Notices
Results 1 to 75 of 75
Like Tree17Likes
  • 2 Post By Strange
  • 3 Post By MagiMaster
  • 1 Post By Markus Hanke
  • 4 Post By Strange
  • 1 Post By Flick Montana
  • 2 Post By Howard Roark
  • 1 Post By Strange
  • 1 Post By uptonryan
  • 1 Post By Markus Hanke
  • 1 Post By Markus Hanke

Thread: Black holes and dark matter

  1. #1 Black holes and dark matter 
    Suspended
    Join Date
    Oct 2013
    Posts
    794
    im not sure if this is the right section to post this... im totally noob on this... but how should I see an black hole... how should I look at it??? and about the dark matter (none knows what it is...) but they say because the light bends... its there... but aint so many things why the light bends... ? especialy small space ? how could this dark matter in best way for a leak and a black hole?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Life-Size Nanoputian Flick Montana's Avatar
    Join Date
    Jul 2012
    Location
    Flatland
    Posts
    5,437
    I am under the impression that dark matter is postulated because there is a requirement for a large amount of invisible mass which contributes to natural forces and allows for our known physical laws. I'm not aware of it having an interaction with photons. I always thought it was more of a requirement of gravitation and mass.

    As for black holes, I'm not sure what you mean by "how should I look at it?" I could asinine and suggest that it doesn't matter because it will just be black...


    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
    Reply With Quote  
     

  4. #3  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by blackscorp View Post
    im not sure if this is the right section to post this... im totally noob on this... but how should I see an black hole... how should I look at it???
    You'll want a very large telescope: New Telescope to Take First-Ever Black Hole Photo | Black Holes & Event Horizon Telescope | Einstein & Theory of General Relativity | Space.com

    and about the dark matter (none knows what it is...) but they say because the light bends... its there... but aint so many things why the light bends... ? especialy small space ? how could this dark matter in best way for a leak and a black hole?
    Dark matter is currently only known through its gravitational effects. It was first proposed to explain the velocities stars in galaxies, and galaxies in clusters (of galaxies). Given the apparent distribution of matter in a galaxy, you would expect velocities to decrease with increasing radius. However, it was found that the velocities of the outer stars (and gas and dust) was roughly constant. This requires there be more mass in the galaxy that we can see (and that it has a particular distribution: denser towards the middle).

    Similar observations apply to galaxy clusters.

    The effects of dark matter can also be seen through gravitational lensing. In particular, "micro-lensing" where it causes small distortions of the background. This has allowed the large scale distribution and density of dark matter to be mapped.

    It is still possible that dark matter is not actually matter (e.g. there is some unknown aspect of gravity) but the lensing thing makes it a lot more likely that it is "stuff". There are many hypotheses for what it is that are currently being tested.

    (Note: despite the "black" and "dark" names, there is no connection between them.)
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  5. #4  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,231
    Just for further clarification: even though dark matter only interacts with normal matter gravitationally, it can still affect the path light follows, since gravity is the curvature of space-time, according to general relativity. Black holes can cause distortions of light for the same reasons.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  6. #5  
    Suspended
    Join Date
    Oct 2013
    Posts
    794
    I just do not understand how I need to look at black holes... (don't forget im noob on that) is it a hole?
    Reply With Quote  
     

  7. #6  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by blackscorp View Post
    I just do not understand how I need to look at black holes... (don't forget im noob on that) is it a hole?
    No, you can consider at as a body so massive that not even light can escape its gravitational influence.
    Reply With Quote  
     

  8. #7  
    Suspended
    Join Date
    Oct 2013
    Posts
    794
    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by blackscorp View Post
    I just do not understand how I need to look at black holes... (don't forget im noob on that) is it a hole?
    No, you can consider at as a body so massive that not even light can escape its gravitational influence.
    so its thin like paper with no body on the down side???
    Reply With Quote  
     

  9. #8  
    Life-Size Nanoputian Flick Montana's Avatar
    Join Date
    Jul 2012
    Location
    Flatland
    Posts
    5,437
    I'm not sure what that means.

