GALILEO’S SENSORY AND EMPIRICAL CONCEPTS OF RELATIVITY
The concept of inertia spawned
Galileo’s concept of relativity: the sensory
and empirical equivalence of the uniform rectilinear velocities of
terrestrial bodies, and the accelerated motions that occur on them. As we learned in Chapter 4, the magnitudes of
force, mass and acceleration vary in such a way (covariantly) that
A. Galileo’s Relativity
In Aristotle’s theory of the cosmos, the Earth was at rest at the center, and the Sun, Moon, planets, and stars all rotated around the stationary Earth once each day. (see Figure 2.1A) Aristotle attempted to justify his theory that the Earth was stationary in space with the following rationalization. When a rock is allowed to fall to the ground from a tall tower, it is observed to land at the foot of the tower. If the Earth was moving through space while the rock was falling, then it should land in a much different place. (Gamow, 1961, pp. 42 – 43)
On the other hand, in the 16th century Nicolas Copernicus theorized that the Sun was at the center of the Cosmos, and that the Earth, Moon and planets all rotated around it. (Figure 2.1B) This heliocentric concept was completely at odds with Aristotle’s theory and Christian dogma, and was vehemently opposed by the Catholic Church. In 1632, Galileo wrote a book that supported the Copernican theory, entitled Dialogues on the Two Great World Systems. (French, p. 67)
In his book,
Galileo “argued that the vertical path of a falling object does not compel one
to the conclusion that the earth is stationary.” (
“He gave, by way of analogy, the example of a rock dropped from the top of the mast of a ship. Whether the ship is at rest or moving with a constant velocity, the rock always lands just at the foot of the mast. Thus an observation of the point of impact on a deck reveals nothing of the ship’s state of motion.” (French, p. 67; see Figure 5.1)
By analogy to the above empirical experiment, Galileo argued that the point of impact on the Earth of the rock that falls from a tower reveals nothing of the Earth’s state of motion.
Galileo also argued that the illusion of being at rest that we sense when we stand on the Earth does not compel the conclusion that the Earth is not moving. He then asserted, by way of analogy, the following thought experiment. If an observer is enclosed in a windowless cabin on a large ship, the observer cannot tell from the uniform rectilinear velocity of the ship and from observing the accelerated motions of the objects inside the cabin, whether he is sailing uniformly straight ahead at a constant speed on calm seas, or whether he is docked in port and at rest relative to the Earth’s surface. Galileo’s actual words were:
“so long as the vessel stands still [the motions of material objects take place in a normal manner. Then] make the ship move with what velocity you please, so long as the motion is uniform and not fluctuating in this way and that. You shall not be able to discern the least alteration in all the forenamed effects, nor can you gather by any of them whether the ship moves or stands still...” (Galileo, Dialogues, 1632 [Gamow, 1961, p. 45])
Ergo being on a body that exhibits uniform rectilinear motion, or observing accelerated motions on such body, reveals nothing of the state of motion of such body (i.e. Earth). Why is this true? Because all observers and all other material objects on any body with any uniform rectilinear velocity empirically have exactly the same experience…a sensory illusion of rest. This concept of the sensory equivalence (relativity) of all uniform rectilinear terrestrial motions, and the empirical equivalence of all accelerated motions which occurs on them, is sometimes referred to as Galileo’s principle of relativity.
Galileo’s Relativity demonstrated that all uniform rectilinear velocities of terrestrial bodies (including the Earth itself) are equivalent states of motion with respect to any mechanical events that occur on such bodies. (Figure 5.2) This concept of mechanical relativity is nothing new. Everyone since the dawn of civilization has experienced this same phenomenon of empirical equivalence (relativity). (Bird, p. 51) But Galileo was possibly the first person to specifically describe it.
(vt) that the ship traveled from the port did not matter, nor did the magnitude
of its uniform velocity (v). 
The only thing that mattered was that in each position of the ship (in dock or
sailing away from the dock) the velocity of the ship was uniform and
Galileo’s example also demonstrated that such uniform rectilinear velocities may either be ‘inertial’ (without applied force, like we perceive the motion of the Earth through space to be); or it can result from the application of a force (like the wind uniformly pushing the sailing ship forward). Therefore, Galileo’s concept of the empirical equivalence (or the ‘relativity’) of uniform velocities is not really dependent upon the concept of inertia…uniform rectilinear motion without applied force.
