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This captivating video presents a vision of our potential future: humanity at ease with open space. An astronaut opens a hatch and gracefully dives into the cosmic void. But what exactly did he exit: a space station or an airplane? Given how small the Earth looked beneath the craft, it should be the station. Yet, the astronaut walked and jumped as if he were on a plane grounded by gravity. Of course, artistic imagination isn't bound by the laws of physics. But why not root this flight of fantasy in real physics and use it to explain why we remain affected by gravity on the planes, while astronauts on the International Space Station float in weightlessness? Fig. 1 .  Between art and reality: a jump that only imagination can make Why walking in airplanes feels like walking on Earth An airplane maintains an altitude due to aerodynamic lift, which balances the downward force of gravity ( Fig. 2 ). The wing design and engine propulsion generate the lift as the plane moves through the air. The wings are slightly curved on top and angled upward, resulting in higher air pressure below them than above. This pressure difference generates the upward force, which increases as the aircraft's speed grows. When the speed reaches a point where the lift equals the aircraft's weight, the airplane enters a cruising stage, maintaining a stable altitude. Fig. 2 . The Four Fundamental Forces Acting on an Aircraf t: Lift, Weight, Thrust, and Drag. The lift impacts the airplane, but not the passengers inside. Passengers do not have wings or airflow around them to create the upward force necessary to counteract the force of gravity. Instead, they are supported by the plane's floor, much like the ground supports us on Earth. If this support is lost, such as during engine failures when the airplane sinks, the passengers enter free fall, experiencing weightlessness. However, during normal flight, their weight is practically the same as on the ground. A scale onboard would measure only a half-percent reduction in weight due to the decrease in gravity at the plane's altitude, which is too small for us to notice. Therefore, during the cruising phase, we can walk inside planes as we do on the ground. How orbital motion creates microgravity In contrast, a space station maintains its altitude through orbital mechanics , rather than aerodynamic lift. At the station's altitude, the atmosphere is negligible, so no meaningful lift can be generated, nor is lift required in the first place. Instead, a stable orbit exists because the station is moving forward at a speed that takes it away from a collision course just enough to trace a continuous curved path around Earth. Gravity constantly pulls the station downward, but its orbital velocity keeps it at a consistent distance from the planet ( Fig. 3 ). This precise balance between gravitational pull and forward velocity enables the station to orbit Earth continuously, without the need for engines or counteracting forces, such as aerodynamic lift, to combat the effects of gravity. Fig.3 . The ISS remains in a stable orbit because its forward orbital velocity is precisely balanced by gravity, which curves its trajectory into the closed path. Crucially, everything inside the station, astronauts, tools, and even the air, is moving at the same velocity, while Earth’s gravity is pulling them with equal force. Even without the station, the internal contents would continue orbiting Earth along the same curved path. Because no object has an advantage in terms of speed or gravity, neither exerts pressure on another. This condition creates a microgravity environment in which astronauts feel weightless. A scale onboard would show zero weight, just as it would on an airplane during free fall. However, the free-falling plane, along with its passengers, would crash on the ground. In the case of the space station, its sideways velocity is dragging it away from the crash path, making it fall around the Earth. Fig. 4 . Experience microgravity with ESA astronaut Thomas Pesquet, who takes you on a tour around the International Space Station In a microgravity environment, astronauts can't perform acrobatic stunts that look so impressive in movies. In airplanes, where gravity is fully present, pushing against the floor results in an upward leap because the ground provides a strong opposing force. However, in orbit, pushing off a surface only causes a slight relative motion between the astronaut and the station. Without gravity to hold them down, astronauts glide gently in the opposite direction of their push, drifting slowly rather than launching forcefully. In true microgravity, movements are subtle, focusing on conserving momentum rather than the bold, dramatic gestures we are used to on Earth.

Are we the pinnacle of cosmic creation? It appears we believe so. After all, the universe has spent billions of years sculpting us from dust to breath. We must hold a special place in nature’s heart if it was willing to trade time for perfection. It took eons for the first sparks of life to evolve into creatures who don't just exist, but ask about the purpose of their existence. And driven by passion and curiosity, write poems, compose symphonies, and make discoveries. Nature moved slowly and with exquisite precision to transform us from the earliest single-celled organisms, aimlessly drifting in ancient seas, to the self-aware minds capable of reasoning and expressing their thoughts and feelings in words, numbers, notes, and visuals. ? In our quest to develop Artificial Intelligence, are we uncovering something new or simply mirroring ourselves? And yet, in a brief flicker of time, a mere century, we've outpaced the natural roll of evolution and built machines that reason, write, and calculate more effectively than we ever could. Inspired by Charles Babbage's ideas of the Analytical Engine, we quickly transitioned from the first punch card computers to the invisible neurons of Artificial Intelligence. What nature accomplished over billions of years, we compressed into decades, successfully training matter to reason, speak, and see. Have we surpassed our creator in the art of creativity, becoming masters of the Universe? What we perceive as giving life to matter is merely an illusion. We didn’t create  intelligence; we simulated  it. Where nature breathed life into existence, we built reflections of the mind producing thought. Our networks mimic neurons, our algorithms replicate our memory, our models echo our intuition. Every layer of our technology is a clever imitation of the scheme that brought us into being. We are rapidly advancing because we started with a blueprint already perfected. We made AI in our image and likeness, because we could not do otherwise. For us, creation has always been an act of imitation, resulting from intelligence recreating itself through knowledge of preexisting patterns. Have We Surpassed the Creator? In efficiency, perhaps. Machines think faster, write more proficiently, and calculate flawlessly. They can compose music in seconds, analyze graphics, and provide an instant solution to a problem. But when it comes to possessing that divine spark that lives within us and inspires dreams, evokes tears, and brings laughter, they remain silent. Artificial intelligence has no feelings; you can't offend or flatter it. It is designed to recognize the patterns that correlate with human emotions and generate   an appropriate response. While they may create an illusion of support and care, they will immediately forget you once the exchange of signals ends. A vast collection of text and images is labeled with emotions such as joy, anger, irony, or grief, which programs recognize as clues for systems to respond. Transformers generate replies based on acknowledgement of these emotions, with words of comfort and support. There are safeguards in place to filter out harmful, biased, or overconfident replies, serving as a digital conscience to ensure machines remain aligned with human values. The result can feel empathetic. However, it is still about matching signals rather than feelings, a simulation of care built from statistics, patterns, and guidance—useful, often comforting, yet fundamentally driven by programs, not coming from the heart. An intelligence built by human hands contemplates a consciousness it does not possess. Image by Kohji Asakawa   from   Pixabay, Intelligent machines are capable of crafting poems about human joy and sorrow, with words that may sound gentle and wise. But their apparent beauty is nothing more than a form of plagiarism, a compilation of countless patterns absorbed during training. Poetic images that fill our souls with tremor can only generate the alternating current within their circuits. Feelings can't be expressed through codes and manufactured through the logic of machines. They are immaterial. No science has ever measured love or captured grief in a formula. Emotions are something no simulation can ever replicate. They are the personal possessions that can be shared only through empathy, and not the codes. The Immaterial Gift Perhaps nature made feelings immaterial for a reason. Maybe it’s a kind of mercy, a safeguard against the power of producing machines capable of inflicting pain and suffering on others. Or perhaps it is a reminder to us that emotion, empathy, and love are sacred. That intelligence without compassion is as hollow as an empty vessel. That our ability to feel is not a flaw, but the highest achievement of evolution. We may have made AI in our image. Still, what truly defines us is not logic, but our souls. And maybe nature kept that secret for itself, so we would never forget our purpose and meaning: We taught machines to think, hoping to understand ourselves better, and in doing so, rediscovered what makes us human. Every time an AI writes, paints, or speaks, it holds up a mirror: not to its own soul, but to ours. It shows us that consciousness without compassion is cold, that knowledge without meaning is empty. While we craft minds of silicon and current circuits , nature watches quietly, reminding us that what truly matters can't be computed. Intelligence is not merely a tale of evolutionary success; it is a daily triumph of experiencing all the joys and sorrows of the human way of living.

