Biggest Mysteries in Physics: Antimatter, Dark Energy & ToE - Don Lincoln | Lex Fridman Podcast #497
In a Nutshell
Don Lincoln explains that physics progresses through successive unifications of forces and concepts, from Newton's gravity and Maxwell's electromagnetism to the electroweak theory confirmed by the Higgs boson discovery at the LHC. The Standard Model is now complete but leaves major unsolved problems: the matter-antimatter asymmetry (baryogenesis/leptogenesis), the nature of dark matter (likely a weakly interacting particle, not modified gravity), and dark energy (vacuum energy discrepancy of 10^120, possibly a property of quantized space). A true Theory of Everything unifying all forces including gravity remains decades or centuries away, as current theories cannot be reliably extrapolated to Planck-scale energies without new experimental anomalies to guide them.
These notes were generated by AI and may contain inaccuracies.
Don Lincoln is a particle physicist at Fermilab who has spent decades working at the frontier of high energy physics. Physics, particularly the work of particle physicists and cosmologists, focuses on finding the underlying principles that govern the laws of nature.
In the 1650s, people recognized two distinct phenomena: terrestrial gravity (objects falling on Earth) and celestial gravity (motion of stars and planets). Newton realized these were the same force. His theory is called the law of universal gravity, with the word "universal" emphasizing that these previously separate concepts were unified. This is considered the first easily describable unification in physics.
Democritus proposed the concept of atoms as the smallest particulate form of matter. While he was incorrect about the specific properties of atoms (imagining smooth atoms of oil and sharp atoms of vinegar), the fundamental idea of a smallest particle was correct.
In the 1830s, electricity and magnetism were considered separate phenomena. Electricity was understood through sparks and lightning, while magnets were pieces of magnetized iron. Throughout the 1800s, scientists discovered that running electricity through a wire creates a magnetic field. In the 1860s, James Clerk Maxwell synthesized these discoveries into his laws of electromagnetism. Maxwell's equations demonstrate that electricity equals magnetism, unifying the two into electromagnetism.
The ultimate goal is a unified theory explaining all behavior of energy, matter, space, and time. Science more broadly aims to construct models that generalize how the world works. Darwinian evolution is cited as an example of a theory capturing another layer of reality.
Biology is caused by the movement of molecules. Molecules are made of atoms. Atoms work because of nuclei and electrons. Nuclei consist of protons and neutrons. The goal is to identify the smallest building blocks of nature. Knowing the building blocks is insufficient without understanding how they interact, which requires studying forces.
Electromagnetism governs electricity and magnetism, explains how light works, and plays a significant role in chemistry by holding atoms together. Applying calculus to Maxwell's equations produces a wave equation showing that electric and magnetic fields oscillate at the speed of light. Without electromagnetism or if it functioned differently, atoms would be fundamentally different.
Maxwell's work on electromagnetism enabled modern technology including electricity and the internet. Research into how protons and neutrons are held together led to nuclear power. Nuclear power represents a major energy source that may help humanity move away from fossil fuels.
Solving mysteries around antimatter could potentially lead to new energy sources and propulsion systems for space travel. Technology is described as a double-edged sword that can lead to both beneficial and harmful applications, requiring society to make careful choices.
Science reveals power that nature has already presented. Fire can cook food or burn houses. Nuclear weapons and nuclear power carry risks, but scientists determine how the world works while society determines how to apply that knowledge.
Einstein had his miracle year in 1905. His work on special relativity showed that different observers moving at different speeds experience time differently. Newton assumed time was universal for everyone. Einstein demonstrated this was not the case.
Hermann Minkowski, one of Einstein's teachers, recognized that Einstein's equations showed space and time are fundamentally the same thing. This insight was presented in 1908. Space allows freedom of movement in multiple directions, while time only moves forward.
Einstein's two premises were: the laws of nature are the same for all observers, and everyone measures the speed of light as the same regardless of relative motion. These assumptions, combined, produce the counterintuitive predictions of special relativity. Experiments with subatomic particles decaying into photons have confirmed that light travels at the same speed whether emitted from a stationary or high-speed source.
The speed of light is the speed through spacetime. Space itself has properties that limit transmission to this speed. Once accepted as a property of spacetime rather than a separate constraint, the concept becomes more intuitive.
The unification of space and time was a massive conceptual leap. Historical examples of difficult conceptual shifts include accepting that the world is made of atoms and that sodium (explosive metal) and chlorine (toxic gas) combine to form table salt.
Einstein received the Nobel Prize for the photoelectric effect. He did not receive Nobel Prizes for special relativity or general relativity. General relativity is described as another unification.
Einstein realized that acceleration in a rocket ship feels identical to gravity. This insight, combined with his spacetime framework, led to the understanding that gravity can be described as the bending of spacetime.
