At the beginning of my career, I organized a meeting of a research project team. I was still an undergraduate student. The project was concerned with the development of thin superconducting films for microwave circuits, and my task in the project was the optimization of those films. But I was not an expert in waveguides and microwave circuits, and I had not developed a feeling for the propagation of waves along the walls of our tiny cavities. I knew Maxwell’s equations, but this is not the same as feeling how a specific electromagnetic system would evolve over time.

Since I was the most inexperienced member of the team, I dared to utter what I considered a really stupid and basic question. At the end of the meeting, after we had discussed all sorts of details and project management issues, I asked:

How exactly do these waves propagate? How are they confined to the waveguides?

There was no answer, except: This is really a question which would make us all sweat.

In that moment, I promised to myself that I will always dare to ask the so-called stupid questions – both to myself and to others. I would not care about damaging my reputation as an expert. I think, that I have been able to keep the promise. But it is hard sometimes  to resist the temptation to cover gaps in basic knowledge by resorting to expert lingo.

Science shows on TV are successful – partly due the unexpected twist in the explanation of simple everyday phenomena. I believe that every student of physics has seen an experienced professor guessing wrong when trying to explain such phenomena. Consider the famous experiment of the helium filled balloon in a car: The balloon is attached to a stiff wire, and the other end of the wire is attached to the seat of the car. The length of the wire is shorter than the height of the car. If the car is not accelerating, the angle between the wire and the bottom plate of the car is 90 degrees. What would happen if the car accelerates?

Intuition might imply that the balloon would move in the opposite direction as the acceleration of the car – like anything else in the car. But actually the balloon moves in the direction of the acceleration. One can find numerous videos on the internet that demonstrate how counter-intuitive this result seems to be. The first idea is not totally wrong: if the car would be evacuated, the balloon would move in the “usual” direction. In an evacuated car, you would simply see the relative movement of the accelerating car and the balloon that is still keeping its velocity (relative to the street or to the earth). The clue of this experiment is that the car is filled with air. “Normally”, when we observe any body moving in air, we neglect its effect. Both the body in the car and the air have inertia. When the car accelerates, any stuff floating inside the car keep its original velocity. In a frame of reference attached to the car the movement of floating bodies inside the car could be described by assigning to them an acceleration pointing in the opposite direction as the acceleration of the car. So air and helium both try to move to the back of the car, driven by the same acceleration. Force is mass times acceleration, and mass is density times volume. The density of air is higher than that of helium, the (pseudo)force per unit volume is higher for air than for helium. The force on each little packet of air is higher than for little packets of helium. Thus air wins and pushes helium aside. Since helium is confined to a certain volume by the balloon, we can only judge from the movement of the balloon what is going on.

There is a different way to explain it which might make it intuitive again: Consider a balloon being under the influence of gravity instead of the influence of the inertial pseudo-force. Isn’t it natural and intuitive that the balloon would move upward in an oxygen/nitrogen atmosphere under the pull of a gravitational field? We’ve seen this often: Helium balloons fly! In a vacuum, however, e.g. on the moon or an asteroid that does not have an atmosphere, the helium filled balloon would be attracted by the celestial body. You might say one needs to be careful in order to prevent the balloon from escaping on the asteroid – just as the astronauts in science fiction movies who take a leap and are propelled into space. But this is something different – this is about the momentum or velocity that allows a body to leave the gravitational field, and it can be calculated by equating kinetic energy at the surface and the potential energy at infinity. But in my experiment, the balloon would be placed gently in mid-air, or better say in mid-vacuum, so that it does not have any velocity before gravity starts to pull on it.

So in summary, our physics intuition is often bad. It took humankind about 2000 years to find out that it is not required to apply a force in order to make a body move in a straight line.

I believe the key is that our so-called intuition is tied to our everyday environment. We fail terribly if we change essential details, such as removing friction – which makes planets moving forever in contrast to anything moving in our everyday world. I also fail to solve such puzzles, and I fully agree with Richard Feynman who states – in his Physics Lectures, part 1, on explaining the gyroscope – that it is very often easier to follow the math step by step than really understanding what is going on. This becomes worse and worse the more abstract the concepts in physics become. All those so-called paradoxa related to relativity and quantum mechanics are due to the fact that we have no intuition whatsoever for velocities near the speed of light or the microscopic world.

I am still determined to sharpen my physics intuition – I have been re-visiting foundations and explanations of allegedly simply phenomena all the time, especially after I encountered the following: Though I “left science” for a period of time I never stopped reading textbooks. I was particularly interested in continuing to learn about theoretical physics to compensate for selecting applied and experimental physics in graduate school. I was actually proud to get into quantum statistics again and follow the math. When was about to go for a degree in energy engineering I re-visited simple thermodynamics, and I had considerable difficulties to develop an intuition (again – if I ever had it…) for discerning quickly how a simple apparatus or machine does work. Yet statistical thermodynamics and those hyper-dimensional spaces felt more familiar to me. I am not yet sure what the reasons are, these are my working hypotheses:

• I have just discovered the differences between physics and engineering – I was just more trained in physics thinking than in engineering reasoning.
• Anything is a matter of training – no matter how simple it is per se. Having worked with functions over abstract spaces requires training, but reading construction plans does as well.