Science topics: Physics
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# Physics - Science topic

Physics related research discussions
Questions related to Physics
Question
It does not matter how you fabricate the surface. I only want to have a physical feeling on how big the reflectance can be. Or if someone has a physical equation to show me, well that's even better.
1
Question
The equation for centrifugal force for fluids needs correction!
We made an experiment and we got some strange results:
When we rotate the drum to some speed with liquid in it, and when we will leave the drum to rotate alone by its momentum it is rotating much longer period of time when the liquid has more heat in it.
All the time the amount of liquid is the same.
The drum diametar is 310mm and the height is 160mm. The axis of the drum is in it's center. The axis is positioned vertically to the ground.
We rotate the drum with electromotor. The drum is made of semi-steel.
The drum uses 3 lagers.
-On the down side of the axis of the drum we have one axial and one radial lager
-One radial lager on the upside of the axis
The fluid is water. The fluid is heated with water-heater.
In the first case we have 10kg water on 8 °C in the drum. We rotate the drum to 2400 revolutions per minute and than we stop rotating it. And than it rotates 7min by it's momentum.
In the Second case we have 10kg water on 90 °C in the drum. We rotate the drum to 2400 revolutions per minute and than we stop rotating it. And than it rotates 10 min and 20s by it's momentum.
In both cases everything is the same, just the difference in the temperatures on the fluid. And at the end we have important difference in the time intervals of rotating the drum by it's own momentum.
There are a number of potential confounding factors here, like the warming of the lubricant or whatever of the mounting for the axle. The friction bringing the thing to rest probably changes with temperature. This can be checked by wrapping a cord around the drum and gently pulling it so that the tension in the cord is constant but there is no angular acceleration. Perhaps more importantly, keep in mind that going from 8 C to 90 C expands three-dimensionally: the diameter of the barrel increases, and for the same rotation speed, the angular momentum is greater which should keep the drum rotating for a longer time.
Question
How "wide" is a photon, if any, of it's electromagnetic fields? Is there any physical length measurement of these two orthogonal fields, E and M, from the axis of travel? When a photon hits a surface, and is absorbed by an electron orbital, this width comes into play, as there could have been more than one electron that could have absorbed the photon?
The single photon coherence length is also a way to define a size for the photon in its direction of travel. There is an interesting discussion with some references here: http://www.physicsforums.com/showthread.php?t=153755
What is the main criteria, if a compound wants to transmit light through it?
Question
Has anybody studied what are the main factors contributed to make a compound which can transmit light through it? For example glass, quartz, diamond and few plastics can transmit the light through them but why can't other materials like ceramics, metals and so on?
Now let's consider a photon moving toward and interacting with a solid substance. One of three things can happen: 1. The substance absorbs the photon. This occurs when the photon gives up its energy to an electron located in the material. Armed with this extra energy, the electron is able to move to a higher energy level, while the photon disappears. 2. The substance reflects the photon. To do this, the photon gives up its energy to the material, but a photon of identical energy is emitted. 3. The substance allows the photon to pass through unchanged. Known as transmission, this happens because the photon doesn't interact with any electron and continues its journey until it interacts with another object. Glass, of course, falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a glass electron to a higher energy level. Physicists sometimes talk about this in terms of band theory, which says energy levels exist together in regions known as energy bands. In between these bands are regions, known as band gaps, where energy levels for electrons don't exist at all. Some materials have larger band gaps than others. Glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another and back again. Photons of visible light -- light with wavelengths of 400 to 700 nanometers, corresponding to the colors violet, indigo, blue, green, yellow, orange and red -- simply don't have enough energy to cause this skipping. Consequently, photons of visible light travel through glass instead of being absorbed or reflected, making glass transparent. wood and other opaque materials fall into the first category and that is the reason why they are not transparent.
What is the main criteria, if a compound wants to transmit light through it?
Question
Has anybody studied what are the main factors contributed to make a compound which can transmit light through it? For example glass, quartz, diamond and few plastics can transmit the light through them but why can't other materials like ceramics, metals and so on?
If you mean light (the visible region) this means that you have a particles characteristics (photons). Consequently, the packing between molecules or atoms plays the major role in transmission phenomenon. In other words, there is a space or gap between the molecules of class as in liquid state which gives the main reason of class transparency.
Question
I would like to clear my concept about half integer spin. Spin Quantum number, we learned are given the values of +1/2 and -1/2 to show their direction. But now I am confused about some other values like 3/2, 5/2 etc. Can anyone please explain?
Dear Hisham, elementary particles can have only integer spins or half integer spins and this fact classifies their statistics, which determines how such a particle would behave in the company of its peers. For example a photon has spin 1and is therefore a boson. It obeys Bose-Einstein statistics and that means a collection of all the bosons in a system can occupy the same quantum ground state called a Bose-Einstein condensate. Laser is a Bose-Einstein condensate of photons at room temperature. W and Z bosons are also bosons with spin 1. Another example of boson is Higgs boson with spin 0, discovered recently at LHC. It is the particle that endows all other massive particles with mass. Graviton is predicted to be another (massless) boson in the theories of quantum gravity with spin 2.
On the other hand if the spin of the elementary particle is a fraction of an integer, then it is called a fermion and a collection of such particles obeys Fermi-Dirac statistics. This means any unique quantum state can only be occupied by a single fermion and a collection of fermions would form a degenerate fermi gas at very low temperature. In some theories of quantum gravity such as supersymmetry, fermions of higher spin are also predicted to occur. For example super partner of graviton is a spin 3/2 particle called gravitino. Off course no one has yet observed either graviton or its extremely massive super partner gravitino. Supersymmetry does not allow fermions with spin greater than 5/2 because they would not couple to the matter particles. There is some hope that LHC might observe gravitino and LIGO might observe gravitons by the end of this decade.
Question
For Bose-Einstein Condensates people care about the chemical potential. In almost all of the papers, articles or books they give the mathematical explanation for chemical potential and its impact in the dynamics of the condensates. But I want to know the physical explanation for Chemical Potential. Why is it called so?
BEC has no practical use today yet. I think it is hard for cold atoms to have use in the near future. However, people often in their paper argue that their proposal with cold atoms or becs will be useful. That is totally trash or just a lie.
I think it is a very bad trend in physics is that physics is no longer physics, but engineering. In main stream journal such as prl, nature, science, there are too many such engineering papers today. No new physics at all.
We do bec just because it is funny. That is enough.
Question
Physics paper Entropy:A concept that is not a physical quantity Published in (SCI-E Journal) PHYSICS ESSAYS (Volume 25, Issue 2 (June 2012)).
"Entropy:A concept that is not a physical quantity"
You can get this paper via Email: uhsgnahz@126.com
Entropy is a physical quantity, although sometimes it is expressed as an information system. Counting of states still qualifies as physical when the states are energy levels filled or unfilled.
Question
I am very confused about the conditions for fluid to be considered as ideal. We were taught that for an ideal fluid, the conditions required are:
1) Streamline
2) Non-viscous
3) Incompressible
However, if we put the viscosity as 0 in the equation of Reynold’s no, it will result in infinity which suggests fluid to be fully turbulent (i.e Re>4000) which then doesn’t support fluid to be ideal. Please clear my confusion.
Reynolds number (no apostrophe, by the way! :o ) is perhaps more clearly understood as a ratio of interial effect to viscous effect. Then the kinematic viscosity is perhaps the more appropriate parameter to consider, where the dynamic viscosity would only tend toward zero when the density also tends toward zero; therefore, the kinematic viscosity is always non-zero in the limit but of a to-be-determined numerical value in practice (in theory, who knows!). Furthermore, the parameter "d" in the inertial effect is arbitrary until a physical system is defined, where it may become a pipe diameter or a film thickness as appropriate, and the velocity is usually neither uniform nor constant through the region of interest but is often taken as some justifiably consistent average suited to the particular study. Finally, as a side note, there are different kinds of "ideal" fluids dependent on the behavior of interest. For the system of your "ideal fluid," the kinetic theory of gases might be better employed and Reynolds number becomes a different equation, as well, based on the conceptual definition mentioned in the first sentence, addressing the issue of the appropriate velocity term and perhaps even redefining kinematic viscosity on more fundamental/molecular terms rather than classic/continuum properties.
Question
In metals, the conductivity increases with decrease in temperature and in semiconductors the conductivity increases with increase in temperature but why in alloys (constantan and manganin) the conductivity practically remain the same with increase in temperature?
Metals, including alloys, have free electrons as charge carriers. Their movement controls by defects. One of defects of usual metal is oscillations of atoms due to temperature. More temperature - more oscillations - more collisions of electrons with atoms - less mobility - more resistivity. In alloys, like constantan, atoms are in disorder so alloys have big resistivity. Their additional disorder due to temperature increase is insignificant. That is why alloys have no temperature dependence of resistivity. The same picture is with metal glasses.
Question
Is it due to the following reasons?
Metals : Due to the formation of more free electrons
Semiconductors: Due to the formation of Cooper pairs and hence the creation of Phonon field
Alloys: No Free electrons or Phonon field created
Hi all,
While I am not an expert in solid state physics, i can propose the following.
First the flow of electricity is due to the transport of mobile charges (carriers) through a material. (aside: We usually view these carriers as electrons because in metals the carrier is the electron. However, in Semiconductors, the carrier can either be an electron or a hole (the absence of an electron).) Most materials contain vast quantities of charges (electrons and protons), however, the majority of these charges are fixed (or non-mobile) and can do nothing for conductance. Think of an insulator (eg glass) where there are plenty of charges (electrons and protons in the constituent atoms) but they cant move, hence the conductivity is very low. In order for a material to conduct, there must be mobile charges. That is, the existing charges in the atoms must be able to easily pop out of their sites in the lattice and move around freely. Since the atoms are too tightly bound to move, that leaves the outermost electron from each atom as the most likely candidate (there is more to say about density of states, but I'm trying to keep this simple)
Thus, we can see three difference classes of materials. Insulators, where all the charges are locked down (large binding energy). Conductors (usually metals), where there is a 'sea of electrons' that are freely able to slosh around in material (zero binding energy). And semiconductors, where the electrons are lightly bound (small binding energy) and, if given enough of a nudge can be made to move around and to conduct.
