# All Questions

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### How much energy is contained in a 40 meter wave?

Consider a wave that stands 40 meters high in the sea on very deep water. How much energy would approximately be contained in this wave if it was 100 meters wide and had been produced by wind? Is ...
314 views

### Why do balls in a spinning ellipsoid move to the minor axis plane?

There is a question concerning the Physics of a small child's tall that has been bothering me for some time now. I have investigated this to a small degree, but I have not been able to find a ...
100 views

### Can the Lamb shift be expressed in more-or-less closed form in terms of the renormalized 2-, 3-,…,n-point VEVs of QED?

I see here that there are three contributions to the Lamb shift, from vacuum polarization (-27 MHz), from electron mass renormalization(+1017 MHz), and from the anomalous magnetic moment (+68 MHz). ...
2k views

### Easy to obtain liquid with freezing point a couple of degrees above 0 Celsius [closed]

Please advice me a liquid whose freezing point is 2-5 Celsius. I know that the solution of salt in water can have freezing point of about -3 Celsius. I would like to know if similar liquid but with ...
272 views

### Crystal Field Theory

I am literally lost with this question: Suppose that within the set of (2L+1)(2S+1) lowest-lying ionic states the crystal field can be represented in the form a(L_x)^2 + b(L_y)^2 + c(L_z)^2, with ...
726 views

### Does Kaluza-Klein theory successfully unify GR and EM? Why can't it be extended to the Standard Model gauge group?

As a quick disclaimer, I thought this might be a better place to ask than Physics.SE. I already searched there with "kaluza" and "klein" keywords to find an answer, but without luck. As background, ...
234 views

### Universe is expanding at enormous speed

I'm not an expert but I've come to understand that the universe is expanding at enormous speed. That means that all of the visible galaxies are moving away from us at great speed. I also came to ...
127 views

### Different results for Magnetic field using different methods

In calculating the magnetic field created by this current at the center point of the loop using Biot-Savart and using the vector potential will there be a difference? If so what is it and why? ...
108 views

### Is the Energy Sharply or Fuzzily Defined in Quantum Mechanics?

According to quantum mechanics, energy of a state is uncertain within a small range in hydrogen atom. But we also know that energy of a state is quantized which is contradictory to the first. Which ...
4k views

### How light years is measured? [closed]

How are light years are measured? I remember the distance between earth and moon are measured by the delay in light which travels and comes back. But how are light years calculated?
1k views

### What are bootstraps?

I've heard occasional mentions of the term "bootstraps" in connection with the S Matrix. I believe it applies to an old approach that was tried in the 1960s, whereby - well I'm not sure - but it ...
107 views

### Could a viable solar system work with a cluster of dwarf stars in center? And would it last longer than a single stellar mass star?

So, I was watching various sci channel shows, and they touch on how extremely massive stars live only 100k years, vs the sun which lives ~10 billion years, and dwarf stars live some unspecified time ...
249 views

### Average speed of seismic waves towards the center of Earth

I'm looking at an approximation of the average speed of seismic wave towards the center of Earth. The horizontal path of the waves that affects us directly is well documented, but there is not much ...
2k views

### Would a high-explosive in a vacuum be less harmful?

Putting aside shrapnel effects, I believe that high-explosives cause damage by producing a shockwave. How do shockwaves work in space? I've managed to convince myself that a high-explosive shockwave ...
13k views

### Why does a gas get hot when suddenly compressed? What is happening at the molecular level?

My guess is that the molecules of gas all have the same speed as before, but now there are much more collisions per unit area onto the thermometer, thus making the thermometer read a higher ...
7k views

### How does the Higgs mechanism work?

I'm not a particle physicist, but I did manage to get through the Feynman lectures without getting too lost. Is there a way to explain how the Higgs field works, in a way that people like me might ...
437 views

### How to explain the Moon halo phenomenon?

Today, here in Brazil, I have observed (and is still observing) an interesting phenomenon. The Moon is near to a big star in the sky, but this is normal. The interesting part is what's around them. ...
119 views

### Derivatives of fluctuations about a condensate

Firstly I am not sure as to whether I am using the word "condensate" in the right context. In QFT contexts I think I see it getting used to mean the space-time independent solution which would solve ...
1k views

### What if we could give photons some mass?

I was reading an article and these paragraphs got me wondering... Before I list the replies, here is some background. The Higgs mechanism describes an invisible field that, it is argued, split one ...
294 views

### Point charge potential (sign problem)

I'm a bit embarrassed, but I'm not able to compute the electric potential at point $P$ (at a distance $R$ from the origin) generated by a positive unitary point charge in the origin with the right ...
167 views

### If a puck on ice deccelarates, is its speed “v = v0 - t * a” or is it “v = v0 -k * v0”?

I've been wondering about a puck sliding on ice or a puck in an air hockey table game. If a puck is hit and we start watching it as soon as it has reached its maximum speed and starts becoming slower, ...
2k views

### Sinusoidal vs exponential wave functions with Schrodinger's equation

When solving Schrodinger's equation, we end up with the following differential equation: $$\frac{{d}^{2}\psi}{dx^2} = -\frac{2m(E - V)}{\hbar}\psi$$ As I understand it, the next step is to guess the ...
439 views

### Why model space with real numbers?

Are there any good papers discussing why we use $\mathbb{R}^{3}$as a model for space? More specifically are there any that explain why we don't use other number systems such as extensions of the real ...
180 views

