Starship Asterisk*

by **dvd.vrl1** » Wed Sep 07, 2016 5:36 am

This picture shows how nothing happens or exists inside a black hole. Space-time bends around this body of mass in what appears to be a perfect circle.

by **Ron-Astro Pharmacist** » Tue Oct 28, 2014 10:04 pm

Lightcone variant (per Wiki link)
In the literature the Kruskal–Szekeres coordinates sometimes also appear in their lightcone variant:

So if UV =O and V = constant = Future (F) then UF = O at some time in the future. (I thought so)

I was close.

Sorry - forgive me my attempt at humor(and don't believe a word I type) except that I really do say thanks for all your comments as I try to digest them. Ron

by **Markus Schwarz** » Tue Oct 28, 2014 6:31 pm

Chris Peterson wrote:
Markus Schwarz wrote:Roughly, once a particle passes the event horizon, it will continue to travel to the "center" of the black hole.
This has always seemed off to me. In reality, I'd expect that a good deal of mass inside the event horizon (maybe all of it) must actually be in relativistic orbits around the center. As it sheds energy to collisions and gravitational waves, it will get closer and closer to the center. But how long does it take a subatomic particle to actually intersect a dimensionless point, and what happens when it does? It's not clear to me how anything ends up in the center.

This is best discussed mathematically in Kruskal-Szekeres coordinates. The wikipedia article does a good job at discussing the physics, IMHO. Notice that in these coordinates the singularity does not appear as a point but as a hyperbola. In these coordinates, showing that any particle inside the black hole reaches the singularity is transformed into showing that a line intersects a hyperbola.

Of course, this applies only to the Schwarzschild black hole. Once you start to consider a rotating black hole (and this if also the more realistic scenario) things get more involved. I don't know if stable trajectories inside it are possible or not.

by **Chris Peterson** » Tue Oct 28, 2014 4:59 pm

Markus Schwarz wrote:Roughly, once a particle passes the event horizon, it will continue to travel to the "center" of the black hole.

This has always seemed off to me. In reality, I'd expect that a good deal of mass inside the event horizon (maybe all of it) must actually be in relativistic orbits around the center. As it sheds energy to collisions and gravitational waves, it will get closer and closer to the center. But how long does it take a subatomic particle to actually intersect a dimensionless point, and what happens when it does? It's not clear to me how anything ends up in the center.

by **Markus Schwarz** » Tue Oct 28, 2014 4:32 pm

rstevenson wrote:Markus, is the singularity simply the spot where the math breaks down? Another way of asking the same question: is there a mass at the center of the black hole which has a diameter > 0, however small it might be?

Rob

The concept of a singularity of a space-time is mathematically well defined. But the definition is rather abstract and not immediately intuitive. Yes, it's the spot where the math/physics goes boink. Roughly, once a particle passes the event horizon, it will continue to travel to the "center" of the black hole. It cannot reverse its course for the same reason we can't go back in time. As a consequence, everything that falls into a black hole will end up at the "center". We then have a finite amount of mass (like 4 million solar masses) at a single point. This creates an infinite energy density (finite mass/0 volume), which in turn, due to general relativity, means also an infinite space-time curvature. But an infinite curvature contradicts the assumption of a smooth space-time, hence general relativity is no longer valid. Another way to view this is to say that there is no center (because it as unphysical properties), but then you can show rigorously that you can fall over the edge of space-time in a finite amount of proper time

As is often the case when infinities arise in physics, it is reasonable to expect that general relativity is just not valid at the "center". Once matter is compressed to such small dimensions, you can't ignore quantum theory. A quantum theory of gravity could then solve all the problems with infinities and have physically reasonable properties. Alas, all attempts to properly combine general relativity and quantum physics have lead to even more infinities...

by **Chris Peterson** » Tue Oct 28, 2014 2:01 pm

rstevenson wrote:Markus, is the singularity simply the spot where the math breaks down? Another way of asking the same question: is there a mass at the center of the black hole which has a diameter > 0, however small it might be?

The math says all the mass is at a dimensionless point at the center. That's what defines a singularity, and that's where the theory breaks down.