The nudge needed to cause electrons in a semiconductor to wander away from their original state is often small enough that room temperature energy can be used to cause them to activate. In fact, the dopants, which are added to silicon to allow it to conduct, are specifically chosen to ensure that these electrons do in fact activate at room temperature. This is the beauty of semiconductors, since one can actually engineer the local conductivity of the material.
For a metal there are plenty of electrons available to conduct, regardless of the temperature (mostly true). As one applies a voltage, the electrons stream through the lattice of the material and we now have the current that we are all familiar with.
These electrons travel through a lattice of fixed atoms, much like a shopper making their way through a crowd of other pedestrians. If the crowd is not too dense one can make their way through the crowd in a series of straight but short lines, turning left and right as one finds a person in their way.
As the electrons flow, they will bump into obstacles just like the pedestrian example above. The obstacles are the atoms in the lattice. The question you are asking is: why would the electrons be more inclined to bump into atoms (i.e. resistance) when the material is hotter than when it is colder. The answer has to due with thermal energy. Think of Brownian motion. Atoms at absolute zero sit locked into their lattice sites and don't move; they are frozen in place. But at any temperature warmer than absolute zero, they bounce and wiggle around, causing them to move slightly out of their lattice sites. These warmer atoms can begin to choke off the channels through the lattice and to interfere with electrons travelling through the channels. The warmer they are, the more they wiggle and the more interfere. Effectively, they start to block electrons on their path, causing electrons to scatter.
If we return to the example of the crowd, now imagine each standing person as wiggling back and forth by a centimeter or two; this would interfere with your travel through the crowd, but only slightly. Now imagine the people in the crowd and wiggling by tens of centimeters, now travel becomes difficult. The more motion, the more interference.
Thus, as we cool a metal, the atoms stay more closely to their centers and it is easier for the electrons to make their way through the channels between the atoms.
While this is a very simplistic explanation, it hits the high points.
In conclusion:
With metals there are plenty of mobile carriers; and the motion of the lattice atoms due to thermal energy causes them to interfere with the transport of mobile carriers through the lattice.
With semiconductors, there are insufficient mobile carriers at low temperatures and resistance is high; but as one heats the material, more and more of the lightly bound carriers escape and become free to conduct. It should be noted that once all the soon-to-be-mobile carriers in a semiconductor are fully activated, it then behaves like a 'metal' and further increases in temperature will cause it to reduce conductivity.
I hope this helps. This is an intentionally high level overview of a fairly esoteric field (Solid State Physics) and I have glossed over a lot of details to make for easier reading. Let me know if anything is incorrect.
Kevin A. Shaw, Ph.D.
Chief Technology Officer
Sensor Platforms, Inc.
Question
I am working on a project with diametral divided barrel and by the experimental results I get different results for liquids than solid bodies
the centrifugal force is the same at the same point, but the pressure field and the shear are of course totally different.
Question
Also tell me the density matrix approximation?
Thank you everybody
Question
Assuming a spherical wave emitted from one point on the optical axis and
passing through a lens with spherical aberration.
If I consider two rays, one along the optical axis and one at an angle, these will intersect in some point on the optical axis somewhat closer to the lens than
the Gaussian image plane.
Is it correct that these two rays (wave portions) are in phase in this point, or in
other words, the two light paths contain the same number of waves?
@Philip: In your initial question you state that the ray inclined to the optical axis hits the optical axis closer to the lens as the Gaussian image plane. All my intuition agrees with that. However, in the famous textbook by Born&Wolf: Principles of Optics, I found the sentence 'The rays from an axial object point which makes an appreciable angle with the axis will intersect the axis in points which lie in front of or behind the Gaussian focus'. . Along the lines of your nice thought experiment, you easily find artificial lens shapes which let the 'outer rays' hit behind: let the outer parts of the lens approach the profile oh a plano-parallel plate. By the way, do you know that the shape of your 'ideal lens' is hyperbolic (conjectured by Kepler, and proved by Descartes)?
Question
In Newtonian mechanics, the dynamic equation F=ma is Galileo invariant. Its quantization equation, Schrodinger equation, is also Galileo invariant.
In Newtonian gravity theory F=Gm_1m_2/r^2, or its field equation is Poisson equation that is also Galileo invariant. The geometrization equation of Poisson equation should keep Galileo invariance.
Both quantization from the Newtonian dynamic equation and geometrization from the Newtonian gravity field equation should be regarded as generalizations from so-called "classical" theories. It seems to keep their original invariances, namely, Galileo invariance.
Why is Einstein field equation Lorentz invariant?
From this point of view, the geometrization field equation of Newtonian gravity theory should has Galileo invariace instead of Lorentz invariance.
I agree with Niz Nl, the main reason that i see for Lorentz invariance is that this is truly the symmetry of nature (Galileo's symmetry is just an aproximation). Therefore, field equations must respect this relativistic symmetry and that's why you should add by hand the requirment of Lorentz invariance.
I would like to add a little detail here. Recall that Einstein field equations have wider symmetry, they are "generally covariant" which means that they are invariant under general coordinate transformations of the space time (technically called difeomorphism of the space time manifold). I dont have a clear answer for whether this transformations should preserve the metric tensor or not (in order to reduce the symmetry group to the so called isometry group of the space time manifold). The reason for this is simple, it is not clear what is the metric if you dont know the solutions to the equations (it is not even clear if the equations have solutions, in a general manifold). From this point of view you can recover "lorentz invariance" just locally.
Greeting
Raúl
Question
X-ray crystallography is explained as scattering of photons on the electrons of a crystal. But what is the force that leads to this scattering? The usual explanation in terms of absorption and re-emission seems a bit phenomenological to me.
This is U(1) gauge field called electromagnetic field. Electron's behavior is governed by Dirac's equation. One of the consequences of this equation is that electron can only couple to a U(1) gauge field and the quantum of this field is the photon. The full theory that takes into account a quantized electron, a quantized photon and the interaction between the two is called quantum electro-dynamics or QED. One of the most useful formulations of this great theory is by Feynman in terms of his famous diagrams.
An electron can absorb or emit a photon depending on the situation. Electron may absorb a photon to jump to higher energy level and then after a while may emit that photon to transition back to the lower energy level.
Question
If the only difference between a matter and antimatter is that they have opposite charges, how can that explain the fact that when they collide they are annihilated leaving nothing but pure energy?
I mean how can the electromagnetic attraction make two particles lose all its mass and convert into energy?
The uncertainty principle is MISUSED to explain why the electron in a hydrogen atom does not plunge into the proton. I'm at IISc for the next couple of weeks (in the old physics bldg), if you all wish to get together to discuss it in detail (mules333@gmail.com)
Question
Does anyone have an idea on how to calculate the density of states in conduction band of SnO2 or how to modify it?
Dear Sandeep,
we did calculations based on density functional theory and many-body perturbation theory for the electronic structure of SnO2. So I would think that can be an option, details can be found here: http://link.aps.org/doi/10.1103/PhysRevB.83.035116
What type of modifications are you thinking of?
Andre
Question
In a previous question of mine, there was given an expression relating the maximum velocity of lignt to the maximum gravitational force. That expression seems to explain why black holes continue to grow in size instead of becoming a deadly singularity.
There is no agreed-upon value, among physicists, for a maximum possible temperature. Under our current best-guess of a complete theory of physics, the maximum possible temperature is the Planck temperature, or 1.41679 x 1032 Kelvins. However, it is common knowledge that our current theories of physics are incomplete, thus leaving open the possibility of still higher temperatures.
I found a great article that could explain why it is so hard to provide a solid answer.
Question
It is well known that there are no natural materials having simultaneously negative values of the dielectric ε and the magnetic μ. In general, we call the materials left-hand Material or double-negative Media. But we have found some special materials in our labs. When the electromagnetic waves go though the left-hand Material by some special frequencies, the waves will be changed from right-hand to left-hand. What I want to ask is that, does a left-hand electromagnetic wave exist in nature? And how can we find it?
You wrote: "what I want to ask is that, does a left-hand electromagnetic wave exist in nature?
And how can we find it?"
As far as I know, left-handed electromagnetic phenomen appears in the artificial materials made of split-ring resonators or wire strips as well as in photonic crystals. Some details you can find here
Question
for example i have got two M-H curves. can by just looking at curves we can say that its ferromagnetic or ferrimagentic in nature?
Ferromagnetic and Ferrimagnetic can be differentiated using neutron diffraction. In case of ferrimagnetic there are extra magnetic structure peaks in the diffraction pattern.
Question
Suposing we stretch a rope between the Earth and the Sun (for example) and we put a dynamometer in each end measuring a tension (T), when we cut the Earth's rope, when will the Sun's dynamometer stop measuring any tension?: instantly?, after 8:19 minutes?
I think there is no need to use relativity in the above example, except for the comment that in relativity there is no such thing as a rigid body. The elements of the rope will be displaced with the velocity of sound.
Question
I want know whether they increase the temperature of the earth's crust if they are in great numbers when they come from the sun?
A short answer: absolutely nothing. The energy of the neutrino flux is only 2 % of that of the photons generated in Sun's fusion reaction; this means that the photon flux dominates in terms of energy. However, even though the neutrino flux transports some of the energy from fusion, the flux barely interacts with Earth - for example, in the water-based neutrino detectors only about 10 neutrinos interact at all with 20 kilotons of water per day! The heating effect of this (as compared to anything else) is totally negligible (the same conclusion applies to Earth's core, which is liquid metal).
Question
Let us consider a charged particle beam travelling in a vacuum with relativistic motion. The total Lorentz force on each beam particle is almost zero to the order of 1/\gamma^2 (\gamma being the relativistic mass factor). Then, the beam will diverge only because of the thermal spreading among the paticles. In fact, the blow up due to the electric part of the Lorentz force is almost balanced by the attractive effect due to the magnetic part of the Lorentz force. However, in this process the rms transverse profile of the beam is formally described by the diffraction caustic as given in the paraxial electromagnetic beam motion.
What is your opinion? The above charged particle beam divergence is a diffusion process (due to the thermal spreading among the beam particles) or it is a diffraction process (due to the fact that its transverse rms profile is fully similar to the diffraction caustic)?
