# Crossing an event horizon



## Bullgrit (Aug 31, 2010)

Am I understanding this correctly?

"Traveller" is moving towards an event horizon.

"Observer" is watching the Traveller.

The Observer sees the Traveller approaching the event horizon, but the Traveller slows, slows, slows such that although he is still moving towards the horizon, he never actually crosses it.

The Traveller sees himself approaching the event horizon, and crosses it with no slow down.

Is this an accurate description?

If so, does the Traveller actually cross the horizon? And when does he cross it -- at the "end of time"?

Bullgrit


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## Plane Sailing (Aug 31, 2010)

The Astrophysics Spectator agrees with you

The Astrophysics Spectator: Falling through the Event Horizon of a Black Hole

Cheers


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## Deset Gled (Aug 31, 2010)

Bullgrit said:


> The Observer sees the Traveller approaching the event horizon, but the Traveller slows, slows, slows such that although he is still moving towards the horizon, he never actually crosses it.
> 
> The Traveller sees himself approaching the event horizon, and crosses it with no slow down.
> 
> Is this an accurate description?




It's a good description for a sci-fi book.  As is often the case with sci-fi physics, the real math gets in the way.

The bottom line is that you can't actually travel at the speed of light.  If you could somehow travel faster than the speed of of light, lots of funny things would happen, but it is not an actualizable consequence.  

The equation for time dilation is:

To = Tt / (sqrt(1-v^2/c^2))

Where To is time seen by the observer, Tt is time seen by the traveller, v is velocity of the traveller relative to the observer, and c is the speed of light.

The key point to notice in this equation is that when v = c, you literally divide by zero.  Once you figure out how to do that, feel free to re-write math and/or physics as we know it.  A similar problem happens when you look at inertial dilation, as your mass becomes infinite at the speed of light.


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## Bullgrit (Aug 31, 2010)

Plane Sailing said:
			
		

> The Astrophysics Spectator agrees with you



Reading that brings up another question that twists my noodle:

Two travellers moving towards the event horizon. (Maybe one slightly ahead of the other?) Traveller 1 sees Traveller 2 slowing and never reaching the horizon, but sees himself going through?

But back to the OP: When does the Traveller cross the horizon? At the "end of time" or some such?

The idea of something stretching to infinity, I can metally grasp, even if in theory. But something happening at infinity, well, that's getting me. Like climbing over an infinitely high wall.

Bullgrit


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## El Mahdi (Aug 31, 2010)

Only Stephen Hawking truly knows...


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## Deset Gled (Aug 31, 2010)

Bullgrit said:


> Two travellers moving towards the event horizon. (Maybe one slightly ahead of the other?) Traveller 1 sees Traveller 2 slowing and never reaching the horizon, but sees himself going through?




Well, you already know my stance on the fact that reaching the event horizon won't actually happen, but the equation I posted above tells you exactly what happens as two observers get near the speed of light.  

Note that v is the velocity between the observer and the traveler, not relative to an absolute point (which, by the way, doesn't exist).  If the observer is accelerating just a bit behind the traveller, it will look the same as if the traveller was accellerating slowly.  To figure out how this looks to another non-accelerating observer, simply use a third person (second observer) to calculate the time dilation relative to both travellers.


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## Umbran (Aug 31, 2010)

Bullgrit said:


> But back to the OP: When does the Traveller cross the horizon? At the "end of time" or some such?




In this case, the question of "when?" must be accompanied by "according to whose clock?"

By the Traveller's clock, they cross the event horizon whenever they should have, as a freely falling object - probably about 20 minutes past Noon.

By the Observer's clock, it is, "at the end of time or somesuch".  Honestly, long before the end of time, light coming off the Traveller to the Observer will be so red-shifted as to be undetectable by the Observer.  

This is assuming a fairly large black hole, without much tidal distortion near the event horizon.  For smaller black holes, both the Traveller and the Observer see the Traveller turned into spaghetti long before the end of time.


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## Umbran (Aug 31, 2010)

Deset Gled said:


> Well, you already know my stance on the fact that reaching the event horizon won't actually happen, but the equation I posted above tells you exactly what happens as two observers get near the speed of light.




Note that to cross the event horizon, the Traveller does not need to be moving near the speed of light.


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## Deset Gled (Aug 31, 2010)

Umbran said:


> Note that to cross the event horizon, the Traveller does not need to be moving near the speed of light.




Thus demonstrating that I can remember formulas from my modern physics courses, but not simple terminology like "event horizon".

Carry on, nothing to see here.


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## Klaus (Aug 31, 2010)

Bullgrit said:


> Reading that brings up another question that twists my noodle:
> 
> Two travellers moving towards the event horizon. (Maybe one slightly ahead of the other?) Traveller 1 sees Traveller 2 slowing and never reaching the horizon, but sees himself going through?
> 
> ...



Maybe the Observer never sees the Traveler entering the Event Horizon because the *image* of the Traveller (ie, light bouncing off the Traveller and into the Observer's eyes) can't escape the Event Horizon, thus freese-framing the Traveller's image?


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## Shag (Sep 1, 2010)

Assumptions:  Both Traveller and Observer are industructible and immortal.  There is nothing else falling into the hole that obscure observers view.


An observer sees the traveller become redshifted and slow down as they approach the event horizon but the traveller never quite reaches it.

The traveller speeds ever faster towards the hole, he looks up and sees massively blue shifted light, but notices the stars visibly moving, some are going nova, some are being born.  As he reaches the horizon the universe ends, time ends.  Basically a black hole is a one way time machine.  The only possible esape for traveller is the off chance that the hole dies before the end of time through losing energy through hawking radiation.  

So actually it would be a great prison for immortals.


