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Ever since Isaac Newton and the falling apple<\/a>, surprises have often pushed physics forward. Many truths about the universe we live in and the particles that make up ourselves and the world around us, as well as the forces that drive them, seemed to come out of left field when they were first discovered. For instance, scientists once thought atoms were the smallest bits of matter<\/a> in existence until they split atomic nuclei to find protons and neutrons, which in turn proved to be made of even smaller fundamental particles, called quarks. And it was less than 100 years ago that researchers found out the Milky Way wasn\u2019t the only galaxy in the cosmos<\/a> but rather one of billions.<\/p>\n The surprises in the history of physics are far too many to comprehensively describe, but we polled a variety of physicists for some of their favorites. A few discoveries, such as the accelerating expansion of the universe<\/a>, were so groundbreaking that multiple experts picked them as top choices. And many of these events occurred relatively recently, showing that the field of physics continues to astound us. Here\u2019s a selection of physicists\u2019 responses on the most amazing, stunning and flabbergasting findings.<\/p>\n One of the most shocking findings in the history of physics was the discovery of dark energy<\/a> just before the turn of the millennium. None of us working in physics saw that coming! The observations that distant supernovae are dimmer than expected led to the idea that the universe is not just expanding but accelerating. These objects are very well understood, no matter how far back in time they are observed, so alternative explanations just don\u2019t work. The name \u201cdark energy\u201d was given to the material that causes this acceleration. After the initial discovery, many other observations of different types confirmed this result, such as studies of the cosmic microwave background, which is the leftover light from the big bang, and studies of clusters of galaxies. The list goes on and on. We now have a standard model of cosmology in which the ordinary matter and energy that we experience in our daily lives\u2014our body, the air we breathe, the walls around us, and all the stars and planets\u2014add up to only 5 percent of the content of the universe. Most of the universe is \u201cthe dark side\u201d: the universe is thought to consist of 25 percent dark matter and 70 percent dark energy. I, for one, am working to identify the nature of these mysterious components.<\/p>\n This discovery of dark energy in particular created a paradigm shift. The simplest explanation [for dark energy] would be a cosmological constant originally introduced by Albert Einstein as a possible term in the equations of the general theory of relativity but then abandoned by him as his \u201cbiggest blunder.\u201d Now it seems he may have been right after all. The trouble is that the predicted value for the cosmological constant from calculations using quantum field theory produces a number that is too large by a factor of 10120<\/sup><\/a>. Editor\u2019s Note: If the constant was this large, the universe would have expanded much, much faster than it did<\/em>.] This conundrum has been known for some time, and theorists conjectured that there must be some physics that drives the number down to zero instead [to match the observed expansion history of the universe]. Now with the discovery of dark energy, however, the number must be driven down to a particular tiny value [rather than zero to explain the accelerating expansion], which is much harder to explain. This cosmological constant problem<\/a> is thought by many to be the deepest unsolved problems in all of modern physics.\u201d \u2014Katherine Freese, University of Texas at Austin<\/em><\/p>\n I think the accelerating expansion of the universe has to be a strong contender. I\u2019ve read references published around 1990 that talk confidently about how we will soon use supernovae to measure the rate at which the expansion of the universe is\u00a0decelerating<\/em> and the curvature of the cosmos and how this will tell us about the ultimate fate of our universe (because closed matter-dominated universes undergo a \u2018big crunch<\/a>,\u2019 while open ones expand forever)\u2014very little of which applies to the dark-energy-dominated, spatially-flat cosmos that we appear to actually live in! I think this also qualifies because even with the benefit of hindsight, it still seems very\u00a0surprising\u00a0that the dark energy\/cosmological constant has its measured value. \u2014Tracy Slatyer, Massachusetts Institute of Technology<\/em><\/p>\n The most spectacular discoveries in fundamental physics since I started graduate school in 1973 have been the following: One of the most surprising discoveries in the history of physics is Karl Schwarzschild\u2019s black hole solution of the Einstein equation. [Editor\u2019s Note: Schwarzschild calculated the first exact solution to Einstein\u2019s field equation of general relativity, and the solution predicted the existence of black holes<\/em>.]