Mach's Principle

Ernst Mach (1838 – 1916) is best known for inventing the Mach Number, which measures the speed of an object relative to the speed of sound. But he invented a few other things, of which the most notorious in scientific circles is Mach's Principle. Most physics professors get red in the face with apoplexy as soon as it's mentioned. Mach's Principle takes the most sacred mantra of classical physics, Newton's Second Law of Motion, and ascribes it to the collective effect of all the matter in the universe. To a scientist, this sounds like mystical mumbo-jumbo, or holistic New Age nonsense.

There was a time, a few years ago, when the Wikipedia entry on Mach's Principle had a box branding it as "Pseudoscientific" (reproduced at left... sadly for the point I'm trying to make, the box is no longer there). But what's so dangerous about "the principle that the inertia of a body arises from its relation to the totality of all other bodies in the universe"? And just what is "inertia" anyway?

The official line on inertia is that it's a fundamental property of matter that tends to oppose the effect of any externally applied force, such as gravity, to a degree that is proportional to the mass of the object (as quantified by Newton's Second Law of Motion). In the presence of rotation, inertia causes an object to experience centrifugal and Coriolis forces... which orthodox scientists insist are merely "fictitious" forces, not real forces at all. One of the commonest descriptions of Mach's Principle takes the form of a thought-experiment involving centrifugal force. If you stand outside on a clear night with your arms hanging loosely by your sides, the stars above will appear motionless. If you start to spin around, the stars will start to rotate above your head, and your arms will start to lift up. You might think the rotating stars are exerting an "anti-gravity" force on your arms. As I'll show in a moment, there's good reason to believe that this is perfectly and literally correct... although most mainstream scientists would jump in front of a freight train before they'd admit it.

Gravity is a force of attraction that exists between any two masses. The equation describing this force is identical to the equation for the electrostatic force between two electric charges (except that, if both charges have the same sign, the electrostatic force is repulsive rather than attractive). But in the electrical case, if one of the charges is moving relative to the other, there is another force as well. This is the electromagnetic force, which (without going into details) is an inevitable consequence of Einstein's Special Theory of Relativity. But Relativity doesn't just affect electric charges -- exactly the same thing should be true of gravity as well. And it is... except that between everyday objects, the "gravimagnetic" force is much too small to detect.

Ordinary gravity (the "gravistatic" force) obeys an inverse-square law, which means that very massive objects that are a long way away (such as distant galaxies) exert a much smaller force on us than less massive objects that are closer to us (such as the Earth). But the gravimagnetic force obeys an inverse-linear law, which has the opposite tendency... in fact the effect only becomes measurable when you include all the matter out to the edge of the universe. And the effect in question, exactly as predicted by Mach's Principle, is Newton's Second Law of Motion! The supposedly fictitious centrifugal and Coriolis forces are nothing other than the gravitational analogues of well-established electromagnetic forces.

The idea that "inertia is to gravity as magnetism is to electrostatics" was, as far as I know, first put on a quantitative basis by Dennis Sciama in the early 1950s... but it was popularized twenty years later by Eric Laithwaite, Britain's most notorious rebel against the scientific establishment!