# All Questions

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### How does Einstein got equivalent mass - energy equation E=mc2? [duplicate]

How does Einstein got equivalent mass- energy equation E=mc2 ?
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### Compact manifold taken as an Einstein Manifold

In Kaluza-Klein theories I often see that the compact space is assumed to be an Einstein manifold, that is, its Ricci tensor is proportional to its metric. So, why is this done?
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### If Space is continuous, why can't we go below Planck Length?

I am not talking about any other attributes of particles, vacuum etc ruling out Uncertainty Principle thing. If talking about pure Space (which is continuous, not discrete, cf. e.g. this Phys.SE ...
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### Compressibility of solids (quadratic approximation)

(according to Steven H. Simon's "The Oxford Solid State Basics") A well-known approach to describe a one-dimensional chain of atoms is to approximate the potential of each of the atoms quadratically: ...
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### Is entropy of all systems zero at absolute zero?

Is the entropy of every system zero at the absolute zero? Or is it taken to be zero at the absolute zero? Are there systems that doesn't reach zero entropy even till absolute zero?
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### What is the relationship between the Schwarzschild radius and Black hole Singularity?

What is the relationship between the Schwarzschild radius and Black hole Singularity? Can the Planck length be the length of singularity? Or is the length of the Schwartzschild shorter than the ...
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### Consider a horizontal surface with or without friction. Ideally, will a wheel rolling without slipping roll forever in both cases?

Suppose a wheel is rolling smoothly on a horizontal plane i.e., it is rolling without slipping. Now let's take the two cases of the horizontal plane: It has friction It is frictionless In the ...
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### Understanding vectors in an example

I just read an example of vectors in my book which is confusing me. Three particles A,B and C are at the vertices of an equilateral trinagle ABC. Each of the particle moves with constant speed v. ...
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### Gravitational wave, Planck length and Schwarzschild radius

I have heard that Schwarzschild radius corresponds to the Planck length when the radius gain the Planck energy. Is the gravitational wave length (amplitude) equal to the Planck length?
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### Why there are no uuu and ddd baryons with spin 1/2?

What is preventing $Δ^{++}$ and $Δ^-$ baryons from going to a lower-energy state with spin 1/2 similar to that of protons and neutrons? I don't think the Pauli exclusion principle can prevent it ...
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### What's a geometric explanation for exponential-falloff fundamental forces?

Gravity and electromagnetism are inverse-square laws. This makes geometric sense -- if you build a spherical shell around a lamp then a shell with twice the radius has four times the surface area and ...