Einstein Centenary.
The United Nations proclaimed 2015 as the International Year of Light.
2015 marks an important milestone in the history of physics: one hundred years ago, in November 1915, Albert Einstein wrote down the famous field equations of General Relativity. General Relativity is the theory that explains all gravitational phenomena we know (falling apples, orbiting planets, escaping galaxies…) and it survived one century of continuous tests of its validity. After 100 years it should be considered by now a classic textbook theory, but General Relativity remains young in spirit: its central idea, the fact that space and time are dynamical and influenced by the presence of matter, is still mind-boggling and difficult to accept as a well-tested fact of life.
The development of the theory was driven by experiments that took place mostly in Einstein’s brain (that is, so-called “thought experiments”). These experiments centred on the concept of light: “What happens if light is observed by an observer in motion?” “What happens if light travels in the presence of a gravitational field?” Naturally, several tests of General Relativity have to do with light too: the first success of the theory and the one that made the theory known to the whole world, was the observation of the light deflection by the Sun. Eddington in 1919 was able to observe, during an eclipse, the effect of the Sun on the light coming from a faraway star. The observed deflection was in perfect agreement with Einstein’s theory while the prediction of the old theory of Newton was off by a factor of 2: a triumph for Einstein! Nowadays, light deflection by astrophysical objects (that is optics with very massive lenses!) is a tool successfully used to explore the Universe: it is called gravitational lensing.
Light remained central even in subsequent tests of the theory. For example in the so-called gravitational redshift: light changes frequency when it moves in a gravitational field, another predictions of General Relativity, experimentally tested since 1959. Actually, the happy marriage between light and General Relativity is important every time we use a GPS device: general relativistic effects are crucial to determine our position with the required accuracy!
But the most amazing prediction of General Relativity has not to do with light, but rather with its absence! Black holes are objects so dense that even light cannot escape their strong gravitational field!. Again it is not science fiction: black holes are by now standard objects that we (indirectly!) observe and study.
On much larger, cosmological scales, the gravitational redshift of light from galaxies and exploding stars (supernovae) constitutes the basic tool that allows us to “map” the Universe and study its “geometry”. It is through these tools that we realized that the Universe is expanding, i.e. all Galaxies are moving away from each other. Even more recently it became clear that this expansion is in fact accelerating! As a consequence we realized that there is new form of (dark) energy present in our Universe! It is worth underlying that all these amazing and surprising discoveries were made possible by studying the light coming from distant astrophysical in the framework of General relativity.
From cosmology comes another connection between light and General Relativity, related to the early moments in our Universe. General Relativity predicts that our Universe comes from a very energetic state, the Big Bang, and a sign of this is imprinted in the so called Cosmic Microwave Background: CMB. The CMB is the light produced in the hot Early Universe in the moment when its decreasing density finally allowed photons to travel freely. This very same light we can see today and provides us with precious information of how the Universe looked like when its age was only 1/30000th of its age today!
What about the future discoveries? We are eagerly waiting (in 2015?) for the first detection of gravitational waves, i.e. “ripples” in the space-time fabric, another fascinating prediction of General Relativity, so crazy that not even Einstein believed in it.
Those produced in the early stages of the history of the Universe could be detected, indirectly, as peculiar patterns in the polarization of the CMB light. Such detection could provide us with invaluable information on the very Early Universe, pushing further back in time our “sight”.
