The Evidence for Modern Physics: How We Know What We Know
Don Lincoln is a Senior Scientist at Fermi National Accelerator Laboratory (Fermilab). He is also a Guest Professor of High Energy Physics at the University of Notre Dame. He received his Ph.D. in Experimental Particle Physics from Rice University.
Dr. Lincoln's research has been divided between Fermilab's Tevatron Collider, until its close in 2011, and the CERN Large Hadron Collider, located outside Geneva, Switzerland. The author of more than 1,000 scientific publications, his most noteworthy accomplishments include serving on the teams that discovered the top quark in 1995 and confirmed the Higgs boson in 2012. He is a fellow of the American Physical Society and the American Association for the Advancement of Science.
His writing at a popular level includes many articles as well as four books: Understanding the Universe, The Quantum Frontier, The Large Hadron Collider, and Alien Universe. His enthusiasm for science education earned him the 2013 Outreach Prize from the High Energy Physics Division of the European Physical Society.
Dr. Lincoln has given hundreds of lectures on four continents to a broad range of audiences. He is a blogger for the website of the PBS television series NOVA, and he also writes a weekly column for the online periodical Fermilab Today.
01: Do You Believe In Atoms?
Dr. Lincoln opens the course with the observation that all matter is made of atoms. But how do we know? The atomic hypothesis goes back to antiquity, although that was just an inspired guess. Survey the contributions of later scientists such as John Dalton and Albert Einstein. Discover why atoms are invisible to light microscopes, but not to the scanning transmission electron microscope.
02: Peering inside Protons and Neutrons
Peer inside atoms to find mostly empty space, along with electrons and a compact nucleus, composed of protons and neutrons. These particles were all discovered indirectly through painstaking but straightforward experiments. Learn how physicists used more complex tools to uncover hundreds of even smaller objects. It took the quark theory to bring simplicity and unity to this seeming chaos.
03: Seeing Light as Wave and Particle
Probe one of the most baffling mysteries of physics: the wave-particle duality of light. Trace the debate over the nature of light to its apparent solution in 1801, when Thomas Young demonstrated that light is a wave. A century later, Einstein proved that light also behaves as a particle. Astonishingly, further work showed that electrons and other matter also have this Janus-faced identity.
04: The Paradox of Quantum Entanglement
Dr. Lincoln boldly confronts the paradox of quantum entanglement, which governs the behavior of particles that share the same quantum state. Discover that the rules of quantum mechanics defy every attempt to explain what seems inexplicable—implying, for example, that a cat could be simultaneously dead and alive in Erwin Schrödinger’s famous thought experiment. Explore other spooky examples.
05: How We Know Special Relativity Is Real
Learn how Dr. Lincoln routinely conducts experiments that show the bizarre effects of Einstein’s special theory of relativity, which come into play at speeds approaching that of light. Like quantum theory, relativity strains credulity, but clocks really do slow down and length contracts at relativistic speeds; we just don’t notice these effects in our relatively slow-moving lives.
06: Why the Speed of Light Is the Speed Limit
How can the speed of light be the same for everyone, regardless of their state of motion? First, investigate how the speed of light is determined. Next, consider the hypothesized medium for light propagation—the aether—which was dealt a fatal blow by the Michelson-Morley experiment in the 1880s. Finally, examine laboratory proof that the speed of light is constant for all observers.
07: Discovering Subatomic Particles
Survey the fundamental particles and forces of the Standard Model, which is the prevailing theory of particle physics. Then focus on nonfundamental particles and the tools that discovered them, such as the cloud chamber. Easily built at home, the cloud chamber reveals the products of radioactive decay, including antimatter—which sounds like science fiction but is an authentic feature of reality.
08: How Do You Weigh a Quark?
Learn the secret for measuring the masses and lifetimes of subatomic particles that exist for roughly a trillionth of a trillionth of a second. Using the Higgs boson and top quark as examples, Dr. Lincoln draws on a simplified version of Einstein’s mass-energy equation and Werner Heisenberg’s uncertainty principle to infer detailed information about truly ephemeral entities.
09: Capturing the Ghostly Neutrino
Hear the story of the neutrino, the ghostly particle that passes through you at the rate of one quadrillion per second, with no ill effects. Neutrinos are created copiously in nuclear reactions and are fiendishly difficult to detect. Pinning them down took great experimental ingenuity, especially since neutrinos turn out to be quick-change artists, often transforming their identities in flight.
10: The Search for the Higgs Boson
As a member of the research team, Dr. Lincoln recounts the discovery of the Higgs boson, one of the major science stories of the past half century. Predicted in 1964, the Higgs particle wasn’t experimentally confirmed until 2012. Trace the path to this triumph, as physicists narrowed down the properties of the elusive particle and utilized powerful particle accelerators in the hunt.
11: Are Man-Made Black Holes Dangerous?