    A black hole would be spherical. It would appear flat to the naked eye from all sides, but the central body would be spherical.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
    Reply With Quote  
     

  10. #9  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by blackscorp View Post
    so its thin like paper with no body on the down side???
    No, you can picture it as a spherical body with a well defined radius, called the Schwarzschild radius. Just bear in mind that this is not a "normal" surface, such as the surface of a planet.
    Reply With Quote  
     

  11. #10  
    Suspended
    Join Date
    Oct 2013
    Posts
    794
    its a hole right??? so then it has to have somebuddy loking from behind right???
    Reply With Quote  
     

  12. #11  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by blackscorp View Post
    its a hole right??? so then it has to have somebuddy loking from behind right???
    "Black Hole" is just a name, because it would visually resemble a hole in space. It is not actually a hole in the sense of anything going "through" it, and out the other side.
    Reply With Quote  
     

  13. #12  
    Life-Size Nanoputian Flick Montana's Avatar
    Join Date
    Jul 2012
    Location
    Flatland
    Posts
    5,437
    No, it's not a hole. There is a supermassive body at the center. If you get into deeper physics and alternate dimensions or universes within black holes you get way outside my understanding of them.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
    Reply With Quote  
     

  14. #13  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by blackscorp View Post
    its a hole right?
    No. "Black hole" is just a name. It doesn't really mean anything. Like "big bang" or "dark matter", they are just vaguely descriptive.

    Let's call it something else: Fred.

    A Fred is a mass that has collapsed to the point that its gravity means that nothing, not even light or information, can escape from it. This means it has an "event horizon", which is like a one-way information barrier, around it. The event horizon is a sphere with radius proportional to the Fred's mass (the Schwarzschild radius referred to earlier). Things can fall in through the event horizon but nothing can come back out. The event horizon would therefore appear black.

    However, because of the intense gravity, the Fred would bend light around it, which means it would distort the pattern of stars behind it (see link in my earlier post).

    Also, because of its powerful gravity, it is likely to have a rapidly spinning disk of material around it which is gradually falling in. This will generate lots of heat and radiation. This is what we see as various forms of "active galaxies" (quasars, blazars, etc.)
    Markus Hanke and adelady like this.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  15. #14  
    Suspended
    Join Date
    Oct 2013
    Posts
    794
    Quote Originally Posted by Flick Montana View Post
    No, it's not a hole. There is a supermassive body at the center. If you get into deeper physics and alternate dimensions or universes within black holes you get way outside my understanding of them.
    that's exactly what makes me think how should I look at black hole... if it aint hole... than why can we "travel" to other dimensions (or like some say teleport) to other galaxys
    Reply With Quote  
     

  16. #15  
    Life-Size Nanoputian Flick Montana's Avatar
    Join Date
    Jul 2012
    Location
    Flatland
    Posts
    5,437
    That's Stephen Hawking's realm of interest. I'd suggest looking up some of his videos or interviews. He's probably a lot better at explaining it than anyone else. He's pretty good at speaking down to us mortals and helping us understand.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
    Reply With Quote  
     

  17. #16  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Any real black hole is likely to be rotating and therefore a Kerr black hole (rather than the simplified Schwarzschild model)

    Kerr black hole
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  18. #17  
    Forum Freshman
    Join Date
    Oct 2013
    Posts
    6
    Hi blackscorp! Check out Nassim Harramein and the Resonance Project.com as he "other" definitions of a "Black Hole" which are more logical than anything physicist's have come up with to date.

    Yeah, I already "know"...nobody likes him BUT, "I DO!"
    Reply With Quote  
     

  19. #18  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    Logical - You keep using that word. I do not think it means what you think it means.
    Strange, adelady and DogLady like this.
    Reply With Quote  
     

  20. #19  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by blackscorp View Post
    that's exactly what makes me think how should I look at black hole... if it aint hole... than why can we "travel" to other dimensions (or like some say teleport) to other galaxys
    Are you thinking of black holes, or wormholes ? These are not the same things. A black hole does not allow you to travel anywhere, since it isn't really a "hole"; a wormhole on the other hand could in principle allow such travel, but there are a number of problems associated with this.
    Reply With Quote  
     

  21. #20  
    Forum Freshman
    Join Date
    Oct 2013
    Posts
    6
    NOPE! Have you checked out Nassim Harramein? So-called, "Black Holes" are DEAD ENDS to stop anyone from discovering the truth about them.

