MigL

Certain properties of the collapsed star that formed the black hole are conserved on the event horizon, not in the possible singularity. Properties like mass ( and so gravity ), angular momentum and charge and a semi-QM argument can be made for the preservation of entropy information ( and so temperature ) on the event horizon. The event horizon has no physicality, you cannot see it or touch it. It is defined as the radial distance from the possible singularity where the velocity needed to escape ( move infinitely far away ) is equivalent to the speed of light. Alternately, an object infinitely far away will, in the absence of any other forces, have accelerated to the speed of light upon crossing the event horizon ( note that this is then an impossibility ).

More like 'a singularity is not allowed when quantum effects are taken into account', since the singularity would, by definition, localize an amount of energy into an infinitely small space, and that is not allowed. I myself like to think of the possible singularity as an edge to space-time. Our current ( non quantum ) theory ceases to be valid at or very close to this 'edge'.

Zapatos, if an object possesses an intrinsic property which affects space-time so as to curve or warp it, and this effect propagates outward at the speed of light, we know what is 'exchanged' between two objects and at what speed the curvature information is exchanged to account for gravity. Using the Newtonian model for gravity, what exactly is exchanged between the sun and earth to compel the earth to follow its orbit ? That is what is called action at a distance. Just to clarify the concept.

Time ceases at the Swartzchild radius of a black hole ??? Tell that to the poor sucker who is yelling at the top of his lungs "STOP ! STOP !" as he plunges into the depths of the gravitational well on his way to meet his demise at or near the singularity. But seriously, time does not stop. We measure time by separation of signals received from the vicinity of the black hole. We measure light signals emitted by the object in the vicinity of the black hole and see the distance travelled per unit time decreasing as the horizon is approached. Unfortunately we are using time signals generated in our 'far-away' frame, if instead we use signals generated by the object in the vicinity of the black hole, we note that they also 'stretch'. In effect if frames are not mixed there is no discrepancy. As a matter of fact. this time stoppage effect arises when using Swartzchild co-ordinates, but it can be made to disappear using other ( I forget the name ) co-ordinates. An excellent popularization of space-time effects in the vicinity of a black hole is Kip Thorne's 'Black Holes and Space Warps; Einstein's outrageous legacy'.

Sorry to hear about your condition PMB, its not often we get to interact with forum members on a personal level. What about J A Wheeler, did you have the opportunity to get to know him ? He is, to me anyway, one of physics' icons of the last century.

Arch2008, the first link describes a specific version of string theorywhere gravity is postulated to fall off with r^9 instead of r^2. This is because gravity falls off with r^(n-1) where n is the number of dimensions. The fact that M-theory is 10 spatial dimensioned, and that in certain versions of M-theory gravitons are the only bosons whose 'string' is not attached at either end to a brane, means that they can pass through all 10 dimensions, leading to a force of gravity which is very much stronger at extremely small distances than the currently accepted theory. To investigate actual Planck scales a collider the size of the galaxy would be needed. I don't think they'll get funding for it anytime soon. I don't understand the significance of the second link.

The thrust of the engine will, as long as it is equal to or greater than the weight of the Harrier, sustain a hover at any height according to momentum conservation laws ( or Newton's laws, whichever you prefer ). However the Harrier and aircraft like it have several factors which affect this hover height. It may seem counterintuitive , but hover close to the ground is very difficult. First off you need low jet exhaust velocity or you risk ground erosion. The high speed jet impinges on the ground and causes debris ( chunks of asphalt and concrete ) and hot air to be recirculated into the intakes. Even without the debris, the hot air will appreciably reduce the thrust generated by the Harrier's Pegasus engine. The AV-8B developed from the Harrier used inward angled nozzles and a system of dams to reduce exhaust gas recirculation. High altitude hover is also very difficult. Again to reduce exhaust velocity which makes the take-offs and landings difficult, most V/STOL aircraft, including the F-35B, use turbofans as opposed to turbojets for their lower exhaust velocity, and don't use reheat ( or afterburning ). Now turbofans produce a lot of their thrust by large, but low compression, fan stages, and as a result loose thrust quickly with altitude. By 40000ft the Pegasus is producing a lot less thrust than it is rated at sea level and probably cannot hover. Aircraft design is a compromise with priorities set by operational requirements. V/STOL aircraft design, even more so as evidenced by the fact that the original Kestrel ( which became the Harrier and AV-8B and could have become the P-1154 ) flew in the late 50s and is only now possibly ( ? ) being replaced by the F-35B.

Either I and the others aren't able to understand eric555 and he is not effectively communicating his ideas, or, he has the wrong ideas. All I can add is to restate that time is relative, ie it is slower in a gravitational well or at relativistic speed compared to time far from a gravitational well or at low speed. BUT there is no absolute reference frame for time.

Maybe I wasn't clear enough in my post. If you are looking at a spacecraft approaching an event horizon, you see it travelling progressively shorter distances according to your clock. But then you are mixing frames. If you use a clock generated by the spacecraft, so that you are not mixing frames, then the intervals between clock pulses get progressively longer. in its own frame it is still travelling at its proper speed. The only time confusion arises is when frames are mixed. As to the difference between time dilation due to gravity as opposed to speed, A spacecraft is a distinct well defined frame ( defined by the outer hull ) and its speed is measured against the background stars, which are in your ( the observer's ) frame. Gravitational time dilation doesn't have a well defined frame as it changes continuously with depth in the gravity well. This is at the root of any observed differences.

