# All Questions

3k views

### Is it possible to calculate weight of person with sensors found in today's smartphone?

Is it possible to calculate the weight of a person by only using his smartphone, some action he must perform (jump, rotate etc.) and some data like his height or age. Current smartphones have the ...
95 views

### Gibbs Ensemble initial densities

I'm trying to calculate coexistence curves and get a rough estimate for the critical point of simple fluids using the well-known Gibbs Ensemble Monte Carlo (GEMC) (c.f. Panagiotopoulos, Molecular ...
102 views

### Physics behind 2 different wall kicks

I'm looking at a situation where a sprinter starts at some distance x from a wall. The goal is to sprint to the wall, hit it, and get back to the start as quickly as possible. There are two ways ...
221 views

### Does light escape when I open my blinds?

When I open my blinds, there is light enough to read a book outside while the lamp is on inside. So when I close my blinds, does the light that otherwise will escape, stay inside and thus being ...
1k views

### canonical and microcanonical ensemble

What does one mean by canonical and micro canonical ensemble in statistical mechanics? Can one elaborate on this in a very simple way with examples? Pardon me, if it is a very simple thing; I am a ...
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Lets consider the Bose-Einstein and Fermi-Dirac statistics: Bose-Einstein statistics: $$\langle n_i\rangle = \frac{1}{\exp{[(\epsilon_i-\mu)/kT]} - 1}$$ Fermi-Dirac statistics: $$\langle ... 1answer 77 views ### Accurate quantum state estimation via “Keeping the experimentalist honest” Bob has a black-box, with the label "V-Wade", which he has been promised prepares a qubit which he would like to know the state of. He asks Alice, who happens also to be an experimental physicist, to ... 2answers 198 views ### Why did population III stars lack planets? Jay Wacker1 (professor of physics at the SLAC National Accelerator Laboratory) stated: The first stars (known as Pop III) were made out of hydrogen and helium. They had no planets. Why couldn't ... 3answers 717 views ### Lorentz force in Dirac theory and its classical limit It is well known that in Dirac theory the time derivative of P_i=p_i+A_i operator (where p_i=∂/∂_i, A_i - EM field vector potential) is an analogue of the Lorentz force: \frac{dP_i}{dt} = ... 2answers 320 views ### Local Charts in General Relativity We may consider a "local" region in curved spacetime (local in respect of the spatial and the temporal coordinates). A "local inertial frame" may be constructed by some transformation that produces ... 1answer 743 views ### Help with Cutkosky cutting rules for fermions I know that a cut boson propagator is replaced with the mass shell delta function. But what happens when you cut a fermion propagator? Do you just replace the denominator with a mass shell delta ... 1answer 734 views ### Conservation of angular momentum for a nonrigid body Question: The sun is not a rigid body but a hot ball of gas. The period of rotation varies from 37 days at the pole to 26 days at the equator. The mean radius of the sun is 7\times 10^8\text{ ... 3answers 490 views ### What are the final particles emitted from an evaporating black hole? Hawking radiation predicts that black holes can slowly evaporate through the effective emission of a particle. This particle is a real particle, as in, it is not a black hole itself. I'll write this ... 2answers 844 views ### Capacitance of Non-concentric Spheres We all know how to obtain the capacitance C=\frac{ab}{b-a} (ignoring constants) for two concentric spheres of radii a,b. I was just thinking to myself, what would happen to the capacitance for ... 1answer 182 views ### Back-of-the-envelope calculation of electron anomalous magnetic moment I wonder if there is an intuitive way to obtain the \frac{\alpha}{2\pi} correction to electron's \frac12 (g-2) just like how Bethe estimated the Lamb shift? Here is an attempt by Drell & ... 0answers 85 views ### How can I model a polyatomic molecule as a system of coupled oscillators? (Classical Mechanics) Let's say I have a polyatomic molecule, what is the best way for finding the equations of oscillations if they are bounded by a torsion spring? 1answer 436 views ### Buoyancy experiment with my child I've read through a few other answers here on buoyancy and I was hoping to get some guidance on discussing it with at 5th grader. So, taking a ball of playdoh clay and dropping it into a container of ... 1answer 390 views ### Proca theory and renormalization What is the simplest physical argument to claim that Proca theory (involving a massive spin-1 boson) is not renormalizable? 4answers 1k views ### Alternate layman's metaphors for illustrating curved space-time The metaphor of a surface (typically a pool table or a trampoline) distorted by a massive object is commonly used as a metaphor for illustrating gravitationally induced space-time curvature. But as ... 2answers 414 views ### Theoretical basis for black hole evaporation What is the basis for black hole evaporation? I understand that Hawking-radiation is emitted at the event horizon, a theoretical result originating in General Relativity and Quantum Field Theory, but ... 2answers 811 views ### Desperately Need Help with Grade 9 Static Electricity I am preparing to teach Grade 9 Static Electricity next week and am going crazy trying to figure out what is happening in one of my experiments. I have a short piece of PVC pipe, 4 inches diameter, ... 0answers 33 views ### status of +4/3 scalar as explanation of t\bar t asymmetry One of the early proposals for the Tevatron asymmetry on t \bar t was a "fundamental diquark" with a charge (and hypercharge) +4/3, either in a triplet or a sextet colour. I am interested on the ... 0answers 98 views ### Convergence and well-definedness of Lorentzian path integrals Wick rotation of quantum field theories to Euclidean path integrals with a nonnegative measure everywhere is a wonderful tool. Not so with Lorentzian path integrals. Events far separated in ... 6answers 572 views ### pros and cons of the anthropic principle The anthropic principle has become a very popular explanation among theoretical physicists lately. Life is unlikely, but only life can observe, so we find ourselves in an unlikely position, so to ... 