# All Questions

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### what about there is no time, only space [closed]

what about a theory where no time exists, only discrete space, steady states and occasion chains. what would speak against this? ;-)
• 133
547 views

### Dangerous dose of I-131?

I was searching a lot and could only find dosages for curing cancer and allowed emission, but no Iodine-131 dose that could be connected with increased thyroid cancer risk (like, 10mSv is the ...
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4k views

### Distribution of forces

(little background: I'm trying to develop a small, quick 'n dirty static physics engine to determine whether a stacking of boxes is stable). If I have a 3D rigid box (with the bottom in the ...
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5k views

### What happens to chemical compounds that include radioactive nuclei, when those decay?

Say you have a chemical compound made up of one or more radioactive nuclei. If these nucleus decay, does the compound decay as well? Possible outcomes I can think of: the compounds continues to exist ...
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196 views

### How to reproduce highway vibration?

I would like to test my hardware under vibration that can appear on a highway gantry. If someone has a model of such vibration. i.e period and amplitude. In my lab I have a motor that can be ...
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22k views

### How does ATP transfer energy to a reaction?

This is a question for which I've found it surprisingly hard to find a good answer. Biology texts talk mystically about the ATP->ADP reaction providing energy to power other reactions. I'd like to ...
10k views

### Why is the Earth so fat?

I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy? The ...
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1 vote
241 views

### Combining chemical elements

Prof Brian Cox mentioned on Wonders of the Universe when discussing chemical elements that Carbon 12 consists of 6 protons and 6 neutrons, he also mentioned that Helium consists of 2 protons and 2 ...
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227 views

### Looking for review articles on big bang nucleosynthesis

Can someone point to a good review article covering contemporary developments in big bang nucleosynthesis, beyond what's covered in Kolb and Turner's The Early Universe? Thanks.
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### Balloon rubbing; where do the electrons go?

If you rubbed a balloon with a towel, where would the electrons go: the balloon or the towel? Why? I'm guessing the electrons would go to the object with a larger mass, but it's just a guess. :)
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43k views

### About the complex nature of the wave function?

1. Why is the wave function complex? I've collected some layman explanations but they are incomplete and unsatisfactory. However in the book by Merzbacher in the initial few pages he provides an ...
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3k views

### Are these two quantum systems distinguishable?

Suppose Stanford Research Systems starts selling a two-level atom factory. Your grad student pushes a button, and bang, he gets a two level atom. Half the time the atom is produced in the ground state,...
• 3,277
703 views

### I lost a factor of two in the electromagnetic field tensor [closed]

I apologize for this simple question, but I lost a factor of 2 and can't find it anymore, so now I'm looking on the internet, perhaps one of you has some information about its whereabouts. :-) ...
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913 views

### Decomposing geodetic/de Sitter effect into Thomas precession and spatial curvature

According to Rindler the geodetic effect can be considered as consisting of Thomas precession combined with the effect of moving through curved space. Wolfgang Rindler (2006) Relativity: special, ...
• 147
1k views

### Why is GR ghost-free?

I wonder how one can show that general relativity is ghost-free? By ghost I mean the negative norm state that breaks the unitarity. I think it is a well-known "fact" but I just couldn't find any ...
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22k views

### Do magnets lose their magnetism?

I recently bought some buckyballs, considered to be the world's best selling desk toy. Essentially, they are little, spherical magnets that can form interesting shapes when a bunch of them are used ...
3k views

### How to tell if QM is really random?

Given a stream of random binary numbers(*) Is there any way to differentiate if they came from a Truly Random or from a formula/algorithm ? how? if there is no way to decide this, then, I can't find ...
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4k views

### How to explain independence of momentum and energy conservation in a 2-body collision in elementary terms?

I'm trying to explain to someone learning elementary physics (16 year old) that linear momentum and energy are conserved independently in a 2-body collision. I'm not a professional physicist and haven'...
12k views

### Does the foam on top of boiling maple sap affect the rate of evaporation?

This is a serious question from someone engaged in evaporating large quantities of water to turn sap into syrup at this time of year. Probably some background will help. When sap boils vigorously it ...
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4k views

### What does "soft" in "soft symmetry breaking" mean?

For example it is stated that if supersymmetry breaking is soft then stability of gauge hierarchy can be still maintained.
4k views

### Angular deficit

If one starts with a flat piece of paper, removes a wedge, and tapes the paper together, you get a cone. The angle of the removed wedge is called the "angular deficit". Now if this is done in 3 ...
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2k views

### Observing the exponential growth of Hilbert space?

One of the weirdest things about quantum mechanics (QM) is the exponential growth of the dimensions of Hilbert space with increasing number of particles. This was already discussed by Born and ...
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12k views

### Photon hitting an atom with higher energy than needed to ionize

Suppose we have an atom with several energy levels (e.g. an hydrogen), and it is hit by photons. I know that in order to have the atom change energy levels, the photon must have an energy level ...
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322 views

### Does the rotational speed of a planet consistently become faster and faster given that there are no conflicting events? [closed]

Does the rotational speed of a planet consistently become faster and faster given that there are no conflicting events?
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4k views

