# All Questions

70 views

### Mistake in Briefer History of Time by Stephen Hawking [on hold]

I was reading A Briefer History of Time by Stephen Hawking and Mlodinow. I found something silly. On page 36 at the bottom, it says the following : If, say, the sun suddenly disappeared, Maxwell's ...
12 views

### Convolve a minimum phase wavelet with delta peak [migrated]

let's assume I have two time series: one with a delta peak at position $x$ and another time series with a minimum phase wavelet that starts at time $y$ with a maximum peak at position $z$. Now, when ...
34 views

### How to find traces of the matrices of the Lorentz group for the Dirac spinors?

How to show that for proper Lorentz transformations the absolute value of the trace for Lorentz boosts is more than 4, and absolute value of trace for rotations is less than 4 for the Dirac ...
28 views

### How many years of expansion occurred during inflation?

During inflation, expansion happens at a very rapid rate. How many years of expansion did it fast forward through? Meaning, if it weren't for inflation, how many years would it take for the ...
33 views

### Basic concepts in quantum theory to start learning semiconductor physics [on hold]

What basic concepts in quantum theory one must known to start learning semiconductor physics?
24 views

### Optical Simulation Tool [duplicate]

I am a student and trying to design an aperture from which a light will pass will project on a sensor. I tried to find some software simulation tool for this purpose where i can draw the aperture ...
138 views

### Does evaporation decrease entropy?

I have a container of water in thermal equilibrium (there are no temperature fluctuations inside the container). Some molecules will evaporate out of the container of water thus decreasing its ...
28 views

### group velocity VS probability current

Think about an electron been accelerated from rest in a static electricfield. If we treat the problem classically, in which the electron is just a point charge. The velocity of the electron would ...
34 views

### What path will an object in a cockpit take?

Situation: A fly is in the cockpit of a fighter jet, and flying directly in front of the pilot. The pilot becomes annoyed and decides to get rid of the fly by executing a sudden, high speed turn. ...
49 views

### In Feynman's “Random Walker” probability example, why is $D^2$ better for illustrating “wandering”?

Taken from Volume 1, Chapter 6, Section 3 of the Feynman Lectures on Physics. Feynman says that in describing random, equally-probable-backwards-or-forwards motion, that, We might therefore ask ...
39 views

### Can a long pipe stop the flow of a liquid from a pump?

If a pump is discharging a fluid into a pipe, is it possible to add a long enough pipe such that the flow of the liquid will be stopped and none will come out of the open end of the pipe?
3k views

### How a half-life works

Carbon-14 has a half-life of 5,730 years. That means that after 5,730 years, half of that sample decays. After another 5,730 years, a quarter of the original sample decays (and the cycle goes on and ...
78 views

### Why does my body create interference with my clock radio while in bed?

I typically listen to NPR in the morning from a clock radio next to my bed. There are days when just moving my body, even my arm or leg, is enough to cause interference with the radio to dramatically ...
99 views

### How does $(c^2 - v^2)$ factor to $c(1 - v^2/c^2)$? [on hold]

I just started studying the ideas leading up to Relativity and as soon as I came up to this equation in the Michelson-Morley Experiment I seemed to have had a brainfart. I'm assuming I'm forgetting ...
22 views

### How does a microscopes optics expose defects in the users vision?

I've got cataracts in both eyes. My vision is correctable to 20/30, so the cataracts are essentially a non-issue in daily living. But when I use a microscope, which I do daily, (binocular, zoom ...
83 views

### Integral of Theta Function with $e^{ix}$ [migrated]

I want to evaluate the integral: $$I=\int_{-\infty}^{\infty}dx_1 \int_{-\infty}^{\infty}dx_2 \ \Theta(x_1-x_2) \ e^{i(ax_1+bx_2)}$$ where $\Theta(x)$ is the Heaviside function. What I was doing now ...
44 views

### How does the Higgs Particle Decay?

Background: It is my understanding that the Higgs Boson is a fluctuation of the Higgs field. It also is not very stable and decays into a plethora of other particles. If all other particles interact ...
31 views

### Buoyancy of a positively charged object inside positively charged fluid

An object with a density of $x$ inside a liquid with a density greater than $x$ would float. If we assume that both of these are positively charged and the object is in the middle of the liquid: ...
62 views

### Is time dilation an illusion? Variation on the twins paradox [duplicate]

Consider the twins paradox with a slight variation: Twins A and B are in separate space ships both capable of going at the speed of light instantly (i.e. without any acceleration). Both ships are ...
37 views

### Can we fire a single femtosecond pulse of light without an associated train of additional pulses? What sort of equipment allows for this?

What sort of mode-locked laser systems allow for the production of single isolated pulse of light (as opposed to a train of more than one pulse), where the individual pulse duration is on order ...
37 views

### How do I calculate stopping power?

Malcolm Gladwell made a claim in a recent talk that a sling with a stone going at 30m/s has the same stopping power as a .45 calibre handgun. How would I verify whether or not this claim is true - ...
28 views

### How does Newton's law apply to a person throwing snow onto their snowbank at the side of their driveway?

We know Newton's three laws: A object at rest will remain at rest, and an object in motion will remain in motion unless a external force acts upon it. If an unbalanced force acts on a object, the ...
138 views

### How long does it take for a black hole to form?

The well-known fable of an astronaut sending signals out to an external observer while falling toward an event horizon states that the time lapse between such signals becomes greater even if in the ...
53 views

### Time dilation in special relativity?

I'm trying to understand the concepts of time dilation and spatial compression. I've been using the classic example of firing a photon of light inside a ship (spaceship, boxcar, etc.) moving at a ...
11 views

