# All Questions

2answers
841 views

### How does a fabric containing 10% stretch material make it stretchy?

Why should adding a small amount of a stretchy material make an otherwise non-stretchy fabric stretch? Shouldn't the non-stretch fibres still constrain the maximum stretch of the fabric?
1answer
100 views

### How is a photon/particle measured when passing through one of the slits of the Double Slit Experiment?

It is stated in many popular science videos that one of the reasons that quantum mechanics is so wacky is that if you measure which slit the photon/particle went through on the way, then you no longer ...
1answer
660 views

### Local versus non-local functionals

I'm new to field theory and I don't understand the difference between a "local" functional and a "non-local" functional. Explanations that I find resort to ambiguous definitions of locality and then ...
5answers
2k views

### Do radio waves travel around the Earth or through it?

Whenever you hear someone illustrating/describing the transmission of radio waves they always make it seem like they'd travel perfectly around the Earth to another distant location. For example, a ...
3answers
281 views

### Why $e$ in the formula for air density?

I am reading a book that says that the density of air is approximately $D = 1.25 e^{(-0.0001h)}$, where h is the height in meters. Why is Euler's number $e$ used here? Was a differential equation ...
1answer
654 views

### wall thickness for a very large, low-pressure vessel?

I'm working on a space colony simulation, and I'm at the point where I need to calculate the thickness of the habitat walls, as a function of air pressure and geometry (and assuming some common ...
0answers
114 views

### -Thought Experiment- At superluminal speeds, to what degree would vision be impaired? [closed]

I was thinking about superluminal travel when thinking about advanced civilisations in the future. I was also thinking about the Doppler effect; so I decided to think about how light may be affected ...
2answers
812 views

### Why does light travel in a straight line through a liquid?

So I was reading a document that stated that when traveling through a material (I'll use a liquid here, maybe water), a photon actually always traveled at 300,000 km/s, it was just that it ...
1answer
355 views

### frequencies of light

Why are available frequency of light continuous ? I keep hearing that atoms absorb (and emit ) photons of particular frequency which correspond to their energy levels of their electron. Where do all ...
0answers
129 views

### Perturbative vs. non-perturbative approaches to a well-defined Yang-Mills theory in 4 dimensions

Another question regarding the Yang-Mills Existence and Mass Gap problem (http://www.claymath.org/sites/default/files/yangmills.pdf). Does the problem require that the "construction" of a four ...
0answers
92 views

### The speed of light during the inflationary period

Introduction: As a thought experiment, suppose I modified the value of $c$ (speed of light) in some local region and attempted to measure it with a clock placed in that same region. I will denote ...
2answers
189 views

### Why do some bound states disappear in a discontinuous way?

Generally, we have the picture that as the parameter (say, the depth of a trap) of a system varies, the bound state gets more and more extended and disappears eventually at some critical parameter ...
8answers
8k views

### Are we slightly lighter during the day and slightly heavier at night, owing to the force of the Sun's gravity?

Using $g = \frac{Gm}{r^2}$, the force on a point mass located at 1 AU from the Sun ($m = 2 \cdot 10^{30} \text{ kg}$) is about ~0.006 N/kg. Does that mean that, e.g., a 70 kg person is ~42g lighter ...
1answer
186 views

### Supersymmetric cancellation of loop contributions in a SUSY gauge theory

It is known that in SUSY models, loop contributions are automatically zero leading to a technically natural solution of the Higgs mass hierarchy problem. In many SUSY books/notes, it is often shown ...
2answers
154 views

### Metric expansion of space and Newton's second law

I wanted to ask the following question: If the expansion of the universe is really accelerating, does that mean a certain force applied on the universe? (According to Newton's second law) What kind ...
2answers
304 views

### Protection of the electron mass by chiral symmetry

In many textbooks it is said that mass renormalization of the electron mass is only logarithmic $\delta m \sim m\, log(\Lambda/m)$ because it is protected by the chiral symmetry. I understand that in ...
2answers
153 views

### Why is $dr/dt= -V$

In the solution, it says we have $dr/dt= -V$ (polar coordinates) How? I can't see how this can be possible, we know that $r(t)=V/\omega(t)$, and that's it.
1answer
81 views

### If non-zero cosmological constant interpreted as a repulsive field, what would be the properties of this field's quanta?

If non-zero cosmological constant interpreted as a repulsive field, what would be the properties of the excitation of such field, i.e. the particle which serves as the field's quantum? What would be ...
2answers
119 views

### Why do I get readings from my radiation meter with magnetized pieces?

What causes those radiation readings in my radiation meter (See the linked video)? Metal pieces were magnetized. Used radiation detector is RADEX RD1503. "Radiation" is not generated if the tube has ...
2answers
315 views

### gravitational waves and inflation theory

I am not a technical guy and I have no scientific knowloedge in physics but I have been reading books, watching videos in order to understand our cosmology and ...
1answer
67 views

1answer
90 views

### Vapor-compression refrigeration

In a vapor-compression refrigeration system I would like to understand the following : Why is there a need for the compressor ? Can't the saturated vapor from the evaporator be cooled directly in ...
3answers
514 views

### Will a prism widen a monochromatic beam?

The representation of the experiment which demonstrates that white light consists of many colors, invariably shows that the beam of white light is broadened inside the prism and is some what more ...
3answers
99 views

