All Questions

8
votes
2answers
2k views

Why is there a minus sign in this wave equation derivation?

My book on quantum mechanics suggests a derivation of the wave equation $$\left(\Delta - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \right) \psi(\bar{r},t) = 0$$ from the photon energy-impulse ...
1
vote
2answers
516 views

Laplace's equation

I have got some mathematical difficulties in the following exercise : Calculate the potential of the polarized sphere along the z-axis. There are no free charges. For this, we need to solve ...
21
votes
4answers
4k views

Applications of the Spectral Theorem to Quantum Mechanics

I'm currently learning some basic functional analysis. Yesterday I arrived at the spectral theorem of self-adjoint operators. I've heard that this theorem has lots of applications in Quantum Mechanics....
4
votes
1answer
533 views

Seiberg Witten theory

I'm currently reading the Seiberg-Witten paper on $N=2$ supersymmetric Yang Mills pure gauge theory (i.e. no hypermultiplets). I have the following question: How does one understand that the metric ...
5
votes
1answer
3k views

Good books on elasticity

Can someone suggest good books/textbooks/treatises/etc on elasticity?
3
votes
2answers
520 views

Effect of spacetime curvature on the sea of virtual particles

Does the nature of the assortment of virtual particles depend upon the warping of spacetime in a direct manner ?
2
votes
1answer
174 views

Does the recent Gravity Probe - B mission mean both the mass of an object and the spin of an object affect time?

I'm a non-engineer interested in the recent GP-B mission results: http://www.engadget.com/2011/05/06/nasa-concludes-gravity-probe-b-space-time-experiment-proves-e/#disqus_thread Is it correct that ...
3
votes
5answers
674 views

Low-temperature hobby rocketry

I work with a lot of Boy Scouts who want to complete the Space Exploration merit badge. It's a great program, and one of the coolest requirements is building, launching, and recovering a model rocket....
2
votes
2answers
676 views

Coffee cooling in a cup

Will coffee in two different size styrofoam cups in the same room conditions cool at the same rate? One cup is 8 oz. and the other would be 12 oz.
8
votes
2answers
1k views

On black holes, Hawking radiation and gravitational atoms

Over the past hour or so I've been following one of my standard physics-based, wanders-through-the-internet. Specifically, I began by reviewing some details of dark energy theory but soon found myself ...
13
votes
4answers
2k views

Helicopter in an Elevator

You buy one of those remote control toy helicopters. You bring it into an elevator. The elevator goes up. Does the helicopter hit the floor or does the floor of the elevator push the air up into the ...
3
votes
3answers
3k views

A professional physics career without a degree? [closed]

I learned programming as a child, for fun. Now I am working as a programmer, even though I got a business major degree. I wonder if there are career paths for doing physics other than becoming a ...
6
votes
4answers
884 views

Far-field intensity from scattering of small particles

Howdy, I'm building a simulation for looking at the light field underwater. In order to verify my simulation, I'm looking for some data showing the far-field intensity that comes from single ...
2
votes
1answer
131 views

experimental setup to measure Center-Of-Momenta of products from spontaneous radiactive decay

This question is an attempt to complement this other question about fluctuations in radiactive decay. This question is completely experimental though: in general, suppose i have certain sample of a ...
1
vote
3answers
389 views

How can I measure the physical properties of a metallic string?

Metallic strings exist in different kinds but I would like to measure for a metallic string: the section/geometry along its length, to a precision of 1/100 mm its elongation when a tension is exerted ...
5
votes
3answers
417 views

How close can two extremal black holes with the same charge can get?

Here's a puzzle I have been pondering over. If we have two extremal black holes with the same charge, the electrostatic repulsion between them ought to cancel the gravitational attraction between ...
1
vote
1answer
302 views

Why is it that the splash pattern for every drop is never the same?

