“When you are courting a nice girl, an hour seems like a second. When you sit on a red-hot cinder, a second seems like an hour. That’s relativity.” ~ Albert Einstein
In 1632, Galileo Galilei stated his principle of relativity: that the laws of physics are the same for all inertial frames. Galilean invariance would underpin Newtonian classical mechanics, which was the mathematic clockwork of classical physics, with an absolute space sharing a universal vector of time.
Galileo and Newton took for granted that reality was circumscribed to the observable dimensions, and that physics could posit properties that were absolute cornerstones of reliance; whence ‘laws of Nature’.
Classical physics modeled the everyday world; a set of rules only for the dimensions we can see. When one’s view veers from the ordinary, onto a path partly paved by Einstein, those rules are rent.
“Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.” ~ Lithuanian mathematician Hermann Minkowski
Einstein’s special relativity confirmed the Galilean relativity that physics laws are inviolable while trashing the Newtonian relativity that space and time are unqualified. Under Einsteinian relativity, only the speed of light is absolute in that it appears the same to any observer, regardless of how fast that observer is traveling. All else is subject to change.
Simply, special relativity put a cap on how fast anything may travel: the speed of light. Time stops there (as it does at the opposite end of the spectrum: in a black hole).
Practically, the speed of light depends upon the medium it travels through. Light can be slowed or bent.
(Even in a vacuum, structuring light can slow it down. While light is usually approximated as plane waves, its structure is considerably more complex.)
Light propagates through transparent materials, such as air or glass, at less than c. The ratio between c and the speed at which light travels in a material (v (phase velocity)) is called the refractive index (n) of the material: n = c / v.
Under special relativity, all uniform motion is relative. Passengers in planes, trains, and automobiles get a visceral sense. But special relativity leads to some counterintuitive predictions for objects traveling at obscene speeds.
1st, relativity of simultaneity: simultaneity is not absolute; instead, it depends upon an observer’s frame of reference. Different observers in relative motion to one another may legitimately disagree as to whether 2 events occurred simultaneously, or one before the other.
2nd, time dilation: that time itself is relative to the relative motion of an observer. In other words, clocks tick at different rates depending on their relative motion. Time dilation has been experimentally demonstrated.
3rd, length contraction: a moving ruler that appears at rest to an observer will measure shorter than otherwise. Length contraction is noticeable when the frame of reference approaches the speed of light.
This principle of relativity is “special” in that it applies only to inertial reference frames. Under special relativity, the maximal speed of light is the only absolute.
As everything is in relative motion, all phenomena are qualified, including space and time. Special relativity posits an adamant bond between space and time: the two are entwined as spacetime. General relativity exposed the mutability of this medium in which existence swims.
“The geometry of spacetime is not given. It is determined by matter and its motion.” ~ Austrian theoretical physicist Wolfgang Pauli
“Gold is a difficult system.” ~ Indian chemist Sourav Pal
Chemistry is mostly concerned with the reactivity of elements, which owes to the number of electrons in the outer shell; the fewer electrons there, the more reactive.
Cesium and gold both have a single electron in their outer shell (the 6th such shell for them). Cesium is the most alkaline of natural elements, and highly reactive: it explodes if dropped in water, and even reacts to ice.
In contrast, gold is stalwart to a fault. Gold does not react to oxygen at any temperature, nor with ozone. Gold is unaffected by most acids and most bases. Hence gold does not tarnish. Special relativity accounts for gold’s stability and its color.
Negatively charged electrons whirl about their atomic nucleus with a speed and tightness corresponding to the intensity of the positively charged protons within, along with the nuclear core’s mass. With 79 protons in gold’s nucleus versus the 55 protons in cesium, and half again as many neutrons, gold’s tightly bound nuclear core has much more pull on its orbiting electrons.
The electrostatic attraction of gold’s nucleus relativistically speeds up, and tucks in, gold’s electrons, making it less reactive, and increasing its light absorption. (The subatomic attraction of gold’s positively charged protons to the negatively charged electrons in orbit both increases the electrons’ speed and increases their mass, causing a relativistic contraction in their orbits because, as an electron’s mass increases, the radius of its orbit with constant angular momentum shrinks proportionately.) Thus, gold soaks up blue light; reflecting the reds and greens which combine into the golden hue we see.
Gold is not the only element under the influence of special relativity. Mercury is another heavy atom, with electrons held close to the nucleus. But the bonds between mercury atoms are weak; hence mercury has a low melting point and is liquid at ambient temperature.
Just as Galilean relativity is a slow-motion approximation of special relativity, special relativity approximates general relativity for weak gravitational fields.
Special relativity is violated by quantum entanglement: simultaneity faster than light. Special relativity applies only within certain dimensional constraints.
“There are really four dimensions, three of which we call the three planes of Space, and a fourth, Time.” ~ English author H.G. Wells in the novel The Time Machine (1895)
Issued by Einstein in 1916, general relativity posits gravity as a geometric property of 4-dimensional (4D) spacetime, based upon the mass of objects. General relativity is a simple theory of gravitation; a generalization of special relativity coupled to Newton’s law of universal gravitation. Under general relativity, objects are deflected when they pass near a massive body, not because of a force per se, but because spacetime itself around the body is warped.
