Rolando Posted January 22, 2015 Posted January 22, 2015 How can it be that intelligent people claim that there are black holes in the universe, e.g., at the center of galaxies, although the universe has only existed for a finite time in the cosmology they believe in? A black hole is an object whose mass is surrounded by an event horizon at which time is dilated infinitely. Such a state cannot be reached in a finite time. An event horizon is infinitely remote from an observer. This alone is sufficient to realize that black holes are merely mathematical constructs that do not exist in reality even if the universe is infinite in age. Isn’t it obvious enough that objects that need more than infinite time to form cannot exist in a universe whose age is finite? In science it happens that incompatibilities arise between predictions based on different foundations, such as general relativity and quantum theory. This is not mysterious, but in the case of black holes vs. a big bang universe, the foundation is the same: general relativity.
Delta1212 Posted January 22, 2015 Posted January 22, 2015 Most people don't expect to actually find the singularity predicted by GR at the center of a black hole, but to have whatever actually happens be governed by a quantum theory do Gravity which we do not currently possess.
Mordred Posted January 22, 2015 Posted January 22, 2015 Time is relative to the observer. From outside the BH it appears time is slowed down. However from the frame of the singularity itself time moves normally. As a side note the Schwartz child radius is when the escape velocity equals c this is also the point that seperates a bh from a neutron star. How we view the BH depends on the reference frame of the observer.
imatfaal Posted January 22, 2015 Posted January 22, 2015 ... A black hole is an object whose mass is surrounded by an event horizon at which time is dilated infinitely. ... This bit is just wrong. Even for a simple S'child blackhole - which there are probably none in the universe - the "divide by zero" at the event horizon is just a flaw in the maths. It is not a physical prediction that time will stop - it is merely an artifact of maths being used incorrectly. If you want to get close to the event horizon of a s'child blackhole and describe it mathematically you must use eddington-finkelstein coordinates and not schwartzchild coordinates.
Rolando Posted January 22, 2015 Author Posted January 22, 2015 Time is relative to the observer. From outside the BH it appears time is slowed down. However from the frame of the singularity itself time moves normally. It is only the outside observer that is relevant in this context. Even for a simple S'child blackhole - which there are probably none in the universe - the "divide by zero" at the event horizon is just a flaw in the maths. It is not a physical prediction that time will stop - it is merely an artifact of maths being used incorrectly. If you want to get close to the event horizon of a s'child blackhole and describe it mathematically you must use eddington-finkelstein coordinates and not schwartzchild coordinates. This was new to me. I have to get informed about eddington-finkelstein coordinates. But do these really resolve the conflict? Most people don't expect to actually find the singularity predicted by GR at the center of a black hole, but to have whatever actually happens be governed by a quantum theory do Gravity which we do not currently possess. The problem arises already at the event horizon.
imatfaal Posted January 22, 2015 Posted January 22, 2015 ... This was new to me. I have to get informed about eddington-finkelstein coordinates. But do these really resolve the conflict?... A few links from an old post .... Firstly the time slows massively when viewed by those in an external accelerated frame and not for those in free fall along side at the same gravitational potential. Secondly you have your coordinate system wrong if you come up with infinite time dilation - near a blackhole's event horizon you should be using Kruskal-Szekeres coordinates, or Eddington-Finkelstein coordinates - these avoid the mathematical singularites that schwarzchild coordinate systems throw up. And some re-written from another post (where it was getting fractious so I won't straight quote) You are linking the event and its observation by a distant third party observer. I know that an observer in an accelerated reference frame will never see Alice cross the event horizon; but this does not preclude the fact that an infaller does fall through the barrier. It is not two separate histories causing a paradox - it is a single history and an observation. An observation of a signal by a distant observer does not affect the passage of an infaller through space - it is the signal that is affected not the history. In order to understand what is happening to an infaller in a physical sense you do not calculate her progress using coordinate time what you must do is calculate her progress using a coordinate independent proper time - and she will reach the singularity (let alone the EH) in a finite amount of time.
