Questions tagged [point-particles]
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3
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1answer
109 views
Two Questions about Path Integral from “Gauge Fields and Strings” by Polyakov
My questions are about worldline path integrals from the book Gauge Fields and Strings of Polyakov. On page 153, chapter 9, he says
Let us begin with the following path integral
\begin{align}
&...
0
votes
2answers
59 views
When would an object not be considered a point-particle in relation to gravity?
When is the Earth, for example, not considered a point in relation to gravity?
I am thinking that if one was asking a question below the crust continously to the core, then this would be an example.
...
1
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0answers
32 views
Shape of electron [duplicate]
Today, in the BBC Science section, a headline reads that the Imperial College of London has determined that the shape of the electron is completely spherical. In a physics book I'm reading now, the ...
-3
votes
1answer
61 views
When/Why Did Physics Discard The Point Charge As An Accurate Representation? [closed]
Is there anything about the implications of the "early" quantum theory of Schrodinger equation, wave-particle duality, or the two slit experiment that conflicts with the idea of a point charge? Did ...
0
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2answers
56 views
Can an electron pass through a gap smaller than an electron?
If an electron (photon) can pass through a gap smaller than an electron (photon) then it is a wave otherwise it it a particle. Is this a correct way of reasoning?
3
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2answers
58 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:
$$
S = -\frac{1}{2} \int d\tau \:\eta \: (\eta^{-2} \dot{x}^\mu \dot{x}^\nu g_{\...
0
votes
1answer
61 views
What was Newton's view of theory of matter? [closed]
Did Newton believe in infinitely small particle theory of matter? Because he talks about axis of rotation, which is locus of the centers of the circles of the rotating body
and my teacher said ...
4
votes
2answers
160 views
Doesn't the fact that elementary particles are not black holes prove they are not point structures?
De Schwarzschild radius of a mass $m$ is defined as
$$r_s=\frac{2mG}{c^2}(\frac m {kg}).$$
So if we insert in this formula the mass of an electron (a point particle, according to mainstream ...
1
vote
3answers
53 views
How does relativity dimensional contraction affect point like particles such as the electron and neutrino?
I might be misunderstanding a basic concept here, so forgive me. I know that the faster an object gets, the more it's dimensions will contract according to the following equation:
$${1\over D} = 1-{V^...
3
votes
1answer
51 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
95 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
107 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 ...
28
votes
3answers
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 ...
0
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0answers
41 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 ...
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2answers
72 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
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2answers
67 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
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0answers
44 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 ...
0
votes
3answers
129 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}})$
...
0
votes
1answer
52 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
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1answer
61 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 ...
1
vote
1answer
77 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 ...
1
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3answers
78 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
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0answers
63 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
31 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
64 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*}
...
1
vote
1answer
180 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}
$$
...
0
votes
1answer
42 views
Spin of 2 oppositely charged fused point charges
If i fused 2 point particles (with opposite charges) together, would it be possible to change the magnitude of this object's spin by applying a torque on it?
Also, if they are joined together, wouldn'...
0
votes
1answer
157 views
Does Radar detect objects or has point cloud
Many times in literature I read that Radar is used to detect objects. How does this work? Are reflections from objects with Radar not point reflections? And how does Radar know that all reflections ...
1
vote
1answer
682 views
Lagrangian of a relativistic free massive particle
Lagrangian for a relativistic free particle can be written as
$$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
455 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 ...
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1answer
57 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
138 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
182 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
75 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 ...
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5answers
595 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.
6
votes
4answers
777 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
228 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
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2answers
65 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
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5answers
4k 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
174 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
1answer
139 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
187 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
377 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
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0answers
49 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
73 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\...
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vote
2answers
683 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
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0answers
246 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?
13
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3answers
612 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?
0
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0answers
16 views
Uniform circular force in Frenet frame
For a while I am curious about the cycloid curve and last day I noticed that the net force applied on a point mass travelling through the cycloid under constant gravity is a uniform circular force in ...
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
...