### Could this open electric circuit work?

Two batteries are interfaced by a resistor. Parameters such as resistance can be set arbitrarily. Does it work?
1answer
806 views

### Are gravitational waves transverse or longitudinal waves, or do they have unique/unknown properties? [duplicate]

Gravitational waves propagate through a medium of space-time. Are they traverse waves or longitudinal waves? Or do they propagate without oscillating?
2answers
163 views

### What caused a fluctuating cloud to form in a jet engine intake on a humid day?

I was on a commercial flight in a 737 stationary at idle on a taxiway. It had recently stopped raining so the relative humidity was likely near 100% and the air temperature was about "light jacket" ...
4answers
160 views

### Would infinite time elapse relative to an outside observer if an object was completely at rest?

Here's my reasoning... time dilation due to velocity: t'=t√(1-v^2) v expressed as a % of the speed of light. If you are moving through distance at the speed of light, to an observer at rest relative ...
3answers
152 views

### internal conversion and the electromagnetic force

I have read that the mechanism behind internal conversion, in which a nuclear transition leads to the ejection of an electron in one of the lower atomic orbitals, is related to the fact that the ...
3answers
186 views

### Can the distance over time of an electron between two measurements be higher than the speed of light?

So measure an electron, take down it's position $p$. Then measure the electron a second time and take down it's new position $p'$. Note the time between measurements, $t$. What does physics say about ...
1answer
215 views

### Could Carbon 14 have been created before the Big Bang? [closed]

Here is my interest in learning about the source of Carbon 14(The Stuff I think they use for Carbon Dating). I might even be asking the question wrong, but here goes. Source of the Elements? Does not ...
0answers
106 views

### Is general covariance a symmetry?

Is general covariance a symmetry? If it is ,what is its symmetry group and corresponding generator?
3answers
281 views

### Does processing for a quantum computer take place in other universes?

Apologies in advance if my question seems misinformed. I am a software developer, and neither quantum mechanics nor physics are my specialties. From http://physics.about.com/od/quantumphysics/f/...
2answers
738 views

### Why isn't a pendulum clock considered a perpetual machine?

Why do people say that a perpetual machine is one that runs for eternity? Wouldn't a machine that runs taking no energy from outside but sometimes needs to be restarted, such as pendulum clock, be ...
0answers
214 views

### Entropy of photons

Do low energy photons have greater entropy than high energy photons, or is the entropy of all photons the same (and the relationship between high and low entropy radiation is just a function of the ...
1answer
4k views

### Why does glass absorb infrared light?

I was under the impression that glass was transparent to light with a wavelength above ultraviolet, but when playing with my latest toy, an infrared thermometer which brought up a question I asked ...
1answer
92 views

### Units kT = 1… why?

Why is it convenient to choose units kT=1? It came up in this paper: http://jfi.uchicago.edu/~leop/SciencePapers/scaling%20and%20universality%20in%20statistical%20physics.pdf in page 4, equation (2.1)....
0answers
71 views

### Physical, intuitive reason for divergence of dielectric constant at electronic percolation transition?

Several papers such as this (warning, PDF) and this (PDF again) talk about how, near the electronic percolation transition for a metallic 2D film, the real part of the dielectric constant diverges (...
3answers
2k views

### Why doesn't a wall move when you push it if there's space behind it?

In the first screen you can see that if a person were to push a wall within a typical household the wall would not move while keeping themselves tractioned to the floor. If you push hard and do not ...
1answer
180 views

### Second fundamental form

How do I calculate the integral of the trace of the second fundamental form on a surface? The formula used in the Gibbons, Hawking, York paper Action integrals and partition functions in quantum ...
1answer
99 views

### Naive unification of scalar QFT and GR is possible?

I am thinking on the Klein-Gordon equation with curved (non-diagonal) metrics. Is it possible? Doesn't have it some inherent contradiction? If yes, what? If no, what is this combined formula?
0answers
875 views

### Are the fast axes on Thorlabs quarter-waveplates mislabeled?

Some members of my lab are performing a polarization-sensitive experiment where they need to use a quarter-waveplate (QWP) with the fast axis in a specific direction. In the process of carefully ...
3answers
848 views

### Is escape velocity the same for all objects?

Would a lighter-than-air craft in the mid atmosphere at 80,000 feet altitude need to achieve the same velocity to escape earth gravity as the space shuttle?
1answer
157 views

### Problem in Grandfather paradox

I am very confused about a paradox and a recent research on Quantum particles. I have read an article which states that time travel is possible for quantum particles. If it is possible then why does ...
2answers
2k views

### Standing wave velocity

My question is simple: How is it that a standing wave has velocity? I mean, it's not travelling... A lot of equations depend on this concept, for example: $f_n = \frac{nv}{2L}$ Here we're ...
4answers
5k views

### How to tell if the collision is elastic or inelastic?

I'm a programmer and a game developer, not a mathematician or a physicist. So please go easy on the math :) I know two things: How to find the new velocities of two objects after an elastic ...
1answer
268 views

### Rigorous QFT on a Torus

The problem description for the Yang-Mills Existence and Mass Gap problem (http://www.claymath.org/sites/default/files/yangmills.pdf) says in its "Mathematical Perspective" section that Some ...

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