Biggest Mysteries in Physics: Antimatter, Dark Energy & ToE – Don Lincoln

Lex Fridman introduces Don Lincoln as a particle physicist at Fermilab who has spent decades working at the frontier of high energy physics. Fridman compares him to Richard Feynman, saying Lincoln can take very complicated ideas and explain them simply without losing the essential insights at their core.

Fridman frames the conversation around the search for a theory of everything. He borrows Lincoln's own framing that "the history of physics can be told effectively as a kind of history of unifications": a centuries-long effort, going back at least to Newton, to show that phenomena that look distinct are linked by unified underlying principles. Lincoln agrees to go through that history.

Lincoln starts with a qualification. There are "lots of different ways to do physics," he says, but the approach of particle physicists and cosmologists is to find the underlying principles that govern the laws of nature.

Lincoln asks the listener to imagine being the most brilliant person around in roughly the 1650s. You would have noticed two unrelated things. When you trip, you fall, which is the everyday experience of gravity. When you look at the heavens, you see the stars march across the sky and the planets move through the stars. That second phenomenon seemed to have "absolutely nothing to do with what happens when you drop your sandwich and the dog grabs it from you."

maybe the moon is falling, but it’s missing the Earth.

Lincoln calls this Newton's key insight. Around 1650, he says, there were in effect two sets of laws: celestial gravity, governing the heavens, and terrestrial gravity, governing the Earth. Today we just say gravity, but at the time the link "wasn’t at all obvious." He points out that the word "universal" in Newton's law of universal gravity is there because Newton realized the two were one and the same. Lincoln calls Newton "arguably one of the most brilliant humans of which I’m ever aware" and describes this as the first unification in physics that is easy to state in a way that makes sense to modern people.

Lincoln notes that the story can be traced back further, into chemistry and the nature of atoms, to Democritus. He says Democritus "was wrong about very many things" but was right that there is a smallest particulate form of matter. He gives examples of the errors. Democritus imagined a smallest atom of oil that was smooth, because oil is smooth. He imagined that atoms of vinegar were "little sharp, pointy things," because vinegar is tart and pricks the tongue. Lincoln stresses that the modern concept of the atom is very different from Democritus's, even though the core idea of a smallest particle survived.

Lincoln's second unification starts around 1830. Scientists then were dealing with two familiar phenomena. Magnets were mostly little pieces of magnetized iron that stuck to steel. Electricity meant small sparks generated for experiments and play or, more broadly, a lightning bolt across the sky. The lightning bolt and the little magnet, he says, "seem to be really unrelated."

Over the 1800s, many scientists explored small parts of the connection. For example, running electricity through a wire seemed to produce a magnetic field. Lincoln does not name these researchers. He says only that "there were a lot of names." Then, in about the 1860s, James Clerk Maxwell took the ideas that had been percolating for the previous 50 years and wrote down the laws of electromagnetism. Lincoln notes that these are differential or integral equations. Basically, he says, they have terms involving electricity on one side and terms involving magnetism on the other.

Electricity equals magnetism.

Lincoln presents this slogan as a deliberate simplification, reached by "forgetting all of the mathematical symbols." He calls it a staggering concept that a lightning bolt and the magnet that holds your kids’ art to the refrigerator are one and the same. With this, he has two examples of unification: terrestrial and celestial gravity, and electricity and magnetism combined into electromagnetism.

Lincoln explains the reach of the theory, which he dates to "1860 or ’70." If you take Maxwell's equations and "apply a little bit of calculus," the laws of electricity and magnetism combine into a wave equation. It shows that electric and magnetic fields oscillate and move as a wave, and the math gives the speed of that wave as the speed of light. Lincoln offers his own hedged judgment that this "had to be, I think, very persuasive" to people at the time. He adds that electromagnetism plays a significant role in chemistry, because atoms are held together by electromagnetic forces. He is careful to say that "There’s more to how atoms work," including quantum mechanics. Still, if electromagnetism were absent or very different, atoms would be very different.

I would have some unified theory that would explain all the behavior of all energy, matter, space, and time, which is a grand goal.

Fridman places this goal in a wider view of science. He says one goal of science beyond physics is to "construct models that can generalize the world." He calls Darwinian evolution a very beautiful theory that captures another layer of reality, and describes a physics theory of everything as capturing a different layer of abstraction about how the universe works.

Lincoln agrees that evolution is remarkable. He calls the significant overlap between human genetics and the genetics of a banana "pretty staggering." But he describes the outlook of his kind of scientist as a chain of explanations. Whatever happens in biology is caused by the movement of molecules. Molecules behave as they do because they are made of atoms. Atoms work as they do because of the nucleus and electrons, and the nucleus is made of protons and neutrons. He counts himself among those who want to "dig down to the very, very bottom" and find the smallest building block of nature.