### How do radio telescopes work?

If I search online for how radio telescopes work, the found articles talk about how RF is on the spectrum, etc, how the parabolic collector is the aperture which contributes to the sensitivity and ...
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97 views

### Experimental limits on anisotropies in the $e/m_{e}$ ratio

Currently the charge-to-mass ratio of the electron is known to 10 orders of magnitude. However, i'm curious if: Are there any experiments trying to bound the anisotropy of this ratio for different ...
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322 views

### Does quantum mechanics allow us to formulate causally sufficient conditions for the occurrence of an outcome?

This is a follow-up question to "In QM, why do the probabilities ... add up to 1?". No actual measurement is perfect. While theorists may ignore this, experimenters know well enough that in many runs ...
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1k views

### Madelung constant list (for surfaces as well)

Searching for this on google proved to be quite tedious, but I reckon that someone working with crystals a lot might know this off the top of his head: Is there a good source that lists the Madelung ...
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11k views

### A circuit with no voltage difference, but current flowing

From Michael on Skeptics Stackexchange: How about a wire that's grounded? Safe to touch, right? WRONG. ...
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2k views

### Gravity and Planetary Differentiation

During solar system formation, many bodies achieved hydrostatic equilibrium, a spherical shape where their self gravitational force was balanced by internal pressure. Many also achieved ...
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584 views

### Is there Bremsstrahlung radiation for a charged massless particle?

This is a follow up question from: Massless charged particles Since by definition such a particle would interact with photons- resulting in some change in momentum- would the particle emit ...
1k views

### Plotting a SHO in matlab [closed]

I have no prior experience of using matlab. My teacher want me to solve this question. I have been trying for a couple of hours now with no luck, please help! The mass of 100 g hanging in a spring ...
771 views

### What is Electromagnetic Induced Transparency?

Please can anyone explain the concepts of electromagnetic induced transparency? I am having problem with the technicality of the explanation on wikipedia. Please I am an engineer with a physics ...
604 views

### Alternative career paths in physics? [closed]

What do you guys think of alternative career paths in physics away from academia free from the usual academic shackles? Examples: Garrett Lisi who spends his time surfing and skiing while not working ...
11k views

### How do electrons "know" to share their voltage between two resistors?

My physics teacher explained the difference between voltage and current using sandwiches. Each person gets a bag full of sandwiches when they pass through the battery. Current = the number of people ...
359 views

### Why aren't D-branes and strings independent degrees of freedom?

A condensate of open strings with both ends attached to the same D-brane can be equivalent to a displacement of the D-brane with no open string condensate. A solution to the D-brane Born-Infeld ...
2k views

### In quantum mechanics, why do the probabilities of the possible outcomes of a measurement add up to 1?

The question assumes the standard formalism with projector-valued measures rather than POVMs. Suppose a measurement has two possible outcomes, and the corresponding probabilities are greater than 0 ...
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496 views

### What happens when particle-antiparticle pairs annihilate in MWI?

The many-worlds interpretation of quantum physics is built around a configuration space, where the position of a particle is three components of the position of that universe. What happens with ...
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8k views

### Is it possible for wind to break the sound barrier?

I understand that in nature wind would never get high enough, but I am just curious as to whether physics would allow this to occur or not.
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10k views

### Why can different batteries with the same voltage send different currents through the same object?

According to an answer in this thread on Skeptics: If you take one of the little 12V garage door opener batteries and short out (directly connect) the two terminals with a piece of wire or ...
• 2,674
854 views

### Are black holes really that special?

Science and science fiction alikeScience fiction describes black holes as these amazingly different entities in space that don't behave according to the same laws of physics that the rest of the ...
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2k views

### Pseudoscalar action in classical field theory

I was reading Landau and Lifschitz's "Classical Field Theory" and came across a comment that the action for electromagnetism must be a scalar, not a pseudoscalar (footnote in section 27). So I was ...
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603 views

### How might a resonant antenna and black body radiation interact?

How does an antenna behave when it is cooled so that its black-body radiation is emitting energy at its resonant frequency? Edit: To clarify, its not how they're related in general, but how might ...
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11k views

### Massless charged particles

Are there any massless (zero invariant mass) particles carrying electric charge? If not, why not? Do we expect to see any or are they a theoretical impossibility?
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651 views

### What direction does the force vector point in regards to Earth's E and B fields?

In regards to the right hand rule, given Earth's electric and magnetic field, in which "direction" would a particle go?
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### How do we know the size of the universe?

Ok, from astronomical observations we can tell that the observable matter is separating - so rewind the clock about 13.7 billion years and it was all at a single point. However, how do we distinguish ...
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595 views

### Fairly Broad Spectrum Light Source Options

I tried this sight before to find filtering options and got fantastic results, so let's try again! I am setting up an experiment that requires light of two different frequencies (445nm and 350nm). ...
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623 views

### Graviton Emission from D-Branes

I'm working through Polchinski's book on string theory, and I ran into something that I don't think I understand. I'm hoping that someone who knows this stuff can help me out. Before calculating the ...
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3k views

### conformal, Weyl transformations, apparent discrepancies and confusions

Because of the apparent discrepancy of how some CFT and GR books define conformal transformation unlike in string theory area, I wanted to get rid of all the confusion from McGreevy's lecture notes: ...
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