0
votes
0answers
20 views

Did a racer on a “restless” course race at some particular average velocity? (1: Starting blocks, finish line moving uniformly wrt. each o.)

Consider two participants, $A$ and $B$ moving uniformly wrt. each other; first having approached each other, then having met and passed each other, and eventually having separated from each other such ...
2
votes
5answers
565 views

Is there a universal rest frame of reference?

I am still struggling with C being a constant and what that implies. So can an experiment be done to find the resting state for the universe? Take a device with an observer and a light source and two ...
1
vote
2answers
819 views

Derivation of relativistic uniformly accelerated motion

I'm trying to understand solution of the following problem from Landau, Lifshitz, Classical Theory Of Fields: (ending skipped). What I see when I "write out the expression for $w^iw_i$", using ...
3
votes
0answers
115 views

Electric field of a capacitor in dielectric medium with weird size

I have been learning gauss's law in capacitor recently, recently I come up with this problem that I couldn't solve myself. If we have a capacitor,and a dielectric medium with half the volume between ...
1
vote
2answers
86 views

What happen if we put permanent potential across conductor?

why if we put conductors under permanent potential like a battery ,it will not be equi-potential ?? and why if we just put it under electric field it is equi-potential although there is a surface ...
0
votes
1answer
128 views

Equation of a flying kite

My question is the following: What is the shape of the rope which holds a kite flying? (Steady state.) I am not a physicist (I am a mathematician), so I can not work the physics part of the ...
2
votes
3answers
97 views

Information about newton's third law

Well I am still small student of physics, but as far as I know, Newton's Third Law states that, "For every action there is equal and opposite reaction." Now let me tell you how I understood ...
2
votes
0answers
112 views

The proof that Dirac's hamiltonian commutes with inversion operator

I tried to check the statement that Dirac free Hamiltonian commutes with inversion operator. For $$ \hat {P}\Psi(\mathbf r , t) = i\hat {\gamma}_{0}\Psi (-\mathbf r , t), \quad \hat {H} = (\hat ...
1
vote
2answers
403 views

Why doesn't a phone charge faster, rather than slower when it is in use

In Physics class, we were building parallel circuits, and as more lights were attached in parallel, they got brighter (as more power was being provided to the lights, and the resistance decreases). ...
0
votes
3answers
552 views

Pole and Barn Paradox

I'm having trouble reconciling two different versions of the Pole and Barn paradox. Version 1: Consider a pole 10 m long and a barn 5 m long with a front and rear door. A runner carrying the pole ...
2
votes
2answers
153 views

Does any material glow, under appropriate conditions?

Given a high enough temperature, or a high enough electrical field, can we make every material emit visible light?
0
votes
0answers
235 views

The Deflecting System in a Hot Cathode Ray Tube

In an HCR-Tube, the deflecting system used to deviate the electron beam is made of positively charged plates. How is this justified? If, due to some malfunction, the electron beam deflects from its ...
1
vote
0answers
65 views

Writing Dirac Mass Term For Massive Neutrinos

How does one write down the following Dirac mass term for a collection of "massive" neutrinos? \begin{equation} -[\overline{(\psi_R)}M_D\psi_L+\overline{(\psi_L})M^\dagger_D\psi_R] \end{equation} I ...
5
votes
1answer
396 views

Mechanisms of mass generation for Dirac neutrinos

If neutrinos are Majorana particles, one way of explaining their small masses is the seesaw mechanism. Now say I'd like my neutrinos to be Dirac, for symmetry to the quark sector. What mechanisms ...
4
votes
7answers
14k views

Why do they consider radioactive matter with long half lives more dangerous than matter with a short half life?

The title says it all. For example why is plutonium considered more dangerous than radioactive iodine?
4
votes
1answer
669 views

Turn-on delay time for Laser diode

Do you know any simple explanation on the reason why the turn-on delay time on a laser diode is reducing while we increase the bias current? Turn on delay,is the time that the laser needs from the ...
-1
votes
1answer
64 views

Can a solid Au nanoparticle rotate (i.e., spin) in a vacuum?

