Questions tagged [point-particles]

The tag has no usage guidance.

Filter by
Sorted by
Tagged with
3
votes
2answers
190 views

Geodesic equations from action with auxiliary field

A textbook says that the geodesic equations (for both massive and massless) can be derived from the following action: $$ S = -\frac{1}{2} \int d\tau \:\eta \: (\eta^{-2} \dot{x}^\mu \dot{x}^\nu g_{\...
0
votes
1answer
62 views

What was Newton's view of theory of matter? [closed]

Did Newton believe in infinitely small particle theory of matter? Because he talks about axis of rotation, which is locus of the centers of the circles of the rotating body and my teacher said ...
3
votes
2answers
284 views

Doesn't the fact that elementary particles are not black holes prove they are not point structures? [duplicate]

De Schwarzschild radius of a mass $m$ is defined as $$r_s=\frac{2mG}{c^2}(m).$$ So if we insert in this formula the mass of an electron (a point particle, according to mainstream physics), which ...
1
vote
3answers
78 views

How does relativity dimensional contraction affect point like particles such as the electron and neutrino?

I might be misunderstanding a basic concept here, so forgive me. I know that the faster an object gets, the more it's dimensions will contract according to the following equation: $${1\over D} = 1-{V^...
3
votes
1answer
71 views

Relativistic action is a constant?

Say that you want to find the equations of motion of a free relativistic massive point particle by minimizing the action $$S=-m\int\mathrm{d}\tau\,\sqrt{\eta_{\mu\nu}\frac{\mathrm{d}x^\mu}{\mathrm{d}\...
1
vote
2answers
294 views

How can 0-dimensional particles or 1-dimensional strings be 3D matter? [closed]

According to the latest information we got String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings....
1
vote
1answer
426 views

2 bodies on an inclined plane, we need to find the force one exerts onto the other as they both slide down

So I've been trying to tackle this for the last few hours, but no dice. (exam practice, by the way) We got two point masses $m_1$ and $m_2$ lying tangent to each other on top of an inclined plane ...
29
votes
2answers
2k views

Can the center of charge and center of mass of an electron differ in quantum mechanics?

Traditionally for a free electron, we presume the expectation of its location (place of the center of mass) and the center of charge at the same place. Although this seemed to be reasonable for a ...
1
vote
1answer
180 views

Is continuum mechanics a generalization or an approximation to point particle mechanics?

Newtonian Mechanics is usually presented as a theory of point particles (and forces). My impression of the status of continuum mechanics is that it is mostly taken as an approximate description for ...
-3
votes
2answers
78 views

Can somebody explain why the action in the picture is true?

I can provide the resource for where this is from. Can somebody explain how to get this expression?
1
vote
2answers
71 views

Does the propagation of light through spacetime, with regards to how we describe it, depend on when a photon is a wave function or point particle?

When for example, a photon is emitted from an atom, does that photon propagate through spacetime in all directions away from the atom in the form of a sphere (Wave Function) and then at some point ...
1
vote
0answers
96 views

Integrating Out Auxiliary Field of point-particle Polyakov Action

The Polyakov action of a point-particle is $$S[X,e]=\frac{1}{2}\int d\tau\left(\frac{\dot{X}^{2}}{e}-m^{2}e\right)$$ with the $(−,+,+,+)$ Minkowski sign convention. How to perform the path-integral ...
1
vote
3answers
539 views

Electric field from disc versus point charge

A uniformly charged disc of radius R and net charge Q with an x-axis through the center of the disc will have an electric field in a point $x_0$ on the x-axis $E=kQ(1-\dfrac{x_0}{\sqrt{x_0^2+R^2}})$ ...
1
vote
1answer
379 views

When and why are we allowed to treat a rigid body as a point mass?

When the subject Mechanics first taught, it is common that we explicitly state that the Newton's laws are valid only for point masses, and then we give examples of rigid bodies colliding with each ...
1
vote
1answer
125 views

From non-relativistic to relativistic action

There is a derivation of relativistic action that treats space and time symetricaly which is just playing arround with the square of kinetic energy in the non-relativistic action and plugging in speed ...
2
votes
1answer
271 views

Point particle with a magnetic dipole?

I have read these questions: Are contravariant basis vectors and basis 1-forms identical? Where John Rennie's answer says that electrons do have an electric dipole moment and we imagine that in math ...
2
votes
3answers
234 views

What are point objects?