It’s like having a whole bunch of Legos, but not knowing how to put them together.

Lincoln uses the Lego analogy to admit a limit of a pure catalogue of particles. Knowing the smallest building blocks "doesn’t tell you the story." You also need to know how they interact, which is why physicists study forces. Electromagnetism is his example of a force with wide explanatory reach. It accounts for electricity, magnetism, how light works and "much of chemistry."

Lincoln asks permission to "get on a small soapbox." He recalls that people once asked, "Well, why are you messing around with magnets and sparks?" His answer is that fundamental research has spinoffs, and that one big spinoff of taming electromagnetism is "our entire technological society." His example: people can hear this podcast through the internet, or simply because electricity runs the computers.

Without being able to govern electricity, we’d still be farmers and shoemakers in cities.

His broader claim is about timescales. Digging into deep, fundamental, not-understood, mysterious things can transform the world "a hundred or two hundred years later."

Lincoln is candid about the limits of that argument for his own field. When people ask what good it is to know how the inside of atoms or quarks works, he says, "I don’t know the answer to that." Instead he gives a more pragmatic historical case. About a hundred years ago, people were trying to understand how protons and neutrons hold together inside atoms, how they split and how they could be combined. That work led to nuclear power. He stays neutral on the policy question, noting that "some people like it and some people don’t," but says it is powerful and will generate energy for humanity. He suggests, as a possibility rather than a prediction, that "it may be that is the path that we take" as humanity moves away from fossil fuels, because humanity is going to need power no matter what.

Lincoln adds that "Nobody is going to go back to the way things were in the 1700s." Modifying the nucleus of atoms once seemed to have nothing to do with anything, yet it is now an enormous energy source available "if we so choose." He adds the condition that using it requires thinking carefully about how to do so, and whether we want to at all.

Fridman calls nuclear fusion and fission clear, near-term ways to unlock the large amounts of energy a civilization needs to flourish. He then turns to longer-term, explicitly speculative possibilities: dark energy, which he raises as a "dark energy crisis" to come later in the conversation, and antimatter. "Maybe," he says, solving the mysteries around antimatter could lead to ways of producing energy, and it "might lead to counterintuitive propulsion systems."

Lex Fridman opens this stretch by imagining that progress in fundamental theoretical physics could one day help humans travel through the universe. He grants that the idea currently seems far-fetched, too expensive, too complicated and too difficult. Still, he suggests that such breakthroughs might unlock new energy sources and technologies for exploration. He adds a caveat: like all technology, these advances would most likely also lead to more dangerous weapons or other sources of harm, and civilization would have to try to do more good than bad with them.

Don Lincoln agrees but recasts the worry. He acknowledges that nuclear weapons are very dangerous and that nuclear power produces waste that has to be dealt with. In his view, though, science is not creating new dangers so much as finding power that nature already offers, and that is nothing new. He points to fire as the oldest example of the same dilemma.

Fire can burn down your house or it can cook your steak.

From this Lincoln draws a division of responsibility. Scientists work out how nature behaves, but decisions about using that knowledge belong to everyone. He argues that conversations about science must involve all of society, because "what scientists can do is figure out how the world works. Society has to figure out how we wish to apply that or not apply that."

Fridman then defends curiosity for its own sake, saying that solving the universe's puzzles is "effing awesome," and Lincoln agrees. Fridman describes asking "How does this work?" as part of what makes us human. In his telling, groups of "apes" poking and shaking at the world eventually produced rockets, roads, bridges and the internet. He then moves the conversation's thread on unification, which had already covered Newton and Maxwell, into the twentieth century and Einstein, the subject of one of Lincoln's books.

Lincoln starts with Einstein's 1905 "miracle year," when Einstein wrote several papers. The best known of these is special relativity, which showed that two people experience time differently. Lincoln says this result "makes no sense to anybody who’s not really dug into it very hard." He also admits a basic gap in physics: "We don’t really understand what time is, which is weird." Physicists know a lot about time, he says, but do not really understand it.

Newton had treated time as universal, the same for someone here, on Mars or at Alpha Centauri. Einstein showed instead that observers moving at different speeds relative to one another experience time differently. Lincoln calls this "absolutely a mind-blowing concept."

Lincoln then corrects a common belief. Many people credit Einstein with inventing spacetime, the idea that space and time are the same thing. Lincoln says that insight actually came from Minkowski, whom he describes as one of Einstein's teachers and as "a little bit more mathematically inclined than Einstein." Minkowski noticed that in Einstein's equations, one person's space and time equal some numbers multiplied by another person's space and time. Lincoln dates Minkowski's full statement of spacetime to 1908.