    Just as "Sun Spots" are said to be, "...on the surface of the Sun..." when in reality they are tunnels penetrating straight-through the center of the Sun as Nassim Harramein has discovered. AND, if you look at the SOHO images of the Sun, it's clear to see he's...CORRECT!

    But, that's "Okay!" You probably believe that "...nothing can go faster than the speed of light...", also...huh!
    Reply With Quote  
     

  22. #21  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by bobby90247 View Post
    NOPE! Have you checked out Nassim Harramein? So-called, "Black Holes" are DEAD ENDS to stop anyone from discovering the truth about them.

    Just as "Sun Spots" are said to be, "...on the surface of the Sun..." when in reality they are tunnels penetrating straight-through the center of the Sun as Nassim Harramein has discovered. AND, if you look at the SOHO images of the Sun, it's clear to see he's...CORRECT!

    But, that's "Okay!" You probably believe that "...nothing can go faster than the speed of light...", also...huh!
    MODERATOR NOTE : Stop spamming this discussion with pseudo-scientific nonsense; this is the main Physics section. If you have a personal theory of your own to present, then please open a separate thread in the relevant section.
    Howard Roark likes this.
    Reply With Quote  
     

  23. #22  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by bobby90247 View Post
    Yeah, I already "know"...nobody likes him BUT, "I DO!"
    You shouldn't let the fact you like him get in the way of your critical thinking skills. You should be more open minded and learn some science.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  24. #23  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by bobby90247 View Post
    But, that's "Okay!" You probably believe that "...nothing can go faster than the speed of light...", also...huh!
    Just because you believe a lot of crap doesn't mean you should assume everyone else is incapable of learning.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  25. #24  
    Life-Size Nanoputian Flick Montana's Avatar
    Join Date
    Jul 2012
    Location
    Flatland
    Posts
    5,437
    It's so infuriating that half of the time that could be allocated to teaching someone about the marvels of our universe has to instead be spent trying to direct them away from this kind of intellectual pollution.
    Markus Hanke likes this.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
    Reply With Quote  
     

  26. #25  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Black holes can be found in several ways. The first is to plot the orbits of the stars and other matter around the black hole. I think this is how we found the supermassive black hole at the centre of the milky way. There was a hole bunch of stars orbiting a point in space at rapid speed that could only be explained by a huge point mass. Another way is to look at the lensing effect. Black holes can bend light from behind it so from the way the light is bent you can calculate the mass of the object. If it is a point and very heavy then it is a black hole. Another way is if it passes in front of a sun you can see the black sphere in front of the sun.
    Reply With Quote  
     

  27. #26  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by uptonryan View Post
    Another way is to look at the lensing effect. Black holes can bend light from behind it so from the way the light is bent you can calculate the mass of the object. If it is a point and very heavy then it is a black hole. Another way is if it passes in front of a sun you can see the black sphere in front of the sun.
    Although these are both possible in principle, neither have been done yet (as far as I know).
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  28. #27  
    Suspended
    Join Date
    Feb 2013
    Posts
    1,774
    Quote Originally Posted by bobby90247 View Post
    NOPE! Have you checked out Nassim Harramein?
    Yes, I actually know him personally. Here.
    Reply With Quote  
     

  29. #28  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    I have seen articles on all methods. I will try and find them sometime later. Another method is to look for Hawking radiation. Apparently Stephen Hawking's theorised that black holes would produce a type of radiation. This arises because on the edge of the event horizon there are particles and anti particles popping into existence. One half is sucked into the black hole while the other radiates outward giving the black hole a less than black appearance.
    Reply With Quote  
     

  30. #29  
    Forum Senior pineapples's Avatar
    Join Date
    Apr 2011
    Location
    Ireland someplace
    Posts
    360
    I’ve a question which should expose my basic misconceptions about black holes (and probably physics in general). In other words, I have a stupid question!

