Thanks to the work of countless scientists dating back millennia, we know a great deal about how nature works. We have an idea of our place in the unimaginably vast universe, an idea of how everything in the universe began and of how it might all end. But we don't know everything. What IS an electron, a photon, a quark or any other elementary particle? How do we visualize the particles of energy and matter, and how deeply have we pondered the fact that matter and energy are ultimately equivalent? We must consider too that particles are never isolated. They exist in situ in Einstein's well-established space-time, and, although the theories of relativity, which describe space-time, are highly successful, physicists speculate there may be more to it. It could be built of many dimensions or of inter-dimensions, and there is evidence that virtual particles bubble in and out of space-time fabric all the time. Our current understanding of space-time breaks down at the extremes of the universe - at its very beginning and at black holes - where gravity appears to be infinitely concentrated and physical space contracts to a point. What is a black hole really? And what came before this universe, if anything? These are the questions that ensure that the universe remains the beautifully complex puzzle it is and they are some of the best fodder for those amongst us who have philosophical leanings. Of all the areas of active research in physics, perhaps quantum mechanics contains the richest treasure of all. In fact, it is difficult to imagine someone who has grappled with quantum mechanics who hasn't been shocked by the unfathomable secret life of our universe.
We are not outside looking in on the mystery. We are made up of it too. Our bodies and our minds operate by quantum mechanical rules. Our furniture, our houses, the trees, soil and animals are all quantum mechanical in nature. Everything - all particles of matter and all particles of energy - at their heart are quantum mechanical. It is easy to forget this fact because the uncertainties inherent in particles of matter virtually cancel each other out when billions upon billions of atoms combine chemically and physically into an everyday macroscopic object. We can observe and measure different aspects of objects without disturbing them because the energy of the photons used to see the object represent a miniscule fraction of the energy, or momentum, of the object as a whole. We can measure both the velocity and the position of an object simultaneously because there is virtually no quantum uncertainty in such large collections of atoms.
None of these measurements work for us at the quantum mechanical scale of matter. It is not because we haven't found instruments fine enough. It is because, by its very nature, matter at this scale does not lend itself to such probing. This strange line drawn in the sand does not mean that quantum uncertainty disappears altogether in everyday sized objects. It just becomes imperceptible. The underlying physics does not change as we go from quantum scale to macroscopic scale; only our perception does.
So what does quantum mechanics tell us about matter and energy? I believe Richard Feynman was correct - all we need to appreciate the mystery of quantum mechanics is to look at how electrons (fermions or particles of matter) and photons (bosons or particles of energy) behave in various versions of Young's double slit experiment. The problem we have digesting the simple but unfathomable conclusions of this experiment has to do more with our minds than anything else.
Consider that we have evolved to interact with our classical everyday, and predictable, world of solid objects separated by empty spaces and permeated by time's one-way flow. Our perception is extremely limited. Our lives are only a micro-thin slip of time in the eons through which the universe has evolved. We can't even perceive evolution or geologic change. We don't move fast enough to see the effects of special relativity and we do not possess the sensory organs to see space-time stretch around our mass, but we do feel it as gravity's pull. In terms of size, we are unimaginably tiny specks in our own solar system, let alone the Milky Way, which is just one of countless galaxies in the universe. And yet, our perception is on far too large a scale to even begin to appreciate just how small an atom is. Not just our bodies are limited by our evolution. Our minds too have evolved to make logical sense of the everyday world, not quantum mechanics. We have extreme difficulty conceptualizing any reality other than our familiar three dimensions of space. What does a four-dimensional building block look like? Einstein's very useful and very well documented four-dimensional construct of space-time, where time is treated mathematically just like an additional spatial dimension, tells us that we exist in a context that is far more complex than our minds would let us believe. Have we digested the simple fact that time and space are complementary and equivalent components of space-time? According to special relativity, space contacts as time spreads out and vice versa. Neither time nor physical space is absolute. As I said we are limited beings, and while it is disconcerting to do so, isn't it good practice as scientific explorers to keep reminding ourselves of what we don't know, of what we can't know?
