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J.D. Steens's Reviews > The Meaning of Relativity
The Meaning of Relativity
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In this book, the lay reader pulls out what one can and moves on.
References to “inertia� have been confusing, but maybe this book clarifies it somewhat. Are there three different meanings to this single word - as a reference system, as a resisting force, and as an accelerating force. There’s an inertial frame of reference. Since everything is in motion, and there’s no absolute reference point like Newton’s space and time (General Relativity he says “frees physics from inertial systems� that “singles out certain coordinate systems among all conceivable ones�), then for purposes of observation and measurement an observer’s frame of reference must be specified. Then there’s a body’s inertial property that, per Newton’s first law, wants to remain at rest if at rest, or in motion if in motion (Einstein refers to this as an “inertial force� - a resistance to being moved).
And there’s the equivalence principle whereby inertial mass and gravitational mass are the same thing (“the unity of inertia and gravitation�):* the mass (energy concentration and, thereby, weight?) of a body determines the force necessary to move it (larger the mass, the more force necessary) from its inertial state, and the mass of a body determines the accelerating force it exerts on another body. Seen this way, a mass (of concentrated energy) both resists acceleration, which is its inertial property, and accelerates (pushes or pulls**, which is a body’s gravitational force), and the force of its resistance and acceleration is in proportion to its relative mass. Interestingly, in Newton’s world, the emphasis is on gravity, as it is the cause for movement, not inertia, which is resistance to movement. Inertia is the default position, the natural state of a body were it not for gravity, but this also tends to put inertia into a subsidiary role whereas, with Einstein, and the equivalence principle, you cannot have one without the other: any body has both an accelerating and resisting property.***
This leads to Einstein’s emphasis in this book about what is perhaps his central concept: the gravitational field. The theory of relativity he writes “can be conceived only as a field theory� that “presupposes the independence of the field concept.� Relationships between bodies in space (and time) involve the constancy of the ebbs and flows between bodies of mass influencing each other - in proportion to their respective masses, a body both accelerates (exerts gravitational force) on another body which, in turn, resists (exerts inertial force) gravitational effects. But “field� presupposes what? What lies between the bodies in space (and time), especially when that space is referred to as a “fabric� that is shaped, per Einstein (and Wheeler) by gravitational masses. Is this dark matter and is dark matter just a concentrated (relatively) form of dark energy (with both lying at the poles of matter-energy that doesn’t reveal itself because of its severely dissipated state)?
The field concept in turn leads to Einstein’s overall view of the cosmos. Everything lies in a gravitational field - of varying strengths per the inverse square law - and the so-called “fabric� of space might be conceptualized as “wavy� - concentrated here and there, and stretched out here and there, creating the movement from low to high concentrations of energy (formerly, Newton’s conception of gravity as an attractive force). But, overall, the field concept leads to the notion of a bounded, closed universe (in contrast with what seems to be the prevailing theory today of an unbounded, infinite cosmos), governed by an overall gravitational field that in the end leads to an overall curvature?**** Given the Hubble findings on cosmological expansion and, its reverse conceptualization, which is a tracing backward to a starting point (the big bang origins), if there’s an overall curvature, would this mean that the outward expansion continues around, back to a starting point, and would curvature also mean, that movement is possibly cyclical: movement back to the beginning (big crunch) is but the starting point for another outward expansion (big bang)?
*Einstein refers to the equality of “inert mass� and “gravitational mass.� By a common definition, inert means the lack of internal power to move, in contrast to the accelerating movement caused by gravitational mass. But that notion of “inert� is confusing, the so-called explosive power of the BB is an outward movement (in effect, internal movement) that is consistent with Newton’s law that a body in motion stays in motion (or stays in rest if at rest via it’s resisting, inertial motion) unless an accelerating force acts on it to deviate from its motion. As Einstein describes it: “A material particle upon which no force acts moves, according to the principle of inertia, uniformly in a straight-line.�
**Commonly, gravity is characterized as an attractive force, which is a “pulling� effect of a large gravitational mass. But such a mass traveling through space also, sometimes, collides with other bodies, thereby exerting an accelerating effect that comes from “pushing,� not pulling. So does that mean that gravity is more than an attractive force. And, while on the topic of “pushing,� doesn’t that mean a “pushing away� as the resisting component of an inertial mass that doesn’t want to deviate from its natural, straight-line motion? Pushing then has a dual meaning - this resistance of an inertial mass, plus the pushing effect of a gravitational mass?
