Most traditional [spiritual] paths were developed in prescientific cultures. Consequently, many of their teachings are expressed in terms of cosmologies or world views which we no longer find relevant. . .The question then naturally arises: Is it possible to incorporate both science and mysticism into a single, coherent world view? . . .Up until the first quarter of the twentieth century science was wedded to a materialist philosophy which was inherently antagonistic to all forms of religious insight. With the advent of quantum physics, however, this materialist philosophy has become scientifically untenable. That is, the evidence of science itself contradicts a purely materialistic account of the universe.
-Challenge and Response 
The primary purpose of this essay is to explain how quantum mechanics shows that the materialistic common sense notion of reality is an illusion, i.e., that the objective existence of the world is an illusion. If the reader is familiar with the teachings of the mystics, this may sound familiar. Consider, for example, the following words written by a little-known Western mystic:
The appearance of an objective world distinguishable from a subjective self is but the imaginary form in which Consciousness Perfectly Realizes Itself.
-Joel Morwood 
Now listen to Niels Bohr, the pioneer of 20th century physics:
An independent reality, in the ordinary physical sense, can neither be ascribed to the phenomena nor to the agencies of observation. 
Consider the words of Shankara, the famous Hindu philosopher:
All things -- from Brahma the creator down to a single blade of grass -- are. . .simply appearances and not real. 
Compare with Werner Heisenberg, the inventor of quantum mechanics:
If one wants to give an accurate description of the elementary particle. . .the only thing which can be written down as description is a probability function. But then one sees that not even the quality of being. . .belongs to what is described. 
These people make the outrageous claim that we normally live in the delusion that there is a real objective world. Since this seems to be in blatant contradiction with both our immediate experience and everything most of us were ever taught, our natural response is to dismiss it as ludicrous.
For a typical modern Westerner, it might be easy to dismiss the radical claims of a few isolated mystics. But can we dismiss the physicists so easily? Niels Bohr and Werner Heisenberg are legends of 20th century science. Nobel prize winners. They are the creators of quantum mechanics, the most precise and far-reaching physical theory ever devised. It explains how the sun shines, how molecules bond together, how iron is magnetized, and even why objects are hard. This is the physics that gives us computer chips, lasers, and atomic bombs. So to dismiss quantum mechanics is to throw out the cornerstone of modern physics and the basic theory at the foundation of all these modern wonders. If we are to be honest with ourselves, we had better think twice before dismissing what Bohr and Heisenberg have to say and take a closer look.
Our experience, for the most part, conforms to the idea that there really is an objective world out there. Most of us take thousands of objects to be real every day and find that there is no contradiction with experience at all. Nevertheless, both the mystics and the physicists claim that the objective world is an illusion. But if this common sense idea of an objective world is wrong, why does it seem so right? To shed some light on this question and its answer, let us digress for a moment and consider the following scenario.
Imagine going back in time 3000 years and encountering people convinced that the world is really a flat disc. When you hear this you tell them that they are mistaken, that the world is really round. But you become quite embarrassed when they ask you to prove it to them and you find that you can not. After all, their experience conforms to the idea that the earth is really flat. They measure out land and make road maps using plane geometry and never find any contradiction with experience at all. So your claim that the earth is really round sounds to them like a delusion and they dismiss you as a crazy mystic -- especially when you start telling them about people from your time who ascend into the heavens in a blaze of fire where they are able to look down upon the whole created world and actually see that it is round. So you get back in your time machine and head back home to the future.
Due to a 500 year miscalculation, you accidentally end up in the year 1495 rather than 1995. You are happy to find that, in this age of adventurous transatlantic navigation, it is widely known and accepted by everyone that the earth is round. Having just been unable to give hard evidence that the earth is round, you go to a nearby university and ask a young astronomy student to tell you what evidence there is for a round earth. He tells you about the recent voyages across the ocean by Columbus and others, and how these voyages were based on the latest navigational technologies and cosmological theories of the day. The flat earth idea simply is not consistent with this new evidence, he tells you. When you object that our direct experience tells us that the earth is flat, he then explains to you that our direct experience is limited. Because the earth is so large and we are normally limited to only a small part of it, the earth appears flat even though it really is not. Our experience today is not as limited as it was in the past. We can now verify the curvature of the earth by subtle astronomical measurements or long ocean voyages.
Although you are quite relieved to have returned to an enlightened age, this young university student tells you in the course of his explanation that the latest cosmological theories assume that the earth is stationary at the center of the universe. It seems that, in this age of enlightenment, there are still some illusions. When you ask him if it is possible that the sun, rather than the earth, is the center of the universe, he says that would mean that the earth is moving, and we don't experience the earth moving. It would also mean that the earth is spinning like a top, and we do not experience that either. And most important of all, we obviously see the sun rise, move through the sky, and set. This is our direct experience, and there is no evidence to contradict it or suggest otherwise. Once again, you find that you are not able to provide any evidence to contradict these illusions of the past. Nevertheless, the young astronomy student seems to consider your suggestion (his name, you later find out, is Nicholas Copernicus). Disappointed, you jump in your time machine and head back to the present.