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## Umbran (Sep 1, 2010)

Klaus said:


> Maybe the Observer never sees the Traveler entering the Event Horizon because the *image* of the Traveller (ie, light bouncing off the Traveller and into the Observer's eyes) can't escape the Event Horizon, thus freese-framing the Traveller's image?




And the Traveler actually slips through unnoticed, leaving behind an immaterial ghost/hologram for all eternity?  No.

Let me reiterate - as far as the Observer is concerned the clock the Traveler is carrying _slows down_ as she falls.  He floats up there, getting old as the years pass, and the Traveler is as young as when she first dropped.  

Here's the kicker - at the last moment of his aged life, if the Observer could yank the Traveler back up, she'd still be young!  It isn't just an image, it's her actual body not changing as time hasn't passed for her.


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## Klaus (Sep 1, 2010)

No wonder Dr. Sheldon Cooper is so well-adjusted...


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## Bullgrit (Sep 1, 2010)

Is gravity instantaneous? I mean, is there travel time between the gravity of an object and its effects on another object a long distance away? 

Say we create a black hole out beyond pluto space. Would the Solar System (planets, Sun) react immediately? Would "sensors" on Earth detect it immediately? Or would there be a delay as the gravity "wave" swept in?

Bullgrit


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## jonesy (Sep 1, 2010)

Bullgrit said:


> Is gravity instantaneous?



Nope.

According to the measurements done by the National Radio Astronomy Observatory, and the University of Missouri, gravity travels at the speed of light.


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## Someone (Sep 1, 2010)

AFAIK gravity "travels" at the speed of light. If some mass magically appeared X light minutes, light hours or light years away from us we'd notice it's gravitational pull X minutes, hours or years after that.


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## Umbran (Sep 1, 2010)

jonesy said:


> According to the measurements done by the National Radio Astronomy Observatory, and the University of Missouri, gravity travels at the speed of light.




Correct.  This also matches the best available theory.



Bullgrit said:


> Say we create a black hole out beyond pluto space. Would the Solar System (planets, Sun) react immediately? Would "sensors" on Earth detect it immediately? Or would there be a delay as the gravity "wave" swept in?




"Gravity wave" is just the sort of thing you should think, yes.  Just like light, gravity moves in waves (and particles - the photon carries electromagnetism, the graviton carries gravity), that move at the speed of light (so, like the photon, the graviton should not have any mass).

Black holes orbiting each other should radiate such waves, and the  the Laser Interferometry Gravitational-wave Observatory (or LIGO) is designed to detect such things.


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## Bullgrit (Sep 1, 2010)

I've read _A Brief History of Time_ several times, but the last was a couple/few years ago, so I may be misremembering. But I thought Hawking at least suggested in that book that gravity affects happen immediately -- that gravity warps space around the source. I think the textual illustration was spheres sitting on and indenting a piece of black cloth.



> Just like light, gravity moves in waves (and particles - the photon carries electromagnetism, the graviton carries gravity), that move at the speed of light (so, like the photon, the graviton should not have any mass).



Wait, huh? Gravitons travel away from their source, and cause what they "hit" to move back towards their source? And if gravitons are particles, how do they escape a black hole? And if photons have no mass, how are they pulled into a black hole?

Honestly, until you mentioned it here, I thought gravitons were a science fiction construct. No lie, the only time I remember reading about gravitons was in a Superman comic -- he supposedly manipulated gravitons to fly. This is why I haven't bothered looking up the term on Wikipedia or elsewhere. I'm going to look it up now, though.

I'm all confusalled. (If someone tells me that gravitons are talked about in _A Brief History of Time_, I'm going to be embarrassed. Really, it's one of my favorite books.)

Bullgrit


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## jonesy (Sep 1, 2010)

Brief History of Time is 20 years old next year. A lot has happened since. When he talked about it the experiments hadn't happened yet.

Gravitons are so far hypothetical particles, but the waves themselves have been measured.


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## Umbran (Sep 2, 2010)

Bullgrit said:


> But I thought Hawking at least suggested in that book that gravity affects happen immediately -- that gravity warps space around the source. I think the textual illustration was spheres sitting on and indenting a piece of black cloth.




It has been ages since I read it as well.  At this time, I don't think Hawking would say gravity works instantaneously.  

The usual analogy is that spacetime is a rubber sheet, and masses rest on the sheet, and deform it.  Now, if you do that with a real rubber sheet, it doesn't deform instantaneously.  It actually takes time for the sheet to change shape.  If you shake one end of the sheet, it takes time for that shaking to get to the other side of the sheet.  Same thing here.



> Wait, huh? Gravitons travel away from their source, and cause what they "hit" to move back towards their source?




They cause what they _interact with_ to move back towards the source.  It isn't exactly like colliding billiard balls here.  

There's nothing new about it, though.  Electromagnetic forces are transferred by photons.  If you bring a positive and negative electric charge near each other, they exchange photons, and are drawn together, too.



> And if gravitons are particles, how do they escape a black hole?




They don't come from inside the black hole.  You can think of them being emitted from the outer surface, if that helps.  The real answer is that sometimes they act like particles, and sometimes they act like waves (again, just like light), and for most concerns, emission from a garden variety black hole is when they're more like waves.



> And if photons have no mass, how are they pulled into a black hole?




The photons have energy, and as Einstein told us, mass and energy are equivalent.  Light is bent by heafy objects, occasionally leading to what astronomers call "gravitational lensing" (something else for you to look up).



> I'm all confusalled. (If someone tells me that gravitons are talked about in _A Brief History of Time_, I'm going to be embarrassed. Really, it's one of my favorite books.)




I don't believe they are mentioned there, so you're safe.   Remember that _A Brief History of Time_ is brief, a short explanation.  It doesn't contain everything.


