Nate Smith

Creation ex nihilo

32 posts in this topic

What bothers me about the whole concept of "size" of the universe, is the yard stick of measurement. What also bothers me is the yardstick of "time" (speed of light) under these circumstances. Just how long does it take for light to travel the width of a universe the size of a foot ball?

Usually what is referred to is the size of the known universe as observed.

A lightyear is a unit of distance, not time. It is the distance that light travels in a 'vacuum' (index of refraction = 1) in one year, which is approximately 186,000 miles/sec * 60 * 60 * 24 * 365 = 5.9 * 10^12 miles = 5,900,000,000,000 miles.

The time in fractions of a second it takes for light to travel the length of a football is the length of football as a fraction of a mile/186,000 miles per second.

I still don't get it. Regardless of the "size" of the universe, doesn't one need to have some yardstick "outside" the universe to speak of size? IOW the idea of "size" makes no sense. Small in comparison to what other aspect of existence I ask?

The size of the observable, known physical universe is a measurement of distance across galaxies within the universe. Every measurement is a measurement within existence of something in existence. You can't measure anything from the perspective of outside existence. Measurements of distances are specified in some number of a unit; distances across galaxies are in terms of some number of a unit (light year) within the universe.

It is true that a light year is a distance. But it is also a measurement of of an event - motion. We know that if light has traveled 186000 miles, what we regard as one second, has elapsed. Since light is supposed to be the only constant, then it must have taken very little time to travel the full distance of this so called 'small' universe.

The light year isn't a measurement of motion, it is a unit of length measurement established in terms of distance and the speed of light. It was selected to be a larger unit than ordinary units like inches and miles so as to be more cognitively appropriate for very large distances (just like you don't use millimeters to specify distances traveled on your vacation). There are many examples of units of measurement defined in terms of physical phenomena not measured by the unit they establish. See Herbert Klein's The Science of Measurement, 1974; Dover reprint 1978, and this thread here on the Forum.

Are we correct in assuming light speed was as we know know it, in the different gravitational conditions billions of years ago?

According to Einstein's general theory of relativity (as opposed to the special theory of relativity which is more limited in scope), the velocity of light outside a local Euclidean coordinate frame of reference depends on the gravitational field: it is not a constant, and its speed depends on direction and can be much greater than the constant c=3*10^8 m/sec = 186,000 mi/sec.

See Max Born's, Einstein's Theory of Relativity, Dover 1962 revision of original 1924 edition. (It ends with a chapter on general relativity and a section on older ideas in cosmology but with no bangs.)

This book may be the best you can find that explains the physics in terms of necessary mathematics, but mathematics selected only for special cases for simplicity of illustration to avoid unnecessary advanced technicalities. (Unfortunately the attempts to replace simple calculus with algebra sometimes makes it more cumbersome, and the approach is even more limited for general relativity than special relativity.)

Born wrote in his 1962 Preface: "[Popular books on relativity] avoid, in general, all mathematical formulae and diagrams and give only descriptions in ordinary language or in philosophical terms -- a way in which, I fear, only a very superficial acquaintance with relativity can obtained... The presentation in [my book] should have an appeal to a considerable number of people, particularly to those who, without knowing higher mathematics and modern physics, remember something of what they learned at school and are willing to do a little thinking."

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What bothers me about the whole concept of "size" of the universe, is the yard stick of measurement. What also bothers me is the yardstick of "time" (speed of light) under these circumstances. Just how long does it take for light to travel the width of a universe the size of a foot ball?

Usually what is referred to is the size of the known universe as observed.

A lightyear is a unit of distance, not time. It is the distance that light travels in a 'vacuum' (index of refraction = 1) in one year, which is approximately 186,000 miles/sec * 60 * 60 * 24 * 365 = 5.9 * 10^12 miles = 5,900,000,000,000 miles.

The time in fractions of a second it takes for light to travel the length of a football is the length of football as a fraction of a mile/186,000 miles per second.

I still don't get it. Regardless of the "size" of the universe, doesn't one need to have some yardstick "outside" the universe to speak of size? IOW the idea of "size" makes no sense. Small in comparison to what other aspect of existence I ask?

The size of the observable, known physical universe is a measurement of distance across galaxies within the universe. Every measurement is a measurement within existence of something in existence. You can't measure anything from the perspective of outside existence. Measurements of distances are specified in some number of a unit; distances across galaxies are in terms of some number of a unit (light year) within the universe.

It is true that a light year is a distance. But it is also a measurement of of an event - motion. We know that if light has traveled 186000 miles, what we regard as one second, has elapsed. Since light is supposed to be the only constant, then it must have taken very little time to travel the full distance of this so called 'small' universe.

The light year isn't a measurement of motion, it is a unit of length measurement established in terms of distance and the speed of light. It was selected to be a larger unit than ordinary units like inches and miles so as to be more cognitively appropriate for very large distances (just like you don't use millimeters to specify distances traveled on your vacation). There are many examples of units of measurement defined in terms of physical phenomena not measured by the unit they establish. See Herbert Klein's The Science of Measurement, 1974; Dover reprint 1978, and this thread here on the Forum.

