# All Questions

0answers
104 views

### Line-shape asymmetry in undergraduate Hydrogen-Deuterium experiment

I'm working as an LA (undergraduate TA) for an undergraduate physics laboratory experiment where students test the Bohr model and use reduced mass to determine the approximate mass of the neutron. In ...
2answers
201 views

### Bertrand's theorem

I found in Goldstein's Classical Mechanics that the condition for closed orbits is given by $\frac{d^2 V_{eff}}{dr^2}>0$.(bertrand's theorem). Can somebody explain to me, how this inequality is ...
2answers
836 views

### How can I solve a circuit of resistances with both a voltage and a current-source in between them?

If I have a circuit that consists out of multiple non-parallel/serial resistances with current/voltage sources in between them like the one below How can I solve a system like that (in this case for ...
7answers
6k views

### What is the difference between a white object and a mirror?

I was taught that something which reflects all the colors of light is white. The function of a mirror is the same, it also reflects all light. What's the difference? Update: But what if the white ...
2answers
320 views

### Reflectance of Titanium as Function of Thin Film Thickness

As far as I know, transmittance equals $e^{-\alpha x}$, where $\alpha$ is absorption coefficient and $x$ is thin film thickness($100-300\,nm$). My team and I have engineered a way to find absorption. ...
1answer
190 views

### Explanation of Superconductivity

I can't get a definitive explanation of why superconductivity happens and I am getting mixed explanations from my textbooks. I will tell you what I know and hopefully you can correct any ...
17answers
3k views

### Can pure maths create new theories in physics or does the “idea” ALWAYS come before the math?

I am in a debate with a friend about the value of string theory in physics. He is concerned that we are wasting valuable intellectual and financial resources on a path that is fanciful and can't ever ...
0answers
22 views

### Does distance really exist at least for observer which moves at the speed of light? [duplicate]

To our time a light year is the distance that light travels in a vacuum but if we consider time of observer which moves at speed of light we found for light time and distance doesn't really exist. ...
3answers
12k views

### Static Water Pressure vs Height [closed]

OK, so I have water at a known pressure, lets say 12psi. I have 2 pipes, installed vertically, one 2" diameter, the other 1/2" diameter. To what height (in feet) will the water raise to in each case? ...
1answer
467 views

### Dimensional regularization: removing more than just logarithmic divergencies?

I have followed two courses on QFT, which both involved renormalization by dimensional regularization. My confusion is that one of the professors claimed that dimensional regularization can only be ...
1answer
103 views

### Electromagnetic field to cool a substance?

I saw somewhere that an electromagnetic field would cause a substance to let off thermal energy, ultimately resulting in the substance to cool really quickly. If this is possible, does the strength ...
2answers
548 views

### Why do neutrinos propagate in a mass eigenstate?

I am aware that flavor $\neq$ mass eigenstate, which is how mixing happens, but whenever someone talks about neutrino oscillations they tend to state without motivation that when neutrinos are ...
2answers
148 views

### Uncertainty in path integral formulation

In Feynman's path integral formulation, in order to calculate the probability amplitude, we sum up all the possible trajectories of the particle between the points $A$ and $B$. Since we know ...
3answers
1k views

### Energy Spectrum of pair of spin-1/2 particles with general Hamiltonian

I found this problem, and so far I am stumped. I was wondering if anyone wanted to solve it with me, or help me calculate eigenvectors, or just give insight on my questions. Consider a system of ...
0answers
155 views

### Saturn V F-1 engines and beyond

I read that the F-1 engine from the 1st stage of the Saturn V rocket is the most powerful engine ever created by mankind, delivering ~200 gigawatts of power. Thus, I have got two questions: Will ...
1answer
432 views

### Explanation of a proper time (an add)

I have read on web that: Proper time $\tau$ is a time interval measured by a clock which at rest relative to the observer. But lets consider this problem: The plane is flying with a speed ...
1answer
348 views

### Composition of solar spectrum

I read some where that there are three types of UV and infrared rays namely UV-A, UV-B, UV-C and near infrared, mid infrared and far infrared. Which is the most abundant among the the three in ...
0answers
112 views

