
#1
May706, 02:26 AM

P: 2,048

Just what the title says. In the book Spacetime Physics, by Taylor and Wheeler, the time coordinate is measured in metres of lighttravel time, but that's just a roundabout way of saying that they are using the second...or am I missing the point.




#2
May706, 04:59 AM

P: 2,048

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.




#3
May706, 05:50 AM

Mentor
P: 6,044

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 



#4
May706, 10:26 AM

P: 2,048

What is the dimension of the spacetime interval? 



#5
May706, 07:37 PM

Sci Advisor
HW Helper
PF Gold
P: 4,108

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



#6
May806, 02:34 AM

P: 2,048





#7
May806, 03:54 AM

Sci Advisor
P: 1,883

Isn´t "Spacetime Physics" exactly the book where they start with the example of length in northern direction has different units than lenght eastwards, despite both describe the same thing?




#8
May806, 08:36 AM

P: 2,048





#9
May806, 08:44 AM

Mentor
P: 6,044

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 



#10
May806, 09:28 AM

Sci Advisor
HW Helper
PF Gold
P: 4,108

http://www.physics.pomona.edu/facult.../tgerrors.html http://www.physics.pomona.edu/sixideas/sicpr.html (see "unit R") 



#11
May806, 09:43 AM

Sci Advisor
P: 1,883

Yes, the book is a good one (maybe pete would disagree see #20 and #22).
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). 


Register to reply 
Related Discussions  
Spacetime interval?  Introductory Physics Homework  1  
Invariant Spacetime Interval  Special & General Relativity  17  
spacetime interval of zero  Special & General Relativity  15  
Simple Question regarding spacetime interval  Advanced Physics Homework  2  
Spacetime interval formula  what's the d?  General Physics  8 