1
vote
0answers
104 views

What is the derivation of $E=mc^2$? [duplicate]

How did Einstein derive his most famous equation: $$E=mc^2$$ Is the above equation a special case of $$E^2=m^2c^4+p^2c^2$$ Its derivation? What is the difference between them?
0
votes
0answers
21 views

Need Quick reference card for scientific notation and symbols for paper reading

Hi can any one lead me to resource for symbols and notation and their meaning used for writing/publishing/scientific writing paper eg: Δ or δ means difference . ∀ means 'for all' x | y pipe mean ...
2
votes
0answers
51 views

What is the output of a quantum simulator?

First off the bat, the context of this question is philosophical, but please don't run away, since a physicist is required to comment on this one: Philosophers sometimes contemplate a hypothetical ...
1
vote
0answers
63 views

Many Worlds or Infinite Worlds?

Looking at the latest paper to deal with the topic: where it purports to show that QM can be recovered from the interactions of a multitude of Newtonian worlds, we have the following statements: ...
1
vote
1answer
139 views

Redshift and conformal time

What is the relationship between redshift and conformal time ? For example in a paper i found: taking $z_e = 3234 $ at the time of radiaton-matter equality yields the conformal time ...
7
votes
4answers
1k views

Why the statement “there exist at least one bound state for negative potential” doesn't hold for 3D case?

Previously I thought this is a universal theorem, for one can prove it in the one dimensional case using variational principal. However, today I'm doing a homework considering a potential like ...
1
vote
0answers
57 views

Why free electron has orbital magnetic moment?

I was about to ask why don't we use electron beam instead of atoms in Stern-Gerlach experiment, then I saw this question and my question become why free electron has orbital magnetic moment...
0
votes
0answers
28 views

Strongest Magnet Material - Theoretical Limits?

In terms of field strength the current best (AFAIK) is NdFeB with small magnets capable of producing strengths on the order of one Tesla. Is there a theoretical upper limit on the field strength of ...
1
vote
1answer
220 views

Magnetic levitation frog example

I'm trying to figure out the strength of the magnetic field needed to levitate the frog, assuming it's spherical, has mass $5$kg and diameter $20$cm. I also assume $\vec{B}$ is uniform. Since it's ...
1
vote
1answer
2k views

How does a plunger (toilet plunger) work?

I don't quite understand the physic involved in operating a plunger. I thought that using a plunger on a clogged sink, for example, would create a difference in pressure inside the tube so that, ...
0
votes
0answers
55 views

Robin boundary conditions for the heat equation

Consider a 3D channel with fluid or gas with walls $\Gamma_1$, inflow part $\Gamma_2$ and outflow part $\Gamma_3$. The temperature is described by the heat equation: $$ \frac{\partial T}{\partial t} ...
-1
votes
3answers
110 views

Why are we not able to move at the speed of light again? [duplicate]

In many many answers here and papers everywhere, it's often stated that no object can move faster than light. Why is that again?
0
votes
1answer
130 views

Most efficient way to cool a two story house with two fans?

Suppose I have two fans in my two story house, one in an upstairs window and another in a downstairs window. I want to cool my whole house; it is cooler outside than inside. Is it more efficient for ...
1
vote
1answer
86 views

Can the strings in string theory be thought of as troughs in a field?

I figure that string theory is a new breed of QFT which looks at fields in terms of a network of strings and also incorporates gravity into its module, however my question is that since elementary ...
0
votes
3answers
180 views

How much further can I throw a baseball (148 grams) than a grenade (453 grams)?

Question: Over a flat plane, how much further can I throw a Major League baseball than I can an M67 grenade? Major League baseball: 148 grams M67 frag grenade: 453 grams For a baseline of how ...
2
votes
0answers
79 views

How does the Darma - Smart Cushion work? Explanation of the new fiber loop ring-down spectroscopy

How does the Darma - Smart cushion monitor your sitting position and obtain the breathing rate, body movements, and BCG (Ballistocardiography) signals? The creators say that they accomplish this by ...
10
votes
1answer
168 views

What does an excitation in a field mean?

