new ideas.

We really don't understand that much about how stars collapse into black holes and neutron stars. What we think we do understand is based mostly on pressure and density rules. You can say whatever you want, it's not OK for you to unintentionally misrepresent what other people are saying.

twofish-quant

And the forces aren't "extinguished", they're overwhelmed. Each stage of collapse produces some quantum-powered opposition to the collapse, as you noted - but that's unstable for large masses.

There are some fundamental reasons why that happens. Suppose you have a ball and you hit it hard enough. Because information that you have hit it can't go faster than light, if you hit it really, really hard so there is a delay in how quickly the ball can bounce back. So what basically happens is that if you start putting tremendous amount of forces on an object, it will start to lose rigidity.

One other way of thinking about it, when the particles in a solid are vibrating near the speed of light, any extra pressure that you put on that object would cause the particles to move faster. Since the particles can't and don't react to oppose the force you put on it, then at really high pressures things start to liquidify. White dwarf matter tends to liquidify first, because white dwarf stars are held up by electron pressure, so when electrons start vibrating near the speed of light, you lose pressure, and then you end up with neutron star material and sense neutrons are much larger objects, they don't vibrate quite as quickly. Eventually if you pile up enough matter they'll start to vibrate at close to the speed of light, and which point that matter will liquidify.

(This is a very rough sketch of what goes on, but it's what happens with the equations.)

The really tricky parts is that it's not clear this liquidification of matter will happen since that depends on the details of the material, but it's going to happen eventually.

twofish-quant

But it may be possible for a neutron star to gain a LOT of mass and then become unstable. This would be due to the pressures of gravity alone, opposing the Strong Atomic force, which keeps the neutrons intact.

That's not what is happening. The strong nuclear force is attractive just like gravity which means that the strong nuclear force isn't resisting gravity. Neutron stars (and for that matter white dwarves) aren't stable because of any of the four forces. What gives them stability is the Pauli exclusion principle.

qraal

Here's the results of computations by Crane and Westmoreland on low-mass black-holes. Things get complicated as the temperature gets very high, so it's not the simple equation anymore.

$$\begin{center} \begin{tabular}{|ccccc|} \text{Radius } 10^{-18} \text{ m} & \text{Mass } 10^{9} \text{ kg} & \text{kT (GeV)} & \text{Power } 10^{15} \text{ W} & \text{Lifetime (yr)} \\ \hline 0.16 & 0.108 & 98.1 & 5519 & 0.04 \\ 0.3 & 0.202 & 52.3 & 1527 & 0.12 \\ 0.6 & 0.404 & 26.2 & 367 & 1 \\ 1.5 & 1.01 & 10.5 & 56.2 & \text{16 − 17} \\ 2.0 & 1.35 & 7.85 & 31.3 & \text{39 − 41} \\ 2.5 & 1.68 & 6.28 & 19.8 & \text{75 − 80} \\ 2.6 & 1.75 & 6.04 & 11.7 & \text{85 − 91} \\ 2.7 & 1.82 & 5.82 & 16.9 & \text{95 − 102} \\ 2.8 & 1.89 & 5.61 & 15.7 & \text{106 − 114} \\ 2.9 & 1.95 & 5.41 & 14.6 & \text{118 − 127} \\ 3.0 & 2.02 & 5.23 & 13.7 & \text{130 − 140} \\ 5.8 & 3.91 & 2.71 & 3.50 & \text{941 − 1060} \\ 5.9 & 3.97 & 2.66 & 3.37 & \text{991 − 1117} \\ 6.0 & 4.04 & 2.62 & 3.26 & \text{1042 − 1177} \\ 6.9 & 4.65 & 2.28 & 2.43 & \text{1585 − 1814} \\ 7.0 & 4.71 & 2.24 & 2.36 & \text{1655 − 1897} \\ 10.0 & 6.73 & 1.57 & 1.11 & \text{4824 − 5763} \\ \end{tabular} \end{center}$$

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gawd.iz.life

]That's not what is happening. [/QUOTE How do you "know" anything that is based on theory? Nothing I've stated is contrary to the Pauli Exclusion Principal. The only thing contrary to me, apparently, is you. If you can't discuss with me, then why respond to my posts? To try to devalue any of my ideas? I still have my ideas and my perspective, and if sharing them with you doesn't provide an interesting discussion for the both of us, then why respond to my posts? To prove some point? That you know more than I? silly

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twofish-quant

How do you "know" anything that is based on theory?

Make a prediction. See if it matches observations. If it doesn't quite match you can try tweaking thing a bit, and if it still doesn't work then give up on the model, and try something else.

Nothing I've stated is contrary to the Pauli Exclusion Principal.

What you are arguing is that what keeps supernova from collapsing is the strong nuclear force, and I'm saying that it isn't. The big problem is that the strong nuclear force is attractive so you can't use it to keep a star from collapsing.

I still have my ideas and my perspective, and if sharing them with you doesn't provide an interesting discussion for the both of us, then why respond to my posts?

Because science is about getting the facts and the models right. The universe doesn't really care what you or I think, so it's a matter of making one's beliefs consistent with observations and models of the universe.

gawd.iz.life

What you are arguing is that what keeps supernova from collapsing is the strong nuclear force, and I'm saying that it isn't. The big problem is that the strong nuclear force is attractive so you can't use it to keep a star from collapsing.

What I said is that the impact that results in a neutron star "overcomes" might be a better word ("extinguishes") the weak nuclear force such that only the strong force and gravity remain. That is true, too.

In questioning mini-black holes, I suggested that if a black hole (sun-sized) lost enough mass mass, it might become unstable, such that the gravity keeping it so densely compact, might "lose it's grip" allowing the strong force to come back into play. This would be an expansion, an explosion. This was just a suggestion.

What I actually believe, is that super massive black holes, spinning very fast (such as quasars), might reach some limit, some threshold. I suggested the speed of it's spin, reaching the speed of light, could "overcome" gravity, the last remaining force of the 4 basic forces of physics. I assert that all the mass would "spin out" to form a galaxy.

If there are forces strong enough to snub out each of the 3 forces, given larger masses and more powerful implosions, why not speculate as to an event, that could also snub out gravity.

Thanks for clarifying what you thought I meant.

Staff Emeritus
2021 Award

I think it may be time to refer everyone to the PF rules on overly speculative posts.

gawd.iz.life, do you have any evidence for these claims?

twofish-quant

What I said is that the impact that results in a neutron star "overcomes" might be a better word ("extinguishes") the weak nuclear force such that only the strong force and gravity remain. That is true, too.

No it's not. If you raise the temperature you can reach a point where the difference between the forces are not important, but if you just increase gravity that's not going to do it.

Think of it this way. If you take a chair and a cup of coffee and heat them to 10,000K, they basically behave more or less the same way. A 10,000K chair and a 10,000K cup of coffee are just going to be gases that tend to act in the same ways. The hotter you make the chair and the cup of coffee, the more similar they are going to be. At 10,000K, they still have different elements, but if you heat things to 10 million K, the chair and the cup of coffee are going to break down into protons and electrons and then they *really* start being similar.

All the talk about "force unification" is an extension to that idea.

But if you crush the chair and the cup of coffee and don't increase the temperature, they are still going to be very different.

You still have the weak nuclear force happening in a neutron star, and the weak nuclear force is *extremely* important in neutron stars since it provides the neutrinos that have something to do with blowing up the star.

If there are forces strong enough to snub out each of the 3 forces, given larger masses and more powerful implosions, why not speculate as to an event, that could also snub out gravity.

Because the *if* is as far as we know incorrect. Gravity doesn't "snub out" the other forces.

Staff Emeritus