AT first, scientists didn’t have much of an idea what light was.
Then, science started studying waves (water surface waves, sound waves, etc.), and soon they noticed something very interesting: Light has a startling number of similarities to those waves:
waves travel at a constant speed, independent of the characteristics of the emitter
waves exhibit doppler shifts when emitted by fast-moving objects
wave speed is dependent on what it’s going through; different substances allow different speeds
waves exhibit refraction effects when passing into a medium of different propagation speed, plus some reflection at the same interface, with the degree of reflection correlating to the difference in propagation speed
waves can carry multiple frequencies that can be separately detected by resonators
wave frequencies can be split (prism-style) by a refractor that supports different propagation speeds for different frequencies
waves can pass through each other without noticeably affecting each other
while occupying the same space, waves can additively cancel each other, producing interference patterns
waves can be blocked by an obstacle, but the effectiveness of this blocking seems to be a function of the size of the obstacle and the frequency of the wave
when a wave is blocked by an obstacle, the portion of the wave that goes past the edge of the obstacle tends to creep into the area that was blocked (the shadowed zone), and the degree of this creep varies depending on the frequency
waves reflect off of a smooth surface at the incident angle, provided imperfections in that surface are substantially smaller than the wavelength
All of these effects can be observed in water waves, sound waves (which may travel through air, liquids, or solids) — and in experiments with light.
That pretty compellingly indicates that light is a wave. And so for quite some time, light was said to be a wave.
Then, later, some new experiments showed something interesting. If you ran a light emitter at extremely low power, then the light energy was delivered to the receiver in very small but discrete, fixed-size units, dubbed “quanta.” And if the light receiver was a sizable screen set up to allow you to see where the light energy was received, then each time a quantum of light energy was received, you could see that it arrived at a discrete spot on the screen. And the spot seems to be randomly located each time.
But if you also set up the experiment so that light from the source should generate a wave interference pattern on the screen (via the “double slit” arrangement), and then you take note of where each individual quantum of energy arrives, then you find that over many repeated trials they add up to a wave interference pattern.
Very interesting indeed. Now — without any immediate way to know exactly what’s really going on, and knowing of the lengthy set of wave properties listed above, how would you describe light after observing this new, quantum energy phenomenon? It seems pretty obvious to me that you would say:
Light is waves with some particle-like properties.
Simple enough, right? But that’s not what they said. Instead, the scientists studying these behaviors of light declared that light was particles with wave-like properties.
This seems rather odd, because the list of coincidental wave behaviors above is far more compelling an indication that waves are at work than the quantum-energy experiment results are that particles are at work. It’s pretty easy to imagine scenarios in which waves exhibit particle-like behavior. For example, light could be a wave, but with photon particles riding (tracking) the wave, with each particle starting out in a random direction from the source, then following the wave’s peak until it reaches a target suitable for energy acquisition. Or, there might not be any particles at all, and light waves, when they strike an object capable of receiving light energy, have a random or pseudo-random probability of delivering a quantum of energy at any particular spot they hit.
But particles with wave-like properties? How would that work, even hypothetically? Nevertheless, that’s what the scientists said. They declared that light was a particle, but one that has (through unknown mechanism) “wave-like properties.” And those properties just happen to include everything in the list at the top of this article! The mountain of evidence that light is waves was collapsed into the incredibly vague adjective “wave-like,” and duct-taped to a particle theory of light.
And it gets worse. Scientists did the double-slit experiment with an electron emitter instead of light emitter, got the same results, and then thought of a cool idea: What if we put a detector just after the slits, so we can tell which slit the electron went through? When they did that, the wave interference pattern disappeared — instead, repeated electron arrivals added up to two large fuzzy spots (one for each slit). OK, what could that mean? Could it mean that the detector is doing something to the wave, or causing a new wave to be initiated after the slits? That seems like a pretty good guess — but no. These results were interpreted as meaning that particles have wave-like properties until they are observed (by the target screen or the detector, whichever comes first), and instantly “collapse” to particle behavior when such observation is attempted. According to this explanation, the electron passes through both slits simultaneously, its location in space essentially undefined, but then acquires a specific location when the detector looks at it.
To me, this mode of thinking suffers from some really puzzling questions. A single wave emitted from one spot (after the slits) can generate a big fuzzy spot on the screen. Isn’t it possible that a new wave was started when the electron was “detected?” And how do you detect an electron flying through space nearby without seriously affecting that very electron?
And most importantly, isn’t it antithetical to science to say that something happens only when we’re not looking? And instantly does something else as soon as we look?!
