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**Question of the Week / Re: QotW - 23.12.08 What are black holes made of? **

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**Yesterday**at 23:38:34 »

What are black holes made of?Black holes are essentially places, not objects. The interior of a black hole is just more spacetime, just like outside, except more curved. If you fell into a large black hole, you'd notice no particular change as you crossed the event horizon. You could go on with you day as usual. All events inside a black hole are causally in the future of events outside, so there would be no way to affect anything outside.

The spacetime curvature would result in a tidal effect which gets stronger the closer to the singularity you get. For small black holes and other dense objects (such as a neutron star), the tidal effects would be fatal as you get anywhere close to it, let alone cross some event horizon. Hence my reference to a large black hole above where the tidal effects are not so noticeable even after crossing the event horizon.

If Escape Velocity is above 186,000 miles/sec, does a Single Photon start to get Stretched?It really makes no sense to talk about any velocity over c. There is no escape velocity from a black hole, not even greater than c.There is no meaning to a photon getting stretched. Sure, it has a wavelength, but that is frame dependent, and photons and everything else would behave pretty normal at (and beyond) the event horizon, per the first postulate of special relativity which says that the laws of physics are the same in any inertial frame, and spacetime at an event horizon is locally Minkowskian (flat), so the physics of inertial frames very much does apply there.

Zer0, the escape velocity is determined by the gravity experienced at the launch site,Escape velocity is a function not of the gravity experienced (how much you weigh there), but rather the relative gravitational potential. So for instance, escape velocity of Earth is about 11.2 km/sec, but from the surface of Uranus you might weigh about 10% less but the escape velocity there is 21.4 km/sec, nearly twice that of Earth. The difference is that the gravitational potential on Uranus is lower than it is on Earth, assuming each is the sole object being escaped respectively.

Escape velocity has little to do with black holes since the concept is meaningless for one. There is no escape, at any speed and any acceleration. It would literally be trying to travel to the past. You just can't go that way.