Tag: Isaac Newton

• Worlds between theory and experiment

  Once Isaac Newton showed that a single gravitational law plus his rules of dynamics could reproduce the orbits of planets that Johannes Kepler had predicted, explain tides on Earth, and predict that a comet that had passed by once would return again, physicists considered Newtonian mechanics and gravitation to have been completely validated. After these successful tests, they didn’t wait to test every other possible prediction of Newton’s ideas before they considered them to be legitimate.

  When Jean Perrin and others carefully measured Brownian motion and extracted Avogadro’s number in the early 20th century, they helped cement the science of the kinetic theory of gases and statistical mechanics that Ludwig Boltzmann and Josiah Willard Gibbs had developed. As with Newtonian mechanics, physicists didn’t also require every single consequence of kinetic theory to be rechecked from scratch. They considered them all to be fully and equally legitimate then on.

  Similarly, in 1886-1889, Heinrich Hertz produced and detected electromagnetic waves in the laboratory, measured their speed and other physical properties, and showed that they behaved exactly as James Clerk Maxwell had predicted based on his (famous) equation. Hertz’s experiments didn’t test every possible configuration of charges and fields that Maxwell’s equations allowed, yet what they did test and confirm sufficed to convince all physicists that Maxwell’s theory could be treated as the correct classical theory of electromagnetism.

  In all these cases, a theory won broad acceptance after scientists validated only a small (yet robust) subset of its predictions. They didn’t have to validate every single prediction in distinct experiments.

  However, there are many ideas in high-energy particle physics that, even as they are derived from other theoretical constructs that have been tested to extreme precision, physicists insist on testing them anew as well. Why are they going to this trouble now?

  “High-energy particle physics” is a four-word label for something you’ve likely already heard of: the physics of the search for the subatomic particles like the Higgs boson and the efforts to identify their properties.

  In this enterprise, many scientific ideas follow from theories that have been validated by very large amounts of experimental data. Yet physicists want to test them at every single step because of the way such theories are built and the way unknown effects can hide inside their structures.

  The overarching theory that governs particle physicists is called, simply, the Standard Model. It’s a quantum field theory, i.e. a theory that combines the precepts of quantum mechanics and special relativity*. Because the Standard Model is set up in this way, it makes predictions about the relations between different observable quantities, e.g. the mass of a subatomic particle called the W boson with a parameter that’s related to the decay of other particles called muons. Some of these relations connect measured quantities with others that have not yet been probed, e.g. the mass of the muon with the rate at which Higgs bosons decay to pairs of muons. (Yes, it’s all convoluted.) These ‘extra’ relations often depend on assumptions that go beyond the domains that experiments have already explored. New particles and new interactions between them can change particular parts of the structure while leaving other parts nearly unchanged.

  (* Quantum field theory gives physicists a single, internally consistent framework in which they can impose both the rules of quantum theory and the requirements of special relativity, such as that information or matter can’t travel faster than light and that our spacetime conserves energy and momentum together, for example. However, quantum field theory does not unify quantum theory with general relativity; that’s the monumental and still unfinished purpose of the quantum gravity problem.)

  For a more intricate example, consider the gauge sector of the Standard Model, i.e. the parts of the Model involving the gluons, W and Z bosons, and photons, their properties, and their interactions with other particles. The gauge sector has been thoroughly tested in experiments and is well-understood. Now, the gauge sector also interacts with the Higgs sector, and the Higgs sector interacts with other sectors. The result is new possibilities involving the properties of the Higgs boson, their implications for the gauge sector, and so on that — even if physicists have tested the gauge sector — need to be tested separately. The reason is that none of these possibilities follow directly from the basic principles of the gauge sector.

  The search for ‘new physics’ also drives this attitude. ‘New physics’ refers to measurable entities and physical phenomena that lie beyond what the Standard Model can currently describe. For instance, most physicists believe a substance called dark matter exists (in order to explain some anomalous observations about the universe), but they haven’t been able to confirm what kind of particles dark matter is made of. One popular proposal is that dark matter is made of hitherto unknown entities called weakly interacting massive particles (WIMPs). The Standard Model in its contemporary form doesn’t have room for WIMPs, so the search for WIMPs is a search for new physics.

  Physicists have also proposed many ways to ‘extend’ the Standard Model to accommodate new kinds of particles that ‘repair’ the cracks in reality left by the existing crop of particles. Some of these extensions predict changes to the Model that are most pronounced in sectors that are currently poorly pinned down by existing data. This means even a sizeable deviation from the Model’s structure in this sector would still be compatible with all current measurements. This is another important reason physicists want to collect more data and with ever-greater precision.

  Earlier experience also plays an important role. Physicists may make some assumptions because they seem safe in some year but new data collected in the next two decades might reveal that they were mistaken. For instance, physicists believed neutrinos didn’t have mass, like photons, because that idea was consistent with many existing datasets. Yet dedicated experiments contradicted their belief (and won their performers the 2015 physics Nobel Prize).