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## GameDaddy (Sep 2, 2010)

If by some remote chance you are not torn asunder by the variable tidal gravitational pull of the black hole yourself... You still won't see them as they cross the event horizon. At that point gravity is sucking in the light as well, allowing no light to escape... no light = no vision. Mo light = color vision.

And they'll be dead already. Just so much compressed jelly. You might get a nice measurable little x-ray burst out of them though. And if you orbit just right you get out of the gravity well before reaching the event horizon yourself being close enough to see the luckless traveller during his/her last moments. 

The gravitational tidal force are off the charts at the event horizon of a black hole.

Thank you. I've just figured something else new out...


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## Umbran (Sep 2, 2010)

GameDaddy said:


> If by some remote chance you are not torn asunder by the variable tidal gravitational pull of the black hole yourself... You still won't see them as they cross the event horizon. At that point gravity is sucking in the light as well, allowing no light to escape... .




There's more to it than that.  The relativistic effects muck with clocks, as we've noted above.  If you're off at a distance, you see her clock slow to pretty much a stop as she falls - from the perspective of the rest of the universe, it takes an infinite amount of time for her to reach the event horizon.



> The gravitational tidal force are off the charts at the event horizon of a black hole.




Not necessarily.  For some, even most, that may be true, but not all.

Specifically, what you say is more true the smaller the black hole is.  For large black holes (like the supermassive ones found in the hearts of galaxies) the event horizon can have only tiny tidal forces,  less than that which the Earth is exerting on you at this moment, small enough to not damage materials entering the hole.

Now, inside the black hole, near the singularity itself, that's another matter.


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## GameDaddy (Sep 3, 2010)

Umbran said:


> There's more to it than that. The relativistic effects muck with clocks, as we've noted above. If you're off at a distance, you see her clock slow to pretty much a stop as she falls - from the perspective of the rest of the universe, it takes an infinite amount of time for her to reach the event horizon.













Umbran said:


> Not necessarily. For some, even most, that may be true, but not all.
> 
> Specifically, what you say is more true the smaller the black hole is. For large black holes (like the supermassive ones found in the hearts of galaxies) the event horizon can have only tiny tidal forces, less than that which the Earth is exerting on you at this moment, small enough to not damage materials entering the hole.
> 
> Now, inside the black hole, near the singularity itself, that's another matter.





How is that possible? At the event horizon the gravitational pull of the black hole is so great, even light can no longer escape. If there is any variation in the density of the black hole it's going to create a gravitational sheer effect. You saying that the black holes in the center of the galaxy are perfectly spherical, and uniformly dense? 

Given that the black hole sucks in mass at a variable rate, how is that mass evenly distributed in the singularity?


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## jonesy (Sep 3, 2010)

If you are falling towards the horizon with the person you are observing, then neither of you experiences a slowdown of time. Relative to each other. But the stars around you would appear to speed up.

The farther away you are from the person you are observing, and the horizon he is falling towards, the greater the slowdown from your perspective, to the person falling in.

The farther in he is, the slower he seems to fall. At some point you will experience the interesting notion that he doesn't seem to be moving at all, because he will be moving inches by aeons. You will never in your own lifetime see him fall all the way through.

From his perspective the person observing him, and all the space around him, would speed up. The farther in he'd be, the faster everything would go.


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## GameDaddy (Sep 3, 2010)

jonesy said:


> The farther in he is, the slower he seems to fall. At some point you will experience the interesting notion that he doesn't seem to be moving at all, because he will be moving inches by aeons. You will never in your own lifetime see him fall all the way through.




Only if you are observing him from a distance. What happens if you are closer?

Closer still.

and even closer?

Unless someone can provide some mathematics or an illustration to prove it is physically impossible to witness an event horizon transition I'll be in the school of: _now you see him_ [He crosses the event horizon, you don't], Now you don't see him anymore... He simply vanishes... ummm like forever.

P.S. one would have to be careful when that close in to an event horizon, as I doubt they are perfectly spherical as well...


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## jonesy (Sep 3, 2010)

GameDaddy said:


> Only if you are observing him from a distance. What happens if you are closer?
> 
> Closer still.
> 
> ...



Did you not read what I just said? I didn't say it was impossible. I said _in your own lifetime_.

If we go with your original assumption of indestructible immortals, then sure, why not. But it's going to be a looooong wait for the observer if he watches the whole thing from beginning to end.

Edit: oh, it was Shag who said it. D'oh.


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## Umbran (Sep 3, 2010)

GameDaddy said:


> image snipped




The scenario you show in this image is complicated, as you have two travelers who both approach the event horizon.  It'll take me a bit to write that up properly.  But, I can handle the next one quickly.



> How is that possible? At the event horizon the gravitational pull of the black hole is so great, even light can no longer escape. If there is any variation in the density of the black hole it's going to create a gravitational sheer effect. You saying that the black holes in the center of the galaxy are perfectly spherical, and uniformly dense?
> 
> Given that the black hole sucks in mass at a variable rate, how is that mass evenly distributed in the singularity?




Perhaps you don't understand what I mean by "singularity".

A black hole forms when no force known can prevent collapse.  Not even light gets away.  Nothing stops the collapse.  Nothing.

I repeat that - nothing.  The thing shrinks down to the event horizon, and _it keeps collapsing_.  The event horizon isn't a physical thing, it is not the surface of the collapsed object.  It is just a demarcation line.  The mass within continues to collapse, because there is no force in the universe that can hold it up, until it takes up _zero volume_.   The density, therefore, is infinite - or more properly, it is undefined.  There is no worry about the distribution of mass being uneven, as the mass isn't taking up any space!

