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oldman
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Does time exist for cosmologists?
Few would deny that time - as we experience it - is a vitally important feature of life and living.
But does time exist for modern cosmology?
There are different ways of gauging time. One measure is age, say that of a horse, gauged with limited accuracy by the state of its teeth (I’ll call this a horse-measure for fun); another is the time of day, say solar time, gauged roughly with a sundial. And telling the time with quartz-oscillator watches reveals that solar time and Greenwich Mean Time (GMT) almost always don’t agree --- for reasons contributors to this forum understand. As precision of gauging increases, subtleties creep into the measuring of time, so important in our lives: relative motion affects the gauging of time, but only to a degree that hardly affects the intervals of time encountered in the everyday life of ordinary folk. The proximity of large masses similarly affects the gauging of time. Such subtleties are in physics described quantitatively with an invented language --- namely mathematics, where abstraction rules.
The judgement of which spatially separate events are simultaneous, what happened in the past, or will occur in the future, then becomes a complex matter described by General Relativity (GR). Such questions are decided by first agreeing how simultaneity is to be defined, then by considering the relative motions of observers (both as in Special Relativity), and lastly by taking into account an observers’s environment of mass/energy with GR. The importance of these factors is small in usual human experience, where simultaneity is defined by what we now see, and where one’s speed, or environment, are relatively unimportant. But accurate experiments and observations show that account must indeed be taken of them.
It has been argued (say by Julian Barbour in his bookThe End of Time) that our best abstract description of the universe, GR, does not require that time exists. In GR, as I understand it, “Now” is represented in an abstract way by hyperplanes in four-dimensional Spacetime, which itself is a sort of 'block-representation' of reality. These sections are not 'plane', but meander across Spacetime like rivers sketched on paper, along paths determined both by an observer’s motion and his/her mass/energy environment. But there are knowledgeable people (my compatriate George Ellis, for example) who believe that a description without time can’t be reconciled with our own experience. I also feel it is too Platonic.
Specifically, consider a simple and practical 'horse-measure' of age that could be used by
cosmologists who accept the current consensus: any such cosmologist --- here or elsewhere in the universe, at any epoch --- could gauge cosmic age simply be measuring (accurately) the temperature of the (almost uniform) cosmic microwave background, which is postulated to be relic radiation from an energetic and compact beginning that permeates the universe. How is the view that time does not have a fundamental existence to be
reconciled with such a simple 'horse measure'? This measure has to do with irreversible physics; ageing and the cooling of the universe, rather than GR!
Few would deny that time - as we experience it - is a vitally important feature of life and living.
But does time exist for modern cosmology?
There are different ways of gauging time. One measure is age, say that of a horse, gauged with limited accuracy by the state of its teeth (I’ll call this a horse-measure for fun); another is the time of day, say solar time, gauged roughly with a sundial. And telling the time with quartz-oscillator watches reveals that solar time and Greenwich Mean Time (GMT) almost always don’t agree --- for reasons contributors to this forum understand. As precision of gauging increases, subtleties creep into the measuring of time, so important in our lives: relative motion affects the gauging of time, but only to a degree that hardly affects the intervals of time encountered in the everyday life of ordinary folk. The proximity of large masses similarly affects the gauging of time. Such subtleties are in physics described quantitatively with an invented language --- namely mathematics, where abstraction rules.
The judgement of which spatially separate events are simultaneous, what happened in the past, or will occur in the future, then becomes a complex matter described by General Relativity (GR). Such questions are decided by first agreeing how simultaneity is to be defined, then by considering the relative motions of observers (both as in Special Relativity), and lastly by taking into account an observers’s environment of mass/energy with GR. The importance of these factors is small in usual human experience, where simultaneity is defined by what we now see, and where one’s speed, or environment, are relatively unimportant. But accurate experiments and observations show that account must indeed be taken of them.
It has been argued (say by Julian Barbour in his bookThe End of Time) that our best abstract description of the universe, GR, does not require that time exists. In GR, as I understand it, “Now” is represented in an abstract way by hyperplanes in four-dimensional Spacetime, which itself is a sort of 'block-representation' of reality. These sections are not 'plane', but meander across Spacetime like rivers sketched on paper, along paths determined both by an observer’s motion and his/her mass/energy environment. But there are knowledgeable people (my compatriate George Ellis, for example) who believe that a description without time can’t be reconciled with our own experience. I also feel it is too Platonic.
Specifically, consider a simple and practical 'horse-measure' of age that could be used by
cosmologists who accept the current consensus: any such cosmologist --- here or elsewhere in the universe, at any epoch --- could gauge cosmic age simply be measuring (accurately) the temperature of the (almost uniform) cosmic microwave background, which is postulated to be relic radiation from an energetic and compact beginning that permeates the universe. How is the view that time does not have a fundamental existence to be
reconciled with such a simple 'horse measure'? This measure has to do with irreversible physics; ageing and the cooling of the universe, rather than GR!