jld
Reputation
2,250
Next privilege 2,500 Rep.
Create tag synonyms
 Oct 14 awarded Necromancer Sep 27 comment $\hbar \rightarrow 0$ in quantum mechanics @UnlimitedDreamer We do the same sort of thing when we take $c \to \infty$ to get from relativistic equations to Newtonian equations. $c$ has a definite value, but by pretending it is infinite our equations become Newtonian. In the same way, when we pretend Planck's constant is zero our equations go from quantum to classical. Edit: Just realized this is an old question. Not sure why it popped up in my recent questions page. Sep 8 reviewed Approve Do solar systems typically spin in the same direction as their galaxy? Sep 8 awarded Custodian Sep 8 reviewed Approve Do solar systems typically spin in the same direction as their galaxy? Jul 2 awarded Enlightened Jul 2 awarded Nice Answer Mar 22 comment Why does the electric field escape a black hole? A field isn't a 'thing' that can be sucked into a BH. A field is a mathematical object that has value(s) at every point in space. There is no reason to ad hoc assume that the EM field outside of a BH is zero. Feb 10 awarded Yearling Jan 22 comment Does potential energy always equal kinetic energy? @StanShunpike $T=V$ under special circumstances. What's important is that $T+V=constant$ over time. There may be some points on a particle's path where $T=V$, but it certainly won't be true everywhere. Jan 22 comment Does potential energy always equal kinetic energy? But $T \neq V$ in general. However $dT/dt = -dV/dt$ in general. Jan 22 comment Confusion about what the Euler-Lagrange equation says I'm not sure I understand what you're asking. Given a Lagrangian, the E-L equations tell you what the equations of motion are. Dec 30 comment Can one of Newton's Laws of motion be derived from other Newton's Laws of motion? @Timaeus x(t)=t^3 does not have a(t)=0. It has a(t)=x''(t)=6t. The only moment where a=0 is at t=0. It is implicit in my answer (and I have now explicitly said it) that I'm setting a(t)=0 for all t, not just at a single instant. a(t)=0 does in fact imply v(t)=const. Dec 30 comment Can one of Newton's Laws of motion be derived from other Newton's Laws of motion? @Timaeus Why the strawman? I clearly never said "a=0 at a single instant is enough to tell you v=const." When I say "set F=0" I'm implicitly doing this for all time, and that is indeed enough to tell you that v=const. You're arguing against a strange strawman that I clearly never stated nor implied in any way. Dec 29 comment Can one of Newton's Laws of motion be derived from other Newton's Laws of motion? @Timaeus Here in the real world we need only consider accelerations. Realistic equations of motion are always first or second-order. Dec 23 comment What are the relative limitations of the Schrödinger, Pauli, and Dirac Equations? Both the Dirac equation and the Klein-Gordon equation fail to make much sense if you attempt to use them as single-particle wave equations. They both require the "correct" interpretation, which is in the context of quantum field theory. When you mix relativity and QM you're inevitably led to many-particle systems, where particles can be created and destroyed. Dec 12 comment What's the Cause of Quantum Entanglement? @Sofia Just so you know, you said "the-function" instead of "wave-function." Dec 5 comment Can Schwarzschild black holes evaporate? The Schwarzschild metric is static by definition. If the metric changes over time then it's not the Schwarzschild metric. Simple as that. Dec 5 comment Find time-parametrization given path and speed of a particle Hint: the speed is the magnitude of $r'(t)$. Dec 1 comment The virtual particles are only a fictive tool in equations? DO they exist or DON'T? And if they exist, why do we call them VIRTUAL? @PeterShor I see what you're saying, that anything we interact with must be a bit off-shell, but I think you're carrying the interpretation of Feynman graphs too far. They are just approximation tools. Particles aren't "really" in momentum eigenstates either. If I was going to describe what's "really" going on I wouldn't use perturbation theory or virtual particles to begin with.