Let me only put forward in this discussion that more than 20 years ago I have developed a quantum-like model for charged particle beam transport, the so called Thermal Wave Model. It describes classical processes in particle beam physics, but using the formalism and the language of Quantum Mechanics.
However, beyond such a model, it would be important to deepen the fundamental physical aspects related to my question. Thank you for your attention.
The attractive magnetic part of the Lorentz force can never compensate the electric repulsion. This is most easily seen considering that whether the beam is spreading or collapsing on itself is a reference-frame invariant characteristic, and in the rest frame of the charged particles there's no magnetic force to counteract the Coulomb repulsion. The observable effect of suppressed spreading is entirely due to the relativistic mass increase and time dilation. I believe that the spreading is a combined effect of the beam's initial emittance (which includes, but usually surpasses the temperature effect) and its space charge. In relativistic beams the particles' "wavelengths" are much too short compared to the typical beam radius to make noticeable diffraction possible.
Question
While I learning the Variational calculations peoples wrote a Lagrangian Density Equation from the Gross-pitaveskii (GP) or NLS equation. From that they found the effective Lagrangian and other evolution equations. My question is how can I form that Lagrangian Density equation from GP equation? Can anyone tell the derivation in clear. [NB: In PRA 81, 043636 (2010) Eq. (10) is Lagrangian Density Equation]
Not sure if I understand your question, but I will give it a shot ...
Of course there is no unique density that is the ancestor of a differential equation, since one can add a total derivative to the density without affecting the Euler equations.
I would start by left-multiplying any linear terms in the NL differential equation in the wavefunction by the complex conjugate of the wavefunction.
For the remaining non-linear terms I would attempt a functional integration over the complex conjugate of the wavefunction. This integration would be the inverse of the Euler-variation of the complex conjugate wavefunction. (It would be easier to write this down mathematically than express in words.)
Is it possible for a sound wave to be coherent?
Question
Can the direction of sound waves be controlled like (laser) light, so they can be guided in certain position?
Do you mean theoretically or in practice? In theory, the wave equations are much the same and it is very much possible. In practive a directive sound source is also possible. In fact at specific frequencies a normal loudspeaker unit will have a very directive radiation pattern. (http://resource.isvr.soton.ac.uk/spcg/tutorial/tutorial/Tutorial_files/Web-further-piston.htm for piston radiation patterns explained) For a loudspeaker driver to become directive, the frequency has to be high. If a certain directivity is desired at a lower frequency, complex sources can be made from a combination of sources. A simple example is a dipole, a complex example is the Yamaha YSP-4000 (http://usa.yamaha.com/products/audio-visual/hometheater-systems/digital-sound-projector/ysp-4000_black__u/?mode=model) Which uses this principle to send rays of sound in specific directions.
Question
Heat capacity as you know is a measure of the amount of energy (heat) required by a certain mass of a material to raise its temperaure by unit degree. The variable heat is required because heat gets absorbed into non-temperature raising modes also (other than kinetic or translational or vibrational). Can we control and hinder say rotational modes so that energy gets redistributed into kinetic ones only? This is very important because it can lead to self raising of the temerature of a given material having same energy, but diffrently divided.
The heat capacity have a contribution from the electronic structure of the material. This contribution can easily be changed by an external field. At room temperature this effect is small compared to lattice vibrations, but at low temperatures the electronic contribution can be dominating.
Regards
Henrik
Is SnS is an ntype semiconductor or p type semiconductor?
Question
In some articles it is mentioned as N-type and some shows it was a P-type material. I don’t know what should I conclude.
SnS has a 1.07 eV indirect band gap with an effective absorption onset located 0.4 eV higher. The effective mass of minority carrier ranges from 0.5 m0 perpendicular to the van der Waals layers to 0.2 m0 into the van der Waals layers. The positive characteristics of SnS feature a desirable p-type carrier concentration due to the easy formation of acceptor-like intrinsic Sn vacancy defects.
Question
How much is the Rayleigh scattering cross section of a Rydberg state atom (or molecular)? For example, the light wavelength is 500 nm.
When the radius of the Rydberg state atom is very large, could the scattering mechnism be Mie scattering?
Your question is too broad. Due to different behavior expressed by different atoms there is no universal answer. The reason for that is scattering cross section depends on both radius and wavelength (to be precisely it depends on coefficient alpha that is proportional to radio of two).
If you need particular examples you can find some of them in the book - P. Hiemenz, Principles of the colloid and surface chemistry, 2nd edition, p. 276
Question
If entanglement is preserved across an event horizon then it is possible to obtain information about the state of a photon inside a Black Hole.
As far as I know entangled particles cannot transmit information to each other. This would mean that they cannot do so if one is inside an event horizon and the other is not.
Question
However, long before the Planck length is reached, around ~10(-25)m the energy density (mass) of the photon exceeds that required to create a Black Hole. No cosmic radiation has been seen with a wavelength smaller then 10(-19).
i 'd say where the electromagnetic (EM) and the weak (W) force become one, i.e. 246 GeV. At higher energies there's no EM but EW. have a look at wiki on the unification scale or http://www.scholarpedia.org/article/Grand_unification
Question
It is clear that the Maxwell Equations are locally gauge invariant with all that this leads to for the conservation of current and electromagnetic coupling to current in (quantum) electrodynamics. How in essence does one consider in a similar way a world of scalar field and current with respect to local gauge invariance?
Referring to the context in which the question is posed, i.e. electromagnetism, the gauge invariance refers to transformations of the field under the group of complex numbers with norm one ( U(1), for people familiar with group nomenclature). A scalar field can transform as a singlet meaning that it does not change at all when the transformation is applied. In that case is does not transform at all and it does not couple to the electromagnetic potentials. I slightly less trivial case is a complex scalar field that picks up a change of phase when applying the U(1) transformation.
Now the field senses the presence of electromagnetic potentials and the coupling is constructed in such a way that the overall local gauge invariance is respected.
This is done by the 'minimal substitution'. You can find the details in introductory books on field theory.
PS: I assumed that your question is not related to attempts to catch the electromagnetic fields (the gauge potentials) in a single scalar field.
Question
It works in any single atom, but what about the two electrons located in two nearby quantum wells? Or, does it work for conduction electrons in metals?
The answer of the question as asked is "Pauli principle is valid regardless of distance". Pauli principle states that the wave function of a system of fermions must be antisymmetric with respect to its arguments. It is valid for any number of fermions in the system, at any distances, interacting or not. As for its practical implications, it only has such when the probability to have more than one fermion in the spatial volume of interest is not negligible.
Question
Current laboratory syntheses of nanoparticles, especially those based on conventional flask-type reactors, are able to deliver only small amounts of particles, usually below one gram, and are plagued by low reproducibility and high synthesis costs. Various factors are involved here, like limitations of the size of the reactor, high costs of the chemicals involved, their toxicity, uncontrollable side effects of chemical impurities and others. A key activity will be the design of new synthesis schemes, for example based on continuous batch synthesis approaches, which will be able to deliver particles in amounts that are suitable for applications in catalysis, photovoltaics and energy storage.
please have a look on the attached paper of Prof. Hyeon's group for the Ultra-large-scale syntheses of monodisperse nanocrystals Nature Materials.
Is it possible to disintegrate a matter by just sound or some other form of waves (other then heat producing types).
Question
Can sound wave disorder the lattice structure of any material ?
You could probably destroy a glass by singing. An opposite effect is known: a palladium complex turns into a gel upon being exposed to ultrasound. http://shootingcupoche.com/publication/7758761_Molecules_that_assemble_by_sound_an_application_to_the_instant_gelation_of_stable_organic_fluids http://cerncourier.com/cws/article/cern/29397 http://www.redorbit.com/news/science/157816/sound_turns_liquid_to_jelly/ Regards, Joachim
Question
It is possible to use it in modern cars and starcrafts to travel to nearest stars.
Oh yeah, I just had to mention: Science fiction writer Larry Niven in his collection of short stories "Tales of Known Space" (1975) pointed out that, if you could create a gigawatt-light-beam with enough momentum transfer to accelerate rockets carrying human beings across space... you would have a hellishly powerful beam-weapon that, properly focused, could slice through any material at extremely long (planetary) ranges. So be careful what kind of rocket engine you wish for... it might turn out to be a "dual use" technology. ;-)
Question
Many times in research, it is very difficult to accept either that the theoretical value is correct or experimental value. It is because theoretical minds says " Every experiment equipment has own limitation and approximation and chances of error starts from being " While 2nd time experimental mind says " Theoretical value can't be correct because you method is working after assuming various approximation" . So my doubt is which one is more correct and on what basis. Is there any limit on absolute error?
There are two ways of doing research. Either we have some experimental observations (facts) we do not understand and therefore we are looking for an appropriate theory to explain them, or - contrary and less often - we have some theory and we are looking for its experimental confirmation, especially its predictions never investigated before. In both cases only the negative result is certain. The funny thing is that quite often we do not need high accuracy to see such negative result. Positive result, on the other hand, is never 100% certain, so it's better to say that "the experimental results are in agreement with such and such theory" or "don't contradict the theory" (but never: "confirm the theory"). If you are the experimenter, then your task is to present any obtained results with *reliable* estimates of uncertainties. Without such estimates the result is simply worthless, see this:
"The number pi (a ratio of circle's circumference to its diameter) is roughly equal to -1".
True or false?
Question
Why does latent heat does not increase temperature?
When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. The potential energy stored in the interatomics forces between molecules needs to be overcome by the kinetic energy the motion of the particles before the substance can change phase.That is why there is no rise in temprature, as all energy applied is consumed in conversion of states.
When something changes phase from solid to liquid, or from liquid to gas, it takes energy to break the intermolecular interactions. These interactions between the water molecules are what make it solid. When you have ice, these interactions are strongest, which is why ice is hard. Then when you have water, the interactions are not as strong, and although the water still "stays together" it is now a liquid and moves and flows freely. Then when all the interactions are broken, it become a gas, or steam, and now none of the water molecules are attached to any other molecules. Whenever it goes through a phase change like this, the energy goes into breaking up these interactions, and so the temperature stays constant until all the interactions are broken. Once all the ice is melted, or all the water has turned to steam, then any added heat will act to raise them temperature again
Why, sometimes, some people do not consider the work in technique and methodology development as research area?
