# Nikolaj K.

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A duck walks into a bar. Animal control is promptly called and the duck is released into a near by park.

 17h comment How rigorous can conservation of energy be made? In physics you try what has worked before until it doesn't. Conservation of energy is a showpiece of the cookbook, hard to get rid of. And people like it, it's tasty. I also asked a related question, and see the related links posted by Cmechanic. Mar12 comment Can Somebody give me Questions You misinterpreted the Q&A concept. Mar10 comment Why quantising gravity necessarily give us gravitons? Do you know if there is any hypotetical way of tweaking the spin degrees of freedom and make the graviton a fermion? Are there physical arguments against that? Mar7 comment How to find different operator representations in QM? @DepeHb: i.imgur.com/IchgG0c.png Momentum is that which is conserved if you translate the system. Turns out the operator $T_d$ which shifts a function $f(x)$ by $d$ can be written as $\exp\left(i\hbar\,d\,\left(\frac{1}{i\hbar}\frac{\partial}{\partial x}\right)\right)$. The generator is the momentum operator $\propto\frac{\partial}{\partial x}$ and so as function of $x$, the eigenstates $\left|p\right\rangle$ of that operator must be $\mathrm{e}^{cpx}$. Mar7 comment Naive visualization of space-time curvature @StanLiou: Then you are assuming you can explain the notion of a local interial frame to a layman. Mar5 comment Is there a limit to acceleration? @Carterini: Not that a factor of 2 is of any relevance if we're already speaking of $10^{51}$, but no, $2.2$ is the value you get if you plug in "(speed of light)^(7/2)/((Plancks constant)*(Gravitational constant))^(1/2)" into wolframapha and so here I bet against your back of the envelope calculuation. And I say it again, I'd just sit down and see how fast momentum can translate between two particles, say with a $\left|r_1(t)-r_2(t)\right|^{-2}$. Mar4 comment Is there a limit to acceleration? Afaik, there is no slogan about maximal acceleration. You could cook up a Planckian acceleration $c^{3.5}/(\hbar G)^{0.5}\approx 2.2\,10^{51}m^2/s$, but I don't see the use for that here. Seems the latter must ask for r'' vs. W. We should maybe make it a little more concrete. Consider a particle at rest with $r_1=0$ at some time and set up a potential $V(|r_1-r_2|)$ with another particle. You can consider a) a free collision (where total momentum is constant) and compute $r_1''(t)$. b) try to compute the energy cost it takes to move the second particle in some way to push the other one. Feb27 comment What's the real fundamental definition of energy? ...and call this the Lagrangian, for the map $t\mapsto q(t)$ it should read action there, alternatively you can speak about the image of that map. Feb26 comment Why do we use $\psi$ instead of a straightforward probability? This thread should answer the question. Feb25 comment Quantum frequency vs classical frequency and Energy dependence From the point of view that a QFT starts by setting up the states and the Hamiltonian $H\sim \nu\ a^Ta$, the QED energy is given like this and is responsible for both of course $\nu$'s. I.e. $\langle H\rangle\propto \nu$ and "$\mathrm{e}^{-itH}\sim>\sin{(t\nu)}$". Feb24 comment Discrete sum over a gaussian function @EmilioPisanty: ListPlot3D Feb23 comment Why is introductory physics not taught in a more “axiomatic” way? @TerryBollinger: Doesn't the abstract S-matrix approach do exaclty that? Feb23 comment Why is introductory physics not taught in a more “axiomatic” way? @TerryBollinger: Uncomfortable similarities? Stop worrying and learn to love the bomb. ;) Feb20 comment What is the difference between translation and rotation? That's vague. Can you give an example for a situation where "a rotation motion corresponds to a force acting on a body towards a direction perpendicular to its velocity." and where you can make precise what the "velocity", acting "force" and "rotation" is? Feb19 comment Does the 1-D poisson's equation have monotonic potentials if $\rho=\rho(\phi(z))$? @Anode: "It's no surprise that $\phi(z)=0$ in your example." What do you mean by that? The solution is $\phi(z)=a·z+b$ and these are valid for all $a,b$. The conclusion you should draw isn't that $\phi(z)=0$ follows. Feb19 comment Does the 1-D poisson's equation have monotonic potentials if $\rho=\rho(\phi(z))$? @Anode: So? $\rho(\phi):=0$ fulfills $\rho(\phi=0)=0$. So I'm pointing out that for the trivial case, the concluded relation is already broken. That's a general tool for investigating mathematics: If you're skeptical of a result obtained with free parameters, go back and choose particular examples for those parameters to see at which point in the derivation the system breaks down. Feb19 comment Does the 1-D poisson's equation have monotonic potentials if $\rho=\rho(\phi(z))$? For $\rho(\phi):=0$ the solution is $\phi(z)=a·z+b$ and the equation reads $0=0$. You say you "integrate up" and end up with $\frac{1}{2}a^2=0$. Magic. Feb11 comment What is the relation between General Relativity and Newtonian Mechanics? I know what you want to say in your first sentence, but what you write isn't information one can work with. And going further, it's not clear what "case ... can be proven means", but you are implying that at least any result obtained in Newtonian spacetime agrees with the corresponding one in general relativity. But that's not true and it's not clear in what sense you'd "arrive" at Newtonian equations. Feb7 comment What justifies dimensional analysis? @KyleKanos: Well, if it's arguing to you then I can understand - otherwise one has the chance to learn. And I don't think people think enough about units to understand them, mostly because the practical approach suffices most of the time. When I say I don't understand physical units I mean in the sense that mathematicians don't understand prime numbers. Feb7 comment What justifies dimensional analysis? @KyleKanos: Of course you must give meaning to it, but it's not like that step is avoided in the case of making sense of $3\cdot 2\ kg\ s^{-1}$. You just effectively define "makes sense" as "no person working in a physics department has been able to publish a paper on it that others find valuable". You don't tell the OP something which he doesn't know.