6,213 reputation
1028
bio website sjbyrnes.com
location Massachusetts
age
visits member for 3 years, 3 months
seen 4 hours ago

Jun
4
answered Why is the phase velocity used in the definition of the refractive index?
Jun
4
comment Why is the phase velocity used in the definition of the refractive index?
No. You cannot define index of refraction using group velocity. "Index of refraction" has a specific meaning in physics. It is a meaning that everybody learns and uses -- 100% of people, not 99%. Likewise, "group index" has a specific (different) definition. Let me ask you: "Why does the word 'velocity' always refer to the time-derivative of position, and never refers to the mass of Jupiter?" The answer is, because it's the way language works. Words have definitions. Otherwise communication would be impossible!
Jun
4
comment Why is the phase velocity used in the definition of the refractive index?
This is a strange question. If you want to talk about "c / group velocity", you call it "group index". If you want to talk about "c / phase velocity", you call it "index of refraction". It's just terminology! Those terms are as good as any. I think what you're really wondering is: "Why are there zillions of formulas that involve index of refraction, and very few formulas that involve group index?"
Jun
2
awarded  Yearling
May
30
accepted Formalism for BEC with short-distance sub-structure “corrections”
May
28
comment total noise power of a resistor (all frequencies)
@endolith -- Yes, I just said it was shorted because I wanted my question to be very concrete and specific. If you have a transmission line, it has a series of modes (standing waves), and in thermal equilibrium each mode has kT of energy (or less at high frequency). These modes exchange energy with a resistor: They give energy via joule heating, and get energy via johnson noise. This quantity 1.893E-12W/K2 is related to how fast the energy is exchanging. But, depending on what exactly you're calculating, you may need to take into account impedance matching etc.
May
28
asked Formalism for BEC with short-distance sub-structure “corrections”
May
13
comment Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
Yes, heating a system increases its entropy largely because the velocity of each molecule has a greater range of possible values. There are other effects too: At higher temperatures, there is more uncertainty in how fast each molecule is rotating, and how much it is stretching or contorting...
May
12
comment Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
The process creates heat. Wherever the heat goes, that's where the entropy increases. If the salad dressing is thermally insulated, the heat stays there, increasing the temperature and (thus) entropy. That's what user26866 is imagining. In the opposite extreme, the salad dressing might have negligible heat capacity compared to the surroundings, in which case all the heat spreads into the surroundings, so the entropy increase would occur in the surroundings. That's what Art Brown is imagining.
May
11
awarded  Nice Answer
May
9
revised What is happening to the electrons, and E & H fields, in an antenna with a standing wave inside?
added 73 characters in body
May
9
revised What is happening to the electrons, and E & H fields, in an antenna with a standing wave inside?
added 37 characters in body
May
9
answered What is happening to the electrons, and E & H fields, in an antenna with a standing wave inside?
May
9
comment Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
you mean exothermic
May
9
answered Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
Apr
27
comment Why do tunneling photons outrace their non tunneling counterparts in vacuum?
You can calculate the group delay (aka "phase time") easily, e.g. using the classical transfer matrix method. And when you calculate the group delay, you'll find that sometimes it happens to be less than the speed of light over the thickness. So that is already a "theory that explains it", if you ask me.
Apr
27
comment Why do tunneling photons outrace their non tunneling counterparts in vacuum?
When they say "no one has any explanation", they mean "no one has any intuitive explanation". Even more accurate: "no one has any explanation that I personally find to be sufficiently intuitive". There is no scientific mystery here: The math and physics of light propagation through multilayer coatings is extremely straightforward and well understood. They are stating a subjective judgment related to pedagogy. (A judgment which I would disagree with.)
Apr
27
comment Why do tunneling photons outrace their non tunneling counterparts in vacuum?
@user45342 -- Yes, I agree with all that.
Apr
27
revised Why do tunneling photons outrace their non tunneling counterparts in vacuum?
added 16 characters in body
Apr
27
revised Why do tunneling photons outrace their non tunneling counterparts in vacuum?
added 9 characters in body