Rolando Posted January 22, 2015 Author Posted January 22, 2015 (edited) Firstly the time slows massively when viewed by those in an external accelerated frame and not for those in free fall along side at the same gravitational potential. Secondly you have your coordinate system wrong if you come up with infinite time dilation - near a blackhole's event horizon you should be using Kruskal-Szekeres coordinates, or Eddington-Finkelstein coordinates - these avoid the mathematical singularites that schwarzchild coordinate systems throw up. It makes actually no difference to the facts which coordinate system is used in for describing black holes, but Kruskal-Szekeres coordinates invite confusing the imaginary with the real. The coordinate system appropriate to my reasoning is that appropriate in flat space (where the observers are). The situation becomes different only when one discards black holes in favour of some alternative, such as gravastars. Edited January 22, 2015 by Rolando
imatfaal Posted January 22, 2015 Posted January 22, 2015 It makes actually no difference to the facts which coordinate system is used in for describing black holes, but Kruskal-Szekeres coordinates invite confusing the imaginary with the real. The coordinate system appropriate to my reasoning is that appropriate in flat space (where the observers are). The situation becomes different only when one discards black holes in favour of some alternative, such as gravastars. No it makes no difference to the facts what mathematical model you are using. But as we are involved in a necessarily theoretical discussion then we have to choose a model to work with - in the case of approaching the event horizon of a s'child black hole you cannot use s'child coordinates. The coordinate system appropriate to my reasoning is that appropriate in flat space (where the observers are) But they are not in flat space - they are in an accelerated non-inertial frame otherwise they too would be falling into the black hole. And even if you take them to be so far away that the acceleration is negligible (which would be too far away to observe them) then the problem becomes that you do not get black holes in otherwise flat spacetime - a black hole is an extreme curvature of spacetime. As per my message above you need to consider that what the observers see and measure is important - but it is not definitive! There is one event and that is poor Alice falling into the black hole; relativity, the idea of frames of reference, lack of simultaneity etc means that different observers will observe at different times - but this does not change the event. If you want to document the event - do it from the free-falling frame of reference that is coincident with Alice.
Rolando Posted January 22, 2015 Author Posted January 22, 2015 (edited) But they are not in flat space - they are in an accelerated non-inertial frame otherwise they too would be falling into the black hole. And even if you take them to be so far away that the acceleration is negligible (which would be too far away to observe them) then the problem becomes that you do not get black holes in otherwise flat spacetime - a black hole is an extreme curvature of spacetime. Evidently, you are thinking of observers in the vicinity of a black hole. This is not the topic here. The question was whether there can be black holes in a universe of finite age. This has to be considered in a frame of reference in which the universe can be said to be at rest. There is no other choice that fits each of the black holes that might be there. Having said this, I realize that in an expanding universe, comoving coordinates would be appropriate. Edited January 22, 2015 by Rolando
imatfaal Posted January 22, 2015 Posted January 22, 2015 Evidently, you are thinking of observers in the vicinity of a black hole. This is not the topic here. The question was whether there can be black holes in a universe of finite age. This has to be considered in a frame of reference in which the universe can be said to be at rest. There is no other choice that fits each of the black holes that might be there. Having said this, I realize that in an expanding universe, comoving coordinates would be appropriate. I am not thinking exclusively about observers at all - think of the infalling object as I have been saying since my first post in this thread. This is your OP and the problem is that one of your premises is incorrect. A black hole is an object whose mass is surrounded by an event horizon at which time is dilated infinitely. Such a state cannot be reached in a finite time. An event horizon is infinitely remote from an observer. This alone is sufficient to realize that black holes are merely mathematical constructs that do not exist in reality even if the universe is infinite in age. Isn’t it obvious enough that objects that need more than infinite time to form cannot exist in a universe whose age is finite? The premise is "A black hole is an object whose mass is surrounded by an event horizon at which time is dilated infinitely." This is incorrect. Time dilation does appear to be infinite if you use the wrong coordinate system - so don't use the wrong coordinate system. Even using the correct coordinates an external observer will still see a huge time dilation - but not infinite But even using the wrong system you are looking at it the wrong way - time is not dilated near the event horizon - time is dilated when an object near the event horizon is observed by an observer in a different frame of reference. Time is never dilated within a single frame - you are never time dilated with reference to yourself; how could you be? And in the black hole example there is nothing to stop an in-falling object from passing through the event horizon and doing whatever nature does with stuff internal to a black hole's eh. The fact that external accelerated observers will not notice this is immaterial. Black holes formed and still form when gravity overcomes outward pressure and compresses a mass to a radius smaller than it's schwartzchild radius. Note there are very complicated quantum mechanical arguments that I have seen here and could not rehearse with any fidelity that claim that this cannot happen - but black holes are predicted by relativity and that's what we are talking about.
