David H
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 Jul 23 awarded Yearling 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 23 awarded Altruist 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. Dec 19 awarded Investor Sep 10 awarded Nice Question 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 23 awarded Yearling 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.). Jul 11 revised How does quantization solve UV catastrophe in black body radiation? What would happen if there was no Planck constant $h$? Added latex; minor grammatical fixes Jul 11 suggested approved edit on How does quantization solve UV catastrophe in black body radiation? What would happen if there was no Planck constant $h$? Jun 1 answered Measuring the nearest order of magnitude 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.