Category: Observation

Tomorrow marks the last lecture of my course, PHY1308 (“Introductory Physics – Electricity and Magnetism”). The topic of the last lecture – topics, really – were chosen by the students of my class. This class is intended for pre-med students, and while not all of them are planning to go to medical school each student is extremely bright and all of them have excelled in various ways in my class. We have struggled, together at times, to come to a better understanding of the natural world through physics.

The title of my last lecture for 2010 is “Beyond Einstein: How Light Led the Way to a Dark Cosmos.” The topics included will be physics in 1890, the early life of Albert Einstein, 1905 and the papers that started twin revolutions (I will demonstrate the photoelectric effect using household items), and the implications of those twin revolutions for our current understanding of the universe.

What lies behind this lecture are an innumerable set of experiments conducted over the past two hundred years. The 1800s were a revolutionary period for our understanding of electricity and magnetism, themselves two faces of a single electromagnetic force described by Maxwell’s Equations. Each symbol in Maxwell’s Equations was determined not just through logic but through the sweaty and often dangerous labor of experimentalists. Combining prowess in the laboratory with a profound grasp of mathematics, men like Faraday, Coulomb, Biot  and Savart pieced together the laws of nature that governed electric and magnetic phenomena. It was Maxwell who extended that work and formed what now call “Maxwell’s Equations.” There, in turn, predicted the existence of electromagnetic waves that traveled without the aid of a medium and did so at the speed of light. Again, the labor and toil of men such as Hertz revealed the existence of such waves, and confirmed that light is such a wave.

I say all of this, because experimental results – asking Nature questions and having the will and the skill to tease answers from her – are ALWAYS behind our most profound understanding of Nature. This understanding does not end with mechanics, thermodynamics, and electromagnetism; experimental work was CRITICAL to making sense of the quantum theory of radiation and matter, as well as the theory of relativity. Without experiment, such ideas would not have been formed; without experiment, such ideas could not have been tested, or even fully honed into their present state. The light of experiment has even led us to understand how little matter and light play a role in the shape and destiny of our cosmos; these most profound issues seem to be ruled by as-yet-unidentified dark matter and dark energy. It will be the twin lights of experimental physics and a deep understanding of mathematics that will again illuminate even these dark corners of our cosmos.

So tonight, as Jodi and I sat in a coffee shop in Allen, she working on her final exam and I working on my last lecture, my ears pricked up when the big bang became a topic of discussion amongst a group of grade-school girls sitting at a table behind us. One of them proudly proclaimed intellectual defiance when her teacher came to teach about the age of the universe and the big bang. “I asked her why I should believe all of that,” the girl said to her friends, “and the teacher basically told me that it was because a bunch of scientists say so. Well, I say that the big bang theory is just that – a theory – and we shouldn’t take it so seriously.”

And I cringed. Because that’s what scientists do when confronted with a person so defiant. She is young, and that was factored into my response. But I also recall that I was once very devout, and yet somehow the fact that the universe was born billions of years ago and evolves according to a set of well-defined laws did not challenge my faith. It added a new dimension to my view of the universe, and I came to understand that it was not idle philosophy or mere speculation that proclaimed such things; it was the weight of observation, the voice of Nature herself speaking in the ear of the experimentalist and the ear of the theorist that gave rise to these understandings.

This girl behind us continued on, talking about how she told her teacher that she would simply be absent from class when the teacher taught about the big bang. She then went on to say how she evangelized her friends in gym class and told them all to go to church.

Why am I saying all of this? My lecture tomorrow will recount what we understand now, from experimental measurement, about the age and fate of the universe. There is no time in 50 minutes to understand all the “whys” – really, just the “whats.” But behind every word – every “what” –  is a string of experiments that have pointed the way, and a mathematical framework that makes predictions and allows for tests. In other words: science. Perhaps this girl was unfairly summarizing her teacher’s response when explaining why her teacher said she should believe in the big bang. Perhaps this girl could never be swayed by facts because her mind is not open to the possibility of Nature behaving in a way other than she would like. Who can say?

I can only really say this: be humble before Nature. She has a lot to teach us, if only we are willing to listen. Shutting Nature out of our lives endangers our economy, our health and well-being, our intellectual prowess, and our ability to innovate and compete. Shutting out Nature, the most significant relic of the creation of the universe, is like cutting out your eyes in order to become an art critic. You can talk about art, and I wager there will be people who listen to you, but can you truly understand it if you cannot see?

