The Most Important Experiment In all of Science

The most important experiment in all of science

Quantum mechanics underlies perhaps the most famous of many strange behaviors of our reality. It may be most notable because it seems to straddle the worlds of science and the mystic. Although quantum theory has become one of the world's most successful scientific theories, most people are either entirely ignorant of it, or don't understand its colossal implications. In the end, many would rather ignore it ,since it is an inconvenient truth that opens the door to what Albert Einstein once called “spooky occurrences” which he eventually acknowledged, but could never adequately explain.

Of course there are entire libraries of information dedicated to this subject, so I will not even attempt a thorough review here. Even when fully described, the actions of quantum mechanics still leave brilliant minds unclear about how things actually behave in our world. Prof. Richard Feynman once famously said “anyone who is not shocked by quantum theory doesn't understand it”.

He also said “ What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school... It is my task to convince you not to turn away because you don't understand it. You see, my physics students don't understand it.... That is because I don't understand it. Nobody does.” (Feynman, Richard P. Nobel Lecture, 1966, 1918-1988, QED, The Strange Theory of Light and Matter)

So we start with the fact that one of the preeminent minds of 20th century physics claimed not to understand quantum physics. What is not in dispute however is the “spooky” behavior of it, and that is what we will cover here.

Best place to start is with the double slit experiment, perhaps the most influential experiment in the history of science.

This experiment attempts to look at the smallest constituents of light to uncover their basic nature. How do they behave? We find through various experiments that sometimes these objects behave like Newtonian billiard balls: they come in individual discrete chunks (quanta), occupy distinct positions, and travel in straight lines until struck by other “billiard balls” which change their speed and direction.

Remember the rule? Every action causes an equal and opposite reaction...

However we see in other experiments that these pieces if light, these objects, behave like waves: they emanate in all directions, their “size” (amplitude) varies an an analog fashion, and they can combine with other waves to increase or decrease in size.

Long ago science had theorized that light was a wave. It travels at a fixed speed (the speed of light) and it contains all the classical properties of waves. However later-on others found out that light comes in discrete packets: quanta. So for example, if you shine a light on a detector that makes a sound when light hits it, and it is very sensitive, and if you keep turning down the light's intensity, you will eventually get to a point where individual clicks (quanta) are detected with more or less frequency as the brightness is turned down or up a little bit. The force or sound caused by each click doesn't get any lower by turning the light down further, it just clicks less often. This demonstrates that light is made of quantum units, what Feynman and others called “corpuscles”. Light is made of particles even though it seems to usually “act” as a wave.

So we ware going to create an experiment to analyze the properties of photons (the constituent “particles” of light) to see if we can learn more about whether protons are are actually “objects” acting en mass like waves or whether they are something else entirely.

The famous “Double Slit Experiment” was originally devised to study light and its odd tendency to sometimes behave like particles and other times like waves. I think you'll agree that the findings were, to say the least, rather spectacular.

Lets start with a large pool of water. Stand on the edge and drop a rock into it. What do you see?

Fig 1

Fig 1 shows wave radiating in familiar pattern outward from where the rock hit the previously calm water.

Fig 2

Now in Fig 2 lets put a square edge around the pool and see what the waves do.

(fig 3 removed)

See how they combine with each other after they bounce off the walls. See how they interfere with each other? See how the multiple wave shapes combine?

Now let's place a wall in the pool with a slit cut in it to let some of the wave through. Can you imagine what you will see? Actually in Fig 4 we have shown a drawing with two slits (one left and one right), but only one of which is open at a time.

Fig 4

A wave will start from either slit and will move outward as before. No surprise here either.

When we open both slits things get more interesting. Fig 5 shows the two sets of waves moving out from the two openings. As one would expect, these waves interfere with each other. This “interference pattern” is caused by the fact that when the two waves interact you will see the sum of the two expressed as a combined wave form. So if the wave from slit A is 1 foot high and it hits the wave from slit B where it is 1 foot high, at that spot the new wave will be 2 feet high. And alternatively if the wave from A is 1 foot high and hits the wave trough from B which is 1 foot low the result will be a zero foot wave, ie: calm water at that point. So the combined “waveform” shape will be the combined version of the two waves at that particular location at that time at each particular location.

