3
votes
1answer
21 views

Power loss due to dipole antenna position mismatch

If we have two dipole antennas, it is well known that in order to transfer maximal power, two dipole antennas should be parallel and on the same height, which means that line that connects their ...
3
votes
1answer
74 views

Why doesn't topological phase transition break any symmetry? Hidden symmetry?

This question may be superficial. However why all people saying this without a proof? Just like the "hidden variables" assumption in quantum mechanics, can one disproof that there is no hidden ...
21
votes
3answers
919 views

Hilbert space of harmonic oscillator: Countable vs uncountable?

Hm, this just occurred to me while answering another question: If I write the Hamiltonian for a harmonic oscillator as $$H = \frac{p^2}{2m} + \frac{1}{2} m \omega^2 x^2$$ then wouldn't one set of ...
5
votes
1answer
379 views

Hilbert space of a free particle: Countable or Uncountable?

This is obviously a follow on question to the Phys.SE post Hilbert space of harmonic oscillator: Countable vs uncountable? So I thought that the Hilbert space of a bound electron is countable, but ...
0
votes
0answers
73 views

Momentum operator in Dirac formalism

Could you derive the momentum operator as follows: Since $\mathcal{T}(\Delta x)=\exp(-ip_{x} \Delta x/ \hslash)$, if we set $\Delta x=x-0$ then it follows that $\left \langle x\right | ...
3
votes
1answer
36 views

What is the best angle to see water

Sometimes when I clean my house, I miss some of the water since it seems invisible, but when I stand in different angle from the water I can see it clearly, whereas some angles offer partial ...
7
votes
1answer
139 views

Is there a formalism for talking about diagonality/commutativity of operators with respect to an overcomplete basis?

Consider a density matrix of a free particle in non-relativistic quantum mechanics. Nice, quasi-classical particles will be well-approximated by a wavepacket or a mixture of wavepackets. The ...
0
votes
3answers
2k views

Normal force of loop-the-loop at the side of the circle

In the loop-the-loop ride a car goes around a vertical, circular loop at a constant speed. The car has the mass of 230 kg and moves with the speed of 300 m/s. The loop-the-loop has a radius R=20 m. ...
3
votes
2answers
184 views

1D Infinite Square Box Discrete Energy levels but Continous Momenta?

In the 1d particle in the box the energy of the particle should be completely determined by the momentum of the particle that you observe correct? So how can you have discrete energy levels and a ...
0
votes
1answer
37 views

Really quick question about the order of operations in the Lorentz force

I'm trying to calculate the Lorentz force for a particle in a uniform electric Field, E and magnetic field B. The formulas is $$F=q(E+v\times B)$$ I'm just wondering what the order of operations is ...
4
votes
2answers
61 views

Can a Storm make the day longer/shorter?

I've read that changes in jet streams can affect the speed that the Earth rotates on its axis, thus making the day longer or shorter?
1
vote
2answers
105 views

Principle of locality and forces

I have a silly confusion about the statement written in the link Einstein and Locality ''external influence on A has no direct influence on B; this is known as the Principle of Local Action.'' ...
2
votes
2answers
61 views

Relation between conformal and topological field theories

The Chern-Simons (CS) theory is a topological quantum field theory (TQFT). The question is, is a conformal field theory (CFT) a topological quantum theory? Or the reverse, topological quantum field ...
-2
votes
0answers
62 views

A new interpretation of QM [on hold]

Do you think that this new interpretation of quantum mechanics has solved the measurement problem completely as it claims? http://article.sapub.org/10.5923.j.ijtmp.20140405.04.html
4
votes
2answers
156 views

Is there any physical quantity that does not have uncertainty?

I saw this video and I got a thought: Is there any physical quantity that does not have uncertainty? Basic models are: for lenght for time end energy (so for mass too) and I realized that ...
1
vote
1answer
46 views

One step in deriving the Einstein-Hilbert action

In this amazing first principles derivation of the Einstein-Hilbert action there is one small manipulation needed to show $$c g_{ab,cd}\left(\eta^{ac}\eta^{bd} - \eta^{ab}\eta^{cd}\right)$$ is ...
1
vote
3answers
152 views
+50

Generic term comprising everything that can be represented with a number and a unit?

I am looking for the generic term comprising all of the following: $23.42\,\text{m}$ $200\,\text{K}$ $123\,\text{MeV}$ $ħ$ with other words, everything that can be reasonably represented ...
2
votes
2answers
51 views

Deriving the Coulomb force equation from the idea of photon exchange?