I think there are two cases, if it has an initial angular velocity, then it is supposed to rotate. If its initial angular velocity is zero, then the particle would not rotate. However, should a solid ...
1
vote
0answers
27 views

Can the electric polarisation lead to a similar effect like the Anomalous Hall Effect

The magnetisation of a material leads to an Anomalous Hall Effect in a material. Is there a similar effect that arises from the electric Polarisation of a material?
1
vote
1answer
939 views

Confusion over the gravitational potential energy inside a sphere

I'm confused over the concept of gravitational potential energy inside a sphere. I understand that the gravitational potential energy inside the sphere is supposed to be a constant $U = ...
2
votes
1answer
112 views

Regarding the possibility of Closed Timelike Curves

I've been looking a lot at Closed Timelike Curves, and how if a theory allows for these curves it doesn't respect causality. I understand that about the curves themselves (Grandfather Paradox), but ...
-1
votes
1answer
45 views

Total energy of a body following circular motion

I learned that when a body rotates, its total energy is, $$energy=\left(\frac12\right)mv^2 + \left(\frac12\right)I\omega^2 $$ However, if an astronomical object is orbiting around the earth, is ...
0
votes
1answer
633 views

The nature of the gravity force: Is it attractive or repulsive between positive and negative mass? [duplicate]

If negative mass is rotating around fixed positive mass, then what will be the nature of force and how?
1
vote
2answers
81 views

Coupling constant is turned off adiabatically?

To me, adiabatic processes are idealisation. What do people mean with statements such as: "turning off the coupling constant (in QED say) adiabatically"?
0
votes
0answers
126 views

Why tritium doesn't undergo $\beta^-$ decay to form $He^3$?

In my textbook it is written that Tritium is an isotope of hydrogen whose nucleus Triton contains 2 neutrons and 1 proton. Free neutrons decay into p + electron + anti-neutrino . If one of the ...
4
votes
2answers
358 views

Why does welding produce UV light?

Looking directly at a welder is dangerous because large amounts of UV light is produced. What makes this light? Is it electrons from the current that excites metal atoms, and these atoms sends out UV ...
5
votes
3answers
763 views

Why is a proton assumed to be always at the center while applying the Schrödinger equation?

Why is a proton assumed to be always at the center while applying the Schrödinger equation? Isn't it a quantum particle?
8
votes
2answers
184 views

What are the current experimental limits on the polarizability of the vacuum?

Many (all?) materials are polarizable at some level; meaning that when we apply an electric field across the material, there is an induced dipole moment $$ \vec{p}=\alpha(\omega)\vec{E}, $$ where ...
1
vote
0answers
24 views

How does beta+ decay actually occur? [duplicate]

I'm having difficulty in understanding beta plus decay. How can a proton which has slightly less mass than neutron transform into a neutron, positron and neutrino? Form where does the extra mass for ...
3
votes
0answers
55 views

The question about MTW 4-momentum integral expression and lorentz nature

In section 5.8 of Misner, Thorne, and Wheeler's "Gravitation" there is a proof that 4-momentum determined as $$ \tag 1 p^{\mu} = \int T^{\mu 0}\,\mathrm{d}^{3}\mathbf r , \quad \partial^{\mu}T_{\mu ...
2
votes
2answers
194 views

How can space expand?

How can space expand when it is only a perception of the separation between at least 2 objects. Isn't saying "space expands" implying it has properties?
2
votes
2answers
225 views

Physics textbooks that distinguish between laws and definitions?

Often when I am learning physics I start to think about whether the laws I'm learning are mere definitions or experimentally determined, and usually the textbook does not make this clear. As Thomas ...
2
votes
1answer
172 views

Hypersurface Normal

Could anyone explain why $$n^{a}n_{a}=\pm1$$ where $n^{a}$ is the normal to the hypersurface
1
vote
0answers
49 views

Coherent State in 2 dimensions

I am looking at a 2D harmonic oscillator $$H=\frac{1}{2m}(p_x^2+p_y^2)+\frac 12m(\omega_x^2x^2+\omega_y^2y^2)$$ Where $\omega_x=5\omega_y$. I am told that the oscillator is prepared in a coherent ...
0
votes
1answer
78 views

What electromagnetic wave produces the strongest electrical current when making contact with an element?

