Читать книгу Quantum Physics is NOT Weird - Paul J. van Leeuwen - Страница 36

The Bohr atom model gained a firmer foothold with the Davisson-Germer experiment, but the idea that particles also had a wave character caused many physicists to experience strong feelings of disbelief and unease. Therefore, many experiments had to be done to confirm this paradoxical behavior. Electrons and photons are very apt for such experiments because they are easy to bundle and fire, but also larger objects such as protons and even large molecules ^[1] are confirmed to show demonstrable wave behavior. Many of those experiments are sophisticated versions of the double-slit experiment.

Figure 5.1: Confusing message: electrons pictured as simultaneous traveling particles and waves.

There is a tremendous amount of confusing information to be found on the paradoxical particle-wave behavior of quantum objects. You will encounter this in popular science literature, on the Internet in countless YouTube videos, and in the better academic media. The above figure 5.1 concerning the Davisson-Germer experiment, found at www.natuurkunde.nl, may serve here as an example. In both situations, upper left and right, electrons are depicted as concrete particles traveling a path, however ultimately bringing interference about. That’s simply confusing and – as we shall see – even misleading.

With such confusing representations – which have their origin in a stubborn material Newtonian image of the world – you are already misled before you even start to delve deeper into quantum physics. We will see that there is much to be said about these material-physical duality representations and that they are not necessary either.

Not only with electrons – repeatedly pictured as tiny matter balls – but also with photons, we encounter often this confusing dual image of particles that are simultaneously acting as matter and wave. Figure 5.2 intended as a clarifying illustration of the photoelectric effect, can be found on Wikimedia Commons. The figure shows a photon traveling in the form of an energy particle – a Newton corpuscle – on its way to the metal surface to release there an electron particle. It is important to realize that this kind of behavior has been never observed by instruments. This is an image intended to aid comprehension; however, the effect is confusion.

Figure 5.2: Misleading and confusing: both photon and electron presented as speeding tiny hard balls.

Source: Wikimedia Commons.

My advice is this. Try to keep, albeit it can be sometimes helpful, the idea of little particles physically traveling along a concrete path – and therefore existing at every point of their path at every point in time – at bay, set it apart from your ideas about reality. If you succeed in such a mental stance, it won’t be mind-shattering when it becomes apparent that this image cannot be correct.

However, the particle image of a quantum object can serve as a helpful mental crutch, an approach that I will often use when analyzing experiments on the following pages. It can be useful, but keep in mind that the image of a persistent material object is not necessarily a presentation of reality. The contradictions that we encounter when we try to maintain the image of persistent particles will demonstrate that the particle image must be wrong. Most mathematicians consider a proof by contradiction ^[2] as valid.

In any case, it should be clear now that, whether we fire photons or electrons at a double-slit, we will observe an interference pattern on the screen behind the slits. This outcome has been confirmed in numerous experiments in many laboratories. An interference pattern showing up in an experiment is considered by every physicist as evidence that the electrons and photons used in their experiments behaved like waves. However, please keep in mind again that this inference is not bulletproof in a strictly logical sense. In logic it is not allowed to derive from proposition A => B (wave thus interference) the proposition B => A (interference so the electron is a wave).

What the actual ‘substance’ is that is behaving like a wave has not been determined. In other words, the photon and the electron exhibit wave behavior but do not have to be these waves themselves, just like the surfer is not the wave. It is unquestionably true that waves do play an important role when we observe interference. I am not disputing that. But this illogical inference has serious consequences for any attempt to understand the implications of quantum physics, it confuses the issue as we will see.

With large numbers of photons or electrons, it is still conceivable that half of them will pass one slit and the other half the other slit and that they somehow reinforce or extinguish each other subsequently at the screen. For that to succeed they should then be paired one-to-one with each other, rather unlikely, but we cannot exclude that possibility without experimental verification. So, what will happen if we restrict the intensity of the beam to single particles fired one at a time at the double-slit? In case you were wondering how to accomplish such a feat: you can diminish the perceived intensity of a light source as much as required by simply placing the source further away.

That is exactly how it was done in practice in the first double-slit experiments. If you increase the distance to a point-like light source for instance by a factor three, keeping the diameter of the detector surface equal, you will then, on average, capture one ninth of the previous number of photons per second. The intensity is inversely quadratic proportional to the distance. This applies also to a diffuse electron source. A practical example: a LED lamp of 1 Watt in New York will deliver about 1 photon per second in someone’s eye pupil in Indianapolis.

You would probably expect that when we fire a single particle per ‘shot’, that this particle now will pass through only one of the two slits and that therefore the interference pattern will disappear. This one-particle-at-a-time experiment ^[3] has been performed countless times with electron microscopes ^[4], leaving the set-up running for at least half an hour because it takes a lot of photons or electrons to get a discernable interference pattern. More recently this one-photon-at-a-time experiment was done with a single-photon sensitive video camera. For the result, see figure 5.3.

Figure 5.3: Results of a one single photon at-a-time double-slit-experiment.

Source: Wikimedia Commons – provided by Dr. Akira Tonomura.

The spots on the screen, where each individual particle hits, look completely random at the start of such an experiment, but the final image nevertheless develops into the usual interference pattern with an intensity distribution as shown in figure 5.4. It is a graph of the electron hit density as pictured in figure 5.3-e.

Figure 5.4: Double-slit hit distribution with a great number of individually fired photons or electrons.

Source: Wikimedia Commons

So, each single particle seems to have gone through both slits to interfere with itself in the space after the slits. Here, our imagination, shaped by classical physics, simply collapses into unimaginability. But, when we close one of the slits, i.e. the left one, the result is an intensity distribution as shown in figure 5.5, the bolder grey curve.

Figure 5.5: Intensity distribution with left slit closed.

Source Wikimedia Commons.

As you can see, now there is obvious no interference, which is to be expected. However, it is still a wave phenomenon and surely not an effect what you would expect when shooting material particles through a single slit. What you see here is the intensity distribution of a single wave expanding from the right-hand slit hitting the screen everywhere but mainly in the middle behind the open right-hand slit.

Next, we open both slits again and now we are going to observe someway through which slit the particle passes. And then it gets weird.