This was just a random thought that crossed my mind, and I'm sure there is a simple physics explanation. Say we have a leaky faucet 20 feet above the ground that lets out the exact same amount of ...
8
votes
5answers
813 views

Why aren't all rocks in space orbiting bigger rocks?

Why do only big rocks (planets) have satellites, and not small ones? Why doesn't cosmic dust orbit rocks that are many times heavier than the dust grains? If dust is still too heavy then what about ...
2
votes
1answer
395 views

Physical interpretation of equation for relativistic aberration

I'm working in a book on relativity. The author states that if $u$ and $u'$ are a velocity referred to two inertial frames with relative velocity $v$ confined to the $x$ axis, then the quantities $...
2
votes
1answer
522 views

Tunneling Rate Constant

I am trying to "decode"/derive an expression for the macroscopic rate constant for the tunneling of protons through a potential energy barrier that I read in a journal article: $$ k_{\rm tun}(T)=(2\...
4
votes
1answer
399 views

Have the correlation functions of the XY spin chain model been calculated using a functional partition function with source terms?

Have the correlation functions of the XY spin chain model, \begin{equation} H=-\sum_l (J_x \sigma_l^x \sigma_{l+1}^x+J_x \sigma_l^y \sigma_{l+1}^y)-B\sum_l \sigma_l^z \end{equation} been calculated ...
0
votes
1answer
5k views

Ohm's Law and Space-Charge Limited Current

I am trying to understand Space-Charge-Limited-Current (SCLC) and the resulting Mott-Gurney Law. My main problem is conceptual; I don't get why SCLC doesn't produce Ohm's law (j $\propto$ V).
0
votes
1answer
865 views

Projectile motion question [closed]

A ball is struck such that its max height is equal to its range. determine the angle its struck at. Air resistance is ignored and the ground is horizontal. my reasoning is that: using vectors Xsin(...
10
votes
2answers
906 views

What is the fastest process or shortest time in nature?

We know about some events that happen very quickly. For example, the dielectric relaxation time is about $10^{-14}\, \mathrm{seconds}$. I'm interested in other processes that switch extremely fast ...
1
vote
1answer
905 views

free adiabatic expansion

My question is mainly an engineering question. Assume I have a turbine in which I adiabatically expand compressed air. The air cools down and does work to its surroundings, which is captured by the ...
2
votes
2answers
810 views

Diffusion across an interface and conservation of mass

I am reading a physiology book chapter (Mathematical Physiology, by Keener --Respiration chapter) about the gas exchange between capillaries and alveoli. It seems that this gas exchange can be modeled ...
2
votes
2answers
2k views

Can you use a laser to measure the speed of light with a rotating mirror?

I have learned that the classical measurement of the speed of light with a rotating mirror does not work with a laser (as opposed to, say, a mercury-vapor lamp). Can you tell me if and how coherency ...
26
votes
4answers
8k views

Why is the angle of the wake of a duck constant?

Why is the angle of a wake of a duck constant? And why are some conditions on the water depth necessary? I realize that this question turns up in google searches, but I did not see a good discussion. ...
0
votes
2answers
466 views

Distance increase between Sun and Venus

Is the distance between the Sun and Venus increasing? What is the measurement? May I request for a formula?
1
vote
2answers
322 views

Can the equivalence principle be tested to high precision in a human-sized lab falling through the horizon of a black hole, in principle?

Can the equivalence principle be tested to high precision in a human-sized lab falling through the horizon of a black hole, in principle? By "human-sized lab" I mean a lab the size of the ...
2
votes
1answer
242 views

Testing General Relativity

Ever since Einstein published his GR theory in 1916, there have been numerous experimental tests to confirm its correctness--and has passed with flying colors. NASA and Stanford have just announced ...
-3
votes
1answer
1k views

Gravity anomaly in Lacnov, Slovakia: Gravity ashamed

http://www.youtube.com/watch?v=4qxwqNqL-Nc http://tvnoviny.sk/sekcia/spravy/technologie/v-lacnove-sa-predmety-kotulaju-do-kopca.html (Sorry: above links in Slovak.) Can someone explain this to me? ...
5
votes
2answers
676 views

In what order should the subjects be studied in order to get to String Theory [duplicate]

I know: Quantum Mechanics (Griffiths Level, currently doing Sakurai Level) Mechanics (Newtonian+ Lagrangian/Hamiltonian but at level lower than Goldstein/Landau) Classical Electrodynamics (Griffiths ...
1
vote
3answers
2k views

Why is heavier object more reluctant to get falling down?

