1
vote
0answers
57 views

Hipothetical universe history with power law distributed matter [closed]

As every physicist knows that the history of universe has been dominated from light, matter and vacuum. All of three are supposed uniform distributed at very large scale. What would been happened to ...
2
votes
1answer
658 views

How does Lunar Orbit Injection (LOI) work?

Just what the title states. I read that LOI was used by the Apollo program (and possibly others) to achieve escape velocity. How does it work? Is it merely a matter of centrifugal/centripetal force?
0
votes
1answer
347 views

Fundamentals of Physics vs Classical Mechanics book and physics undergraduate course

There are books like Halliday Fundamental of Physics and there are books titled Classical Mechanics. To me, these two types of books seem to cover the same thing. Is there any difference? (If ...
3
votes
2answers
813 views

Vortex street and Reynolds number

Its been stated in Wikipedia regarding Reynolds number(Re) that "laminar flow occurs when Re<2300 and turbulent flow occurs when Re>4000. In another wiki file related to Kármán vortex street it has ...
2
votes
2answers
232 views

If I lift a submerged hose over 35 feet above the water's surface, what's in the top foot?

I've been arguing with a friend about the whole "perfect vacuum" concept. He and I agree that the most powerful vacuum pump in the world couldn't pump water more than ~34 feet above the surface of a ...
4
votes
1answer
479 views

Help me understand the first equation in Landau & Lifshitz's Quantum Mechanics

While I've covered a basic course in Quantum Mechanics, I'm self-studying Landau & Lifshitz's book to help me understand what's going on. Unfortunately, I'm stuck on the very first equation in ...
3
votes
4answers
536 views

Why are coordinates and velocities sufficient to completely determine the state and determine the subsequent motion of a mechanical system?

I am a Physics undergraduate, so provide references with your responses. Landau & Lifshitz write in page one of their mechanics textbook: If all the co-ordinates and velocities are ...
0
votes
1answer
70 views

are there any resource for the Abundance ratio of nuclear isotopes in early earth

The abundance ratio of a nuclide is the ratio of the number of atoms of one isotope to the number of atoms of another isotope of the same chemical element. It may also refer to the ratio of one ...
6
votes
1answer
109 views

Critical bathymetric profile to maximize surge and minimize breaking?

Reading about storm surge, I found it fascinating that the gradual slope of the Gulf Coast of Florida resulted in a much higher storm surge but much lower energy release in breaking waves. Is there ...
0
votes
2answers
95 views

Why is the thomson cross section constant over cosmological time?

In all astrophysical calculations I'm aware of, the thomson cross-section (for electron scattering) is taken as a constant in time, why is this the case? I have only weak experience in cosmology and ...
3
votes
2answers
615 views

Is the escape velocity at Geosynchronous Earth Orbit 0km/hr?

Follow-up question to How long must escape velocity be maintained? Is the escape velocity at GSO 0?
0
votes
1answer
86 views

Space station gains enough mass to lose orbit?

I.S.S is constantly being improved (add-ons). Will the space station need to be moved to a higher orbit at some point?
7
votes
6answers
4k views

How long must escape velocity be maintained?

Escape velocity from Earth's surface is 11.2 Kilometres/second. How long would one need to maintain this escape velocity to actually escape Earth's gravitational pull? Must this 11.2 km/s velocity ...
2
votes
1answer
119 views

Are Quantum Physics and statistical theory always the same as semiclassical approximations?

Quantum Mechanics and Statistical physics is a bit hard , could we then study only the WKB approximation ? In the form: replace $ \sum_{n=0}^{\infty}exp(- \beta E_{n})=Z(\beta)\sim\iint ...
3
votes
1answer
171 views

Does recharging a battery at a lower temperature lower its internal resistance?

Does temperature affect the internal resistance of batteries? And does charging a "frozen" battery allow it to charge faster than a warm or room temperature battery?
8
votes
1answer
1k views

Why does photon have only two possible eigenvalues of helicity?