Be humble before Nature, but have the courage to question your assumptions and the responsibility to learn how to answer your questions. This is really what college is all about – not just about filling your heads with facts – and in the end the ability to think will serve you better than any skill in life.

My “Modern Physics” class has come to one of the most crucial and important insights that has been made into the natural world: waves of probability seem to lie at the heart of the behavior of matter. “Paradigm shift” is a phrase overly applied in the modern world, but it applies extremely well to the wave nature of matter. Like all such punctuated bits of evolution, this evolution in thinking came because facts opposed notions, and was resisted as if the lives of all physicists involved in the conversation depended on being right in spite of the data. The early 1900s, when the wave nature of matter became a thing of serious consideration, were turbulent and revelatory in the physics community. With implications not just for physics, but philosophy and technology, the wave nature of matter is both alarming (at first) and crucial.

What does all of this mean? We think of waves and particles as distinct things. Of particles, we ask: where are you, where are you going, when will you get there? Of waves, we ask: how often do you repeat in space and in time, what is your maximum amplitude, where are you zero, and how spread out are you? At first, putting these two phenomena together seems impossible. But then you start to think about “relevant dimensions,” and then you start to cease to see distinctions between the two.

What are “relevant dimensions?” The textbook I am using, Harris’s “Modern Physics,” handles this quite well: simply put, these are the relative sizes scatterer and the scatteree. If an electron is to scatter off of atoms spaced far apart, the relevant dimensions are large compared to the electron; if the spacing of the atoms is extremely close, then perhaps the dimensions are small compared to the electron.

Harris gives a nice analogy, which I modified a bit for my class. Imagine taking a boat out to the middle of a lake, then laying down in the boat (so you can’t see the water around you) and staring up at the sky. Somewhere far across the lake, at a construction site, a crane drops a large concrete or steel structure. What happens?

If the structure hits the beach and then rolls into the water, the waves it creates can have wavelengths that are meters long. The waves travel across the lake and reach your boat. Your boat is not that big, so the waves begin to raise, then lower, the boat. You would report that the boat experienced wave motion – the dimensions of the wave are much larger than the boat, so the boat only responds to a small part of the overall wave, rising and falling as the wave passes.

But what if the crane drops the structure straight into the water? Then the energy goes into one short, sharp wave. Its wavelength may be half-a-meter or less. When it reaches your boat, what happens? Rather than the boat responding to some small part of the wave, the whole of the wave strikes the boat at almost the same time. All that energy is deposited into the boat at nearly the same time. What do you report? You are likely to think another boat has hit you, or that you have drifted into a rock. You would report a “particle collision”.

The relevant dimensions – the wavelength compared to the boat – affect the interpretation of the event. Particle-like behavior manifests when the wavelength is short compared to the dimensions of the object, and wave-like behavior is apparent when the wavelength is long compared to the dimensions of the object.

But for matter, what is “waving”? If electrons, protons, atoms, etc. are waves – what is waving? The answer was not originally arrived at easily, and to this day nobody is sure if there is a better answer. But the answer that works is PROBABILITY. The particle itself is not oscillating or wiggling or vibrating. Rather, its associated wave is one of probability amplitude – in one place in space or time, the wave amplitude is lower, so the chance of finding the particle there is small, while in another the amplitude is large, making the chance of finding the particle there high. To see the particle, you have to measure it, and when you measure it the particle nature manifests (to find it, you have to detect it, and to detect it, you either have to bounce something off of it or make it bounce off something). Until the measurement is made, you only know where the particle is likely to be or not be.

This is among the hardest things in physics to accept. You accept it only because it works – it makes predictions that time after time have born out. The probability wave interpretation of matter tells us not only why atoms work the way they do, why electrons can be used to measure subatomic structure, and why transistors and diodes work, but gives us a means to explore new possibilities regarding the nature of reality.

Is the wave nature of matter true? Is it right? Those are difficult words for a physicist. Certainly, experiments don’t (yet) suggest otherwise. But perhaps one day we’ll find a better explanation, a bigger explanation, that makes this seem more sensible to us. Until then, I am excited that my class gets to explore the wave nature of matter, and I am excited that I get to teach it.

Homework, anyone?