Michael Dine - This Way to the Universe: A Theoretical Physicists Journey to the Edge of Reality

It seems to be an extraordinary moment. On the one hand, we face daunting challenges: climate change, global pandemics, the threat of nuclear war. On the other hand, as a species, we have knowledge of the worldand the universearound us beyond anything humans might have imagined even a century ago. No matter what happens, we have an unprecedented understanding of the natural world of which our daily experiences sample only a tiny corner. Our lives play out on scales of centimeters, meters, kilometers, perhaps thousands of kilometers. But we know about nature on smaller scalesfar, far smaller than the size of an atomic nucleus. We also know about the universe out to unimaginably large distances. Even more amazing is what we knowreally knowabout events billions of years ago, and we can make statements with near certainty about what will happen to the universe for the next few tens of billions of years. An extraordinary moment indeed.

Most of us have heard about faraway stars and galaxies, have some inkling that the universe emerged from a big bang billions of years ago. But precisely how large and how old is the universe? Where did it come from? What is its ultimate fate? How do we find answers to these questions?

We are aware of atoms and maybe somewhat aware of things smaller than atoms. How can we possibly know about atomic nuclei that are far too small to see with the most powerful microscopes? How do these tiny things control the operation of the universe at large, as well as events like making a sandwich, using a credit card, or driving to work? From the largest scales to the smallest scales, our universe can seem impossibly mysterious. Can we do more than speculate about the architecture of the cosmos and its building materials? Can we construct experiments that will answer our questions about reality at such fantastic scales?

As I write this, we are still confronting the Covid-19 pandemic. From this ordeal, were all now familiar with the significance of powers of 10. In the early stages of the outbreak, the number of cases was growing by nearly a factor of 10 every week. Here is what that meant for projections about cases in the United States.

March 10, 2020

1,000

March 18, 2020

10,000

March 25, 2020

100,000

April 3, 2020

1,000,000

April 7, 2020

10,000,000

From 100 people to 10 million people sick in a matter of five weeks. After that, in this case, the growth would have slowed, only because it would have been harder for the virus to encounter people who had not already been infected. Fortunately, states and local communities, to a large extent, adopted shelter-in-place restrictions within a few days of March 11, 2020, a little less than two weeks after the exponential growth began. Two weeks later, about the time from exposure to the virus to visible symptoms, the effects of the partial lockdown started to be felt. So on March 10, there were 994 cases, just under our thousand expected, 9,307 on March 18, somewhat further below our 10,000 expected. But by March 25, the effects of social distancing became visible, with 68,905 cases. On April 3, 250,000 cases, and on April 11, 509,000a factor of 200 less than our worst-case scenario. The drastic measures we took as a society saved millions of lives. Had we acted earlier, even more would have been saved; had we waited longer, an even greater catastrophe would have unfolded. Indeed, states and localities that acted earlier generally did better. Around the world, similar stories played out. Subsequent months saw waxing and waning of the virus, tied to behaviors, improved treatment strategies, and the eventual rollout of vaccines.

But powers of 10 need not always tell such a grim story. They are a valuable tool for thinking about nature. We humans occupy a tiny planet in a vast universe. At the same time, there is a world of far tinier thingsmolecules, atoms, protons, neutrons, and electrons. Powers of 10 are also a useful concept in these happier pursuits. In 1977, while a graduate student visiting the Smithsonian Institution with my brother, I watched the video Powers of Ten, by Charles and Ray Eames (a couple best known for their work in industrial design). This beautiful film summarized our understanding of nature at that time, on the largest and smallest scales. Starting with a couple enjoying a beautiful spring day, occupying a space maybe two meters across in each direction, it explored scales progressively larger by factors of 10parks, cities, states, nations, the planet, the solar system, galaxies, and clusters of galaxies. It then proceeded in the other direction, describing smaller scalesparts of the human anatomy, then cells, atoms, and the nuclei of atoms. It summarized for me pretty well what I was learning in my studies. To be honest, there was a good deal I didnt yet know in that film.