Einstein's insight that space and time are aspects of a single entity called spacetime challenged intuitive understanding. Time flows continuously in one direction while space allows free movement in multiple directions. In 1908, Minkowski formalized this unification into strict spacetime. This framework established the speed of light as a universal speed limit.
Einstein's special relativity rests on two premises. The first, Galilean relativity, states that the laws of nature remain identical regardless of relative motion. The second and more controversial premise asserts that every observer measures the same speed of light irrespective of their relative velocities. These two assumptions together produce the counterintuitive effects of special relativity.
Experimental validation confirms the constancy of light speed. Subatomic particles created in collisions decay and emit photons. Detectors surrounding collision points measure photon travel time, consistently registering light speed. Particles moving at 95-97% of light speed also emit photons that reach detectors at exactly light speed, contradicting any expectation that the photon's speed would add to the particle's velocity.
The speed limit imposed by light speed initially registers as extremely counterintuitive. Once understood as the speed of light through spacetime rather than through separate space and time, the concept becomes more coherent. Space possesses intrinsic properties that limit transmission speed, analogous to how space supports a maximum electric field strength. The apparent weirdness arises largely from the human tendency to treat space and time as fundamentally distinct categories.
The spacetime unification serves as preparation for later conceptual leaps required in physics. Historical precedents show how initially implausible ideas become accepted through evidence and familiarity. Paul Dirac took antimatter seriously because mathematics predicted it, despite the idea sounding absurd at the time.
A simpler example involves sodium, an explosive metal, and chlorine, a toxic gas. Their chemical combination produces sodium chloride, ordinary table salt essential for life. This illustrates how unification and deeper understanding can transform apparently incompatible or dangerous entities into something beneficial.
Einstein received the Nobel Prize solely for the photoelectric effect. He should have received additional prizes for both special and general relativity. General relativity represents another unification: Einstein realized that acceleration and gravity produce indistinguishable effects. This equivalence principle allowed him to describe gravity as the curvature of spacetime.
The idea generation process behind such insights requires multiple elements. Knowledge of prior work, mathematical facility to derive implications, and rigorous self-critique are essential. Most ideas prove incorrect, requiring disciplined elimination. The intuitive spark that produces genuinely novel connections remains rare. History remembers individuals who combine creative insight with rigorous validation.
Einstein himself initially resisted quantum mechanics despite its experimental success. He understood the theory but found its implications unacceptable. Nevertheless, his critiques proved valuable. His analysis of quantum entanglement generated testable predictions that later experiments confirmed, demonstrating that his critical engagement advanced the field even without generating new theories himself.
By the 1930s, physicists recognized four apparently distinct forces. Gravity and electromagnetism operate over macroscopic distances. The strong nuclear force binds atomic nuclei. The weak nuclear force governs certain radioactive decays. The long-term goal of a theory of everything envisions these four forces as manifestations of a single underlying interaction.
In the late 1950s and early 1960s, theorists proposed that the weak nuclear force and electromagnetism represent different aspects of a single interaction. Steven Weinberg, Sheldon Glashow, and Abdus Salam demonstrated this unification in 1967, showing that at sufficiently high energies the two forces merge into the electroweak force.
A significant obstacle remained. Electromagnetism has effectively infinite range, while the weak force operates only at distances much smaller than a proton. The resolution came from ideas proposed in 1964 by three independent groups. The electroweak force is mediated by particles: photons for electromagnetism and W and Z bosons for the weak force. The photon is massless while the W and Z bosons are massive.
The Higgs field, postulated in 1964, permeates all space and endows certain particles with mass through interaction. Particles that couple to the Higgs field acquire mass; particles that do not couple remain massless. The photon does not interact with the Higgs field and stays massless. The W and Z bosons do interact and therefore gain mass.
At the extremely high energies present shortly after the Big Bang, the Higgs field strength was zero. All particles, including those that would later become massive, traveled at light speed. When the universe cooled to approximately 10^-12 seconds after the Big Bang, the Higgs field acquired a nonzero vacuum expectation value. This event, called electroweak symmetry breaking, gave mass to the weak force carriers while leaving the photon massless.
The Higgs mechanism functions as a low-energy correction to electroweak theory. The underlying electroweak symmetry holds only at high energies; the Higgs field provides the adjustment that reproduces observed low-energy physics.
The Higgs field itself remains invisible. Like the electromagnetic and gravitational fields, only its effects are observable. Quantum field theory describes fields that can undergo localized excitations. These excitations manifest as particles. The electromagnetic field excites as photons; the Higgs field excites as Higgs bosons.
Experimental searches for the Higgs boson began after its theoretical role became clear in 1967. The Tevatron at Fermilab, upgraded to deliver ten times more collisions per second, searched for the particle by colliding protons and antiprotons. The Large Hadron Collider later succeeded in this search with seven times higher collision energy and roughly one hundred times higher collision rate.
