Philip Gibbs - inactive
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Dirt energy, possible?
2 votes

The laws of physics that makes it difficult to convert dirt into pure energy are the conservation of lepton and baryon number. Each proton or neutron has a baryon number of one and a lepton number of ...

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How can I prevent my son building up static on his trampoline?
2 votes

The charge is produced by rubbing between the socks (or feet) and the bed of the trampoline, and by cloths rubbing on skin as he jumps. This is called the triboelectric effect. Grounding the frame of ...

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Surface tension of N non-mixing fluids
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2 votes

This is a very complex problem to solve so you will probably want to start with some simplifying assumptions such as N=2 to make it more tractable. You will be looking for a minimum energy solution ...

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Does the stress-energy tensor contain the equations of motion?
2 votes

Simple answer, your equation has only four components, but there is one equation of motion for every field variable in the system which is more than four, so the answer is no.

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Frame dragging around a black hole
2 votes

If a black hole is orbited by even just one other black hole, the event horizon bulges out towards it and increases in area. If a spaceship is inside an event horizon it cannot escape no matter what ...

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Is it outside science to figure out the precise string compactification of our universe?
2 votes

There is an real life analogy that provides a nice answer to this question In 1999 Lord Monckton marketed a toy called the "Eternity Puzzle". It was a tiling problem with 209 pieces. He thought it ...

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Does the discreteness of spacetime in canonical approaches imply good bye to STR?
2 votes

Loop Quantum Gravity is an attempt to quantise four dimensional general relativity directly by canonical quantisation methods. If it succeeded it would have to be able to recover the original curved ...

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Is it physically meaningful to talk about the arrow of time in other universes?
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The possibilities depends on how diverse the alternative universe can be. Some alternatives may have two or more macroscopic time dimensions, or they may be just euclidean spaces with no time ...

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The contribution to mass from the dynamical breaking of chiral symmetry
2 votes

The extra mass of the proton and neutron is not due to chiral symmetry breaking. It is due to the energy in the electromagnetic and strong force fields. If chiral symmetry were an exact symmetry of ...

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Is the law of conservation of energy still valid?
1 votes

Energy has been shown to be conserved in all circumstances where it is currently possible to test it experimentally. It is also conserved according to theory in any system with time translation ...

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Can a spacetime solution in GR have no Killing vector fields?
1 votes

The existance of Killing fields even just for a small region is a special property of the metric. For general metrics you cannot expect to find Killing vectors. Notice that the Killing equation which ...

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Was the Big Bang actually cold?
1 votes

In the standard homogeneous cosmological models the total energy in an expanding volume is zero. This is true for positive, negative or zero curvature and it must take into account the gravitational ...

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How is dark energy consistent with conservation of mass and energy?
1 votes

"I presumed the source of this energy was not coming from the conversion of other types of energy to dark energy, so it must violate conservation." This is where you go wrong. The positive dark ...

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How early might a moon-sized asteroid be detected?
1 votes

To give an indication, comet ISON was seen first by two amateur astronomers when it was at magnitude 18.8, so a large body approaching from a distance might be found at about the same magnitude. ...

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Question about simple permutation of covariant derivatives
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1 votes

$[[D_{\mu}, D_{\nu}],D_{\lambda}]A^{\rho} = [D_{\mu}, D_{\nu}]D_{\lambda}A^{\rho}-D_{\lambda}[D_{\mu}, D_{\nu}]A^{\rho}$ $=-R^{\tau}_{{\lambda}\mu \nu}D_{\tau}A^{\rho}+R^{\rho}_{\sigma \mu \nu}D_{\...

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Weighing a flying bird
1 votes

The weight will be the same on average over time. The bird is supported by the air which in turn is supported by the box. To every action there is an equal and opposite reaction so the force ...

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Weight of a tensor density
1 votes

The weight would be the power of the Jacobian used in the transformation law. For an ordinary tensor the weight is then zero and for a tensor density it is one. In principle you can define quantities ...

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Is "dark clothes for winter, light for summer" relevant?
1 votes

For most circumstances this is a myth and the opposite is more true. If you are standing in direct sunlight it would best best to wear white clothes to reflect the heat. The direct radiation from the ...

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Conservation of momentum
1 votes

The term $\lambda\vec e$ added to each argument in the potential is just a translation under which the potential is known to be invariant. So the derivative must be zero. But if you use the chain ...

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Conservation of energy
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1 votes

It is true that by Noether's theorem energy is conserved when the action is depednent on position only and not time. However, I think they want you to write down the equation of motion (use $F=ma$) ...

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Can a diver swim a short distance in great depths without being physically crushed by the pressure?
1 votes

The compressive effect on the body itself is not a problem because we are composed of incompressible fluids and solids. All gas filled cavities in a healthy person are connected by tubes to the ...

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Have red shifted photons lost energy and where did it go?
1 votes

The short answer is "yes". The energy lost from the photons is taken up by the energy in the gravitational field. Of course energy is a relative concept but if you take the simplest case of a ...

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Is the total energy of the universe constant?
1 votes

The only thing that prevents us defining a total conserved energy for the entire universe is that if the universe is infinite then the total energy could be infinite or indeterminate. The statements ...

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How do swimfins work?
1 votes

Bifins work in a way similar to a fishes tail, with one essential differences. A fishes tail propels the fish equally when moving in either direction, whereas bifins are designed to give a push mainly ...

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How to concile flat spacetime and big bang?
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1 votes

Your reasoning contains some errors and unjustified assumptions. The first error is to think that the universe started out with a size of about the Plank length. This may not be the case. If it is ...

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Would a person not be able to distinguish left from right in outer space?
1 votes

Left and Right can be used in different contexts. In most cases they work equally well in space but not quite always. Often when people talk about left or right they mean something relative to a ...

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What are the prerequisites to studying general relativity?
1 votes

Special Relativity is the first prerequisites, obviously. That should include the necessary linear algebra, group theory and classical field theory, because GR is itself a field theory. After that ...

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Conservation law of energy and Big Bang?
1 votes

Energy is indeed conserved perfectly well in General Relativity, even in the extreme conditions of the early big bang. It is not merely true by definition, it is true as a result of the dynamics of ...

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Can gravitational potential energy be released in a fire?
1 votes

The question is about the effects of gravity on energy and can only be answered properly in the context of general relativity. The answer is that more energy is released when the wood is burnt on the ...

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Size of our inflationary bubble
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1 votes

The answer depends on how long inflation lasted, how rapidly it expanded, and how big the bubble was to start with. These parameters are obviously model dependent and there are many models none of ...

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