Reputation
2,156
Next privilege 2,500 Rep.
Create tag synonyms
Badges
5 28
Impact
~45k people reached

Oct
10
answered Is there a geometric interpretation of the spacetime interval?
Oct
10
comment What is the meaning of “a straight line”?
John Henckel: "[...] no concept of "straight" except for the geodesic itself." -- That's an overstatement. No: "straightness" can be understood simply as "distances adding up" between three given points (but without any "geodesic itself" existing between these three points, due to "some points missing from the space"). Or: "straightness" can be understood as "curve and its tangent running parallel" perhaps at even only one point. I'm very interested in whether these two notions might indeed be inequivalent in some settings. But I don't know whether that's the topic of the book mentioned.
Oct
9
revised Special Relativity and Gravity
Unstucked the "n".
Oct
9
suggested approved edit on Special Relativity and Gravity
Oct
8
revised To what extent can the superconducting order parameter be thought of as a macroscopic wavefunction?
corrected spelling the surname of H. Fröhlich; cmp. https://en.wikipedia.org/wiki/Cooper_pair https://en.wikipedia.org/wiki/Herbert_Fr%C3%B6hlich and http://physics.stackexchange.com/questions/17994/how-to-write-the-fr%C3%B6hlich-hamiltonian-in-one-dimension
Oct
8
suggested approved edit on To what extent can the superconducting order parameter be thought of as a macroscopic wavefunction?
Oct
1
comment Deduce time dilation from a Minkowski diagram?
Atriya: "does [...] represent time dil.?" -- As a construction of/in Euclidean geometry, the given diagram is indeed suggestive of$$\begin{align}~&\frac{\text{d[ blue dot, pink dot ]}}{\text{d[ blue dot, intersection of green lines ]}}=\cr~&\sqrt{1 - \left( \frac{\text{d[ blue dot, pink dot ]}}{\text{d[ blue dot, intersection of green line and red line ]}}\right)^2}.\end{align}$$It doesn't show, however, how to decide which participants belonged together to one "(inertial) reference frame", how to compare durations, how to compare (chronometric) distances etc. for deducing SR time dil.
Sep
28
asked Given the intervals of a punctured flat spacetime how to reconstruct the intervals of the complete flat spacetime?
Sep
28
comment Are there Planck units for weak or strong “charge”, similar to the electromagnetic Planck charge $\sqrt{4~\pi~\epsilon_0~\hbar~c}~$?
zooby: "You don't need c and h to define these numbers [...]" -- Comparing with $$q_P^{el.-mag.} = \sqrt{4~\pi~\epsilon_0~\hbar~c} = \frac{e}{\sqrt{\alpha_{el.-mag.}}}$$ from the definition of (el.-mag.) Planck charge, what's stopping us from expressing $$q_P^{weak} = \sqrt{4~\pi~\epsilon_{weak}~\hbar~c} = \frac{Y_W}{\sqrt{\alpha_{weak}[~0~]}}$$, where $Y_W$ is the weak hypercharge, or $$q_P^{strong} = \sqrt{4~\pi~\epsilon_{strong}~\hbar~c} = \frac{1}{\sqrt{\alpha_{strong}[~0~]}}$$?
Sep
28
comment Are there Planck units for weak or strong “charge”, similar to the electromagnetic Planck charge $\sqrt{4~\pi~\epsilon_0~\hbar~c}~$?
zooby: "All are dimensionless constants." -- The Planck units for weak or strong "charge" are dimensionless constants? What are the (experimentally determined) values of these two (real) constant numbers?? What are the concrete expressions for these two numbers, involving (by definition) Planck's constant ($h$; or the "reduced form" $\hbar$)? (Btw. Planck's constant $h$ and the reduced Planck constant $\hbar$ are a dimensionful quantities.) Do these expressions also involve corresponding "permittivities $\epsilon$" of suitable dimension?
Sep
27
comment Do relativistic events need to match if accounted for time dilation and length contraction?
@dmckee: "For time-like separated event[s] $\Delta s$ is [...]" -- But how to call this quantity, as such, besides giving it some notation? I like: "spacetime chord between the two events (such and such)". How about you? "some authors (Landau and Lifshitz [...]) call ${\rm d}s$ the interval" -- That would be rather a "chord line element"; and not to be confused with a "line element". "is the proper time" -- Duration of whom?, between what? ...
Sep
26
comment Invariant mass spectrum to a transverse momentum distribution
Samuel: "Thank you !" -- You're welcome. "But I don't really see how you get the last formula ?" -- Actually, I typed some equivalent of the second, third, and fourth equation of my answer in my equation solver program, solving for $\|\mathbf p_{lab}^{trans~H}[~\gamma~]\|$ (which is the same value for both photons), along with $\theta_1$ and $\theta_2$. (The version history of my answer shows that I also had made some mistakes in the process.) Now your question made me think again ... So I've simplified and checked:Solve[ M^2 + (Sqrt[ e^2 - t^2 ] + Sqrt[ f^2 - t^2 ])^2 == (e + f)^2, t ].
Sep
26
comment Muon demonstration of time dilation
Javier: "I haven't met in my life a single person who would be confused by the use of the word "time" for a duration." -- Well, next time you meet a person who is using the word "time" in what seems (to you) the specific sense of "duration", at least you have the opportunity to ask. And next time someone is using the word "time" in what seems (to you) not the specific sense of "duration" then you have the (verbal) means to avoid confusion yourself.
Sep
26
revised Does the stellar MK classification fix the apparent luminosity?
corrected spelling the surnames of J. Stefan and L. Boltzmann; cmp. https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant
Sep
26
suggested approved edit on Does the stellar MK classification fix the apparent luminosity?
Sep
26
comment Do relativistic events need to match if accounted for time dilation and length contraction?
dmckee: "There is a value related to the two events that all observers can agree on. The "interval" $(\Delta s)^2$ [...]" -- I have two questions regarding your suggested notation for the "interval". 1. If the value is related to (or depending on) two specific events, then should its notation not include the names of these two events explicitly as two arguments? 2. By using the pair of parentheses in your notation you're suggesting that "$\Delta s$" denotes by itself a meaningful quantity whose square is the "interval". How do you call this quantity "$\Delta s$" itself?
Sep
26
revised Question on Shockley's diode equation
copy-editing
Sep
26
suggested approved edit on Question on Shockley's diode equation
Sep
26
answered Muon demonstration of time dilation
Sep
26
comment Muon demonstration of time dilation
Javier: "What the text calls half-life should really be mean lifetime." -- The PDG actually calls (each value of) this quantity "mean life"; see pdg.lbl.gov/2014/tables/rpp2014-sum-leptons.pdf Specificly, it is a duration; being the mean value of individual life durations of muons of a suitable sample. Correspondingly, the values of mean life are stated in units of "second" which is in turn declared as a duration. (The word "time" is used very broadly and it is depreciated when referring specificly to durations.)