For example, when two water waves intersect, a new wave pattern is set up. Two wave peaks can coincide and form an amplified peak. Likewise, two wave troughs can coincide to form a deeper trough in a relationship known as constructive interference. Where a peak and trough coincide, however, they cancel each other out in a phenomenon known as destructive interference. On Young's screen, bright bands or lines would evidence constructive interference; dark lines or bands would evidence destructive interference.

Wave Interference

Young’s experiment thus was as follows:

Young’s experiment clearly demonstrated that light behaved as a wave, creating constructive and destructive interference, thus giving the multi-banded results one would expect from wave interaction. And so for the next 85 years, the scientific community abandoned Newton’s particle theory of light in favor of the wave theory of light.

However, by classifying light as a wave, a puzzling issue was reopened. If light was a wave, then according to wave theory, it must move through a medium. However, light was observed moving through the vacuum of space, a place that is apparently devoid of a medium. Thus the search for the mystical Æther began again in earnest.

In 1881, the American scientist Albert Michelson invented a device so sensitive that it was thought to be capable of detecting the Æther. His invention, the interferometer, was based on two principles of waves. The first, interference, when two waves are out of alignment, they interfere with each other, either destructively or constructively, (as demonstrated in Young’s experiment), hence forming a distinctive interference pattern.

The second wave property is that of a wave moving in a medium that is at motion. To an observer looking at a wave moving in a stream, the wave will appear to move more slowly when going upstream, and more rapidly when moving downstream. Michelson figured that if the universe was filled with this Æther, with the earth moving through it, then there should be an observable change in the speed of light as it moved through what he termed the “Æther wind”.

Michelson’s interferometer was devised to detect this difference in the perceived speed of light, no matter how small that difference might be. To do so, he simply used a focused beam of light and split it with a mirror so that half the beam went one way, and the other half went off at a right angle. Michelson then brought the two beams back together in such a way that each beam had traveled exactly the same distance when they were rejoined. The difference between the two halves of the beam was that one of the beams had traveled in one direction relative to the Æther wind, and the other had traveled in a perpendicular direction. Thus, there would always be a difference in the motion of the light waves relative to the Æther wind. If either beam slowed down or speeded up in the slightest relative to the other beam, rejoining the two beams would create an interference pattern.

Michelson, however, never found any interference that couldn’t be accounted for by known inaccuracies in his equipment. Thus, Michelson's first tests called into question the presumption of an Æther wind that would speed up or slow down the light in any measurable way.

By 1887, Michelson and a new collaborator named E.W. Morley had improved their instrument’s sensitivity, but were not able to identify ANY discernable change in the speed of light as it passed through the Æther wind.

The Michelson-Morley experiment called into serious question the existence of the Æther, forcing the scientific community to accept that light was a wave, without a medium.

From Photons to Electrons

By the turn of the century, most scientists were convinced that light was fully described as a wave, despite the lack of a medium. However, while the wave model amply explained the interference, diffraction, and polarization properties of light, it did little to explain the reflective, refractive, and photoelectric properties observed in experimentation. Only by assuming that light was a particle could these latter properties be properly explained.

In 1900, physicist Max Planck showed that certain effects in physics could only be explained by light being a particle. In 1921, Albert Einstein was awarded the Nobel Prize in physics for his work showing that the particle nature of light could explain the “photoelectric effect.”

Then electrons were found to also demonstrate this dual nature.

The electron was discovered in 1897. It was a particle that could be isolated, could not be further subdivided, and thus proved to be an elementary unit. Every electron is just like every other electron, and there are no partial electrons. It was a physical particle that one could envision (unlike the photon), holding in the palm of your hand. It orbited the nucleus of the atom (albeit not in as simple a manner as first postulated by Niels Bohr’s shell model). It was when the electron confronted the double-slit experiment that the weirdness of quantum physics began.

The apparatus for the double-slit experiment using electrons is basically the same as Young’s original light experiment; however, the screen has been replaced by a phosphorescent screen capable of glowing with a small dot when struck by an electron.

Dr. Richard Feynman

This is not a particle distribution, but an interference pattern like we observed with light. The electrons are behaving as if they are a wave. If the electrons are behaving like a wave, but we are shooting one electron at a time, what is it interfering with? In this experiment, there are only four possible ways that the electron could be traveling:

• The electron went through the right slit.
• The electron went through the left slit.
• The electron went through both slits.
• The electron went through neither slit and reached the screen by some other path.

The option of traveling through both slits seems very possible based on our observations, since to create an interference pattern, something must radiate from both slits to create the interference. However, we already know from our discussions of quanta, that there is no such thing as a half of an electron, and we know the electron gun only shoots one electron at a time.

So what is happening? To answer this question, we place electron detectors on each slit. These are sensitive enough to detect a single electron. Now we will be able to determine exactly where the electrons are going as they pass through the slits and head towards the screen. When we rerun the experiment, we notice that half of the electrons travel trough the right slit and half of the electrons travel through the left slit. No electrons went through both slits, and all electrons went through one of the slits. But when we look at the screen to see the pattern that was formed, something surprising happens: No interference pattern is formed. Instead, we see the expected clumping results that we would predict if electrons behaved as particles.

Maybe the electron detectors are causing the problem, so we turn them off (but leave them in place), and rerun the experiment. Low and behold, the interference pattern returns.

By detecting the electrons at the slits, we forced them to behave as particles, and as such, we obtained the resulting particle distribution.

Now, perhaps the actual functioning of the detectors altered the results, so again we try the experiment, but this time we leave the detectors on, but do not gather the data about which slit the electron went through. The result: The interference pattern is observed, and the electrons behaved as waves.

