Recent speculations

A more recently proposed view of black holes might be interpreted as shedding some light on the nature of classical white holes. Some researchers proposed that when a black hole forms, a big bang occurs at the core, which creates a new universe that expands into extra dimensions outside of the parent universe. See also Fecund universes.

The initial feeding of matter from the parent universe’s black hole and the expansion that follows in the new universe might be thought of as a cosmological type of white hole. Unlike traditional white holes, this type of white hole would not be localized in space in the new universe, and its horizon would have to be identified with the cosmological horizon.

— Wikipedia on White hole

2011.01.14 Friday ACHK

In quantum mechanics, the black hole emits Hawking radiation, and so can come to thermal equilibrium with a gas of radiation. Since a thermal equilibrium state is time reversal invariant, Stephen Hawking argued that the time reverse of a black hole in thermal equilibrium is again a black hole in thermal equilibrium. This implies that black holes and white holes are the same object. The Hawking radiation from an ordinary black hole is then identified with the white hole emission. Hawking’s semi-classical argument is reproduced in a quantum mechanical AdS/CFT treatment, where a black hole in anti-de Sitter space is described by a thermal gas in a gauge theory, whose time reversal is the same as itself.

— Wikipedia on White hole

2011.01.13 Thursday ACHK

How do we know that this assumption is incorrect? Well, classical theories may lead to anomalies that imply that no quantum theory with a given classical limit may exist. More generally, quantization of classical theories often leads to non-renormalizable theories which are no good as a starting point for predictions.

These insights are robust conclusions of a paramount section of theoretical physics, the renormalization group, a theoretical machinery that is important both in high-energy physics as well as condensed-matter physics and other fields.

— Loop quantum cosmology

— Lubos Motl

2011.01.12 Wednesday ACHK

But that would take us too far. The main lesson here is that general relativity is not a theory that requires physical objects or fields to propagate in a pre-existing translationally invariant spacetime. That’s why the corresponding energy conservation law justified by Noether’s argument either fails, or becomes approximate, or becomes vacuous, or survives exclusively in spacetimes that preserve their “special relativistic” structure at infinity. At any rate, the status of energy conservation changes when you switch from special relativity to general relativity.

— Lubos Motl

2011.01.11 Tuesday ACHK

The third way in which the explanation can be misleading is due to the nonlocal nature of a quantum state. Sometimes, two particles can be entangled, and then a distant measurement can be performed on one of the two. This measurement should not disturb the other particle in any classical sense, but it can sometimes reveal information about the distant particle. This restricts the possible values of position or momentum in strange ways.

— Wikipedia on Uncertainty principle

2011.01.07 Friday ACHK 

Uncertainty principle and observer effect

Today, logical positivism has become unfashionable in many cases, so the explanation of the uncertainty principle in terms of observer effect can be misleading. For one, this explanation makes it seem to the non-positivist that the disturbances are not a property of the particle, but a property of the measurement process — the particle secretly does have a definite position and a definite momentum, but the experimental devices we have are not good enough to find out what these are. This interpretation is not compatible with standard quantum mechanics. In quantum mechanics, states which have both definite position and definite momentum at the same time just don’t exist.

This explanation is misleading in another way, because sometimes it is a failure to measure the particle that produces the disturbance. For example, if a perfect photographic film contains a small hole, and an incident photon is not observed, then its momentum becomes uncertain by a large amount. By not observing the photon, we discover indirectly that it went through the hole, revealing the photon’s position.

— Wikipedia on Uncertainty principle

2011.01.06 Thursday ACHK

Today, centuries after the search began for the fundamental constituents that make up all the complexity and beauty of the everyday world, we have an astonishingly simple answer — it takes just six particles: the electron, the up and the down quarks, the gluon, the photon and the Higgs boson.

— Gordon L. Kane, The Dawn of Physics Beyond the Standard Model

2011.01.05 Wednesday ACHK 

Gluons (from English glue and the suffix -on) are elementary expressions of quark interaction, and are indirectly involved with the binding of protons and neutrons together in atomic nuclei through the strong force. The antiparticle of a gluon is another gluon (see Eight gluon colors below).