Rolando Posted January 22, 2015 Author Posted January 22, 2015 But even using the wrong system you are looking at it the wrong way - time is not dilated near the event horizon - time is dilated when an object near the event horizon is observed by an observer in a different frame of reference. Time is never dilated within a single frame - you are never time dilated with reference to yourself; how could you be? What you say is correct but beside the point. Let me restate my question in other words: "Can there be objects that are further remote from us than the big bang is?" (Remoteness in time or in space along a geodesic.)
imatfaal Posted January 22, 2015 Posted January 22, 2015 What you say is correct but beside the point. Let me restate my question in other words: "Can there be objects that are further remote from us than the big bang is?" (Remoteness in time or in space along a geodesic.) Most cosmologist think that the big bang is a good explanation of the early moments of the universe - and I agree. I don't think there is anything older than 13.78 ish Gyrs. But this has nothing to do with blackholes - at least one will be forming about now somewhere in the universe; they are not supremely old unchanging objects that predate even the era of last scattering (where we can see upto) let alone closer to the bigbang Black holes are dynamic, hawking and bekenstein even showed that they change and radiate/absorb. There isn't a problem with black holes in relativity - quite the reverse as the equations of GR predict black holes (and more problematically a physical singularity at the centre). Please be specific what is your reason for doubting that black holes exist? - are you still standing by the notion that time is infinitely dilated at the EH? - or do you have further reasoning?
Rolando Posted January 22, 2015 Author Posted January 22, 2015 (edited) Most cosmologist think that the big bang is a good explanation of the early moments of the universe - and I agree. I don't think there is anything older than 13.78 ish Gyrs. But this has nothing to do with blackholes - at least one will be forming about now somewhere in the universe; they are not supremely old unchanging objects that predate even the era of last scattering (where we can see upto) let alone closer to the bigbang If black holes exist empirically, they must have formed prior to the big bang (and somehow survived it). An object that collapsed later cannot yet have contracted sufficiently to have reached the event horizon, since in our view, this will require an eternity. Right or wrong? Edited January 22, 2015 by Rolando
Mordred Posted January 23, 2015 Posted January 23, 2015 If black holes exist empirically, they must have formed prior to the big bang (and somehow survived it). An object that collapsed later cannot yet have contracted sufficiently to have reached the event horizon, since in our view, this will require an eternity. Right or wrong? I agree wrong. For the reasons provided.
Strange Posted January 23, 2015 Posted January 23, 2015 Also, the time dilation seen near an existing event horizon doesn't seem relevant to the process of formation of the event horizon in the first place. 1
imatfaal Posted January 23, 2015 Posted January 23, 2015 If black holes exist empirically, they must have formed prior to the big bang (and somehow survived it). An object that collapsed later cannot yet have contracted sufficiently to have reached the event horizon, since in our view, this will require an eternity. Right or wrong? In agreement with others - the above is wrong. An object that collapsed later cannot yet have contracted sufficiently to have reached the event horizon,since in our view, this will require an eternity. Please advise where this comes from - it is commonly held view - but it is wrong. I have explained the trouble with Schwarzchild co-ordinates and the divide-by-zero (mathematical singularity) at the event horizon - do you still disagree with me? or is there another reason for your claims?
