All Questions

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Presence of a nucleus for a photon to produce electron-positron pair

Is the presence of a nucleus always necessary for a gamma-photon to produce electron-positron pair? Is Pair production possible with two photons head on collision, in the absence of nucleus or virtual ...
36 views

Cosmological Significance of Uncertainty [on hold]

Is it fair to say that quantum uncertainty is the most fundamental and primordial characteristic of the universe? Wouldn't uncertainty have predated the initial expansion of spacetime, and not be ...
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Concept of Projectile Motion [on hold]

A projectile is thrown from a platform at a height 10m with a velocity of 20m/sec . At what angle should the projectile be thrown to reach the farthest point from o , which is vertically below the ...
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If the four velocity of a photon is undefined, what can we say about the velocity of a photon? Is Brian Greene right about motion through spacetime? [on hold]

Edit: THIS QUESTION IS ASKING ABOUT THE FOUR VELOCITY OF A PHOTON. THE PREVIOUS QUESTION DOES NOT. PLEASE ANSWER THIS QUESTION, WHICH HAS NOT BEEN ASKED. If the four velocity of a photon is ...
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What is the optimal distance to sit from a TV? [on hold]

I was wondering how to calculate the optimal distance to sit away from a TV. I don't quite know the full set of parameters it will depend on, I would suspect it to include the following: Size of the ...
80 views

Derivation of continuum expression of the first law of thermodynamics

Continuum expression of first law of thermodynamics: $$\frac{D E_t}{D t}=\nabla\cdot({\bf \sigma\cdot v}) - \nabla\cdot{\bf q}$$ (I've seen it in my physics book) How this equation is derived? ...
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Hexagonal shape of snow flakes [duplicate]

As we know snowflakes has hexagonal shape. My question is why is that? and Is there any mathematical model which can explain that particular geometric shape of the snowflakes?
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Does a scale weigh a bee flying inside a box?

This question is very similar to this one or even this one except some minor differences. The box is much larger than the bee. The box has no top cover. The bee is flying still in the middle of the ...
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The origin of the universe and its current movement [duplicate]

Our universe started with the big bang, which was not within our universe, since it didn't exist. Let's assume it was the result of a quantum fluctuation in time. I have 2 questions: The common ...
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How can I measure the security of Quantum Key Distribution (QKD)?

QKD protocol is a technique that allow two parties to share common secret key for cryptographic purpose and defined as being secure, by definition. But I am still wondering how can we measure that QKD ...
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Kerr Metric and Asymptotically Static Frame

Suppose we are given a Kerr spacetime (e.g. containing a single uncharged rotating black hole). How does one know that the coordinates chosen is rotating or non-rotating as seen from infinity? And how ...
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Energy stored in a charged capacitor [on hold]

There is a circuit with a 12.0 V cell and two capacitors with a switch, the switch in its initial position connects the cell and the first capacitor(4.7uF) (P) the switch in its second position ...
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Significance of wave number?

Till now all I know about the wave number is its formula i.e. ${\frac{2\pi}{\lambda}}$. I always wanted to know what it really means. So can anyone please, explain me its physical significance?
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Sound waves - physics [on hold]

A closed organ pipe of length L is in resonance with tuning fork. If a hole is made at a distance L/4 from the closed end will there be resonance again? If so when?
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Can astronomers observe neutron stars optically?

Are there any neutron stars near enough for astronomers to observe them optically? If not, then how close are we to having the technology to do so?
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Do electronic energy levels of an atom effectively shift for scattering processes if the atom is moving in the lab frame?

