# All Questions

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### Exact diagonalization to resolve ground state degeneracies

I am studying a perturbed Toric Code model that is not analytically solvable. On a torus the ground state degeneracy of the unperturbed model is 4. Once we turn on the perturbation there is a change ...
667 views

### What does 1/k represent regarding Newtons Law of Cooling?

What does 1/k represent regarding Newtons Law of Cooling? I know k represents the cooling constant. I think the inverse of k is the time taken for the liquid to cool from its maximum temperture to ...
417 views

### Heisenberg XXX time evolution operator for three qubits

I've a problem to reproduce the result in equation (4) on page three of this paper: http://arxiv.org/abs/0802.2588. So far I've understood that they apply a Heisenberg XXX interaction between ...
160 views

### Are there two different ways to express the position operator $x$ in terms of the creation and annihilation operator?

As we known, to express the position operator $x$ in terms of the creation and annihilation operator $a^{+}$ and $a$, one way is: $$x= \sqrt{\frac{\hbar}{2\mu\omega}}(a^++a);$$ p= ...
6k views

### Electric potential is zero but non zero electric field?

What is the physical significance of such a point where electric field is non-zero but electric potential is zero? I mean, how can we understand this concept without mathematics?
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I've read in a book, that in general case energy $E$, momentum $\textbf{p}$ and angular momentum $\textbf{M}$ of a closed system are the only additive constants of motion, that is, if I have $N$ ...
123 views

### Paradox - Laws of physics are the same in all inertial reference frames vs Equivalence Principle (Pictures Added)

According to the equivalence principle, no experiment should exist that one can perform to determine whether one is in an accelerating elevator, or in a gravitational field. I will outline two ...
576 views

### Does a photon travel through space exactly at the speed of light?

In practice, given that in the vacuum of the universe is not perfectly empty, does the photon actually reach exactly c? Is it even possible? If it travels at exactly c, then from the photon ...
133 views

### How does $p_x$ commute with $p_y$, i.e. $[p_x,p_y]=0$? [closed]

I know it's a simple and basic question but would someone show me how to evaluate $[\hat{p}_x,\hat{p}_y]$?
151 views

### Relativistic Time dilation and travel to exoplanets

NASA recently discovered the first habitable exoplanet with the Kepler space telescope. Recently I was on this site: http://www.cthreepo.com/lab/math1/ With the Long Relativistic Journeys ...
74 views

### Spinor representation of $SO(d+1,1)$

I have been looking over the internet for a resource that tells me the number of dimensions of a spin $s-1$ spinor representation of $SO(d+1,1)$, but unfortunately have yet to be able to find it. In ...
115 views

### Do COBE, WMAP, or Planck data show time-variance of the CMB's anisotropies? [duplicate]

What limits on the time-variance of the CMB do the COBE, WMAP, or Planck data put? In other words, I am looking for a peer-reviewed paper that would answer this question: If I made maps of, say, WMAP ...
495 views

### Dropping a ball in a train moving close to the speed of light?

Suppose a train is moving very close to the speed of light, say 0.999c relative to a stationary observer on Earth. Now a stationary observer on Earth will observe clocks on the train to tick slower ...
171 views

### A simple question about special relativity [closed]

Assume that the table moves with the velocity $\vec{v} = v\hat{i}$. For the observer at $x=0$, the event at $(ct',x')=(0,k)$ is observed at $(t,x)=(k\gamma\beta, k\gamma)$ using Lorentz's ...
160 views

### If tachyons exist, what would they do to spacetime?

Mass (Positive) bends space-time generating gravity. If Tachyons are supposed to have imaginary mass, what would they do to spacetime (if they do exist).
95 views

### Raising and lowering 3D harmonic oscillator state

A good solution to the 3D harmonic oscillator is shown here. This gives the basis states $|n,\ell\rangle$ My question is if there are some operators comparable to the 1D SHO that will raise either n ...
330 views

### Why does hydrogen give up its electron to a platinum catalyst?

All descriptions of a Hydrogen fueled fuel cell (such as this one) Start with $H_2$ giving up its electrons to a platinum coated anode. Then the $H^+$ ions (protons really) travel through the ...
297 views

### Entropy and the uncertainty principle

According to the second law of thermodynamics, the total entropy of the Universe must always increase after any interaction (as I understand). So in the hydrogen atom, the electron has a high ...
154 views

### What does a constant signal sound like?

Say I was sampling a sound incorrectly and it produced a constant signal as below: What would this signal sound like? In Matlab, it plays nothing. Is this correct?
163 views

### Why does a halocline form?

I was doing an experiment over the last couple of days to try to crystallize alum using a thermal gradient. The idea was that solute at the bottom of my container would be dissolved at a higher ...
195 views

### How is ionization explained in Quantum Mechanics?

I remember in my highschool chemistry classes, they taught that an atom can be ionized when it loses a valence electron and becomes positively charged. In quantum mechanics, if electrons aren't really ...
169 views

### Can the equivalence principle be shown to follow from special relativity?

Einstein showed in various papers that the energy content of a body was a measure of its inertial mass. If you increase the internal energy of a system while keeping the center of energy at rest, its ...
425 views

### Representation of SU(3) generators

Let's discuss about $SU(3)$. I understand that the most important representations (relevant to physics) are the defining and the adjoint. In the defining representation of $SU(3)$; namely ...
214 views

