Departament de Física i Química
La importància, o el pes, que l’Òptica ha tingut en la física, es pot veure en una enquesta realitzada al maig de 2002 sobre quin havia sigut, segons el parer de la comunitat científica, l’experiment més bonic de la física.
La classificació dels 10 experiments més votats pel col•lectiu de físics, ordenats de major a menor pel nombre de vots aconseguits són :
1. La difracció d’electrons per una doble escletxa (Claus Jönssons, 1961)
2. L’experiment de la caiguda lliure dels cossos ( Galileu Galilei, 1600)
3. La determinació de la càrrega de l’electró amb gotes d’oli ( Robert Milllikan, 1909)
4. La descomposició de la llum solar per un prisma( Isaac Newton, 1665)
5. L’experiment de la interferència de la llum ( Thomas Young, 1801)
6. La mesura de la força gravitatòria amb una balança de torsió ( Henry Cavesdish, 1798)
7. La mesura de la circumferència terrestre ( Eratóstenes, segle III)
8. La caiguda dels cossos per un pla inclinat ( Galileu Galilei, segle XVII)
9. El descobriment del nucli atòmic ( Ernest Rutherford, 1911)
10. L’experiència del pèndol de Foucault ( Jean Foucault, 1851)
Resulta significatiu que, en el top ten dels experiments de física, hagueren sigut seleccionats dos experiments i mig d’òptica.
Amb l’Òptica es pretén que aprengues :
- A distingir quan la llum es comporta com una partícula i quan com una ona
- Què es la reflexió i com es comporta un raig de llum quan arriba a un espill pla, convex o còncau.
- A conèixer què és la refracció i el comportament de la llum quan travessa lents convergents i divergents.
- A conèixer el funcionament d’una càmera obscura
- Perquè és forma l’arc de Sant Martí.
- A conèixer el fonament del telescopi i del microscopi.
The Nineteenth Century Mystery of Light and the Thomas Young Double-Slit Wave Mechanics Experiment
In the year 1800, exactly a hundred years before the birth of the Quantum Theory and the wave-particle duality of light, the world of physics was already getting a taste of this phenomena, which was to become one of the main mysteries of the New Physics a century later. At the time the debate on the nature of light focused on whether light was corpuscular, or particles as the theory of Sir Isaac Newton (1642-1727) postulated and the ondulato¬ry, or wave, as the theory of Dutch physicist Chris¬tiaan Huygens (1629-1695) suggested. Though neither theory was definitely established nor entirely successful in explaining all of the known phenomena of light, Newton’s was generally accepted.
It was then in 1801, the first years of the nineteenth century, that the British physicist, physician, and Egyptologist, Thomas Young (1773-1829), began his study of light with these two theories in the background. By a¬llowing light to pass through two closely set pinholes onto a screen, Young found that the light beams spread apart and overlapped, and, in the area of overlap, bands of bright light alternated with bands of darkness. With this demonstration of the interference of light, Young definitely established its wave nature. In 1817 he then proposed that light waves were transverse (vibrating at right angles to the direction of travel), rather than longitudinal (vibrating in the direction of travel) as had long been assumed, and thus explained polarization, the alignment of light waves to vibrate in the same plane. Young’s work was received by most English scientists as illogical, unscientific, and some¬how unpatriotic, This was partly due to the idea that any opposition to a theory of New¬ton’s was unthinkable. It was only with the work of the French physicists Augustin J. Fresnel (1788-1827) and Francois Arago (1786-1853) that Young’s wave theory finally achieved acceptance in Europe.
However, it would be a hundred years after Young’s experimentations, during the early years of the New Physics at the onset of the the twentieth century, that his double slit experiment would take physics deeper into the bizarre mysteries of quantum mechanics and its wave-particle duality than most any other experiment. That is the mystery which we shall be looking at here in this paper, but before we get into the thick of this, we shall first familiarize ourselves with the ordinary double slit experimentation, but for this demonstration we have “hijacked” the double slit experiment graphics from the Professor Stephen Hawking’s book: A BRIEF HISTORY OF TIME. Here then is our story of this most famous of man’s dialog es with nature.
