The Double Slit Experiment
The double slit experiment or sometimes called the experiment with two slits was first performed by Thomas Young in the early nineteenth century. The original purpose of his experiment was to show that light is composed of both a wave and particles. Sometimes, these characteristics of light are referred to as the complementary nature of light or simply Complementarity.
Young used a piece of card board with two slits cut into it as the apparatus and shone a beam of light on it thus casting some shadows onto a screen. At the centre positions directly facing the two slits, the light beam cast bright spots whereas for positions further away from the centre spots darker shades ( umbra and penumbra ) appeared. These represent interference effects caused by light waves passing through the two slits which obstruct the continuous light waves in different ways causing them to go out of steps with one another. The rationale behind the experiment is that only waves interfere with one another. In the centre spots, particles of light do not form shades because they do not affect one another as the particles are localised and pass directly through the centre of the slits to form bright spots on the screen.
The double slit experiment or sometimes called the experiment with two slits was first performed by Thomas Young in the early nineteenth century. The original purpose of his experiment was to show that light is composed of both a wave and particles. Sometimes, these characteristics of light are referred to as the complementary nature of light or simply Complementarity.
Young used a piece of card board with two slits cut into it as the apparatus and shone a beam of light on it thus casting some shadows onto a screen. At the centre positions directly facing the two slits, the light beam cast bright spots whereas for positions further away from the centre spots darker shades ( umbra and penumbra ) appeared. These represent interference effects caused by light waves passing through the two slits which obstruct the continuous light waves in different ways causing them to go out of steps with one another. The rationale behind the experiment is that only waves interfere with one another. In the centre spots, particles of light do not form shades because they do not affect one another as the particles are localised and pass directly through the centre of the slits to form bright spots on the screen.
In modern times this experiment is refined to reveal the peculiar and
mystical nature of light or any other form of electromagnetic waves. The two slits are
modified to two holes and the beam of light is replaced by a photon or electron gun
capable of shooting a single photon or electron at a time. Since there is only one photon
( or quanta of light ) being released at a time, the single photon must either pass through
one hole or the other. The experiment is further refined to allow for the installation of a
more sophisticated monitoring screen opposite the two holes with the photon gun behind
the two holes on one side and the screen on the other side. As each photon is fired
towards the holes and onto the screen we should not expect any interference patterns to
appear on the screen. This is because only waves create such patterns due to their wave-
lengths not being in step after passing through the holes which obstruct the waves in
different ways. To our surprise, the same interference patterns still appear on the screen
even though photons are fired singly towards the two holes. How can this be ? As
mentioned above, the single photon can, according to common sense, only pass through
one or the other hole but not both. There cannot be the situation where half a photon
passes through each hole and interfere with each other because the photon is the smallest
possible unit ( or quanta ) of light. There also cannot be the situation of the single photon
interfering with itself because it takes two to tango. So what is happening at the atomic or
quantum level ? Is there such a thing as a ghost photon interfering with the real photon ?
These inteference patterns with single photons are puzzling enough. But wait till you cover up one of the holes to continue the experiment. To your even greater surprise, the interference patterns disappear and clear little individual dots or images of particles ( cast by single photons ) prominently appear on the screen. It was as if the photons knew that one of the holes was covered when they started their journey. Advanced technology has made it possible to refine the experiment even further by delaying the choice of the experimenter after each photon has left the photon gun. One of the holes is covered only after each photon has started its journey. The same results are seen. It seems that each photon can foretell future events. That is whether one or both holes are opened. The unmistakable conclusion to be drawn from such surprising experimental results is this. When given a chioce of two holes a single photon ( or for that matter any elementary particle from the electromagnetic wave spectrum ) will take all possible routes to reach the destination ( in this case the screen ) so that there are interferences among these numerous POSSIBLE routes or POSSIBILITIES of a SINGLE photon. Whereas if it is being watched ( or given only one option, or measured, or observed , or monitored ) such as in the case of being 'forced ' to take only one path to that destination with only one hole opened it will oblige and 'materialise' or localise and give up all other choices or possiblities. Now, you can try again and open up both holes. Sure enough, the interference patterns show up on the screen as before. It would appear, then, that atoms and elementary particles can assume indefinite or uncertain positions ( or technically called superpositions ) until they are observed or measured. They can be ANYWHERE when unobserved ! It would appear that they can foretell the future, too!