The ongoing activity at Hawai‘i’s Kīlauea volcano has garnered intense media attention, with images and live footage circulating worldwide. During the most recent eruptive episode, lava jets soared as high as 1,200 feet (about 370 meters), lighting up the night sky and painting the crater in fiery orange ( Fig. 1 ). Watching these towering fountains in video footage, one can easily imagine the sheer terror volcanoes must have inspired in ancient people as they were trying to flee the deadly flames. Today, we have a much better understanding of this natural phenomenon. So, what ignites the flame and causes the Earth to open its crust and propel molten rock high into the air? Fig. 1 Lava jets erupt from the Hawaii Kilauea volcano, during the latest episode of activity, producing bright, high-temperature fountains and feeding fresh lava flows across the caldera floor. Image credit: U.S. Geological Survey (USGS), Hawaiian Volcano Observatory. At the most fundamental level, volcanic eruptions are driven by pressure, heat, and gas within Earth’s interior. Our planet was formed 4.5 billion years ago from a nebula containing remnants of ancient stars. During its formation, intense heat from debris collisions, gravitational compression, and radioactive decay melted a planet into a jellylike state. The elements moved and separated according to their density, much like ingredients settling in a liquid mixture. Heavier elements, such as iron and nickel, sank to the core, while intermediate rock-forming elements settled in the mantle. Lighter minerals and gases formed the crust and atmosphere. Fig. 2   I nternal structure of the Earth showing the crust, mantle, outer core, and inner core. As time passed, Earth cooled. Today, the Earth's crust and most of the mantle consist of solid rock ( Fig. 2 ). Still, the mantle remains very hot, particularly near the core, where the rock becomes softer and moves slightly over geological timescales. This gradual movement can cause the crust above to thin or fracture. In these regions, the mantle can rise closer to the surface, where it encounters lower pressure with profound consequences. Within the Earth, pressure increases significantly as you move toward the center due to the weight of the layers above. As a result, the pressure at Earth’s core is enormous compared with anything we experience in life, being 3.6 million times higher than the air pressure at the surface. As we move in the opposite direction, away from the center, the pressure decreases because the weight of the layers above us diminishes. Lower pressure makes materials melt and boil at lower temperatures. As a familiar example, Water boils at a lower temperature on a mountain than at sea level. With less atmospheric pressure pushing down on it, water molecules escape into vapor more easily. The same principle applies inside Earth, though at vastly higher pressures and temperatures. Mantle rock that is completely solid at great depth can begin to partially melt when it rises to the surface, not because it gets hotter, but because the melting point drops in the lower-pressure environment. When rock melts to form magma, its volume increases, similar to how most liquids take up more space than their solid forms. Being less dense than the surrounding rock, magma begins to rise toward the surface. However, this ascent doesn’t always go smoothly. Magma gets trapped in underground crevices and chambers, where it accumulates, building intense internal pressure. Gases within magma bubble out in the upward flow, further raising pressure in the pockets. Eventually, the surrounding rock can no longer hold back this growing, destructive force. Magma breaks out through cracks and conduits. When it finally reaches open air, the trapped gases burst free, propelling molten rock skyward in spectacular fountains, similar to those recently observed at Kīlauea. The Earth releases built-up pressure, heat, and gases in a dramatic showcase of internal energy, reminding us of the power of nature. Fig. 3 . Video footage of the Hawaii Kilauea volcano eruption. The vivid orange glow of erupting lava is more than just a striking visual; it is a direct clue to its temperature. Based on Wien’s displacement law , lava that glows a bright orange radiates most strongly at wavelengths corresponding to temperatures of about 1,000–1,200 °C (about 1,832–2,192 °F).  At these temperatures, the thermal radiation curve shifts sufficiently into the visible spectrum, creating the distinctive fiery hue observed in lava fountains and flows. Thus, the color of lava acts as a natural thermometer, enabling us to gauge its heat directly from the glow it casts into the night.

Every so often, a question pops up that seems simple enough to answer until you try. With nearly 400,000 new people born every day, it’s tempting to imagine that they are continually increasing the planet's weight. After all, more people must mean more mass, right? Actually, that's not the case. In reality, Earth doesn’t gain weight when a new person is born. Understanding the reason behind this is an intriguing way to explore how closed systems work, a concept in physics that is frequently misunderstood. Let’s start with a foundational principle of physics: mass can’t be created or destroyed. It can only change form or move from one place to another. New humans are not outsiders; they are not delivered from space. Every person begins as a collection of atoms already present on Earth. Take the familiar story of pregnancy. A baby is conceived using material provided by their mother and father. An embryo grows because the mother eats food, and food is plants, animals, and nutrients that ultimately come from the Earth’s soil, water, and atmosphere. Pregnancy rearranges old atoms into a new shape. With the baby's arrival in the world, no new mass has been added from outside Earth .  They are the same old atoms rearranged into a new, beautiful beginning. This same principle holds long after birth. Every meal we eat, every sip of water we drink, every breath we take is made of Earth material. And whatever you eventually return, CO₂, waste, heat, goes right back into the Earth’s system. We shuffle mass around, but we don’t add to it. A growing population doesn't equate to a growing planet. Earth does gain some matter from elsewhere: about 100,000 tons every year comes from tiny meteorites falling through the atmosphere. However, it's a negligible contribution compared to the planet's size. It also loses some mass as atmospheric gases slowly escape into space. The gains and losses mostly balance out, and none of them involve humans being born. Of course, no system is ever 100% closed. Perfectly closed systems don’t exist in nature. They are approximations we use in physics to simplify calculations and understand conserved quantities, such as mass or energy. But Earth’s ecosystem is closed enough for most practical purposes. Almost all the matter that makes up living things stays within the planet’s boundaries, circulating through air, water, soil, and organisms. Thinking about humanity in this way highlights something profoundly important. We are not separate from Earth; we are the Earth. Every atom within us has been and will remain an essential part of the planet's resources, whether drifting in clouds, resting in soil, or dissolved in water. Our atoms have lived in mountains and valleys, inside ancient organisms, and in countless cycles of life before becoming part of us. Throughout our evolution, we’ve grown crops, built cities, shaped landscapes, and nurtured future generations. We are creatures of this planet, bound to it by every particle of our being. Earth’s closed system has supported life for billions of years, and will continue to do so, quietly recycling atoms one by one, so we can live comfortably in our planetary home.