Question
This subject has made me think about which is considered a research area. I have seen people saying that methodology development may not be a research area and that you need to have a sample or system to study. So, for them, the sample/system is the subject of research. From my point of view, there are some areas in Physics where the methodology and technique development is a research area. So, I would like to know what do you think about it.
Question
How does the density of states(DOS) affect the transport properties?
Please find attached some articles which tackle this problem in various systems:
There is also a Wikipedia contribution on Mott's theory which addresses this problem
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Newton introduced differential equations to physics, some 200 years ago. Later Maxwell added his own set. We also have Navier-Stokes equation, and of course - Schroedinger equation. All they were big steps in science, no doubts. But I feel uneasy, when I see, for example in thermodynamics,
differentiation with respect to the (discrete!) number of particles. That's clear abuse of a beautiful and well established mathematical concept - yet nobody complains or even raises this question. Our world seems discrete (look at STM images if you don't like XIX-th century Dalton's law), so perhaps we need some other mathematical tool(s) to describe it correctly? Maybe graph theory?
One of my math professors used to dream about the universal differential equation that would describe everything in the universe. This notion is of course utterly ridicoulus, as present modern theories gives us computable limits on predictability. But the Liebnitz ideas die slowly.
Math is in my opinion best wieved as a modeling language that enables us to make models of reality which allow us to extrapolate and make predictions in the physical world. The models have to be simple enough to make a mathematical solution possible, and this in turn sets limits on how far the model can extrapolate known physical results.
Wether a differential equation is the best model depends on the real world phenomena that it is going to model, It is going to describe reality with varying accuracy, but never fully. But it may or may not exactly describe the simplified description of reality made. Sometimes other models are better descriptions of reality. For large ensembles of particles, such as gases, differential equations are quite good. The 3 body problem on the other hand is a classical example of a seemingly simple problem that is very dificult to solve.
Question
We know that electrons have a dual nature just like EM waves(of course all the material are said to have dual nature; noticeable or not). So looking at wave nature of electron and comparing this with photons, is it possible to make it's kinetic energy equal to zero?
The group velocity of an electron can be zero. It is a standing wave in this case. But according to the Heisenberg principle you cannot define both momentum and position precisely. If you want a momentum of exactly zero and not a momentum spectrum centered around zero (standing wave), your electron would have the wavelength infinity and would be delocalized across the entire universe.
Is the pressure value higher at laminar or turbulent flow region?
Question
Is the pressure at its maximum at the laminar flow region or turbulent flow region? Thank you.
According to Bernoulli's principle where the Velocity increase w.r.to pressure decrease. Based on this law turbulent region velocity is higher than laminar, then how will be pressure higher at turbulent region? Thank you
Question
Not very often we can see magnetic hysteresis loops of type-II superconductors in their entirety. The published figures mostly present only their upper and lower branch, but their "ends" are usually outside the figure. Yet, in situation when the exciting field amplitude falls between Hc1 and Hc2, the discontinuity (jump) is observed in sample magnetization, even its sign is changed. Anything like this never happens in ordinary magnetic materials: here the two branches are always coinciding at the return point, i.e. at the maximum of the external field. What could be the possible reason for such an amazing behavior?
In a type II superconductor, if the applied field is less than Hc2, then the material will still show diamagnetic magnetization. Now if the field is reversed (ie, when the applied field is less than Hc2), the superconducting shielding currents have to change the direction and hence the magnetization will shift to the positive values. This will result in a discontinuity in the hysteresis loops
Question
I conducted practical class for students, In Hall effect set up, I measured the readings, Hall voltage as a function of Hall current at constant magnetic field, Hall voltage as a function of magnetic field at constant Hall current . i am not able to measure the readings Hall current as function of Hall voltage at Constant magnetic field and then Hall current as a function of magnetic field at constant voltage
According to the definition of Hall effect, when a current carrying conductor is placed in a transverse magnetic field, an electric field is produced (charges are separated) in a direction perpendicular to both the current and magnetic field. The produced electric field is called Hall voltage. From the applied current and applied magnetic field in a conductor, the charges are separated and we get the Hall voltage. Therefore, Hall voltage is the result of current and magnetic field applied through the conductor.Hence, the Hall voltage changes according to the applied current and applied magnetic field. Hall voltage is the result of current and magnetic field. So, we cannnot measure current as a function of Hall voltage at constant magnetic field or current as a function of magnetic field at constant Hall voltage.
Question
I think this phenomenon involves a weak interaction, because as in beta decay the parity of the muon is not conserved.
A reaction is proceeded as µ + µ gives κ + κ . this reaction is by which decay.
If by weak decay then why? As parity is conserved in this reaction . But in weak decay the parity is not conserved. And tell me how to see the isospin conservation in this reaction?
Question
In 2006, Philip Marston predicted that an object illuminated by a Bessel beam could scatter some of the beam's transverse momentum into the axial direction, thereby increasing the axial momentum density. The object thus would recoil backward up the beam by conservation of momentum. This would be an implementation of a tractor beam. More recently, this idea has been picked up by several groups who have proposed related schemes for implementing tractor beams based on the properties of Bessel modes. As far as I can see, none of these recoil-based implementations have been reduced to practice. Is that right?
Hi David --- it is rare that I submit and answer to RG, mainly because I don't know -- this is no exception albeit I have an admin task I should be doing and replying to this is more fun!
If I am honest I have never been very surprised over Bessel Beams tractors. Standard optical tweezers, in which you lead the world, also exert forces on the particle directed back towards the source. If seems to me that the important parameter is the Numerical Aperture of the the optical system (Gaussian beam) and for a Bessel beam the equivalent number is the cone angle. Depending upon the scattering force (Fresnel reflection from the particle surface) there is a lower limit to this NA, of order 0.5. I think that this tells us straight away that for a Bessel beam to exert a reverse force on the particle its cone angle needs to be similar. For a Bessel beam I think this suggests that the range over which the reverse force can be exerted cannot exceed roughly the aperture of the optical element which creates the beam.
However, I guess the question remains how might this range compare to a Gaussian beam?
When comparing a Bessel to a Gaussian I note two things. Firstly if one takes the Rayleigh range of the Gaussian beam as a metric then we see that the depth over which the Bessel beam nicely propagates is multiplied by a factor roughly equal to the number of concentric rings in the beam. Secondly that the power contained in the central spot of the Bessel is, compared to the equivalent Gaussian, reduced by the same factor. Taken all together, this suggests to me that the range over which one can observe a pull force (greater than the scattering force) is primarily related to the aperture of the beam forming element and the power of the beam - albeit over this range, the Bessel beam will have a more uniform performance, which I accept may be important. There may also be factors of two or so to play with but I doubt any massive scaling advantage.
If I were looking to make a tractor in this way I would start with a very high power laser and create a very high cone angle Bessel (ie k_r approx = k_z) containing as many rings as possible, and ensuring that the central maximum contains >10s mW. I suspect then one could indeed obtain "fracturing" over a range comparable to the aperture of the beam forming optic.
I doubt I'm saying anything here that you did not know -- but if you think I have missed something then I'd be very keen to hear.
BTW I liked very much your comment in PRL last year :)
Kind regards
Miles
Question
The sintering of particles with a size of a few µm with a 1064 nm laser shows unpredictable results, there is either no reaction or evaporation and ablation of material. My suggestion is that I get poor laser absorption, due to the small particle size, as soon as a few particles melted their size grows due to surface tension and the absorption coefficient increases, resulting in an avalanche effect which leads to material evaporation, instead of sintering. Can anyone confirm or negate this theory or point out papers where this has been observed?
The amount of energy a particle absorbs from a beam of light depends on the size and shape of the particle, and also on the particle's composition. To a very rough approximation, the absorption cross-section is proportional to the particle's projected area. Larger particles thus absorb more. Once a particle starts to melt, moreover, it is possible that their absorption coefficient may change due to chemical changes. Oxidation of organic materials, for instance, tends to increase the absorption coefficient, leading to just the sort of run-away effect you describe. A classic text on the subject is Bohren and Huffman, "Absorption and Scattering of Light by Small Particles," (Wiley, New York, 1983).
Question
While studying Schrödinger's equations of wave, I had this question:
This equation deals with waves, right then, can we apply it in the case of electromagnetic wave, but the equation deals with one particle system. What does this mean?
No, you can't apply it...
-Matter wave is represented by de-Broglie relation(particle-wave duality),
-EM wave is represented by Maxwell's equations(electromagnetic form of energy),
-Schrodinger's equation wave is interpreted by Born as its absolute square represents probability of finding the system in some region of space
Thus you have THREE different kinds of waves...
Particle is not wave and wave is not particle, but they both have some common characteristics like wave behavior of particle(diffraction of electrons) and particle behavior of of photon(Compton effect)
Schrodinger equation solution gives probability of finding particle of atomic system in some space with specific energy(the well-known four quantum numbers;principal,orbital,magnetic,spin)
Question
Before answering this post, make yourself sure you are not a quack, else you shall be deleted. Please check:
@Bob, you're distorting a very simple message here. There's nothing wrong in expressing opinions, even uneducated ones, but pushing crank ideas ("relativity is wrong") without any factual proof or basis is simply an elementary mistake and not tolerated here.
Question
Energy of a light wave is hv . When it is polarized and divided into two parts then there energy will also hv and total energy will be 2hv. How it is possible. I have some conclusions. If you have any please tell.
When you say polarized light wave....1st it means the photon has a new quantum no. call it polartization (actually it is spin +1 or -1) hence, a polarized wave has all photon in one of the states either +1 or -1 and all has energy hv.
Question
Hello friends,
I am conducting a survey about string theory and in understanding the impacts of a simple space-time.That question lingered in my mind there is some probability.
I guess not: in a explosion the energy dissipates in a large area. If you have a very huge implosion could it be possible?
Question
Can you help me? I am doing research on this subject.
Please frame your question in other way so that one can think of help.
positron forms Positronium (Ps) a bound state with electron after penetrating the material . Ps has a similar structure to that of a hydrogen atom, and of same size, but is 1000 times lighter. The formation of the Ps atoms occurs mainly in molecular media that usually have a relatively open structure. Several scientists are working on it.
Question
Is it true in all cases, that the more precise one property is measured the less precise
the others can be controlled, determined or known?