The singularity is the point (and I mean that in the literal, mathematical sense) where we divide by zero, and the normal rules of physics no longer apply.

There is a volume of space between the event horizon and the singularity - all paths within the event horizon eventually lead to the singularity.


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## Pbartender (Sep 3, 2010)

Umbran said:


> The density, therefore, is infinite - or more properly, it is undefined.  There is no worry about the distribution of mass being uneven, as the mass isn't taking up any space!
> 
> The singularity is the point (and I mean that in the literal, mathematical sense) where we divide by zero, and the normal rules of physics no longer apply.




Or to look at it from another point of view...

It's perfectly spherical, perfectly dense, and perfectly even in a way that only nothing can be.


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## Umbran (Sep 3, 2010)

Okay, so about that picture.

We have two travellers going near to the event horizon - one is falling straight in, the other is traveling so he skims just over the surface, and the question is how close does the skimmer have to be to see the other fall in.  

Now, we have a problem.  In order to skim by the black hole hole and not fall in, you have to be traveling quickly.  The closer you want to go and still not fall in, the closer to the speed of light you need to go.  To skim just above that surface, you have to be just below the speed of light.  

The skimmer cannot actually be at the surface - that would require him to move at the speed of light, which he cannot, as he has mass.  Only massless things can move that fast.

So, the person falling is at the event horizon, and the skimmer is above it.  This is now no different from the case where one person is in a ship high above, and watching the other fall in.  It doesn't matter how far above the observer is - if the observer isn't also falling in, the same clock-slowing will occur.

The math is kind of wonky that way.  An inch is as good as a yard is as good as an astronomical unit.


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## Umbran (Sep 3, 2010)

Pbartender said:


> It's perfectly spherical, perfectly dense, and perfectly even in a way that only nothing can be.




So long as it is not spinning, anyway.


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## GameDaddy (Sep 3, 2010)

Umbran said:


> So, the person falling is at the event horizon, and the skimmer is above it. This is now no different from the case where one person is in a ship high above, and watching the other fall in. It doesn't matter how far above the observer is - if the observer isn't also falling in, the same clock-slowing will occur.




So Luckless won't actually fall in then, until the end of time? For Luckless, it's just a minute or so, however for visitors nearby, another person can flyby and see him sitting just above the event horizon, and a billion years later, yet another (somewhat evolved hopefully) person can flyby and also see Luckless still easing on into ...that black hole.

And we are not doing SETI searches around Black holes, why? exactly?....

Also... if we are really on our way to the beginning of time instead of the end of time (As people commonly believe), it would mean Luckless is on his way to the beginnings of creation, yes?


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## jonesy (Sep 3, 2010)

GameDaddy said:


> So Luckless won't actually fall in then, until the end of time? For Luckless, it's just a minute or so, however for visitors nearby, another person can flyby and see him sitting just above the event horizon, and a billion years later, yet another (somewhat evolved hopefully) person can flyby and also see Luckless still easing on into ...that black hole.



If we are still assuming indestructible immortals, perhaps. As for the end of time, well, are you talking multiverse theory or just the time in this universe? That stuff is so complicated my head hurts just reading summaries of theories. And then someone throws quantum universes into the mix and my brain starts to bleed.



> And we are not doing SETI searches around Black holes, why? exactly?....



Well, you know how fashion from a couple of decades ago gets a lot of flak from people? Can you just imagine how bad the fashion must have been a million years ago? It's just dreadful. All those neon leggings and shoulder length mullets. 


Here's an x-ray image of Cygnus X-1, a most famous black hole:




How much do you see? And the bright white thing? That's not it. That's the star it orbits.

Here's a better image:




Can you see it now?

Let's zoom in:




Notice what the problem is? 



> Also... if we are really on our way to the beginning of time instead of the end of time (As people commonly believe), it would mean Luckless is on his way to the beginnings of creation, yes?



I have no idea what that means, or why you would say it's commonly believed.


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## Bullgrit (Sep 3, 2010)

Does all this mean that nothing ever actually has fallen into a black hole? No matter how much stuff a black hole has sucked towards it, none of it has yet crossed the (any) event horizon?

Bullgrit


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## jonesy (Sep 3, 2010)

Bullgrit said:


> Does all this mean that nothing ever actually has fallen into a black hole? No matter how much stuff a black hole has sucked towards it, none of it has yet crossed the (any) event horizon?



Nope. But it does mean that there's a lot of stuff near the horizon which has been in a state of "falling" for longer than humans have existed. As an example.


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## Bullgrit (Sep 3, 2010)

OK, wait a minute. Am I not understanding "infinite" correctly?

Bullgrit


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## Umbran (Sep 3, 2010)

GameDaddy said:


> So Luckless won't actually fall in then, until the end of time? For Luckless, it's just a minute or so, however for visitors nearby, another person can flyby and see him sitting just above the event horizon, and a billion years later, yet another (somewhat evolved hopefully) person can flyby and also see Luckless still easing on into ...that black hole.




Sort of.  As Luckless falls down the hole, light gets further and further shifted to the red end of the spectrum - down past red, infrared, microwave, radio, and further.  Eventually (actually, pretty quickly by our standards) it'll be shifted down to the point where we cannot detect it.  



> And we are not doing SETI searches around Black holes, why? exactly?....




Aside from jonsey's excellent demonstration you mean?  

We can add to jonesy, by the way, by noting that small black holes have nasty tidal forces near their event horizons - the fact that your clock ticks slowly is not a comfort as you are turned into spaghetti.  Really big black holes don't have that problem, but they are generally found in the center of galaxies - very busy places, even harder to find small things in them.

