Philip Gibbs - inactive
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What is the actual significance of the amplituhedron?
70 votes

There was a presentation of the idea at SUSY2013 by Nima Arkani-Hamed which is available on video at http://susy2013.ictp.it/video/05_Friday/2013_08_30_Arkani-Hamed_4-3.html The amplituhedron is a ...

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Why is the sky not purple?
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60 votes

This is from the Physics FAQ article that I wrote 15 years ago: If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with ...

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Is this pseudo science or real: code found in superstring
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47 votes

The work being described is by Prof S. James Gates and it has a serious basis. He has noted that the supersymmetric equations of string theory contain some binary codes built in. These are the same as ...

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Why is Graphene Transparent?
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22 votes

Graphene is only transparent because it is very thin (one atom thick). If it absorbs 2% per layer then just a few hundred layers would absorb almost all light and that would still be a very thin sheet ...

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Does the scientific community consider the Loschmidt paradox resolved? If so what is the resolution?
22 votes

Loschmidt's paradox is that the laws of thermodynamics are time asymmetric because entropy always increases, but the underlying laws of physics are symmetric under time reversal. It should not ...

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What experiment would disprove string theory?
22 votes

It is very hard to probe the scales of quantum gravity experimentally so there are very few possibilities for testing if string theory is incorrect as a unified theory encompassing quantum gravity. ...

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Was the universe a black hole at the beginning?
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21 votes

Here is a copy of an answer I wrote some time ago for the Physics FAQ http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html Is the Big Bang a black hole? This question can be ...

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Do we move at the speed of light relative to light?
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16 votes

When we say that object A is moving at speed v relative to an object B it means that there is a reference from where B is at rest and A is moving at speed v in that reference frame. If A is a photon ...

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Is the total energy of the universe zero?
13 votes

The claim that the total energy in the universe is zero can be rigorously justified. To answer your specific questions: General Relativity is required. It does not apply for Newtonian gravity. It ...

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What would actually happen to a person jettisoned into space?
13 votes

Most of what is shown in sci-fi on this is believed to be false. You do not freeze to death and your blood does not boil. Provided you do not try to hold your breath there are not likely to any ill ...

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Could dark matter be a kind of Goldstone boson?
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13 votes

A Goldstone boson is a generic type of particle formed when symmetries are spontaneously broken. If you want to suggest that dark matter is a Goldstone boson then that says very little unless you ...

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How do ice spikes form?
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12 votes

I think what is happening in rough qualitative terms is that the water freezes around the sides and the top first leaving a hole in the centre. Ice expands by 4%-9% when freezing so as the water below ...

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Newton's cradle
12 votes

It conserves both energy and momentum in the collision at the same time. By design, when the balls collide the strings that hold them up are vertical (assuming balls are only swung from one side). ...

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Why is there no absolute maximum temperature?
11 votes

If there is a maximum possible physical temperature it is well above anything we can reach experimentally and would require a complete theory of quantum gravity to understand it fully. Neutron stars ...

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The age of the universe
11 votes

You can define the age of the universe roughly as the proper time for a hypothetical observer who is comoving with the galaxies and not too near a strongly gravitating object. This is imprecise ...

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Is nature symmetric between particles and antiparticles?
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11 votes

The anti-particle for any particle is obtained by charge C and parity P conjugation. C is the operation that interchanges positive and negative charges and P is the operation that reflects in a mirror....

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Why can't echoes be heard inside a room?
10 votes

Sound does echo inside a room but you might not notice it much for two reasons. Firstly the time for the echo to return is very small so you will not hear a long sound repeated as an echo, instead ...

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What is the symmetry which is responsible for preservation/conservation of electrical charges?
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10 votes

Remember that voltage is always expressed as a "potential difference." You can't measure the absolute value of voltage because everything is invariant when you add a constant voltage everywhere. That ...

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What would happen if you put your hand in front of the 7 TeV beam at LHC?
10 votes

Each bunch of protons circulating in the LHC at full power will have about 120 billion protons of 14TeV each, i.e. $1.6 \times 10^{24}$ eV which is about 250000 Joules. Compare that with a high ...

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Energy conservation in General Relativity
10 votes

Energy conservation does work perfectly in general relativity. The overall Lagrangian is invariant under time translations and Noether's Theorem can be used to derive a non-trivial and exact conserved ...

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Noether charge of local symmetries
9 votes

First I will say something about the paranthetical point at the end of the question. A local symmetry is defined as invariance under transformations that are parameterised by field variables, i.e. ...

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Conservation of Energy vs Expansion of Space
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9 votes

Dark energy is a form energy that appears to have a constant density in space even as the universe expands. The simplest model for this is a cosmological constant in the gravitational field equations. ...

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Why does no physical energy-momentum tensor exist for the gravitational field?
9 votes

It is true that no energy-momentum tensor for the gravitational field exists, however, it is easy to understand why not and then derive a perfectly correct formulation for conservation of energy and ...

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What made the keys in my pocket so hot?
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9 votes

Mobile phones transmit at microwave frequencies so they can induce currents in metals and other conductors. The energy dissipates as heat. The principle is the same used by microwave ovens. The phone ...

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Can a planet cover the whole sky as seen from its satellite
9 votes

If the planet and satellite are perfect spheres, or any convex shape then the answer is NO. If the planet is close to the satellite and you stand on the point nearest to it then you will only be able ...

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Are the second and third generations of matter required or optional?
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8 votes

This question can be split into two parts: How do three generations affect the way the universe runs within the standard model, and how do they affect beyond standard model physics? It is likely that ...

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What is known on violations of unitarity or locality?
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8 votes

The authors of the work on the amplituhedron (Trnka and Arkani-Hamed) have said that locality and unitarity are "emergent" in this formalism for N=4 Super Yang Mills. The way that locality and unitary ...

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Are matrices and second rank tensors the same thing?
8 votes

Strictly speaking matrices and rank 2 tensors are not quite the same thing, but there is a close correspondence that works for most practical purposes that physicists encounter. A matrix is a two ...

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Can one submit/publish a paper between postdocs?
8 votes

There is no reason why you should not publish papers between postdocs. Journals do not require authors to have affiliations. However, not having an affiliation may mean that your work is given more ...

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What is the difference between "kinematics" and "dynamics"?
8 votes

Kinematics is about the range of movement or change a system can undergo, or the state space in which it acts. Dynamics is about the movement it undergoes according to the laws of motion. For ...

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