9
votes
1answer
316 views

evidence on the equation of state for dark energy?

If dark energy contributes mass-energy density $\rho$ and pressure $p$ to the stress-energy tensor, then you can define $w=p/\rho$, where $w=-1$ gives a cosmological constant, $w<-1$ gives a big ...
4
votes
2answers
207 views

How much solar energy hits a rooftop in Israel?

I was asked to today about the available plans (in Israel) to install photo-voltaic receptors on the roof and sell the energy. This is a legitimate plan and there are several companies that do this. ...
11
votes
2answers
96 views

How are interplanetary trajectories found?

How are interplanetary trajectories that involve gravity assist maneuvers found? Examples: the MESSENGER spacecraft made flybys of Earth, Venus, and Mercury before getting into orbit the Juno ...
0
votes
1answer
502 views

Helmholtz Free Energy relation

Ok so in a lecture my professor gave us this definition: $dF=\left(\frac{\partial F}{\partial T}\right)_{V,N}dT+\left(\frac{\partial F}{\partial V}\right)_{T,N}dV+\left(\frac{\partial F}{\partial ...
-1
votes
1answer
150 views

Radiation dosage problem [closed]

A large room is filled with air polluted with Caesium-137 at an average density of 1 Cs-137 particle per $\text{cm}^3$. A human is sitting inside the room for duration of $T$. What is the total energy ...
10
votes
5answers
1k views

Dimensional analysis restricted to rational exponents?

After some reading on dimensional analysis, it seems to me that only rational exponents are considered. To be more precise, it seems that dimensional values form a vector space over the rationals. My ...
3
votes
0answers
116 views

How does one place QFT's reality in context with that of other areas of physics? [closed]

Okay, so I'm learning some QFT, I read through Bogoliubov, Shirkov Introduction to Quantized Fields up to the section on renormalization, and then wanted to see a more modern point of view- so I ...
3
votes
1answer
187 views

Pressing a fluid through leaks

Say I have a cylindrical container of some volume $V$, that I fill with water or some other fluid with dynamic viscosity $\mu$. Here, the bottom of the container is milled such that some fraction, ...
11
votes
5answers
859 views

Examples where an ill-behaved function leads to surprising results?

In mathematical derivations of physical identities, it is often more or less implicitly assumed that functions are well behaved. One example are the Maxwell identities in thermodynamics which assume ...
3
votes
1answer
135 views

3 current loops with non-conservation of momentum, when one specifically considers SIGNs

Thought experiment I. Consider two loops of wire, 2 small dipoles B and C , with a common axis z (facing each other) and (say) 30 cm apart B to C. At the speed of light, information (including a ...
4
votes
3answers
703 views

Vacuum and repulsive gravity

How can one show from General Relativity that gravity is attractive force, and under which conditions it becomes repulsive, also why positive energy vacuum drives repulsive gravity?
11
votes
1answer
199 views

Theorems on instability of classical systems of charged particles?

Classically, a hydrogen atom should not be stable, since it should radiate away all its energy. I remember hearing from my favorite freshman physics prof ca. 1983 about a general theorem to the effect ...
13
votes
3answers
5k views

How Earth communicates with Voyager I?

After taking a basic signals & systems class and learning about the frequency domain, I started wondering: How the heck do scientists still communicate with Voyagers I and II??   Do they ...
5
votes
1answer
327 views

2 electromagnetic loops, with light-speed constraints: Is Newton's Third Law violated?

I'm a retired Physics Professor, but I admit I'm puzzled by this... Consider two loops of wire, 2 small dipoles B and C , with a common axis z (facing each other) and (say) 30 cm apart B to C. At ...
4
votes
1answer
63 views

Is accelerating particles through a chain of accelerators a continuous or batch process?