If some critter wants to see the end of the Universe, it might crawl down into the gravity well of a black hole to wait it out, sure.  But in order to get a message out as, say, a radio wave, it would have to emit it as gamma rays or other high-energy electromagnetic radiation.

And the clock differential presents a data transfer rate problem.  Their time is very slow compared to ours.  What seems like a screaming high rate of data transmission looks like a crawl to us.  If your entire life goes by in what they see as a fraction of a second, you aren't going to get much data from them.



> Also... if we are really on our way to the beginning of time instead of the end of time (As people commonly believe), it would mean Luckless is on his way to the beginnings of creation, yes?




Being on our way to the beginning of time isn't one of the common theories.



Bullgrit said:


> Does all this mean that nothing ever actually has fallen into a black hole? No matter how much stuff a black hole has sucked towards it, none of it has yet crossed the (any) event horizon?




There are some unanswered questions of quantum effects - in a classical view, we out here would say they never reach the event horizon, Zeno's paradox style.  However, quantum mechanics may provide a way for them to finally pop over the edge when we aren't looking.


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## jonesy (Sep 3, 2010)

Bullgrit said:


> OK, wait a minute. Am I not understanding "infinite" correctly?



Depends on what you're applying it to. And how long it actually takes. And whether all of the matter that the star had would instantly become consumed by the event horizon. And how long it would take for, say, a companion star or planet to get sucked in. And I think Umbran understands that part much better than I do.

The math, it burns. It burns.


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## Bullgrit (Sep 3, 2010)

So it takes an infinite amount of time, assuming a sufficiently small quantity of infinity? 

"To Infinity, and beyond!"
-- Buzz Lightyear

Bullgrit


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## jonesy (Sep 3, 2010)

Bullgrit said:


> So it takes an infinite amount of time, assuming a sufficiently small quantity of infinity?



See? You do understand.


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## Joker (Sep 3, 2010)

Bullgrit said:


> So it takes an infinite amount of time, assuming a sufficiently small quantity of infinity?
> 
> "To Infinity, and beyond!"
> -- Buzz Lightyear
> ...




There's nothing beyond infinity.  Except maybe +1.


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## Shag (Sep 4, 2010)

There is about a 50-50 split in what cosmologists think about spin.  I'm the in belief that you can't actually say whether it spins or not, since I consider the EH out of the universe so it doesn't matter or have any effect one way or another.  But! the matter around an EH is spinning quite fast.  As fast as the core of a supermassive star spins (pulsar speeds). In fact some kinds of what were thought of as pulsars are in fact supermassive galactic black holes (Quasars).  If a black hole takes alot of matter, the matter will be heated until it basically becomes a superheated plasma that gives off massive gamma rays, the black hole will then emit these as gamma ray bursts along its poles.

Its really pretty much as I said in my last post.  Observer will see traveler slow and redshift and never quite touch the EH.  Traveller will see the universe speed up and then as he touches the EH the universe ends.  If traveler is immortal the only escape is the slim possibility that the BH bleeds out first through Hawking radiation.


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## Bullgrit (Sep 6, 2010)

> There are some unanswered questions of quantum effects - in a classical view, we out here would say they never reach the event horizon, Zeno's paradox style. However, quantum mechanics may provide a way for them to finally pop over the edge when we aren't looking.



Not to be snarky, but honestly asking: what is "infinite" in regards to black holes? If matter is falling through a black hole's event horizon, where time is supposedly frozen, how long is it taking the matter to move across the line?

Are the other "infinite" aspects of black holes (density, volume, etc.) also not really infinite?

Bullgrit


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## Someone (Sep 6, 2010)

Bullgrit said:


> Not to be snarky, but honestly asking: what is "infinite" in regards to black holes? If matter is falling through a black hole's event horizon, where time is supposedly frozen, how long is it taking the matter to move across the line?
> 
> Are the other "infinite" aspects of black holes (density, volume, etc.) also not really infinite?
> 
> Bullgrit




The answer probably is some variation of "not yes or no, but something completely opposite of those" Honestly, when you start reading about quantum physics you realize that God was full of peyote when He created the universe.


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## jonesy (Sep 6, 2010)

Bullgrit said:


> Not to be snarky, but honestly asking: what is "infinite" in regards to black holes? If matter is falling through a black hole's event horizon, where time is supposedly frozen, how long is it taking the matter to move across the line?
> 
> Are the other "infinite" aspects of black holes (density, volume, etc.) also not really infinite?



The volume is zero.

The density of the singularity is infinite because it is placed under infinite gravity which is caused by infinite curvature of spacetime. This means that inside the singularity space and time as we know them are effectively removed from the equation.

Every time that you'd move closer to the event horizon time would move slower. The closer you get, the slower you move. This is effectively a variation of Zeno's Paradox. You would need to be travelling at light speed to catch the singularity. The problem with that is that because you have mass your mass increases the closer you get to light speed, and the harder it becomes the closer you get.

The closer you get, the farther it seems.


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## Shag (Sep 6, 2010)

Not sure why people arn't getting this very important point.  You can never cross an event horizon, you can only ever slowly (from an outside observers POV) approach it.  You have to remember that for something travelling at the speed of light there is no time.  For a photon, the beginning and end of its journey are the same time.  
I watch all kinds of shows on black holes and stuff and even the talking head scientists often get this wrong or at least oversimplify what they are saying and therfore give the wrong impression.  There is a reason these guys are on TV and not in the lab.  In talking to you Krauss and Tysen!

Lets go for a PS.  There doesn't have to be infinities involved in any of this. The BH doesn't have to be infinetly dense just so dense its easier to say infinit.  Time isn't infinit, its just until the end of the existence of this Brane, or until the BH bleeds out.


