What is Particles: Definition and 1000 Discussions

In the physical sciences, a particle (or corpuscule in older texts) is a small localized object to which can be ascribed several physical or chemical properties such as volume, density or mass. They vary greatly in size or quantity, from subatomic particles like the electron, to microscopic particles like atoms and molecules, to macroscopic particles like powders and other granular materials. Particles can also be used to create scientific models of even larger objects depending on their density, such as humans moving in a crowd or celestial bodies in motion.
The term 'particle' is rather general in meaning, and is refined as needed by various scientific fields. Anything that is composed of particles may be referred to as being particulate. However, the noun 'particulate' is most frequently used to refer to pollutants in the Earth's atmosphere, which are a suspension of unconnected particles, rather than a connected particle aggregation.

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  1. QuasarBoy543298

    I Problem with the idea of identical particles in QM

    assume i have a gass made from N identical particles in a box and i want to calculate the probability for k out of N particles to be in the left side of the box. the problem is ,that if we treat the N particles as identical , each state in which exacly k of the N particles are in the left side...
  2. B

    B Can quantum entangled particles be split and sent into a black hole

    I am new here so apologies in advance. When a virtual particle and anti particle appear at the event horizon of a black hole, before they destroy each other, they are split with one being sucked into the black hole and the other becoming exhaust. Are these the same particles as the quantum...
  3. Lunct

    B Can virtual particles have an imaginary mass?

    I was talking to a physicist who said to me that virtual particles can have a mass of a constant times by i ,as in the root of -1. I have been thinking about this more and it intrigues me. I have done some research into this and can't find further details. If they have an imaginary mass does...
  4. dRic2

    The work function and mutual forces between particles

    I tried to apply the chain rule $$X_{ik} = \frac {\partial U}{\partial \xi_{ik}} = \frac {\partial U}{\partial x_{i}} \frac {\partial x_i}{\partial \xi_{ik}} = \frac {\partial U}{\partial x_{i}} $$ and I got the force x-component of the force acting on ##P_i## I guess. but I do not know what...
  5. W

    I Relativistic Quantum Mechanics & Localized Particles

    A lecturer today told the class that relativistic QM for single particles is flawed by showing us that for a state centered at the origin, it was possible that ##Pr(\vec{x}>ct)>0##. He said that this was down to the fact that we should be considering multi-particle states in relativistic...
  6. R

    I How Is Particle Distribution in a Solid Angle Derived in Mechanics?

    I'm reading Mechanics by Landau and Lifshitz, chapter IV, and trying to understand how in a (closed) center of mass system, with randomly distributed and oriented particles that disintegrate, "the fraction of particles entering a solid angle element ##do_{0}## is proportional to ##do_{0}##, i.e...
  7. K

    I Confused about virtual particles

    Hello! I am a bit confused about the distinction between virtual and real particles. For example a Z boson, which has a very short lifetime, in all experiments will decay to some other stable particles (i.e. it is detected through its decay). This means that it will always appear as a...
  8. e2m2a

    A The Higgs Field, constant velocity, and the acceleration of particles

    I need some clarification on how the Higgs field work. The popular science explanation explains the effect on moving particles the way an object would be impacted by moving through a medium of maple syrup. I understand this is a very bad, inaccurate analogy. What I want to understand is does...
  9. M

    I Why are 'virtual particles' allowed to be 'off-shell'?

    If virtual particles are supposed to be some sort of Green's function excitation of a field following a particular Lagrangian or PDE (induced by the presence of another particle, free or virtual), then why are they allowed to be "off shell"? (Especially: Why are they allowed to be nonzero...
  10. Cryo

    Hamiltonian for Charged Particles + EM-Field

    Summary: I have found the Hamiltonian for the free particles and the electromagnetic field (##\mathbf{E}## - electric, ##\mathbf{B}## - magnetic) to be (non-relativistic !): ##H=\sum_i \frac{m \dot{r}_i^2}{2} + \int d^3 r \left(\frac{\epsilon_0}{2} E^2 + \frac{1}{2\mu_0} B^2\right)## (1)...
  11. J

    I Can You Help Me Couple 3 Spin 1/2 Particles?

