Top new questions this week:

Why can't Humans run any faster?

If you wanted to at least semi-realistically model the key components of Human running, what are the factors that determine the top running speed of an individual? The primary things to consider would …

forces energy simulation biophysics estimation

asked by Bogdanovist 13 votes answered by zhermes 9 votes

How can Ohm's law be correct if superconductors have 0 resistivity

Ohm's law states that the relationship between current ( I ) voltage ( V ) and resistance ( R ) is $$I = \frac{V}{R}$$ However superconductors cause the resistance of a material to go to zero, and …

electricity superconductivity

asked by Loourr 10 votes answered by Schlomo Steinbergerstein 16 votes

Can vorticity be destroyed?

I have a professor that is fond of saying that vorticity cannot be destroyed. I see how this is true for inviscid flows, but is this also true for viscous flow? The vorticity equation is shown below …

fluid-dynamics conservation-laws viscosity

asked by user2018790 10 votes answered by Mark Rovetta 3 votes

The notion of an adiabatic process in thermodynamics -vs- quantum mechanics

I'm confused about the terminology in the two contexts since I can't figure out if they have a similar motivation. Afaik, the definitions state that quantum processes should be very slow to be called …

quantum-mechanics thermodynamics terminology adiabatic

asked by Siva 9 votes answered by joshphysics 3 votes

General definition of an event horizon?

Horizons are in general observer-dependent. For example, in Minkowski space, an observer who experiences constant proper acceleration has a horizon. Black hole horizons are usually defined as …

general-relativity black-holes event-horizon

asked by Ben Crowell 8 votes answered by DarenW 1 vote

Trace and adjoint representation of $SU(N)$

In the adjoint representation of $SU(N)$, the generators $t^a_G$ are chosen as $$ (t^a_G)_{bc}=-if^{abc} $$ The following identity can be found in Taizo Muta's book "Foundations of Quantum …

homework quantum-field-theory group-representations lie-algebra

asked by soliton 7 votes answered by Peter Kravchuk 3 votes

What is the current state of research in quantum gravity?

I was browsing through this and was wondering what progress in quantum gravity research has taken place since the (preprint) publication. If anyone can provide some helpful feedback I would be …

reference-request quantum-gravity

asked by UGPhysics 7 votes answered by Ben Crowell 4 votes

Greatest hits from previous weeks:

Why does kinetic energy increase quadratically, not linearly, with speed?

As Wikipedia says: [...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $mv^2/2$. Why does this not increase linearly with speed? Why does it take so much …

newtonian-mechanics energy kinematics speed

asked by Generic Error 35 votes answered by Gerard 40 votes

Why does the atmosphere rotate along with the earth?

I was reading somewhere about a really cheap way of travelling: using balloons to get ourselves away from the surface of the earth. The idea held that because the earth rotates, we should be able to …

newtonian-mechanics fluid-dynamics atmosphere relative-motion

asked by claws 29 votes answered by Mark Eichenlaub 33 votes

Can you answer these?

Wald problem 11.4

Consider a stationary solution with stress-energy $T_{ab}$ in the context of linearized gravity. Choose a global inertial coordinate system for the flat metric $\eta_{ab}$ so that the "time direction" …

homework general-relativity

asked by WannabeNewton 2 votes

The potential and the intensity of the gravitational field in the axis of a circular plate

Calculate the potential and the intensity of the gravitational field at a distance $x> 0$ in the axis of thin homogeneous circular plate of radius $a$ and mass $M$. Could anybody describe how to …

homework gravity newtonian-gravity

asked by user50222 1 vote

Lecture Notes confusion: Constructing the Einstein Equation

This question is on the construction of the Einstein Field Equation. In my notes, it is said that The most general form of the Ricci tensor $R_{ab}$ is $$R_{ab}=AT_{ab}+Bg_{ab}+CRg_{ab}$$ …

general-relativity tensors tensor-calculus

asked by Clarice 1 vote