### Supermassive Black Holes = expansion origin?

(Sorry, couldn't resist the title) An article on Physics Today brings news that there is a new record for largest black hole (not) seen in space, some 300 million light-years away. It hints at ...
542 views

### How is the Hanbury-Brown and Twiss effect used to measure the size of stars?

I understand what an Hanbury Brown and Twiss (HBT) interferometer does, but how can this be used to measure the apparent angular diameter of some object? What is the mathematical explaination?
395 views

### How can we know, today, that there's something from 100 light-years from here?

In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we ...
523 views

### How do I get the focal length from a camera matrix?

Given the camera matrix, can I find the focal length of this camera ?
867 views

### Tensor product of Hilbert spaces and non-interacting particles

Consider a system of N quantum mechanical particles described on Hilbert spaces $\mathcal{H}_1,...,\mathcal{H}_N$ and with Hamiltonians $H_1,...,H_N$. The Hamiltonian operator $H_1$ acts on the ...
565 views

### The role of resistor in e.g. an AND gate

What is the role of the resistor in e.g. an AND gate like this one? : One often sees lots of resistors in electric circuits, but I haven't really understood their role.
141 views

### How Is Entanglement Created Among Qubits?

How are qubits entangled? I understand the basics of entanglement but what I do not get is how it occurs in nature or in the lab. What causes entanglement to occur or what is done to the particle to ...
406 views

### Is the total energy of a quantum system the sum of the particle energies?

I really should know this off by heart (this is my field...) but I never really grasped the difference between the total wavefunction of a system and the wavefunctions of particles within it, so it ...
251 views

### Uniqueness of eigenvector representation in a complete set of compatible observables [duplicate]

Possible Duplicate: Uniqueness of eigenvector representation in a complete set of compatible observables Sakurai states that if we have a complete, maximal set of compatible observables, ...
142 views

### Gravitational sphere

I was reading this article that mentions a blackhole as having a gravitational sphere of 4,000 light-years. I'd not heard of the term (gravitational sphere) before ...
132 views

### Space expansion effect on wavelengths across two points in space

Is the expansion of space taken into consideration when calculating light or any (Radio to Gamma) wave length distance and speed? I know C is a constant, but my concern is if "space expansion" is ...
39 views

### A question from Ticcati's red QFT textbook.

From Ticcati's textbook, he asks to show that from the axioms of position operator we get that: $$\text{e}^{-ia\cdot P} |x\rangle = |x+a\rangle$$ where the axioms are: $$X=X^{\dagger}$$ If ...
478 views

### electric dipole momentum calculation

I'm studying the linear electric susceptibility, using Schroedinger equation and perturbation theory of the interaction potential $$V=-\mu \cdot E$$ and the book arrive to an expression where ...
904 views

### Second law of thermodynamics and a bunch of magnets

Say I put a bunch of powerful square magnets on a nearly frictionless table in a disordered fashion. The second law of thermodynamics states that the system shall spontaneously get more disordered, ...
972 views

### Why are anticommutators needed in quantization of Dirac fields?

Why is the anticommutator actually needed in the canonical quantization of free Dirac field?
311 views

### What material can a lens be made from?

I need to make a camera lens that can easily withstand temperatures of about 1000 Fahrenheit. What should I make the lens out of?
85 views

### Is there any chance we could eventually observe the supernova of the first (Population III) stars

In an answer to my previous question about the first stars it was stated they probably formed at Z=20 to Z=60 and may have had a mass between tens to 100s to 1000s of times the mass of the Sun. Given ...
324 views

### What happens to a rotating rod that breaks in two?

I know that the approximation for the moment of inertia of an infinitely thin rod of mass $m$ and length $L$ spinning around an axis perpendicular to its own axis at its center is $\frac{mL^2}{3}$: ...
321 views

### Is it necessary to embed a 4D surface in 5D space?

Lets consider the line element: $$ds^2=dr^2+r^2[d\theta^2+\sin^2\theta d\phi^2]$$ There are three variables r,theta and phi. If we use a surface constraint like r=constant the number of independent ...
259 views

### In astronomy what phenomena have theory predicted before observations?

As far as I know, astronomy is generally an observational science. We see something and then try to explain why it is happening. The one exception that I know of is black holes: first it was thought ...
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### Is there a limit to the resolving power of a mirror telescope?

Like, if you hammered out the asteroid 16 Psyche into a 1 mm thick iron foil disc telescope mirror with 2.4x the radius of the Sun, could you resolve details on the surface of an exoplanet? At what ...
417 views

### Defining the ground state energy of a QFT

I would like to hear of some general discussion on how is the ground state and its energy defined in QFT and how does one go about finding it. (..at least in some simple cases I have seen the use of ...
70 views

### How are exoplanets confirmed?

In reference to the Kepler 22b news: The Kepler team had to wait for three passes of the planet before upping its status from "candidate" to "confirmed". This is possible because the planet ...
214 views

### Relativity and predictability

My old theoretical physics professor used to say that (already) (special?) relativity shows that "the future is not predictable". Any ideas how this should be interpreted?
7k views

### How does evaporation affect the temperature of the air?

A wet object or a volume of water will decrease in temperature due to the effect of evaporation. We understand this to be because of the molecular kinetics, where the faster water molecules escape ...
Sakurai states that if we have a complete, maximal set of compatible observables, say $A,B,C...$ Then, an eigenvector represented by $|a,b,c....>$, where $a,b,c...$ are respective eigenvalues, is ...