The Rogues Gallery

Curious Quantum Questions

We recently received the following email question:

I have been studying quantum physics for a bit now and was hoping that Bob could give his explanation of The Uncertainty Principle and Wave/Particle duality.

My first question is…
Is the drawing of a wave (on paper) just a 2D representation of a more abstract idea of what a wave really is? or can we literaly imagine a particle flying through space oscillating up and down along it’s amplitude.(quantum oscillation?)

My second question is how this relates to Photon slit experiments…
Does the photon display the properties of a wave as it is flying towards the slit, and then display the properties of a particle when it hits the wall? IS this why we cannot predict where the photon will land? Because it is displaying the characteristics of a wave and therefore exist anywhere within the oscillating amplitude, even if all places at one?

…If you please, I would like to hear Bob and the SGU’s understanding of Wave/Particle duality, The Uncertainty Principle, and Photon slit experiments.

Dan K.

Thanks for the question Dan, I’m no quantum physicist but I do play one on a podcast so I’ll give it a go.

First a little background.

Wave Particle Duality refers to the bizarre dual nature of light. Some consider this the central mystery of Quantum Mechanics. Light is not either a wave OR a particle but it is somehow a fusion of both. Some experiments show its wave nature and others its particle nature.

Two classical examples of its wave nature are diffraction and interference. Only waves exhibit these behaviors.

On the other hand, the photoelectric effect in which incident light knocks out electrons from matter is that iconic experiment which shows that light is also a particle.

Most interestingly, this dual nature extends to atoms and molecules and even to all macroscopic objects. So yes, you and I have a wavelength but they are so incredibly small that our wave properties aren’t seen.

Dan also mentions the Uncertaintly Principle in his question so I’ll give a little background on this as well.

Heinsenberg’s Uncertainty Principle (also known as the Indeterminacy Principle), is the notion that since matter ultimitely has this spread-out wave-like nature, it is impossible to pin it down precisely at the quantum scale. There is then a fundamental limit to the extent we can examine nature.

For example, objects of study often consists of pairs of quantities like position and momentum, so-called conjugate variables.

Heisenberg said we cannot know both of these quantities with arbitrary precision. The more we know one, the less clear the other becomes. If we know position with absolute precision, then there’s nothing we can know about its momentum.

It is important to stress that this limitation is not due to primitive technology or brain structure but is impossible IN PRINCIPLE. Think of any uber-advanced sci-fi aliens
The Q
Organians
Krell
pan-dimensional mice

None of them can investigate nature any deeper than allowed by this principle.

My favorite example of this limitations is weather prediction.

Generally speaking, If we knew the precise locations and momenta of every particle in the atmosphere, we could extrapolate future interactions and therefore the weather deep into the future. Because we can’t in principle know this information, then any model we create is by definition an average with unavoidable round-off errors no matter how tiny. These errors double and double over time eventually overwhelming any attempt at detailed prediction past a handful of days. This same limitation applies to all chaotic systems in which there is this sensitive dependence on initial conditions.

Dan’s first question is…

“Is the drawing of a wave (on paper) just a 2D representation of a more abstract idea of what a wave really is? or can we literaly imagine a particle flying through space oscillating up and down along it’s amplitude.(quantum oscillation?)”

I don’t think we can internalize the dual nature of light enough to visualize it as it truly is. This may just be a fundamental limitation to how humans perceive the world and our intelligence. Like trying to teach a dog algebra. The smartest dog can’t hope to understand it.  Depending on the context of the visualization, I think you can legitimately visualize it in a number of ways. Each one is a valid way to visualize if it makes sense in the context. You can visualize it as a simple 2D wave and that would be sufficient for many applications. If you want to visualize it in 3 dimensions then imagine an oscillating magnetic field 90 degrees away from an electric field. Each field is creating the other letting it fly through even the vacuum of space, no ether required.

The other obvious way you can visualize it is as discrete photon or packet of energy. If you just wamt to try to imagine it as it really is then my favorite compromise is a small tear-drop shape with a wave shape inside it.

Dan’s second question related this to the famous photon slit experiments…

“Does the photon display the properties of a wave as it is flying towards the slit, and then display the properties of a particle when it hits the wall? IS this why we cannot predict where the photon will land? Because it is displaying the characteristics of a wave and therefore exist anywhere within the oscillating amplitude, even if all places at one?”

This refers to one of the most famous experments of all time. Young’s Double-Slit experiment from the early 1800s. This clearly shows not only Duality in which light is somehow both particle and wave but also Complementarity in which only one aspect can be revealed at a time.

Here’s the experimental setup: Imagine light shining on a barrier with 2 slits in it allowing the light to shine through and onto a far wall. Light coming thru 2 slits creates 2 sources of light which then interact with each other like water waves creating an interference pattern of bright and dark spots instead of big and small waves.

The really weird stuff happen when you shine light so dim that only one photon is traversing the slits at a time. Even in this case an interference pattern builds up. It seems then that the single photon of light is entering both slits at once and interfering with itself.

This simple experiment is pretty much the poster boy for Quantum Mechanics. In fact, physicist Richard Feynman famously said once that all of QM could be derived from the implications of this one experiment.

Dan asks why we can’t predict where the photon will land. We cannot predict this because quantum phenomena are inherently unpredictable. They are for all intents and purposes a-causal events. The best we can do is assign probabilities as to where they will go. This inherently statistical nature of reality is what upset Einstein so much prompting his God Does Not Play Dice quote and removing him from cutting edge physics for the last decades of his life.

The fact is that God does play D&D every chance he gets.