The term "field excitation" is used a lot especially when I hear about the Higgs boson. However, I cannot find an explanation of what precisely that means. I have a few questions relating to this. ...
1
vote
1answer
66 views

Why is electricity so successful (how to explain the variety of transducers we have discovered)

From piezoelectric crystals, to loudspeakers, microphones, hydrophones, strain gauges; electricity possesses the amazing ability to transfer energy/information to and from so many different media. I ...
1
vote
1answer
98 views

Time dilation: faster or slower?

I was wondering about Time-dilation in Special Relativity. I am still a middle school student who wonders so please excuse me if I missed any important aspects. Let us assume we have a system of ...
12
votes
3answers
935 views

How to prevent water droplets becoming larger on the tip of a nozzle?

I use an ordinary syringe to generate a single water droplet. However, it is not usually able to make a droplet smaller than 1 mm (because of surface tension forces, inner diameter of needle, etc.) ...
0
votes
1answer
35 views

Applying the time evolution operator as a form of molecular dynamics

I had a kind of weird idea. In molecular dynamics, long timescale simulations (like protein folding) are a really hard problem because you can't "skip steps" of the simulation without huge ...
-1
votes
1answer
57 views

Problem Boltzmann distribution [closed]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
0
votes
1answer
70 views

Solving kinetic energy exercise through electrical work formula instead of voltage formula?

While studying electric potential, I run into an issue on exercise 43 of Chapter 24 of Fundamentals of Physics 8th ed. Vol. 3 (Halliday et al.). The exercise states the following, paraphrased: A ...
0
votes
0answers
28 views

Boltzmann Distribution [duplicate]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
16
votes
3answers
2k views

Is it a postulate or a well proven fact that speed of light remains constant w.r.t any observer?

We usually heard that speed of light in vacuum $c$ remains same no matter how observer is moving? I am wondering whether is it taken as a postulate or a proven phenomenon that $c$ is constant ...
1
vote
0answers
47 views

Pair production and initial separation

I was looking at the wiki article on electron-positron pair production (http://en.wikipedia.org/wiki/Pair_production) and have a question. The article states that the photon energy needs to exceed ...
3
votes
1answer
205 views

Distribution of current of a rotating cone

If I have a hollow cone (surface with no bottom cover ) as the one in the picture. The cone has surface charged density $\sigma$. It rotates around the symmetry axis with an angular velocity ...
4
votes
2answers
512 views

What type of energy is Dark Energy?

What type of energy is Dark Energy? Just as the title says. Is it kinetic/potential or some other type?
5
votes
2answers
425 views

Electron's self-energy in QED in arbitrary gauge

Recently I've tried to evaluate electron's self-energy in QED in the second order of perturbation theory by using dimensional regularization. Corresponding 1PI-diagram leads to $$ \Sigma_{1loop} = ...
1
vote
0answers
37 views

What is the exact renormalization regularization for divergent harmonic serise?

given the harmonic series $$ \sum_{n=0}^{\infty}\frac{1}{n+a} $$ what is the correct option for the regularization ? a) $ \sum_{n=0}^{\infty}\frac{1}{n+a}= -\Psi (a) $ Digamma function b) $ ...
0
votes
2answers
127 views

Only differences in chemical potential have physical meaning?

Is the statement in the title true? Does the chemical potential of a single pure substance that does not undergo any chemical reactions or phase changes have any physical meaning?
1
vote
5answers
1k views

How to find total force on an object?

If there are a multitude of forces acting on an object in different directions, how do we find the TOTAL force? I know we add up the $ x $- and $ y $-components of the forces individually, but how do ...
0
votes
0answers
29 views

maximun work in a reversible process

I often read maximum work done is by a reversible process. Shouldnt the more appropriate sensence should be maximum work done per unit of heat is maximum in a reversible process. ?
1
vote
0answers
47 views

Electric field at a point [closed]

We consider a thin disk whose center is $O$ and radius is $R$, which has an electric surface charge density of $\sigma$. We want to find the expression of the electric field $\vec{E}$ at a point ...
1
vote
0answers
45 views

What does “nonlocal in time” mean?

