Einstein’s Special Theory of Relativity (STR) is broadly misunderstood by the public. In most popular science books, relativity theory is hailed for introducing relativity to physics. Whereas Newtonian physics embraces absolute space and time—the narrative goes—Einstein proved that space and time are relative. Fair enough. However, the relativity of space and time is the consequence of STR, not its first principle. The cornerstone of STR is the absolute nature of light or, more precisely, the invariance of the speed of light, which is the same (in a vacuum) in all inertial frames of reference. As Tim Maudlin, a leading contemporary philosopher of physics, writes:
The fundamental feature of the Special Theory is not what it makes relative but what it makes absolute. The speed of light is an invariant quantity under the transformation of inertial frames.
Einstein’s theory of relativity could have been more appropriately titled “the theory of absolute light.” And this absolute light—the light above (spiritual light) and the light below (physical light)—is the subject of this essay.
When we call physical light “absolute,” we mean to say that the light propagates with the same velocity in any inertial frame of reference—that is, in any frame of reference that moves with constant velocity along a straight line. Suppose we were to represent a frame of reference (a physical representation of a vantage point) as a coordinate system (a method of labeling points in space and time). In that case, as is done in STR, the absolute nature of light expresses itself in the fact that the speed of light is the same in any inertial reference frame. This is very counterintuitive.
Suppose you were riding a train and shot a gun in the direction of the train’s movement. As far as you are concerned, the bullet’s velocity would be the proper velocity of such a bullet as it leaves the muzzle of the gun you fired. However, for an observer on the ground alongside the path of the train, the bullet’s velocity would be the sum of the train’s velocity and the bullet’s velocity, in that the moving train gives the bullet a “boost” beyond its own velocity. If, however, you turn on a flashlight, shining it in the same direction you were traveling, the speed of the ray of light would be precisely the same for you (on the train) and for an observer alongside the train track. The speed of light is the same for all observers in all inertial reference frames. You cannot give the light a “boost.” It turns out that the speed of light in a vacuum is a fundamental constant of physics—an absolute.
Kabbalah and Chasidic writings use light as the primary metaphor for the Divine Emanation, given that light shares many of its properties. Light illuminates and warms and is a source of energy. Plants convert sunlight into energy via photosynthesis, and animals and humans receive that energy by eating plants. Without this process, life would be impossible. Therefore, it is not an exaggeration to say that light is a source of life. The same can be said about the Divine Emanation—the ultimate source of life—which sustains, vivifies, and illuminates the physical. Moreover, just as light reflects the properties of its source (in fact, we use spectroscopy to analyze the chemical composition of stars or other sources of light by studying the color spectrum of the light), so too does spiritual light (the Divine Emanation) reflect its source in the Divine. Thus Rabbi Schneur Zalman of Liadi (the Alter Rebbe) stated, ohr mein hameor (“light reflects the Luminary”). For all these reasons, we call the Divine Emanation Ohr (“Light”) or, more specifically, Ohr Ein Sof (“Infinite Light ”).
The prophet states:
For I am the Lord—I do not change. Malachi 3:6
Let us combine this prophecy with the principle formulated by the Alter Rebbe, stating that light reflects the Luminary—its source, that is, G‑d. What follows is that the Divine Light is as immutable as its source—the Divine Light does not change. Isn’t this the most explicit and unambiguous hint at the invariant property of the speed of light? Indeed, the speed of light does not change!
Some Kabbalists objected to calling G-d “Ein Sof” because it might mislead someone into thinking that, while G-d does not have an end, He may have a beginning, which is heresy. Indeed, Chasidic philosophy accepts this objection and only uses the term Ein Sof to refer to infinite light, not to G-d as Luminary (i.e., source of the Divine Light), and certainly not to the essence of G-d. The description of Divine Emanation as Ohr Ein Sof is highly significant because herein lies another hint at the absolute property of physical light.
