# Tagged Questions

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### 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 ...
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### 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 ...
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### Why the action of a relativistic point particle is considered to be negative? [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) Is there a ...
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### If particles are points, then aren't atoms empty space?

Zero dimensional points do not take up space, so then wouldn't everything in the universe be literally empty? Or is there something that I'm missing?
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### What is the meaning of the size of an elementary particle in QFT? What is the meaning of a point particle? [duplicate]

I have often seen people refer to the size of a particle being at most a given value, or a particle being a point particle, in the context of quantum field theory. Examples are the Wikipedia entry on ...
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### Why do physicists say that elementary particles are point particles?

For example, an electron, it has mass and charge, but is considered to have point mass and point charge, but why? Why are they assumed to have charge and mass in a single infinitely small point in ...
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### Why don't people use Hamilton's equations for a relativistic free charged particle?

A charged relativistic free particle has the Hamiltonian in general: $$\mathcal{H} = \sqrt{p^2c^2+m^2c^4}.$$ I read somewhere that says, it is possible to go further and say that the EoM are ...
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### Is the self energy divergence problem of point charge resolved in the context of general relativity?

The point charge model of electron became problematic in the context of electrodynamics/special relativity, because if we calculate the mass/energy of the electric field, it becomes divergent in the ...
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### Is a blackhole just a neutrino with much more mass?

If there was a blackhole that had a mass similar to that of a neutrino ($0.320 ± 0.081\,\mathrm{eV/c^2}$), would we still be able to differentiate the blackhole from the neutrino? Is there any ...
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### Is it possible for an object to have mass but zero volume?

Can there exist a particle/object in the universe having mass but no volume? Is it possible that mass can exist without volume and density? We think we know that matter is anything having mass and ...
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### Evidence electrons are fundamental particles [duplicate]

Is there any evidence that entities like electrons and quarks are indivisible, i.e. not composed of other, smaller entities? And have I just made a category error by even asking this question?
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### Is there any evidence to suggest that subatomic particles have 3D physiscal size? [duplicate]

What I'm asking is do subatomic particles like a proton have a volume or any 3 dimensional size? Or are they just points?
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### Parameterisation of the equation of motion for a relativistic massive point particle

The equation of motion for a relativistic massive point particle is given by: $$\frac{dp_{\mu}}{d \tau} = 0,$$ where $p_{\mu}$ is the four-momentum defined by $p_{\mu} = m \frac{dx_{\mu}}{ds/c}$, ...
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### What really is a particle?

In Classical Mechanics we consider particles as things whose internal structure for the purpose of studying some phenomenon might be neglected. In that setting we associate particles to points and ...
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### How to prove or disprove that elementary particle has no spatial extention?

We are told that elementary particles has dimension zero and take up no space. For example, the electron is a point particle that have a negative unit charge, also has mass and spin, but no size. My ...
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### What is the smallest observable structure in the universe?

I've been wondering about the Planck length recently, but it is not observable. What is the smallest actually observable structure in the universe?
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### Definition of the Lagrangian for a relativistic point particle in curved space

I have read that the Lagrangian in GR is defined as $L=\frac{\mathrm{d}s}{\mathrm{d}u}$, where $\mathrm{d}s = g_{ab}\mathrm{d}x^a\mathrm{d}x^b$ is the line element with the metric tensor $g_ab$ and $u$...
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