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1

From my point of view, there are two definitions I find "definitive" and helpful. I hasten to add that I am neither a particle physicist nor quantum field theorist; my interest in the former comes from an application of Lie groups (which I am interested in), in the latter from a professional career in optics where I have had to gather a sound working ...


1

In the quantum mechanical regime "particles" are observed and classified according to their behavior, they should be called "quantum mechanical entities". In some experimental situations elementary particles behave as classical particles and sometimes as probability waves, the probability defined by the square of the wavefunction which is a solution of ...


1

Particles are defined by the physical situation that you have on your hands. In Classical Celestial Mechanics, the planets themselves will oft be referred to as "particles". In Quantum Mechanics the constituents of matter ( electrons, protons, neutrons, quarks, etc ) are also referred to as particles. In cosmology, galaxies are particles. In kinematics, a ...


0

By removing all the spaces between nuclei you are converting the matter to degenerate matter, which is what happens at the centre of a neutron star. The density of matter at the centre of a neutron star is $6 - 8 \times 10^{17}$ kg/m$^3$. The density of a human body is around $10^3$ kg/m$^3$ (a bit less when you inhale and a bit more when you exhale) so your ...


1

In the center of mass frame, if two particle beams have the same energy, using energy-momentum 4-vectors we get: $s =(P_1 + P_2)\cdot(P_1 + P_2) = (E + E, \vec{p}-\vec{p})\cdot(E + E, \vec{p}-\vec{p}) = (2E, 0)\cdot(2E,0) = 4E^2$ Therefore the $E_{CM} = \sqrt{s} = 2E$ For a fixed target ($E_b$ = Energy of the beam and $m_t$ = mass of the target): $s ...


1

I'm not particularly confident with experimental Physics, nevertheless I will try to answer to your interesting question. Not every scattering experiment in Particle Physics needs the acceleration of particles in opposite directions, there are a lot of experiment (for example Rutherford scattering) in which a fixed target is used. However in doing so the ...


1

Electromagnetic waves don't always travel in straight lines. They can bend when they encounter a changing wavespeed at non-normal incidence, this is how a lens works. Mechanical waves act in the same way. They travel in a straight line until they encounter a change in the material parameters (such as a change in density) which changes the speed of the ...


2

Like electromagnetic waves, mechanical waves, and (in fact) everything travels in a straight line until something acts on it causing it to stop going straight or until the medium it's travelling in changes. Light will bend due to gravity, refraction, reflection. Without an outside influence and in the same medium, everything travels in a straight line. ...


3

No. Just like in Chemistry and Thermodynamics, we never get anything for free. On a mechanistic level, it's important to recognize that zero-point (vacuum) energy represents the lowest energy state waveform. I remember thinking that because the EM fields are everywhere and quantized, that there was some sort of magic taking place. Realistically, ...


3

The answer kinda is "You can, but why would you". It is indeed possible to extract energy from the vacuum. It has been studied, both theoretically and experimentally, using a variety of metal plates and other Casimiresque gizmos. The problem is just that it basically acts like a spring. To put the Casimir effect in action, you must first approach together ...


13

Whether you can extract energy from this or not (and I strongly suspect not) the Casimir effect is a consequence of vacuum fluctuations. Essentially when two metallic plates are very close to each other, the wavelengths of virtual particles that can be created between the plates is restricted and hence there are fewer particles between the plates and ...


20

The energy is borrowed from the Heisenberg Uncertainty Principle to create virtual particles and has to be paid back in a very short time. $\Delta{t} \geq \frac{\hbar}{2\Delta{E}}$ This is why virtual particles live for very short times (i.e pop in and out of existence). We cannot manipulate this energy.


3

For this sort of task, it's easier to check through Wikipedia's list of baryons and list of mesons. Each article has a table listing the properties, including mass, of the known particles of the appropriate type, so you can just scan down the table and find the particle that matches your mass. In addition to mesons and baryons, in general, you would need to ...


1

In QFT, the Dirac spinor will also be promoted to a field, whose oscillation mode coefficients are creation and annihilation operators. BUT: For the Dirac spinor it is possible to well-define a probablility density and current: $$\rho^\mu \propto \bar\psi (\partial^\mu \psi) - (\partial^\mu \bar \psi) \psi $$ This current's zero component is positive ...


1

When a type of quantum (elementary) particle is absent it just means that the field of that particle is in the quantum "vacuum state". But "vacuum state" does not mean absence of everything concerning that field. The vacuum state has various physical properties in spite of its name: It is nothing but a possible state or quantum configuration of the field, ...


8

Obviously, the smallest particle that scientists have ever seen directly is a photon. The question is a bit silly because it tries to eliminate simple device like a photographic plate, but the human eye, its nerves and the visual cortex together are far more complicated.


18

Taking your question literally, you can see a single barium ion: The TRI┬ÁP group has achieved capturing a single barium ion in a Paul trap. The images show Coulomb crystals formed by a decreasing number of laser-cooled ions as detected with an EMCCD camera. This forms an important step towards the planned experiments on single radium ions to measure ...


4

A simple version of this is bremsstrahlung, i.e. an electron that decelerates and produces electromagnetic radiation / photons. By your reasoning the energy of the electron should only be able to go into other electrons: maybe it should radiate other electrons, maybe a single electron shouldn't lose energy as it travels. But the electron can transfer some ...


1

There are two mechanisms. Which one is more important depends on the colloid. A large number of colloidal particles have ionisable groups on their surfaces. These are usually salts of carboxylic acids. The vast majority of organic colloids (e.g. milk) are in this class, as are colloids prepared from acidic monomers like the acrylates. In water the surface ...


2

John Rennie will probably have more details on the matter, but in general colloids (such as oil dispersed into soap water) are not so much stabilized by a net total charge of the mixture, but rather are stabilized by repulsions from separated charges. For example: This is a cartoon representation of what an oil droplet in soap water looks like; note that ...



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