Solar Physics



Is time travelling (going to the past/future)possible ? (Fazil)

Although there are speculations about travelling through time using "wormholes" in space, there has been no serious mechanisms proposed for space travel. Given that we have not actually seen any time travellers from the future, it seems quite unlikely that time travellers really do exist.   Top

Is there any end of physics ? If so, why ?What are the arguments of Prof Hawking ? (N P Abraham)

I believe that Professor Hawking thinks that we will have a Grand Unified Theory which will bring all the four forces (gravity, strong force, weak force, and electromagnetic force) under one model. In effect, we will have a single equation which describes all of physics! Of course,these claims have been around since the dawn of physics and we are still continuing.    Top

How does the density of the substance influence its gravity ? (R T Mangala Cheliyan)

The gravitational force is determined only by the total mass and the distance from the centre of the mass. If the density is very high, this mass will be concentrated in a very small volume so that the gravitational field very near the object is large.   Top

What is space time curvature ? What is space and time ? (Prakash)

This is too broad a question to answer effectively. Please try this link. Britannica.com   Top

What is imaginary time ? (Arjun Shankar)

Imaginary Time arises from a "Wick rotation" and is used in the path integral quantization of gravity. It is a computational tool carried over from relativistic quantum field theory (in flat space-time) in which the contour integration that is required to evaluate the Feynman propogator is replaced by an integral in imaginary energy (the contour is "Wick rotated" in
the complex energy plane). This is a mathematically well defined contour deformation and is equivalent to defining the path integral in Euclidean space or in "imaginary time". The final answers must be given in real time of course, so a "wick rotation" back to real time is necessary, but it can be carried out at the end of the computation.

In quantum gravity, the imaginary time formulation implies that one is interested in space-times that are solutions to the Euclidean Einstein equations. Only solutions that are sufficiently regular (eg. free of conical defects) need be considered. This, for example, is why one requires periodicity in the time variable of euclidean black hole solutions, which shows eventually that there must be a "temperature" associated with them.   Top

Is space time quantized ? (Dimitra Atri)

The point is, that spacetime quantization should happen on extremely small length scales. Quantum gravity predicts that the discrete nature of space time is manifested only on length scales of the order of a Planck length, which is ~ 10-33 cms. This is twenty orders smaller (a billion trillion times smaller) than an atomic nucleus. We need extremely high energies (far far beyond our present accelerators) to probe such small regions of space time. There is no current experimental evidence that space time is quantized out to distances less than 10-18 metres. However, on Planck length scales or maybe a few orders larger than that space time could well be quantized.   Top

Could you give details on polytropic index ? (Sumith)

A polytropic index is used in defining a general relation between pressure and density (so called equation of state), that is P = Kx dn (P is the pressure, d, the density & n is the so called polytropic index, K is a constant).

This equation of state is used in studying the internal structures of various astronomical objects (mainly stellar objects). Chandrasekhar used n=3/2 & 3 in his studies of white dwarfs. n=0,corresponds to constant pressure, that is, incompressible flow. n=5,denotes an infinitely extended object (with a central condensation). n=1,corresponds to a perfect gas law P=k x d. For an exhaustive discussion on polytropes (configurations described with a polytropic index, see S Chandrasekhar, "Principles of Stellar Structure".   Top

Why is the speed of light absolute and not relative ? Is it absolute even if we travel at a significant fraction of this speed ? (Arjun)

The spectral theory of relativity states that the speed of light is absolute. No matter what your velocity is with respect to the source of light the speed will be the same. However, the energy changes. This is a consequence of the photon being massless.   Top

Is it possible to reduce the speed of light to zero, experimentally ? If so, how ? Is it also possible to increase the speed of light ? Does it not violate the special theory of relativity ?
(Srikanth)

Light's speed in a medium is given by c/r, where c is the in vacuo velocity of light (about 300,000 km/s) and r the refractive index of the medium. Thus an arbitrarily large r can slow down the speed of light _in the medium_. Witness the recent work of Harvard physicist Lene Vestergaard Hau. But current theory says that it is impossible to exceed c.   Top

As we all know, earth revolves round the sun in a definiteorbit. We shall imagine that at a moment, the sun's mass has been dropped to zero instantaneously (by someway). After this, does the earth move tangentially to the orbit instantaneously or does it take a definite time to move. If so, is it greater than 8.5 minutes or less than 8.5 minutes ?

The answer is that the situation presumed by the question is invalid, because, as a theory that conserves energy and momentum, General Relativity (GR) does not permit the sudden disappearance of mass !

However, on a related vein, at least in the linear limit GR we know that gravitational perturbation, which is in fact ripples in the spacetime fabric, propogates at speed c.   Top

Last updated on: February 20, 2024