Question
How to explain this in a very simple manner.
The density of hydrogen is lower than that of helium, so it would probably fly a little higher. Approximate densities are He 0,2g/L, H2 0,09g/L and air 1. 2g/L at sea level. As far as I remember most record setting balloons have been filled with He for safety reasons. Weather balloons and recreational gas balloons use H2 most of the time for cost reasons. Use of H2 of course requires electrostatic and many other safety precautions.
Can anyone really say that to Physics? Physics is not always the seamless subject that it pretends to be, or it is a ramshackle tower of Babel.
Question
I think to a large extent that is true, the study at the frontline of theoretical physics in this day and age has a philosophical aura to it. But if in some way, future applied physicists manage to design an experiment to prove the thoery, then it would not be. For example, it took more than half a century before experiments could be made to experimentally prove special relativity and similarly, it might take a better part of a century before experiments can be conducted to uncover information to prove string theory (or other theories that may come to usurp it).
Question
For a long time, that puzzled physicists. A neutrino's variety determines how it interacts with matter. Physicists built experiments to detect the flavour coming out of the Sun only to find far fewer than they expected.
In 2001, that mystery was solved when they discovered that the missing neutrinos had flipped, or oscillated from one flavour to another, during their journey from the Sun to the Earth.
Since then physicists have scrambled to understand neutrino oscillations in more detail. It turns out that the effect is sensitive to the distance that the neutrinos have traveled and also to the amount of matter the particles have passed through.
The current theory is thus that the oscillations in a beam of neutrinos created at one point on the Earth and beamed through the crust to another point, ought to reveal information about any change in density along the way. This technique ought to be able to spot cavities some 200 km across or larger filled with water, iron-based minerals, oil accumulations or even regions of charge accumulations.
Is this actually possible/feasible? How would one create such an intense bean of neutrinos?
Ref: arxiv.org/abs/1201.6080 : Searching For Cavities Of Various Densities In The Earth’s Crust With A Low-Energy ν¯e β-Beam.
Attached is the full text publication for the above.
Usually neutrino beams are created at particle accelerators, e.g. CERN. (Anti-)neutrinos are also produced in great numbers in power plants, but they are not collimated and very low energy. Now this paper that you mention seems to indicate it is feasible, they rely on MeV energies and a high intensity electron beam.
Basically, there are two kind of mechanisms to learn about the interior.
One is to look for osciallation behavior and from that deduce how the matter looks like (as described in the paper).
This is usually feasible in MeV-GeV energies.
The other is to look for absorption (this is independent of oscilations). But you usually need higher energies (TeV or higher).
In IceCube( http://icecube.wisc.edu/) , which detects neutrinos at such high energies (usually coming from cosmic ray interactions in the atmosphere), there are actually people trying to accomplish this. Alot of uncertainties are involved, though.
Also neutrino oscillations are reachable with the DeepCore extension of IceCube, but
to really understand matter with those oscillations, the lower energy threshold is probably too high (the minimum energy accessible for IceCube/DeepCore is around 10 GeV).
Question
It has been observed in many theories related to space and time that time can be warped but my question how would we able to control the wormhole for a time interval for our warp if possible?
Yes, but this process would be necessary enormous energy.
Question
If we store photons or directly sunlight then we can use it for many purposes like use for lightning at night, to generate electricity at night by using solar cells and the time of availability of light may increased about 24 hours of the day.
I thought the same way when I was a first year physics student and I could not find why not. I think the constant loss on successive reflections which are too many lets it lose intensity or number of photons. I also used to wonder why a room filled with light on switching on a light bulb suddenly becomes dark on switching it off. After all the room was filled with photons. They could stay their for a while. I think the surface of the walls is not a could elastic reflector.
Question
A few years ago, physicists showed that it might be theoretically possible for neutrons to make the leap from our universe to a different one if the gravitational potential of an entire galaxy were involved. Lucky for us, we happen to live in a galaxy so if we look closely enough, it should be possible to spot these neutrons jumping out of our galaxy.
By watching a bunch of neutrons all at once, we might be able to experimentally verify whether any of them are disappearing off to alternate universes.
How Neutrons Might Escape Into Another Universe:
Attached is the full text for the publication:
Experimental limits on neutron disappearance into another braneworld.
Michael Sarrazin, Guillaume Pignol, Fabrice Petit, Valery V. Nesvizhevsky
"Recent theoretical works have shown that matter swapping between two parallel braneworlds could occur under the influence of magnetic vector potentials. In our visible world, galactic magnetism possibly produces a huge magnetic potential. As a consequence, this paper discusses the possibility to observe neutron disappearance into another braneworld in certain circumstances. The setup under consideration involves stored ultracold neutrons - in a vessel - which should exhibit a non zero probability p to disappear into an invisible brane at each wall collision. An upper limit of p is assessed based on available experimental results. This value is then used to constrain the parameters of the theoretical model. Possible improvements of the experiments are discussed."
Thoughts?
Yes, mathematically through A4 projections,
Question
Does antimatter behave differently in gravity than matter? This could possibly explain why the universe seems to have no antimatter and why it is expanding at an ever increasing rate.
Question
E=mc squared establishes that energy is related to mass in proportion to the speed of light squared. Since that speed is calculated by dividing distance in space by time, it raises the question of whether there are any limits to the spacetime scale in which the equation must hold. If one makes the assumption that energy must equal mass times the speed of light squared on all spacetime scales and follows it to its logical conclusions, it seems to lead to a Theory of Everything that explains the nature of many of the aspects of the universe that physicists don't understand, like dark energy and dark matter for example. I have written a paper that describes this theory.
Also E=mc^2 is the low-momentum approximation, the full equation is E^2 = m^2 c^4 + p^2 c^2 where p is the relativistic momentum. Gravity and quantum mechanics can alter the results given by special relativity.
Question
It is argued that the Markovian Brownian motion of a particle immersed in an environment does feature continuous but not differentiable paths. As far as non-Markovian effects are concerned, do such Brownian trajectories hold non-differentiable?
Discrete stochastic processes are of course mathematical idealizations of real physical fluctuation phenomena, but within this framework stochastic trajectories (whether Markovian or non-Markovian) are usually not differentiable. This is somewhat strange question to ask - do you have a specific application in mind?
Question
For CS2 it is high but we find new material better than it.
At sufficiently high intensity, everything goes non-linear. You can make white light with a fs pulse in water.
Question
?
Sharanya,
Simply, in general a closed orbits is an ellipse. One only has a circular orbit when certain criteria for momentum and energy are met.
Here's a thought experiment.
You are high above the Earth, and are carrying a suitable rocket engine. If you do not fire the engine, you will fall (slowly at first, but with rising speed) till you hit the Earth. So you fire the rocket engine a little, pointing it 90° away from the Earth. If you repeat this experiment, but fire the engine a little longer each time, then you will repeat the classic figure shown here.
Unless the impulse given by your rocket is a very particular amount, you *will* have an elliptical orbit, and your starting point will either be the perigee or apogee, depending on how long that rocket engine fired.
No need to consider general relativity, this is basic Newtonian mechanics at work. May I recommend wikipedia (Orbit) or any good undergraduate text? (I used Kleppner and Kolenkow 25 years ago, and still have that copy)
If you're feeling brave, read up on the 'vis viva' equation. It contains the gist of the matter, all covered under 'central force' models in elementary mechanics courses in physics.
Question
For example: How do vary kinetic energy of atom electrons when atomic mass increases؟ For example this equation (E at Z) is liner, power, exponential or, logarithmic؟
for more details you can read "Liquid drop model and semi-empirical mass formula" from any nuclear physics book (e.g. nuclear physics by S.B. Patel ). I hope it will help you.
Question
i was just reading a book on laser and I found that in the book its written that Einstein's Postulates are as follows:
1.light energy is not emitted continuouslybut intermittently by indivisible amount of energy called quantum of energy.
2.these quanta of energy travel in space in definite bundles and are called photons.
but as far as my knowledged leads to me, I believe that I have read these in Bohr's Atomic model.
in my opinion Einstein postulated in theory of relativity.
according to that all the fundamental laws of physics have the same form in all inertial frames of reference and second is: speed of light in vaccum is same in all inertial frames of reference.
It was actually Max Planck who first proposed that the energy in electromagnetic waves exists only in dicrete 'packets' of energy, and this theory was later developed further by Einstein, even before Bohr's conception of his atomic model.
You are correct that Einstein also postulated the existence of special relativity, based upon the two postulates:
i) Light waves appear to propogate at the same speed in all directions in a vacuum whe for all inertial observers.
ii) The laws of physics are invariant under Lorentz transformations.
Question
Quantum ESPRESSO(opEn-Source Package
for Research in Electronic Structure, Simulation, and Optimization)
thanks ..
but i want work in ibav 6!
can i work with ibrave 6 and Should chenge it to 14???Can you explain ???
Question
Non-locality is built into the theory with noncommuting space coordinates. One might wonder if quantum entanglement can naturally arise from noncommutative spaces, or whether they can play any important role at the Planck length?
How can one comment on the nature of observables without observing? I am not sure we mean the same thing when we use words like "fundamental" and "principle". I can not comment on the nature of an observable without observing it.
This is going in a tangent and I would like to come back to the topic. Existence of Plank scale points to the fact that one might need to figure out quantum physics on noncommutative geometries in order to develop a quantum theory of gravity [in the same way the UV catastrophe pointed out that one might have to discard the smooth phase-space and replace it with nonzero finite delta(x)delta(p) cells].
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This discussion continues the preceding forum on neutrinos
If the STR is in problem than the Maxwell’s equations will also be because STR is the direct consequence of Maxwell’s equations.
Question
The value of the electrical band gap always found to be slightly higher than the optical band gap. When material excites by the electric field the transition mainly occur via defect states because of such transitions the ideality factor in the V-I characteristics varies (practical identification of the existence of charge states) , while in the optical band gap transition occur between two states only, there is no effective grabbing of the charges happen in the defective states (uncharged trap states).
additionally the band gap value of the materials strongly depends on the polarity of the charges in the materials, let say if the material is polar in nature (c-GaN) in this case the two would be approximately same.