Are we correct in assuming light speed was as we know know it, in the different gravitational conditions billions of years ago?

According to Einstein's general theory of relativity (as opposed to the special theory of relativity which is more limited in scope), the velocity of light outside a local Euclidean coordinate frame of reference depends on the gravitational field: it is not a constant, and its speed depends on direction and can be much greater than the constant c=3*10^8 m/sec = 186,000 mi/sec.

See Max Born's, Einstein's Theory of Relativity, Dover 1962 revision of original 1924 edition. (It ends with a chapter on general relativity and a section on older ideas in cosmology but with no bangs.)

This book may be the best you can find that explains the physics in terms of necessary mathematics, but mathematics selected only for special cases for simplicity of illustration to avoid unnecessary advanced technicalities. (Unfortunately the attempts to replace simple calculus with algebra sometimes makes it more cumbersome, and the approach is even more limited for general relativity than special relativity.)

Born wrote in his 1962 Preface: "[Popular books on relativity] avoid, in general, all mathematical formulae and diagrams and give only descriptions in ordinary language or in philosophical terms -- a way in which, I fear, only a very superficial acquaintance with relativity can obtained... The presentation in [my book] should have an appeal to a considerable number of people, particularly to those who, without knowing higher mathematics and modern physics, remember something of what they learned at school and are willing to do a little thinking."

You agree that the universe can only be measured from within itself. You also agree that the common measurement of length is determined by the distance light travels in a year. Since you say that "the velocity of light outside a local Euclidean coordinate frame of reference depends on the gravitational field: it is not a constant, and its speed depends on direction and can be much greater than the constant c=3*10^8 m/sec = 186,000 mi/sec."

Can you see why I say that the concept of different size universes can have no meaning without a yard stick. Once again, what yardstick (yardstick = standard of measure) does one use if the speed of light is not constant?

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The unit of length used is a multiple of units you are familiar with: about 5,900,000,000,000 miles. It is based on the constant number c but does not depend on what light is doing under some particular circumstances. The complex physics involved in making any particular distance measurement is a separate problem from what unit the results are expressed in. You can in principle convert any length measurement, including the size of your toenail, to any units of length: feet, light years, furlongs, etc.

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You can in principle convert any length measurement, including the size of your toenail, to any units of length: feet, light years, furlongs, etc.

Here's Admiral Grace Hopper, a computer pioneer and gifted teacher, doing such a conversion to explain what a nanosecond is.

Sorry, but this is the best quality video I could get,

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You can in principle convert any length measurement, including the size of your toenail, to any units of length: feet, light years, furlongs, etc.

Here's Admiral Grace Hopper, a computer pioneer and gifted teacher, doing such a conversion to explain what a nanosecond is.

The point was that all units of length are commensurable among themselves. One does not need a 'yardstick' (or some other material sample of length representing a length unit) to measure the huge distances across galaxies. More advanced principles of physics can be used to infer the distances, but the different units used are related as a multiple of lengths, with the same concept of 'length' for all of them. Employing the speed of light as a means to establish a more appropriate unit of measurement does not alter the concept of distance employed.

There are a lot of common examples showing how to visualize vastly different magnitudes in relation to each other, such as the popular examples of how high (somewhere in outer space) a pile of hundred dollar bills would have to be to pay the national debt (which is why Richard Feynman once said that with the enormous increase in spending and debt in Washington, instead of referring to the debt as "astronomical", distances in space should be called "economical" -- and he made that comment about a half century ago.)

But Hopper's explanation in the utube video cited by Bets of a very small duration of time in terms of the speed of light and a perceivable length of wire is similar to the problem of grasping enormous distances in light-years in terms of ordinary experiences of time.

The matter of how to grasp the relationships of enormous differences in distance were discussed in this thread here on the Forum already referred to above. Watch the video referred to there five years ago as on a DVD -- now it is on

. But read the conceptual discussions on the Forum first.

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The unit of length used is a multiple of units you are familiar with: about 5,900,000,000,000 miles. It is based on the constant number c but does not depend on what light is doing under some particular circumstances. The complex physics involved in making any particular distance measurement is a separate problem from what unit the results are expressed in. You can in principle convert any length measurement, including the size of your toenail, to any units of length: feet, light years, furlongs, etc.

This is not answering my question. To say the universe was once the size of a football is meaningless unless you tell me how it was measured, or could in theory be measured..

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The unit of length used is a multiple of units you are familiar with: about 5,900,000,000,000 miles. It is based on the constant number c but does not depend on what light is doing under some particular circumstances. The complex physics involved in making any particular distance measurement is a separate problem from what unit the results are expressed in. You can in principle convert any length measurement, including the size of your toenail, to any units of length: feet, light years, furlongs, etc.

This is not answering my question. To say the universe was once the size of a football is meaningless unless you tell me how it was measured, or could in theory be measured.

With no humans in existence, nothing can be measured, but characteristics still exist metaphysically. If something has a length it can "in principle" be measured, just like we do now by various means, directly or indirectly.

You don't have to measure something to know that it had a length and what its value was if you can infer it from known principles. That is a different question than empirically validating the principles used. It is also a different question than notions of actual singularities or things coming from "nothing".

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