1answer 231 views ### Laws of physics and general relativity I'm reading that general relativity let's us describe physics from the point of view of both inertial and accelerated observers. What does that actually mean in terms of doing actual physics? For ... 2answers 306 views ### Do amorphous metals undergo conchoidal fracture? Amorphous metals are often referred to as metallic glasses due to their quenched atomic disorder. Do they fracture in the same fashion as silicate glasses? If not, what failure mode(s) do they have? 1answer 2k views ### Constant velocity in siphon? I was recently reading the explanation for the behaviour of a siphon on Wikipedia, which uses Bernoulli's equation in its proof. The argument is generally pretty easy to follow, except for one key ... 1answer 139 views ### If a car appears in horison and within 2 seconds passes you by, whats the speed it's doing? While watching the first 4 seconds of driving at 745 km/h is ludicrous from any angle wondered If we knew the curvature of the Earth in a "flat" desert, what would be the speed of the car? ... 2answers 5k views ### What's the difference between dim and bright light? When comparing two light sources, for example, a light bulb at 20W and a light bulb at 100W, what is it about the incoming light that makes the latter look brighter than the former? Are there ... 0answers 527 views ### How do I find equations for the distance of the planets relative to one another? I know I should be able to piece together some basic Newtonian equations for this, but I'm not sure where to start. I want to be able to choose one planet as the center and calculate its distance ... 4answers 518 views ### What was the motivation behind suggesting the trichromatic theory of vision? Background In this thread, I asked whether it is true that the colors red, green and blue, through additive mixture, can make up any color. Turns out they can't. However, when reading about the ... 2answers 945 views ### Equation of motion for the reduced density matrix The equation of motion for the density matrix of a many body isolated quantum system is the von Neumann's equation: \dot{\rho }(t)=i[\rho (t),H]. How about the equation of motion for the reduced ... 1answer 970 views ### Ping-pong ball pontoon Imagine a vertical pipe (both ends opened) in the water. Drop several ping-pong balls into pipe and cover them with a cylinder. When you have enough balls, the cylinder will float. Now start adding ... 3answers 882 views ### Non-computable numbers in Physics It is known that there are real numbers that can't be calculated (non-computable numbers). Quite probably that some physical phenomena (it is possible still undetected) depend on this numbers. Whether ... 4answers 2k views ### Inertial frames of reference I'm struggling with the notion of an inertial frame of reference. I suspect my difficulty lies with the difference between Newtonian and relativistic inertial frames, but I can't see it. I've read ... 1answer 762 views ### Simplified partial trace of two operators If I have two operators A and B living in the Composite Hilbert Space H_I \bigotimes H_{II}  and I want to take the partial trace of C=AB over the subspace H_I, i.e., Tr_I[AB], is there any ... 1answer 157 views ### Quantum mechanical gravitational bound states The quantum mechanics of Coloumb-force bound states of atomic nuclei and electrons lead to the extremely rich theory of molecules. In particular, I think the richness of the theory is related to the ... 1answer 247 views ### Lifetime of a quasiparticle I'm having a rather technical question concerning the calculation of the lifetime of a quasiparticle. The reference I use is: ... 1answer 402 views ### False vacuum in axiomatic QFT There is an elegant way to define the concept of an unstable particle in axiomatic QFT (let's use the Haag-Kastler axioms for definiteness), namely as complex poles in scattering amplitudes. Stable ... 1answer 175 views ### Phase diagram of simplified QCD Consider QCD with a single generation of massless quarks (u, d). This is probably the simplest variant of QCD which bears some relation to the real world. The theory has the following exact global ... 1answer 1k views ### Chiral anomaly and decay of the pion I am told that if all classical symmetries were reflected as quantum symmetries, the decay of the neutral pion$$\pi^0 ~\longrightarrow~ \gamma\gamma$$would not happen. Why would the conservation of ... 1answer 274 views ### Additional accelerating force during take off of a rocket? During the take off of a rocket, the exhaust produces some pressure below the rocket, which gives an additional force. How large is this force in comparison to the force produced directly by the ... 1answer 199 views ### Red shift and time distortion Superman throws a light emitting object away from himself fast enough to notice a red-shift. The object passes through a region in which time runs more slowly. From Superman's perspective, does the ... 2answers 89 views ### Resources for current thought on time/spacetime? Are any of the big-name physicists associated with the time in the same way that Hawking and Penrose are associated with black holes? I'm interested in some good books that focus on the topic. 1answer 128 views ### Question regarding escape velocity Haliday-Resnick-Walker state that the escape velocity will cause a projectile to move upward forever, theoretically coming to rest only at infinity. I was wondering what the forces forcing the body ... 1answer 516 views ### Where to find experimental/theoretical value of (charge) radius of particles? Where to find experimental/theoretical value of (charge) radius of particles like proton, neutron, pi^+, etc. ? 1answer 527 views ### A physical understanding of fractionalization all! Is there a physical understanding of fractionalization in condensed matter physics? The textbook approach is theoretical, not physical. I'm thinking of spin-charge separation for electrons, the ... 2answers 1k views ### Generating Pink Noise I've generated some white noise in Excel by using the formula$$2*\mathrm{Math.RAND}()-1 I would now like to create some pink noise. I believe this is done by applying some sort of filter to the ...
I know the one-dimensional case in which the force is proportional to $e^{-R}$ and the force is attractive for solitons with opposite charge and repulsive for solitons with same charge. I was ...