Again, you head to a local university to find out what evidence there is that the sun is the center of the solar system. You learn about the telescope that Galileo invented and used to discover the phases of Venus, and about how Newton's laws of motion explain why we do not feel the earth moving. You are also told about the recent voyages through the solar system by space probes, whose precise course between the planets was calculated using the heliocentric theory. So, although our limited experience may suggest that the heavens revolve around the stationary earth, if we expand our experience with telescopes and space travel, then this view is no longer consistent with experience.
If cultures in the past have been so deluded about reality, how can we be so sure that our present world view is so real? Is it not foolish to imagine that we are somehow immune to such illusions, that we have figured it all out once and for all? Could it be that our idea of the objective world is an illusion, just as much an illusion as the idea of a flat or stationary earth? It is clear from the foregoing story that we can not be certain that there must be an objective world just because that is consistent with our experience. That would be like arguing 3000 years ago that the earth must be flat just because that fits with the limited experience of that era, or arguing 500 years ago that the earth must be stationary just because it fits with the limited experience of that age. How can we know that no wonders lie beyond the limits of our present experience? How can we be certain that another reality does not lie hidden beneath our present illusions?
By analogy with these examples from the past, we can reconcile our experience of a real objective world with the shocking claims of Heisenberg and Bohr. The solution is to recognize that, just like all other people in the past, our experience is limited. Due to this limited experience, we often fall prey to the illusion that what seems to be absolutely true is absolutely true. Just as the belief that the world is flat or stationary is at best a useful fiction, and not at all real, the belief that the world has objective existence is also no more than a useful fiction, an imagination, an illusion. Of course, these fictions fit much of our ordinary experience. But the moment we depart from the limitations of the ordinary and expand our experience to include more subtle observations, these fictions are revealed to be just approximations to a more comprehensive reality.
To quote Bohr and Heisenberg once more,
As our knowledge becomes wider, we must always be prepared. . .to expect alterations in the point of view best suited for the ordering of our experience. 
The existing scientific concepts cover always only a very limited part of reality, and the other part that has not yet been understood is infinite. Whenever we proceed from the known into the unknown we may hope to understand, but we may have to learn at the same time a new meaning of the word 'understanding'. 
Having understood how, in spite of our experience to the contrary, it is possible that the objective existence of the world is an illusion, let us now look at the quantum mechanical evidence itself. Keep in mind, however, that this evidence, like the evidence showing the earth is not flat and the evidence showing the earth is not stationary, will necessarily draw from phenomena that lie outside the usual limitations of our experience.
Around the turn of the century, experiments were beginning to open a window to the world of the atom, a world that had before been hidden completely from view. But the common-sense ideas of classical physics contradicted what was observed in this new world. Just to give one important example, the colors of light emitted by atoms did not agree at all with what classical physics predicted. Max Planck discovered that the observed colors can be explained if it is assumed that light is emitted in chunks that he called quanta. This was quite a paradox, since many experiments seemed to indicate that light was a wave. To make matters worse, this strange wave-particle paradox also affected material particles like electrons.
To demonstrate this strange situation, imagine that we have a source of particles behind a wall with two slits in it. On the other side of the wall is a screen that can detect the particles coming through the two slits. Since particles are localized in space, each one is emitted from the source, travels through one hole or the other, and hits the screen. The pattern we expect, therefore, is two clusters of points corresponding to particles that went through each of the two slits. Note that these results are consistent with the assumption that the particles had a definite path through one slit or the other just as any objectively existing thing. With large objects of our ordinary experience, this is actually what happens when the experiment is performed.
Next imagine that the experiment is performed again, only instead of localized particles traveling through empty space, the whole space is permeated with some medium, such as water, and the source disturbs this medium, continuously emitting waves in all directions. These waves are not sent out in chunks like particles, and they are not localized in space like particles. As a result, a wave does not go through one slit or the other, like a particle, but goes through both slits simultaneously. Another essential difference is that waves can interfere with each other. When the two waves from the two slits come together out of phase, they can cancel each other out, leaving nothing. This is very different from what a particle does! And the results reflect this difference: the screen shows a wave interference pattern, with peaks where the waves add up and troughs where they cancel out. Remember also that these waves are continuous, so we see a smooth disturbance of the screen, not chunks plopping on the screen like particles. This is exactly what we observe when the experiment is done with ordinary waves.
When this experiment is done on a microscopic scale with quanta, however, the results are a weird mixture of waves and particles. The source emits particles in chunks, and the screen detects particles in chunks. But the pattern on the screen is the pattern for waves! We can not say it is a wave since it arrives in chunks. We can not say it is a particle since it has a wave pattern.