Since Newton's law of gravitation can be gotten out of Einstein's field equatons as an approximation, I was wondering whether the same applies for the electromagnetic force being the exchange of ...
0
votes
1answer
25 views

Net Electric Field of Multiple Charges [on hold]

Question: Four charges, each of magnitude +Q are placed at the corners of a square of side a. Obtain an expression for the electric potential at the centre of the square and at the mid point of one ...
0
votes
1answer
46 views

Finding the spectrum of a curious hamiltonian

I wish to analyse the following hamiltonian, i.e. find its eigenvalues and eigenstates. $$H = \frac{1}{2}\epsilon(\sigma _z \otimes \mathbb{1} + 1\otimes \sigma _z) - \Delta (\sigma _x \otimes \sigma ...
4
votes
1answer
109 views

Poincaré maps and interpretation

What are Poincaré maps and how to understand them? Wikipedia says: In mathematics, particularly in dynamical systems, a first recurrence map or Poincaré map, named after Henri Poincaré, is ...
4
votes
1answer
63 views
+50

Solving the Liouville equation for classic harmonic oscillator via method of characteristics

I'm interested in solving Liouville's equation $$\frac{\partial W}{\partial t} + \{ H,W\}=0$$ with $$H=\frac{p^2}{2m} + \frac{1}{2}m\omega^2x^2$$ using the method of characteristics. However I ...
2
votes
2answers
56 views

Why is an electron negatively charged, and what is the difference between negative and positive charges?

Nobody has yet defined the actual meaning of a charge, or why a negative charge is different from a positive charge. Everybody knows that positive charge is due to protons and negative charge is due ...
0
votes
0answers
26 views

Interpretation of angular momentum in semi-classical vector model

My Professor said that when we calculate the total angular momentum of multiparticles, in this case i.e. 2 particles, we can add total angular momentum by thinking this way, if both spins allign they ...
4
votes
1answer
99 views

Deducing instability growth rates from the Hamiltonian for the nonlinear schrodinger equation

Consider the following nonlinear Schrodinger equation (NLSE): $$A_t+iA_{xx}+i|A|^2A = 0$$ where $A$ is a complex valued function of $(x,t)$. A solution to this equation is $A=a_oe^{-ia_o^2t}$. We ...
0
votes
0answers
19 views

How to calculate the atomic form factor of Lithium? [on hold]

I have a density function, $\rho=e^{\frac{-2r}{\pi a^3}}$, and I have also two numbers, but I don't know what they mean. They are: $a_k=0.2 \mathring A$ angstrom, $a_l=1.6 \mathring A$ . And teacher ...
4
votes
5answers
4k views

Why do two bodies of different masses fall at the same rate (in the absence of air resistance)?

I'm far from being a physics expert and figured this would be a good place to ask a beginner question that has been confusing me for some time. According to Galileo, two bodies of different masses, ...
5
votes
1answer
147 views
+100

Geodesic deviation equation - why does the ordinary second derivative give the correct answer?

I've calculated the correct answer to my problem, but don't understand one of the assumptions I made when doing so. I used the geodesic deviation equation ...
3
votes
2answers
216 views

Does relativistic mass exhibit gravitiational effects?

Groundhog Day Update, 2014 The simple and dumb way to ask my main question is this: If something like a neutron start goes sailing by at very close to the speed of light, say fast enough to double ...
2
votes
1answer
370 views

Force required to move a fluid out of piston at depth

I have a cylinder filled with water in it at the bottom of the ocean. Say 100 meters down. In this cylinder is a piston that moves up and down. Its job is to squish the water out the bottom of the ...
1
vote
1answer
104 views

Gravitational interactions by energy or by mass?

The mass-energy equivalence, first established by Einstein is an important and highly discussed phenomenon in physics. Without claiming much knowledge about high-end discussions on this topic, I would ...
3
votes
1answer
363 views

How quickly does gravity extend from created mass? [duplicate]

I apologize in advance if this is a stupid or off-topic question. Since Energy can be converted into matter according to $E=mc^2$, how fast does it take for the gravity of that new matter to extend ...
0
votes
2answers
78 views

Properties of an object whose collision with Earth would completely disintegrate Earth

This picture got me thinking, whether such collision would be possible, and would it really look like that from the moon. What would be the properties (mass, size, velocity etc.) of an object, whose ...
1
vote
1answer
49 views

Increasing matter density in the Friedman equations?

