Science on This Day: November 21 | Hot-Air Balloons, the Phonograph and Einstein’s Mass-Energy Equation
What connects a balloon floating over Paris, a machine that records sound and one of the most famous equations in physics? Each represents a change in how people understood or interacted with the physical world. On November 21, 1783, humans made an early untethered balloon flight. In 1877, Thomas Edison announced the phonograph, demonstrating a new way to record and reproduce sound. And on November 21, 1905, a paper by Albert Einstein exploring the relationship between mass and energy was published.
These three events make useful starting points for middle and high school science lessons. They connect physics to engineering, technology and the history of scientific ideas, while offering students opportunities to ask questions, interpret evidence and consider how inventions change what people can do.
1783: Humans take to the sky in a hot-air balloon
On November 21, 1783, Jean-François Pilâtre de Rozier and François Laurent d'Arlandes made an early untethered human flight in a hot-air balloon over Paris, France. The balloon was based on the work of Joseph-Michel and Jacques-Étienne Montgolfier, the brothers whose experiments helped establish the potential of hot-air ballooning.
The flight lasted roughly 25 minutes and covered several kilometres. Unlike earlier tests in which balloons were attached to the ground, this journey allowed the passengers to travel freely through the air. It was a major milestone in the history of aviation, demonstrating that humans could leave the ground and travel through the atmosphere using a lighter-than-air craft.
The underlying physics is a useful example of how a scientific principle can become a practical technology. Heating the air inside a balloon makes it less dense than the cooler air outside, provided the pressure is approximately the same. The surrounding air then exerts a net upward buoyant force on the balloon system. If that force is greater than the total weight of the balloon, its occupants and its equipment, the balloon rises.
This does not mean that hot air has no mass or that the balloon rises simply because heat moves upwards. The balloon rises because of the difference in density between the air inside and outside, and the resulting buoyant force. The envelope, basket, fuel and passengers all contribute to the total mass that must be lifted.
A practical classroom connection: Why does a balloon rise?
Ask students to explain why a hot-air balloon rises without using the phrase “hot air rises” as their entire answer. They should consider temperature, density, buoyancy and the weight of the whole system.
A simple demonstration using warm and cool air can introduce the idea that gases expand when heated. For older students, this can lead into density calculations or a discussion of how engineers balance the mass of the balloon, its payload and its fuel against the lift available.
Students can also compare hot-air balloons with helium balloons. Both rely on buoyancy, but the gas inside the balloon differs. In a hot-air balloon, the air is heated to reduce its density; in a helium balloon, helium is used because it is less dense than the surrounding air under comparable conditions.
A useful discussion question is: What design changes would allow a hot-air balloon to lift a heavier payload? Students might consider the volume of the envelope, the temperature of the air inside, the mass of the materials and the energy needed to maintain the temperature.
For further reading, Britannica's overview of hot-air balloons provides historical and technical context.
1877: Thomas Edison announces the phonograph
On November 21, 1877, Thomas Edison announced the phonograph, a device that could record and reproduce sound. The announcement marked an important moment in the development of sound recording. For the first time, sound could be captured as a physical record and played back later, rather than being heard only as it happened.
The early phonograph used a recording mechanism that translated sound vibrations into physical grooves on a rotating surface. In Edison's original design, a diaphragm responded to sound waves, causing a stylus to make a corresponding pattern in a rotating cylinder covered with foil. During playback, the stylus followed the recorded pattern, making the diaphragm vibrate and recreating sound.
The principle connects directly to the physics of sound. Sound travels through a medium as variations in pressure. When these pressure changes reach a microphone or another recording mechanism, they can be converted into a signal or a physical pattern. A playback system then uses the stored information to generate vibrations that travel through the air to a listener's ears.
Modern digital recording works differently from the original phonograph, but the central idea remains: information about a changing sound wave is captured, stored and later used to reproduce the sound. Digital systems represent sound as numerical measurements, while early mechanical devices stored a physical pattern.
Edison's invention also changed how people could experience music, speeches and other sounds. Recordings could be replayed, shared and preserved, helping to transform entertainment and communication. The history of sound recording is therefore about both engineering and the social effects of technology.
A practical classroom connection: How can sound be stored?
Ask students to sketch how they think a sound recording system works, from the moment someone speaks to the moment another person hears the recording. Then compare their diagrams with the mechanical process used by an early phonograph and the digital process used by a modern phone.
Students could investigate how sound frequency and amplitude affect the appearance of a waveform. Frequency is related to perceived pitch, while amplitude is related to the strength of the pressure variation and often to perceived loudness. A recording system needs to preserve enough information about these changes for the sound to be reproduced recognisably.
For a simple activity, students can record a short sound using a digital device and compare its waveform or spectrogram with another sound. They can ask which features differ and what information a recording must preserve. This can be extended into a discussion of sampling, storage and the limitations of recordings.
A useful question is: What information must a machine preserve to reproduce a sound accurately? Students may consider pitch, loudness, timing and the complexity of the original sound.
The Library of Congress collection on Edison Company sound recordings offers historical material for exploring how sound recording developed.
