Thanks to all the tutorials, however, I think this video is dramatically better than the previous one. There are still a number of issues I'm unhappy about, notably the timing, which I find hard to get right. Also, the lip-synching is poor. Not to mention that I still can’t draw and hence my cartoon child looks like a giant zombie hamster.
Listening to it again, the voiceover seems too fast to me and would have benefited from a few breaks. In summary, there’s room for improvement.
The other day, my daughter asked me “What is physics?”
She’s six. My husband and I, we’re both physicists. You’d think I had an answer. But best I managed was: Physics is what explains the very, very small and very, very large things.
There must be a better explanation, I said to myself.
The more I thought about it though, the more complicated it got. Physics isn’t only about small and large things. And nobody uses a definition to decide what belongs into the department of physics. Instead, it’s mostly history and culture that marks disciplinary boundaries. The best summary that came to my mind is “Physics is what physicists do.”
But then what do physicists do? Now that’s a question I can help you with.
First, let us see what is very small and very large.
An adult human has a size of about a meter. Add some zeros to size and we have small planets like Earth with a diameter of some tenthousand kilometers, and larger planets, like Saturn. Add some more zeros, and we get to solar systems, which are more conveniently measured with the time it takes light to travel through them, a few light-hours.
On even larger scales, we have galaxies, with typical sizes of a hundred-thousand light years, and galaxy clusters, and finally the whole visible universe, with an estimated size of 100 billion light years. Beyond that, there might be an even larger collection of universes which are constantly newly created by bubbling out of vacuum. It’s called the ‘multiverse’ but nobody knows if it’s real.
Physics, or more specifically cosmology, is the only discipline that currently studies what happens at such large scales. This remains so for galaxy clusters and galaxies and interstellar space, which fall into the area of astrophysics. There is an emerging field, called astrobiology, where scientists look for life elsewhere in the universe, but so far they don’t have much to study.
Once we get to the size of planets, however, much of what we observe is explained by research outside of physics. There is geology and atmospheric science and climate science. Then there are the social sciences and all the life sciences, biology and medicine and zoology and all that.
When we get to scales smaller than humans, at about a micrometer we have bacteria and cells. At a few nanometers, we have large molecular structures like our DNA, and then proteins and large molecules. Somewhere here, we cross over into the field of chemistry. If we get to even smaller scales, to the size of atoms of about an Angstrom, physics starts taking over again. First there is atomic physics, then there is nuclear physics, and then there is particle physics, which deals with quarks and electrons and photons and all that. Beyond that... nobody knows. But to the extent that it’s science at all, it’s safely in the hands of physicists.
If you go down 16 more orders of magnitude, you get to what is called the Planck length, at 10^-35 meters. That’s where quantum fluctuations of space-time become important and it might turn out elementary particles are made of strings or other strange things. But that too, is presently speculation.
One would need an enormously high energy to probe such short distances, much higher than what our particle accelerators can reach. Such energies, however, were reached at the big bang, when our universe started to expand. And so, if we look out to very, very large distances, we actually look back in time to high energies and very short distances. Particle physics and cosmology are therefore close together and not far apart.
Not everything in physics, however, is classified by distance scales. Rocks fall, water freezes, planes fly, and that’s physics too. There are two reasons for that.
First, gravity and electrodynamics are forces that span over all distance scales.
And second, the tools of physics can be used also for stuff composed of many small things that behave similarly, like solids fluids and gases. But really, it could be anything from a superconductor, to a gas of strings, to a fluid of galaxies. The behavior of such large numbers of similar objects is studied in fields like condensed matter physics, plasma physics, thermodynamics, and statistical mechanics.
That’s why there’s more physics in every-day life than what the breakdown by distance suggests. And that’s also why the behavior of stuff at large and small distances has many things in common. Indeed, methods of physics can, and have been used, also to describe the growth of cities, bird flocking, or traffic flow. All of that is physics, too.
I still don’t have a good answer for what physics is. But next time I am asked, I have a video to show.