# All Questions

1k views

### How do particles become entangled?

A person asked me this and I'm just a lowly physical chemist. I used a classical analogy (how good or bad is this and how to fix?) Basically, light has a net angular momentum of zero, insofar as ...
878 views

### Is my interpretation of how a gravitational wave is formed correct?

I'm sure many here are familiar with the following image showing the 2D representation of how the fabric of spacetime is warped by the presence of mass:- Can this fabric be interpreted as an ...
107 views

### Question about equation 2.27 from Pachos's Introduction to topological quantum computing

http://quince.leeds.ac.uk/~phyjkp/Files/IntroTQC.pdf above is the PDF that is hosted on his website. The equation is on page 22 (pg 30 in the pdf). In chapter 2. It is the second equation of the ...
11k views

### How do you find the uncertainty of a weighted average?

The following is taken from a practice GRE question: Two experimental techniques determine the mass of an object to be $11\pm 1\, \mathrm{kg}$ and $10\pm 2\, \mathrm{kg}$. These two ...
10 views

### Find the current/ potential difference across each bulb [on hold]

I need help solving this problem: Two identical bulbs are connected in parallel to a different source, one with E = 8.0 V and internal resistance 0.8 ohm. Each light bulb has a resistance R = 2.0 ohm ...
23 views

### Calculating the distance that a telescope can detect [on hold]

A typical 4-meter telescope is capable of detecting stars with apparent ￼magnitude of +24 with an exposure time of 5 minutes. Using this exposure time, ￼out to what distance (in pc) can this telescope ...
24 views

### Dynamics of modifing a Hamiltonian

I am working on quantum mechanics right now, and I am wondering if there is a qualitative way to think about off diagonal term of a Hamiltonian matrix? I know that we can diagonalize a matrix, then ...
136 views

### Why is the orbital angular momentum of a pi electron along the axis of two atoms' molecule one?

I'm reading quantum chemistry. The book says that the orbital angular momentum of a $\pi$ electron along the symmetry axis of a molecule made up of two atoms is $\pm 1$. I think this is a primary ...
62 views

### Landau's Problem - Poisson bracks of a spherical symmetry function and angular momuntum in z axis

In landau's Mechanics, there's a problem: I think, if the function has the property spherical symmetry, or: $\phi(r,p)=\phi(-r,-p)$ The form suggested by Landau follows this property, but I can't ...
22 views

### Pedagogical introduction to vertex, domain wall, and kink

Recently, Majorana fermion becomes hot in condensed matter physics. The concepts: vertex, domain wall, and kink often appear in these articles about Majorana fermion. I have no idea about the ...
16 views

### Electric Field Resulting from a Conductor

In preparation for an upcoming exam, I've been attempting to visualize the electric field produced by a conductor in a variety of situations. I asked myself the following question and attempted to ...
137 views

### Can two single particles interfere with each other?

Groups of particles can interfere with one another; In the double slit experiment when measuring single photons at the screen each one arrives at the screen in a random manner and they only show the ...
2k views

### Do Category Theory and/or Quantum Logic add value in physics?

I know they have their adherents, but do more or less esoteric branches of mathematics such as Category Theory and/or Quantum Logic provide powerful tools for new theory development or are they just ...
16 views

### Local unitary transformation that maximizes overlap

Could anyone point me in the right direction (reference to papers would suffice) regarding the following: Given two quantum states $|\psi\rangle ,|\phi\rangle \in (\mathbb{C}^d)^{\otimes n}$, where ...
46 views

### Surface acoustic wave

Why does a surface acoustic wave (SAW) propagate only in the surface leyer of the solid, let's assume a semiconductor? One can create such a SAW through an interdigital transducer (IDT) as I read, ...
7k views

### Why does ice have a lower density than water?

Can someone explain me why is ice lighter than water? As I know, all solids are usually heavier than the liquids (correct me if I am wrong).
255 views

### Center of mass error - calculating systematic error in change in PE

Suppose we have to calculate systematic error in change in PE. Let's suppose systematic error due to scale is 1%. I'm confused about the center of mass error. \begin{align} \Delta PE = m*g*h_1 - ...
42 views

### What's the difference between insulators and topological insulators?

What's the difference between insulators and topological insulators? When I asked some people about this, they told me that "because the topological insulators have gapless edge states,...", but what ...
116 views

### What Did Nobelaureate Smoot Mean by “Modern Efforts to Find Violations of Special Relativity”

In George Smoot's 2006 Nobel Lecture, having won the Nobel prize for his research on the Cosmic Microwave Background (CMB), he refers to the possibility of the CMB being a special frame stating ...
22 views

### Right Hand Rule for Proton Magnetic Field

I thought that right hand rule was for find the magnetic field generated by a current, where your thumb point in direction of the current. However, I was watching something that said we can curl our ...
57 views