Question
There are different methods for growing crystals. 1. Growth from solution ( Both High temperature solution growth and Low temperate solution growth )2. Growth from vapor (eg Molecular beam epitaxy, CVD) 3 Gel Growth 4 Growth from melt. Bridgeman, Czochralski Kyropoulos ,Verneuil are crystal growing methods from melt.
In Bridgman (pulling method) technique the material is melted in a vertical cylindrical container, tapered conically with a point bottom. The container is lowered slowly from the hot zone of the furnace in to the cold zone. The rates of movement for such processes range from about 1 – 30 mm/hr. Crystallization begins at the tip and continues usually by growth from the first formed nucleus. This technique is best suited for materials with low melting point.
In Czochralski method, (pulling method) the material to be grown is melted under a controlled atmosphere in a suitable non-reacting container. By controlling the furnace temperature, the material is melted. A seed crystal is lowered to touch the molten charge. When the temperature of the seed is maintained very low compared to the temperature of the melt, by suitable water cooling arrangement, the molten charge in contact with the seed will solidify on the seed. Then the seed is pulled with controlled rate. The majority of crystals is produced by pulling from the melt. Crystals of dimensions 10 to 40 cm have been grown using this method An alternative approach to solve, or at least minimize, the axial segregation and the striation problem is the co-rotating ring Czochralski (CRCZ) method which induces a hydrodynamic double-crucible effect: by optimized rotation rate of crystal and counter-rotation rate of the crucible, a nearly convection-free melt fraction is achieved below the growing crystal, whereas the larger fraction of the melt is homogenized by combined forced and natural convection
In Kyropoulos technique, (top seeded method) the crystal is grown in a larger diameter. As in the Czochralski method, here also the seed is brought into contact with the melt and is not raised much during the growth, i.e. part of the seed is allowed to melt and a short narrow neck is grown. After this, the vertical motion of the seed is stopped and growth proceeds by decreasing the power into the melt. The major use of this method is growth of alkali halides to make optical components.
In the Verneuil technique, (flame fusion) a fine dry powder of size 1-20 microns of the material to be grown is shaken through the wire mesh and allowed to fall through the oxy-hydrogen flame. The powder melts and a film of liquid is formed on the top of the seed crystal. This freezes progressively as the seed crystal is slowly lowered. The art of the method is to balance the rate of charge feed and the rate of lowering of the seed to maintain a constant growth rate and diameter. By this method ruby crystals are grown up to 90 mm in diameter for use in jeweled bearings and lasers. This technique is widely used for the growth of synthetic gems and variety of high melting oxides.
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Neutrinos run faster than light...
A visual summary of the experiment.
Question
A particle immersed in a given environment (a fluid, for instance) undergoes a jittering movement dubbed Brownian motion. As far as quantum effects are concerned, could we describe the quantum Brownian motion of this particle through a Schrödinger function? In other words, is the wavefunction description of quantum phenomena universal?
Thanks to A.O. Bolivar for posting his paper. Dr. Bolivar may be overstating his case, though. I suspect that a wave function description of Brownian motion is possible (with rapidly vanishing off-diagonal elements in the state operator), but worthless because it is so much simpler to treat all the randomness in the problem statistically! I suppose we can use quantum mechanical scattering theory to describe in great QM detail the behavior of a gas of, say, 100000 helium atoms, but why bother when statistical mechanics is there to help with the job. Still, there are aspects of Brownian motion where quantum behavior is relevant--perhaps Taplo Ala-Nisslla's paper referenced above.
David Bohm has an interesting footnote in his book "Causality and Chance in Modern Physics," Ch. 4, Section 2, showing that a particle undergoing ordinary Brownian motion obeys an indeterminacy relationship for momentum and position uncertainties analogous to the Heisenberg uncertainty principle.
Question
To understand the scope of the question, suppose that there are only two particles A and B in the Universe.
If A changes its position, then B must also change, because the barycenter of the system must remain unaltered. However, if the distance between both particles are very large, when the information from the position of A reaches B some time T has passed, and meanwhile A changes again. Perhaps, the existence of a single point as barycenter instead of a fuzzy region requires an instantaneous information, which contradicts relativity theory.
You are right, the concept must be defined. In the initial question, I have changed center of gravity by barycenter, that is to say, the center of mass in order to improve accuracy. In other words: By barycenter it must be understood, the position average of all particles in the Universe, weighted by their masses. The question is to know whether such a definition is consistent, and by consistent it must be understood that it can be satisfied by a point at each time, disregarding whether or not such a point can be determined. Indeed, according to Newton's laws such a point exists, but its existence it is not clear under relativity or quantum mechanics.
Question
Why it referred to as the acoustic equivalent of a photon?
Dear Anjaly,
phonons are a way to describe the vibrational degrees of freedom in periodic structures (lattices): the vibrational motions can be modelled as harmonic oscillators, so they can be quantized nearly in the same way. The result of the quantization (among other consequences) is the appearence of "quanta" of oscillations: the phonons. In this sense, the phonon is the quantum of structural vibrations much in the same way as a photon is a quantum of oscillation of electromagnetic field.
Phonon is considered a quasi-particle, beacuse it can exist only in solids as a consequence of vibrational motions: they cannot propagate in vacuum.
Regards,
Giovanni
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Ans
if i am not wrong iodine tincture contains water in some amount so it is better to use the same with hydrophobic dyes and moreover composition of iodide/iodine matters. so according t my opinion u can use it but without much efficiency..
Question
A new class of exact solutions to the classical Maxwell's equations of wave propagation have been found, which prove that there can be light beams that bend around a circular path without diffraction
I was not aware about it, but as a particle physicist my first reaction is - great, it will be (maybe) possible to construct a photon storage ring to study, in a clean way, photon-photon interactions ? Actually such photons following a circular path can be forseen to exist only in the vicinity of a black hole.
Question
I know that steel has large elasticity but don't know why.
Steel. Elasticity is measured as ratio of stress to strain. For a given stress (stretching force per unit area) strain is much smaller in steel than in rubber and hence the answer.
Question
How muon neutrons are changed into Tau neutrons?
Dear Anjaly,
I think what you are asking is about 'neutrinos', not neutrons. Neutrinos were first proposed in 1930's by pauli to explain the missing energy in beta-decay. From the conservation principles it was inferred that it should be a massless spin-1/2 particles. With experiments it appeared that if electron-neutrinos interacts with matter, it produces electron only. The same is true for mu- and tau- neutrinos. So, the neutrinos appear to conserve their 'flavor', i.e. its electron-ness or muon-ness. But a few years back 'neutrino-oscillation' were observed. That is, if you start with a pure muon-neutrino beam , after sometime some tau-neutrinos appear, as if from nowhere. This can only mean one thing that, the neutrinos we see in our real world is not those which like to maintain their 'flavor". In a more technical language, the 'flavor' eigenstates and the 'mass' eigenstates are different. According to QM this can only happen if the neutrinos have a finite mass (more precisely, mass difference). There are several lecture notes in the internet on neutrino oscillation, if you are interested and have a basic knowledge of QM, you will understand them.
Hope it helps!!!
Question
Suppose we place a detector in the forward direction (practically not possible) to collect the scattered particles only. Will it collect infinite no. of particles for a finite incident flux?
Am I missing some mathematical subtlety?
(The statement I gave above is based on the definition of scattering cross section)
Dear Dipanker Das,
As I thought about your question for a few more minutes, and I wrote down the integral of the Rutherford (Coulomb) scattering cross-section over the full solid angle, I saw that you were indeed correct: the integral of the standard Rutherford scattering cross section formula over the full solid angle does indeed diverge to an infinite flux. However as we noted earlier in this thread, this cannot be physically correct. So I began to review the assumptions of the mathematical model carefully, and I realized that there is an approximation in the model that is a reasonably good approximation for most angles but not for very low angles or very high angles (near total-backscatter, angle = pi or angle = 180 degrees.)
The approximation in the model is that the incident particle is much less massive than the target particle (for example in Rutherford's experiment the helium nucleus is much less massive than the gold nucleus) and therefore the conservation of momentum results in very little energy transfer from incident particle to target particle. In the simple Rutherford cross-section, the assumption is that ZERO energy is transferred to the target particle (the target particle remains stationary during the whole interaction). If you eliminate this assumption and formulate the interaction correctly, you should get a result that still has zero in the denominator at deflection angle = 0 (the physical meaning here is that the area of space far away from the target particle is "infinitely large" compared to the area near the particle) but its integral over the full solid angle must converge to probability P = 1, that is, flux in = flux out.
Unfortunately the mathematical expression for the correct trajectory (including the resultant motion of the heavier particle and the energy transfer) is going to be horrendously complex and not integrable using symbolic mathematics: you would have to integrate the resulting expression numerically using a computer and accept an approximate numerical solution as the result.
As an example of how the correct treatment, including energy transfer and the resultant motion of the target particle, can transform the divergent (infinite) flux of the Rutherford approximation into a convergent (finite) flux in a physically real scattering system, see the discussion on this web page:
Even the case discussed there is an oversimplification. The physicist who wrote that article, realizing that a full two-body treatment of the Coulomb scatter interaction would be unsolvable symbolically, chose a simpler energy-transfer model. He chose to model the case where there is a Coulomb interaction at a distance, and then at a certain finite (very small) radius there is an effective energy transfer as the incident particle changes direction near a spherical "surface", using an equation from the well-known mechanics textbook by Landau and Lifschitz. Using the resultant energy transfer to derive an approximate correction to the Rutherford scattering formula, he shows that this correction removes the divergence and allows the total cross-section to be normalized to a finite flux.
Well-spotted. Your intuition was correct that there was something wrong with the standard "Rutherford scatter cross-section" formula for the Coulomb interaction for small deflection angles.
Question
I'd like to hear opinions about the utilization of the terms 'principle' and 'law' assigned to statements or equations in physics. What characterise a principle as a 'principle' or a law as a 'law'? What's the difference between these concepts?
Interesting question. In my opinion, 'principles' are more specific than 'laws'. So in other words a law can have numerous principles in order to aid a laws proof. A principle is a sort of guiding logic.
Laws would then involve mathematical proofs in most cases while the principle is not really mathematical.
Question
Is it 0 or c?
Let us reformulate the problem. What is the speed of light in the reference system moving in the direction of photon with speed of light?
