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. Elvis 123456789

    System of particles with non-degenerate energy levels

    Homework Statement A system has three non-degenerate energy levels with energies 0, ε, and 2ε. a) Calculate the entropy of the system if the three levels are populated by two distinguishable particles such that the total energy is U=2ε. b) Calculate the entropy of the system if the three...
  2. quantumfunction

    B Do we measure the particle or the wavefunction?

    It seems to me that we don't measure a particle because a particle doesn't have an objective existence independent of the wave function or does it? The wave function in this case would have to be real because you can't have probability without the underlying possibility of a specific outcome...
  3. J

    B Relativistic Simulation of Charged Particles: How a Physicist Would Go About It

    Supposed i wanted to do a relativistic simulation of charged point particles moving at different velocities and interacting with each other. My simulation would give me the x,y,z coordinates of each particle seen from an arbitrary observer's point of view, at a given t. The t given however, is...
  4. weezy

    I Degeneracy of energy levels greater than no. of particles?

    From statistical mechanics in zeemansky's book . He states that it's easy to see that for a closed system the no. Of degenerate states ##g_i## for energy level ##E_i## is greater than the number of particles ##N_i##occupying that energy state. I can't find a mathematical proof for it. Can I...
  5. M

    Stokes Number for Particle Motion?

    Hello, I have a question... With the Stokes Number you can derive if particles follow a streamline or not, right? Let's say I am in a windtunnel, the wind is coming from the east. And I have a tube in the wind tunnel (horizontally located) which is 180° opposite to the flow (the opening of the...
  6. D

    A Quantum Field Theory - Why quantise fields?

    As I understand it, the need for quantum field theory (QFT) arises due to the incompatibility between special relativity (SR) and "ordinary" quantum mechanics (QM). By this, I mean that "ordinary" QM has no mechanism to handle systems of varying number of particles, however, special relativity...
  7. K

    B Gravitational Filed and Particles

    (Sorry for my bad English.) Suppose we choose the surface of the Earth at a point as having a zero potential energy. Now if there're two particles of equal mass at rest, one of them just an heigh above the another, the one which is at the ground will not have potential energy. Since the...
  8. LarryS

    Particle Accelerator for Uncharged Particles?

    Particle accelerators use the electromagnetic field to accelerate charged particles. Would it be possible to build a particle accelerator for uncharged particles? Say we are given a narrow beam of neutrons. How could we accelerate them so that the accelerated particles were still in a...
  9. F

    # of gas particles hitting one face of a cubic container

    Homework Statement [/B] 1 mol of gas at temperature T is contained in a cubic container of side L. Estimate the number of collisions per second between the atoms in the gas and one of the walls of the cubic container. My book gives this formula for that quantity \frac{N_A}{6L}\sqrt{\frac{3 k...
  10. A

    I Localization of particles and disorder at the quantum level?

    Hi all, Based on a previous post on Anderson Localization, I'm posting a more general question here: given that there is substantial disorder in many systems in general (e.g., the body, or the Earth), why do we not see effects like Anderson Localization more frequently - that is, why aren't...
  11. hsdrop

    B The path of particles in the L.H.C. pics

    I was looking at the diagrams from the lhc of the path of particles and notice that the path of the particles always craved parabolically not traveled strat outward from the point of impact. Why does this happen explachely when the particles are traveling at near light speeds thank you ahead...
  12. felici95

    A Emergence of spin 2 particles from lowers spin particles

    I was referring a lecture series by Professor Hong Liu. In his first lecture on Emergence of gravity he says, "massless spin-2 particles (gravitons) can arise as bound states in a theory of massless spin-1 (photons, gluons) and spin- 1 particles (protons, electrons)". What does it exactly...
  13. Student100

    I Gravitating particles and entropy

    <Moderator's note: this is a spin-off from another thread> Matter isn't spreading out because of gravitational attractions between matter. If you took a bunch of gravitating particles and sealed them in container, the highest state of entropy is one where all the particles coalesce, which...
  14. P

    B Uncertain position and momentum -- A property of particles?

    I could completely understand the fact it it was just a limit to our observations but how can it be a property of the microscopic particle itself? Here's how I understand about probabilities: Before a die is thrown, the probability of a certain number coming up is 1/6. But, it's before the die...
  15. Thomas Eaton

    B Virtual Particles; Making the Universe an Open System?

    The question here is simple; would the existence of virtual particles make the universe an open system? And if not, why? I thank all who consider this question for their time and would love to hear an answer.
  16. A

    B What would happen if nothing was like it is

    HI guys! I was thinking... What would happen if the atom were considered the smallest thing of the universe, or just that electrons, protons e neutrons were that? That would impact in semiconductors technologies? Or plasma? Or nanotech? How? Thanks in advance.
  17. A. Neumaier

    I Entangled particles in the thermal interpretation

    Ah. It was skepticism about publishing, not about giving a comprehensive account! Yes, I am preparing a book on quantum mechanics, which will contain an account of the thermal interpretation - but primarily to macroscopic, nonrelativistic reality, where it is obvious that it gives the correct...
  18. L

    I Degrees of freedom of elementary particles

    The EM wave and the photon have two degrees of freedom. Their polarization directions and spin states, respectively. But they move in space, too. I mean light has the freedom to go in all directions in space. Like a macroscopic ball in 3-D space, which can go all three directions, if there are...
  19. Blockade

    Is the total electric field of two opposite charges 0?