1905: Einstein publishes a paper on mass and energy
On November 21, 1905, a paper by Albert Einstein titled Does the Inertia of a Body Depend Upon Its Energy Content? was published in the journal Annalen der Physik. The paper explored a remarkable consequence of his work on relativity: mass and energy are related, and a change in a system's energy can correspond to a change in its mass.
This idea is now widely associated with the equation (E = mc^2), where (E) represents energy, (m) represents mass and (c) is the speed of light in a vacuum. Because the speed of light is an extremely large number, squaring it gives an enormous conversion factor. Even a small amount of mass corresponds to a very large amount of energy.
The equation is often introduced through nuclear reactions, where small differences in mass between the initial and final systems correspond to energy released or absorbed. It is important, however, not to interpret the equation as saying that ordinary matter can easily be converted into useful energy in everyday situations. The physical processes involved matter, and the energy associated with a given mass is not necessarily available for extraction.
Einstein's 1905 paper was one of several influential publications he produced that year. It helped establish a connection between mass and energy that became central to modern physics. The result also illustrates how scientific ideas can develop through mathematical reasoning, even when their consequences are not immediately obvious from everyday experience.
For students, the equation offers an opportunity to distinguish between mass and weight, and between energy stored in a system and energy transferred during a process. It can also help them appreciate why nuclear reactions can release far more energy per unit mass than ordinary chemical reactions. Chemical reactions primarily involve changes in the arrangement of electrons and chemical bonds, whereas nuclear reactions involve changes in atomic nuclei.
A practical classroom connection: Making sense of (E = mc^2)
For older students, begin by asking what each symbol represents and why the speed of light appears in the equation. Students can explore how the energy equivalent changes when the mass changes, using small hypothetical values and appropriate units.
The central mathematical point is that energy is proportional to mass, with the proportionality factor (c^2). Because (c^2) is so large, even a very small mass corresponds to a substantial amount of energy.
Students can also compare chemical and nuclear processes conceptually. They do not need to calculate the details of nuclear reactions to understand that the scale of energy involved is different. Ask them what evidence scientists would need to compare the energy released by two processes fairly, perhaps by considering the energy released per reaction or per kilogram of fuel.
A useful discussion question is: Why can a small change in mass correspond to a large amount of energy? Encourage students to explain the relationship rather than simply memorise the equation.
For historical context, the American Institute of Physics' chronology of Einstein's 1905 work explains when the mass-energy paper was submitted and published. The Library of Congress guide to Einstein's 1905 papers also provides an introduction to the work that helped reshape physics.
Bringing science history into the classroom
These three events are very different, but they share a useful teaching theme: scientific understanding becomes powerful when it helps people explain observations, test ideas or develop new technologies.
The balloon flight provides a real-world introduction to buoyancy and density. The phonograph shows how knowledge of sound and mechanical systems can lead to a new technology. Einstein's paper demonstrates how mathematical ideas can reveal relationships that are not obvious from everyday experience.
A short science-history starter can be as simple as displaying an event and asking students to write one fact they know, one question they have and one piece of evidence that would help them investigate further. Alternatively, students can work in small groups to explain an event to their classmates, identify the science involved and consider how the event influenced later developments.
If you want ready-to-use examples for this approach, the November Science on This Day resource set includes 109 STEM stories across the month. It comes in several formats: an interactive HTML version with filters and classroom display features, editable Google Slides with discussion questions, a filterable spreadsheet and a PDF version. Teachers can choose an event to suit their current topic or use the collection for a regular science-history warm-up.
You can also explore the Teaching Astrophysicist resource store for free and paid science teaching materials in a range of formats.
Looking ahead, the December Science on This Day resource set offers 132 STEM items across the 31 days of December, providing another month of ready-to-use stories and discussion prompts.
Looking ahead: November 22, 23 and 24
November 22 offers a connection to physics and biology through the birth of Louis Néel in 1904. His work on antiferromagnetism and ferrimagnetism helped scientists understand how magnetic moments can be arranged within materials, creating opportunities to discuss why different substances respond differently to magnetic fields.
November 23 brings chemistry and physics together through the birth of Johannes Diderik van der Waals in 1837. His work improved the ideal-gas model by accounting for the finite size of molecules and the attractions between them. It is a useful reminder that scientific models can be powerful without describing every detail of reality, and that improved evidence can lead to improved models. The same date also marks the birth of Henry Moseley in 1887, whose X-ray measurements helped establish atomic number as the correct basis for ordering the elements in the periodic table.
On November 24, students can explore the history of evolution through the publication of Charles Darwin's On the Origin of Species in 1859. They can also learn about the discovery of the fossil known as Lucy in 1974 and consider how fossil evidence helps scientists investigate human evolution. Apollo 12 returned to Earth on November 24, 1969, offering another opportunity to discuss lunar samples and the scientific value of bringing materials back from space.
Science history can help students see that scientific knowledge is built over time, through observations, experiments, mathematical reasoning and collaboration. Whether the topic is buoyancy, sound, relativity, magnetism, chemistry or evolution, a historical event can be the starting point for a practical question and a deeper scientific conversation.
Thanks for reading
Cheers and stay curious
Oliver - The Teaching Astrophysicist