Of course, we have no idea if there is a physical singularity in the center of a black hole, or some sort of exotic particle, or mass in some exotic state, or what.

by **rstevenson** » Tue Oct 28, 2014 1:47 pm

Markus, is the singularity simply the spot where the math breaks down? Another way of asking the same question: is there a mass at the center of the black hole which has a diameter > 0, however small it might be?

Rob

by **Markus Schwarz** » Tue Oct 28, 2014 8:38 am

ta152h0 wrote:re black holes still considered a singularity ?

At the "center" of a black hole the theory that describes them, general relativity, breaks down because of a singularity. Everywhere else is fine, in particular at the event horizon, which marks the "point of no return". This is also what allows one to simulate today's APOD.

ta152h0 wrote:And has the star movement been deciphered at the center of our own galaxy ?

Yes. From the orbit of stars close to the galactic center (see, for example, Fig 13 in the reference) one can deduce that there is a black hole at the center with a mass of about 4 million solar masses.

by **MarkBour** » Tue Oct 28, 2014 4:06 am

I'll take a stab at this, in hopes that I understand Ron's question. Imagine a compact, spherical, massive body in space and that nothing much else is there to confuse the simple situation. Draw the plane that passes through its center and the observer's eyes. The diagrams in your post graph the potential energy level at the points in that plane. If you arbitrarily give a zero value to the potential at some given distance from the body, then the diagrams plot a graph of points above or below zero at that point. The "up" direction in the diagram is simply the direction used to plot increasing potential, which is being graphed. The curvy sheets are the surface of the potential function.

So, a tiny traveller moving in the plane I described, would undergo the changing levels of potential energy with respect to the gravity well as depicted in the warped sheets of those diagrams. Of course the diagram just depicts life in one plane. If you draw any other plane through the massive object, the potential diagram needs to be re-oriented for it. So, you're right, of course, it is the same from any direction. But I have no idea how someone could make a diagram that depicted that, but was still a useful, legible graph depicting much of anything clearly.

by **Ron-Astro Pharmacist** » Mon Oct 27, 2014 11:04 pm

If a picture is worth a thousand words in literature then an equation is probably worth a thousand pictures in astrophysics. Thanks Chris!!

Good luck with your hyper sphere – I'm looking forward to the answer to "It's a Bird, It's a Plane" in the Open Space forum. It'll probably be U = FO but we can always hope.

by **Chris Peterson** » Mon Oct 27, 2014 10:12 pm

Ron-Astro Pharmacist wrote:I was asking wouldn't the "falling in" really be occurring in all directions; not just down as in the picture? I know the above is just an attempt to visualise the concept but when used over and over I wonder if we, perhaps, misrepresent the spacetime distortion in our minds to be uni-directional.

This image is treating space as two-dimensional with a fictitious gravity-like force acting "downwards". In reality, the distortion of spacetime that is responsible for gravity operates in four dimensions (three spatial dimensions). There is no obvious "down" in that situation.

The point is, the intent is to introduce the concept of warped space. That is useful for understanding certain things about the nature of gravity, but not particularly for understanding it from a behavior standpoint. In that sense, it's nothing more than an attractive force that operates on mass and follows an inverse square law for magnitude.

by **Ron-Astro Pharmacist** » Mon Oct 27, 2014 8:57 pm

I guess the best way I "might" describe it as analogous to hyperspace being the next dimension up from our three sensory perceptions except in reverse – a negative "hypo space". The only way I've been able to mentally imagine hyper space is the hyper cube.

Hence a black hole's gravity "might" be better represented by a negative hypo-sphere if that' possible to think of. Sorry this is off the cuff - I'm not trying to invent a concept. And don't ask me to draw one. That would actually be worse than me trying to describe it but not nearly as hard to believe.

Thinking of it terms of potential energy is actually much easier than my way. Thanks!!

by **geckzilla** » Mon Oct 27, 2014 8:20 pm

I have no doubt that my brain's attempt at conceptualizing spacetime fails utterly but I'm still not understanding what you mean. Instead of thinking about it spatially, think of it in terms of potential energy.

by **Ron-Astro Pharmacist** » Mon Oct 27, 2014 8:13 pm

geckzilla wrote:
Ron-Astro Pharmacist wrote:I've never understood why they depict material falling into a black hole as going down. Is that just a random direction because we perceive gravity as pulling something downhill? It seems to me that it's just as rational to have it go up or sideways or all directions simultaneously.
I'm not sure what you mean. Falling into any gravity well, even a black hole, is still falling. The same thing happens on Earth, even if the scale is so large we don't think that someone falling in Australia is moving "up" relative to someone in Canada.

