# Century of theory of relativity

### MILESTONE

A 100 years ago, a paper published by Albert Einstein revolutionised the understanding of space and time. Quite naturally, all the cosmologists and physicists are eager to celebrate the 100th year of this turning point. The well-understood, textbook rendering of gravity becomes only a special case of Einstein’s theory. However, the underlying mathematics generally flies above the head of a layperson.

The ‘absolute’ concept

The long believed concepts of an eternal constant universe, with an absolute time, absolute space had also an absolute frame of reference, the earth.  The revolutionary ideas of Copernicus, Galileo and finally Newton questioned these fundamental
concepts. Newton’s Principia provided the new physics – or ‘the natural philosophy’, as it was called, and the doctrines therein ruled for over two centuries – till some of the fundamental constants figured in another branch of physics. The velocity of light in Newtonian physics demanded an absolute frame of reference, which could not be ascertained in the context of electromagnetism. The idea of ‘ether’ provided the required frame of reference.

The universality of it all
Einstein approached this problem from a different perspective, questioning the meaning of simultaneity. The frame of reference is essential to decide this – therefore an observer A moving relative to another observer B will not agree that the two events declared by B as simultaneous. Thus time is not absolute. The four coordinates x,y,z for space and t for time are to be treated together as space time. Although it looks too far-fetched at first sight, one of the extensions of this argument resulted in the famous equation E=mc2, identifying mass and energy mutually transformable. Thus universality came to be associated with space time.

Bending mass, bending theories
Geometry is the best way express universality. The 4-dimensional space-time can be geometrically expressed as curved; gravitational field (because of a mass) increases the amount of curvature. The huge mass of the sun “bends” space-time; the earth, obliged to move along a ‘straight’ path, curves around to move in an elliptical orbit. Thus the space-time curvature dictates the path of the planets like earth, rendered as an approximation of the theory of relativity.

The prediction of the General Theory of Relativity (GTR) has found its place in broader areas, especially in the branch of astrophysics. Light is said to ‘bend’ around the sun – a classic experiment done during a total solar eclipse proved this. However, GTR states that it is the space that bends at the sun’s gravitational field, because of which the light appears to bend although, it still moves in its own straight path.

The last few decades have witnessed many of the predictions of GTR come true – gravitational lensing is one where the distant galaxies appear as distorted or multiple images owing to the gravitational field of another massive object. Two neutron stars going round each other lose energy in an unusual way – by creating waves in the space.

Relating it generally
The observations by Edwin Hubble in 1929 that the distant galaxies are all
receding from us posed a challenge for an explanation. GTR came to rescue here again by providing an apparently homogeneous and isotropic universe which is also dynamic.  The observed expansion of today can be traced back to the ‘Big – Bang’, which happened once upon a time when ‘there was no time’. It makes the question “what was there before the Big Bang?” totally irrelevant.  Further, the predictions using GTR relate to the existence of a relic glow termed the cosmic microwave background radiation (CMBR), emitted when the universe was very young, in its infancy. One can also calculate the relative abundances of elements in the first three minutes after the Big – Bang.

Physical implications
The physical meaning of a hypothetical scenario was successfully provided by GTR in the context of black holes. Here the quantum mechanical principles had to be accommodated. The right conditions are manifested in the stellar collapse when the gravitational force can be only countered by that derived from quantum mechanical Pauli Exclusion Principle.

Black holes are regions of extreme curvatures of space time, not letting even photons to escape. Although the situation seems to be still hypothetical the universe seems to be offering good number candidates for study including one at the centre of our own galaxy.

Revolutionary idea needed?
With more and more sophisticated technologies available for us the finer  details can be fetched to cross examine the GTR. The signals from the artificial satellites of the earth to the relics of stellar explosions, we have gadgets to measure the minute deviations. However some of the questions on our notions of space time and explanations for example of the inhomogeneities in the CMBR, or a union of principles of quantum mechanics and so on, remain. Even as all the cosmologists are looking at 1915 paper as the trend setter, some ask the question – do we need yet another revolutionary idea?