  (Aside: High-energy particle physics uses large machines called particle colliders to coerce subatomic particles into configurations where they interact with each other, then collect data of those interactions. Operating these instruments demands hundreds of people working together, using sophisticated technologies and substantial computing resources. Because the instruments are so expensive, these collaborations aim to collect as much data as possible, then maximise the amount of information they extract from each dataset.)

  Thus, when a theory like the Standard Model predicts a specific process, that process becomes a thing to test. But even if the prediction seems simple or obvious, actually measuring it can still rule out whole families of rival theories offering to explain the same process. It also sharpens physicists’ estimates of the theory’s basic parameters, which then makes other predictions more precise and helps plan the next round of experiments. This is why, in high-energy physics, even predictions that follow from other, well-tested parts of a theory are expected to face experimental tests of their own. Each successful test can reduce the space for new physics to hide in — or in fact could reveal it.

  A study published in Physical Review Letters on December 3 showcases a new and apt example of testing predictions made by a theory some of whose other parts have already survived testing. Tests at the Large Hadron Collider (LHC) — the world’s largest, most powerful particle collider — had until recently only weakly constrained the Higgs boson’s interaction with second-generation leptons (a particle type that includes muons). The new study provides strong, direct evidence for this coupling and significantly narrows that gap.

  The LHC operates by accelerating two beams of protons in opposite directions to nearly the speed of light and smashing them head on. Its operation is divided into segments called ‘runs’. Between runs, the collaboration that manages the machine conducts maintenance and repair work and, sometimes, upgrades its detectors.

  One of the LHC’s most prominent detectors is named ATLAS. To probe the interactions between Higgs bosons and leptons, the ATLAS collaboration collected and analysed data from the LHC’s run 2 and run 3. The motivation was to obtain direct evidence for Higgs bosons’ coupling to muons and to measure its strength. And in the December 3 paper, the collaboration reported that the coupling parameters were consistent with the Standard Model’s predictions.

  So that’s one more patch of the Standard Model that has passed a test, and one more door to ‘new physics’ that has closed a little more.

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  Featured image: A view of the Large Hadron Collider inside its tunnel. Credit: CERN.

  2025.12.06

• The molecule that was also a wave

  According to the principles of quantum mechanics, you’re a wave – just like light is both a particle and a wave. It’s just that your wavelength is so small that your wave nature doesn’t matter, and you’re treated like a particle. The larger an object is, the smaller its wavelength, and vice versa. We’re confused about whether light is a particle or a wave because photons, the particles of light, are so small and have a measurable wavelength as a result. Scientists know that electrons, protons, neutrons, even neutrinos have the properties of a wave.

  But while the math of quantum mechanics says you’re a wave, how can we know for sure if we can’t measure it? There are two ways. One, we don’t have any evidence to the contrary. Two, scientists have been checking if larger and larger particles, as far as they can go, exhibit the properties of a wave – and at every step of the way, they’ve come up with positive results. Both together, we have no reason to believe that we’re not also waves.

  Such tests reaffirm the need for quantum mechanics to understand the nature of reality because the rules of classical mechanics alone don’t explain wave-particle duality.

  On September 23, scientists from Austria, China, Germany and Switzerland reported that they had measured the wavelength of a group of molecules called oligoporphyrins. Specifically, they used “oligo-tetraphenylporphyrins enriched by a library of up to 60 fluoroalkylsulphanyl chains”. Altogether, they consisted “of up to 2,000 atoms”, becoming the heaviest object directly known to exhibit wave-like properties.

  

  According to the scientists’ peer-reviewed paper, the molecules had a wavelength of around 53 femtometers, about 100,000-times smaller than the molecules themselves.

  * * *

  We have known since at least the 11th century, through the work of the Arab scholar Ibn al-Haytham, that light is a wave. In 1670, Isaac Newton propounded that light is made up of small particles, and spent three decades supplying evidence for his argument. His push birthed a conflict: was light wave-like or made up of particles?

  The British polymath Thomas Young built on the 17th century Dutch physicist Christiaan Huygens to devise an experiment in 1801 that definitively proved light was a wave. It is known widely today as the Young’s double-slit experiment. It is so simple even as its outcomes are so immutable that it has become a mainstay of modern tests of quantum mechanics. Physicists use upgraded versions of the experiment to this day to study the nature and properties matter-waves.

  (If you would like to know more, I highly recommend Anil Ananthaswamy’s biography of this experiment, Through Two Doors At Once; here’s an excerpt.)

  In the experiment, light from a common source – such as a candle – is allowed to pass through two fine slits separated by a short distance. A sheet of paper sufficiently behind the slits then shows a strange pattern of alternating light and dark bands instead of just two patches of light. This is because light waves passing through the two slits interfere with each other, producing the famous interference pattern. Since only waves can interfere, the experiment shows that light has to be a wave.

  

  The particulate nature of light would get its proper due only in 1900, when Max Planck stumbled upon a mathematical inconsistency that forced him to conclude light had to be made up of smaller packets of energy. It was the birth of quantum mechanics.

  * * *

  The international group’s test went roughly as follows: the scientists pulsed a laser onto a glass plate coated with the oligoporphyrins to release a stream of the molecules; collected them into a beam using collimators; randomly chopped the beam into smaller bits; passed each bit through diffraction gratings to split it up; then had the two little beams interfere with each other. Finally, they counted the number of molecules striking the detector while the detector registered the interference pattern.