From what I understand, the Friedman equation is $H^2=\frac{8\pi G}{3} \rho$ and the $H$, the Hubble constant, is a measure of the rate of expansion of the universe. What I am confused about is that ...
1
vote
0answers
41 views

Why can't we reach the ends of rainbow?

Rainbows are spectacular things. But I imagine why we can't reach to the ends of rainbows. Do rainbows have no ends ? If so, why can't we reach at them? If you go closer,they will go further. Why?
11
votes
4answers
555 views

How Earth protected from the gamma rays generated by Sun?

Sun is generating energy by nuclear fusion. And nuclear fusion will emit energy in the form of gamma rays. Normally earth's ozone filtering the Ultraviolet radiation. And magnetic fields of earth ...
0
votes
3answers
55 views

Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?

In classical physics we often cast an analogy between translational and rotational systems Force < > Torque Energy < > Rotational Energy Momentum < > Angular Momentum and considering SI ...
0
votes
0answers
22 views

Calculating time of being in a lift only by weight & travel distance

If someone is in a lift and normally weighs 90kg, when the lift starts accelerating the person weighs 100kg. After the lift has maintained a constant speed for a while it starts slowing down. While ...
16
votes
1answer
560 views

Is general relativity holonomic?

Is it meaningful to ask whether general relativity is holonomic or nonholonomic, and if so, which is it? If not, then does the question become meaningful if, rather than the full dynamics of the ...
3
votes
2answers
396 views

Polyakov action: difference induced metric and dynamical metric

The Polyakov action is given by: $$ S_p ~=~ -\frac{T}{2}\int d^2\sigma \sqrt{-g}g^{\alpha\beta}\partial_{\alpha}X^{\mu}\partial_{\beta}X^{\nu}\eta_{\mu\nu} ~=~ -\frac{T}{2}\int d^2\sigma ...
1
vote
5answers
234 views

Detecting absolute motion inside a box

This is not a contradiction and I know it is impossible but still consider a thought experiment by me and point out if something is wrong. See the following picture and then the explanation follows. ...
0
votes
0answers
15 views

A car collides with a net [on hold]

A little car of 500 grams with a speed of 2 m/s collides with a net with a spring constant of 400 N/m. If the force is bigger then 40 N, the net breaks. 1) Prove that the net brakes 2) What is the ...
8
votes
2answers
1k views

Would using Cherenkov radiation for lighting be feasible?

Could Cherenkov radiation be used for general illumination, for example, to replace LEDs, light bulbs etc? I.e. are there, or could there be, methods to produce substantial amount visible light with ...
1
vote
2answers
69 views

Finding sphere of influence in multibody system

I would like to draw a boundary illustrating the sphere of influence of each body in a 3 body system. What is the best way to find such a boundary? My understanding is at the edge an SoI is the ...
2
votes
0answers
25 views

What prevented the early universe to become a black hole before inflation? [duplicate]

I am an ignoramus on this subject, and I assume that these statements quoted from wikipedia are more or less accurate:(http://en.wikipedia.org/wiki/Inflaton) 1) Prior to the expansion period, the ...
0
votes
0answers
20 views

primordial perturbations

According to cosmic inflation, primordial perturbations are adiabatic. As in, the fractional number density of all types of particles is the same, n_baryons = n_photons etc. But what would it mean if ...
2
votes
3answers
60 views

In terms of physics, does the phrase “time slows down” mean the same thing as “things happen more slowly?”

The common definition of "time" is a type of measurement, like size. But the sentence "size gets bigger" doesn't make any sense. Is "time slows down" an odd phrasing of "events occur more slowly" or ...
4
votes
1answer
80 views

Engineering new particles

I am not a physicist, but I have a somewhat philosophical question regarding particle physics. In chemistry, and biology, there is a notion of synthesis, which has led to the creation of novel ...
1
vote
2answers
75 views

Why does the Earth rotate on its axis?

I know that the earth moves around the sun because of the gravity force, because the spacetime around the sun is curved. But why does the earth rotate on its axis, and which parameters can affect ...
1
vote
2answers
163 views

Why do corn flakes clump together in milk when stirred? [duplicate]

The Cheerios Effect is the name the scientific community has given to this phenomenon. When cornflakes or oats are mixed with milk and stirred, they tend to clump together. Why does this happen? Can ...

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