# All Questions

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### How accurate are our calculations about distant stars keeping in mind their enormous distances?

Since many stars are hundreds of light years away from the Earth and therefore, what we observe of them today is really their distant past, how can we say anything with certainty about their ...
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### How scientists could say that such meteorite comes from Mars

How could scientists affirm that a meteorite comes from Mars and not from another source ? This is a probability or an absolute certainty ? How much percent ?
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### How to comoving volumes depend on the evolution of the Universe?

I'm reading a paper which states that Neutron star binary merger rate at redshift $z$ per unit observer time interval per unit volume is $\dot{n}_{m} = \dot{n}_{0} (1+z)^{2} (1+z)^{\beta}$, ...
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### What exactly is the difference between a sychrotron light source and a free electron laser?

Cyclotron radiation is radiation emitted by a charged particle being bent by a magnetic field. The only difference between cyclotron and synchrotron radiation is that the latter occurs at relativistic ...
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### Orientation in GaAs

I can't find the precise definition of what is the orientation of a GaAs lattice. Being the superposition of two fcc lattices (one of Ga, the other of As), I would think that it is the direction of ...
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### Chiral perturbation theory: what is the Quark Condensate? why expand in $U$ rather than Goldstone fields?

I'm studying Chiral Perturbation Theory ($\chi PT$) from Scherer's Introduction to Chiral Perturbation Theory. What I am currently having some trouble understanding are two things: The quark ...
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### Relationship between frequency and wavelength [closed]

I am currently writing up a report for science class on the relationship between frequency and wavelength. And so I was wondering if anyone knew where I could find published results (literature value) ...
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### What are the problems in quark-gluon plasma? [closed]

Someone (T.D.Lee?) said that quark-gluon plasma would (after 1980?) be very important to understand high-energy physics experiments. I read some description of this new state in wikipedia ...
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### Projectile motion problem with upward acceleration and horizontal velocity [closed]

An electron in a cathode-ray tube is traveling horizontally at 2.10×10^9 cm/s when deflection plates give it an upward acceleration of 5.30×10^17 cm/s^2 . B.) What is its vertical displacement during ...
241 views

### Redshifted photons

When a photon is emitted from a far away source and then measured by an observer, there is a loss of energy or redshift which takes place. Why does this happen? I have read this similar post, however ...
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### Finite amount of matter and space

(Please bear with me.) If the universe included multi-universes, and black holes transferred matter between them, AND yet there was a large, but FINITE amount of matter and space, How would that ...
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### 3D movie glasses making white light look red and blue

While waiting for a 3D movie to start, I was playing with the glasses they give you. I understand each lens has different polarized filters, so the left and right superimposed images on the screen go ...
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### The status / acceptance of block time?

What is the current status or acceptance of block time as it relates to Einstein's theory of relativity? Has quantum mechanics ruled it out or is it still the favored view of the world? Perhaps there ...
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### Typical operators in tight binding

Let the tight-binding Hamiltonian be $\sum\limits_{ij} {{t_{ij}}\left| i \right\rangle \left\langle j \right|}$. Where ${\left| i \right\rangle }$ is the atomic orbit at lattice site $i$. My question ...
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### Do I need to have an airtight container to siphon boiling water?

I'm attempting to siphon boiling water into another container for a school project. I have researched the physics of a siphon but I can't seem to make my boiling water siphon out of its initial pot. ...
368 views

### Beam waist at extremely long distances

A beam of light of wavelength $\lambda$ and width $W$ needs to be focused at a distance $D$ to a spot not bigger than $w_S$, which stands for 'width of sail'. Now, the diffraction limit says it ...
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### How can I tell if a system has a quadrupole moment?