Is it because of the upward force that stops the object? for example-
1
vote
3answers
560 views

Electric field at a point being an $n^{th}$ derivative of electric (or magnetic) field at some other point

This is a theoretical question for which i would like to know an answer with an example. I'd like to know if its possible to create a setup where the electric field at a point $P$ is $n^{th}$ ...
2
votes
2answers
360 views

Gravitational wave energy

Electromagnetic energy can be related to it's frequency via $E=h\nu$. Is there a comparable relationship between gravitational wave energy and frequency?
17
votes
2answers
2k views

Would a submarine float in the atmosphere of Jupiter?

Would a submarine float in the atmosphere of Jupiter, or would it get crushed?
2
votes
1answer
2k views

How radio waves penetrate through buildings?

For example how radio signals of a base transceiver station (BTS) penetrate through buildings?
8
votes
3answers
13k views

Video lectures on graduate level Classical Electrodynamics

This is a rather broad question. Does anyone know of good video lectures for graduate level classical electrodynamics?
4
votes
2answers
331 views

Do all of our discoveries of black holes in nature depend on the validity of GR?

In the question Is there a black hole in the centre of the Milky Way? the answer by Motl seems to all but say the existence of that black hole is a fact (see also Evidence for black hole event ...
2
votes
3answers
314 views

A question on Galilean Relativity

In a test I took recently in my beginning Physics class there was a question that really irked me: A man is swimming upstream at 10 mph. The water is flowing downstream at 5 mph. How fast is ...
2
votes
3answers
1k views

What is the probability that a half dollar coin lands on heads?

I recently found a half dollar coin in my basement. It's strange but I think when you flip it, that heads is a big favorite to land face up! I really could have sworn I heard that somewhere before; ...
3
votes
1answer
253 views

The vacuum as trigger

Do the apperance in the atomic nucleus of virtual matter-antimatter particle pairs play a role in the random nature of radioactive decay?
3
votes
4answers
4k views

Max rocket speed in interstellar space?

Interstellar space propulsion...if a spaceship were to get beyond our Sun's gravitational pull and since there is no atmosphere/wind/friction in space...does that mean, if an engine was constantly ...
0
votes
1answer
234 views

What is 656 Beagle?

What kind of object is 656 Beagle (1908BU)? I know it's a minor planet, but that includes a large array of different stuff. Specifically, I am looking at the general chemistry/geology of the object.
2
votes
1answer
509 views

How does inflation drive Ω close to 1?

I'll keep it simple. How does inflation drive Ω close to 1?
14
votes
3answers
599 views

Strange modulation of radioactive decay rates with solar activity

Recently I found out this strange article about nuclear decay rates somehow showing seasonal variations with a high correlation with sun activity. Has this been experimentally confirmed/disproved? An ...
21
votes
2answers
6k views

How can something finite become infinite?

How can the universe become infinite in spatial extent if it started as a singularity, wouldn't it take infinite time to expand into an infinite universe?
43
votes
4answers
26k views

Why do electrons occupy the space around nuclei, and not collide with them?

We all learn in grade school that electrons are negatively-charged particles that inhabit the space around the nucleus of an atom, that protons are positively-charged and are embedded within the ...
3
votes
2answers
3k views

Energy of unmixing

Mixing of two different fluids is associated with an increase of entropy. Conversely, separation of two gases must be associated with a decrease of the entropy of the two fluids. Is there a minimum ...

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