Fig 5

So using this as a model, we have a clear picture of what a wave looks like when passed through one, or two slits.

Now lets look at a similar setup to see how particles would act when going through slits.

In order to see how particles would act in a similar situation, we will use a scatter gun for our experiment this time instead of a rock dropping in the water. Like a machine gun it will shoot lots of pellets one after the other. Our gun is very inaccurate though so the pellets will kind of scatter around a bit instead of hitting one spot on the target which is located on the back of the laboratory (which is the particle analog of the glass side at back of the pool).

Fig 7 shows the somewhat random distribution of the shots and the pattern it leaves on the target.

Fig 7

Fig 8 shows what would happen if we set up a wall with a single slit opened in it. Just like in our wave experiment, the wall containing the slit will block most of the pellets but the slit in the wall will let a few pellets through. The shot pattern on the target will show an unsurprising pattern. A band of pellet marks in the general shape of and directly in the path of the open slit.

Fig 8

Fig. 9 shows that with a second slit open, a second band of pellet marks will be found on the target, just as you would expect. Notice that there is a band of impacts behind the left slit and one behind the right. The main difference between the pellets and the water waves is that with two open slits, the water demonstrates a wave-like interference pattern and the pellets show and particle-like pattern with two bands of markings. The pellets are acting as Newton would predict: as distinctly independent objects, whereas the waves are doing what waves do: interacting and combining. This all fits with what we would expect from our experiences in the world.

Fig 9

What we have shown so far is that pellets act like particles and water acts like waves, and we have shown experimentally how to see the characteristic difference between the two. Particles will tend to go through the either of the two slits and hit the target behind each opening. Waves will also go though a slit, but will then spread out in a regular arc, and if multiple slits of open they will interact and and cause a repeating and more pattern quite unlike the regular wave shape from the single slit: an interference pattern.

Now that we have tested our setup and test methods, and have learned to tell the characteristic differences, the fingerprints, of particles and wave patterns, let's get down to some real work.

We now need a new scatter gun. Instead of shooting pellets, it shoots units of light (photons). This is not a mystical device. If you take apart an old TV that uses a picture tube you will find one of these photon “guns”.

Just like before, the first experiment shoots photons at the target with no intervening slit wall in place. We see a random pattern as shown in fig 10. Once again, no surprise.

Fig 10

Then we set up a wall with a very small slit. We see in fig 11 the same “band” pattern that we saw with our single slit pellet experiment. This is different from what we would have seen with a wave. A wave would have been smeared out to both sides. Eureka! We have confirmed that photons are particles because they have the same pattern as our particle experiment and do not show the distinctive visual fingerprint of waves going through a single slit.

Fig 11

So lets finish this part of the experiment and open the second slit in our wall.

Fig 12

What we find is in fig 12. 

But wait a minute... Notice that on the target under the L and R where we would expect the two bands of particle impacts we see nothing. But now we see various bands starting in the center and occurring to the left and right. This is an interference pattern!

This characterizes photons as waves.

After running the tests over and over again we see that this is consistent. With either the left or right slot open, we see a band (particle) pattern directly behind the open slit, and with both slits open we see the interference (wave) pattern. Wow. It looks like photons can act like both particles and waves depending on the test setup. This is referred to as the wave-particle duality of light by the way.

On closer analysis, maybe somehow the two streams of particles are interacting with each other on the way to the target screen. Maybe the wave behavior is more about particle properties than something mysterious. Maybe it's about electric or magnetic fields, or maybe gravity? Or perhaps occasionally a particle hits the edge of the slit as it passes through and is diverted, kind of like how a stream of water is diverted as it comes out of a faucet and you touch your finger to it? Perhaps this could explain why the particles are “acting like waves” when they pass through two holes.

Ok, so let's back to our test setup. Let's modify the gun so that only one photon is shot every time we pull the trigger. This will guarantee that as each particle travels to the target there will be no other particles in flight that could possibly interfere with it. Maybe this will fix things and show the expected particle.

When we run the photon test again by firing millions of single photons one at a time we find the following. With no slit wall after many firings we get the same random pattern of markings as before.

With a single slit we see a single band of marks, just as we would expect. Same is true if we have a double slit but alternately cover one or the other slit.

