5. The Mystery of Light
For the rest of my life I want to reflect on what light is.
My decision to study theoretical physics along with experimental psychology was most fortuitous. Theoretical physics took me closer to the ultimate truths of the physical world, while my pursuit of experimental psychology was a first step towards uncovering truth in the inner world of consciousness. Moreover, the deeper I went in these two fields, the closer the truths of the inner and outer worlds became.
The bridge that linked them was light.
Both relativity and quantum physics, the two great paradigm shifts of modern physics, started from anomalies in the behavior of light. And both led to radical new understandings of the nature of light. Light, it seemed, occupied a very special place in the cosmos; it was in some ways more fundamental than space, time or matter.
Of these two paradigm shifts, the theory of relativity fascinated me the most. Back in high school I had pondered its implications for the nature of space and time. At university, it was my favorite part of the physics syllabus. And more recently I have realized that relativity points in exactly the same direction as Kants arguments.
The theory of relativity sprang from the curious character of the speed of light. According to classical physics, measurements of the speed of light should vary according to the motion of the observer. Such variations happen all the time in everyday life. If, for example, you are cycling along a road at 20 m.p.h., and a car traveling at 30 m.p.h. passes you, then, relative to you, the car would be traveling at 10 m.p.h.. If you were to pedal a little faster, until you were also moving at 30 m.p.h., the cars speed relative to you would be zero, and youd be able to have a conversation with the driver.
Light moves millions of times faster than a bike, so you wouldnt expect to notice any significant differences in its speed relative to you. Nevertheless, you would expect the same principal to apply. The faster you traveled, the slower would be the speed of light relative to you. But when physicists tried to detect these changes, they obtained puzzling results. Whether you traveled towards the light or away from it, the relative speed of light was always the same.
Perplexed by these findings, two American scientists, Albert Michelson and Edard Morley, designed an experiment that could detect variations in the speed of light to an accuracy of two miles per second, which was about a hundred times more accurate than the expected variation. Yet they still came up with exactly the same result. The observed speed of light never varied.
For the existing scientific paradigm, this was a major anomaly. Why did light not obey the same laws as everything else? It just didnt make sense.
Einstein's Paradigm shift
Enter the young Albert Einstein. Having failed his college entrance examinations in electrical engineering, and having been turned down for various teaching posts in mathematics and physics, he had finally gained employment as an "assistant, third class," in the Swiss patent office. During his spare time he pondered various mathematical and physical problems, including the inexplicable results of the Michelson-Morley experiment.
In 1905, at the age of 26, and virtually unknown to the scientific community, he published two seminal papers, one on the quantum nature of light, which we will come to shortly, and one on the "Electrodynamics of Moving Bodies," in which he proposed a radical resolution to the problem of the speed of light, laying the foundations for his Special Theory of Relativity.
The basic premise of relativity was not new. Two hundred and fifty years earlier, Galileo had realized that if you were in a closed room, with no windows, there would be no way of telling whether the room was at rest or moving with a steady velocity; any experiment you were to perform in a moving room would have the same results as one performed in a stationary room.
Imagine, for example, you are flying in a plane and you drop a tennis ball. The ball will fall vertically (from your perspective) to the floor and bounce up again towards your hand. It does not slam into the rear of the plane at 500 miles per hour. Relative to you, the ball behaves in the same way as it would if you were standing on the ground. You cannot tell from the balls motion alone whether the plane is moving or at rest.
Galileos theorynow known as classical relativitystates that the laws of physics are the same in all uniformly moving frames of reference. The phrase "uniformly moving" is important. It means moving at a steady speed in a steady direction. If the plane were accelerating or turning, you could tell that you were moving. The ball would roll across the floor, and you might feel changes in the pressure of the seat against your body.
Classical relativity referred to the motion of physical objects; it said nothing about light. Einstein took classical relativity and brought it up to date. He proposed that the principle of relativity should be valid for all the laws of physics, including those governing light. These, too, should be the same in all uniformly moving frames.
In 1864, James Clerk Maxwell had proposed that light consisted of electromagnetic waves, with their own equations of motion. These equations specified a precise value for the speed of light of 186,282 miles per second (thats about 670,000,000 miles per hour). If, as Einstein argued, these equations are the same in all uniformly moving frames of reference, then the speed of light must be the same in all such frames.
