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9
votes
2answers
17k views

Scattering vs bound states

Why are these states called as such, and how do they differ? I vaguely understand that when $E > 0$ you obtain a scattering state, but when $E < 0$ you have a bound state.
2
votes
2answers
907 views

IRIS alternative on mac?

I need to process some images taken from a telescope to determine the intensity of an astroid. This way I can determine the rotation period of this asteroid. The pictures were taken the usual way (...
3
votes
4answers
7k views

Calculating current used by individual resistor?

I just recently finished a test on Physics, and when receiving my scores back I missed a question. Here's what it was if I can remember it correctly: Three resistors (10.0, 15.0, and 6.00 ohms) ...
-4
votes
1answer
62k views

What is the extent of Earth's gravitational pull? [closed]

Where the gravitational pull of Earth exist up to? What distance from Earth it will be zero? How do the skydivers fly at a same altitude? Won't they feel gravitational pull? What is the Earth's ...
2
votes
2answers
44k views

Dimensonal analysis of damping constant?

What are the units of the damping constant from the following equation by dimensional analysis? $$\zeta = \frac{c}{2\sqrt{mk}}$$ I'm assuming the units have to be s^-1, as the damping constant is ...
7
votes
2answers
1k views

Why don't we consider centrifugal force on a mass placed on earth?

Let us say a block of mass is placed on the surface of earth. Then while drawing the forces on that body, we say: Force $F = mg$ acting towards the center of Earth. Normal reaction $N$ offered by the ...
6
votes
0answers
652 views

Descent equation and anomaly polynomial [closed]

I am just reading Ryu, Moore and Ludwig's paper on classifications of topological insulators and quantum anomaly. They are trying to relate the quantum anomaly as a signal of the presence of a ...
1
vote
2answers
743 views

Particle Accelerator Energy and Luminosity

Both beam energy and luminosity are important for succesful particle accelerator experiments. The LHC's nominal design is for 7 TeV beams and 1E34 cm^-2 sec^-1 luminosity, while the SSC's nominal ...
15
votes
1answer
1k views

Does de Sitter space admit an asymptotic S-matrix?

From the Penrose diagram of de Sitter space, we see it has a future and past conformal boundary, and they are both spacelike. So, does de Sitter space admit an asymptotic S-matrix? Sure, in the usual ...
13
votes
4answers
12k views

What is “pure energy” in matter-antimatter annihilation made of?

I used to read the term "pure energy" in the context of matter-antimatter annihilation. Is the "pure energy" spoken of photons? Is it some form of heat? Some kind of particles with mass? Basically,...
8
votes
5answers
2k views

What's my weight when jumping on a bicycle from a higher to a lower ground?

Before I start defining the situation and asking a question, I'd like to make a few things clear: this is not a homework, merely a matter of personal interest and enthusiasm I am neither a physicist, ...
3
votes
1answer
153 views

potential energy of an object due to other two objects

consider object A with mass $m_{A}$ and positional vector $\overrightarrow{r_{A}}$ object B with mass $m_{B}$ and positional vector $\overrightarrow{r_{B}}$ object C with mass $m_{C}$ and positional ...
32
votes
3answers
1k views

The Pioneer anomaly finally explained?

Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately ...
127
votes
2answers
19k views

Does the Planck scale imply that spacetime is discrete?

On a quantum scale the smallest unit is the Planck scale, which is a discrete measure. There several question that come to mind: Does that mean that particles can only live in a discrete grid-like ...
3
votes
3answers
7k views

Solids with very large specific heats

I've noticed that for many solids, specific heat is around 1 Kj/Kg*K is there a theoretical explanation for this? What common solids have the highest heat capacity per mass and per volume?
7
votes
3answers
955 views

Is there a way to decrease the rate of nuclear Beta decay?

In that question and its answers it was mentioned that you could trigger radioactive decay by bombarding atoms with gamma rays of the right energy level (there may be other solutions I do not know ...
5
votes
2answers
344 views

Very High Power Light Beams

Consider a collimated light beam in a vacuum (I am being unspecific about the frequency, anything from radio to gamma). If the beam power/cross section was increased indefinitely would new phenomena ...
8
votes
6answers
42k views

Is fire matter or energy?