Any theory of physics should be at least crudely simulable via a computer program. (If it has a random factor, fine, you can use the computer’s pseudo-random number generator as part of your program.) I’m not talking about using a computer to predict which slit a real electron will go through or anything like that — the input data would be impossible to gather, plus any random factor would ruin the prediction — I’m just talking about making a computer simulation that behaves in a similar manner to the real thing; i.e. that allows you to duplicate the results of well-known experiments, as you would duplicate them with real equipment in a lab.
Making a simulation of waves with particle-like behavior would be easy. We already can simulate waves quite readily (here’s a good simulator written by Paul Falstad). Throwing in particles that semi-randomly track wave crests, or creating probabilistic whole-quantum energy delivery whenever a wave impacts a wave-absorbing target, would be pretty easy.
But how would you write a computer simulation of “particles with wave-like properties?” How would you write code that simulates particles that exhibit additive properties highly reminiscent of wave effects? I can’t even imagine how you would begin to do that. Probably, you would wind up writing a program just like I described above (i.e. waves with particle-like properties), make the program keep the waves and particles hidden and show only what you would see in a real lab — then simply call your simulation “particles with wave-like properties.” And hope no one studies your sourcecode and realizes you really simulated waves with particle-like properties.
Of course, I’m aware that computer simulation doesn’t prove that the process being simulated is an accurate depiction of what happens in the real world. But it at least demonstrates that you have a coherent theory of what is going on. If you can’t even do that, then what the hell is your theory, really? Just some vague phrases, loosely re-describing the results of your experiments?
Why would scientists interpret these experiments the way they did? A few answers seem likely.
First, most scientists want to discover something new — and the newer, the better. The list of wave behaviors at the top of this article, and their applicability to light, is an old discovery, made by yester-century’s scientists. The scientists who discovered quantum theory wanted something different. Plus there’s something really special and revolutionary about seeming to overturn the beliefs of previous generations of scientists. It makes the current discoveries (and their discoverers) a lot more important.
But even more significantly: Particles with spookily undefinable, wave-like properties had something else going for them. They were a lot more compatible with Einstein’s relativity, which had gained wide acceptance by that point. Einstein asserted that all motion is relative, meaning that objects move only relative to each other, not relative to any such thing as “absolute space.” But if there is no such thing as absolute space, then there can be no such thing as a wave propagation medium for light (and other energies and forces) to propagate through, for that would itself be an absolute space. In effect, Einstein was proposing that light gets from emitter to receiver without actually propagating through a medium, yet still somehow exhibits a long list of wave-identical characteristics.
How it does that, Einstein didn’t really say. But the weight of Einstein’s successes in physics carried the day, and by the time quantum theory’s discoverers had to choose between waves with particle-like properties or particles with wave-like properties, the latter seemed the natural fit to the Einsteinian revolution. When quantum theory was developed, the idea that light actually is a propagation wave was viewed as a quaint anachronism, with the term Luminiferous Aether forever welded to wave theory, precisely for reasons of its archaic, almost alchemy-like tone. It didn’t occur to these scientists, apparently, that Einstein might be right about some things, but wrong about others. It didn’t occur to them that some of Einstein’s pronouncements might be based on philosophical persuasion, not empirical science.
Specifically, I’m referring to the philosophical mandate that there be nothing significant outside this universe — either absolutely nothing, or some perpetually ephemeral, unknowable-by-definition “mind of God.” Einstein was a Deist and may have leaned toward the latter. But either way, the idea that this world is actually a mechanistic thing in the same category as a computer simulation or video game that humans often create, is considered almost anti-science by those of this slant. The ideology at work here holds that anything we can’t examine in a lab isn’t just outside science’s current purview — it’s actually required to not exist when formulating scientific theories. If it’s really hard to tell what an electron is doing because you can’t watch it directly the way you can watch a football, then it’s not really doing anything! It only does something when you can observe it — when it arrives at the target screen or detector. If we can’t examine creators or their mechanisms, then they don’t exist — or exist only as an undefinable mind from whence our physical laws (the thing we can directly observe) spring forth with no mechanism of implementation.
Now you may be wondering, what difference does it make? Wave-like particles? Particle-like waves? As long as we’re all talking about the same effects, what’s the difference? Either way, can’t we still use known effects and behaviors in our technologies?
Yeah, we sure can. And that’s fine, as far as that goes. But it’s still a problem because:
(a) It has the potential to stifle future discoveries by shutting down avenues of exploration, and
(b) It teaches new generations of budding scientists that protecting philosophies of the acceptable and unacceptable is a more important part of science than following the most obvious indications of the empirical evidence.
Update 2011.02.06 — Wording of sixth (prism) bullet corrected. “Several answers” changed to “A few answers”.
Theory As Simulation