I can't seem to get the idea of point mass into my head. Why are equations of physics applicable on only point masses and should be altered while dealing with object that has a collection of points? ...
0
votes
0answers
110 views

Energy of single particle is equal to energy of multiparticle system it consists of?

The solution to a problem of atom fission led me to this question. In this problem the mass of original atom nucleus, masses of two child atom nuclei as well as zero kinetic energy of first atom ...
0
votes
1answer
35 views

Real points vs. Physical points and what replaces them

Quantum field theory and relativity share the need for point particles (besides we have learned how to deal with extended objects with more or less success). Heisenberg uncertainty principle ...
2
votes
1answer
99 views

A question about the Lagrangian for a massive or massless point particle

In my lecture I learned that the action that can be applied to the light ray is written like below: \begin{equation*} S[x;e]=(1/2)\int [(1/e)g_{ab}\dot x^a\dot x^b-m^2e]ds \tag{1} \end{equation*} ...
4
votes
2answers
310 views

Doubt with Relativistic Lagrangian Expression

I'm trying to learn by myself some Lagrangian mechanics, and I found on Wikipedia this relativistic Lagrangian $$L=\frac{1}{2}mu_{\alpha}u^{\alpha}, \tag{1}$$ where $$u^\alpha=\frac{dx^{\alpha}}{d\...
1
vote
1answer
356 views

Lagrangian formulation of free massive point particle in special relativity

I wonder if there is a way to reproduce the 4-force generalization for Newton's equation for a free particle i.e. $$ m\frac{d^2x^\mu}{d \tau^2} = 0, \qquad \text{ for } \, \mu =0,1,2,3, \tag{1} $$ ...
0
votes
1answer
45 views

Spin of 2 oppositely charged fused point charges

If i fused 2 point particles (with opposite charges) together, would it be possible to change the magnitude of this object's spin by applying a torque on it? Also, if they are joined together, wouldn'...
0
votes
1answer
226 views

Does Radar detect objects or has point cloud

Many times in literature I read that Radar is used to detect objects. How does this work? Are reflections from objects with Radar not point reflections? And how does Radar know that all reflections ...
1
vote
1answer
2k views

Lagrangian of a relativistic free massive particle

Lagrangian for a relativistic free particle can be written as $$L=-m_0c^2\sqrt{1-\frac{v^2}{c^2}} .\tag{1}$$ It gives correct expression of Hamiltonian which is $$H=\sqrt{p^2 c^2+m_0^2c^4}.\tag{2}$$ ...
-1
votes
2answers
1k views

Are quarks/electrons different sizes (physically/in terms of mass)? [duplicate]

i.e. is an electron bigger/smaller than a strange quark? Is a strange quark bigger/smaller than an up quark? Or are they all the same size? Leading on, are electrons the same size (as in physical ...
-1
votes
1answer
63 views

Meaning of 'Particle of Mass'

The following quote is from Section II at http://suppes-corpus.stanford.edu/articles/physics/431.pdf A simple pendulum consists of a particle of mass $m$ hanging from a... My understanding is ...
-3
votes
1answer
270 views

What is the smallest particle? What is the building block of an electron? [duplicate]

I'm curious. Why when I was 5 years old did I rip things apart in half... Always trying to get another half... is it endless? I took a leaf and ripped it in half, and kept on going... God?
2
votes
3answers
236 views

About fermionic particles and bosonic particles in RQFT, ST [closed]

In relativistic quantum field theory, we can observe that the Dirac equation is a square root of Klein-Gordon equation. But, we can get the Dirac equation by defining Dirac spinor as $(1/2,0)\oplus(0,...
1
vote
1answer
129 views

What is the induced electromagnetic field of a point charge?

If I move a point charge on some trajectory, then it will produce an electric field as well as a magnetic field. As the charge is moving, and as a point charge can not produce a steady current, then ...
5
votes
5answers
991 views

Do point charge really exist?

Do point charge really exist experimentally? Am I right with definition of point charge? According to me point charge is a charge having 0 (zero) mass and have 0 (zero) volume.
7
votes
4answers
1k views

How to find a particle's dynamics in general relativity?

About a year ago, I took a course on general relativity. It isn't until now that I realize that, given a metric, I am unsure how to find a particle's dynamics. What I mean by that is, normally I ...
1
vote
1answer
704 views

With point particles being smaller than their Schwarzschild radius are they forever cloaked in a black hole?