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## Umbran (Sep 6, 2010)

Shag said:


> There is about a 50-50 split in what cosmologists think about spin.  I'm the in belief that you can't actually say whether it spins or not, since I consider the EH out of the universe so it doesn't matter or have any effect one way or another.




You should be able to actually say: if it has significant spin, the event horizon will not be perfectly spherical.  Like planets, it should be be slightly "oblate" - a little pressed in at the poles of spin and fatter at the equator.  At least so says the math.



Bullgrit said:


> Not to be snarky, but honestly asking: what is "infinite" in regards to black holes?




The density.  The asymptotic slowing of clocks as you approach the event horizon.



> If matter is falling through a black hole's event horizon, where time is supposedly frozen, how long is it taking the matter to move across the line?




Classically speaking: If you're out here in the rest of the fairly normal universe, it takes "forever".  If you're the one falling, it takes as long as it would take you to fall, like you were falling anywhere else.  

Quantum mechanically speaking: As above, but there might be a chance for you to "tunnel" through the event horizon at some point.  The chance of you tunneling through should increase the closer you are to the horizon - and there's never certainty that you will pop across the barrier.  So, maybe forever, maybe some absurdly long time. 



> Are the other "infinite" aspects of black holes (density, volume, etc.) also not really infinite?




Given that we've only ever observed these things at great distances, and don't have an adequate theory of how gravity and quantum mechanics work together, we cannot say with absolute surety.  We can only speak by way of current understanding.  



Shag said:


> The BH doesn't have to be infinetly dense just so dense its easier to say infinit.




Actually, it does.  If it isn't infinitely dense, if there is some force that does keep it from complete and utter collapse, you don't get an event horizon.  Existence of such a force would imply the existence of something that can escape the grip of the object's gravity, and the hole is no longer "black".



> Time isn't infinit, its just until the end of the existence of this Brane, or until the BH bleeds out.




This is why we often say, "until the end of time" - whatever end that might be.  Mind you, brane theory is not tested (and so far is untestable) and not widely accepted - and depending what version you're talking about, our universe and our brane are not equivalent.


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## Bullgrit (Sep 6, 2010)

So, crossing the event horizon will never happen, but it happens all the time. 

Nothing can escape a black hole, but radiation is put off by it all the time.

I'm beginning to think black holes are just imaginary constructs scientists use to confuse the lesser minds. They're just a big inside joke among you smarty pants. ;-)

Bullgrit


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## Umbran (Sep 6, 2010)

Bullgrit said:


> So, crossing the event horizon will never happen, but it happens all the time.




Generally speaking, we talk of things falling into a black hole, which is a protracted event.  We only occasionally talk about things actually crossing the event horizon.



> Nothing can escape a black hole, but radiation is put off by it all the time.




Matter that's falling in to a black hole tends to form an "accretion disk" - a whole bunch of stuff swirling around on its way down.  As this dust and gas collects, it compresses and heats up, and emits radiation.  This is the in-falling gas and dust, not the black hole itself.

The phenomenon of Hawking Radiation is also not actually the black hole radiating anything, so much as radiation from just outside the hole failing to fall in.

The black hole may radiate gravity waves if it is orbiting a star or some other large object.



> I'm beginning to think black holes are just imaginary constructs scientists use to confuse the lesser minds. They're just a big inside joke among you smarty pants.




No.  They just happen to be some of the more extreme objects out there.  If you want everything to be simple and easily comprehensible to a brain developed for hunting and gathering... well, that's just too bad.


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## jonesy (Sep 6, 2010)

Bullgrit said:


> I'm beginning to think black holes are just imaginary constructs...



That's pretty much what theoretical science is. The pursuit to discover a perfect model of how reality behaves.

The pursuit might not be realistically possible because of our limited perspective within reality itself, but even a slightly perfect model would be immensely useful.

Black holes are a good example of the attempt to make different fields of science connect with each other. If it would be possible to form a single model for black holes which works in both classical physics and quantum physics it would advance both branches by great leaps.


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## El Mahdi (Sep 6, 2010)

Bullgrit said:


> ...I'm beginning to think black holes are just imaginary constructs scientists use to confuse the lesser minds. They're just a big inside joke among you smarty pants...




If it's any consolation, Einstein would have probably agreed with you. He didn't believe that singularities were possible, and even wrote a paper attempting to prove this. I don't know if he ever changed his mind about this before his death, but there were a lot of aspects of quantum physics, including black holes, that he really didn't like.


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## Umbran (Sep 6, 2010)

El Mahdi said:


> If it's any consolation, Einstein would have probably agreed with you. He didn't believe that singularities were possible, and even wrote a paper attempting to prove this. I don't know if he ever changed his mind about this before his death




Not publicly, he didn't.  But he died in 1955, before many of the major bits of work on them were completed.  The very term "black hole" is post-Einstein.



> but there were a lot of aspects of quantum physics, including black holes, that he really didn't like.




Yes, well, the existence of black holes owes more to Einstein's own General Relativity than to quantum mechanics.  In essence, all QM does is fail to provide forces sufficient to prevent gravitational collapse.  The singularity that Einstein didn't like is in his own equations of General Relativity, not in QM.


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## Mustrum_Ridcully (Sep 7, 2010)

I was confused for a moment about this notion that an object cannot cross an event horizion. It absolutely can do that. What doesn't happen is that we actually observe this happening. All signals informing us of the events are slowed down and delayed so much we will never see them.


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## Umbran (Sep 7, 2010)

Mustrum_Ridcully said:


> I was confused for a moment about this notion that an object cannot cross an event horizion. It absolutely can do that. What doesn't happen is that we actually observe this happening. All signals informing us of the events are slowed down and delayed so much we will never see them.