The expression, Ohr Ein Sof, is sometimes mistranslated as the “Light of the Infinite.” However, the literal translation is “Infinite Light.” This implies that this divine light possesses some properties of infinitude. Indeed, if the divine source of this light is infinite—Ein Sof—according to Alter Rebbe’s principle that light is just as the luminary, and thus light must be infinite as well. Now recall a simple arithmetic property of infinity—any number added to infinity results in infinity, that is, a + ∞ = ∞ (where a is any number and ∞ denotes infinity). Precisely the same formula holds for the speed of light in the Special Theory of Relativity: v + c = c (where v is a velocity of an inertial frame of reference of the observer, and c is the speed of light in vacuum). We see from here that the speed of light plays the role of infinity in STR, and the invariance of the speed of light represents the absolute nature of light.
I don’t know if these ideas inspired Albert Einstein when he postulated the invariance of the speed of light. It is unlikely. What is certain is that the description of light in the STR deepens the parallel between physical light below and Divine Light above, making physical light an even better metaphor for Divine Light than was previously understood.
 An inertial frame of reference is one that moves with a constant velocity along a straight line relative to another inertial frame of reference. This begs the question, What makes the other frame of reference inertial? Is there at least one inertial frame relative to which we can define all other inertial frames? A better way to define an inertial frame of reference is to say that an inertial frame of reference is one in which the First Law of Newton holds true.
 Tim Maudlin, Quantum Non-Locality & Relativity, Metaphysical Intimations of Modern Physics, 3rd ed. (Wiley Blackwell, 2011), p. 40.
 A frame of reference (or reference frame) is a physical concept representing a vantage point from which a particular physical phenomenon (such as the movement of a particle) is considered. A reference frame requires an observer equipped with measuring tools to measure at least space and time. Measurements obtained in one reference frame can be correlated with measurements obtained in another reference frame moving at a certain velocity with respect to the first reference frame. In classical (Newtonian) mechanics, such a correlation can be made using Galilean transformation and, in STR, using Lorentz transformation.
 Contrary to common misconception, reference frames are not coordinate systems. Unlike a reference frame—a physical concept—a coordinate system is an abstract mathematical concept, which has no physical meaning. Coordinate systems are simply a method of labeling points in space. One can assign numbers to points or names. It is similar to labeling houses in a city. In Japan, it is customary to display the name of the house owner written on a wooden plank next to the entrance to the house. However, this is a very cumbersome method that makes it difficult to find a particular house. In most countries, cities have streets (labeled with names or numbers) and the houses have numbers. This makes it easy to identify a house. However, if the street is renamed, or the houses are renumbered, nothing changes in the physical world—houses do not move because someone decided to rename a street. The same is true about labeling points in space. In three-dimensional space, we need three numbers to uniquely identify every point in space. We typically assign these numbers by selecting a coordinate system—three, preferably orthogonal, axes—and choosing the projection of the point on each axis. This choice is arbitrary, just as naming streets of the city are—both are matters of convention. In the Special Theory of Relativity, which concerns itself only with inertial frames of reference corresponding to flat spacetime, no harm is done when using coordinate systems as a proxy for reference frames. In the general theory of relativity, this confusion can lead to serious problems. See my paper, “On the Ontology of Spacetime in a Frame of Reference” (2014), presented at the International Conference on Ontology of Spacetime (https://www.academia.edu/80323856/On_the_Ontology_of_Spacetime_in_a_Frame_of_Reference, retrieved 3/12/2023).
 In the language of mathematics, the speed of light is invariant under Lorentz transformation, that is, under coordinate transformation in Minkowski spacetime.
 Rabbi Schneur Zalman of Liadi, Tanya, Igeres Hakodesh ch. 20; Likutei Torah, Parsha Acharei, p. 24c, Parsha Behar, p. 44d. See also Rabbi Shalom Dovber, Hemshech Samach Vav and Ayn Beis.
 Unlike the Luminary, however, the emanated light has a beginning. However, this does not detract from its infinitude.
Originally published on 03/16/2023 on QuantumTorah.com.