    Is it the singularity at the centre of the black hole that is causing the force of gravity, or is it the force of gravity that is causing the singularity? Almost like the chicken and the egg question I suppose.
    Reply With Quote  
     

  31. #30  
    Suspended
    Join Date
    Feb 2013
    Posts
    1,774
    Quote Originally Posted by pineapples View Post
    I’ve a question which should expose my basic misconceptions about black holes (and probably physics in general). In other words, I have a stupid question!

    Is it the singularity at the centre of the black hole that is causing the force of gravity, or is it the force of gravity that is causing the singularity? Almost like the chicken and the egg question I suppose.
    neither, the singularity is a mathematical artifact
    pineapples and Bad Robot like this.
    Reply With Quote  
     

  32. #31  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    It is the mass of the black hole which causes the black hole. Currently the only theory we have says that the mass will collapse to a point of infinite density (a "singularity"). I don't think many people think the singularity has a physical existence; it is more likely to be an indication that our theory no longer works under those (extreme) conditions.

    Edit: xyzt put that so much more succinctly...
    pineapples likes this.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  33. #32  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    No such think as a stupid question Black holes are where the mathematics breaks down. No one really knows what happens there. There are several parts to a black hole. If we take the supermassive black hole at the centre of the milky way. There is normal space. Then there are objects orbiting the black hole. As you travel closer the orbits can get rather hot and dangerous. If the black hole is feeding then things can get violent. However the Milky Way Black hole is dormant and there is a dead space around it. You will then be spaghettified as the gravitational tidal forces on your feet will be far greater than your arms. The black hole will then wrap you around itself thinner and thinner. Some of you will be ejected as light speed jets at the top and bottom of the black hole. If you travel closer still you will cross the event horizon. You won't notice that but its the point at which nothing can escape, not even light. The rest will be crushed to a singularity. A singularity is a very small point. At the singularity the force of gravity is stronger than the nuclear forces of atoms. So even atoms are crushed. It may be that this punches a hole in space time or that space time changes phase to a different state. Sort of like water changes into steam. It may be that quantum effects, normally reserved for the very small suddenly exhibit macro world effects. The arrow of time may not point in all the same directions etc.
    pineapples likes this.
    Reply With Quote  
     

  34. #33  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by uptonryan View Post
    Black holes are where the mathematics breaks down. No one really knows what happens there.
    The mathematics does not break down. They are very well described by the appropriate solutions to the Einstein Field Equations (Schwarzschild, Kerr, Reissner–Nordström, Painlevé-Gullstrand, etc).
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  35. #34  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    The mathematics does not break down.
    have you been there?
    Reply With Quote  
     

  36. #35  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by uptonryan View Post
    The mathematics does not break down.
    have you been there?
    How is that relevant? Does that entitle you to make incorrect statements?
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  37. #36  
    Forum Senior pineapples's Avatar
    Join Date
    Apr 2011
    Location
    Ireland someplace
    Posts
    360
    Quote Originally Posted by xyzt View Post
    Quote Originally Posted by pineapples View Post
    I’ve a question which should expose my basic misconceptions about black holes (and probably physics in general). In other words, I have a stupid question!

    Is it the singularity at the centre of the black hole that is causing the force of gravity, or is it the force of gravity that is causing the singularity? Almost like the chicken and the egg question I suppose.
    neither, the singularity is a mathematical artifact
    I didn’t know that. I thought it was a quantum mechanic type thingy! However, I’ve started looking into it a bit more as a “mathematical artefact” so I'll see what I can learn from that. Thanks.
    Reply With Quote  
     

  38. #37  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    How is that relevant? Does that entitle you to make incorrect statements
    We all make mistakes, that is how we learn. I like the way Madam Currie discovered Radium. Why are you so antimistakes? Don't you like to learn? In any event Black Holes are on the edge of our understanding. Surely you would agree with me there.
    Reply With Quote  
     