It isn't just the signals are delayed.  The actual event is delayed.  As far as the universe far away from the hole is concerned, the clocks down near the event horizon tick so slowly that things never get around to crossing.

As I noted up thread, (as far as classical physics goes) if you could attach a rope to someone, and push them in, let them sit down there for 1000 years of our time (or whatever arbitrary length of time you wish), and haul them back up.  They will have aged less than the rest of the universe you know.  If you can haul them back up, they obviously didn't cross the event horizon yet.


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## jonesy (Sep 7, 2010)

If only we had a supply of indestructible immortals to throw at black holes. They could hold hands and form a chain into it. 

Richard Gott calculated that it only takes a tenth of a second for your avarage human to go from 'I don't feel too good' to 'shredded to atoms' when he closes with a black hole, regardless of how big the black hole in question is.

If you drop someone into a black hole with the mass of the Sun it would take, from his perspective, 2 months to fall all the way in.

The interesting part is that that 2 months would, from our perspective, be so long that it's hard to know whether the universe would have enough time left for that to actually happen.

So it's not about whether it's possible, it's about whether there's enough time for it.

Obviously the matter that a star had before it became a black hole did enter the event horizon. The event itself was matter crushing itself into a tiny space.


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## Umbran (Sep 7, 2010)

jonesy said:


> Richard Gott calculated that it only takes a tenth of a second for your avarage human to go from 'I don't feel too good' to 'shredded to atoms' when he closes with a black hole, regardless of how big the black hole in question is.




Supermassive black holes don't have strong tidal effects near their event horizons - they don't shred you as you approach.  Once you are across the horizon, that's another matter.



> Obviously the matter that a star had before it became a black hole did enter the event horizon. The event itself was matter crushing itself into a tiny space.




Technically, no - the event horizon doesn't exist before the collapse.  It only forms once sufficient mass is crushed into a small enough space.  You could imagine the object falling in on itself, it's normal surface shrinking down, light coming off it red-shifting as it goes, until it is then shrouded in a cloak of blackness.


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## jonesy (Sep 7, 2010)

Umbran said:


> Supermassive black holes don't have strong tidal effects near their event horizons - they don't shred you as you approach.  Once you are across the horizon, that's another matter.



For supermassives the killlzone is inside the event horizon, yes. But if you could get there the same effect would apply. As far as gravity affecting you is concerned, you'd be fine all the way up to the crush and then you'd go pop.



> Technically, no - the event horizon doesn't exist before the collapse.  It only forms once sufficient mass is crushed into a small enough space.  You could imagine the object falling in on itself, it's normal surface shrinking down, light coming off it red-shifting as it goes, until it is then shrouded in a cloak of blackness.



Does that mean that when a black hole is formed all of the matter that the star has instantly becomes the singularity? There won't be any, umm, leftovers to briefly hover above it?


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## Umbran (Sep 7, 2010)

jonesy said:


> For supermassives the killlzone is inside the event horizon, yes. But if you could get there the same effect would apply. As far as gravity affecting you is concerned, you'd be fine all the way up to the crush and then you'd go pop.




So long at the thing is not rotating, yes.  Rotating black holes, if they exist and follow the math, may provide an out, a way to avoid the singularity.  But then we start talking about time-travel, and things get very messy. 



> There won't be any, umm, leftovers to briefly hover above it?




Briefly, from whose point of view?  Anything left outside the event horizon will be subject to the same time dilation effects as someone dropped in after the hole forms.  From out point of view far from the hole, the hovering may be anything but brief.


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## Shag (Sep 7, 2010)

I'm gunna leave this thread because an M-theory war might break out and I don't want any trouble!


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## jonesy (Sep 7, 2010)

Shag said:


> I'm gunna leave this thread because an M-theory war might break out and I don't want any trouble!



"I had retired to the Martian Empire when the man and the dog walked into my bar and ordered JP5 with a twist. They looked a mysterious duality. He winked and drew a laser. I dodged and the wall behind me burned to a hole. Which was interesting. It would be called the first day of the M-Theory Wars and everyone wanted me dead. So began my quest to defeat.. the Murky Rangers."


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## GameDaddy (Sep 8, 2010)

Mustrum_Ridcully said:


> I was confused for a moment about this notion that an object cannot cross an event horizion. It absolutely can do that. What doesn't happen is that we actually observe this happening. All signals informing us of the events are slowed down and delayed so much we will never see them.




I'm not certain about this, just yet. If someone crossed the event horizon eons ago, we should be able to observe that. 

Matter of fact, we should be able to see it all the time, especially with the older black holes, if only that background radiation generated by the accretion disk, and all the rest of the junk that manages an orbit that won't bring it across the event horizon, wasn't obstructing our view.

We should be able to see into the past clearly enough, we certainly do in all other regards...


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## freyar (Sep 8, 2010)

It seems like maybe some people are confusing the time measured by the person falling into the black hole and the time measured by the observers who manage to stay outside.  You might find the Penrose diagrams to be helpful.  The idea of a Penrose diagram is that light rays make 45-degree lines (so massive objects follow trajectories that are always more vertical than 45 degrees), and the ones we'll look at have the angular directions suppressed, just showing radius from some center and time.  As an example, look at the diagram for Minkowski spacetime -- the normal flat spacetime of special relativity with trivial gravity -- it's at the top of the page. You'll notice that the sides of the diamond are "lightlike infinity" (all light rays end up on these lines), "spacelike infinity" at the two left-right points of the diamond (where you are if you are an infinite distance away from the center at a fixed finite time), and "timelike infinity" at the top-bottom points (where you are when you're infinitely far into the future or past but at a finite distance from the "center").