At the Advanced Photon Source, they use two accelerators before injecting the electrons into the large storage ring. Is the addition of particles to the storage ring done in "batches" (however ...
2
votes
1answer
623 views

Physics needed to build a top down billiards game [duplicate]

Possible Duplicate: How are these balls reflected after they hit each other? I was wondering what sort of physics equations would I need in order to build a top down billiards game? I tried ...
-4
votes
3answers
884 views

Can we model gravitation as a repulsive force? [closed]

This question is actually related to my earlier question ("what is motion"). The fact that objects move a lot in the universe and that the universe is expanding, can imply that gravity is a repulsive ...
5
votes
1answer
362 views

Shaping a wire such that a bead sliding on it has exactly isochronous oscillations

Let a wire be shaped according to some even function $y=f(x)$, with $f'(0)=0$ and $f''(0)>0$, and let a bead of negligible size slide frictionlessly on the wire. Let the bead oscillate ...
6
votes
3answers
790 views

Point particle moving on a frictionless semicircular hill

Consider an point particle moving on a frictionless semicircular hill (curve). The particle's initial kinetic energy is equal to the potential energy on the top of the hill, i.e it has the necessary ...
14
votes
1answer
1k views

Mechanics + Thermodynamics: Bouncing Ball

In preparation for an exam, I'm revisiting old exam questions. This one seems neat, but also quite complicated: A soccer ball with Radius $R=11cm$ is inflated at a pressure of $P =9 \times 10^4 ...
0
votes
1answer
323 views

How does the average Mars-Jupiter distance compare with the average Earth-Jupiter distance?

One thing is certain: Mars can be further from Jupiter than Earth can ever be (when they're on opposite sides of the solar system), but Mars can also be closer from Jupiter than Earth can ever be. ...
1
vote
0answers
406 views

Rocket engines design & materials [closed]

Does anyones know which materials (or type of steel) are used to make rocket engines nozzles & combustion chambers? What are the tricks to reduce temperature of the engine except trying to cool ...
1
vote
1answer
89 views

Creating the opposite of an optical lattice

Is it possible to create periodic potentials that instead of creating a well for an atom to be trapped in, repulsed by that specific location? If yes, can we use this as a means to make artificial ...
3
votes
1answer
170 views

Smoothed particle hydrodynamics in cosmological N-body simulations

What is the role of smoothed particle hydrodynamics (SPH) in cosmological N-body simulations like the Millenium Run (performed with Gadget-2)?
4
votes
1answer
78 views

What is the mass of the LEGO figurines being launched with spacecraft Juno?

What is the mass of the LEGO figurines being launched with spacecraft Juno? How much additional fuel will be needed to get them to Jupiter?
-4
votes
2answers
219 views

Could dark matter be expanded atoms, and dark energy expanded radiation? [closed]

After watching Lawrence Krauss' lecture 'life, universe, and nothing', an interesting idea lighted up in my mind. Is it possible that dark matter is just spread out matter waves and dark energy ...
4
votes
1answer
410 views

Which Friedmann equation is redundant?

For flat FLRW cosmology, we can write down two Friedman equations and one matter equation: (1) $H^2=\frac{8 \pi G}{3} \rho$ (2) $\frac{\ddot{a}}{a} = -\frac{4 \pi G}{3} (\rho +3p)$ (3) $\dot{\rho} ...
6
votes
4answers
2k views

Can an object accelerate to infinite speed in FINITE time (Newtonian)?

Obviously this is impossible in relativity; however, if we ignore relativity and use only Newtonian mechanics, is this possible? How (or why not)?
4
votes
3answers
360 views

Laser beam shaping

Hi I like to build a laser pointer that reflects different shapes for a project I am working on. I know this can be done with Diffractive Optical Elements. Are there other options? DOEs are expensive. ...
14
votes
2answers
3k views

Particle physics plots

I'm having a hard time understanding what some of the plots that are presented by ATLAS/CMS actually show. See for example: http://resonaances.blogspot.com/2011/07/higgs-wont-come-out-of-closet.html ...
1
vote
4answers
2k views

How can a laser pointer have range of several kilometers in atmosphere?

Laser pointers manufacturers claim that some pointers have range of several kilometers. Okay, they use a powerful laser, but that powerful laser usually has power less than one watt. Okay, the laser ...
3
votes
2answers
387 views

Pade Approximant

I have some questions about Pade approximants. Given a divergent power series $ \sum_{n >0} a(n)x^{n} $ can we use a Pade Approximant to it $ R(x)$ so we can obtain a SUM of the series for every ...
4
votes
1answer
227 views