  They had insulated the whole device, about 2m long, from extremely small disturbances, like vibrations, to prevent the results from being corrupted. In their paper, the scientists even write that the final interference pattern was blurred thanks to Earth’s rotation, and which they were able to “compensate for” using effects due to Earth’s gravity.

  

  To ascertain that the pattern they were seeing on the detector was in fact due to interference, the scientists performed a variety of checks each of which established a relationship between the shapes on the detector with the properties of the components of the interferometer according to the rules of quantum mechanics. They were also able to rule out alternative, i.e. classical, explanations this way.

  For example, the scientists fired a laser through the cloud of molecules post-interference. Each molecule split the laser light into two separate beams, which recombined to produce an interference pattern of their own. This way, scientists could elicit the molecules’ interference pattern by studying the laser’s interference pattern. As they varied the laser power, they found that the visibility distribution of the molecules more closely matched with quantum mechanical models than with classical models, confirming interference.

  

  What these scientists have achieved isn’t only a feat of measurement. Their findings also help refine the border between the classical and the quantum. The force of gravity governs the laws of classical mechanics, which deals with macroscopic objects, while the electromagnetic, strong nuclear and weak nuclear forces rule the microscopic world. Although macroscopic and microscopic objects occupy the same universe, physicists haven’t yet understood how classical and quantum mechanics can be combined into a single theory.

  One of the problems standing in the way of this union is knowing where – and how – the macroscopic world ends and the microscopic world begins. So by observing quantum mechanical effects at the scale of thousands of atoms, scientists have quite literally pushed the boundaries of what we know about how the universe works.

  2019.12.12

• Why JC Bose isn’t Isaac Newton

  David Beerling, a botanist at the University of Sheffield, writes in Nature Plants this week that Isaac Newton knew in the 17th century how water moved up from the soil through a plant and onto a leaf centuries before modern botanists discovered the mechanism. Historians found the corresponding notes in a small notebook at the Cambridge University Library whose contents Newton’s executor had judged unfit to publish. The transcription reads:

  Suppose a b the pore of a Vegitable filled with fluid mater & that the Globule c doth hitt away the particle b, then the rest of subtile matter in the pores riseth from a towards b. & by this meanes juices continually arise up from the roots of trees upward leaving dreggs in the pores & then wanting passage stretch the pores to make them as wide as before they were clogged. which makes the plant bigger untill the pores are too narow for the juice to arise through the pores & then the plant ceaseth to grow any more.

  What Newton means to say is that when particles of light, one identified as ‘Globule c’, knock away a molecule of water (‘particle b) from the surface of a leaf, sap rises up through the roots of trees. His words also suggest that the loss of water on leaves pulls up more from the roots. This is in keeping with the now-widely accepted theory of water rising up through tissues in the plant thanks to capillary action: water molecules adhering to the walls of the tissues and pulling up other water molecules using cohesive bonds. This idea, in its modern form, was proposed in the last decade of the 19th century.

  A discussion of unnoticed achievements in botany brings to mind the work of the Bengali physicist Jagdish Chandra Bose. Bose was more precisely a biophysicist. His experiments in the first decade of the 20th century involved studying plants’ reactions to electrical and chemical stimuli. Based on them, he found that plants, like animals, could also feel pain. Using a device of his making called the crescograph, he also studied plants’ responses to rays of light and microwave radiation, and was able to conclusively establish that their responses to stimuli were guided through chemical pathways.

  While his work in botany is widely recognized to be pioneering, his work in physics is mired in controversy. Nothing eminent like Newton’s battle with Gottfried Leibniz to claim the discovery of the methods of calculus, Bose was engaged in a multi-sided war over who had invented the radio. A paper of his dated 1899 describes a mercury coherer that was used in 1901 by Guglielmo Marconi for his famous demonstration of transatlantic radio communication for the first time. A similar instrument was reportedly demonstrated by the Russian scientist Alexander Popov in 1895. Marconi went on to become identified as the discover of the radio while Popov’s contributions (if any) and Bose’s discoveries went unnoticed in the process.

  At the same time, notwithstanding Marconi’s and Popov’s claims, what undermines Bose’s claims is the lack of a respected institution in India to vouch for his achievements, that has made any systematic attempts to preserve his notes and reports, and, most importantly, which has professionally engaged in dispelling claims that would disabuse Bose of his claims to primacy. While his books and papers are easily available – on the web or in print – there is a conspicuous lack of efforts in situating them in historical contexts and in communicating such assessments to the people.

  The absence of such institutions is becoming obvious by the day, as is the ignorance with which their authority is being undermined (the latest casualty was the Indian Science Congress 2015). In the almost 290 years since Newton’s death in 1727, institutions like the Cambridge University Library have been responsible for preserving his legacy in the public consciousness, so much so that he’s still able to take a smidgen of credit for thinking of capillary action in plants in the 1660s or 1670s. JC Bose, on the other hand, continues to be endangered by the possibility that Newton thought up what he did two centuries earlier.

  2015.02.02