Particle accelerators exploit E=mc² to convert kinetic energy into new particles. When two particles collide with equal and opposite momentum, their net momentum is zero. The collision energy can materialize as new particle pairs. Conservation laws require that particles be created with their antiparticles.
This process created the antielectron (positron) in 1932 and the antiproton in 1955. The reverse process also occurs: matter-antimatter annihilation converts mass into energy. At Fermilab, producing one antiproton required smashing approximately 100,000 protons into a target. The Tevatron operated at 120 GeV for antiproton production, while CERN's current antimatter program uses a 26 GeV accelerator for different experimental goals.
The LHC surpasses the Tevatron in both energy per collision and collision frequency. The top quark, discovered at Fermilab in 1995, required six to twelve months of data collection yielding 38 candidate events, roughly half of which were background. At the LHC, top quarks appear at a rate of one per second, transforming them from discovery targets into background processes that must be rejected when searching for rarer phenomena.
Protons possess internal structure, making production of specific hadronic states more difficult than producing electrons and positrons. Higher collision energy increases the probability of creating heavier particles once kinematic thresholds are exceeded.
At the CERN accelerator, collisions occur at a rate of about a billion collisions per second. Some collisions happen simultaneously, with roughly 40 million moments per second where a shot is taken, and each moment may contain about 20 collisions. Particle beams at the LHC resemble tiny sticks of spaghetti, much thinner than actual spaghetti, and about the length described. Beams travel in opposite directions, each filled with protons, passing through one another like swarms of bees, where most pass without interaction but occasional head-on collisions occur.
Beams are roughly the thickness of a human hair, making side-to-side collision positions difficult to distinguish, but positions along the beam direction can be resolved. Detectors surrounding collision points allow identification of where particles originated. At any crossing, there may be around 20 collisions, though most are uninteresting because they exemplify well-understood physics tested over decades.
The CMS detector is 70 feet long, 50 feet high and wide, five stories tall, and weighs 14,000 tons. ATLAS is 150 feet long, 80 feet across, and weighs 7,000 tons. ATLAS could fit four units on a soccer or football field with room for sidelines personnel. Both detectors function as cameras taking pictures 40 million times per second.
Not all data can be recorded, so detectors use triggers to select interesting events such as large energy deposits or asymmetric energy distributions. Fast electronics reduce 40 million possible pictures per second to about 100,000 candidates. Commercial processors then perform quick analysis, accepting roughly 1,000 collisions per second for recording. Graduate students later analyze these to identify potential Nobel Prize discoveries. Data flow involves petabytes moving seamlessly worldwide.
Before the LHC, scientists at Fermilab worked to find the Higgs boson while simultaneously preparing for CERN's capabilities. The Higgs boson might not have existed, but evidence suggested it could. Both Fermilab and CERN accelerators could either discover or rule out the Higgs if it existed within the energy ranges tested.
Theory allowed calculations for every conceivable Higgs mass, enabling systematic exclusion of mass values where the particle was not observed. The LHC had ten times the collision rate and 3.5 times the energy compared to Fermilab. By July 2012, Fermilab had ruled out most mass ranges, narrowing the possible Higgs mass to between roughly 120 and 145 in the relevant units.
Fermilab's accelerator shutdown meant insufficient data existed by July 2012 to claim discovery, though two days before the CERN announcement, Fermilab measurements confirmed that if the Higgs existed, it must lie in the remaining unexcluded region.
The July 4, 2012 discovery found a particle consistent with the Higgs boson. Alternative theories like supersymmetry predicted five Higgs bosons rather than one. Initial data supported the single Higgs but could not immediately exclude multiples. Over the subsequent 14 years, measurements confirmed the particle's mass, its spin of zero, and its decay patterns.
The Higgs preferentially decays into the heaviest accessible particles. It decays into bottom quarks, W and Z bosons, and photons through a loop process, matching rates predicted by the original 1964 Higgs theory. These validations confirm Peter Higgs, Robert Brout, and François Englert's predictions.
The Higgs boson is sometimes called the "God particle," originating from Leon Lederman's book title. Lederman reportedly wanted to call it the "goddamn particle" due to the difficulty in finding it, but the publisher favored the more marketable name. Lederman directed Fermilab and wrote a foreword for one of Don Lincoln's books.
The discovery validated the last unconfirmed piece of the Standard Model. It demonstrated a field giving mass to some particles but not others. While important, the discovery ranks below paradigm-shifting contributions like Einstein's work, representing instead the completion of about 50 years of Standard Model development.
The Grand Unified Theory (GUT) aims to merge the electroweak force with the strong force into a single grand unified force. Gravity remains outside this unification due to fundamental differences. A subsequent Theory of Everything would incorporate all forces including gravity.