I am studying MOND theory. At some point Milgrom says the theory needs to be nonlocal in time in order to enjoy Galilei invariance, but I don't understand the meaning. Which are the major ...
2
votes
0answers
31 views

Measure of interaction of two quarks and Casimir operators [closed]

Let's have two quarks, which refers to representations of $r_{1}$ and $r_{2}$ of color symmetry group. They create bounded state which refers to the representation $r$. There is a statement that ...
3
votes
1answer
126 views

Rigorous version of field Lagrangian

In Classical Mechanics the configuration of a system can be characterized by some point $s\in \mathbb{R}^n$ for some $n$. In particular, if it's a system of $k$ particles then $n = 3k$ and if there ...
1
vote
1answer
94 views

Wavefront sensor specifications Shack-Hartmann

Why is the dynamic range of Shack-Hartmann wave-front sensors (As seen in the first image below) quoted in wavelengths and not in angles? I thought that one of the most important aspects of the ...
4
votes
3answers
123 views

Spontaneous symmetry breaking to subspace not giving massless bosons

I'm currently trying to understand spontaneously broken in general and have stumbled upon a weird result which doesn't seem to correspond to my knowledge about broken gauge symmetries. Suppose we ...
0
votes
0answers
63 views

Heisenberg approach of Quantum Electrodynamics

I am reading the book of Gunar Kallen "Quantum Electrodynamics" and in the Chapter VI he study the Vacuum polarization. He computes the experimental observable current ...
9
votes
2answers
320 views

Is reflection instantaneous?

I was wondering while reading "On the Electrodynamics of moving bodies" by Albert Einstein (1905) (Translated to English). In the paper, he describes the time as being: by definition that the ...
0
votes
0answers
73 views

Direction of particle's vibration.

I have seen a question related to the the direction of the vibration when a wave moves. I see there two opposite picture that represents contradictory notion of getting the accurate direction of the ...
0
votes
0answers
47 views

Arrow of Time in Information transfer

I am writing a sci-fi script and need some legitimate theory to back up a central story element (so there's no real world application): Could there be a logically consistent theory supporting the ...
1
vote
0answers
44 views

Is it possible to predict the weather like John Locke? [closed]

In series 1 of the TV show Lost, the character John Locke seems to be able to predict the weather preternaturally well; he says, during a monsoon, that the rain will stop in a minute, and lo and ...
4
votes
2answers
755 views

Understanding the Jacobian Matrix

Taking the example of a two dimensional system, desribred by the following ODE's: \begin{align} \frac{dx_1}{dt}&=f_1(x_1,x_2)\\ \frac{dx_2}{dt}&=f_2(x_1,x_2) \end{align} The Jacobian Matrix ...
5
votes
0answers
264 views

Zamolodchikov's c-theorem paper

I am reading the 1986 paper [1] where Zamolodchikov proves the c-theorem and I would like to understand how equations (7a), (7b) and (8) are derived from the Callan-Symanzik equation. For ...
1
vote
2answers
67 views

The concept of torque among other unknowns

Someone asked me this question and I am going to try to ask it as 'proper' as possible. Imagine there is a door and it needs a mass on the floor at the edge of the door to keep it from swinging shut. ...
2
votes
2answers
162 views

How to accurately mark with arrows an electric circuit

As I understood, we choose to mark the current with arrows going from the plus pole to the minus pole (even though we know that in reality it is the contrary). As I'm looking at electric circuits, I ...
0
votes
1answer
55 views

Defects of smart glass (electrochromic glass)?

Smart glass is a type of glass that uses electrochromic mechanism to change it's light transmission properties in response to voltage, and it's made by sandwiching certain materials between two panes ...
1
vote
0answers
44 views

Can multiple lasers be concentrated in a circular area using some optical arrangements?

Can multiple lasers be concentrated in a circular area using some optical arrangements such as using a paraboloid surface? If yes, why do in high laser power systems do they use only 2D arrays of ...

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