The Great Courses Daily
A Search For the Theory of Everything
FNAL Don Lincoln
From a lecture series by Professor Don Lincoln, Ph.D. & Head Scientist at FermiLab
The unifying theories of physics are among the greatest and most complex in all of science; their progression toward ever-grander insights will transform our understanding of the universe, and is nothing less than a search for the theory of everything.
No image caption. No image credit.
“Dream no small dreams for they have no power to move the hearts of men.”
This quote by Johann Wolfgang von Goethe is still powerful today, two centuries after he first wrote it down. It doesn’t matter whether you’re trying to broker an international peace treaty or cure a disease or change a society, it’s not the incremental improvements that stir the blood; it’s the big ideas.
There is a class of scientists who who live by these words. They keep thinking big and asking “why,” with each answer resulting in yet another question. They do that over and over and over again, and the hope is that, one day, there will be no more questions, because we understand the reasons for everything. That is dreaming big!
Our mastery of the atom made chemistry possible and also allowed us to build electronics and computers that can calculate faster than human imagination. No image credit.
In science, humanity has had great success over the centuries. Isaac Newton’s amazing ideas about gravity were the first major scientific steps toward a theory of everything, ideas that we still use to guide our space probes to distant targets, like when the New Horizons spacecraft buzzed by Pluto.
NASA/New Horizons spacecraft
Our mastery of the atom made chemistry possible and also allowed us to build electronics and computers that can calculate faster than human imagination.
Each of these achievements is big in its own way, but they aren’t the biggest possible. While there’s no denying that these ideas originated from a grand dream, each represents merely a single facet of human knowledge. The ultimate goal of science is much bigger. The ultimate goal of science is nothing less than an understanding of the fundamental rules of the universe itself. That’s a pretty ambitious goal and it depends crucially on the idea, which seems to be a fact, that all of the phenomena we see around us are interconnected and arise from even deeper causes.
The Standard Model
While nobody claims that science is done in their search, you can regard the standard model as the current best guess of a grand unified theory.
The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
That’s why it’s so important to understand it and what it signifies. For one thing, whatever the final theory of everything looks like, the standard model will be part of it.
The key components of the standard model consist of:
Quarks – found inside protons and neutrons in the center of atom;
Leptons – the lightest of the subatomic particles, the most familiar one is the electron is found in the outskirts of every atom;
Force-carrying particles, sometimes called gauge bosons – responsible for transmitting three of the four known forces;
Higgs Boson – a particle whose existence was confirmed in 2012, the final missing piece to the standard model.
Over the last few decades, science has unified forces that historically have seemed distinct. That’s incredibly exciting, but it also leads to a bit of confusion, so let’s clear that up a bit, by talking about the five forces — the third item in the components of the standard model.
Earth gravity. ThinkstockPhotos
The Five Forces
The five forces are as follows:
Gravity, which keeps us firmly planted on the ground and guides the planets through their trajectories
Electromagnetism, which includes electricity, magnetism, light and chemistry
The strong nuclear force, which binds protons and neutrons together in the nucleus of atoms
The weak force, which is responsible for forms of radioactivity
The Higgs field, which gives mass to subatomic particles
But why then, in some cases, does science refer to only three or four forces? Well, in the late 1960s, physicists showed that the weak force and electromagnetism were really two facets of a single thing, much in the same way that electricity and magnetism turned out to be two facets of something that we now call electromagnetism.
Therefore, scientists often talk about an electroweak theory, so they might say that the forces are gravity, the electroweak force, the strong force, and the Higgs field. On the other hand, the Higgs field is inextricably tied with the electroweak force, so maybe it can get tucked under the electroweak umbrella. Under that way of thinking, there are but three: gravity, the strong force, and the electroweak complex.
And how about the term forces? A better word for these would be interaction, because the word interaction means that some change is caused, like changing a particle’s identity without actually moving it. However, the word force is ingrained in the literature, so let’s stick with that word most of the time.
The Strong force is used to explain why the sun burns at such high temperatures. No image credit.
The strong force is the strongest of the known forces. For example, it’s the force that explains why the sun burns so very hot. But it also has a weird behavior. It’s incredibly strong over very short ranges—say, the size of a proton. Once two particles are separated by a distance much larger than that, the strong force goes to zero. It’s a little like Velcro. If two pieces of Velcro are touching, they’re strongly bound together, but once they’re separated, they feel no attractive force at all. That particular facet plays a big role in understanding the large range observed in the mass of atoms. That’s the strong force.
The next strongest force is electromagnetism, which unifies electricity and magnetism into a single force. It’s much weaker than the strong force, but it has a different behavior as far as distance is concerned. Two particles experiencing the electromagnetic force will, in principle, feel a force between one another even if they are located on opposite sides of the universe. Granted, that force will be very small, but it won’t be mathematically zero, because electromagnetism has an infinite range.
Because of the difference in how the two forces change with distance, you have to be very careful to specify distances when you compare electromagnetism to the strong force, so you traditionally pick a separation distance of about the size of a proton, which is a femtometer, or 10−15 meters. At that separation distance, the strong force is about 100 times stronger than electromagnetism. Of course, given the short range of the strong force and the infinite range of electromagnetism, if two particles are separated by just a meter, or even a millimeter, electromagnetism is actually much stronger.
The next weakest force is the weak force. The natural range of the weak force is about 1/1000 the size of a proton. However, if we ask how strong it is at the separation of a femtometer, it’s about 100,000 times weaker than the strong force. When we look at the weak force at its natural scale, we see that it’s actually similar to electromagnetism, and that was the beautiful insight that allowed for electroweak unification.
Then there’s gravity. It has an infinite range like electromagnetism, but at the femtometer distance scale, gravity is approximately like 1040 times weaker than the strong force. That’s a one over a one followed by 40 zeros. “Approximately” because you get a different answer if you’re talking about the gravitational force between two protons, two electrons, or a proton and an electron, but the 1040 number gives you the right message: gravity is crazy weak. And, indeed, gravity is so weak that we’ve never figured out a way to study it on these super-small scales. If we tried, the measurements would just get swamped by the effects of the other forces. So gravity is not covered by the standard model.
The Higgs field is a bit different — it actually gives mass to particles, so it’s not a force in the way that the others are. Therefore, it isn’t discussed in quite the same way because we don’t know how its strength compares to the others. This is one of the times where the word interaction is more apt. Because of its interaction, the Higgs field turns massless particles into massive particles.
CERN CMS Higgs Event
CERN ATLAS Higgs Event
The standard model is amazing, and we’ve only bareley discussed one of it’s four components. With this standard model, science can explain basically everything we see, from why cells divide, to how stars burn, to why objects move in a particular manner, and on and on. The hope is that one day, we will be able to unify the electroweak and strong forces into a single force called a grand unified theory, which I’m certain we will discuss in a later article.
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