Why is Galileo’s concept of relativity true? Because the uniform motion of the ship is common to the observers and the other material objects relatively at rest on it.  In other words, the ship and its contents all experience the same common uniform velocity. Empirically, the motions of such material objects are the same as when the uniformly moving ship was lying in the harbor at rest relative to the Earth’s surface. (Figures 5.2A and 5.2C)  The different magnitudes of uniform velocity of two moving bodies (the Earth and the ship) are not sensed or perceived by an observer or by any other material object in either situation. All that is perceived by an observer in both equivalent situations is a sensory illusion of rest. Again, the observer and his material objects have exactly the same experience in both situations of perceived rest.
To summarize, the principle of Galileo’s Relativity was comprised of two separate but related concepts:
1. All uniform rectilinear (inertial) velocities of terrestrial bodies are sensorally and empirically equivalent states of motion; and
2. The (covariant) accelerated motions of
terrestrial objects on such inertially moving bodies demonstrate the
Without both concepts, there is no principle of Galileo’s Relativity.
One might ask at this point: What relevance or importance does Galileo’s sensory and empirical concept of relativity have with respect to Einstein’s Special Theory of Relativity? The answer is that Einstein adopted a very different mathematical variation of Galileo’s Relativity (which we shall call “Galilean translational relativity’) as the basic mathematical framework for his Special Theory, and he needed to characterize this very different mathematical variation as merely an extension of Galileo’s Relativity so that his ad hoc mathematical Special Theory might appear to have some semblance of an empirical foundation. (see Chapters 13, 14, and 24) This subterfuge by Einstein was an attempt to disguise the fact that his mathematical Special Theory had no empirical foundation at all and was in fact totally ad hoc.
B. Mathematical Variations of Galileo’s Relativity Concept
years after 1632,
“The motions of bodies included in a given space are the same among themselves, whether that space is at rest, or moves uniformly forward in a right [straight] line…A clear proof of this we have from the experiment of a ship; where all motions happen after the same manner, whether the ship is at rest, or carried uniformly forwards in a right [straight] line.”  (Newton, Principia [Motte, Vol. 1, p. 21])
Very importantly, Newton left out inter alia the sensory illusion of rest which is perceived by the inertial observer, so that he cannot determine whether he is moving or not.
During the 18th
and 19th centuries, the above empirical concept described by
equivalence of every uniformly and rectilinearly moving frame on the Earth
for the same algebraic operation of
There never was a question that Newton’s second law of motion (F = ma) worked the same way and did not change its algebraic form where the uniform velocity of the terrestrial laboratory was common to the inertial motion of the Earth, such as two different acceleration experiments conducted at different locations on the surface of the inertially moving Earth. As Galileo stated in 1632, “no man doubts that, so long as the vessel stands still [on the surface of the Earth, such accelerated motions of objects] ought to take place in [the normal] manner.” (Galileo, Dialogues, 1632 [Gamow, 1961, p. 45])
The only possible remaining question was: Does Newton’s second law of motion work the same way and does it retain the same algebraic form on two different uniformly moving bodies where the different uniform rectilinear velocities are only equivalent? (see Figure 5.3) Thus, by the early 18th century, the only possible purpose of Galileo’s Relativity was to theoretically demonstrate the equivalence (empirical ‘covariance’) of accelerated motions on two different and spatially separated uniformly moving bodies, and thus to theoretically demonstrate the empirical ‘invariance’ of Newton’s second law of motion on all equivalent terrestrial frames with different uniform velocities anywhere on the planet. (Figures 5.2A and 5.2C)
By the latter
part of the 18th century there was no doubt that
mentioned, notably omitted from
For all of the above reasons, during the score of years between 1885 and 1905, Galileo’s simple sensory and empirical principle of relativity underwent a dramatic theoretical and mathematical metamorphosis. It was first modified into an abstract ‘relativistic’ concept of co-moving inertial reference frames by Ludwig Lange in 1885 (Chapter 13), and thereafter mathematically described and applied by what were called the ‘Galilean transformation equations.’ (Chapter 14) In 1904, Lorentz mathematically modified such Galilean transformation equations in a very radical way so that they became ad hoc Lorentz transformation equations, and Poincaré attempted ad hoc to generalize and apply Galileo’s complete concept of Relativity (including the inability of inertial observers to tell whether they were uniformly moving or at rest) to electromagnetics (the constant velocity of light) and optics as well as mechanics. (see Chapter 16)