I wanted to know that if FIRE is Matter or Energy. I know that both are inter-convertible. but that doesn't mean there is interconversion taking place just like nuclear fusion or fission. If it is ...
5
votes
1answer
2k views

What is the value of the fine structure constant at Planck energy?

At low energy, 511 keV, the value of the fine structure constant is 1/137.03599... At Planck energy $\sqrt{\frac{\hbar c^5}{G}}$, or 1.956 $\times$ 109 Joule, or 1.22 $\times$ 1028 eV, it has a ...
2
votes
2answers
824 views

Why is $\frac{dx}{dt}=0$ in this average momentum calculation?

In the following excerpt from S. Gasiorowicz's Quantum Physics, he derives an expression for the average momentum of a free particle. $\psi(x,t)$ is the wave function of a free particle, $\psi^*$ ...
17
votes
2answers
14k views

Why is the Yang–Mills existence and mass gap problem so fundamental?

Ref: Yang–Mills existence and mass gap Can anyone please explain why the Yang–Mills Existence and Mass Gap problem is so important / fundamental to contemporary mathematics (and, presumably, ...
1
vote
2answers
515 views

A lightning protection device physics

Here is a description of the principle of the operation of a new lightning device: During a storm the ambient electric field may rise to between 10 to 20 kV/m. As soon as the field exceeds a thresold ...
3
votes
2answers
1k views

What is the risk from radiation on imported food from Japan

I'm currently resident in Hong Kong a country which appars to import heavily from Japan. Last saturday 7th May I went to a restaurant: http://www.openrice.com/english/restaurant/sr2.htm?shopid=39760 ...
4
votes
2answers
767 views

How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?

Feynman diagrams are ordinarily the usual method of perturbative analysis for weakly interacting quantum field theories. However, over a de Sitter background, the total number of particles isn't even ...
1
vote
1answer
534 views

resistance of a flywheel to change of angular momentum

I've got a flywheel with angular momentum $\vec L$. Now I'd like to change the orientation of $\vec L$ by an angle $φ$ (in degrees). How do I calculate the angular momentum $\vec M$ for that change?
2
votes
1answer
520 views

Paradox of the Relativistic Record Player [duplicate]

Possible Duplicate: Invariant spacetime - distance - Circular Motion This is a question that I thought up a few years ago when I was taking mechanics. I asked the professor but didn't really get ...
36
votes
4answers
16k views

The meaning of action

The action $$S=\int L \;\mathrm{d}t$$ is an important physical quantity. But can it be understood more intuitively? The Hamiltonian corresponds to the energy, whereas the action has dimension of ...
1
vote
3answers
1k views

Noether's theorem and “translations” of the Hamiltonian function

In a nutshell, Noether's theorem states that for every continuous symmetry a corresponding conserved quantity exists. Now, the Hamiltonian equations of motion (let's talk about a classical system ...
2
votes
1answer
2k views

What gauge is used in the Lagrangian for a non-relativistic point particle in an electromagnetic potential

For the Lagrangian $$L = \frac{1}{2}mu^2 - q(\phi - \frac{\vec{A}}{c}\cdot \vec{u})$$ of a non-relativistic point particle in an electromagnetic potential, what gauge is used for the electromagnetic ...
9
votes
4answers
3k views

What happens if you try to freeze water in an water tight container

If I have a container that is full of water and I attempt to freeze the water by freezing the container, what would happen if the container is strong enough to prevent the water expansion? Could the ...
27
votes
4answers
19k views

Is it pions or gluons that mediate the strong force between nucleons?

From my recent experience teaching high school students I've found that they are taught that the strong force between nucleons is mediated by virtual-pion exchange, whereas between quarks it's gluons. ...
15
votes
3answers
14k views

Protons' repulsion within a nucleus

Do the protons inside the nucleus repel each other by the electrostatic force? If they do, why doesn't the repulsion drive the protons apart so that the nuclei get disintegrated?
1
vote
3answers
2k views

cm^3/g as a unit of adsorption

I recently saw cm^3/g as a unit for amount adsorbed. Usually, you see either kg adsorbate/kg adsorbent or mole adsorbate/kg adsorbent. Does anyone know the meaning of this unit?
4
votes
1answer
178 views

Why is the critical state in the Mean Field Bak-Sneppen Model a global attractor?