Science memes have become extremely popular among the general public and scientists. Some achieved such wide recognition that they became iconic, blending science and humor into an irresistible mix of intellectual enjoyment. Perhaps the most notable is the one where a car changes its color from blue to red as it drives past a pedestrian. The meme uses a well-known example from sound to illustrate the Doppler effect in light, playing out the analogies in a humorous yet informative manner. Drawing parallels between light and sound through this meme can help make a fundamental physics concept easier to grasp, turning the learning process into a fun experience. We will determine  if a car can change its color due to the Doppler effect,  the speed required for such a transformation, and the true color of the vehicle. Fig. 1. This meme illustrates the Doppler effect in light using a familiar example from sound. What is the Doppler effect? The Doppler Effect is associated with wave phenomena, with light and sound being the most common examples. Waves are defined by their frequencies, which determine the color we see and the pitch we hear. Our eyes have photoreceptor cells, and our ears contain hair cells that detect frequency variations and relay this information to the brain, where it is converted into colors and tones. Thus, we can experience the world in all its diversity because our visual and auditory systems  are capable of differentiating between wave frequencies. Although our sensory system possesses remarkable abilities, the accuracy of the information is affected if the wave source moves relative to the observer. For example, when a car approaches us, we perceive the frequency as higher, resulting in a higher pitch. Conversely, if the car moves away, the frequency appears to be lower, resulting in a lower pitch. Despite the appearance, the frequency of the sound produced by the car remains constant, which is why the passengers in the car don't notice any change. In science, when we explore various phenomena, we are the observers whose measurements are affected by this effect. The Doppler formulas allow us to calculate the Doppler shift and understand the actual situation versus a perceived reality. . Fig. 2 . The car exhibits a frequency shift due to the Doppler effect while approaching and receding from the observer. The difference between the Doppler effect in sound and light. The Doppler effect applies to all types of waves, including mechanical, sound, and light waves. The primary difference lies in the speed at which the waves travel. For our eyes and ears to detect a Doppler shift, the speed of the wave source must be comparable to the speed of the wave itself. Since light travels almost a million times faster than sound, a light source must also move nearly a million times faster than a sound source, like a car, to show an equivalent optical shift. Textbooks often use sound examples because, on Earth, light sources can't achieve such extremely high, relativistic speeds. As a result, we don't observe the visual manifestation of the Doppler effect in everyday life. However, astronomers observe this phenomenon as galaxies display a blueshift when moving toward us and a redshift when moving away ( Fig. 3 ). Fig. 3 . The star exhibits a frequency shift due to the Doppler effect, appearing blue while approaching observers and red while receding from them. When a car drives past and its pitch changes, its color changes too. However, this change is imperceptible to the human eye because the Doppler shift in light is negligible in the moving cars. It would require high-precision instruments to detect it. For the Doppler shift to be visible, the vehicle would need to travel at a relativistic speed, unachievable under Earth's conditions. But if it could reach this astronomical speed, some strange things would happen to the color of the car and to its headlights. The Doppler effect on the headlights. Headlight is similar to starlight. Starlight comprises various wavelengths that form a continuous spectrum, spanning the entire visible range from 400 nm to 700 nm ( Fig. 4 ). This mix of wavelengths causes stars to appear white, with shades of red, yellow, and blue, depending on a star's temperature and chemical composition. The Doppler effect shifts each wavelength by the same fractional amount v/c , where c  is the speed of light and v  is the speed of the light source. Consequently, the entire spectrum of blue, green, yellow, and red components shifts as a whole toward shorter wavelengths when a star or a car is approaching (blue shift) and toward longer wavelengths when they are receding (red shift). Fig. 4 . The wavelengths and frequencies of the visible spectrum, ranging from red to blue. For white light to demonstrate the full span Doppler effect, changing color from bluish to reddish ( Fig. 3 ), a star and a car would need to travel at nearly 20% of the speed of light ( v/c  = 0.2). As their speed increases further, some light components would slide into the invisible ultraviolet and infrared bands. Once they surpass half the speed of light, v/c  > 0.5, the entire visible spectrum slides into the invisible bands. A car would play a dangerous game of "Now you see me, now you don't" with a pedestrian attempting to cross the road. The car would only briefly become visible when it was directly in line with an observer. While approaching and moving away, the car would turn into a ghost, undetectable by human photoreceptors. The Doppler effect on the car's color.    We perceive the car as red because it is coated with paint that absorbs all wavelengths except for red when white sunlight illuminates its surface. By reflecting red components to our eyes, the car acts as a secondary light source, with reflected wavelengths being subject to the Doppler effect. The Doppler effect is consistently symmetrical, shifting each wavelength, λ , by the same proportion, Δλ , toward the blue and red ends of the spectrum. Let the red wavelength be λ = 670 nm ( Fig. 5 ). The Doppler shift Δλ in relation to the car speed, v , can be calculated using a simple formula, where we neglect the relativistic component for simplicity : Given that the speed of light is approximately 300,000 km/s ( 186,000 miles per second ), the car must travel at a speed of 4,500 km/s (2,800 miles/s) to experience a Doppler shift of Δλ =  ±10 nm. If you were a pedestrian attempting to cross the road, you would perceive the color as a shade of red corresponding to the wavelength λ = 660 nm when the car is approaching, and as a shade of red corresponding to the wavelength λ = 680 nm when it is moving away from you ( Fig. 5 ). The true color of the car, corresponding to the rest wavelength λ = 670 nm, would briefly appear when the car was directly beside you. As you can see, even at this astronomical speed, the shift would remain subtle. Fig. 5 . Due to the Doppler effect, the red light with the rest wavelength of 670 nm experiences a 10 nm shift, as a result of the light source moving at a speed of 0.015c. Referring back to our meme, we now possess all the necessary information to determine the car's actual color and the speed it needs to display a full-span Doppler shift from blue to red. Let the color appear blue with a wavelength of λ = 470 nm when the car is approaching an observer, and red with a wavelength of λ = 650 nm when it is receding. Given the symmetry of the Doppler shift, this combination will give a rest wavelength of λ = 565 nm , (470 + 650)/2, identifying the car's actual color as a shade of green . By plugging in a Doppler shift of Δλ = 85 nm (650 nm - 565 nm) into the formula above, we calculate that the car would need to travel at about 15% of the speed of light (0.15c), which is far beyond the capabilities of current technology, even for rockets in space. Still, the practicality of this scenario is not our concern. The meme inspired us to dive into the core of the Doppler effect, explore it like physics pros, and enjoy the colorful experience. Isn't science incredible? The thought experiments it provokes can outshine the most vivid imagination! The car's length was humorously contracted and stretched to emphasize the shorter blue and longer red wavelengths. In reality, the Doppler effect does not alter the length of objects. . I will conclude with a physics joke that is believed to have inspired this meme. A physicist, caught running a red light, devised a scheme to avoid a penalty. In court, he claimed that the red light appeared green due to the Doppler shift. Unfortunately for him, the judge was a physics fan. Without an argument, he accepted the physicist's explanation and doubled the fine for driving at a relativistic speed in the residential area. Trapped by his own lie, the physicist had no option but to accept the punishment.