Strange Posted January 23, 2015 Posted January 23, 2015 An object that collapsed later cannot yet have contracted sufficiently to have reached the event horizon, since in our view, this will require an eternity. The event horizon is created at the center of the collapsing supernova and expands outward (at, or close to, the speed of light) to encompass more mass within the growing Schwarzschild radius. I'm curious; do you think that all the cosmologists and physicists who study these things are going to go, "Doh! Why didn't we think of that!" when they hear your unique insight? (The only reason I am aware of the many reasons it is wrong is because it has been brought up so many times on science forums and the flaws explained, often by experts in the field.)
Rolando Posted January 24, 2015 Author Posted January 24, 2015 (edited) I'm curious; do you think that all the cosmologists and physicists who study these things are going to go, "Doh! Why didn't we think of that!" when they hear your unique insight? (The only reason I am aware of the many reasons it is wrong is because it has been brought up so many times on science forums and the flaws explained, often by experts in the field.) Since the problem I raised is so close at hand, I am not surprised that it has been brought up many times before, but I did not know this. I rather thought that those who claim black holes to exist in the Universe, just express themselves sloppily and do not mean classical black holes, from which no radiation can reach us, but rather any object that is close to being a black hole, be it a collapsed star or the central region of a galaxy. Even if there is just Hawking radiation, the hole is no longer black. In astronomy, such sloppiness may be tolerable, since the distinction mostly makes no difference. The outside gravitational field remains the same, being given by the mass, irrespective of its state of contraction. In response also to Mordred and imatfaal: From your reactions, I conclude that it was less appropriate to involve the formation of black holes in the statement of my problem. This complicated the reasoning unnecessarily. However, it is still completely clear to me that there can be no classical black holes anywhere in our view if the Universe has only existed for a finite time. In a system of reference in which we are at rest, it requires no less than an eternity for light to reach us from the position of an event horizon. This is immediately clear from the Schwarzschild metric and the definition of an event horizon. There can be holes in our view, but these will not be as deep as required if the Universe is finite in age. Edited January 24, 2015 by Rolando
Mordred Posted January 24, 2015 Posted January 24, 2015 And yet we detect blackholes, we also detect the changes in their feeding rates. http://www.sciencedaily.com/releases/2015/01/150122114555.htm We've also spotted one in its birth moments http://news.discovery.com/space/astronomy/black-hole-birth-spawned-record-breaking-blast-131122.htm The Schwartzchild metric is a very specific vacuum non rotating solution. As pointed out the likely hood of ever seeing a Schwartz child non rotating BH is incredibly remote. Kerr rotating blackholes are however common. Imaatsfal has already pointed out The correction to the Schwartz child metric.
