And God said, “Let there be light,” and there was light. God saw that the light was good, and he separated the light from the darkness. God called the light “day,” and the darkness he called “night”. (Genesis. 1: 3-5).
This is a remarkable beginning of the history of creation, indeed. The creation of light, so the Bible says, is just a first step. It is only through the separation of light and dark that day and night and thus time are created. Semi-darkness and the endless twilight do not know any time. Light has to be separated from the dark in order to be recognized in the same way as the good needs the contrast to the bad, the conscious to the unconscious and knowledge to ignorance. Today’s psychology would state: ‚Without darkness no light.
Yet, the separation of light and darkness has its price. If the two no longer part of the same unity, light has to part the universe with darkness both in time and in space. This results in our experience of the eternal alteration between light and darkness, between day and night. This rhythm has mad us conscious of a temporal structure and thus of the finite nature of our life. The period change between day and night has given us the basic unit of time, a unit which mankind was able to recognize long before clocks were developed.
First, day and night together formed something larger which as it was the common wisdom before Copernicus corresponds to one complete cycle of the sun turning around Earth. Today we call this period of 24 hours one Day, yet at the same time us the word ‘day’ still for the bright part of the cycle (“it is day”) which is a sign of man’s understandable preference (of life in general) for light. Without light no life, no plant growth through photosynthesis.
Talking about growth we should not forget a second unit of time which, besides the day, played an important role for man’s survival, especially for agricultural societies: the year and the seasons. With the development of agriculture, in most cultures certain artefacts or edifices which although of ritual origin should help man to recognize the sequence of the seasons.
(As a remark on the side we just point out that further subdivision of the time unit day (hours) probably has played a role already early in human cultures whereas the invention of minutes and seconds are products of modern societies and are closely related to navigation on the great oceans. Navigation has also triggered the invention of highly precise clocks.)
In the seventeenth century Christian Huygens pointed out for the first time that light is a kind of wave. This theory produced yet another relationship between light and time. A wave is characterized by its wavelength and its frequency. The product of the two gives the velocity of wave propagation. Yet, mankind had to wait for another two hundred years until the real nature of this wave became clear. Light is an electromagnetic wave and that part of a much more general physical phenomenon to which, for instance, radio waves and X rays belong as well. That part of the electromagnetic wave spectrum which is visible for our eye with typical wavelengths of the order of one thousandth of a millimeter and frequencies of the order of thousand trillion oscillations per second represents just a very tiny area of the extremely rich ‘zoo’ of all electromagnetic waves.
The fact that in the vacuum all electromagnetic waves propagate with a constant velocity which cannot be surpassed by any physical phenomenon represents one of the central pillars of modern physics. 
Every electromagnetic wave is kind of a vehicle for the transport of energy. This is also true for light. The origin of nearly all the light which hits Earth is the sun. The sun radiates not only in the visible spectrum, that is with those wavelengths which can be detected by our eye, but also in the range of shorter (ultraviolet) or longer (infrared) waves. In fact, the visible light brings the major part of the solar energy. Of course, this is not by coincidence but the product of evolution which with good reason made the retina of our eye sensitive to that part of the wave spectrum which offers the largest part of the solar energy flux.
Light is energy. This principle represents one of the central pillars of evolution. With the process of photosynthesis plants have invented a method to store light energy for their later use. This capability have neither animals nor man; they have developed their own trick how to make use indirectly of the solar energy flux, that is by eating plants or other animals and thus by profiting of the energy stored by those organisms which are able to photosynthesis. The former species are called heterotrophic (using other nutrients) in contrast to the latter which are called autotrophic (using their own nutrients).
Man is a heterotrophic organism. In order to sustain its physiological processes humans need about 10 million Joule (equivalent to about 2,4 Kilowatt-hours) per day corresponding to an average energy demand per unit time (physicists call this power demand) of 100 Watt.
Thus man as a machine driven by an external energy flux needs about the same energy as a 100 Watt electric light bulb which is burning permanently. At least that was the case until about two hundred years ago when humans began to make use of great amounts of fossil energy, coal first, later oil and natural gas that is those energy reserves which during millions of years have been accumulated by the photosynthesis of plant. To say it with other words: In the course of the last two centuries the heterotrophic biological species homo sapiens steadily increased its energy demand beyond its physiological needs to a value which today has reached a global average of 2000 Watt per person, that is the twentyfold of the energy which humans take up by food. In the industrialized world the technological energy demand amounts to about 5000 to 10’000 Watt per person, and some among us (people who frequently travel by airplane) need 100’000 Watt or more.
In fact, today the additional energy demand of modern civilization is based overwhelmingly on fossil solar energy except for some quantitatively minor parts like nuclear energy. Thus, the light from the sun is not only the basis of our biological (or physiological), but also of our technological energy demand.
The flux of light from the sun is the external energy supply of the global ecosystem like electricity from the socket guarantees the running of a washing machine or a computer. In a certain sense in the course of evolution Nature has learned to respect the limits of this energy supply and to live with them much in the same way as we have learned to limit the number of different items plugged into the same socket in order to avoid the fuse to blow. In other words: The solar light flux has governed the speed of life since millions of years. Slow light, understood as life matched to the natural energy flux from the sun, represents the opposite of the fast light of today’s technological society which reverts to the reserve of solar energy (fossil fuels) put back during millions of years and thus to use this analogy - bypasses the fuse of solar light.
How does this rapidity manifest itself concretely? For instance in the fact that regarding their geographical mobility humans have overcome their physiological limits both in terms of distance and speed. To give an example, a large aircraft which has reached its cruising altitude consumes about ten thousand liter of kerosene per hour corresponding to a power of 100 Megawatt or if attributed to 300 passengers about 300 Kilowatt per person. This is three thousand times more than our physiological energy demand. If related to the solar energy flux in medium latitudes averaged over the 24 hours of the day, the large plane what require the solar energy flux from an area of nearly one square kilometer. In fact, taking into account the final efficiency of solar panels the area would even be about ten times larger.
Of course, Nature is capable for great power outputs as well; e.g., a hunting leopard can reach tremendous speeds, but only for a short time and by storing (solar) energy gained from the natural (‘slow’) energy fluxes.
To summarize: In the void, light is propagating at a constant speed of about 300’000 km/h. In so far as light can move through matter over greater distances, the speed of light may be somewhat reduced, but none the less it is obvious that with the term slow light something different is expressed as the reduced speed of light.
On our search for slow light we came across two different links. The first one is related to the daily as well as annual light cycle which results from the rotation of the Earth and the geometry of the circular movement of our planet around the sun. Both cycles have impressed their characteristic seal to our life to life in general. Slow in contrast to fast light means that this natural seal or rhythm is not ignored.
Second, light represents the energy resources which have created life and keeps it going. In the same sense as the natural measure of time, the day and the year, could be overcome by technology, it would be (and is) possible for humankind to become independent from the limits of the solar energy flux, at least for some time. Slow light thus stands as a symbol for the renewed reflection on the natural limits of light and energy.
 This explains the way towards the end of 2011 physicists became extremely nervous when a group of scientists from CERN pointed out that in a recent experiment they had found evidence that neutrinos (elementary particles without charge and rest mass) may move with a velocity which is larger than the speed of light. A few months later the strange phenomenon found its natural explanation: A cable connection used for the transmission of data did not working properly.
Dieter Imboden has been President of the Research Council of the Swiss National Science Foundation (SNSF) since 2005 and full Professor of Environmental Physics in the Department of Environmental Sciences at the ETH Zurich since 1988.