The philosophical implications of the quantum behaviour of elementary particles are both enormous and profound. There had been a lot of debate on how to interpret Qantum Theory since its inception in 1900 with no definitive answers. Even an important pioneer and contributor to the theory like Albert Einstein could not accept its philosophical implications. So far, there had been three major versions on the interpretation of the theory. First, there was the formal version in 1928 known as the Copenhagen Interpretation. Then, Einstein had his own Hidden Variables Interpretation which actually meant that the theory was as yet incomplete. The third and more modern version was the Many Worlds Interpretation advanced by Hugh Everett in 1957.
These inteference patterns with single photons are puzzling enough. But wait till you cover up one of the holes to continue the experiment. To your even greater surprise, the interference patterns disappear and clear little individual dots or images of particles ( cast by single photons ) prominently appear on the screen. It was as if the photons knew that one of the holes was covered when they started their journey. Advanced technology has made it possible to refine the experiment even further by delaying the choice of the experimenter after each photon has left the photon gun. One of the holes is covered only after each photon has started its journey. The same results are seen. It seems that each photon can foretell future events. That is whether one or both holes are opened. The unmistakable conclusion to be drawn from such surprising experimental results is this. When given a chioce of two holes a single photon ( or for that matter any elementary particle from the electromagnetic wave spectrum ) will take all possible routes to reach the destination ( in this case the screen ) so that there are interferences among these numerous POSSIBLE routes or POSSIBILITIES of a SINGLE photon. Whereas if it is being watched ( or given only one option, or measured, or observed , or monitored ) such as in the case of being 'forced ' to take only one path to that destination with only one hole opened it will oblige and 'materialise' or localise and give up all other choices or possiblities. Now, you can try again and open up both holes. Sure enough, the interference patterns show up on the screen as before. It would appear, then, that atoms and elementary particles can assume indefinite or uncertain positions ( or technically called superpositions ) until they are observed or measured. They can be ANYWHERE when unobserved ! It would appear that they can foretell the future, too!
The philosophical implications of the quantum behaviour of elementary particles are both enormous and profound. There had been a lot of debate on how to interpret Qantum Theory since its inception in 1900 with no definitive answers. Even an important pioneer and contributor to the theory like Albert Einstein could not accept its philosophical implications. So far, there had been three major versions on the interpretation of the theory. First, there was the formal version in 1928 known as the Copenhagen Interpretation. Then, Einstein had his own Hidden Variables Interpretation which actually meant that the theory was as yet incomplete. The third and more modern version was the Many Worlds Interpretation advanced by Hugh Everett in 1957.
Because the elementary particles do not
have definite positions until they are observed they are said to be in a Superposition when
unobserved. This concept leads to a very absurd situation. This famous episode is known
as Schrodinger's Cat. Erwin Schrodinger is the Austrian scientist who invented the special
set of equations called Wave Mechanics Equations in 1926 which can predict the
probability of the momentum and positions of elementary particles. More precisely, they
provide a mathematical desciption of the wave-like ( or probability ) properties of such
particles. Again, I must emphasize that the solution to these equations will only lead to the probabilities and not absolute certainties of the above variables. In Schrodinger's
terminology, the act of observation on the elementary particles will cause a Collapse of the
Wave Functions and lead to the materialisation or localisation of the particles in a
particular probability momentum and position instead of the infinite number of possibilities.