<\/p>\n It is apocryphally said that when Einstein discovered his highly nonlinear equation, he thought an exact solution would never be found, but Schwarzschild proved him wrong only months later. Yet the structure of the solution was so surprising that many thought black holes did not exist. Einstein himself wrote in 1939 that [\u201cthe \u2018Schwarzschild singularities\u2019 do not exist in physical reality\u201d].\u00a0It is only a century later, with the recent direct LIGO<\/a> [Laser Interferometer Gravitational Wave Observatory] and EHT [Event Horizon Telescope] observations of black holes<\/a> that the last shreds of disbelief have been stamped out.\u201d\u00a0\u2014Andrew Strominger, Harvard University<\/em><\/p>\n It\u2019s got to be the flexibility of spacetime. Let\u2019s say I hop on a really fast rocket or go very close to a black hole and then return to where I started. If I go fast enough on the rocket or go close enough to the black hole, I can have only 10 minutes go by on my watch while 10,000 years go by for Earthlings. This is an experimentally verified time machine that lets you travel to the future<\/a>!\u00a0\u2014Edgar Shaghoulian, University of California, Santa Cruz<\/em><\/p>\n I think my favorite event in physics was the prediction of the existence of the neutrino<\/a> [a subatomic particle with no charge and very little mass] because so much of our fundamental approach to physics today grew out of that moment. The neutrino prediction by Wolfgang Pauli was one of the first examples of taking energy and momentum conservation seriously\u2014you must either explain nuclear beta decay [a common radioactive process] by violating this conservation law or by introducing a new particle. The neutrino would be the first new particle predicted that wasn\u2019t obvious in everyday life. Today predictions for new ghostlike particles are almost a dime a dozen, but in the early part of the last century, introducing potentially unobservable particles simply wasn\u2019t done. When Enrico Fermi introduced the interaction explaining why the neutrino was so unlikely to be observed, he predicted the first new force [the weak nuclear force] beyond the two that are obvious in everyday life (gravity and electromagnetism). Today physicists consider many new types of forces all the time, but back then that just wasn\u2019t in the picture. The idea of unifying forces, which is so essential to physics today, grew out of the discovery of Fermi\u2019s \u2018weak force\u2019 that the neutrino feels. One of the most amazing examples that shows quantum mechanics makes sense as a theory, because it can happen on kilometer scales, where we can really see it, comes from neutrino physics. So that moment, when Pauli predicted the neutrino, is my favorite surprise because of all the paths it led to in physics. \u2014Janet Conrad, Massachusetts Institute of Technology<\/em><\/p>\n I would say the discovery of neutrino oscillations<\/a> is up there for me. Neutrinos themselves were predicted to exist by Pauli and subsequently discovered in a great demonstration of the power of theory. But what makes neutrinos incredibly interesting little particles is the fact that they have mass and can change flavors, which requires a modification of the Standard Model of particle physics. \u2014Sanjana Curtis, University of Chicago<\/em><\/p>\n Long ago two ancient Greek savants, Democritus and Leucippus, argued that matter consists of atoms, a notion that would be confirmed more than two millennia later. I recently coined the\u00a0word\u00a0\u2018leucippity<\/a>\u2019 to characterize those speculative hypotheses that wait many years for widespread acceptance. My new\u00a0word\u00a0honors the elder of the two proponents of the atomic\u00a0hypothesis, Leucippus.