### IR divergence and renormalization scale in dimensional regularization (part 2)

This is in continuation of my previous question, IR divergence and renormalization scale in dimensional regularization. Lubos gave a nice answer there but I want to get to a very specific example ...
0answers
57 views

### Blocking electro-magnetic waves between two near field antennas

Its been a while since I studied physics, so pardon my ignorance. I have two near field antennas. The range of the antennas is about 2-3 cm. I am looking for a way where I can block and unblock the ...
1answer
446 views

### Calculate vector forces on object being affected by wind

I am wondering how you calculate the vector forces of the wind on an object? The information I have is the bearing direction of the wind and the speed of the wind. So how do I calculate the x and y ...
1answer
345 views

### Integral representation of Thomas-Fermi Equation

The Thomas-Fermi equation with dimensionless variables is identified as; $$\frac{d^2\phi}{dx^2} = \frac{\phi^{3/2}}{x^{1/2}}$$ with the boundary conditions as $$\phi(0) = 1 \\ \phi(\infty) = 0.$$ ...
2answers
312 views

### Effect of space time relativity on the age of the universe?

So we all heard about the twins paradox to explain einstein's time space relativity. Wikipedia Quote :" In physics, the twin paradox is a thought experiment in special relativity involving identical ...
1answer
303 views

### Dynamic simulation of surface tension [closed]

Is there any substantial body of work in physics on dynamically simulating effects of surface tension on liquids? The texts i found so far on fluid dynamics all seem to ignore surface tension, ...
1answer
149 views

### How to multiply Celsius and kilograms? [closed]

I have something like this: $$\frac{0.04\,\text{kg} \times 90\,^{\circ}\text{C} + 0.06\,\text{kg} \times 50\,^{\circ}\text{C}}{0.1\,\text{kg}} = T_k$$ Where $T_k$ is a temperature, in Celsius. But ...
1answer
237 views

### Online physics diagram tool

What is a suitable tool for drawing physics diagrams online? I would love to know if there is a program like the excellent schematic drawing tool that is integrated into the electrical engineering ...
1answer
386 views

### What is the evidence for the super-rotation of Earth's inner core?

What are the geophysical observations that support (or contradict) the hypothesis that the Earth's inner core rotates at a faster rate than the Earth's mantle? Summary of Answers: 1) Studies of ...
3answers
275 views

### Why does light seem to occlude objects in front of it

My question: why does light seem to occlude an object in front of it, especially when viewed from a distance? E.g. imagine a grid-like structure (for example an empty gasometer) viewed from a ...
0answers
51 views

### can we break the speed of light [duplicate]

I know this is impossible but I just want to know where I went wrong. here's my proposal: Let us imagine that we have a source of light and a switch that are far one light year from us. so it would ...
2answers
3k views

### Do radio waves go faster than the speed of light? [closed]

My science teacher used to say a lot of weird stuff, but I'm just making sure on this one.
3answers
1k views

### Physical interpretation of Poisson bracket properties

In classical Hamiltonian mechanics evolution of any observable (scalar function on a manifold in hand) is given as $$\frac{dA}{dt} = \{A,H\}+\frac{\partial A}{\partial t}$$ So Poisson bracket is a ...
2answers
438 views

### Classical Limit of Commutator

In Dirac's book Principles of quantum mechanics (4th ed., pgs 87-88), he seems to give a very elementary argument as to how the commutator $[X,P]$ reduces to the Poisson brackets ${x,p}$ in the limit ...
0answers
212 views

### The correspondence between Poisson bracket and Commutators in Quantum Mechanics

I don't understand canonical quantization. In passing from classical to quantum, one replaces the Poisson brackets with the commutators. I don't really understand this. How can we generally show that ...
4answers
2k views

### On a bicycle, why does my back tyre wear so much more quickly than the front?

This question is cross-posted from Bicycles.SE, but it is really one for those that know a bit about physics. Why does the back tyre of a bicycle wear out quicker than the front tyre? I have my ...
1answer
288 views