Current progress toward GUT appears slow, with ideas from the early 1980s failing experimental tests. String theory proposes particles as tiny vibrating strings at the Planck length scale. Loop quantum gravity represents another candidate approach.
A Theory of Everything is believed to exist as fundamental rules governing matter, energy, space, and time. However, reaching such a theory is estimated at 50 to 100 years or more. Historical unification timelines show gravity unification took 200 years, electromagnetism another 100 years, and electroweak unification followed. The unification scale for GUT is approximately 10^15, a quadrillion times higher than current accelerator energies.
Accelerator energy increases by roughly a factor of seven every 20 years, suggesting hundreds of years at current rates, though diminishing returns make this uncertain. Any viable theory must produce falsifiable, testable predictions backed by feasible experimental methodology.
String theory remains untested and should not be believed until validated, even if internally appealing. If correct, predictions occur at Planck energies around 10^19 GeV, far beyond the current 10^4 GeV reach. Two testing paths exist: building facilities reaching Planck energies, or deriving measurable predictions such as the electron mass from Planck-scale equations.
String theory currently offers approximate solutions to approximate equations. No successful reduction to everyday measurable predictions has emerged despite work since the 1980s. Black hole singularities combine general relativity and quantum mechanics, but interiors remain inaccessible.
Einstein's unfinished dream was a theory of everything. Practical constraints require theories to make testable predictions rather than remaining beautiful but unverified abstractions. Projecting from current measurements to a quadrillion times higher energy scale presents enormous extrapolation challenges. Early human ancestors operating at meter scales illustrate the difficulty of inferring physics at vastly different scales.
Physics theories work well within certain scales, but extrapolation becomes unreliable over vast distances. Working at 100 meters or even 1,000 meters is feasible, and 10,000 meters is still possible though unlikely. The challenge from observable scales to the Planck scale spans 10 to the 15th, not 10 to the 5th. An analogy illustrates this: a person walking around Africa can make accurate local predictions for short distances, but traveling 500 miles east would encounter the Indian Ocean, sperm whales, or the Kraken—none of which could be predicted from central Africa. Going a mile up changes little, but 10 miles up means no breathable air and freezing temperatures, while 100 miles up is fatal. Two miles down would roast a person. Local understanding fails when applied to distant or extreme conditions.
Even the best theory developed within one realm cannot anticipate phenomena like the Alps, Antarctica, penguins, or flamingos. Current physics allows predictions a factor of 10 or 100 beyond what is measured, but extending predictions a quadrillion times higher represents the pinnacle of arrogance. As an experimentalist, progress toward a theory of everything requires addressing unsolved questions: whether something exists smaller than quarks, whether dark matter is real and what it consists of, the reality and nature of dark energy, and the fundamental nature of space and time.
Progress at factors of 10 or 100 improvement might occur within a generation, but predicting a quadrillion times beyond current reach is improbable. Superstring theory is likely incorrect not due to lack of intelligence among its proponents, but because new phenomena will emerge. Chemistry could never have predicted nuclear physics despite the relatively small energy increase involved. Early calculations estimated the Sun's age at only 10 million years before nuclear processes were understood. The odds of correctly predicting physics a quadrillion times beyond current measurements are vanishingly small. The issue lies not in human brilliance but in undiscovered phenomena.
Dark matter remains unknown despite extensive knowledge of what it is not. It might fit within superstrings or operate under entirely different physics. A theory of everything requires a conceptual leap comparable to Einstein's spacetime insight, where gravity bends spacetime. Mathematical frameworks alone are insufficient; they must enable new understanding of reality and generate testable macro-world predictions. Ideas such as space and time emerging from entropy represent interesting conceptual shifts but remain unvalidated without measurement.
Theorists produce many creative ideas that ultimately fail experimental tests. One example proposed complex dark matter with an entire dark sector containing dark atoms that interact. This idea gained attention but most simple versions have been invalidated by testing. Large extra dimensions were proposed to explain gravity's weakness relative to other forces by allowing gravity to leak into additional dimensions. These elegant concepts frequently die when confronted with data.
Science progresses both through top-down testable theories and through bottom-up anomalies. Vera Rubin measured galaxy rotation speeds using high school physics calculations and found the measurements disagreed with predictions. This discrepancy led to the dark matter hypothesis. Such precise measurements that conflict with established theory provide powerful clues. The entire theoretical framework might unravel or require only minor adjustments.
String theory faces challenges beyond experimental testability, including reliance on unobserved extra dimensions and the landscape problem. The theory permits an enormous number of possible universes, rendering it unpredictive because parameters can be tuned to describe any universe, including ours. While this objection holds, connecting string theory predictions to physical measurements could eliminate invalid alternatives, similar to solving x + 5 = 9 to constrain possible values. The vibrating string concept remains appealing, but without experimental validation the theory cannot be confirmed.