In 1905, Einstein adopted Lorentz’s ad hoc transformation equations and Poincaré’s ad hoc concepts of relativity and modified, reinterpreted and applied them so that inter alia they described an impossible absolute propagation velocity of light at c, an impossible contraction of matter, an impossible dilation (expansion) of time, an ad hoc increase in mass with velocity, etc., etc. Einstein asserted that he was merely reconciling Galileo’s mechanics concept of relativity with Maxwell’s concept for the constant velocity of light at c, in order to arrive at his Special Theory of Relativity. However, as we will demonstrate in later chapters, the radical principle of relativity that Einstein was referring to had nothing to do with Galileo’s simple sensory and empirical concept of relativity, nor with the algebraic version of relativity, nor with Maxwell’s concepts of light. By the time Einstein was through mathematically fiddling with Galileo’s Relativity, Galileo most likely would not have been able to recognize his own simplistic sensory and empirical concept. Not surprisingly, Galileo’s Relativity, the algebraic version of relativity, Lange’s Relativity, Lorentz’s Relativity, Poincaré’s Relativity, and Einstein’s Special Theory of Relativity are all very different concepts. (see Memo 24.3)
C. Is Galileo’s Relativity a fundamental law of nature?
Galileo’s analogy in favor of the Copernican theory, that the observers in a closed cabin on a uniformly moving ship could not physically sense such motion and that the objects on such uniformly moving ship empirically accelerated in the same manner as when the ship was at rest in port (relative to the Earth), helped Galileo in 1632 to persuade others that the apparently stationary Earth might actually be moving. It remains an interesting phenomenon and relationship for purposes of discussion. However, such analogy was irrelevant to the rest of Galileo’s concepts of relativity and to the relative motions that they described. If the observers went up on deck they would sense the wind in their faces and see the boat moving relative to the water. If they looked through a telescope they would realize that the Earth was moving relative to the other planets. If they constructed a Foucault pendulum on board, they could observe the motion of the Earth around its axis. Thus, we must ask the question: Is Galileo’s Relativity even a fundamental law of motion?
Because of the strict theoretical limitations (i.e. uniform rectilinear velocity) of Galileo’s Relativity, and because it substantially ignores the ubiquitous force of gravity, Galileo’s concept of relativity in any form is really just an approximation of reality and an impossible idealization. Even the Earth does not meet these strict requirements. Its surface is curved, its elliptical path around the Sun is curved and results from gravitational forces, and its elliptical velocity is not uniform. On Earth, the uniform velocity of terrestrial objects is only sustained by a continuous application of force; therefore such velocity is not inertial. For these reasons, Galileo’s Relativity most likely does not even qualify as a fundamental law of mechanics, much less a fundamental law of physics or nature.
If two trains accelerate unequally over the same
circular track, are not their accelerated motions still covariant? Does not
From all of the above discussions,
it becomes apparent that the 17th and 18th century
mechanics concept of Galileo’s Relativity should not be characterized as a
fundamental law of motion, much less a fundamental law of physics or nature. Nor should its mathematical counterparts,
such as the Galilean transformation equations, or Lange’s, Lorentz’s or
Einstein’s modified relativistic models of Galileo’s Relativity, be
characterized as fundamental laws of motion.
Although originally based on a fundamental law (inertia), Galileo’s
concept of relativity remains just a sensory and empirical illusion of rest and
a manmade convention that was convenient for early scientists as an
intuitive theoretical demonstration of the empirical covariance and invariance
Whatever their current relevance, Galileo’s original principle of relativity and its mathematical variations should be confined to the realm of mechanics. Contrary to the assertions of Poincaré and Einstein, the very different phenomena of electromagnetism, radiation, and the constant velocity of light in a vacuum have absolutely nothing to do with inertia, nor with the uniform rectilinear motions of matter, nor with the accelerated motions of matter, nor with mechanics, nor with material frames of reference, nor with coordinate transformation equations, nor with Galileo’s Relativity. (see Chapters 23 and 24)
 For example, the Earth uniformly rotates on its axis each day at a uniform velocity of approximately 0.5 km/s. It more or less uniformly orbits around the Sun at a uniform velocity of about 30 km/s (Born, p. 67), and along with the Sun it more or less uniformly orbits around the core of the Milky Way Galaxy at a uniform velocity of about 225 km/s. (see De Sitter, 1932, p. 95; Hubble, 1942, p. 103) Yet we on Earth do not notice nor distinguish any of these great uniform velocities. (Gamow, 1961, p. 174) “All mechanical events on the earth occur as if this tremendous forward motion did not exist.” (Born, p. 68)
 The velocity of any moving ship (body) may be very different, as long as such velocity is uniform and rectilinear.