My understanding of point particles is that they have mass and are dimensionless. If this is so then wouldn't the mass cause the particle to have a Schwarzschild radius which would then make the ...
0
votes
2answers
105 views

Coordinates for the Thomson problem

Is there a resource that lists the coordinates for known solutions to the Thomson problem? Wikipedia lists the energies, and I'm having trouble solving for $N=78$. The closest I can get is ...
0
votes
5answers
6k views

Direction of the electric field of a negative point charge?

How can the direction of the electric field of a negative point charge be going in towards itself (radially), when the charge itself is generating the electric field? Is it generated at infinity then? ...
1
vote
3answers
201 views

Why do we invent non-physical concepts (like e.g. point particles) to study physical phenomenons?

There is nothing exist like point particles in reality then why did we invented the notion of point particles and how does it relate to real world?
5
votes
2answers
247 views

Why does it make sense to talk about the first-quantized BRST formulation of a relativistic point particle?

My question is about the BRST quantization of a point particle in Polchinski, Vol.1, Section 4.2. The BRST quantization starts from the effective action for the gauge-fixed path-integral. But for the ...
2
votes
1answer
217 views

Finding a 'vector potential' such that $\mathbf E = \nabla\times \mathbf C$ for a point charge

Supposedly, "Any divergence-free vector field can be expressed as the curl of some other divergence-free vector field" over a simply-connected domain. So, what is one such vector potential which ...
0
votes
2answers
706 views

How to calculate static electric field produced by multiple point charges at a point (simple…)

[I've seen several related questions but I ask for a confirmation here.] I have a set of point charges to model atoms, say: $q_1$ at $(x_1,y_1,z_1)$ $q_2$ at $(x_2,y_2,z_2)$ ... $q_n$ at $(x_n,...
2
votes
0answers
62 views

Motion of a catapulted point mass moving freely inside a spherical bucket

I am trying to study the motion of a point mass inside the bucket of a catapult. The catapult is shooting downward (i.e. describing a rotation of 180° from the horizontal axis) and I would like to ...
0
votes
1answer
75 views

Moving point particles in quantum mechanics

Given a quantum system with a hamiltonian $H$, if initial state of a physical object is $|\psi\rangle $, then state after time $dt$ is $e^{iH\cdot dt}|\psi\rangle$. It is easy to see that $|\langle\...
1
vote
2answers
980 views

Force on point mass vs extended mass

Can anyone help me put some equations on this problem: Some point mass A (mass mA) produces a gravitational field. Compare the force exerted on a point mass B (mass mB) to the force that would be ...
2
votes
0answers
300 views

Idealizations of an object as a point particle

Why is it that an object can be idealized as a point particle or 'particle like' to solve problems? What are the limits of such a tool?
14
votes
3answers
653 views

How do we prove or disprove that a particle has no internal structure? [duplicate]

In many pop physics books I have read that an electron has no internal structure. How do we know that and how can we rigorously prove that it has no such structure at all?
1
vote
1answer
2k views

Volume charge density of an electric dipole

My attempt: $$\rho(\overrightarrow r) = Q\delta^3 (\overrightarrow r)$$ $\overrightarrow r =$ separation vector from the origin to $\overrightarrow a$ $Q$ = the total charge of the electric dipole ...
0
votes
1answer
581 views

Volume charge density of a single point

The original problem: I read this one, but there's something that is still unclear, even after going through all their answers and comments. Why doesn't the expression use the 3-dimensional Dirac ...
27
votes
5answers
3k views

Where is the evidence that the electron is pointlike?

I'm writing a piece about the electron, and I'm having trouble finding evidence to back up the claim that the evidence is pointlike. People tend to say the observation of a single electron in a ...
9
votes
2answers
512 views

What does a QFT particle state have to do with a classical point particle?

In the question Can one define a “particle” as space-localized object in quantum field theory? it is said that in quantum field theory, a particle state is a state with well defined energy and ...
4
votes
2answers
1k views

Validity of point mass approximation in Newton's laws of motion

Chapter 2 of Kleppner & Kolenkow's An Introduction to Mechanics reads as follows - Newton's Laws describe the behavior of point masses. In the case where the size of the body is small compared ...
0
votes
1answer
138 views

What is the intuitive concept of the action of a relativistic point particle? [duplicate]

The action of a relativistic point particle is its negative rest energy along its worldline, the parameter being its own proper time. $$ S = - mc^2 \int d\tau $$ (see Wikipedia) Action is energy ...