Answer; this reference system cannot exist. In the other words, you can not make a frame moving with c. If you consider the system moving with high but less с speed, the velocity of light is c.
What is the quantitative difference of strength between nuclear interaction and the electromagnetic interaction?
Question
Strong nuclear force supercedes all known forces in strength. The electromagnetic force is just below in the ladder of strength. Many constants are also employed in both of them in the same senses. Therefore, it is natural to inquire about the numerical difference of strength.
The Wiki (and the other link) give exactly the answer for the relative strength of the interactions that you're after so there's no need to wonder about this :-). Contrary to what you say, EM and gravitational interactions (on the classical level) do have the same mathematical structure. Other forces don't.
Do all electrical appliances obeys ohms law ?
Question
We know from v=iR all the devices which work based on electricity shoud obey it
Ohm's law is not fundamental in the same sense as Maxwell's equations (of EM), and thus many materials display non-ohmic behavior, i.e. the current is not linearly proportional to the voltage difference. You can (again) find more information in the Wiki and numerous other sites.
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Is the Warp Drive fact or fiction?
Can the Warp Drive be fact or fiction?
Warp Drives are Exotic solutions of the Einstein Field Equations of General Relativity that "allows" Faster Than Light Space Travel within the framework of General Relativity
Note that the word "allows" is between quotes.
The first of these Exotic solutions was discovered by the Mexican mathematician Miguel Alcubierre in 1994. The second was discovered by the Portuguese mathematican Jose Natario in 2001.
Alcubierre is PhD from University of Wales in Cardiff UK and Natario is PhD from University of Oxford UK.
Their works are here:
The Warp Drive is an entire family of solutions of the Einstein Field Equations of General Relativity like the Schwarzschild or the Reissner-Nordtstrom solutions are for Black Holes but Warp Drives, although mathematical elegant solutions faces some serious problems
an excellent description about the problems Warp Drive faces can be given by
I would like to discuss in ResearchGate the Warp Drive. Using the scientific works of arXiv or HAL to start to show how this theme can be interesting for people in General Relativity
abstract of arXiv:0710.4474
The General Theory of Relativity has been an extremely successful theory, with a well established experimental footing, at least for weak gravitational fields. Its predictions range from the existence of black holes, gravitational radiation to the cosmological models, predicting a primordial beginning, namely the big-bang. All these solutions have been obtained by first considering a plausible distribution of matter, and through the Einstein field equation, the spacetime metric of the geometry is determined. However, one may solve the Einstein field equation in the reverse direction, namely, one first considers an interesting and exotic spacetime metric, then finds the matter source responsible for the respective geometry. In this manner, it was found that some of these solutions possess a peculiar property, namely 'exotic matter,' involving a stress-energy tensor that violates the null energy condition. These geometries also allow closed timelike curves, with the respective causality violations. These solutions are primarily useful as 'gedanken-experiments' and as a theoretician's probe of the foundations of general relativity, and include traversable wormholes and superluminal 'warp drive' spacetimes. Thus, one may be tempted to denote these geometries as 'exotic' solutions of the Einstein field equation, as they violate the energy conditions and generate closed timelike curves. In this article, in addition to extensively exploring interesting features, in particular, the physical properties and characteristics of these 'exotic spacetimes,' we also analyze other non-trivial general relativistic geometries which generate closed timelike curves.
from this forum
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Warp Drives May Come With A Killer Downside
Planning a little space travel to see some friends on Kepler 22b?
Thinking of trying out your newly-installed FTL3000 Alcubierre Warp Drive to get you there in no time? Better not make it a surprise visit - your arrival may end up disintegrating anyone there when you show up.
"Warp" technology and faster-than-light (FTL) space travel has been a staple of science fiction for decades. The distances in space are just so vast and planetary systems - even within a single galaxy - are spaced so far apart, such a concept is needed to make casual human exploration feasible (and fit within the comforts of people's imagination as well… nobody wants to think about Kirk and Spock bravely going to some alien planet while everyone they've ever known dies of old age!)
While many factors involving FTL travel are purely theoretical - and may remain in the realm of imagination for a very long time, if not ever - there are some concepts that play well with currently-accepted physics.
The Alcubierre warp drive is one of those concepts.
Proposed by Mexican theoretical physicist Miguel Alcubierre in 1994, the drive would propel a ship at superluminal speeds by creating a bubble of negative energy around it, expanding space (and time) behind the ship while compressing space in front of it. In much the same way that a surfer rides a wave, the bubble of space containing the ship and its passengers would be pushed at velocities not limited to the speed of light toward a destination.
Of course, when the ship reaches its destination it has to stop. And that's when all hell breaks loose.
Researchers from the University of Sydney have done some advanced crunching of numbers regarding the effects of FTL space travel via Alcubierre drive, taking into consideration the many types of cosmic particles that would be encountered along the way. Space is not just an empty void between point A and point B… rather, it's full of particles that have mass (as well as some that do not.) What the research team - led by Brendan McMonigal, Geraint Lewis, and Philip O'Byrne - has found is that these particles can get "swept up" into the warp bubble and focused into regions before and behind the ship, as well as within the warp bubble itself.
When the Alcubierre-driven ship decelerates from superluminal speed, the particles its bubble has gathered are released in energetic outbursts. In the case of forward-facing particles the outburst can be very energetic - enough to destroy anyone at the destination directly in front of the ship.
"Any people at the destination," the team's paper concludes, "would be gamma ray and high energy particle blasted into oblivion due to the extreme blueshifts for [forward] region particles."
In other words, don't expect much of a welcome party.
Another thing the team found is that the amount of energy released is dependent on the length of the superluminal journey, but there is potentially no limit on its intensity.
"Interestingly, the energy burst released upon arriving at the destination does not have an upper limit," McMonigal told Universe Today in an email. "You can just keep on traveling for longer and longer distances to increase the energy that will be released as much as you like, one of the odd effects of General Relativity. Unfortunately, even for very short journeys the energy released is so large that you would completely obliterate anything in front of you."
So how to avoid disintegrating your port of call? It may be as simple as just aiming your vessel a bit off to the side… or, it may not. The research only focused on the planar space in front of and behind the warp bubble; deadly postwarp particle beams could end up blown in all directions!
Luckily for Vulcans, Tatooinians and any acquaintances on Kepler 22b, the Alcubierre warp drive is still very much theoretical. While the mechanics work with Einstein's General Theory of Relativity, the creation of negative energy densities is an as-of-yet unknown technology - and may be impossible.
Which could be a very good thing for us, should someone out there be planning a surprise visit our way!
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from arXiv
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The Alcubierre warp drive allows a spaceship to travel at an arbitrarily large global velocity by deforming the spacetime in a bubble around the spaceship. Little is known about the interactions between massive particles and the Alcubierre warp drive, or the effects of an accelerating or decelerating warp bubble. We examine geodesics representative of the paths of null and massive particles with a range of initial velocities from -c to c interacting with an Alcubierre warp bubble travelling at a range of globally subluminal and superluminal velocities on both constant and variable velocity paths. The key results for null particles match what would be expected of massive test particles as they approach +/- c. The increase in energy for massive and null particles is calculated in terms of v_s, the global ship velocity, and v_p, the initial velocity of the particle with respect to the rest frame of the origin/destination of the ship. Particles with positive v_p obtain extremely high energy and velocity and become "time locked" for the duration of their time in the bubble, experiencing very little proper time between entering and eventually leaving the bubble. When interacting with an accelerating bubble, any particles within the bubble at the time receive a velocity boost that increases or decreases the magnitude of their velocity if the particle is moving towards the front or rear of the bubble respectively. If the bubble is decelerating, the opposite effect is observed. Thus Eulerian matter is unaffected by bubble accelerations/decelerations. The magnitude of the velocity boosts scales with the magnitude of the bubble acceleration/deceleration.
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in Alcubierre Warp Drive the energy is located above and below the ship....in the front of the ship there exists empty space..or the region where spacetime contracts in the front.....a photon sent to the front of the ship will suffer from blueshifts there is nothing to prevent the photon to suffer a blueshift
but fortunately we have another Warp Drive geometryy..the Natario Warp Drive in this Warp Drive the energy surrounds all the ship the ship is in the center of a sphere of negative energy....a photon sent to the front will be deflected by this field of negative energy so the blueshift do not occurs
but the analysis the work arXiv:1202.5708 made for Alcubierre Warp Drive perhaps needs to be repeated..for the Natario Warp Drive
for details on Horizons or Doppler blushifts see this one
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Although both Alcubierre and Natario Spacetimes belongs to the same family of Einstein Field Equations of General Relativity and both have many resemblances between each other ,the Energy Density distribution in the Alcubierre Warp Drive is different than the one found in the Natario Warp Drive.The Horizons will arise in both Spacetimes when approaching Superluminal(Warp) speeds however due to a different distribution of Energy Density the Natario Warp Drive behaves slightly different when compared to the Alcubierre one. The major differences between the Natario and Alcubierre Warp Drive Spacetimes occurs when we study the Infinite Doppler Blueshifts that affect the Alcubierre Spacetime but not affect the Natario one because while in Alcubierre Spacetime the Negative Energy is distributed in a toroidal region above and below the ship perpendicular to the direction of the motion while in front of the ship.the space is empty having nothing to prevent a photon to reach the Horizon because in this case the Horizon lies on empty space,in the Natario Spacetime the Energy Density is distributed in a spherical shell that covers the entire ship and a photon sent to the front will be deflected by this shell of Negative Energy before reaching the Horizon because the Horizon also lies inside this shell and not on "empty" space.This shell avoids the occurrence of Infinite Doppler Blueshifts in the Natario Warp Drive Spacetime.We examine in this work the major differences between both Natario and Alcubierre Spacetimes outlining the repulsive character of the Negative Energy Density.The creation of a Warp Bubble in Alcubierre or Natario Spacetimes is beyond the scope of Classical General Relativity and will have to wait until the arrival of a real Quantum Gravity theory that must encompass Superluminal Non-Local Quantum Entanglement Effects in order to deal with the Horizon problem added to the Geometrical features of Classical General Relativity plus it must also provide a way to generate large outputs of Negative Energy Densities.Since this theory is ahead of our scientific capabilities,we discuss in the end of this work an approach that could be performed by our science in a short period of term.to increase our knowledge about the Warp Drive as a Dynamical Spacetime.