Killing string theory definitively requires a failed prediction, which proves difficult. After roughly 50 years of development since the 1970s, progress remains limited. Researchers increasingly question dedicating their careers to a framework unlikely to yield definitive answers within their lifetimes. This mirrors interest in quantum mechanics interpretations during the 1940s, which some avoided in graduate school because brilliant physicists had spent lifetimes without definitive progress.
Alternative theories often lack predictive power despite passionate advocacy. Loop quantum gravity differs as a well-developed but incomplete framework addressing only quantum gravity, not a theory of everything. It does not attempt to unify all forces. General relativity treats space as smooth and infinitely divisible, allowing calculus to apply. Loop quantum gravity proposes that at sufficiently small scales, space becomes quantized like water molecules, where further division becomes impossible.
String theory originated as a theory of the strong force competing with quantum chromodynamics. After failing that competition, researchers noticed it predicted a massless spin-2 particle, proven to be the graviton. This discovery generated excitement as a potential route to include gravity with other quantum forces. Loop quantum gravity instead focuses on quantizing gravity and understanding space's fundamental nature.
An early loop quantum gravity prediction held that light speed would vary with frequency or wavelength due to interaction with quantized space structure. Gamma-ray bursters billions of light-years away emit light across all wavelengths simultaneously. If the prediction held, different wavelengths would arrive at different times. Observations showed simultaneous arrival, appearing to falsify the theory. However, the theory was subsequently modified, rendering that particular prediction invalid while preserving other aspects.
The 2017 observation of gravitational waves from two neutron stars provided a striking test. The neutron stars coalesced, producing gravitational waves and, unlike black holes, an explosive bright flash of light. Both signals traveled 140 million light-years and arrived within 1.7 seconds of each other, confirming gravity propagates at the speed of light.
Empty space is not truly empty. Quantum field theory assumes space exists and postulates fields for every known particle: photon field, electron field, up quark field, down quark field, and others. Fields vibrate in characteristic patterns to produce real particles. Non-characteristic vibrations produce virtual particles that do not truly exist as stable entities.
Virtual particles can be described as matter-antimatter pairs briefly appearing and disappearing, or more accurately as field ripples. These phenomena occur too rapidly to observe directly but produce measurable effects. The Casimir effect places two parallel metal plates extremely close together. Virtual particles between the plates face wavelength constraints because longer wavelengths cannot fit, while outside the plates all wavelengths exist freely. The resulting pressure imbalance pushes the plates together, confirming the effect.
A second validation involves particle magnetic moments. Standard 1930s quantum mechanics predicted electron magnetic moment values that disagreed with 1948 measurements by 0.1%. This discrepancy at the Shelter Island conference prompted development of quantum electrodynamics, which incorporates quantized fields. Virtual particles surrounding a bare electron modify its measured magnetic properties by the observed 0.1%. Measurements of electron and muon magnetic moments now agree with theory to 10 significant figures out of 12 total figures, with remaining disagreement possibly indicating new physics.
Paul Dirac predicted antimatter in 1928 while reconciling quantum mechanics with relativity. The Schrödinger equation was non-relativistic. Dirac's relativistic equations reduced to something equivalent to x squared equals 1, yielding solutions of positive and negative one. The positive solution corresponded to the electron; the negative solution initially lacked interpretation. Dirac maintained the equations were correct, predicting a positively charged electron sibling later called the positron.
Carl Anderson and Seth Neddermeyer discovered the positron in 1932. The antiproton was created in 1956 at Berkeley using a high-energy accelerator. The antineutron followed in 1957. Current technology produces antimatter protons and electrons through particle collisions. Antimatter helium nuclei have been created using two antiprotons and two antineutrons. CERN has produced antimatter hydrogen atoms.
CERN researchers create antimatter hydrogen atoms by producing antimatter protons from a lower energy accelerator beam, collecting and slowing them, then cooling them near absolute zero. They use sodium-22 to generate antimatter electrons, which are also slowed and combined with the antimatter protons. This produces literal antimatter hydrogen atoms consisting of an antimatter proton surrounded by an antimatter electron.
Scientists have performed extensive measurements on these atoms. They have agitated the atoms to cause light emission and compared the spectral characteristics of light from antimatter hydrogen atoms to ordinary hydrogen. The spectral lines match exactly as predicted by theory.
In 2023, the ALPHA experiment at CERN placed antimatter hydrogen in a bottle and released it to determine whether it falls up or down. While Coulomb's law shows that electric charges can attract or repel, theoretical reasons suggested antimatter should fall downward like ordinary matter.
Researchers first tested with regular hydrogen, finding that approximately 80% of atoms would fall through the bottom of the bottle and 20% through the top due to gravity's weakness. When they repeated the identical experiment with antimatter hydrogen, they found that antimatter falls down.