  39. #38  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by pineapples View Post
    I didn’t know that. I thought it was a quantum mechanic type thingy! However, I’ve started looking into it a bit more as a “mathematical artefact” so I'll see what I can learn from that. Thanks.
    To briefly address your question as well - the black hole and its gravity are a result of the gravitational collapse which initially led to its formation. Our currently accepted model of gravity is called General Relativity - the issue is that this is a purely classical theory in that it does not account for any quantum effects; there comes a stage during the gravitational collapse where quantum interactions between elementary particles become important, and that is where General Relativity breaks down. If you naively try to extrapolate General Relativity all the way to the end of the collapse, you end up with a singularity; however, we do not expect to actually find a singularity inside a black hole, or anywhere else in the universe. In that sense it is a mathematical artefact born out of General Relativity's failure to account for quantum effects. We do not yet possesses a full model of quantum gravity - this is currently an area of ongoing and very active research.
    pineapples likes this.
    Reply With Quote  
     

  40. #39  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by uptonryan View Post
    We all make mistakes, that is how we learn.... Why are you so antimistakes? Don't you like to learn?
    I have tried to politely point out some of your errors so we can all learn from them. But you insist on vociferously defending them. Inevitably the discussion has got a bit heated.

    In any event Black Holes are on the edge of our understanding. Surely you would agree with me there.
    They are certainly extreme environments and therefore good for testing our current theories of relativity and quantum physics (and how they relate).
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  41. #40  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    They are certainly extreme environments and therefore good for testing our current theories of relativity and quantum physics (and how they relate).
    I agree.
    Reply With Quote  
     

  42. #41  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    dark matter on the other hand is matter that weakly interacts with the Higgs Field. Some people think that dark matter provides a form of antigravity and that is responsible for an anomaly in the rotation of galaxies as well as the increased expansion of the universe. A galaxy usually has a shadow of dark matter chasing it and around it. A moving galaxy will be followed by that shadow. We know the shadows are there because we can see their gravitational effects.
    Reply With Quote  
     

  43. #42  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by uptonryan View Post
    dark matter on the other hand is matter that weakly interacts with the Higgs Field.
    If, as some propose, they are "weakly interacting massive particles" (WIMPs) then presumably the "massive" part means they are interacting strongly with the Higgs field?

    Some people think that dark matter provides a form of antigravity and that is responsible for an anomaly in the rotation of galaxies
    That doesn't seem right. Do you have a reference for these "some people"? My understanding is that dark matter is hypothesized to provide the extra mass (with normal gravity) in the galaxy to explain observed rotation speeds. If it had anti-gravity, then it would have the opposite effect.

    as well as the increased expansion of the universe.
    That would be dark energy, surely.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  44. #43  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    How can dark energy be a source of anti gravity? I do have some references but I think it might be that no one really has defined this stuff yet and so it is seen as able to rectify all the issues. Like why is the universe expanding at faster and faster rates. And why do the galaxies not collapse on themselves etc. The have built massive swimming pools to find WIMPS. I saw the French have a deep sea experiments to find WIMPS even. The French string a line of detectors into the deep sea to try and detect WIMPS and their interactions. Its amazing stuff.
    Reply With Quote  
     

  45. #44  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by uptonryan View Post
    How can dark energy be a source of anti gravity?
    My (very limited) understanding is that presence of dark energy causes negative pressure, rather than negative gravity.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  46. #45  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    I personally think its a weird form the of casimir effect. The universe that we know is at a certain energy level. The universe is expanding into a lower energy level. Sort of like a bucket of water spreading across a dry cloth. The water spreads across the cloth and there is a force that sucks the water into the dry parts of the cloth. However how can that be proved?
    Reply With Quote  
     

  47. #46  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    How can dark energy be a source of anti gravity?
    It is not. Anti-gravity would be go hand-in-hand with a certain form of space-time curvature, but on global scales the universe appears flat as far out as we can see it. Dark energy can best be thought of as an intrinsic property of space-time itself, which acts as a form of "negative pressure", thereby accelerating the metric expansion of space.
    Reply With Quote  
     

