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- This states that an object will not move or keep moving forever at a constant speed, unless a force acts on it. Let’s explain what this means with a simple example. When a parachutist jumps out of a plane, they accelerate to the ground because the force of gravity is pulling them down.
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Jan 26, 2017 · Einstein's theory of relativity is as simple as it is groundbreaking, but how exactly does it work? Let's take a very quick look.
- Profound implications. Relativity is one of the most famous scientific theories of the 20th century, but how well does it explain the things we see in our daily lives?
- Electromagnets. Magnetism is a relativistic effect, and if you use electricity you can thank relativity for the fact that generators work at all. If you take a loop of wire and move it through a magnetic field, you generate an electric current.
- Global Positioning System. In order for your car's GPS navigation to function as accurately as it does, satellites have to take relativistic effects into account.
- Gold's yellow color. Most metals are shiny because the electrons in the atoms jump from different energy levels, or "orbitals." Some photons that hit the metal get absorbed and re-emitted, though at a longer wavelength.
- Overview
- The mechanical universe
- Light and the ether
relativity, wide-ranging physical theories formed by the German-born physicist Albert Einstein. With his theories of special relativity (1905) and general relativity (1915), Einstein overthrew many assumptions underlying earlier physical theories, redefining in the process the fundamental concepts of space, time, matter, energy, and gravity. Along with quantum mechanics, relativity is central to modern physics. In particular, relativity provides the basis for understanding cosmic processes and the geometry of the universe itself.
(Read Einstein’s 1926 Britannica essay on space-time.)
“Special relativity” is limited to objects that are moving with respect to inertial frames of reference—i.e, in a state of uniform motion with respect to one another such that an observer cannot, by purely mechanical experiments, distinguish one from the other. Beginning with the behaviour of light (and all other electromagnetic radiation), the theory of special relativity draws conclusions that are contrary to everyday experience but fully confirmed by experiments. Special relativity revealed that the speed of light is a limit that can be approached but not reached by any material object; it is the origin of the most famous equation in science, E = mc2; and it has led to other tantalizing outcomes, such as the “twin paradox.”
“General relativity” is concerned with gravity, one of the fundamental forces in the universe. (The others are electromagnetism, the strong force, and the weak force.) Gravity defines macroscopic behaviour, and so general relativity describes large-scale physical phenomena such as planetary dynamics, the birth and death of stars, black holes, and the evolution of the universe.
Britannica Quiz
Physics and Natural Law
Relativity changed the scientific conception of the universe, which began in efforts to grasp the dynamic behaviour of matter. In Renaissance times, the great Italian physicist Galileo Galilei moved beyond Aristotle’s philosophy to introduce the modern study of mechanics, which requires quantitative measurements of bodies moving in space and time. His work and that of others led to basic concepts, such as velocity, which is the distance a body covers in a given direction per unit time; acceleration, the rate of change of velocity; mass, the amount of material in a body; and force, a push or pull on a body.
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The next major stride occurred in the late 17th century, when the British scientific genius Isaac Newton formulated his three famous laws of motion, the first and second of which are of special concern in relativity. Newton’s first law, known as the law of inertia, states that a body that is not acted upon by external forces undergoes no acceleration—either remaining at rest or continuing to move in a straight line at constant speed. Newton’s second law states that a force applied to a body changes its velocity by producing an acceleration that is proportional to the force and inversely proportional to the mass of the body. In constructing his system, Newton also defined space and time, taking both to be absolutes that are unaffected by anything external. Time, he wrote, “flows equably,” while space “remains always similar and immovable.”
Newton’s laws proved valid in every application, as in calculating the behaviour of falling bodies, but they also provided the framework for his landmark law of gravity (the term, derived from the Latin gravis, or “heavy,” had been in use since at least the 16th century). Beginning with the (perhaps mythical) observation of a falling apple and then considering the Moon as it orbits Earth, Newton concluded that an invisible force acts between the Sun and its planets. He formulated a comparatively simple mathematical expression for the gravitational force; it states that every object in the universe attracts every other object with a force that operates through empty space and that varies with the masses of the objects and the distance between them.