    Let's say you have two particles that are the same in magnitude but have opposite charges like the equation down below: E1 = -q*k/r^2 E2 = q*k/r^2 ETotal = q*k/r^2 + -q*k/r^2 = 0 Does this mean that the electric field of both these charges cancel out each other? Then what is the electric...
  20. Renu420

    I Quark and anti quark neutral particles question

    Hi there ppl! I have a question ! I learned that neutral miuon ( μ0) is made of a quark and his anti quark (can't remember which) which explains it's very little lifetime(around 10-24s if I'm not wrong). Now i wonder how two particles ,that are meant to destroy each other,form for a little time...
  21. J

    I Loop Quantum Gravity and Virtual Particles....

    Hi all, Following up on another post - for a layman, can someone describe the status of virtual particles in Loop Quantum Gravity models? Since LQG avoid UV divergences, and has a different structure from field-based theories, are virtual particles still talked about? (in the context of closed...
  22. john baez

    Insights Struggles with the Continuum - Conclusion - Comments

    john baez submitted a new PF Insights post Struggles with the Continuum - Conclusion Continue reading the Original PF Insights Post.
  23. john baez

    Insights Struggles with the Continuum - Part 7 - Comments

    john baez submitted a new PF Insights post Struggles with the Continuum - Part 7 Continue reading the Original PF Insights Post.
  24. Elvis 123456789

    Another relativistic particle decay question

    Homework Statement Unstable particles cannot live very long. Their mean life time t is defined by N(t) = N0e−t/τ , i.e., after a time of t, the number of particles left is N0/e. (For muons, τ=2.2µs.) Due to time dilation and length contraction, unstable particles can still travel far if their...
  25. K

    I What's so strange about entangled particles?

    In a thought experiment, there is a spin-0 source emitting particles. (I suppose you already know what is that experiment.) Two observers in opposite sides along the same axis measure opposite spin components. If one observer measure, say, spin up, then the second observer will certainly measure...
  26. Elvis 123456789

    How Fast Must Muons Travel to Reach a Distant Target Without Decaying?

    Homework Statement Unstable particles cannot live very long. Their mean life time t is defined by N(t) = N0e−t/τ , i.e., after a time of t, the number of particles left is N0/e. (For muons, τ=2.2µs.) Due to time dilation and length contraction, unstable particles can still travel far if their...
  27. A

    I Is the SM of particles physics and Cosmology SM

    I'm in a deep discussion with a friend. He says that the Standard Model of particle physics is actually known by Standard Model of cosmology and that both are the same and that the SM of particle physics is in the Minkousky geometry... I disagree about this, I do think that the SM of particle...
  28. entropy1

    I The mechanism of entangled particles

    First let me ask this: Consider a pair of entangled photons fired at a respective detector after passing respective polarisation filters. If a photon passes a polarisation filter, is it in a superposition of having passed and not having passed? Is the measuring device (that detects the...
  29. ohad

    A Density waves in gravitationally interacting particles

    My question is about system of infinite number of point like particles with gravitation interaction (classic Newtonian interactions). If the infinite system can be considered to be in equilibrium, uniform density, in 3D, did anyone analyzed this system with regard to propagation of density...
  30. C

    B World made up of 2nd & 3rd Generation particles

    This is a common question asked but I can't find a good reference. Supposed the first generation particles (electron, quark up, down) vanish and we only had STABLE second generation particles composing of muon, strange and charm quarks that suddenly became stable (don't decay).. would they form...
  31. S

    Quantum Particles & Quantum Fields - Hagen Kleinert

    I've discovered a potential treasure horde tucked away in the deep dark folds of the world wide web. A 1625 page mammoth on all aspects of quantum field theory by Prof. Hagen Kleinert. There's a draft ed. for free available here -...
  32. gasar8

    Two particles with spin - measurements

    Homework Statement We have got two particles with S_1=1 and S_2=1. We know that S_{1z}|\psi_1\rangle=\hbar |\psi_1\rangle and S_{2x}|\psi_2\rangle = \hbar |\psi_2\rangle. a) Find wave function |\psi_1\rangle in S_{1z} basis and |\psi_2\rangle in S_{2z} basis. b) We measure S^2 of total...
  33. gasar8

    Two particles in magnetic field QM

    Homework Statement Two particles with spin S_1={1 \over 2} and S_2={1 \over 2} are at t=0 in a state with S=0. a) Find wave function at t=0 in S_{1z},, S_{2z} basis. b) Second particle is in a magnetic field B = (\sin\theta,0,\cos\theta), the Hamiltonian is H=\lambda \vec{S_2} \cdot \vec{B}...
  34. Sophrosyne

    I Wavelength of photons exchanged between charged particles

    When two electrons approach each other, there is a repulsion between them by the exchange of a photon as the electromagnetic force carrier. Is there a general range of wavelength of such photons? Does it depend on how rapidly these electrons are approaching each other?
  35. tomdodd4598

    I Physical eigenstates of systems of n particles of spins sᵢ?

