Question from raxacoricofallapatorius: Why orbits don’t eventually decay?
Response from anna v:
You are right, the planetary model of the atom does not make sense when one considers the electromagnetic forces involved. The electron in an orbit is accelerating continuously and would thus radiate away its energy and fall into the nucleus.
One of the reasons for “inventing” quantum mechanics was exactly this conundrum.
The Bohr model was proposed to solve this, by stipulating that the orbits were closed and quantized and no energy could be lost while the electron was in orbit, thus creating the stability of the atom necessary to form solids and liquids. It also explained the lines observed in the spectra from excited atoms as transitions between orbits.
Question from anna v: How does uranium from supernovae explosions end up in mineral veins in a planet?
Response from Martin Beckett:
Mostly because they are heavy.
Rocks erode putting their constituents into solution, the heavy stuff settles out in river/sea beds, and metals are heavy.
For many metals hydrothermal process are more important. Super hot water deep in the earth dissolves the rock containing the minerals, it moves along cracks in the rock and cools depositing the salt and metals as lines in the rock.
In an asteroid with no geological process the metals are found in their raw state having cooled directly from the original ball of primeval gas
Question from jcw: Why don’t metals bond when touched together?
Response from Hasan:
I think that mere touching does not bring the surfaces close enough. The surface of a metal is not perfect usually. Maybe it has an oxide layer that resists any kind of reaction. If the metal is extremely pure and if you bring two pieces of it extremely close together, then they will join together. It’s also called cold welding.
Question from Nogwater: How does gravity escape a black hole?
Response from Vagelford:
Well, the information doesn’t have to escape from inside the horizon, because it is not inside. The information is on the horizon.
One way to see that is from the fact that from the perspective of an observer outside the horizon of a black hole, nothing ever crosses the horizon. It asymptotically gets to the horizon in infinite time (as it is measured from the perspective of an observer at infinity).
On how big a bubble would have to be for us to live inside? http://physics.stackexchange.com/questions/67970/surviving-under-water-in-air-bubble
Why do airplanes fly? (sersiously) http://home.comcast.net/~clipper-108/lift.pdf
Sun’s light “to be” in phase, or not in phase? http://physics.stackexchange.com/questions/69929/stupid-yet-tricky-question-why-do-we-actually-see-the-sun
Event horizon vs black hole: http://physics.stackexchange.com/questions/95366/why-does-stephen-hawking-say-black-holes-dont-exist