<\/p>\n Isaac Newton concluded that light consists of particles in 1672; Christiaan Huygens\u00a0developed his wave theory of light<\/a> six years later. Who got it right? The question lingered for two centuries until\u00a0James Clerk Maxwell\u2019s profound and leucippitous\u00a0discovery that light favors Huygens\u2019s wave theory. (Later on\u00a0Einstein would have his say on this matter.) Leucippity abounds in science. Alfred Wegener\u2019s\u00a0prescient \u2018geopoetry\u2019<\/a> of drifting continents emerged as the mature science of plate tectonics half a century afterward. More recently, the discovery of a boson<\/a> [the Higgs boson] first imagined by Peter Higgs and a few others in 1964 was triumphantly announced at CERN [the European laboratory for particle physics near Geneva] on\u00a0July 4, 2012. Lastly, the\u00a0gravitational waves produced by mergers of black-hole pairs<\/a> were detected by LIGO in 2015, a full century after their existence had been proposed by Einstein. Leucippity again! \u2014Sheldon Lee Glashow, Harvard University<\/em><\/p>\n In my opinion, one of the most incredible and surprising experimental findings in physics resulted from when the pioneer of helium liquefaction, Heike Onnes, performed experiments in which he cooled metals such as gold, platinum and mercury to liquid helium temperatures. On the same day that he found that the electrical resistance of mercury dropped to effectively\u00a0zero at liquid helium temperatures, he also found that [using a vacuum pump] on a normal liquid helium sample caused the liquid\u00a0to further cool and aggressively\u00a0boil before suddenly becoming placid. This is incredible! On the same day Onnes discovered both the phase transition to a state of superconductivity in mercury and the phase transition to the state of superfluidity in helium. \u2014Charles Brown, Yale University<\/em><\/p>\n Two discoveries\u2014Bell\u2019s theorem<\/a> and the Michelson-Morley interferometry experiment<\/a>\u2014upended our understandings of space, time and the nature of reality, so I can\u2019t resist voting for them both.<\/p>\n The American Physical Society calls the Michelson-Morley experiment\u00a0\u201cwhat might be regarded as the most famous failed experiment to date<\/a>.\u201d\u00a0Until the experiment was performed in 1887, scientists believed that light waves propagate through a medium that scientists called the luminiferous aether. After all, sound waves propagate through air, and surfers\u2019 waves propagate through water. But Albert Michelson and Edward Morley provided strong evidence that light is different; it needs no medium. This lack paved the path for Einstein\u2019s special theory of relativity (nothing can travel more quickly than light, E = mc2<\/sup><\/em><\/a> [the c<\/em> stands for the speed of light in a vacuum], how short an object looks depends on how quickly you\u2019re moving relative to it, etcetera), which led to his general theory of relativity (spacetime has a shape).<\/p>\n Bell\u2019s theorem [named after John Stewart Bell] revealed that quantum systems have wonky relationships with information and with each other. Ordinarily, if you know everything about a pair of systems\u2014say, everything about a pair of people named Audrey and Baxter\u2014then you know everything about each individual\u2014everything about Audrey and everything about Baxter. But if Audrey and Baxter are labels of quantum particles, then you can know everything about the pair without knowing anything about the individuals. Information can be not in one particle and not in the other but sort of in the relationship between the two: the whole is greater than the sum of its parts in quantum physics. Bell\u2019s insight paved the path for the quantum computers and networks now under construction across the world. \u2014Nicole Yunger Halpern, University of Maryland, author of\u00a0<\/em>Quantum Steampunk<\/p>\n Here are a few\u00a0surprising\u00a0discoveries that pop into my mind, in no particular order:<\/p>\n (1) Special relativity: the fact that the speed of light is constant, irrespective of the frame of reference.<\/p>\n (2) General relativity<\/a>: the fact that gravity represents a curvature of spacetime.<\/p>\n (3) The expansion of the universe, the ensuing big bang model and the fact that the expansion is accelerating.<\/p>\n (4) The \u2018unreasonable\u2019 effectiveness of mathematics<\/a> in formulating the fundamental laws of nature.<\/p>\nDark Energy<\/h2>\n
Expanding Universe<\/h2>\n
Charmed Quarks and Accelerating Cosmos<\/h2>\n
\n(1) The discovery in October 1974 of the J\/psi particle, interpreted in terms of a new quark, the charmed quark<\/a>, which gave dramatic confirmation to the then emerging Standard Model of particle physics.
\n(2) The discovery in the late 1990s that the expansion of the universe is accelerating, apparently because of a tiny but nonzero energy density of the vacuum, upending many of our ideas about the cosmos. \u2014Edward Witten, <\/em>Institute for Advanced Study, Princeton, N.J.<\/em><\/p>\nBlack Holes<\/h2>\n
Spacetime<\/h2>\n
Neutrinos<\/h2>\n
Oscillations<\/h2>\n
Leucippity<\/h2>\n
Phase Transitions<\/h2>\n
Bell and Michelson-Morley<\/h2>\n
Top Five<\/h2>\n