You would measure the speed of the spacecraft by measuring distance travelled per time interval. If you are using a time interval generated by the spacecraft approaching the black hole, the time intervals received by you, the far-off observer, will get progressively longer and approach infinite. Gravitational time dilation works by time shifting all signals, including light, rising out of the gravity well. Some of the other members of this board are much more knowledgeable of GR than I am; Elfmotat, AJB or the absent DrRocket come to mind.

Not a big surprise Arch, since the LHC isn't capable of the energies needed to investigate Planck scale particles and events. Nor compacted extra dimensions, also of the same scale. A much bigger collider is needed for Plank scales and the only one we know of is called the Big Bang.

I thought Witten had shown that all FIVE string theories were part of a whole, ie subtheories of the ONE theory. Five, because Bosonic was not a true string theory but a failed attempt by Veneziano ( ? ) to model the strong interaction which QCD did much better.

No, you're confusing speed with time. I you could see inside a spaceship moving at an appreciable fraction of c, you would see all motion inside the spaceship as well as their clocks slow down. The spaceship and anything moving along with it, is one frame, while you, stationary on the earth, are another frame.

It is the same 'mechanism' for any gravitational time dilation. From the frame of a distant observer, clear of the gravity well, an object sinking into the gravity well experiences time dilation. Conversely an observer in a gravity well will observe a distant object's time appear to speed up. This gravitational time differential predicted by GR, verified experimentally and used in every GPS, is not encountered just in the vicinity of black holes, but also our sun and even between earth orbit and surface. The only difference is that at a black hole's event horizon, a far observer will 'see' ( if he could see loonger and longer wavelengths approaching infinite, of light ) the infalling object time dilation approaches infinite, ie time appears to stop at the event horizon to a far observer. This is purely based on the chice of reference frame, however, since an infalling object or person, sees no slow-down in their subjective time. They see them selves continue through the event horizon with no slow down or stopping, just as we on the surface of the earth don't see a time slowdown compared to astronauts. As has been mentioned before light always travels at c, and so light is not trapped inside the event horizon, unable to escape. Rather the infalling light ( or alternately any light emitted by the interior 'structure' of the event horizon ) is red shifted to infinitely long wavelength and so cannot be seen or detected. It is this absence of any visible emissions which give rise to the term black hole, its not really a hole in space.

Welcome back AJB, it's been a while.

I still remember the HOTOL, Sanger and forget the name of the French concept, for a reuseable launch vehicle in the closing years of the last century. Shuttle type launch vehicles became a maintenance nightmare and used technology from 1980 ( intel i286 processors ?? ). New technology would solve a lot of the problems and make them a viable choice rather than going backwards with the Ariane ( or Russian ) launcher.

No apologies necessary. As you've no doubt noticed, I have bad days too and tend to snap at people. Forgive, forget and move on.

You said "Does this help ?" and I said "Not really." , for the reasons that I explaned. One surface, wing or sail will not have a force component at an angle less than 90deg to the incident airflow no matter what the geometry ( ? ). So, yes, I did think about what you said, but it made no sense. If you aren't prepared to explain, or don't know, just say so and I'll research it myself.

The quasar IS the black hole's polar jets. Only 'active' black holes have accretion discs, which is mass spiralling and infalling along the black hole's plane of rotation. This superheated ionized plasma generates elecromagnetic radiation, usually very energetic gamma rays, at angles perpendicular to the plane of rotation ( along spin axis ). Galactic core black holes, when active, generate tremendous amounts of energy by this method, rivalling the intensity of billions of stars and outshining galaxies. Active galactic cores were more common billions of yrs ago, they have mostly settled down in current times. Neutron stars have similar effects, but because of the lower energies involved, produce x-ray polar jets and we call them pulsars.

Not really. Using simple vectors you can show that a lifting surface, sail or wing, has no component in the direction of its motion, ie. into the wind. Otherwise you could use a wing for propulsion. I've been told, by sailors, that it has to do with sail/keel interplay but I've been too lazy to actually investigate the phenomenon.

Given a gravitating mass and a radius you should be able to determine the acceleration due to gravity, Airbrush. Its simple Newtonian gravity and it does not change as you approach, reach or pass through the event horizon. Work it out for youself, I believe the absent DrR worked it out in another thread. The energy to accelerate a given mass is supplied by the gravitating body ( conservation laws remember ? ). The infinite energy needed to accelerate a body with mass to light speed would have to be supplied by an infinite sized black hole. Do you see where your argument falls apart ?

You seem quite knowledgeable on the subject. Is the interaction between the keel and the sail responsible for the ability of a sailboat to tack against the wind ? I could never understand how that was possible.

I think we may be stuck in a causality ( time ) loop, We keep having the same discussion over and over with the same result.

The timelines are very specific because they occurr at the times of symmetry breaking of the early universe when the original force of GUT and electroweak split into today's strong weak and EM forces. These two symmetry breaking events take place at very specific energies and so the time can be derived.

You may be right about these differences, but consider then the vertical stabilizer on an aircraft. It has a symmetric rounded leading edge/ sharply pointed trailing edge to which all my coments apply. A lot of aircraft wings are symmetric these days, especially any that spend any time dealing with supersonic flow, they generate lift with leading and trailing edge flaps and other devices, and angle of incidence. Even subsonic airliners or business jets can have locally supersonic flow over the top of the wing under certain conditions.