This discussion is mental preparation for working toward your own personal interpretation of quantum mechanics. You may have researched quantum mechanics and read about what some physicists have to say on the matter in the two previous articles in this series, or on your own. In this article I lay out my own interpretation for you to consider and critique. I tend to like to stick to the most basic raw data I can find while trying to resist any interpretive embellishment, a tactic I think most scientists use. For quantum mechanics this forces us into a quandary. Something has to give. Either you bend the data to fit into something you can conceive or you loosen your grip on common sense and logic and let your mind bend to the data. As I've explored quantum mechanics, I've experienced a slow migration from the former tactic to the latter one. Therefore, I personally resist ideas of hidden variables and other efforts to retain locality and realism when I ponder the experimental data. Most importantly, while I lay out my perspective, I leave your decision to you. Ponder the evidence and ponder my and other people's positions. Be brutal - find holes in arguments and consider new ways of looking at the data.
There are three basic experiments - Young's double slit experiment, the delayed choice experiment and the EPR thought experiment - that I tend to rely on as the basis for my interpretation of quantum mechanics. I try to allow myself to hear what they say in their most naked form. I explored the double slit experiment and the delayed choice experiment in the article What Is An Electron REALLY? and I covered the EPR thought experiment in Interpretations of Quantum Mechanics, but I think it is worth revisiting the first two experiment's results with an ear to what they say and, more importantly, what they don't say about the quantum behaviour of particles.
What Young's Double Slit Experiment Says
To refresh our memory, this 9-minute video by professor Jim Al-Khalili demonstrates how the double slit experiment works:
The double slit experiment demonstrates that particles such as photons and electrons have both a wave nature and a particle nature. A laser beam or an electron beam (red arrow) strikes a plate pierced by two slits, as shown below right.
If light is used, the light shines on a screen behind the plate. If electrons are used, the screen behind the plate is an electron detector, detecting the charge of each electron that strikes it. In both cases, an interference pattern appears (see below left bottom image). Both light and the electrons pass through the two slits as if they were waves. If a single slit is used, then no interference pattern is observed (see below left top image). These are the same results you would expect of classical mechanical waves, if you used water instead of particles. You create waves in a water tank and set up a barrier with either one or two slits and then observe the wave pattern behind that barrier.
And yet, both light (photons) and electrons are always detected as discrete particles on the detection screens. This is where classical physics and quantum physics part ways. The interference pattern is created by the varying density of discrete bright spots as these particles strike the screen. These results show very clearly that particles have both a wave and a particle nature, called wave-particle duality. This experiment has been done with atoms, molecules and even objects as large as buckyballs, proving that particles and objects of matter have a wave nature. When compared, they even their own specific frequencies. The probability of detection is mathematically the modulus squared of the wave-function. This gives you a real number that describes the probability density of finding a particle in a certain place at a certain time. Where waves cancel each other out, a particle merely shows up in another location at the detector.
This is not all, however. This experiment also shows us that particles are intrinsically probabilistic. If a beam of photons or electrons is slowed down so that just one particle is shot out at a time with a pause in between firings, which can be a millisecond or a year long, the interference pattern still shows up. Particles shot individually interfere with themselves. The images below left show how electrons shot individually gradually build up an interference pattern over time. This pattern build-up is completely random, so that if this experiment is repeated, the first few dots will appear in different random locations on the screen and so on, but the same interference pattern always builds up.
This tells us that somehow the particles "know" where to go in order to build up a pattern and that this seeming pre-arrangement process is statistically purely random.
Even more interesting is the fact that if we keep two slits open and place particle detectors at the slits, telling us which slit a particle goes through, the interference pattern disappears. Upon being measured, the particle's wave nature collapses into particle nature in other words. Recent experiments show that some information about a particle's position at the slits can be obtained and still retain some of the interference pattern. It is not an all or nothing situation. Disturbing the particle just a little bit (and obtaining a slightly unreliable answer as to which slit it passed through) will retain some (reduced) wave interference. An undisturbed particle acts like a wave. A disturbed particle collapses into its particle nature. A measurement is like a collision to a particle. In order to "see" the particle something must bounce off it such as a photon or electron. This means that a photon can never be seen in flight because it cannot be detected with 100% certainty until it reaches the end of its current life, until it is absorbed, for example, by an electron in a photomultiplier detection screen.
