Black holes -> why are they black?

zenith · Oct 20, 2004

Basically.. how can a black hole "suck in" light, if light is massless? and how is it that the light cannot escape also?

doogsy · Oct 20, 2004

yes, i've got a similar question Black holes do not give off light as their escape velocity is greater than the speed of light. (correct?) But if light always travels at 3x10^8 ms then gravity should have no effect on light 'escaping'. Im lead to beleive light does has mass, a very small amount (thats how solar sails work?), but then how do photons travel at the speed of light if no mass can??? Maybe I should have started my own thread <img src="/images/icons/blush.gif" />

zenith · Oct 20, 2004

well photons can travel at the speed of light BECAUSE they are massless... which brings us right back to my question.. if they are massless, then how can gravity effect them? or is it that black holes also give out EM radiation, which "attracts" the photons?.. but then that would mean that the photons have a charge?

CalliArcale · Oct 20, 2004

The black hole itself is not really a hole; it's just a fantastically dense object. In fact, it is so fantastically dense, concentrating all of its considerable mass in an unbelievably tiny space, that it warps the fabric of space. Actually, everything warps the fabric of space, but only a black hole warps it so much that light cannot escape. Basically, light travels in a straight line, but if space is curved, the light will follow that curve. A black hole curves space so severely that even light's trajectory will be bent back on itself. Einstein predicted this effect, though he did not postulate the black hole itself. (Roger Penrose postulated black holes as an extension of Einstein's thinking, which were later found to actually exist.) The effect of light being bent as it travels near a massive object is called gravitational lensing. It has been observed around the Sun -- background stars appear from behind it slightly sooner than they ought to, because their light is being bent around the Sun. It was not observed until after Einstein's death, but gravitational lensing is real and has allowed astronomers to see very distant galaxies much better than they could using only Earth-based telescopes. Basically, a gigantic galaxy cluster acts as a lens, allowing astronomers to see very distant galaxies beyond it. There's no way to aim this lens, of course, so it's pot luck what they can see with it. But here's an example of a gravity lens: Abell 2218: A Galaxy Cluster Lens. Light is bent by gravity; this has been observationally proven. So, logically, if you bend light far enough, you should be able to trap it. And that's what a black hole does. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

zenith · Oct 21, 2004

ahh i seee now, very good.. but does a black hole warp the space in every direction around it? If so, what would the space lines look like around the hole?

Saiph · Oct 21, 2004

yep, all the directions. Imagine a string heading out from the singularity, turning around at the event horizon, and heading back the way it came. Okay, now, make that 3d. They come from all angles, and actually make a 3d volume (yes, that's a volume that loops back on itself...it's wierd but we're working with more than 3 dimensions here...) <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

zenith · Oct 21, 2004

intruiging... just how dense is the super dense matter?

Saiph · Oct 21, 2004

in a black hole? Beyond meaningful description. Physics knows its really, really dense, but is unable to fully describe the state due to current limitations in quantum mechanics and general relativity. Basically, before it breaks down you end up with the singularity being smaller than an atomic nucleus, and a mass typically greater than the sun (often billions of times greater). This state is often refereed to as having an infinite density, since physics actually predicts it to take up zero volume. But as I said, physics can't quite deal with the values near the singularity with any real accuracy. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

zenith · Oct 22, 2004

&%$#@!e.. now that is dense... that is almost as dense as some of my friends..

Saiph · Oct 23, 2004

Well, You're completely right to be confused since: They do gain mass if they absorb other material. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

newtonian · Oct 24, 2004

Calli & Saiph - Excellent answers, sorry I was too busy too post for a while. Saiph - I would question the zero volume infinite density model. While I do agree the volume could have a diameter less than planck length, I do not agree that it could be zero. Unless we are indeed dealing with more dimensions than we are currently familiar with, so the 3D dimensions could be a point, or singularity, whereas other dimensions could be above zero intersecting that point.

Saiph · Oct 24, 2004

well, for a non-spinning, non-charged, uniformly created BH...current physics doesn't give us another choice. If it's spinning, we get a disk shaped singularity. If its charged, you get something else. And if it's both, and the creation wasn't completely smooth (basically a given in reality) then you get a chaotic singularity. A roiling, twisted mass of stuff and spacetime. Small, but not infinitely so. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

newtonian · Oct 24, 2004

Saiph - Thanks for your spin on this! Well, according to ice skaters laws of motion, arms in, wouldn't there be an incredibly fast spin approaching the speed of light? At least in the faster spinning black holes? And are the singularity's magnetic field lines also prevented from escaping the event horizon? Or, alternately, do we observe different magnetic fields coming from black holes? And what of other dimensions? Is it possible that other dimensions we do not observe because of our large (macroscopic) observations play a much more important role in the very small location of a black holes mass? I have reference not only to String theory and theories involving collisions of branes, etc., but also to the simple concept of other dimensions intersecting our 3-D planck length locations at intersecting points of nearly zero 3-D dimensions.

CalliArcale · Oct 26, 2004

<blockquote>In reply to:<hr />Well, according to ice skaters laws of motion, arms in, wouldn't there be an incredibly fast spin approaching the speed of light? At least in the faster spinning black holes? <hr /></blockquote> Quite probably. Pulsars, which are stellar remnants a bit too small to have become black holes, spin extraordinarily fast. An object the size of Earth (but more massive than our Sun) can go around in less than a milisecond in some cases! So black holes might be spinning very fast indeed. I wonder what effect frame-dragging would have in that situation? <blockquote>In reply to:<hr />And are the singularity's magnetic field lines also prevented from escaping the event horizon?<hr /></blockquote> Totally wild guess -- I don' tthink so. I think the magnetic field, if it's still being generated, should extend beyond the event horizon. Of course, it should get incredibly tangled, and it's quite possible that a black hole is so dense that it cannot generate a magnetic field anyway. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

Saiph · Oct 27, 2004

yes, spinning black holes spin fast, at ~.5c. Faster than that, and they rapidly lose their angular momentum to their surroundings, IIRC. How do they lose it? Their electric and magnetic fields do extend beyond the EH, and cause the spinning BH to act as the spinning dynamo in a magnet. They can induce currents and motion in particles near the BH. you also have frame dragging (dragging space time around with the BH) which slows it down as well. Calli: Rule of thumb, the denser the material, the stronger a field it can generate. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

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