With double slits open, when we fire over and over one photon at a time, find find something very odd. We again see an interference pattern emerge!

Think about it: if we fire one proton at a time through the one open slit and we do it over and over again, we get a band behind the slit. However if we fire one proton at a time at two open slits, each proton will do one of three things: randomly hit the slit wall, or go through either one slit, or the other. We would expect to see two bands of impact marks on the far wall. But what we see is: when we fire one proton at a time and then look, and it over an over again an interference pattern emerges! This is definitely not what we saw when we fired the pellets.

Let's look at this in detail:

One photon at a time is being shot at the target... It either hits the wall (because doesn't go through a slit at all), or goes through one slit or the other, right? If the photon is a particle, we would expect it to end up in one of the two individual bands. What could be causing a single photon in flight, going through only one of the two open slits, to be redirected so it doesn't land in the "band" behind the slit it passed through? There are no other particles in flight which could interfere with it. It makes no sense. If the photon was hitting the side of a slit, and being re-directed, it would also do so in the single slit test, and since that is not happening we can eliminate this factor.

The fact that it is creating an interference pattern says that it quite literally MUST be interacting with something because the place where the proton impacted is not in a straight line from the gun to the slit. But what? There is no other photon with which to interact.

So we have this amazing occurrence that each individual photon is acting as if it is being interfered with by other photons as if part of a wave. We understand that with a huge number of particles in flight at the same time perhaps it is possible for them to “act” as a wave. After all isn't that the case with water? Trillions of individual molecules behave en-mass as a wave. But in this case how can you explain wave behavior, when there is only one photon at a time being shot at the target?

We are scientists. We can continue to modify the experiment to figure this situation out.

Here's what we'll do. We'll put a sensor device to “watch” each slit as the photon goes through. For each time we fire, it can tell us which slit each photon passes through, and then we can look at the target and see where it lands. This way we can figure out for each individual photon we fire, what path is taken. We can then, perhaps figure our where and how the photon is being misdirected.

Once again, we start with a single slit open. We fire each photon and the sensor correctly senses it and we find it impacts just where it should: at the target behind the slit we sensed it went through. We do this first with only the right slit open and then with the only the left open and after some time and many single photons fired, we end up with two bands of spots on the target: two bands, just as before without the sensors.This is good. Seems like our sensors are working and not affecting the protons flight pattern.

Then we open the two slits and fire one photon. In this case, the sensor shows it went through the right hand slit and it strikes the target on the right hand side. Then the next photon, randomly hits the slit wall and no photon is sensed going through either slot. Again and again we fire single photons and record which slit each went through, and where each one struck the target. In every case a photon either hit the slit wall (not going through either slit) or it went through the right or left slit and on to the target. For every case where a photon goes through either slit, it ended up striking the target in a band just behind the right or left slit. So now with the two slits open, and sensors activated, the protons decide to behave like particles again? What the heck?

If you've been paying attention you may have noticed a problem here. Without the slit sensors installed, we found that with both slits open, and shooting one photon at a time, we got the "unexpected" appearance of an interference pattern: the unmistakable sign of wave behavior. Now while we are monitoring the slits with our sensors, and we run the exact same procedure, we find the two separate bands of marks on the target: the sign of a particle.

We might presume that our sensors are affecting the test. Even today, decades after these tests first took place, most in the scientific community believe that the change in behavior of the photons must be the result of the sensors interacting with the particles. (After all we posit, in order to sense something we must disturb it at least a little) The sensors must be straightening out the photons somehow and causing them fly straight to the target. Over the years scientists have played around with different sensors and different ways to detect the photons with no change. We always see the same results: sensing the photons makes them behave like particles and not sensing them makes them behave like a wave. And also remember: with one slit covered, when we use the sensor, we get the band pattern. If the sensor was somehow affecting the flight pattern of the light it would affect it in this case also, but it doesn't. 

After decades of testing and analysis, quantum science has concluded an amazing thing: the sensors are not physically interacting enough with the protons as they go through the slits to re-direct their path. Here is the current working theory of what is going on:

Since the objects we are talking about are so small as to be invisible, and without any sensors are also undetectable, they act according to probability, not according to physical movement. For the period of time after we fire the electron and before we try to detect its position, the particle is in an unknown state: it is said to be in quantum superposition.