  48. #47  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Space is not flat it is curved. That means if you travel very far in one direction you will eventually return to the point you started. The expansion does not occur at the edges. There is not edges. Just like there are not edges to the planet earth. The expansion bubbles up through the void between galaxies. Sort of like a loaf of bread with raisins in it expands in the oven.
    Reply With Quote  
     

  49. #48  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Also relativistic effects mean that as you travel towards the edge strange things happen in time. You might leave the earth at close to the speed of light C. however the Earth will observe you as travelling slowly through time. If you then loop back to the earth you will find decades have passed while you have only aged years. This also means that as you look out towards the edges of the universe you look further and further back in time.
    Reply With Quote  
     

  50. #49  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    Space is not flat it is curved. That means if you travel very far in one direction you will eventually return to the point you started.
    This isn't an empirical, observational fact, but merely one of numerous possibilities for the global geometry of the universe. Currently we have insufficient data to decide what global geometry the universe has - all we can say is that as far out as the observational horizon, it appears flat, or at least very nearly so. Setting aside some non-trivial topologies, we have three basic possibilities :

    1. A completely flat universe
    2. A universe with constant positive curvature ( the one which you were referring to )
    3. A universe with constant negative curvature ( "saddle shape" )

    Like I said, at the moment we have insufficient data to decide which one it is.
    Reply With Quote  
     

  51. #50  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    Also relativistic effects mean that as you travel towards the edge strange things happen in time
    There is no boundary ( "edge" ) to the universe, regardless of what the global geometry actually is.
    Reply With Quote  
     

  52. #51  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    How can you make options 1 and 3 comply with the big bang?
    Reply With Quote  
     

  53. #52  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by uptonryan View Post
    Also relativistic effects mean that as you travel towards the edge strange things happen in time
    There is no boundary ( "edge" ) to the universe, regardless of what the global geometry actually is.
    That is correct. I meant as you travel away from your origin.
    Reply With Quote  
     

  54. #53  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    How can you make options 1 and 3 comply with the big bang?
    I am not sure what you mean by that - metric expansion would take place in any one of these scenarios, so there is no incompatability with the Big Bang. Remember also that we are dealing with 4-dimensional space-time here, so don't try to visualise this as an explosion with an expanding sphere afterwards.

    I can sort of see where you are coming from - you are thinking how can an infinite universe start in just one point ? This problem arises if we try to visualise the universe as a sheet with extrinsic curvature; you can do this, but then you need to remember that the Big Bang happened everywhere on that sheet, after which all points on the sheet started to recede from one another. This is hard to explain and picture, but mathematically rigorous.
    Reply With Quote  
     

  55. #54  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    There are more than 4 dimensions. The universe started with lots and a number curled up smaller than we can see. The number changes but this link says there are 11 dimensions. M-theory - Wikipedia, the free encyclopedia However they might just be there to make the maths work out.
    Reply With Quote  
     

  56. #55  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    There are more than 4 dimensions. The universe started with lots and a number curled up smaller than we can see. The number changes but this link says there are 11 dimensions. M-theory - Wikipedia, the free encyclopedia However they might just be there to make the maths work out.
    Ok, what you are referring to is M-Theory, but this is currently merely a hypothesis - we simply don't know yet whether or not that model has any physical content.
    Reply With Quote  
     

  57. #56  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    we simply don't know yet whether or not that model has any physical content.
    Quite a few theories are like that in physics. It was only recently that relativity was definitively proved. String theory is notoriously hard to prove because it has very few real world effects. Would be interesting if we could expand some of those tiny dimensions though. http://home.web.cern.ch/about/physic...ny-black-holes
    Reply With Quote  
     

  58. #57  
    ...matter and pixie dust wegs's Avatar
    Join Date
    Jul 2013
    Posts
    2,058
    Spaghettification - Wikipedia, the free encyclopedia This is the most fascinating thing to me, relating to blackholes.
    Reply With Quote  
     