In other words, however fast you are moving you will always measure the speed of light to be 186,282 miles per secondjust as Michelson and Morley had found. Even if you were to travel at 186,281 miles per second, light would not pass by a mere 1 mile per second faster; it would still zoom by at 186,282 miles per second. You would not have caught up with light by even the tiniest amount.
This goes totally against common sense. But in this instance it is common sense that is wrong. Our mental models of reality have been derived from a lifetimes experience of a world where velocities are far below the speed of light. At speeds close to that of light, reality is very different.
The Relativity of Space and Time
That the speed of light is the same for all observers, however fast they are moving, is strange enough, but even stranger things are in store for our notions of space and time.
Einsteins equations of motion predict that moving clocks will run slower than clocks that are at rest. At the speeds we usually encounter, the difference is negligible. But as we approach the speed of light the effect becomes quite noticeable. If you were to travel past me at 80 percent the speed of light, I would observe your clocks running one third the speed of mine. This slowing applies not just to man-made clocks, but to all physical processes, to all chemical processes, and to all biological processes. Your whole world appears to run slower than mine. Time itself is running slower.
Weird as this may seem, experiments have shown that this slowing of time does actually happen. Very sensitive atomic clocks have been flown round the world, and they have been found to run slow by exactly the predicted amount. The change is very smalla factor of about one in a trillionbut it is there.
Nor is it just time that changes; space is also affected. As an observer approaches the speed of light, measurements of length (that is, measurements of space in the direction of motion) get shorter, and in exactly the same proportion as time slows. If you were passing by me at 80 percent the speed of light, your measurements of length would have shrunk to one third of mine.
Again this seems to defy common sense; space, like time, seems fundamental and fixed, not something that changes according to your speed. Nevertheless, experiments with subatomic particles traveling at speeds close to that of light have verified the effect. The faster you go, the more compressed space becomes.
Henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.
The Realm of Light
For an observer actually traveling at the speed of light, the equations of Special Relativity predict that time would come to a complete standstill, and length would shrink to nothing. Physicists usually avoid considering this strange state of affairs by saying that, since nothing can ever attain the speed of light, we dont have to worry about any weird things that might go on at that speed.
When physicists say nothing can ever attain the speed of light, they are talking of things with mass. Einstein showed that not only do space and time change as speed increases, so does mass. In the case of mass, however, the change is an increase rather than a decrease; the faster something moves, the greater its mass becomes. If an object were ever to reach the speed of light its mass would become infinite. However, to move an infinite mass would take an infinite amount of energymore energy than there is in the entire universe. Thus, it is argued, nothing can ever attain the speed of light.
Nothing, that is, except light. Light travels at the speed of light. And it does so because it is not a material object; its mass is always precisely zero.
Since light travels at the speed of light, lets imagine a disembodied observer (pure mind with no mass) traveling at the speed of light. Einstein's equations would then predict that, from light's own point of view, it travels no distance and takes zero time to do so.
This points towards something very strange indeed about the light. Whatever light is, it seems to be in a realm where there is no duration; no before, and no after. There is only "Now."
The Quantum of Light
More hints as to what light isand what light is notare found in the other great paradigm shift of modern physics, quantum theory. As with relativity, the anomaly that sparked this shift concerned light.
When you raise the temperature of a metal rod it begins to glows a dull red. As it gets hotter, the color brightens and changes from red to orange, then to white and finally takes on a bluish tinge. But why should this be? According to classical physics all glowing bodies should radiate the same color, whatever their temperature.
In 1900, the German physicist Max Planck realized he could account for these changes in color if energy was not radiated in a continuous smooth flow, as had previously been supposed, but came in discrete packets, or quanta (from the Latin word quantum, meaning "amount"). He proposed that any energy change, whether it be an electron in an atom changing its orbit, or the warming of your skin from sunlight, consisted of a number of whole quanta. It could involve 1, 2, 5, or 117 quanta; but not half a quantum or 3.6 quanta. When Planck applied this constraint to the light radiated from a glowing object he found it led to precisely the changes in color that are observed.