Most quantum researchers claim that the particles don't even exist physically during this period: that in this state the photons are actually a "wave function" and not a particle at all.This state will continue until the wave function interacts with something that “collapses” the wave function and forces the particle's position to be “revealed”.

So let's follow through with this alternative (quantum) way of looking at things.

A photon is emitted from a photon gun. Because of its tiny size and our inability to determine its location without any sensors, for a time it exists only as a probability, a wave function. For the moment it exists in quantum superposition. For each photon, we could calculate the probability that it will pass through the slit and strike the target, or not.

Actually each photon, while in superposition contains a probability wave function that describes every possible path it could have taken, an infinite number of paths. The particle exists as a probability wave until it is forced to “choose” a position by an interaction with something that collapses the state.

So for example, when a proton hits the target and it is sensed by the impact with a photographic plate, it must “reveal” a position. Since this is the first time the particle has encountered anything, its location is determined by the fact that it has been a probability wave since it left the photon gun so it's impact pattern on the target will reflect this previous probability wave behavior. It will not reflect what slit it went through because while in superposition in went through neither slit, or more accurately both and neither, as it traveled an infinite number of paths (every possible path in fact) probabilistically. So its impact location on the target is determined by this combined probability wave, and not a Newtonian path through a slit, on the way to the target.

Alternatively, if a sensor had been placed at the slit, it would have identified the location of the photon as it passed through. The wave function for the photon would have then been collapsed by the detection process, ending superposition at that point. With superposition concluded, the particle would then behave as a “Newtonian particle” and travel straight to the target in the classical Newtonian fashion. In this case the photon would travel in a straight path from the slit to the target and impact directly behind behind whichever slit it passed through. It would have left a mark close to all of the other marks from other photons which were identified as having gone through that particular slit.

What the scientists are saying is that when objects can't be detected they behave according to probability, not according to "normal rules" with characteristics like mass, speed, momentum and physical location. While being expressed as a probability function (wave) objects act like wave.

What gives them the property of a wave is rather interesting. Scientists theorize that they are blinking in and out of existence (yes this is really what tiny particles like electrons are theorized to do). So imagine something moving very quickly and blinking in and out of existence. They would appear and disappear in a regular pattern which could be plotted as a waveform. Hence, this is how a particle could possibly act as a wave. And why a particle, when in superposition, can exhibit characteristics such as interference patterns rather than exhibit traditional particle behavior. Different colors of light have different energy levels and exhibit different wavelengths reflecting the fact that the electrons blink in and out a varying frequencies.

Unfortunately this quantum probabilistic behavior is so foreign to we humans that many of us refuse to believe it. It just won't sink in because it doesn't make rational sense to us. We can more easily imagine that our slit sensor is pushing the photon onto a different trajectory than to think there is some still not understood machinery that manages whether an object needs to “reveal” it's location.

To make things even stranger: many scientists contend that the mechanism at play which collapses the wave function is consciousness itself. So the wave function is not collapsed by “hitting a target” or even by the fact that a camera witnessed it, but by the fact that a conscious being became aware of the results: for example, by witnessing the camera's image. Had no conscious entity been there to witness the camera, the quantum superposition would not have been disturbed. This has been proven in many ways over the decades.

Had enough? Well let take it up one more crazy notch. There are tests that show this effect is independent of time. i.e. that an event can change the results of a previous event.

Consider the following. In recent years scientists have devised a more cunning test that doesn't involve a sensor to detect which slit was involved. This way the argument that we must be somehow physically interfering with the proton as it transits the slit, can be avoided all together. It is called the Delayed Choice Experiment. I won't describe the details here but if interested the reader should see the links I've provided.

This test does sense the slit but it does it a very clever way. Once the photon goes through the slit it goes through a splitter mirror that spawns two twin photons which go through two very different parts of the test apparatus. One part of the apparatus collects target impact location data, the other part detects slit origin ½ the time and no slit origin the other ½ the time. The "choice" to collect slit data done randomly by a special silvered mirror. The trick is the silvered mirror which makes the "choice" is located such that it is struck AFTER it's twin photon already struck the target. So the choice to determine the path of the photon is made AFTER the target has already been struck.

