# All Questions

134,907 questions
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 ...
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### 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 ...
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### 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....
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### 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 ...
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### Good books on elasticity

Can someone suggest good books/textbooks/treatises/etc on elasticity?
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### 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 ?
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### 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 ...
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....
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### 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.
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### 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 ...
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### 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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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### Would a submarine float in the atmosphere of Jupiter?

Would a submarine float in the atmosphere of Jupiter, or would it get crushed?
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### How radio waves penetrate through buildings?

For example how radio signals of a base transceiver station (BTS) penetrate through buildings?
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?
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 ...
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 ...
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; ...
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?
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 ...
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.
509 views

### How does inflation drive Ω close to 1?

I'll keep it simple. How does inflation drive Ω close to 1?
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### 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 ...
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?