Light passing on to a screen through one slit
In this arrangement light behaves as composed of courpulses, or particles and supports the explanation Newton postulated.
Anticipated passage of light on to a screen through two slits
This is what some of us have anticipated, but not what happens and can thus be disregarded.
Light actually passing on to a screen through two slits
In this arrangement light behaves in an undulato¬r manner, or as a wave, as the explanation of Dutch physicist Chris¬tiaan Huygens suggested.
Here is the double slit experiment light interference graphics in two forms, the firs is the drawing that Thomas Young made in order to explain the double slit phenomena, the next is a more recent graphical explanation for the IN and OUT of phase effects in theinteraction of two waves.
The Thomas Young Original Drawing of Wave Interference
In and Out of Phase Wave Interference
This represents the double slit experiment in its simplest form and is pretty much the story of nineteenth century wave mechanics in physics, but here is already a hint of the underlying mystery of the bizarre reality that appears in the quantum mechanical experiments that began to emerge in the beginning of the twentieth century. This is the second part of this paper and it deals with the still conclusively un-resolved questions by physics.
“I think I can safely say that nobody understands quantum mechanics… Do not keep saying to yourself, if you can possibly avoid it. ‘But how can it be like that?’ because you will go ‘down the drain’ into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that”.
Professor Richard Feynman
The Twentieth Century Mystery of Light and the ThomasYoung Double Slit Particle-Wave Mechanics Experiments
In the year 1900, exactly a hundred years after the birth of the double slit experiment by Thomas Young, the New Physics of Quantum Mechanics was born on the 14th of December, the birthday of Nostradamus who had prophesied about this. This took place whenProfessor Max Karl Ernest Ludwig Planck (1858-1947) presented his research finding into black body radiation to the German Physical Society. This science would deepen the wave-particle duality of light mystery even further as the world of physics was beginning to get a new understanding and taste of this phenomena. This came with the discovery of Planck that energy came in discrete packages–particles–and with Professor Albert Einsteins (1879-1955) Special Relativity in 1905 and the the Photon in his explanation of thePhotoelectric Effect. This lead to the first double slit experiments in 1090 where just one photon was allowed to pass through the slits at at time. The mystery was again deepened with the parallel concept-that matter also exhibits the same duality of having particle like and wave like characteristics-was developed in 1925 by the French physicist Louis Victor, prince de Broglie (1892-1987) and the mathematical studies of the wave mechanics of orbiting electrons published in 1926 by the Austrian Professor Erwin Schrödinger (1887-1961).
We shall now look at the mysteries that appears in the experimentation where just one particle is allowed to pass through the slits at at time, with considerable time-intervals before the nest one was allowed to pass. In these experimentation it does not matter whether we are using individual particles such as photons, electrons or even such massive particles as neutrons. However, for the sake of clarity, before we look at the behavior of “quantum reality” we begin by looking at reality as we know it in our everyday experience. This is the tainted golf balls experiment.
The Double Slit and the Individual Particle Interaction
This lead to the first double slit experiments where just one photon was allowed to pass through the slits at at time, with considerable time intervals before the nest one was allowed to pass.
Anticipated outcome of individual photons being fired–with some time-intervals on–on to a screen, through two slits
This is what some of us have anticipated, but not what happens, but the results is described by many as the strangest phenomena in all of physics. In this experiment a photon (or a fundamental particle such as an electron or a neutron) of light is discharged individually through the double slit onto a photosensitive paper that collects the individual photon hits. Since there is no other photon to interfere with the photon being discharged, we are ready to wage that there will only be two slit-patterns appearing on the photosensitive paper. However, instead of seeing two slit-patterns appearing on the photo- sensitive paper, we observe the many line interference pattern as if we were conducting the normal experiment with a flood of photons. It is no wonder that this result gave the physicists who first carried out this experiment a real surprise and a shock, but many stories are associated with their reactions, including that this experiment is supposed to have turned physicists to becoming gardeners. The experiment revealed that somehow an individual particle managed to interfere with itself, which to our “common sense” is something utterly absurd, but in-numerous untestable suggestions for explanations have been offered. This is the quantum mechanical realties “one hand clapping” as Professor John D. Barrow so elegantly describes it. Here the most popular suggestion for explanation was offered by the late ProfessorRichard Feynman, which postulates that particle, be it a Boson (energy carrying particle) or a Fermion (matter manifesting particle), while traveling at high velocity through the vacuum, must “split it self-up” and traverse all possible paths. Figure #08 shows this with the “Feynman paths” of the particle being drawn on to the graphic on the left.