***Does this alter the way we understand force? Physicists commonly state that there are four forces - gravity, electromagnetic, and the strong and weak nuclear force. Gravity is also commonly stated as an attractive force, with the implication that there is no resisting (inertial) force, and there’s some confusion about what is meant by forces that push or pull, since gravity only attracts (pulls) but doesn’t resist (push away) With Einstein’s equivalence principle and understanding of gravity, isn’t a body both a gravitational force that accelerates (pushes or pulls other bodies in proportion to its relative mass) and an inertial force that resists (being pulled or pushes itself away) acceleration?
****In countering what he refers to as “quasi-Euclidean� view of the universe, Einstein states that “it is improbable that the mean density of matter in the universe is actually zero,� and hence, he argues “against the conception of a space-infinite, and for the conception of a space-bounded, or closed, universe,� adding that “From the standpoint of relativity, to postulate a closed universe is very much simpler than to postulate the corresponding boundary condition at infinity of the quasi-Euclidean structure of the universe.� But Einstein also writes that “there are finite regions, where, with respect to a suitably chosen space of reference, material particles move freely without acceleration�.� How this matches up with his overall cosmic curvature conception is a question, for, if there is overall curvature due to gravitation, wouldn’t there be acceleration?
Again, is the way force has been defined, muddling. On the cosmic scale, are there not just two forces at work - an outward, expansion that creates movement via the dissipation of heat from high to low, and an inward movement toward a gravitational center, and that both of these forces work within Einstein’s concept of curvature: outward expansion moving in a curvature continues around to become an inward movement toward the gravitational center?
References to “inertia� have been confusing, but maybe this book clarifies it somewhat. Are there three different meanings to this single word - as a reference system, as a resisting force, and as an accelerating force. There’s an inertial frame of reference. Since everything is in motion, and there’s no absolute reference point like Newton’s space and time (General Relativity he says “frees physics from inertial systems� that “singles out certain coordinate systems among all conceivable ones�), then for purposes of observation and measurement an observer’s frame of reference must be specified. Then there’s a body’s inertial property that, per Newton’s first law, wants to remain at rest if at rest, or in motion if in motion (Einstein refers to this as an “inertial force� - a resistance to being moved).
And there’s the equivalence principle whereby inertial mass and gravitational mass are the same thing (“the unity of inertia and gravitation�):* the mass (energy concentration and, thereby, weight?) of a body determines the force necessary to move it (larger the mass, the more force necessary) from its inertial state, and the mass of a body determines the accelerating force it exerts on another body. Seen this way, a mass (of concentrated energy) both resists acceleration, which is its inertial property, and accelerates (pushes or pulls**, which is a body’s gravitational force), and the force of its resistance and acceleration is in proportion to its relative mass. Interestingly, in Newton’s world, the emphasis is on gravity, as it is the cause for movement, not inertia, which is resistance to movement. Inertia is the default position, the natural state of a body were it not for gravity, but this also tends to put inertia into a subsidiary role whereas, with Einstein, and the equivalence principle, you cannot have one without the other: any body has both an accelerating and resisting property.***
This leads to Einstein’s emphasis in this book about what is perhaps his central concept: the gravitational field. The theory of relativity he writes “can be conceived only as a field theory� that “presupposes the independence of the field concept.� Relationships between bodies in space (and time) involve the constancy of the ebbs and flows between bodies of mass influencing each other - in proportion to their respective masses, a body both accelerates (exerts gravitational force) on another body which, in turn, resists (exerts inertial force) gravitational effects. But “field� presupposes what? What lies between the bodies in space (and time), especially when that space is referred to as a “fabric� that is shaped, per Einstein (and Wheeler) by gravitational masses. Is this dark matter and is dark matter just a concentrated (relatively) form of dark energy (with both lying at the poles of matter-energy that doesn’t reveal itself because of its severely dissipated state)?