What Delayed Choice Experiments Tell Us
According to the double slit experiments, a particle seems to travel in one of two guises - wave and particle. When two slits are open, the particle seems to decide to travel as a wave as soon as it is emitted. When one slit is open, the particle seems to decide to travel as a particle as soon as it is emitted. When photons are used it seems clear that the photon acts like a wave as it goes through two slits simultaneously, interferes with itself, and then manifests as a particle when it is detected as a localized point of brightness on the detector screen. When a photon is shot through only one slit it seems to act like a particle the whole time, since no interference pattern is seen.
The question then is when does the particle decide which guise it's going to use? Another way of putting this is to ask if a particle senses the experimental apparatus it will travel through and decide right off the bat whether to act like a particle or wave accordingly? Or, does a particle remain in an indeterminate superimposed state throughout the experiment, and depending on what questions are asked of it, respond by "looking" like a wave or "looking like a particle? John Wheeler asked these questions in 1978 and set about answering them by designing a series of delayed choice thought experiments which were later carried out.
Delayed choice experiments showcase an especially interesting aspect of quantum behaviour. A common example of such a design is the simple interferometer. To explore particle behaviour, this experiment is done using two slightly different setups - one without a second particle beam splitter (setup #1) and one with a second beam splitter (setup #2). A diagram of setup #1 is shown below.
|adapted from Patrick Edwin Moran;Wikipedia|
Now we change the setup as shown in the image below.
|adapted from Patrick Edwin Moran;Wikipedia|
We can refine this setup further by removing or placing the second beam splitter after the photon has gone through the first beam splitter but before it has reached the second beam splitter. Photons obviously travel fast so in this case the distances between the mirrors and splitters can be lengthened, up to several kilometres as desired. When we do this, we should see whether a photon, which is set up to show particle effects, for example (starts with no second beam splitter), can reverse its decision en route and show interference (after fast insertion of the second beam splitter). As it turns out, the photon always shows particle effects when the second beam splitter is absent at the end and it always shows wave effects when the beam splitter is present at the end, even when the beam splitter is removed or put in just before the photon gets there. Does this mean that the photon somehow went backward in time to change its mind? Or is there another possible explanation for these results?
A Brief Look At the Effects of Relativity
Three particle behaviours made apparent by the delayed choice experiment - instantaneous information sharing, what seems to be retrocausality, and the particle's ability to be in two places at once - absolutely run counter to common sense. However these are not the only phenomena that don't follow our common sense rules.
Consider relativity. It is well documented experimentally that when objects approach light speed, time for them slows down relative to us, the observer. For example, unstable particles created in accelerators that are travelling very near light speed have been shown to exist for much longer than their non-relativistic lifespans. What if one of these particles could observe us, the observers? Our timeframe, or our "clock," would seem to be set on fast-forward, while its "clock" would run normally, at least from its perspective. And from our perspective, its "clock" would appear to run slow. This is time dilation in special relativity. Because of time dilation, within the limit of light speed, a past event for one observer can be a future event for another, and events separated by time in one frame of reference can appear simultaneous in another one. Now consider a particle such as a photon traveling exactly at light speed. To it, our clock would be running so fast our lives would pass in an instant before it. In fact the entire lifespan of the universe would pass in an instant. To us, the photon's "clock" would appear be stopped forever.