Actual outcome of individual photons being fired–with some time-intervals–on to a screen, through two slits
The other grand mystery that appears, both in multi particle and single particle discharges, is the fact that as soon as we try to register it’s passage on the “downwind-side” through either slit, the wave-interference effect disappears. The particle “traverses from the gun” to the screen as a particle and its wave properties disappear.
The photon is being detected as it is actually passing through either of the two slits on to a screen
When a photon, electron, or neutron is detected passing through either of the two slits, the interference pattern disappear
The golf ball experiment is large scale, but we have found that the description by Professor John D. Barrow in his brilliant book THE WORLD WITHIN THE WORLD, to be one of the best analytical description of these two phenomenas and have therefore “hijacked” the following passages from his book.
“Suppose we now see what happens when subatomic particles like neutrons are fired towards the two slits (instead of the golf balls). If we place a photographic film across the target then we find the striking result shown in Figure #08. The neutrons behave like the golf balls in the sense that each hit on the target film produces a definite mark. But as more and more neutrons are fired at the screen the individual hits build up a picture that has the characteristics of a wave interference pattern. There are bands where there is a high development of the target, evenly interspersed with underdeveloped bands each possessing some statistical scatter. Although the neutrons arrive at the target as distinct objects, like the golf balls, the probability that they hit a particular point on the target is determined by a wave intensity. If we close one of the slits then this produces a single wave-intensity distribution with no interference just as in the ¬case with the light-waves. Hence the neutrons manifest particle and wave properties at the same time: they arrive at the target as distinct `hits’, but with an intensity pattern characteristic of a wave.
There are further peculiar aspects of the wave interference pattern produced at the target screen by the neutrons, which make it subtler in nature than the ‘ordinary’ interference pattern produced by the light waves. If we fire the neutrons slowly, one at a time, towards the screen so that we can watch the film developing neutron by neutron, and so avoid any obvious interaction between different neutrons which would lead to interference, then we still find the interference pattern being built up bit by bit. More striking still, we could set up many identical versions of this experiment all over the world and fire just one neutron towards the slits in each of them at a prearranged moment. If we add together the results from all these completely different experiments we would find that the net result would look like the wave interference pattern! The single neutrons seem to be able to interfere with themselves. This is indeed ‘one hand clapping’. We could have arranged for the different experiments to be huge distances apart and synchronized the performance of the different experiments so that in the time that it takes for the neutron to get from its source to the target no signal could travel, even at the speed of tight, between one experiment and another to cause a correlation of their results in some way. The result is the same the individual neutron-hits add to form a correlated interference pattern. How does each neutron know which role to play in order to produce the ‘right’ big picture of wave interference?.
An even more perplexing fact about the microscopic two-slit experiment with the neutrons is that any attempt to unravel the wave-particle ambiguity and discover through which slit a particular neutron actually passed en route to the target invariably destroys the interference pattern seen at the target. Neutrons are rather delicate. If we set up a photoelectric cell at each slit in the screen to be triggered when a neutron passes through that slit, then we might expect to discover through which slit each individual neutron passes on its way to the target. Unfortunately this ambition can never be realized. The intervention of the light from the photoelectric cells alters the behavior of the neutrons in a manner ¬that destroys the wavelike result of the experiment. If we are ever able to determine through which slit a neutron goes, then the pattern seen at the target screen is changed from the light-wave pattern of Figure #03 into the particle-like result of the golf balls in Figure #06. Whenever we decide to examine whether a neutron is behaving like a particle and determine through which slit it passes, then, and only then, it is found to behave as a particle. If we do not attempt to determine if it is behaving as a particle, then, and only then, it manifests itself as a wave. It is not possible to construct any device, which can determine through which slit a neutron has passed without destroying the wave interference pattern at the target.
