33,317 reputation
241100
bio website joshphysics.com
location Los Angeles
age 29
visits member for 2 years, 7 months
seen 10 hours ago

New project: phermi.com

Let me know if you know of any hard physics problems with clever solutions. (email listed to the left)

Personal website: joshphysics.com

Currently a lecturer at the UCLA Department of Physics and Astronomy.

Ph.D. theoretical high energy physics, UCLA.

BA/BS in physics/math, UC Berkeley.


2d
awarded  Enlightened
2d
awarded  Nice Answer
Aug
31
comment Mathematics of the Virtual Displacement
Potentially helpful: physics.stackexchange.com/q/129786
Aug
28
awarded  Enlightened
Aug
28
awarded  Nice Answer
Aug
16
comment Why define four-vectors to be quantities that transform only like the position vector transforms?
@Physicslover I'm not quite sure what you mean in this context. Could you explain what it means for a representation of the Lorentz group to be Lorentz-invariant?
Aug
13
comment How can I interpret $P(t) = \frac{1}{Q(t)} \frac{dW(t)}{dt}$ physically?
You should be careful with the way you defined $W(t)$. Notice that the integral on the right hand side has no residual $t$-dependence because you are integrating over $t$. You should re-write it as a definite integral like $W(t) = \int_{t_0}^t P(t) Q(t)\, dt$, and if you do this, you see that you have to introduce some initial time $t_0$. This makes sense since how much work is done depends on when the guy checks into work.
Aug
11
revised Active versus passive transformations
deleted 220 characters in body
Aug
8
awarded  Good Answer
Jul
30
comment Heat transfer between two surfaces
@Stefan I agree. There must be an error. I haven't looked at this in a long time, so I'll need to re-read this answer when I have a bit of time to see what I did wrong.
Jul
30
comment General proof of formulas of geometric optics?
@CarlWitthoft I knew it!
Jul
29
comment General proof of formulas of geometric optics?
If I'm not mistaken, the OP wants to know how one can use snell's law and some geometry to demonstrate that provided one uses certain sign conventions, one can predict e.g. image distances using the lens equation. I'm confident that someone can recommend a reference on geometric optics that does this sort of thing in detail.
Jul
28
comment Can the uncertainty principle be redefined for different standard deviations?
Would you happen to have a good reference on this? I'd be especially interested to understand if there is a precise sense in which this is the "most general" form of the principle.
Jul
28
comment Can the uncertainty principle be redefined for different standard deviations?
@annav What does "defined in any way the problem defines" mean? I'm completely confused by that phrase.
Jul
25
awarded  Nice Answer
Jul
22
awarded  Nice Answer
Jul
15
revised What does it mean to transform as a scalar or vector?
added 135 characters in body
Jul
7
comment Are there any fully analytically solvable nonlinear oscillators?
@docscience This is matter of taste, but the definition of "exact solution" you're using seems unduly restrictive to me. I (and I think many physicists) don't have much of a problem calling solutions written in terms of, say, "special" functions (like elliptic functions) exact since they are, well, "exact" solutions in the sense that they give the solution to arbitrarily high precision, just like if the solution were written in terms of "elementary" functions.
Jul
4
comment Is particle number a problem for formulating statistical physics in a mathematically rigorous manner?
I find myself a bit skeptical of the statement "...in order for expressions like ... to be meaningful, you have to be using the grand canonical ensemble...in which particles are able to enter and leave the system." Do you object to Arnold Neumaier's answer below? Although there is certainly no issue in the grand canonical ensemble, isn't is a bit strong to say that the derivative doesn't make sense in the thermodynamical limit of the microcanonical ensemble? Don't I get the correct answer in the thermo limit using a fixed energy and particle number if I use $S = -k\ln \Omega$?
Jul
2
reviewed Reject If starting speed is faster than terminal velocity then what?