We know that gravitational waves are emitted (at least in GR) when the system has a time-varying quadrupole (or higher) moment. My question is Is it possible to easily tell (e.g. just by looking) if ...
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I have two related questions on the representation of the momentum operator in the position basis. The action of the momentum operator on a wave function is to derive it: $$\hat{p} ... 3answers 418 views ### Partition function of a gas of N identical classical particles Partition function of a gas of N identical classical particles is given by$$ Z~=~\frac {1}{N! h^{3N}} \int \exp[-\beta H(p_1.......p_n, x_1....x_n)]d^3p_1...d^3p_n,d^3x_1...d^3x_n $$in this ... 1answer 306 views ### Why is the Wick contraction in HFB or BCS equal to a single-particle density? I'm trying to understand how in Hartree-Fock-Bogoliubov (HFB) or BCS theory we can write a product of creation/annihilation operators as single-particle densities under the guise of "Wick's theorem". ... 1answer 535 views ### What is the mathematical background needed for quantum physics? [duplicate] I'm a computer scientist with a huge interest in mathematics. I have also recently started to develop some interest about quantum mechanics and quantum field theory. Assuming some knowledge in the ... 4answers 1k views ### The real meaning of time dilation Is this true or false: If A and B have clocks and are traveling at relative velocity to each other, then to B it APPEARS that A's clock moving slower, but A sees his own clock moving at normal speed. ... 5answers 3k views ### When driving uphill why can't I reach a velocity that I would have been able to maintain if I started with it? Consider these two situations when driving on a long straight road uphill: Starting at a high velocity v_h, which the car is able to maintain. Starting at a lower velocity v_l, and then trying ... 1answer 242 views ### Why does only one twin travel in the twin paradox? The wikipedia page repeatedly says that the twin travelling in space is the only one which travels, and also is the only one which faces acceleration and deceleration. So it does not age, while the ... 1answer 637 views ### How does light get into a stable optical cavity in the first place? It is supposedly possible to trap a beam of light bouncing back and fourth between two mirrors in a stable configuration. As I understand it, this means the configuration will prevent further spread ... 1answer 116 views ### How reflected objects are composed and who is responsible for that? Please refer to this image. The scene contains an object close to a mirror in the wall and a window, note that the reflected object is receiving more light than the object itself. I read some ... 3answers 857 views ### Difference between spinor and vector field [duplicate] How do we distinguish spinors and vector fields? I want to know it in terms of physics with mathematical argument. 1answer 831 views ### Minimal Kinetic energy for particle in a box This is driving me crazy! The question goes as follows: A proton is enclosed in a zone of length 2pm along the x-axis. The minimal kinetic energy of the proton lies closest to: 5000eV 0.5eV 50eV ... 2answers 186 views ### Why wouldn't Hubble's Law be directly in units of frequency? Maybe using this as an example: The energy of a particular color of yellow light is 3.44 \times 10^{-22} kJ So if I want to find frequency of that light, I take that number, divide by h ... 1answer 159 views ### Special relativity, acceleration and being in two places at once (How to resolve this paradox?): Imagine I am on Earth I have a clock which measures time t. When my Earth clock reads time t, then when I look at a moving spacecraft's clock I see the time t'. Let us assume time is measured ... 2answers 549 views ### If we are travelling with the speed of light, can we see what's behind us? [closed] If we are travelling with the speed of light, can we see whats behind us(like if we are moving away from earth can we able see the earth)? And how we see the things that we are approaching with speed ... 2answers 136 views ### Why is the turbulent energy cascade described as function of a wavenumber? In all the literature I've seen the turbulent energy spectrum described as E(k) instead of E(L), i.e. as a function of a wave number not eddy size. The connection via k=2\pi/\lambda is clear, ... 2answers 859 views ### In the diode equation, why the exponential \exp and the ideality factor n are there? What do they represent & what is their significance? In the Shockley diode equation, why the exponential \exp and the ideality factor n are there? What do they represent & what is their significance? I have to work on Solar Photovoltaics, and ... 0answers 406 views ### How to solve fixed-fixed beam with finite difference method? What equations to use on this system to form a matrix A with dimensions [n,n] and load vector q with dimension [n] ? I am trying to get vertical displacement w.$$w = A^{-1}\times q$$... 1answer 274 views ### Electrostatic charge leakage What are the ways electrostatic charged objects leak charge in humid conditions? Can airborne particles pick up charge by contact, then be repelled hence removing charge? If so would it be a ... 1answer 107 views ### Car tires wearing in dependence on breaking system Lets say I have two cars. They are identical in every way, except that Car A has a normal breaking system, where most of the breaking power is inflicted on the front wheels, and some on the back, and ... 2answers 100 views ### How does the correlation length of weather emerge? The question is pretty simple: If I know the weather where I stand, I can estimate the weather 5 meters or 1 km away away pretty well, but I'll have a hard time guessing what the weather is, say, 50 ... 1answer 454 views ### Compressible fluid flow through a branched pipe junctions Backdrop: Designing a dust extraction system (LEV) with a branched junction. What principles allow for the calculation of volume flow rate and pressure at the inlets if the volume flow rate and ... 0answers 160 views ### A question on charge renormalization in QED Let us work with charge renormalization in QED. Consider 2-point photon correlation function \Pi_2(q^2) at one loop level. We normalize the coupling constant at q^2=0 (point of normalization). ... 2answers 147 views ### Would a magnetic rod through Mars shield it from solar wind? Would a magnetic rod going all the way through Mars shield it from solar wind? 1answer 263 views ### What are the odds of 2012 DA14 hitting a satellite or the moon? Asteroid 2012 DA14 will pass by close to Earth. Very close. So close, in fact, that it's inside the orbit of the moon and even inside the orbit of geostationary satellites, as shown by this ... 1answer 134 views ### Magnetic field line length If I had a magnetic bar in space that was 2cm wide and 2cm high and was one Earth diameter long and had the same magnetic strength as the Earth, would the distribution of field lines be the same ... 3answers 158 views ### How do I determine where I can no longer hear sound? A teacher stands well back from an outside doorway that is 95 cm wide and blows a frequency of 600 Hz. The door faces north. Assume the speed of sound is 340 m/s and that there are no reflections. At ... 1answer 74 views ### Reflected light from pulsars If I point one telescope at a pulsar and record the image and then I point a second telescope at a mirror that has the image of the pulsar on it and record it, will the two recordings be different? ... 1answer 145 views ### Redshift and shapes of galaxies How does peculiar velocities lead to the observed elongation of galaxy clusters in redshift space, otherwise known as the Fingers of God effect? I have read the relevant Wikipedia page, but cannot ... 3answers 535 views ### Room temperature and fan orientation [duplicate] So I'm in a tiny dorm room and I normally point my fan blowing outside the window to cool my room off. I've been in some debates on blowing air out or in is more effective, so I'm hoping to get some ... 1answer 1k views ### Does photon have size measurement because of its particle nature Does photon have size measurement because of its particle nature like electron's 3.86*10^-13m etc.. 0answers 157 views ### Semiflexible discrete polymer chain Suppose we have a 2D polymer model described by a set of 2D vectors {\mathbf{t}_i} (i=1,2,\dots N) of length a. The energy of the polymer is given by:$$ ...
In condensed matter physics, one often encounter a Hamiltonian of the form \mathcal{H}=\sum_{\bf{k}} \begin{pmatrix}a_{\bf{k}}^\dagger & a_{-\bf{k}}\end{pmatrix} \begin{pmatrix}A_{\bf{k}} ...