Under the Microscope/Prof William Reville: This year has been nominated World Year of Physics to mark the centenery of 1905, the "miraculous year" when Einstein published five papers that shattered cherished scientific beliefs and established him as the world's leading physicist.
The only other period of comparable individual scientific achievement was 1665 and 1666 when Isaac Newton, confined at home to avoid plague, began to develop the calculus, his law of gravitation and his theory of colours.
The discoveries of Newton and Einstein were so great that they each left worldviews behind them. The Newtonian worldview describes a universe of absolutes, whereas the Einsteinian worldview describes a world of relativities. In the Newtonian world time invariably flows at the same rate, effects have precise causes and the future is predictable from the past. In the Einsteinian world the rate at which time flows depends on the observer and innate uncertainties at the subatomic level mean that the future can be predicted only as probability, not certainty.
Newton published his laws of motion and of universal gravitation in the monumental Principia in 1687. Newton's majestic scope covered all physical phenomena from the motion of pendulums to the behaviour of colliding billiard balls to the trajectories of planets, and all these behaviours were shown to conform to mathematical laws. For the first time it was clear that the entire physical world was open to human understanding.
The two best-known of Einstein's five 1905 papers laid the foundation of special relativity. They showed that the absolute time of the Newtonian worldview had to be discarded, but replaced by another absolute - the speed of light. They also showed the equivalence of mass and energy which led to the famous equation E=MC2, where E represents energy, M represents mass and C is the speed of light. Two other 1905 Einstein papers demonstrated the existence of atoms, a still controversial idea in some quarters, and showed how to determine their size. The fifth paper challenged the wave theory of light and suggested light could be viewed as a collection of particles. Einstein won the 1921 Nobel Prize for ideas in this paper, but his greatest intellectual achievement was yet to come.
In Principia, Newton described gravity as an ever-present tug that all objects exert on other objects. The greater the mass of an object the stronger its tug, and increasing the separation between two objects weakens the attractive tug. Newton described this gravitational tug relationship mathematically and explained for the first time why the planets orbit the Sun. Today we can use Newton's equations to send astronauts to the Moon and to explain all everyday observations of gravity.
Newton didn't know the mechanism whereby gravity works. Two hundred and fifty years later, Einstein described how gravity works in his General Theory of Relativity (1916). Gravity is a consequence of the effect of mass on space-time.
What is space-time? Einstein realised that one's motion in everyday three dimensions (length, breadth, depth) of space also affects one's measurement of the passage of time. This becomes readily noticeable only when you travel at a significant fraction of the speed of light. For example, if you travel very fast through space, your wristwatch will tick more slowly than the watch you left behind on Earth. Space and time are inseparable. Change your motion through space and you automatically change your motion through time and vice-versa. Space and time are woven together into four-dimensional space-time.
The key equation of general relativity describes the relation between the curvature of space-time and the mass (or energy) located in it. In summary, mass warps space-time and the warped space-time tells mass how to move. Space-time is often visualised as a large rubber mattress. A heavy object, such as our Sun, distorts this space-time fabric, just as a bowling ball would bend the surface of a mattress. Now, if you flick a marble onto the mattress it will roll down towards the bowling ball because of the curvature of the mattress. Similarly, an asteroid near the Sun will fall towards the Sun because the curvature of space-time dictates its movement.
In 1919 it was observed (using an instrument built by the Dublin optics and telescope manufacturer, Grubb) that the Sun's gravitational field deflected rays of starlight passing through it during a solar eclipse. This was a prediction of general relativity and the first experimental confirmation of the theory.
Newton famously said hypotheses non fingo - "I frame no hypotheses" - meaning he theorised only on the basis of observable facts. Einstein overturned the primacy of empirical study and experience.
He formulated hypotheses and logical systems in his own mind that were only later tested by experiment. All our ordinary experience tells us that time moves at a uniform rate - yet this is not true. Einstein began an era in physics in which we have to move beyond sense perception and common sense in order to understand nature.
William Reville is associate professor of biochemistry and director of microscopy at University College Cork