When trying to simulate a black hole's or any gravity, one way to depict the thought is to use as picture such as:

I was asking wouldn't the "falling in" really be occurring in all directions; not just down as in the picture? I know the above is just an attempt to visualise the concept but when used over and over I wonder if we, perhaps, misrepresent the spacetime distortion in our minds to be uni-directional.

by **ta152h0** » Mon Oct 27, 2014 8:00 pm

https://www.youtube.com/watch?v=krD4hdGvGHM

by **geckzilla** » Mon Oct 27, 2014 6:49 pm

Ron-Astro Pharmacist wrote:I've never understood why they depict material falling into a black hole as going down. Is that just a random direction because we perceive gravity as pulling something downhill? It seems to me that it's just as rational to have it go up or sideways or all directions simultaneously.

I'm not sure what you mean. Falling into any gravity well, even a black hole, is still falling. The same thing happens on Earth, even if the scale is so large we don't think that someone falling in Australia is moving "up" relative to someone in Canada.

by **Ron-Astro Pharmacist** » Mon Oct 27, 2014 6:38 pm

It's odd that Leonard Susskind used this exact same illustration of a black hole in his lecture "The World As Hologram."

https://www.youtube.com/watch?feature=p ... DIl3Hfh9tY

I've never understood why they depict material falling into a black hole as going down. Is that just a random direction because we perceive gravity as pulling something downhill? It seems to me that it's just as rational to have it go up or sideways or all directions simultaneously.

by **morr** » Mon Oct 27, 2014 4:57 pm

Boomer12k wrote:
userloser wrote:In case someone missed it.

http://www.wired.com/2014/10/astrophysi ... lack-hole/

IF...a Black Hole looked like that....we would be able to SEE ALL OF THEM.....no doubt....and then they would not be called "Black Holes"...but AWESOME HOLES.....

:---[===] *

They're too small to see at any distance. Even the SMBH at the center of the galaxy has an event horizon smaller than our solar system. Stellar mass black holes would be tinier (planet sized). Get close enough to one, and it may look something like that.

by **Ron-Astro Pharmacist** » Mon Oct 27, 2014 3:42 pm

In "Black Holes Explained" , Alex Filippenko describes a simulation Robert Nemiroff developed to show a black holes effect on space time. I was happy to see it referred to again under the link "visual distortions." I also noticed, while looking for it myself, Dr. Nemiroff's efforts to further astronomy education and research had been recognized by Michigan Tech's Research Magazine back in 2013.

http://www.mtu.edu/research/archives/ma ... rch-award/

"Quote from above link" - The passion for education and outreach evident in the Astronomy Picture of the Day manifests itself throughout Nemiroff’s working life. “He really cares about teaching,” said Jay Norris of Boise State University. “Bob’s quest to understand Nature—and transfer that understanding to others—is unflagging.”

It's great to see that his influence has reached our neck of the woods. Many cudos!!

by **Chris Peterson** » Mon Oct 27, 2014 2:13 pm

Roland wrote:If you are too close to the black hole for there to be a star between, then why is there no gravitational lensing in the background stars. you can't have it both ways.
Nice image.

Let's see. Do I go with a rigorous mathematical simulation by people who are experts in general relativity and tensor calculus, or do I go with your gut feeling?

by **Roland** » Mon Oct 27, 2014 1:32 pm

If you are too close to the black hole for there to be a star between, then why is there no gravitational lensing in the background stars. you can't have it both ways.
Nice image.

by **geckzilla** » Mon Oct 27, 2014 4:04 am

Guest wrote:Well, possibly the gravity well of a black hole is able to partially capture light from 360/360 degree directions that is able to spiral out of the gravity well because it was just not close enough to be caught. Logically, a 'black hole' (or the 'surface' just just outside the event horizon) seen from close up may appear fairly bright, if not brilliant, from the distance shown. And appearances of such a gravity well may appear non-circular given the variety of light sources perceived from source. Really, we must consider all possibilities, not just assume an 'all round knowledge' of all things that require imagination and insight of other possible explanations.