Meanwhile, another effect called Lorentz contraction in special relativity means that for a particle travelling near light speed, space contracts relative to an observer. Since it is impossible to imagine what a photon "looks" like in flight, let's imagine watching a space ship fly by us at close to light speed. That space ship would appear shortened in the direction it was going, although people in it would notice no such effect. For a photon flying past us at light speed, this would mean that physical space contracts to a flat sheet that has no width from our perspective. It would appear that the photon, with its "clock" stopped, would have no distance to travel, relative to us. From the photon's perspective, however, its "clock" and distance would appear unaffected. This doesn't mean that special relativity explains the photon's quantum behaviours, and it clearly doesn't significantly impact the same behaviour observed for electrons traveling through the experiments at speeds well below light speed, but it does remind us to exercise caution when we place our everyday-world assumptions on the quantum behaviours of particles.
I personally don't believe that backward time travel (retrocausality) is involved in the delayed choice experiment. Wheeler himself considered the photon to be neither particle nor wave at any given point in time. Instead it was intrinsically undefined, until the moment it was measured. As an undefined particle, it is not necessary to invoke retrocausality because the decision to define itself is made right at the detector (where the particle's wave function collapses) and not before. This would mean that in both setups, the photon travels in an undefined state even though it appears to us to travel as a particle in setup #1 and as a wave in setup #2. And when a second beam splitter is placed and removed, the photon simply makes its decision right there to act one way or the other. I find that Wheeler's assumption of an undefined state fits most simply with the results.
Conclusions of These Two Experiments
Both the double slit experiment and the delayed choice experiment tell us that photons (and electrons) travel in an undefined state as neither particle nor wave, or they travel as a superposition of both particle and wave, but they are never just particle or just wave alone as they travel. Once a particle hits a detector or is measured by some other mechanism, it must out of necessity experience a collision (either with an electron in matter or another photon) and a collision ends that particle's undefined or superimposed state. The particle's wave function collapses into a particle state. We can make a choice between neither and both at this point. It seems to me that both or "superimposed" is the better choice of the two possible undefined states. The beauty of the double slit experiment is that it clearly shows evidence of the particle's wave nature even though each particle hitting the detector hits it as a point-like particle. We cannot directly witness any particle travelling along as a wave but in this experiment we see clear evidence that it does act as a wave somehow as it travels.
Maybe the most spellbinding conclusion of these experiments is that, while traveling in a superimposed state where wave nature is always there, particles have no problem whatsoever traveling two different routes at the same time. Recall that one photon somehow must travel through both slits in order to interfere with itself as a wave. Where it ends up individually is random even though a wave pattern builds up as particles strike the detector. In fact, the double slit experiment suggests that an infinite number of simultaneous routes are possible as the interference pattern randomly builds up. This tells us that undetected particles are spread out in physical space as waves (and this space can be astronomically large). This spread out state acts according to the rules of probability only, and it is this aspect of their nature that lends to Einstein's spooky action at a distance (quantum entanglement) as well as the intrinsic unpredictability of all particle behaviour.
This means that, to the particle, there is no "here" or "there." Consider the second delayed choice scenario with the additional beam splitter. A particle split by a beam splitter travels two vastly separate routes simultaneously (as a wave). As it does so, it always retains the reality of its "twin" so that it always ends up at the detector interfering with itself as two waves. When no second beam splitter is present, I argue that the photon still travels in the same superimposed state but its wave nature is not apparent. Now consider the EPR experiment discussed in a previous article. Two quantum-entangled particles, separated by even astronomical distances, retain the reality of their twin so that when one particle's wave-function collapses, its twin particle's wave-function collapses immediately as well. Each particle retains the other particle's reality. Spin or location can also demonstrate this. They are always found to be complimentary to the entangled partner. For example, when one particle collapses as spin-up, its partner immediately collapses as spin-down and vice versa. Before the collapse neither particle has a defined quantum spin orientation. Do these experiments tell us that faster than light communication or backwards through time communication is going on between particles? Or do they tell us that distance (physical space) itself is an illusion? I personally think, once again, that it is simplest to accept the second choice. The EPR experiment illuminates the inescapable nature of non-locality in quantum mechanics. You can tweak the definition a bit and call it "non-separability" just as accurately. In fact, I believe that this also defines particles as interdependent because the reality of two entangled particles is spread out as a single wave-function, which describes them both as a single more complex entity. And when we consider the probabilities of all the wave-functions of all particles in the universe spreading out to infinity, it seems that, out of necessity, every particle must overlap with every other particle's wave-function, even though the chance of a particle across the universe interacting with a particle right here would be infinitesimally small.