What if we record the detector results, but erase the data before we look at the results? The result: The interference pattern is observed, and the electrons behaved as waves. But notice that in this scenario, the experiment is complete in every respect when we decide to erase the recorded slit data. Up until that moment, there is absolutely no difference in this test and the one run earlier, which produced particle clumping when we looked at the slit data. But we obtained different results. It seems that by removing the slit data completely, after the fact, it has actually changed the outcome of the already completed experiment.

So let’s take this to the extreme. This time, we record the data at both the slits and at the screen, but we encrypt the data so that it is absolutely meaningless. We then create a program that will either:

1. Delete the slit data and decrypt the screen data
2. Decrypt both the slit data and the screen data

We rerun the experiment, then send the data and the program off to the Congress to vote on which choice they would have us select.

Months later, let’s say they choose option 1. When we run the program, we see that months earlier the experiment yielded the wave interference pattern. However, if they choose option 2, we see that months earlier the experiment yielded the particle clumping pattern. It seems that we can choose the result months, or even years later. And, by changing the result, we mean that this arbitrary, delayed choice will affect the actual location of the electron hits as recorded by the electron detector at the back wall, representing an event that was supposed to have happened days, months, or even years in the past. An event that we suppose has taken place in the past (an electron striking the detector screen) will turn out to be correlated to a choice that we make long after the experiment is completed.

What causes this difference?

It turns out that the results of this experiment, in so much as scientists have been able to determine, are dependent on whether we examine the results on the screen with knowledge of the electron activity at the slits or not. Thus, the act of a sentient being in seeking a measurement will cause the thing to have a property which can be measured, and thereby produce a definite property that can be measured.

Evidently, when we look at what is going on at the slits we cause an irreversible change in the behavior of the electrons. This is usually called the "Heisenberg Uncertainty Principle." The conclusion of all this is that there is no experiment that can tell us what the electrons are doing at the slits that does not also destroy the interference pattern. This seems to imply that there is no answer to the question of what is going on at the slits when we see the interference pattern. The path of the electron from the electron gun to the screen is not knowable when we see the interference pattern. As Heisenberg said, "The path [of the electron] comes into existence only when we observe it."

We can think of the probability of where the electron is as a wave, that is, the greater the possibility of expecting to find the electron a one point has a peak, while a point where the electron is least expected to be as a trough, then when we don't look the probability wave has two peeks at the slits, representing the fact that there is a 50% chance the electron went through the right slit and a 50% chance it went through the left slit. These two probability waves from the two slits, then, recombine at the screen and cause the interference pattern.

A wave of probability

It turns out that the electron (or even the photon of light first discussed) is a wave before it is measured. But it is not a wave in the ocean-wave sense. It is not a wave of matter but rather, it turns out that it is apparently a wave of probability. That is, the elementary particles making up the trees, people, and planets - what we see around us - are apparently just distributions of likelihood until they are measured (that is, measured or observed).

The shock of matter being largely empty space may have been extreme enough – that if an atom were the size of a large stadium, then the electrons would be dust particles floating around at all distances inside the stadium, while the nucleus, or center of the atom, would be smaller than a grape. But with quantum physics, the atom itself does not really exist until it is measured. One might rightly ask, then, what does it mean to measure something? And this brings us back to the Uncertainly Principle first discovered by Werner Heisenberg. Dr. Heisenberg wrote, “Some physicist would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist independently of whether we observe them. This however is impossible."

Since 1927, the standard quantum mechanical explanation for the difference between results in the double slit experiments is that in one set of experiments, we know which slit the electron went through; and in the other set of experiments, we don't know which slit the electron went through. This conclusion is part of the "Copenhagen interpretation" of quantum mechanics.

According to quantum mechanics, all quantum units exhibit the properties we have seen in the double slit experiments. We have already seen that photons and electrons, which seem to have nothing to do with each other, behave in exactly the same way. Everything in the entire universe is made up of quantum units. That being the case, what exactly does the skeptic have in mind when he or she speaks of the "real world" as though it were somehow different from a bunch of electrons in the laboratory? What we are seeing in the laboratory is the "real world"; it is, in fact, the real "real world." The double slit experiment with photons or electrons seemed fairly straightforward; but each of those multiple observations actually was following the precise rules of quantum mechanics, rather than our "intuitive" rules of how things should behave, based on the observations of our day-to-day experience. Matter, all matter, thus behaves like both a wave (when unobserved) and as a particle (when observed). This has been given the name Wave-Particle Duality by Quantum physics.

Scientists continue to ‘scale-up’ the experiment by subjecting larger and larger objects to the double-slit experiment. Protons, which are made up of three quanta, have been subjected to the double slit tests with the same results as photons and electrons. Recently, helium atoms, consisting of two electrons, two protons and two neutrons all bound together, have been put through the double slit experiment with the same results. Even more recently, 60 atoms of carbon have made the journey in exactly the same way. Quantum physics states that ALL matter will exhibit the same results, even your cup of coffee.

So what does this mean?

The trees, cars, houses, in fact, all the objects you observe in your everyday life are nothing more than a collection of waves of probability until they are actually measured or observed. And it doesn’t matter whether you observe them today or tomorrow or next year, time has no meaning. It is through our observations, our sentient knowledge of ‘reality’ that we force the many waves of probability to collapse into an outcome that we can observe. In other words:

We Create Our Reality From All That Is Possible

So take a closer look at that cup of coffee, and imagine the possibilities.