### Why is the Oort cloud spherical?

Having read up a bit about the Oort cloud hypothesis, I find I don't quite understand this. Why is the Oort cloud spherical while the rest of the solar system is disk-shaped?
60 views

### How to calculate the photoelectric effect with photon efficiency?

I am not sure exactly where to go with this problem. The problem states: "A $1.3 \, \mathrm{mW}$ laser ($\lambda = 545 \, \mathrm{nm}$) shines on a cesium photocathode ($\phi = 1.95 \, \mathrm{eV}$). ...
2k views

### Producing photons with same frequency, different amplitude wave [duplicate]

I don't understand how two photons of the same frequency can have different amplitudes, neither how to produce them. I know that classically the square of the amplitude is proportional to the energy, ...
8k views

### Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why ...
33 views

### Dry Ice Sublimation time, practical calculations [on hold]

I would like to send a parcel to a family abroad with frozen (to the temperature of -18 ° C.) products.The products are small balls having a diameter of 4 cm. Balls are made from pizza dough.Total ...
145 views

### Understanding gyroscopes

Considering the typical situation of a rotating bicycle wheel held by one end of its axle by a rope tied to the ceiling: gravity torque is the time derivative of the angular momentum, and in this case ...
21 views

### Relativistic Doppler Effect and the Sagnac effect

What I think I know about the Sagnac effect can be found here. It occurs to me that the equations for calculating the time do not take the Doppler effect into account. I'm familiar with the very ...
26 views

### How do we 'watch ' atomic interactions?

I'm relatively new to objective physics and it's measuring and imaging abilities.. Specifically I don't know if we can 'watch' inter/or intra molecular exchanges like the photonic and electronic. I ...
148 views

### Can we draw analogy between em power flow through free space and ac power flow through a transmission line?

Knowing that the free space has a characteristic impedance (which is purely resistive, measured in ohms) I was wondering if I can model the free space as an infinitely long transmission line- ...
74 views

### Why do we believe in a “force” driven universe? [on hold]

Why do we not believe in the potential for a "unified force field" universe, to the exclusion of the belief in the potential for a mechanical, gear driven universe, if the correct shape for the gear ...
201 views

### Verifying radiation measurement smart phone applications

I've stumbled upon a strange class of Android applications lately. (And I'm sure such applications are available for other platforms too.) These apps claim the ability of detecting radiation. The ...
22 views

### Circuit Diagram [on hold]

Three resistor are put into play, two in parallel and another in series to them. One of the resistors in parallel is decreased. what will happen to the voltage and current.
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### How can light have the same speed for all observers? [duplicate]

I'm a little confused on this. If you're travelling at, say, 10% of the speed of light then light is travelling at 3x10^8 ms^(-1) relative to you. If you're moving at 80% of the speed of light and ...
5k views

### Is it possible to start fire using moonlight?

You can start fire by focusing the sunlight using the magnifying glass. I searched the web whether you can do the same using moonlight. And found this and this - the first two in Google search ...
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I have an RC circuit to which I am going to apply voltages of 5V every 100ns. So every 100ns, the capacitor will change its "state" from charging up to uncharging. I have been trying to find the a ...
31 views

### Freezing or melting Brine

In the freezing of brine, is it possible to do it at a rater or in such a way that the water only freezes or the ice is somehow isolated? Or conversely is this possible in the melting of brine-ice?
32 views

### Theory on what would happen if a proton touches anouther [on hold]

I came up with a theory on what would happen if two protons touched each other. I want to know if it is scientifically valid. If two protons repel each other, then the closer you bring them together, ...
40 views

### Is Ballentine's description of the Berry phase (in his book _Quantum Mechanics_) flawed?

Ok, so I'm looking at Ballentine's Quantum Mechanics right now, 7th reprint (2010). On page 363, he starts with 12.7 Adiabatic Approximation and quickly moves on to explain Berry's phase on page 365. ...
2k views

### Why doesn't air freeze?

I am in no way experienced in the Physics field so this question may seem a bit silly but i'd appreciate an answer :) Why doesn't air freeze?
41 views

### Which inertial observer should I choose when using $\Delta U = -\Delta K$?

I just read and undertood that For any conservative force $F$ (which is not necessarily constant), we can define a potential energy change $\Delta U$ of an object on which the force exerts as ...
161 views

### What's the efficiency of real heat engines?

Real heat engines always have lower efficiency than the Carnot efficiency. I wonder how efficient real engines can be? Can their efficiency get anywhere near the Carnot-limit?
180 views

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
52 views

### Safety glasses for a CO₂ laser

I am working on a CO$_2$ laser project and I know glass blocks the wavelength of the laser, 10,600 nm, well. So, should normal glasses work good as safety glasses or do I need to buy a specific one? ...
83 views

### Can light produce weak gravitational waves?

I have read online that light can produce a weak gravitational field (for example antiparallel beams should, in principle, attract weakly). This made me wonder if light can produce minute ...