Is the superimposed state of a particle (which we cannot directly observe, measure or experience) the reality? Or, is the collapsed state where the particle is defined by its particle nature the reality? It is interesting to speculate on these choices when we consider that the particle in flight (the photon of light flying through space or the electron flying around the nucleus, for example) is dynamic BUT undefined and the collided or absorbed particle is static, like an afterimage, but defined. When we observe and measure, what are we observing or measuring? When we think of the endless flow of transformation and evolution in the universe, is that activity encapsulated in the still snapshots we take with our instruments or is better encapsulated in the superimposed flowing reality that escapes our notice?
What is an electron then? Mathematically it is a wave-function, but does that wave-function have any physical reality? It seems, based on the results above, that it transforms into what we would normally call a physically real locatable object only when its wave-function collapses. Does it appear then ex nihilo? The wave function is mathematically predicted to extend in all directions without end, although the probability density of a particle's location is centered on one location and very rapidly diminishes from that point. Is this wave function also the "real" particle, and if so where does each particle end? Each wave-function seems to be filled with the smallest probability of the entire universe's matter. The experimental results of the double slit experiment suggest to us that there is no such thing as absolutely empty space. Space seems to be a pregnant "empty" filled with wave-particle-position-spin-velocity potential. Otherwise, we must consider space to be a true vacuum with particles appearing out of nothing. To me, at least, the former possibility - a potential-filled space - seems to fit the quantum experimental results best.
I think many of us expect that, with effort and time, we can reach an understanding of the workings of the universe, and that somehow it all has to fit together like a puzzle does. In that puzzle, each piece should have its own definable separate physical reality. Quantum mechanics tells us, very clearly, that none of this is true. It's a slap in anyone's logical face, and it can feel very defeating.
There are clear merits of, and subscribers to, all the interpretations of quantum mechanics investigated in the last article. However, I personally think it is simplest to accept reality at its deepest level as a potential-filled emptiness. What we perceive of this reality seems to me to resemble a series of moving flashcards creating a moment-to-moment forward flow of time. Each "flashcard" is a slightly changed collapsed state of particles. Our perception is like the act of flipping through the cards, and this progression of subtle change after subtle change is what we perceive as the flow of time. I imagine our perception as being a bit like the changing views of this cute kitten stalking the camera:
We perceive the cards to be real but they are a facsimile of a deeply complex and truly dynamic potential-filled "space." Each card shows us our subjective reality at a particular moment in time. What is shown on it depends entirely on how we make our observation.
My interpretation disagrees with the Copenhagen interpretation not because I assume that the wave-function is real, but because I consider neither it nor the collapsed particle state to be real. An atom therefore has no intrinsic reality. An atom of matter, as a complex wave function, has no permanent properties. Its mass can be converted entirely to energy and vice versa. The locations, velocities, spin and all other quantum properties of its constituent particles - electrons and quarks - depend on how they are observed and have no immutable intrinsic reality. So, then, is the atom just a product of relationships, all of which have no fundamental reality? I would answer yes. If there is any fundamental reality, I think it is the potential-filled "space" of the universe. I use the word "space" with some skepticism because space too can transmute into time and vice versa (as shown by special relativity), suggesting that it too has no ultimate reality. To the particle in the experiments, it too seems that physical space and distance have no meaning.
The photon and electron and all particles of matter and energy are alive as superimposed states in space-time, which seems to be inexorably connected to them. I think we are just not built to perceive it.
By extension, if particles of matter have potential but no tangible reality, then macroscopic objects also have no tangible reality, even though we experience them as reliable, stable and solid separate entities. The desk, the moon, our bodies and the galaxies, are illusions or mirages. Some of these mirages last for billions of years, but they are no more physically real than the particles/wave-functions that make them up.
Check out the next article: Sometimes Passions Run High, a response to the overwhelming response to this article.