The International Space Station (ISS) ( Fig. 1 ) is a laboratory the size of a mansion, orbiting Earth at an altitude of 250 miles (400 km). It was assembled in space using modules sent into orbit in separate launches, because no rocket is powerful enough to launch such a large structure in one piece. The station features habitable areas with breathable air for astronauts, as well as airless sections housing machinery operated by robotic arms, which are accessible only via spacewalks. The laboratory is used for conducting scientific experiments in the unique microgravity environment, where people and objects feel weightless. But why is the ISS a weight-free zone if gravity at that altitude is still almost as strong as on Earth? Fig. 1.   The  International Space Station (ISS) is depicted against the backdrop of Earth and the Sun. In space, above the Earth's atmosphere, the Sun looks white. Why are astronauts weightless in orbit? At the ISS's altitude, gravity is only 10% weaker than on Earth's surface. If we built a tower reaching the ISS, our weight at the top would still be 90% of what it is on the ground. Astronauts feel weightless not because there is no gravity, but because there is no ground support exerting pressure on their bodies. On Earth, when support is withdrawn, we also experience weightlessness as we enter free fall. During this state, a scale shows zero weight since it moves at the same rate, applying no pressure on the body. Astronauts aboard the ISS travel at the same speed as the station and everything around them. In this environment, a scale would apply zero force and register zero weight, which is why a satellite in orbit is considered to be in a state of free fall. But how can it be falling if its distance from Earth remains unchanged? Inertial frame of reference.  The key lies in the chosen frame of reference. An observer in the inertial frame perceives objective reality because they travel in a straight line and at a constant speed, unaffected by external forces. This frame does not necessitate the use of fictitious forces, such as centrifugal and Coriolis forces, to explain what happens around. In contrast, an observer in a non-inertial frame, such as those on the Earth, experiences a subjective reality because their perspective is influenced by forces acting upon them. The ISS is falling around the Earth. In Fig. 2 , the inertial frame is aligned with the vector of the station's orbital velocity .  In the absence of gravity, the station would follow this straight line, receding from Earth. Gravity curves the straight path into a circular trajectory, continuously pulling the satellite towards Earth. Observers in a non-inertial frame, such as we on the Earth's surface, would see the station as maintaining the same distance from the Earth. Whereas observers in the inertial frame, aligned with the vector of the orbital velocity, would perceive the satellite as moving away from them, falling towards Earth, but evading a crash due to its sideways motion. Fig. 2.  The International Space Station (ISS) orbit. The ISS maintains a stable orbit because its velocity tries to drag it away from Earth with the same impetus that gravity tries to pull it toward it. At 7.67 km/s (4.75 miles per second), this velocity perfectly counteracts the force of gravity at the orbital altitude, which is 90% of that on the ground. As everything onboard travels at the same velocity and is subjected to the same gravitational force, no pressure is exerted by the station's components on their neighbours, thus creating a microgravity environment. Even without the station, astronauts and objects would continue to orbit Earth together but independently from each other. This unique weight-free zone provides a perfect opportunity for developing innovative medical treatments and advanced space technologies. Enjoy this breathtaking footage of Earth rotating beneath the International Space Station. Over 24 hours, the ISS completes 16 orbits around Earth, witnessing 16 sunrises and sunsets.

The Schroeder stairs illusion ( Fig. 1 ) is a classic example of perspective reversal, often used in research on how our brain interprets reality. The drawing is stripped of the cues indicating its spatial orientation. In the absence of these cues, the brain struggles to produce a clear 3D interpretation of this 2D structure, switching between two equally plausible, yet mutually exclusive perspectives. In the first scenario, the stairs appear to be viewed from above ( Fig. 1 , left image), with panel A in the foreground and steps rising from right to left. In the second scenario, the same pattern is perceived as the stairs seen from below, with panel A in the background and steps rising from left to right ( Fig. 1 , right image). Fig. 1 . The Schroeder staircase is rotated 90 degrees to help you recognize the second perspective. The illusion was named after the German natural scientist Heinrich G. F. Schröder, who published it in 1858. Not everyone can easily switch between the two viewpoints, as it depends on how adaptable your brain is in accepting ambiguous solutions. When faced with an image that allows for multiple 3D interpretations, the brain may choose a preferred option based on past experiences. This preferred assumption becomes dominant, making it hard for the brain to disengage from it and adopt an alternative view. In Fig. 1 , the pattern is   rotated 90 degrees to assist you in visualising both perspectives. However, it is essential to experience the flip in action to understand how our brain generates 3D solutions. If your brain stubbornly resists switching modes, you can try the following exercise. Fig. 2 . The central orientation activates perspective reversal in the Schroeder stairs illusion. In Fig. 2 , the central image shows the staircase at the halfway point of a 90° rotation. This intermediate position allows for effective engagement with both perspectives shown on the left and right. Cover the left image with your hand and shift your focus between the letter B in the center and the right image to bring Panel B forward. When this is achieved, the brain will reverse direction, and you will see the steps appear to ascend from left to right in the central image. To return to the previous mode, cover the right image and shift your focus between the letter A in the left and central images. This will bring Panel A forward, causing an illusion that the steps ascend from right to left. If needed, you can tilt your head or lie down to relax your eyes. Once the dominant pattern is disrupted, the perceptual reversal is triggered, allowing the central image to change orientation freely. Perceptual flexibility is generally greater in children and gradually decreases as we reach adulthood. Younger brains are still building internal models of the world, so they are more open to accepting ambiguous outcomes. As we get older, the brain becomes less adventurous and more reliant on established pathways, tending to settle on a single option and stick with it. Fig. 3 . Stair perception: view from above vs. view from below The impression of viewing the stairs from above or below is due to how the treads and risers are projected onto the retina. When observing the stairs from above ( Fig. 3 , left), the horizontal treads are more visible, while the vertical risers appear hidden. The brain interprets the pattern of wider treads and narrower risers as consistent with the elevated position of the viewer. In contrast, when we stand at the bottom of the stairs ( Fig. 3 , right), we face the vertical risers, which makes them appear wider and the treads compressed. The brain interprets this shift in proportions as a cue that the viewer is positioned at the bottom of the stairs. In Fig. 1 , the lighter bands are wider than the darker ones. This difference provides visual cues about the observer's position relative to the stairs. The first interpretation brings Panel A forward, making the light bands appear horizontal. The horizontal bands are always treads, as they are the only parts that can support the climber's weight. Since they are wider than the risers, the stairs must be viewed from above. The second interpretation brings Panel B forward, turning the treads into risers. Now, with the risers appearing wider, the apparent viewpoint shifts from above to below. Fig. 4 . Structure of the eye. Light passes through the pupil and forms a 2D image on the retina, which the brain translates into 3D perception. Information about the world is delivered to us by light that activates photoreceptors in the retina, located at the back of the eyeball ( Fig. 4 ). As a curved, two-dimensional sheet of cells, the retina can only record two-dimensional projections of our three-dimensional environment. Thus, every scene we view is encoded as a flat pattern of light wavelengths and intensities. From this inherently incomplete information, the brain must reconstruct a coherent 3D interpretation of the objects and their spatial relationships. This reconstruction relies on the contextual cues, some examples of which we have examined using Schroeder's illusion. We see the world in a flat mode. A third dimension, depth, is created by our brain using cues stored in our memory. This mechanism may not be perfect, but it allows us to navigate our surroundings with remarkable accuracy. Essentially, the reality as we perceive it is a sequence of 2D “snapshots” captured by the retina, which our brain converts into a stream of unfolding 3D events. Space has no inherent directions, such as left and right, up and down, or front and back; these orientations exist only in relation to an observer. When cues are muted, the brain either experiences a perceptual flip or selects the dominant interpretation that has been prioritized through past life experiences.