When an electron of low energy scatters off an atom it interacts with one of the electrons in its shell and can transfer energy to it. The energy transfers would be discrete, since the shell electron ...
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Will Project Avogadro introduce a new constant? [on hold]

Will the Avogadro Project introduce a new constant? It seems to me that the aim of the project is to define the kilogramme as some constant number of silicon atoms. What would this constant be ...
Calculate the excess pressure for a surface located to plans a) $e_{1}$ b) $e_{2}$ for the flow specified by : $v_{1} =-c(x_{2} + x _{1})$ , $v_{2} = c(x_{2} - x_{1})$ , $v_{3} = 0$ where $c = 0,5 \... 1answer 53 views Determination of nature frequency and differential equation of vibration of Hartnell governor? I found this solution for the nature frequency but here it does not include the Ball weight and centrifugal force in the moment balance equation about the pivot (O), it is wrong answer...is not it? ... 0answers 17 views How much heat does a Thermoelectric generater need to produce and how much would it produce [on hold] How much heat (Fahrenheit) does a Thermoelectric generater need to produce electricity and how much would it produce 1answer 28 views Difference between fermi energy level in metals and semiconductors [duplicate] It is not exactly a question its a kind of confusion ... Here my confusion starts... what is meant by Fermi energy level?? The only definition that i found was the energy level at which the ... 0answers 60 views Is there a fundamental picture for 'the speed of light'? I am curious about the profound reason for the concept of 'the speed of light'. My considerations are: (1) It's related with the structure of spacetime (for example in Penrose's twistor plan),so ... 3answers 75 views Size and density of neutron stars Most of the books which I looked at give approximately 10 km as the radius of a neutron star. Just yesterday I looked at a book by Dave Goldberg titled The Universe In the Rearview Mirror (2013) which ... 0answers 8 views Chromatic confocal microscopy (CCM) and stylus probe I read research that compared between chromatic confocal microscopy (CCM) and stylus probe in wear test but the research didn't clearfield what is chromatic confocal microscopy and what is the stylus ... 1answer 70 views One particle states in an interacting theory Question: What is the general definition of one particle states$|\vec p\rangle$in an interacting QFT? By general I mean non-perturbative and non-asymptotic. Context. 1) For example, in Weigand'... 1answer 59 views +50 How to derive entropy from density of states? I'm trying to derive the entropy of a black hole, given the density of states of a bosonic string (the details are not relevant). The density of states is $$\omega(E) = E^\alpha e^{\beta E}$$ The ... 0answers 23 views Graphene sheet “cuts”? I'm no expert in physics at all, so this is probably a very simple question for this site's regulars. When reading a book or browsing paper sheets, I've sometimes had the skin of my finger cut (it ... 1answer 20 views Are there interference effects for light that is reflected at an angle? I am imagining a glass substrate with a thin coating that has a 1/4th wave optical thickness. I understand how this acts as a basic anti-reflective coating for the one wavelength for which it acts as ... 0answers 49 views $\left< \frac{\partial (xp)}{\partial t} \right> = 0$When is this true? Is this always true in quantum mechanics? $$\left< \frac{\partial (xp)}{\partial t} \right> = 0$$ I encountered this when working problem 3.31 in Griffiths Introduction to Quantum Mechanics II.... 0answers 93 views Are strings in string theory actually little black holes? [on hold] I sometimes read that strings in string theory are actually little black holes, or can be interpreted that way. Is this true? How is that consistent with that the particle that a string represents ... 1answer 68 views Significance of symplectic form in classical field theory I'm trying to understand the significance of construction presented to me in field theory class. Let me first briefly describe it and then ask questions. Given two solutions$\phi_1$,$\phi_2$of the ... 0answers 16 views Why must the separation constant be real in a time dependent wave function? I'm not sure if I'm asking this right. I'm reading ''Introduction to Quantum Mechanics'' by Griffiths and in the chapter 2 exercises he asks to prove that the separation constant,$E$, must be real. ... 