    Hello, I'm trying to couple 3 spin 1/2 particles. So far, I have been able to find the coefficient for the other states but I can't get the results for ##j_{12} = 0## to ##j_3=1/2##. Here is my attempt: 1) Using CG table...
  12. I

    I Instantly reformable wave packets instead of "particles"?

    Hello, I am a high school physics teacher, and I have been thinking about a way to model quantum mechanics in an intuitive way in order to teach it better, but I don't want to lead my students down the wrong path. I am certainly no expert in quantum theory. In looking at the guidelines, I...
  13. J

    A Is the Klein-Gordon equation a quantization of classical particles?

    The Schrödinger equation can be derived from the path integral quantization of the Lagrangian of classical, non-relativistic particles. Can the Klein-Gordon (and maybe the Dirac) equation be derived from the path integral quantization of a given classical (supposedly relativistic) Lagrangian of...
  14. C

    B Are massless particles truly massless?

    Most of the mass of matter comes from energy of strong force interactions between quarks. However the quarks still have intrinsic mass. Other particles have no intrinsic mass but still have energy. So according to mass-energy equivalence, these particles should still have effectively mass, to my...
  15. B

    Do magnetic lenses do work on particles flowing through them?

    Do magnetic lenses do work on charged particles flowing through them? Intuitively I would think yes because the magnetic field produced by the electric current in the coil is applying a force on the particles flowing through the lens, so therefore an electric current (the beam of particles to...
  16. C

    Relativity - energies of particles in circular motion

    Summary: What is energy of proton, deuteron and alpha particle in circular motion of the same radius. Hello, I have a problem. Here is the content of an exercise: In some experiment, proton with energy of 1MeV is in circular motion in isotropic magnetic field. What energies would have...
  17. barbs00

    Comet's acceleration due to ejected particles

    What I did was consider a circular orbit around the Sun. We'll have the comet's velocity tangential to its movement (vorb) and a radial velocity pointing towards the sun due to the ejected particles (vej). Therefore we can find the total velocity to be vtot=√(vorb^2 + vej^2). What I did after...
  18. binbagsss

    Applying LSZ reduction - scattering particles - quantum theory

    Above
  19. A. Neumaier

    Insights Clarifying Common Issues with Indistinguishable Particles

    Continue reading...
  20. E

    Calculate the kinetic energy of two alpha particles

    if lithium 7 3 captures proton, it is divided into two alpha particles. Calculate the kinetic energy of the alpha particles. the proton energy can be neglected and the mass of the nucleus of Li is 7,01601u. I only know how to write the equation: 3Li7 + 1H1 → 2He4 + 2He4 Can anyone help me? How...
  21. A

    Trajectory of charged particles in a magnetic field?

    A charge is entering magnetic field perpendicularly! Griffiths says it motion will be circular! But it is accelerated so it must radiate energy and it's motion should be spiral inward?
  22. TerranIV

    I Particles w/ 0 Inertial Mass Traveling Faster than Light?

    I was just curious if there were any known (theoretical) particles with zero inertial mass that don't travel at light speed. Is this even possible?
  23. Lunct

    Is Brownian Motion actually random or does it follow determinism?

    If you have enough information could you not determine with certainty the movements of pollen particles in water? In other words, if you were able to measure the movements the particles, then repeat the exact same experiment, with all things controlled, would the particles move in the same way...
  24. JD_PM

    Calculating the Fraction of Time Particles Spend Outside a Potential Well

    I want to compute the fraction of time both particles spend outside the finite potential well. All I can get is the probability to find them outside. The wavefunction outside the potential is: $$\frac{d^2\psi}{dr^2} = -L^2 \psi$$ Where: $$L = \sqrt{\frac{2mE}{\hbar^2}}$$ Solving the...
  25. L