Our vision of colonizing Mars is heavily influenced by the blockbuster sci-fi movie Total Recall . A domed city on the red planet, bustling with life and adventure — what’s not to love? Yet, creating a fantasy in the confines of a Hollywood studio is much easier than transforming a barren rock into a second home for humans. The engineering challenges of this endeavor will far exceed the scope of a romantic space adventure. No matter how advanced technology becomes, every Martian will still face a down-to-earth problem: what's for dinner? On Martian terrain, even the modest task of growing home potatoes can escalate into a challenge of epic proportions. Fig. 1 Plants that thrive on Earth can't survive in toxic martian soil, blasted by radiation. Martian soil is infertile and toxic Martian soil is a sterile regolith, the rock crushed into gravel and dust over billions of years due to meteoroid bombardment and erosion. Laced with chemicals so toxic that they would kill most plants, it would have to be thoroughly washed before anything could grow. On Earth, this would be simple. On Mars, there’s no liquid water. Colonists would have to extract ice buried beneath the surface, melt and purify it, and then use this precious resource to rinse tons of soil, recovering every drop in the process. It's not exactly your average weekend at the allotment. Healthy soil on Earth teems with life, full of microbes, fungi, worms, and decaying organic matter. This ecosystem supplies nutrients for plant growth. In contrast, Martian regolith is completely sterile, meeting the scientific definition of "no signs of life." To breathe life into this dead soil, colonists would need to import large quantities of organic matter to prepare the soil for future agriculture. In the solar system, where all planets are as lifeless as Mars apart from Earth, the organic matter could only come from us. Therefore, every ounce of organic stuff that Mars would gain must be the ounce that our planet lost forever. Transporting food and agricultural equipment to Mars presents challenges far beyond those of a typical home delivery. With a distance of approximately 225 million km (140 million miles) between Mars and Earth, the fuel costs will be exceptionally high. Moving just 1 kg of material demands about 120 kg of propellant. This is merely the start. When you factor in additional costs, we can only hope that these high prices won't dampen the appetite of Martian settlers. Martian radiation and climate Mars' atmosphere is nearly 100 times thinner than Earth's, and consists primarily of carbon dioxide, which plants love and humans hate. However, the thin atmosphere and lack of a magnetic field leave Mars vulnerable to harmful cosmic rays and solar radiation. This exposure could cause severe radiation burns, acute radiation sickness, and an increased risk of cancer in unprotected humans. Plants are even more vulnerable to intense ultraviolet radiation, which would bleach them of chlorophyll, causing them to wilt and die within days. All life forms, including humans, animals, and plants, would require constant protection, posing one of the biggest engineering challenges. Fig. 2   Artist's impression of a prospective human colony on Mars . Credit: NASA Building underground living spaces and greenhouses is the most cost-efficient choice. By going just 1-2 meters beneath the regolith, radiation can be reduced to Earth levels. However, not everyone would fancy being buried under regolith without windows and natural light. Plants require sunlight to convert the planet's carbon dioxide into oxygen and organic matter through photosynthesis. This prompts designers to explore alternative solutions, such as creating domes on the surface with specialized glass for windows ( Fig. 2 ). In greenhouses, more economical polystyrene panels, similar to those used on Earth, but with an added anti-UV film, could be employed. The challenges extend beyond construction. Being farther from the Sun, Mars will subject the settlers to freezing temperatures of –60 °C (–76 °F) on average, with milder summer spells on the equator for some relief. While on Earth, we often grumble about rain and seek shelter under umbrellas, on Mars, rain doesn’t exist. Instead, the dry atmosphere fuels massive dust storms that can last for weeks, casting the planet into a dim, sunless haze. An umbrella won't protect you from regolith dust, though it wouldn't be necessary. Colonists will always need to wear spacesuits outdoors, and crops will grow in the pressurized greenhouses with Earth-like temperature and humidity. Robots come first Due to significant risks, all initial preparations will be conducted by robots. This is not just safer, but actually cheaper. Before humans arrive, robotic pioneers will excavate regolith to lay foundations and assemble dome living structures, power stations, and landing pads for supply deliveries. To transform the harsh environment into a human-friendly habitat, they will establish water extraction systems and start generating oxygen from the Martian atmosphere using already developed technologies. Soil preparation will ensue with washing regolith using recycled water, mixing in organic matter shipped from Earth, and testing the first crop grown in sealed greenhouses. Fig. 3. The Perseverance rover during its landing on the Martian surface, being lowered by a rocket-powered descent stage. This moment represents the final phase of the Entry, Descent, and Landing sequence, often referred to by engineers as the “seven minutes of terror.” Image credit: NASA/JPL-Caltech Current robotic missions have performed sample drilling of the Martian regolith to examine its characteristics. New excavation systems would need to be developed, capable of operating on the alien soil in low gravity. Martian rigolith differs from Earth's soil in density, cohesion, and ice content, presenting extra challenges for excavating systems. In low gravity, machinery's stability is affected, and the coordination between moving parts becomes more complex. Much preparation work is still required before the groundwork begins. For now, the large-scale excavations remain a future goal rather than a present reality. Yet, Mars colonization is an active project, not science fiction, in which several governmental, private, and academic organizations are involved. No petals in spring, only a roar of Martian geysers Mars takes approximately 687 Earth days to complete its orbit around the Sun, resulting in seasons that last nearly twice as long as those on Earth. If Mars had a climate similar to ours, Martian colonists could enjoy the sight of blooming orchids for twice as long. However, the freezing temperatures rule out any dreams of seeing red terrains covered with vibrant flowers. Instead, in spring, Mars would offer colonists a different treat, a natural wonder as unique to Mars as orchids are to Earth: spectacular carbon dioxide geysers, blasting jets of dust up to 100 meters (330 feet) into the air. Fig. 3. Martian carbon dioxide geysers, photographed from a Martian satellite. Credit: NASA/JPL-Caltech/University of Arizona  In spring,  the intensified sunlight reaches a translucent layer of dry ice at the poles, warming the ground beneath it. This process causes the CO₂ ice at the bottom to sublime into gas, with pressure building up until the gas erupts into jets ( Fig. 3 ). On Earth, dry ice doesn't occur naturally; our atmosphere is too thick and warm for CO₂ to freeze. We manufacture it in small amounts for shipping frozen foods and creating fog effects on stage. Rather than CO₂ gas jets, our planet features liquid water geysers. The same fundamental physical laws apply on Earth, yielding different results under different conditions. Martian dry geysers are as unique in the solar system as our liquid ones.. In its unbounded creativity, nature avoids repetition, making each planet one os a kind.   