1answer 50 views Is Biot-Savart Law valid for time-varying currents unlike Ampere's law? I have just finished learning the basics of magnetism, and it should be noted that I am not very familiar with Maxwell's equations. Note: In the question, when I say "Ampere's Law", I am referring ... 1answer 46 views What is the difference between worldsheet supersymmetry and spacetime supersymmetry? What is the difference between worldsheet supersymmetry and spacetime supersymmetry? For worldline formulation of fermions quantum mechanics, there is a supersymmetry. But the corresponding spacetime ... 0answers 24 views symetric of lissajous figure [on hold] I want to know if this lissajouss figure is symmetric with phase between 3-5 and angle between 35° and 70° 1answer 48 views +100 Trace of raman tensors A raman tensor$\gamma_m$is defined as the derivative of the polarizability tensor$\alpha$with respect to a raman mode$Q_m$, so $$\gamma_m =\frac{\partial\alpha}{\partial Q_m}$$$\gamma_m$will ... 0answers 39 views Pendulum is symmetric [on hold] I need to find the plane phase for a pendulum of 0.8 joules of energy, is symmetric the figure that it shows? why? http://www.sc.ehu.es/sbweb/fisica/oscilaciones/pendulo2/pendulo2.htm 0answers 4 views Minima and maxima of a three variable function [on hold] I have a three variable function$f(x,y,z)$and I want to find its minima and maxima. I found its critical points and at one of its critical point, Hessian matrix look like $$H = \... 2answers 54 views Why isn't proton radiation by decay on earth known in nature? Perhaps asking for why isn't appropriate in physics, but as there is neutron and alfa radiation what causes proton radiation not to be very common in nature (in laboratory it is seen although) 8answers 2k views Prove that an electron in a hydrogen atom doesn't emit radiation [duplicate] According to electrodynamics, accelerating charged particles emit electromagnetic radiation. I'm asking myself if the electron in an hydrogen atom emits such radiation. In How can one describe ... 1answer 39 views General expression of internal energy change in a closed system let's say we have a classical closed system. What I've understood is that during a reversibile process the internal energy E undergoes a change which could be expressed (as generally as possible) as ... 1answer 30 views Why does length of objects seem different in different configurations of object and observer? Today, I had gone to my friend’s garden. There was a dried water well in his garden. We got curious that know how much its depth is. First, we stood on top of the well and estimated its depth. We ... 1answer 59 views Lorentz Transformations in Minkowski space If \Lambda represents the Lorentz transformation matrix, then the transformation of contravariant components x^\mu is given by$$x'^\mu=\Lambda^{\mu}{}_{\nu} x^\nu$$and that of the covariant ... 0answers 28 views Virtual Particles in QCD Vacuum [on hold] I would like to ask what are the virtual particles in the QCD vacuum. Are they QCD bound states or they are just quarks? In fact, I am puzzled by the logics explained in Peskin and Schroeder book, ... 2answers 40 views Why alloy have more resistance? Is there any simple way to understand why alloy have more resistance than metals? My teacher ask this, I answer that, there might be more free electrons in metals than an alloy, but she said you are ... 4answers 111 views If a photon has no mass why doesn't it have infinite speed? [duplicate] Please help a naïve layperson understand -- if a photon has no mass, why is its velocity limited at all? Shouldn't a particle with no mass be able to travel at an infinite speed? 6answers 5k views How can a battery charge up another battery to a higher percentage? Say I have my phone on 5% and a large battery pack on 35% and I charge the phone. By the end the phone is on 100% and the pack is on 12%. How can the battery pack charge the phone up to a higher ... 1answer 19 views By which equivalent circuit (RC serie or RL serie) can I substitute the following circuit? I was trying to solve the following question: "Assuming that 𝐑 = 𝟏𝐤𝛀, 𝐂 = 𝟏𝛍𝐅, 𝐋𝟏 = 𝟔𝐦𝐇, 𝐋𝟐 = 𝟑𝐦𝐇, 𝐞 𝛚 = 𝟏𝟎𝟒 rad/s, tell if the circuit can be substituted by an equivalent RC ... 1answer 71 views Where does the 0.61 come from in R \sin \theta = 0.61\lambda ? In my A2 Physics textbook it says that In the estimation of the nuclear radius by using electron diffraction the equation$$ R \sin \theta = 0.61\lambda$$where:$R$= radius of the nucleus$\...
In quantum mechanics if two quantities $A$ and $B$ are said to be coupled what does this actually mean? I would guess that it means we have a term like $A\cdot B$ in the Hamiltonian but this is only ...