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Does Fine Structure Constant play any role in gravitation?
Question
Fine structure constant is often used in nuclear physics and electromagnetism. Sometimes, it is looked as a mysterious quantity. Its role and importance in Gravitation are to be clearly defined.
Hey, The fine structure constant is the coupling constant determining the strength of electromagnetic interactions. This does not have a role in gravitational interactions. For gravity there is a corresponding constant called the gravitational coupling constant.
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What is the reason for its variable nature?
Michael C., the influence of the Sun's field to that of the Earth has been discussed earlier in this forum, and the effect is practically negligible. In fact, the Sun's field does not penetrate Earth's at all, but the plasma current from the Sun is deflected by Earth's field. We are in grave danger when Earth's field eventually changes its direction in the future.
Another fact is that the Sun's magnetic field changes direction about every 11 years, and this has very little practical effect: http://www.dailygalaxy.com/my_weblog/2012/04/the-great-switch-suns-magnetic-field-does-a-complete-reverse-every-11-years.html.
You should not post nonsense here, when facts can be trivially checked from Wiki or other sources: http://en.wikipedia.org/wiki/Earth's_magnetic_field.
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According to the canonical quantization scheme, a given classical physical system is said to be quantized as long as quantization conditions are imposed on its Hamiltonian or Lagrangian functions. So, no Hamiltonian! No quantization!
Are there quantum effects on non-Hamiltonian systems?
Dear Bolivar,
Well, I see that the statement "There is no such thing as non-Hamiltonian systems" is incorrect. I was not familiar with this use of the term "non-Hamiltonian" (roughly speaking the systems where we do not have enough information to write down the Hamiltonian governing the system.).
Thanks.
Question
Like the way we point the light using lenses , can we have a device to focus the sound ???
Yes we can as Alexander has mentioned. I saw the attached video demonstrating it.
Inventor Woody Norris has developed a revolutionary technology called Hyper Sonic Sound that can direct sound the way a laser directs light. In the sound beam's direct line, you hear the audio signal as if through headphones, regardless of background noise. Outside the beam, you hear nothing. HSS works by generating two types of ultrasonic waves, both inaudible to the human ear. Once those waves reach an object (like your head), they crash together and re-create the original sound. Ultrasonic waves also conserve sound for 150 yards without distortion or volume loss.
Question
The equation is Energy = Force * Area^(1/2) or E^2 = F^2 * A
Is the above listed equation physically relevant in the classical domain?
What physical behavior does the equation describe?
Other researchers and I experimented with this formula and discovered that it accurately determines the amount of energy that is transferred when a force sensor with a certain area head bounces after going down a ramp and bouncing off a solid "immovable" object.
Our experiment showed that the formula was accurate between 10-50N. We believe this equation has ballistic applications as well.
Out of curiosity we seek the opinion and ideas of other scholars on ResearchGate.
Oh and the height of course comes from the standard E=mgh on the slanted ramp.
Question
Let's say an iron rod connects the moon to earth, and a machine connected to one side pulls and pushs it a bit to transmit some message, then a computer connected to the other side read those pushes and pulls. So can we affirm that the information travelled instantaneously between these 2 computers?
Speed of sound in iron rod would be the transmission rate.
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I think a useful starting point is the definition of temperature itself:
1/T = dS/dU,
where S = entropy, U = total energy, T = temperature
To to have positive temperature simply means that dS/dU > 0, which means that adding energy to the system increases its entropy. (I think this makes physical sense for most systems - you'd normally expect your bunch of gas molecules to get more "disordered" if you give it more energy to run around, and conversely sucking energy out will cause your molecules to move slower and less.)
However, it's possible to imagine situations where increasing the total energy actually causes a decrease in entropy (dS/dU < 0, so T < 0). For instance, consider the classic textbook example of an ensemble of two-state systems that do not interact with each other.
For concreteness, let's imagine having N distinct, non-interacting electron spins in an external magnetic field, where spin up has energy +1, and spin down has energy 0.
For a given total energy E, one can derive that the entropy is S = k log (W), where W = N! / { E! (N-E)! }. It's possible to show mathematically that when E > N/2 you'll get dS/dU < 0, but you can argue equally well from physical intuition:
When you start off with everything spin down, adding energy will greatly increase the total number of ways you can allocate your energy to the N electrons - at very small E, you'll pretty much have almost N ways of allocating each energy quantum you have. However, as you approach E = N/2, you get less and less extra ways of allocating energy with each extra quantum...
And when you reach E > N/2 you basically have more spin ups than spin downs - you'll be stuck with having more and more spin ups, and hence less ways of "allocating spin downs", as you gain more energy. The total entropy of the system thus decreases as you add more energy, when E > N/2, giving rise to negative temperature.
PS: The wikipedia article on negative temperature brings out an interesting point: T < 0 objects will behave as if they're hotter than anything with T > 0 (-1 > +infinity!!) when they're in thermal contact.
Question
Water has density, diffusion, specific heat, compressibility unusual behaviors. Why?
Because it is not a simple atomic liquid, but a molecular liquid. All the extraordinary structural and thermodynamic properties of water (and ice) follow from the strong directional bonding between individual water molecules. A good reference on this is Ariel Ben-Naim's new book on water (or my paper J. Chem. Phys. 131, 054505 (2009)).
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Vinod
Cahos, making a new science J.Glieck, a very good and famous vulgarization.
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Water boils at 100 degrees Centigrade.
Water freezes at zero degrees Centigrade.
The temperature of a black hole is determined by the 'black body radiation temperature' of the radiation which comes from it. (e.g., If something is hot enough to give off bright blue light, it is hotter than something that is merely a dim red hot.)
For black holes the mass of our Sun, the radiation coming from it is so weak and so cool that the temperature is only one ten-millionth of a degree above absolute zero. This is colder than scientists could make things on Earth up until just a few years ago (and the invention of of a way to get things that cold won the Nobel prize this year). Some black holes are thought to weigh a billion times as much as the Sun, and they would be a billion times colder, far colder than what scientists have achieved on Earth.
However, even though these things are very cold, they can be surrounded by extremely hot material. As they pull gas and stars down into their gravity wells, the material rubs against itself at a good fraction of the speed of light. This heats it up to hundreds of millions of degrees. The radiation from this hot, infalling material is what high-energy astronomers study.
What does directional and non-directional in materials mean?
Question
Under the topic bonds in solids ( ionic, covalent .... ) what does directional and non-directional property mean...
By directional I assume you're referring to anisotropic and isotropic materials properties. A directional (anisotropic) property is one that varies in different directions in the crystal. For instance, a material may possess an anisotropic strain response, in which case it would respond differently to stresses from different directions. However, a non-directional (isotropic) material, is one that shows the same response, regardless of direction. Covalent bonds show directionality because of the shape of the electron wavefunction in space. The probability of finding an electron is higher along certain directions that correspond to occupied orbitals. Ionic bonds involve the transfer of an electron and so generally have less directionality associated with them.
Can anyone explain X-Ray Diffraction by Rotating Crystal Method
Question
Which book to refer
u can understand this topic in a very clear way from X-ray diffraction by B.D.Cullity.....
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Today in class I heard the following affirmation:
"In just the same way the photon field A_i has two helicities related by parity, the graviton, associated to the g_ij field, must have its physical degrees of freedom related by parity."
The metric tensor g_ij is defined by a solution of Einstein's field equations, which are pairs (M,g) where M is a four dimensional Lorentzian manifold. Well, if you accept that definition, then parity is not a global symmetry, simply because M does not necessarily have a "global parity transformation" or more precisely a Z_2 action. Therefore, if you can in some way make sense of some parity like symmetry in a general manifold M, it shoud be a local one (unless M has a Z_2 action), and then that would mean that those degrees of freedom coming from the local parity are not physical at all, therefore the graviton doesn't exist as a spin 2 particle in a general spacetime manifold M.
Hi, Raul, I don't understand well about your question also. But Giovanni's explanation seems close to mine. As a field theory, one need to define background space-time, and particles are perturbations of this background (or more precisely, the vacuum). However, in a generic curved space-time, how to define a global vacuum is difficult since it will evolution as time going. The more difficult thing is to define a particle in a curved space-time since it will decay! So the only thing we can do is doing the perturbaton on some static background, such as, Minkowski space-time. When the background is given, then one can do the usual field quantization, which is, write down the action( for gravity, this is Einstein-Hilbert action), find conjugate variables, define time-equivalent commutation relation, canonical or path integral quantization. The parity means for this action, it preserve this symmetry, that is called "global parity", this symmetry is not a really Global symmetry, but just for the action itself! If one has done the quantization, and renormalization and then get an effective theory, if this "global symmetry" still preserve, then one can say that this symmetry is a real symmetry of this theory. If it is broken(usually, it will), then it is not a local symmetry, that is called anomaly. Since for the quantum gravity theory, Graviton propagates on the space-time, it will bring the "local" breaking down to everywell on this space-time and of course, there should not be a global parity symmetry. However, if there are no local breaking down of the defined global symmetry, it will survived from the quantization and then be a real global symmetry.
However, these global symmetries have nothing done to the spin or fermionic particles. The defination of a spinful particle is an alternating speaking of a spin representation of the isomorphic group of the space-time. Spin symmetry is not up and down symmetry. For example, in Minkowski space-time, the iso group is SO(1,3), but the motion group is Poincare group, which is SO(1,3) half-producting Gallilea group. SO(1,3) actully is a local group of the motion group of the manifold. spinful particles are local excitations, so they are spin representations of SO(1,3), eg. Dirac particles are spin 1/2 representations of SO(1,3), Gravitino (Graviton's super parterner) is spin 3/2 representaitions of SO(1,3) etc.
I do not like that saying that photon has two helicities which related to each other by parity, I prefer to say that photon has two polarization directions, on this polarization plane, electronic and magetic fields excite each other vertically. since these two fields are always vertical to each other, one can define a Z_2 symmetry to transform between them. A massless particle can have D-2 polarization directions, where D is the dimention of spacetime, eg. in 9+1 dimention (as string theory) massless particles are representations of SO(8) which is the little group of SO(1,9). For massive particles, the independent polarization directions are D-1 (there is an additional backward polarization which makes the particle Massive).
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