Day 46 : Book Excerpt : Making Starships and Stargates

Ernst Mach, an Austrian physicist of the late nineteenth and early twentieth centuries, is now chiefly known for Mach “numbers” (think Mustang Mach One, or the Mach 3, SR71 Blackbird). But during his lifetime, Mach was best known for penetrating critiques of the foundations of physics. In the 1880s he published a book – The Science of Mechanics – where he took Newton to task for a number of things that had come to be casually accepted about the foundations of mechanics – in particular, Newton’s notions of absolute space and time, and the nature of inertia, that property of real objects that causes them to resist changes in their states of motion.

Einstein, as a youngster, had read Mach’s works, and it is widely believed that Mach’s critiques of “classical,” that is, pre-quantum mechanical, physics deeply influenced him in his construction of his theories of relativity. Indeed, Einstein, before he became famous, had visited Mach in Vienna, intent on trying to convince Mach that atoms were real. (The work Einstein had done on Brownian motion, a random microscopic motion of very small particles, to get his doctoral degree had demonstrated the fact that matter was atomic). Mach had been cordial, but the young Einstein had not changed Mach’s mind.

Nonetheless, it was Mach’s critiques of space, time, and matter that had the most profound effect on Einstein. And shortly after the publication of his earliest papers on General Relativity Theory (GRT) in late 1915 and early 1916, Einstein argued that, in his words, Mach’s principle should be an explicit property of GRT. Einstein defined Mach’s principle as the “relativity of inertia,” that is, the inertial properties of material objects should depend on the presence and action of other material objects in the surrounding spacetime, and ultimately, the entire universe. Framing the principle this way, Einstein found it impossible to show that Mach’s principle was a fundamental feature of GRT. But Einstein’s insight started arguments about the “origin of inertia” that continue to this day. Those arguments can only be understood in the context of Einstein’s theories of relativity, as inertia is an implicit feature of those theories (and indeed of any theory of mechanics). Since the issue of the origin of inertia is not the customary focus of examinations of the theories of relativity, we now turn briefly to those theories with the origin of inertia as our chief concern.

Einstein had two key insights that led to his theories of relativity. The first was that if there really is no preferred reference frame – as is suggested by electrodynamics – it must be the case that when you measure the speed of light in vacuum, you always get the same number, no matter how you are moving with respect to the source of the light. When the implications of this fact for our understanding of time are appreciated, this leads to Special Relativity Theory (SRT), in turn, leads to a connection between energy and inertia that was hitherto unappreciated. The curious behavior of light in SRT is normally referred to as the speed of light being a “constant.” That is, whenever anyone measures the speed of light, no matter who, where, or when they are, they always get the same number – in centimeter-gram-second (cgs) units, 3 10 10 cm/s. Although this works for SRT, when we get to General Relativity Theory (GRT) we will find this isn’t quite right. But first we should explore some of the elementary features of SRT, as we will need them later. We leave detailed consideration of Einstein’s second key insight – the Equivalence Principle – to the following section, where we examine some of the features of general relativity theory.

Mention relativity, and the name that immediately jumps to mind is Einstein. And in your mental timescape, the turn of the twentieth century suffuses the imagery of your mind’s eye. The principle of relativity, however, is much older than Einstein. In fact, it was first articulated and argued for by Galileo Galilei in the early seventeenth century. A dedicated advocate of Copernican heliocentric astronomy, Galileo was determined to replace Aristotelian physics, which undergirded the prevailing Ptolemaic geocentric astronomy of his day, with new notions about mechanics. Galileo hoped, by showing that Aristotelian ideas on mechanics were wrong, to undercut the substructure of geocentric astronomy. Did Galileo change any of his contemporaries’ minds? Probably not. Once people think they’ve got something figured out, it’s almost impossible to get them to change their minds. As Max Planck remarked when asked if his contemporaries had adopted his ideas on quantum theory (of which Planck was the founder), people don’t change their minds – they die. But Galileo did succeed in influencing the younger generation of his day.


~~Making Starships and Stargates- The Science of Interstellar Transport and Absurdly Benign Wormholes -by- James F. Woodward