Albert Einstein is probably the most well-known scientific genius. His creative ability allowed him to dream of new physics and create scientific revolutions, including his masterpiece, the theory of general relativity. While people around the globe instantly recognize Einstein’s image, many still have not had an occasion to learn some of the astonishing details and amazing implications of his most monumental discovery.
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. The International Society for General Relativity and Gravitation has declared 25 November 2015 as Einstein Centenary Day.
The idea of relativity had been studied almost three centuries earlier by Galileo, when he stated the principle of relativity in 1632 (that the fundamental laws of physics are the same for all bodies in uniform motion). Later in the 17th Century, Sir Isaac Newton also took the principle of relativity for granted, asserting that if his famous laws of motion held in one inertial frame, then they also held in a reference frame moving at a constant velocity relative to the first frame.
Einstein’s theories of relativity are somewhat more involved, even if his starting point was in many respects the same. His ground-breaking theories take into account the speed of light, the structure of space-time and the equivalence of acceleration and gravity. They have led to some remarkable consequences, including the dilation of time, the contraction of length, mass-energy equivalence and the bending of light, as well as the prediction of the existence of black holes, wormholes and the “birth” of the universe in a Big Bang.
Einstein’s theories still hold up today, after exhaustive experimentation and testing, and have been described as the single most important contribution by one man to science.
Einstein did a kind of science that very few people understand. His earth-shaking physics formula, Rµv–½gµvR=(8πG/c4)Tµv, is daunting to many of us.
Although the math behind the General Relativity is awesomely daunting, the underlying concept is simple and elegant: the spacetime of the universe with no matter around (as in an empty universe) is just flat, and the light rays propagate in straight lines. Instead, in presence of a massive body (for example, a star), the spacetime right around it will be distorted. In a two-dimensional analogy, the spacetime can be represented by a billiard table: in the empty universe case, a ball that was thrown will roll smoothly over it, following a straight line. In the same analogy, the massive object, the star, might be depicted as a dip in the middle of the table. The closer you get to it, the more curved the surface will be: the ball will now deviate from a straight line trajectory, and the closer it rolls to the dip, the more it will deviate.
Leaving metaphors aside, if a light ray happens to pass close to a massive object such as a star, it will be forced to bend in order to follow the curved spacetime around it, as it cannot travel anywhere else: it has to comply with the warps of the spacetime and cannot just “detach” from it, as there’s nothing else, “outside” it.
Einstein’s theory revealed that time runs more slowly near a strong source of gravity — an idea that revolutionized physics when first presented to the Prussian Academy of Sciences in November 1915, but would have no practical applications for decades, because the technologies that could make use of the theory did not yet exist. Today, for example, Einstein’s discovery makes it possible to ensure GPS devices sync up properly with satellites far from the Earth’s center of gravity.
His theory broke away from the Newtonian concept of absolute space and time in which natural phenomena just “happen” in favour of a more comprehensive scenario in which the space and time are tied to each other and the resulting space-time is shaped by the matter (and therefore the energy) it contains.
It took Einstein eight years after publishing his theory of special relativity to expand that into his theory of general relativity, in which he showed that gravity can affect space and time, a key to understanding basic forces of physics and natural phenomena, including the origin of the universe.
In 1905, his so-called “miraculous year”, Einstein published three papers. The first (dealing with the so-called “photoelectric effect”) gave a very strong impulse to quantum theory, and got him the Nobel prize in 1921. The second dealt with the movement of small particles in a fluid (Brownian motion). The third paper of 1905 was called “On the electrodynamics of moving bodies”, and it changed the face of physics and the way we understand nature.
The Special Theory of Relativity has two main postulates: firstly, that physical laws have the same mathematical form when expressed in any inertial system (so that all motion, and the forces that result from it, is relative); and secondly that the speed of light is independent of the motion of its source and of the observer, and so it is NOT relative to anything else and will always have the same value when measured by observers moving with constant velocity with respect to each other. Not such a scary proposition at first glance, perhaps, but it does lead to some rather interesting implications.
The drawback to Einstein’s Special Theory of Relativity is that it is “special” in the respect that it only considers the effects of relativity to an observer moving at constant speed. Motion at constant speed is clearly a special case, and in practice bodies change their speed with time. Einstein wanted to generalize his theory to consider how a person sees another person who is accelerating relative to them. To do this, he had to take on Sir Isaac Newton’s Law of Universal Gravitation, which had stood undisputed since 1687.
Einstein’s ground-breaking realization (which he called “the happiest thought of my life”) was that gravity is in reality not a force at all, but is indistinguishable from, and in fact the same thing as, acceleration, an idea he called the “principle of equivalence”. He realized that if he were to fall freely in a gravitational field (such as a skydiver before opening his parachute, or a person in an elevator when its cable breaks), he would be unable to feel his own weight, a rather remarkable insight in 1907, many years before the idea of freefall of astronauts in space became commonplace.
Einstein devised a completely new description of gravity. First, he realized that objects in the universe exist in three dimensions of space and one of time. He then combined these into a four-dimensional spacetime. The motion of an object throughout its entire history in the universe could then be fully described by its trajectory in spacetime. The English astronomer Arthur Eddington confirmed Einstein’s predictions of the deflection of light from other stars by the Sun’s gravity using measurements taken in West Africa during an eclipse of the Sun in 1919, after which the General Theory of Relativity was generally accepted in the scientific community. Eddington was able to observe, during the eclipse, the effect of the Sun on the light coming from a far away 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.