The law of gravity was brilliantly successful in explaining the mechanism behind Kepler’s laws of planetary motion, which the German astronomer Johannes Kepler had formulated at the beginning of the 17th century. Newton’s mechanics and law of gravity, along with his assumptions about the nature of space and time, seemed wholly successful in explaining the dynamics of the universe, from motion on Earth to cosmic events.
However, this success at explaining natural phenomena came to be tested from an unexpected direction—the behaviour of light, whose intangible nature had puzzled philosophers and scientists for centuries. In 1865 the Scottish physicist James Clerk Maxwell showed that light is an electromagnetic wave with oscillating electrical and magnetic components. Maxwell’s equations predicted that electromagnetic waves would travel through empty space at a speed of almost exactly 3 × 108 metres per second (186,000 miles per second)—i.e., according with the measured speed of light. Experiments soon confirmed the electromagnetic nature of light and established its speed as a fundamental parameter of the universe.
Maxwell’s remarkable result answered long-standing questions about light, but it raised another fundamental issue: if light is a moving wave, what medium supports it? Ocean waves and sound waves consist of the progressive oscillatory motion of molecules of water and of atmospheric gases, respectively. But what is it that vibrates to make a moving light wave? Or to put it another way, how does the energy embodied in light travel from point to point?
For Maxwell and other scientists of the time, the answer was that light traveled in a hypothetical medium called the ether (aether). Supposedly, this medium permeated all space without impeding the motion of planets and stars; yet it had to be more rigid than steel so that light waves could move through it at high speed, in the same way that a taut guitar string supports fast mechanical vibrations. Despite this contradiction, the idea of the ether seemed essential—until a definitive experiment disproved it.
In 1887 the German-born American physicist A.A. Michelson and the American chemist Edward Morley made exquisitely precise measurements to determine how Earth’s motion through the ether affected the measured speed of light. In classical mechanics, Earth’s movement would add to or subtract from the measured speed of light waves, just as the speed of a ship would add to or subtract from the speed of ocean waves as measured from the ship. But the Michelson-Morley experiment had an unexpected outcome, for the measured speed of light remained the same regardless of Earth’s motion. This could only mean that the ether had no meaning and that the behaviour of light could not be explained by classical physics. The explanation emerged, instead, from Einstein’s theory of special relativity.
- 1895: Running Beside a Light Beam. By this point, Einstein’s ill-disguised contempt for his native Germany’s rigid, authoritarian educational methods had already gotten him kicked out of the equivalent of high school, so he moved to Zurich in hopes of attending the Swiss Federal Institute of Technology (ETH).
- 1904: Measuring Light From a Moving Train. It wasn’t easy. Einstein tried every solution he could think of, and nothing worked. Almost out of desperation, he began to consider a notion that was simple but radical.
- May 1905: Lightning Strikes a Moving Train. Einstein’s revelation was that observers in relative motion experience time differently: it’s perfectly possible for two events to happen simultaneously from the perspective of one observer, yet happen at different times from the perspective of the other.
- September 1905: Mass and Energy. That first paper wasn’t the end of it, though. Einstein kept obsessing on relativity all through the summer of 1905, and in September he sent in a second paper as a kind of afterthought.
Jan 31, 2024 · Einstein's theory of relativity proved to provide solutions to some of the phenomena that scientists had been unable to reconcile with classical physics. Einstein's relativity theory unified the three basic theory complexes: the law of gravity, electrodynamics, and thermodynamics.
For example, a fast-moving person measures time passing more slowly than a relatively stationary one. And a fast-moving object appears shorter than a slow-moving one. Special relativity also produced the most famous equation ever written: E = mc2.
Relativity is the study of how observers in different reference frames measure the same event. Modern relativity is divided into two parts. Special relativity deals with observers in uniform (unaccelerated) motion, whereas general relativity includes accelerated relative motion and gravity.