So after test completion one can inspect the impact data on the target and see if it was caused by a photon for which slit data is known or not.

What we find is that for those photons who were eventually identified as having gone through a particular slit, the impact pattern was two bars. For those photons who were not linked to a particular slit they showed as having an interference pattern.

The points to emphasize are:

1- The path of the photon which eventually stuck the target was never sensed in any way, so there is no way to claim that the path of travel was interfered with at all.

2- The twin photon which eventually was sensed as having gone through a slit made "its choice" to show or not show its slit data AFTER its twin already struck the target. So the decision about whether to collect slit data happened after the target location data was collected.

See the following Youtube video that does a good job illustrating this experiment.

http://www.youtube.com/watch?v=hSRTvKgAs9c&list=PLD823D27CAF0732C0&index=4

It seems like magic, but it is really how our universe works.

The point of all this is to demonstrate that although you may experience our world as a clockwork mechanism where action and reaction make logical sense, you are not perceiving what is really going on. You are not seeing the true nature of things.

Here's another quick example of the impossible: it is called "quantum entanglement"

Part of the delayed choice experiment involves a single photon striking a crystal, which annihilates the original photon but emits two photons. These two photons are in a way twins. They share the same attributes. For example: like other electrons, photons have a property called spin than can be measured.

It has been discovered that these photons are linked by what is called “quantum entanglement”. What this means is, whatever property one has, the other has: like spin, although in this case their spin would always be opposite of each other.

The odd bit is this: once entangled, if the property of one changes the other immediately changes to match. So if for example if one of the two photons is directed by a mirror to the far end of the galaxy and one is kept around locally, and we were to change the spin of the local twin, the spin of the other photon which could be thousands of light-years away would change instantly. This action is immediate so it is in violation of the speed of light, which is what caused Einstein to refer to it as “spooky actions at a distance”. There is no "physical" connection between the two objects and no force or communication that interacts between them. So how can this be?

It looks like this phenomenon of entanglement may be the functioning mechanism behind the whole mystery of the double slit experiment.

 Is it possible that this mechanism also interconnects us all in some fashion? Could this be the basis of things like ESP or other psi phenomenon? Perhaps we are in some way all entangled at some quantum level?

This quantum mystery is somehow a part of the world that exists, and has been measured scientifically, but is outside of our everyday experience. Again- the point is: the world is not what you think it is.

It is important to consider this fact as we proceed and uncover other odd aspects of reality. If you find yourself having a hard time swallowing some new way of looking at existence, try reminding yourself what you just learned in these sections. Hopefully we've shown you that you don't really understand how the world works in the first place. So perhaps you should not hold so tightly to what you think you understand, and be open to other explanations that are a better fit with all the facts.

The effects shown in the double slit experiment and the effect of quantum entanglement are examples of things that shouldn't, -couldn't-, happen in a billiard ball world: a world where we are totally separate individuals which are in turn separate from the objects being tested. The data seems to show that there is something interconnecting us (possibly via consciousness itself) to everything. Unfortunately we are generally unable to perceive and therefore to discover the nature of this interconnection.

Interestingly, this sense (that there is a hidden nature tho things that is “under the surface) is exactly what many personally intuit. Perhaps this “feeling” many have is more than just a personal thought or emotion, but rather a sensation of a real thing?

Perhaps what mankind has called a “sixth sense” is in actuality just as “real” as touch or taste. Perhaps it is even more so? Perhaps touch, taste and the other senses are the illusions?

Here's where we are left after this experiment-
We have seen that there are interactions that are shown to occur that:
- violate the speed of light
- that seem not to care about time or even just the order of occurrences (chronology)
- that show interactions at great distances instantaneously
- and, most incredibly, are perhaps somehow linked to consciousness

I'll say it again: we may be seeing that not only is our world not a gigantic clockwork machine, but that it is, in fact, in some fashion connected to our consciousness!

Now, with this more informed sense of the possibilities, perhaps we can take a closer look at some of those odd things that happen in our world that seem to defy scientific explanation. Perhaps with a mind more open to oddities of how are reality actually works, we can make better sense of things.