 Born and other scientists have asserted that: ‘the root of this [relativity] law is clearly the law of inertia.” (Born, p. 69) But these assertions are not rigorously correct as we have seen from Galileo’s example, and also because inertial motion without applied force is only an idealization (due to the ubiquitous force of gravity). A more correct statement might be that the root of Galileo’s Relativity is uniform rectilinear velocity (v).
 Galileo’s actual words were: “…the cause is that the ship’s motion is common to all the things contained in it and to the air also…” (Galileo, Dialogues, 1632 [Gamow, 1961, p. 45])
 The fact that all inertially (or uniformly and rectilinearly) moving bodies on Earth are equivalent is an absolute terrestrial concept; a constant phenomenon that is the same for every terrestrial observer at sea level. (Goldberg, p. 80) The observer is therefore irrelevant with regard to any attempts to distinguish one uniform terrestrial velocity from another.
 We on Earth share these common uniform velocities with the uniformly moving mother ship Earth. Relative to the Earth and to each other, when we stand on the Earth we are all relatively at rest. As Born put it: “A system of bodies all of which travel through space with the same constant velocity is…at rest as regards their mutual position…” (Born, p. 69)
 Nevertheless, in his 1905 Special Theory, Einstein substituted the constant speed of a ray of light with respect to inertial bodies for the accelerated motions of material objects on inertial bodies, and incorrectly characterized this scenario as Galileo’s Relativity, which of course it was not. (see Chapter 19)
 The reasons why ‘Galilean Translational Relativity’ (usually called the ‘Galilean transformation equations’) is so different from Galileo’s Relativity are described in Chapters 13 and 14.
 For example, “the billiard player in a closed box car of a train moving uniformly along a straight track cannot tell from the behavior of the balls what the motion [uniform velocity] of the train is with respect to the ground.” (Resnick, 1968, p. 13) The train may be going east or west, north or south, at any uniform velocity; or it may be stopped and just sitting on the tracks. The billiard balls exhibit exactly the same laws of mechanics (i.e. force, acceleration, momentum, conservation of momentum, inertial resistance and equal and opposite actions) in all of these equivalent situations.
 Born referred to Galilean Relativity as “the law of the relativity of mechanical events…” (Born, p. 69)
theoretical demonstrations also applied to
 By 1687, the Copernican heliocentric theory of the Solar System was widely accepted. Therefore, Newton (unlike Galileo) did not need to use the sensory and empirical equivalence of uniform rectilinear moving bodies for an observer in order to demonstrate and justify the idea that the apparently stationary Earth could be moving, and thereby to advance, defend and justify the Copernican theory.
 A modern statement of this additional concept “is that no mechanical experiments carried out [or deduced] entirely in one inertial frame can tell the observer [in that frame] what the motion [magnitude of velocity] of that frame is with respect to any other inertial frame.” (Resnick, 1968, p. 13)
 The term ‘relativistic’ means that there are two abstract frames (i.e. ships) in uniform rectilinear motion relative to one another, rather than Galileo’s empirical concept of two positions of the same ship: one position in port and one position moving uniformly away from port.
 Of course the reader should ask the question: What relevance does the constant velocity of light have with respect to the accelerated motions of matter or with respect to inertial motions? The obvious answer is: none.
 Kepler discovered in 1609 that the Earth speeds up when it is near to the Sun, and slows down when it is further from the Sun.
 On the other hand, Einstein attempted to characterize Galileo’s Relativity as a fundamental empirical law of physics and nature, because he needed its empirical qualities in order to justify his Special Theory, and to give it an empirical foundation. (see Einstein, Relativity, pp. 15 – 18; Chapters 23 and 24)