    Problem about identical particles

    Can someone help me with this excercise?: Consider two electrons constrained to move in one dimension. They interact through the potential $$ V(x) = \begin{cases} 0, & \text{ if } |x| > a \\ -V_{0}, & \text{ if } |x| \leq a \end{cases} $$ where ##x## is the relative coordinate. The total spin is...
  26. L

    I Two identical spin 1/2 particles

    I am studying identical particles and I have some doubts. Considerer two identical spin 1/2 particles interacting through a central potential ##V##. In the rest of CM, the hamiltonian is $$ H = \frac{\textbf{P}^{2}}{2M} + \frac{\textbf{p}^{2}}{2\mu} + V(r),$$ where ##\textbf{P}## is the momentum...
  27. R

    SR: Finding the speed of particles wrt the laboratory

    Homework Statement Two particles in a high-energy accelerator experiment approach each other head-on with a relative speed of 0.890c. Both particles travel at the same speed as measured in the laboratory. What is the speed of each particle, as measured in the laboratory? Homework Equations...
  28. K

    I Free Particles & Geodesics: Do They Go Hand-in-Hand?

    Is it correct to say that free particles always follow geodesics?
  29. A

    Spin-##\frac{1}{2}## particles in infinite square well

    Homework Statement Construct the four lowest-energy configurations for particles of spin-##\frac{1}{2}## in the infinite square well, and specify their energies and their degeneracies. Suggestion: use the notation ##\psi_{n_1,n_2}(x_1, x_2) |s,m>##. The notation is defined in the textbook...
  30. fight_club_alum

    Speed of two charged particles repelling each other

    Homework Statement 28. Particle A (mass = m, charge = Q) and B (mass = m, charge = 5 Q) are released from rest with the distance between them equal to 1.0 m. If Q = 12 μC, what is the kinetic energy of particle B at the instant when the particles are 3.0 m apart? a . 8.6 J b. 3.8 J c. 6.0...
  31. W

    I Normalisation constant for identical particles

    In my lecture notes, the normalisation for such a bosonic state was given by However, I can't quite seem to grasp how the normalisation factor came about. Could someone walk me through it? Many thanks in advance!
  32. M

    Potential/Kinetic Energy of Particles in Harmonic Oscillator

    Homework Statement I'm trying to reconcile the answers to two questions regarding the average potential and kinetic energies in simple harmonic oscillator Question 1: The average potential energy of the vibrational motion in the ground state of a diatomic molecule is 12 meV. The average...
  33. T

    I Why do particles around us have finite momenta?

    Since there is no privileged inertial frame, I would have expected the first particles in the universe to have no particular bias in their momenta. Relative to an observer I would expect the distribution to be uniform and unbounded. The mean momentum of the initial particles relative to an...
  34. L

    Potential Energy and Potential -- Systems versus Particles

    When I first learned about these subjects, I did what was intuitive to me and treated particles as if they carried potential energy. I would do this similarly for rigid bodies where I would also treat them as a particles with their body's mass at the center of mass. This wasn't helped by...
  35. DennisN

    David Kaplan lecture on Particles and the Nature of Nothing

    I just saw this public lecture and I really enjoyed it (the lecture begins at about 18 minutes into the video). He is an engaging and rather funny speaker. At the end there is a Q&A in which he for instance goes into some of the misconceptions about quantum mechanics. David Kaplan is also one of...
  36. HotFurnace

    B Effects of Cooling and Heating in a Cloud Chamber

    Good day everybody! As we have already known the structure and operation of the cloud chamber, can someone explain to me why the cooling is done at the bottom of the chamber and the heating at the top, but not vise versa?? How this affect the function of the cloud chamber?
  37. universal2013

    Beginning through the Pedrotti optics book -- I have an energy question

    Hello. I started to work on pedrotti optics book (2nd edition) and i got confused about what is relativistic mass and why we use it rather than kinetic energy (1/2mc^2)? Also in the beginning of these explanations there is one equation i barely understand nothing out of it. Could you please...
  38. D

    Time of closest approach between two particles

    Homework Statement Two objects ##1## and ##2## move at constant speeds ##v_1## and ##v_2## along of two mutually perpendicular lines. At the moment ##t = 0## the particles are located at distances ##l_1## and ##l_2## from the point of intersection of the lines. At what time will the two objects...
  39. L

    I What is the relation between chemical potential and the number of particles?

