Fiery crushy burning death

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ST47
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Fiery crushy burning death

Postby ST47 » Sat Dec 20, 2008 4:48 am UTC

I hope we've read Dr. Plait's new book. I seem to have missed a few things, and I can't find an easy answer online.

The first is either a really petty nitpick or physics weirdness. Page 46 says a solar flare shot protons at us at about one-third the speed of light, and it took them 15 minutes to get here. Google tells me that that would be about half the speed of light. Are there some weird proton magics going on that lets this happen, or is it an innocent mistake?

Black holes. How big are they, and with the effects they wreak upon space itself, what does size mean anyway? Black holes start out as supercompressed degenerate neutrons, which then manage to get compressed more and more. Does this stop at some point, or does it continue until the neutrons occupy a literal point in space? I've heard it said that an object the size of our solar system and the density of air could, under its own gravity, become a black hole. Once it does so, do we still measure its size as the size of its event horizon?

Now, if black holes do shrink down to a point. A note on page 138 states that rotating black holes can bulge out. I haven't seen any points floating in space recently, but I'd be rather surprised if they had any concept of shape. Now, the event horizon can't really 'bulge out', since it's defined as a sphere with a fixed radius, the radius r that satisfies the equation cr2=GMblack hole. Right?

Ooh. I hit submit instead of preview there. I wasn't finished. Supermassive black holes. By coincidence, in Chem the other day we were discussing death and destruction and mayhem, and someone asked 'what would happen if I dropped [something, I don't remember what] into a black hole'. Natural question, I suppose. I'd rather drop, let's say, a smallish galaxy into a black hole though. Because of tidal forces, the poor stars and planets would wind up becoming rather thin. However, this would happen before they actually passed the event horizon. Nuclear fusion only occurs in a star's core, naturally, except for one weird type of star that actually circulates and uses up all its hydrogen. Would these tidal forces ever be enough to force fusion outside of the core? Perhaps even for something like a large planet, which can't do any fusion on its own but could become a star in its dying moments?

See, it isn't enough to just go and gobble up stars and planets. You need to actually do some damage to them before you eat them.

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Re: Fiery crushy burning death

Postby Carnildo » Sat Dec 20, 2008 7:56 am UTC

ST47 wrote:The first is either a really petty nitpick or physics weirdness. Page 46 says a solar flare shot protons at us at about one-third the speed of light, and it took them 15 minutes to get here. Google tells me that that would be about half the speed of light. Are there some weird proton magics going on that lets this happen, or is it an innocent mistake?


Could be a mistake, could be an odd effect: protons and other charged particles can be accelerated by magnetic fields, and there are plenty of strong magnetic fields in the vincinity of the sun.

Black holes. How big are they, and with the effects they wreak upon space itself, what does size mean anyway? Black holes start out as supercompressed degenerate neutrons, which then manage to get compressed more and more. Does this stop at some point, or does it continue until the neutrons occupy a literal point in space?


General relativity says that the matter collapses to a point, but once you get inside the event horizon of a black hole, our current physics equations stop working. Nobody knows what goes on in there.

Now, if black holes do shrink down to a point. A note on page 138 states that rotating black holes can bulge out. I haven't seen any points floating in space recently, but I'd be rather surprised if they had any concept of shape. Now, the event horizon can't really 'bulge out', since it's defined as a sphere with a fixed radius, the radius r that satisfies the equation cr2=GMblack hole. Right?


"Shape" for a black hole is usually defined as the shape of the event horizon. Once you get a rotating black hole, you get all sorts of odd effects that can change the shape, and you really need general relativity to analyze it -- Newtonian mechanics just won't cut it.

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Re: Fiery crushy burning death

Postby You, sir, name? » Sat Dec 20, 2008 1:51 pm UTC

ST47 wrote:The first is either a really petty nitpick or physics weirdness. Page 46 says a solar flare shot protons at us at about one-third the speed of light, and it took them 15 minutes to get here. Google tells me that that would be about half the speed of light. Are there some weird proton magics going on that lets this happen, or is it an innocent mistake?


An error of merely 60% is pretty small in astrophysics. Usually you're happy to be in the right power of 1000.
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Re: Fiery crushy burning death

Postby danpilon54 » Sat Dec 20, 2008 3:16 pm UTC

For the first one. Which reference frame? I don't have time to do out the math, but to a proton it might feel like 15 mins if its going 1/3c. Then again Im sure the quoted number is in earth frame time, since that is the only useful one. So I dunno...
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Re: Fiery crushy burning death

Postby thoughtfully » Sat Dec 20, 2008 5:14 pm UTC

Carnildo wrote:General relativity says that the matter collapses to a point, but once you get inside the event horizon of a black hole, our current physics equations stop working. Nobody knows what goes on in there.

The physics on one side of the event horizon are pretty much identical as on the other side. In theory, of course, but physicists are pretty confident about this one. The trickiness comes into play when the observer/victim approaches the center, where quantum effects become important.