Schrodinger invented a thought experiment which put a cat in a sealed
box together with a small container holding some radio active material which could kill the
cat if released by a connecting triggering device. He purposely made the triggering device
in such a way that it would release the radio active substance at random. A press of the
control button can either release or withhold the toxic substance. Therefore, one cannot
know whether the cat is dead or alive after activating the control until the box is opened
and the condition of the cat observed. Mathematically speaking the cat has a fifty-fifty
chance of being dead or alive after the control has been activated. Schrodinger argued
that before the box was opened the cat is in a superposition of being 50% dead and 50%
alive. He insisted that unobserved, the cat can neither be dead nor alive. In other words,
the event of the cat being dead or alive cannot happen without an observation being
carried out. This is an absurd proposition in relation to the macro world and an example of
applying the rules of quantum mechanics beyond their boundaries of the subatomic world.
As mentioned earlier, a physical object more or less stays where it is. This is because the
enormous number of atoms it contains will average out the probabilities of the fuzzy
locations of the atoms so that the entire object would stay in one place where the highest
probabilities of all the atoms being present is concentrated. The same rationale should
apply to events such as the life and death of a cat cited in Schrodinger's thought
experiment that involves large physical objects such as the cat.
Despite the apparent theoretical and academic nature of the Superposition concept, it has found very important application in the field of computer engineering. As described in the section on Quantum Evolution about the Origin of Life ( Eccentric Ideas section in chapter (ii) ), the computing powers of the conventional computer can be greatly enhanced by introducing the Quantum concept of Superposition. The current system of using binary digits of 0 and 1 as on-off switches in digitizing information can be vastly expanded by adding the Superposition concept of being neither on nor off using elementary particles such as electrons as computer switches which obey subatomic quantum rules. The technical details are very complicated and far beyond the scope of this book. Suffice it to say that the greatly increased numbers of the positions of computer switches will enhance the computing capacity of Quantum Computers by possibly millions of folds. Recently, the application of this Superposition quantum rule has been successfully tested in laboratory experiments in principle. According to present estimates, the commercial quantum computer may be on the market within a decade. The main obstacle in achieving a working quantum computer seems to be the difficulty in converting the results of the quantum computation back into classical state or the macro-world format which obeys classical Newtonian rules rather than quantum rules. You may be aware by now that under quantum rules an observation made on the end results of the quantum computation will precipitate a collapse of their related wave functions and thus causes them to lose their superpositions. While this may not create a big problem with the end results it does represent a big hurdle in the input stage where data have to be treated quantum mechanically. The big challenge seems to be the problem of how to avoid a collapse of the related wave functions of the input data when handling them at the input stage so as to keep their superpositions intact .
Despite the apparent theoretical and academic nature of the Superposition concept, it has found very important application in the field of computer engineering. As described in the section on Quantum Evolution about the Origin of Life ( Eccentric Ideas section in chapter (ii) ), the computing powers of the conventional computer can be greatly enhanced by introducing the Quantum concept of Superposition. The current system of using binary digits of 0 and 1 as on-off switches in digitizing information can be vastly expanded by adding the Superposition concept of being neither on nor off using elementary particles such as electrons as computer switches which obey subatomic quantum rules. The technical details are very complicated and far beyond the scope of this book. Suffice it to say that the greatly increased numbers of the positions of computer switches will enhance the computing capacity of Quantum Computers by possibly millions of folds. Recently, the application of this Superposition quantum rule has been successfully tested in laboratory experiments in principle. According to present estimates, the commercial quantum computer may be on the market within a decade. The main obstacle in achieving a working quantum computer seems to be the difficulty in converting the results of the quantum computation back into classical state or the macro-world format which obeys classical Newtonian rules rather than quantum rules. You may be aware by now that under quantum rules an observation made on the end results of the quantum computation will precipitate a collapse of their related wave functions and thus causes them to lose their superpositions. While this may not create a big problem with the end results it does represent a big hurdle in the input stage where data have to be treated quantum mechanically. The big challenge seems to be the problem of how to avoid a collapse of the related wave functions of the input data when handling them at the input stage so as to keep their superpositions intact .
