While it’s often fascinating to glance at the science of now, sometimes it’s just as fascinating to look back. It’s perhaps a sense of wonder at the reality we inhabit which drives such a desire to keep, at least, a passing acquaintance with the cutting edge of physics. Sometimes, however, it’s at least as fascinating—perhaps even more so—to take a look at the science of yesterday.
Discoveries made by great and insightful minds to glimpse, even if for a moment, at the world they saw. This is, of course, not to say that the discoveries made by Sir Isaac Newton or Johannes Kepler offered nothing to our modern understanding of the Universe. Far from it, in fact. They, and their ideas, are indeed the very foundations of modern physics.
For this, momentary, retrospective we’ll take a look at the works of another. He was perhaps the greatest mind of last century and between 1905 and 1916, German physicist Albert Einstein made incredible contributions to our understanding of the Universe. For his efforts, he won the Nobel Prize in 1921.
What is Relativity?
Generally (at least within the confines of the scientific meaning) when someone mentions “relativity”, they refer to two theories, themselves halves of a greater whole, proposed by Albert Einstein. The first, published on September 26th of 1905 in Annalen der Physik, was the Special Theory of Relativity (it was part of a series of four papers by Einstein throughout that year). The second was a much larger undertaking requiring eight years of his life (from 1907 to 1915) before finally being published in 1916. This was the General Theory of Relativity.
Especially Relative; the Twin Paradox
One of the bizarre things to come out of the Special Theory of Relativity is the inconsistency of time. According to the conventional wisdom of the past, time is immutable and, irrespective of a person’s inertial frame of reference, was measured the same by everyone. Einstein, however, noted that this is not the case. According to the Special Theory of Relativity, the speed of light in a vacuum (represented by the constant c) is as much a universal speed-limit as it is unchanging. In all inertial frames of reference, the speed of light will be measured the same and, as a result, the experienced time might vary.
In order to explain this bizarre flow of time (an effect called time dilation), a thought experiment was conceived. It was called the Twins Paradox. In succinct form, it works simply;
There are two twins. One decides to remain on Earth, while the other boards a space ship and flies away at near the speed of light. Some distance away, the space ship turns around and returns. When the two twins a reunited, the twin who remained on Earth is much older than the twin who went on a trip.
According to Special Relativity, as an object approaches the speed of light, the slower time moves for the object from the perspective of an outside observer. In this case, the object is the space ship. It might be argued that, in this scenario, there are two inertial frames of reference (ie, the Earth and the ship) which leads to contradicting results. For the twin on Earth, time on the ship seems to run slower. For the other twin, however, looking back at Earth it seems as though it is the Earth moving, and not the ship, at near the speed of light—the twin on the ship sees Earth’s time as moving slower. This, of course, is why it’s called the Twins Paradox.
There is a solution, however, and it has to do with the fact that one of the objects changes trajectory. One of the objects (either the ship or the Earth) was not, itself, an inertial frame of reference.
For a more detailed look at time dilation (and the Twins Paradox in particular), visit this page from the University of New South Wales or this video from ozmoroid.
The General Theory of Relativity is what marries three dimensional space and time into the single, four dimensional, spacetime. It provides a geometric description of gravity by generalizing Special Relativity and Newton’s Law of Universal Gravitation. It is the most accurate description of gravitation today and, so, still stands tall in modern physics.
According to General Relativity, space-time is curved by the presence of matter. It is this curvature of space-time which causes the attractive force of gravity—objects will move along these curves, essentially falling toward another along a slope of space-time.
General Relativity can be used to accurately describe the motion of planets (though Newton’s version works almost as well), stars and even galaxies, and from it come some rather unusual predictions. One such example of odd happenings is gravitational lensing, where light is stretched and bent as if being passed through a lens. This lensing is more than just a prediction of the model, it has also been observed and stands, among many other things, as one of the most powerfully persuasive pieces of evidence which supports the theory.
That leads us to another odd happening with General Relativity. When an object has sufficient mass to provide a gravitational force greater than the escape velocity of light, then light, itself, is trapped by gravity. At this point, you have a black hole.
Black Holes; Breaking Space-time and Understanding
It was thought, for quite a while, that black holes couldn’t actually exist. Modern astronomers, however, have observed indirect evidence of their existence—most notably at the centre of our own galaxy. In addition, other great minds have come along since Einstein’s time to weigh in. We now understand that sufficiently large stars (at least three times the size of our own Sun) can collapse into black holes at the end of their lives. It turns out that these monsters aren’t just mathematical anomalies with Einstein’s field equations; they actually exist.
What exactly is a black hole though? Well, this appears to be where General Relativity and another important pillar of modern physics, Quantum Mechanics, come into conflict. They provide contradicting predictions. This tells us that there is something fundamentally wrong, that our understanding of physics is most definitely incomplete.
Many great minds, including Einstein himself, have worked to try to unify Quantum Mechanics and Relativity to little success. Stephen Hawking, Michio Kaku and many, many others tackle with this conundrum even today. Is a Grand Unified Theory possible in our lifetimes? It’s possible and, more importantly, exciting.