### Did a racer on a “restless” course race at some particular average velocity? (2: Starting gate and finish line being rigid to each other)

Consider two participants, a starting gate ($A$) and a finish line ($B$), in a (sufficiently) flat region, both undergoing uniformly accelerated motion, such that $A$ finds constant ping duration ...
21 views

### Relationship between diffused light and intensity

I was wondering if the relationship of the intensity of diffused light is a linear correlation to the diffusing material. Basically will the intensity of the diffused light change in a linear fashion ...
54 views

### Feynman diagram for pair production in matter

I'm aware that electron pair-production from a single photon requires the presence of matter — say some large nucleus — able to absorb momentum, as in the process \begin{align} N \gamma \rightarrow N ...
53 views

### What is the difference between a QCD “glue ball” and a Higgs Boson?

Although they have never been observed, theoretically gluons can exist outside of the nucleus of an atom in the form of a "glue ball". Since the predominant mechanism of Higgs creation at the LHC was ...
19 views

### How much electricity can be generated from ambient sources?

I know that recent advances in technology are doing amazing things with very low amounts of electricity, with RFID being one of the most familiar examples, but I'm not very familiar with the ...
17 views

### What happens to the electrons that escape an atom when matter is converted to a plasma state?

Lets say we have a device that generates plasma fields; if this device creates plasma by heating a gas, where do the electrons that escaped the "orbits" of the atom go? Would the electron physically ...
49 views

### Focused beam of light

I'm trying to understand what happens to photons when a beam of light is focused down to its waist. In the image attached, do photons take the path 1 or 2. That is, do the photons cross or just get ...
35 views

### Tension on a string between two objects on a friction less surface [on hold]

I just need my work checked here. Please let me know if I am correct. I had this problem in one of my test in which I did not get full credits. I am re-doing this(hopefully correctly this time) just ...
20 views

### Aerosol size distribution

When the size distribution of particles in an aerosol is obtained, there is always a bimodal distribution. The small peak accounts for the particles that are in sub-nanometer size range and the larger ...
51 views

### If most satellites in orbit were destroyed from debris, would we ever be able to recover?

Suppose enough satellites were destroyed in orbit to cause so much debris from the explosions that a chain reaction destroying other satellites took over to generating more and more debris (even ...
33 views

### Energy in a Solenoid?

Consider a circuit consisting of a battery, a resistor and a solenoid inductor. Then, the emf $\mathcal{E}$, is defined as: $$\mathcal{E} = L\frac{di}{dt} + iR$$ Multiplying both sides by ...
41 views

### Apparent and Event Horizons

Does the apparent horizon expand and become the event horizon, or vice versa? I've gotten myself a little confused! Ultimately, does this paragraph make sense? This trapped horizon can be thought ...
44 views

### Question about the vacuum bundle on A- and B-model

Let us consider the topological string A- and B-model (twisted SUSY non-linear sigma model on CY 3-manifold $X$). They are realization of $N=2$ SCFT and there are ground-states vector bundle ...
37 views

### Inductance of a solenoid?

My textbook is using Faraday's law to explain the self inductance that happens in a solenoid with changing current. According to Wikipedia, Faraday's Law is: The induced electromotive force in ...
19 views

### How to practically find the RA-DEC coordinates of a star?

How are RA-DEC (Right Ascension & Declination) coordinates of a star measured i.e. if scientist spot a new star in the sky how do they accurately measure it's coordinates?
57 views

### Why is this mass equation giving me mass = weight when clearly it isn't?

Convert the weight measured in pounds to the equivalent in Newtons. In the formula for determining mass based on weight, mass is measured in Newtons. Weight is measured in Kilograms, and the ...
40 views

### What happens when you pass a current through a coil made of a ferromagnetic substance?

I know when you pass a current through a length of copper coil or a solenoid, there is the induction of a magnetic flux. But what of the coil made of the ferromagnetic material, the permanent type ...
28 views

### Determine the damping ratio of a RLC circuit [on hold]

How to determine the damping ratio of a RLC circuit. There are given LC, magnitude ratio and frequency. Thanks :)
40 views

### Strain energy density in index notation

The strain energy density is defined as $$dU = \int_0^{\epsilon_{ij}} \sigma_{ij} d \epsilon_{ij}$$ (see Reddy "Energy Principles and Variational Methods in Applied Mechanics", 2nd Ed, 4.11). Assuming ...
42 views

### How is the width of a slit related to the intemsity of light passing through it?

Here's a question I got in my final exam this morning. "If in a Young's double slit experiment setup, the ratio of intensity of the bright spot to the dark spot is 25:9, what is the ratio of the width ...
64 views

### What makes an abstract physical system describable by a “fluid” equations of motion?

We can describe (some of) the dynamics of many systems using fluid mechanics. Of course these include classical fluids like water, more exotic fluids like photon gases and the universe as a whole and ...
106 views

### Quantization of electrostatic $\vec E$ field?

Can a electrostatic field $\vec E=\vec E(x,y,z)$ (time-independent) or electrostatic potential $\phi=\phi(x,y,z)$ be quantized? If yes, will these quanta be photons again? But we don't have an ...
40 views

### Time inversion for Euler equation

Consider Euler equation for continuum body: $$\frac{\partial u^i}{\partial t}+\mathbf u\cdot \nabla u^i=- \frac{1}{\rho} \frac{\partial p}{\partial x^i}$$ where $\rho$ is the mass density, $p$ is ...
18 views

### Total radiation exposure [on hold]

Technologist stands next to a 99Mo generator yielding an exposure rate of 38mR/hr for 7 min. What is the total radiation exposure to that technologist? Please provide detailed step by step.