David H
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 Apr 2 comment Could any object have zero mass? My feeling is that the assignment of null rest mass to photons can have physical meaning, at least in a sense. With mass-energy equivalence, this is essentially the statement that rest-photons do not exist. Jan 12 comment What's the boundary of microscopic world and macroscopic world? The boundary between the microscopic world and the macroscopic world is the mesoscopic world of course! Jan 11 comment How do we stabilise satellites so precisely? They use yo-yos. :) Dec 20 comment Dependence of kinetic friction on relative velocity Well, for starters do you understand why it shouldn't really depend upon relative position (like gravity)? If so then velocity is the next obvious candidate. Dec 20 comment Dependence of kinetic friction on relative velocity In different regimes you might approximate the dependency by some power law, e.eg., proportional to $v$ or $v^2$, but in general there is no simple mathematical function you can write down that will represent the exact dependency. Dec 20 comment How would gravitons couple to the Stress-Energy tensor? This question runs a high risk of ordering outside the domain of mainstream physics. Since we don't yet have a quantum theory of gravity, providing a complete account of the behavior and interactions of gravitons is just not something we can do at this time. Aug 9 comment Why is it more convenient to consider space or time as a continuum? @bobie Because plancke units are too large. Suppose you have a circle with radius equal to one plancke unit. If you try to calculate the area of the circle of this unit circle via "integration" with $dA$ equal one plancke unit-squared, you get an area of 4 units-squared instead of $\pi$. Aug 9 comment Why is it more convenient to consider space or time as a continuum? The primary benefit of regarding space as a continuum is calculus. Jul 12 comment Is the universe infinite? @Sayans25 That said, there is a wealth of partial results on the conditions for / implications of a finite/infinite universe. See en.m.wikipedia.org/wiki/Shape_of_the_Universe#Global_geometry Jul 12 comment Is the universe infinite? We don't know the answer yet, and on some level we're not even completely certain the questionable answerable. It would sure be neat if someone figured it out though. Jul 11 comment Why time is considered a dimension? I think the question the OP is trying to ask but doesn't know how to is why the dimension of time is considered to be a geometric dimension as opposed to just being a parameter off in its own category (like the dimensions of charge, mass, temperature, etc.). Jun 1 comment Measuring the nearest order of magnitude @Freddy You obviously can't stop at just the formula that gives $L$ as a function of $d$, because the problem asks you to give a number for $L$, not a function, and you don't have a number yet. So it's time to guess a value for $d$. This step makes some students uncomfortable at first because the worry over how to guess "the right value". Take a deep breath and guess anyway. You can always guess again. Jun 1 comment Measuring the nearest order of magnitude What rob said. Moreover, that's the entire purpose of questions like these. The question intentionally forgets to tell you how big $d$ is, forcing you resort to a guess and deal with the uncertainty that unavoidably enters into your result. This is the difference between an actual physics problem as opposed to mere applied math problems. May 26 comment Icecube experiment at the North pole instead of the South? The biggest reason is probably because the North Pole is in the middle of the Arctic Ocean. The IceCube laboratory was able to be installed 1 kilometer below the surface of the Antarctic ice sheet because the depth of the ice sheet goes down almost 3 kilometers, and below that is solid land. At the North Pole, the ice is a mere 10 feet thick in most places before you hit liquid water, and even that 10 feet might melt altogether come summertime. May 21 comment time dependent current/ magnetic field You could in principle use Jefimenko's equations, but I imagine that's a bit overkill here. May 4 comment “Periodic Table” of Particles of the Standard Model? @Geremia The inner circle is the set of particles that interact with gluons (strong force), the middle circle is the set of particles that interact with photons (electromagnetic force), and the outer circle is set of particles that interact with W's and Z's (weak force). We know some of the particles the Higgs interacts with but we're not sure if there are more or not so no circle as of yet. May 3 comment “Periodic Table” of Particles of the Standard Model? @Geremia Your table is out of date. It has 17 particles now. upload.wikimedia.org/wikipedia/commons/0/00/… May 2 comment Why is the Coulomb taken as the unit positve test charge, though $1 C$ charge is a high value? And note that 1 ampere of current (~ 1 light bulb) is neither an extremely large nor extremely small amount of current. In fact it's a very ordinary amount. Apr 26 comment Weak interaction and Neutrino You heard wrong. All known fermions interact through the weak interaction. That includes all leptons and all quarks. Apr 21 comment Schroedinger equation. Mass. Charge You could say the same thing about $F=ma$.