Evaluate three alarmist scenarios for a physics experiment gone horribly wrong. Some theorists predict that exotic phenomena such as strangelets, a false vacuum, and miniature black holes could be produced by new particle accelerators, leading to the destruction of Earth and even the universe! The risk, however small, hardly seems worth it. But Dr. Lincoln gives you good reasons to sleep soundly.
12: How We Know What Stars Are Made Of
Scientists did not know the exact composition of the Moon until astronauts brought back rocks. So how do we know what the unimaginably more distant stars are made of? Get a short course in astrophysics as you explore the secrets of starlight, which reveal stellar temperature and elemental composition to observers on Earth. Then apply the lessons of nuclear physics to the life cycle of stars.
13: Forming the Milky Way and Other Galaxies
Until 100 years ago, our Milky Way galaxy was thought to comprise the entire universe. Now we think there are roughly a trillion galaxies of various sizes and shapes in the observable universe. Investigate how astronomers reached this conclusion and how they mapped the structure and contents of the Milky Way, discovering a supermassive black hole at its center—among other galactic attractions.
14: Finding Planets around Distant Stars
Planets beyond our solar system weren’t discovered until the 1990s. Since then, thousands have been confirmed around nearby stars, and billions likely populate the Milky Way Galaxy. Planets are so dim compared to the stars they orbit that observers had to come up with clever techniques to infer their presence. Focus on the “wobble” and “shadow” methods, which have been remarkably productive.
15: The Awesome Evidence for General Relativity
Dr. Lincoln introduced Einstein’s special theory of relativity in Lesson 5. Now he covers general relativity, which incorporates gravity and predicts the warping of spacetime around massive objects. Study three phenomena that prove general relativity: an anomaly in the orbit of Mercury, the bending of starlight passing near the Sun, and the slowing of clocks in regions of stronger gravity.
16: The Hunt for Gravitational Waves
General relativity predicts that titanic events such as colliding black holes cause the fabric of spacetime to ripple with gravitational waves. Join the search for these signals produced by rare events that are all but undetectable by the time they reach Earth. The existence of gravitational waves was inferred from observations in the 1970s and finally confirmed by detectors in 2015.
17: How We Know the Universe Began with a Bang
The Big Bang is one of the few scientific concepts that has entered popular culture. But where did the idea come from? Trace this gripping detective story to attempts by a young female astronomer in the early 1900s to measure distances to stars. Her success set the stage for others to discover that the universe is expanding, as if from an initial “big bang.” More clues filled in the picture.
18: The Case for Cosmic Inflation
Unlike the well-founded theories you’ve studied so far in this course, turn to one that is as-yet-unproven—but mindboggling in its implications. Cosmic inflation proposes that a period of explosive expansion occurred in the first instants of the Big Bang. This startling idea accounts for two puzzling features of today’s universe: the observed uniformity of matter and the flat geometry of space.
19: How We Know Dark Matter Exists
Dark matter is the conjectured substance that outweighs ordinary matter by five to one. However, we can’t see it, nor can anyone say what it is—at least, not yet. The first clues to the existence of dark matter turned up in observations of stars and galaxies in the 1930s. Since then, the evidence has mounted. Consider alternative explanations and reasons to believe that dark matter is indeed real.
20: How We Search for Dark Matter
Dig deeper into the quest to understand dark matter. Start by ruling out plausible early explanations, including that dark matter is invisible ordinary matter like cold hydrogen gas or rogue planets. Also rebut some popular exotic theories. Then Dr. Lincoln outlines current experiments to pin down this elusive substance, among them his own work with high-energy particle accelerators.
21: How We Know the Universe Is Accelerating
Investigate evidence that the expansion of the universe reversed its gradual slow-down and stepped on the accelerator 5 billion years ago. “Dark energy” is the term given to this mysterious force that is expanding space at an ever-increasing rate. Learn how this remarkable phenomenon was discovered and explore its link to the cosmological constant proposed by Einstein a century ago.
22: Measuring the Size and Age of the Universe
Draw on the astonishing facts about the universe you have learned since Lesson 15. Then add observations from recent satellite missions, and reach exact conclusions about the size and age of the universe. One thing you discover is that the diameter of the entire universe is at least 500 times larger than the visible universe. Since we can’t see that far, how do we know? Dr. Lincoln explains.
23: What We Know about Quantum Foam
Few claims of physics are as absurd as that empty space is writhing with “virtual” particles—a foam of particles, antiparticles, and photons that appear and disappear with riotous abandon. Learn how Heisenberg’s uncertainty principle gives rise to this phenomenon of getting something from nothing, and discover that it is a crucial consideration for engineers creating microelectronics.
24: Are Space and Time Quantized?
Finish the course with a leap into one of the most speculative realms of physics—the quest to understand gravity at the quantum scale. Examine why Einstein’s theory of gravity—general relativity—is incompatible with quantum mechanics. Then consider what a quantized theory of gravity would imply. One thing it means for sure is a future filled with bold theories and big surprises!