Now look at the second diagram and check out the example for the basic black hole (labeled Static "Grey" Wormhole in the diagram) in the second diagram.  The person falling into the black hole passes the event horizon and hits the timelike singularity in a finite amount of time according to their watch.  However, if they're sending out light pulses as they fall in, you can trace out those 45-degree lines.  You'll see that no one can see the last pulse emitted along the horizon unless they're at timelike infinity -- one of the points at the corner of the timelike singularity adjoining the "our universe" diamond.  The Real Black Hole is also relevant; the star is collapsing, and eventually it forms an event horizon -- we observers only actually see the event horizon swallowing the star at an infinite time in the future.

A note on the Electrically Charged And/Or Rotating Wormhole: the wormhole tends to collapse if you put any actual mass through it, so you can't really get through to the other universes.


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## Umbran (Sep 8, 2010)

GameDaddy said:


> Matter of fact, we should be able to see it all the time, especially with the older black holes, if only that background radiation generated by the accretion disk, and all the rest of the junk that manages an orbit that won't bring it across the event horizon, wasn't obstructing our view.




Well, that, and the fact that the nearest known/suspected black hole is 1600 light years or so from Earth, and we don't have the technology to make out anything like surface detail at that range, even if nothing were in the way.


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## GameDaddy (Sep 9, 2010)

Umbran said:


> Well, that, and the fact that the nearest known/suspected black hole is 1600 light years or so from Earth, and we don't have the technology to make out anything like surface detail at that range, even if nothing were in the way.




We have other things more relevant at the moment that requires our attentions... I noticed with interest a report on CNN this morning, about two large rocks that flew by earth that were only spotted three days ago on Sunday... Cutting it kind of close, ey?

Reference: In One Day, (Yesterday now...) Two Asteroids buzz Earth.
In one day, two asteroids buzz the Earth - CNN.com

I went ahead and took a look at the Catalina Sky Survey, Which is now finding more than one rock/Iceball a day that makes a rather close transit bwteen the Moon and Earth.

Better yet, On another blog, I came across this video... All the orbits of all the rocks detected since about 1980 or so.. If you can, view the video at high-res at 1080 px...

These are just the ones that have been spotted with their orbits charted, it's not counting the eccentric orbits and the deep orbits that only get close to us infrequently... Also not on this video, The rogue rocks that are coming in from other star systems...

[ame="http://www.youtube.com/watch?v=S_d-gs0WoUw&feature=player_embedded"]YouTube - Asteroid Discovery From 1980 - 2010[/ame]

We are going to get tagged hard long before anyone has a chance to spot something crossing an event horizon of a black hole...


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## jonesy (Sep 9, 2010)

There's talk of meteor death in the other thread:
http://www.enworld.org/forum/media-lounge-off-topic/287979-how-far-we-colonizing-off-earth.html



GameDaddy said:


> We are going to get tagged hard long before anyone has a chance to spot something crossing an event horizon of a black hole...



We get tagged on avarage 50,000 times every year. But it's all small stuff. The Sun and Jupiter act as pretty efficient vacuum cleaners.


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## Joshua Randall (Sep 17, 2010)

Minor thread necromancy. (The body isn't that cold yet.)

Assuming you had a spaceship with superpowerful engines made of superstrong material, could you "hover" outside the event horizon, like in the classic Disney movie _The Black Hole_?

And would you see/detect anything interesting at the accretion disk? (I know you can't see the hole itself, by definition.)


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## Umbran (Sep 17, 2010)

Joshua Randall said:


> Assuming you had a spaceship with superpowerful engines made of superstrong material, could you "hover" outside the event horizon, like in the classic Disney movie _The Black Hole_?




In short - Yes.  At least until you ran out of fuel.  

For smaller black holes, there's a point where no matter how strong the material of the ship might be, your body wouldn't take the tidal stresses. See the Larry Niven short story "Neutron Star" for a description.



> And would you see/detect anything interesting at the accretion disk?




Well, you'd see material undergoing compression heating possibly to the point it gives off X-rays.  You'd see dynamics of an accretion disk, which we've never seen up close before.  

But really wild and wacky stuff?  I can't think of any offhand.


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## jonesy (Oct 17, 2010)

Spam post reported.

Also, this just in:
Astronomy.com - Astronomer leverages supercomputers to study black holes and galaxies

Those gas density maps look really interesting.


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## freyar (Oct 17, 2010)

Massive numerical simulations like that are really becoming an important part of mainstream astrophysics, especially when it comes to understanding the structure and formation of galaxies.


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## jonesy (Nov 17, 2010)

freyar said:


> Massive numerical simulations like that are really becoming an important part of mainstream astrophysics, especially when it comes to understanding the structure and formation of galaxies.



Speaking of which:
HubbleSite - Detailed Dark Matter Map Yields Clues to Galaxy Cluster Growth

Jet Propulsion Lab using the Hubble telescope have been making a map of dark matter. Well, more like a map of mass in space. They also think they've figured out gravitational lensing.

The images look really intersting:
HubbleSite - Detailed Dark Matter Map - Images


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## freyar (Nov 17, 2010)

jonesy said:


> Speaking of which:
> HubbleSite - Detailed Dark Matter Map Yields Clues to Galaxy Cluster Growth
> 
> Jet Propulsion Lab using the Hubble telescope have been making a map of dark matter. Well, more like a map of mass in space. They also think they've figured out gravitational lensing.
> ...



The theory of gravitational lensing in general relativity is quite well understood in that, if you tell me where the mass is, I can tell you how it lenses light (well, maybe not _me_, but you get the point).  The so-called inverse problem is trickier: we see how light is lensed and want to use that to figure where the mass is.  People have been making maps like this for years now, but this team has found an improved way to solve this inverse problem.  In any case, it seems like the results may be interesting for comparison to simulations, as well.


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