### Magnetic field resistance material: are there any? [duplicate]

I was wondering with a question for a quite long time, thought to ask here. I need to know is there any material or element which can block magnetic field? I mean I am searching for such material or ...
90 views

### Doing a spring experiment, and need to find the stretch required

I'm doing an experiment where I have to calculate the stretch of a spring needed for it to launch itself into a trashcan located some distance away. The spring is located on the edge of a ramp that is ...
240 views

### Why can't an excess of electrons or holes by themselves cause current flow?

I am a beginner in electrical engineering. Often times (most cases actually), the underlying physics aren't really explained to us and we are just left to assume that it works "because it works." This ...
183 views

### Lorentz invariance?

What exactly is meant by Lorentz invariance? Is it just an experimental observation, or is there a theory that postulates it? What quantities do we expect to be Lorentz invariant? Charge? Charge ...
347 views

### How do we know if a formulation of classical mechanics is correct?

For example, the Lagrangian formulation. I may be missing something, i.e. not having done it in enough detail, but here is my issue: from the definition of the lagrangian ($\mathcal{L}$) and from ...
985 views

### Why does the n=0 Landau level in graphene have half the degeneracy of the other levels?

I've looked through several papers that talk about the anomalous integer quantum Hall effect of graphene (such as http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.95.146801), and they all state ...
791 views

### How loud is the thermal motion of air molecules?

In other words, given a magical room with walls that produce no vibration and transmit zero vibration from the outside, and nothing on the inside except room temperature air, what would be the noise ...
371 views

### Confusion about Length Contraction in special relativity [closed]

so I'm working on length contraction in relativity theory. I feel pretty confident time dilation and have not really gone over Lorentz Transformations that much. The question itself lies at the ...
492 views

### Electron Spin Resonance (ESR) Experiment: the two peaks

I am unable to figure out how we get two peaks in the Electron Spin Resonance (ESR) experiment, in which we use Helmholtz coils as B source with modulation, and a basic unit as a source of radio ...
42 views

### Do COBE, WMAP, and Plack have design flaws? [duplicate]

MRI expert Pierre-Marie Robitaille on the design flaws of COBE, WMAP, & Planck and the violation Kirchhoff's Law: Here are his talks: Pierre-Marie Robitaille: The Cosmic Microwave Background ...
171 views

### Can the wave function of the hydrogen atom be written as a superposition of plane waves with a single frequency?

The solutions of the Schrödinger equation for a hydrogen atom have definite energy. Does this mean that they could be written as a superposition of plane waves of a single frequency - corresponding to ...
57 views

### Extending a solution for $\left(H_\text{osc} + \delta^{(3)}(\vec r) \frac{\partial}{\partial r} r \right) \Psi(\vec r)$ to Gaussian potentials

I am a undergrad and currently trying to understand and use the following paper: Th. Busch u. a. Two Cold Atoms in a Harmonic Trap. 1997. URL: ...
57 views

### Stop Motion 3D model of airflow over an airfoil

I am looking for a 3D stop motion image of air represented as molecules over a wing. Does anyone know where I can find an image that I can build a model from?
127 views

### Physical properties of cigarette smoke

I've been trying to find papers that list various properties of idealized cigarette smoke( ex. the molar mass, gas compressibility factor, density of smoke exiting a cigarette) but I have had very ...
4k views

### If I were standing on the surface of the sun, how far could I see?

Assuming it would be possible to handle the intense heat... Is the surface even? If not, would average surface elevation be an appropriate definition of "surface"?Does the sun have an atmosphere that ...
119 views

### Is there a way to compute (trivalent) Feynman integrals inductively from smaller diagrams?

Suppose that I would like to compute the Feynman integral associated to the trivalent graph One can argue that this diagram comes from two copies of the smaller diagram glued together at the ...
60 views

### Does the doppler shift have any effect on the energy that the atom absorbs?

Does an atom moving towards the light that is falling on it, absorb more energy than when it is stationary? The atom should a see blue shift due to Doppler effect so the atom will see a light of ...
7k views

### Proof of Thevenin and Norton theorem

How can I prove Thevenin's and Norton's theorem? Thévenin's theorem can be used to convert any circuit's sources and impedances to a Thévenin equivalent.
177 views

### Flux on a Gaussian surface between two charged plates

If we have two parallel charged plates, equal and opposite in charge: What is the flux felt on a Gaussian surface between them? surely it sum to 0 as each amount of flux will enter and then leave? ...
407 views

### Does Fermi-Dirac Statistics explain anti-particles?

I wondered whether the Fermi-Dirac Statistics describes the anti-fermion particles. Does it include the anti-particles?
311 views

### Capillary in a slanted tube with a bend

So I've read a question on here about Capillary action in a slanted tube, but my question goes a little further. Firstly though, could someone explain to me what is meant by the only answer to that ...
714 views

### “Consciousness causes collapse”-interpretation or free will excluded by quantum zeno effect? [closed]

This question is if the von Neumann–Wigner interpretation, aka "consciousness cause collapse" (see e.g. John von Neumann's 1932 book The Mathematical Foundations of Quantum Mechanics) is compatible ...
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### Unknown Mass Spring Problem [closed]

A student conducts an experiment to measure the motion of two unknown masses (A and B) along a smooth table. A constant force of 1 N was applied by means of a string connected by a falling weight. A ...