The field concept in turn leads to Einstein’s overall view of the cosmos. Everything lies in a gravitational field - of varying strengths per the inverse square law - and the so-called “fabric� of space might be conceptualized as “wavy� - concentrated here and there, and stretched out here and there, creating the movement from low to high concentrations of energy (formerly, Newton’s conception of gravity as an attractive force). But, overall, the field concept leads to the notion of a bounded, closed universe (in contrast with what seems to be the prevailing theory today of an unbounded, infinite cosmos), governed by an overall gravitational field that in the end leads to an overall curvature?**** Given the Hubble findings on cosmological expansion and, its reverse conceptualization, which is a tracing backward to a starting point (the big bang origins), if there’s an overall curvature, would this mean that the outward expansion continues around, back to a starting point, and would curvature also mean, that movement is possibly cyclical: movement back to the beginning (big crunch) is but the starting point for another outward expansion (big bang)?
*Einstein refers to the equality of “inert mass� and “gravitational mass.� By a common definition, inert means the lack of internal power to move, in contrast to the accelerating movement caused by gravitational mass. But that notion of “inert� is confusing, the so-called explosive power of the BB is an outward movement (in effect, internal movement) that is consistent with Newton’s law that a body in motion stays in motion (or stays in rest if at rest via it’s resisting, inertial motion) unless an accelerating force acts on it to deviate from its motion. As Einstein describes it: “A material particle upon which no force acts moves, according to the principle of inertia, uniformly in a straight-line.�
**Commonly, gravity is characterized as an attractive force, which is a “pulling� effect of a large gravitational mass. But such a mass traveling through space also, sometimes, collides with other bodies, thereby exerting an accelerating effect that comes from “pushing,� not pulling. So does that mean that gravity is more than an attractive force. And, while on the topic of “pushing,� doesn’t that mean a “pushing away� as the resisting component of an inertial mass that doesn’t want to deviate from its natural, straight-line motion? Pushing then has a dual meaning - this resistance of an inertial mass, plus the pushing effect of a gravitational mass?
***Does this alter the way we understand force? Physicists commonly state that there are four forces - gravity, electromagnetic, and the strong and weak nuclear force. Gravity is also commonly stated as an attractive force, with the implication that there is no resisting (inertial) force, and there’s some confusion about what is meant by forces that push or pull, since gravity only attracts (pulls) but doesn’t resist (push away) With Einstein’s equivalence principle and understanding of gravity, isn’t a body both a gravitational force that accelerates (pushes or pulls other bodies in proportion to its relative mass) and an inertial force that resists (being pulled or pushes itself away) acceleration?
****In countering what he refers to as “quasi-Euclidean� view of the universe, Einstein states that “it is improbable that the mean density of matter in the universe is actually zero,� and hence, he argues “against the conception of a space-infinite, and for the conception of a space-bounded, or closed, universe,� adding that “From the standpoint of relativity, to postulate a closed universe is very much simpler than to postulate the corresponding boundary condition at infinity of the quasi-Euclidean structure of the universe.� But Einstein also writes that “there are finite regions, where, with respect to a suitably chosen space of reference, material particles move freely without acceleration�.� How this matches up with his overall cosmic curvature conception is a question, for, if there is overall curvature due to gravitation, wouldn’t there be acceleration?
Again, is the way force has been defined, muddling. On the cosmic scale, are there not just two forces at work - an outward, expansion that creates movement via the dissipation of heat from high to low, and an inward movement toward a gravitational center, and that both of these forces work within Einstein’s concept of curvature: outward expansion moving in a curvature continues around to become an inward movement toward the gravitational center?
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