### Why strong and weak forces are short range? [closed]

Why are the strong and weak nuclear forces short range? Are quarks confined or welded together? Why are elementary particles confined at short range? Or is color confinement color welding? the ...
1answer
151 views

### What is a set of minimal assumptions needed to interpret general relativity?

Next semester, I am going to lecture about (the mathematics of) general relativity and I am still thinking hard how to organize and even more importantly how to motivate all the stuff. I am wondering ...
0answers
39 views

### What is the volume of electron? [duplicate]

I know that electron has mass , and that is particle( a body which has only mass and whose size is negligible) but can we ever calculate the volume of the electron . if yes how much it is . if no why? ...
1answer
56 views

### What is the $\delta$ in fluctuation model [closed]

I found this formula : $G-G_0=\frac{-nkT}2\delta^2$ on this Site and I'd like to ask what does the $\delta$ stand for?
1answer
165 views

### What is volume * mass be called? [closed]

I need a denotation for a rather unusual unit. $$\frac{m}{V}$$ is called density. So what is $$m \cdot V$$ called? Is there a denotation for it? I'd like to get a denotation that could somehow ...
1answer
173 views

### Can massive fermions have zero modes?

I'm confused about fermion zero modes in relation to instantons. I understand that instantons can create fermion zero modes, but it's not clear to me when a fermion has a zero mode. For example, ...
2answers
126 views

### Computing $\Delta m$ in $\beta^-$ decay

$\require{mhchem}$What is the energy $Q$ released when $\ce{^131_53I}$ decays and $\ce{^131_54 Xe}$ is formed? The atomic mass of $\ce{I}$ is $130.906118~u$ and the atomic mass of $\ce{Xe}$ is ...
3answers
386 views

### How much choice did Einstein have in choosing his GR equations?

General relativity was summarised by Wheeler as "Spacetime tells matter how to move; Matter tells spacetime how to curve". I have a fairly good mental picture of how the first part works. However, I ...
2answers
3k views

### Why Silver atoms were used in Stern-Gerlach experiment?

For the Stern-Gerlach experiment done in 1922: Why were silver atoms used? Silver atoms contain many electrons in different shells (with different angular momemtum quantum numbers. Why are those not ...
1answer
172 views

### Superconductor: What form of paramters (like London penetration depth) to use?

I am dealing with thin film and LN2 temperature, and am trying to calculate values such as the Pearl Length, but am trying to find clarification as to which value of $\lambda_L$ to use in this case. ...
1answer
2k views

### Time required for water to freeze

recently I was wondering if there is any specific formula in order to calculate how long it takes for certain liquids to freeze (especially water). I know this depends on: the volume of the liquid, ...
1answer
102 views

### Electron Spin Resonance and Free electrons

When performing an experiment to observe electron spin resonance, we use DPPH molecules as they contain an unpaired electron on one of the N atoms. My question is, why cant free electrons be used in ...
1answer
220 views

### does matter radiate energy? [duplicate]

Since we know that all accelerated charges radiate energy and we also know that all matter is made up of protons and electrons which are all the while doing accelerated motion.So from this can i ...
2answers
603 views

### What makes radio waves refract?

What materials cause radio waves to refract? What are the radio IOR's of these materials?
1answer
187 views

### Is there a heuristic explanation for the derivation of Heisenbergs Uncertainty Principle from String Theory?

Heisenberg famously derived his uncertainty principle by considering the disturbance that a measurement would have on a small enough system. Of course in the mathematical formalism of Quantum ...
1answer
233 views

### Is this a Fata Morgana?

I took this photo. Above the horizon there is a thick layer that looks like a cloud, but it surely isn't. Is this a Fata Morgana? What do you think? EDIT: I think the more appropriate name is ...
1answer
323 views

### Electron in Magnetic Field can lead to non-Hermitian Hamiltonian?

Consider a charged quantum particle in a magnetic field. The Hamiltonian can be written using minimal coupling: $$H = \frac{1}{2m} \left( \mathbf{p} - \frac{e}{c} \mathbf{A}(\mathbf{x}) \right)^2$$ ...

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