All Questions

293 views

why the difference between $\langle \hat p^2 \rangle_{\psi}$ and $\langle \hat p \rangle_{\psi}^2$ is NOT zero?

Well, the difference between the two expressions $\langle \hat p^2 \rangle_{\psi}$ and $\langle \hat p \rangle_{\psi}^2$ is exactly $\Delta p^2$ , i.e. the squared uncertainty (variance) of the ...
117 views

What is the importance of Vacua in Field Theory?

I understand that defining the Vacuum is important in Field Theory, why? Is this because it is the 'ground' state, before particles are added, so defines the 'background'? I assume its not important ...
540 views

Is String Theory a Field Theory?

Is String Theory a Field or Quantum Mechanical Theory of the String rather than a Particle? I should know this having studied this for a term, but we jumped into the deep end, without really ...
768 views

Which formula for entropy is correct? (OR Is the fundamental thermodynamic equality always right?)

Quick question, in the lecture notes to a thermodynamic course I'm taking, $$d\underline{S}=\frac{d\underline{U}}{T}+\Big(\frac{\partial P}{\partial T}\Big)_{\underline{V}} d\underline{V}$$ But ...
2k views

How does String Theory predict Gravity? [duplicate]

Firstly, General Relativity states that Spacetime is dynamic and is consonant with the distribution of matter/energy. How does String Theory predict gravity, when it is background dependent, that is ...
242 views

what is the relationship between the dynamical casimir effect and virtual particles?

Since virtual particles are disturbances in a field, and not particles in any sense, as explained here, how is it that true photons arise from them when excited with kinetic energy via the dynamical ...
575 views

What in Newton's three laws of motion is original and not a paraphrasing of his predecessors?

The three laws are: First law: The velocity of a body remains constant unless the body is acted upon by an external force. Second law: The acceleration a of a body is parallel[disambiguation needed ...
17k 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? ...
173 views

On the Existence of Interstellar Meteoroids

A meteoroid in orbit around the Sun has a heliocentric velocity in the vicinity of Earth of about 42.1 km/s. So if a meteoroid has a heliocentric velocity exceeding that, it will have an open ...
3k views

Differences between X-Ray and Electron Diffraction

I would want to know what the differences between these two techniques in cristalography. I research on internet, but it is not clear to me what each one does. When it's better to use one and not ...
256 views

Hairy ball theorem: references to applications

I'm looking for references to applications of the Hairy ball theorem. This is a result of mathematics (topology), but I am interested in applications. I already visited wikipedia and cited ...
249 views

Why do we stop using optics for photons above a certain energy?

I'm reading about how the soon-to-be-launched NuSTAR is on the cutting edge of focusing x-rays, which captures 5 to 80 keV radiation by focusing them with optics that have a 10.15 meter focal length ...
702 views

Would the rate of ascent of an indestructible balloon increase as function of it's altitude?

Assume a balloon filled with Hydrogen, fitted with a perfect valve, and capable of enduring vacuum (that is to say, it would retain it's shape and so well insulated that the extremes of temperature at ...
294 views

What is the optical power level of common fiber optics lasers?

In the book Nonlinear Optics, it is stated that the nonlinear effects start to become a problem in WDM systems (around 1550 nm) after about 1 mW of optical power. However, I measured the optical power ...
584 views

Relation between linear momentum and translational kinetic energy

The momentum $m v$ of a particle is formally the same as the derivative its translational kinetic energy $\frac{1}{2} m v^2$ with respect to $v$. Similarly the angular momentum $I \omega$ is the ...
803 views

How can the Huygens-Fresnel principle be derived from the Maxwell equations?

The Huygens-Fresnel principle states that every point to which a luminous disturbance reaches becomes a source of a spherical wave. I have been trying to understand this considering a infinite screen ...
3k views

Transforming a sound wave into a literal light wave. Is it possible? [duplicate]

Literally transforming sound into an actual light wave seems almost impossible. But transforming the sound wave into a light wave while containing a single mass? I know that sound can contain a mass ...
340 views

Non-relativistic spinors

Even in a non-relativistic theory Spinors can arise as irreducible representations of the rotation subgroup of the symmetries of the theory. Why do people then put so much emphasize on the role of ...
1k views

Book covering Topology required for physics and applications

I am a physics undergrad, and interested to learn Topology so far as it has use in Physics. Currently I am trying to study Topological solitons but bogged down by some topological concepts. I am not ...
2k views

Would a fast inter-stellar spaceship benefit from an aerodynamic shape?

Some (generous) assumptions: We have a spaceship that can reach a reasonable fraction of light speed. The ship is able to withstand the high energies of matter impacting at that speed. Given the ...
75 views

How to apply Guass Law to Voltages

So, I know $\oint E\centerdot dA = 4\pi Q_{enc}$ I'm trying to solve for a TEM mode with two concentric (infinite) cylindrical wave guides of radius a and b, $a<b$. I know that for TEM modes, I ...
665 views

How were the heavy elements from iron to uranium made? [duplicate]

Possible Duplicate: Age of the Earth and the star that preceded the Sun How were the heavy elements from iron to uranium made? References: ...
888 views

Linear charge density, surface charge density and volume charge density

What is the difference among: linear charge density, surface charge density, and volume charge density?
545 views

Does a magnetic field induce an emf in a loop of wire

Is it true that any magnetic field induces an emf in a loop of wire? If It it is true then kindly explain me with an example
428 views

Can anyone estimate what proportion of water remains after I flush a toilet?

