Theory of relativity and Quantum theory are, undoubtedly, two of the finest creations of human mind. Einstein single-handedly formulates theories of relativity whereas it takes many people to formulate Quantum mechanics.
Max Planck in 1900 first propounded the quantum hypothesis, according to which radiation consists of streams of particles called quanta, now known as photons. Each photon carries energy equal to the product of a constant, called Planck’s constant, and frequency. Evidently, a photon of higher frequency carries more energy than a photon of lower frequency. That is, a high frequency photon is more energetic than a low frequency photon.
Einstein in 1905 first published series of papers on what is later on known as special theory of relativity and by 1915 he completed the formulation of General theory of Relativity. The choice of this name is rather unfortunate because it led many people including the philosophers to the notion that according Einstein’s theory of relativity every thing is relative. This is not true. If every thing is relative, there is nothing for them to be relative to. On the other hand the special theory attempts to achieve something like the following.
Suppose an event occurs at some point in space and time. Let this event be observed by two observers. One observer is at rest. The other observer is moving uniformly with respect to the first observer who is at rest. The two observers shall describe the event as seen from their respective place of observation. The theory of relativity attempts to answer the question: Can one know the observation of the moving observer by knowing the observations of the observer at rest or vice-versa. The answer is yes.
It is found that the two observations are connected by equations of transformation known as Lorentz transformations. Einstein arrives at these equations on the basis of the following postulates.
1. The speed of light in vacuum has the same value in all inertial frames.
2. The laws of physics are the same in all inertial frames.
A frame may be defined as the place of observation. An inertial frame is one in which an observer will feel no force. In such frames light will travel with the same speed in vacuum and the laws of physics shall be the same. On the basis of such simple postulates Einstein revolutionised our concept of space, time, mass and energy. One important result is that measurements on length and time are different for different frames. However, if the measurement is done in kind of space in which time also become a dimension of this space, then the measurement is the same. Such a space is called spacetime. According to Einstein, spacetime is a continuum and is smooth everywhere.
Can Einstein be wrong? Is it possible that the spacetime is not smooth?
Let us consider jointly postulates of special theory of relativity with quantum hypothesis. Then it is clear that light, a form of radiation, travels with the same speed regardless of its frequency. That is, an energetic photon shall travel with the same speed as that of a less energetic photon.
In a quantum theory of gravity the fine scale shape of spacetime may fluctuate. High energy photons (photons with high frequency or short wavelength) are more sensitive to such fluctuations than the low energy photons (photons with low frequency or long wavelength). If spacetime is something like roads in Imphal dotted with portholes and bumps a truck with large tires may easily navigate these perturbations whereas a car with smaller wheels may spend longer time in their journey. Similarly two photons of different energies will not take the same time in travelling the bumpy parts of spacetime. Therefore, to test if spacetime is bumpy or not a race should be conducted between photons of different energies and the arrival times of these photons at a destination, say a laboratory must be recorded. If the race is a long one the difference in arrival times at the laboratory may be high and can then, be accurately measured.
Fortunately, nature conducts such a race every time a gamma-ray burst occurs. The burst releases pulses of photons of various energies. These photons shall travel billions of light years to reach us. The race is a long one because light takes billions of years to reach us.
GLAST, a space telescope to be launched by NASA this year, can scan the sky and surely shall pick up such photons from bursts at several distances. It is capable of resolving the arrival times of these photons. If GLAST detects a difference, then spacetime has a foamy structure. Therefore, Einstein’s assumption that spacetime is a continuum may be wrong.
(The author, Prof. Th. Jekendra Singh, is a senior faculty, Department of Physics, Manipur University).