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Bruce Perry
Member #270
April 2000
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I love threads like this So, outside a black hole's event horizon, you have freedom to move around in space, but have no choice but to move forwards in time. If you somehow find yourself inside a black hole's event horizon, you have no choice but to travel towards the centre. Do you find yourself free to move back in time? If you watch something fall into a black hole, it never actually appears to cross the event horizon. As it approaches the event horizon, light takes longer and longer to escape the gravitational pull and reach us. There will never be a time when there isn't still some light on its way from it to you (notwithstanding light coming in quanta etc. etc.). Does this mean it never actually entered the black hole? Do people walk through walls inside black holes? Evert said: Measurement of orbits gives you the gravitational mass, measurement of light gives you the luminous mass (ie, stars). These two numbers disagree, indicating that there is a substantial amount of "dark matter" contributing to the gravitational mass of galaxies. Do you mean to say dark matter has the distinction that its gravitational field attracts matter (e.g. planets) but doesn't attract photons? axilmar said: B sees O's past, but O can not see B's future Is it possible you're thinking of B and O as two separate observers? I think they're actually supposed to be two instants in time for the same observer - so there is ONE observer who was at B (and received the message) and then later was at O (and sent the message). The difficulty is that this observer can then receive the message and deliberately choose not to send it - you get the standard paradox. (Am I right in thinking the diagram is missing the x=±y lines that represent light speed?) -- |
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Thomas Fjellstrom
Member #476
June 2000
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Bruce Perry said: Do you mean to say dark matter has the distinction that its gravitational field attracts matter (e.g. planets) but doesn't attract photons? They used gravitational lensing to try and find what might be dark matter. So far it has popped up some possibilities. So darkmater's gravity effects photons and other matter (well more like it effects space), but the dark matter itself doesn't directly effect anything. Or thats how I saw it.. -- |
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alethiophile
Member #9,349
December 2007
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Bruce Perry said: Do you mean to say dark matter has the distinction that its gravitational field attracts matter (e.g. planets) but doesn't attract photons? I think what he means is that if you take a rough light-emitted-per-mass quotient of a star, then do the same for the galaxy, you get very different numbers, implying that much of the mass in the galaxy is not emitting light. -- |
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Arthur Kalliokoski
Second in Command
February 2005
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Bruce Perry said: Do you mean to say dark matter has the distinction that its gravitational field attracts matter (e.g. planets) but doesn't attract photons? IIRC, the theory of relativity says that mass warps space, so any gravitational field that attracts matter should also affect photons. The extra mass could be explained by bodies of matter too small to initiate nuclear fusion (self-illuminating stars), whether they're microscopic motes of dust or brown dwarfs. They all watch too much MSNBC... they get ideas. |
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Bruce Perry
Member #270
April 2000
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Arthur Kalliokoski said: IIRC, the theory of relativity says that mass warps space, so any gravitational field that attracts matter should also affect photons. That was my understanding too. Evert's phrasing made me question it - hence the question. Quote: The extra mass could be explained by bodies of matter too small to initiate nuclear fusion (self-illuminating stars), whether they're microscopic motes of dust or brown dwarfs. Earlier he said dark matter was an intrinsic curvature of space-time, not actual matter as we know it. alethiophile said: I think what he means is that if you take a rough light-emitted-per-mass quotient of a star, then do the same for the galaxy, you get very different numbers, implying that much of the mass in the galaxy is not emitting light. Error in thread Whats-outside-a-donut OriginalExplanationInsufficientForValidationException Another thought - wouldn't an intrinsic curvature of space-time be evenly distributed and not cause any concentration in any specific location? How, then, can it explain galaxies staying in one piece? -- |
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Thomas Fjellstrom
Member #476
June 2000
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Bruce Perry said: Earlier he said dark matter was an intrinsic curvature of space-time, not actual matter as we know it. I think thats the explanation he prefers -- |
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Arthur Kalliokoski
Second in Command
February 2005
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Bruce Perry said: Earlier he said dark matter was an intrinsic curvature of space-time, not actual matter as we know it. Wouldn't that imply the "energy" of the curvature (in the sense of mass and energy being equivalent) then is more mass? Definition of Recursion: See Recursion [EDIT] They all watch too much MSNBC... they get ideas. |
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Evert
Member #794
November 2000
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Bruce Perry said: Does this mean it never actually entered the black hole? That depends on your frame of reference. Quote: Do you mean to say dark matter has the distinction that its gravitational field attracts matter (e.g. planets) but doesn't attract photons? No - because that's not how gravity works. Photons as well as matter follow the curvature of space time. You can't attract one or the other, it's always both. Quote: (Am I right in thinking the diagram is missing the x=±y lines that represent light speed?) Yes. alethiophile said: I think what he means is that if you take a rough light-emitted-per-mass quotient of a star, then do the same for the galaxy, you get very different numbers, implying that much of the mass in the galaxy is not emitting light. That's right. Bruce Perry said: Earlier he said dark matter was an intrinsic curvature of space-time, not actual matter as we know it.
Yeah, ok, sorry. Confusing, I know. Arthur Kalliokoski said: The extra mass could be explained by bodies of matter too small to initiate nuclear fusion (self-illuminating stars), whether they're microscopic motes of dust or brown dwarfs.
The problem is that the best estimate of the non-luminous baryonic matter content of the galaxy (or the universe) doesn't give you enough to explain galactic rotation curves. |
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alethiophile
Member #9,349
December 2007
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Lots of black holes? -- |
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Johan Halmén
Member #1,550
September 2001
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Inside a black hole Allegro 5 is released. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Years of thorough research have revealed that what people find beautiful about the Mandelbrot set is not the set itself, but all the rest. |
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Noone quite knows what it is, and one of the possibilities is some form of normal or strange matter.
Of course, then you wouldn't be able to see things behind them...