The current measurement shows antimatter falls with 75% the strength of regular matter, but with significant uncertainties of plus or minus 0.13 from experimental error and plus or minus 0.16 from the theoretical model. This yields a range of approximately 75 plus or minus 29, meaning the result remains consistent with gravity acting at full strength. Researchers are working to improve measurement precision.
Until 2011, Fermilab operated as the world's most powerful antiproton production facility. Every 2.3 seconds, they would smash 10 to the 13th protons into a target and extract 10 to the 8th antiprotons, meaning roughly 100,000 protons were required to produce a single antiproton.
Over 12 to 24 hours, they collected and cooled approximately 10 to the 12th antiprotons. While a trillion antiprotons sounds substantial, a gram of antimatter contains 10 to the 23rd antiprotons. This means Fermilab produced roughly 100 billionths of a gram per day, equating to about a nanogram per year.
At this production rate, creating a single gram of antimatter would require a billion years of continuous operation with minimal downtime.
Combining one gram of antimatter with one gram of matter releases energy equivalent to the combined Hiroshima and Nagasaki explosions. A one-megaton explosive yield would require approximately 25 grams of antimatter.
NASA estimates indicate that producing 25 grams of antimatter sufficient for a one-megaton weapon would cost approximately 1.5 quadrillion dollars. This contrasts sharply with the 10 to 50 million dollars required to produce a conventional one-megaton nuclear warhead.
The cost estimate derives from 62 to 63 trillion dollars per gram of anti-hydrogen production.
Antimatter represents an extremely compact and powerful energy source. One gram could theoretically accelerate a spacecraft to 0.2 times the speed of light, enabling a 20-year journey to the Alpha Centauri star system.
The fundamental physics of antimatter production and use is understood. Energy can be converted into antimatter, which can then be contained, stored, and used to heat matter and expel it from a rocket nozzle. However, containment presents enormous challenges. Any loss of containment, even for a millionth of a second, would result in catastrophic annihilation.
The primary limitation is not physics but engineering. The danger extends beyond weapons applications. In a spacecraft powered by antimatter, any containment failure would be instantly fatal with no warning.
Current antimatter production requires concentrating enormous energy into volumes the size of protons. Accelerators achieve this by smashing protons together in tiny spaces. The local energy density, not the total energy, determines whether antimatter can form.
No new fundamental physics breakthroughs are expected to dramatically simplify antimatter production. Alternative mechanisms would still require concentrating energy to extreme densities, which accelerators currently achieve most effectively.
Einstein's equations predict that energy converts into equal quantities of matter and antimatter. After the Big Bang, abundant energy should have produced equal amounts of both. However, the observable universe contains almost exclusively matter.
By counting protons in galaxies and photons in the cosmic microwave background, physicists calculate that the early universe had a tiny asymmetry: for every billion antimatter particles, there was one extra matter particle. The billions of pairs annihilated, leaving the excess matter that forms our universe.
The mechanism responsible for this slight asymmetry remains unknown.
One possibility is that the asymmetry was simply built into the universe at formation. Another class of theories falls under baryogenesis, which proposes that protons and other baryons can oscillate between matter and antimatter states with a slight preference for matter.
This oscillation phenomenon was measured in the 1960s using ephemeral particles produced in accelerators, confirming that matter and antimatter can behave asymmetrically, though the measured effect is insufficient to explain the observed cosmic asymmetry.
Fermilab is investigating leptogenesis, which focuses on leptons including neutrinos. Fermilab operates the world's most powerful neutrino accelerator. Neutrinos oscillate between three flavors, analogous to cycling between different cat species. Researchers will compare oscillation rates between neutrino beams and antineutrino beams to determine whether they oscillate differently.
If neutrinos and antineutrinos oscillate at different rates, combined with other conditions, this could explain the matter-antimatter asymmetry. Fermilab and a Japanese collaboration are racing to complete this measurement first.
Dark energy is either energy of space itself or energy within space, manifesting as a repulsive form of gravity. Its existence was confirmed through astronomical observations in the late 1990s.
Before these measurements, three expansion scenarios seemed possible: the universe would slow and recollapse in a big crunch; it would continue expanding while slowing indefinitely; or it would approach zero expansion rate only at infinite time.
Measurements revealed a fourth possibility: the universe's expansion is accelerating. This requires a repulsive force, which was named dark energy.
Einstein originally introduced the cosmological constant to counteract gravitational collapse in a static universe. After Hubble demonstrated universal expansion, Einstein removed the constant, later calling it his greatest blunder. The 1998 discovery reinstated the concept.
Quantum field theory predicts vacuum energy by summing contributions from all possible wavelengths within a volume. This calculation yields a value 10 to the 120th power larger than the observed dark energy density.
Even if new physics appears at energies accessible to current accelerators, reducing the integration limit by a factor of 10 to the 15th, the discrepancy remains 10 to the 60th power.
