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Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding the [energy-conservation] tag if it is specifically about its conservation.

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1 answer
55 views

Can we see the rest-energy of a mass as its kinetic energy in the $ct$-direction?

A mass $m$ at rest has an energy $E=mc^2$. Can we say this is its kinetic energy in the $ct$-direction? …
Il Guercio's user avatar
2 votes
1 answer
64 views

How fast do the neutrinos in the neutrino background radiation move through the universe?

The expansion of space drains energy from particles. That's why the CMBR contains less energy nowadays than in the early stages of the universe. …
Il Guercio's user avatar
0 votes
0 answers
68 views

How fast must an incompressible ball, with mass $M$ and radius $R$, rotate to form a black h...

Will this happen when the kinetic energy of the mass equals the mass to form a black hole with radius $R$, which then is $R_s=\frac{2GM}{c^2}$? … What is the total kinetic energy contained in the sphere? …
Il Guercio's user avatar
1 vote
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88 views

What means Alan Guth's Free Lunch Principle for the universe?

Does he mean that the total energy of the universe is zero? So the total energy of all particles is the negative of the total gravitational energy of these particles? … If so, does this explain that the energy of the photon background radiation can be diminished by the expansion of the universe without violating energy conservation? …
Il Guercio's user avatar