Five years later, in the same year as he published his Theory of Special Relativity, Einstein came to a similar conclusion. He was exploring the newly discovered photo-electric effect, in which light shining on a metal can trigger the release of electrons. The only way he could explain the rate at which electrons appeared was to assume that light was transmitted as a stream of particles, or photons. Each of these photons of light was equivalent to one of Plancks quanta, or packets of energy.
Light as Action
A quantum may the smallest packet of energy that can be transmitted, but the energy contained in a quantum varies considerably. A gamma-ray photon, for example, packs billions of times more energy than an infra-red photon. This is why gamma rays, X-rays, and even ultraviolet light to some extent, can be so dangerous. When these photons hit your body, the energy released can blow apart the molecules in a cell. On the other hand, when an infra-red photon is absorbed by the body, the energy released is far less; all it does is vibrate the molecules, warming you a little.
Although the amount of energy in a photon varies enormously, there is one aspect of the quantum that is fixed. Each and every quantum has a constant amount of action.
Mathematicians define action as an objects momentum multiplied by the distance it travels; or the objects energy multiplied by the time it is travelingthe two are equivalent. The amount of "action" in a ball thrown across a football field, for example, would be greater than the same ball thrown half the distance. Double the balls mass, and you double the action. Or imagine yourself running at a constant rate of energy output. If you run for twice as long, there will be twice the actionwhich makes intuitive sense.
The actual amount of action in a quantum is exceedingly small, about 0.00000000000000000000000000662618 erg.secs (or 6.62618x10-27 erg.secs in mathematical shorthand)but it is always exactly the same amount.
All matter is just a mass of stable light.
This is called Plancks constant (after its discoverer). It is the second universal constant to emerge from modern physics. Like the firstthe speed of lightit is a constant of light. Light always comes in identical units of action.
Like relativity, quantum theory also points to light being beyond space and time. We may think of a photon being emitted from some point in space and traveling to another point where it is absorbed. But quantum theory says that we know nothing of what happens on the way. The photon cannot even be said to exist in between the two points. All we can say is that there is a point of emission and a corresponding point of absorption, and the transfer of a unit action between the two.
Light at No Speed
The materialist metaparadigm assumes that space, time and matter are the primary reality. Both quantum theory and relativity suggest that light is even more fundamental. If so, then some of the difficulties science has with light may stem from our trying to treat light as if it were part of the material world.
Take, for example, the speed of light. As we have seen, for light itself, time and length both shrink to zero. From the photons point of view, it travels no distance, and takes no time to do so. It therefore has no need of speed.
Why then, does light appear to us to have a very definite speed?
When we observe a photon from our frame of reference, in a sense, we draw out the zero space and zero time of the photons frame of reference into a definite amount of space and a corresponding amount of time. If we are traveling close to the speed of the photon, we see a little bit of space and a little bit of time between its point of emission and its point of absorption. The slower we travel, the more space and time we observe the photon to have crossed.
If we observe the photon to have crossed space and time, then it appears to us to have a speed. But it is not really a speed at all. What we are observing is the ratio in which space and time manifest in our frame of reference. For every 186,282 miles of space that manifests, there always manifests one second of time. It is this ratio that is constant for all observers, however fast they are moving.
Kant argued that the noumenonthe "thing-in-itself," the physical reality that is apprehended by the senses and interpreted by the mind, but never experienced directlytranscended space and time.
A hundred and twenty years later, we find Einstein lending support to Kant. Time and space are not absolutes. They are but two different appearances of a deeper reality, the spacetime continuumsomething beyond both space and time, but with the potential to manifest as both space and time. But the spacetime continuum itself, like Kants noumenon, is never directly known.
If we think we can picture what is going on in the quantum domain, that is one indication that we've got it wrong.
Light, too, has unknowable qualities. We never see light itself. The light that strikes the eye is known only through the energy it releases. This energy is translated into a visual image in the mind. Although the image appears to be composed of light, the light we see is a quality appearing in consciousness. What light actually is, we never know.
Light seems to lie beyond reason and any commonsense understanding, a finding that again parallels Kants conjectures. Reason, he said, was not an intrinsic quality of the noumenon, but was, like space and time, part of the way the mind made sense of things. If so, it should not be that surprising that our minds find it so hard to comprehend the nature of light. It may be that we will never be able to make sense of it. With light we may have reached the threshold of knowability.