  59. #58  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    It was only recently that relativity was definitively proved.
    Just out of interest - what makes you think that ? Relativity ( both SR and GR ) were well verified and part of the scientific mainstream already back in th 1920s, at least to the best of my knowledge. All "classic" tests of relativity date to around that time.
    Reply With Quote  
     

  60. #59  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by uptonryan View Post
    It was only recently that relativity was definitively proved.
    Just out of interest - what makes you think that ? Relativity ( both SR and GR ) were well verified and part of the scientific mainstream already back in th 1920s, at least to the best of my knowledge. All "classic" tests of relativity date to around that time.
    This experiment here Gravity Probe B - MISSION STATUS
    Reply With Quote  
     

  61. #60  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    This experiment here Gravity Probe B - MISSION STATUS
    Ok, this relates to two predictions of GR, geodetic precession and frame dragging. They are certainly key tests, but GR was accepted as the mainstream model for gravitation long before that. The last outstanding test now is the direct detection of gravitational waves.
    Reply With Quote  
     

  62. #61  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    yeah there has been a lot of problems with
    direct detection of gravitational waves
    . I remember them working on that when I went through uni in 97. One of the main problems is that there is just too much noise here on earth. That means expensive space missions. IMPERIAL COLLEGE LONDON -Upcoming space mission could herald new type of astronomy - HispanicBusiness.com Some people thing that gravity just doesn't work like that at large distances. It would mean that every point in space feels some sort of gravitational tug from all the matter in the universe. That would cause random tidal tugs on matter at random directions and pull everything apart. Gravity has to taper off very quickly to prevent that
    Reply With Quote  
     

  63. #62  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    One of the main problems is that there is just too much noise here on earth.
    Yes, that's one of the issues, the other is of course that the signal itself is extremely weak, so even in space it is a major engineering challenge.

    It would mean that every point in space feels some sort of gravitational tug from all the matter in the universe. That would cause random tidal tugs on matter at random directions and pull everything apart.
    No, for the same reason that it is hard to detect - its effects are very, very tiny.
    Reply With Quote  
     

  64. #63  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Plus if string theory is right and there are 11 dimensions and not 4. Gravity might be getting lost in 11 and not 3 dimensions. That would mean that gravity decreases a lot more rapidly than we expect.
    Reply With Quote  
     

  65. #64  
    Forum Freshman Lizard7Bo's Avatar
    Join Date
    Oct 2013
    Location
    South Dakota
    Posts
    19
    Sorry, but what? since when did gravity become a wave? or has everything that is taught just been a load of crap.. Last i checked gravity was a curvature of spacetime where objects of greater mass had a larger distortion and all objects interacted with each other according to the distortion of the objects around them based on their own distortion of spacetime. Or am i wrong?
    Reply With Quote  
     

  66. #65  
    Forum Freshman Lizard7Bo's Avatar
    Join Date
    Oct 2013
    Location
    South Dakota
    Posts
    19
    But on the black hole topic, it is theorized that all the information that is absorbed by the black hole can be measured by the surface area of the black hole. Which makes sense if you think about it, being that gravity does have its end limit to how far it can compact matter. If we could ever measure how dense a black hole is we would be able to calculate how much matter it had absorbed over its lifetime.
    Reply With Quote  
     

  67. #66  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Lizard7Bo View Post
    Sorry, but what? since when did gravity become a wave? or has everything that is taught just been a load of crap.. Last i checked gravity was a curvature of spacetime where objects of greater mass had a larger distortion and all objects interacted with each other according to the distortion of the objects around them based on their own distortion of spacetime. Or am i wrong?
    No, you are not wrong at all, so don't worry, you don't have to relearn. Gravity is a geometric property of space-time. But suppose the source of gravity isn't stationary and constant - what would happen ? The curvature of space-time wouldn't be constant either. Imagine a very still pond of water, with an initially flat surface; now drop a stone into this pond to disturb the surface - what will happen is that ripples will spread outwards from the point of the disturbance. Space-time works much the same way - take a mass distribution that is not constant, such as two very massive objects orbiting a common barycenter ( i.e. a binary system ). What happens here is that the curvature of space-time oscillates in tune with the revolutions of the two bodies, creating "waves of curved space-time" spreading outward. Here is a schematic what such gravitational radiation looks like ( two dimensions removed ) :