The feeling of fuzz on the display surface of a Cathode Ray Tube Television

I was always wondering where the feeling of fuzz comes from when touching a CRT (Cathode Ray Tube) television on its display screen when the TV is turned on. Can someone please explain why I feel ...
3
votes
2answers
4k views

oscillations of blocks connected by a spring

Imagine two blocks of masses $m_1$ and $m_2$ joined together by a spring of spring constant $K$. Now let the spring be stretched by a distance $X$ and then the system is released. suppose during the ...
10
votes
2answers
1k views

Interpretation of basic free boson propagator (Euclidean action)

In section 2.3.4 of Di Francesco, Mathieu, Senechal's Conformal Field Theory (and I would guess in many other introductory discussions of QFT) one finds a simple sample computation of a propagator ...
8
votes
1answer
585 views

Physical Explanation for “Kolmogorov -5/3 spectrum” in Fluid Mechanics

According to Kolmogorov, the energy spectrum function of a turbulent fluid is given as, $E(k)=C\epsilon^{\frac{2}{3}}k^{\frac{-5}{3}}$ where $\epsilon$ is the energy flux and $k=\frac{2\pi}{r}$ ...
50
votes
10answers
6k views

What is a field, really?

There was a reason why I constantly failed physics at school and university, and that reason was, apart from the fact I was immensely lazy, that I mentally refused to "believe" more advanced stuff ...
2
votes
1answer
473 views

How to know the time a disc takes to stop from rotating and the numbers of revolutions

Assuming I rotate a disk, I want to know how long it takes to completely stop, and the number of revolutions it made since I removed my fingers off the disk. Lets say a DVD I rotate with my fingers. ...
9
votes
1answer
540 views

Discrete gauge theories

I'm trying to understand a particular case of gauge theories, namely discrete spaces on which a group G can act transitively, with a gauge group H which is discrete as well. From what I've already ...
0
votes
1answer
13k views

What did Marie Curie do for atomic theory?

There appears to be a distinct lack of agreement in the physics community on what exactly Marie Curie did for atomic theory. Many journals state that Curie was responsible for shifting scientific ...
5
votes
3answers
12k views

Can pockets of air exist underwater?

If yes, why don't they fill up with water, and can you breathe the air there? Like, it's not exactly atmosphere there, but an underwater cave with higher ceiling. P.S. Possible that it has a ...
13
votes
3answers
924 views

Where the angular momentum has gone?

I came across an interesting problem when I prepared for the preliminary exam on electromagnetism. Below is the problem in its original words: A metallic sphere of mass, $m$, and radius, $a$, ...
2
votes
1answer
16 views

How can we be sure that liquid iron/titanium droplets can form around particulates on hot Jupiters - just as they do on Earth and Venus?

I know that we have some solar system analogs. But is there a limit to it?
3
votes
1answer
30 views

Is CO2 really a potent greenhouse gas for planets around M dwarfs?

The thing with global warming is that it absorbs infrared (IR) radiation from the planet and reradiates much of it back to the planet (whereas the Sun's peak flux is in the visible region, that is ...
1
vote
1answer
46 views

How did micrometeorite flux change with the age of the solar system?

Do we also have functions for the micrometeorite flux on the Moon, Mars, and even any random body in space as a function of solar distance too?
5
votes
1answer
174 views

Why are Wilson lines the effective degrees of freedom for high energy partons?

I've read that Wilson lines are the effective degrees of freedom for high-energy partons, when you consider collinear gluon emission. But I'm struggling to find a readable account of this topic. Does ...
2
votes
2answers
460 views

Simulating eye diagrams

I'm trying to figure out how to simulate eye diagrams for communications systems using Python. I'm not sure I have the theory down completely, though. From what I could figure out using some old ...
3
votes
2answers
1k views

How to make mirrors give more authentic image of myself

My friend bought two normal mirrors from some shop probably IKEA, looking like this one: My images are different from the two. One is a little fatter and shorter, and the other a little thinner and ...
2
votes
1answer
140 views

Measuring concentration

I am trying to measure the concentration of a Volatile Organic Compound (VOC) inside a liquid using Gas Chromatography (GC). The liquid contains particles that cannot be sent to a GC, so I am using ...
10
votes
1answer
228 views

Can Fermionic symmetries be fully integrated into geometric deformation complexes or symplectic reduction?

How should a geometer think about quotienting out by a Fermionic symmetry? Is this a formal concept? A strictly linear concept? A sheaf theoretic concept? How does symplectic reduction work with odd ...

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