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There's something from earlier in the thread I would like to return to. How can you get a black hole if it doesn't go into runaway, that is like having an atomic bomb without critical mass ... think about it In order to ensure we were talking about the same thing, you would have to be quite clear as to whether you are treating the centre of a BH as a singularity, or not. We would also have to be sure that by “singularity” we understood the same thing.
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In order to ensure we were talking about the same thing, you would have to be quite clear as to whether you are treating the centre of a BH as a singularity, or not.
We would also have to be sure that by “singularity” we understood the same thing. Can I suggest we do this a different way and we start with massive black holes because they are easier to follow and understand than a collapsing sun forming a black hole. For example if we created a latex film and enclosed the area out to saturn with the sun as a central point and filled it with air to the atmospheric pressure of earth, a black hole would open up at the sun. Really doesn't take much matter in a confined area to form one. That is why anywhere that matter accumulates you almost always get a black hole, such at the centre of galaxies. So the creation is interesting and in a lot of ways more interesting than the black hole itself and tells you things. It's up to you but the singularity argument is actually a side issue to other problems you will face. The one I would first like to look at is infinite gravity and what it actually means and will it kill you. Hint: GR says if you moved towards a supermassive black hole you would easily pass thru the event horizon and it's huge gravity probably barely noticing it to inevitably die horribly some time later. What does gravity on a human body actually mean and how much does it take to kill a human hint 2: The gravity at the 4200 miles up the international space station circles at is 89% what is on earth. However the special path the ISS takes makes conditions there almost zero gravity. Could you do such a path around a black hole and what is the maximum gravity you could thus be exposed to, see gravity as a force is tricky something layman don't often get and an infinite force may not mean anything special Reference if you need it: http://profmattstrassler.com/articles-an...-space-station/Hint 3: If you believe in singularities in a black hole. It is a general fact you will be torn apart by a black hole approximately a tenth of a second before you hit the singularity, independent of the mass of the black hole. Why?
Last edited by Orac; 08/02/15 01:14 PM.
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Can I suggest we do this a different way and we start with massive black holes because they are easier to follow and understand than a collapsing sun forming a black hole. That’s OK with me. All I was doing was seeking clarification so as to avoid singularities causing problems later. Do super-massive not have singularities at the centre?
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They are the same as solar mass black holes in make-up just very very big and theoretically born a different way. If solar mass black stars contain a singularity then so would super massive black stars. As I said I would first like you to look at is very very large gravity sources (bordering on infinite) and what it actually means and will it kill you as it might shed light on what infinite gravity actually means. The second issue I would like to cover is what a singularity looks like in a static black hole versus a rotating black hole do you know the layman science magazine view on the difference? If you don't here is the reference:https://en.wikipedia.org/wiki/Ring_singularity
Last edited by Orac; 08/03/15 03:40 AM.
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As I said I would first like you to look at is very very large gravity sources (bordering on infinite) and what it actually means and will it kill you as it might shed light on what infinite gravity actually means. I know what you mean, so I’ll not argue with “bordering on infinite”. you introduced rotating and non-rotating BHs as a second point, but it might be useful to include them under the first heading. It might be good to check first that my thoughts/understanding about these are bordering on correct. Kerr BHs rotate; Schwarzschild BHs do not. A Schwarzschild BH can have a “point” singularity at its centre. A Kerr BH has a central ring, sometimes called a ring singularity. A stellar BH must rotate, because the original star would have been rotating, and angular momentum is conserved. Galaxies rotate, so it seems reasonable to assume that the BHs at their centres also rotate. To date, there are no known examples of non-rotating BHs. This links to the first point in that if there are Schwarzschild BHs, and if they have singularities at their centres, any close approach to the singularity would be fatal. ….I would first like you to look at is very very large gravity sources (bordering on infinite) and what it actually means and will it kill you… Get too close to a point singularity, where gravity goes to infinity, and it will turn you into spaghetti.
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Kerr BHs rotate; Schwarzschild BHs do not. A Schwarzschild BH can have a “point” singularity at its centre. A Kerr BH has a central ring, sometimes called a ring singularity. A stellar BH must rotate, because the original star would have been rotating, and angular momentum is conserved. Galaxies rotate, so it seems reasonable to assume that the BHs at their centres also rotate. To date, there are no known examples of non-rotating BHs. Absolutely correct to all. We can add that we have never see a sun or planet that doesn't rotate, many consider it impossible for a celestial body to form as a direct consequence of the laws of gravity and conservation of angular momentum. It would follow a collapsing star should be rotating and one may ponder that there really could be such thing as a static back hole. This is the sort of subtle problem I have with the usually naive discussions around black holes, reality and logic are usually the first victim of the discussion. I wouldn't waste my time discussing static back holes unless you first convince me how one could form or that there is evidence of one. This links to the first point in that if there are Schwarzschild BHs, and if they have singularities at their centres, any close approach to the singularity would be fatal.
.. snip
Get too close to a point singularity, where gravity goes to infinity, and it will turn you into spaghetti. Correct you have the basics the thing that will kill you is the rate of change of gravity creating a tidal difference in you not gravity itself. Most layman don't get that there is no absolute value of gravity you could not survive provided you were in free fall. The formula is given in the wiki discussion https://en.wikipedia.org/wiki/SpaghettificationSo I completely agree with everything you have said but now want to look at more mundane physics infinities. Lets start with the a simple one which is the formula for induction in electricity .... V=L(di/dt). That says voltage induced across a coil equals the inductance multiplied by the change in current, divided by the change in time. If the change in time goes zero the voltage produced goes to infinity. That formula is correct to all know values and you see it in your car ignition system, all switch mode power supplies and most TV's and monitors where breaking current flowing in an inductor is used to create very high voltages and usually even extended higher by use of transformer as the inductive component. Theoretically if you could open a switch instantly with (dt=0) you would and could perfectly insulate you would indeed generate infinite voltage. This is point I am trying to get you to think about infinities require theoretical conditions that can not exist in the real world. The fact a theory or law predicts an infinity does not mean it exists. We saw just the subtle change of rotation changed what GR said about the singularity. GR is just another physics theory that predicts an infinity, for me I don't expect the infinity to ever be realized because I think there will be a number of factors in the real world that stop it. Unfortunately practical reality doesn't sell glossy science magazines. What would be interesting would me trying to sell the story to layman, that their car ignition coil could open it's points in zero seconds and the atmosphere might be perfect for insulation and so they could die to the infinite voltage. No scientist can say I am wrong because that is what the theory and law says they would instead argue real world modifies the prediction So I take a fairly standard approach when I see infinity in theories that it just means some high value that would be moderated by the real world. Hence I don't take all the hype and ventilation over gravity singularities too seriously, I would need to see experimental results saying it exists in reality.
Last edited by Orac; 08/04/15 05:12 AM.
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I wouldn't waste my time discussing static back holes unless you first convince me how one could form or that there is evidence of one. No way! I can’t see how they could be a physical reality. I just wanted to be sure that if we were talking about singularities, they were ring singularities. Lets start with the a simple one which is the formula for induction in electricity .... V=L(di/dt). That says voltage induced across a coil equals the inductance multiplied by the change in current, divided by the change in time. If the change in time goes zero the voltage produced goes to infinity.
That formula is correct to all know values and you see it in your car ignition system, all switch mode power supplies and most TV's and monitors where breaking current flowing in an inductor is used to create very high voltages and usually even extended higher by use of transformer as the inductive component. Therefore, voltage can be infinite for a period of no time. I have difficulty making a distinction between something that happens for a period of no time, and something that does not happen at all. Infinite voltage is evidently theoretically possible, but is not possible in our 3+1 dimensional Universe. Theoretically if you could open a switch instantly with (dt=0) you would and could perfectly insulate you would indeed generate infinite voltage. This is point I am trying to get you to think about infinities require theoretical conditions that can not exist in the real world. The fact a theory or law predicts an infinity does not mean it exists. This is exactly what I have been criticised for saying for years. We saw just the subtle change of rotation changed what GR said about the singularity. GR is just another physics theory that predicts an infinity, for me I don't expect the infinity to ever be realized because I think there will be a number of factors in the real world that stop it. The major one being that the “real world” which we experience is finite, so cannot “contain” infinity. Hence I don't take all the hype and ventilation over gravity singularities too seriously, I would need to see experimental results saying it exists in reality. Good luck with that.
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I would say we probably have about the same view on this stuff. My view is the more mundane infinities science run across they can easily identify the real world restrictions that will prevent it. In much of the spacetime discussion we don't know the problem well enough to know the restrictions but that doesn't mean there are no restrictions. The telling point to me is we don't waste time arguing over infinite voltage, yet some sections of science waste time arguing over singularities. I think they missed the lesson that most theory predictions assume ideal unrealistic conditions in the real world your answer will be approximated If it's any consolation we are not alone http://www.askamathematician.com/2012/09/q-what-are-singularities-do-they-exist-in-nature/Just our practicality doesn't provide stories for glossy science magazines or web sites.
Last edited by Orac; 08/05/15 02:55 AM.
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You should know the answer to that question with all your discussion we had above it's not hard. The problem for a photon and time in classic physics is the E & B (electric and magnetic) fields can't be described in a reference frame of the photon at least not in a manner that makes any sense in our reference frame. So a photon can't be used as a reference frame for itself much less anything else in classic physics including GR. You can resolve the paradox in Quantum Field Theory (QFT) because the photon is nothing but a manifestation of the fields. Time for the fields remains real and it can be used for a reference frame ... try it ... you should get an answer close to one you were given with hand waving using classic physics Sometimes QM/QFT gives obvious answers that make more sense than classic physics. You disappointed me with not rethinking that problem. Trying to describe the world from a rainbow is always problematic You like the matrix, the photon never experiences anything because it isn't real ... it is the spoon !!!!!!
Last edited by Orac; 08/05/15 04:28 PM.
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The interesting thing about that thread is the variety of things it threw up, and the fact that “the Physicist” vanished. It is a question/answer forum, rather than a general discussion, so I think those who ask questions should have a reasonable expectation of an expert opinion, not just the thoughts of another lay-person. You can resolve the paradox in Quantum Field Theory (QFT) because the photon is nothing but a manifestation of the fields. Time for the fields remains real and it can be used for a reference frame ... try it ... you should get an answer close to one you were given with hand waving using classic physics. I suspect it would be a valuable exercise, but I’d need some direction if I were going to “try it”.
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I suspect it would be a valuable exercise, but I’d need some direction if I were going to “try it”. Well lets start with "time" which in the classic physics you can conjecture if it exists for the photon or not. QM does not give you a "no time" choice, that photons behaviour is described by a probability wave (as is everything) and that wave requires time. You do not get to conjecture if "time exists" it is implicit in the description. So when you send the photon thru a double-split experiment QM can predict what is going to happen because it is implicit time exists for the photon which describes it's probability wave ... no time ... no QM ... no probability wave ... no predicted answer. That is why I get cross at people who say they accept QM but then talk about time being something that is "experienced" like the classic physics trash version. I had a go at Bill G a couple of times over that when he tried to pull his consensus science garbage. I don't care how many science magazines and/or sites run trash like that it's wrong unless you can overturn QM. So if you accept QM then a photon experiences time it isn't up for conjecture. As to why GR breaks down I gave you the answer the photon or anything moving at the speed of light isn't a classic physics object and the theory isn't valid for it, hence SR/GR exclude a reference frame at the speed of light. If you want it in science speak: Quantum physics sees time as an intrinsic variable and not extrinsic as in classical relativistic physics.Now in QFT you have fields everywhere and the probability waves must traverse that worldscape so time at a minimum must be everywhere the field is. You can also describe the E/B fields even when its moving at the speed of light (Quantum electrodynamics) something all classic theories fail at. Now I can't help some people don't get classic physics is wrong and often misleading An illusion can not describe itself hence my rainbow joke, what does a rainbow look like to itself? Do you see why a photon can not classically describe itself? Now you spent the time to follow the logic and evidence and you should now get the current science position on time. You can play silly games with time in classic physics but don't mistake that for thinking it is something science says is consistent with ALL THE DATA, which is a key point of science. My complaint of the classic physics zealots is they exclude data and are no better than religious zealots excluding data they don't like. If one is a quantum zealot at least you are consistent with all the data and it's still science
Last edited by Orac; 08/06/15 04:13 AM.
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I think I get that, but as resident devil's advocate I'll have to give it some consideration. There must be something I can question.
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LOL no problem ... I am open to changing my mind if you can construct a good argument and highlight a breakdown in the logic.
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I did say "questions", not logical arguments. I'd appreciate some maths help here, please. A particle travelling at 0.9c from emitter to detector is timed by observer 1, who is stationary relative to both emitter and detector, as taking 1 sec to cover that distance. Observer 2 is travelling (in the direction from detector to emitter) at 0.8c. What time does observer 2 record for the particle? What equation is used to determine this? Of course you can see where this is going, but that's OK, its not a "try to catch Orac" exercise.
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Not hard to catch me out on Saturday The setup on the experiment isn't very good because technically the particle is going to be observed flying at observer 2 at one speed and then moving away at another (Plays stereotypical racing car noise of approach an recede). However ignoring you bad setup the formula for time dilation for observer 2 is dilation time = time / sqrt(1-(v2/c2)) plugging in dilation time = 1s / sqrt(1-((0.8*0.8)/(1*1))) dilation time = 1s / 0.36 = 1.6667 sec So observer 2 sees the particle take 1.6667sec emitter to detector (ignoring bad setup) The other way of saying this is observer 2 would say the particle was doing 0.54c not 0.9c that observer 1 is seeing, so he is seeing it redshifted by quite a way.
Last edited by Orac; 08/08/15 12:17 PM.
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I left the other answer but I just realized from what you said you actually wanted the calculation of the horrible setup you have.
Ok you will need to break the sitation into two the blue shift approach and the red shift recede
So lets map the distances from the emitter and get the time observer 2 passes the particle
Particle: (Simply moves away with time) distance from emitter = 0.9c * t
Observer 2: (He starts at detector 0.9c away and reduces down) distance from emitter = 0.9c - 0.8c * t
So they pass each other when time "t" is equal so we have (0.9c * t) = 0.9c - (0.8c * t)
rearranging (0.9c * t) + (0.8c * t) = 0.9c
simplifying steps 1.7c * t = 0.9c t = 0.9c / 1.7c
t = 0.5294 seconds
So if observer 2 passes receiver at time particle leaves emitter they meet 0.5294 sec in journey
Lets just double check that
The particle will move 0.9c for 0.5294 = 0.47646c from the emitter The observer will move 0.8c for 0.5294 = 0.42352c from detector
Add the two together and hopefully we get our 0.9c the emitter and detector are apart ... 0.47646+0.42352=0.89998 ..... woot no maths fail for me
So observer 2 will see the particle blue shifted for 0.5294 sec then it will pass him and red shift away
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Thanks Orac. Lots to think about there, not least the tendency of experts to over complicate! What I said was: "Observer 2 is travelling (in the direction from detector to emitter) at 0.8c." I didn't say he was travelling between the two at any point. However, I take your point that I should have been more specific. I didn't say if observer 2 was travelling towards, or away from the set-up. My bad.
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I predicted that you would see where this was going. It was nothing more serious than than to ask:
In relativity time dilation/length contraction works to the point where the speed of light is involved. It then breaks down.
The same rules of time dilation/length contraction seem also to apply to QM.
How does what happens at "c" differ between SR/GR and QM?
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Back at the start of this thread you were looking for a “theory”. I don’t have theories; I lack the necessary maths/science to formulate one effectively. However, I’m willing to have a go at an idea. If nothing else, I can learn from having it dismantled.
Why do we not have runaway gravity?
This is the wrong question; it should be, “Do we have runaway gravity”. This has a straightforward answer: “Yes”.
Notwithstanding the fact that the Universe is expanding at an increasing rate, gravity is operating locally to cause matter to aggregate. One of the ways in which we see this happening is by black holes accreting material. True, this is a slow process, but on the timescale of the Universe processes do not need to be fast. The range of gravity is infinite, so however far apart the galaxy groups become, gravity will eventually catch up. This may be indistinguishable from the closed universe concept, but it has nothing to do with the basic geometry of the Universe. It is runaway gravity, caused by the fact that gravity creates more gravity, so gravity will eventually overcome any basic geometry.
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