In technical terms, they are vector gauge bosons that mediate strong color charge interactions of quarks in quantum chromodynamics (QCD). Unlike the electrically neutral photon of quantum electrodynamics (QED), gluons themselves carry color charge and therefore participate in the strong interaction in addition to mediating it, making QCD significantly harder to analyze than QED.

— Wikipedia on Gluon

2011.01.04 Tuesday ACHK

A symmetric explanation is qualitatively different from an asymmetric explanation. It therefore deserves a comparable prior probability.

Because a symmetric theory is more constraining, an agreement with observations adds more units of evidence in favor of its validity than it does to the competing asymmetric theory.

— Lubos Motl

2011.01.03 Monday ACHK

In physics, a covariant transformation is a rule (specified below), that describes how certain physical entities change under a change of coordinate system. In particular the term is used for vectors and tensors. The transformation that describes the new basis vectors in terms of the old basis, is defined as a covariant transformation. Conventionally, indices identifying the basis vectors are placed as lower indices and so are all entities that transform in the same way.

The inverse of the covariant transformation is called the contravariant transformation. In order that a vector should be invariant under a coordinate transformation, the components of a vector must transform according to the contravariant rule. Conventionally, indices identifying the components of a vector are placed as upper indices and so are all indices of entities that transform in the same way. The summation over all indices of a product with the same lower and upper indices are invariant to a transformation.

— Wikipedia on Covariant transformation

2011.01.02 Sunday ACHK

Under certain conditions, the Gibbs canonical ensemble maximizes the von Neumann entropy of the state subject to the energy conservation requirement.

— Wikipedia on Quantum statistical mechanics

2010.12.30 Thursday ACHK

When Andy was a student, his advisor Roman Jackiw would insist that Andy and others had to convert everything to a harmonic oscillator. And you know, field theory showed that this thing can be done to all elementary particles (and also to the Hydrogen atom and other things!). Andy is now what Roman used to be, and he insists that the students transform everything to a black hole. :-)

— Lubos Motl

2010.12.28 Tuesday ACHK

Consequently, as cs increases to c, the absolute speeds v_e and v_a of the emitter and absorber relative to the fixed medium merge into a single relative speed u between the emitter and absorber, independent of any reference to a fixed medium, and we arrive at the relativistic Doppler formula for waves propagating at c for an emitter and absorber with a relative velocity of u:

— 2.4  Doppler Shift for Sound and Light

— Reflections on Relativity

— mathpages

2010.12.21 Tuesday ACHK

After all, it’s a main conclusion of quantum gravity that the qualitative distinction between elementary particles and black holes fades away. However, small black holes are “different” elementary particles – especially because they tend to decay “isotropically” to many other particles – those that we call the Hawking quanta if the black holes are large.

— Lubos Motl

2010.12.16 Thursday ACHK

As of July 2010 LIGO has never conclusively detected any gravitational waves at all. Since Einstein’s General Relativity requires gravitational waves and the LIGO specifications ensure it is sensitive enough to detect them if they exist, LIGO’s failure forces the question whether General Relativity is a valid theory.

— Wikipedia on LIGO

2010.12.15 Wednesday (c) All rights reserved by ACHK

Working on string theory using the methods from loop quantum gravity was a lot of fun.

On one of the few occasions when I talked to Richard Feynman, he said that many theoretical physicists spend their careers asking questions that are only of mathematical interest. “If you want to discover something significant,” he told me, “only work on questions whose answers will lead to new experimental predictions.”

The key thing that Amelino-Camelia and others realized is that we can use the universe itself as an experimental device to probe the Planck scale.

— Loop Quantum Gravity

— Lee Smolin

2010.12.14 Tuesday ACHK

It is a complete surprise that gravity emerges in string theory. Indeed, none of the vibrations of the classical relativistic string correspond to the particle of gravity. It is a truly remarkable fact that we find the particle of gravity among the quantum vibrations of the relativistic string.

This book will explain in detail how string theory, at least in its simplest form, is nothing but the quantum mechanics of classical relativistic strings.

— A First Course in String Theory

— Barton Zwiebach 

2010.12.11 Saturday ACHK