    Chemical potential is defined as the change in energy due to change in the number of particles in a system. Let we have a system which is defined by the following Hamiltonian: $$H = -t \sum_i^L c_i^\dagger c_{i+1} + V\sum_i^L n_i n_{i+1} -\mu \sum_i^L n_i$$ where ##c^\dagger (c)## are creation...
  40. W

    B Do Helium Atoms Decay in Complete Isolation?

    Thought experiment: If we had one kilogram of Helium in a nonpermeable box in the middle of the void between sol and our nearest Stellar neighbor and we left it there with no other interractions (all other things being equal) what happens to those electrons protons and neutrons ASSUMING no high...
  41. T

    Connected particles on an inclined plane

    Homework Statement Mass of m1=90kg Inclination is 40degrees. (see the attachment) a) What must be the size of m2 , to hold the system in balance (no movement) ? b)what must be the size of m2 at least, to make m1 move upwards. The static friction coefficient between m1 and the plane is 0.3...
  42. L

    A How to choose the number of particles per site in Fermionic DMRG?

    I am doing DMRG (in traditional formalism, not MPS) for Hubbard model H = -t ∑i ∑σci,σ ci+1,σ + U∑ini,σni,σ- In every iteration we add two sites to the system, but how do we set that how many particles are allowed in the system?
  43. arda

    I Why particles have group velocity?

    I just confused about it.Why can't we discribe a particle just one wave function instead of wave packet(group of waves with different phase velocities)?
  44. Geofleur

    I Qualitative description of particles in QFT

    I would like to be able to qualitatively describe what particles are according to quantum field theory without saying anything wrong. Is it correct to say the following things about, say, an electron? (1) Electrons are excitations of an underlying quantum field, the electron field. There is a...
  45. J

    B Are all particles and photons entangled?

    1) are all particles/photons theoretically entangled at birth but we just never or will know? Or is it under specific conditions? 2) how much of the photons that the sun emits entangled particles/photons?
  46. F

    I Space and time dependence of entangled particles

    It seems that the entanglement of two particles does not change with time and can cross long distanced as long an neither particle decoheres with the environment. This makes me wonder if the wave function for that entanglement can have any time or space dependence? I only did a brief search for...
  47. Z

    Why do alpha particles experience large angle deflections?

    Homework Statement I would like to understand why alpha particles experience large angle deflections and if this is the same thing as recoil of alpha particles? Do large angle deflections occur near the end of the alpha particle track or can they occur at any point along the trajectory...
  48. F

    I Gluons: Particles or Not? Evidence for their Particle Nature

    What evidence is there that gluons are even particles? They were originally proposed to supply an extra quantum number to comply with the Pauli exclusion principle. So gluons are supposed to carry a color-anticolor charge in order that the quarks in baryons would add up to the color white. But...
  49. T

    Calculate the range of sizes of the particles

    Calculate the range of sizes of the particles. 
 ν= velocity of decent d = diameter ps = density η = viscosity P1 = density v= (d^2 (p_s-p_1 )g)/〖18〗_η Rearrange for the diameter. v= (d^2 (p_s-p_1 )g)/〖18〗_η 〖18〗_η v= d^2 (p_s-p_1 )g (〖18〗_η v)/(p_s-p_1 )g= d^2 √((〖18〗_η...
  50. alantheastronomer

    A Can String Theory Explain Fundamental Particles Through Standing Waves?

    I've heard in a general way that string theory can describe the properties of the fundamental particles through standing waves or resonances of strings. Is this in fact the case, and if so, can someone provide a link to the relevant research papers?
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