Photon is a spin-1 particle. Were it massive, its spin projected along some direction would be either 1, -1, or 0. But photons can only be in an eigenstate of $S_z$ with eigenvalue $\pm 1$ (z as the ...
1
vote
0answers
164 views

Hooke's Law and the shape of coils

I've learned in school that the force in a coil is $F=kx$, linear on how much the coil is stretched. Two questions: Is it always linear for every shape of a coil? Does it remain linear if we ...
1
vote
1answer
577 views

What is the difference between observer, frame of reference, and gauge?

It seems to me that there is considerable relationship between the three concepts: frame of reference, observer, and gauge. How do they overlap? My current understanding is that an observer with a ...
1
vote
1answer
210 views

Inverse of a sum of two easy matrices

Let $A$ be a symmetric positive semidefinite matrix and $I$ the identity matrix. Given the linear equation $$ y = A(A + \sigma^2I)^{-1} x $$ Write $A$ in terms of its eigenvectors $|u_i\rangle$, ...
4
votes
1answer
240 views

Lorentz invariance and the vacuum expectation value of fields with spin > 0

I had a question about Moduli space, which I was reading about here, but then I read this sentence: "Lorentz invariance forces the vacuum expectation values of any higher spin fields to ...
6
votes
2answers
459 views

How does universal inflation fit with the Planck length?

If the universe is undergoing inflation, and there is a minimum scale that things can exist at (the Planck length), does that mean that new Planck-sized domains have to be continuously popping into ...
9
votes
0answers
282 views

Gauge invariant but not gauge covariant regularization

I'm not sure if someone's already asked this before, but I was wondering, in field theory, when we say that a certain field is gauge invariant but not gauge covariant, what does this mean? In ...
1
vote
2answers
51 views

Locating a Comet, how is it done?

How are the comet-hunters able to precisely locate/know about the required comets ? How can they distinguish between comets from such a far distance ? How are they able to estimate the exact time ...
1
vote
2answers
198 views

Effect of Charged Particles trapped in Magnetic Field on that Field

Given a stream of moving charged particles that encounter a uniform magnetic field such that they are trapped in a circular orbit, what effect do these particles have on the net magnetic field over ...
2
votes
2answers
1k views

Parabolic or Hyperbolic?

How can astronomers find the difference between a parabolic and a hyperbolic comet ? What are the criteria that helps them distinguish these ? Can a parabolic comet switch over to become a hyperbolic ...
5
votes
2answers
923 views

Does the Hilbert space of the universe have to be infinite dimensional to make sense of quantum mechanics?

Does the Hilbert space of the universe have to be infinite dimensional to make sense of quantum mechanics? Otherwise, decoherence can never become exact. Does interpreting quantum mechanics require ...
2
votes
1answer
238 views

Photon-photon scattering matrix element

I have to write the matrix element $i\mathcal{M}$ given the Feynman rules of QED for the $\gamma \gamma$ scattering as the example above. Suppose that time is the vertical. There are two times, $t, ...
4
votes
3answers
4k views

Proving angular momentum is conserved for a particle moving in a central force field $\vec F =\phi(r) \vec r$

A problem I am trying to work out is as follows: A particle moves in a force field given by $\vec F =\phi(r) \vec r$. Prove that the angular momentum of the particle about the origin is constant. ...
1
vote
0answers
21 views

On Bolte's semiclassical law

i have seen on internet the following, for $ E >> 1 $ the Eigenvalue Staircase can be approximated by $ N(E)= \frac{1}{\pi}argZ(1/2+i \sqrt E ) $ ...
1
vote
1answer
98 views

Complex Atom Manipulation?

Is it theoretically possible for humans to manipulate Atoms to the extent that we can recreate anything we want? e.g. 'Computer, Milky Way Bar please [or other].' (out pops a Milky Way Bar after some ...
12
votes
1answer
530 views

Do any naked-eye stars have planets

Are there any known exoplanets around naked-eye stars? I know that Fomalhaut has a planet, and Vega has a dust belt that may be a protoplanetary disk or even analogous to our Kuiper belt. What else is ...
4
votes
3answers
1k views

Can human push earth away from the sun?

It's said the earth is approaching the sun. Can we push it away little by little? Although we can emit rocket towards the sun, but it's too expensive to make a rocket: So I think maybe we can ...
1
vote
1answer
407 views

Density of universe during recombination

How can I calculate the density of the Universe at recombination?
0
votes
1answer
293 views

Why is the mass of even the upper limit of the possible Higgs boson is less than the top quark?