More specifically, what electromagnetic waves can we utilize to generate electrical power and out of those waves, which one would produce the most power? Additionally, which one would be the most ...
0
votes
2answers
85 views

It's possible to create a proton colliding a positron with enough energetic photons?

If a proton is supposed to decay in a positron and gamma ray photons is possible to obtain the opposite process colliding enough energetic photons with a positron and create a proton ?
0
votes
1answer
201 views

How to obtain the distance traversed by a free falling body equation?

I know that the distance that a free falling body has traverse through time is given by $d=0.5*g*t^2$. I would like to know how to get to this equation to study a bit more how it was obtained. I have ...
1
vote
4answers
100 views

Wrong positioned Ampere-meter and Voltmeter [closed]

I'm dealing with a problem here and even that I'm trying to solve it i can't It says: In what figures the voltmeter and ampere-meter are wrong positioned? I think that all the the others are ...
0
votes
1answer
35 views

Query regarding interference

In Young's Double Slit experiment, why is it that two waves vibrating perpendicular to each other doesn't show interference? I know that for interference to happen, the waves must be coherent ( i.e., ...
8
votes
1answer
840 views

Glass that only allows light of specific amplitude to pass through it

Is there a type of glass, mirror or lens that allows only lights of amplitude equal to or greater than a fixed value to pass/reflect through them?
1
vote
0answers
178 views

Divergent path integral

What does it mean to have a divergent path integral in a QFT? More specifically, if $$\int e^{i S[\phi]/\hbar} D\phi (t)=\infty $$ What does this mean for the QFT of the field $\phi $? The field ...
1
vote
2answers
146 views

Electromagnetic radiation lenses

This will probably be a very basic question, but looking for a simple answers. What I know the visible light is a form of electromagnetic radiation with a defined wavelength. the full spectrum ...
2
votes
1answer
44 views

QM state probability

What does $|<\psi|\psi>|^2=1$ physically mean ? Can we use born's rule for superpositioned states ? Does it carry any physical meaning ? Why don't wquwe ever observe a superosotioned state then? ...
13
votes
1answer
647 views

How to prove that a spacetime is maximally symmetric?

In Carroll's book on general relativity, I found the following remark: In two dimensions, finding that $R$ is a constant suffices to prove that the space is maximally symmetric [...] In higher ...
11
votes
6answers
1k views

What causes polarised materials to change colour under stress?

Our physics teacher showed the class a really interesting demonstration. He used two polarised filters in opposite orientations, then he took some antistatic tape and stretched it under the two ...
38
votes
8answers
3k views

Is there something similar to Noether's theorem for discrete symmetries?

Noether's theorem states that, for every continuous symmetry of a system, there exists a conserved quantity, e.g. energy conservation for time invariance, charge conservation for $U(1)$. Is there any ...
0
votes
1answer
129 views

Can walking provide electrical energy?

This is more of a theoretical question, but is it possible to convert the energy you use to walk into electrical energy to say, power a light bulb?
0
votes
1answer
44 views

What is the physical significane of Complex Time Evolution in EM Waves? [duplicate]

So, I have been having a hard time understanding why there is even a complex phase for EM waves: $$\phi=\exp[i\omega t]=\cos(\omega t)+i\sin(\omega t)$$ Don't understand why it is there? Any one ...
7
votes
2answers
940 views

Is Einstein's Special Relativity completely inclusive of Newton's 3 laws of motion?

Relativity has always been explained to me (in books I've read, etc) as a superset of newton's laws - that is; it encapsulates all of Newton's mechanics in addition to other effects (observer effect, ...
0
votes
2answers
250 views

Why is no EMF being induced in this ring passing through a magnetic field?

The book's logic is that there is no induced EMF because flux is constant as it passes through the magnetic field. Which makes sense, but this seems counter-intuitive to what I previously learned. ...
1
vote
0answers
66 views

Derivation of cylindrical line heat source problem?

I have a line heat source embedded inside a cylinder and I am trying to find the temperature distribution T(r,t). By using the similarity variable, the solution to the differential equation is ...

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