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 May 26 comment What does it mean to say that the electron is a near-perfect sphere? Jerry Schirmer is correct, but just to rephrase his answer so it answers the question as asked: "What does it mean that the electron is [almost a perfect] sphere?", it means that the electric dipole moment has been measured to be very small. May 23 comment Is there a “Size” Cutoff to Quantum Behaviour? Re: the baseball example, if you want to make a double-slit for a baseball, the slits have to be a few cm wide (otherwise the baseball can't get through). To see interference fringes from slits separated by a few cm, you'll need the deBroglie wavelength of to be on the order of a few cm. To get that, the baseball has to be moving incredibly slowly. So slowly that it'll take longer than the age of the universe to go through the slits. Waiting that long isn't technologically feasible. I've glossed over some math, and there are other issues as well, but I'll skip those for now. May 23 comment Is there a “Size” Cutoff to Quantum Behaviour? Re: the Reynolds-like cutoff: no, there isn't a known one, as described in the answer. May 23 answered Is there a “Size” Cutoff to Quantum Behaviour? May 23 comment Why is travelling around the speed of light a problem? Just to be a wiseass, I'll point out it's certainly possible to get light propagating with a phase velocity faster than c (where c=speed of light in vacuum). It's also possible to get light propagating with a group velocity faster than c. Folks do it all the time. Of course, this doesn't answer your question, but I'm not sure anything will. Does Jerry Schirmer's formula help you understand why light travels at the speed of light any more than pointing out "that's the way our universe is"? It's an experimental "fact" that light in vacuum always travels at the same speed. May 23 comment How many boats does it take to find an acoustic buoy by Doppler shift? I'll try to edit my answer appropriately once I figure out what's going on. In the boat position/velocity example you give, I think the location of the buoy will be unambiguous if it a distance greater than 1 from the origin (and for certain locations within that circle as well). So (and I think this is what you were getting at in your 2nd comment) by putting the boats very close together you can get an unambiguous answer. Of course, in the limit of zero separation you can't locate the buoy. So if this "close boats" solution isn't allowed, I'd have to think about how many you'd need. May 23 comment How many boats does it take to find an acoustic buoy by Doppler shift? @dmckee: Thank you for the example. You are correct. May 19 comment In a low pressure (near vacuum), -20°C vessel, would sublimated moisture rise to the top or fall to the bottom? I agree with the answer, but to be explicit: it will fill the vessel uniformly in the limit that mgh << kT. If you were to build a vessel that was a few 10's of kilometers tall (it wasn't specified) the answer would change. May 17 comment How many boats does it take to find an acoustic buoy by Doppler shift? dmckee: I don't understand your objection, but perhaps that's because I'm not envisioning the same case you are (I don't know what it means for boats to "diverge"). Could you give a specific example, such as the x and y coordinates of the two boats, their velocity vectors, and the location of the buoy? May 16 comment How many boats does it take to find an acoustic buoy by Doppler shift? I disagree that 3 boats is not sufficient for the general case. I think we agree that 2 boats (with a 3rd to measure frequency) will give two "lines of position" extending out each boat. But these lines will only intersect at one point, not 4. They are not infinite lines, but only "half-infinite" (with one end at the boat of origin, the other at infinity). May 16 answered How many boats does it take to find an acoustic buoy by Doppler shift? May 14 answered What is an observer in quantum mechanics? May 13 answered How do you explain spinning tops to a nine year old? May 10 comment stability of hypothetical lunar atmosphere Dr Motl: how do you get the "100 seconds for a molecule to try another value of its velocity" figure? May 10 comment Low-temperature hobby rocketry EAMann: your objection to the water rocket doesn't make any physics sense to me. The energy is stored in compressed air contained by the rocket itself (at least it was in the water rockets I played with as a kid). Sure, that energy came from somewhere else, but that's no different than saying the gasoline in my car came from an external source). If you don't like water rockets because they're not as cool as the black powder rockets, then I'm with you, but the "self-contained" objection is malarky. Maybe a mini-C02 cylinder could be a viable middle ground? May 10 comment Electric field at a point being an $n^{th}$ derivative of electric (or magnetic) field at some other point I totally don't understand the question. The electric field at some point CAN'T be equal to the derivative at some other point: a field and its derivative have different units. Do you mean proportional to its derivative (at some other point)? If that's the case, and you're really only referring to two points and a single value of n, then pretty much ANY distribution would satisfy it. What am I missing here? May 10 comment UV reflective surfaces Then I'll just repeat what Martin Beckett said: aluminum is a decent reflector at 390-400nm as well as in the visible. The folks who professionally make mirrors out of aluminum can get you R>90% at 400 nm, and R>80% for most of the visible. If you wanted 99% reflectivity, you'd need a broadband dielectric, but from the sound of it aluminum foil would do the job for you. It'll get you R>50% (and even do your diffusing if you use the "other side") and costs less than \$5. May 10 answered Physics of scaling up an animal: the neck May 10 comment UV reflective surfaces Probably, but how easy or difficult it will be depends on the wavelength. The shorter the wavelength, the harder it will be. If you're talking "Vacuum ultraviolet" it will be quite difficult to make a diffusor out of anything. But because you've got LED's, I'm sure you're not talking about vacuum UV. What is the wavelength you care about? And how efficient of a reflection do you need? May 9 answered How many digits of Pi are required in physics?