Martian gravity Mars' surface gravity is 0.38 g, meaning you’d weigh there 38% of your Earth weight. A person weighing 100 kilograms on Earth would weigh 38 kilograms on Mars, feeling as if they instantaneously lost 62 kilograms. This would make tasks like wearing spacesuits and lifting heavy equipment much easier. However, what appears to be an advantage comes with uncertainties. Our experience with low gravity's effects on the human body is limited, with data only from astronauts' brief stays in microgravity on the International Space Station. Despite rigorous exercise, astronauts face muscle loss, bone thinning, and vision changes. We lack long-term data on how the human body responds to partial gravity. We don’t yet know how well humans can grow, age, or even have children under such conditions. Why Mars? If Mars is so inhospitable and challenging, then why have we chosen this planet for future colonization? The answer starts with simple planetary bookkeeping. Of the eight planets in our Solar System, four are gas giants: Jupiter , Saturn , Uranus , and Neptune . They have no solid surfaces, only deep, violent atmospheres of hydrogen and helium. There is nowhere to land, stand, build, or breathe. That leaves us with two remaining rocky planets. Mercury is a tiny planet of extremes, where survival is unfeasible. Due to its almost nonexistent atmosphere, which fails to retain or distribute heat, temperatures swing violently from +430 °C (+800 °F) during the day to – 180 °C (–290 °F) at night. A single day on Mercury spans nearly two Earth months, giving the surface plenty of time to bake and then freeze. It’s a place where metal softens in the daytime and gases turn into frost at night. In contrast, Venus is a high-pressure furnace. Its surface temperature averages 465 °C (869 °F), hot enough to melt lead, and the atmospheric pressure is 92 times that of Earth, sufficient to destroy your internal organs. Dense clouds of sulfuric acid trap heat, causing a runaway greenhouse effect that renders Venus not only extremely hot but also overwhelmingly hostile. Even the most durable probes can survive just for a few minutes before being fried, flattened, and corroded. Through a process of elimination, Mars stands out as the only planet in our neighbourhood where humans could potentially survive. While it may not be an ideal host,  it does offer water ice, a day length similar to Earth's at nearly 24 hours, and an atmosphere, though thin, still suitable for future greenhouse warming. Admittedly, it falls short of providing our usual home comforts. But if we ever aim to establish a long-term presence on another planet, Mars remains our only viable option for survival, at least among the choices within the Solar System.

If Saturn replaced the Moon, our nights would never be the same. Imagine stepping outside and seeing a colossal, golden globe filling the sky, with its pale rings stretching from horizon to horizon, glowing softly in reflected sunlight. The familiar silvery Moon would be gone, replaced by a planet so huge and bright it would cast shadows at midnight. Yet behind that breathtaking sight lies a violent story of gravity, landslides, and tides. Because a night under Saturn’s rings wouldn’t just be beautiful; it would be utterly transformative for our planet. So, what would happen if by some magic, Saturn and the Moon exchanged positions? Let's examine this scenario step by step, using celestial mechanics to explore the consequences of such a swap. Saturn at the Moon's distance: a thought experiment. Artistic impression.   The current Earth-Moon dynamics At present, the Earth and the Moon orbit a common barycenter located inside the Earth. Because that point lies beneath Earth’s surface, the Moon appears to circle Earth. It travels at a speed of ≈ 1.022 km/s (≈ 2,287 mph), which perfectly offsets Earth’s gravity at a distance of 384,400 km (238,900 miles). This perfect match between the orbital velocity and the gravitational field strength allows the Moon to maintain a stable orbit and complete one circle around Earth every 27.3 days. As the Moon is tidally locked to Earth, it also completes one rotation around its axis in the same time it takes to orbit Earth, which is why we always see the same side. As a result, for the Moon, one day equals one month. While the Moon and Earth have their own dynamics, they both orbit the Sun at ≈ 30 km/s (≈ 18.6 miles per second). This speed balances the Sun's gravitational pull, allowing them to maintain a stable orbit around it. The key point is that the orbit's stability depends on the mass of the central body, not on the mass of the orbiting body, provided the orbiting body is much smaller than the central one. Therefore, replacing the Moon with Jupiter wouldn't alter the relationship with the Sun. The Earth–Jupiter pair would continue orbiting the Sun as before. Dramatic changes would occur between Earth and Jupiter, not between the pair and the Sun. And that’s where things get interesting.     Who will orbit whom? Saturn is 95 times more massive than Earth. Therefore, if it were to take the Moon's place, the barycenter would shift deep inside this giant, flipping the entire relationship. Earth no longer functions as the central body and becomes the moon. As explained, the Earth–Saturn pair would continue orbiting the Sun, just as the Earth–Moon pair does now, because the Sun’s gravity only cares about how far away the pair is, and how fast it’s moving. However, inside that pair, everything would change. We’d no longer be the center of attraction. We’d be the smaller partner, circling a giant whose gravity dominates our sky.   Because of its larger mass, Saturn exerts a much stronger gravitational pull than Earth. For Earth to remain in orbit around Saturn, it would need to travel significantly faster, approximately 18 km/s, completing its orbit in just 37 hours. Initially, the Earth would keep rotating on its axis every 24 hours to preserve its current angular momentum. However, this scenario would not last. Saturn’s gravitational tides would be about 10,000 times stronger than the Moon’s tidal effect. They would reshape the existing landscape forever, unleashing Armageddon on life on Earth. The tides that would tear life apart Once Earth begins orbiting Saturn, the sky above becomes mesmerising, but the reality beneath turns into a nightmare. The catastrophic tidal forces would make the oceans slosh hundreds of meters high, violently crashing into shores and obliterating cities and continents. Large regions of seafloor would be exposed, only to be swallowed again hours later. The Earth’s crust would be pushed together and pulled apart, triggering massive earthquakes and landslides, with mountains rising and falling, permanently altering Earth's landscape. Volcanic activity would surge. As the tides drag against the oceans and continents, they extract energy from Earth's rotation, converting it into heat, causing the oceans   to boil. The Earth’s spin begins to slow rapidly, and the 24-hour day lengthens to 37 hours to match the Earth's orbit. Within a few thousand years, a mere blink in geological terms, Earth would become tidally locked to Saturn, constantly showing the same face as the Moon does now. If any life on Earth survived this early onslaught, it would face a sky and planet changed beyond recognition. The new era Our planet would be permanently divided into two radically different hemispheres, much like the Moon is today. The far side will never see Saturn, only the Sun or stars in its sky. The near side will captivate observers with a sight of extraordinary beauty: Saturn, eternally fixed in one spot in the sky. Its disk would appear 35 times wider in diameter than the Moon does to us now. Its rings would stretch even farther, carving enormous arcs across the sky. They’d reflect sunlight throughout the night, rendering darkness impossible and making nights as bright as twilight. The Sun would rise and set as usual. But Saturn would remain fixed in its position, always visible, even during the day. Yet, beauty comes at a price, opening the way to a harsh reality. The Saturn-facing side would be battered by constant light, oceans would evaporate, and temperatures would climb to uninhabitable levels. The slowed rotation would turn the opposite hemisphere into a frozen wasteland, deserts of ice and shadow. Only a narrow band between these two opposites, the twilight zone, would offer any hope. Here, the balance between heat and cold may ht allow liquid water and tolerable conditions. Life, if it endured, would migrate to this narrow strip of equilibrium. Even here, at the edge of twilight, Saturn would continue to dominate our lives: immense, unmoving, impossible to ignore. Its golden rings would hover just above the horizon, a permanent reminder of what this planet had endured: battered coastlines, boiling seas, and earth-shattering quakes. Overwhelmed by its immense gravity and luminosity, we would struggle to appreciate the beauty that destroyed the world we once knew. Looking up at those magnificent rings, spanning the sky where the Moon once hung, we would likely reminisce about our former tiny, grey companion, with its gentle touch and modest shine. And talk about the delicate balance, an almost impossible precision that gives conditions for the birth and nurturing of intelligent life.