Just as a bowling ball dents a canvas, a massive object such as the sun significantly bends spacetime in the solar system. As Einstein showed, relatively small objects, such as planets and comets, moving in the curved spacetime of a massive object, like the sun, will be deflected into curved paths, instead of traveling on straight lines. This is not because of an invisible force that pulls the small objects toward the massive one, but because the latter is curving the fabric of spacetime on which the small objects must move. In this sense, mass tells spacetime how to bend and spacetime tells mass how to move.
General relativity is the prevailing modern theory of gravity. It describes the motion of all large-scale objects, including stars, planets, and galaxies. Sir Isaac Newton described gravity as an instantaneous and invisible force between two objects. He imaged that matter simply pulls on other matter across empty space. His laws of motion and universal gravitation are still relevant today because objects still obey these laws approximately in everyday human experience. But Newton’s laws are inaccurate when describing the gravity produced by very massive objects, such as black holes or neutron stars.
Einstein’s theory has been proven remarkably accurate and robust in many different tests over the last century. The slightly elliptical orbit of planets is also explained by the theory but, even more remarkably, it also explains with great accuracy the fact that the elliptical orbits of planets are not exact repetitions but actually shift slightly with each revolution, tracing out a kind of rosette-like pattern. For instance, it correctly predicts the so-called precession of the perihelion of Mercury (that the planet Mercury traces out a complete rosette only once every 3 million years!!), something which Newton’s Law of Universal Gravitation is not sophisticated enough to cope with.
Stephen Hawking and Roger Penrose’s singularity theorem of 1970 used the General Theory of Relativity to show that, just as any collapsing star must end in a singularity, the universe itself must have begun in a singularity like the Big Bang (providing that the universe does in fact contain at least as much matter as it appears to). The theorem also showed, though, that general relativity is an incomplete theory in that it cannot tell us exactly how the universe started off because it predicts that all physical theories (including itself) necessarily break down at a singularity like the Big Bang.
The theory has also provided endless fodder for the science fiction industry, predicting the existence of sci-fi staples like black holes, wormholes, time travel, parallel universes, etc. Just as an example, the notionally faster-than-light “warp” speeds of Star Trek are based firmly on relativity: if the space-time behind a starship were in some way greatly expanded, and the space-time in front of it simultaneously contracted, the starship would find itself suddenly much closer to its destination, without the local space-time around the starship being affected in any relativistic way. Unfortunately, however, such a trick would require the harvesting of vast amounts of energy, way in excess of anything imaginable today. One day!
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?” Several tests of General Relativity have to do with light as well – 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 by Eddington in 1919.
Light remained central in subsequent tests of the theory such as the so-called gravitational redshift: light changes frequency when it moves in a gravitational field, another prediction of General Relativity, experimentally tested since 1959.
But the most amazing prediction of General Relativity has not to do with light, but rather with the absence of light. Black holes are objects so dense that even light cannot escape their strong gravitational field. It is no longer science fiction: black holes are now standard objects that astrophysicists (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 astrophysicists to “map” the Universe and study its “geometry”. It is through these tools that astrophysicists realized that the Universe is expanding, and that all Galaxies are accelerating away from each other. As a consequence they realized that there is a new form of (dark) energy present in our Universe. All these amazing and surprising discoveries were made possible by studying the light coming from distant astrophysical events 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 temperature 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 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.
Gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe, such as the collision of two black holes or shock waves from the cores of supernova explosions. Gravitational waves are emitted by accelerating masses much as electromagnetic waves are produced by accelerating charges. These ripples in the space-time fabric travel toward Earth, bringing with them information about their cataclysmic origins, as well as invaluable clues as to the nature of gravity.
Albert Einstein predicted the existence of these gravitational waves in his 1916 general theory of relativity, but only now in the 21st Century has technology advanced to enable their detection and study by science. Although not yet detected directly, the influence of gravitational waves on a binary pulsar (two neutron stars orbiting each other) has been measured accurately, and was found to be in good agreement with original predictions. Scientists therefore have great confidence that gravitational waves do exist. Joseph Taylor and Russel Hulse were awarded the 1993 Nobel Prize in Physics for their studies in this field.
But the direct detection of gravitational waves is another story. Such an experiment is incredibly hard because these elusive gravitational waves are predicted to be very faint – too tiny for all but the most recently developed instruments to detect – even for the strongest waves generated in the collisions of the most massive objects in the universe. The precision required by these detectors is equivalent to measuring distances as small as one thousandth the size of a proton.
Earlier this year scientists — who in March last year announced evidence of cosmic inflation (the ballooning of the universe in the first 10-35 seconds after the Big Bang, which smoothed everything out), a potentially Nobel-worthy find — threw handfuls of dust on the grave of their own results. The official paper they published this year tells the story of how they mistook cosmic dust for “primordial gravitational waves”. Now that the dust has settled, the search for gravitational waves can continue.
The way forward for physics now rests with attempts to combine the theory of relativity (the theory of the very large, which describes one of the fundamental forces of nature, gravity) with quantum theory (the theory of the very small, which describes the other three fundamental forces, electromagnetism, the weak nuclear force and the strong nuclear force) in a unified theory of quantum gravity (or quantum theory of gravity), the so-called “theory of everything”. Some physicists prefer their space stringy (superstring theory) and others prefer their space loopy (loop quantum gravity) 🙂 however both still need to overcome major formal and conceptual problems.