Carnildo wrote:"Shape" for a black hole is usually defined as the shape of the event horizon. Once you get a rotating black hole, you get all sorts of odd effects that can change the shape, and you really need general relativity to analyze it -- Newtonian mechanics just won't cut it.

The event horizon is defined by the surface from which the escape velocity equals the speed of light. For a rotating black hole, the escape velocity is reduced by the centrifugal force (or something analogous, the usual notion of centrifugal force probably doesn't correspond perfectly)
If a black hole is rotating fast enough, the event horizon can disappear entirely. This is a "naked singularity." There is much debate as to whether such circumstances are physically possible.

Keep in mind that singularities aren't. Infinite density and zero size are failures of theory, and inconsistent with Quantum Mechanics. The Uncertainty Principle will smear them out to some finite size.
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Re: Fiery crushy burning death

Postby doogly » Sat Dec 20, 2008 5:42 pm UTC

ST47 wrote:Now, if black holes do shrink down to a point. A note on page 138 states that rotating black holes can bulge out. I haven't seen any points floating in space recently, but I'd be rather surprised if they had any concept of shape. Now, the event horizon can't really 'bulge out', since it's defined as a sphere with a fixed radius, the radius r that satisfies the equation cr2=GMblack hole. Right?


Oh it's bulgy, you get the Kerr solution! This is a fun solution.

The outer event horizon is bulged. That is the surface within which even light must rotate in the same direction as the black hole in order to not fall in. The inner surface is still a sphere, which is the surface from which light can never escape, no matter how much rotating it does. The singular part isn't a weirdly shaped point, it's actually a ring in this case. Inside the inner surface though, you are not doomed to fall into the singularity. This is because the radial spacial direction and time have swapped signs twice, and thus things are normal.

One of the fantastically excellent things you can do with a Kerr black hole is shoot in a rocket full of trash, dump the trash, and get your rocket back with more total energy than you sent it in with. A net energy gain and a dumping of waste would be perfect, if only we had a black hole nearby to do it with. This is the Penrose Process, if you want to look up details.
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Re: Fiery crushy burning death

Postby ST47 » Sat Dec 20, 2008 10:16 pm UTC

This one I thought about while reading another post. Let's do the whole hole through the earth thing. The gravity at distance r from the center of the earth is based on what mass? The mass of all earth below the plane which is perpendicular to the line connecting you and the center of the earth? The mass of that, minus the mass of the earth above that plane? (That is, does the mass above you pull on you and 'offset' some of the gravity from the mass below you, or is it just ignored?)

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Re: Fiery crushy burning death

Postby jmorgan3 » Sat Dec 20, 2008 11:11 pm UTC

ST47 wrote:This one I thought about while reading another post. Let's do the whole hole through the earth thing. The gravity at distance r from the center of the earth is based on what mass? The mass of all earth below the plane which is perpendicular to the line connecting you and the center of the earth? The mass of that, minus the mass of the earth above that plane? (That is, does the mass above you pull on you and 'offset' some of the gravity from the mass below you, or is it just ignored?)

Imagine that you are partway down this hypothetical hole, a distance h from the center of the earth. Now imagine that the earth is in two parts. The first part is a sphere centered at the earth's center with a radius of h. That is, if you are 1,000 km from the center of the earth, this sphere is right below you and has a radius of 1,000 km. It is really easy to find out the gravitational force exerted on you by the sphere, because this situation is analogous to just standing on the surface of the earth. The remaining part of the earth is a hollow shell, like a pingpong ball, with an internal radius of h and and external radius equal to the radius of the earth. According to Newton's shell theorem, this shell exerts no net gravitational force on any object inside it, which includes you.

tl;dr If you are a distance h away from the center of the earth, the "big M" in the gravity equation is the total mass of the earth that lies within a distance h of the center of the earth.
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Re: Fiery crushy burning death

Postby Sir_Elderberry » Sat Dec 20, 2008 11:50 pm UTC

If you've done any E&M, think Gauss's Law. Now with gravity!
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Re: Fiery crushy burning death

Postby meat.paste » Sun Dec 21, 2008 3:45 am UTC

ST47 wrote: Supermassive black holes. By coincidence, in Chem the other day we were discussing death and destruction and mayhem, and someone asked 'what would happen if I dropped [something, I don't remember what] into a black hole'. Natural question, I suppose. I'd rather drop, let's say, a smallish galaxy into a black hole though. Because of tidal forces, the poor stars and planets would wind up becoming rather thin.


For a supermassive black hole (galactic size and larger), the tidal forces at the event horizon are not particularly large. No thin planets. For a 10 m tall spaceship falling into a galactic black hole, the tidal force is ~1 kg from the top to the bottom (if I did the maths correctly.)

I do not think black holes require a singularity. If a cluster of galaxies were heavy enough and close enough together, then space could fold enough to make it look like a supermassive black hole from outside the cluster. Inside the cluster, everything would continue to evolve as usual. Of course, now I will have to go do some math to check on this idea. Oh bother.
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