Quick, pick a side: Is light a wave or a particle? This question is best known for its power to set crowds of undergraduates a-tittering. But it’s more than a pop science brain-teaser. Scott Holmstrom, a physicist at the University of Tulsa and former professor of mine, is convinced that this sort of super basic question can propel critical minds into physics careers — if we’d only let that happen.
Holmstrom studies light. For years he has been working on detecting toxic gases using light confined in small structures called waveguides — analogous to fiber optics. Unsurprisingly, the theories that describe light’s movement through waveguides describe light as a wave — even though the light in question was usually generated by a laser, a device that is always described using the particle theory.
So that’s a head scratcher.
I talked to Holmstrom about light, quantum mechanics and physics education. Happy to say, we made some waves.
Inverse: One premise of your research is that you’re dealing with waves. Where would you even start thinking about light in particle terms?
Prof. Holmstrom: There are things that go on in my research that I would typically use the particle theory of light to describe, and there are things in my research that I would typically use the wave theory to describe. But it’s all based on a matter of convenience.
It depends on what you are trying to describe?
Right. If I wanted to describe how a laser works, I would probably pull out quantum mechanics [which describes light as a stream of particles called photons]. To describe that you’d say, “One atom releases a photon in the direction of the highly reflecting mirrors of the laser cavity and in passing through the material it induces other atoms to release photons in the same direction with the same polarization.” So that’s this idea of “Light Amplification by Stimulated Emission of Radiation” (LASER). The concept of stimulated emission is almost always described using the photon picture of light.
Because it has to be or because that’s the most convenient option?
There are very few things that need to be described using the photon picture of light. One of them is emission from atoms.
What does this mean for the perennial pop science question, “Is light a wave or a particle?”
The answer to that question is, it’s neither a wave nor a particle. Light is light. Light behaves like light. We do have theories that treat things as waves, and we do have theories that treat things as particles. And we can sometimes use one or sometimes use both to one degree of difficulty or another. It’s all basically a matter of convenience. There are only a couple of things where I’ve got to use the particle nature of light to describe it, spontaneous emission being the most important of them.
So with that statement, people could easily say that light is a particle, period. Just because there is one thing that we know of for sure that can’t be described with our current wave picture. There isn’t the opposite. We know of no one thing that can’t be described by the quantum nature of light. Having said that, I’ve never tried it all. I’ve never seen somebody recreate all of the classical optics experiments using quantum electrodynamics.
So the jury is out?
No. The jury is back and the jury has declared it a victory. I’m telling you as an outside observer that the jury probably doesn’t have all the information it should have had before it made that judgment. So in the end, maybe the particle theory, even though it’s highly touted as the best and most practical theory, blah blah blah — nobody ever uses it to describe even the simplest experiments.
This couples onto a larger beef that you have with the way we think about models and theories about the world.
We tend to think we understand the world around us when we get to the point where we’re comfortable talking about a model, that usually somebody else dreamt up, about nature. We tend to think we understand nature when we can use a model, when we can spout off interesting facts about a model. My biggest pet peeve is the atom. The thought experiment that’s very important to me is the concept of letting an electron and a proton come into proximity with one another and what happens. Will they form an atom? That whole thought experiment is just a thought experiment, because nobody has ever held a proton.
There’s this problem. We think of these electrons as being little balls. If an electron is a particle, why can’t I hold it? I can understand why I can’t hold a wave. But if it’s a particle, why can’t I hold it? I know there’s this uncertainty principle and this sort of stuff, but that has to do with waves. We think we understand atoms, like there’s a proton and there’s an electron. Well how the hell did those things get there? I assume at some point in time, there was a lonely proton and a lonely electron and they found one another and they entered into this relationship.
It just gets tossed off kind of casually that, “Oh, small stuff doesn’t obey the same rules.” But you would think, hypothetically, that a “complete” description of the world, a “totally accurate” model, would not have such a disjunct between small stuff and big stuff.
Right. We’re touching on another problem I have with physics. A thing people don’t look at is the realm of applicability of the models. And I guess that’s what we’ve been talking about this whole time. As a physicist, as someone who teaches budding physicists, one of the most important things when I’m teaching is to help them understand the limitation of the model.
I was just talking in my Math Physics class today. I was complaining that the way we’re taught physics — it doesn’t inspire anybody. Because what you’re doing is you take Physics 1 and Physics 2, and you’ve got this survey of all these physics concepts that are so watered down and the details brushed over that it won’t inspire anybody. So you’re almost excluding finding your best physicists by making them take that course. I barely stayed a physics major when I was taking those courses. As a person with a physics mind, you want to know the details.
How would you restructure a physics curriculum to inspire people?
Find a good physics for poets book and start talking about it. The conversation we’re having right now should be the walk-in-the-door conversation with potential physics majors. What we need in physics are minds that are not sheep-like, that are investigating and questioning, and answering the simple questions. We have so much drive in physics to get funding for our research, to follow the great white light of funding into the tunnel with the oncoming train, but we never stop and think, “Why is it I can’t hold an electron?”
There’s all kinds of little things in all of our theories that could be further investigated, it’s just hard to investigate and there’s no funding out there for it. As physicists, we’re always chasing the dollar and looking for the interesting physics bit along the way.