Motion

• 1. Introduction

  ›

  Light moves at 299,792,458 metres per second in a vacuum, and nothing in the universe can outrun it. That single number sets the outer boundary for motion itself. Motion, in physics, is the change in position of an object relative to a reference point over a given span of time. It is also one of the strangest ideas in science, because it is never absolute. Modern physics holds that there is no absolute frame of reference. Isaac Newton's concept of absolute motion cannot be determined at all. So where does that leave the rest of us? If everything in the universe can be considered to be in motion, what does it mean to stand still? How do scientists describe a wave, a galaxy, and a single electron with the same handful of ideas? And how can something appear to move faster than light when nothing truly can? The answers begin with the difference between describing motion and explaining its cause.

• 2. Two Sciences Of Movement

  ›

  Kinematics is the branch of physics that describes how objects move without asking why. It works in displacement, distance, velocity, acceleration, speed, and frame of reference, measuring how a body shifts against a chosen observer's frame as time changes. Dynamics asks the harder question: it studies forces and their effect on motion. Together they let physicists map any moving body. An object that is not in motion relative to a given frame is described many ways: at rest, motionless, immobile, stationary, or holding a constant, time-invariant position with respect to its surroundings. The reach of this vocabulary is vast. Motion applies to objects, bodies, matter particles, matter fields, radiation, radiation fields, radiation particles, curvature, and space-time itself. It applies to images, shapes, and boundaries too. The configuration of a quantum particle, for instance, is the set of probabilities of it occupying specific positions. That probabilistic picture is exactly where classical certainty starts to break down.

• 3. Newton's Three Laws

  ›

  On the 5th of July 1687, Sir Isaac Newton first published Philosophiæ Naturalis Principia Mathematica, and classical mechanics had its foundation. Newton, along with Euler, formulated three laws that describe how forces relate to the motion of a body. The first law, the law of inertia, says an object at rest stays at rest and an object in motion keeps moving in a straight line at constant velocity, unless a net force acts on it. The second law, in an inertial reference frame, sets the vector sum of forces on an object equal to its mass multiplied by its acceleration. If the resultant force is not zero, the body accelerates in the same direction as that force. The third law states that when one body exerts a force on a second body, the second simultaneously pushes back with equal magnitude and opposite direction. These laws did something no earlier model had managed. They gave the first accurate mathematical description of bodies orbiting in outer space, unifying the motion of celestial bodies with the motion of objects on Earth.

• 4. When Newton Runs Out

  ›

  Speeds approaching that of light break the comfortable world Newton described. Classical mechanics gives very accurate results for macroscopic objects moving far slower than light, from projectiles and machine parts to spacecraft, planets, stars, and galaxies. It remains one of the oldest and largest scientific descriptions in all of science, engineering, and technology. Modern kinematics grew out of the study of electromagnetism and measures every velocity against its ratio to the speed of light. Velocity is then read as rapidity, a hyperbolic angle, and acceleration changes that rapidity according to Lorentz transformations. This is special relativity. W. K. Clifford and Albert Einstein worked to fold gravity into this picture, using differential geometry to describe a curved universe with gravity. That study is called general relativity. A different frontier opens at the atomic scale. Quantum mechanics governs molecules, atoms, electrons, protons, neutrons, and smaller particles such as quarks. There, matter and radiation show both wave-like and particle-like behaviour, the wave-particle duality. The Heisenberg uncertainty principle forbids knowing a subatomic particle's location and velocity at the same time. Quantum mechanics even illuminates large-scale effects like superfluidity, superconductivity, smell receptors, and protein structure.

• 5. The Motions We Cannot Feel

  ›

  Humans are in constant motion in ways almost impossible to sense. Beyond obvious limb movements and walking, the body carries hidden motions that only special tools and careful observation reveal. Two reasons keep the largest of these motions invisible. Newton's third law prevents us from feeling motion on a mass we are attached to, and there is no obvious frame of reference to show us we are moving. The Earth spins on its axis, giving the equator an eastward velocity of 0.4651 kilometres per second, and it orbits the Sun at an average of about 30 kilometres per second, completing one orbit in roughly 365 days. The Sun itself circles the dense Galactic Center, carrying every planet and moon with it. Away from the central bulge, typical stellar velocity runs between 210 and 240 kilometres per second. The whole Milky Way moves through space at roughly 600 kilometres per second relative to nearby galaxies, or around 582 kilometres per second when measured against the Cosmic microwave background. Even the ground beneath our feet drifts. Plate tectonics carries continents on convection currents in the mantle, with the Cocos Plate advancing 75 millimetres per year, the Pacific Plate 52 to 69 millimetres per year, and the slow Eurasian Plate near 21 millimetres per year. Beneath the largest of these is the strangest motion of all: the fabric of the universe stretching.

• 6. The Stretching Universe

  ›

  Spacetime, the fabric of the universe, is expanding, so everything in it stretches like a rubber band. This is the most obscure motion of all, because it involves no physical movement but a fundamental change in the universe's nature. Edwin Hubble supplied the primary verification, showing that all galaxies and distant astronomical objects are moving away from Earth. That relationship became known as Hubble's law, the predicted signature of a universal expansion. The expansion stretches the very distances between things rather than carrying them through fixed space. It is a motion you cannot point to, yet it shapes the destiny of every galaxy in view.

• 7. Motion Inside The Body

  ›

  The human heart contracts on a steady rhythm to push blood through the body. In the larger veins and arteries blood travels at roughly 0.33 metres per second, though peak flows in the venae cavae have been measured between 0.1 and 0.45 metres per second. The smooth muscles of hollow organs move too. Peristalsis forces digested food along the digestive tract, and an average passage through the human small intestine runs at 3.48 kilometres per hour. The lymphatic system shifts excess fluids, lipids, and immune products, with lymph creeping through a skin capillary at about 0.0000097 metres per second. Inside each cell, the motion continues at a finer grain. Cytoplasmic streaming carries molecular substances through the cytoplasm, while motor proteins act as molecular motors, walking along substrates such as microtubules. These motors run on the hydrolysis of adenosine triphosphate, ATP, converting chemical energy into mechanical work. Vesicles they propel move at roughly 0.00000152 metres per second. Below even this, the laws of thermodynamics insist that every particle of matter moves randomly so long as the temperature is above absolute zero. That vibrating, colliding motion is what we feel as temperature.

• 8. At The Edge Of Light

  ›

  Inside the atom, electrons occupy a region called the electron cloud around the nucleus. Bohr's model gives them high velocity, faster for larger nuclei. If they orbited in strict paths like planets around the Sun, they would need to exceed the speed of light, which is impossible. So physicists drop that strict picture and treat electrons as particles that capriciously exist within the bounds of the cloud. Protons and neutrons inside the nucleus probably move too, driven by electrical repulsion and angular momentum. The speed of light marks the absolute ceiling. It is the speed of all massless particles and fields in a vacuum and the upper limit for energy, matter, information, or causation. It is also invariant, holding the same value regardless of the observer's position or speed, which makes it a fundamental constant of nature. In 2019, the speed of light was redefined alongside all seven SI base units in what the revision calls the explicit-constant formulation, fixing the metre by setting light's value to exactly 299792458 in metres per second. Yet some motion still appears to break the limit. Bursts of energy along relativistic jets, thought to be ejected by black holes, can show proper motion seeming greater than light. Light echoes can do the same. The illusion comes from the finite speed of light and the shrinking time delay as an object moves toward Earth. The naive calculation overestimates the speed of an approaching object and underestimates a receding one, a reminder that what we see is never quite when it happened.