    These gravitational waves propagate at or below the speed of light, and what they are is quite simply time-dependent changes of space-time curvature at each point. Since these waves carry energy-momentum ( let's disregard technical issues here for the moment ) away from the binary system, the orbits of these bodies will gradually decay over time; this orbital decay is observable and measureable, as for example in the Taylor-Hulse binary system.

    You can find some further information in the Wikipedia article on gravitational waves.
    Lizard7Bo likes this.
    Reply With Quote  
     

  68. #67  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Lizard7Bo View Post
    If we could ever measure how dense a black hole is we would be able to calculate how much matter it had absorbed over its lifetime.
    Trying to define a "density" for black holes is largely meaningless, since the geometry ( and possibly topology ) of space-time in the interior region is non-trivial. However, you don't need density - you can calculate its total mass ( which is equal to all energy-momentum that ever entered the black hole ) via its horizon surface, or quite simply through its gravitational interactions with its surroundings.
    Reply With Quote  
     

  69. #68  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Black holes evaporated via Hawking radiation. That evaporation rate may not be uniform.
    Reply With Quote  
     

  70. #69  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    That evaporation rate may not be uniform.
    It isn't, because the BH's temperature is inversely proportional to the surface of its event horizon. In other words, the smaller it gets, the hotter it is.
    Reply With Quote  
     

  71. #70  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    I meant the evaporation may be random and depend on whether the blackhole is feeding or not.
    Reply With Quote  
     

  72. #71  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by uptonryan View Post
    I meant the evaporation may be random and depend on whether the blackhole is feeding or not.
    Hawking radiation is function of the black hole. It is quite separate from the radiation formed by matter falling into the black hole.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  73. #72  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    I meant the evaporation may be random and depend on whether the blackhole is feeding or not.
    Like I said, the temperature ( and thus radiation ) is a function of the event horizon area only.
    Reply With Quote  
     

  74. #73  
    Suspended
    Join Date
    Oct 2013
    Posts
    794
    for me this topic may be closed
    Reply With Quote  
     

  75. #74  
    Forum Sophomore
    Join Date
    Oct 2013
    Posts
    111
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by uptonryan View Post
    I meant the evaporation may be random and depend on whether the blackhole is feeding or not.
    Hawking radiation is function of the black hole. It is quite separate from the radiation formed by matter falling into the black hole.
    I would disagree with your assertion. What proof do you have that the two are separate? I would think that matter falling into a black hole would interfere with Hawking radiation, in some way. Even if it is only a reduction to state that such violent interactions would not be disruptive is a pretty brave claim.
    Reply With Quote  
     

  76. #75  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by uptonryan View Post
    I would disagree with your assertion. What proof do you have that the two are separate?
    Hawking radiation originates from QFT processes near the event horizon and is almost perfect black body radiation; the thermal radiation of in-falling matter is obviously not. As such, these are very different effects; a black hole will emit Hawking radiation even if it is isolated and no matter is falling into it.
    Reply With Quote  
     

Similar Threads

  1. Black holes, dark matter & dark energy
    By Cuete in forum Astronomy & Cosmology
    Replies: 12
    Last Post: May 3rd, 2013, 05:33 PM
  2. black holes-dark matter (a moronic non-genius theory)
    By orion6676 in forum Astronomy & Cosmology
    Replies: 11
    Last Post: June 29th, 2010, 06:42 PM
  3. Black holes vs Dark stars
    By gc in forum Personal Theories & Alternative Ideas
    Replies: 7
    Last Post: November 6th, 2009, 05:52 PM
  4. The relationship between anti-matter and Black holes
    By TheyoungSkeptic in forum Physics
    Replies: 15
    Last Post: March 20th, 2009, 03:49 PM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •