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Author Topic:   Black Holes, for Eta Carinae
RingoKid
Inactive Member


Message 46 of 53 (83233)
02-05-2004 12:54 AM


can you se my confusion...
...it's picturing the shape of the expansion cos it's predetermined by the analogy of the balloon hence spherical
take this
it implies a curved bubble like structure as a membrane with a hole sinking into it leading to...
...if this gets any more confusing I'm gonna have to start another thread

Replies to this message:
 Message 47 by NosyNed, posted 02-05-2004 12:58 AM RingoKid has not replied
 Message 51 by Beercules, posted 02-05-2004 10:26 PM RingoKid has replied

  
NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 47 of 53 (83235)
02-05-2004 12:58 AM
Reply to: Message 46 by RingoKid
02-05-2004 12:54 AM


Ya know, the whole thing keeps getting more and more confusing and harder and harder to wrap your head around.
I've talked to some physicists who say the only way is to deal directly with the math. Otherwise they say you get the wrong idea. Others say that you should be able to get an idea of it without the math. Einstein was one who pictured and even "felt" some of what he was thinking about. I guess different people manage in different ways.
But I think it is probably sure that you don't really have more than a weak, fuzzy image of what the physics is really about without the math. Unfortunately a lot of that is out of reach for a lot of us.

Common sense isn't

This message is a reply to:
 Message 46 by RingoKid, posted 02-05-2004 12:54 AM RingoKid has not replied

  
RingoKid
Inactive Member


Message 48 of 53 (83264)
02-05-2004 2:00 AM


I got time on my side and the interweb...that's the difference

  
Mike Holland
Member (Idle past 483 days)
Posts: 179
From: Sydney, NSW,Auistralia
Joined: 08-30-2002


Message 49 of 53 (83592)
02-05-2004 7:19 PM
Reply to: Message 39 by Sylas
02-04-2004 5:21 PM


Chris, I am taking this in good humour. Refer my comments at the start of my previous post regarding being flogged or worse!
Thanks for those two equations for a falling light ray and a falling body at 0.5c. OK, they have convinced me that only a limited set of 'later' light rays would reach him before he reaches the event horizon. So he will see a little blue shifted light, but not the whole future of the universe. I am still putting figures into those equations to get a better feel for what happens. It seems crazy that he takes forever to reach the event horizon (in a Schwarzschilkd metric), and the light is travelling twice as fast (at least in normal space), but it doesn't catch up. Something like the tortise and hare paradox.
Obviously if he were stationary, he would see the whole future (but no blue shift). I will try lower velocities for the observer, to see where it changes.
But I still disagree with Eta -
Yes you observe an object falling in to slow down (and get redshifted) BUT the object in reality fell in long ago.
But then if what you say about choosing a metric to allow you to talk about the time at a remote place is correct, then either/neither of us is 'right'.
Regarding Arp and other misfits, don't be so hard on them. Sometimes the 'cranks' are right, eg. Peter Wegener and continental drift. And there is plenty of room for speculation when current theories propose that the universe is 90% dark matter, that we know nothing about as yet! I also like the Aquatic Ape theory of Elaine Morgan and co. Much more fascinating than the standard theories of human evolution.
Mike.

This message is a reply to:
 Message 39 by Sylas, posted 02-04-2004 5:21 PM Sylas has replied

Replies to this message:
 Message 50 by Sylas, posted 02-05-2004 9:54 PM Mike Holland has not replied

  
Sylas
Member (Idle past 5259 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 50 of 53 (83708)
02-05-2004 9:54 PM
Reply to: Message 49 by Mike Holland
02-05-2004 7:19 PM


Mike Holland writes:
Thanks for those two equations for a falling light ray and a falling body at 0.5c. OK, they have convinced me that only a limited set of 'later' light rays would reach him before he reaches the event horizon. So he will see a little blue shifted light, but not the whole future of the universe. I am still putting figures into those equations to get a better feel for what happens. It seems crazy that he takes forever to reach the event horizon (in a Schwarzschilkd metric), and the light is travelling twice as fast (at least in normal space), but it doesn't catch up. Something like the tortise and hare paradox.
The light in the Schwarzchild metric also gets "frozen" before reaching the event horizon; they both take forever. This is an artefact of the metric, which is such that it gives a divergence at points in spacetime which cannot send signals back to the outside observer.
Obviously if he were stationary, he would see the whole future (but no blue shift). I will try lower velocities for the observer, to see where it changes.
An observer hovering outside the event horizon for any length of time would see much more blue shift, because they have all the gravitational blue shifting effect, and none of the velocity red shifting effects.
To see the entire future of the universe, an observer has to remain above the event horizon forever, according to their own clock. This requires an enormously powerful retro rocket. There are no stable orbits within 1.5 Schwarzchild radii; to remain somewhere between 1.5 and 1 Schwartzchild radii for any length of time (in the observer's own experience) requires continuous application of other forces to overcome gravity. Inside the event horizon no force can save you at all.
But I still disagree with Eta -
Yes you observe an object falling in to slow down (and get redshifted) BUT the object in reality fell in long ago.
I agree with you; good catch. The term "in reality" is not a good one, it sneaks in the idea of a privileged time scale. The phrase "long ago" in reference to events at a distance implies a metric. It cannot be an absolute statement about reality; and I'll bet that on reflection, Eta would agree.
There are some absolute statements we can make about events. For example, suppose you are in a parent ship holding steady somewhere outside the event horizon by virtue of retro rockets, and you release a probe which drops into the hole. In principle, at all times into the future you might get a signal back from the probe. If you get a signal, it must have been released by the probe before it crossed the event horizon. However, photons take a long time to climb out of the gravity well. They also get red shifted into invisibility. In fact, the red shift of signals from the probe rapidly (in the frame of the parent ship) reach a point where you can't really tell the difference between Hawking radiation and signals sent by the probe. Bear in mind that the peak wavelength of Hawking relation is around the length of the Schwarzchild radius, I think.
On the other hand, by your own clock there is an identifiable time very shortly after release of the probe where you lose contact, in the sense that you can no longer hope to send signals to the probe. Any photons you release after that point will never reach the probe. There is also a short interval of time (by the parent ship's clock), in which signals may be sent that will reach the probe, but no response is possible because the signal reaches the probe inside the event horizon. (This corresponds to the falling probe seeing the stationary observer for a brief interval of time between crossing the event horizon and being crushes in the central singularity.)
That is, from the perspective of the outside observer:
  • There is a small finite elapsed time after which you can no longer send signals to a dropped probe.
  • There is a slightly smaller finite elapsed time after which you can no longer send signals to a dropped probe and get a response.
  • Towards the end of this very small interval in time in which signals can be sent with the hope of a response, there is no limit to the time you might need to wait before the reflected signal returns, and the red shift of signals from the probe increase without bound.
  • For any physically realistic photon detector, there is a small finite interval of time beyond which you can no longer detect signals from the probe; it is practically lost to sight even though in theory it should be visible at very long wavelengths.
  • The black hole itself is not actually black; it shines with Hawking radiation that will eventually outshine signals from the probe! This gets into quantum effects, and when these are taken into account the probe may indeed end up inside the event horizon within Schwarzchild time, I think. I'm not sure; I've just passed the limits of my competance.
These are absolute times, because they refer to time measured by your own clock of events at your own location. The metric is needed only if we want to speak of the time of an event at some distance.
From the perspective of the falling probe:
  • There is a small finite time before you get crushed at the central singularity.
  • There is a slightly smaller finite time after which you can no longer hope to send signals back to the parent ship. The point at which you can no longer send signals is the event horizon; after that your signals will simply follow you into the hole.
  • There is a smaller finite time value which corresponds to the last glimpse you have of the clock on the parent ship.
Regarding Arp and other misfits, don't be so hard on them. Sometimes the 'cranks' are right, eg. Peter Wegener and continental drift. And there is plenty of room for speculation when current theories propose that the universe is 90% dark matter, that we know nothing about as yet! I also like the Aquatic Ape theory of Elaine Morgan and co. Much more fascinating than the standard theories of human evolution.
Science relies on people throwing up new and revolutionary ideas from time to time. Often such ideas are initially rejected. Often this is the right thing to do; the ideas were wrong. Sometimes the ideas are new and useful insights, and they get confirmed in time. Science is basically about trying to tell the difference. But sometimes individuals get attached to bad ideas and never ever let them go; that is part of science also.
There is always room for speculation. As time passes, we learn more about the universe. This gives new scope for new matters on which we can speculate, but it also limits the scope for reasonable speculation on specific matters. Uninformed speculation continues, of course; but I don't find that particularly fascinating.
I stand by all my comments with respect to Halton Arp. I didn't call him a crank; I called him a maverick. This was deliberate.
In Halton Arp's areas of interest, there is a very relevant example of a maverick who prevailed against othodoxy to establish new and better views of reality: Hannes Alfven. Read the link. You'll like it. Alfven had many revolutionary ideas, some of which panned out. He won the physics Nobel in 1970. He was also a critic of Big Bang cosmology. His alternative ideas on cosmology are an example of a case where his ideas did not pan out; he was wrong. People like Eric Lerner continue to try and press his ideas on cosmology, but they can only do so by ignoring and misrepresenting accumulated evidence which bears upon the matter. On the other hand, in my very amateur view it seems there is still scope for Alfven's ideas on plasma physics to play a role in improving our understanding of galaxy formation and development. Time will tell.
Although scientific knowledge is never complete or formally proven; we can still get better and better understandings over time. Dark matter is a good case in point. It was first detected (indirectly) in 1933 by Fritz Zwicky. In principle there might be other ways to explain observations; but pretty much everyone in the field is convinced it is dark matter; and the question is "What is it?". That is still the question; but the observations have continued to improve. For example, here is an article from last year showing a Map of Dark Matter developed using observations with Hubble. The method is to observe gravitational bending of light to map the masses which cause this bending.
In this image, the pale blue is dark matter. The blue is not actually visible light; it is the inferred map of dark matter superimposed on the image of a galaxy cluster. The link is to associated discussion and more images.
(Image Discussion link)
The aquatic ape is a case where speculations moved from maverick to ridiculous. What someone finds fascinating is, of course, subjective. I find human evolution and what we can learn about it to be fascinating. I find the aquatic ape itself irrelevant. That is, of course, a whole other topic for debate. The relevance to creationism is tenuous; but there are some parallels. I think a discussion might be interesting; but not in this thread.
Best wishes -- Chris

This message is a reply to:
 Message 49 by Mike Holland, posted 02-05-2004 7:19 PM Mike Holland has not replied

  
Beercules
Inactive Member


Message 51 of 53 (83721)
02-05-2004 10:26 PM
Reply to: Message 46 by RingoKid
02-05-2004 12:54 AM


The Einstein Mattress
The confusion arises because only the surface of the mattress is part of the analogy. You know the difference between a surface and a volume, right?

This message is a reply to:
 Message 46 by RingoKid, posted 02-05-2004 12:54 AM RingoKid has replied

Replies to this message:
 Message 52 by RingoKid, posted 02-06-2004 7:57 AM Beercules has not replied

  
RingoKid
Inactive Member


Message 52 of 53 (83841)
02-06-2004 7:57 AM
Reply to: Message 51 by Beercules
02-05-2004 10:26 PM


Re: The Einstein Mattress
yes yes...
...the surface is the surface and the volume is how loud a sound the balloon makes when you pop it or the mattress springs make when you jump up and down on it cos it depends on how much air is inside the balloon or how many springs in the mattress and what they're made out of
can you tell me how thick the mattress is ???

This message is a reply to:
 Message 51 by Beercules, posted 02-05-2004 10:26 PM Beercules has not replied

  
Beercules
Inactive Member


Message 53 of 53 (83944)
02-06-2004 12:45 PM


It doesn't matter how thick the whole mattress is, because the surface of the mattress does not have any thickness at all. That is the whole point of the analogy. Forget about the 3D object and just focus on the 2D aspect of it's surface.
The mattress appears to be flat (overall) while the balloon is curved. But these are not incompatible. If you could zoom into a region of the surface, it would appear to be flat like the surface of the mattress. Zoom back out, and you can see that while appearing flat on small scales, the overall surface is in fact curved.

  
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