The interference pattern tells us that the particle is not localized in space as it travels from the source to the screen. Like a wave, it travels through both slits. We can even test this by looking to see if we can tell which slit the particle goes through. When we look, we can see it go through one slit or the other, just like a particle, but then the pattern we see on the screen is not a wave pattern anymore, but a particle pattern. So the only way to see the wave pattern is to not look at which way it went; and when we look, we do not see the wave pattern anymore. Therefore, when we do not look at it, the particle is a non-local wave! Moreover, this is not a physical wave like a water wave, but a wave of probability describing the potential for the particle to exist. It is not that the particle has a real position and we just do not know what it is. It positively does not have a position in space and time. Its existence is not that of an ordinary object which we think of as having a definite and objective position in space. It is a non-local wave of probability, with no definite or objective existence in the ordinary sense.
Here is another similar experiment. A beam of quanta are split and sent in two different directions, and are detected by two detectors. Half are detected at one and half at the other. So far, this is consistent with the idea that each one followed one path or the other.
Now we put in another beam splitter where the two beams intersect. This splits the two beams into four, but it is positioned so that they are paired up. If the quanta were actually particles, then we would still get half at one detector and half at the other. We actually get all of them at one detector and none at the other! This happens because the probability waves interfere constructively at one detector and interfere destructively at the other. Yet the detectors detect particles in chunks.
We can even do this one particle at a time. Without the mirror, half the time the particle goes to one detector and half the time it goes to the other detector. We can then say that it took one route or the other. But with the mirror, it will always be detected at one of the detectors and never at the other. This can not be explained by thinking that it took one path or the other. We can even delay the choice of putting the mirror in until the last instant. So even in the first case, we can not really say it took one path or the other, even though the results let us think so if we want. In fact, the particle does not take one route or the other, does not exist as a localized particle in space when we are not looking.
These experiments can be done with electrons, photons, protons, neutrons--any subatomic particle you like. The result will be the same. It unequivocally demonstrates that all subatomic particles do not have definite material existence when they are not being observed. These effects can also be observed with atoms and small molecules. When millions and millions of molecules are clumped together into a macroscopic object, however, the effects are not usually noticeable. This does not mean, however, that the weird quantum reality is not there anymore. It is just that it is not noticeable anymore. It is like when we consider only a small area of the earth: just because the curvature effects are not noticeable does not mean that the earth is not round anymore. As Heisenberg said,
The statistical features of natural laws are ubiquitous and a matter of principle. It's just that these quantum-mechanical features are far more obvious in atomic structures than in the objects of daily experience. 
So all matter is really this way. Even large macroscopic chunks of matter. The independent existence of the material world is an illusion. This is very startling--or it should be very startling. As Niels Bohr once said,
Those who are not shocked when they first come across quantum theory cannot possibly have understood it. 
Schrödinger, one of the inventors of quantum mechanics, understood just how radical this is. To dramatize it, he imagined putting a cat in a box with a radioactive atom and a detector that would trigger a bottle of poison to break when the atom decayed. Since the whole thing is unobserved in the box, the state of the atom, and consequently the state of the cat, is not determinate. Just as the photon does not take one route or the other unless it is observed, the cat is not alive or dead unless it is observed. This is so outrageous that even some quantum physicists find it hard to swallow. Yet that is exactly what quantum mechanics itself says. There is even hard evidence now that large macroscopic objects can exist in these strange non-local quantum states. Examples are superconductors, superfluids, and lasers. These are the exceptions, though. For most macroscopic objects, the quantum effects are not noticeable. They are there, to be sure, but they are not noticeable.
The reason the quantum nature of most objects is not noticeable is because of a phenomenon called decoherence. When the one wave went through the two slits, the resulting two waves had a phase relationship with each other that created the interference pattern. Such waves are called coherent. When millions and millions of particles are involved, though, there are so many of these phase relationships interfering that they appear on the macroscopic scale to average out, or decohere. This is analogous to how the curvature of the earth appears to disappear in a limited area of its surface. The decoherence effect is the reason we can usually get away with ignoring the quantum coherence effects of macroscopic objects. It is the reason classical physics works so well for ordinary objects.
It is important to remember that, like the earth appearing flat, this decoherence effect is only an appearance and is not real. The quantum coherence is really still there--it is just hidden in the microscopic details and is not noticeable on the macroscopic scale. So, in principle, macroscopic objects, just like microscopic objects, do not have objective existence in definite locations while they are not observed. Nevertheless, because of this decoherence effect, the macroscopic world appears in a manner that is consistent with objectively existing matter. So, although the objects of the world do not actually exist objectively, they appear as if they did.
Compare this situation with the words of philosophers Franklin Merrell- Wolff and Immanuel Kant:
Just as we must regard the presence of objects as a seeming, and no more, so is the vanishing only a seeming. The nonderivative Reality is unaffected in either case. 
All existence and all change in time have thus to be viewed as simply a mode of the existence of that which remains and persists. . .The unity of experience would never be possible if we were willing to allow that new things, that is new substances, could come into existence. 
Buddhist philosophers Longchenpa and Nagarjuna echo the same idea:
What appears never becomes what it seems to be. 
The ultimately reality is unmade; it will never be other than what it always is. 
Philosophers and mystics, East and West, are therefore in agreement with modern quantum theory: the objective existence of the world is an illusion. Just what is the Reality behind this illusion, then? That important question is a topic deserving substantial treatment on its own and so will be left for later.