Rolando Posted January 24, 2015 Author Posted January 24, 2015 (edited) And yet we detect blackholes, we also detect the changes in their feeding rates. http://www.sciencedaily.com/releases/2015/01/150122114555.htm We've also spotted one in its birth moments http://news.discovery.com/space/astronomy/black-hole-birth-spawned-record-breaking-blast-131122.htm Those who claim to have seen blackholes are likely to express themselves sloppily. The Schwartzchild metric is a very specific vacuum non rotating solution. As pointed out the likely hood of ever seeing a Schwartz child non rotating BH is incredibly remote. Kerr rotating blackholes are however common. Imaatsfal has already pointed out The correction to the Schwartz child metric. Right, Kerr blackholes are the Kerr metric is much more realistic. Kerr blackholes are similar to Schwarzschild blackholes only if looked at in a polar direction. Hovever, this is not really crucial for my argument. The presence of an event horizon alone is sufficient. Edited January 24, 2015 by Rolando
Mordred Posted January 24, 2015 Posted January 24, 2015 Who is being sloppy, the person pushing a metric artifact or the professional scientists involved in spotting that GRB? Are you claiming those scientists are wrong simply because of your misunderstanding of the Schwartz child metric? Here this covers what is known as a coordinate singularity. "Before exploring the behavior of test particles in the Schwarzschild geometry, we should say something about singularities. From the form of (7.29), the metric coefficients become infinite at r = 0 and r = 2GM an apparent sign that something is going wrong. The metric coefficients, of course, are coordinate-dependent quantities, and as such we should not make too much of their values; it is certainly possible to have a coordinate singularity The simple solution is change the coordinates. Exactly as Imaatsfal stated. http://preposterousuniverse.com/grnotes/grnotes-seven.pdf There is an expression you should consider. "The universe doesn't care how we measure it". What this means is there is a limit to how far you can take any metric. 1
MigL Posted January 24, 2015 Posted January 24, 2015 (edited) Let me see if I can explain it simply. Consider falling into a black hole. You are an astronaut who if falling and approaching the event horizon. Do you note any change in the passage of time ? .Of course not. You pass right through the ( mathematical demarcation ) horizon, and are never seen again. But say you have a flashlight with you on your journey, and every second, you shine that flashlight outwards to another astronaut on your far away spaceship. That spaceship is a different frame, and the light from the flashlight, or the flash per second signal ( information ) has to climb out of a steep gravitational well, which becomes infinitely curved. In effect, time increasingly dilates, becoming infinitely dilated at the event horizon. This doesn't mean that you, in your local frame, don't ever cross the event horizon. It means you do, but your signal/information/light, being viewed from a distant frame, cannot reach that distant observer anymore. The flashlight goes 'black' at the event horizon, because the light or signal or information can no longer get 'out'. So how does that prevent the formation of black holes ? Edited January 24, 2015 by MigL
Rolando Posted January 24, 2015 Author Posted January 24, 2015 (edited) Let me see if I can explain it simply. Consider falling into a black hole. You are an astronaut who if falling and approaching the event horizon. Do you note any change in the passage of time ? .Of course not. You pass right through the ( mathematical demarcation ) horizon, and are never seen again. But say you have a flashlight with you on your journey, and every second, you shine that flashlight outwards to another astronaut on your far away spaceship. That spaceship is a different frame, and the light from the flashlight, or the flash per second signal ( information ) has to climb out of a steep gravitational well, which becomes infinitely curved. In effect, time increasingly dilates, becoming infinitely dilated at the event horizon. This doesn't mean that you, in your local frame, don't ever cross the event horizon. It means you do, but your signal/information/light, being viewed from a distant frame, cannot reach that distant observer anymore. The flashlight goes 'black' at the event horizon, because the light or signal or information can no longer get 'out'. Your description of falling into a black hole agrees with how this uses to be described, except that "infinitely steep" has to be substituted for "infinitely curved". However, we on Earth are not falling into a particular black hole. We are just looking into deep gravitational wells that are claimed to be black holes. It takes more time for light to reach us from a gravitational well, the closer it comes to a black hole in its properties. An object turns into a black hole only when it has contracted to its event horizon. At that point, it takes an eternity for light to reach us from its event horizon. If the Universe has only existed for a finite time, it cannot contain such black holes and event horizons. Edited January 24, 2015 by Rolando
Strange Posted January 24, 2015 Posted January 24, 2015 However, we on Earth are not falling into a particular black hole. We are just looking into deep gravitational wells that are claimed to be black holes. If an object is a black hole, it takes an eternity for light to reach us from its event horizon. If the Universe has only existed for a finite time, it cannot contain such black holes and event horizons. Even if we ignore the flaws in that description, all you are saying is that we cannot observe objects at the event horizon, not that the event horizon doesn't exist. But then nothing is at the event horizon for very long, so that hardly matters.
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