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*Spacetime Physics*, by Taylor and Wheeler, the time coordinate is measured in metres of light-travel time, but that's just a roundabout way of saying that they are using the second...or am I missing the point.

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Ok...it took me sometime to realise that [itex]ds^2 = c^2dt^2 - dx^2 - dy^2 - dz^2[/itex] that has the dimensions of length . But now another question came up...why length? Isn't spacetime a a union of space *and* time. Even if we divide the whole expression by [itex]c^2[/itex], we get a dimension of time only.

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George Jones

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Yes. In the first case, everything can be considered to be measured in units of length, e.g., metres, where 1 metre of time is the time taken for light to travel a distance of 1 metre, and, in the second case, everything can be considered to be measured in units of time, e.g., seconds, where 1 second of distance is the distance traveled by light in 1 second of time.neutrino said:But now another question came up...why length? Isn't spacetime a a union of spaceandtime. Even if we divide the whole expression by [itex]c^2[/itex], we get a dimension of time only.

Most relativity books use the former, but I have seen the latter used. In cosmology the latter is often used, i.e., (light)years and years.

Regards,

George

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Yes, I realise that. I'm just wondering why this quantity (ds²), which says something about the unity of space and time does not have a dimension made up of a combination of length and time.George Jones said:Yes. In the first case, everything can be considered to be measured in units of length, e.g., metres, where 1 metre of time is the time taken for light to travel a distance of 1 metre, and, in the second case, everything can be considered to be measured in units of time, e.g., seconds, where 1 second of distance is the distance traveled by light in 1 second of time.

Most relativity books use the former, but I have seen the latter used. In cosmology the latter is often used, i.e., (light)years and years.

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The quantity involved in ds² that "unifies" space and time (namely, the speed of light) has the dimensions of "length/time".neutrino said:Yes, I realise that. I'm just wondering why this quantity (ds²), which says something about the unity of space and time does not have a dimension made up of a combination of length and time.

Similarly, the quantity that "unifies" momentum and energy has the dimensions of "momentum/energy".

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Thanks...now it's all clear.robphy said:The quantity involved in ds² that "unifies" space and time (namely, the speed of light) has the dimensions of "length/time".

Similarly, the quantity that "unifies" momentum and energy has the dimensions of "momentum/energy".

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Ich

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Yes, that's the one. I especially like their spacetime-first approach.Ich said:

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George Jones

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This is the book from which I first learned special relativity. Great book.Ich said:Isn´t "Spacetime Physics" ...

I recommend also "A Traveler's Guide to Spacetime: An Introduction to Special Relativity, which is the book from which I lifted the accelerometer that I used in the "A falling object" thread.

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George

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Here are some useful supplements for Moore's book:George Jones said:This is the book from which I first learned special relativity. Great book.

I recommend also "A Traveler's Guide to Spacetime: An Introduction to Special Relativity, which is the book from which I lifted the accelerometer that I used in the "A falling object" thread.

Regards,

George

http://www.physics.pomona.edu/faculty/prof/tmoore/tgerrors.html [Broken]

http://www.physics.pomona.edu/sixideas/sicpr.html [Broken] (see "unit R")

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Ich

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Yes, the book is a good one (maybe pete would disagree https://www.physicsforums.com/showpost.php?p=737202&postcount=20").

And their point is: c is simply an arbitrary conversion factor from time units to length units. "meter" and "second" are two units where you need only one. Comparable with inches and meters. Two units for the same thing.

The difference between time and space is then not the units, but the metric (-1 1 1 1 instead of 1 1 1 1).

And their point is: c is simply an arbitrary conversion factor from time units to length units. "meter" and "second" are two units where you need only one. Comparable with inches and meters. Two units for the same thing.

The difference between time and space is then not the units, but the metric (-1 1 1 1 instead of 1 1 1 1).

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