Mach's Principle
Through Gravity Objects Far Away Affect How Things Move Nearby
It was Isaac Newton who suggested that space was an absolute backdrop. Newton’s vision of space contained an engraved set of coordinates and he mapped all motions as movements with respect to those coordinates. Ernst Mach disagreed with this though, believing instead that motion was only meaningful if measured with respect to another object, not to Newton’s coordinates. Mach believed that motion ultimately depended on the distribution of matter, or its mass, not on the properties of space itself.
Mass
Simply put, mass is a measure of how much matter an object contains. It was Albert Einstein who proved that energy and mass are interchangeable, that mass can be converted into pure energy.
Inertia
Proposed by Galileo Galilei in the 17th century, the principle of inertia states that if an object is left alone and no forces are applied to it, then its state of motion is unchanged. If the object is moving, it continues to move at the same speed and in the same direction. If the object is standing still then it continues to do so. This principle of inertia was refined by Isaac Newton to form his first law of motion.
Mach Refines Newton’s Bucket
Newton identified that masses attract one another. He used the example of an apple falling from a tree to the ground. The apple fell because it was attracted to the Earth’s mass. The Earth was likewise attracted to the apple’s mass although measuring the microscopic shift in position of the Earth towards the apple would be near impossible. Newton also proved that the strength of gravity decreases quickly as the distance between objects increases.
Newton tried to further understand the relationships between objects and movement by visualising a spinning bucket of water. When the bucket is first turned, the water stays still even though the bucket is moving. Then the water starts to spin too. The surface of the water dips as the liquid moves up the sides of the bucket but it is kept in place by the bucket’s confining force. Newton believed that the water’s rotation could only be understood if seen in the fixed reference frame of absolute space and judged against his coordinates.
Ernst Mach also considered Newton’s bucket argument. He wanted to know if it would be possible to know if the bucket was spinning if the bucket itself was the only thing in the universe. He suggested that it would be equally possible to say that the water was rotating in relation to the bucket. Mach believed that the only way to make sense of it would be to introduce another object into the bucket’s universe.
According to Mach, motion requires external reference objects for it to make sense and that therefore inertia would be a meaningless concept in a universe with just one object in it.
Influence of Mach’s Principle
The ideas of relative versus absolute motion that are expressed in Mach’s Principle have inspired many other physicists. Einstein used the idea that all motion is relative as the basis for his theories of special and general relativity.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
Ernst Mach
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