What I tell laymen who ask me about SR

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Garth said:
It would not be a major part of my lecture but another way of trying to explain the 'meaning of relativity'. I'd first ask what rate does time flow? With the obvious answer one second per second, in order to talk about time dilation you have to compare one clock against another. However if space and time are combined in space time, with one second of time equivalent to 186,000 miles then 'one second per second' can become '186,000m.p.s' or light speed.
Garth
But [tex]d\tau / d\tau[/tex] is not equal to the velocity c, it's equal to the dimensionless number 1. Anyway, I don't see how the notion of spacetime implies that "one second of time is equivalent to 186,000 miles" unless you use the "time = imaginary distance" convention I mentioned earlier...if you measure time and space in different units, then although you are free to pick a unit system where 1 second and 1 light-second have the same numerical value in that unit system, you are also free to pick a unit system where they don't, I don't think there'd be any unit-independent physical truth expressed by the statement "one second of time is equivalent to 186,000 miles".
 
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JesseM - back to you on that reference - The Title of the Book is "Stephen Hawking" by Kitty Ferguson 1991 Cambridge U Press and some other publishers also - at page 100, the notion movement through time and then space is discussed with the aid of a drawing.
 
JesseM said:
But [tex]d\tau / d\tau[/tex] is not equal to the velocity c, it's equal to the dimensionless number 1. Anyway, I don't see how the notion of spacetime implies that "one second of time is equivalent to 186,000 miles" unless you use the "time = imaginary distance" convention I mentioned earlier...if you measure time and space in different units, then although you are free to pick a unit system where 1 second and 1 light-second have the same value in that unit system, you are also free to pick a unit system where they don't, I don't think there'd be any unit-independent physical truth expressed by the statement "one second of time is equivalent to 186,000 miles".
The notion of spacetime implies nothing until you introduce a coordinate system. Once you do choose a coordinate system and assign units of measure to anyone vector, you automatically impose that measurement system upon all other coordinates [dimensionless numbers].
 
yogi said:
JesseM - back to you on that reference - The Title of the Book is "Stephen Hawking" by Kitty Ferguson 1991 Cambridge U Press and some other publishers also - at page 100, the notion movement through time and then space is discussed with the aid of a drawing.
So was the explanation of relativity in terms of "speed through spacetime" Kitty Ferguson's own way of explaining it, or was she recounting something that Hawking had said?
 
JesseM - the book is a sort of interview - part of his life and partly his ideas - there are quotes and chapters on various subjects - I can't say if she embellished upon what was communicated - but the book has been around for a long while - apparently not the one that is being complained about - and if hawking didn't like it he has had 14 years to discredited or qualify what was said.
 
Brian Greene reprises this explanation of SR in "Fabric of the Cosmos." Before using Bart and Lisa Simpson as characters in an imaginary experiment, he says essentially what you said, Crosson, although with some distinctions.

He emphasizes that, because everything moves through space-time at a constant speed, the name for relativity is somewhat misleading. And that in fact Einstein originally wanted to call it "invariance theory." This may be something to interest the layman (although, in my case it has only confused me to the point of wanting to become a physicist just to understand everything... and considering I'm in my senior year of economics, it's not exactly productive to be considering a career change)!

Also, he says that a parked car has devoted all of its constant motion to motion through time. But, when it starts to move it has transferred some of that motion to space and now time will slow down for it, as it has taken some of its motion away from moving through time.
 
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