Imagine I have a clean toilet with some amount of water in the bowl. When I flush the toilet much of that water will be displaced by the tank's water. I want to work out (or model really) what amount ...
210 views

The ${\cal N} = 3$ Chern-Simons matter lagrangian

This question is sort of a continuation of this previous question of mine. I would like to know of some further details about the Lagrangian discussed in this paper in equation 2.8 (page 7) and in ...
59 views

is cosmic expansion related with IR divergencies?

This question is related to renormalization, but in the IR limit. It is assumed that unitarity does take care of IR divergencies in interacting theories like QED. But how would one interpret ...
248 views

How many stars within 5 parsecs?

I've ran some scripts on some star catalogs, which counted the number of stars with a parallax greater than 200 mas, which should mean everything closer than 5 pc. The results are as follows: ...
32k views

Gravity on the International Space Station

We created a table in my physics class which contained the strength of gravity on different planet and objects in space. At altitude 0(earth), the gravitational strength is 100%. On the moon at ...
3k views

Determine the acceleration and angular acceleration of a disc

The question is: A 90kg disc is floating in a frictionless vacuum. A 150N force is applied to the outer rim of the disc. The disc has a radius of 0.25m and a radius of gyration of 0.16m. What ...
1k views

Why isn't the symmetric twin paradox a paradox?

Two twin sisters synchronize their watches and simultaneously (from the earth frame) depart earth in different directions. Following a predetermined flight plan, each sister accelerates identically to ...
41 views

bond energies database [closed]

I’m looking for a public database of bond energies of biochemical compounds. Do you know about any? Otherwise, which is the standard computational methodology to calculate them? Any references would ...
542 views

Is Schrodinger's Cat a real conceptual problem or just a problem with approximations?

In this thought-experiment a cat is placed in a box set with a bottle poison that will release and kill it depending on whether or not a certain radioactive particle decays. The box is kept closed and ...
165 views

How is extra information encoded in digital radio?

Earlier today, I was listening to my digital radio, and was intrigued as to how they are able to encode the information displayed on the LCD screen (which was basically a short string of text giving a ...
78 views

Unitarity and quantum cosmology

By studying quantum cosmology I was asking myself if the fact that the universe is expanding, so space is expanding and with it I would say that phase space is also expanding, so it's a non-unitary ...
11k views

How can I theoretically describe the potential between two capacitors in series?

Suppose to have two capacitors in series: The voltage in the middle point will be: $$V_X = V_1 \frac{C_1}{C_1+C_2}$$ How can this be explained? It's been asked in electronics, and explained in ...
354 views

Two identical rockets, time dilation, and possible weirdness [duplicate]

Possible Duplicate: Why isn't the symmetric twin paradox a paradox? Suppose there are two identical rockets, each carrying one of two identical clocks and one of two identical ...
13k views

Why do people recommend wider tyres in car for better road grip?

Tyre companies boast of their wider tires for better grip on road. Also, the F1 cars have broad tires for better grip. But as far as I know Friction does not depend on the surface area of contact ...
1k views

Why is the center-of-mass of 2 bodies at the focus of their elliptical orbits?

Why is the center-of-mass of 2 bodies (which interact only via Newtonian gravity) located at a focus of each of the elliptical orbits? I know that when there are no external forces, the center of ...
461 views

How is traveling back in time possible in theory according to some scientists? [duplicate]

Possible Duplicate: Is it possible to go back in time? Is time travel possible? I get the idea of traveling to the future and it makes perfect sense as we'd be somehow trapped in a ...
70 views

spectral eigenvalue staircase and quantum system

in a d-dimensional system of Quantum physics , does the Eigenvalue staircase $N(E)= \sum_{E_{n}\le E} 1$ determine ALL the properties of Quantum System ?? for example, let us assume that the ...
134 views

Change in the Higgs mechanism

The most important thing about the Higgs mechanism is that the higgs mechanism is one of the simplest mechanisms (certainly the simplest way to break the electroweak symmetry) of elementary particles. ...
4k views

Spring compression and Momentum

I am asked to rate a series of elastic collisions in order of greatest time of max compression to least time of max compression for several vehicles with varying masses and velocities, which strike a ...
363 views

Why can i replace a gauge field by the current it couples to in the calculation of a greens function?

I am reading about anomalies in QFT at the moment and have a related question. Often people calculate the time ordered expectation value of some fields (in QED for example) by replacing the field ...
621 views

How much thrust would be needed to turn a hobbyist weather balloon into a deep space probe?

I was reading the article Weather Balloon Space Probes that says you can put your own balloon probe at 65,000 ft temporarily. Is it even remotely possible to raise the probe high enough using ...
2k views

Make a semi transparent mirror with copper

The question: How would you make a semi transparent mirror (50% reflection, 50% transmission) with glass with a layer of copper. For light $\lambda$ = 500nm Try to be as realistic as possible What ...
231 views

Propagator of the Klein-Gorden equation

Does this integral converge? My question is related to this one: Free particle propagation amplitude calculation I am reading the book of Peskin and Schroesder. In the second page of their chapter ...
767 views

Finding instantaneous speed in MPH given acceleration, RPM and gear ratio

I am still trying to figure out how to (semi) accurately model instantaneous speed after having found acceleration. I have found that at higher RPMs, the resultant acceleration will be lower. I was ...