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 Feb 21 comment Keeping air in a well For what it's worth: Mars's Hellas Planitia is 23km below datum and has 30km-high walls, and the atmospheric pressure at the surface at the bottom is 1155 Pa; normal atmospheric pressure is only 610 Pa. Not good enough for humans either way. :( Feb 19 revised Problem on nuclear physics radioactivity edited tags Feb 17 comment What is the Correlation Between Solar Wind Velocity/Density and Sunspot Count? iopscience.iop.org/0004-637X/587/2/818/fulltext/16823.text.html Feb 15 comment 17 Joules of Energy From a Mouse Trap I was going to make this a joke comment but then I realized that it's an answer :) Feb 15 answered 17 Joules of Energy From a Mouse Trap Feb 13 revised If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion? deleted 2 characters in body Feb 13 comment If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion? This is fair enough, I will edit my answer. Thanks. Feb 12 answered If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion? Feb 9 comment units and nature What you are saying, @RonMaimon, is that basically all the units are like the candela. They're just there to make things convenient for human experience. Feb 9 answered units and nature Feb 4 answered Physically what happens during Avalanche breakdown to the pn junction? Feb 4 answered Physics of the Internet? Feb 4 comment Physics of the Internet? This user's profile indicates he is a HS student. I am willing to give him the benefit of the doubt. Feb 2 answered Are matrices and second rank tensors the same thing? Feb 2 comment Energy required to reach 1 wavelength Thanks, @Community♦ for catching my error in the sign of the exponent. Feb 2 comment Energy required to reach 1 wavelength As @Martin alludes to above, it's not correct to think of photons as "using" or "needing" energy to travel. They always travel; that's what they do, and always at the speed of light. A photon doesn't need to "reach" a frequency: when it is emitted, it already has that frequency. It's not like a mechanical wave. Photons essentially are energy. And absent hitting something, a photon will keep travelling to infinity. Feb 1 answered Energy required to reach 1 wavelength Jan 31 comment What experiment would disprove string theory? I like the metaphor of the extension of the positive integers to the signed integers as a model for the extension of relativity+quantum theory to string theory. But I think a better metaphor is the extension of Euclid's axioms. It turned out there were multiple consistent (even "correct") extensions of the axioms. But of course, only one extension is "true" (or "real) for any given universe. This is rather the question being asked here: is there any experimental way to know which extension reflects reality? Jan 31 comment What experiment would disprove string theory? Your indication of the importance of quantum gravity to this question is a great point. Jan 29 comment How can I measure the conductivity of a copper rod? Well, you could always measure the length and the resistance with a ruler and ohmmeter, and just calculate $\sigma=1/(\rho * l$)