Lincoln explains why the idea is hard to accept. People know time at "a visceral level and an experiential level," even without a formal understanding of it. Time is what "makes today today and not yesterday or tomorrow." Space feels different, because you can walk somewhere and walk back. In his words, "You can always move forward in time. It’s just moving backwards that turns out to be a little more difficult." He says that recognizing space and time as "pretty much the same thing" caused everybody to think about the world very differently.

When Fridman says the work led to the speed of light as a speed limit, Lincoln clarifies that the constancy of light's speed was a premise, not a result. He describes two premises:

The laws of nature are the same for everybody, so either of two people moving relative to each other can say they are the one at rest. Lincoln calls this uncontroversial Galilean relativity, an idea hundreds of years old. Everybody measures the same speed of light, however they are moving relative to each other. Lincoln says this was the controversial part, and very different from what Newton, Galileo "or any of the old guys" would have said.

Lincoln says the combination of these two premises produces "all of the weirdnesses of special relativity." He grants that a skeptic could call the second premise "just dumb," and says that is why testing matters. He separates two kinds of evidence. At first, the support was indirect: equations built on the two assumptions predict the behavior of everything "perfectly well," and because "The predictions are true, so the assumption is true." Direct experiments showing that the speed of light is the same for everybody came later.

He then describes how particle physicists make the direct measurement. Some subatomic particles emit light when they decay. Researchers collide two things together, which tells them when a particle was created, and surround the collision point with a detector. They then time how long the light takes to reach the detector. When the particle is at rest, the light arrives at the speed of light, "which it should be."

The key comparison uses particles that leave the collision at high speed, which Lincoln puts at "ninety-five or ninety-seven or very large fraction of the speed of light" before they decay into photons. If Einstein's conjecture were wrong, light from such a particle should travel at, as Lincoln puts it, "say, two times the speed of light or something like that" and reach the detector in about half the time. It doesn't. The light from moving particles still arrives at the speed of light.

Lincoln calls this "a hard, serious measurement": light from a particle created at rest and light from a moving particle both travel at the speed of light. He notes a historical limitation. This kind of measurement "is not something that was possible in Einstein’s day," so Einstein's premise rested at first on its successful predictions, not on this direct test.

Fridman asks where the speed limit ranks "in the full ranking of weirdness that is physics." Lincoln admits that when he first encountered it, it was "pretty freaking weird," saying it "pegs the weird meter." He says the idea becomes comfortable once you see the speed of light as "the speed of light through spacetime." Then, he says, everything falls much more into place.

Lincoln offers his own interpretation and labels it as such ("That’s, that’s how I view it"). To him, an ultimate speed is not that shocking. It is simply a property of space, in the same way that space can transmit an electric field of a certain strength and support certain things. He is open about what remains unknown:

Whatever space is, and we don’t know what space is, but whatever it is, it has the capability of transmitting these things at that one speed through space or time, and everything else comes from our insisting that we keep space and time different.

Fridman explains what he is after with these questions. He wants to imagine how it felt to live before space and time were unified and to judge how difficult that leap was. Lincoln answers in one word: "Huge." Fridman links this to the present. Twenty-first-century physics will need similar leaps, so he asks which unifications we are failing to see "in front of our eyes."

As an example, Fridman cites Paul Dirac, a figure who appears often in Lincoln's work and lectures. Dirac took antimatter seriously because the mathematics pointed to it, which Fridman says "just sounds insane." Lincoln agrees. Fridman calls spacetime unification "a good warm-up," since in his view the twentieth century gets "weirder and weirder, even with Einstein himself."

Lincoln replies with "an even more basic example" from chemistry. Sodium is an explosive metal; in water it "doesn’t quite explode, but it gets hot, and it pops around." Chlorine is a gas that is "going to kill you." Combined, they form the salt people put on their food. Lincoln uses this to illustrate both unification and deeper understanding: two dangerous substances become something "not only innocuous, but necessary for human life." His conclusion is that "this is not unusual, what you’re describing."

Lincoln adds that we tell children the world is made of atoms, even though most people have never seen one, and he calls that "crazy." He suggests that people accept atoms mostly through familiarity: the culture slowly accepts the idea, and it becomes real to people "even without the evidence." He qualifies this right away. He calls "How do we know there are atoms?" a valid question, a reference to his course on how we know what we know, and says "there are ways we do." He does not describe those ways in this section.

Fridman closes by asking to go through how physicists identified the universe's building blocks, all the way to quarks. He describes a path from a long-standing philosophical idea of atoms, to observations he says prove or strongly indicate that atoms exist, and then to the nucleus, electrons, photons and quarks. He notes that it "gets weirder and weirder" and that a current mystery is whether there are building blocks even smaller than these.