Quantum Astronomy: The Double Slit Experiment

This is a series of four articles each with a separate explanation of different quantum phenomena. Each of the four articles is a piece of a mosaic and so every one is needed to understand the final explanation of the quantum astronomy experiment we propose, possibly using the Allen Array Telescope and the narrow-band radio-wave detectors being build by the SETI Institute and the University of California, Berkeley.

With the success of recent movies such as "What the &$@# Do We Know?" and the ongoing -- and continuously surprising -- revelations of the unexpected nature of underlying reality that have been unfolding in quantum physics for three-quarters of a century now, it may not be particularly surprising that the quantum nature of the universe may actually now be making in-roads into what has previously been considered classical observational astronomy. Quantum physics has been applied for decades to cosmology, and the strange "singularity" physics of black holes. It is also applicable to macroscopic effects such as Einstein-Bose condensates (extremely cold conglomerations of material that behave in non-classical ways) as well as neutron stars and even white dwarfs (which are kept from collapse, not by nuclear fusion explosions but by the Pauli Exclusion Principle - a process whereby no two elementary particles can have the same quantum state and therefore, in a sense, not collapse into each other).

Well, congratulations if you have gotten through the first paragraph of this essay. I can't honestly tell you that things will get better, but I can say that to the intrepid reader things should get even more interesting. The famous quantum physicist Richard Feynmann once said essentially that anyone who thought he understood quantum physics did not understand it enough to understand that he did not actually understand it! In other words, no classical interpretation of quantum physics is the correct one. Parallel evolving universes (one being created every time a quantum-level choice is made), faster-than-light interconnectedness underlying everything, nothing existing until it is observed, these are a few of the interpretations of quantum reality that are consistent with the experiments and observations.

There are many ways we could go now in examining quantum results. If conscious observation is needed for the creation of an electron (this is one aspect of the Copenhagen Interpretation, the most popular version of quantum physics interpretations), then ideas about the origin of consciousness must be revised. If electrons in the brain create consciousness, but electrons require consciousness to exist, one is apparently caught in circular reasoning at best. But for this essay, we shall not discuss quantum biology. Another path we might go down would be the application of quantum physics to cosmology -- either the Inflationary origin of the universe, or the Hawking evaporation of black holes, as examples. But our essay is not about this vast field either. Today we will discuss the scaling of the simple double-slit laboratory experiment to cosmic distances, what can truly be called, "quantum astronomy."

The laboratory double-slit experiment contains a lot of the best aspects of the weirdness of quantum physics. It can involve various kinds of elementary particles, but for today's discussion we will be talking solely about light - the particle nature of which is called the "photon." A light shining through a small hole or slit (like in a pinhole camera) creates a spot of light on the screen (or film, or detector). However, light shown through two slits that are close together creates not two spots on the screen, but rather a series of alternating bright and dark lines with the brightest line in the exact middle of this interference pattern. This shows that light is a wave since such a pattern results from the interference of the waves coming from slit one (which we shall call "A") with the waves coming from slit two (which we shall call "B"). When peaks of waves from light source A meet peaks from light source B, they add and the bright lines are produced. Not far to the left and right of this brightness peak, however, peaks from A meet troughs from B (because the crests of the light waves are no longer aligned) and a dark line is produced. This alternates on either side until the visibility of the lines fades out. This pattern is simply called an "interference pattern" and Thomas Young used this experiment to demonstrate the wave nature of light in the early 19th Century.

However, in the year 1900 physicist Max Planck showed that certain other effects in physics could only be explained by light being a particle. Many experiments followed to also show that light was indeed also a particle (a "photon") and Albert Einstein was awarded the Nobel Prize in physics in 1921 for his work showing that the particle nature of light could explain the "photoelectric effect." This was an experiment whereby low energy (red) light, when shining onto a photoelectric material, caused the material to emit low energy (slow moving) electrons, while high energy (blue) light caused the same material to emit high energy (fast moving) electrons. However, lots of red light only ever produced more low energy electrons, never any high-energy electrons. In other words, the energy could not be "saved up" but rather must be absorbed by the electrons in the photoelectric material individually. The conclusion was that light came in packets, little quantities, and behaved thus as a particle as well as a wave.

So light is both a particle and a wave. OK, kind of unexpected (like Jell-O) but perhaps not totally weird. But the double slit experiment had another trick up its sleeve. One could send one photon (or "quantum" of energy) through a single slit at a time, with a sufficiently long interval in between, and eventually a spot builds up that looks just like the one produced when a very intense (many photons) light was sent through the slit. But then a strange thing happened. When one sends a single photon at a time (waiting between each laser pulse, for example) toward the screen when both slits are open, rather than two spots eventually building up opposite the two slit openings, what eventually builds up is the interference pattern of alternating bright and dark lines! Hmm... how can this be, if only one photon was sent through the apparatus at a time?

The answer is that each individual photon must - in order to have produced an interference pattern -- have gone through both slits! This, the simplest of quantum weirdness experiments, has been the basis of many of the unintuitive interpretations of quantum physics. We can see, perhaps, how physicists might conclude, for example, that a particle of light is not a particle until it is measured at the screen. It turns out that the particle of light is rather a wave before it is measured. But it is not a wave in the ocean-wave sense. It is not a wave of matter but rather, it turns out that it is apparently a wave of probability. That is, the elementary particles making up the trees, people, and planets -- what we see around us -- are apparently just distributions of likelihood until they are measured (that is, measured or observed). So much for the Victorian view of solid matter!

The shock of matter being largely empty space may have been extreme enough -- if an atom were the size of a huge cathedral, then the electrons would be dust particles floating around at all distances inside the building, while the nucleus, or center of the atom, would be smaller than a sugar cube. But with quantum physics, even this tenuous result would be superseded by the atom itself not really being anything that exists until it is measured. One might rightly ask, then, what does it mean to measure something? And this brings us to the Uncertainly Principle first discovered by Werner Heisenberg. Dr. Heisenberg wrote, "Some physicist would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist independently of whether we observe them. This however is impossible."

Perhaps that is enough to think about for now. So in the next essay we will examine, in some detail, the uncertainty principle as it relates to what is called "the measurement problem" in quantum physics. We shall find that the uncertainty principle will be the key to performing the double-slit experiment over astronomical distances, and demonstrating that quantum effects are not just microscopic phenomena, but can be extended across the cosmos.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Contributing Writer

Laurance Doyle is a principal investigator for the Center for the Study of Life in the Universe at the SETI Institute, where he has been since 1987, and is a member of the NASA Kepler Mission Science Team. Doyle’s research has focused on the formation and detection of extrasolar planets. He has also theorized how patterns in animal communication, like those of social cetaceans, relate to humans.