    I am relatively well versed when it comes to systems of spin, or doing the maths for them at least, but am unsure whether all of the {L2, Lz, (other required quantum numbers)} basis eigenstates for a general system of n particles of spins si, where si is the spin of the ith particle, can...
  36. LarryS

    I Why must force-carrying particles be virtual?

    Why must photons, for example, that carry the EM force be virtual? Is it because they are tied to gauge invariance and that is not observable? Thanks in advance.
  37. Chris Frisella

    I Force Between Moving Charge Particles: Does Speed Matter?

    You have two electrons held at a fixed distance X from each other. You measure the opposing force is Y. Now you bring the electrons closer to each other, and then let them go so they can fly apart. As the electrons move apart and reach distance X, will the opposing force still be Y, or will it...
  38. F

    I Understanding Thermalisation in Particle Systems

    I've recently been reading a bit into the thermalisation of a system of particles and I'm unsure on a couple of concepts. Firstly, if a system of particles are out of mutual thermal equilibrium, does this essentially correspond to the particles in the system having randomly distributed momenta...
  39. I

    B Shape of elementary particles in QFT, etc?

    Hello, I hope this is not a stupid question as I am not a physicist. But I was curious about how contenders for the so-called Theory of Everything view the shape of the elementary particles. I know that the basic idea of string theory is related to the shape of elementary particles as one...
  40. K

    A Dark matter does not contain certain axion-like particles

    this was widely reported in the past, but not commented on here https://www.sciencedaily.com/releases/2016/04/160422115320.htm based on http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.161101 Search for Spectral Irregularities due to Photon–Axionlike-Particle Oscillations with...
  41. D

    Scattering of two charged particles

    Homework Statement Two identical charged particles, each of mass m and charge e, are intitialy far apart. One of the particles is at rest at the origin, and the other approaching it with velocity v along the line x=b, y=0 where b=e^2/2 \pi \epsilon_0 mv^2. Find the scattering angle in the CM...
  42. mfb

    Insights LHC Part 4: Searching for New Particles and Decays - Comments

    mfb submitted a new PF Insights post LHC Part 4: Searching for New Particles and Decays Continue reading the Original PF Insights Post.
  43. T

    Collisions between 3 particles (perfectly elastic)

    Homework Statement Three perfectly elastic particles A, B, C with masses 4 kg, 2 kg, 3 kg respectively, lie at rest in a straight line on a smooth horizontal table. Particle A is projected towards B with speed 15 m/s and after A has collided with B, B collides with C. Find the velocities of the...
  44. Auburn2017

    What are the forces acting on a car on a slope at an angle of θ=33.2°?

    Homework Statement Please refer to both figures. The result of the example problem was θ=33.2° Homework Equations ΣFn=man The Attempt at a Solution I am stumped. I thought you could solve the forces in the z direction, straight up from the car, and set that to zero since there is no...
  45. Quantum of Solace

    I Delayed choice, pseudo-retrocausality, massive particles

    Double-slit experiments have been done to death, Obtaining and preserving which-path information (or otherwise) for a particle or photon causes its entangled partner to show the probability distribution with or without the interference pattern, violating Bell's inequality and thus proving...
  46. Auburn2017

    What is the solution to the Kinematics of Particles Homework?

    Homework Statement Refer to figure please. Homework Equations ∑Fy=0 before string is cut The Attempt at a Solution I tried summing the two forces in the y direction before the strings were cut which would be the two tensions at a sin40° and minus the weight of the ball(mg). That gave me T in...
  47. A

    I Clarify interaction between real and virtual particles....

    Hi all, I've read a number of posts on virtual particles (including one of my own), but I'm still confused about one aspect - the interaction between real and virtual particles. Say we have an electron moving from position A to position B. As the electron moves, there should be virtual...
  48. F

    B Are photons particles or waves?

    I've heard of photons being described as a wave/particle duality. But what evidence is there that individual photons behave like anything other than a particle? I can see how photons en masse can display wave/particle characteristics, but what evidence is there that any individual photon...
  49. B

    I Why do particles move along longest proper time trajectories

    Hi, I am working my way thought Hartle's Gravity. In Section 5.4 he states that "The straight lines along which free particles move in spacetime are paths of longest proper time" and proceeds to proof that "in flat space time the proper time is a curve of extremal proper time". Can someone...
  50. DuckAmuck

    I Black Holes and Charged Particles

    What happens when charged particles fall into a black hole? Say like N electrons fall in, giving the black hole a net charge of -N. Since light cannot escape the event horizon, I imagine electric fields cannot either, since they are mediated by photons. So is that charge effectively lost until...
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