This is quite unlike anything ever-encountered in classical physics. It confronts all philosophical positions regarding the character of-the laws of Nature and underlying reality with a totally novel challenge. It appears that the observer of the world plays a crucial role in determining what can be observed, but in a way that is subtly different from the old idealist view that everything is in the eye of the beholder. The naive realist would hold to the belief that there is an objective world that exists whether we like it or not, and which possesses definite properties that exist independent of any measurement of them. Unfortunately this does not stand up to its first encounter with the quantum laws of Nature. The observed phenomenon together with the act of observation together determine what can be observed in the two-slit experiment. This does not mean that one should conclude that everything observed is observer-created, in the sense that the idealist or the solipsist might claim. There is no reason to suspend belief in an underlying reality. It is just that the steps we take to establish it determine what it wilt be found to be. Reality is contextual. We must also recognize that, at the very least, this reality which dictates goings-on in the micro-world of photons and neutrons is very different from the approximate impression of it that we have assumed from our contact with large objects whose quantum wavelengths are ¬minute, and whose wave-like properties are for all practical purposes indiscernible. It is not that golf balls do not possess wavelike attributes. They do. But golf balls are so large compared with neutrons and the length of their wave attributes so small, that wave interference effects are indiscernible by the human eye”.
With all this in view it is not unexpected that this has lead to concepts of multitudes of universes, or what is known as the “Many Worlds Interpretation of Quantum Mechanics” or “Parallel Universes”. However, this is not the way the QF-theory interprets the nature and fabric of reality. For that interpretation we shall have to take into account the QF-interpretation for the fabric of space and the QF-interpretation for the interaction of the particles of energy (Bosons) and matter (Fermions) with space. These comprise then the last part of this paper.
“No point is more central than this, that empty space is not empty.
It is the seat of the most violent physics”.
Professor John Archibald Wheeler
The first half of the simplistic QF-explanation is the suggestion that the two infinitely small and invisible “virtual matter-energy V-poles”of space, the V-negative and the V-positive, represent the fabric of space at the quantum level. They represent the “reality” out of which the the matter-energy of the Universe emerged during the first part of the first second of creation in the Big Bang, out of which our creation begins some 14.000.000.000 years ago. This suggestion is thus for a twofold infinite reality of space, or twofold infinity; one with a positive nature and the other with a negative nature. It is then through this reality which the fundamental particles move by making their famous”quantum leaps”, or by jumping from one pole to the other. In this they do not jump just between the one kind of pole, but between both the V-negative and the V-positive and thus through the two fundamental realities, or dimensions, of space. This does not suggest an otherUniverse, or other Universes but is simply a fundamental property of our own Universe. It is this property that comes into play when fundamental particles travel in their quantum leaps through the vacuum as Feynman suggested resulting in “all possible paths”. In addition to this the QF-theory suggests that as the particle is going through its quantum leaps traveling through the vacuum, it’s V-polesalso force it to manifest its opposite spin image and thus produce the wave that produces the interference. In the case of the photon this becomes manifested in the famous Aspen experiment where the second collapsed photon wave always has the opposite spin to the first.
The photon, which is the energy state of matter traveling at the speed of light, is the only fundamental particle that is both its matter and antimatter particle at the same time. With the photons being fired individually in the double slit experiment with some time intervals, it is the antimatter side of the photon that waves in the V-positive poles and the matter side that waves in the V-negative poles and thus a twofold wave is produced with a single photon. This is how the interference wave mechanics come about with a single photon being fired at intervals, but obviously the collapsing of the wave for each side will manifest opposite spins for each collapsed wave-package. Whether larger and many times more massive particles such as electrons and neutrons in this experiment are traveling at the speed of light and producing an antimatter wave is not clear.
The QF-theories description of individual photons, electrons or neutrons being fired on to a screen, through two slits
The graphic shows a single photon- path producing the interference patt- erns as it passes through the slits. The photon is also it s own antimatter particle.
This is the actual wave pattern for the individual photon passing through two slits with time intervals, where the single photon produces two wave-packages.