It took a half-century for the implications of general relativity to be appreciated. After an initial burst of excitement following confirmation of the theory – a 1919 announcement of light-bending by gravity – general relativity was ignored for decades. Only during the 1960s did appreciation of this gravitational theory sink in.
Mass is the fundamental physical property of matter. Mass is measured in terms of its inertia: a body’s resistance to change in motion (acceleration).
In everyday parlance mass and weight are used interchangeably, but weight is a measure of inertia within a specific gravitational field. 2 objects with the same mass have different weights on the surface of Earth and the surface of the Moon, as these celestial bodies have their own gravitational pull.
Mass determines the degree to which an object is affected by or generates a gravitational field. Mass creates gravity and is affected by bodies with greater mass.
In providing both the gravitational power that conducts cosmic motion and the sense of solidity at ambient scale, mass is the maestro of materiality; its effects achieved by entropy and inertia.
Though inertial mass and gravitational mass are conceptually distinct, there is no actual difference between them. Repeated experiments since the 17th century demonstrate their identicalness.
Oddly, the concept of mass in general relativity is amorphous and problematic. Mass is a measure of motion, which means that the mass of a physical system is within a reference frame of momentum. Hence, mass is always relative to the system in which it is observed.
Subatomic particle mass is a homonym to the concept understood in classical physics: same word, different meaning. A practical conception of subatomic particle mass is the threshold energy at which a certain particle may appear. Quantum mass has nothing to do with gravity, and is only tangential to inertia, in that subatomic particles hesitate to display themselves until sufficiently prodded energetically.
There are various theories which attempt to explain how mass is generated. None succeed beyond some fancy mathematics that fit within a system of equations. Some mass generation mechanisms (as such theories are called) involve gravity, which presents an insolvable chicken-or-egg problem, as mass and gravity are manifestations of the same thing.
“Space is not a lot of points close together; it is a lot of distances interlocked.” ~ Arthur Eddington
The first evidence that Einstein was on the right track with general relativity came from space. By the end of the 19th century, slight perturbations in Mercury’s orbit indicated that Newton’s gravitation theory was off. What Newton could not account for Einstein could.
Atomic clocks on GPS satellites tick faster than they would on Earth by about 45,900 nanoseconds per day. This is because they experience less gravity out in space.
This uptick is somewhat compensated by satellites orbiting the planet rather than being stationary on it; a slower tick by ~7,200 ns/day, owing to special relativity.
Cosmological objects themselves experience relativistic effects. Earth’s core is 2 1/2 years younger than its crust thanks to gravity. (Relative youth has nothing to do with why Earth’s crust is wrinkly and its core is not.) So too the Sun: its center is some 40,000 years younger than its surface.
“General relativity is a description of the whole universe as a closed system.” ~ Canadian theoretical physicist Lee Smolin
As general relativity predicted, gravity manifests as a curvature of 4D spacetime. To capture this distortion geometrically requires extra dimensionality (ED), an obvious implication Einstein studiously ignored.
The warpage of spacetime caused by an orbiting body is termed the geodetic effect. A feebler distortion, in which a spinning body yanks and twists surrounding spacetime, is the frame-dragging effect. These effects have been verified via satellite observations of Earth.
Though classical in its assumption of the universe as a 4D closed system, general relativity propelled predictions that differed from classical physics, notably the geometry of space, the passage of time, the motion of bodies in gravity-influenced free-fall, and the propagation of light.
“Spacetime tells matter how to move; matter tells spacetime how to curve.” ~ American theoretical physicist John Wheeler
General relativity is not the only relativistic theory of gravity, but it is the most mathematically simple theory to explain the experimental data which validates it.
“General relativity is the basis of our understanding of gravity. But 21st-century work in cosmology and particle theory strengthens the belief that it is an incomplete description.” ~ American astronomer Gary Wegner
In his quest for a unified field theory, Einstein spent the back half of his life trying to incorporate electromagnetism into relativistic spacetime. He never succeeded. An unrelenting bias against higher dimensionality and mediocre mathematics acumen made Einstein ill-suited for the task he had set himself.
William Kingdon Clifford
English mathematician and philosopher William Kingdon Clifford literally worked himself to death, succumbing at 34. In his short life, Clifford blazed an illuminating trail.
“He with great ingenuity foresaw in a qualitative fashion that physical matter might be conceived as a curved ripple on a generally flat plane. Many of his ingenious hunches were later realized in Einstein’s gravitational theory.” ~ Hungarian physicist Cornelius Lanczos
Clifford published in 1870 On the Space Theory of Matter, where he advanced the concept of reality as particles in space, though appearing from a higher dimensionality; matter as non-Euclidean disturbances viewed from a perspective of “flat” (noncurved) 3D space.
Clifford envisioned fields (electric, magnetic, gravitational, et cetera) expressed geometrically, and that particles interacted by means of these fields.
Relatively little was known about the composition of matter at the time, and so Clifford’s explanations lacked sophistication. Working from received wisdom, Clifford presaged the most advanced theories of modern physics.
Among other musings, Clifford developed the notion of consciousness as being formed from a composite of information (“mind-stuff”); and the basis of moral law as being founded upon social interdependence (“tribal self”).
“It is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence.” ~ William Clifford