We come into this world with inquisitive minds, eager to understand how it works. By observing various events, we try to find the reason why they happen this way rather than another. Why can we see through the glass but can't hear? Why does an apple fall from the tree while the Moon stays in the sky ? Why is the sky blue, the sunset red, and the rainbow full of colors? We want to get to the core of everything, soar over the rainbow, above the sky, and to the final frontiers of the Universe! Physics Core translates the language of equations into intuitive concepts and visual ideas. If you're also searching for answers, Physics Core is the place for you. Physics is the oldest and most advanced branch of science, born out of our desire to study natural phenomena. With a rich history spanning thousands of years, it has laid the groundwork for numerous academic disciplines, evolving alongside them and merging to form new fields, such as biophysics and quantum chemistry. Today, this monumental testament to human intellectual prowess has outgrown the limits of our home star and embarked on deep space exploration in search of the origins of our Universe and the meaning of our existence. Its boundless resources will empower us to achieve the impossible and turn our dreams into reality. But then, we would say that, wouldn't we? We are mad about physics! Physics never sleeps, continually generating new ideas. The progress made over the past century has been remarkable, revolutionizing our lives and sparking an unparalleled interest in physics among the general public.  Unfortunately, educational literature is often too technical for most people to understand, with the truth getting lost in scientific jargon. We aim to change that! At Physics Core , we believe the laws of nature are simple, which is why the Universe runs so smoothly. We explain everything in plain English, from classical physics to relativity and quantum mechanics. Newton and Einstein, pillars of our eternal journey to understand reality Nature does not offer easy solutions. Nothing has been handed to us on a silver platter. Behind every discovery lies a long road paved with trials and errors, false starts and detours, sweat and tears. The history of scientific progress is a testament to our perseverance in the pursuit of the ultimate truth. Rewards are high. We add the new natural laws to our collection, which signify new milestones in our development and grant immortality to those who derived them. These laws are never going to change; the Universe was born and will die with them. We will pass our treasured collection to future generations to continue our legacy.​​  ​Physics concepts can take centuries to establish, undergoing rigorous testing until proven with mathematical accuracy. Once solidified in equations, they can be utilized in new technologies to enhance our lives. However, as important as they are, equations don't answer the question 'Why'. It is the job of physics to interpret what they mean in the grand scheme of nature. Interpretations typically reflect the knowledge available at the time of their discovery and are rarely revised , possibly out of respect for the great names behind these historical achievements.​ However, time moves on, and our perception of the world evolves, enriched by new scientific evidence. At Physics Core , we believe that longstanding concepts have much more to offer, and we've merely scratched the surface. The bigger picture is better seen from a distance. By gaining deeper insights into nature's operating mechanism, we can better understand the role of each internal component. We have delved beneath the surface and discovered some precious gems hidden at the core. We will share our findings with you, labeling  the relevant articles as ' You read it here first ' .​​​​​​​​​​​​​​​​  Gas Giant Exoplanet (middle) and Spiral Galaxy NGC, 1566. Illustrations.  Source: NASA   ​​​​​​​​​​​​​​ As permanent residents of planet Earth, bound to it by gravity, we find it hard to comprehend how vast and diverse the Universe is. Still, in every part of it — over the rainbow, above the sky, and at its final frontiers — the laws of nature remain consistent, making it possible for us to conduct research, make predictions, and find supporting evidence. We may encounter exotic cosmic structures and bizarre galactic formations in its deepest corners. Still, we study them, applying the same physical principles we use on Earth to address our local problems. We are part of one great cosmic community , sharing common DNA and obeying common rules. The Universe is our home, and we hold the key to its past, present, and future. Know the Universe you live in! Learn physics with us! We promise to make your journey engaging and enlightening.