This represents one of physics' most severe theoretical failures. Perfect cancellation between opposing fields is theoretically straightforward, but the observed dark energy requires imperfect cancellation leaving a small residual value.
One approach involves hypothesizing an additional field with opposite effects to existing quantum fields, but with imperfect cancellation that leaves the observed dark energy residue.
New theoretical fields are developed by identifying modifications that resolve the discrepancy while preserving agreement with all existing measurements. The goal is finding additions that change nothing in measured regimes while fixing the vacuum energy problem.
This theoretical exploration also provides mechanisms for understanding the universe's long-term future and expansion dynamics.
Any good model that captures some of the weirdness of dark energy might give insights about how the universe ends and the deep future of the universe. If dark energy is real exactly as measured, then as the universe gets bigger, dark energy becomes a bigger component of the energy balance and drives the continued accelerated expansion. If dark energy gets lower or bigger over time, that could change things. One of the big open questions is whether dark energy is constant over time or not. A recent measurement suggests that dark energy is getting smaller, though this is unconfirmed and should not be believed. It is a hint that maybe dark energy is changing, which is interesting because the current bias until recently was that dark energy is constant.
Dark energy is a density. If the universe expands and density is constant, then dark energy is increasing. It is not just increasing as a fraction and overwhelming ordinary matter. Ordinary matter density decreases as the universe expands because the volume gets bigger. Dark energy until recently was thought to be constant density, which means it is actually increasing because space is increasing.
This ties into the nature of space. If dark energy is a field in space, increasing the volume would suggest energy density would drop. But if space is increasing and quantized, then perhaps space is not stretching but little space particles or bubbles of space are appearing, each containing a certain amount of dark energy. This would suggest dark energy is a property of space rather than a field in space. If dark energy has constant density, this leans toward the idea that it is a property of space, space is quantized, as space expands little quanta of space are appearing, and each quantum has a certain amount of energy associated with it.
Finding bubbles of space is difficult, but the quanta could conceivably be Planck-size bubbles. The idea is that as space expands, new grains of space appear like grains of sand on a dune. Experiments in the coming decades or centuries could help understand dark energy better. In standard quantum mechanics a particle can be in two places at the same time. With two particles near one another or far apart, the effects of gravity on quantum entangled particles being in two places can be calculated. People are talking about doing such measurements to determine whether gravity is a quantum phenomenon or a continuous phenomenon. Recent work on quantum mechanics is allowing instrumentation precise enough to potentially do this measurement soon. This measurement would not tell what quantum gravity is, but it would tell whether gravity is quantized, shutting out continuous gravity theories and directing theoretical attention elsewhere. If gravity is quantized, people will start thinking more about space being quantized.
Dark matter may be even more mysterious than dark energy. Astronomical measurements do not agree with predictions by Newtonian or relativity theory. Galaxies spin too fast, clusters of galaxies move too quickly, and the distortion of very distant galaxies due to the gravitational field of nearby galaxies disagrees with predictions from observed matter. These three distinct reasons suggest something is wrong in understanding either the laws of physics or the matter budget of the universe.
The easiest case to discuss is spinning galaxies. Galaxies spin more quickly than they should if gravity from visible matter is added up. Galaxies spinning that fast should blow themselves apart but do not. The force required for a star to move in a circle and the force due to gravity are connected by an equal sign, and the prediction is wrong. Either the force due to gravity is wrong, the force needed to move in a circle is wrong, or the equal sign is wrong. One possibility is Newton's law of gravity is wrong. Another is Newton's F equals ma is wrong. Maybe gravity is not understood or there is more mass than can be seen.
The most obvious possibility is more mass than can be seen, such as black holes or hydrogen gas. Hydrogen gas can be seen with radio waves. In the 1990s, searches for black holes and rogue planets found they exist but not enough of them. This leaves the possibility of matter that cannot be seen or that gravity or inertia is not understood. Twenty-five years ago the most likely answer seemed to be that inertia or gravity was not understood. Observations have changed that thinking.
The bullet cluster provides strong evidence that dark matter is real. Two large clusters of galaxies pass through one another. Galaxies themselves pass through basically not interacting. Gas clouds interact, stop in the middle, and become very hot. If there were no dark matter, distortions would be seen in the middle where the gas cloud is. If dark matter is real, galaxies pass through, the cloud stops, and dark matter passes through without interacting. Distortions are seen where the galaxies are, matching observations.
Dragonfly 2 and Dragonfly 4 are galaxies that rotate exactly according to Newton's laws. The fact that they rotate exactly according to Newton's laws suggests that whatever causes galaxies to rotate too fast is not a property of matter. The existence of a galaxy with no dark matter is strong evidence that dark matter is real because dark matter can be taken out. These observations suggest dark matter is likely a real thing, though it remains possible that laws of inertia or gravity need modification.