On 7 March 2012, the DØ and CDF collaborations announced that, after analyzing the full data set from the Tevatron accelerator, they found excesses in their data that might be interpreted as coming ...
2
votes
1answer
424 views

What is the Lie algebra of the Galilean group and what is the structure of it?

I read Freeman Dyson's article Missed Opportunities, in which he talked about the mathematical attractiveness of the Lorenz group compared to the Galilean group. I am reading Florian Scheck's book on ...
2
votes
2answers
4k views

Finding acceleration for a car after finding torque

I am trying to calculate the acceleration of a vehicle after finding the torque $\tau$. Assuming my Horse Power is 130 and my RPM is 1000, I calculated torque as: $$\tau = \frac{130 \times ...
3
votes
1answer
81 views

SOC and the butterfly effect

We knows that in a critical system and self organized criticality we have long range interaction due power law decay in correlation. Is this fact equivalent to the butterfly effect?
8
votes
1answer
232 views

Who added $\frac{3}{2} \partial^2 c$ to the virasoro BRST current (and why)?

I've been looking at the literature on quantizing the bosonic string, and I noticed that there was a change made in the definition of the BRST current around 1992. However, I haven't found any ...
1
vote
0answers
87 views

Geometry as Physics [closed]

What would be good introductory and follow-up references to understand the close ties between physics and geometry. I'm a retired engineer with the math background to handle Shankar's Principles of ...
4
votes
0answers
82 views

Known properties of a specific class of quantum states

Recently, I have been studying a quantum protocol for the "Hidden Matching" problem that makes use of states that can be expressed as $|\psi\rangle=\frac{1}{\sqrt{n}}\sum_{i=1}^n ...
6
votes
0answers
526 views

General equation of motion for elementary particles

Elementary particles can be grouped into spin-classes and described by specific equations, see below: Is there a general Lagrangian density from which all these equations can be derived? A ...
6
votes
1answer
245 views

What would gravity be like on a hemispherical planet?

On a hemispherical world half the size of Earth (ignoring that the planet should break apart and become a smaller sphere) what would the gravity be like? How would it change as you traveled from pole ...
0
votes
1answer
109 views

Entropy, Mass and Brane gravity

Does string theory state as vibrational entropy increases, mass increases? Related: What is a D-brane? Reference: Cambridge Relativity
4
votes
2answers
227 views

What counts as a measurement?

In quantum mechanics, an elementary particle does not have a well defined position until a measurement is performed on it (right?). Such a "measurement" is any sort of interaction with other ...
18
votes
3answers
801 views

Direct observations of a black hole?

I'm not very knowledgeable about physics generally, but know that nothing can escape a black hole's gravitational pull, not even light (making them nearly invisible?). My question is: What has been ...
2
votes
1answer
302 views

Power dissipated in resistor

Suppose one has a circuit consisting of an inductor $L$ and resistor $R$ in series where $L$ and $R$ are known, passes an alternating voltage of frequency $\omega$ through it and that one wishes to ...
8
votes
2answers
610 views

Do you pay more for gas when the day is warmer?

Found this at the gas station yesteday - got me thinking...
12
votes
1answer
605 views

Is this a quaternion representation of the equations of motion of General Relativity?

In The Quaternion Group and Modern Physics by P.R. Girard, the quaternion form of the general relativistic equation of motion is derived from $du'/ds = (d a / d s ) u {a_c}^* + a u ( d {a_c}^* / ...
3
votes
0answers
168 views

Some more questions on conformal spinors of $SO(n,2)$

This is somewhat of a continuation of my previous question. I had stated there that a conformal spinor ($V$) of $SO(n,2)$ can be created by taking a direct sum of two $SO(n-1,1)$ spinors $Q$ and $S$ ...
1
vote
1answer
226 views

why does perpendicular motion to the direction of someone' s approach does not affect the distance between them

I was reading about the the four turtles/bugs math puzzle Four bugs are at the four corners of a square of side length D. They start walking at constant speed in an anticlockwise direction at all ...

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