The first force discovered by the first physicists was gravity. Over four hundred years ago our understanding that gravity was a force at work throughout the Universe, was explained by Sir Isaac Newton. Nearly 100 years ago we modernized our understanding with Albert Einstein’s General Theory of Relativity. And despite all this homework, gravity is still the force least understood by physics.
Over the centuries three other basic classes of forces were discovered:
Electromagnetism is familiar to us and pushes currents into our houses and electrons through our electronic devices. Lucky for us, it holds atoms and molecules together.
The “strong” force holds nuclei together. The atomic nucleus is made of neutrons (no electric charge) and protons (positive electric charge). Since same-sign charges repel, the atomic nuclei should all blow apart. They are trying to, but another force, the strong force, holds them together. The strong force is stronger than the electromagnetic force in the nucleus. But only at short distances. That’s why we don’t experience the strong force on our human scale. Its range determines the size of the biggest nuclei we can make. We have never made a nucleus larger than about 300 protons and neutrons. Since they are packed in a sphere and protons and neutrons are about the same size, you can calculate the radius using simple geometry: Volume =4/3πr3.
So the radius of the largest nucleus is about 4 protons and neutrons in radius (8 in diameter). That’s about the range of the strong force, about 4 protons’ width. That is 0.000000000000004 meters. (We’ve since learned that this “strong” force is generated by a more elementary force at work inside the protons and neutrons themselves, called “quantum chromodynamics”, but that’s a story for another day.)
The final force on our list is the so-called “weak” force. The weak force is so weak it doesn’t hold anything together (that I know of). But it still operates at the subatomic level. Its strength would be comparable to the electromagnetic force but its range is even smaller than the strong nuclear force. It’s range is almost 1000 times shorter. It’s important though because it is the only force that can change protons into neutrons and other transitions. Without it, the Sun could not fuse 4 hydrogen atoms (4 proton) into helium (2 protons and 2 neutrons) and would not have an energy source.
But these last three, while discovered later, are the best understood. These three play nicely with the over-arching framework of quantum mechanics. You may often hear that quantum mechanics is the domain of processes that are very small. That isn’t really the right story. Quantum mechanical effects have been seen to operate over distances the size of the Earth as one type of neutrino morphs into another. And that’s not the limit. I once co-authored a paper about a possible quantum mechanical effect that takes place over a distance of 1 billion light years.
So it is an over-simplification to say quantum mechanics is the physics of very small distances. It misses a key feature. Quantum mechanics actually uncovers a special relationship between distance and momentum. So it is really when the product of the characteristic distances and momenta are small that quantum mechanics is important.
Just as quantum mechanics reveals a special relationship between distance and momentum, the theory of relativity, which describes gravity tells us about a special relationship about time and space. And that’s not all. Quantum mechanics also describes a special relationship between energy and time. Meanwhile relativity is concerned with how momentum and energy are related. To summarize…
The two theories are about different relationships. It is when we try to deal with all these relationships at once, that gravity gets into trouble.
One of the biggest dramas in physics of the last 50 years of physics has been to reconcile the relationships of quantum mechanics with the theory of relativity. And three out of four times we were ultimately successful. There are good working theories of the quantum mechanical behavior of the electromagnetic, strong and weak forces. They are tested every day in the laboratory to high precision.
But gravity has not yielded. Much of the theoretical work in physics works on forming a consistent formulation of quantum mechanics and gravity. These are the goals of string theory and loop quantum gravity that Leonard and Leslie once argued about. Other attempts are underway. So far though, none has passed an experimental test, most don’t even make a distinguishing experimental prediction.
And that, finally, brings us to the science in today’s show. Sheldon’s seminar is not just about “thermodynamic fluctuations”, an elementary part of our theories of heat and energy. Rather, it is how gravity might emerge from a process that looks a lot like them. A new theory arguing this point came out recently from a respected theorist in the Netherlands, Erik Verlinde. The relevant equations were on tonight’s apartment whiteboards.
(Meanwhile, the whiteboards in Leonard’s lab were about something different…the search for hypothetical particles called axions. I promise that had nothing to do with Sheldon’s editorializing with what he wrote on the board above it.)
So it is a great embarrassment of physics, that the first force we discovered, gravity, is still the least understood. But I have hope. After all, Sheldon is on the job.