This picture is already made using known theory, logic, and not assumptions. Adding imagination or guessing would only make it less accurate. It might be possible to make it more "realistic" with an improved simulated environment but that's a fault of the simulated environment and not the simulated physics of the black hole itself.

by **alter-ego** » Mon Oct 27, 2014 3:52 am

Guest wrote: ...
Well, possibly the gravity well of a black hole is able to partially capture light from 360/360 degree directions that is able to spiral out of the gravity well because it was just not close enough to be caught. Logically, a 'black hole' (or the 'surface' just just outside the event horizon) seen from close up may appear fairly bright, if not brilliant, from the distance shown. And appearances of such a gravity well may appear non-circular given the variety of light sources perceived from source. Really, we must consider all possibilities, not just assume an 'all round knowledge' of all things that require imagination and insight of other possible explanations.

Given the simplest Schwarzschild black hole, the mathematics describing all the observed phenomena (in the APOD) is on solid ground within the framework of general relativity, including non-circular Einstein Rings. I'm not sure what you meant about "may appear non-circular given the variety of light sources perceived from source". Extended, non-circular light sources behind the black hole are distorted in a very predictable way, and in fact visible in the image. Assuming no accretion process, as is the case for this APOD, no photons can escape so that they appear from within the black region. (But how fuzzy are the edges?)

Although future observations and data approaching 1.5 Schwarzschild Radii (we currently have none) may reveal new phenomena that require revisiting GR, there is no data today to suggest anything "out of the ordinary"

The current GR theory is holding strong and not cracking yet. I'm anxiously awaiting the start of upcoming VLBI observations of the Sag A* (Milky Way's BH) event horizon (or at least to the Innermost Stable Circular Orbit). The telescope consortium is the Event Horizon Telescope a good S&T article about it can be found here.

by **Guest** » Mon Oct 27, 2014 2:17 am

Chris Peterson wrote:
MikeS wrote:
We're far too close to this black hole for there to be a star between it and the observer.
Maybe that was the intent, but it looks extremely artificial. The perfectly rounded circle looks fake as well.
Well, we're looking at the darkest, roundest thing in nature, something with no edge to scatter light. I don't know what else it would look like.

Well, possibly the gravity well of a black hole is able to partially capture light from 360/360 degree directions that is able to spiral out of the gravity well because it was just not close enough to be caught. Logically, a 'black hole' (or the 'surface' just just outside the event horizon) seen from close up may appear fairly bright, if not brilliant, from the distance shown. And appearances of such a gravity well may appear non-circular given the variety of light sources perceived from source. Really, we must consider all possibilities, not just assume an 'all round knowledge' of all things that require imagination and insight of other possible explanations.

by **alter-ego** » Mon Oct 27, 2014 12:39 am

Boomer12k wrote:
userloser wrote:In case someone missed it.

http://www.wired.com/2014/10/astrophysi ... lack-hole/

IF...a Black Hole looked like that....we would be able to SEE ALL OF THEM.....no doubt....and then they would not be called "Black Holes"...but AWESOME HOLES.....

:---[===] *

It's likely that most do look something like that, at least if the simulations are reasonably accurate and if you're looking a the accretion disk nearly edge on.

The APOD shows the simplest BH condition: Non-rotating and no accretion disk. This case is probably not the norm. Today, the primary point of interest is the visible field distortion around a black hole using an actual star field in the simulation. The more general BH cases have frame dragging (spatial distortion due to rotation) and a messy accretion disk (that you probably can't see through anyway).

The video below show a nice simulation of viewing black hole with accretion disk (fixed view angle) and key feature descriptions. The link at the bottom takes you to a simulation showing an accretion disk ad different inclination angles (as well as intensity line-profiles at different angles). Userloser's wired.com image shows something close to what you find in that link.

Click to play embedded YouTube video.

http://jila.colorado.edu/~pja/black_hole.html

Starship Asterisk*

APOD: Too Close to a Black Hole (2014 Oct 26)

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