Simulations of the Bak-Sneppen Model of Species Evolution (introduced in http://link.aps.org/doi/10.1103/PhysRevLett.71.4083) show that it exhibits Self-Organized Criticality where after a transient ...
9
votes
2answers
517 views

Sterile Neutrinos as Dark Matter

There has been recent activity by astrophysicists to determine whether a fourth flavor of neutrino, a sterile neutrino, exists. It would likely be more massive than electron, muon or tau neutrinos. ...
-2
votes
1answer
475 views

Space-time hospitality

Are there any reasons to assume that organic processes cannot arise in relatively flat spacetime environments? An argument for the existence of a critial value or window.
2
votes
1answer
199 views

equivalence principle and nontrivial compactifications

it is commonly argued that the equivalence principle implies that everything must fall locally in the same direction, because any local variation of accelerations in a small enough neighbourhood is ...
5
votes
2answers
4k views

Shape of Fan Blades

How is the shape of the blades of an air fan determined? Trial and error, or is there a theory behind it? What are they trying to maximize, volume of air dislocated per rotation?
3
votes
2answers
2k views

Weyl exponential form of the Canonical Commutation Relations

What is the physical meaning of the $c$-numbers $Q, P\in \mathbb{R}$ in the exponent of the Weyl system $\exp\left[\frac{i}{\hbar} Q \hat{p}\right]$ and $\exp\left[\frac{i}{\hbar}P\hat{q}\right]$? ...
7
votes
1answer
718 views

7 GeV dark matter particle: how particle accelerators missed it?

This posting is regarding the recent confirmation of the DAMA results that might be due to underlying differences in proton and neutron cross section with the dark matter particles, which reflect on ...
18
votes
2answers
1k views

Correlation between outstanding hints in experimental particle physics

The 115 GeV ATLAS Higgs with enhanced diphoton decays has gone away but there are several other recent tantalizing hints relevant for particle physics, namely CoGeNT's 7-8 GeV dark matter particle ...
6
votes
3answers
990 views

Recently publicized experiment on destructive interference between two laser beams

Recently I've had several non-physicist friends ask me, independently of each other, about an experiment where two collinear laser beams destructively interfere along a certain length. Everybody wants ...
2
votes
2answers
952 views

Mechanical energy problem - please help [closed]

Quite new to all this - simple question. "A ball starting from rest rolls down a hill and reaches 10m/s at the bottom. If the same ball starts rolling down the hill from an initial velocity of 5m/s, ...
7
votes
4answers
5k views

How many digits of Pi are required in physics? [closed]

In other words: which physics experiment requires to know Pi with the highest precision?
23
votes
4answers
42k views

What do ants see?

After watching some ants in my garden today, and then looking at this very illuminating demonstration, I got to wondering, about what they would see. Not specifically ants (I understand their ...
13
votes
4answers
28k views

Why do power lines sag when they are heated up?

I was reading some information about the 2003 power blackout in the Northeastern US. Beginning early in the afternoon of August 14, 2003 big transmission lines began to fail in First Energy's ...
10
votes
2answers
384 views

Is this a weather phenomenon or an instrumental artifact?

The radar image of the midwest provided by Weatherunderground at 10:30 PM Central time, May 8 2011 has odd patterns. Are these patterns real? Perhaps caused by large scale convection over cities? Or ...
6
votes
1answer
541 views

Neutrino Speed in Supernova

I've read that neutrinos in supernova can be affected by "neutrino refraction." Is this analagous to the refraction of light, and if so, is the speed of these neutrinos similarly reduced from their ...
3
votes
2answers
8k views

Glycerol: refractive index & absorption spectra in 0.2-0.4um range

Could anyone suggest where can I find absorption spectra & refractive index of Glycerol? I am specifically interested in UV range, 200-400nm, everything I was able to find out was for standard ...
6
votes
2answers
466 views

$\nabla ^2\psi$ equals $\psi -$ average value of $\psi$ at neighboring points

Let $\psi (x,y,z)$ be a scalar field. I found the following statement in Morse & Feshbach Methods of Theoretical Physics: The limiting value of the difference between $\psi$ at a point and the ...

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