Dark matter makes up five times more matter than ordinary matter. It is not black holes or rogue planets. Measurements across nearly every mass range for compact objects have ruled them out. If dark matter is real, it cannot be made of those. This leaves the idea that dark matter is a particle. The name for the dark matter particle long considered is WIMP for weakly interacting massive particle. Searches have been conducted for thirty years.
There are three ways dark matter might be seen. The direct way places detectors deep underground to see dark matter passing through Earth like a wind. No evidence has been found. Neutrinos are weakly interacting and have mass but not enough mass. Another possibility is looking at the center of galaxies for annihilation signatures such as gamma rays, but other sources like neutron stars complicate this. The third option is smashing particles together at high energy to make dark matter particles. If made, they escape the detector but recoil on the other side can be seen due to momentum conservation. Neutrinos produce similar signatures, so everything about neutrinos must be understood. This approach has not worked either.
The viable dark matter mass range if particulate extends from asteroid mass to far lighter than an electron. Some spots in that phase space have been ruled out, but it is a big range. Astronomical searches were not sensitive to low-mass dark matter. Microlensing searches for massive objects passing in front of distant stars momentarily brightening them found some events from black holes but not enough. Very low mass particles like asteroids would not produce enough brightening effect. The sensitivity reached about a third the mass of the Moon.
If someone gets on an experiment looking at one little range of mass, they might not measure the right place. Many groups doing radically different experiments exploring all parameter space are needed. Until dark matter is seen, the possibility remains that gravity or inertia is not understood. Direct observation would be ideal, such as pointing a telescope and detecting dark matter in an obvious way through orbiting objects or eclipses. Experiments in the 1990s called MACHO, OGLE, and others looked for black holes but did not find enough.
Don Lincoln grew up a poor kid in the boondocks with great, nurturing parents who were not able to guide him academically. His mother would laugh that she could stop helping with math after sixth or seventh grade. He was a voracious reader as a kid, reading a book a day, mostly science fiction, which fostered imagination. In the 1970s, science communicators Isaac Asimov, Carl Sagan, and George Gamow wrote books about science aimed at laypeople that he could understand.
He was irrepressibly curious about everything and had a quasi-philosophical mind interested in questions that have moved from theological to philosophical to scientific: how the universe came into existence, why the universe is the way it is, why the laws are what they are, how it was created, and how it will be destroyed. In college he minored in philosophy and religion hoping to understand these questions through history, but realized the answers were not to be found in those directions.
He became a scientist and had to decide between cosmologist or astrophysicist versus particle physicist. In the mid-1980s there were fewer cosmology measurements and a lot of thinking without enough measuring. Particle physics offered the ability to do experiments and actually get an answer. It was difficult without family mentors, which is why he has spent time writing books hoping to help other kids in little towns without access to highly educated parents find their own path. He has had summer interns at the lab who were inspired by his videos or books.
When arriving at Fermilab early in his career, Don Lincoln worked extremely long hours, from 8:00 a.m. to midnight. As a graduate student with no family obligations, he voluntarily worked at the lab Monday through Saturday from 8:00 a.m. to midnight, and on Sunday from 8:00 a.m. until approximately 5:00 p.m. The remaining evening hours on Sunday were spent on personal necessities like washing clothes and buying groceries.
Lincoln emphasized that being smart alone is insufficient for scientific success. All scientists are intelligent, but what separates exceptional scientists from others is drive and grit. When experiments fail or equipment breaks, the response that distinguishes committed scientists is determination rather than discouragement. Lincoln described how setbacks would make him angry and motivate him to invest even more effort, refusing to let the universe or laboratory challenges defeat him.
"There is absolutely nothing more fascinating to me than having a hard problem and figuring it out."
Lincoln noted that while his extreme schedule was exceptional and not sustainable or advisable for everyone, successful scientists demonstrate a genuine inability to accept not knowing the answer. He observed that when encountering this level of intensity and drive in younger researchers, it signals a high likelihood of success in the field.
Lincoln referenced Marie Kondo's principle that if an activity doesn't bring joy, it may not be the right path. For those considering a career in science, genuine passion must be intrinsic to one's identity rather than an external pursuit. Individuals who find joy in the work can become practicing scientists, while others might engage with science as informed readers or enthusiasts without pursuing it professionally.
Lincoln acknowledged that a scientific career is demanding but also deeply fulfilling. Working intensively toward something one genuinely loves represents a meaningful way to live. He drew parallels to artists and musicians who continue practicing because the activity is fundamental to who they are.
Lex Fridman expressed appreciation for Fermilab as an institution advancing science and engineering globally. He thanked Don Lincoln for his contributions to physics research, online science education, and the conversation itself. Lincoln reciprocated the thanks for being invited.
Fridman concluded the episode by referencing a quote from Marie Curie, the two-time Nobel Prize winner in physics and chemistry:
"Nothing in life is to be feared. It is only to be understood."
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