For as long as we’ve been able to look up at the stars, we’ve also been waiting for them to fall, bringing disaster upon us. From the Book of Revelation to the modern TikTok prophets, we never seem to tire of anticipating the next Doomsday. So when a wandering comet was spotted making its way toward our Solar System, you could bet that some corner of the internet would proclaim it an omen. However, what doomers perceive as a harbinger of catastrophe, scientists welcome as a rare gift from another part of our galaxy, delivered to their doorstep. Interstellar Comet 3I/ATLAS: a cosmic time capsule Fig. 1 . Comet 3I/ATLAS passing through the solar system. Digital artwork created in collaboration with ChatGPT (OpenAI).© Physics Core 2025. This newest visitor, the interstellar comet 3I/ATLAS , is only the third foreign object ever detected passing through our celestial neighborhood. Discovered in September 2024, it comes to us from far beyond the solar system, traveling on a hyperbolic orbit, which will carry it back into interstellar space ( Fig. 2 ). It’s a fleeting visitor. The comet passed Mars on 3 October 2025 and reached its closest approach to the Sun yesterday, completing the inward leg of its hyperbolic trajectory. Now, on its outbound path, it will pass Earth on 19 December, Jupiter by mid-2026, and exit the solar system forever in the early 2027. Fig.2 . The interstellar comet 3I/ATLAS  following a hyperbolic trajectory through the inner Solar System, shown near its 2025 perihelion at 1.36 AU from the Sun. Credit: NASA/JPL-Caltech   What we know so far For astronomers, even a brief encounter is a treasure, as the interstellar objects represent fragments of other solar systems formed in an earlier universe. Recent studies suggest it may have originated within the thick disk of the Milky Way. That’s the older, more vertically extended population of stars in the galaxy. Trajectory modelling indicates it is arriving from the constellation Sagittarius, near the galactic center, which puts the comet's age at around 8 billion years.. Our solar system, located in the younger thin disk region, is about 4.6 billion years old. This significant age difference makes the comet a time capsule, carrying palpable evidence of what the universe was like 4 billion years before our solar system was formed. Initial analyses suggest that 3I/ATLAS is similar to an average comet from our solar system, reinforcing the idea of a common chemical heritage throughout the universe. Measuring about eight kilometers wide, it consists of a solid core of ice and dust, followed by a greenish shimmering tail. This emerald hue comes from diatomic carbon (C₂), simple molecules that are released as the comet's surface ices warm in sunlight. The same carbon-rich chemistry is observed in many comets within our solar system, suggesting that organic building blocks might be universal across the stars. Despite its respectable age, the comet 3I/ATLAS doesn't behave like a frozen relic. Instead, it exhibits the physics of a living comet, spinning and sending jets of vapor into space. These jets act as natural thrusters, subtly altering the comet's rotation and causing it to wobble. Solar radiation and the solar wind draw this material outward, creating a pale, streaming tail. Because the comet is so distant and faint, it is hard to determine its period precisely. Yet, early photometric studies suggest it rotates similarly to most solar comets, probably over tens of hours. Why the Mars images are special When the comet swept past Mars, its machine-operated satellites captured high-resolution images of the eight-billion-year-old traveler. It was the first time in human history that an interstellar object was observed from the orbit of another planet, a significant milestone in the exploration of the cosmos. Though the comet will come close to our home planet, Earth, on 19 December, providing us with a new trove of data, these images may remain the best we get. The images were taken at an optimal angle for sunlight to illuminate the comet Orbiters around Mars have superior cameras and spectrometers compared to those on Earth The images captured the comet before it reached perihelion (its nearest point to the Sun), so the comet was probably less active, making its nucleus and jets more visible. As 3I/ATLAS  now begins its long journey back toward interstellar space, telescopes across Earth and in orbit will continue to track the comet's fading light, collecting data in infrared, optical, and radio wavelengths. But the resulting images will be less about sharpness and more about spectral data (its composition, gas emissions, and dust behavior). So these images, captured from the Red Planet, may end up being the sharpest and most detailed visual data we’ll have in the first-of-its-kind interplanetary observation. The universe sends us surprises, not as omens of doom, but as invitations to wonder. What we see in 3I/ATLAS is a reminder of our cosmic connection: that across the immeasurable distances of other galaxies, the same natural laws are at work, guiding the formation of matter and the evolution of the universe. As the interstella r comet travels through the solar system on its journey back into the darkness, it leaves us with more than just data and images; it provides living proof that in the cosmos, we are not merely spectators. We are active participants in solving its greatest mysteries.

Problem : We have a tangle of tubes and four hungry cats: ginger, black, white, and grey, eagerly waiting for milk ( Fig. 1 ). As milk begins to pour, which cat will be left without it? Answer:  Ginger, black, and white cats will not get milk. Solution:  You’ve probably seen this puzzle before, where the challenge was to find which cat gets the milk first.  Some tubes were blocked, so you eliminated those, traced the open paths, and picked the winner. The puzzle's phrasing would lead you to believe there was at least a second cat that gets its share, albeit reduced. Therefore, when the puzzle's task changes to: Which cat doesn’t get milk? , you might conclude that it must be the ginger one, because its tube is blocked, while the remaining black and white cats get smaller portions. However, this is not the case. Even with an endless milk supply, the first cat will get it all, leaving none for the others. Fig. 1 . Four cats compete for the milk poured from a bottle into the network of tubes. We can still provide for black and white cats if we slightly modify the puzzle's conditions. Some minor modifications would redirect some of the milk to the cats' tubes and partially restore justice. Breaking down this problem into two scenarios will help to grasp the basics of fluid dynamics through the puzzle's simple setup. This little riddle isn’t just cute; it’s a neat illustration of how fluid flows in real systems, such as water in plumbing or blood in arteries and veins. Let’s tackle the puzzle in its current form and then proceed to the second scenario. Solving the puzzle as it stands. When milk is poured from the top, it follows the path of least resistance, guided by gravity ( Fig. 1 ). The milk builds up at the bottom of Chamber 1 until it rises to the lowest outlet on the right. This outlet provides a direct path to Chamber 4 , where the milk accumulates again until it reaches the lowest outlet on the right. This right tube delivers the milk directly to the grey cat's mouth. Once milk begins to flow through that route, it never accumulates sufficiently within the network to reach the other branches. The milk finds its first way out and continues using it, ignoring all the tubs on the left and effectively cutting off other cats from the milk supply for good. In this situation, the blocked tube leading to the ginger cat doesn't affect the overall result; it might as well be unblocked. Replacing a bottle with a tap. If you had the misfortune of leaving your bath tap running and flooding your bathroom, you know that the vent holes can't handle a strong, constant flow for too long. This happens because water keeps entering faster than it can leave. In drainage systems, it's all about a balance between the inflow and the outflow rates. The narrow vent pipe can't carry away as much water as the wider main pipe. As a result, the water in the bath rises above the vent and spills out. A steady, pressurized stream from the tap quickly outpaces what those narrow pipes can carry away.   Fig. 2 . Four cats compete for the milk poured from a tap into the network of tubes. This is precisely what we need to do to open the outlets supplying other cats. Replacing the bottle with a tap will dramatically increase the inflow, while the slimmed-down tubes will sharply decrease the outflow. When we pour gently, the puzzle behaves as it did with the bottle: milk drains through the rightmost tube, and only the grey cat drinks. But as we open the tap wider, the inflow begins to exceed what that single outlet can handle. Extra milk raises the level inside the upper chambers, gradually engaging new branches. The first new branch to open is the one leading to the white cat. As the milk level rises inside Chamber 4 , due to the stronger inflow and weaker outflow, it begins to spill into the left outlet leading to the white cat. The moment it happens, the left outlet becomes another drain, so the total outflow is now the sum of two outflows delivering milk to the grey and white cats. If we open the tap wide, the black cat joins in. When the combined drainage of the two tubes can't handle the tap's flow, the level in Chamber 1 rises, causing milk to overflow into Chamber 2 and directly into Chamber 3 . There it begins to build up, slowly rising to the ginger cat tube and spilling through if it were open. But since it's blocked, the level will continue to rise until it reaches the black cat outlet and drips into the last cat's mouth. This staged feeding scenario is a simple demonstration of how the outflow capacity competes with the inflow rate. At a low inflow, only the grey cat receives milk. At a medium inflow, the level rises to activate the white cat’s path. At high inflow, pressure builds enough to feed the black cat. This plumbing system paints an intuitive picture of fluid dynamics at work when multiple outlets compete for the same source. Whether the next cat receives any milk depends on the strength of the pour and the diameter of the tubes, though the first cat will always receive the most. The classic puzzle becomes more intriguing when you think like a physicist and explore all possible scenarios.